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Chapter 12

HEAVY MINERALS IN SHALES

MATTHEW W. TOTTENa AND MARK A. HANANb

aDepartment of Geology, Kansas State University, Manhattan, KS 66506, USAbDepartment of Geology and Geophysics, University of New Orleans, New Orleans,

LA 70148, USA

ABSTRACT

Although shales dominate the sedimentary rock record, when compared to sandstones their heavy

mineral fraction has received little attention. A principal reason for this disparity is the difficulty

of density separations in rocks with high clay contents using organic liquids. These liquids work

well for clean sandstones, but their effectiveness in clay-rich samples is limited by the adsorption

of organic molecules onto clay minerals. Additionally, the small size of heavy mineral grains

within shales demands advanced observational conditions, and considerable expertise that an-

alysts must develop before undertaking these investigations.

In this study heavy minerals were separated from the clay and light-mineral matrices of over

100 shale samples using lithium metatungstate (LMT), a non-organic heavy liquid. The sample

sets include sites from Ordovician through Mississippian shales from the Ouachita Mountains of

Oklahoma and Arkansas, USA, and Miocene through Pleistocene shales from the subsurface

Gulf of Mexico (GOM). Heavy minerals include variable amounts of zircon, tourmaline, rutile,

apatite, baryte, monazite, and xenotime. Opaque minerals are Fe, Ti, Fe-Mn, Ba-Mn, and Cr-Fe

oxides, as well as Fe-rich micas. Comparisons with heavy-mineral contents of interbedded sand-

stones with the surrounding deep-water shales show that heavy minerals are equally plentiful and

diverse in the finer-grained clastics.

Tectonic and provenance interpretations based on trace-element geochemistry rely upon the

assumption that signature elements are quantitatively transferred from the same source as the

bulk of the sediments. Trace-element analyses of whole-rock and light-mineral separates indicate

that heavy minerals are sometimes the dominant sites for some of the trace elements in shales.

The potential control of signature trace elements by an accessory mineral phase, perhaps from a

secondary, low-volume source terrain, needs to be addressed.

We stress the importance of using the heavy-mineral fraction to complement the geochemical

study of shales. Heavy minerals in shales can be as effective in determining sediment provenance

as they have proven to be in sandstones. A source of some of the heavy minerals within shales

could be from resistate mineral inclusions within both light and heavy host minerals. This is

Developments in Sedimentology, Vol. 58, 323–341

r 2007 Elsevier B.V. All rights reserved.

ISSN: 0070-4571/doi:10.1016/S0070-4571(07)58012-X

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dx.doi.org/10.1016/S0070-4571(07)58012-X.3d

Chapter 12: Heavy Minerals in Shales324

particularly important for zircon, rutile, and monazite, based upon their observed size in shales.

This should be considered when using heavy minerals in shales for provenance studies.

Keywords: heavy minerals; shale; Ouachita Mountains; Gulf of Mexico; trace-elements

1. INTRODUCTION

Shales comprise at least two-thirds of the sedimentary record and their lowpermeability is favourable for the preservation of heavy minerals. The potential ofheavy mineral analyses of fine-grained rocks (shales) for broadening our under-standing of sedimentary systems was first proposed by Blatt 35 years ago (Blatt andSutherland, 1969) and was more recently outlined by Schieber and Zimmerle (1998).Their arguments are valid even today.

Some of our preconceptions regarding shales may impede their study. We learn inintroductory-level sedimentation classes about hydraulic equivalency, and the associ-ation of silt-sized heavy minerals with sand-sized clastic sediments. When we make thetransition to shales, which by definition have a mean grain size in the clay to silt range,the initial response is that there should not be any heavy minerals large enough toseparate and work with, or they are so minor in abundance that they can be ignored.They certainly are rarely mentioned in chapters covering shales in sedimentary petrol-ogy texts, with the exception of Boggs (1992) who cites the presence of heavy mineralswithin shales but adds that little is known about their occurrence or abundance.

A number of previous studies hint at the potential utility of heavy minerals inshales, even though we do not yet fully understand the mechanism by which they weredeposited. Of particular interest are studies that, while not directly looking for heavyminerals, attempted to explain whole-rock geochemical variations of certain traceelements by implicating heavy minerals. The most commonly used normalising con-stant for sedimentary geochemistry, the North American Shale Composite (NASC),is consistently not constant for certain elements. Gromet et al. (1984) reported sig-nificant heterogeneity in different aliquots of NASC for certain trace elements, andargues that this variability was controlled by variable amounts of heavy minerals suchas zircon. Condie (1991) was more specific and used trace-element ratios, likely in-fluenced by specific heavy minerals (e.g., La/P2O5 for apatite; Zr/Hf for zircon), toexplain the observed whole-rock ratios. Comparative studies provide evidence for theconcentration of some signature trace elements in heavy minerals (Preston et al.,1998, 2002). These studies are the exception, with the majority of sedimentary geo-chemists citing clays as the mineralogical source of most trace elements (Cullers et al.,1975). The reported correlation between clay-mineral percentage (or Al2O3 as aproxy) and many trace elements (e.g., TiO2) seemingly supports this conclusion(Condie, 1991; Totten and Blatt, 1993). An alternative explanation for this obser-vation is the dilution of trace elements and clay minerals primarily by quartz, themajor framework component of siliciclastic sediments.

Shale heavy minerals are a robust source of potential insight into sedimentprovenance, in exactly the same way that sandstones have proven to be over the last100+ years. Their study could solve significant stratigraphic problems in shales in amanner similar to Mange et al. (2003) for coarser-grained rocks. The potential of

3. Methodology 325

individual heavy-mineral species in determining shale provenance (Morton, 1991;Schneiderman, 1995) is also untapped. The major goal of reviewing the findings of ourprevious work is to raise (excite) interest in the heavy-mineral fraction of mudstones,inviting especially colleagues who study heavy minerals in coarser-grained rocks.

2. UNITS OF STUDY

Heavy minerals have been separated from shales of two distinct depositionalbasins from formations ranging in age from Ordovician to Pleistocene. The initialsample set was collected from the Stanley Formation (Mississippian) of the OuachitaMountains. Sample locations are described in Totten et al. (2000). Pre-Carboniferousshale samples from the same general area as the Stanley Formation include thosefrom Arkansas, including the Novaculite Shale (Devonian), Missouri MountainShale (Silurian), and Polk Creek Shale, Big Fork Chert, Womble Shale, Blakely Shale(Ordovician). These shales are marine deep-water clastics.

The Cenozoic shale samples were collected from well cuttings offshore GOMfrom a single well in the Ship Shoal area. Their age was determined by comparisonwith published palaeontological markers.

3. METHODOLOGY

3.1. Heavy Minerals from Shales

The early work by Blatt and others—on separating the heavy-mineral fraction fromfine-grained sediments—relied upon organic heavy liquids such as tetrabromethaneand bromoform. Not only are these liquids toxic but are especially problematic whenused with clay-rich rocks. The highly polar organic molecules strongly adsorb ontoclay-mineral surfaces, often creating an almost gel-like substance that rafts othergrains, and make it difficult to reclaim the liquid (Nelson, 1971). Blatt and Sutherland(1969) used bromoform to separate heavy minerals from Tertiary shales, although theydecanted the smaller than 10mm fraction before separation. Their method avoided theproblems associated with the interaction of organic liquids and clays, but ignored thefiner grained,o10mm, heavy minerals. These studies are the exception, and the overallimpression seems to be that organic heavy liquids do not work with well-indurated,clay-rich sedimentary rocks (Schieber and Zimmerle, 1998). Personal experience by thesenior author, working in Blatt’s sedimentation lab in the late 1970s (and early 1990s),confirmed this observation, and prompted us to test more suitable heavy liquids.

We have developed a technique that uses a non-toxic heavy liquid, LMT, whichdoes not react with clay minerals. It is a water-soluble salt that has a working densityrange between 2.0 and 3.1 g/cm3. The ability to adjust LMT to precise densities, itsdeflocculating behaviour with natural clays, and the ease with which it may bewashed off of clay-mineral surfaces allowed us to separate the common species ofclay minerals from one another and recover the liquid (Totten et al., 2002). It worksextremely well in separating heavy minerals from shales. Details of the LMTseparation technique are given in Hanan and Totten (1996).


3.2. The Challenge of Identifying Fine-Grained Heavy Minerals

Blatt and Sutherland (1969) reported the difficulty in identifying heavy-mineralgrains finer than 10 mm using the petrographic microscope. Tourmaline, because ofits strong pleochroism and prismatic morphology, is an exception and often is rec-ognisable down to 5 mm. Because of these limitations, and because we were alsointerested in the opaque fraction, we relied upon micro-beam imaging to identify themineralogy of the heavy fraction.

We have examined different shale sample sets for heavy minerals, and althoughthere have been slight variations as the technique developed, we always relied on thescanning electron microscope (SEM) to identify and quantify the presence of the heavyminerals. SEM images combined with energy dispersive spectra (EDS) on grains (andgrain areas within rock fragments) reveal details that would be difficult to see with anoptical microscope, especially on grains less than 10mm in size. The analytical tech-niques for mineral identification include EDS on individual grains, elemental X-raymaps, and backscattered electron imaging. Combinations of these techniques may beparticularly effective for identifying opaque heavy minerals and for finding less abun-dant, but important, heavy minerals that are known to sequester specific trace elements.

The mineralogy of the heavy fraction was further refined using the SEM-EDSto identify the opaque heavy minerals. Photomicrographs of the plain-light grainimages allowed comparison to backscattered images of grain areas at the SEMworkstation. Backscattered X-ray intensity and EDS spectra of grains intersectingthe polished surface of the grain mount were matched to opaque and non-opaquegrains in the projected slide. Matching grains seen by the SEM with their opaqueimage on the slide was time-consuming. Although all of the grains are visible by thelight microscope, only grains that intersect the polished surface can be imaged on theSEM. SEM grain images are the result not only of the grain size but also of the depthof the polished surfaces. Many grains are below the surface of the epoxy and notimaged by backscattered electrons or their outlines are very small.

Due to the fine-grained nature of shales it is important not to exclude the finestsize fractions of heavy minerals as is routinely done in traditional heavy-mineralstudies on sandstones. The reason for our focus on the entire population of heavy-mineral sizes is because of the fine-grained nature of shale samples, and a desireto overcome misconceptions about the presence of the heavy minerals in shales.Currently, we are still testing and assessing better methods for a complete extractionand identification of the entire population of heavy minerals present.

4. HEAVY-MINERAL SEPARATION FROM FINE-GRAINED TURBIDITESOF THE OUACHITA MOUNTAINS

4.1. Sandstone and Shale Pairs

Heavy minerals were separated from interbedded sandstone and shale pairs collectedthroughout the Mississippian Stanley Shale from the Ouachita Mountains ofOklahoma and Arkansas, USA (Totten and Hanan, 1998). The heavy minerals wereas abundant in the shales as they were in the sandstones. A somewhat unexpected

4. Heavy-Mineral Separation from Fine-Grained Turbidites of the Ouachita Mountains 327

result was that, on an average, the shales have a higher weight percentage of heavyminerals (0.95%) than the interbedded sandstones (0.65%). Fig. 1 shows thevariation in the heavy-mineral yields from the sandstone/shale pairs and illustratesthis relationship.

The overall median grain-size of the heavy minerals in the shales is approximately25mm less than that of the heavy minerals in the sandstones. Most shale samples showthat the largest portion of the heavy minerals are concentrated in the finer than 10mmsize-fraction, grains that would have been lost using the method of Blatt andSutherland (1969). Fig. 2 is a plot of the median grain-size of the sandstone and shalepairs.

More heavy minerals might be expected in the sandstones because of the higherenergy of the depositional environment and hydraulic equivalency of the silt-sizedheavy minerals with coarser-grained quartz and feldspar grains. The apparenthydrodynamic disequilibrium of the shale heavy minerals with primarily clay-sizedlight minerals might be explained by non-sphericity of the heavy (Fe-rich) micas,rafting of heavy-mineral grains on or within clay-mineral flocculants, and perhapsauthigenesis of heavy minerals (mainly Fe-oxides). It is also possible that the sand-stones had more heavy minerals initially but intrastratal dissolution has reducedtheir amount in the more permeable sandstones.

Rafting of heavy minerals by light minerals was certainly a problem during theheavy-mineral separation procedure. Although the amount of rafting of heavy min-erals by clay floccules during transport and deposition of fine-grained sediments isunknown, it is possible that the tendency of the much more abundant light fractionto raft the sparse heavy fraction is as important to the understanding of the dis-tribution of heavy minerals as is the hydrodynamic equivalency of grains in shales.

Fig. 3 shows a composite photomicrograph of the 20–30mm size heavy minerals in asandstone/shale pair from the Stanley Shale under the same magnification. Fig. 3 servesto reinforce the findings of Blatt and Sutherland (1969) for Texas Gulf Coast shales and

0.0

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5 11 12 15 24 28 33 41 42 43 57 58 302 1002sample numbers

sandstone mudrock

acce

ssor

y m

iner

al w

t%

Fig. 1. Relative abundance of heavy minerals within interbedded sandstone–shale pairs from

the Mississippian Stanley Formation of the Ouachita Mountain foldbelt.

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2

1med

ian

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of h

eavy

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eral

s (µ

m)

11 12 24 33 41 42 43 57 58 302 1002

shales sandstones

sample number

Fig. 2. Median grain-size of the heavy minerals in the interbedded sandstone–shale pairs

from the Mississippian Stanley Formation of the Ouachita Mountain foldbelt.

Fig. 3. Photomicrographs of heavy minerals in the 30–62 mm fractions of shale and inter-

bedded sandstone from the Stanley formation of the Ouachita Mountain foldbelt. Both pho-

tos taken at 200 power.



illustrates that there are sufficient coarse-grained heavy minerals in these ancient shalesto enable a ‘classical’ heavy-mineral approach to provenance determinations using thelight microscope. The high percentage of finer-sized material and the widespreadavailability of SEM technology provides opportunities to go much farther.

4.2. Image Analyses of all Size Composites

The average mineralogy of the heavy fraction from 66 Stanley Shale samples isillustrated in Fig. 4. Data for the Stanley Formation in Arkansas and Oklahomawere collected from grain mounts that included the entire size fraction of the heavy-mineral separates. Each sample was examined using an Amray 1820 digital SEMequipped with an EDS. Heavy minerals were identified by comparing their EDSelemental spectra to EDS spectra obtained from known heavy minerals. The cross-sectional area of each grain was determined using the image analysis capabilities ofthe Iridium IXRF software. The relative proportions of heavy minerals present andtheir size distribution were calculated from these data.

Fe-oxides, Ti-oxides, and Fe-bearing biotites are the dominant minerals in theheavy-mineral fraction of the Stanley Shale samples. The common non-opaquesinclude apatite, zircon, tourmaline, garnet, and monazite. Except for monazite thismineralogy is similar to the heavy minerals found in the Stanley interbedded sand-stones reported by Bokman (1953).

The quantity and diversity of the finer-sized heavy minerals seen in grain mountsindicate that these are important constituents of the total heavy-mineral fraction.Even though these very fine-grained heavy minerals are abundant, it is not feasible tomeasure and count all of the small grains in the fields of view. Moreover, small grains

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Fe-

oxid

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rutil

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biot

ite

apat

itehi

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-alte

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iteal

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otite

chlo

rite

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azite

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xeno

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mus

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ibol

eky

anite

silli

man

iteba

ryte

Average Stanley Shale

%

Fig. 4. Average distribution of specific minerals contained within the heavy mineral fraction

of shales from the Stanley shale of the Ouachita Mountains.


are often observed within part of a larger rock fragment. In fact, monazite mostoften occurs intergrown with quartz as is shown in the composite grain in Fig. 5.Although the total cross-sectional area of the smaller grains does not comprise amajor portion of the entire grain area, we are concerned that this method of de-termining mineral abundances might overlook significant minerals within the veryfine-grained fraction.

The occurrence of small monazites in rock fragments is important and suggeststhat some portion, a sub-population, probably remains in the light fraction duringseparation (i.e., the small heavy-mineral grains become locked within light-mineralcomposite grains) and we need to modify our methods to ensure a more completesegregation of this heavy mineral sub-population. The work of Caggianelli et al.(1992) and Gromet et al. (1984) suggests that even intense disaggregation may notrelease all the heavy minerals because the extremely small (2 mm or less) grains staylocked up within the light-mineral rock fabric of shales.

4.3. Pre-Carboniferous Shale Samples from the Ouachita Mountain Flysch

There is also geochemical evidence that a portion of the heavy minerals remain in thelight fraction, even after intense disaggregation. A suite of shale samples from thepre-Stanley Ouachita flysch (Ordovician to Mississippian) were analysed usingstandard XRF techniques. Fine-grained powders were prepared by milling in aSPEX shatter box for 30min. According to information provided by SPEX, thisproduces a powder with grain sizes between 1 and 4 mm. Aliquots of these sampleswere processed using LMT and the powdered heavy-mineral fractions were thenremoved. The resulting heavy-mineral grains are too small to be identified; however,their chemistry may be indirectly determined by assessing the amount of each

Fig. 5. Backscattered SEM image of composite monazite–quartz grain. Bright area of com-

posite grain is caused by the high atomic number of monazite, which was further confirmed by

its characteristic EDS spectra.


element removed with the heavy fraction. The light fractions of these powders wereanalysed by XRF using the same settings as used for the original whole-rock pow-ders. Not surprisingly, the major element concentrations were nearly identicalbetween the whole-rock and light-mineral fraction for each sample. Many traceelements, however, show a distinctly lower concentration in the light-mineralfraction when compared to the whole-rock concentrations. Figs. 6–8 show thatthe removal of heavy minerals reduced the concentrations of La, TiO2, and Zr, in theXRF powders, respectively. Sample no. 25 is particularly interesting because ofthe large amounts of Zr, La, and TiO2 related to the heavy-mineral fraction. Sampleno. 25 contains approximately 50% La, 33% TiO2, and 73% Zr, all associated withthe heavy-mineral fraction. This suggests that there is a strong control of heavyminerals on the concentration of these elements, contained dominantly by monazite,rutile, zircon, and other heavy minerals in the pre-Carboniferous Ouachita shales.

For the pre-Carboniferous shale samples we prepared two sets of heavy-mineralseparates. The SEM samples were prepared using extremely short milling times(20–30 sec) so as to leave relatively large heavy-mineral grains intact for identifica-tion and imaging. The XRF samples were prepared by long milling times (30min) toproduce homogeneous powders for the analyses. Inspection of the samples showedthat the amounts of heavy minerals were generally higher in the separates than in thepowdered material for XRF analyses.

In the heavy-mineral fraction of sample no. 25 from the Polk Creek Shale weexpected that in the SEM analysis zircons would be common because of the high

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Stanley

ArkansasNovaculiteMissouri MountainShalePolk Creek

Bigfork

Womble

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Collier

0% in heavy minerals

Lanthanum

60%

hea

vy m

iner

als

rem

ove

d (

pp

m)

whole rock (ppm)

80%


40%

20%

Fig. 6. Whole-rock Lanthanum concentrations versus Lanthanum concentrations in the light

fraction after heavy-mineral separations from Ordovician–Mississippian shales of the Oua-

chita foldbelt. The difference between the two values is presumed to reside in the heavy-

mineral fraction.

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Bigfork

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Blakely

Mazarn

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Collier90% in heavy minerals


80%

60%

20%

40%

spl 25

hea

vy m

iner

als

rem

ove

d

whole rock

TiO2

Fig. 7. Whole-rock TiO2 concentrations versus TiO2 concentrations in the light residue after

heavy-mineral separations from Ordovician–Mississippian shales of the Ouachita foldbelt.

The difference between the two values is presumed to reside in the heavy-mineral fraction.

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spl 25

Zirconium

hea

vy m

iner

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ove

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whole rock



Bigfork

Womble

Blakely

Mazarn

Crystal Mt

Collier

+

Fig. 8. Whole-rock Zirconium concentrations versus Zirconium concentrations in the light

residue after heavy-mineral separations from Ordovician-Mississippian aged shales of the

Ouachita foldbelt. The difference between the two values is presumed to reside in the heavy-

mineral fraction.



amount of Zr, concentrated by the density separation (anticipated to be in zircons).However, we found zircons to be rare. This may be the result of the short millingtimes that were insufficient to free the small-sized zircon population from the rockfragments in this sample. To determine the abundance of zircon in the XRF powderwe prepared a grain mount of the powdered heavy-mineral separate for X-ray map-ping. The X-ray map of the Polk Creek Shale sample no. 25 revealed a significant andconsistent occurrence of tiny zircons (near 1mm) throughout the powder. Apparently,the longer milling times yielded higher amounts of zircon by crushing rock fragmentscontaining zircon (or minerals with zircon inclusions) and releasing a population oftiny zircons. An analogous situation could arise with other heavy minerals thatcommonly occur as inclusions within other minerals (e.g., monazite).

The heavy-mineral control of important trace elements often used for provenancediscrimination is illustrated in Fig. 9, which shows an element variation diagramfrom a sample of the Womble Shale. The whole-rock curve in Fig. 9 is plottedconsistently above the light-mineral fraction in the right side of the chart and istypical of the pre-Carboniferous Ouachita Flysch samples. Significant portions ofthe high-field strength elements (HFSE) Zr, Hf, Ti, Nb, Ta, Hf, V are separated outwith the heavy minerals. Petrogenetic discrimination schemes based on HFSE ofwhole-rock analyses of shales (e.g., Bhatia and Crook, 1986; Garver and Scott, 1995)might actually be measuring the presence of late-stage, highly evolved, resistateheavy minerals that sequester these HFSE.

The results of the heavy-mineral separations on powdered shale samples stronglyimply that many heavy minerals are contained in rock fragments with lighter min-erals, and have an average density lower than that of LMT. Fig. 5 clearly shows a

Womble Shale (Ordovician)

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3.00

K(%) Rb Sr U P(%) Cs Ba V Cr Ni Nb Ti Zr Y La Ce Th

Whole Rock

Heavy MineralsRemoved

Fig. 9. (Spider) Diagrams showing elemental distribution of whole-rock and in the heavy-

mineral-removed fraction in a shale sample from the Ordovician Womble Shale of the Oua-

chita foldbelt. Note the distinct shift of trace elements on the right side of the diagram, even

though only less than 1% of the rock was removed during heavy mineral separation.


composite grain of monazite and quartz that sank with the heavy fraction. A moreintense milling process for the XRF powders frees many more of these grains fromthe rock fragments, allowing them to be separated. Clearly, it is important to definethe goal of each separation to determine the best method to use. Milling destroys theoriginal petrographic information but is necessary for geochemical analyses.Producing a light fraction entirely free of heavy minerals will require long millingtimes before LMT separation but this will destroy the shape and grain size distri-bution of the heavy minerals. Because it is important to identify heavy-mineralcontents and grain sizes, perhaps a better technique would be to perform LMTseparation on samples subjected to both short and long milling times.

5. CENOZOIC SHALES FROM THE GULF OF MEXICO

Heavy minerals were separated from shale samples, collected from an oil welldrilled in the GOM, offshore Louisiana. Our goal was to compare the results ofheavy-mineral separations from these younger shales with the older Palaeozoicshales from the Ouachita Mountains. These two basins are often compared in theliterature, especially for outcrop analogies to the deeply buried fine-grained turbid-ites in the Gulf (Slatt et al., 1994). Both are shale-rich successions, with intermittentinterbedded sandstones and similar rates of deposition.

The drill cuttings used in this study are from a vertical well in the Ship Shoalprotraction block. Sample collection began at 1500m and continued to the totaldepth penetrated at 4146m. These samples range in age from Pleistocene to lateMiocene, based upon micropalaeontological picks. Ten samples were selected forheavy-mineral separation from an approximately 300m interval. Shale sections werechosen using wire-line logs, recorded during drilling operations.

The weight percentages of heavy minerals separated from these shales range from0.21 to 0.73%, comparable to the amounts recovered from the shales of the OuachitaMountains. Fig. 10 compares the average heavy-mineral distribution in the >10mmsize fraction in the GOM samples to the Palaeozoic Stanley data. The heavy-mineralsuite of the GOM shales was very similar to the heavy minerals found in the StanleyShale. The major difference between the two basins is in the mineralogy of the iron-richsuite. The most abundant heavy mineral in the GOM shales is pyrite, in contrast to theabundant iron oxides and rare pyrite in the Stanley. Whether this is the result ofdifferent oxidation potentials in the depositional environment or diagenetic alteration isunknown at present. An additional contrast between the two basins is the presence ofcarbonate minerals in the GOM samples, represented by ankerite and dolomite. Theywere present at every sampled interval but both decreased in abundance with depth.

For provenance identification, the individual heavy-mineral species were groupedinto mineral assemblages based upon the main rock suites in which they commonlyoccur: felsic igneous rocks are represented by tourmaline, biotite, muscovite, zircon,rutile, monazite, sphene, and apatite (Boggs, 1992). The metamorphic assemblageincludes kyanite-andalusite-sillimanite, staurolite, wollastonite, and garnet.

The abundance of the metamorphic and the felsic igneous heavy-mineral groupversus depth is shown in Fig. 11. The latter group shows a consistent increase withdepth from less than 10% to nearly 40%. The metamorphic minerals show a more

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Average Paleozoic StanleyAverage Cenozoic GOM

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mus

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amph

ibol

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anite

silli

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iteba

ryte

pyrit

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carb

onat

e

Fig. 10. Average distribution of heavy minerals contained within the heavy mineral fraction of

late Miocene through Pleistocene shales from the Gulf of Mexico, compared to the average from

the Mississippian Stanley Shale of the Ouachita Mountains. The major difference is seen in

Fe-sulfides in place of Fe-oxides, and a significant carbonate component in the GOM samples.

6. Discussion 335

moderate increase with depth. This dual provenance mirrors the lithology of sourceareas that provide sediment to the modern GOM, with a metamorphics-dominatedAppalachian source and an igneous-rich western source. During Miocene time, thefelsic igneous source was dominant but has gradually diminished relative to a meta-morphic source during the Pliocene.

6. DISCUSSION

6.1. The Occurrence of Heavy Minerals in Shales

The results of our work to date confirm the presence of a significant heavy-mineralfraction in shales. The diversity of our set of sample shales over a wide age range andfrom multiple basins supports our conclusion that this is not an isolated occurrence. Infact, the quantity of heavy minerals in shales is comparable to that in many sand-stones. The mineralogy of the heavy fraction is also comparable to those of sand-stones. The major difference is the fine grain size of heavy minerals in the shales. Ourresults stress the importance of the o10mm fraction, which however presents manychallenging technical problems for separation, identification, and quantification.

6.2. Are Heavy Minerals an Important Source of Trace Elements in Shales?

Fig. 12 shows an example of the strong correlation between Al2O3 and TiO2 fromStanley Shales of the Ouachita Mountains (Totten and Blatt, 1993). There are many

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felsic igneous/volcanic suite(tourmaline, biotite, zircon, rutile,monazite, sphene, apatite)

metamorphic suite (garnet,kyanite-andalusite-sillimanite,staurolite, wollastonite)

dept

h (m

)

%

Fig. 11. Provenance-controlled variation of heavy mineral assemblages with depth from a

well in Ship Shoal area of the Gulf of Mexico.


examples using similar comparisons that are generally used to conclude that specifictrace elements must reside on clays. It is worth noting that the y-intercept of TiO2 isnon-zero, suggesting the presence of a titanium phase that is separate from the clayfraction, similar to what was reported by Condie (1991). The overwhelming im-pression, however, is that trace elements occur within the clay-mineral fraction ofshales (Chaudhuri and Cullers, 1979).

The implications of this for the study of shales are vital. Because quartz containsextremely low concentrations of most trace elements, and because quartz dominatesthe silt-size range in both shales and sandstones (Charles and Blatt, 1978; Blatt andTotten, 1981), what these correlations might be testing is not whether trace elementsreside in clays but, rather, that they do not reside in the quartz population. As thepercentage of quartz increases, both the percentage of clay and the concentration oftrace elements decrease. The two latter components appear to correlate, but theactual cause of this correlation is an equivalent variation in quartz percentage. Anend-member of this reasoning would be a pure quartz-arenite, which would containextremely low concentrations of trace elements. The opposite end-member, a verypure clay-mineral sample (which may be rare in nature) is often presumed to havesignificant trace-element concentrations adsorbed onto clay-mineral surfaces.However, we have consistently separated significant amounts of heavy minerals inclay-mineral standards available through the Clay Mineral Society’s repository(Totten et al., 2002). Published chemical analyses of the clay-mineral standards are

0.0

0.2

0.4

0.6

0.8

1.0

1.2

%T

iO2

%Al2O3

r = 0.97

0 5 3025201510

Fig. 12. TiO2 versus Al2O3 for Stanley shales. Y-intercept greater than zero suggests a ti-

tanium mineral separate from the clay mineral fraction. assuming Al2O3 exists primarily

within the clay-mineral fraction of these shales.

6. Discussion 337

available, but these are whole-rock data and include the fraction we have removed. Ifnot completely controlled by clays, and obviously not in quartz, where do the bulk oftrace elements reside? Our results, and those of Preston et al. (2002), suggest that theheavy-mineral fraction is an important source. Specific heavy minerals can sequesterspecific trace elements in very high concentrations, easily influencing the overallwhole-rock chemistry of shales. In this case, a little truly does go a long way!

6.3. Implications for whole-rock shale geochemistry

We have constructed a simple mixing model to illustrate the contribution of even minoramounts of heavy minerals to the whole-rock trace chemistry (Totten et al., 2000). Inthis model, monazite was used as a control on the overall rare earth element (REE)pattern of a hypothetical fine-grained sediment (Fig. 13). We intentionally chose twovery different components, NASC and a mid-oceanic ridge basalt (MORB). As ex-pected, a 50/50 mixture of these components results in an REE pattern intermediatebetween those of the two parents. The addition, however, of only 70ppm of monaziteto the 50/50 mix results in a calculated pattern that is indistinguishable from NASC.Monazites contain such a high percentage of light rare earth elements (LREE) thateven a trivial amount of the mineral can greatly influence the whole-rock values of theseelements that generally occur in trace amounts.

Gromet et al. (1984), while analysing the NASC, found HF-resistant zircon grainsand surmised that the heterogeneity between aliquots of a homogenised and powderedsample involved these zircons. They also presumed another minor phase that concen-trates LREE. Based on our finding monazites with the SEM in all of our shale samples,we believe that this presumed minor phase was monazite. Our results also confirm the

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

50/50+monazite

NASC

50/50

N-MORB

80

60

40

30

20

10

100

Fig. 13. Chondrite normalized REE distributions for N-MORB, NASC, a 50/50 mixture of

N-MORB and NASC, and a mixture containing 70ppm monazite (0.007% monazite,

49.9965% N-MORB, and 49.9965% NASC by weight). Modified from Totten and Hanan,

(1998).


control of certain trace-element concentrations by zircon. The REE contents of com-mon heavy minerals range from one to several orders of magnitude greater than thoseof the average shale (Taylor and McLennan, 1995). One other implication involves thewidespread use of whole-rock Sm/Nd isotopic signatures in determining provenance.These two isotopes are members of the LREE that can be dominantly controlled byvariable amounts of specific heavy minerals such as monazite. In these cases, theisotopic signature would indicate the provenance of the monazite, and not necessarilythe whole rock. Authigenic monazite growth, as reported by Evans and Zalasiewicz(1996), would further complicate whole-rock signatures.

The implications for determining provenance, based upon whole-rock trace-element geochemistry, are important. If minor phases can control overall chemistryout of proportion to their abundances, an understanding and consideration of thisfact is vital to sedimentary petrologists. Consequently, it is essential to research andlearn more about the distribution of heavy minerals in shales.

6.4. The Potential for Size Bias

The potential for size bias, associated with the disaggregation of rocks in preparationfor heavy mineral separation can be problematic for coarse-grained sediments, and

References 339

the bias may be even more important to consider in fine-grained shales. Currently, weare experimenting to develop techniques that will allow to separate and quantify theentire grain-size population of heavy minerals in shales. Although very important forprovenance studies and for textural information (e.g., the presence of reworked,rounded heavy minerals), the sub-population of coarser-grained (>10 mm) heavyminerals in shales may not be representative of the entire heavy mineral population.The dilemma associated with shale heavy minerals, as mentioned earlier, is that lightmilling/disaggregation helps to preserve larger grains and facilitates identification,but it also has a drawback because more rock fragments will appear in the heavymineral separations (e.g., intergrown heavy/light minerals, forming compositegrains). With light milling (i.e., short times in the shatter box) some unknown pro-portion of heavy+light mineral-containing rock fragments, with densities lower than2.85 g/cm3, will float in the light fraction. This suggests that a significant proportionof very fine-grained heavy minerals may remain locked up with light minerals and/orrock fragments during separation. The geochemical data from whole-rock samplesand from light-mineral fractions suggest that small heavy minerals, present as inclu-sions within other mineral phases, can easily escape settling in the heavy liquid. Ineither case, whether they are discrete heavy minerals that are separable from a rock,or inclusions within a mineral phase, heavy minerals are important components thatsequester many trace elements and significantly influence whole-rock chemistry.

7. SUMMARY

Our ongoing research does not yet allow us to conclude firmly how increasedunderstanding of the heavy mineral control on trace elements (especially the HFSE andREE) will effect future research, especially on clastic sediment provenance, or whereexactly a further study of shale heavy minerals will take us. If important tectonic orprovenance signatures in shales appear to be controlled largely by particular, resistant,heavy minerals, as we suspect, then the question arises how do these minerals, whichare nearly insignificant by volume or weight, correlate in age and provenance with thebulk of clay minerals, quartz, and other light minerals? The pervading presence of traceamounts of monazites and zircons found in the analysed shale samples also raises thequestion whether Sm/Nd model ages, based on whole-rock shale samples, are merelymeasures of the composite ages of these minerals. A fundamental observation of ourstudy on the occurrence of heavy minerals in shales is that for XRF analysis all whole-rock shale samples should be powdered as long as possible to avoid heterogeneityproblems introduced by the resistant heavy-mineral suite.

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