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Western Australian School of Mines, Curtin University of Technology, Bentley Campus, P.O.Box U1987, Bentley, 6845, Western Australia
* E-mail: J.P.Vaughan{at}curtin.edu.au
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ABSTRACT |
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 TOP ABSTRACT IntroductionGeology and gold deposits...Analytical methodsProcessing of gold oresSulphide oresArsenical oresPyritic oresDiscussionClassification of refractory...AcknowledgementsReferences |
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Pyritic ores are free-milling to mildly refractory, or rarely moderately refractory. Pyrite in pyritic ores contains negligible to low levels of submicroscopic gold (<5 ppm). Other reasons for refractory behaviour in pyritic ores include very fine-grained native gold inclusions in pyrite, or the presence of gold-bearing tellurides.
It is concluded that submicroscopic gold is incorporated into the crystal lattices of arsenopyite and arsenical pyrite at sub-greenschist to lower greenschist-facies temperatures, and is progressively expelled as inclusions and fracture fillings of native gold in sulphides, and ultimately into the gangue, as recrystallization proceeds through upper greenschist- into amphibolite-facies temperatures, during deformation and burial. Submicroscopic gold is expelled more rapidly from pyrite than arsenopyrite. Pyrrhotite progressively replaces primary pyrite at higher temperatures, but rarely contains gold.
Finally, a metallurgical classification scheme for refractory ores is presented which incorporates the above mineralogical conclusions.
KEYWORDS: gold ores, Archaean greenstones, metallurgy, Western Australia
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Introduction |
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TOPABSTRACTIntroductionGeology and gold deposits...Analytical methodsProcessing of gold oresSulphide oresArsenical oresPyritic oresDiscussionClassification of refractory...AcknowledgementsReferences |
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). The timing of gold mineralization in the Craton has been dated at ~2.60–2.65 Ga. The ores form part of a worldwide episode of gold mineralization that took place in the late Archaean, which is now represented by gold deposits on many continents.
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The mineralogy and processing characteristics of these refractory gold ores have been investigated in a number of projects over the past several years (Vaughan et al., 1989; Vaughan, 1991; Vaughan and Corrans, 1992; Bacigalupo-Rose, 1992; Kyin, 1995;). This work has found that submicroscopic gold in sulphides is by far the most common reason for refractory behaviour in gold ores of the region, with the exception of the large Kalgoorlie Golden Mile deposits, which contain abundant gold tellurides. The present paper draws on the above work and presents new data to explain the mineralogical basis of refractory behaviour in Western Australian Archaean gold ores. In addition, this paper gives further insights into the incorporation of submicroscopic gold into sulphide minerals, together with the evolution of gold ores during their deposition, metamorphism and recrystallization. In the final part of the paper a classification scheme for refractory gold ores is suggested, based on the conclusions of this work.
Submicroscopic gold
The role of submicroscopic gold (or invisible gold) as a major contributor to refractory gold ores has long been known or suspected (e.g. Bürg, 1930; Archibald, 1949; McPheat et al., 1969). However, detailed investigations of the nature of submicroscopic gold, its distribution in sulphides and the mechanisms of incorporation of this gold have been extensively studied only over the past ~15 y (e.g. Cabri et al., 1989; Cathelineau et al., 1989; Cook and Chryssoulis, 1990; Fleet et al., 1993), coinciding with the development of better instrumental techniques to deal with the problem (Chryssoulis et al., 1987, 1989). A wide variety of gold deposits from all geological ages are found to contain submicroscopic gold, structurally bound into arsenopyrite and arsenian pyrite, the most important host minerals. Cathelineau et al.(1989) studied arsenopyrites in Palaeozoic gold ores from France and found two types: a submicroscopic gold-rich type which they suggested was deposited at lower temperatures (200±50°C, from fluid inclusion data), and a gold-poor type which they suggested was deposited at higher temperatures (300–500°C). Fleet et al.(1993) found that submicroscopic gold was associated exclusively with As-rich growth bands in arsenian pyrite, and suggested that the gold was incorporated into pyrite by chemisorption onto As-rich growth surfaces.
Mumin et al. (1994) described mesothermal gold ores from the Proterozoic Ashanti Belt in West Africa in which submicroscopic gold is present in sulphides within ores from structurally shallower areas, but has been remobilized into particles of native gold within structurally deeper ores. However, Oberthür et al. (1997) also investigated gold ores from the Ashanti Belt and found gold in two ore types: free-milling gold in quartz veins and refractory sulphide ores (arsenopyrite + pyrite ± pyrrhotite ± marcasite) containing submicroscopic gold. They concluded that gold in quartz veins was not formed by recrystallization of refractory sulphides; rather that the formation of the two ore types was broadly coeval, from multiple pulses of mesothermal ore fluids. Larocque et al. (1995) measured maximum submicroscopic gold concentrations of ~10 ppm in pyrite in the Mobrun VMS deposit, north-west Quebec, and found evidence for remobilization of gold during recrystallization of pyrite under greenschist-facies metamorphic conditions.
Fleet and Mumin (1997) published a comprehensive account of invisible gold in sediment-hosted gold deposits from the Carlin area, including hydrothermal synthesis of gold-bearing sulphides, and concluded that incorporation of gold into sulphides proceeds via a chemisorption process, most favourably into As-rich arsenopyrites and pyrites at lower temperatures. In both minerals submicroscopic gold deposition is facilitated by chemisorption onto As-rich growth surfaces.
Tarnocai et al. (1997) found high concentrations of submicroscopic gold in arsenopyrite, and lower concentrations in pyrite, at the Campbell gold mine, Ontario, and concluded that, on the basis of textural evidence in arsenopyrite, garnet and gahnite, gold was deposited during peak metamorphic conditions (upper greenschist). Genkin et al.(1998) studied arsenopyrites in four mesothermal gold ores from Siberia and concluded that there were two main stages of gold deposition –early chemically-bound submicroscopic gold in arsenopyrite and pyrite, and later native gold in veins and as fracture fill or overgrowths on auriferous arsenopyrites. In contrast to Fleet and Mumin (1997), they found that gold-bearing arsenopyrite was depleted in As. They concluded that there was no evidence that late-stage native gold originated from recrystallization of early submicroscopic gold-rich arsenopyrite.
Simon et al. (1999b) used X-ray absorption near edge structure spectroscopy (XANES) measurements to study the oxidation state of gold and arsenic in gold-bearing arsenian pyrite concentrates from Carlin-type deposits and concluded that gold was present as both Au0 microinclusions and Au1+ in the lattice. However, as this work was carried out using a large-diameter beam on sulphide concentrates, it is possible that small inclusions of native gold were recorded in addition to submicroscopic gold. In a related study on arsenian pyrite from the Twin Creeks deposit, Nevada, Simon et al. (1999a) found that the highest concentrations of gold and arsenic (595–1465 ppm and 1.05–2.43 wt.%, respectively) were present in very fine-grained pyrite (up to 2 µm), formed at low temperatures (120–200°C). The lowest gold and arsenic (17–60 ppm and <1 wt.%, respectively) was contained in coarser-grained pyrite (10–30 µm), formed at higher temperatures (250°C). Cabri et al. (2000) also used XANES microbeam measurements on individual sulphide grains to investigate the chemical speciation of submicroscopic gold in arsenopyrite. They concluded that gold occurs in arsenopyrite in two mutually exclusive chemical forms: chemically bound (covalent Au1+) and elemental (nanometer scale Au0 particles). They speculate that the two forms might behave differently in metallurgical processing.
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Geology and gold deposits of theYilgarn Craton |
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TOPABSTRACTIntroductionGeology and gold deposits...Analytical methodsProcessing of gold oresSulphide oresArsenical oresPyritic oresDiscussionClassification of refractory...AcknowledgementsReferences |
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). Gold mineralization is widely distributed throughout the greenstone belts, which also contain important deposits of nickel sulphides, nickel laterites and base metals. The Yilgarn Craton has been informally subdivided into four provinces: (1) the Eastern Goldfields Province, containing the important Norseman-Wiluna Belt; (2) the Southern Cross Province; (3) the Murchison Province; and (4) the Western Gneiss Terrains. The first three of these are granite-greenstone provinces, while the Western Gneiss Terrain consists of higher-grade metamorphic gneiss complexes. Subdivision of the granite-greenstone provinces is essentially on the basis of differing proportions of major rock types in the greenstone belts. More recent tectonic interpretations propose that the Craton has been assembled from a number of smaller tectonic units known as terrains and superterrains (Myers, 1997). Gold deposits are overwhelmingly located within greenstone belts in the granite-greenstone provinces. The highest concentration of gold deposits is within the Eastern Goldfields Province, especially within a graben-like north-north-west corridor of distinctive rock types, including tholeiites, calc-alkaline volcanics and komatiites, known as the Norseman-Wiluna Belt.
Most gold deposits in the Yilgarn Block are classified as mesothermal ores, reflecting emplacement at moderate crustal depths, in contrast to shallower epithermal deposits of younger terrains. They form a coherent group with a number of common features (Groves et al., 1990a,b). These include structural control, gold-only type, distinctive wall-rock alteration haloes and greenschist to amphibolite metamorphic grade. Greenstone host rocks are mainly ultramafic, mafic and felsic volcanic rocks and Fe-rich sedimentary rocks (e.g. iron formations). Most deposits were formed under sub-amphibolite conditions (250–400°C, 1–3 kbar; Mikucki and Groves, 1990) at inferred depths of perhaps 2 to <10 km, while a few were formed under amphibolite to granulite conditions (up to about 750°C and 631 kbar) at inferred depths up to ~20 km (Perring et al., 1990).
Deposits investigated
Gold deposits that were investigated for this study include New Celebration, Sons of Gwalia, Wiluna, Lancefield, Ora Banda, Mickey Doolan, Paddington, Coolgardie and Exhibition (Fig. 1).
The ‘New Celebration’ gold mine lies between Kalgoorlie and Kambalda in the Eastern Goldfields Province (Fig. 1). Gold mineralization is structurally controlled by the regional-scale Boulder-Lefroy Shear Zone, and is hosted in upper greenschist-facies felsic porphyries, mafics and ultramafics, accompanied by extensive quartz veining and carbonate alteration (Norris, 1990). Pyrite is the overwhelmingly dominant sulphide mineral present in the ore, accompanied by only trace amounts of a few other sulphides (e.g. chalcopyrite and galena). The New Celebration ore is mildly refractory and is treated by finer milling of a pyrite gravity concentrate, prior to cyanidation.
The ‘Sons of Gwalia’ gold mine is located a few kilometres from the town of Leonora towards the northern end of the Norseman Wiluna Belt (Fig. 1). Host rocks are greenschist-facies high-Mg basalts and lesser dolerites. Gold mineralization is located within a zone of intensely sheared mafic volcanics, now represented by alternating mica and quartz-carbonate rocks (Kalnejais, 1990). Gold mineralization is hosted by quartz-chlorite-sericite schists containing disseminated sulphides. The sulphide assemblage is dominated by pyrite, with minor pyrrhotite, chalcopyrite, arsenopyrite, galena and sphalerite. The siting of gold is described as free gold in quartz veins in relatively pyrite-rich domains, or associated with pyrite (Skwarnecki, 1990). The ore is free-milling and is processed by conventional cyanidation.
The ‘Wiluna’ gold deposits are located ~5 km south of the town of Wiluna at the northern end of the Norseman-Wiluna Belt (Fig. 1). The orebodies are hosted in a folded sequence of weakly metamorphosed basalts, intercalated felsic volcanics and quartz dolerite sills (McGoldrick, 1990). Gold deposits are structurally controlled and consist of older quartz reef systems along the contacts of mafic flows, together with lode systems in cross-cutting shear zones. Ore mineralogy consists of a pyrite-arsenopyrite-quartz-dolomite ± fuchsite ± sericite ± chlorite ± stibnite assemblage in very low-metamorphic grade (prehnite-pumpellyite) host rocks (Hagemann, 1990). The Wiluna deposits are very refractory and have been processed by roasting in the past, and in more recent times by a large bacterial oxidation plant.
The ‘Lancefield’ deposit is located ~8 km north of the town of Laverton in the north-eastern Eastern Goldfields Province. It lies near the base of a major cycle of mafic to ultramafic volcanics, which is separated from underlying cycles by thin units of banded iron formation, carbonaceous shale and chert (Hronsky et al., 1990). Locally the deposit is hosted by carbonaceous schist and minor basalt and dolerite, metamorphosed to mid-greenschist facies. The lodes are structurally controlled by brittle-ductile reverse shear zones, localized along carbonaceous schist units (Hronsky, 1990). The sulphide assemblage is dominated by pyrrhotite, pyrite and arsenopyrite, with minor sphalerite and galena. Eighty percent of the gold is described as being associated with arsenopyrite (probably as solid solution), the remainder with pyrite and in the gangue (Hronsky, 1990). The Lancefield ore is refractory and in the recent past was treated by roasting.
The ‘Mickey Doolan’ deposit is one of several orebodies of the Paddy’s Flat district, located immediately to the east of the town of Meekatharra in the Murchison Province. Host rocks are a regional sequence of komatiitic basalts, ultramafics and minor banded iron formations, which have been weakly metamorphosed to prehnite-pumpellyite facies. Gold mineralization is hosted by a regional shear zone within which the Mickey Doolan deposit is hosted by an intensely altered zone of carbonate-fuchsite-quartz rocks (Alexander et al., 1991). Gold in the primary ore is associated with disseminated sulphides, including pyrite, arsenopyrite and gersdorffite, this latter sulphide being the product of reaction between As-bearing ore fluids and Ni-rich ultramafic host rock. Gold production from the Mickey Doolan open pit has been mainly from deeply weathered oxide-zone ore; deeper sulphide ore has not been mined.
The ‘Ora Banda’ deposit is located ~65 km north-west of Kalgoorlie in the Norseman-Wiluna Belt. It is hosted in weakly metamorphosed basalts (lower greenschist facies) of the Kalgoorlie greenstone belt (Harrison et al., 1990). Gold lodes at Ora Banda are hosted by dextral shears which intersect the basalt sequence at high angles, and consist of fine-grained alteration zones of quartz, calcite, sericite, chlorite and disseminated sulphides, which are cut by quartz-calcite-sulphide veinlets. Sulphides are dominated by pyrite, pyrrhotite and arsenopyrite, with minor chalcopyrite and tellurides. Ora Banda sulphide ore has been blended with oxide and other ore sources before processing in a CIL plant.
The ‘Paddington’ gold deposit is located ~35 km north of Kalgoorlie in the Norseman-Wiluna Belt. It lies on the eastern limb of the north-north-west trending Bardoc-Broad Arrow syncline, which is a sequence of strongly deformed metasediments, mafic and ultramafic volcanics in a series of fault-bounded slices (Hancock et al., 1990). The main host rock to gold mineralization is a unit of granophyric quartz dolerite which cross-cuts a volcanic unit. Gold mineralization is hosted by a stockwork of quartz-carbonate-scheelite-sulphide veins within a variably altered host rock consisting of a sericite-carbonate-quartz-chlorite-green mica-pyrite-arsenopyrite assemblage. Sulphides are mainly pyrite and arsenopyrite, with pyrrhotite becoming increasingly abundant at depth. Gold is fine-grained (1–50 µm) and is strongly associated with quartz, and as inclusions and along microfractures and grain boundaries in arsenopyrite (Hancock et al., 1990). The Paddington ore is essentially free-milling, and after finer grinding to account for the fine-grained native gold, is processed in a CIL plant.
The ‘Coolgardie’ ore investigated in this study was collected from the Lindsays and Consuls areas, in the vicinity of the old Bayleys mine located ~1.5 km north of the town of Coolgardie, in the Norseman-Wiluna Belt. The succession in the Coolgardie area consists of a folded sequence of mafic and ultramafic rocks, minor sedimentary units, and felsic porphyry dykes and sills, metamorphosed to amphibolite facies grade (Swager, 1990; Knight et al., 1993). Orebodies are pipe-like bodies within the ultramafic quartz reefs, and are hosted by laminated and bucky quartz reefs. Ore minerals are pyrrhotite, pyrite and arsenopyrite that occur both within the quartz reefs and in the adjacent wall rocks. Wall rock alteration adjacent to the orebodies consists of a biotite-actinolite-chlorite assemblage, together with sulphides. The Coolgardie ores are free-milling and processed by conventional cyanidation.
The ‘Exhibition’ deposit is located ~2 km east of the town of Marvel Loch in the Southern Cross Province of the Yilgarn Craton. It is one of a series of deposits that lie along a north-northwest trending ductile shear zone within the Southern Cross greenstone belt (Rolley and Baxter, 1990). Regionally, the deposits are hosted by a sequence of mafic to ultramafic volcanics and interlayered sediments that have been metamorphosed to amphibolite facies (garnet-biotite-andalusite metapelites). Mineralization at North Exhibition is characterized by disseminated arsenopyrite and pyrrhotite in strongly silicified zones with biotite alteration, while that in the Exhibition Pipe consists of laminated quartz veins cutting intensely silicified, diopside biotite-chlorite altered wall rocks (Brabham and Johnson, 1995). The Exhibition ore is free-milling and processed by a carbon-in pulp cyanidation plant.
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Analytical methods |
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TOPABSTRACTIntroductionGeology and gold deposits...Analytical methodsProcessing of gold oresSulphide oresArsenical oresPyritic oresDiscussionClassification of refractory...AcknowledgementsReferences |
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Polished sections, polished thin sections and grain mounts of sulphide concentrates were examined using reflected light microscopy, scanning electron microscopy (SEM), electron microprobe and secondary ion mass spectroscopy analysis (SIMS). Several hundred analyses of each type (microprobe and SIMS) were carried out over a period of several years. Electron microprobe analyses for major and minor elements in sulphides were carried out using a JEOL 6400 EDS instrument at the University of Western Australia Centre for Microscopy and Microanalysis, and a CAMECA SX-50 WDS instrument at CSIRO, Perth. In both cases analyses were carried out using pure element standards and secondary sulphide standards. The operating conditions generally were 20 kV and 30 nA.
Submicroscopic gold in sulphides was determined using secondary ion mass spectrometry (SIMS) at Surface Science Western, University of Western Ontario, and subsequently AMTEL, London, Ontario. In general, the methods described in Chryssoulis et al. (1987, 1989) were followed. Calibration of the measurements was by way of mineral-specific gold-implanted standards. Primary beam spot size was 25 to 40 µm. Detection limits for submicroscopic gold varied slightly during the several analytical sessions, but were of the order of 150 ppb in pyrite and 48 ppb in arsenopyrite.
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Processing of gold ores |
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TOPABSTRACTIntroductionGeology and gold deposits...Analytical methodsProcessing of gold oresSulphide oresArsenical oresPyritic oresDiscussionClassification of refractory...AcknowledgementsReferences |
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). The Yilgarn Craton in Western Australia varies from moderately to deeply weathered, and it is common for the upper 50 to 100 m of outcropping gold deposits to be oxidized ore. In this zone all pre-existing sulphides have been destroyed, and any gold originally present as submicroscopic gold in sulphides, or gold tellurides, is remobilized and re-deposited as native gold. Silica, iron oxides and clays dominate the remaining gangue mineralogy. Some of the gold in the oxide zone is secondary, which has been remobilized by interaction of primary gold with saline groundwater solutions, in places resulting in gold nugget formation. Gold-enriched supergene zones are developed in deposits where large amounts of secondary gold accumulate near present or former water tables. Oxide ores are invariably free-milling and usually give good gold recoveries (typically 90 to 95%). However, gold in supergene zones may be more difficult to treat, often because of the presence of cyanide-soluble secondary sulphides and other minerals.
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Sulphide ores |
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TOPABSTRACTIntroductionGeology and gold deposits...Analytical methodsProcessing of gold oresSulphide oresArsenical oresPyritic oresDiscussionClassification of refractory...AcknowledgementsReferences |
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In terms of processing, sulphide ores vary from free-milling to highly refractory. A free-milling ore may be defined as one that gives >95% gold recovery under standard conditions (80% passing 75 µm, coarse gold removed by gravity, cyanide concentration about 0.05% and leach residence time about 24 to 36 h). Mildly refractory ores are here defined as those which give between 80 and 95% gold recovery, moderately refractory (50 to 80%) and highly refractory ores give <50% gold recovery. These sub-divisions are necessarily subjective; however the gold recovery figures quoted are convenient in that they reflect broad sub-divisions of treatment methods within the industry. Free-milling ores require no special treatment; ores with gold recoveries <~80% usually require roasting, bacterial oxidation or some other method to treat refractory sulphides. Ores in the 80–95% gold recovery range are problematical in that construction of refractory treatment plants often cannot be justified economically.
Many sulphide gold ores in the Yilgarn Block can be considered to be mildly refractory. However, in most cases mining companies accept less than ideal gold recovery when weighed against the capital expenditure required for a process such as roasting or bacterial oxidation, which might improve recoveries by only a few percent. In those deposits where gold recoveries fall below ~80%, further processing of the ore is usually justified where the grade and tonnage of the orebody are considered adequate for a reasonable mine life, e.g. Golden Mile ores and Lancefield (both roasters), and Wiluna (bacterial oxidation plant).
Sulphide ore types
In terms of processing, sulphide-associated ores can be sub-divided into three main types: pyritic, arsenical and antimonial. Pyritic and arsenical ores are by far the most abundant types, with antimonial ores of relatively minor importance. As with all geological classifications, the dividing lines between each type are not sharp, and there is a degree of overlap between types. For example, the Wiluna ore is mainly arsenical with a small antimonial component. However, from a processing standpoint it is convenient to discuss the ores under these groupings. The three main types can be further subdivided as follows:
Antimonial ores are difficult to process because of the presence of aurostibite, which is poorly soluble in cyanide. In addition, it appears that stibnite and other Sb-bearing sulphides interfere with cyanidation (Hedley and Tabachnick, 1968). However, antimonial ores are relatively uncommon in the ores under consideration, and will not be dealt with further. The Blue Spec mine near Nullagine is the best known of these deposits in Western Australia (Gifford, 1990). It has been treated by a batch pressure oxidation process in the past.
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Arsenical ores |
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TOPABSTRACTIntroductionGeology and gold deposits...Analytical methodsProcessing of gold oresSulphide oresArsenical oresPyritic oresDiscussionClassification of refractory...AcknowledgementsReferences |
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shows the As content of several arsenical ore samples (expressed as a proportion of the sulphide fraction) plotted against gold recovery by standard cyanidation (i.e. without attempting to recover the refractory component; from Vaughan et al., 1989; Vaughan and Corrans, 1992). It is clear that both the As contents of the ores and their gold recoveries vary over a wide range. However, the refractory behaviour of the ores is not related to increasing As content, as perhaps might be expected. Therefore, factors other than the amount of As in the ore exert a primary control on gold recovery.
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Several deposits contain pyrite + arsenopyrite + pyrrhotite assemblages, for example Lancefield, Paddington and the Coolgardie deposits. The most notable features of the Lancefield ore are strong banding and fine grain size of the sulphides (Kyin, 1995). Fine-grained shear-hosted arsenopyritepyrite-pyrrhotite-gold mineralization cuts an earlier pyrite ± pyrrhotite assemblage (Fig. 4), which was most probably deposited syngenetically in the host carbonaceous sediments (Kyin, 1995). Grain size (longest dimension) of early pyrite varies up to ~500 µm, while that of arsenopyrite averages ~10–25 µm. Additional evidence for arsenopyrite being later than pyrite includes peripheral inclusions of fine arsenopyrite in larger pyrite grains adjacent to arsenopyrite shear bands, but not in pyrite away from such bands (Kyin, 1995). Native gold is present as small inclusions in pyrite and gangue.
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).
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). However, Kyin (1995) suggests that pyrite may be a later replacement of earlier pyrrhotite. Native gold occurs as inclusions in arsenopyrite and the gangue, and also as stringers along arsenopyrite fractures and grain boundaries, similar to Paddington (Fig. 7). Importantly however, it does not occur as stringers along pyrrhotite or pyrite fractures.
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). Therefore, it is likely that arsenopyrite + pyrrhotite was the highest-temperature sulphide assemblage formed during amphibolite-facies metamorphism. This is supported by arsenopyrite compositions, which are discussed below. Arsenopyrite in the Exhibition deposit is coarse-grained (up to several mm), and most grains consist of a poikiloblastic core region, with inclusions of metamorphic silicates and sulphides, surrounded by an idioblastic rim zone (Figs 9 and 10). Poikiloblastic core zones are strongly plastically deformed, indicating deformation and recrystallization of pre-existing arsenopyrite during metamorphism (Fig. 9).
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Submicroscopic gold in arsenical ores
The concentration of submicroscopic gold in arsenopyrite and pyrite was determined by SIMS analysis, measured mainly on grain mounts of sulphide concentrates (Table 1a). The large ranges of values and high mean confidence intervals illustrate the heterogeneous distribution of submicroscopic gold in the sulphides. Numerous SIMS images confirm the heterogeneous nature of submicroscopic gold distribution, often concentrated along crystal margins (e.g. Figs 11 and 12).
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are all refractory and contain average levels of submicroscopic gold in arsenopyrite of between 64 and 276 ppm. For Lancefield, the average level of submicroscopic gold in arsenopyrite is 276 ppm, while that in the accompanying pyrite is negligible (~1 ppm). For Wiluna, Mickey Doolan and Ora Banda, submicroscopic gold in arsenopyrite varies between 64 and 102 ppm, while average levels in the accompanying pyrite vary between 20 and 43 ppm. In all cases the level of submicroscopic gold in pyrite is less than in the accompanying arsenopyrite.
Relative proportions of sulphide minerals in the four refractory concentrates are given in Table 1b (from Vaughan and Corrans, 1992) which, when combined with the concentration of submicroscopic gold in individual sulphides (Table 1a), gives the absolute distribution of gold between arsenopyrite and pyrite (Table 1c). Therefore, while both Lancefield and Mickey Doolan both have high levels of submicroscopic gold in arsenopyrite (>100 ppm), most of the submicroscopic gold at Lancefield is contained within arsenopyrite, but at Mickey Doolan it is mostly contained within pyrite, because of the large difference in pyrite/arsenopyrite ratio between the two concentrates.
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In addition to the concentrations of submicroscopic gold contained in arsenopyrite and pyrite, the type of submicroscopic gold is also important in understanding these ores. Cabri and Chryssoulis (1990) have shown how the shape of the SIMS depth profile can be used to demonstrate the type of gold present, with smooth flat profiles indicating solid-solution gold and spiky profiles indicating colloidal gold (i.e. gold microinclusions). In the present investigation, which has involved the measurement of several hundred depth profiles of individual sulphide grains, both solid-solution and colloidal gold were observed. In general, sulphides carrying high levels of submicroscopic gold (i.e. refractory) contain solid-solution type gold, while sulphides which carry low levels of gold (i.e. non-refractory) contain both solid-solution and colloidal type gold.
Arsenopyrite compositions
The variation in As/S ratios of arsenopyrites from several refractory (Lancefield, Wiluna, Ora Banda and Mickey Doolan) and free-milling ores (Paddington, Coolgardie and Exhibition) is plotted in Fig. 13. Compositions were determined by electron microprobe analysis of sulphide concentrates in polished mounts. Therefore, points analysed are a random selection of core to rim compositions for each ore. Arsenopyrite compositions fall into two distinct but overlapping fields –gold-bearing arsenopyrites (~27 to 32 mol.% As) and gold-poor arsenopyrites (~30 to 35 mol.% As). Most of the arsenopyrites are As-deficient relative to stoichiometric arsenopyrite (As = 0.333, S = 0.333). However, gold-bearing arsenopyrites are more As-deficient than gold-poor varieties. The wide range of compositions plus the degree of overlap between the two fields indicate zoning in the arsenopyrites, and also suggest that there is probably considerable disequilibrium in the arsenopyrite assemblages.
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). The average grain size of arsenopyrite in Coolgardie and Mt Martin ores (free-milling) is ~0.5–0.7 mm, while that in Lancefield and Wiluna ores (refractory) is ~20 µm.
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, including data from Vaughan et al., 1989; Vaughan and Corrans, 1992; Kyin, 1995). Gold associations in Table 3 have been subdivided into inclusions in pyrite, inclusions in arsenopyrite, gold stringers along arsenopyrite fractures and inclusions in silicates. Some of these data are from polished sections of sulphide concentrates, and thus may underestimate the amount of gold in gangue minerals (Mickey Doolan is the one deposit which contains data from sulphide concentrates only). It should be noted that data in Table 3 are not comparing the same volumes of ore or concentrate for each deposit, nor do they take into account different ore/concentrate grades. Therefore the data are not a strict comparison between ores. However, despite these limitations, Table 3 gives a reasonable indication of particulate gold distribution between the ores, especially where gold is abundant or virtually absent from particular sulphides.
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that refractory gold ores contain very little particulate gold associated with arsenopyrite. However, the accompanying pyrite contains numerous inclusions. Conversely, free-milling ores have most of their particulate gold associated with arsenopyrite, either as inclusions or stringers along fractures in arsenopyrite (cf. Figs 5 and 7). The accompanying pyrite contains very little particulate gold. Both ore types have varying amounts of particulate gold associated with silicates. In particular, the Exhibition deposit contains a large number of particulate gold grains in silicate gangue, which will be discussed later in this paper. |
Pyritic ores |
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TOPABSTRACTIntroductionGeology and gold deposits...Analytical methodsProcessing of gold oresSulphide oresArsenical oresPyritic oresDiscussionClassification of refractory...AcknowledgementsReferences |
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Submicroscopic gold in pyritic ores
The submicroscopic gold content of pyrite in two pyritic ores was measured using SIMS analysis (Table 4). The Sons of Gwalia pyrite contains small but significant amounts of submicroscopic gold, some of it in colloidal form (as indicated by SIMS depth profiles). New Celebration pyrite contains virtually no submicroscopic gold. However, the New Celebration ore is mildly to moderately refractory and employs a gravity concentration of pyrite followed by finer milling to improve gold recoveries. In this case the lower gold recovery is related entirely to fine-grained inclusions of native gold (
15 µm) locked in pyrite (Vaughan, 1991). Gold inclusions of this type are amenable to finer milling, which is employed at the New Celebration metallurgical plant.
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. The As values in Fig. 14 are an average of between 5 and 10 microprobe analyses; submicroscopic gold values are as in Tables 1a and 4. There is a clear relationship between increasing average arsenic content and increasing average submicroscopic gold content of pyrite for these deposits. However, it should be noted that the ranges for both Au and As values are very high for each point on the diagram.
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Discussion |
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TOPABSTRACTIntroductionGeology and gold deposits...Analytical methodsProcessing of gold oresSulphide oresArsenical oresPyritic oresDiscussionClassification of refractory...AcknowledgementsReferences |
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Type 1
Lower As content (~27–32 at.% As), finer-grained, containing greater average levels of submicroscopic gold (~60–270 ppm), but lacking inclusions or fracture fill of native gold.
Type 2
Higher As content (~30–35 at.% As), coarser grained, containing negligible or small average levels of submicroscopic gold (up to a few ppm), and containing inclusions and fracture fill of native gold.
The wide range of arsenopyrite compositions for individual deposits is probably due to zoning and disequilibrium in at least some of the assemblages. Using the data from Fig. 13, arsenopyrite geothermometry (Kretchmar and Scott, 1976; Sharp et al., 1985) suggests temperatures of <300 to ~425°C for Type 1 arsenopyrites (pyrite + arsenopyrite ± pyrrhotite assemblages) and ~310–560°C for Type 2 arsenopyrites (pyrite + arsenopyrite ± pyrrhotite and arsenopyrite + pyrrhotite assemblages). Depositional temperatures for specific deposits are summarized in Table 5. These temperature estimates are broadly consistent with estimates from fluid inclusion data by Cathelineau et al. (1989) who propose that Au-rich arsenopyrites crystallize at temperatures of 200±50°C and Au-poor arsenopyrites crystallize at temperatures of 300–500°C. However, the lower temperature range for Au-rich arsenopyrites estimated by Cathelineau et al. (1989) is significantly lower than that estimated from this study.
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Other estimates of ore formation temperatures for the ores in Table 5 include that by Hagemann et al. (1991), who used fluid-inclusion data to estimate temperatures of 250±50°C for the Wiluna deposit. Hronsky (1990) also used fluid-inclusion data to estimate a temperature range of 245–330°C for the Lancefield ore. These estimates for the two refractory ores are consistent with those in Table 5, and in particular, agree closely with the lower temperature range estimated by Cathelineau et al. (1989). Thermodynamic data were used to estimate a temperature range of 280–350°C for the Paddington deposit (quoted in Knight et al., 1996), which is closer to the lower end of the temperature range estimated from arsenopyrite compositions (Table 5). Finally, garnet-biotite geothermometry was used to estimate peak metamorphic temperatures of 590±40°C in the Marvel Loch area, in the same area as the Exhibition deposit (quoted in Dalstra et al., 1999), which is in reasonably good agreement with the temperature range estimated using arsenopyrite compositions (Table 5).
Pyrite is an essential component of both arsenical and pyritic gold ores. It is concluded that the submicroscopic gold content of pyrite is directly proportional to the As content, for both ore types (cf. Fig. 14). For pyritic ores such as New Celebration, which contain virtually no arsenic, it appears that gold never entered the crystal lattice of pyrite, but rather was deposited as small inclusions in pyrite, the majority of which are <~50 µm in size (Vaughan, 1991). Where the ore contains small amounts of arsenic, e.g. at Sons of Gwalia, pyrite contains small concentrations of submicroscopic gold. In these deposits it appears that there was insufficient arsenic in the ore fluid to form significant amounts of arsenopyrite, but rather entered the pyrite lattice. As a consequence, small amounts of submicroscopic gold were incorporated into the pyrite crystal lattice, but most was deposited as metallic gold inclusions in pyrite or gangue.
Where the ore fluid carries greater concentrations of As, both pyrite and arsenopyrite are formed. Submicroscopic gold is incorporated into both arsenopyrite and pyrite, but only if the pyrite carries significant As (e.g. Wiluna, Mickey Doolan and Ora Banda). At Lancefield, pyrite has very low As, and consequently only minor amounts of submicroscopic gold. It is not clear what controls the level of As in pyrite at Lancefield, but it is probable that fine arsenopyrite (and hence As) is significantly later in the paragenetic sequence than the bulk of the pyrite (Kyin, 1995).
Evolution of the gold ores is interpreted in a similar way to that described by Mumin et al. (1994). Lower-temperature ores, such as Wiluna, Mickey Doolan and Ora Banda are interpreted to form at shallower depths and lower temperatures (<300 to ~420°C), in sub-greenschist- to lower greenschist-facies rocks. At Wiluna, which appears to have the lowest formation temperature, fine-grained arsenopyrite and arsenical pyrite both contain high levels of submicroscopic gold (91 and 43 ppm, respectively). Arsenopyrite at Wiluna contains very few particulate gold inclusions, while arsenical pyrite contains numerous small gold inclusions. The amount of submicroscopic gold in pyrite is controlled by the amount of arsenic in pyrite, which is dependent on the partitioning of arsenic between arsenopyrite and pyrite during sulphide formation. Fleet and Mumin (1997) have shown that for Carlin-type ores, if pyrite contains enough arsenic, then it can carry very high concentrations of submicroscopic gold.
Recrystallization of gold ores deposited under lower-greenschist conditions probably began shortly after deposition, during structural evolution and burial to deeper levels in the crust, as proposed by Mumin et al. (1994). Ores such as Paddington and Coolgardie began recrystallization at temperatures of <400°C, extending to ~550°C and above. Both arsenopyrite and pyrite were recrystallized into much larger grains, at the same time expelling submicroscopic gold from their crystal lattices. There is evidence from the sulphide assemblages that pyrrhotite began to form by reaction between pyrite and arsenopyrite, leading to the formation of more As-rich arsenopyrite.
The expulsion of submicroscopic gold from the crystal lattices of arsenopyrite and pyrite resulted in the deposition of native gold along arsenopyrite microfractures and grain boundaries, as inclusions in arsenopyrite and, progressively, into the gangue minerals. At both Paddington and Coolgardie, pyrite in the ores contains very few inclusions of native gold (Table 3), and very low levels of submicroscopic gold (Table 1a), suggesting that recrystallization of pyrite has resulted in almost complete remobilization of gold from pyrite into gangue minerals. It is rare to see stringers of native gold along microfractures in pyrite, a relatively common texture in coarse-grained arsenopyrite. Pyrite in lower-temperature ores (Wiluna, Ora Banda and Sons of Gwalia) contains both submicroscopic gold and gold inclusions (Table 3), thus it appears that gold is expelled from pyrite at a faster rate than arsenopyrite, during early recrystallization.
In contrast to pyrite, arsenopyrite in Paddington and Coolgardie ores contains both inclusions of native gold plus native gold stringers along microfractures and grain boundaries (Table 3). Paddington and Coolgardie arsenopyrites contain very low levels of submicroscopic gold, but a higher proportion of this gold appears to be in colloidal form, as indicated from SIMS depth profiles. Taken together, this is interpreted as evidence of the migration of submicroscopic gold from the arsenopyrite lattice into inclusions, microfractures and along grain boundaries of arsenopyrite. Colloidal gold in arsenopyrite is interpreted to be submicroscopic gold in the process of migration from the crystal lattice, before aggregation into visible inclusions. Unlike pyrite, migration of gold from arsenopyrite has not proceeded to complete expulsion of native gold from the sulphide grains.
The Exhibition deposit represents the highest temperatures of recrystallization for the deposits investigated in this study (~490–560°C). According to the phase relations of Kretchmar and Scott (1976), arsenopyrite + pyrrhotite is the stable assemblage at temperatures above 490°C. The sulphide assemblage at Exhibition is arsenopyrite + pyrrhotite + pyrite, but there is evidence that pyrite is formed by later hydrothermal, lower-temperature replacement of pyrrhotite (Kyin, 1995). In addition, all of the arsenopyrite compositions from Exhibition plot on the arsenopyrite + pyrrhotite tie line of Kretchmar and Scott (1976). Therefore, it is concluded that arsenopyrite + pyrrhotite was the highest-temperature assemblage formed during evolution of the Exhibition deposit, and pyrite later replaced some of the pyrrhotite at lower temperatures.
The most notable feature of gold deportment at Exhibition is the very large number of native gold grains in the silicate gangue (Table 3). A small number of gold grains are located as inclusions in arsenopyrite or along arsenopyrite microfractures (Fig. 10). Native gold was not observed associated with pyrrhotite or pyrite, nor is there any submicroscopic gold in sulphides (Table 1). This deportment of gold is interpreted as evidence of almost complete expulsion of gold from sulphides into the silicate gangue, at higher recrystallization temperatures than the Coolgardie and Paddington deposits. It is consistent with the conclusion of Mumin et al. (1994) that with continuing recrystallization of sulphides, gold is ultimately expelled into the gangue, where it has a long residence time.
The above interpretation for the evolution of gold ores differs from those of OberthüOberthür et al. (1997) and Genkin et al. (1998) for similar gold ores in Ghana and Siberia. Oberthür et al. (1997) propose that both free gold and submicroscopic gold are deposited from evolving hydrothermal gold-bearing fluids, and that free gold does not result from recrystallization of submicroscopic gold in sulphides. Genkin et al. (1998) suggest two stages of gold deposition: an early stage of submicroscopic gold in arsenopyrite and pyrite, followed by a later stage of native gold in veins or as fracture fillings and overgrowths on gold-bearing arsenopyrites. That is, arsenopyrites from Siberian deposits contain both submicroscopic gold and native gold in fractures and as overgrowths on arsenopyrite.
Evidence for the interpretation presented here is that arsenopyrite in the Western Australian ores contains either submicroscopic gold or gold inclusions and fracture fillings, but not both to any significant degree. However, as noted previously, there is evidence of small amounts of remnant submicroscopic gold, some of it in colloidal form, in both Paddington and Coolgardie arsenopyrites. In addition, pyrite and pyrrhotite in higher-temperature ores such as Paddington and Coolgardie do not contain inclusions or fracture fill of native gold. If native gold were deposited as a separate high-temperature event, it should occupy microfractures and occur as inclusions in pyrite and pyrrhotite, in addition to arsenopyrite. But this is not the case, and there appears to be a clear and logical evolution from submicroscopic gold in fine-grained arsenopyrite and, to a lesser extent, pyrite at sub-greenschist- to greenschist-facies temperatures, followed by progressive recrystallization and coarsening of sulphide grains, together with progressive expulsion of submicroscopic gold from the sulphide lattices. Native gold, which forms as a result of this expulsion, deposits first in nearby fractures and grain boundaries in arsenopyrite, and to a lesser extent pyrite, and is finally expelled completely into the silicates (Fig. 15).
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In the ore deposits under investigation in this study, the submicroscopic gold content of arsenopyrite and pyrite is considerably less than reported for synthetic sulphides. The maximum average submicroscopic gold content of arsenopyrite is 276 ppm at Lancefield, and the maximum single SIMS analysis on one grain is 1069 ppm, also at Lancefield. This is broadly similar to levels measured in Carlin arsenopyrites (Fleet and Mumin, 1997). The maximum average submicroscopic gold content of pyrite is 43 ppm at Wiluna, and the maximum single SIMS analysis on one grain is 627 ppm, at Mickey Doolan. This is much less than reported in Carlin pyrites (Fleet and Mumin, 1997). In all cases, arsenopyrites in this study have higher average submicroscopic gold contents than co-existing pyrite.
Arsenopyrite compositions from the Western Australian ores are almost all As-deficient with respect to stoichiometric FeAsS (As = 0.333 and S = 0.333 on Fig. 13). However, arsenopyrites carrying submicroscopic gold are more As-deficient than those which do not carry gold. This is in marked contrast to the conclusions of Fleet and Mumin (1997) who found that arsenopyrites from Carlin were all As-rich compared to stoichiometric FeAsS. However, it is consistent with observations by Genkin et al. (1998) that gold-bearing arsenopyrites from Siberia are As-deficient. All of the arsenopyrites from Western Australian gold deposits are also very slightly Fe-deficient, consistent with observations from other localities (Cathelineau et al., 1989; Fleet and Mumin, 1997).
The submicroscopic gold content of pyrite from Western Australian gold ores is directly related to their As content, which ranges up to a maximum of ~2.5 wt.% (Fig. 14). The gold content of pyrite crystals is directly related to As content (Fig. 14). This is also consistent with observations from other deposits (Fleet et al., 1993; Fleet and Mumin, 1997). However, As contents of pyrite from Western Australian gold ores do not approach maximum levels of 9.3 wt.% recorded in synthetic pyrite (Fleet and Mumin, 1997).
The reasons for the quite different patterns of incorporation of gold in Carlin-type epithermal ores (very low T) and Archaean-type mesothermal ores (sub-greenschist to amphibolite Ts) may reflect the different formation temperatures. Fleet and Mumin (1997) report a range of sulphide compositions between FeAsS and FeS2 for low-temperature sulphide minerals at Carlin, suggesting that, at low temperatures, sulphide compositions may be much more variable than at higher temperatures. Elements such as As and Au may be more easily incorporated into sulphide structures by adsorption onto high surface area, fine-grained sulphides, and this may explain the very high levels of arsenic and submicroscopic gold in lower-temperature Carlin-type ores. At higher temperatures (lower greenschist) the grain size of pyrite and arsenopyrite increases, their compositions approach stoichiometry and some submicroscopic gold is exsolved, leading to the lower gold concentrations found in sulphides of mesothermal ores. This is not to suggest that mesothermal ores are higher-temperature analogues of Carlin-type ores, but it could imply that initial deposition of at least some mesothermal ores proceeded via a lower temperature, finer-grained precursor ore type.
Several other observations concerning gold in epithermal and mesothermal ores can be explained if the above interpretation is correct. Firstly, a high proportion of gold in Carlin-type ores is contained in fine-grained sulphides in submicroscopic form, and relatively little is present as native gold. Secondly, pyrite in Carlin-type ores contains much more submicroscopic gold than arsenopyrite (Fleet and Mumin, 1997). In Wiluna-type ores (at the lower end of the temperature range for mesothermal ores), pyrite contains some submicroscopic gold, but much gold is present as inclusions in pyrite. This latter gold may have exsolved from finer-grained pyrite during recrystallization at relatively low temperatures. On the other hand, arsenopyrite at Wiluna still retains high levels of submicroscopic gold, supporting the contention that arsenopyrite is a more refractory sulphide.
The mechanism of incorporation of submicroscopic gold into sulphides is still not well understood. Fleet and Mumin (1997) propose that gold is incorporated into sulphides by chemisorption onto As-rich, Fe-deficient surfaces of pyrite, marcasite and arsenopyrite, where it exists as a metastable solid solution. Simon et al. (1999a,b) conclude that gold is present in arsenian pyrite as Au0 microinclusions and Au1+ in the lattice, with Au0 concentrated in finer-grained, lower-temperature pyrite. Cabri et al. (2000) conclude that gold is present in arsenopyrite in two forms: covalent Au1+ and Au0 microparticles. The present study provides some support for the existence of two forms of submicroscopic gold in sulphides. The presence of colloidal gold in non-refractory, higher-temperature ores from Western Australia (as indicated by SIMS depth profiles) possibly indicates the presence of Au0 micro-particles, whereas non-colloidal gold in the lower-temperature ores may correspond to Au1+ in the lattices.
Of the four gold deposits studied by Cabri et al. (2000), only the Olympiada deposit is hosted by amphibolite-facies rocks (Genkin et al., 1998); the other three (Sentachan, São Bento and Sheba) are hosted by greenschist- or sub-greenschist-facies rocks (Genkin et al., 1998; Thorman et al., 2001; Wagen and Wiegard, 1986). Olympiada is the only deposit that contains Au0 microparticles. It is possible that submicroscopic gold is incorporated into arsenopyrite as covalent Au1+ at sub-greenschist/greenschist-facies temperatures, but is progressively expelled from the arsenopyrite lattice as Au0 microparticles at upper greenschist- to amphibolite-facies temperatures, which then aggregate and recrystallize as larger inclusions of native gold and stringers along microfractures in arsenopyrite, as observed in the Western Australian ores.
The studies by Simon et al. (1999a,b) find both types of gold in Carlin-type deposits, although Au0 microparticles in arsenian pyrite are associated with lower-temperature, finer-grained ores, while Au1+ occurs in higher-temperature, coarser-grained pyrite. This appears to be the opposite of what has been observed for arsenopyrite, although the two deposit types (mesothermal and Carlin) are fundamentally different in form and origin. The present investigation indicates that submicroscopic gold appears to be exsolved much more rapidly, and at lower temperatures, from pyrite than from arsenopyrite in the Western Australian ores.
With regard to the timing of gold mineralization, this investigation provides some evidence for pre-peak metamorphic deposition of gold at shallower crustal levels, followed by deeper burial and recrystallization and remobilization of gold at higher metamorphic temperatures. That is, native gold in some deposits that may appear to be texturally late, possibly could be remobilization of submicroscopic gold in pre-peak metamorphic ores.
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Classification of refractory ores |
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TOPABSTRACTIntroductionGeology and gold deposits...Analytical methodsProcessing of gold oresSulphide oresArsenical oresPyritic oresDiscussionClassification of refractory...AcknowledgementsReferences |
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). The ores are sub-divided into pyritic, arsenical and antimonial types, and in broad terms they become more refractory from the top of the table to the bottom (with the exception of Type 2A ores which are essentially free-milling). Many pyritic ores are free-milling, and those that are refractory are only mildly or at most moderately refractory. This is related to the generally lower concentrations of submicroscopic gold in pyrite or in the case of Golden Mile ores, the presence of gold tellurides. Arsenical ores may vary from free-milling to highly refractory, dependent on the amount of submicroscopic gold in arsenopyrite, which is in turn related to the degree of recrystallization of the ore. Gold ores that contain high levels of Sb are often highly refractory because of the presence of aurostibite and cyanide-soluble antimony. Therefore, the ease of processing of pyritic and arsenical sulphide gold ores essentially is related to thermal maturity of the ores, and demonstrates the relationship between natural and synthetic processing of gold ores, where lower-temperature refractory ores require roasting (or some other oxidation process) to simulate metamorphism in higher-temperature, free-milling ores gold from the latter.
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Acknowledgements |
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TOPABSTRACTIntroductionGeology and gold deposits...Analytical methodsProcessing of gold oresSulphide oresArsenical oresPyritic oresDiscussionClassification of refractory...AcknowledgementsReferences |
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TOPABSTRACTIntroductionGeology and gold deposits...Analytical methodsProcessing of gold oresSulphide oresArsenical oresPyritic oresDiscussionClassification of refractory...AcknowledgementsReferences |
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[Manuscript received 4 April 2003: revised 10 October 2003]
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References |
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TOPABSTRACTIntroductionGeology and gold deposits...Analytical methodsProcessing of gold oresSulphide oresArsenical oresPyritic oresDiscussionClassification of refractory...AcknowledgementsReferences |
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