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Indicator mineral methods in mineral exploration


Workshop 21 August 2011

25th International Applied Geochemistry Symposium 201122-26 August 2011 Rovaniemi, Finland

Beth McClenaghan, Vesa Peuraniemi and Marja Lehtonen

Publisher: Vuorimiesyhdistys - Finnish Association of Mining and Metallurgical

Engineers, Serie B, Nro B92-4, Rovaniemi 2011

McClenaghan, B., Peuraniemi, V. and Lehtonen, M. 2011. Indicator mineral methods in mineral explora-tion. Workshop in the 25th International Applied Geochemistry Symposium 2011, 22-26 August 2011 Ro-vaniemi, Finland. Vuorimiesyhdistys, B92-4, 72 pages.

Cover – Irma VarrioContent – Elizabeth Ambrose

ISBN 978-952-9618-70-5 (Printed) ISBN 978-952-9618-71-2 (Pdf)ISSN 0783-1331

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Printed in:Painatuskeskus Finland Oy, Rovaniemi


Beth McClengahan1, Vesa Peuraniemi2 and Marja Lehtonen3

1 Geological Survey of Canada, E-mail: [email protected] University of Oulu, E-mail: [email protected] Geological Survey of Finland, E-mail: [email protected]

Abstract

This one-day workshop builds on the success of the indicator mineral workshop held at the 25th International Applied Geochemistry Symposium. This workshop will review principles, methods, and developments in the application of indicator mineral methods to mineral exploration. In the 1970s and 1980s, indicator mineral methods were used mainly for tin and tungsten and gold prospecting. Since playing a key role in the discovery of the Lac de Gras diamond field in northern Canada in the 1990s, indicator mineral methods have risen in prominence. The scope of the method has expanded, and the range of commodities being sought has broadened to include base and rare metals, as well as diamond and precious metals. The workshop consists of presentations by some of the most experienced practitioners in the field. Indicator mineral methods applied to exploration for gold, porphyry Cu, rare metals, magmatic Ni-Cu and diamonds will be presented as well as heavy mineral recovery methods, the application of Fe oxide mineral chemistry, and the value of sulphides and sulphate minerals.

Workshop 3 Program Sunday, 21 August 2011, Hotel Santa Claus, Rovaniemi

8:30 - 8:45 am Registration

8:45 - 9:00 am IntroductionBeth McClenaghan and Vesa Peuraniemi (Conveners)9:00 - 9:30 am Sample processing methods for recovery of indicator minerals from surficial sediments, Beth McClenaghan, Geological Survey of Canada, Canada

9:30 - 10:00 am Indicator minerals in diamond exploration: a case study from eastern Finnmark, Arkhangelsk and the Devonian Belt (Estonia, Lithuania, Novgorod and Pskov), Pavel Kepezhinskas, Kimberlitt AS, Norway

10:00 - 10:30 am Exploring for RE and REE mineralization using indicator minerals Marja Lehtonen, Geological Survey of Finland, Finland

10:30 - 10:45 am COFFEE BREAK

10:45 - 11:15 am Placer gold microchemistry in conjunction with mineralogy and mineral chemistry of heavy mineral concentrates to characterise bedrock sourcesNorman Moles, University of Brighton, UK

11:15 - 11:45 pm Applicability of sulphide and sulphate minerals in heavy mineral studies, Vesa Peuraniemi, University of Oulu, Finland

11:45 - 1:00 pm LUNCH

1:00 - 1:30 pm Application of iron-oxide discriminant plots in mineral exploration, Georges Beaudoin, Université Laval, Canada

1:30 - 2:00 pm Porphyry Cu indicator minerals in glacial till samples around the giant Pebble Porphyry Cu-Au-Mo deposit, Alaska,Bob Eppinger, US Geological Survey, USA

2:00 - 2:30 pm Heavy mineral assemblages in Devonian sandstones and Quaternary sediments in Latvia, Vija Hodireva, University of Latvia, Latvia

2:30 - 2:45 pm COFFEE BREAK

2:45 - 3:15 pm Kimberlite indicator mineral based diamond exploration programmes in Karelia, North-West Russia, Vladimir Ushkov, Karelian Geological Expedition, Russia

3:15 - 3:45 pm Till indicator mineral and geochemical signatures of magmatic Ni-Cu deposits, Thompson Nickel Belt, central Canada, Beth McClenaghan, Geological Survey of Canada

3:45 - 4:00 pm Discussion and closing remarks

Content

IntroductionBeth McClenaghan & Vesa Peuraniemi

Overview of processing methods for recovery of indicator minerals from sediment and bedrock samplesM. Beth McClenaghan 1

Indicator minerals in diamond exploration: A case study from eastern Finnmark, Arkhangelsk and the Devonian Belt (Estonia, Lithuania, Novgorod and Pskov)Pavel Kepezhinskas 7 Exploring for RE and REE mineralization using indicator minerals Marja Lehtonen, Jukka Laukkanen & Pertti Sarala 13

Placer gold microchemistry in conjunction with mineralogy and mineral chemistry of heavy mineral concentrates to characterize bedrock sourcesNorman Moles & Rob Chapman 19

Applicability of sulphide and sulphate minerals in heavy mineral studiesVesa Peuraniemi & Tiina Eskola 27

Application of iron-oxide discriminant diagrams in mineral explorationGeorges Beaudoin, Céline Dupuis, Beth McClenaghan, Jennifer Blain & Isabelle McMartin 35

Exploration case study using indicator minerals in till at the giant Pebble porphyry Cu-Au-Mo deposit, southwest Alaska, USARobert G. Eppinger, Karen D. Kelley, David L. Fey, Stuart A. Giles & Steven M. Smith 41

Heavy mineral assemblages in Devonian sandstones and Quaternary sediments in LatviaVija Hodireva & Denis Korpechkov 49

Kimberlite indicator mineral-based diamond exploration program in Karelia, northwest RussiaVladimir Ushkov 57

Indicator mineral signatures of magmatic Ni-Cu deposits, Thompson Nickel Belt, central CanadaM.B. McClenaghan, S.A. Averill, I.M. Kjarsgaard, D. Layton-Matthews & G. Matile 67

Indicator Mineral Methods in Mineral Exploration, Workshop 3, 1-6. 25th International Applied Geochemistry Symposium

The concentration of heavy minerals and recovery of indica-tor minerals from surficial sediment is one of the oldest explo-ration methods, being first applied to stream sediments inRoman times. The application of indicator mineral methods tomineral exploration has expanded and developed significantlyover the past two decades. They are now used around theworld to explore for a broad spectrum of deposit types includ-ing kimberlites (diamonds), lode gold, magmatic Ni-Cu-PGE,metamorphosed volcanogenic massive sulphides (VMS), por-phyry Cu, uranium, tin, tungsten, and rare metals (e.g. Averill2001). Indicator minerals, including ore, accessory and alter-ation minerals, are usually sparsely distributed in their hostrocks. Indicator minerals may be sparser in derived sediments,thus sediment samples must be concentrated in order torecover and examine them. Most indicator minerals have amoderate to high specific gravity, thus most processing tech-niques concentrate indicator minerals using some method ofdensity separation, often in combination with sizing and/ormagnetic separations. The presence of specific indicator min-erals in unconsolidated sediments provides evidence of abedrock source in the provenance region and in some cases,the chemical composition of the minerals is associated withthe ore grade of the bedrock source. As few as one sand-sizedgrain of a particular indicator mineral in a 10 kg sample may besignificant. To recover such potentially small quantities (equiv-alent to ppb) of indicator minerals, samples are processed toreduce the volume of material that must be examined. Inreducing the volume of material, processing techniques mustbe able to retain the indicator mineral(s) and do so withoutcontaminating the sample, without losing indicator minerals,and at a reasonable cost.

Indicator minerals can be recovered from a variety of sam-ple media, including stream, alluvial, glacial, beach or eoliansediments and residual soils. They are also recovered fromweathered and fresh bedrock as well as mineralized float. Thecombinations of processing techniques used by explorationcompanies or government agencies for recovering indicatorminerals are quite variable (e.g. Gregory & White 1989;Peuraniemi 1990; Davison 1993; Towie & Seet 1995; Chernetet al. 1999; McClenaghan et al. 1999). These workshop notes area summary of a more detailed paper describing common pro-cessing methods used to reduce sample weight, concentrateheavy minerals, and recover indicator minerals (Fig. 1)(McClenaghan in press). The methods used will depend on thecommodities being sought as well as cost per sample. Mostoxide and silicate indicator minerals are medium to coarse sandsized (0.25 to 2.0 mm). Thus, concentration techniques thatrecover the sand-sized heavy minerals can be used. In contrast,approximately 90% of gold grains, platinum group minerals(PGM) and sulphide minerals are silt sized (<0.063 mm), thusconcentration of these indicators requires a preconcentration

technique that includes recovery of the silt- as well as the sand-sized fractions.

SAMPLE WEIGHTThe weights of material collected for indicator mineral stud-

ies will depend on the type of surficial sediment collected, thegrain-size characteristics of the sample material, the commod-ity being sought and shipping costs (Table 1). For example, inglaciated terrain clay-rich till samples may be as much as 20 to30 kg (or more) in order to recover a sufficient weight of sand-sized heavy minerals (Table 2 - #5) (e.g. Spirito et al. 2011).Coarse-grained silty sand till, typical of shield terrain, requiressmaller (10 to 15 kg) samples because it contains more sand-sized material in the matrix (Table 2- # 1 to 4) (Spirito et al.2011). Alluvial sand and gravel samples collected for recoveryof porphyry Cu indicator minerals (PCIM) need only be ~0.5kg because porphyry Cu alteration systems are large and rich inindicator minerals (Averill 2007). Bedrock and float samplesusually vary from 1 to 10 kg.

Overview of processing methods for recovery of indicator minerals from sediment and bedrock samples

M. Beth McClenaghanGeological Survey of Canada, 601 Booth Street, Ottawa, Ontario, Canada K1A 0E8

(email: [email protected])

Final concentration

Lithology examination>2 mm or >1 mm

Indicator Mineral Identification:

Bulk sediment sample:10 to 40 kg

Heavy Fraction:Ferromagnetic

Separation

Nonferromagnetic fraction:Dry sieve to

specific size fractions

Sieve <2 mm or <1mm

Preconcentrationjig, spiral, shaking table, Knelson Concentrator,

DMS, or pan

3 kg split for geochemical analysis

& archive

heavy liquids, and/ormagstream

Magnetic fraction:Examine or store

2. Chemical analysis: electron microprobe, LA-ICP-MS, Ion microprobe

Indicator mineral selection

Store under-/over-sized fractions

1. Examine & photograph

< 2mm

Fig. 1. Generalized flow sheet showing the steps in sample process-ing that are used to reduce sample weight, concentrate heavy miner-als, and recover indicator minerals.

2 M.B. McClenaghan

BEDROCK PREPARATIONBedrock or float (mineralized boulders) samples often need tobe disaggregated or crushed prior to processing to reduce rockfragment/mineral grain size to <2 mm. Electric pulse disag-gregation (EPD) using an electric current from a high-voltagepower source in a water bath is an efficient means of liberatingmineral grains from rock (Cabri et al. 2008). The major advan-tage of this method is that individual mineral grains can berecovered in their original shape and form regardless of grainsize. Conventional rock crushers may also be used, however,they are more difficult to clean between samples and thus posea higher risk of cross contamination, often break rock frag-ments across grain boundaries, and mark/damage grains asthey are liberated. Barren quartz should be disaggregated orcrushed as a blank between routine rock samples to reduce andmonitor contamination.

PRECONCENTRATIONIf sample shipping costs are an issue, samples may be partlyprocessed in the field to reduce the weight of material shipped

to the processing laboratory. Samples may be sieved to removethe coarse (>1 or >2 mm) fraction, which may reduce weights by a few % to 30% (e.g. Table 2-columns B-C).Preconcentrating, using a pan, jig, sluice box or Knelson con-centrator, also may be carried out in the field to further reducethe weight of material to be shipped. Preconcentrates may beexamined in the field, significantly reducing the time to obtainresults for follow up. However, preconcentraing in the field canitself be expensive and time consuming and the available meth-ods may not provide optimal recovery of the indicator miner-als of interest. Field setup of concentrating equipment may bemore rudimentary than at the processing laboratory, thus extracare is required to avoid cross contamination or material lostduring the pre-concentration procedure completed in the field.

Whether sieved off in the field or in the laboratory, thecoarse >2 mm fraction may be examined to provide additionalinformation about sample provenance and transport distance.The <2 (or <1 mm) fraction is preconcentrated most com-monly using sieving and/or density methods (e.g. jig, shakingtable, spiral, dense media separator, pan, Knelson concentra-

Target

Typical Sample Weight (kg)

Table Micropan Ferro-magneticseparation?

Ferro-magneticseparation?

A. Sediment SamplesGold 10 Single Yes 3.3 YesKimberlite 10-30 Double No 3.2 YesMassive sulphides(Ni-Cu-PGE, BHT, VMS,IOCG, MVT, skarn)

10 SingleYes

(PGMonly)

3.2 Yes

Porphyry Cu 0.5 No No 2.8, 3.2 YesUranium 10 Single Yes 3.3 YesHeavy mineral sands(grade evaluation) 20 Triple No 3.3 Yes

Tampering(investigation) Variable Optional Yes 3.3 Yes

B. Rock Samples

Gold, PGE, base metals 1 Optional Yes 3.3 YesKimberlite 1-10 Optional No 3.2 YesTampering(investigation) 1 No Yes 3.3 Yes

Required Separations

latot:AerutxeTnoitacoLelpmasthgiew

)gk(

:Bthgiew

mm2>stsalC

)gk(

thgiew:Ctupelpmas

ssorcagnikahs

)gk(elbat

thgiew:Delbatgnikahs

etartnecnoc)g(decudorp

thgiew:EyvaeHdiuqiLthgiL

)g(noitcarf

thgiew:Fcitengam

noitcarf)g(

-nonthgiew:Gyvaehgam

larenim)g(etartnecnoc

mm0.2-52.0

9.744.635.4019.51010.210.30.51llitdnasytlistleBiNnospmohT.19.810.316.2041.52114.96.50.51llitydnasyrubduS.21.822.58.9131.3535.93.28.11llitdnasytlispmacdloGsnimmiT.38.530.220.7737.8346.82.18.9llitdnasytlisetilrebmikBelpirT.45.116.52.532,10.703,10.564.24.76llityeyalcatreblAnrehtroN.5

Table 1. Examples of variation in sample weight and processing procedures with sample and target type at Overburden Drilling ManagementLtd.’s heavy mineral processing laboratory (Averill & Huneault 2006).

Table 2. Weight of each fraction generated by a combination of tabling and heavy liquid separation to reduce till sample weight, concentrateheavy minerals, and recover indicator minerals: A) initial sample weight; B) sieving off <2 mm; C) & D) tabling; E) heavy liquid separation; F) magnetic separation; G) final heavy mineral concentrate weight. Till samples are from 1) the South Pit of the Thompson Ni Mine, Thompson,Manitoba; 2) Broken Hammer Cu-PGE deposit, Sudbury, Ontario; 3) Pamour Mine, Timmins, Ontario; 4) Triple B kimberlite, Lake Timiskamingfield, Ontario; and 5) Buffalo Head Hills, northern Alberta.

3Overview of processing methods for recovery of indicator minerals from sediment and bedrock samples

tor) to reduce the weight of material to be examined withoutlosing indicator minerals. Some of the more common precon-centration techniques are described below.

PanningPanning is the oldest method used to recover indicator miner-als, primarily for gold and PGM. Sediment is placed in a panand shaken sideways in circular motion while being held justunder water; heavy minerals sink to the pan bottom and lightminerals rise and spill out over the top (Silva 1986; English etal. 1987; Ballantyne & Harris 1997). Pans are of various shapes(flat bottomed or conical) and sizes, and can be made out ofplastic, metal or wood. The advantages of this techniqueinclude that it can be a field or laboratory-based operation, isinexpensive in terms of equipment costs, and if used in thefield it reduces sample shipping weight and thus cost. Panningis often used in combination with other preconcentrationmethods to recover silt-sized precious metal grains (e.g. Grantet al. 1991; Leake et al. 1991, 1998; Ballantyne & Harris 1997;Wierchowiec 2002). The disadvantages of this method are thatit is slow, is highly dependent on the experience and skill of theoperator and therefore requires consistent personnel to per-form the panning. It is considered to be a rough concentratingmethod when used in the field and is followed up with furtherlaboratory-based concentration techniques (Stendal &Theobald 1994).

Tabling Preconcentration using a shaking (Wilfley) table is one of theoldest methods for concentrating and separating heavy miner-als on the basis of density. It recovers silt- to coarse sand-sizedheavy minerals for a broad spectrum of commodities includ-ing diamonds, precious and base metals, and uranium (Averill& Huneault 2006). A brief description of the method is sum-marized below from Sivamohan & Forssberg (1985), Stewart(1986), and Silva (1986).

The table consists of a deck covered with up to 1 cm highriffles covering over half the surface. A motor mounted on oneend drives a small arm that shakes the table along its length. Aslurry of <2.0 mm sample material is put across a shaking tableto prepare a preconcentrate. If kimberlite indicators are tar-geted, the sample is tabled twice to ensure higher recovery ofthe key lower density minerals (Cr-diopside and forsteriticolivine) and the coarsest grains. The advantages of this methodare its moderate cost, ability to recover indicator minerals for abroad spectrum of commodities, and ability to recover silt- aswell as sand-sized indicators. It is a well established method forthe recovery of precious metal mineral grains as well as kim-berlite indicator minerals (e.g. English et al. 1987; McClenaghanet al. 1998; 2004). The disadvantages of this method includethe loss of some coarse heavy minerals during tabling, thelonger time required to process each sample, and that thetabling procedure is dependent on the skill of the operator.

Dense Media SeparatorA gravity method used to preconcentrate kimberlite indicatorminerals is the micro-scale dense media separator (DMS). Anoverview of this method is summarized below fromBaumgartner (2006).

Heavy mineral concentration is carried out using a gravity-fed high-pressure cyclone. The <1 mm fraction of a sample ismixed with fine-grained ferrosilicon (FeSi) to produce a slurrythat has a controlled density. The slurry is fed into a cyclonewhere the grains travel radially and helically, forcing the heav-ier particles toward the wall of the cyclone and the lighter par-ticles toward the centre. The lighter and heavier particles exitthe cyclone through different holes, with the light fraction dis-carded and the heavy fraction collected on a 0.25 or 0.3 mmscreen. The heavy mineral concentrate is collected on a 0.25mm screen and is then dried and screened to remove residualFeSi. A Tromp curve is used to define the efficiency and preci-sion of the DMS separation. The DMS is calibrated to recoverthe common kimberlite indicator minerals that have a specificgravity (SG) >3.1 : pyrope garnet, chrome-spinel, Mg-ilmenite,Cr-diopside, forsteritic olivine and diamond. It is tested usingsynthetic density tracers before processing samples. The den-sity settings and cut points are checked once per day. Theadvantages of the micro DMS system are that it is fast, less sus-ceptible to sample contamination than other heavy mineralconcentrating techniques and not operator dependent. Themethod, however, is more expensive than other methodsdescribed here and it does not allow for the recovery of silt-sized precious and base metal indicator minerals.

Knelson concentratorThe Knelson concentrator is a fluidized centrifugal separatorthat was originally designed for concentrating gold and plat-inum from placer and bedrock samples. However, in recentyears it has also been used to recover kimberlite indicator min-erals from sediment samples (e.g. Chernet et al. 1999; Lehtonenet al. 2005). The concentrator can handle particle sizes from>10 microns up to a maximum of 6 mm. The general pro-cessing procedure is summarized below from the Knelson con-centrator website (http://www.knelsongravitysolutions.com).

Briefly, water is introduced into a concentrate cone througha series of holes in rings on the side of the cone. The sampleslurry is then introduced into the concentrate cone from a tubeat the top. When the slurry reaches the bottom of the cone, itis forced outward and up the cone wall by centrifugal forcefrom the spinning cone. The slurry fills each ring on the insideof the cone wall to capacity to create a concentrating bed. Highspecific gravity particles are captured in the rings and retainedin the concentrating cone. At the end of the concentrate cycle,concentrates are flushed from the cone into the sample collec-tor. The advantages of the Knelson concentrator are that it isfast, inexpensive, and can be mobilized to the field to reducethe weight of material to be shipped to the laboratory.However, recovery of kimberlite indicator minerals from silt-poor material, such as esker sand or stream sediments, is diffi-cult due to the absence of fine-grained material to keep theslurry in suspension (Chernet et al. 1999). Knelson concentra-tors are optimal for recovery of gold and PGM.

Spiral concentratorHeavy minerals can be recovered using a rotary spiral concen-trator that consists of a flat circular stainless steel bowl withrubber ribs that spiral inward, a detailed description of whichis reported by Silva (1986). A spiral concentrator is mountedon a frame so it can be tilted and has a water wash bar extend-

4 M.B. McClenaghan

ing laterally from one side of the bowl to the centre. As thebowl spins, water is sprayed from the bar and heavy mineralgrains move up and inward along the spirals to the centralopening where they are collected in a container behind bowl.Water washes light minerals down to the bottom bowl. Theheaviest minerals are recovered first. The advantages of thespiral concentrator are that it can be field based and thusreduce sample weight to be shipped, it is inexpensive to acquireand operate, it is fast if the material is sandy, and it recoversindicator minerals across a broad size range from silt- to sand-size grains. The method, however, is dependent on experienceand skill of the operator, the lower density threshold is vari-able, there is some loss of heavy minerals and the method isslow if the sample is clay-rich. It is used mainly for gold recov-ery (e.g. Maurice & Mercier 1986; Silva 1986; Sarala et al. 2009)but in the past 10 years it also has been used for the recoveryof kimberlite indicator minerals (e.g. Sarala & Peuraniemi2007).

JigsJigging is one of the oldest gravity concentration methods andseparates heavy minerals based on differential settling veloci-ties of mineral grains in water (Stendal & Theobald 1994).Jigging is performed by hand or by mechanically jerking a par-tially filled screen of material up and down underwater for sev-eral minutes. While submersed in water, mineral grains separatethrough suspension and gravity effects into layers of varyingspecific gravity. Heavier grains concentrate on the surface ofthe screen, with the heaviest generally concentrated towardsthe centre of the screen forming an ‘eye’. Very heavy minerals,such as ilmenite and magnetite, will be found at the very cen-tre of the screen and lighter heavy minerals, such as garnet andpyroxene, will concentrate at the periphery of the eye.Diamonds tend to concentrate towards the centre despite theirmoderate specific gravity (SG 3.51). A spoon is used to removethe heavy minerals in the eye for more detailed examination.For optimal recovery, the jig tailings should be re-jigged 2 to 3times until no eye exists. The method is typically used forrecovery of gold (e.g. Silva 1986) and kimberlite indicator min-erals (Muggeridge 1995). The advantages of using a jig are thatit can be field based and thus reduce sample weight to beshipped, is inexpensive to operate, is relatively fast and worksbest for fine to coarse sand-sized grains. It is best used in afixed, laboratory-based setting with an experienced operator.

FINAL CONCENTRATIONHeavy liquid separationA preconcentrate is usually further refined using heavy liquidsof a precise density to further reduce the size of the concen-trate prior to heavy mineral selection (Table 2-column E).Heavy liquid separation provides a sharp separation betweenheavy (sink) and light minerals (float) at an exact known den-sity. It is slow and expensive and therefore not economical forlarge volumes of sample material, hence the preconcentrationprocedures described above that are used to prepare a precon-centrate before this step (Stendal & Theobald 1994). The mostcommon heavy liquids used include methylene iodide (MI)with a SG of 3.3 and tetrabromoethane (TBE) or the low-tox-icity heavy liquid lithium heteropolytungstates (LST) both withSG of 2.9. The density required of the heavy liquid will depend

on the indicator minerals being sought. Some laboratories usea combination of both heavy liquids, separating first using thelower density heavy liquid at about SG 2.9 to reduce the vol-ume of material to be further separated at SG 3.2 or 3.3. (e.g.de Souza 2006; Le Couteur & McLeod 2006; Mircea 2006).The recovery of kimberlite and magmatic Ni-Cu-PGE indica-tor minerals requires heavy liquid separation at SG 3.2 usingdilute methylene iodide to include the lowest density indicatorsCr-diopside and forsteritic olivine. Recovery of porphyry Cuindicator minerals requires separation at SG 2.8 to 3.2 torecover the mid-density indicators tourmaline (dravite), alunite,jarosite, and turquoise (Averill 2007). Some indicator minerals,such as apatite and fluorite, are of intermediate density but arerecovered mainly from the mid-density rather than the heavyfraction.

Magnetic separation Magnetic separation may be used to further refine heavy min-eral concentrates and reduce concentrate volume for picking ofmineral species with specific magnetic susceptibilities (Towie &Seet 1995). The most common magnetic separation is splittingthe ferromagnetic from the nonferromagnetic fraction becausethe ferromagnetic minerals can comprise a considerable por-tion of the concentrate (e.g. Table 2-column F). Removing theferromagnetic minerals decreases concentrate size prior toindicator mineral selection and removes any steel contaminantsderived, in most instances, from sampling tools. The ferro-magnetic fraction may then be (1) set aside, (2) examined todetermine the abundance and mineral chemistry of magnetite(e.g. Beaudoin et al. 2011), pyrrhotite or magnetic Mg-ilmenite,as is the case for some kimberlites (e.g. McClenaghan et al.1998), or (3) analyzed geochemically (e.g. Theobald et al. 1967).A hand magnet or plunger magnet is most commonly used tocarry out this separation.

A specific size fraction of the non-ferromagnetic heavymineral fraction may be further separated electromagneticallyinto fractions with different paramagnetic characteristics tohelp reduce the volume of material to be examined for indica-tor minerals (Averill & Huneault 2006). Minerals such as dia-mond are nonparamagnetic, pyrope garnet, eclogitic garnet,Cr-diopside and forsteritic olivine are nonparamagnetic toweakly paramagnetic, and Cr-spinel and Mg-ilmenite are mod-erately to strongly paramagnetic (see Table 1 in McClenaghan& Kjarsgaard 2007). If the non- or paramagnetic portion ofthe concentrate contains a significant amount of almandinegarnet it may be processed through a magstream separator toseparate the orange almandine from similar looking eclogitic orpyrope garnets. Magstream separation divides the concentrateinto (1) a fraction containing most of the silicates (e.g. pyropeand eclogitic garnet) and no almandine, and (2) a fraction con-taining ilmenite, chromite and other moderately magnetic min-erals such as almandine (Baumgartner 2006).

INDICATOR MINERAL SELECTION AND EXAMINATION

The non-ferromagnetic fraction is commonly sieved into twoor three (e.g. 0.25-0.5 mm, 0.5-1.0 mm and 1.0-2.0 mm) sizefractions for picking of indicator minerals, however the finalsize range will depend on the commodity sought. For example,kimberlite indicator minerals are most abundant in the 0.25-0.5

5Overview of processing methods for recovery of indicator minerals from sediment and bedrock samples

mm fraction (McClenaghan & Kjarsgaard 2007) and thus tomaximize recovery and minimize counting time and cost, thisfinest size fraction is most commonly picked.

Indicator minerals are selected from non-ferromagneticheavy mineral concentrates during a visual scan, in most cases,of the finer size (e.g. 0.25-0.5 mm, 0.3-0.5 mm or 0.25-0.86mm) fractions using a binocular microscope. The grains arecounted and a selection of grains are removed from the sam-ple for analysis using an electron microprobe (EMP) to con-firm their identification. Methods for examining a sample forcounting/picking vary from rolling conveyor belts todishes/paper marked with lines or grids. If a concentrate isunusually large, then a split is examined and the indicator min-eral counts are normalized to the total weight of the concen-trate. If a split is picked, the weight of the split and the totalweight should both be recorded. Not all grains counted in asample will be removed for EMP analyses. If this is the case,the total number of grains counted and the number of grainsremoved should both be recorded.

Indicator minerals are visually identified in concentrates onthe basis of colour, crystal habit and surface textures, whichmay include features such as kelyphite rims and orange peeltextures on kimberlitic garnets (Garvie 2003; McClenaghan &Kjarsgaard 2007). Scheelite and zircon in a concentrate may becounted under short-wave ultraviolet light. Gold and PGMgrains may be panned from concentrates that were prepared insuch a way that the silt-sized fraction has been retained (e.g.tabling). The grains may be counted and classified with the aidof optical or scanning electron microscopy. Commonly, goldgrains are classified according to their shape/degree of wear(e.g. DiLabio 1990, Averill 2001), which can provide informa-tion about relative transport distances (McClenaghan & Cabriin press).

INDICATOR MINERAL CHEMISTRYMineral chemical analysis by EMP, scanning electron micro-probe (SEM), laser ablation-ICP-MS, or secondary ion massspectrometry (SIMS) may be carried out to determine major,minor and trace element contents of specific indicator miner-als because mineral chemistry is used to confirm identity,establish mineral paragenesis, and in some cases deposit grade(e.g., Ramsden et al. 1999; Belousova et al. 2002; Scott 2003;Heimann et al. 2005). For example, kimberlite indicator miner-als are characterized by a specific range of compositions thatreflect their mantle source and diamond grade (e.g. Fipke et al.1995; Schulze 1997; Grütter et al. 2004; Wyatt et al. 2004). Gold,PGM and sulphide grains may be analyzed to determine theirtrace element chemistry or isotopic compositions (e.g. Grant etal. 1991; Leake et al. 1998; Chapman et al. 2009). Prior to indi-cator mineral grains being selected from a heavy mineral con-centrate, newer techniques such as mineral liberation analysis(MLA), computer-controlled scanning electron microscopy(CCSEM), or quantitative evaluation of materials by scanningelectron microscopy (QEMSCAN) may provide quantitativemineralogical analysis and identification of indicator mineralsin a portion of the heavy mineral concentrate that has beenprepared as a polished epoxy grain mount, in the 0.25 to 2.0mm fraction of the rarely examined <0.25 mm fraction. Thesemethods can be used to identify indicator minerals of interestand prioritize grains for further detailed and more costly EMPanalysis, thus reducing EMP analytical costs. The cost per sam-

ple for these new techniques is, in general, more expensive thanconventional methods.

QUALITY CONTROLProject geologists may use a combination of blank samples (noindicator minerals), spiked samples (known quantity of specificindicator mineral species), and field duplicates, as well asrepicking of 10% of the heavy mineral concentrates to moni-tor a laboratory’s potential for sample contamination and qual-ity of mineral grain selection. In addition, heavy mineral pro-cessing and identification laboratories can be asked to reporttheir own quality control monitoring procedures and testresults. Quality assurance and control measures are beingimplemented at the Geological Survey of Canada for projectsusing indicator minerals (Spirito et al. 2011).

SUMMARYThese workshop notes describe some of the procedures avail-able for processing surficial media and rocks to recover indica-tor minerals for mineral exploration. The processing methodused will depend on sample media, commodities being sought,budget, bedrock and surficial geology of the survey area, andprocessing methods used for previous batches. When report-ing indicator mineral results in company assessment files, gov-ernment reports, or scientific papers, it is helpful to report thelaboratory name, processing methods used, and sampleweights. Monitoring of quality control is essential at each stagein the processing, picking, and analytical procedures describedhere and should be monitored both by the processing labora-tories and clients. Geologists are encouraged to visit process-ing and picking laboratories so that they have a clear under-standing of the procedures being used and can discuss cus-tomizations needed for specific sample batches.

ACLMPW;EDGEMENTSS. A. Averill of Overburden Drilling Management Ltd., T. Nowicki and M. Baumgartner of Mineral Services Canada,and L. Cabri of CNT-MC are thanked for providing informa-tion about the procedures used in specific heavy mineral pro-cessing labs. GSC Contribution No. 20110083.

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Indicator minerals have been successfully used in diamondexploration for more than 50 years worldwide. Kimberlitescontain a rich mineral inventory that includes olivine, peri-dotitic and eclogitic garnets, chrome spinel, chrome diopside,Mg-ilmenite, enstatite, phlogopite, and periclase among others.Kimberlite indicator minerals (KIMs) for the purposes of thispresentation can be subdivided into two groups: 1) typical orcommon and 2) exotic. Common kimberlite indicator mineralsbelong to two mantle assemblages – peridotitic and eclogitic(Stachel & Harris 2008; Gurney et al. 2010). Peridotitic KIMsinclude pyrope garnet (of lherzolitic or G9 and harzburigitic-dunitic or G10 chemical affinity), Mg-rich ilmenite, Cr-richspinel, Cr-rich diopside, enstatite, and forsteritic olivine.Eclogitic indicator minerals primarily consist of eclogitic (Na-rich) garnet and Na-rich omphacitic clinopyroxene (commonlywith elevated K contents reflecting the high pressures involvedin their formation). Exotic kimberlite minerals include suchphases as corundum and uvarovite (Ca-Cr-rich garnet).

Corundum is present in kimberlites and as inclusions in dia-monds but also occurs in peraluminous metamorphic rocksand peralkaline syenites and pegmatites. Corundum – espe-cially with elevated Mg, Cr, Ni, Ti, and Fe concentrations – isan important indicator mineral in kimberlite explorationbecause it is most probably derived from eclogite xenoliths fre-quently carried by kimberlite melts (Hutchinson et al. 2004).Kimberlite-borne eclogitic xenoliths are commonly associatedwith impressive diamond grades - they average 14,000 cpt, fre-quently up to 90,000 cpt (Spetsius 2004). One eclogitic xeno-lith (66 grams) from Udahcnaya Pipe in Siberia contains 74microdiamonds implying grade of 144,000 cpt.

Uvarovite essentially is a crustal mineral, but when found inkimberlites is positively correlated with diamond grade of indi-vidual pipes (as exemplified by several pipes in Siberia andCanada). Uvarovite is typically associated with chromites andCr-rich ore formations including certain skarns, but in theabsence of such rock formations within certain explorationterritory, it can be successfully used – together with otherKIMs – to locate primary diamond sources.

Another potential “exotic” indicator mineral of kimberliticassemblage is moissanite (SiC), which has been documented inkimberlites and glacial and alluvial sediments from Siberia,Canada, China, Western Australia, Tersky Bereg of KolaPeninsula, and Latvia (Hodireva et al. 2003; Shiryaev et al.2011). Moissanite has also been reported as inclusions in dia-monds from Fuxian kimberlite in China (Leung 1990).

Besides chemical and optical identification of kimberliteindicator minerals, detailed studies of their crystal shape andother surficial features (such as cleavage, surface sculptures,formation of secondary alteration production such askelyphitic rims on garnets) are of great importance in diamondexploration, specifically for estimating the mode and distances

of transport of these indicator minerals. The purpose of thismineralogical exercise ultimately is determination of proximityof primary magmatic sources of kimberlitic indicator minerals,which means the ground locations of kimberlitic pipes.Presence of KIMs with so-called “diamond chemistry” (e.g.minerals formed under P-T-Of conditions compatible with dia-mond formation and stabilization in cratonic lithosphere) iscrucial for location of diamond-bearing kimberlite pipes asopposed to barren diatremes.

Kimberlitt AS’s exploration in eastern Finnmark establishedthe presence of olivines and chrome diopsides with diamondwindow chemistry as well as corundum, uvarovite, and Mg-spinel in till, lake, and river sediments. Some diamond windowKIMs in eastern Finnmark show complete preservation ofsuch crystal features as cleavage, which suggests derivationfrom local kimberlite sources (within <1 km). These data arecompared with exploration results from the Arkhangelsk diamond province and the Devonian Belt of the EasternEuropean Craton (Lithuania, Estonia, Novgorod, and Pskov)and possible implications for location of diamondiferous kim-berlitic pipes through regional exploration are discussed.

EASTERN FINNMARK (NORWAY)Eastern Finnmark is underlain by thick Precambrian continen-tal crust which, on the basis of our recent geochronologicalstudies, reveals complex crustal evolution history. Oldest inher-ited zircon grains from basement grey gneisses yield EarlyArchean ages of 3.69 and 3.2 Ga, suggesting the presence of avery old crustal component under this part of the EastEuropean craton. The peak of crustal production in EasternFinnmark happened around 2.8-2.9 Ga, which was followed byanother episode of thermal re-activation, high-grade metamor-phism, and granitic magmatism at 2.5 Ga. This crustal chronol-ogy suggests that Eastern Finnmark retains a protracted his-tory of evolution of mature continental crust that is broadlysimilar to terrains in other Northern Atlantic cratons, such asFinland, Greenland, and Canada.

Kimberlitt AS carried a reconnaissance sediment samplingsurvey within a part of Eastern Finnmark covering an areaover 3,000 km2. Several populations of indicator minerals havebeen subsequently recovered including olivines, chrome diop-sides, Mg-spinel, corundum, and uvarovite. Olivines have Focontents of 86 to 92 and elevated Ni contents of up to 0.6wt.%, comparable with those typical of kimberlitic olivines(Kamenetsky et al. 2008). Chrome diopsides display low-Al(under 1 wt.% of Al2O3) and Cr contents of 1.5 wt.%. Someclinopyroxenes also have elevated Na concentrations coupledwith high Cr contents, which is typical of mantle-derived diop-sides. These chrome diopsides from Eastern Finnmark plotinto a field of diamond inclusion clinopyroxenes on a Cr2O3-

Indicator minerals in diamond exploration: A case study from eastern Finnmark, Arkhangelsk and the Devonian Belt

(Estonia, Lithuania, Novgorod, and Pskov)Pavel Kepezhinskas

Kimberlitt AS, Tolbugatta 24, Oslo, Norway (e-mail: [email protected])

Al2O3 discrimination diagram. Corundum exhibits high Cr,Fe, Mg, and Ni contents, consistent with their kimberliticprovenance (Hutchison et al. 2004). So far the regional miner-alogical signature in Eastern Finnmark is chromediopside>olivine>>corundum>Mg-spinel>uvarovite.However, it is the appearance of the KIM grains that might behelpful in unlocking the diamond potential of EasternFinnmark.

Fairly significant populations (locally up to 20-30 percent ofthe total number of heavy minerals recovered from till or riversediment) of indicator mineral grains from Eastern Finnmarkretain primary magmatic crystal habits and are characterized bya minimal degree of chemical abrasion and physical wear.Some chrome diopsides show features such as mineral cleavageand some forms or sculptures, which are indicative of growthin igneous melts (Fig. 1), suggest of very limited transport influvio-glacial systems, and possible origin from local kimberlitesources. Cleavage is a particularly important diagnostic featurein relation to localization of primary magmatic sources ofthese clinopyroxene grains since it can not be preserved indiopside grains during transport by water or ice over distancesof more than several hundreds of metres. Other chrome diop-sides exhibit various degrees of chemical abrasion (Fig. 2) thatare indicative of more complicated transport histories andadvanced travel distances. Magnesian spinel shown in Figure 3has retained its original octahedral crystal shape and magmaticcrystal growth features, which – as in the case of the chromediopside grain shown in Figure 1 – is also an indication of itsderivation from local magmatic sources.

ARKHANGELSKThe Arkhangelsk region of northwest Russia is underlain byPrecambrian continental crust composed of Late Archean toProterozoic granulites, gneisses, granites, and greenstone belts(terrains). Mantle conditions beneath the Arkhangelsk regionare comparable to classic diamondiferous cratons worldwide,such as the Kaapvaal craton in South Africa and the Slave cra-ton in Canada. Single clinopyroxene thermobarometry definesa cool continental geotherm similar to the 36 mW/m2 geot-herm calculated by Kukkonen et al. (2003) for the 600 million

year old Karelian craton. These mantle conditions arefavourable for diamond formation within the Arkhangelskmantle at depths of 110 to 224 kilometres (corresponding tothe diamond stability field) and are further confirmed by geo-chemical signatures of pyropes and chromites from a high-grade Arkhangelskaya pipe in the Lomonosov diamonddeposit (Lehtonen et al. 2009). Kostrovitsky et al. (2004) reportpressures of 28 to 68 kbars derived from a megacryst suite(garnet, clinopyroxene, Mg-ilmenite, phlogopite, and garnet-pyroxene intergrowths) in the diamondiferous Grib pipe,which again is supportive of kimberlites sampling diamond-rich mantle within a cool cratonic keel beneath theArkhangelsk region. Low-hematite content of ilmenitemegacrysts suggest low oxygen fugacity and overall reducingconditions in the mantle during its formation, which indicatesan environment favourable for diamond preservation(Kostrovitsky et al. 2004).

Indicator mineral studies in the northern part of theArkhangelsk region are abundant and have led in the past tosuccessful discoveries of more than 30 kimberlite pipes, sevenof which – 6 pipes of Lomonosov kimberlite cluster and the

8 P. Kepezhinskas

Fig. 1. Chrome diopside from Eastern Finnmark with primary mag-matic features (such as cleavage) indicative of short transport distances (Jakobselv Target Area, sample 1083).

Fig. 2. Partially resorbed and chemically altered chrome diopside sug-gesting a more complex transport history.

Fig. 3. Unabraded Mg-spinel grain retaining primary magmatic features.

9

Grib pipe in the Verkhotina kimberlite field – carry economicdiamond mineralization. These pipes include the spectacularGrib pipe with a net present value of 9.3 billion US dollars inthe ground and a high-grade (0.85 to 1 cpt) Arkhangelskayapipe within the Lomonosov kimberlite cluster that is currentlybeing mined by the Russian diamond producer ALROSA.

The regional study discussed in this presentation is focusedon the central and southern Arkhangelsk region and was car-ried out in 2006 to 2008 by a now defunct private diamondexplorer, Russian Diamonds plc.

Abundant indicator minerals were collected within theTokshenga property in the central Arkhangelsk region (Fig. 4).Indicator minerals are dominated by pyrope garnets (both G9and G10 types), olivine (Fo content of 87 to 95, Ni content upto 0.4 wt.%), chromite, and low-Al, high-Cr clinopyroxene.

Spatial distribution patterns, morphology of individual KIMgrains combined with local ice-flow directions suggest thepresence of kimberlitic sources within the western portion ofthis area (Fig. 4).

Indicator minerals within the southern part of theArkhangelsk region (Lachsky property) are dominated bypyrope garnets. Based on microprobe analyses of more than70 grains collected from Quaternary river sediments, it appearsthat a vast majority of diamond grains have originated withinthe diamond stability field (Fig. 5). Co-variation of Cr2O3 andCaO in these garnets indicates that most of the grains are lher-zolitic G9 garnets with harzburgitic G10 garnets forming asubordinate but prominent population (Fig. 5), suggesting thatkimberlite source rocks of these pyropes might be associatedwith diamond mineralization. Although some garnet grains


110 Depth (m) to Target

Pyrope (1-8 grains)

Chromite (1-2 grains)

CPX (1-50 grains)

National Park

Carboniferous

Vendian

N

Ultramafics &Greenstone Belts

Devonian

KIM Distrubution

Archean

0 30km

Tokshenga LicenseQuaternary IceDirection

Olivine (1-3 grains)

Fig. 4. Distribution of indicator minerals in central part of the Arkhangelsk region (Russian Federation).

Fig. 5. Chemical composition and crystal morphology of indicator minerals in southern part of the Arkhangelsk region (Russian Federation).

show clear signs of chemical abrasion (pitting) and physicalwear (extensive rounding) indicative of large transport dis-tances, a number of low-Ca pyropes exhibit angular formswith well preserved elements of original chemical textures,suggesting that they were derived from local kimberlitesources.

THE DEVONIAN BELT – BALTIC STATESThe Baltic States are located within a geological feature knownas the Devonian Belt, which is essentially a significant areawithin the western portion of East European Craton domi-nated by Devonian sedimentary cover. Kimberlite indicatorminerals were reported earlier from Latvia (Hodireva et al.2003) and were later discovered by us in Lithuania and Estoniaunder the auspices of a regional reconnaissance study fundedby De Beers in the early 1990s.

Indicator minerals were recovered from both Devonian andQuaternary sediments in Lithuania. KIM populations areclearly dominated by chrome spinels with subordinate garnetsand minor picroilmenite (chrome spinel>>garnet>Mg-ilmenite). Chrome spinels form a typical “regional array” onthe Cr2O3-MgO discrimination plot with approximately 10percent of all chemical analyses (185 analyses) plotting withinor near the diamond inclusion field (Table 1). Garnets aredominated by lherzolitic compositions; however some border-line G10 chemistries have been also detected (Table 1).Magnesian ilmenites are characterized by fairly high MgO con-

tents and are consistent with the overall derivation ofLithuanian indicator mineral suites from kimberlitic sources.Indicator minerals from Devonian sedimentary rocks (Skervelesuite) are mostly heavily abraded, suggesting their processingthrough one or more secondary sedimentary collectors, whileindicator grains from modern river sediments contain a healthypopulation (about 30 percent) of grains showing only minorchemical alteration or no signs of any significant transport atall.

Indicator minerals in Estonia are less abundant compared toLithuania, but it also might reflect some sampling bias (lesssamples collected in Estonia due to certain time limitations).Indicator mineral populations in modern river sediments aredominated by garnets with subordinate spinels. Several grainsof corundum with elevated Ni contents (45-65 ppm) were alsorecovered. Garnets are mostly lherzolitic G9 and borderlineG9/G10 varieties. Spinels have moderate to high chromechemical compositions with elevated FeO contents and Fe/Mgratios. Overall recovered indicator minerals in Estonia wereinterpreted as broadly kimberlitic in origin. However, most ofthe grains show extensive degrees of chemical abrasion andnearly total absence of primary magmatic features, which sug-gests to us that these indicator minerals were probably deliv-ered into modern river drainage from secondary sedimentarycollectors of Devonian/Carboniferous/Permian and evenpossibly Paleogene/Neogene age and reconstruction of theirtransport distances and, hence, localization of primary kim-

10 P. Kepezhinskas

garnet garnet garnet ilmenite spinel spinel spinel spinel garnet garnet spinelSiO2 40.94 40.11 41.2 n.d. n.d. n.d. n.d. n.d. 40.21 41.19 n.d.TiO2 0.63 0.52 0.26 47.05 0.09 0.0 0.03 0.0 0.3 0.26 0.18Al2O3 20.55 20.11 20.55 0.22 4.63 7.09 8.43 4.55 19.21 19.22 6.57C O 4 59 5 16 5 18 1 23 64 98 63 22 61 07 65 17 5 59 4 69 60 9

OxideLithuania Estonia

Cr2O3 4.59 5.16 5.18 1.23 64.98 63.22 61.07 65.17 5.59 4.69 60.9FeO 7.88 7.17 7.12 42.34 19.5 16.08 19.84 16.53 6.71 7.03 20.97MnO 0.33 0.23 0.26 0.34 0.36 0.33 0.41 0.36 0.32 0.31 0.42MgO 20.56 21.21 20.59 8.21 10.01 13.86 9.43 13.79 22.05 22.13 10.25CaO 4.85 4.55 4.68 n.d. n.d. n.d. n.d. n.d. 4.78 4.46 n.d.Na2O 0.0 0.05 0.12 n.d. n.d. n.d. n.d. n.d. 0.21 0.12 n.d.Total 100.33 99.12 99.96 99.39 99.77 100.58 99.21 100.4 99.38 99.41 99.29

Note: n.d. - not determined

Table 1 . Chemical composition of indicator minerals in modern sediments from Lithuania and Estonia.

tenragtenragtenragenexoryponilcenexoryponilcetinemlietinemlietinemlitenragtenragtenragOiS 2 .d.n.d.n.d.n71.1477.0436.04 93.55 92.0444.0410.2438.55OiT 2 28.00.031.00.00.075.4470.4446.848.00.00.0lA 2O3 35.6124.6121.7119.036.045.095.086.098.6128.6159.41rC 2O3 10.961.0155.821.142.16.062.015.030.751.880.21

61.665.643.570.125.130.2495.6468.9317.740.761.7OeF53.04.043.00.00.044.03.0064.084.03.0OnM97.0282.0216.1294.6132.6196.777.884.887.9148.9110.91OgM76.594.517.470.5253.52.d.n.d.n.d.n31.673.630.5OaC

aN 2 .d.n.d.n.d.n86.094.0.d.n.d.n.d.n.d.n.d.n.d.nO26.9957.9918.9964.00163.00116.9985.00171.8949.9974.9961.99latoT

denimretedton-.d.n:etoN

edixOnoigervoksPnoigerdorogvoN

Table 2. Chemical composition of indicator minerals in modern sediments from Novgorod and Pskov regions of Russian Federation.

11

berlitic sources in reality represents an unrealistic, if notimpossible, task.

THE DEVONIAN BELT – NOVGOROD ANDPSKOV REGIONS OF RUSSIAN FEDERATION

Indicator minerals were reported in high concentrations (up to300-500 grains per 20 litre sample) throughout the centralEuropean Russia (Novgorod, Pskov, and Tver regions).Diamonds – both unabraded cubic crystals and fragments –were also found in the Novgorod region in association withkimberlitic garnets and spinels (Table 2). Garnets are mostlychrome-rich pyropes, some with well developed kelyphiticrims, suggesting short transport distances and local kimberliticsources. Other garnets show variable degrees of chemical abra-sion, suggesting complex transport history and possible resi-dence in one or more Paleozoic sedimentary collectors.Follow-up on several strong pyrope/chrome spinel indicatortrains led to a discovery of a metakimberlite pipe in the centralNovgorod region, however no information on its diamondcontent is available at this time.

CONCLUSIONSThe case studies presented above highlight the importance

of regional and local surveys of indicator minerals in diamondexploration. Local geochemical signatures – predominance ofcertain minerals over other indicators – can be establishedthrough such surveys and can be used to characterize individ-ual kimberlite clusters or larger kimberlite provinces on a cra-ton scale. Not all traditional kimberlite indicator minerals –peridotitic and eclogitic garnets, chrome spinels, Mg-ilmenites,chrome diopsides, forsteritic olivines – should and can be pres-ent, but the presence of indicator minerals with diamondinclusion chemistry is essential for ultimate exploration suc-cess. Some kimberlite provinces, fields, and clusters are associ-ated with “exotic” indicator minerals, such as corundum (withelevated Cr, Ni, Fe, and Ti concentrations) and green Ca-Cruvarovitic garnet, which can provide valuable information onthe location of primary kimberlitic sources and even providesome possible insights into the potential diamond grade ofthese kimberlitic sources (e.g. uvarovite). Finally, crystallo-graphic appearance and degree of chemical alteration and

physical wear (degree of rounding, preservation of primarycrystallographic features, cleavage, original magmatic growthforms on crystal planes, kelyphitic rims on garnets, etc.) ofkimberlite indicator minerals are important in deciphering oftheir transport histories, distances from primary sources, andpotential residence in Phanerozoic secondary sedimentary col-lectors. All these factors are essential in ensuring that each andevery diamond exploration effort leads to ultimate success, notultimate failure.

REFERENCESGURNEY, J.J., HELMSTAEDT, H.H., RICHARDSON, S.H. & SHIREY, S.B. 2010.

Diamonds through time. Economic Geology, 105, 689-712.HODIREVA, V., KORPECHKOV, D., SAMBURG, N., & SAVVAITOV, A. 2003. Sources

of kimberlitic minerals in clastic sediments of Latvia and some problemsin the succession of formation of supposed kimberlites. Geologija, 42, 3-8.

HOOD, C.T.S. & MCCANDLESS, T.E. 2004. Systematic variation in xenocrystmineral composition at the province scale, Buffalo Hills kimberlites,Alberta, Canada. Lithos, 77, 739-747.

HUTCHINSON, M.T., NIXON, P.H. & HARLEY, S.L. 2004. Corundum inclusionsin diamonds – discriminatory criteria and a corundum compositionaldataset. Lithos, 77, 273-286.

KAMENETSKY, V.S., KAMENETSKY, M.B., SOBOLEV, A.V., GOLOVIN, A.V.,DEMOUCHY, S., FAURE, K., SHARYGIN, V.V. & KUZMIN, D.V. 2008. Olivinein the Udachnaya-East kimberlite (Yakutia, Russia): types, compositionsand origins. Journal of Petrology, 49, 823-839.

KOSTROVITSKY, S.I., MALKOVETS, V.G., VERICHEV, E.M., GARANIN, V.K. &SUVOROVA, L.V. 2004. Megacrysts from the Grib kimberlite pipe(Arkhangelsk Province, Russia). Lithos, 77, 511-523.

KUKKONEN, I.T., KINNUNEN, K.A. & PELTONEN, P. 2003. Mantle xenoliths andthick lithosphere in the Fennoscandian shield. Physics and Chemistry of theEarth, 28, 349-360.

LEHTONEN, M.L., O’BRIEN, H.E., PELTONEN, P., KUKKONEN, I.T., USTINOV, V.& VERZHAK, V. 2009. Mantle xenocrysts from the Arkhangelskaya kim-berlite (Lomonosov mine, NW Russia: Constraints on the compositionand thermal state of the diamondiferous lithospheric mantle. Lithos, 112,924-933.

LEUNG, I.S. 1990. Silicon carbide cluster entrapped in a diamond from Fuxian,China. American Mineralogist, 75, 1110-1119.

MCCLENAGHAN, M.B., 2005. Indicator mineral methods in mineral explo-ration. Geochemistry: Exploration, Environment, Analysis, 5, 233-245.

NIXON, P.H. & HORNUNG, G. 1968. A new chromium garnet end member,knorringite, from kimberlite. The American Mineralogist, 59, 1833-1840.

SHIRYAEV, A.A., GRIFFIN, W.L. & STOYANOV, E. 2011. Moissanite (SiC) fromkimberlites: polytypes, trace elements, inclusions and speculations on ori-gin. Lithos, 122, 152-164.

SPETSIUS, Z.V. 2004. Petrology of highly aluminous xenoliths from kimberlitesof Yakutia. Lithos, 77, 525-538.

STACHEL, T. & HARRIS, J.W. 2008. The origin of cratonic diamonds – con-straints from mineral inclusions. Ore Geology Reviews, 34, 5-32.



Exploration for the RE and REE and other high-tech metals(Ga, Ge, In, Li, Nb, Ta, and Ti) is presently active in Finland.Also, the Geological Survey of Finland (GTK) has launched afour-year project (2009-2012) to investigate the country’spotential for high-tech metals. As a part of the project, soilgeochemistry and indicator mineral methods have been used totrace potential sources for mineralized bedrock. Methodologyused for the exploration high-tech metals is basically the sameas for other commodities, such as base metals, gold, and PGE.Also, new methods have been developed and tested during therecent years.

Till geochemistry is a key method for exploration inglaciated terrains. A regional till geochemical database andnumerous target-scale datasets collected by GTK are the foun-dation for estimating the potential for different regions inFinland. Chemical analyses done using ICP-AES and/or ICP-MS methods include several elements applicable to high-techmetal potentiality estimations. For example, elevated concen-trations of La, Li, Sc, Ti and Y can be used for targeting stud-ies or for choosing samples for re-analysis with special meth-ods. New sampling and detailed fieldwork can be done usingtest pit surveys and percussion drillings.

RE and REE anomalies in till exist only in ppb or ppm lev-els, but normally occurring as heavy minerals they can be con-centrated using gravity separation methods, such as theKnelson concentrator and heavy liquid. The minerals can beconcentrated further by magnetic separation. The coarsefractions (>200 μm) can be investigated by hand picking underoptical microscope. However, the microscopic identification ofRE and REE minerals is challenging and requires a lot of man-

ual SEM-EDS (scanning electron microscope + energy disper-sive spectrometer) work to analyze the selected individualgrains for chemical composition.

The searching for and analysis of the indicator minerals canbe also automated by using a modern SEM and suitable soft-ware. One of the most exciting new applications is the fullyautomated study of the till fine fraction (<63 μm) mineralogyby the Mineral Liberation Analyzer (MLA). The MLA consistsof a modern FEI Quanta 600 SEM, two energy dispersive X-ray detectors, and JKTech quantitative mineralogy software,which allow quantitative analysis of mineral and material sam-ples (Fig. 1). The analyses are done from the monolayer pol-ished sections of the heavy mineral concentrates.

In this paper, two case studies from different parts ofFinland are presented to demonstrate the applicability of MLAto detect RE and REE minerals in soil.

CASE STUDY 1: REVONKYLÄ AREA INILOMANTSI, EASTERN FINLAND

In the Ilomantsi study area (Fig. 2), a pyrochlore anomaly hadbeen discovered in till during an earlier heavy mineral surveyfor diamond indicator minerals (Lehtonen et al. 2009).Altogether approximately 100 pyrochlore grains were detectedin the 0.25-1.0 mm fractions of three Knelson-concentratedoriginally 60 kg basal till samples. The grains were analyzed bya Cameca SX100 electron microprobe, and compositionallythey could be classified into (1) yttro-betafite-(Y), (2) yttro-pyrochlore-(Y), and (3-4) uranobetafite varieties (Fig. 3). Thesource for the grains remains unknown. Based on the micro-analytical data, both pegmatitic and alkalic source rocks arepossible. In the context of the GTK high-tech metals project,additional sampling was carried out in the area.

The sample processing flow sheet is given in Figure 4. Forthe traditional indicator mineral work the samples wereprocessed as in the previous study, using the standard Knelson-based protocol for diamond indicator minerals (Lehtonen et al.2005). -63mm sample material was subjected to geochemical(ICP-AES, ICP-MS) and MLA analyses. For the MLA, theheavy mineral fractions were concentrated using HMS (heavymedia separation) and LIMS (low-intensity magnetic separa-tion), followed by mounting in epoxy. The analyses were car-ried out on polished monolayer mounts, where graphite-pow-der was used to separate the grains. From each sample 2 to 4mounts were made, corresponding roughly to 300,000 to800,000 individual mineral grains. Energy-dispersive X-ray(EDX) analyses were taken from all heavy mineral grains withdensity higher than that of zircon (4.7). The selection wasbased on backscatter electron (BSE) imaging.

Exploring RE and REE mineralization using indicator mineralsM. Lehtonen1, J. Laukkanen2 & P. Sarala3

Geological Survey of Finland1. P.O. Box 96, FI-02151 Espoo, Finland

2. Tutkijankatu 1, FI-83500 Outokumpu, Finland3. P.O. Box 77, FI-96101 Rovaniemi, Finland

E-mail [email protected]

Fig. 1. The Mineral Liberation Analyzer (MLA). Photo: GeologicalSurvey of Finland.

The results of geochemical analyses show no anomalouscontents of RE and REE compared to the regional back-ground. The MLA analyses were carried out on those sampleswhere pyrochlore grains had been found by hand picking of

the coarser (0.25-0.5 mm) Knelson-concentrated heavy min-eral fractions, and samples taken adjacent to them. Pyrochlorewas detected in all analyzed samples, 24 grains per sample inmaximum (Table 1). The results correlate well with those byhand picking, the most enriched samples coincided by bothmethods. It can be also estimated, that the number of mineralgrains studied by both methods was roughly in the same orderof magnitude.

In addition to pyrochlore, the MLA detected other REE-bearing minerals (columbite-tantalite, monazite, xenotime andallanite). Many of them were found in even higher concentra-tions than pyrochlore. By hand picking only monazite wasdetected, which was by far the most common REE-bearingmineral in the sample material. The absence of the other REE-minerals in the hand-picked concentrates can probably beexplained by their difficult recognition under optical micro-scope.

CASE STUDY 2: SAARISELKÄ-PORTTIPAHTA AREAIN CENTRAL LAPLAND

In the other target selected for this paper, the Saariselkä-Porttipahta area (Fig. 5), samples collected for gold explorationhad showed anomalously high concentrations of lantanides(e.g. yttrium and cerium) in till and weathered bedrock, whencompared to the regional background. The aim was to find outinto which minerals the lantanides are bound, and moreover,the reason for the geochemical anomaly on the northeast sideof Porttipahta. During the exploration, only monazite grains

14 M. Lehtonen, J. Laukkanen & P. Sarala

Fig. 2. The Revonkylä sampling area (1), Ilomantsi, Eastern Finland. The sampling sites are indicated by dots, the arrow shows the main ice-flowdirection in the region. © National Land Survey of Finland licence no MML/VIR/TIPA/217/11.

Fig. 3. The microanalyses of the Ilomantsi pyrochlore grains shownin the compositional classification diagram by Hogarth (1977). Thenumbers 1 to 4 correspond to (1) yttro-betafite-(Y), (2) yttro-pyrochlore-(Y), and (3-4) uranobetafite.

15

had been detected in the heavy mineral concentrates of theoriginally ~20 kg samples, but no other REE-bearing mineralshad been found. However, in the mobile metal ion (MMI)studies conducted in the area there were clearly traces of ele-vated high-tech metal concentrations (Sarala et al. 2008).

The remaining sample material from the Saariselkä-Porttipahta area was subjected to further investigation. Thecoarser fractions (0.25-1.0 mm) were concentrated by heavyliquid and studied under binocular microscope and SEM-EDS.The fine fractions (-63 μm) were analyzed by the MLA using

the same procedure as for the Ilomantsi samples. From eachsample 2 polished mounts were made, corresponding roughlyto 300,000 to 400,000 mounted mineral grains. Interestinggrains identified by the MLA were subsequently analyzed by aCameca SX100 electron microprobe.

No REE-bearing minerals were detected during micro-scopic work of the coarse fraction, but the MLA investigationrevealed several of them (Table 2). The samples containedabundant monazite (max. 12,000 grains/sample) and anothermineral (Fig. 6) close in composition to it (max. 20,000

Exploring RE and REE minalization using indicator minerals

Till sample60kg

KnelsonConcentrator

Dry screen0.25mm

Grain sizeanalysis

Wet screen -63μmBisection

Geochemicalanalysis

HMS d>3.3LIMS

Wet screen -63μmBisection

HMS d>3.3LIMS

Hand picking

SEM/EDS

EPMA

2 subsamples

MLA analysis

Wet screen -1mmLIMS

Geochemicalanalysis

+0.25mm

-0.25mm

HMS d>3.3LIMS

MLA analysis

(á 2kg)

Fig. 4. The processing flowsheet for the Ilomantsi till samples. LIMS = low-intensity magnetic separation, HMS = heavy media separation, SEM-EDS = scanning electron microscope + energy dispersive spectrometer, EPMA = electron probe micro analyzer, MLA = mineral liberation analyzer.

90hoP_U_62490seK_U_62490niP_U_62490hoP_U_1E62490niP_U_1E624RM71_U624RM01_U624RM60_U624lareniM tnuoCniarGtnuoCniarGtnuoCniarGtnuoCniarGtnuoCniarGtnuoCniarGtnuoCniarGtnuoCniarG

422142113223)Y(-erolhcoryp-orttY11serolhcoryp_rehtO

60157512etilatnat_etibmuloCdloG 11

htumsiB 1etiyeldeH 13

4211123212etileehcS529251032361149etirohT

nocriZ *26861*82751488221623etiyeleddaB982171315363040279510656401656etizanoM037211652361415etisantsaB1856841301401655737emitoneX72026211etinallA

etilonocriZ 11111etisomahC

00291779718234105226912764811157latoT

mm0.1-52.0fognikcipdnaH66008126600)Y(-erolhcoryp-orttY

.tnemerusaemehtnidedulcnisawnocriZ-*

Table 1. The MLA-results of the Ilomantsi heavy mineral concentrates, fraction -63 μm.

grains/sample). This water-containing mineral is moreenriched in Y and Nd and less enriched in Ce compared tomonazite. The mineral was identified as rhabdophane based onmicroanalytical data. In addition to monazite and rhabdo-phane, xenotime was the third Y-bearing mineral in the sam-ples (max. 2000 grains/sample).

When crosschecking the geochemical and MLA data, a cor-relation can be detected between the Ce and Y concentrations,

and the number of monazite, rhabdophane and xenotimegrains (Fig. 7). This strongly implies that these minerals are themain carrier phases of REE within the samples. When consid-ering the source rock for these minerals, the most likely alter-natives are granite pegmatites, where rhabdophane represent alater crystallization phase than monazite.


72_soP_8002K_70_

72_soP_8002P_70_

75_soP_8002JP_PR_02_

soP6002RMA52,68

68soP6002PDRC

soP6002PRM01,19

soP6002PPR02,19

tnuoCniarGtnuoCniarGtnuoCniarGtnuoCniarGtnuoCniarGtnuoCniarGtnuoCniarGlareniM338200526295823859755183582etizanoM666169010751816381912526etizanomderetlayltraP37494720130901069911921enahpodbahR18100717591899261emitoneX637433432364815594175997469806nocriZ

13etiyeleddaB6124620445etitapA517812237350683448849027120341179latoT

Fig. 5. The Saariselkä-Porttipahta sampling area (2), Central Lapland. The Y contents of the regional geochemical survey of till (ICP-AES) andthe samples selected for the MLA studies are indicated by red dots and black squares, respectively. © National Land Survey of Finland licenceno MML/VIR/TIPA/217/11.

Table 2. The MLA-results of the Saariselkä-Porttipahta heavy mineral concentrates, fraction -63 μm.

17

DISCUSSION AND CONCLUSIONSThe results of the two case studies demonstrate that the MLAhas many advantages compared to the traditional heavy min-eral work, when considering exploration for high-tech metals.

One of them is the very fast speed and efficiency in mineralobserving, far superior even to the most experienced micro-scopist. As a result, the combined sample concentration andthe MLA analysis can detect very small contents of RE and

Exploring RE and REE minalization using indicator minerals

Fig. 6. Backscattered electron images by the MLA of the Saariselkä-Porttipahta samples. The light coloured weathered-looking mineral grainsare rhabdophane.

REE, that cannot be detected by traditional geochemical meth-ods, which can be seen in the Ilomantsi case study. Moreover,much less sample material is needed for the mineralogicalinvestigation by the MLA than by optical microscope becausethe instrument can analyze in a short time hundreds of thou-sands of fine mineral particles that are too small for hand pick-ing. This allows considerable reduction of the original samplesize needed for the heavy mineral work, meaning significantsavings in time and effort spent on sample processing.

The Saariselkä-Porttipahta case study demonstrates how theMLA can be used to solve carrier mineral phases of the REsand REEs detected by geochemistry. The MLA detected REand REE-bearing individual mineral grains can be easily ana-lyzed for more accurate chemical composition using the elec-tron microprobe by taking advantage of a cross-referencedcoordinate system.

Regardless of its advantages, the MLA cannot be used in amass scale to study soil sample mineralogy. For this reason, it

is crucial to screen the samples for the detailed mineralogicalinvestigation using some other methodology, such as geo-chemistry and pre-existing heavy mineral data.

REFERENCESHOGARTH, D.D. 1977. Classification and nomenclature of the pyrochlore

group. American Mineralogist, 62, 403-410.LEHTONEN, M.L., MARMO, J.S., NISSINEN, A.J., JOHANSON, B.S. & PAKKANEN,

L.K., 2005. Glacial dispersal studies using indicator minerals and till geo-chemistry around two eastern Finland kimberlites. Journal of GeochemicalExploration, 87 (1), 19-43.

LEHTONEN, M., KORKEAKOSKI, P., MARMO, J., NISSINEN, A., O'BRIEN, H.,JOHANSON, B., PAKKANEN, L., 2009. Results from a reconnaissance scale heavymineral survey of kimberlitic indicator minerals in eastern Finland. 10 s. + 171 liites.Geological Survey of Finland, Archive report, M41.2/2009/55.

SARALA P. (EDIT.), HARTIKAINEN, A., SARAPÄÄ, O., ILJINA, M., KORKIAKOSKI,E., KOUSA, J., HEIKURA, P., PULKKINEN, E. & TÖRMÄNEN, T., 2008. MobileMetal Ion (MMI) -menetelmän testaus malminetsintätutkimuksissa Itä- ja Pohjois-Suomessa vuonna 2007. 62 s. + 3 liites. Geological Survey of Finland, ArchiveReport, S44/2008/37.


1

10

100

1000

10000

100000

2006-POS 000086 25C

2008-POS 000027 07

2006-POS 000086 25AMR

2008-POS 000057 20Rp

2006-POS 000091 10MR

2008-POS 000027 07P

2006-POS 00091 91,20RP

sniargforebmunro

gk/gm

Ce mg/kg (308M) Y mg/kg (308M) Monazite (MLA) Monazite altered (MLA) Rhabdophane (MLA) Xenotime (MLA)

Fig. 7. A correlation diagram of the Saariselkä-Porttipahta samples, showing the fine fraction (-63 μm) geochemistry (Ce, Y) by ICP-MS and thenumber of monazite, rhabdophane, and xenotime grains detected in the corresponding grain size by the MLA.


The study of placer gold grains is an important part of goldexploration, principally because they are often observed in panconcentrates during stream sediment geochemical surveys.Generally gold occurs as discrete sand-sized particles that arereadily concentrated in heavy mineral separates. The panningmethod allows recovery of even a few grains from a large bulksample, which is equivalent to ppb-level geochemical concen-trations in bulk sediment (Moles & Schaffalitzky 1992;McClenaghan 2011). Visual confirmation of the presence ofgold focuses interest and allows immediate follow-up panningand prospecting. Occasionally this may lead to the discovery ofnearby in situ gold mineralization, but often the bedrocksource is not apparent due to poor exposure and other explo-ration methods may or may not be deployed.

The shape (morphology or condition) of placer gold grainshas been studied extensively and can provide a vector to thelocation of bedrock sources (e.g. Knight et al. 1999a, Townleyet al. 2003; Crawford 2007). Alloy compositions have been usedto investigate placer–lode gold relationships (e.g. Knight et al.1999b) and Youngson et al. (2002) combined these approachesto establish that a population of placer gold in Otago, NewZealand was derived from multiple sources. In addition, min-eralogical characteristics of the gold grains can be used in con-junction with alloy chemistry to generate a ‘microchemical sig-nature’, which can indicate the likely type of the source miner-alization (e.g. Chapman et al. 2009). Our approach gains infor-mation from populations of gold grains collected from placerlocalities and, where possible, from any known bedrock miner-alization, together with other minerals in heavy (panned) con-centrates. This methodology can provide information on min-eralization present within a study area at an early stage in theexploration / reconnaissance process and at a relatively lowcost.

Microchemical characterization of gold grains (strictly, theseshould be referred to as gold alloy or ‘electrum’) was firstdeveloped by the British Geological Survey (BGS). Previousworkers had studied the alloy compositions of gold grains andlinked them to potential bedrock sources but the BGSapproach was the first to augment these data by systematicallyrecording the opaque mineral inclusions within the grains(Leake et al. 1992). Studies of alluvial gold from throughout theCaledonides of Scotland and Ireland (Chapman et al. 2000b)showed that the suite of inclusions within a population of goldgrains reflected the mineralogy of the gold source.

There remains a widely held view that the alloy compositionof placer gold is unrelated to that of the hypogene source as aconsequence of ‘nugget growth’ (see discussion in Hough et al.2009). Whilst the formation of authigenic gold has been welldocumented (e.g. McCready et al. 2003), we have seen no evi-dence whatsoever for grain augmentation by authigenic gold toan extent that would comprise an important addition to the

economic potential of the placer inventory. This subject is dis-cussed fully in Chapman et al. (2009).

METHOD OF MICROCHEMICAL CHARACTERIZATION OF PLACER GOLD

The technique involves bulk sampling to obtain a large numberof gold grains from each site. Specialized field techniques aredeployed involving bulk sampling, sluicing, and panning, whichare best undertaken with a team of 2 or 3 operators. Typically3 sites are sampled per day depending on the number and skillof the operators, ease of access and abundance of gold.Generally it is better to sample small streams (first-order tribu-taries where the streams are <5 km in length) rather than largerrivers, because the former yield more valuable information onthe provenance of placer gold than samples from large rivers.The size of the gold grains is not particularly important; eachgrain has equal value whether large or small, though in practicegrains less than ~50 μm are too small to use and small grainstend to contain fewer inclusions. The number of gold grainsnecessary to characterize a population varies according to theincidence of inclusions, which varies considerably. For exam-ple, samples collected in the Klondike district of the YukonTerritory, Canada typically yielded inclusions in 2% of the pop-ulation whereas around 40% of grains collected from EurekaCreek, immediately to the south, contained inclusions. For aplacer population derived from a single source, approximately10 inclusions are normally sufficient to indicate the nature ofthe original host mineralogy and the number of grains requiredvaries accordingly.

As gold grains are physically modified (usually abraded,rounded, and flattened) during alluvial transport, their mor-phology may also used as a discriminator. Crawford (2007)established a clear and reproducible relationship betweentransport distance and morphology for placer gold in theYukon, and this approach is currently being combined withmicrochemical characterization in areas where multiple goldsources contribute to the placer inventory. A more simpleapproach involves visual separation into sub-populations of‘rough’ (relatively pristine) and ‘flaky’ (flattened) grains fromthe same locality, and this can help to identify the microchem-ical features of proximal and distal types (e.g. Chapman &Mortensen, 2006).

After completion of fieldwork, gold grains are carefullymounted into resin blocks, ensuring that grains from each sitecan be distinguished, and the blocks are ground and polishedto provide a cross-section through every grain. Microchemicalcharacterization combines analyses (using an electron micro-probe) of alloy compositions of gold grains, with identification(using a scanning electron microscope) of the mineral inclu-sions contained within them.

Placer gold microchemistry in conjunction with mineralogy and mineralchemistry of heavy mineral concentrates to characterize bedrock sources

Norman Moles1 & Rob Chapman2

1. School of Environment & Technology, University of Brighton, UK ([email protected])2. School of Earth & Environment, University of Leeds, UK ([email protected])

20 N. Moles & R. Chapman

When reporting alloy compositions of samples of goldgrains from different localities, it is usual practice to quotemedian and mean values. However, these statistics are of lim-ited value when dealing with samples that exhibit a wide rangeof Ag, particularly if the population is clearly bi- or tri-modalwith respect to the alloy compositions. A more informativeapproach is the use of cumulative percentile plots for eachpopulation. Using this form of presentation, the Ag range andrelative proportion of grains of similar Ag contents may beeasily seen, and comparisons between samples quickly estab-lished (Fig. 1). The term ‘population’ is used here to mean a setof grains that have similar Ag contents and that plot as a gen-tly sloping straight line on the cumulative percentile plots, sep-arated from other populations by gaps in the range of alloycontents represented by inflexions or steep lines connectingsample points. Populations are generally considered to repre-sent placer gold derived from a single bedrock source, whereasdifferent populations in a sample may represent multiplebedrock sources.

ORE MINERAL INCLUSIONS WITHIN PLACER GOLD GRAINS

Even within small gold grains, inclusions of ore minerals areoften present and may be abundant. The ore minerals includesulphides, tellurides, selenides, or sulphosalts. Placer gold doesnot usually have other ore minerals adhering to the outside ofthe grains, although observation of this feature indicates closeproximity to source.

Depiction and characterization of inclusion assemblagesmay be done in several ways. Often the construction of a table

of mineral species observed and their abundance immediatelyshows differences between populations. Alternatively, individ-ual points on the cumulative curve can be identified with a keylinked to an inclusion species such that covariance of alloycompositions with inclusion assemblage may be observed (Fig.1). However, the most useful approach is the generation of ter-nary plots with axes selected to emphasize the differencesbetween individual populations or compositional fields derivedfrom multiple populations (Fig. 2).

The term ‘signature’ is used to describe specific associationsof alloy compositions and inclusion mineralogies, e.g., agalena-bearing low-Ag population (Fig. 1), or a populationcontaining Ni- and Co-bearing sulphide minerals (Fig. 2). Thesuite of inclusions within a population of gold grains reflectsthe mineralogy of the bedrock mineralization from which theplacer grains were derived. In many cases placer populationsare derived from more than one source, each of which may beidentified by its signature.

The inclusion suites present within populations of placergold are coeval with gold precipitation and consequently theymay be used to deduce the chemical conditions of the miner-alizing environment. In the early stages of exploration it maybe advantageous to establish the sulphidation state of thesource mineralization, particularly if epithermal gold is the tar-get. The Au(HS)2- ion is more soluble at high values of aS2 andhence the capacity for gold transport is greater. Sulphidationstates are defined by sulphide mineral assemblages (e.g. thebornite/pyrite–chalcopyrite equilibrium) but are also indicatedby the mole % FeS in sphalerite, where sphalerite is in equilib-rium with pyrite. Hannington & Scott (1989) proposed a broadrelationship between FeS in sphalerite and gold grade, although

30

35

Ballygarret placer

Ballygarret float

25

yg

Huntingtown placer

15

20

%A

g

5

10

0

5

0 20 40 60 80 100

cumulative percentile

0

5

10

15

20

25

0 20 40 60 80 100%

Ag

cumulative percentile

Goldmines River, Red hole

Brideswell stream

Avoca stream

Fig 1. Cumulative Ag plots to illustrate single and multiple populations within samples. A) Ag content of placer gold from two rivers in Wexford(southeast Ireland) and of gold from a loose rock sample (float) at one of the sites. Note the narrow range of Ag values in the float comparedto the placer gold at Ballygarret, which generally has lower Ag. However, placer gold from a nearby stream (Huntingtown) includes a small pop-ulation with matching values. B) Contrasting signatures of placer gold from three streams within a 3 km radius in the Goldmines River area inWicklow (southeast Ireland). Goldmines River has relatively fewer Ag-poor gold grains and a flatter profile due to a dominant population of goldcontaining 8-12% Ag. This is population is combined with a ‘regional’ signature of gold with predominantly <10% Ag represented by theBrideswell sample. Symbols indicate inclusion species within grains of a specific Ag content: squares = galena, triangles = arsenopyrite, diamonds= pyrite. Note the absence of galena inclusions in gold grains with >10% Ag.

A B

21Placer gold microchemistry in conjunction with mineralogy and mineral chemistry of HMC to characterize bedrock sources

they acknowledged that individual values vary considerablybetween sphalerite grains within the same mineralizing event.Nevertheless, for the volcanogenic Au-Zn association, thehighest gold grades appear to be associated with very low(<1%) FeS contents of sphalerite. Microchemical characteriza-tion of placer gold permits an application of this approach byanalyzing sphalerite inclusions in populations of gold grains.Evidence for sphalerite-pyrite equilibria may be provided bythe presence of pyrite inclusions in gold grains from the samelocality, and other mineral inclusions can also help to define thesulphidation state (e.g. bornite vs. chalcopyrite). We are cur-rently testing this approach using placer gold derived from dif-ferent styles of mineralization.

INTERPRETATION OF MICROCHEMICAL SIGNATURES

Populations of placer gold grains collected from an area withno known bedrock mineralization can be compared to otherpopulations from sources whose origins are known. Table 1shows signatures associated with different styles of gold min-eralization, based on geographically extensive studies and adatabase of over 25,000 analyses.

Many populations of gold grains are binary Au-Ag alloys.However in some cases Cu, Hg, or Pd may be present. Thepresence of Pd in low-Ag Au alloy was explained in terms ofthe chemical environment of Au transport and precipitation byChapman et al. (2009), but the controls on Hg and Cu withinthe alloy remain unclear. Nevertheless their presence may beuseful as a discriminator between populations of different alloychemistry (e.g. Chapman et al. 2000; Chapman et al. 2010).

Fig. 2. Ternary plots to illustrate contrasting signatures, showing the relative abundance of inclusion species containing (A) Sb-S-As and (right) Cu-Pb-(Ni+Co) in gold grains collected from two regions and two specific sites in Wexford, SE Ireland. Both spe-cific sites are located in the northeast region. However the inclusion assemblage in gold from the Ballykale site differs from othernortheast sites in that it contains no Sb-bearing minerals. Boley lower has no Ni- or Co-bearing minerals in contrast to other north-east sites. Localized variations such as these may provide useful insights to styles of mineralization.

A B

North East

South West

Boley lowery

Ballykale

gA%ygolarenimdetaicossAslatemrehtOegnar

:cinegorOciozorenahP

,KU ,aisyalaM,adanaCailartsuA

ilaM,anahGbS±sA,S%01otgHeraR02-4

noitairavelttilswohsgAlaudividninihtiw

secnerrucco

:cinegorOnaeahcrA

wol:lamrehtipEnoitadihplus iB±eT±sA±S%01otgH001-1dnaltocS,adanaC nihtiwelbairavylhgihgA

secnerruccolaudividni

detalernoisurtnI ,dnalerInrehtroNadanaC iB±eT±sA±S%2otuC02-8 otatadtneiciffusnI

ezilareneggnizidixO

lamrehtordyh,cilbupeRhcezC,KU

lizarB SoN.eT,eS%21otgHro/dnadP6-0 dP-uA;wolyrevyllausugAnommocsyolla

cinegihtuA weN,ailartsuAanitnegrA,dnalaeZ elbigilgenrotnesbagAenoNenoN0

foelytSnoitazilarenim

stnemmoCsnoitacoLerutangislacimehcorciM

evobasAiB±eT±sA,S%3otgHeraR05-5anahG,ewbabmiZ

Table 1. Microchemical signatures associated with different styles of gold mineralization (adapted from Chapman et al. 2009).


Populations of gold grains that exhibit a single homogenousalloy composition indicate uniform conditions of precipita-tion. These are commonly associated with orogenic (mesother-mal) gold, precipitated as a consequence of a uniform mecha-nism of Au(HS)- destabilization, e.g., H2S loss through sulphi-dation of the wall rocks (Morisson et al. 1991). Chapman et al.(2010) showed that the systematic variation in gold grain chem-istry within an orogenic hydrothermal system can be ascribedto variation in the chemical controls on electrum composition.However, more complex depositional environments involvingAu transport by chloride complexes together with bisulphide,and metal precipitation through pH change, redox change,temperature change, and wall-rock interaction, may generate arange of electrum compositions either between homogenousgrains form the same locality or within individual grains. Thesecharacteristics are associated both with epithermal Au(Morrison et al. 1991; Chapman & Mortensen 2006) and withvolcanogenic-hydrothermal massive sulphide Au (Morisson etal. 1991). It is also probable that this scenario is applicable tothe porphyry environment.

Some populations of placer gold are heterogeneous as aconsequence of containing individual grains from differentsource types or mineralizing events. In these cases, differentalloy compositions may often be correlated with mutuallyexclusive inclusion suites. However, where the same inclusiontypes are present throughout the range of alloy compositions,it is generally assumed that the gold grains are derived from asingle source.

COMPOSITIONAL HETEROGENEITY WITHIN INDIVIDUAL GOLD GRAINS

Individual placer gold grains may exhibit internal heterogene-ity in alloy composition, evident in polished sections whenviewed in back scattered electron mode using the SEM (Fig. 3).Placer gold grains from many localities worldwide commonlyexhibit a gold enriched rim, the formation of which has beenascribed by Groen et al. (1990) to a process of electroless plat-ing. Generally these secondary rims are easily distinguishedfrom primary internal heterogeneity, and in our studies suchrims are ignored. The primary textures may provide informa-tion on the ore-depositing processes, and in some cases it hasbeen possible to interpret different component alloy phasesand their unique inclusion assemblages to show a progressivechange in the mineralizing environment or different episodesof mineralization and alteration (Chapman & Mortensen 2006;Chapman et al. 2009).

Huston et al. (1992) note that metamorphic remobilizationof gold (which differs from the initial depositional environ-ment because both Au and Ag are transported as thio com-plexes) generates Au-rich electrum. Grain-penetrating Au-richtracks, common in gold grains from metamorphic terrains (seeFig. 3), may represent partial re-equilibration of the originalelectrum particles, and homogenous high-Au grains may rep-resent their complete re-equilibration. However Hough et al.(2009) suggest that such features may be a result of supergeneprocesses where Au-rich electrum is the alloy in equilibriumwith near surface conditions. However the observation byLeake et al. (1992) that Ag-rich electrum can also form similarfilms, suggests that at least some of these features are hypo-gene.

HEAVY MINERAL CONCENTRATES (HMC):ANALYSIS AND INTERPRETATION

Heavy minerals recovered by the same sampling technique mayhelp to constrain the style of source mineralization for associ-ated placer gold. For example, the occurrence of scheelite sug-gests a felsic igneous-hydrothermal association (greisens, por-phyry or skarn mineralization) (e.g. McMartin et al. 2010).Caution is necessary as the mineral grains may be derived fromdifferent bedrock sources and may be transported differingdistances and by differing mechanisms to the gold, e.g., placergold sourced directly from local bedrock may co-exist in theheavy mineral concentrate with indicator minerals transportedfrom distant sources. Mechanically resistant minerals, such asgarnet, may be derived from glacial sediment transported sev-eral kilometres or tens of kilometres from the bedrock source.Conversely, an association of placer gold with minerals that aretransported relatively short distances in fluvial environments,such as barite and cinnabar, can indicate the presence of prox-imal bedrock mineralization – particularly if these mechani-cally fragile minerals are abundant in the heavy concentrate.Sulphide and sulpharsenide minerals are rarely found in HMCbecause these minerals rapidly oxidize in fluvial environments;for this reason their presence may indicate proximal bedrockmineralization. However such minerals may be preservedunder reducing conditions in glacial sediments and are morecommonly found in HMC from till samples.

Aspects of the bedrock geology of catchments may beinferred from abundances of minerals indicative of particularlithological associations, such as chromite (mafic/ultramaficigneous rocks) or cassiterite (granitic rocks). This can informdiscussion on the possible type of mineralization from whichassociated placer gold may be derived: for example, an absenceor scarcity of igneous indicator minerals would imply that vol-canogenic mineralization is unlikely to occur within the catch-ment area.

In the context of exploration for bedrock gold mineraliza-tion, the objective of analyzing HMC samples is to determinetheir bulk mineralogy and spatial variation within the samplingareas, and any mineralogical association with placer gold abun-dance and characteristics. Simple laboratory-based techniquescommonly used to establish the mineralogy and proportions ofdifferent minerals in the samples include1. Removal of the remaining light mineral component of

panned concentrates by flotation using a heavy liquid,such as di-idiomethane (methylene iodide), that has a spe-cific gravity of 3.2 at room temperature;

2. Testing with a hand magnet for magnetite (and ferrousiron contamination)

3. Visual examination of HMC sub-samples with a x10 tox40 incident-light binocular microscope;

4. X-ray diffraction (XRD) analysis of powdered sub-sam-ples followed by mineral matching and semi-quantitativeprocessing of spectra;

5. X-ray fluorescence analysis, either using a portable metalsanalyzer (PXRF) or benchtop ED-XRF system withappropriate reference materials.

These techniques do not reveal minerals that are present intrace amounts (<1–2%) or the mineral chemistries of heavymineral species. To achieve this it is necessary to prepare pol-ished grain mounts of HMC sub-samples that can be used inmore advanced techniques. These include mineral liberation


analysis (MLA), computer-controlled scanning electronmicroscopy (CCSEM), and quantitative evaluation of materialsby scanning electron microscopy (QEMSCAN). These meth-ods can provide fully quantitative mineralogical analysis andmineral chemistry information (e.g. Keulen et al. 2009;McClenaghan 2011).

HMC mineral chemistry data can help to further refine thestyle of bedrock mineralization, again with the caveat thatmechanically and chemically resistant mineral grains may befar-travelled. For example, Nb-Ta, Mn-Fe2+-Fe3+ and W-Fe3+

can substitute isomorphously for Sn in cassiterite (Möller et al.1988) and the type of substitution may be used to distinguish

Fig. 3. Internal features of gold grains. A) Example of a gold-enriched rim in a gold grain from Glengaber Burn, southern Scotland; reflectedlight image showing the characteristic porous texture of a rim (10 μm thick) which also penetrates flaws in the grain. B–F) Back-scattered elec-tron microscope images of gold grains from Wexford, SE Ireland. B) Thicker Au-rich phase surrounding Ag-rich core: the texture is suggestiveof augmentation of the original core. C and D) Gold grain showing Au-rich phase surrounding multiple cores, which themselves show anincrease in Ag content towards the edges (detail in D). E) subtle variation in primary alloy phase within a grain: The diffuse boundaries betweenalloy compositions suggest an evolving depositional environment, whereas the sharp boundary with Au-rich gold indicates a separate phase ofAu deposition. F) Highly heterogeneous grain showing variation within the high Ag core, and Ag-rich tracks within the major Au-rich phase.Minor addition of low-Ag alloy has again occurred along grain boundaries.


different mineralizing environments (e.g. Sighnolfi et al. 2008).Andy Tindle (pers.l comm.) has shown that in samples fromthe UK and Ireland, cassiterite Nb+Ta contents discriminate apegmatitic from a hydrothermal-magmatic origin (Fig. 4). Ahydrothermal-magmatic association may be more prospectivefor gold mineralization. Cassiterite grains associated with low-abundance placer gold occurrences in the Mourne Mountainsof northern Ireland show a mixed signature, the majority lyingclose to the hydrothermal trend (Fig. 4), whereas cassiteritefrom southeast Ireland and southwest England plot on thetrend of pegmatite-derived cassiterite.

CONCLUSIONS: APPLICATION TO EXPLORATION FOR BEDROCK-HOSTED

MINERAL DEPOSITSMicrochemical characterization of gold can inform the explo-ration process in several ways:1. Identification of the style of source mineralization

Mineralogy diagnostic for specific styles of mineralizationis reflected in the inclusion suite, and an indication of thesulphidation state of the source mineralization may beobtained from diagnostic sulphide species and/or the Fecontent of sphalerite. Furthermore, the variation in alloycomposition from a single mineralizing event indicates thevariation in the chemistry of the depositional environ-ment that can be related to the style of mineralization. Forexample, grains from an orogenic source usually exhibit asimilar Ag content.

2. Characterization of placer gold in a local areaThe technique can identify (i) whether a population ofgrains is the same or different from a nearby locality; (ii)whether a population is derived from a single or multiplemineralizing events; and (iii) the point of influx of a dif-ferent gold type into a watercourse.

3. Characterization of placer gold in a region

Microchemical signatures of placer gold have been usedto identify the most important gold types in areas of mul-tiple sources. This process can aid definition of the mostattractive targets for subsequent exploration, whether lodegold or VHMS or other deposit type.

Mineralogical analysis of heavy mineral concentrates fromwhich placer gold grains have been extracted can also provideuseful information for the exploration program. The presence(particularly if abundant) of metalliferous minerals that aretransported relatively short distances in fluvial environmentssuggests proximity to bedrock mineralization. The presence orabsence of minerals characteristic of particular styles of min-eralization (e.g. scheelite) or of particular host rock types (e.g.granite, or ultramafic rocks) can also indicate the likelihood ofa style of mineralization within the catchment area.Quantitative analyses of the compositions of specific indicatorminerals (such as cassiterite) can also be useful in diagnosis ofthe style of gold-bearing mineralization within a study area.

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analysis of alluvial gold grains in mineral exploration: experiences inBritain and Ireland. Journal of Geochemical Exploration, 71, 241–268.

CHAPMAN, R J., LEAKE, R.C., MOLES, N.R., EARLS, G., COOPER, C.,HARRINGTON, K. & BERZINS, R. 2000b. The application of microchemi-cal analysis of gold grains to the understanding of complex local andregional gold mineralization: a case study in Ireland and Scotland. EconomicGeology, 95, 1753–1773.

CHAPMAN, R.J., LEAKE, R.C., BOND, D.P.G., STEDRA, V. & FAIRGRIEVE, B. 2009.Chemical and mineralogical signatures of gold formed in oxidizing chlo-ride hydrothermal systems and their significance within populations ofplacer gold grains collected during reconnaissance. Economic Geology, 104,563–585.

CHAPMAN R.J., LEAKE, R.C., WARNER, R.A., MOLES, N.R., CAHILL, M.C.,SHELL, C.A. & TAYLOR, J.J. 2006. Microchemical characterization of natu-ral gold and artefact gold as a tool for provenancing prehistoric gold arte-facts: A case study in Ireland. Journal of Applied Geochemistry, 21, 904–918.

CHAPMAN, R.J. & MORTENSEN, J.K. 2006. Application of microchemical char-acterization of placer gold grains to exploration for epithermal gold min-

0.04

0.06

0.08

0.10

Nb+

Ta

Trend of pegmatite-derivedcassiterite

0.00

0.02

1.86 1.88 1.90 1.92 1.94 1.96 1.98 2.00

Trend of hydrothermal cassiterite

Fig. 4. Compositional variation of cassiterite in HMC from the Mourne Mountains of Northern Ireland (filled symbols; each type of symbolrepresents a different locality) and from southeast Ireland (crosses). Units are atomic proportion formula units. Apart from Nb and Ta, the mainelements that substitute for Sn in these cassiterites are Fe and Mn. Data kindly provided by Andy Tindle, Open University, UK.


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CHAPMAN, R.J., MORTENSEN, J.K., CRAWFORD, E.C. & LEBARGE, W. 2010a.Microchemical studies of placer and lode gold in the Klondike District,Yukon, Canada: 2. Constraints on the nature and location of regional lodesources. Economic Geology, 105, 1369–1392.

CHAPMAN, R.J., MORTENSEN, J.K., CRAWFORD, E.C. & LEBARGE, W. 2010b.Microchemical studies of placer and lode gold in Bonanza and Eldoradocreeks, Klondike District, Yukon, Canada: evidence for a small, gold-rich,orogenic hydrothermal system. Economic Geology, 105, 1393–1410.

CRAWFORD, E.C. 2007. Klondike placer gold: New tools for examining morphology, com-position and crystallinity. M.Sc. thesis, University of British Columbia, Canada.

GROEN, J.C., CRAIG, J.R. & RIMSTIDT, J.D. 1990. Gold–rich rim formation onelectrum grains in placers. Canadian Mineralogist, 28, 207–228.

HANNINGTON, M.D. & SCOTT, S.D. 1989. Sulfidation equilibria as guides togold mineralization in volcanogenic massive sulfides: evidence from sul-phide mineralogy and the composition of sphalerite. Economic Geology, 84,1978–1995.

HOUGH, R.M., BUTT, C.R.M. & FISCHER-BUHNER, J. 2009. The crystallography,metallography and composition of gold. Elements, 5, 297–302.

HUSTON, D., BOTTRILL, R. S., CREELMAN, R.A., ZAW, K., RAMSDEN, T.R., RAND,S.W., GEMMELL, J.B., JABLONSKI, W., SIE, S.H. & LARGE, R.R. 1992.Geologic and geochemical controls on the mineralogy and grainsize ofgold-bearing phases, eastern Australian volcanic–hosted massive sulfidedeposits. Economic Geology, 87, 542–563.

KEULEN, N., HUTCHINSON, M.T. & FREI, D. 2009. Computer-controlled scan-ning electron microscopy: a fast and reliable tool for diamond prospect-ing. Journal of Geochemical Exploration, 103, 1-5.

KNIGHT, J.B., MORISON, S.R. & MORTENSEN, J.K. 1999a. The relationshipbetween placer gold particle shape, rimming and distance of fluvial trans-port; as exemplified by gold from the Klondike District, Yukon Territory,Canada. Economic Geology, 94, 635–648.

KNIGHT, J.B., MORTENSEN, J.K. & MORISON, S.R. 1999b. Lode and placer goldcomposition in the Klondike District, Yukon Territory, Canada:Implications for the nature and genesis of Klondike placer and lode golddeposits. Economic Geology, 94, 649–664.

LEAKE, R.C., BLAND, D.J. & COOPER, C., 1992. Source characterization of allu-vial gold from mineral inclusions and internal compositional variation.

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MCCLENAGHAN, M.B. 2011. Gold and Platinum Group Element indicatorminerals in surficial sediments. Geochemistry: Exploration, Environment,Analysis, in press.

MCCREADY, A.J., PARNELL, J. & CASTRO, L. 2003. Crystalline placer gold fromthe Rio Neuquen, Argentina; implications for the gold budget in placergold formation. Economic Geology, 98, 623–633.

MCMARTIN, I., CORRIVEAU, L., BEAUDOIN, G., AVERILL, S.A. & KJARSGAARD,I.M. 2010. Heavy mineral signature of the NICO Co-Au-Bi deposit, GreatBear magmatic zone, Northwest Territories, Canada. Geochemistry:Exploration, Environment, Analysis, in press.

MOLES, N.R. & SCHAFFALITZKY, C.1992. Gold exploration methodologies andtarget definition in the Donegal Dalradian. In: BOWDEN, A.A., EARLS, G.,O’CONNOR, P.G. & PYNE, J.F., The Irish Minerals Industry 1980–1990. IrishAssociation for Economic Geology, Dublin, 119–134.

MÖLLER, P., DULSKI, P., SZACKI, W., MALOW, G. & RIEDEL, E. 1988.Substitution of tin in cassiterite by tantalum, niobium, tungsten, iron andmanganese. Geochemica Cosmochemica Acta, 52, 1497–1503.

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The heavy mineral study of surficial deposits is basically an oldprospecting method. The contrast between anomaly and back-ground values is better in heavy mineral concentrates than indry-sieved fine fraction (Stendal & Theobald 1994). Heavymineral method is best suited for searching minerals with highspecific gravity and which are stable in surficial weatheringenvironment (e.g. gold, diamonds, oxides, phosphates, andwolframates). Heavy mineral till geochemistry has been usedmostly in prospecting for gold, cassiterite, and scheelite inFinland and in Sweden (Brundin & Bergström 1977; Toverud1982, 1984; Peuraniemi 1990; McClenaghan 2005). Sulphideminerals are instead susceptible to chemical and mechanicalweathering, especially in oxidizing conditions above thegroundwater surface. Therefore heavy mineral method usingtill samples quite seldom has been used in prospecting for sul-phides (Thompson 1979; Averill 2001; Sarala & Peuraniemi2007).

KUMPUSELKÄ AND HONKANEN STUDY AREAS The study areas described here (Fig 1.) are situated 20 kmnorth from the city of Oulu, northern Finland. The area islocated in the central part of the former Fennoscandian icesheet.

Geology The area is a part of the Palaeoproterozoic Kiiminki schistbelt. The main rock types of the schist belt are mica schists,mafic volcanics, black schists, dolomitic limestones and skarnrocks. The Quaternary deposits covering the bedrock surfaceare composed of late Weichselian glacigene till deposits andglaciofluvial esker chains. Till occurs as various morphologicalforms, as cover moraine, small drumlin fields, and hummockymoraine trains. Postglacial deposits are quite thin silt and claydeposits and peat bogs. There have been two main Weichselianice movement directions, the older one from the northwestand the younger one from west-northwest. Observations havealso been made from other directions based on weaker stria-tions, whose age is unknown.

The Honkanen hummocky moraine field is situated in thecentral part of the vast mica schist area (Fig. 1). The morainefield is a 30 km long and 4-5 km broad train in a west-north-west – east-southeast direction. The field is composed ofroundish hummocks and ridges transverse to the ice-flowdirection. Mostly the moraine hummocks have the normalcontents of boulders on their surface, only some hummockshave a boulder-rich surface. The thickness of till in morainehummocks is 8-10 m and within depressions between hum-mocks only 2-3 m. Many features suggest that the Honkanenhummocky moraine field could be classified as an active-icehummocky moraine. The Kumpuselkä esker chain goesthrough a hummocky moraine field. Gravel and sand havebeen quarried and exploited for road construction material

during the last twenty years. As a result of exploitation, thegroundwater surface was exposed and forms small ponds(Eskola & Peuraniemi 2008).

FIELD AND LABORATORY STUDIESThe ore exploration project of the Department ofGeosciences, University of Oulu exposed the bedrock surfaceat Kumpuselkä and uncovered a 1-2 m thick quartz vein min-eralized with Cu, Zn, and Pb sulphides, Ni-Co arsenides, andAu. Samples from bedrock were taken with a hammer (chipsamples), using pneumatic drilling (rock flour samples) and dia-mond drilling (core samples). Five sections of the Quaternarydeposits (Kumpuselkä I – V) were cleaned and studied and 5samples (one sample 10 l) from till (KUM 2, 4, 5, 6, 8) and 3samples of esker gravel (KUM 1, 3, 7) were taken for fine frac-tion geochemistry and heavy mineral studies. All other samples,except KUM7, originally occurred under the groundwater sur-face. Figure 2 was taken from a helicopter flying at a height of300 m. The pneumatic drilling profiles and Au contents in rockflour samples and the location of till sampling sections I – Vare shown in this figure. Samples KUM 1 and 2 are from sec-tion I, KUM 3 from section II, KUM 4 from section III, KUM5 - 7 from section IV and KUM 8 from section V. Secondarysulphate precipitations found later at the bottom of the pitwere also sampled for analysis. Eh and pH (Metrohm 704) ofgroundwater ponds were measured in the field.

Applicability of sulphide and sulphate minerals in heavy mineral studiesVesa Peuraniemi* & Tiina Eskola

Department of Geosciences, P.O. Box 3000, 90014 University of Oulu, Finland(*corresponding author: [email protected])

Fig. 1. The study areas of the Kumpuselkä and Honkanen hum-mocky moraine.

28 V. Peuraniemi & T. Eskola

Two pneumatic drill holes, 1.5 km west of the Kumpuselkämineralized vein, are situated on the moraine hummock of theHonkanen field (Fig. 1). The thickness of the till is 9.4 m inhole 430, and 4.7 m in hole 431. Till samples (3l each) weretaken every metre down to the bedrock surface.

The fine fraction (-0.06 mm) of the till and gravel sampleswas dry-sieved. Heavy fraction was separated by wet-sieving (-2 mm) and by Goldhound spiral concentrator. The magneticfraction was removed with a hand magnet and the non-mag-netic fraction was split into two parts. Both the fine fractionand one part of the non-magnetic heavy fraction were analyzedchemically by AAS (Varian SpectrAA-300) for Cu, Co, Ni, Zn,Pb, Au, and Ag in the laboratory of the Department ofGeosciences, University of Oulu. The mineralogical composi-tion of the heavy fraction and sulphate precipitations was stud-ied by stereomicroscope, X-Ray diffraction (SiemensDiffractometer D-5000), and scanning electron microscopewith EDS (JEOL JSM-6400) at the Department of ElectronMicroscopy of the University of Oulu. Heavy minerals werestudied both as separate three-dimensional grains mountedonto carbon film and as polished thin sections.

RESULTSGlacial stratigraphy in the Kumpuselkä and HonkanenareasThe material in the Kumpuselkä esker is stony and sandygravel, which is quite poorly sorted in places like till. Pebbles in

the esker material have rounded edges. Between the eskergravel and the bedrock surface there is a 0.8-1.2 m thick till bed,which can be divided into lower and upper facies. The lowerfacies is grey, very tightly packed stony till. The upper facies isgrey, loose stony till with thin sand layers and stripes. Both tillfacies are composed largely of small angular pebbles. The petrographical composition of pebbles, both in the esker andin the till, is very monotonous, comprising 90% mica schists,5% vein quartz and 5% far-transported granites. Material in themoraine hummocks of the Honkanen field is composed of 2-3 till beds. All till beds are brownish grey or grey, denselypacked sandy till with large numbers of angular pebbles.

Values of pH and Eh of the groundwater ponds atKumpuselkäWater in the groundwater ponds at the bottom of the esker pitis very acidic with pH value of 3.7-3.8 and Eh values arebetween 420 and 470 mV.

Geochemistry and heavy minerals in the till and eskergravel at KumpuselkäThe base metal analyses of the fine and heavy fraction of thetill and esker gravel samples are presented in Tables 1 and 2.The Cu, Zn, and Pb contents are clearly higher in heavy min-eral concentrates than in the fine fraction of the till samples.The opposite is true of with gravel samples, where metal con-tents are higher in the fine fraction than in the heavy mineralconcentrates (Zn is an exception in two of the samples, KUM3 and KUM 7). Till sampling sections Kumpuselkä IV and Vare located 20 m from the mineralized vein in the direction ofthe latest ice movement (WNW-ESE). Samples KUM 5, KUM6, and KUM 7 from section Kumpuselkä IV and sample KUM8 from section Kumpuselkä V were studied very carefully.Heavy mineral fractions of these samples contain so muchcoarse-grained pyrite that they could be classified as pyrite con-centrates. Mostly the pyrite grains are unweathered, thoughsome of the grains are mechanically broken (Fig. 3) and someshow alterations to goethite and Fe sulphates.

Copper, Zn, Pb, and in some heavy mineral samples, Ni andCo contents were so high that more detailed examination ofthe samples was undertaken to determine which sulphides ofthose metals were present. First we picked separate grainsunder stereomicroscope of what were assumed to be chal-copyrite, sphalerite, and galena. When these grains were exam-ined with SEM+EDS, all yellow grains turned out to be pyrite.

LegendAu ppb

0.00-20.00

20.00-50.00

50.01-400.00

400.01-800.00

800.01-3029.00

Till Sections

0 7.5 15 22.5 30metres

Fig. 2. An aerial photograph showing the Kumpuselkä pit and pneu-matic drilling profiles and till sections I – V.

KUM 1 (gravel)

KUM 2 (till)

KUM 3 (gravel)

KUM 4 (till)

KUM 5 (till)

KUM 6 (till)

KUM 7 (gravel)

KUM 8 (till)

Cu ppm 306 40 427 396 42 70 146 87Zn ppm 291 162 172 511 269 260 190 273Pb ppm 192 11 174 185 17 26 77 15Ni ppm 65 54 63 197 78 79 61 53Co ppm 73 16 21 52 21 23 19 16ppAg ppm 3 2 3 3 2 2 3 3Au ppb 0 0 0 0 0 0 0 0As ppm 83 6 124 90 10 29 38 13Bi ppm 7 2 4 4 2 3 4 2U ppm 12 2 9 3 1 1 1 3Fe % 19.16 4.62 13.74 8.82 4.19 5.07 10.15 5.48S % 0.16 0.02 0.1 1.62 0.49 0.98 0.38 0.15

Table 1. Base metal analysis of the fine fraction from gravel and tillsamples.

KUM 1 (gravel)

KUM 2 (till)

KUM 3 (gravel)

KUM 4 (till)

KUM 6 (till)

KUM 7 (gravel)

KUM 8 (till)

Cu ppm 88 131 92 656 732 88 22090Zn ppm 176 239 750 4755 3593 775 1696Pb ppm 0 0 0 31 32 0 1465Ni ppm 168 186 104 675 665 104 197Co ppm 36 42 28 245 269 39 139Ag ppm 2 2 2 5 6 2 32Au ppb 0 0 5 6 0 0 0

Table 2. Base metal analysis of the heavy fraction from gravel and tillsamples.

29Applicability of sulphide and sulphate minerals in heavy mineral studies

Some of the pyrite grains have nice cubic and octahedral crys-tal forms (Fig. 4). All black and brown grains turned out to beFe and Ti oxides. It was concluded that when heavy mineralconcentrates with grain size under 1-2 mm contain a largenumber of pyrite grains it is very difficult to identify chalcopy-rite using only a stereomicroscope.

Polished thin sections studied by SEM+EDS gave mostinformative results. Sample KUM 5, from the lower till facies(section Kumpuselkä IV), contains many coarse-grained pyritegrains (diameter 1-2 mm) and only one chalcopyrite grain(diameter 1.5-2 mm; Fig. 5) with typical composition (34.05wt.% Cu, 29.58 wt.% Fe, 34.46 wt% S, SUM 98.09 wt%). Noother sulphides were found. The heavy mineral sample is quitesmall and therefore it was not possible to get the chemicalanalysis of the whole fraction. Sample KUM 6, from upper tillfacies, is more diverse and interesting in its heavy mineral com-position. Pyrite grains are also coarse with diameters 0.3-1.5mm. Only one separate chalcopyrite grain (size 0.8 x 0.5 mm)with pyrite inclusions was found. Two separate sphaleritegrains (size 0.3 x 0.5 mm; 62.60-62.43 wt% Zn, 32.72-32.24wt% S, 1.03-0 wt% Fe, SUM 96.36-94.67 wt%) were found(Fig. 6). Because the amount of heavy minerals in this sampleis high, these three grains alone do not completely explain thehigh Cu and Zn contents of the heavy fraction. When thepyrite grains were studied with higher magnification, many ofthem were found to contain small-size inclusions (from a fewmicrons to 50 microns in diameter) of chalcopyrite and spha-lerite (Figs. 7 and 8). Also very small-size (0.5-1 micron) galenagrains (63.24 wt% Pb, 8.43 wt% Fe, 3.90 wt% Se, 16.48 wt% S)were found as inclusions in some pyrite grains (Fig. 9 ) as wellas some rutile inclusions. On the basis of these results, it is nosurprise that a stereomicroscope was not able to find chal-copyrite, sphalerite, or galena grains; small-size inclusions inpyrite cannot be seen with the magnifications of the stereomi-croscope.

Sample KUM 7, from esker gravel (section KumpuselkäIV), also contains large numbers of pyrite grains. Some grainshave very sharp cubic crystal form, but many grains are wornand broken. Their grain size is smaller (0.1-0.5 mm) than thatof the pyrite grains in underlying till samples (KUM 5 andKUM 6). Sample KUM 7 also contains a lot of ilmenite grainswith rounded edges than the pyrite grains. Sample KUM 8,from lower till facies (section Kumpuselkä V), contains a lot ofcoarse pyrite grains, with grain size up to 2 mm. Sphalerite was

Fig. 3. A mechanically broken pyrite grain from sample KUM 4.Polished section, SEM, backscattered electron image.

Fig. 4. The octahedral crystal form of a pyrite grain from sampleKUM 5. SEM, secondary electron image.

Fig. 5. A chalcopyrite grain from sample KUM 5. Polished section,SEM, backscattered electron image.

Fig. 6. A sphalerite grain from sample KUM 6. Polished section,SEM, backscattered electron image.


Fig. 7. Small bright chalcopyrite inclusions in a pyrite grain from sam-ple KUM 6. Polished section, SEM, backscattered electron image.

Fig. 8. Small bright sphalerite inclusions in a pyrite grain from sam-ple KUM 6. Polished section, SEM, backscattered electron image.

Fig. 9. Two very bright galena inclusions and one light grey chal-copyrite inclusion in a pyrite grain from sample KUM 6. Polished sec-tion, SEM, backscattered electron image.

Fig. 10. A bright galena grain from sample KUM 8. The darker grainin the left corner of the photo is pyrite. Polished section, SEM,backscattered electron image.

Fig. 11. Cobaltian arsenopyrite grains from sample KUM 4. The twodark grey grains are pyrite. Polished section, SEM, backscattered elec-tron image.

Fig. 12. A worn and porous Ni-bearing cobaltite grain from sampleKUM 4. Polished section, SEM, backscattered electron image.


identified as small inclusions (grain size 10-50 microns) inpyrite grains. One separate galena grain was found (grain size0.45 mm, Fig. 10). Altogether 10 coarse chalcopyrite grains(grain size up to 1 mm) were identified from the polished thinsection. The heavy fraction of till sample KUM 8 yielded veryhigh Cu contents of 2.2%, indicating that the sample is rich inchalcopyrite. However, none of these grains could be identi-fied with a stereomicroscope.

Sample KUM 4 is from till (section Kumpuselkä III) on thesouthern side of the moraine hummock pit, next to the esker.Glacial dispersal from the mineralized vein has not beendirected in any phase to this site. Sample KUM 4 contains quitea few pyrite grains with grain size a little bit smaller than isfound in samples KUM 5 and KUM 6. Some pyrite grains arebroken. Only two small sphalerite grains (10 and 50 microns)were found. Chalcopyrite was not identified. Two types ofarsenides were found: cobaltian arsenopyrite (Fig. 11) and Ni-bearing cobaltite (Fig. 12). The cobaltite grain is worn andporous. Some monazite grains were also identified.

During the dry summers of 2004-2010, white frost- orsnow-like material was found at the bottom of theKumpuselkä pit (Fig. 13). It occurs as thin rims surroundingsmall pebbles, as encrustations on twigs fallen onto ground,and as clusters of grains between sand and gravel particles.These secondary precipitates were analyzed and found to con-sist of long and narrow prismatic fibres of gypsum (Fig. 14),the spines of pickeringite (MgAl2(SO4)4•22H2O, Fig. 15.), andat least two types of Mg sulphates, which on the basis of SEM and XRD results were interpreted as hexahydrite(MgSO4•6H2O) and kieserite (MgSO4•H2O). Some Mg-sul-phate grains and grain aggregates have high Cu contents (up to18.05% Cu) and along with elevated concentrations of Zn andNi (1-3 %). The bulk sample of the sulphate precipitate con-tains 4436 ppm Cu, 5994 ppm Zn, 2358 ppm Ni, 739 ppm Co,and 13 ppm Pb. Thus, when part of the primary sulphide min-erals have been weathered, the liberated base-metal cations(with the exception of Pb) are bound and enriched to second-ary sulphates. It could be possible to use such sulphates as indi-

Fig. 13. White frost- or snow-like material in the Kumpuselkä pit in the summer of 2006. Photo V. Peuraniemi.

Fig. 15. Spines of pickeringite and grains of Mg-sulphate in the sec-ondary sulphate precipitate at Kumpuselkä. SEM, secondary electronimage.

Fig. 14. A long fibre of gypsum in the secondary sulphate precipitateat Kumpuselkä. SEM, secondary electron image.


cators of primary sulphide mineralizations, however, there isone problem, these secondary sulphates are quite unstableminerals and in rainy seasons they are easily dissolved by rain-water.

Geochemistry and heavy minerals in the till ofHonkanen morainic hummockMaterial in drill hole 430 on the moraine hummock ofHonkanen is composed of grey sandy till. Groundwater sur-face is at a depth of 7.9 m. The maximum Pb content in thefine fraction of the till (179 ppm) occurs just in the uppermostsample, at a depth of 1.0-1.6 m, above groundwater surface inthe oxidized zone of till. The next sample at a depth of 2-3 m,also in the oxidized zone of till, contains in its fine fraction 45ppm Pb. The contents of other metals in these samples arequite normal (74-87 ppm Cu, 147-149 ppm Zn, 49-66 ppm Ni,12-26 ppm Co, 0-3 ppb Au, 18-20 ppb Ag). The heavy mineralconcentrate of sample 430 (1.0-1.6 m) contains a fair amountof pyrite grains with a grain size of 0.3 to 1 mm. Some grainsare unweathered and sharp-edged, while some are altered tohydrous Fe oxides and sulphates. One chalcopyrite grain wasfound. Two galena grains with a size of 0.1 to 0.5 mm wereidentified (Fig. 16). Their edges have been altered to secondaryPb-Fe sulphate, the chemical composition of which is similarto that of plumbojarosite (PbFe6(SO4)4(OH)12). Sphaleritewas found as small inclusions (10-20 microns) in pyrite grains;small inclusions of rutile were also found.

The heavy mineral concentrate of sample 430 (2-3 m) con-tains quite a few pyrite grains. Some pyrite grains have nicecubic crystal forms (Fig. 17). Alteration to hydrous Fe oxides(goethite, FeO(OH)) and hydrous Fe-K sulphate (jarosite,KFe3(SO4)2(OH)6) is seen in some pyrite grains. Chalcopyrite,galena, and sphalerite were not found as independent grains,but they occur as tiny inclusions in many of the pyrite grains(Fig. 18). Chalcopyrite is most abundant, followed by galenaand then sphalerite.

Material in drill hole 431 on the moraine hummock ofHonkanen is composed of grey sandy till. The groundwatersurface is at a depth of 3.0 m. The uppermost sample is froma depth of 0.3-1 m, in the oxidized zone. Its fine fraction con-

Fig. 16. A bright galena grain with a grey alteration rim of plumbo-jarosite in sample 430 (1.0-1.6 m). Polished section, SEM, backscat-tered electron image.

Fig. 17. The cubic crystal form of a pyrite grain from sample 430(2.0-3.0 m). Polished section, SEM, backscattered electron image.

Fig. 18. Three small and bright galena inclusions and one bigger lightgrey chalcopyrite inclusion in a pyrite grain from sample 430 (2.0-3.0 m). Polished section, SEM, backscattered electron image.

Fig. 19. Bright chalcopyrite inclusions in a pyrite grain from sample431 (0.3-1.0 m). Polished section, SEM, backscattered electron image.


tains 148 ppm Cu, 244 ppm Zn, 59 ppm Ni, 26 ppm Co, 0 ppbAu, and 12 ppb Ag. The heavy mineral concentrate of thatsample contains an abundance of pyrite grains, many of whichare coarse-grained. Chalcopyrite was not found as independentgrains, but many pyrite grains contain a large numbers of smallchalcopyrite inclusions (with the grain size of 10-70 microns,Fig. 19).

CONCLUSIONSGeochemistry and heavy minerals from till and esker gravel atKumpuselkä near a mineralized vein and those in the oxidizedtill of the Honkanen moraine hummock were studied in orderto find out how sulphide minerals have survived in surficialdeposits and how they could be used in an indicator mineralmethod in ore exploration. Samples from Kumpuselkä containa large number of coarse pyrite grains, most of which areunweathered and show nice cubic crystal forms. Base metalcontents are high in heavy mineral concentrates, but with astereomicroscope it was not possible to identify any chalcopy-rite, sphalerite, or galena grains. Careful study with SEM+EDSwas more informative. Only a few separate chalcopyrite, spha-lerite, and galena grains were found, but they were found tooccur also as small-sized inclusions in pyrite grains. In additionto a large number of pyrite grains, Sample KUM 4 containsone sphalerite grain, one cobaltian arsenopyrite grain, and oneNi-bearing cobaltite grain. Sulphide minerals from the sampleKUM 4 in the till section III are an indication of unknownmineralization to the west of the Kumpuselkä pit. Secondarysulphate precipitates at the bottom of the gravel pit are com-posed of gypsum, pickeringite, and Mg sulphates. Copper, Zn,Ni, and Co, liberated by weathering, are enriched in these pre-cipitates.

Heavy mineral samples from oxidized till in the morainehummock at Honkanen contain numerous pyrite grains. Someseparate chalcopyrite and galena grains were found, but mainlychalcopyrite, galena and sphalerite occur as small inclusions inpyrite grains. Also identified were secondary minerals goethite,jarosite, and plumbojarosite.

When the grain size of sulphide minerals in till is big orwhen the base metal (Cu, Zn, Ni, Co, Pb) sulphides occur astiny inclusions in coarse pyrite grains, the conventional fine-

fraction (-0.06 mm) till geochemistry cannot be used as a reli-able indication to the mineralization. This is very clearlydemonstrated by till samples KUM 5, 6, and 8, whose basemetal contents are quite normal but the heavy sand fractioncontains plenty of sulphide minerals. In this case, the use ofheavy minerals is a better method. The mode of occurrence ofsulphide minerals and their alteration products in heavy min-eral concentrates are best studied by SEM+EDS.

The results show, that sulphide minerals could have survivedeven in the oxidized zone of till and they can be used inprospecting for sulphide ores using a heavy mineral method.Secondary sulphates could be indicators, too, but there areproblems associated with their solubility in water.

REFERENCESAVERILL, S. A. 2001. The application of heavy indicator mineralogy in mineral

exploration with emphasis on base metal indicators in glaciated metamor-phic and plutonic terrains. In: MCCLENAGHAN, M.B., BOBROWSKY, P.T.,HALL, G.E.M. & COOK, S.J. (eds.) Drift Exploration in Glaciated Terrain.Geological Society, London, Special Publications, 185, 69-81.

BRUNDIN, N.H. & BERGSTRÖM, J. 1977. Regional prospecting for ores base onheavy minerals in glacial till. Journal of Geochemical Exploration, 7, 1-19.

ESKOLA, T. & PEURANIEMI, V. 2008. Secondary sulphate precipitates in thegravel pit at Kumpuselkä esker, northern Finland. Mineralogical Magazine,72, 415-417.

MCCLENAGHAN, M.B. 2005. Indicator mineral methods in mineral exploration.Geochemistry: Exploration, Environment, Analysis, 5, 233-245.

PEURANIEMI, V. 1990. Heavy minerals in glacial material. In: Kujansuu, R. &Saarnisto, M. (eds.) Glacial Indicator Tracing, A.A. Balkema, Rotterdam, 165-185.

SARALA, P. & PEURANIEMI, V. 2007. Exploration using till geochemistry andheavy minerals in the ribbed moraine area of southern Finnish Lapland.Geochemistry: Exploration, Environment, Analysis, 7, 195-205.

STENDAL, H. & THEOBALD, P.K. 1994. Heavy-mineral concentrates in geo-chemical exploration. In: Hale, M. & Plant, J.A. (Eds.) Drainage Geochemistry,Handbook of Exploration Geochemistry, 6, 185-225.

THOMPSON, I.S. 1979. Till prospecting for sulphide ores in the Abitibi Clay Beltof Ontario. Canadian Institute of Mining and Metallurgy Bulletin, 72(807), 65-72.

TOVERUD, Ö. 1982. Geochemical prospecting for tin in upper central Swedenusing heavy-mineral concentrate of glacial till. In: DAVENPORT, P.H. (ed.)Prospecting in Areas of Glaciated Terrain 1982. Canadian Institute of Mining,204-212.

TOVERUD, Ö. 1984. Dispersal of tungsten in glacial drift and humus inBergslagen, southern central Sweden. Journal of Geochemical Exploration, 21,261-272.


Iron oxides (hematite and magnetite) are common mineralsfound in magmatic and metamorphic rocks and can be majorto accessory minerals in a range of mineral deposit types. Therange in chemical composition of iron oxides in the variousdeposit types is related to different environments and condi-tions of formation, such that this compositional variety can beused to fingerprint mineral deposit types. The natural variationof mineral chemistry of iron oxides from a range of mineraldeposits will be reviewed, from which we have derived dis-criminant diagrams using the trace element chemistry of mag-netite and hematite. These diagrams are useful to distinguishvarious types of mineral deposits and provide a method toidentify the sources of grains from the ferromagnetic fractionof heavy mineral concentrates from bedrock and derived gla-cial sediment samples. Application of the method will be illus-trated by case studies for Ni-Cu deposits in the ThompsonNickel Belt (Canada), for iron oxide Cu-Au deposits in theGreat Bear Magmatic Zone (Canada), and for regional metal-logeny in Mauritania.

DISCRIMINANT DIAGRAMSA set of discriminant diagrams has been proposed by Dupuis& Beaudoin (2011). The discriminant diagrams are based onanalysis of more that 2000 iron oxide grains from 113 mineraldeposits of 13 deposit types. Mineral composition was meas-ured using a highly efficient and sensitive electron microprobemethod optimized for trace element analysis in iron oxides(Dupuis & Beaudoin 2011). We summarize here the use of thediscriminant diagrams to fingerprint several types of mineraldeposits. The data screening method uses an interpretationflow sheet that facilitates identification of the mineral deposittypes investigated so far.

First, the Ni+Cr vs. Si+Mg diagram (Fig. 1A) is efficient inseparating the average magnetite composition of Ni-Cudeposits from the average composition of magnetite and/orhematite from all other deposit types. Second, magnetite andhematite plotting outside the field for Ni-Cu deposits in theNi+Cr vs. Si+Mg diagram, are compared with compositionsthat are typical of volcanogenic massive sulphide (VMS)deposits using the Al/(Zn+Ca) vs. Cu/(Si+Ca) diagram (Fig.1B), which efficiently separates VMS deposits from all otherdeposit types.The exception being Ni-Cu deposits alreadyidentified using the Ni+Cr vs. Si+Mg diagram. Therefore, aniron oxide plotting in the field for Ni-Cu deposits in Figure 2should not be plotted on the Al/(Zn+Ca) vs. Cu/(Si+Ca) dia-gram (Fig. 1B). Magnetite from the Faro Vangorda clastic-dominated Pb-Zn deposit (Yukon, Canada) plots in the fieldfor VMS deposits. More studies are required to distinguish thecompositional characteristics of iron oxides from VMS andclastic-dominated Pb-Zn mineral deposits. The final step uses

the Ni/(Cr+Mn) vs. Ti+V (Fig. 1C) and the Ca+Al+Mn vs.Ti+V (Fig. 1D) diagrams to discriminate IOCG (iron-oxide-copper-gold), Kiruna, BIF (banded iron formation), porphyryCu, skarn, and Fe-Ti-V deposits. These two diagrams, however,do you not discriminate for Ni-Cu and VMS deposits, suchthat iron oxide compositions that plot in the fields for Ni-Cu(Fig. 1A) and VMS (Fig. 1B) deposits should not be plotted ineither the Ni/(Cr+Mn) vs. Ti+V (Fig. 1C) or the Ca+Al+Mnvs. Ti+V (Fig. 1D) diagrams. The field outlined for the com-position of BIF deposits in the Ca+Al+Mn vs. Ti+V diagramis preliminary and displays overlap with Opemiska Cu veinsand Archean porphyry Cu-Au deposits. Skarn deposits displaya range of compositions plotting in broad fields in both theNi/(Cr+Mn) vs. Ti+V (Fig. 1C) or the Ca+Al+Mn vs. Ti+V(Fig. 1D) diagrams.

FERROMAGNETIC FRACTION OF TILL SAMPLESFROM THE THOMPSON NICKEL BELT,

MANITOBA, CANADAIn addition to their ubiquity in mineral deposits, magnetite andhematite have potential to be used for mineral exploration inareas covered by recent sediments. Magnetite and hematitehave a high specific gravity (5.18 and 5.26, respectively) andmay be easily separated from the heavy mineral fractions ofsurficial sediments. Magnetite and hematite are resistant tochemical weathering and mechanical abrasion during transport,an important feature of indicator minerals (Belousova et al.2002). Because of their resistance to weathering, high specificgravity, and chemical composition variability, the iron oxidescan be useful indicator minerals for exploration in weatheredterrains.

The <2.0 mm ferro-magnetic fraction of till is routinelyrecovered from exploration samples during heavy mineral pro-cessing at commercial laboratories and set aside for archivingor future examination (McClenaghan, in press). In theThompson Nickel Belt (TNB), Manitoba, Canada, for exam-ple, till samples contain between 1 and 80 g of magnetite per10 kg of sample, on average 8 g. This translates into 1000s ofgrains of magnetite per 10 kg of till. The large number ofgrains in the ferro-magnetic fraction of a till sample requiresfurther splitting before selecting grains for electron micro-probe analysis.

Figure 2A shows the result of a test where 5 sub-samplesfrom the 0.5-1.0 mm size ferromagnetic fraction of till sample06-MPB-61 were split using a plastic riffle. Each sub-samplecontained between 33 and 59 grains and weighed between 0.07and 0.10 g, which is approximately 10-5 the weight of the orig-inal till sample, such that each grain corresponds to less than 1ppm of the sample weight (2 x 10-7). Of the grains separatedinto each sub-sample, however, only 45 to 70% plot in the dia-

Application of iron-oxide discriminant diagrams in mineral explorationGeorges Beaudoin1, Céline Dupuis1, Beth McClenaghan2,

Jennifer Blain1 & Isabelle McMartin2

1. Département de géologie et de génie géologique, Université Laval, Québec, Québec Canada G1R 0A62. Geological Survey of Canada, 601 Booth Street Ottawa, Ontario Canada K1A 0E8

36 G. Beaudoin, C. Dupuis, B. McClenaghan, J. Blain, & I. McMartin

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37Application of iron-oxide discriminant diagrams in mineral exploration

gram: other grains are titano-magnetites of various composi-tions that are not considered further because the Ti content istoo high. The five sub-samples shown in Figure 2A display asimilar range of values in the Ni+Cr vs. Si+Mg diagram, andthe average composition of each sub-sample is similar. Thissimilarity indicates that a small sub-sample of the 0.5-1.0 mmsize fraction can be separated to provide representative infor-mation on the chemical composition of the iron oxides.

The chemical composition of the 0.5-1.0 mm ferromagneticfraction of till sample 06-MPB-61 contains magnetite grainsthat plot in the field for Ni-Cu deposits. Sample 06-MPB-61was collected 200 m west of the shoulder of the Pipe Mineopen pit (Thompson Nickel Belt, Manitoba, Canada), down-iceof the ore zone. For each sub-sample, between 1 and 7 grainsplot in the field for Ni-Cu deposits, which represents 5 to 39%of the grains analysed: 20 of the 131 analyses (15%) plot in thefield for Ni-Cu deposits. Sample 06-MPB-58 was collected 600m west from the Pipe Mine open pit, further down-ice fromsample 06-MPB-61. Figure 2B shows that 5 sub-samples of the0.5-1.0 mm size fraction display similar compositional charac-teristics, a feature also shown for sample 06-MPB-61. Figure2B shows that only 8 of 314 (3%) magnetite analyses plot inthe field for Ni-Cu deposits. The number of magnetite grainsfrom till 600 m down-ice is lower than that 200 m down-icefrom the outcropping Ni-Cu mineralization. More studies arerequired to better document the relationship between mag-netite grain abundance and distance to source from erosion ofNi-Cu mineralization shown by this preliminary test.

FERROMAGNETIC FRACTION FROM BEACH SAND, MAURITANIA

The coast of Mauritania, Africa, contains black sands rich inilmenite, rutile, zircon, monazite, and magnetite in well sorted,fine- to medium-grained sand in coastal dunes, beaches, andinlets. Samples collected form layers rich in heavy minerals inbeach and aeolian dunes near El Msid, 80 km north ofNouakchott, Mauritania, are used here to asses use of thechemical composition of magnetite and hematite for evalua-tion of metallogenic potential of the hinterland. The black

sand samples were split in different grain size sub-samples, andthe ferromagnetic fraction was separated with a magnet.

The sedimentary origin of the heavy mineral layers has notbeen studied in detail, but geological observations indicate acomplex history of beach and dune deposits reworked duringseveral episodes of marine transgression and regression.Dominant dune orientation is northeast-southwest. West, andnortheast, from the littoral dune ribbon, a wide sebkha plain,above Mesozoic and Cenozoic sedimentary rocks of theCoastal basin, overlies to the west of the Neoproterozoic toPalaeozoic Maurinatides Belt. The Mauritanides containknown IOCG deposits such as Guelb Moghrein, and numberof small VMS and Ni-Cu massive sulphide occurrences. Westof the Mauritanides, the Paleoproterozoic to PalaeozoicTaoudeni Basin consists of a thick succession of sedimentaryrocks. The Mauritanides and the Taoudeni Basin overly thePrecambrian Reguibat shield, which crops out to the north andhost important BIF deposits near Zouerate and orogenic golddeposits at Tasiast. The Reguibat shield consists of a complexassemblage of Archean and Paleoproterozoic belts of plutonicand volcano-sedimentary rocks.

The composition of magnetite and hematite grains, fromthe size fraction larger than 0.315 mm, plotted on the discrim-inant diagrams described in Figure 1, is shown in Figure 3. InFigure 3A, 2 of 59 grains (3.4%) plot in the field for Ni-Cudeposits on the Ni+Cr vs. Si+Mg discriminant diagram. Inboth grains, the magnetite analyzed contains oriented hematiteexsolutions. The small proportion of grains plotting in thefield for Ni-Cu deposits is consistent with the rare occurrenceof small Ni-Cu showing in the Mauritanides (Gunn et al.,2004). After removing the data from these 2 grains, the remain-ing 2 of the 57 grains (3.5%) plot in the field for VMS depositson the Al/(Zn+Ca) vs. Cu/(Si+Ca) discriminant diagram (Fig.3B). Here also, the small proportion of analyses plotting in thefield for VMS deposits is consistent with the few and smallVMS occurrences in the Mauritanides (Gunn et al. 2004).

The Ni/(Cr+Mn) vs. Ti+V (Fig. 3C) and the Ca+Al+Mn vs.Ti+V (Fig. 3D) discriminant diagrams are used to asses poten-tial for IOCG, Kiruna-type and porphyry Cu deposits. Dataplotting in the fields for Ni-Cu (Fig. 3A) and VMS (Fig. 3B)

200 m west of Pipe pit

Ni+

Cr (

wt.%

)

.01 .1 1

.01

.1

1Ni-Cu

MPB-61A

MPB-61B

MPB-61C

MPB-61D

MPB-61E

Si+Mg (wt.%) Si+Mg (wt.%)

Ni+

Cr (

wt.%

)

600 m west of Pipe pit

.01 .1 1

.01

.1

1

MPB-58AMPB-58BMPB-58CMPB-58DMPB-58E

Fig. 2. Composition of till sampled down-ice from the Pipe Mine, Manitoba, Canada. A) Composition of 5 sub-samples from sample 06-MPB-61. Filled symbol is the average composition for each sub-sample. B) Composition of 5 sub-samples from sample 06-MPB-58. The field for Ni-Cu deposits is from Dupuis & Beaudoin (2011).

A B

38 G. Beaudoin, C. Dupuis, B. McClenaghan, J. Blain, & I. McMartin

deposits are not considered here. The two diagrams showslightly different abundance of data points in the fields forIOCG, Kiruna-type and porphyry Cu deposits. In theNi/(Cr+Mn) vs. Ti+V diagram (Fig. 3C), 2 analyses (3.4%)plot in the field for IOCG deposits, 1 analysis (1.8%) plots isthe field for Kiruna-type deposits, and 1 analysis (1.8%) plotsat the edge of the field for porphyry Cu deposits. In theCa+Al+Mn vs. Ti+V diagram (Fig. 3D), respectively 6(10.9%), 3 (5.5%), and 2 (3.4%) analyses plot in the fields forIOCG, Kiruna, and porphyry Cu deposits. The potential forporphyry Cu deposits is low in the Mauritanides, but IOCG(e.g. Guelb Moghrein) and Kiruna-type deposits and occur-rences are known in the Mauritanides Belt (Gunn et al. 2004).The Guelb Moghrein deposit, in particular, is northwest fromthe samples location, along the dominant wind direction. Noanalyses plot in the field for BIF in the Ni/(Cr+Mn) vs. Ti+Vdiagram (Fig. 3C), whereas 3 data plot at the margins of theBIF field in the Ca+Al+Mn vs. Ti+V diagram (Fig. 3D). Thisresult is perhaps unexpected considering the major irondeposits in northern Mauritania, near Zouerate. A likely expla-nation for this discrepancy lies in the sample locations beingseveral hundred kilometres south of the intersection of theMauritania coast with the dominant southeasterly winds blow-ing from Zouerate. This hypothesis could be tested by analyz-

ing the composition of magnetite and hematite from the blacksands deposits in northern Mauritania.

CONCLUSIONS1. Iron oxides display composition characteristics that allow

identification of the mineral deposit type in which theyformed.

2. Aliquot sub-samples of the ferromagnetic fraction fromtill heavy mineral concentrate carry magnetite composi-tion representative of the sample.

3. Sand-size grains (0.5-1.0 mm) from the ferromagneticfraction of till samples down-ice from Ni-Cu mineraliza-tion contain magnetite derived from erosion of this typeof mineralization.

4. Magnetite and hematite composition from the ferromag-netic fraction of heavy mineral concentrates from blacksand deposits from the coast of Mauritania identifieshigher potential for IOCG deposits, and low potential forNi-Cu, VMS, and porphyry Cu deposits, consistent withknown deposits.

5. The detection of mineral deposits in the hinterlandappears to be controlled by the dominant wind direction,which transports iron oxides towards the coast.

7A

Ni+

Cr (

wt.%

)N

i+C

r (w

t.%)

Ca+

Al+

Mn

(wt.%

)A

l(Zn+

Ca)

.01 .1 1

.01

.1

1Ni-Cu

3A

4A

5A

.01 .1 1 10

.1

1

10

VMS

.01 .1 1 10

.1

1

10

Fe-Ti

Kiruna

PorphyryIOCG

BIF

(Skarn)

Opemiska

.01 .1 1 10.01

.1

1

Fe-Ti

Kiruna

Porphyry

IOCGBIF

Skarn

Ti+V (wt.%) Ti+V (wt.%)

Cu/(Si+Ca)Si+Mg (wt.%)

Fig. 3. Composition of iron oxides from beach sand from El Msid, Mauritania. A) Ni+Cr vs. Si+Mg discriminant diagram. B) Al/(Zn+Ca) vs.Cu/(Si+Ca) discriminant diagram. C) Ni/(Cr+Mn) vs. Ti+V discriminant diagram. D) Ca+Al+Mn vs. Ti+V discriminant diagram.

A B

C D

39Application of iron-oxide discriminant diagrams in mineral exploration

REFERENCESBELOUSOVA, E.A., GRIFFIN, W.L., O'REILLY, S.Y. & FISHER, N.I. 2002. Apatite

as an indicator mineral for mineral exploration: Trace-element composi-tions and their relationship to host rock type. Journal of GeochemicalExploration, 76, 45-69.

DUPUIS, C. & BEAUDOIN, G. 2011. Discriminant diagrams for iron oxide traceelement fingerprinting of mineral deposit types. Mineralium Deposita, 46,319-335 (http://dx.doi.org/10.1007/s00126-011-0334-y).

GUNN, A.G., PITFIELD, P.E.J., MCKERVEY J.A., KEY, R.M., WATERS, C.N. &BARNES, R.P., 2004. Notice explicative des cartes géologiques etgîtologiques à 1/200 000 et 1/500 000 du Sud de la Mauritanie. Volume 2– Potentiel Minier. DMG, Ministère des Mines et de l’Industrie,Nouakchott.

MCCLENAGHAN, M.B. in press. Overview of common processing methods forrecovery of indicator minerals from sediment and bedrock in mineralexploration. Geochemistry: Exploration, Environment, Analysis.


The Pebble deposit in southwest Alaska (Fig. 1) contains oneof the largest resources of copper and gold in the world. Itincludes a measured and indicated resource of 5,942 milliontonnes (Mt) at 0.42% Cu, 0.35 g/t Au, and 250 ppm Mo(0.30% copper equivalent, CuEQ, cut off) and contains signif-icant concentrations of Ag, Pd, and Re (Northern DynastyMinerals 2011). The deposit remains open at depth.

The Pebble West zone was discovered in 1989 by ComincoAmerican. In 2005, Northern Dynasty Minerals Ltd. (NDM)discovered Pebble East, and in July 2007, NDM partnered withAnglo American to form the Pebble Limited Partnership(PLP). The U.S. Geological Survey began collaborative investi-gations with PLP in 2007 to identify techniques that willimprove mineral exploration in covered terranes. The Pebbledeposit is an ideal location for such a study because the deposit

is undisturbed (except for drilling), is almost entirely concealedby post-mineral volcanic rocks and glacial deposits, andbecause its distribution is well constrained in the subsurface byPLP’s drill-hole geology and geochemistry.

An exploration method developed by Averill (2007) that uti-lizes porphyry copper indicator minerals (PCIM®) in glacial tillsamples was applied at Pebble; samples were collected up- anddown-ice (of former glaciers) from the deposit. The distribu-tion of several PCIMs identifies the deposit, which suggeststhat PCIMs may be useful in exploration for other concealedporphyry deposits in the region. In this study, we compare theefficacy of PCIMs relative to that of pond and stream sedi-ments also collected in the deposit area.

The Pebble deposit is located 380 km southwest ofAnchorage, in the Bristol Bay region of southwest Alaska.

Exploration case study using indicator minerals in till at the giant Pebble porphyry Cu-Au-Mo deposit, southwest Alaska, USA

Robert G. Eppinger*, Karen D. Kelley, David L. Fey, Stuart A. Giles & Steven M. SmithU.S. Geological Survey, PO Box 25046, MS 973, Denver, CO, USA

(*corresponding author: [email protected])

Kvichak stade moraine

52 Zone

308 Zone

Sill25 Zone

38 Zone

37 Zone

G-N Zone

Pebble WestPebble East

ntMilutkoK

KaskanakMtn.

SharpMtn.

GroundhogMtn.

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.rCkroF.Silutko

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reppU

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N’5

5°9

5N’

05°

95

155°30’W 155°10’W

ALASKA

Glacial till sample site

Ice direction Mineral occurrence

PLP boundary

Sediment sample site

Fig. 1. Locations of glacial till,stream, and pond sediment samplesites surrounding the Pebble deposit,and the distribution of glacial tillsdeposited during the Kvichak stade.Simplified from Hamilton &Klieforth (2010) and Detterman &Reed (1973). Boundaries for PebbleWest (black dotted) and Pebble East(black solid line) zones are based on>0.3% CuEQ and >0.6% CuEQ,respectively.

42 R.G. Eppinger, K.D. Kelley, D.L. Fey, S.A. Giles & S.M. Smith

There is no road network and access to the study area is by hel-icopter. The deposit is situated in a broad glacially sculptedtopographic low at the head of three drainages, Talarik Creek,North Fork Koktuli River, and the South Fork Koktuli River(Fig. 1). The study area is in a zone of discontinuous per-mafrost and is masked by lichen-rich tundra vegetation.

GEOLOGY AND MINERALIZATIONThe Pebble district comprises Jura-Cretaceous andesiticargillite, siltstone, and wacke, cut by diorite sills and otherintrusions of diverse composition (Bouley et al. 1995).Subalkalic granodiorite intrusions (91-89 Ma) include theKaskanak Batholith (Kaskanak Mountain) and smaller satellitebodies that are genetically related to Cu-Au-Mo mineralization(Lang et al. 2008).

The surficial geology is dominated by the effects of latePleistocene glaciation (Hamilton & Klieforth 2010). The initialand most extensive glacial advance, the Kvichak stade, resultedin distinctive sets of moraines that are present through most ofthe Pebble area lowlands (Fig. 1). Ice-flow directions weredominantly to the southwest and northwest (Hamilton &Klieforth 2010). Glaciers periodically blocked each of the threemajor drainages in the Pebble project area, resulting in tran-sient ice-dammed lakes that filled lowlands in headward partsof each drainage (Fig. 1). Glacial features include terminal andlateral moraines, meltwater channels, outwash deposits, numer-ous kettle depressions now occupied by ponds, and ice-damglacio-lacustrine deposits. Unconsolidated glacial deposits varyfrom 0 m locally at Pebble West to 50 m thick over parts ofPebble East and West (Hamilton & Klieforth 2010).

The Pebble West mineralized zone extends from the surfaceto about 500 m depth. The East zone extends to at least 1700m depth and is concealed by a wedge of Late Cretaceous toEocene volcaniclastic sedimentary rocks (Bouley et al. 1995;Lang et al. 2008). Mineralized rock is found in potassium-sili-cate alteration zones and in stockworks of quartz-carbonate-sulphide veins. Ore minerals include chalcopyrite, molybdenite,and native gold that is found mostly within chalcopyrite. High-grade, bornite-bearing mineralization is present in the core ofthe East zone (Lang et al. 2008). The upper portion of thePebble West zone includes a supergene enrichment zone, a fea-ture not present at Pebble East.

The Pebble porphyry deposit is the first significant mineral-ized porphyry system to be discovered in the region and sug-gests the possibility of a major new porphyry province insouthwestern Alaska. Pebble has been identified as an exampleof a major porphyry deposit that occurs in isolation or distal,compared to the more common occurrence of multipledeposits within orogen-parallel belts and provinces (Sillitoe2010). However, the apparent isolation of Pebble might be aconsequence of extensive cover and/or the relative infancy ofexploration in the region. Nearby small concealed occurrencesbeing explored by PLP include porphyry (38, 308, 52, and G-N (?) zones), skarn (37, G-N?), and epithermal (25 zone andSill) occurrences (Fig. 1).

METHODSSeventy glacial till samples (6-15 kg) were collected up- anddown-ice from the deposit over a total distance of about 42 km

(Fig. 1). Shallow till samples (30-50 cm deep) were collectedfrom moraine ridges and consist of poorly sorted to unsortedmaterial ranging from muddy gravel to sandy coarse gravel.Pebbles >2 cm were removed by hand. Samples were air dried,disaggregated, and sieved to obtain a <2 mm fraction. Goldgrain counts and grain morphology were determined opticallyafter the samples were fed through a shaking table. The <2 mmsamples were then subjected to heavy liquid (S.G. 2.8-3.2 and>3.2) and ferromagnetic separations, and the non-ferromag-netic heavy mineral concentrate fraction was separated into the0.25-0.50 mm, 0.5-1.0 mm, and 1.0-2.0 mm fractions, whichwere examined optically to determine the abundance ofPCIMs. Details about the methodology are included in Kelleyet al. (2011).

Bottom sediments from ponds (80) and bedload stream sed-iments (14) were collected from the study area. Sampling wasconcentrated near known deposits, but a few samples were col-lected at regional background sites (Fig. 1). Sites sampledinclude open-system lakes and ponds with inlets and outlets,and closed-system seepage lakes and ponds supplied by groundwater, as well as small first-order streams. Most sediment siteswere situated in glacial drift (kettles), but some were in glacialbeach, deltaic, or lacustrine deposits. At each site, about 20subsamples were composited using a long-handled PVC scoop.Sample material included detrital gravel, sand, and silt, andcommonly contained high amounts of organic debris. Thematerial was washed and screened on site through a 2 mm (10mesh) stainless steel screen. Subsequently, samples were airdried, sieved to <0.18 mm (80 mesh), and the minus-80-meshfraction was ground for analysis. Samples were dissolved usingboth a near-total, four acid (hydrochloric, nitric, perchloric, andhydrofluoric acids) digestion and a total sodium peroxide sin-ter, and analysed for 60 elements by ICP-AES and -MS meth-ods. Details concerning sample collection, dissolution, andanalysis, and a detailed quality control assessment are inAnderson et al. (2011).

RESULTS AND DISCUSSIONOur study shows that the most effective indicator minerals rel-ative to the Pebble deposits are andradite garnet, Mn-epidote,gold, and jarosite. Other indicator minerals are present in oneor more samples but a direct association with mineralized rockof the Pebble deposit is not evident (Kelley et al. 2011).

Garnet and Mn-epidoteHigh abundances of andradite garnet (Ca3Fe2Si3O12) havebeen identified in the surficial sediments associated with nearlyevery porphyry Cu deposit tested for PCIMs, even if not iden-tified in associated bedrock (Averill 2007). Results of our workaround the Pebble deposit are consistent with these patterns.Garnet was not observed in bedrock samples from Pebble(Kelley et al. 2011), yet it is abundant in till samples (0.25-5.0mm fraction). The highest garnet counts (>40 grains) occur insamples southwest and down-ice of the Pebble deposit in closeproximity to the smaller 308 and 38 zones, and the 37 zone, askarn occurrence (Fig. 2). The distribution of Mn-epidotemimics that of garnet (Kelley et al. 2011) and, with the garnetresults, indicates the presence of skarns associated with theporphyry deposits.

43Exploration case study using indicator minerals in till at the giant Pebble porphyry Cu-Au-Mo deposit, southwest Alaska, USA

Gold and jarositeAll till samples from the study, including those 8 to 10 km up-ice, contain some gold grains (Fig. 3), which suggests that mul-tiple sources have contributed gold to surficial deposits.However, tills immediately adjacent to Pebble West contain 3to 30 times the number of gold grains compared to samplesup- or down-ice, and the total number of grains decreases inthe down-ice direction. Abundant gold grains are also presentin tills near the porphyry and skarn occurrences southwest ofPebble (Fig. 3). The gold grains range from 15 to 150 micronswith a few larger grains.

Gold grain morphology (degree of rounding, polishing, andflatness) provides information about distance to the source. Allsamples near Pebble contain at least 65% pristine (primaryshapes and surface textures) or modified (edges damaged andgrains striated) gold grains, whereas distal samples containmostly reshaped grains. Pristine grains may reflect either anearby bedrock source with little modification during transportor liberation of gold grains from rock fragments during in situweathering of transported gold-bearing sulphide grains(McClenaghan 2005). Because most of the gold is contained inchalcopyrite (Lang et al. 2008), the second option seems morelikely, and it implies that post-glacial till oxidation was sulphidedestructive. In fact, no sulphide minerals were identified in thetills.

The distribution of jarosite in till samples is more restrictedthan that for gold (Fig. 4). Jarosite is most abundant in samplesdown-ice from Pebble and in samples close to the 37, 52, and

308 zone occurrences. Most grains are variably worn, suggest-ing that jarosite formed prior to glaciation during supergeneenrichment of the upper part of Pebble West.

Jarosite grains associated with the Pebble deposit andnearby occurrences are among the first identified in PCIMstudies of till samples (Stu Averill, pers. comm. 2008). Theiroccurrence and preservation probably reflects two main fac-tors: (1) many deposits never develop a supergene enrichmentzone; and (2) the level of bedrock erosion in glaciated areas istypically below the oxide cap development zone (i.e. below thejarosite zone) so jarosite is not available for incorporation intothe till. A supergene cap did develop at Pebble (Lang et al.2008), and the presence of jarosite in till samples suggests thatthe deposit was not deeply eroded.

CinnabarNumerous till samples contain grains of cinnabar, ranging

from a few grains to over 100 grains (Fig. 5). Samples withabundant cinnabar appear to be clustered around the area ofthe Pebble deposit, but careful data examination reveals thatthe highest grain counts are in samples from till that flanksKoktuli Mountain, where Tertiary epithermal mineralizationhas been identified at the Sill prospect. High cinnabar graincounts are also present in till samples near the 25 zone, anotherTertiary epithermal occurrence to the west (Fig. 1). Thecinnabar is likely related to unidentified Tertiary deposits onand around Koktuli Mountain because (a) mercury concentra-tions are inherently low in drillcore samples from the Pebbleore body (J. Lang, pers. comm. 2009) and in soils and waters

52 Zone

308 Zone

Sill25 Zone

38 Zone

37 Zone

G-N Zone

<10 grains 413-1866 grains10-20 grains 21-40 grains 41-125 grains

GARNET


ntMilutkoK

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155°30’W 155°10’W

ALASKA Fig. 2. Distribution of andradite garnet in tillsamples (# grains/10 kg sample). Stars aremineral occurrences. Heavy black arrowsindicate ice directions. Boundaries for PebbleWest and East zones are defined in Figure 1.


52 Zone

308 Zone

Sill25 Zone

38 Zone

37 Zone

G-N Zone


GOLD

Heavy black line around symbol indicates >65% of grains are pristine or modified


ntMilutkoK

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SharpMtn.

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kiralaT

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ilutkoK

reppUkroF.N

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5°9

5N’

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95

155°30’W 155°10’W

ALASKA Fig. 3. Distribution of gold in till samples(#grains/10 kg sample). Stars are mineraloccurrences. Heavy black arrows indicate icedirections. Boundaries for Pebble West andEast zones are defined in Figure 1.

52 Zone

308 Zone

Sill25 Zone

38 Zone

37 Zone

G-N Zone

JAROSITE

<3 grains 4-47 grains 400-900 grains 901-2500 grains


ntMilutkoK

KaskanakMtn.

SharpMtn.

GroundhogMtn.

kiralaT

.R

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K

reppU

kroF.N

.RilutkoK

50 km

N’5

5°9

5N’

05 °

95

155°30’W 155°10’W

ALASKAFig. 4. Distribution of jarosite in till samples(#grains/10 kg sample). Stars are mineraloccurrences. Heavy black arrows indicate icedirections. Boundaries for Pebble West andEast zones are defined in Figure 1.


over the ore body (Anderson et al. 2011), (b) the highestcinnabar grain counts are from tills flanking Koktuli Mountain,and (c) these same till samples do not contain high grain countsfor gold, jarosite, garnet, or epidote. Cinnabar is widespread insurficial deposits throughout much of southwest Alaska and isrelated to Late Cretaceous to Tertiary epithermal activity (Grayet al. 2000; Gray & Bailey, 2003). While speculative, the wide-spread cinnabar identified in the present study appears toextend the geographic distribution of mercury in southwestAlaska.

Pond and stream sedimentsStatistical summary box and whisker plots for selected ele-ments in pond and stream sediments indicate that the widestvariation in concentration is found for Au and Cu (Fig. 6).Relative to the entire sediment dataset, several pond sedimentsamples proximal to Pebble West have anomalous concentra-tions of Au and Cu (Figs. 7 and 8, respectively). Elements hav-ing similar patterns include Mo, S, Sb, and Pb. The high metalconcentrations likely reflect the shallow nature of the ore bodyat Pebble West, where cover is thin and mineralized rock ispartly exposed. Samples with the highest copper concentra-tions (>448 ppm, Fig. 8) are from naturally acidic ponds (pH< 4.0) that contain dominant ferrihydroxide mineral precipi-tates in the sediment, and attendant adsorbed metals(Anderson et al. 2011; Eppinger et al. in press). Several pondsediments at Pebble West also have high K and Rb concentra-tions, likely a reflection of potassium-silicate alteration.Copper concentrations show a dispersion train along the SouthFork Koktuli River (= three sites plotted along the river in Fig.

8) that is apparent below the Pebble deposit for 10 km, the dis-tal limit of sediment sampling along the river. However,absolute Cu concentrations along the South Fork are notextreme, ranging only from 65 to 94 ppm. Similar low-level ele-

52 Zone

308 Zone

Sill25 Zone

38 Zone

37 Zone

G-N Zone

CINNABAR



ntMilutkoK

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155°30’W 155°10’W

ALASKAFig. 5. Distribution of cinnabar in till sam-ples (#grains/10 kg sample). Stars are min-eral occurrences. Heavy black arrows indicateice directions. Boundaries for Pebble Westand East zones are defined in Figure 1.

5000

500

50

5.0

0.50

0.050

0.0050

tcp,S

mpp,gH

mpp,U

mpp,bS

mpp,W

mpp,sA

mpp,oM

mpp,bP

tcp,grO-

C

mpp,nZ

mpp,bR

mpp,uC

bpp,uA

Median75th percentile25th percentileMaximumMinimum

Fig. 6. Box-and-whisker plots showing summary statistics for selectedelements in pond and stream sediments.


25 Zone

38 Zone52 Zone

37 Zone

308 Zone

Sill

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Silutko

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50 km

<0.0011 not detected0.0012 to 0.012 < 75th percentile0.013 to 0.032 75th to 90th percentile

0.033 to 0.068 90th to 95th percentile0.069 to 0.253 95th to 97.5th percentile0.254 to 0.701 > 97.5th percentile

Au, ppm (Fire Assay)

G-N Zone

Fig. 7. Distribution of Au in pond andstream sediment samples. Boundariesfor Pebble West and East zones aredefined in Figure 1.

.R

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50 km

3.2- to 65.1 < 75th percentile65.11 to 198 75th to 90th percentile198.1 to 448 90th to 95th percentile

448.1 to 744 95th to 97.5th percentile744.1 to 2,400 > 97.5th percentile

Cu, ppm (ICP-MS)

25 Zone

38 Zone52 Zone

37 Zone

308 Zone

Sill

G-N Zone

Fig. 8. Distribution of Cu in pond andstream sediment samples. Boundariesfor Pebble West and East zones aredefined in Figure 1.


ment concentrations of Mo, Tl, Sb, and Fe also show disper-sion along the South Fork. Low-level concentrations of Hg insediments (all are <0.32 ppm, Anderson et al. 2011) likelyreflect cinnabar entrained in tills.

A cluster of samples from the western edge and immedi-ately southwest of the Pebble ore body outline (Fig. 7) haveanomalous Au concentrations that are >0.033 ppm. This clus-ter of samples also has anomalous Cu abundances, albeit atlower level concentrations (Fig. 8). The slightly displaced (rela-tive to the ore body outline) Au and Cu anomalies likely reflectore minerals in glacial till that were scraped from the top of theore body and deposited immediately downstream. This dis-placed anomaly in mineralized till is evident in the PCIM mapsof gold and jarosite (Figs. 3 & 4), as well as in the water chem-istry results (Eppinger et al. in press).

The distribution of pond and stream sediment samples isuneven, being more dense over the Pebble deposit area thanthe surrounding region. Few ponds and streams in the vicinityof the outlying occurrences were sampled, and are limited tothe 38 zone and the Sill occurrence (Fig. 1). Low-level anom-alous Cu is present in the sample from the 38 zone (Fig. 8) andvariable anomalous Au is found at both the 38 zone and Silloccurrence (Fig. 7). Samples from these two occurrences alsohave low-level anomalous concentrations of Mo, Sb, and Fe,while samples near the Sill occurrence have in addition low-level anomalous concentrations of Pb, Tl, and Zn.

Comparing PCIMs in till and pond sediment chemistryThe uneven distribution of sediment samples hinders a directcomparison with PCIMs in till, although a few observations arepossible. Element abundances in sediment samples decreasesignificantly within a few km south of the Pebble ore body.Generally, only minimally anomalous (75th to 90th percentile)element concentrations are present downstream of the depositalong the South Fork Koktuli River. Samples from the 38 zoneand Sill occurrence also have minimally anomalous concentra-tions. Dispersion trains for PCIMs are longer, perhaps due inpart to the more extensive down-ice distribution of till sam-ples. Significantly, the PCIM data provide direct mineralogicalinformation that more clearly indicates the presence of con-cealed porphyry and related skarn deposits; results for the sed-iment data are not equally effective. Further, the locally pristinemorphology of gold grains in the PCIM samples providesdirect clues concerning proximity to mineralized rock. Boththe PCIM and sediment data indicate the presence of shallowconcealed mineralized rock at Pebble, but the suite of mineralsidentified in the PCIM data provide a stronger clue to the por-phyry nature of the deposit.

CONCLUSIONSThe PCIM method is a powerful means for identifying con-cealed porphyry and skarn deposits in till-mantled areas ofsouthwest Alaska. Stream and pond sediment data reveal thepresence of the Pebble deposit, with anomalous concentra-tions of Au, Cu, and several additional elements, as well aspotassic alteration. However, down-stream dispersionprocesses quickly dilute the strength of the geochemical anom-alies, and a mineralogical connection is lacking. Specific bene-fits of the PCIM method are

1) The source of gold and distance from the source can beestimated based on the gold grain abundance and degreeof wear.

2) Gold grain abundances in conjunction with andradite andepidote are present along a long reach of the South ForkKoktuli River (~25 km) below Pebble, making the methoda powerful regional exploration tool.

3) The use of multiple indicator minerals increases the likeli-hood that the correct deposit type can be identified. Inisolation, gold grain abundances could reflect any numberof different deposit types. However, the presence ofandradite, Mn-epidote, and gold almost certainly reflectsporphyry and/or skarn deposits as opposed to otherdeposit types.

4) Abundant jarosite in till may be an indicator of past super-gene development over a porphyry deposit and/or thedepth to which the deposit has been eroded. This infor-mation is important when considering the extent thedeposit has been preserved and the likelihood that thedeposit includes an enriched (higher grade) Cu compo-nent due to supergene processes.

ACKNOWLEDGEMENTSWe thank the Pebble Limited Partnership, particularly JimLang, Mark Rebagliati, Keith Roberts, Lena Brommeland,Robin Smith, Gernot Wober, and Sean Magee for logistical andscientific support for this work. Stu Averill (OverburdenDrilling Management Limited) offered helpful discussions.Comments by reviewers Edward du Bray and David B. Smithimproved the manuscript. The use of trade, product, or firmnames in this article is for descriptive purposes only and doesnot imply endorsement by the U.S. Government.

REFERENCESANDERSON, E.D., SMITH, S.M., GILES, S.A., GRANITTO, M., EPPINGER, R.G.,

BEDROSIAN, P.A., SHAH, A.K., KELLEY, K.D., FEY, D.L., MINSLEY, B.J. &BROWN, P.J. 2011. Geophysical, geochemical, and mineralogical data from the PebbleCu-Au-Mo porphyry deposit area, southwest Alaska: Contributions to assessmenttechniques for concealed mineral resources. U.S. Geological Survey, Data Series608, 45.

AVERILL, S.A. 2007. Recent advances in base metal indicator mineralogy.Explore, 134, 2-6.

BOULEY, B.A., ST. GEORGE, P. & WETHERBEE, P.K. 1995. Geology and discov-ery at Pebble Copper, a copper-gold porphyry system in southwest Alaska.In: SCHROETER, T.G. (ed.) Porphyry deposits of the northwestern Cordillera ofNorth America. Canadian Institute of Mining, Metallurgy and PetroleumSpecial Volume 46, 422-435.

DETTERMAN, R.L & REED, B.L. 1973. Surficial deposits of the Iliamna quadrangle,Alaska: U.S. Geological Survey Bulletin 1368-A, 64 p.

EPPINGER, R.G., FEY, D.L, GILES, S.A., KELLEY, K.D. & SMITH, S.M. in press.An exploration hydrogeochemical study at the giant Pebble porphyry Cu-Au-Mo deposit, Alaska, using high-resolution ICPMS. Geochemistry:Exploration, Environment, Analysis.

GRAY, J.E. & BAILEY, E.A. 2003. The southwest Alaska mercury belt. In: GRAY,J.E (ed.) Geologic studies of mercury by the U.S. Geological Survey. U.S.Geological Survey Circular 1248, 19-22.

GRAY, J.E., GENT, C.A. & SNEE, L.W. 2000. The southwestern Alaska mercurybelt and its relationship to the circum-Pacific metallogenic mercuryprovince. Polarforschung, 68, 187-196.

HAMILTON, T.D. & KLIEFORTH, R.F. 2010. Surficial geologic map parts of the IliamnaD-6 and D-7 quadrangles, Pebble project area, southwestern Alaska. AlaskaDivision of Geological and Geophysical Surveys Report of Investigation2009-4, 1 sheet, scale 1:50,000. Available online athttp://www.dggs.dnr.state.ak.us/pubs/id/20621 (last access 12/4/2011).

KELLEY, K.D., EPPINGER, R.G., LANG, J., SMITH, S.M. & FEY, D.L. 2011, inpress. Porphyry copper indicator minerals (PCIMs) in glacial till samples

as an exploration tool: example from the giant Pebble porphyry Cu-Au-Mo deposit. Geochemistry: Exploration, Environment, Analysis.

LANG, J.R., REBAGLIATI, C.M., ROBERTS, K. & PAYNE, J.G. 2008. The Pebblecopper-gold-molybdenum porphyry deposit, southwest Alaska, USA.Australian Institute of Mining and Metallurgy, Proceedings of the PACRIMCongress 2008, Gold Coast, Queensland, Australia. AUSIMM, 27-32.

MCCLENAGHAN, M.B. 2005. Indicator mineral methods in mineral exploration.Geochemistry: Exploration, Environment, Analysis, 5, 233-245.

NORTHERN DYNASTY MINERALS LTD. 2011. The Pebble deposit. Available onlineat http://www.northerndynastyminerals.com (last access 31/3/2011).

SILLITOE, R.H. 2010. Porphyry copper systems. Economic Geology, 105, 3-41.



Sedimentation in the Devonian and Quaternary periods wasvery different, even though the main source of clastic materialduring both periods was the same location in Fennoscandia,north of the territory of Latvia. Some stratigraphically welldefined layers with specific mineral assemblages that can begenetically linked to the kimberlitic rocks of the EastEuropean Platform have been discovered in the UpperDevonian sandstones, which are known to have formed in ashallow sea. Modern beach placers formed due to washing oflarge amounts of Quaternary glacial clastic material. The kim-berlitic mineral assemblages in the siliciclastic rocks have beeninvestigated in Latvia since the 1980s. Mineralogical and geo-chemical research has been carried out in the territory of theBaltic States – Estonia, Latvia, and Lithuania. Through thisresearch, several areas with potential to host kimberlite wereidentified.

MATERIAL AND METHODS By mineralogical and geochemical research (Harkiv 1978), sev-eral areas in the territory of Latvia (Hodireva et al. 2002,2003a,b; Korpechkov et al. 2005) were identified as possiblycontaining kimberlitic indicator minerals. Samples of theweakly cemented Devonian sandstone and the Quaternary sed-iments in the area were sampled by shovel. The initial samplesize was more than 40 kg, which in some cases was increasedto 1 m3. The siliciclastic material was preconcentrated in thefield through the use of simple equipment. The heavy mineralpreconcentrates, a few hundred grams in weight, were sepa-rated using heavy liquid (bromoform, density 2.85), after whichmagnetic separation was used. The nonmagnetic heavy mineralconcentrates were hand-picked under binocular microscope(grain size fraction 0.1-0.25 mm and 0.25-0.5 mm) for indica-tor mineral grains, which were all subsequently analyzed with aJeol JSM840A electron microprobe. The number of pyropegrains found in the 40 kg samples varied considerably, from nograins in some samples to 300 grains in others. Similarly, thenumber of chromite grains varied from none to 200 grains.

In order to study the overall mineralogy of the heavy frac-tions, the smaller grain sizes (<0.25 mm) were observed undermicroscope by immersion using polarizing transmitted light.This size fraction contained mainly silicates and opaque miner-als: ilmenite, hematite, magnetite, rutile, and chromite. Themost dominant silicate minerals were tourmaline, zircon, andgarnet. Other silicates frequently present were staurolite, kyan-ite, sphene, amphibole, and epidote. The phosphate mineralsobserved were apatite and monazite.

RESULTS AND DISCUSSIONAt present the kimberlitic mineral assemblages in theDevonian sandstones, (pyropes, chromite, picroilmenite, Mg-olivine, chromian diopside, moissanite) are considered to beredeposited. A portion of these minerals is considered to berelated to modern beach placers of the Baltic Sea and the Gulfof Riga, as well as to basal till and alluvial deposits. These min-eral assemblages differ in chemical composition and mineralgrain morphology. In the area of this study, the variability ofthe chemical composition of the kimberlitic minerals (Sobolev1974), especially pyrope and chromite, suggests separatesources for these heavy minerals. The chemical composition ofsome mineral grains suggests that they belong to diamondassemblages (after Sobolev 1964 and Dawson & Stephens1975; Dawson 1980).

Some stratigraphically well defined layers in the UpperDevonian sandstones contain specific mineral assemblagesthat are genetically related to the magmatic rocks of kimberliticfacies of the East European Platform. This type of mineralparagenetic assemblage has been identified over a wider geo-graphic area and has been investigated in detail in Russia(Konstantinovskii & Shcherbakova 1998; Ladygina & Nesterov2002; Ladygina et al. 2005), Belarus, Ukraina, as well as thenearest part of the Fenoscandia crystalline basement inFinland (Lehtonen & Marmo 2002) and western Russia. Thisadditional research has identified four separate stratigraphiclevels in the sedimentary rock sequence containing mineralswith very specific typomorphism that is different from themain portion of the accessory minerals present in the sand-stones (Chernisheva 1989).

Upper Devonian Gauja, Ogre (Frasnian Stage), and Ketleriformations (Famenian Stage) have been the subject of detailedresearch and a new, non-typical heavy mineral association hasbeen recognized in the sedimentary rocks of Latvia. Theprovenance of these minerals must be bedrock with composi-tions similar to the kimberlites that are thought to have eruptedduring the Late Devonian. The location of the source rockshas not been determined, although studies have narrowed thepossibilities.

Weakly cemented sandstones of the Gauja Formation,Frasnian stage, are widespread in Latvia but, until now, kim-berlitic mineral assemblages have only been recognized innortheast Latvia (Vidzeme). First found in the 1970s in theBale sand quarry (Prokopchuk et al. 1980), a few grains ofpyrope and chromspinel have been studied. Recently, pyropehas also been detected in outcrops at Brasla and Springuiezis innortheast Latvia. Pyrope of the Gauja Formation is observedas small (0.15-0.4 mm) semi-angular pale violet or pinky violet

Heavy mineral assemblages in Devonian sandstones and Quaternary sediments in LatviaVija Hodireva1 & Denis Korpechkov2

1. Faculty of Geography and Earth Sciences University of Latvia, Alberta str 10, Riga, LV 1010, Latvia (e-mail: [email protected])

2. Institute of Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry (IGEM), Russian Academy of Sciences,Staromonetnyi per. 35, Moscow, 119017 Russia (e-mail: [email protected])

50 V. Hodireva & D. Korpechkov

Fig. 1. SEM secondary electron images Micrographs ofpyrope grains from the Ogre formation (A–B) and theKetleri formation (C–F) of the Upper Devonian in westLatvia, Kurzeme.

A B

C D

E F

51Heavy mineral assemblages in Devonian sandstone and Quaternary sediments in Latvia

fragments with a refractive index of 1.734 (Prokopchuk et al.1980).

A kimberlitic mineral suite of high-pressure and high-tem-perature petrofacies has been identified in the Ogre Formation,Frasnian stage, in western Latvia (Kurzeme) – some hundredsof kilometres away from the location of the indicative heavymineral association in the Gauja Formation. The bedrock unitwas studied at outcrops along the middle of the Abava Rivervalley in southwestern Latvia (Sorokin et al. 1992). The con-centration of kimberlite minerals, which is represented bypyrope (Fig. 1), is significantly lower in the underlyingLielvarde member because of the different direction of mate-rial transport in these members (from the north and northeastfor the Lielvarde member and from the east and east-northeastfor the Rembate member) (Sorokin et al. 1992). The pyropecontent in the Ogre Formation was up to 100s of grains in a40 kg sample.

More abundant chrome pyropes and chromspinelides werefound in sandstones of the Ketleri Formation, Famenian stage,in western Latvia, Kurzeme (Fig. 1). These mineral accumula-tions peak in the siliciclastic rock and bone (fossil remains)breccia of the middle Ketleri subformation. The size of suchdeposits decreases up-section and becomes tens of timessmaller in the overlying Shkervelis Formation. It was discov-ered that the concentration of kimberlitic indicator mineralsincreases in the basal members of cycles and local erosion sur-faces with pyrope being the most widespread kimberlitic min-

eral in the Ketleri intermediate unit. The pyrope content in thisformation was found to be up to 100s of grains in a 40 kg sam-ple (up to 300 grains in some places) in the northeastern areaand decreases to 10s of grains in a 40 kg sample in the south-ern and eastern areas. In contrast to the Ogre variety, theKetleri pyrope is usually angular and finer grained with grainsize varying from 0.08 to 0.4 mm (generally ~0.15 mm).However, the chemical composition of some of these grainsshows Ca depletion and plots in the field of the harzburgite-dunite paragenetic association (Fig. 2 and Fig. 3).

The Ketleri chrome spinel can be plotted on the magmaticevolution trend (Prokopchuk et al. 1980) and compared withchrome spinels of kimberlites. In addition to pyrope andchrome spinels, small grains of olivine, chrome diopside(Sorokins 1997), and moissanite are also found in the Ketleriintermediate unit. The chemical composition of minerals issimilar to that found in minerals of kimberlite paragenesis.Relatively low chromium and titanium content is connectedwith the genesis of the source rocks, mainly from deep-form-ing rocks.

Detailed mineralogical investigations have provided infor-mation on the occurrence of potential heavy mineral collectorswithin the Upper Devonian siliciclastics that were deposited ina shallow epeiric sea basin.

The primary composition of the Quaternary sediments inthe western Latvia zone indicates their origin was the Baltic IceStream. While composition of sediments in the zone of the

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Fig. 2. Features of the chemical composi-tion of the pyrope grains from Gauja,Ogre, Keteri Formation of the UpperDevonian. Formations: 1 - Gauja, 2 - Ogre,3 – Keteri. Fields of the diagram: I –wehrlite paragenesis, II – lherzolite para-genesis, III – dunite-harzburgite paragen-esis, IV – diamond assemblage.

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Fig. 3. Chemical composition ofpyropes occurring in the Quaternarydeposits (diagram Cr2O3 – CaO inweight percents) for the following sam-ples: 1 - Baltic Sea Kurzeme beach plac-ers, 2 – Gulf of Riga Kurzeme beachplacers, 3 – alluvium of West Kurzeme, 4– alluvium of the left bank of the tribu-taries of the Venta River, 5 – alluvium ofCentral Kurzeme rivers, 6 – alluvium ofZemgale rivers, 7 – alluvium of NorthKurzeme rivers, 8 – alluvium ofLithuania rivers. Fields of the diagram: I – wehrlite paragenesis, II – lherzoliteparagenesis, III – dunite-harzburgite par-agenesis, IV – diamond assemblage.


Gulf of Riga, central Latvia, indicates they were formed by theice stream of the Gulf of Riga. The beach placers formed asthe result of abrasion of the deposits on the shore and upperpart of submarine slope and therefore reflect their composi-tion (Savvaitov et al. 1998; Hodireva et al. 2000).

The Quaternary sediments are characterized by a greaterdiversity of kimberlitic minerals as compared to UpperDevonian units. They are dominated by pyrope (Fig. 4) andchrome spinel. Some samples include picroilmenite, chromediopside, and moissanite. The distribution of pyrope grains inthe Quaternary sediments of Latvia is extremely irregular, withmaximum concentrations (more than 100s of grains in a 40 kgsample) recorded in western Latvia, while minimal concentra-

tions are observed in the southeastern area, where pyrope israre.

Compared to Devonian varieties, the pyrope grains from theQuaternary units are more diverse in colour. Quaternarypyrope grains from different regions of Latvia (Fig. 5) can beidentified by these colour variations, and have refractive indicesthat range from 1.737 in the pale coloured varieties to 1.767 inthe dark coloured varieties. Relationships between geneticallydifferent surface sculptures (Moral Cardona et al. 2005) indicatethat the grains probably underwent mechanical processingboth before and after chemical dissolution during the transportby glaciers or in present-day water currents (Fig. 6).

Fig. 4. SEM secondary electron images showing different types of the pyrope grain surface textures from alluvial sediments of Latvia.

A B

C D

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12a2b

Fig. 5. Features of chemical composi-tion of pyropes from Quaternarydeposits (diagram Cr2O3 – CaO wt.%).Pyropes from: 1 - Gulf of Riga Vidzemecoast placers, 2 - Central and NorthVidzeme alluvium (from: a – Savvaitovset al. 2000; Hodireva et al. 2002; b –Krivopalov et al. 1987; 1992). Fields ofdiagram: I – wehrlite paragenesis, II –lherzolite paragenesis, III – dunite-harzburgite paragenesis, IV – diamondassemblage.


Quaternary sediments of Latvia are characterized by theabundant development of chrome spinels (1-100s ofgrains/40 kg). They are commonly observed as well preservedoctahedral crystals that are 0.1 to 0.2 mm in size (up to 1.0-1.5 mm in rare cases) with low Ti concentrations but mediumand high Cr concentrations. In terms of chemical composition,

they correspond to the magmatic and metamorphic evolutiontrends. Chrome and Al concentrations for some chromitegrains plot within the field of diamond association (Fig. 7 andFig. 8).

Ilmenite with increased Cr or Mg concentrations has not yetbeen identified in Devonian sandstone, which is known to con-

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Fig. 8. Comparison of composition ofchromspinel from clastic sediments ofLatvia and chromspinel of diamond par-agenesis. Ratio of Cr2O3 and TiO2 is inweight percents (wt.%). Legend: seeFigure 7.

Fig. 6. The chromspinelide grains with different types of surface features from beach deposits of the Gulf of Riga, (A, B) and Vidzeme allu-vial deposits and (C) in Latvia (grain micrographs).

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Fig. 7. Comparison of composition ofchromespinel from clastic sediments ofLatvia and chromespinel of diamondparagenesis. Ratio of Cr2O3 and Al2O3in wt.%. Chromespinelides of Devoniansiliciclastic rocks: 1 – Arukila (NorthKurzeme), Burtnieki and GaujaFormation; 2 – Ogre Formation (WestKurzeme); 3 – Ketleri Formation;chromspinelides of Quarternarydeposits: 4 – Kurzeme alluvium andbeach deposits; 5 – the Gulf of Rigabeach deposits and Zemgale alluvium; 6 – Vidzeme alluvium; 7 – Rāzna lakebeach deposits in Latgale; 8 – chrome-spinel of diamond paragenetic associa-tion (by publications).


tain ilmenite as one of the most common accessory minerals,though picroilmenite has been recognized in recent alluvialdeposits.

Different regions of Latvia are characterized by specificassemblages of kimberlitic assemblage minerals, the concen-tration of these minerals, and the typomorphic features associ-ated with these individual minerals. Taking into considerationthese distinctions and the dominant trends of the glacial andalluvial transport in the Quaternary period, we can identifythree kimberlitic mineral assemblage zones: western Latvia,Kurzeme; Gulf of Riga; and northern Latvia, Vidzeme (Fig. 9).In addition, the sources of potential kimberlitic minerals canbe subdivided into sources from long-distance and short-dis-tance transportation (Korpechkov et al. 2007). Detailed mineralgrain surface investigation gives additional data to help esti-mate the transportation distance of the clastic material.

CONCLUSIONSTrends of kimberlitic mineral assemblage distribution, as dis-cussed above, can be explained by the presence of several kim-berlite-type sources located both within and outside theLatvian territory. Six types of kimberlitic associations in theLatvian territory can reliably be identified by mineral composi-tion and typomorphic properties of individual minerals:Devonian Gauja, Ogre, and Ketleri formations, as well as thewestern Latvia, Gulf of Riga, and northern Latvia zones ofthe Quaternary intermediate collector. With respect to the dia-

mond potential of the kimberlitic sources in the Latvian terri-tory, northeastern Latvia has the highest potential whilesources in western Latvia have a lower diamond potential.

The typomorphic features of heavy minerals from theUpper Devonian siliciclastic rocks indicate that specific min-eral associations occur in sedimentary rocks of the northeast-ern East European Platform, including the area of Latvia.

Detailed investigations have yielded new information on theoccurrence of three potential kimberlite mineral-bearing unitswithin the Upper Devonian siliciclastics that were deposited ina shallow epeiric sea basin. An increase in mineral concentra-tions in the basal members of Devonian sedimentation cyclesand local erosion surfaces have been observed. Indicator min-eral assemblages in the Devonian sandstones likely have haddifferent provenances.

Specific mineral grain morphology (mainly pyrope andchromspinel) is an important feature required to verify infor-mation about two possible methods of material transportationfrom potential magmatic kimberlite complexes: very shorttransportation (30-40 km, Latvia), or longer transportation(Baltic Sea bottom, nearest areas of Finland, and West Russia).

REFERENCESCHERNISHEVA, L.V. (ed.). 1989. Typomorphism of minerals (Tipomorfizm

mineralov). The directory. Nedra, Moscow. (in Russian)DAWSON, J.B. 1980. Kimberlites and their xenoliths. Springer-Verlag, Berlin.DAWSON, J.B. & STEPHENS, W.E. 1975. Statistical classification of garnets from

kimberlites and associated xenoliths. Journal of Geology, 83, 589-607.

Fig. 9. Schematic location of the kimberlitic mineral assemblage haloes and their bedrock sources in the Latvian territory. (1) Rare pyrope in theGauja Formation; (2) the kimberlitic mineral assemblage in the Ogre Formation; (3) the kimberlitic mineral assemblage in the Ketleri Formation;(4) beach placers with kimberlitic mineral assemblage; (5) area with high kimberlitic mineral concentration in the modern alluvium; (6) approxi-mate boundaries of the kimberlitic mineral assemblage distribution in the Quaternary units: (WKZ) western Kurzeme zone, (GRZ) Gulf of Rigazone, (NVZ) northern Vidzeme zone, (ELZ) eastern Latvian zone; (7) location of supposed sources of kimberlitic mineral assemblages and theirnumbers; (8) direction of terrigenous material transport in the Devonian; (9) direction of glacial flows in the Quaternary Period.


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PROKOPCHUK, B.I., KONSTANTINOVSKIJ, A.A. & SOCHNEVA, E.G. 1980. Thefind of pyrope in Upper Devonian quartz sands of the Baltic Region(Nahodka piropa v verhnedevonskih kvarcevih peskah Pribaltiki).Proceedings of Academy of Sciences of USSR, 254, 1217-1218. (in Russian)

SAVVAITOV, A., VEINBERGS, I., NULLE, U., SAMBURG, N. & STINKULIS, G. 1998.New data about mineral composition of Latvian beach placers. Baltica, 11,52-58.

SOBOLEV, N.V. 1964. Paragenetic types of garnet (Parageneticheskije tipi granatov).Moscow, Nauka. (in Russian)

SOBOLEV, N.V. 1974. Mantle-generic ksenolites in kimberlites and problem of chemicalcomposition of upper mantle (Glubinnije vkluchenija v kimberlitah i problema sostavaverhnei mantii). Nauka, Novosibirsk. (in Russian)

SOROKIN, V.S., KRIVOPALOV, J.A., MŪRNIEKS, A.E., SAVVAITOVA, L.S. &SAMBURG, N.K. 1992. Sobytijnaya stratigrafiya i perspektivy almazonos-nosti zapadnoj Latvii (Eventual stratigraphy and perspectives of diamondpotencial of Western Latvia). In: Paleontology and stratigraphy of phanerozoic ofLatvia and the Baltic Sea. 77-84. (in Russian)


The sampling of Quaternary glacial sediments for diamondhost rock associated heavy minerals based on the dispersal ofindicator mineral grains is a leading technique in many explo-ration programs. It played a principal role in the discovery ofmany major diamond fields in glaciated terrains, includingthose in Canada, Finland, and Russia. Effectiveness of themethod in similar geological environments prompted anAustralian company, Ashton Mining Limited, to apply in 1994for the combined exploration and mining license covering allof the 150 000 sq. km Archean Karelian Сraton. The licensewas subsequently granted but with the exclusion of the natureprotection territories and two areas where some prospectivefinds were already known, namely a 2 500 sq. km area inNorthern Karelia with pyrope and diamond mineral anomaliesin glacial sediments and a 800 sq. km “Kostomuksha area” witha series of lamproite dyke clusters. The Karelian GeologicalExpedition, based in the town of Petrozavodsk, was commis-sioned by Ashton Mining (a subsidiary of Rio Tinto Limitedfrom 2000) to conduct the actual exploration, commencing in1992, on the special permit by the Karelian Government, andcontinuing, with some interruptions, until 2007. This explo-ration led to the discovery of the diamondiferous Kimozerokimberlite and a number of indicator mineral anomalies sug-gesting other potential sources.

METHODOLOGY AND RESULTSAt the initial regional stage of the work, which was performedin 1992-1994, samples were collected within bands 10 km widearranged across the general ice-flow direction (Fig. 1). Eachband was sampled at a density of 1 sample per 15 sq. km, withadditional heavy mineral samples collected from field-locatedstream-sediment trap sites. In total about 3,500 samples werecollected. All samples were taken manually by shovel, wetscreened in the field to <2 mm size, then 40-80 kg sampleswere transported to a laboratory in Petrozavodsk. On receipt,the samples were pre-concentrated, by double passing on a3.45×1.45 m Wilfley shaking table, to a few hundred grams inweight. The pre-concentrates were then dry sieved for 0.4 mm,+0.4-0.5 mm, +0.5-0.8 mm, and +0.8 mm grain fractions andseparated in the heavy liquid methylene iodide. The final +0.4mm concentrated fractions, averaging 30 grams in weight, werestudied under binocular microscope for indicator minerals,which were identified visually by their physical properties. Thetargeted indicator minerals were pyrope, chromite, picro-ilmenite, and diamond. Chrome diopside, though picked upalong with other indicators when recognized, was not consid-ered as statistically informative because of losses into the lightfractions after methylene iodide separation. All stages of sam-

ple processing and analysis were controlled by using speciallyprepared indicator spikes. The initial survey revealed a generalbroad low count indicator dispersal with a few areas of moresignificant concentrations. Chromite was the dominant indica-tor mineral throughout the survey area, with trace sporadicpyrope, and picro-ilmenite in noticeable quantities only alongthe State border with Finland. About 10% of all picked mineralindicator grains were analyzed for their chemical composition.

The follow-up survey, carried out mainly beyond the areasof complex sequences of glacial accumulations, increased thedensity of sampling, both around singular anomalous samplesand within the areas of elevated concentrations, and led to thedefinition of a number of clear individual indicator fans andtrains, including down ice of the saucer-shaped 2 km long x800 m wide by 20-40 m deep Kimozero kimberlite body on theZaonezhsky Peninsula (Fig. 2) (Ushkov et al. 2008). In total,5,000 samples were collected at the follow-up stage, preferablyfrom the basal till but in many cases only upper layers of the tillcover could be sampled. About 50% of chromite grains and allpyropes and picroilmenites were microprobed. During the sur-vey process mineralogical anomalies were continuously rated,both for their quantitative and qualitative mineralization levelsand the most prospective of these were investigated further.

Kimozero indicator trainThe Kimozero indicator train (Fig. 3) (Ushkov 2009) has beentraced for 26 km from the north-west margin of the Kimozerokimberlite body to the shore of Onega Lake. It varies in widthfrom 400 m in its head segment to 3 km at its end.

The discovery of the Kimozero kimberlite body was basedon the follow up of a reconnaissance stream heavy mineralsample containing 105 chromite grains, including high-chromeones, adjacent to which a kimberlite breccia outcrop wasfound. Detailed sampling down ice of Kimozero was com-pleted over several years as an attempt to delineate other kim-berlite bodies in the area. Quaternary deposits in the northern6 km of the indicator train are represented by the thin (1-3 km)till cover that was dug through by most of the pits excavated.In comparison, Quaternary deposits of the southern 20 km ofthe train are dominated by glacio-lacustrine sediments withthicknesses up to 50 m making interpretation of indicatoranomalies within the train farther than 6 km from theKimozero body unreliable. The train is almost exclusivelyformed by chromites with rare pyropes and single diamondgrains in 5 samples. Chromites are dominantly kimberlitic withchemical composition similar to those sourced directly fromthe Kimozero kimberlite, including low to moderate Cr2O3,variable MgO, and low TiO2 typical of the overlapping

Kimberlite indicator mineral-based diamond exploration program in Karelia, northwest Russia

V.V. Ushkov1, I.A. Alexeev2, Yu. S. Shelukhina2, & R.G. Norris3

1. Karelian Geological Expedition, Al. Nevsky Avenue, 65, Petrozavodsk, 185030 Russia ([email protected])2. Saint-Petersburg State University, Geological Faculty, Department of Mineral Resources. 199034, 7/9 Universitetskaya

emb., Saint-Petersburg, Russia ([email protected], [email protected])3. Rio Tinto Exploration Limited, 1 Research Avenue, Bundura, Victoria, 3083 Australia ([email protected])

58 V.V. Ushkov, I.A. Alexeev, Y.S. Shelukhina & R.G. Norris

Fig. 1. Heavy mineral sampling results map. 1- pyropes, 2- chromites, 3- picroilmenites, 4- negative samples, 5- diamonds, 6- ice direction, 7- natural reserves, 8- «Kostomuksha area».

59

chemistries of diamond stable and lower mantle sourcedchromite. The high Cr2O3 population, though significant, isonly just on laps into the diamond inclusion field and does notinclude any high MgO varieties typical of depleted diamond-bearing sources. Pyropes are G9 lherzolitic with few G1, G2,G3, G10, and G11s. Similar to the chromites, harzburgitepyropes are only weakly depleted. Chromites are generally wellpreserved with composite grains with adhering host rocksmatrix traced down ice to the end of the train along the shoresof Onega Lake. Kimberlite fragments from small 3-5 mm upto 0.3-1.2 m in size are variably present. The largest floats wereencountered in the upper part of the till 4.2 and 4.4 km downice from the Kimozero body. Most of them, both small andlarge, display little or no evidence of rounding. The fragmentson the whole, with very few exceptions, correspond to the rockvarieties of the Kimozero body. A peculiar feature of the trainis the contrast variation in abundance of mineral indicatorsalong its course. In the initial 1 km stretch down ice from theKimozero body, the concentration of chromites in the tillremains relatively constant at tens to hundreds of grains persample, approximately the same comparatively high level assamples collected over the Kimozero kimberlite bedrock.Further down ice, the chromite content rapidly decays toalmost complete disappearance but after the 2nd km and 3-5km interval content rises and then gradually decreases onceagain in a cyclic manner. Also down ice of the 3-5 km interval,the concentration of chromites appears to form separatedetached flows within the train boundaries. This dispersal pat-tern may be conditioned by the local rugged relief splitting thetrain into separate flows or be controlled by the existence ofother sources. The latter possibility may be supported by thefact of the appreciable inconsistency between the dominantdirection of the ice flow (SE azimuth 120-125º) and the direc-tion of the Kimozero train as a whole (average SE azimuth135º), suggesting that the train might have been formed as asum effect of a number of smaller individual fans with theirown kimberlite sources emplaced along the common structuralfabric. Test drilling of a number of targets in the area wasinconclusive with no additional kimberlitic sources identified.It is assumed that, if present, any untested sources in this areaare likely to be small and potentially dyke-like in form.

Munozero indicator halo The Munozero indicator halo (Ushkov 2005) is located onZaonezhsky Peninsula, 14 km west from the Kimozero, andcovers an area of only 4 sq. km (Fig. 3). Samples of the halogroup were collected from the basal horizon of the till with athickness of 0.5-2.0 m. The halo is formed predominantly bychromites with rare pyrope grains and is clearly ringed by neg-ative samples. The compact location of the halo along with thepristine character of indicator minerals suggests local deriva-tion. The number of chromite grains generally varies from sin-gles to 80, with 3 samples reporting from 150 to 700 grains.Chromite chemistries show clear fractionation trends with 40-60% Cr2O3 and variable MgO of 2-18%. There is, however,some overlap into the kimberlite-mantle field with few grainscorresponding to the diamond-stability field. This in additionto 6 grains of G9 and 2 grains of weakly depleted G10 pyropeindicates a mantle source for at least part of the indicator-min-eral population. All indicators have a well preserved appear-ance, including one pyrope retaining a kelyphite rim. No spe-cific source for these indicators was defined, although finechromite grains from crushed samples of a sandy dolomitelocated in the core of the Munozero syncline and centre ofthis target were reported. Further, one thin section of thisdolomite described a thin chlorite-biotite veinlet containingchromite grains with fuchsite coating. Geochemistry of therock is characterized by elevated Cr and Ni contents with indi-vidual maxima 768 and 60 ppm, respectively. No kimberlite orother mantle-derived magmatics were identified.

Ukshozero indicator fanThe Ukshozero indicator fan (Ushkov 2009) (Fig. 4) has beendefined 65 km southwest of the Munozero halo. Quaternarydeposits in the area are dominated by till overlain locally byglacio-fluvials with a total thickness of not more than 10 m.Bedrock, which is exposed in a quite a few outcrops, is repre-sented by picrite basalts. The indicator minerals are chromitesfrom single to 50 grains per sample, with one sample returning270 grains. Most chromites are well preserved, with few occur-ring as composite grains with host rock adhesions. Rarepyropes with few or no chromite are reported in samples col-lected along the western margin of the fan.

Two single 0.4 mm diamonds were found in samples situ-ated 5.5 km apart with no significant increase in indicator con-tent in the vicinity of these diamond occurrences. Most of theprobed chromites show strong fractionation trends and areundoubtedly sourced from the picrite basalts, however someoverlap into mantle-kimberlite-derived fields is apparent. This,in addition to the occurrence of pyrope, dominantly G9 lher-zolitic with 2 grains marginal to the harzburgitic field, and theoccurrence of two diamonds attests to a separate primarysource. Analysis of the dispersal pattern of indicators withinthe fan area suggests at least two discrete sources. The prove-nance of the diamonds, pyrope, and kimberlitic chromites,however, remains questionable with separate distal sources notruled out. Exploration works within the Ukshozero prospect,including ground magnetic surveys, did not identify anysources of consequence.

Vartsila indicator trainThe Vartsila indicator train (Ushkov 2009), located in thesouthwest of Karelia, runs from the border with Finland par-


Fig. 2. Schematic geological map and cross section of Kimozerokimberlite.


Fig. 3. Kimozero indicator train (I) and Munozero indicator halo (II).

61Kimberlite indicator mineral-based diamond exploration program in Karelia, northwest Russia

Fig. 4. Ukshozero indicator fan.


Fig. 5. Vartsila indicator train.

63

allel to the main ice-flow direction for 30 km at a width of 2 to5 km (Fig. 5). The Quarternary deposits in the area are domi-nated by till and occasional glacio-fluvials, each with thick-nesses of 0.5 to 5 m.

The indicators are dominated by pyrope and lesser chromiteand picroilmenite. The maximum number of pyrope grains ina sample is 10, with a total of 127 garnets confirmed as pyropeincluding 57% lherzolitic (G9 and lesser G11), 30% megacrys-tic (G1+G11+G9), 11% wehrlitic (G9+G11) and the remain-ing 4% harzburgitic (6 grains G9-G10). Over 50 grains ofchromite were identified in 10 samples, however probing ofthese grains indicates the majority to be fractionated and notkimberlite associated. Picroilmenite in single grains was provedin 7 samples. Based on spacial associations and similar miner-alogical suites and chemistries, the Vartsila train may be inter-preted as a down-ice extension of a train that extends 150-170km to the Kuopio-Kaavi fields located on the territory ofFinland. Localized increase in indicator mineral counts atVartsila may be associated with reworking of a major terminalmoraine that occurs immediately over the Russian-Finland bor-der and the head of the Vartsila (Smith & MacDonald 2002).

The deviation of the general axial line of the train on theRussian territory (at an azimuth of SE 135º) from the well doc-umented direction of glacial transportation (at an azimuth ofSE 145-150º), together with the definition within the train ofat least four individual “second-order” fans (the orientation ofwhich coincides with the ice direction) and the existence ofmoderately well preserved pyrope and picroilmenite may, how-ever, support more proximal sources. It should be noted thaton the whole, the count of indicators is relatively low and noexponential increase in their numbers within the second-orderfans was observed. No primary kimberlite targets were identi-fied from the extensive exploration work, which includedground magnetic surveys, that was completed in this area.

Kostomuksha group of anomaliesThe Kostomuksha group of anomalies (Ushkov 2009) (Fig. 6)is situated in western Karelia near the state border, just acrossfrom the Kuhmo-Lentiira kimberlite-lamproite fields inFinland. Numerous lamproite dykes and a few small kimberlitebodies are also known in the “Kostomuksha area”.

Quaternary deposits are represented by a 0.5-5 m thick tillcover and narrow discontinuous fluvio-glacials, the latteremphasizing the latitudinal and south-western ice-flow direc-tion. Although a large portion of indicators within this area arearguably from distal sources, a number of discrete pyrope-chromite anomalies were defined that are considered to havean equal probability of having a localized source. In the anom-alous halo nearest to the state border, 10 pyrope grains havepositive chemistry of matching portions of lherzolitic andmegacrystic G9, G1 and G11 and a single eclogitic grain. Ofthree confirmed chromites, two are plotting within the dia-mond-inclusion field and one has been interpreted as an oxi-dized kimberlite-associated grain. In 39 samples from otherhalos and trains to the southeast of the Kostomuksha area, 25pyropes and 54 chromites were probed. Pyrope chemistry isstrongly lherzolitic with 19 G9 grains; the remaining grains areweakly harzburgitic. Approximately 30% of the chromites ploton the margin between mantle-derived and diamond-inclusionchemistries and are undoubtedly kimberlitic, the majority ofthe remaining grains are non kimberlite sourced. Indicator

mineral morphologies indicate some evidence for localizedsources in the area, though fluvial reworking of the Quaternaryglacial deposits and more distal sources cannot be ruled out.Notably, picro-ilmenite, although present in the Kuhmo-Lentiira kimberlites, is absent in the Kostomushka area. Thismay in part be due to the lower mechanical resistivity of picro-ilmenite compared to pyrope and chromite, or may attest to thepotential for other picro-ilmenite-poor sources in the area.Infill heavy mineral sampling combined with airborne andground magnetic surveys failed to define any discrete targetsand consequently no further work was conducted on theseanomalies.

Rugozero indicator haloThe Rugozero indicator halo (Ushkov 2009) (Fig. 7) occurs inthe central part of Karelia in the area of complex Quaternaryglacial accumulations at the confluence of three terminalmoraines.

Glacial features associated with each of these terminalmoraines consist of various fluvio-glacial sediments, includingmoraine complexes, tangential moraine eskers, and esker deltasin addition to fluvially derived sediments. Thickness ofQuaternary cover ranges from 0-5 metres in areas of primarytill to 50 and more metres in the areas of terminal morainesand their associated sediments. Accordingly the samples col-lected in the area were taken mostly from upper layers of thetill- and moraine-associated sediments. The area attractedattention mainly because of the presence of harzburgiticpyropes and diamond-associated chromites of a differentdegree of wear from very fresh looking to strongly mechani-cally weathered. However, the positive samples, returning 1-5pyrope grains and up to 26 chromite grains, were sporadic andthere was no evidence of the development of consistent min-eral trains in either the till or fluvio-glacial deposits. The air-borne and ground magnetic surveys demarcated a number ofpipe-type targets, 5 of which were drill tested but only revealedmagnetite-bearing gabbroic rocks.

CONCLUSIONThis paper shows how the project operators, Ashton MiningLimited and Rio Tinto Limited, have successfully utilized indi-cator mineralogy, surface morphology, and mineral chemistrycombined with a deep understanding of the Quaternary geol-ogy and detailed mapping and analysis of indicator mineraltrains to decipher and prioritize targets for further exploration.The diamond heavy mineral exploration program in Kareliafollowed the Ashton Mining positive experience in Finland inlaying out a similar sampling grid and employing similarmethodology. It led to the discovery of the diamondiferousKimozero kimberlite and delineation of a number of welldeveloped indicator anomalies, and indicator-mineral fans andtrains. Despite relatively intensive exploration, no definitivesources have been identified within these other target areas andthe provenance of the indicator minerals forming theseremains contentious. At Munozero, the occurrence of finechromite in crushed sandy dolomite in addition to petrologicaland geochemical evidence suggests potential for a carbonatiteor similar source at this location, although this is yet to beproven. The occurrence of kelyphitized pyrope grains, com-posite chromite grains with matrix attachments, and fresh sur-



Fig. 6. Kostomuksha group of anomalies.

65Kimberlite indicator mineral-based diamond exploration program in Karelia, northwest Russia

Fig. 7. Rugozero indicator halo.

face textures on some of the chromites at several of the targetareas suggests proximal sources but, equally, liberation of indi-cators from kimberlite boulders transported and deposited inthe glacial environment would produce similar features.Conversely, the broad dispersal pattern and typically low count(often singular grain) of indicator minerals in addition to mix-ing of different mineral populations supports a distal prove-nance. In several of the examples presented, it is quite proba-ble that localized proximal dispersal trains are overprinted by amore broad distal population, with proximal sources beingmore dominant at Kimozero and distal sources becoming pro-gressively more dominant at Munozero, Ukshozero, Vartsila,Kostomuksha, and Rugozero.

REFERENCESSMITH C.B. & MACDONALD I.A. 2002. Overview of Ashton’s work in Finland. Rio

Tinto Internal Memorandum.USHKOV V.V. 2005. Report on the results of diamond exploration in Karelia, North West

of Russia in 1992-2004. Petrozavodsk, Karelia (Unpublished).USHKOV V.V. 2009. Report on the results of diamond exploration in Karelia, North West

of Russia in 2006-2009. Petrozavodsk, Karelia (Unpublished).USHKOV V.V., USTINOV V.N., SMITH C.B., BULANOVA G.P., LUKYANOVA L.I.,

WIGGERS DE VRIES D. & PEARSON D.G. 2008. Kimozero, Karelia; a dia-mondiferous Paleoproterozoic metamorphosed volcaniclastic kimberlite.9th International Kimberlite Conference Extended Abstract.



Few case studies have been published that document the indi-cator mineral signatures of magmatic Ni-Cu deposits. Toaddress this knowledge gap, the Geological Survey of Canada(GSC) through its Targeted Geoscience Initiative 3, in collab-oration with the Canadian Mining Industry ResearchOrganization (CAMIRO) and the Manitoba Geological Survey(MGS), collected and analyzed a suite of bedrock and till sam-ples from around the Thompson and Pipe ultramafic Ni sul-phide deposits in the north part of the Thompson Nickel Belt(TNB), northern Manitoba, Canada (Fig. 1). The objective ofthe study was not to define the dispersal trains from eachdeposit but to characterize the mineralogical signature of Ni-Cu mineralization at the deposit- and camp-scale at varying dis-tances down-ice and to define background levels in the area(McClenaghan et al. 2009).

GEOLOGICAL SETTINGThe TNB is a 10 to 35 km wide belt of variably reworkedArchean basement gneisses and Early Proterozoic cover rocksalong the northwest margin of the Superior Craton (Zwanziget al. 2007). It hosts several world-class magmatic Ni-Cudeposits that have been strongly structurally and metamorphi-cally modified. Nickel sulphide mineralization is associatedalmost exclusively with, or localized within, ultramafic bodieswithin the lower part of the Proterozoic rocks (Bleeker &Macek 1996). Primary (i.e. non-supergene) ores are dominatedby pyrrhotite, pentlandite, pyrite, and millerite. Chalcopyrite,magnetite and ferro-chromite are ubiquitous minor phases(Layton-Matthews et al. 2007, 2010)

The most recent glaciation, during the Wisconsin, resultedin ice that flowed southwest from an ice centre in Keewatinand then west from ice centred in Hudson Bay (Klassen 1986)Both ice flow events eroded and transported metal-rich debrisfrom the TNB deposits. In general, till across the region is thin(<3 m thick) and has a silty sand (54% sand) matrix. As theLaurentide Ice Sheet melted back 7800 years BP, the region wasinundated by glacial Lake Agassiz for approximately 100 years,during which time rhythmically bedded clay and silt weredraped over bedrock and till, in places up to 40 m thick(Thorliefson 1996). As a result, the region is a flat-lying com-prising a relatively low relief, poorly drained clay plain domi-nated by organic deposits with few bedrock outcrops. An indi-cator mineral dispersal train defined by Cr-diopside in till (Fig.2) extends up to 300 km southwest of the Belt (Matile &Thorleifson 1997). The elevated Cr-diopside abundances in tilloverlying the TNB are accompanied by local occurrences ofchalcopyrite, hercynite, chromite and loellingite in till proximal

to the Ni deposits. Historically, till sampling methods have notbeen used to explore for Ni-Cu deposits in the TNB.

METHODSBedrock (5 kg) and till samples (15 kg) were collected acrossthe northern TNB in 2005 and 2006, most closely spacedaround the Thompson, Birchtree, and Pipe Ni-Cu deposits(Fig. 1) and at varying distances up- and down-ice (southwestand west) of the deposits. Bedrock samples were collectedfrom various lithologies within or adjacent to the TNB, andmineralized and unmineralized ultramafic intrusions. In addi-tion, archived heavy mineral concentrates from till samples col-lected across the region in 1996 (Matile & Thorleifson 1997) aspart of a reconnaissance survey for kimberlite indicator min-erals were re-examined as part of this study.

Indicator mineral signatures of magmatic Ni-Cu deposits, Thompson Nickel Belt, central Canada

M.B. McClenaghan1, S.A. Averill2, I.M. Kjarsgaard3, D. Layton-Matthews4 & G. Matile5

1 Geological Survey of Canada, 601 Booth Street, Ottawa Ontario, Canada K1A 0E8 (email: [email protected])2 Overburden Drilling Management Ltd., 107-15 Capella Court, Ottawa, Ontario Canada K2E 7X1

3 Consultant, 15 Scotia Place, Ottawa, Ontario, Canada K1S 0W24 Department of Geological Sciences, Queen’s University, Kingston, Ontario, Canada K7L 3N6

5 Manitoba Geological Survey, 360-1395 Ellice Avenue, Winnipeg, Manitoba, Canada R3G 3P2

29BMS sdipaRdnarG

ekaLwonS

Ponton

Wabowden

THOMPSON 55.8°N

97.9°W

ellivretsaE

Kisseynew

Flin Flon

PikwitoneiroirepuS-llihcruhC

enoZyradnuoB

Gods Lake

Molson Lake

kilometres

010 5020304050

PaleozoicCover

Thompson Nickel Belt

CANADA

Mines and Deposits

1

23

4

56

7

1. Moak2. Thompson3. Birchtree4. Pipe

5. Soab North6. Soab South7. Manibridge

391

280

6

60

6

39

373

Fig. 1. Location of the Thompson Nickel Belt in northern Manitobaand location of the Thompson, Birchtree, Pipe and Soab deposits(modified from Layton-Matthews et al. 2007).

Bedrock samples were examined in polished thin sections(PTS). Bulk till and crushed bedrock samples were processedusing a combination of tabling, panning and heavy liquid sep-aration to produce ferro- and non-ferromagnetic heavy mineralconcentrates. Indicator minerals were examined and counted in

the 0.25 to 2.0 mm fraction of the non-ferromagnetic and fer-romagnetic concentrates. Pan concentrates were also exam-ined. Electron microprobe analyses were carried out onchromite and olivine in PTS and selected loose grains picked

68 M.B. McClenaghan, S.A. Averill, I.M. Kjarsgaard, D. Layton-Matthews & G. Matile

104º48º

94º48º

56º94º104º

56º

km

100 2000

Cr-diopsidegrains/sample

0 - 12

3 - 5

6 - 10

12 - 15

44 - 87

Fig. 2. Abundance of Cr-diopside in till samples across central and southern Manitoba. Location of the Thompson Nickel Belt is highlightedby the thick dashed line (modified from Averill 2001).

69

from concentrates from till and bedrock. In addition, selectedCr-diopside and Cr-corundum from till samples were analyzed.

The <0.063 mm fraction of till was analyzed using aquaregia and lithium borate fusion/nitric acid digestions andinductively-coupled-plasma emission- and mass-spectrometry(ICP-ES/MS) techniques. Gold, Pt, and Pd were determinedby fire assay/ICP-MS. Data are reported in McClenaghan et al.(2009, 2011).

INDICATOR MINERAL RESULTSNickel-copper mineralization indicator minerals in the 0.25 to0.5 mm non-ferromagnetic fraction, pan concentrates and/orPTS of bedrock and/or in till include pentlandite, pyrrhotite,sperrylite (Fig. 3), chalcopyrite, pyrite, millerite and loellingite,along with trace amounts of gersdorffite, cobaltite, sphalerite,nickeline and volynskite. Pyrrhotite is also abundant in the fer-romagnetic fraction of mineralized bedrock and proximal tillsamples. Thousands to tens of thousands of these sulphideminerals occurs in the 0.25 to 0.5 mm heavy mineral fractionof till proximal (<1 km) to sulphide mineralization. Proximaltill samples also contain tens to hundreds of pentlandite andchalcopyrite grains in the coarser 0.5 to 2.0 mm heavy mineralfraction. Of these minerals, chalcopyrite and sperrylite are themost likely to survive glacial transport and subsequent post-

glacial weathering (Averill 2009, in press) and thus, will be themost useful for till sampling surveys.

Mineral indicators of potentially fertile ultramafic intrusionsin the region include chromite, Cr-diopside, forsterite, ensta-tite, and Cr-corundum. Chromite is most abundant in till sam-ples proximal to mineralization (Fig. 4). Mineral abundancepatterns for chromite, as well as Cr-diopside, forsterite, andenstatite, also indicate that some of these grains are derivedfrom ultramafic rocks east of the TNB, some of which maywarrant further investigation for their potential to host Ni-Cumineralization.

The composition of chromite in bedrock ranges from fer-rochromite (FeCr2O4) with >60 wt.% Cr2O3 to Cr-rich mag-netite (Fe(Fe,Cr)2O4) with >10 wt.% Cr2O3. A second trendranges from magnesian chromite ((Fe,Mg)(Cr,Al)2O4) with >60wt.% Cr2O3 to spinel sensu stricto (MgAl2O4). A striking featureof the magnetite-trend chromites is their elevated Zn content(2-18 wt.% ZnO) that is restricted in occurrence to mineralizedbedrock at the Thompson, Birchtree and Pipe mines and prox-imal (<500 m) till samples. Zinc-rich chromite was expected, asother researchers have reported its presence in TNB deposits(e.g. Chen 1993; Paktunc & Cabri 1995), as well as in othermagmatic Ni-Cu deposits around the world (e.g. Peltonen &Lamberg 1991). The cause of the Zn enrichment in chromitein the TNB ores is likely related to the high grade of meta-

Indicator mineral signatures of magmatic Ni-Cu deposits, Thompson Nickel Belt, central Canada

Fig. 3. Secondary electron images of angular sperrylite grains recovered from the pan concentrate of till sample 05-MPB-010 collected from thewest shoulder of the South Pit, Thompson Mine.

A B

C D

10 μm 20 μm

10 μm 20 μm


morphism that has affected portions of the belt (Couëslan etal, 2007) combined with the presence of Zn-rich minerals suchas sphalerite. The Thompson deposit is at the highest meta-morphic grade (granulite) of all the deposits in the belt and themost Zn-rich chromites of this study were found here.

Few olivine grains were recovered from rocks in this study.However, unpublished GSC data for olivine in association withmineralized rocks in the TNB has a restricted compositionrange >Fo78 and <3000 ppm Ni, making it another potentialindicator mineral.

20

kilometres

0 40

Mesozoic

Paleozoic

Archean granites and gneisses

Proterozoic granites and gneisses

Mafic and ultramafic intrusive rocks

Metagreywacke

Metasedimentary rocks

Mafic and felsic metavolcanic rocks

7

82

No. samples

46

2

No. grains chromitenormalized to 10 kg

(0.25-0.05 mm)

22

3

201-373

0

1-25

101-200

26-50

51-100

Range

SnowLake

LakeWinnipeg

Thompson

Soab

Birchtree

PipeWintering Lake

Paint Lake

Cuthbert Lake Dyke

Fig. 4. Abundance of chromite in the 0.25-0.5 mm heavy mineral fraction of till normalized to 10 kg sample weight across northern Manitoba.

71Indicator mineral signatures of magmatic Ni-Cu deposits, Thompson Nickel Belt, central Canada

The bedrock source of the 10s to 100s of Cr-diopsidegrains in till across the region has not yet been found. In spiteof the predominance of pyroxenites and peridotites in theTNB, remnants of primary Cr-diopside are scarce becausemost rocks have been metamorphically and metasomaticallyaltered such that serpentine has replaced olivine and orthopy-roxene, and amphibole, chlorite, talc and carbonate havereplaced clinopyroxene. Because the Cr-diopside source isunknown, their compositional range and relationship to Ni-mineralization cannot be determined.

Historically, the <0.25 mm fraction of heavy mineral con-centrate of till is too fine-grained to visually examine and selectindicator minerals from in a cost-effective way. As a test of thisnew rapid SEM scanning method, mineral liberation analysis(MLA) was carried out on a split of the <0.25 mm fraction ofsample 05-MPB-010, from the west shoulder of the South pit,Thompson Mine. This fraction was processed using aHydroseparator HS-11, at CNT Mineral Consulting Inc.,Ottawa, and was found to contain pyrrhotite, pyrite, pent-landite, gersdorffite, violarite, chalcopyrite, galena, arsenopy-rite, skutterudite, sphalerite, irarsite and sperrylite. This initialtest indicates that both hydroseparation and MLA have poten-tial to be a useful tool for identifying silt-sized Ni- and PGE-ore minerals in till in a cost effective manner.

TILL GEOCHEMICAL RESULTSNickel-copper ores in the TNB are dominated by pentlandite,pyrrhotite, pyrite and millerite (Burnham et al. 2003; Layton-Matthews et al. 2007, 2010) and thus local metal-rich till is Ni-rich (up to 3760 ppm). The full suite of till pathfinder elementsfor Ni-Cu deposits in the TNB include Ni, Cu, Pt, Pd, Co, As,Ag, Cd, Sb, Bi, S, Se, and Te (McClenaghan et al. 2011). Till

samples that contain the highest Ni contents also contain up to50,000 pentlandite grains/10 kg in the 0.25 to 0.5 mm fraction.Till with elevated Cu values (215 ppm) at the Thompson andPipe mines contain up to 2500 chalcopyrite grains/10 kg.Thompson Nickel Belt ore also contains Te, As, Sb, Co, Cd, Seand Bi-bearing mineral species (McClenaghan et al. 2011),which are likely the source of elevated concentrations of theseelements in till proximal to the deposits. A variety of PGEminerals occur in the ores (McClenaghan et al. 2011). Sperrylitegrains were recovered from nine till samples collected near theThompson and Pipe mines. Thus, it is likely that PGE miner-als are the source of the elevated Pd (up to 98 ppb) and Pt (upto 13 ppb) concentrations in till.

Platinum and Pd values for the <0.063 mm fraction of tillwere used to check which samples were most likely to containvisible sperrylite grains. Initially six till samples were reportedto contain visible sperrylite grains in the heavy mineral frac-tion. Based on the elevated Pd and Pt values for samples 05-MPB-008, 05-MPB-030, and 05-MPB-035, the fine fraction ofthese three heavy mineral concentrates were repanned andsperrylite grains were found (Table 1).

IMPLICATIONS FOR EXPLORATIONThe advantages of using indicator mineral methods to explorefor magmatic Ni-Cu deposits over conventional till geochemi-cal methods are (1) deposit signatures defined by robust indi-cator minerals such as chromite are not significantly affectedby postglacial surface weathering, (2) as little as a few indicatormineral grains in a 10 kg till sample may be significant; this isequivalent to ppm detection limit, (3) indicator mineral signa-tures are detectable farther down-ice from mineralization thantill geochemical signatures, which is well suited to the larger

elpmaS noitacoL )m(ecnatsiD morfnoitceriD forebmuNforebmuNforebmuN uA tP dP uC iN rCenozeroenozeromorf etilyrreps

gk01/sniargetidnaltnepgk01/sniarg

etirypoclahcgk01/sniarg

mppmppmppbppbppbpp

700-BPM-50 tiphtuoS-nospmohT tsewhtron05 91910 1< 8.0 4.1 51 57 33800-BPM-50 tiphtuoS-nospmohT tsew05 0010046 3 8.2 3.21 05 235 95900-BPM-50 tiphtuoS-nospmohT tsew001 0052000,052 8 3.01 9.79 512 0673 34010-BPM-50 tipB-nospmohT revo0 0070535 5 2.3 6.45 66 948 84110-BPM-50 tipC-nospmohT tsae002 3220 1 2.0 5.0< 62 711 94210-BPM-50 tipC-nospmohT tsew002 970 1< 8.0 8.1 61 24 14310-BPM-50 edistsae-epiP revo0 0300 2 5.3 7.1 04 601 36410-BPM-50 edistsae-epiP tsae052 2300 1 9.1 5.1 53 47 44510-BPM-50 edistsae-epiP tsae002 0200 2 3.1 1.1 43 28 05030-BPM-50 nospmohT tsew051 053040 3 3.7 7.32 87 6331 04130-BPM-50 nospmohT tsew05 61045 1 4.1 4.2 81 95 73330-BPM-50 edishtuos-epiP tsewhtuos001 1101 2 4.2 3.3 03 951 06430-BPM-50 edishtuos-epiP htuos002 000 1 4.2 4.2 32 49 74530-BPM-50 edistsew-epiP tsew004 3051 9 1.31 8.91 421 9571 902140-BPM-50 eniMbaoS tsew005 1400 1 7.2 8.2 53 791 74850-BPM-60 edistsew-epiP tsew005 201 5 9.0 7.0 52 511 15950-BPM-60 edistsew-epiP tsew005 200 5 9.1 5.2 75 943 15060-BPM-60 edistsew-epiP tsew057 102 5 6.3 3.4 28 896 37160-BPM-60 edistsew-epiP tsew003 3103 5 1.2 6.6 55 803 18260-BPM-60 edishtuos-epiP htuos005 700 5 0.2 9.1 34 831 45

Table 1. Number of grains of chalcopyrite and pentlandite in the 0.25-0.5 mm heavy mineral fraction and sperrylite in the pan concentrate +0.25-0.5 mm fraction compared to selected matrix (<0.063 mm) geochemistry for till samples <750 m from Ni-Cu mineralization in the northThompson Nickel Belt (modified from McClenaghan et al. 2011).


sample spacing of reconnaissance- and regional-scale studies,(4) evidence of anthropogenic contamination, which in thecase of the TNB consists of airborne smelter particles, is visi-ble and can be quantified, (5) indicator mineral methods can beused to explore for a broad range of commodities simultane-ously, including diamonds, base-metals, gold and uranium.

Implications specific to exploration in the TNB include thefollowing:• Elevated chromite abundances combined with the pres-

ence of Ni-Cu sulphide minerals are strong indicators ofpotential magmatic Ni-Cu mineralization. Chalcopyriteand sperrylite are the most useful metallic indicator min-erals as they are the most likely to survive glacial transportand postglacial weathering. Elevated Zn content (>2 wt.%ZnO) in chromite is only found in strongly mineralizedrocks and is the strongest indicator for mineralizationother than the actual ore minerals. Zn content in chromitefrom Ni-deposits in regions with lower metamorphicgrade might be considerably lower and this feature thusdoes not have universal application.

• Till geochemistry of <0.063 mm fraction is a useful toolfor Ni exploration in the TNB and surrounding region.Pathfinder elements include Ni, Cu, Pd, Pt, Co, As, Cd,Ag, Sb, Bi, Se, S, and Te. Till geochemistry should be usedin combination with indicator mineral methods.

• For Ni exploration in the TNB and surrounding areas, tillsamples in thin drift areas can be collected from the flanksof bedrock outcrops, and from till exposures in road cutsand along lake shorelines and river banks. In areas ofthicker cover, backhoe trenching and overburden drillingcould be utilized to obtain till below the potentially thickclay cover.

ACKNOWLEDGMENTS Funding was provided under the Geological Survey ofCanada’s Targeted Geoscience Initiative 3 (2005-2010),CAMIRO Project 04E01 and the Manitoba Geological Survey.J. Macek (MGS) M. Doherty (ALS Chemex) and T. Lane(CAMIRO) assisted with the planning and sampling in 2005.Vale is thanked for providing access, confidential information,samples, and assistance with fieldwork. L. Cabri (CNT MineralConsulting Inc.) and R. Lastra (CANMET) are thanked fortheir R&D efforts with hydroseparation and grain mounting ofthe <0.25 mm fraction. I. McMartin is thanked for contribut-ing to the data plotted in Figure 4.

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AVERILL, S.A. in press. Viable indicators in surficial sediments for two majorbase metal deposit types: Ni-Cu-PGE and porphyry Cu. Geochemistry:Exploration, Environment, Analysis.

BLEEKER, W. & MACEK, J. 1996. Evolution of the Thompson nickel belt, Manitoba:setting of Ni-Cu deposits in the western part of the Circum Superior Boundary zone.Geological Association of Canada, Field Trip Guidebook A1.

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