wilde century pmd-crc 2006

predictive mineral discoveryCOOPERATIVE RESEARCH CENTRE

Final Report

Finding the next Century Deposit:Geochemical Studies

Project G14Confidential Report for Zinifex Ltd

Dr Andy Wilde

Back-scattered electron images of primary ore samples. A - Angular ("framboidal") early pyrites crosscut by sphalerite veinlet. B -Transgressive galena veinlet clearly post-dating pyrite. C - Ferroan carbonate grains with interstitial sulphide. D – Early layer-parallel porous sphalerite and cross-cutting transgressive galena veinlet. E - Early layer-parallel porous sphalerite interstitial to fine-grained pyrite and cross-cutting transgressive galena veinlet. F – Chlorite gangue (note extremely fine grain size < 10 microns). G - Porous sphalerite in contact with quartz and chlorite sheaves. H - Part of Zn-poor layer, showing clastic

quartz and muscovite flakes with interstitial phengite. Coloured images were enhanced by applying a colour map in ImageJ

G14: Finding the next Century Deposit: Geochemical Studies

Final Report May 2006

Compiled by: Andy Wilde School of Geosciences PO Box 28E Monash University Victoria 3800 Australia Ph: 03 9905 1140 Email: [email protected]

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Table of Contents

1 Executive Summary ....................................................................................................... 1 2 Introduction..................................................................................................................... 3 3 Geological Setting .......................................................................................................... 4 4 Mineralogy of Primary Ore and Unaltered Rocks........................................................ 7 5 Mineralogy of Weathered Zone ................................................................................... 11 6 Bulk Chemical Characteristics.................................................................................... 14

6.1 Lady Loretta and McArthur River........................................................................ 14 6.2 Century ............................................................................................................... 16 6.3 The impact of near surface oxidation (weathering)............................................. 20 6.4 Soil iron anomalies ............................................................................................. 22

7 Carbonate Composition............................................................................................... 23 8 Stable Isotopes ............................................................................................................. 26 10 Infra-red Response....................................................................................................... 29

10.1 Minerals detected in IR Spectra.......................................................................... 30 10.2 Variation in Spectral Parameters ........................................................................ 32

11 Illite Crystallinity........................................................................................................... 35 12 Magnetic Susceptibility................................................................................................ 37 13 Density........................................................................................................................... 40 14 Geochemical Process Models..................................................................................... 42

14.1 Questions to be addressed................................................................................. 42 14.2 Constraints.......................................................................................................... 42 14.3 Basic Controls on Pb and Zn Solubility............................................................... 44 14.4 Mixing of Brine and Seawater (Exhalative Model) .............................................. 46 14-5 Mixing of Brine and H2S Gas ............................................................................. 47 14.6 Mixing of Brine and CO2 Gas ............................................................................. 48 14.7 Mixing of Brine and Ch4 Gas.............................................................................. 49 14.8 Mixing of Brine and Gas Mixtures....................................................................... 50 14.9 Fluid-rock Reaction - Dolomite ........................................................................... 50 14.10 Fluid-rock Reaction - Carbon.............................................................................. 51 14.11 Fluid-rock Reaction – Lawn Hill Shale ................................................................ 52 14.12 Metal Sources and Depletion.............................................................................. 53 14.13 Sulphur Source/s ................................................................................................ 55 14.13 Source of Chlorine .............................................................................................. 56

15 Conclusions .................................................................................................................. 57 15.1 The Century Footprint......................................................................................... 57 15.2 Chemical Process Model .................................................................................... 57 15.3 Recommended Further Work ............................................................................. 58

16 References .................................................................................................................... 59 17 Appendix 1 – Data Disk................................................................................................ 61

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pmd*CRC Final Report – G14 Finding the next Century Deposit: Geochemical Studies

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1 Executive Summary • Geochemical modelling best approximated the ore-bearing shales by interaction of oxidised brine and

unaltered carbon-bearing shale, although massive dolomite also precipitated galena and sphalerite. Reduction is thought to be a key depositional process. Many rocks of the McNamara Group could therefore host ore, including carbonaceous dolomite. The key constraint on ore formation could be permeability and porosity rather than chemical composition.

• Interaction of hot hydrothermal fluid and pre-existing carbon (liquid hydrocarbon?) would have

modified the precursor (hydro-) carbon and imparted characteristics such as enrichment in poly-aromatic hydrocarbons (PAH) as observed at the McArthur River mine (Chen et al., 2003). This mechanism could also have generated increased porosity and permeability leading to ore grade accumulations of zinc.

• Furthermore, there is no need to invoke a separate reduced fluid and fluid mixing. It is possible that

sulphur was introduced as sulphate and could therefore be derived from dissolution of anhydritic evaporites at depth. In which case exploration should be confined to areas underlain by evaporitic sediments.

• Siderite-rich siltstones probably result from hydrothermal processes, consistent with enrichment in Zn

and apparent restriction to the Century orebody. Two chemical models approximated formation of the sideritic siltstones: mixing of oxidised brine and contemporary seawater (in a rock-absent environment) or mixing of brine with CO2 gas. The siltstones may therefore represent a precipitate at the ocean floor or the product of mixing sub-surface under conditions of high porosity and permeability.

• The source of metal remains a matter for conjecture, but a possible depletion zone in Lawn Hill

formation rocks has been identified extending up to 6km from Century. Data are insufficient to speculate on the volume of depleted rocks and metal available for transport.

• The ore zone is characterized by bulk chemical enrichment in Fe, Mn and S due to abundance of

ferroan carbonate (primarily zincian siderite) and pyrite. Thus, rocks of the ore zone have higher density and magnetic susceptibility than unmineralised rocks of the hanging wall and footwall. Forward modelling of the Century ore and host-rocks is warranted in order to establish whether gravity or gravity gradiometry (airborne gravity) could be useful exploration tools. Susceptibility measurement is suggested as a rapid and low cost tool for identifying altered rocks in drillcore and chips. It should be noted, however, that sampling is very biased towards the pit area and more samples should be measured beyond the pit, to add confidence to these conclusions.

• Reassessment of analytical data for carbonate minerals disproves the concept of a siderite “halo”

about Century but shows that the rare Fe-Mg carbonates sideroplesite and pistomesite occur widely in the region. Zincian siderite sensu stricto is, however, limited in occurrence to the pit area. The possibility that partial extraction of rock carbonate could be more effective than conventional bulk analysis should be investigated.

• An important question that remains unresolved is the extent of the hydrothermally-altered rocks

beyond the pit area. Thus far, none of the holes sampled revealed hydrothermally altered rocks comparable to those in the pit. It is rare however for the host Pmh4 unit to be intersected beyond the pit area. On the other hand at least some of the layering characteristics of Pmh4 – notably sideritic siltstone layers are likely to be due to hydrothermal processes. Some resolution of this issue may be obtained by a comparable study of the McArthur River deposit, where the transition from ore zone to alteration zone may be unambiguously preserved.

• Infra-red (MicaAlOH) and illite crystallinity (IC) anomalies are more extensive than bulk chemical and

carbonate anomalies. All >5% iron in soil anomalies fall within these mineralogical anomalies. Identification of mineralized rocks in drillcore and chips could be aided by use of an infra-red spectrometer such as HYCHIPS or PIMA. This possibility that outcropping alteration zones could be identified by remote-sensed hyperspectral imagery should be assessed in the next year of this project.


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• This study has focussed on the expression of the Century deposit, but further work should also examine the nature of the (thus far) smaller structurally-controlled deposits such as Silver King and determine whether the alteration associated with these deposits is distinctive. Radiometric dating should also be attempted in order to establish whether these deposits are separated in time from the main Century mineralization event.


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2 Introduction This report summarises the results of research into chemical aspects of ore deposition and hydrothermal alteration at the Century Zn deposit, conducted between 2004 and 2006 as part of project G14 of the pmd*CRC. More detailed accounts of aspects of the research can be found in various reports by the author detailed in the reference section. A compilation of existing data together with all new data collected is presented as an ACCESS database bound with this report. A summary of the database structure and its contents is included as Appendix 1.


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3 Geological Setting Two main Proterozoic units are recognized in the Century area (Fig. 1), the older McNamara Group, host to the ore at Century and the unconformably overlying South Nicholson Group (Roper Superbasin of Jackson et al., 1999). The stratigraphy of the McNamara Group is summarized in figure 2 and a more complete account can be found in Andrews (1998). The Century deposit appears to be unusual in comparison to other major Pb-Zn deposits of the region in being hosted by relatively deep-water sediments (Broadbent et al, 1998). No igneous intrusions are known within 25 km, and the only volcanic rocks are sparse outcrops of the Kamarga Volcanics, approximately 25km east of Century. Volcanic rocks may occur at depth below the Century mine at the base of the McNamara Group.

Figure 1: Simplified geology of the area around Century. Oolitic ironstones of the Mullera Formation are shown in red. Stars show

small sub-economic occurrences of Pb and Zn.

Figure 2: Stratigraphy of the McNamara Group with detail of the mine sequence.


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The Lawn Hill formation attains a maximum thickness of 2.2 km and has been divided into five members (Andrews, 1998). It is unit 4 that hosts the ore. Figure 2 shows part of unit 4, and illustrates an important characteristic of the ore sequence, namely the alternation of carbonate-rich siltstones and carbonaceous shales, the latter containing the bulk of the economic ore. The South Nicholson Group ranges in thickness to over 2km but only a few hundred metres are preserved in the vicinity of the Century mine (Jackson et al., 1999). The basal unit west of Century is the Constance Sandstone, which is dark red at outcrop reflecting the abundance of hematite and other iron oxides/hyroxides. Overlying the Constance Sandstone is the Mullera Formation, which contains unusual and extensive oolitic ironstones, which outcrop over 50km of strike. These ironstones are estimated to contain 322 million tonnes of iron ore at approximately 47% Fe (Harms, 1965). The Century deposit is partly overlain by unconformable Cambrian limestone. The nature of the contact is complex, and calcite-cemented breccia dykes intrude the ore sequence. Feltrin et al. (2003) describe 2 – 3cm wide veins of quartz, siderite, sphalerite, galena and pyrite that “pervade the nodular zone in the Cambrian limestone”. A large body of mineralized Proterozoic rock occurs as a megaclast (of 1 million tonnes) in the limestone sequence (Broadbent et al., 1998). Proterozoic rocks in the vicinity of Century are folded into an open synform with a north-south trending axis, the Page Creek Syncline, but elsewhere in the region folding can be tight, as is apparent from elongate map patterns north of Century (Fig. 1). North-south trending folds are correlated with the major Isan orogenic (D2) event of the Mount Isa region (Broadbent et al., 1998). The most obvious fault in the area is the NNW-SSE trending Termite Range fault, which extends over tens of kilometers. It is thought to be long-lived and to have controlled sedimentation of the McNamara Group and basin evolution. NE-SW faults are also prominent and locally host structurally controlled deposits such as Silver King. Major faults within the pit area are the north-dipping Magazine Hill and Pandora's faults. Displacement across the former is of the order of 300m and varies from almost 0 to 200m across Pandora's fault. The NE-SW faults are correlated with D3 faults of the Mount Isa region (Broadbent et al., 1998) which are implicated in copper ore formation in this area.

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Figure 3: Histogram of K-Ar model ages, Ar-Ar plateau ages and Rb-Sr isocron ages of rocks from the Lawn Hill region. (Sources detailed in text).


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Figure 4: TEM image of mica in carbonaceous material in a sample of the Lawn Hill Formation distal to Century. K-Ar ages of these

micas are in the range 1175±50Ma (Uysal et al., 2004). Compare this with Fig. 7 below.

Two samples of tuffaceous layers within the Lawn Hill formation have been dated by the SHRIMP zircon technique at 1616±5 and 1611±4 Ma (Page et al. 2000). A tuff sample from LH539, 5km east of Century yields a significant number of younger ages in the range 1400 to 1000 Ma. The significance of these ages are unclear to Page et al. (2000) but this age range is also recorded in K-Ar, Ar-Ar and Rb-Sr systems (Fig. 3). Zircons from feldspathic sandstone of the Constance Sandstone yielded an age of 1591±10, apparently contradicting evidence of a major unconformity (Jackson et al., 1999). This paradox has been reconciled by interpreting the ages to reflect redeposition of zircons sourced in the Lawn Hill Formation (Jackson et al., 1999). A better estimate of the depositional age of the South Nicholson Group is based on zircons from units higher up the sequence which yielded ages of 1492±4 and 1493±4Ma (Jackson et al., 1999). K-Ar and Rb-Sr ages reflect one or more post-McNamara thermal events (Fig. 3; Kralik, 1982; MacDougall et al., 1965; Uysal et al., 2004). There are distinct maxima of model K-Ar ages at 1300 – 1350 and 1150 – 1200 Ma. Two Ar-Ar determinations on mica from the Flat Tyre deposit yielded a plateau age of 1562±34 Ma and distinct plateau ages of 1310±6 and 1355±6 Ma in the other (P. Polito, unpub. data). Six K-Ar determinations on fine-grained mica intimately associated with carbonaceous material in rocks of the Lawn Hill Formation distal to ore yielded ages of 1175±50 Ma (Uysal et al., 2004). The fine grain size of the analysed micas (<1 µm), however, raises concerns about possible radiogenic argon loss and artificially young ages. Nevertheless, there is additional evidence of a relatively late hydrocarbon migration event in hydrocarbon-bearing quartz-calcite veins that cut a mafic dyke which intrudes rocks of the Roper superbasin (Volk et al, 2005). The age of the mafic dyking event is poorly defined at between 1220 and 1280 Ma by the K-Ar method on plagioclase and 1150 Ma on pyroxene (errors not given; McDougall et al., 1965). There is textural and other evidence for multiple lead and zinc depositional events (e.g. veins in Cambrian rocks) and the possibility that Zn- and C-rich ore is of different age to the lead-rich transgressive veins (see below). In which case Pb isotopic data could be dating the veining event but not the primary ore. This uncertainty will be resolved by: • Dating the age of Lawn Hill sedimentation using the Re-Os technique on carbon assumed to be syn-

sedimentary (Monash University, Bruce Scheaffer) • Dating the carbon-rich ore also using Re-Os technique on carbon assumed to be hydrothermally

modified (Monash University, Bruce Scheaffer) • Dating the late transgressive ore using Rb-Sr (Melbourne University, Roland Maas)


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4 Mineralogy of Primary Ore and Unaltered Rocks There are at least two end-member ore types at Century. One is stratiform, zinc- and carbon-rich the other is transgressive, veinlet-controlled and lead-rich. These ore types may occur together at the hand specimen scale. Small occurrences beyond the Century pit area appear to be mainly of this second type. XRD data for 77 samples of stratiform ore with sphalerite > 1 vol% show that the primary ore is dominated by quartz, sphalerite, siderite, white-mica and pyrite (Fig. 5).

0

10

20

30

40

50

Albite

Calc

ite

Chlorit

e

Dolomite

Gale

na

Kao

linite

Musc

ovite

K fe

ldspar

Pyrite

Quart

z

Rutile

Sphalerit

e

Siderite

Ave

rage

Vol

ume

% M

iner

al

Unaltered Shale (n = 9)

Primary Ore (n = 77)

Figure 5: Average mineral composition of 77 samples of primary ore (defined as sphalerite > 1% by volume)

Quartz occurs as what appear to be relict clastic grains, but which often have irregular and even lobate outlines suggestive of partial dissolution (Fig. 6g,h). Sphalerite occurs as so-called porous sphalerite (Fig. 6e,g) which occurs as laminae parallel to presumed sedimentary layering and in veinlets transgressive to and probably later than the porous sphalerite (Fig. 6d,e). Pyrite in the primary ore samples typically occurs as distinctive angular or rounded grains typically less than 10 microns in diameter (Fig. 6a,b,e). This clearly pyrite predates "porous" sphalerite, which envelopes and encloses it. There is no textural evidence that the sulphur in this early pyrite was used to form the sphalerite (i.e. the abundance, shape and surfaces of the pyrite seem to be the same regardless of whether sphalerite is present). Veinlets of galena postdate framboidal pyrite and porous sphalerite (Fig. 6b,d,e). Fig. 6f,g shows the occurrence of sheaves of probable chlorite. The identification of chlorite is tentative based on Mg and Al in X-ray spectra, but another clay phase could be present. Fig. 6h shows a portion of sample AW04-009 that is more representative of the host-rock. Detrital quartz and mica are apparent and pore-filling of extremely fine-grained material (< 5 microns) possibly phengite. This clearly illustrates one of the problems in studying these rocks: the very fine-grained nature. TEM studies of Century ore have been published by McKnight and Broadbent (1993) and Broadbent. and McKnight (1993). Some of this work is shown as Fig. 7 (courtesy of Stafford McKnight). Some conclusions from this work are that sphalerite is extremely fine-grained (< 1 µm) strongly suggestive that the ore has not been recrystalised (e.g. during metamorphism). Furthermore, “porous” is not an appropriate description, the ore consisting of myriad crystallites of ZnS “floating” in bitumen. Not all organic matter displays the same level of maturation and some appears to have a higher rank than would be expected from other measures of geothermometry (e.g illite crystallinity). As with many orebody studies the mineralogy of unmineralized host-rocks is less well established than that of the ore. In this case, few samples of shale equivalent to mineralized shale have quantitative XRD mineralogy. The average volumetric mineral compositions of unaltered shale are shown in Fig. 5. This


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figure suggests that the differences between ore and host-rocks are that the ore rocks have greatly reduced amounts of quartz and muscovite, due to addition of sphalerite, ferroan carbonate and pyrite. Dolomite is present in the unaltered rocks but virtually absent in the mineralised rocks consistent with the EMP evidence cited below. Chlorite is also present in unaltered rocks but virtually absent in the mineralised rocks, although is present in the oxidised hematitic rocks above the ore. This leads to the important conclusion that the ore and unaltered host-rocks are not mineralogically very different. The main distinction appears to be the presence of siderite and pyrite in ore.


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Figure 6: Back-scattered electron images of primary ore samples. A - Angular ("framboidal") early pyrites crosscut by sphalerite veinlet. B Transgressive galena veinlet clearly post-dating pyrite. C Ferroan carbonate grains with interstitial sulphide. D – Early

layer-parallel porous sphalerite and cross-cutting transgressive galena veinlet. E - Early layer-parallel porous sphalerite interstitial to fine-grained pyrite and cross-cutting transgressive galena veinlet. F – Chlorite gangue (note extremely fine grain size < 10 microns).

G Porous sphalerite in contact with quartz and chlorite sheaves. G Part of Zn-poor layer, showing clastic quartz and muscovite flakes with interstitial phengite. Coloured images were enhanced by applying a colour map in ImageJ


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Figure 7: TEM Images of Century ore from studies of McKnight and Broadbent (1993) and Broadbent and McKnight (1993). A, B

Typical “non-porous” Century sphalerite. “Cryptocrystalline or “ultra-fine polycrystalline” are better descriptors.C, D - Myriads of crystallites of ZnS “floating” in bitumen (paler material). E, F - Highly bituminous “porous” sphalerite x30,000. Paler material is

bitumen. G, H - amorphous organic matter (OM) with sphalerite crystallites. Images courtesy of Stafford McKnight.


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5 Mineralogy of Weathered Zone The upper parts of the Century pit contain rocks that contrast markedly with the primary ore rocks just described. These rocks are pale greenish gray to blood red, depending on the mass of hematite. The interface between such hematitic rocks and dark-coloured primary ore generally defines the upper limit of economic zinc (Fig. 8). Several samples of the hematitic material were examined to determine whether these rocks represent a facies of primary hydrothermal alteration, or whether they represent a post-ore event such as surface-related oxidation (weathering).

Figure 8: Interface between primary ore (dark gray) and hematitic zone – red dotted line. Samples are yellow crosses (prefixed AW04-). Rod Anderson provides a scale. Note the enormous drop in grade below and above the line.

Texturally, the hematitic rocks generally appear similar to the primary ore, in that layering is well preserved, with siltstones being preferentially hematised. Calcite-cemented breccias and calcite veinlets are quite common in this zone. Calcite can be coarse and euhedral possibly indicative of open space filling. NITON portable XRF analysis (Wilde, 2004) reveals that hematitic rocks are more Zn-rich than adjacent hematite-poor rocks. From XRD data it is clear that calcite, kaolinite, hematite and chlorite are more abundant in the rocks of the hematitic zone while sphalerite and K-feldspar were not detected. No evidence of replacement of sphalerite-rich layers is apparent in the hand specimens or in the open pit. SEM images of the textures present in the hematitic rocks are shown in Fig. 10. Iron oxide is often spatially associated with fine-grained chlorite (consistent with the XRD evidence of increased chlorite concentration relative to primary ore). Oxide (hematite) is often best developed at the contact between host-rock fragments and calcite-rich veins (as in Fig. 10c). In one instance oxide was seen to be replacing sphalerite in a sphalerite-quartz vein (Fig. 10d) but this appears to be rare. The irregular contact between calcite and oxide/chlorite and the fact that the latter is enclosed in the former suggests that calcite postdates chlorite and Fe oxide. Hematitic alteration could be interpreted as a primary feature. In this case Zn would have been deposited in reduced (carbon-rich) rocks close to the interface with oxidized, hematitic, rocks. This interface would have been discordant to layering, and could explain why economic grades occur only at and beneath this interface. Hematite visually similar to that in the Proterozoic rocks is, however, present in Cambrian rocks, suggesting that hematite formation post-dates deposition of the unconformable Cambrian cover. Alternatively detrital hematite could have been incorporated into these sediments, but the textures do not support this interpretation. I found no evidence that discordant post-ore hematitic alteration has removed primary zinc, and this could be used as an argument against a secondary origin. The base of this alteration may however simply reflect a primary feature of the orebody. The presence of abundant sulphide and carbon in the primary ore would have rapidly reduced any oxidized surface-derived fluid.


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Figure 9: Transmitted light photomicrograph of Cambrian hematitic sandstone, showing what appears to be interstitial hematite.

AW05-034. Field of view approximately 10mm.


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Figure 10: SEM images of hematite-rich rocks. A AW04-010. Note no obvious texture indicating sphalerite replacement. (Fine bright white grains are sphalerite). B-D AW04-012. Textures such as B suggest that calcite and chlorite may not be in equilibrium. Calcite apparently postdates chlorite and Fe oxide. E - AW04-014. Shows quartz, iron oxide (hematite?) and sphalerite vein. F. AW04-016

from the Termite Range fault showing chlorite enclosing and ? replacing quartz.


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6 Bulk Chemical Characteristics

6.1 Lady Loretta and McArthur River Bulk chemical and aspects of mineralogical variation around the Lady Loretta deposit were documented by Carr (1984) based on 1,218 samples. Carr (1984) found that the proportion of pyrite increases from about 2% in host-rocks 150m below the deposit to over 90% in the immediate footwall to the deposit. There is also significant pyrite enrichment in the hanging wall (to 20%) for up to 100m above the deposit. Clearly, ore is spatially coincident with unusually pyritic rocks. Ferroan carbonate is abundant in the ore and host-rocks in an "aureole that extends for 75m beneath to 50m above the ore" (Figs. 11,12). Ferroan carbonate contains up to 13 mol% ZnCO3 and 32% MgCO3 and should therefore more correctly be referred to as sideroplesite (molar MgCO3 5 - 30%) or pistomesite (molar MgCO3 > 30%). Carr (1984) observed that there is an antipathetic relationship between the abundance of Zn and Mg in carbonate minerals, Mg increasing with distance from ore. Bulk-rock Zn, Hg, Pb, Ag and Ba show "extensive primary dispersion within the host-rocks". Within the plane of sedimentation, haloes vary "in width" from 50m to 1.5km and these dispersions are thought to be "dependent on the shape of the sedimentary basin floor at the time of sedimentation". Zn and Hg show the most extensive primary dispersion with anomalous values extending up to 100m into the footwall and at least 50m into the hanging wall. Large and McGoldrick (1998) collected 108 new samples and developed several geochemical "indices". They found that bulk chemical haloes extend for several hundred metres across strike and up to 1.5 km along strike (corroborating Carr's earlier conclusions). Details of these haloes, such as 3D extent and threshold values, however, were not provided. An inner "sideritic" envelope to primary ore is surrounded by a zone of ankerite and ferroan dolomite that grades outwards to low Fe dolomite. Fe-rich carbonate in the inner envelope includes both siderite and PISTOMESITE (Fe0.6Mg0.4CO3). The zincian nature of these iron carbonates first documented by Carr (1984) was confirmed. Pb, Cu, Ba and Sr show dispersion of <50m across strike, Zn and Fe moderate dispersion (< 100m) and Mn and Tl show greatest dispersion at < 200m. Cu, Mg and Na are depleted in the sideritic halo relative to surrounding sediments, while there is little systematic change in K. Two bulk chemical vectors were developed (applicable to dolomitic rocks): SEDEX metal index: Zn + 100Pb + 100Tl and SEDEX alteration index: (FeO-10MnO)100/(FeO + 10MnO + MgO). A third vector recognised systematic changes in the Mn content of carbonate with proximity to ore: MnOd = (MnO*34.41/CaO) These data suggest that deposition of Pb and Zn was accompanied by major bulk addition of iron and manganese to the host-rocks or that ore is spatially coincident with unusually Fe- and Mn-rich host-rocks. The timing of Fe and Mn enrichment relative to Zn and Pb introduction is not well defined. Lambert and Scott (1973) collected 160 samples from around the McArthur River deposit and their data show that Fe enrichment is also a feature of the hanging wall sediments here. Iron commonly exceeds 10%, probably reflecting large masses of pyrite. Conversely, distal to the orebodies samples containing over 10% Fe are rare. Mineralized host rocks contain dolomite with moderate to high Fe and Mn content. Siderite, however, has not been reported. The apparent absence of siderite or pistomesite as key host-rock phases is apparently a major difference between McArthur River and Lady Loretta. Mn in dolomite probably accounts for bulk enrichment of Mn in the ore zone and immediate footwall to over 0.5% Mn. Considerable enrichment of carbon in ore zone shales (to 13%) was noted. There is excellent correlation between C and Fe and between C and S. Mineralized rocks are generally poorer in K than unmineralized rocks (values in excess of 5% are common in the distal holes sampled by Lambert and Scott, 1973). This conclusion can also be drawn from the data of Large et al., 2000. Host rocks contain "significant Pb, As and Hg anomalies". Up to 0.7% Zn and 800 ppm Pb were noted, however, in drillhole Myrtle 1 drilled about 20 km from McArthur River. Clearly, elevated Pb and Zn are not confined to adjacent to orebodies. High As was attributed to solid solution in pyrite. Hg


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in the ore is in the range 0.7 to 1.7 ppm compared to 0.05 to 0.15 ppm "background". Tl is "concentrated preferentially in the ore deposit" where values of up to 100 ppm "are not uncommon". Dispersion was not defined, although "low pyrite shales" were found to contain < 10 ppm (INAA analysis). Au, B, Bi, Ba, Cr, Ga, La, Li, Mo, P, Rb, Sc, Sr, Th, Ti, U, V, Y and Zr were also analysed for but "showed no significant association with sulphide mineralization". The absence of anomalies in Ba and Sr contrasts with Lady Loretta. Additional data were collected by Large et al. (2000). They sampled two holes remote from the deposit and compared them to six holes previously sampled by Lambert and Scott (1973). Analysis was for major elements and "selected trace elements" (that remained unspecified). They concluded that: "a broad Zn-Pb-Tl halo ... extends laterally along the favourable pyritic black shale facies of the Barney Creek Formation for at least 15km west of the deposit". Manganiferous carbonate forms the most pronounced and laterally extensive halo at HYC and is offset from a ferroan dolomite/ankerite halo. Sedimentary rocks of the Barney Creek Formation have a relatively simple mineralogy of dolomite, ankerite, illite, quartz, K-feldspar, pyrite, chlorite and organic matter. Calcite occurs rarely and siderite has not been identified at HYC or regionally within the McArthur Basin. Strongly pyritic shales extend up to 200m above the ore zone, but there is a sharp drop to <1% below the deposit. Downhole K2O is controlled by illite and K-feldspar. Illite is dominant around the ore zone. K-feldspar is dominant in holes remote from HYC (Barney 3 and BMR 2). I would argue that sampling two holes remote from an orebody does not permit the definition of a halo which is by definition present on all sides of the orebody. Furthermore, it is not proven that background values have been attained, so that the enrichments may be regional. Furthermore, Mn in carbonate is to a large degree a reflection of bulk rock Mn and analysis of carbonates for exploration is probably not warranted. Resolving this uncertainty should be an objective of the I7 project in FY06-07.

Figure 11: Isopachs of the Lady Loretta ore horizon and Lower Siderite Unit (from Carr, 1984)


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Figure 12: Distribution of pyrite in two holes from Lady Loretta 500m apart (Carr, 1984).

6.2 Century In assessing the differences between mineralized rocks of the ore sequence and other rocks of the McNamara Group I have compiled a total of 779 analyses of unmineralised rocks from various sources (documented in the ACCESS database). Partial analyses of 13,779 grade control samples are available in the PCM database (Tables 1, 2), and various studies notably Johnson (2000) provide additional data on ore samples.

Table 1: Statistics for samples from Century in the PCM database containing >1% Zn. Method or methods of analysis are not known.

Fe Mn Pb S TOC TOEC Zn SiO2 CaO K2O MgO P2O5Number of values 11605 11184 13775 9072 671 1589 13779 1228 116 116 116 45Minimum 0.4 0.0 -0.2 -0.1 0.0 0.0 1.0 4.3 0.2 0.7 0.1 0.2Maximum 42.7 7.8 50.0 25.7 12.4 12.0 53.0 85.0 0.8 2.6 1.1 0.5Mean 7.9 0.9 1.4 7.3 3.2 3.2 9.0 50.6 0.5 1.5 0.5 0.4Median 7.4 0.8 0.4 7.2 3.3 3.3 5.6 50.1 0.5 1.4 0.4 0.4Third quartile 10.5 1.4 1.4 11.3 4.5 4.3 14.8 59.9 0.6 1.6 0.7 0.4Standard deviation 4.6 0.8 2.8 4.9 1.8 1.6 8.0 12.8 0.1 0.4 0.2 0.1


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Table 2: Average composition of units from holes PCM350, 354 and 356 using bulk XRF (Source: Zinifex).

Iron and manganese enrichment is a distinctive feature of the ore zone at Century (Tables 1, 2; Fig. 13). The average Fe and Mn concentrations of samples with > 1.0% Zn are 7.9% and 0.9% respectively. It is rare for any distal rocks of the McNamara Group to have iron concentrations in excess of 6.0% and Mn in excess of 0.5%. The siltstone layers of the mine sequence are significantly more Fe- and Mn-rich than the shales averaging in excess of 9.0% Fe and 1.0% Mn compared to values in the shales of between 3.0 and 7.0% Fe and 0.3 to 0.7% Mn (Table 2). Distal shales of the Lawn Hill formation have corresponding average values of 3.7% Fe and 0.12% Mn, while siltstones have averages of 5.1% and 0.45%. Iron enrichment at Century reflects abundance of iron-bearing carbonate minerals and pyrite (see below). The average MgO and CaO contents of 116 ore zone samples are 0.6 and 0.5% respectively. Average MgO and CaO of 129 samples of distal shales of the Lawn Hill formation are 2.4% and 1.4% respectively, while 49 samples of siltstones average 2.3% and 2.3%. These bulk chemical differences reflect the fact that dolomite is present distal to the ore zone but is rare within it. This can be attributed to dissolution and/or replacement of dolomite by siderite in the ore zone, alternatively primary deposition of siderite rather than dolomite. Note that siderite is not abundant at McArthur River, and so the possibility that siderite is not directly related to ore formation could be entertained (see also chemical modelling presented below) The mean K2O content of nearly 116 zinc-rich (Zn > 1%) grade control samples is 1.5% (median 1.4%), significantly less than average values for unmineralized rocks of the Lawn Hill Formation (Fig. 14). The mine sequence is depleted in K2O relative to the footwall rocks, which average over 4% (Table 2; Fig. 15). The dominant and only K-bearing mineral in the ore zone is white mica, typically extremely fine-grained (< 10 microns). Mica and K-Feldspar are abundant in distal unmineralized rocks, suggesting K depletion represents loss of mica and K-Feldspar.

Unit Rocktype Count Pb% Zn% Fe% S% SiO2% Mn% Ctot% Ag Comments

155 Siltstone 2 0.01 0.25 9.9 0.1 69.6 0.10 0.4 2160 Shale 1 0.71 0.04 10.4 9.5 56.5 0.01 4.6 29165 Siltstone 2 0.01 0.20 9.4 0.1 69.7 0.12 0.8 2170 Shale 1 3.69 0.11 19.0 20.7 38.7 0.01 4.9 75175 Siltstone 1 0.89 0.05 6.7 4.4 74.5 0.02 1.8 14

180 Shale 1 0.01 0.01 1.3 0.0 77.2 0.04 0.3 3 The high Si suggests that this may not be a shale.

185 Siltstone 1 0.01 0.12 8.1 0.2 71.0 0.08 0.9 3190 Shale 1 1.73 0.31 14.6 12.8 44.7 0.53 5.7 33195 Siltstone 1 1.29 1.45 10.2 0.9 58.4 1.80 3.0 5

200 Shale 5 2.33 11.17 3.6 8.4 56.1 0.16 4.8 136

311 Siltstone & shale 1 0.28 21.18 10.3 12.3 30.8 1.96 3.6 59312 Siltstone & shale 1 0.52 14.19 9.1 7.5 43.4 1.67 3.4 27320 Siltstone 5 0.51 3.13 17.1 1.7 43.0 1.88 4.1 3

410 Shale 7 1.36 15.82 5.1 9.8 43.2 0.35 4.5 26

420 Massive Dolomite 5 0.23 0.57 8.7 0.5 56.2 1.52 4.2 2 Note high Si content. Probably not massive dolomite

430 Shale 3 0.36 13.37 8.1 9.9 46.2 0.76 4.1 36440 Shale 1 0.01 1.65 12.9 0.8 48.8 2.44 4.1 0450 Shale 7 0.05 3.91 8.6 7.4 51.1 0.20 3.4 29460 Shale 6 0.01 5.15 9.4 7.6 48.1 0.82 3.6 9

UFHM Massive dolomite 1 0.01 0.01 3.0 0.7 67.9 0.06 1.6 4 Note high Si content. Probably not massive dolomite

UFW Siltstone & shale 6 0.01 0.44 5.2 1.5 64.2 0.42 2.6 0UFW10 Siltstone & shale 2 0.01 0.33 8.1 2.0 55.8 0.92 3.2 3UFW100 Siltstone & shale 6 0.01 0.43 4.3 1.6 66.5 0.22 2.3 2UFW20 Siltstone & shale 2 0.01 0.11 2.5 1.0 68.1 0.09 2.6 0


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Figure 13: 3D perspective of pit area (major faults in gray) and sampled "intermediate" (LH) drillholes around Century. Yellow

shows area with Fe > 10% and red > 12.5% (generated using Leapfrog). Looking NE. Field of view is approximately 10 km. Termite Range fault to right, Magazine Hill to left.

Figure 14 shows the variation of Fe and K with rocktype1. The diagram is useful for discriminating between “shales” (high K), siltstones and sandstones (moderate K) and dolomite (low K). Several unmineralised rock units stand out as having unusually high Fe, these are 16 samples of Riversleigh Siltstone and 13 samples of the Torpedo Creek Quarztite. The latter derive from GSQ LH-3 where there are signs of hydrothermal silicification and chloritisation as well as elevated Cu (to 415 ppm). The high Fe is therefore attributed to alteration at the unconformity between the basal McNamara Group and underlying Yeldham Granite. The high-iron rocks of the Riversleigh Siltstone have been confirmed by NITON analysis (Wilde, 2005b) and come from an interval in petroleum well AMOCO 83-1 that may represent a favourable stratigraphic horizon.

Figure 14: Comparison of Fe% and K2O% in mineralized and unmineralized rocks. The diagram at left presents the average values for unmineralized rocktypes and units, whereas the diagram at right presents data points for mineralized rocks (note that very few analyses of ore samples have major element determinations for both K and Fe). Note the anomalous composition of the Torpedo

Creek Quartzite in hole GSQ-LH3.

High Ba appears to be a feature of the Lady Loretta Formation, while high Mn is apparent in the Lawn Hill Formation. High P appears to be another feature of the Lawn Hill Formation, but high P is also seen in the Torpedo Creek Quartzite dolomite and Gunpowder Creek sandstone. The origin of the P enrichment remains uncertain. It could be a primary depositional feature or

1 The term shale is used here to describe all pelitic rocks although shale is probably inappropriate for these rocks that have undergone lower greenschist-facies metamorphism and are somewhat more massive than a typical shale.


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hydrothermal. Alteration associated with unconformity-type uranium, for example, involves deposition of various phosphate minerals.

Figure 15: Graphic log of drill hole LH494 showing % variation in SiO2, Fe, Mn, S and Zn and ppm variation of Ag. 208 analyses

from Johnson (2000).

The bulk chemical characteristics of the ore zone relative to all other units of the McNamara Group can be summarized thus: • Enrichment in Fe and Mn (as at Lady Loretta and McArthur River) due to abundant pyrite (in

shale layers) and siderite (in siltstones). • Depletion in SiO2. Zn shows a good inverse correlation with SiO2, with rocks containing 20%

Zn invariably having SiO2 lower than 45%.


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• Depletion in K2O. The median K2O content of high grade ore is 1.5%. K depletion is probably due to white-mica and K-spar dissolution, since both are more abundant in unmineralised rocks.

• Elevated TOC. Correlation between C and Fe as documented at McArthur River is apparent in holes from Century

• CaO and MgO in the mineralized samples are significantly lower, reflecting the paucity of dolomite relative to unmineralized rocks.

Bulk chemistry is evidently a tool that will allow the identification of rocks that have similar characteristics to the host-rocks at Century, McArthur River and Lady Loretta. What is not well established is whether the elemental variations allow the delineation of a “halo”, whether the distribution is more irregular (e.g. fault controlled) or whether such rocks are regionally extensive.

Table 3: Average chemical compositions of unmineralized rocks from the Century area

6.3 The impact of near surface oxidation (weathering)

One hundred bulk chemical analyses of outcropping rocks from around the Century mine are stored in the ACCESS database (Wilde, 2004). These were compared with sub-surface samples in order to assess bulk chemical changes due to weathering. The surface samples appear to be biased towards sandstones and siltstones presumably because these rocks outcrop rather better than shales.

Formation Lithology # SiO2% TiO2% Al2O3% Fe% Mn% MgO% CaO% Na2O% K2O% P2O5% As Ba Cu Ni Pb Zn

Lawn Hill Formation Cappucino Rock 8 66.5 0.3 8.7 4.3 0.65 2.3 2.9 0.22 2.67 0.44 9 273 38 16 39 52

Dolomite 1 27.2 0.2 3.3 1.8 0.10 6.2 28.7 0.50 0.93 0.03 3 121 3 4 5 48Interlayered Clastics 64 68.2 0.4 10.5 4.0 0.26 2.3 2.5 0.39 3.57 0.46 8 382 31 20 8 181

Sandstone 37 80.8 0.3 8.1 2.2 0.11 0.8 0.5 0.23 2.73 0.22 6 365 17 13 24 129Shale 129 62.4 0.5 13.1 3.7 0.12 2.4 1.4 0.37 3.93 0.18 21 350 30 32 38 116Siltstone 49 66.0 0.3 8.9 5.1 0.45 2.3 2.3 0.18 2.85 0.36 12 270 31 18 35 98Tuff 25 72.7 0.3 12.3 2.1 0.18 1.2 0.5 0.07 4.41 0.07 4 477 8 8 16 62

Riversleigh Siltstone Breccia 1 62.3 0.5 12.7 5.7 0.02 1.4 0.2 0.86 4.56 0.11 55 536 28 36 37 46

Pyritic shale 4 58.0 0.5 12.7 7.5 0.02 1.2 0.2 0.11 4.44 0.13 87 700 27 26 63 54Sandstone 8 73.6 0.3 10.5 2.4 0.05 1.9 1.0 2.01 3.17 0.08 9 587 18 12 18 84Shale 11 68.4 0.4 14.1 7.4 0.02 1.7 0.3 0.95 2.55 0.10 66 342 23 20 43 58Siltstone 12 68.5 0.4 12.4 2.2 0.08 2.1 2.1 1.22 4.06 0.09 17 629 14 12 15 46

Termite Range Formation Dolomite 8 5.0 0.2 1.0 0.25 18.2 29.4 0.14 0.14 0.14 4 15 0 3 6 18

Sandstone 46 81.3 0.2 6.0 1.1 0.08 1.4 2.0 0.07 2.29 0.05 8 258 27 7 20 20Siltstone 3 1.7 0.06 1.3 3.83 5 449 22 7 10 22

Lady Loretta Formation Dolomite 11 21.6 0.1 3.8 1.0 0.07 15.3 19.6 0.14 1.10 0.03 7 1143 14 7 24 18

Interlayered Clastics 7 59.2 0.4 13.9 3.4 0.09 3.0 3.2 0.13 6.12 0.10 17 839 11 9 35 134

Sandstone 1 82.3 0.1 10.3 0.4 0.08 0.3 0.0 0.01 1.25 0.05 18 572 5 6 37 240Shale 22 59.3 0.4 14.7 2.5 0.09 3.0 3.5 0.14 5.71 0.13 27 766 9 13 61 116Siltstone 54 50.8 0.3 9.7 3.4 0.14 5.4 8.4 0.13 4.31 0.10 29 1464 10 9 24 63

Paradise Creek Formation Dolomite 75 21.2 0.1 1.7 0.9 0.06 16.3 22.8 0.13 1.24 0.02 2 379 12 5 13 10

Gunpowder Creek Formation Dolomite 116 22.6 0.1 2.0 1.6 0.09 15.9 21.1 0.05 1.28 0.06 7 358 20 4 12 21

Interlayered Clastics 24 50.0 0.3 7.7 2.8 0.09 6.6 9.2 0.06 5.02 0.19 24 219 34 11 16 9

Quartzite 5 311 44 10 20 12Sandstone 7 73.0 0.1 3.4 1.6 0.06 3.3 5.8 0.03 2.45 0.72 22 280 173 21 22Shale 9 62.1 0.3 8.1 1.7 0.04 4.0 5.8 0.06 5.86 0.18 24 557 101 24 15Siltstone 6 43.7 0.2 5.4 1.8 0.09 9.3 13.7 0.08 3.46 0.14 17 429 39 31 40

Torpedo Creek Quartzite Dolomite 1 56.3 0.2 3.0 6.4 0.19 7.1 5.0 0.02 1.53 0.74 30 163 20 26 23

Quartzite 7 42 18 25Sandstone 12 19.6 0.25 6.5 1.30 28 85 49 10 14 12Shale 1 2.4 0.01 0.4 7.27 19 351 19 23 15 144Siltstone 4 3.3 0.05 8.8 1.92 26 143 415 17 12 25


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Table 4 demonstrates dramatic differences between unweathered and weathered samples. The latter are much enriched in SiO2 and depleted in Al2O3, MnO, MgO, CaO, Na2O and K2O. In some cases there also appears to be minor loss of Fe. These chemical trends are is consistent with secondary silicification, and loss of carbonate minerals, perhaps related to development of silcrete in the region. Loss in K2O reflects conversion of muscovite to kaolinite for white there is infra-red evidence (see below). An important conclusion, therefore, is that the bulk composition of surface samples may not be a reliable guide to the degree of primary hydrothermal alteration, since key minerals have been removed.

Rocktype Type Count SiO2 TiO2 Al2O3 FeO Fe2O3 MnO MgO CaO Na2O K2O P2O5 TOC Ag As Ba Co Cu Ni Pb U Zn

LAWN HILL FORMATIONInterlayered silt & shale Unweathered 77 68.7 0.4 10.3 3.2 0.04 3.0 2.9 0.9 3.0 0.4 0.9 11 489 13 24 22 18 3 93

Weathered 1 78.4 0.4 8.9 2.5 0.01 0.8 0.4 0.0 2.1 0.5 0.0 2 349 12 26 29 102 3 215Mineralized Shale Unweathered 10 64.3 0.5 12.1 3.5 2.0 0.40 2.3 1.7 0.4 3.8 0.3 3.9 18 371 258 42 33 182 8 285

Weathered 1 77.9 0.3 8.7 5.4 0.01 1.0 0.3 0.0 1.7 0.5 0.1 19 373 22 34 52 63 2 433Sandstone Unweathered 54 77.8 0.3 8.5 3.3 0.8 0.28 0.9 0.3 0.1 3.3 0.1 -0.3 11 331 31 4 7 9 3 48

Weathered 6 87.6 0.2 5.0 2.4 0.08 0.2 0.1 0.2 1.6 0.1 0.0 -0.5 4 502 46 14 6 110 -1 61Siltstone Unweathered 31 69.4 0.3 7.7 1.4 5.2 0.30 2.8 3.2 0.3 2.3 0.3 0.4 -0.1 13 326 13 27 21 11 2 56

Weathered 11 76.6 0.4 7.9 6.7 0.04 1.0 0.2 0.0 1.7 0.3 0.1 -0.5 8 299 13 29 16 82 -2 58Tuff Unweathered 47 74.4 0.2 11.5 2.0 0.4 0.21 1.0 0.6 0.1 5.0 0.0 -0.5 3 686 6 2 3 14 6 40

Weathered 32 78.4 0.2 8.5 0.8 1.1 0.12 1.0 1.2 1.8 3.1 0.0 -0.5 2 811 24 8 3 12 2 21

RIVERSLEIGH SILTSTONESandstone Unweathered 8 68.1 0.3 12.5 4.5 0.07 2.7 1.2 1.8 3.8 0.1 -0.5 11 570 26 16 14 13 2 85

Weathered 42 89.1 0.1 5.3 1.2 0.02 0.1 0.0 0.2 2.4 0.0 -0.5 8 648 43 13 5 18 1 39

TERMITE RANGE FORMATIONSandstone Unweathered 81 69.1 0.2 5.1 0.8 0.6 0.13 4.1 6.2 0.1 2.0 0.1 -0.4 7 226 45 16 6 14 2 17

Weathered 60 88.9 0.2 5.8 0.9 0.01 0.2 0.0 0.3 1.4 0.0 -0.5 5 775 63 15 3 6 -2 15

Table 4: Comparison between bulk chemical composition of material from drillcore (unweathered) and surface samples (weathered)


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6.4 Soil iron anomalies Given the significance of iron enrichment in the ore zone, soil data were examined to determine whether evidence existed of iron-rich rocks beyond the pit area. An arbritary threshold of 5% was applied to the data and yielded several coherent anomalies (depicted in Fig. 16; as well as many isolated points not shown). Many of the anomalies appear to be oriented NNW-SSE, but this needs to be confirmed by more detailed sampling. Given the strong evidence of iron enrichment accompanying Zn emplacement these anomalies could indicate the intersection with the surface of a mineralized system or systems.

Figure 16: Iron in soil in excess of 5% (data supplied by Zinifex). The anomalous points have been interpreted to define NNW-SSE trends and fall in four main areas (isolated high soil values have been excluded). Note that the Fe anomaly also defines a circular

pattern, possibly indicating the presence of an intrusion at depth?


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7 Carbonate Composition The PhD theses of Broadbent and Andrews provide 2,535 electron microprobe analyses of carbonate minerals from the Lawn Hill Formation. Figure 17 shows the compositional range of carbonate minerals from drillholes in the pit area (proximal) and kilometers from the pit (distal). There is clearly more dolomite, ferroan dolomite, calcite and ferroan calcite in the distal holes, whereas carbonate minerals at Century are mainly Fe-Mg solid solutions ranging from siderite to sideroplesite to pistomesite, with minor ankerite.

Figure 17: Fe v Mg composition of carbonate minerals from Century (mole%). Note that most should be described as sideroplesite or pistomesite rather than siderite. “Proximal” are samples from drillholes in the pit area, “distal” are from drillholes remote from the

pit area.

Another feature of the Fe-Mg carbonate minerals at Century is high Zn content, ranging up to 14 mole% with an average of 1.48%. The average Zn content of distal carbonates is 0.05% with a maximum of 0.58%. The most zinc-rich carbonates are the poorest in magnesium (Fig. 18).

0

2

4

6

8

10

12

14

16

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Molar Fe/(Fe + Mg)

Mol

ar Z

n%

Figure 18: Mole% ZnCO3 in carbonates from the Century pit.


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Figure 19: Pie charts show the volumetric proportion of different carbonate types as determined by EMP analyses. These data

show the widespread occurrence of pistomesite and sideroplesite, but that zincian siderite is restricted to the pit area. Siderite also makes up a higher proportion of carbonate in the pit area. Note virtual absence of dolomite.

Figure 19 demonstrates that there is no evidence of a siderite halo as proposed by previous studies. The Fe-Mg carbonates pistyomesite and sideroplesite are widespread in rocks distal to Century and form a high proportion of analysed carbonate minerals. Zincian siderite, by contrast, is restricted to the pit area and also constitutes a higher proportion of analysed carbonate here. Thus the presence of zincian siderite indicates close proximity to ore.


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This raises the possibility that partial extraction and analysis of bulk rock samples for Fe, Mg, Mn, Ca and Zn might prove to be a valuable exploration tool, potentially providing better peak to background than conventional bulk rock analysis. The key uncertainty is the extent of such anomalism.


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8 Stable Isotopes Stable isotopic data for carbonate mineral have been used previously to constrain ore depositional models (Andrews, 1998; Broadbent et al. 1998; Polito et al., 2004) and variation in carbonate isotopic composition has been used to define the extent of hydrothermal alteration at McArthur River (Large et al, 2001). The analysed carbonates at Century show an extraordinary range in major element composition within individual thin sections. This suggests that the isotopic composition determined by bulk analysis will result only in a meaningless average of multiple carbonate compositions. This notion is supported by the data in figure 20 which shows a systematic variation in δ18O and δ34C relative to compositional range, i.e. samples containing more magnesian carbonates have a lower δ18O and more positive δ34C.

Figure 20: Range of carbonate composition (mol%) determined by EMP in five samples collected by Broadbent (1999; C007, 042,

073, 151 and 206). δ34C and δ18O are shown in bold red, with sample number and number of EMP analyses in black.

Figure 21: Stable isotopic data for carbonates from Century (Broadbent, 1999; Andrews, 1998).

Figure 21 shows the Century data relative to data from McArthur River and Mount Isa (and carbonatites). The Century data show a greater spread which is attributed here to the problems of compositional variability and bulk analysis.


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A relatively small proportion of the data from Century are from outside the pit area (Fig. 22). The analyses from the ore zone appear to a have a much greater spread in both δ18O and δ34C, but this may simply be a function of the lack of samples beyond the ore zone.

Figure 22: Oxygen isotopic composition of carbonate minerals from the Century pit and surrounding drillholes.


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9 Lead Isotopes Lead isotope data for the region around Century were provided by Graeme Carr. The sample locations and minimum 206/204Pb ratios are shown in Fig. 23. Values below 16.4 are found within the illite crystallinity/mica AlOH anomalies and also in the vicinity of Anglo American and Little Banner and Silver Queen deposits. Other locations have higher values.

Figure 23: Minimum 206/204Pb ratios of samples from the Lawn Hill mineral field (Carr, unpub. data). Note that minimum values less

than 16.4 seem to define the area around Century and coincide with IC and mica ALOH anomalies.

There is a suggestion that the isotopic data define an anomaly that includes Century, and therefore may be useful as an exploration guide. Alternatively the data may be reflecting the age of lead emplacement and identifying the cluster of structurally controlled deposits around Silver Queen as being of different timing. This could be resolved by collection of additional isotopic data and attempting to data some of the structurally controlled deposits, for example using Rb-Sr on sphalerite.


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10 Infra-red Response The database of infra-red measurements of rocks from Century contains 1,314 spectra, which have been interpreted and parameterised by Ausspec. The spectra include: • 499 surface samples from project I1 (none from Century, but includes spectra from structurally

controlled deposits at Lilydale and Watson's Lode) • 32 surface samples collected by Graeme Broadbent used in XRD analysis • Four "distal" drillholes: Amoco 83-1, Amoco 83-2, Argyle Creek 1 and GSQ LH3 • Eight "intermediate" drillholes: LH191, 195, 203, 218, 376, 418, 658, 691 (samples were supplied

direct to the CSIRO lab by Rod Anderson, except LH203, which was sampled by Graeme Broadbent). • Three "proximal" drillholes from the stage 4 open pit: PCM350, 354 and 356 Early analyses were conducted with PIMA (portable infra-red mineral analyzer) which provides data in the short-wavelength infra-red (SWIR) region. Later analyses utilized HYCHIPS at CSIRO’s North Ryde laboratories, and this method yields additional data in the visible and near infra-red range (VNIR). The latter is useful for identification of oxide, hydroxide and sulphide minerals.

Figure 24: Location of surface samples showing MicaAlOH parameter. The orange line defines areas in which micaAlOH > 2210

(i.e. mica is dominantly phengitic). Purple areas are anomalous iron in soil (>5%). Red shows area of illite crystallinity > 0.65. Note the possible NNW-SSE trend in MicaALOH values extending over 20Km north of Century. This is also a trend occupied by many of

the smaller structurally-controlled deposits.


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The locations of the 531 surface samples are shown in Fig. 24. There are 783 spectra from drillholes, mostly of unweathered material.

10.1 Minerals detected in IR Spectra White mica is the dominant mineral detected in most unweathered and unmineralized samples of the Lawn Hill and Termite Range Formations and Riversleigh Siltstone, including samples from drillholes adjacent to the pit area but carbonate and chlorite were also detected in a large proportion of these samples (Fig. 25). Dolomites of the Gunpowder Creek Formation and dolomitic siltstones of the Riversleigh Siltstone yield spectra dominated by carbonate (dolomite and ankerite) with subordinate white-mica, as do the Lady Loretta and Paradise Creek Formations. Quartz- and/or carbon-rich rocks, however, yield poor and noisy spectra. The Constance Sandstone samples (Fig. 25) have low absorption probably because of the abundance of quartz. The resulting weak spectra are dominated by kaolinite and chlorite and some contain dickite making this unit spectrally distinctive. Chlorite has been confirmed as an abundant phase in samples of the sandstone from the Constance Range iron deposits by SEM analysis. Thus it is difficult to differentiate stratigraphic units using infra-red mineralogy, but shale is readily distinguished from dolomite and sandstone. This implies that remote-sensed data should be useful for lithological mapping. White mica was also detected in nearly every spectrum of unweathered rock from the Century pit, both shales and siltstones (e.g. Fig. 25b), while indications of carbonate and chlorite were found in approximately half. The mineralized and unmineralized samples, are not spectrally distinctive in terms of minerals that are detected. The relative intensity of the various illite peaks, however differ markedly (compare Fig 25a and b) which translates to massive variation in the micaXT parameter (see below). HYCHIP analysis permits identification in siltstone samples from the ore zone of a broad absorption feature at approximately 1150 nm with two distinct minima (Fig. 25d, and also one spectrum in Fig. 25b). This feature is tentatively identified as due to ferroan carbonate2. Although the analysis of the distal samples does not extend to the VNIR range, it seems unlikely that this feature is present as the edge of the feature would be recorded by PIMA. There are also several absorption features in the range 500 – 700 nm which are due to either pyrite or hematite (in weathered samples). Minerals detected in surface samples but generally not abundant in sub-surface samples include gypsum3, alunite and jarosite. The latter minerals were found in samples from abandoned workings as noted by the I1 team, but are certainly not restricted to the workings. Plotting the distribution of these minerals using hyperspectral imagery may be a useful exercise. Kaolinite and montmorillonite are much more frequently encountered in the surface samples, while carbonate is relatively uncommon. These reflect inferences on weathering from bulk chemistry discussed above.

2 This feature accounts for the low values of Fe slope recorded in the ore zone (see next section) 3 Gypsum was found in samples from intermediate holes and Ausspec cautioned that this might be a secondary product (e.g. due to recent oxidation of the samples post-drilling).


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Figure 25: Selected infra-red spectra. See text for discussion. Note that figures to the left were generated using PIMA which measures only in the short-wavelength region, whereas spectra to the right were generated with HYCHIPS which measures down

into the near visible and near infra-red region.


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10.2 Variation in Spectral Parameters As noted previously, unweathered samples from the ore zone are characterised by significantly higher values of micaXT ("mica crystallinity") and lower Fe slope (Fig. 26; Wilde, 2005) compared to those in distal holes (Amoco 83-1, Amoco 83-2, Argyle Creek 1, GSQ LH3). Only one of the intermediate drillholes (LH658) returned comparable values (from about 280m depth). None of the other unweathered samples intermediate or distal drillholes contained spectra with this signature. MicaXT in surface (weathered) samples shows almost no overlap with the mineralised population (Figs. 26 and 27). Samples from north of Century have uniformly low values of Mica XT at <1.5. Values in excess of 2 occur at Century (in the weathered portions of PCM holes) and around the Termite Range fault, at least as far as 15 km from Century. There is no systematic relationship between MicaXT and the presence of kaolinite, which is recorded in virtually every analysed sample. Five of the six highest values, however, occur in samples in which alunite was also inferred (possibly indicating former presence of sulphide minerals).

Figure 26: Infra-red MicaXT versus Fe Slope parameters from samples from drillcore. Note the extremely good separation between

ore zone and distal samples.

Figure 27: Infra-red MicaXT versus Fe Slope parameters from surface samples. The trend towards higher micaXT is due to

kaolinite admixture.


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The MicaAlOH parameter shows a quasi-systematic change from north to south, with higher values encountered to the south and west of the Termite Range fault (Fig. 28). This is somewhat consistent with an increase in illite crystallinity from north to south (see Wilde, 2005). The MicaAlOH parameter does, however, not appear at this stage to be a direct guide to the presence of mineralization, and could be an indication of hydrothermal alteration, of metamorphic grade or the presence/absence of detrital mica. A more systematic (grid-based) set of samples from the vicinity of Century is required, however, in order to clarify this. The 32 surface samples collected by Graeme Broadbent have both SWIR micaXT and illite crystallinity determinations from conventional XRD (carried out by Stafford McKnight; Fig. 29). There is poor correlation between the two datasets, reflecting the fact that the XRD and IR methods are recording different aspects of the mica composition and structure. The micaXT value is affected by the presence of kaolinite, for example. The IC value determined by XRD is to some extent a reflection of the degree of smectite interlayering.

Figure 28: Variation in MicaXT parameter in surface samples around Century. Sampled drillholes as gray circles.


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It would appear that the unusual infra-red response (MicaXT and Fe Slope parameters) of fresh rocks is confined to the immediate host-rocks to ore and to the pit area. This conclusion must however be qualified by the fact that none of the sampled distal and intermediate holes intersected rocks that are time equivalents of the mineralized Pmh4 horizon. The data permit two hypotheses: • The unusual spectral response is limited to the ore zone • The unusual spectral response extends beyond the pit area but has not been detected

because of the absence of time-equivalent rocks in the sampled drillholes Resolving this question is of critical importance for exploration, particularly in view of the possibility of using hyperspectral imagery for direct targetting. Another important consideration is the role of weathering4 in modifying the spectral signature of the fresh ore rocks. Comparison of surface and sub-surface responses suggests that weathering has produced some mineralogical changes. Kaolinite and montmorillonite are commonly detected in weathered samples, but are virtually absent in fresh samples. Alunite and jarosite and possibly gypsum have been detected in surface and near-surface rocks (although gypsum could be due to oxidation after drilling).

Figure 29: Comparison of illite crystallinity as determined by conventional XRD (vertical axis) and from SWIR (horizontal axis).

There is poor correlation between the two determinations.

Surface data show apparently systematic variation in the MicaXT and possibly MicaAlOH parameters with respect to the ore zone. There are currently too few surface samples from with in a few kilometers of Century, however, to be confident that these parameters can be used in routine exploration. I suggest that high priority be given to acquisition of hyperspectral imagery (e.g. HYPERION) in order to effectively delineate zones of high MicaAlOH and MicaXT. Detection of Jarosite and alunite in hyperspectral imagery may also facilitate targetting of this style of ore.

4 Weathering probably includes a number or near-surface events involving circulation of oxidised fluids. There is some evidence of a Cambrian event and possibly one or more Tertiary events as documented elsewhere in Australia.


35

11 Illite Crystallinity The crystallinity of fine-grained white-mica (''illite'') can be estimated from X-ray diffraction (or PIMA) and can be related to temperature of equilibration. 156 illite crystallinity determinations of samples from the Century mine collected by Graeme Broadbent were completed by Stafford McKnight at the University of Ballarat. Additional data were collected by Golding and co-workers as part of AMIRA project P552, mainly in petroleum exploration holes. It is not clear whether the two datasets are comparable, however, as details of any standardisation process are sketchy. Figure 30 illustrates the data obtained from 35 surface samples. It can be seen that IC in excess of 0.65 is restricted to the vicinity of Century. The area so defined falls within the MicaAlOH anomaly described above, and encompasses all of the areas in which Fe in soil exceeds 5%.

Figure 30: Distribution of surface samples used in illite crystallinity determinations. Red polygon defines the area in which IC > 0.65.


36

The distribution of samples from drillholes is shown in figure 31. These data, which are restricted to the vicinity of Century, confirm the surface sampling and in drillhole LH239 show that IC decreases with distance from the orebody in a vertical sense.

Figure 31: Surface and sub-surface samples with illite crystallinity determinations. The drillhole samples show a systematic

increase in IC towards the deposit.

Data from the AMIRA P552 project are shown in figure 32. These data show that several samples from distal drillholes exceed 0.65, particularly in Desert Creek 1. The significance of these data remain uncertain, partly because of the issue of standardization.


37

Figure 32: IC data from AMIRA project P552, in various petroleum wells near Century.

12 Magnetic Susceptibility Magnetic susceptibility data from drillholes from the pit area (sourced from the PCM database) are summarized in figure 33 and Table 5. Table 5 shows that the ore sequence has generally low susceptibility but the average is significantly higher than the other units, except for carbonate breccias. This conclusion is reinforced by plotting the variation in susceptibility downhole, as in figure 34.

Ore Sequence UFW HWD BCS BLS CBXSamples 619 320 1382 126 30 13

Average 23 14 14 9 11 61

Median 11 10 10 9 10 18 Table 5: Magnetic Susceptibility measurements of samples from the Century pit area


38

Figure 33: Box and whiskers plot showing the variation in magnetic susceptibility of rock units from the pit area based on data in the

PCM database.


39

Figure 34: Variation of density and magnetic susceptibility with respect to the ore zone in drillhole LH298. SGA, SGT and BDC are

all measurements of specific gravity. Source of data: PCM database. Pink bars show Zn >10%.


40

13 Density Density data from drillholes from the pit area (sourced from the PCM database) are summarized in Fig. 35.

Figure 35: Variation in density of rock units from the pit area using two measurement techniques based on data in the PCM

database (Table x).

Unit Description Count SGA SGT BDC

Immersion Titrimetric Well-Logs440 Unit 4.4 137 2.99 2.84170 Unit 1 shale (band 3) 130 2.98 2.81430 Unit 4.3 shale 258 2.96 2.80190 Unit 1 shale (band 5) 107 2.94 2.79 3.05180 Unit 1 shale (band 4 - high gn) 119 2.93 2.83 3.55450 Unit 4.5 shale 455 2.93 2.78 2.79311 Zinc rich bit in 310 94 2.92 2.811SH Unit 1 Shale 190 2.90 2.80 2.76410 Unit 4.1 shale 275 2.89 2.74 2.72165 Unit 1 siltstone 223 2.88 2.78145 Siltstone interburden 35 2.88 2.72160 Unit 1 shale (band 2) 130 2.87 2.77 3.16140 Predominantly weakly mineralised s 32 2.87200 Unit 2 shale 255 2.86 2.71 2.96CBX Carbonate Breccia 38 2.85 2.73 2.82150 Unit 1 shale (band 1) 191 2.85 2.77 2.78320 Unit 3.2 251 2.83 2.741ST Unit 1 Siltstone 354 2.80 2.74 2.67312 Zinc poor bit in 310 187 2.80 2.68 2.89SMU Stratiform Mineralised Unit 9 2.78 2.84175 Unit 1 siltstone 130 2.78 2.67CLS limestone 38 2.78 2.48 2.70UFW Pmh4 Upper FW siltstone-shale 115 2.78 2.721MS Shale and Siltstone - undifferentiate 86 2.77 2.74 2.46155 Unit 1 siltstone/minor shale 244 2.77 2.70 2.62185 Unit 1 siltstone 119 2.77 2.72420 Unit 4.2 178 2.75 2.66 3.15HWD Pmh4 HW siltstone-shale 240 2.75 2.72 2.63195 Unit 1 siltstone 103 2.74 2.68 2.90100 Unit 1 siltstone/shale 3 2.69FDR Fault Disrupted Rock 11 2.67 2.71BCS Pmh4 carbonaceous shale 9 2.62 2.72HWB Pmh5 sandstone, shale lenses 14 2.53HWS Pmh5 sandstone 19 2.58

Table 6: Average density determinations for various units from the pit area (source: PCM database)


41

The data are strongly biased towards mineralized rocks as might be expected. There are, however, sufficient data from the unmineralised rocks (HWD, BCS, FDR. UFW) to conclude that the density of the ore zone rocks (particularly the shales) is significantly higher than that of unmineralised rocks. Most are significantly denser than the Cambrian limestone, although the density of this unit appears to be dramatically different depending on the method used.


42

14 Geochemical Process Models

14.1 Questions to be addressed A series of key questions were presented to project G14 researchers by Zinifex personnel. Questions that relate to chemical process modelling are reproduced below: • What parts of the stratigraphy have the most potential? Kamarga, Flat Tyre and Grevillea are

all lower in the stratigraphy. Are there other sections of the stratigraphy which are anomalous [prospective]?

• Are there any special host-rock ages, e.g. rocks packages that were semi-consolidated [at the time of ore formation]?

• What were the source rocks? • Can an area of depleted sediments be identified and used to identify fluid flow? • What was the size of fluid cells and amount of metal available for leaching? • What is the significance of siderite? The modelling and discussion below attempt to answer some of these questions. 14.2 Constraints Knowledge of the temperature (and to a lesser extent pressure) is a key constraint on geochemical modelling of the Century ore system. Fluid inclusion data for sphalerite were presented by Polito et al. (2004; Table 7).

Paragenetic Stage Samples Measure-ments

Te ice (°C)

Tm ice (°C)

Salinity (wt% NaCl equiv.)

Homogenisation Temperature

(°C) Main hydrothermal phase 1 5 -59.9 -18.8 21 90

“Transgressive, crackle vein and breccia phase” 4 32 -49.1 -16.3 20 99

Post mineralization 1 2 -48.3 -3.4 6 111

Table 7: Summary of fluid inclusion data for Century samples collected from the open pit (Polito et al. 2004; AMIRA P552). Te – eutectic temperature (ice/hydrohalite final disappearance). Tm – initial melt temperature of ice or hydrohalite. Salinity calculated

assuming all cations present are Na+. Homogenisation is presumed to be by vapour disappearance.

It should be noted that it is assumed rather than demonstrated that the inclusions of the “main hydrothermal phase” are primary or pseudosecondary and therefore sample fluid related to sphalerite formation (and are also unaffected by post-ore deformation). Given that a substantial portion of the Century sphalerite is extremely fine-grained the inclusion data of Polito may in any case relate to relatively late coarse sphalerite and hence may sample late hydrothermal fluids. Homogenisation of fluid inclusions was observed at about 90 - 100ºC. If, the inclusions are indeed primary or pseudosecondary then this temperature range implies either extreme temperatures near the sediment-water interface (exhalative model) or formation at substantial depth (perhaps even > 3 km assuming a geothermal gradient of 35ºC/km). The data also suggest that sphalerite deposition occurred from a brine close to halite saturation. Vitrinite reflectance gives an indication of the maximum temperature experienced by the rock and is widely used in exploration for petroleum as a means of quantifying the thermal character of sedimentary basins. Average vitrinite reflectance (Ro) measurements for drillholes in the Century area are plotted in Fig. 36. Plotting the average in this way smoothes some of the variation that is apparent with depth, but nevertheless gives an indication of regional thermal gradients. It can be seen that the data are concentrated around Century mine. Given the possibility of multiple thermal events we need to be circumspect in relating the maximum temperatures as manifested in vitrinite data to the ore-forming process. Polito et al. (2004) suggested that the Ro temperature values from the vicinity of Century require a pressure


43

correction of the order of 60-70°C to the fluid inclusion data, implying sphalerite deposition at several kilometers depth. If the fluid inclusions represent exhalative brines, however, then the homogenisation temperatures reflect the entrapment temperatures (pressure correction is minimal). The composition and source of fluids involved in metal transport and deposition is one of the main uncertainties in geochemical process modelling and in ore deposit studies generally. There are no analytical data on fluid composition at Century other than the salinity estimates based on fluid inclusion microthermometry (Polito et al., 2004) and these cannot be unambiguously related to the ore-forming event. An hypothetical brine was used by Cooke et al. (2003) in numerical simulation and this is tabulated in Table 8. Cooke et al (2003) advocate the role of an oxidized brine as the metal carrier and assume a salinity of 25 wt%. The other fluid that has been implicated in ore formation is seawater, assuming that the process of ore deposition involves mixing of hot brine with cool seawater. The main uncertainty here is the degree to which modern seawater resembles Proterozoic seawater.

ppm T pH log ¦O2 TDS wt% Na+ Ca++ Mg++ K+ Fe++ Mn++ Al SiO2aqBrine 150 4.5 -43 25 71,919 9,051 3,710 13,139 1467.5 302.1 0.00167 85.27Seawater 5 4 10,800 411 1,290 392 0.0034 0.0004 0.0010 2.9000

ppm Cl HCO3- HS- AuCl2- F- SO42-Brine 152009.1 11493 85.27 8E-07 0.106 70.66Seawater 19,400 152 nd 0.00001 13 2,740

ppm Zn++ Cu+ Pb+ Ag+ Sr++ Ba++ Sb(OH)3Brine 212.2 1.48 198.7 0.072 75.78 9.678 0.369Seawater 0.00500 0.00090 0.00003 0.00028 8.10000 0.02100

Table 8: Composition of brine used in chemical modelling by Cooke et al. (2003) and contemporary seawater as well as parameters calculated in this study. NB the composition of Proterozoic seawater is likely to have been different to that of today.

Possible chemical mechanisms for ore formation at Century include: • Mixing of hot brine and seawater at or above the seafloor (exhalative model) • Mixing of hot brine and another fluid below the seafloor (e.g. mixtures of CH4 H2S and CO2

gas) • Cooling as brine moves upwards through the Lawn Hill Formation • Phase unmixing as brine moves upwards through the Lawn Hill Formation • Wall-rock reaction as brine moves upwards through the Lawn Hill Formation A major assumption is that lead and zinc are carried by an oxidised brine (log fO2 -43 at 150ºC; Cooke et al. 2003).


44

Figure 36: Image showing average Ro values for sampled drillholes in the Century area. Note that within the IC and MicaAlOH

anomalies the Ro values are lower than those outside. There is, however, some variation within individual drillholes.

14.3 Basic Controls on Pb and Zn Solubility The solubility of Zn and Pb in natural systems is dependent on a number of factors, including temperature, Cl- concentration, oxidation state and pH. As illustrated in Figs 37 and 38, oxidised and acidic conditions favour solubility of both metals, as well as extreme oxidised and alkaline conditions. Oxidation states below the sulphate-sulphide boundary would only permit substantial solubility at extremes of pH.


45

Figure 37: Log fO2 vs pH diagram illustrating basic controls on Zn solubility. The composition of the idealized ore-forming brine of

Cooke et al. is indicated.

Figure 38: Log fO2 vs pH diagram illustrating basic controls on Zn solubility. The composition of the idealized ore-forming brine of

Cooke et al. is indicated.

These diagrams suggest that deposition of Zn sulphides is most likely due to reduction, if the estimate of fluid log fO2 is correct. Neutralization of the brine would generate non-sulphide minerals unless neutralization occurred at log fO2 close to the sulphate-sulphide buffer. Neutralization is most effective at below the sulphate-sulphide buffer, but metal transport under these conditions requires extremely acidic fluids. Evidence of such fluids at Century (e.g. abundant kaolinite or pyrophyllite) has yet to be found. Nevertheless the pH and oxidation state of incoming fluids remain a significant uncertainty.


46

14.4 Mixing of Brine and Seawater (Exhalative Model) Exhalation has been proposed for other sediment-hosted Pb-Zn deposits of the North Australia Proterozoic and requires mixing of hot oxidized brine flowing upwards from depth and reduced seawater on or about the sediment-water interface. This mechanism is attractive because it neatly explains how large masses of Pb and Zn can be emplaced into what is now impervious and impermeable rock, and the regularity and continuity of mineralized beds. This scenario was modelled by Cooke et al. (2003, using the software "Chiller") by titrating 10 grams of seawater at 63ºC and pH of 4.4 into a kilogram of brine (Table 8; Fig. 39). The composition of Proterozoic seawater used in the calculation was not stated and it is not clear that the assumptions of a temperature of 63ºC and pH of 4.4 are valid.

Figure 39: Simulation of exhalation of oxidized brine into seawater by Cooke et al. (2003) using the software Chiller. The model

appears to have been set up as a titration, where small increments of one fluid are added to a “reservoir” of the other fluid. Note that both scales are logarithmic, suggesting that the assemblages are extremely sensitive to small masses of reduced seawater.

Nevertheless, Cooke et al. (2003) concluded that this model demonstrated that exhalation is a viable mechanism for precipitating base-metal sulphides, although they conceded that this mechanism is not relevant to Century since its ore assemblages are not reproduced. Mixing of contemporary seawater and the hypothetical brine of Cooke et al. (2003) was undertaken for this study using the software Geochemist's Workbench. This used the default database (from Lawrence Livermore National Laboratory, supplemented with new data for Zn and Cu species). The "flash" option of GWB allows mixing of all proportions of the two end-member fluids, unlike the treatment in Cooke et al. (2003). The first calculations used the brine composition shown in Table 8 at a temperature of 150ºC and contemporary seawater at 25ºC. These models failed to generate significant amounts of siderite. Thus the brine was modifed by increasing the amount of Fe by a factor of five. The result is shown in Fig. 40. At a temperature of 125ºC and over the mixture is saturated with hematite, below that siderite forms and is the most abundant mineral. Sphalerite precipitation does not occur until 50ºC5. While this model reproduces some aspects of the Century ore system it doesn't mirror the association of Zn sulphide with shale rather than sideritic siltstone. Advocates of the exhalative model could of course argue that this is because minerals such as quartz and mica are detrital in origin and therefore shouldn't be predicted in such models.

5 Modelling of the system is hampered because there are no thermodynamic data for zincian carbonate.


47

20 40 60 80 100 120 140

-5

-4.5

-4

-3.5

-3

-2.5

-2

-1.5

-1

Temperature (C)

Som

e m

iner

als (

log

cm3 )

Modified Cooke Brine & Turekian Seawater

Hematite

QuartzPyrite

Barite

Siderite

Anhydrite

Fluorite

Sphalerite

Figure 40: Minerals produced during mixing of hypothetical brine (Cooke et al., 2003) and seawater at 25-150°C.

The model also illustrates an order of magnitude decrease in Zn solubility as temperature declines from 120 to 25ºC. Temperature decrease could therefore contribute to deposition of Zn sulphides. 14.5 Mixing of Brine and H2S Gas A model proposed by Ord et al. (2002) involves mixing of a reduced H2S and/or CO2 rich fluid and an oxidized metalliferous brine. The former is thought to have flowed along the Termite Range fault and presumably its movement is predominantly upward in the vicinity of Century. The source of H2S in these models is either from: • Thermochemical reduction of anhydrite in the McNamara Group • Bacterial sulphate reduction (sulphate source being seawater?) • Deep crustal devolatilization • The mantle Zn, Pb, Fe and sulphate were thought to have been transported to the site of deposition in oxidized metalliferous brine (“circulating crustal brines”) via a separate aquifer. Ore deposition in this model is a consequence of reduction and suphidation of oxidized metalliferous brine by the H2S-rich fluid. This would imply that a specific host-rock is NOT NECESSARY as long as the host- rocks had the requisite porosity and permeability. A key aspect of this model is the hydrodynamic regime that allows mixing of the two end-member fluids in the requisite mass ratio for a protracted period of time. A significant impediment to modelling this mixing scenario is that we no data whatsoever on the composition of the reduced phase, and we can only speculate on the composition of the oxidized phase. Cooke et al. (2003) modelled mixing of oxidized brine and H2S gas (Fig. 41). This generated assemblages inappropriate to Century or indeed any sediment-hosted Pb-Zn deposit, with native sulphur precipitating over much of the reaction path. The reason for this is that the H2S gas condenses into the brine driving pH to extremely acidic values (a process similar to the condensation of hot sulphur-rich gases into cold groundwater above epithermal precious metal deposits). At such acidic conditions metals remain in solution and carbonate minerals are unstable.


48

Figure 41: Simulation of mixing of oxidized brine and 100 grams of H2S gas

0 10 20 30 40 50 60 70 80 90 100-2.5

-2

-1.5

-1

-.5

0

.5

1

H2S(g) reacted (grams)

Som

e m

iner

als (

log

cm3 )

Mixing Cooke Brine & H2S gas

Pyrite

C

S

Figure 42: GWB simulation of mixing of oxidized brine and 100 grams of H2S gas. Pyrite is present throughout the path, while sulphur is saturated only after 45 grams of H2S is added. Note the linear x axis compared to Fig. 41 which has a logarithmic axis.

Traces of covellite are predicted in the GWB simulation but have not been shown in this figure. Chalcopyrite is saturated in the initial reaction step as in Fig. 41. Thus the two simulations are remarkably similar!

A similar calculation was carried out with GWB (Fig. 42). The results reproduce those of Cooke et al. (2003) in that the three main phases are predicted to be pyrite, native sulphur and carbon. 14.6 Mixing of Brine and CO2 Gas Another mixing model calculated with GWB used the hypothetical brine and 100 grams of CO2 gas (Fig. 43).


49

0 10 20 30 40 50 60 70 80 90 100

-5

-4.5

-4

-3.5

-3

-2.5

-2

-1.5

CO2(g) reacted (grams)

Min

eral

s (lo

g cm

3 )

Cooke Brine and CO2

Hematite

Nontronite-Mg

Chalcopyrite

Pyrite

Siderite

Figure 43: GWB simulation of mixing of oxidized brine and 100 grams of CO2 gas.

Siderite is the most abundant mineral produced, but is dissolved away with continued CO2 addition. Addition of low masses of CO2 could explain the formation of siderite-rich rocks at Century. Figure 44 illustrates another control on siderite stability, namely oxidation state. The abundance of siderite in siltstones would imply a CO2-rich and relatively reduced fluid. In this model pH varies from 3.9 to 4.2 which is too acidic for sphalerite to precipitate.

Figure 44: Stability of Fe Phases in the system Mn-O-H-C-S at 100ºC. Log CO2 fugacity = 1.

14.7 Mixing of Brine and CH4 Gas Broadbent et al. (1998) and Broadbent (1999) suggested that ore formation occurred as the result of interaction of brine carrying Zn, Pb and Fe and a transported hydrocarbon accumulation (implying that the carbon was introduced as a liquid). The exact process of metal precipitation was not specified but presumably involved reduction of the brine by gaseous CH4 or heavier hydrocarbons (Broadbent et al., 1998) or some process involving mixing of an oil-rich phase and a brine phase. To test this model I added 100 grams of CH4 gas to the hypothetical brine thus simulating interaction with a mobile hydrocarbon gas phase.


50

0 10 20 30 40 50 60 70 80 90 100-6

-5

-4

-3

-2

-1

0

1

CH4(g) reacted (grams)

Som

e m

iner

als (

log

cm3 )

Cooke Brine & CH4

Chalcopyrite

Pyrite

C

Ferrostilpnomelane

Magnetite Sphalerite

Galena

Minnesotaite

Ag

Greenalite

Figure 45: GWB simulation of mixing of oxidized brine and 100 grams of CH4 gas.

In this case pH was driven to a more neutral value of five hence precipitating both galena and sphalerite. The minerals greenalite and minnesotaite (ferroan talc) were also predicted, and large masses of carbon precipitated. This process seems to have potential for reproducing aspects of the ore assemblage, if the mass of greenalite and Fe talc could be reduced. The model further suggests the possibility that some of the carbon at Century is an hydrothermal precipitate rather than transported hydrocarbon accumulation. Adding an equal mass of H2S to the gas phase, results in saturation in only two minerals, carbon and pyrite reflecting very low pH as in previous calculations. 14.8 Mixing of Brine and Gas Mixtures Adding CO2 to the H2S gas had little effect. Both gas mixtures generate extremely acidic fluids and so promote metal solubility. Chiller calculated pH of the H2S gas scenario to be 1.7 while GWB calculated it to be <1. This scenario therefore appears to be unlikely at Century, although the model fails to account for the presence of rock and its buffering effect on the combined fluid composition and represents a geologically unrealistic scenario (i.e. no rock is present, mixing occurs in a cavity). 14.9 Fluid-rock Reaction - Dolomite In order to resolve the issue of extreme pH generated by mixing H2S with the brine, Cooke et al. (2003) added dolomite as a reactant, effectively changing the model from a mixing scenario to one of fluid-rock interaction. This generated more geologically realistic mineral assemblages (Fig. 46). Increasing the mass ratio of dolomite to H2S (to 50:50) resulted in dolomite being present (saturated) throughout the reaction path (Fig. 47) and minor additional galena precipitating but otherwise the results of the two calculations are similar. This model produces results that are closer to the observed assemblages at Century after 10 grams of reactant are added except that the abundant siderite is not replicated (the Fe concentration of the brine could be too low) and sulphur, graphite and alabandite precipitate at the end of the reaction path. Silicate phases are apparently ignored in these calculations, and this is problematic since these could have controlled pH (or in the case of Fe-chlorite - redox).


51

Figure 46: Simulation of mixing of oxidized brine and 100 grams of H2S gas and dolomite (mass ratio 90:10)

Figure 47: Simulation of mixing of oxidized brine and 1 kg of H2S gas and dolomite (mass ratio 50:50)

14.10 Fluid-rock Reaction - Carbon Another way of simulating the interaction of oxidised brine and a liquid (or solid) hydrocarbon phase is to add graphite to the brine (Fig. 48). This of course neglects issues of miscibility between liquid hydrocarbon and aqueous fluid. A GWB model presented below (Fig. 48) generated only carbon (most abundant), minor siderite and pyrite and traces of chalcopyrite and muscovite. The absence of sphalerite and galena is explained because the pH remained quite acidic.

0 10 20 30 40 50 60 70 80 90 100

-6

-5

-4

-3

-2

-1

0

1

2

C reacted (grams)

Som

e m

iner

als (

log

cm3 )

Cooke Brine & Carbon

Chalcopyrite

Pyrite

C

Siderite

Muscovite

Figure 48: Simulation of mixing of oxidized brine and 100 grams of carbon


52

14.11 Fluid-rock Reaction – Lawn Hill Shale Titration is not the ideal method of simulating fluid-rock interaction as the mass of fluid remains more or less constant as increments of rock are added, rather like drops in a bucket. The "flush" option of GWB keeps the rock mass more or less fixed, while fluid increments are added, displacing an equivalent mass at each reaction step. In this way, flow is better simulated. Figure 49 illustrates the effects of "flushing" 1,400 kg oxidised brine through 2.8 kg unaltered shale at 50ºC (in increments of approximately 1 kg fluid). Sphalerite and galena precipitation occur in response to reduction by carbon present in the rock. Sphalerite mass exceeds that of galena until about 650 kg of fluid flow, when the oxidation state of the rock-fluid system increases substantially due to consumption of all graphite in the rock. At this point barite and pyrite also start to precipitate, perhaps simulating the late vein controlled mineralization. New siderite replaces dolomite in the early stages of reaction progress but dissolves completely after 600 kg of fluid flow (as oxidation state increases). Silver as native silver or acanthite precipitate throughout. The same input conditions except for a temperature increase to 100ºC result in dolomite being the dominant carbonate mineral, but otherwise the results are similar. The pH of the fluid drops from an initial value of 5.9 (higher than that of the input fluid) to 4.9, while log fO2 drops to -65 during the reaction path. As seen from the diagrams above the pH change would be likely to promote metal solubility and the most likely cause of metal precipitation is reduction.

0 200 400 600 800 1000 1200 1400

-5

-4

-3

-2

-1

0

1

2

3

Mass reacted (kg)

Som

e m

iner

als (

log

cm3 )

Sphalerite

Ag

Muscovite

GalenaC

Quartz

Siderite

Pyrite

WitheriteAcanthite

Barite

Figure 49: Mineral masses produced by flushing 1400kg brine into 2.8 kg of unaltered Lawn Hill shale.

This model therefore appears to simulate reasonably well the ore-bearing shales at Century. From it we can conclude that wall-rock reaction is a viable depositional mechanism. Also that ore formation was likely to have occurred at less than 100ºC if the oxidised brine composition is appropriate. Another model was identical in every respect except that carbon was removed from the rock. In this case no sphalerite or silver precipitated, but galena did. pH in this model was buffered at between 4.7 and 4.8 for much of the reaction path and this appears to be the key control on whether sphalerite or galena dominates the ore assemblage. Log fO2 was buffered at between -50 and -51, indicating substantial reduction of the incoming fluid, but not to the same extreme as when carbon was present.


53

0 200 400 600 800 1000 1200 1400

-6

-5

-4

-3

-2

-1

0

1

2

3

Mass reacted (kg)

Som

e m

iner

als (

log

cm3 )

Hematite

Dolomite-ord

Muscovite

Bornite

Galena

Pyrite

Figure 50: Mineral masses produced by flushing 1400kg brine into 2.8 kg of unaltered Lawn Hill dolomite.

If monominerallic dolomite is used instead of shale, galena and bornite precipitate, but not sphalerite. If carbon is added to the dolomite rock then galena and sphalerite precipitate, with sphalerite less abundant than galena. These models emphasise the importance of rock composition in controlling fluid pH and therefore Zn/Pb/Cu. They also suggest that the nature of the host-rock is not critical for zinc precipitation as long as it contains carbon as a reductant. Dolomite could be a suitable host rock on the Lawn Hill platform! 14.12 Metal Sources and Depletion The source of metal in the Century deposit is unknown. The average base-metal content of various units of the McNamara Group based on 550 samples from drillcore is summarized in Figure 51 and Table 9.

Figure 51: Average base-metal content of various units of the McNamara Group by rocktype, from Table 9. Circle size is a function

of Zn/(Zn + Pb) - the large circles represent more zinc-rich rocks.


54

Formation Lithology Count Pb U Zn Cu Pb + Zn Zn/(Zn+Pb) TOC%Lawn Hill Formation Interlayered Clastics 4 39 3 124 25 163 0.76 0.7Torpedo Creek Quartzite Shale 1 15 144 19 159 0.91Constance Sandstone Sandstone 3 14 136 25 150 0.90Lawn Hill Formation Shale 95 39 11 88 31 128 0.69 0.9Riversleigh Siltstone Shale 12 43 9 57 23 100 0.57Lawn Hill Formation Sandstone 17 45 2 52 8 97 0.54 0.0Riversleigh Siltstone Breccia 1 37 10 46 28 83 0.55Lawn Hill Formation Siltstone 23 20 1 56 27 76 0.74 0.4Riversleigh Siltstone Siltstone 15 17 2 59 13 76 0.77Riversleigh Siltstone Sandstone 25 17 1 50 13 67 0.75Lady Loretta Formation Dolomite 3 48 19 45 67 0.29Torpedo Creek Quartzite Interlayered Clastics 9 11 4 47 71 58 0.81Lawn Hill Formation Dolomite 1 5 3 48 3 53 0.90 0.4Torpedo Creek Quartzite Quartzite 7 18 7 25 42 42 0.58Kamarga Volcanics Mafic volcanic rock 2 10 7 32 6 42 0.76Lawn Hill Formation Tuff 37 13 4 29 6 41 0.69Torpedo Creek Quartzite Siltstone 3 13 24 21 37 0.65Riversleigh Siltstone Tuff 2 5 4 30 25 34 0.86Termite Range Formation Siltstone 3 10 22 22 32 0.69Gunpowder Creek Formation Quartzite 5 20 12 44 32 0.37Termite Range Formation Sandstone 89 11 0 16 16 27 0.60Torpedo Creek Quartzite Sandstone 11 14 7 12 17 26 0.45Basement Granitic Gneiss 3 10 7 13 8 23 0.56Gunpowder Creek Formation Interlayered Clastics 21 15 1 8 25 23 0.36Paradise Creek Formation Dolomite 74 13 10 12 22 0.43Gunpowder Creek Formation Dolomite 84 12 0 8 9 19 0.40

Table 9: Average base-metal content of various units of the McNamara Group by rocktype.

The data fall into two groups coloured red and green in figure 51. The green group falls on a linear trend that includes the few available samples (3) of basement rocks. This group is interpreted as reflecting sedimentation with the base metal ratios controlled by that of the source region. A similar relationship was observed in Canning Basin sediments (Wilde et al., 2005). The red group includes rocks that have much higher Zn and Zn/Pb ratios, and these include all lithologies of the Lawn Hill Formation. Given that Century is enriched in Zn one possible explanation of these data is that these rocks reflect hydrothermal enrichment rather than sedimentary source. Circulating fluids may have depleted rocks in base-metals relative, thus it is important to consider the spatial variation in base-metal content. The distribution of samples around Century is not ideal, however for such a study. For example all data for the Gunpowder Creek and Paradise Creek Formations are from two drillholes – GSQ-LH3 and 4. Furthermore, surface samples are likely be affected by weathering processes.

Figure 52: Average Zn concentration of sediments of the Lawn Hill Formation with distance from Century. The high Zn is found in

hole LH355 (the average of 3 samples). The low Zn is the average of 16 samples from LH195.


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Figure 52, however, shows average zinc values for the Lawn Hill Formation (all units) various holes outside the pit plotted against distance from the pit. Apart from highly anomalous Zn in LH355, three holes close to Century apparently have depleted Zn values relative to those more distal and which average about 100 ppm Zn. This may indicate a depletion zone extending to about 6km from Century, but this is far from definitive. 14.13 Sulphur Source/s Deposition of sphalerite requires as much sulphur as zinc, and thus the availability of sulphur is as critical to the generation of ore grade and tonnage as availability of metal. There is evidence of abundant pyrite in the rocks at Century that predates sphalerite but no compelling evidence that this pyrite was consumed to generate the sphalerite, although perhaps textural evidence of this process could be quite subtle and unrecognised. Possible sources of sulphur as sulphate include seawater and dissolution of anhydrite in sediments of the McNamara Group. Magmatic sulphur is improbable because of the lack of evidence of igneous intrusions associated with Century. Reduced sulphur could be derived from the mantle and deep crust or from thermochemical sulphate reduction (TSR) of anhydrite in the McNamara Group. δ34S for sulphides from Century show a range from +4 to +30‰. The

tendency is for paragenetically later sulphides to have heavier δ34S. Mantle-derived sulphur might be expected to generate a more limited range of δ34S close to 0. Such values have yet to be found at Century. Conversely it could be argued that mixing of mantle sulphur with a sulphate from a crustal brine might obscure such primitive values (J. Walshe, pers. comm. 2005). Broadbent et al. (1998) appealed to in-situ TSR, presumably of aqueous sulphate although the details of this process were not specified. An alternative model involves migration of H2S generated through TSR of anhydrite-bearing sediments beneath the level of the deposit. An analogy for this process can be seen in Sichuan, China (Cai et al., 2004) where sour gas (rich in H2S) correlates with the abundance of anhydrite in the sub-surface (Fig. 53).

Figure 53: Distribution of gas fields and "sour"(H2S-rich) gas in Sichuan, China (Cai et al., 2004). Note the extreme variation in 34S

of the sour gas. The gas is so potent that a gas blow-out lead to the deaths of over 200 people.

Sulphur isotopic analyses of the gases which show that there is a major variation from 0 to +20 (Cai et al., 2004) correlating with the proportion of H2S in the gas phase. Presumably high positive values reflect complete consumption of anhydrite during TSR, whereas the lower values represent equilibrium fractionation between gas and anhydrite. Thus the range in pyrite isotopic signature at Century could be a reflection of this process. These data also suggest that isotopic values of 0 do not necessarily indicate a mantle origin! If this process is relevant it would become valuable to map the occurrence of evaporites in the subsurface as a first order means of restricting search areas. Also, presence or absence of


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anhydrite evaporites would be a useful terrane-selection criterion. This would also be true if dissolution of anhydrite contributed sulphate to an oxidised brine (see below). Another possibility is bacterial sulphate reduction (BSR). BSR is presumably a means of fixing sulphur, and doesn’t really explain the source of sulphur. The implication for ore formation at Century might be that the large masses of carbon reflect former bacteria and effectively an in-situ supply of H2S. Sulphur isotope data are not definitive enough for this model to be confirmed or denied. Perhaps a better approach would be to lookm ore carefully at the carbon itself. 14.13 Source of Chlorine Equally important to the story of ore formation, but often neglected is the source of chlorine in the ore-forming brines. Without the chlorine metal mobility would be greatly inhibited. Two sources are often appealed to: • Dissolution of halite • Involvement of brines enriched during surface evaporation (bittern brines) The ratio of Cl/Br/I in fluid inclusions can sometimes be used to determine which of these sources is most appropriate. Such studies however ignore the possibility of fractionation during fluid rock interaction, and in any case no such data exist for Century. It is worth considering the possible involvement of halite evaporates in the formation of Century, both from the point of view of providing a source of chlorine and from the perspective of creating structural architecture and fluid pathways.


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15 Conclusions

15.1 The Century Footprint The ore sequence at Century is mineralogically and chemically distinctive, and several techniques permit recognition of the mineralized sequence in fresh and weathered rocks, in addition to conventional bulk-rock analysis for Zn, Pb and Ag. The ore zone is characterized by enrichment in Fe, Mn and S reflected in the abundance of ferroan carbonate (siderite) and pyrite. The abundance of these minerals results in the rocks of the ore zone having higher density and magnetic susceptibility than unmineralised rocks of the hanging wall and footwall at Century. Forward modeling of the Century ore and host-rocks is warranted in order to establish whether gravity or gravity gradiometry (airborne gravity) could be useful exploration tools. Susceptibility measurements would be a rapid and low cost tool for identifying altered rocks in drillcore and chips. It should be noted, however, that sampling is very biased towards the pit area and more samples should be measured beyond the pit, to add confidence to these conclusions. MicaAlOH and IC anomalies are more extensive than bulk chemical and carbonate anomalies and include all iron in soil anomalies. White mica in the pit area is distinctive compared to that in unmineralised rocks of the McNamara Group as manifested in infra-red spectra and in X-ray diffraction illite crystallinity. The former permits identification of mineralized rocks in drillcore and chips using an infra-red spectrometer such as HYCHIPS or PIMA. MicaXT (equivalent to illite crystallinity - IC) appears to be the best parameter to recognize alteration in fresh rocks. Despite the fact that surface rocks have been modified by weathering the micaAlOH parameter defines an outcropping zone that encloses Century and many of the small occurrences in the region. Curiously, this mirrors observations of gold deposits of the Yilgarn. This surface anomaly raises the possibility that outcropping alteration zones could be identified by remote-sensed hyperspectral imagery and this should be assessed in the next year of this project. Reassessment of analytical data for carbonate minerals throws doubt on the concept of a siderite “halo” about Century but shows that the rare Fe-Mg carbonates sideroplesite and pistomesite occur widely in the region. Zincian siderite sensu stricto is limited in occurrence to the pit area. This raises the possibility that partial extraction analysis aimed at the carbonate content of rocks could be more effective (higher peak to background) than conventional bulk analysis. An important question that remains unresolved is the extent of the hydrothermally-altered rocks beyond the pit area. Thus far, none of the holes sampled revealed hydrothermally altered rocks comparable to those on the pit. It is rare however for the host Pmh4 unit to be intersected beyond the pit area. On the other hand at least some of the layering characteristics of pmh4 – notably sideritic siltstone layers are likely to be due to hydrothermal processes. Thus, the absence of altered rocks may be because they were never there in the first place because hydrothermal activity was restricted to Century itself or because the relevant stratigraphic level has been reached beyond the pit. Some resolution of this important issue may be obtained by a comparable study of the McArthur River deposit, where the transition from ore zone to alteration envelope may be unambiguously preserved. 15.2 Chemical Process Model Processes involving mixing, cooling and fluid-rock reaction have been modelled. Phase unmixing has not, because of the lack of evidence for such a process (e.g. rapid cooling textures, pervasive brecciation and stockwork formation, evidence of silica super-saturation etc). Furthermore, current software and thermodynamic data do not allow modelling of partitioning of metals between aqueous and gaseous phases. The scenario that best approximated the ore-bearing shales was interaction of oxidised brine and unaltered shale. Continued flow also resulted in higher Pb/Zn thus reproducing a key feature of the Century orebodies. Interaction of hot hydrothermal fluid and pre-existing carbon (liquid


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hydrocarbon?) would have modified the precursor (hydro-) carbon and imparted characteristics such as enrichment in poly-aromatic hydrocarbons (PAH) as observed at the McArthur River mine (Chen et al., 2003). If this assumption proves correct then the carbon composition might be expected to vary with proximity to ore, potentially providing another exploration guide. Reaction between brine and hydrocarbon may also have generated increased permeability and porosity, facilitating accumulation of ore-grade zinc. This model further suggests that there is no need to invoke a separate reduced fluid and fluid mixing. Sulphur in this model is introduced with the hydrothermal fluid as sulphate and could therefore be derived from dissolution of anhydritic evaporites at depth (among sources). In which case exploration could be confined to areas underlain by evaporitic sediments. Another significant conclusion is that the mineralogical nature of the host-rock is not crucial since most rocks of the McNamara Group (including massive dolomite) could precipitate sphalerite if some carbon is present. Thus permeability and porosity may be more important controls than chemical composition. The siderite-rich siltstones probably result from hydrothermal processes, consistent with enrichment in Zn (in the siderite lattice) and restriction to the Century orebody. Two chemical models approximated formation of the sideritic siltstones: mixing of oxidised brine and contemporary seawater (in a rock-absent environment) or mixing of brine with CO2 gas. The siltstones may therefore represent a precipitate at the ocean floor or the product of mixing sub-surface under conditions of high porosity and permeability. The source of metal remains a matter for conjecture, but a possible depletion zone in Lawn Hill formation rocks has been identified extending up to 6km from Century. Data are insufficient to speculate on the volume of depleted rocks and metal available for transport. 15.3 Recommended Further Work The following work is recommended in the next year of this project: • Collection of density and magnetic susceptibility data from unmineralised rocks distal to

Century (ideally using drillholes already sampled for bulk chemistry and infra-red spectrometry)

• Acquisition of hyperspectral data over Century (e.g. Hymap or Hyperion) and process in order to emphasise key aspects of the alteration as defined in this study. This should be done in conjunction with the Queensland Geological Survey’s Smart Mapping initiative

• Collect systematic (grid-based) surface samples for further infra-red spectrometry and XRD in order to delineate more fully the limits of MicaAlOH and illite crystallinity anomalies, and provide additional input into the interpretation of hyperspectral imagery. Use of the Hychips technique rather than PIMA would permit examination of oxide/hydroxide minerals and sulphides.

• An orientation survey using partial extraction aimed at dissolving only the carbonate portion of the rock to determine whether this is more effective at delineating the elusive alteration halo than other techniques.

• Possibly more esoteric, but potentially useful for identifying major fluid flow zones is systematic sampling of quartz veins for oxygen and lead isotopes

• A study of the Cl/Br/I composition of fluid inclusions in suitable samples might help establish the role if any for halite dissolution versus bittern brines

Furthermore, this study has focussed on the expression of the Century deposit, but further work should also examine the nature of the (thus far) smaller structurally-controlled deposits such as Silver King and determine whether the alteration associated with these deposits is distinctive. Radiometric dating should also be attempted in order to establish whether these deposits are separated in time from the main Century mineralization event.


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16 References Andrews, S.J., 1998, Stratigraphy and Setting of the Upper McNamara Group, Lawn Hill Region, Northwest Queensland: Economic Geology, V.93, pp. 1132-1152. Broadbent, G.B. and McKnight, S.W., 1993, Microstructures of Ore Minerals from the Century Deposit, Northern Queensland. Abs. p11. Specialist Group in Economic Geology, Geol. Soc. Aust. 2nd National Meeting, Armidale, NSW Feb, 1993 Broadbent G.C., Myers R.E. and Wright J.V., 1998, Geology and Origin of Shale-Hosted Zn-Pb-Ag mineralization at the Century Deposit, Northwest Queensland, Australia: Economic Geology, V.93, pp.1264-1294. Cai C., Xie Z., Worden R.H., Hu G., Wang L., He H., 2004, Methane-dominated thermochemical sulphate reduction in the Triassic Feixianguan Formation East Sichuan basin, China: towards prediction of fatal H2S concentrations: Marine and Petroleum Geology, V21, pp. 1265-1279. Carr, G.R., 1984, Primary Geochemical and Mineralogical Dispersion in the Vicinity of the Lady Loretta Zn-Pb-Ag Deposit, Northwest Queensland: Journal of Geochemical Exploration, 22, pp. 217-238. Chen J., Walter M.R., Logan G.A, Hinman M.A., Summons R.A., 2003, The Paleoproterozoic McArthur River (HYC) Pb/Zn/Ag deposit of northern Australia: organic geochemistry and ore genesisL: Earth and Planetary Science Letters 210, pp. 467-479 Cooke D.R., Bull S.W., Donovan S and Rogers J., 1998, K-Metasomatism and Base-metal Depletion in Volcanic rocks from the McArthur Basin, Northern territory – Implications for base-metal Mineralization: Economic Geology, V.93, pp. 1237-1263. Cooke D.R., Bull S., Large R., 2003, Processes of ore formation in the stratiform sediment-hosted Zn–Pb deposits of Northern Australia: testing the Century model, Journal of Geochemical Exploration V78– 79, pp. 519– 524 Feltrin L., Oliver, N.H.S., Kelso, I., King S., 2003, Basement Metal Scavenging during Basin Evolution: Cambrian and Proterozoic Interaction at the Century Zn-Pb-Ag deposit, Northern Australia: J. Geochem. Expln., V78-9, pp. 159-162. Frakes L.A., and Bolton, B.R., 1984, Origin of manganese giants: sea level and anoxic-oxic history: Geology, V12, pp. 83-86. Harms, J., 1965, Iron Ore Deposits of Constance Range, pp. 264-269. Hutton L.J., 1983, Stratigraphic Drilling Report GSQ Lawn Hill 1-4, Queensland Government Mining Journal, June 1983, pp. 228-240. Herrmann, W., Berry, R. F., 2002, MINSQ; a least squares spreadsheet method for calculating mineral proportions from whole rock major element analyses, Geochemistry - Exploration, Environment, Analysis, 2/4, p. 361-368 Jackson M.J., Sweet I.P., Page R.W., Bradshaw B.E., 1999, The South Nicholson and Roper Groups: Evidence for the Early Mesozoic Roper Superbasin: AGSO Record 1999/19, pp. 36-45 Johnson, L.C., 2000, Sedimentological Controls on Ore Genesis, century Zinc deposit, Northwest Queensland: QUT Honours Thesis, 89 pp. Kralik M., 1982, Rb-Sr Age Determinations on Precambrian Carbonate Rocks of the Carpentarian McArthur Basin, Northern Territories, Australia: Precambrian Research, V.18 pp.157-170. Lambert I.B. and Scott, K.M., 1973, Implications of Geochemical Investigations of Sedimentary Rocks Within and Around The McArthur Zinc-Lead-Silver Deposit, Northern Territory: Journal of Geochemical Exploration, 2, pp. 307-330


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Large, R.R. McGoldrick, P.J., 1998, Lithogeochemical halos and geochemical vectors to stratiform sediment hosted Zn–Pb–Ag deposits. Part 1. Lady Loretta Deposit, Queensland: Journal of Geochemical Exploration 63, pp. 37–56 Large, R.R., Bull S.W., McGoldrick, P.J., 2000, Lithogeochemical halos and geochemical vectors to stratiform sediment hosted Zn–Pb–Ag deposits, Part 2. McArthur River, Northern Territory: Journal of Geochemical Exploration 68, pp. 105-126. Large, R.R., Bull, S.W., Winefield, P.R., 2001, Carbon and oxygen isotope halo in carbonates related to the McArthur River (HYC) Zn-Pb-Ag deposit, North Australia; implications for sedimentation, ore genesis, and mineral exploration: Economic Geology, V.96, p. 1567-1593. McKnight, S.W. and Broadbent, G., 1993, Transmission Electron Microscopy Study of Bitumen occurring in the Century Deposit, N. Queensland. International Congress on Applied Mineralogy. Perth. Pub. 61 McKnight, S.W. and Wilde A.R., 2005, Application of High Voltage Transmission Electron Microscopy to Ultra-Fine Grained Ores: The Century Deposit, Queensland: Abstracts Conference on Structure, Tectonics and Ore Mineralization Processes: Townsville Aug 29 – Sept 2. McDougall I., Dunn P.R., Compston, W., Webb A.W., Richards J.R., Bofinger V.M., 1965, Isotopic Age Determinations on Precambrian Rocks of the Carpentaria Region, Northern Territory, Australia: J. Geol. Soc. Aust., V12/1, pp. 67-90. Tyson, R.V., 1987, The genesis and palynofacies characteristics of marine petroleum source rocks: In Brooks J.R.V., Fleet, A.J., (eds.) "Marine Petroleum Source Rocks" Geol. Soc. Spec. Pub., #26, pp. 47-68. Uysal, T., Glikson, M., Golding, S.D., Southgate, P., 2004, Hydrothermal Control on Organic Matter Alteration and Illite Precipitation, Mt Isa Basin, Australia: Geofluids V4, pp. 131-142. Volk H., George S.C., Dutkiewicz A., Ridley J., 2005, Characterisation of Fluid Oil in a Mid Proterozoic Sandstone and Dolerite (Roper Superbasin, Australia): Chemical Geology, V223, pp. 109-135. Wignall, P.B., 1994, Black Shales, Oxford Monographs on Geology and Geophysics #30. Wilde, A.R., 2004a, F1 - Century Project: ACCESS Database: pmd*CRC project F1 report Wilde, A.R., 2004b, F1 - Century Project: Data Compilation and Review: pmd*CRC project F1 report Wilde, A.R., 2005a, Definition Of Hydrothermal Alteration Related to Zn-Pb Mineralisation in Sediments of the McNamara and McArthur Groups: 1 - Publsihed Bulk Chemical Data: pmd*CRC Report Wilde, A.R., 2005b, Definition Of Hydrothermal Alteration Related to Zn-Pb Mineralisation in Sediments of the McNamara and McArthur Groups: 2 - PIMA Data: pmd*CRC Report Wilde, A.R., 2005c, Definition Of Hydrothermal Alteration Related to Zn-Pb Mineralisation in Sediments of the McNamara and McArthur Groups: 3 - NITON Portable XRF Analysis: pmd*CRC Report. Wilde, A., McPhail D, Brugger, J, McKnight S and Garnett D., 2005, Preliminary Study of the Source of Base Metals in MVT Deposits of the Canning Basin, Western Australia: in Mao J. and Bierlein F. (eds), Mineral Deposit Research: Meeting the Global Challenge: Proc. 8th Biennial SGA Meeting Beijing China, Springer, pp. 195 - 198. Wilde A.R. and McKnight S., 2005, Mineralogical and Chemical Composition of Ore and Hematitic Rocks: pmd*CRC project G14 report


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17 Appendix 1 – Data Disk Data pertinent to the geochemical evolution of the Century orebody were compiled into an ACCESS database in order to facilitate evaluation and identification of data gaps. Digital data were provided by Graeme Broadbent and Steve Andrews (Rio Tinto) and Michael Whitbread (IO Geochemistry). Other data were digitized. The key table is “sample details” which is linked to all others by a unique sample identifier (sample ID) or drillhole ID. I have used in most cases the sample numbers provided in the source, except where there were no sample numbers provided, and sample identifiers of Kralik (1982) have been renamed to avoid duplicating other sample numbers. Drillhole details were imported from the PCM drilling database. This required some modification of the drillhole notation from the original sources in order to conform with the naming convention in the PCM database. For example some holes appear as DD99LH263 in the data sources, whereas this hole would be recorded in the PCM database as LH263. Stratigraphic nomenclature in some cases was modified to conform with the PCM/Zinifex conventions. The database contains a number of data tables: • Sample Details (4,276) • Bulk chemical analyses (1,885) • Carbonate composition from microprobe analyses (2,535) • Drillhole locations, extracted without modification from the PCM database (1,048) • Fluid Inclusion Microthermometry (212) • Illite Crystallinity from Stafford McKnight (162) • Modes, mainly of ore-grade samples (247) • Silicate mineral analyses from microprobe analyses (57) • Stable isotope analyses (320) • Sulphide analyses from microprobe analyses (1,397) • SWIR parameters, as supplied by Ausspec (1,314) • Vitirnite Reflectance Ro values mainly from Steve Andrews PhD work and Miriam Glikson's

contribution to AMIRA P552 (305) Details of analytical methods (where available) and data source are given in each table. More details on the database can be found in Wilde, 2004a. The data CD also contains all data pertaining to samples collected in this study and a large number of images and CorelDraw 9.0 illustrations. The data are grouped in folders as follows:

Drillhole data contains all data collected as part of this project including: NITON bulk chemistry, infra-red spectra, wireline data (from GA), images of core and graphic logs. Conventional bulk chemical data are also available for some of the holes. Images and CorelDraw contains a number of images of rocks and the open pit and various illustrations in CorelDraw version 9 format.


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Infra-red data contains the raw data files, either as individual FOS format files, or collectively as The Spectral Geologist Format (TSG). These files require the software such as The Spectral Geologist or ERDAS-IMAGINE to be read. Project G14 samples include images of many of the samples collected by Andy Wilde, including images taken with the scanning electron microscope. Other data including GADDS files are also included. The published data compilation is self-explanatory and most data are presented as Excel spreadsheets. These data have all been imported into the ACCESS database. Reaction Path Modelling contains all the files used in geochemical modelling and are in Geochemist's Workbench format (version 4). Rock Properties contains visual logs of a number of holes from Century using data from the PCM drilling database. XRD data presents quantitative XRD data for samples collected in this study and from samples collected by Graeme Broadbent.

wilde century pmd-crc 2006

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