Mineralogical Magazine; April 2004; v. 68; no. 2;
p. 255-277; DOI: 10.1180/0026461046820186
© 2004 Mineralogical Society of Great Britain and Ireland
Refractory gold ores in Archaean greenstones, Western Australia: mineralogy, gold paragenesis, metallurgical characterization and classification
J. P. Vaughan* and
A. Kyin
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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Mesothermal gold ores in the Archaean Yilgarn Craton of Western Australia are dominated by a pyrite ± arsenopyrite ± pyrrhotite sulphide assemblage. Many of these ores are refractory to varying degrees and require treatment by roasting, bacterial oxidation or finer milling. The most common sulphide ore types can be sub-divided broadly into pyritic (pyrite±pyrrhotite) and arsenical types (pyrite+arsenopyrite±pyrrhotite). Arsenical ores vary from highly refractory to free-milling. Arsenopyrite in highly refractory ores is finer grained, As-deficient (27–32.5 at.% As), contains high average concentrations of submicroscopic gold (60–270 ppm), but does not contain inclusions of particulate gold. Arsenopyrite in free-milling ores is coarser grained, less As-deficient to slightly As-rich (30–35 at.% As), contains low or negligible concentrations of submicroscopic gold, but contains inclusions and fracture fillings of particulate gold. In some refractory arsenical ores, pyrite also contains moderately high levels of submicroscopic gold (20–40 ppm), the concentration of which is directly related to As content of the pyrite.
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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THE Archaean Yilgarn Craton in southern Western Australia contains hundreds of gold deposits ranging in size from small workings to the giant Golden Mile deposits near Kalgoorlie, the regional centre of the Eastern Goldfields province (Fig. 1
). 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.
Gold ores of the Yilgarn Craton are characterized in their upper parts by deep weathering and the development of extensive oxide and super-gene deposits. From a metallurgical perspective, oxide ores are invariably free-milling and usually give good gold recoveries (typically 90–95% or higher). Supergene gold ores are sometimes more difficult to treat. Below the oxide and supergene ores, primary gold ores are mainly sulphide-associated, and some are refractory to varying degrees. Gold recoveries for these refractory ores are typically in the range 30–80%. As a consequence several of these ores are, or have been treated by roasting, bacterial oxidation or finer milling to maximize gold recovery.
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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The Yilgarn Craton is a large granite-greenstone terrain of late Archaean age located in the southern half of Western Australia (Fig. 1
). 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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Samples collected from the gold ore deposits included drill-core material, milled ore samples and sulphide concentrates. Polished sections of sulphide concentrates were used in determining the compositions of many of the sulphide minerals. This was mainly because many of the earlier projects carried out as precursors to this paper were to investigate the refractory behaviour of sulphide ores (e.g. Vaughan et al., 1989; Vaughan and Corrans, 1992). Sulphide concentrates were often collected from processing plants as run-of-mine ore. Consequently they usually represent samples of mixed ore from the one deposit, rather than ore from a single hand specimen or piece of drill core. Statistically, analyses of milled sulphide fragments would represent the range of both core and rim compositions of individual grains, but with no indication of which end of the range was the core or rim composition.
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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On a broad scale, Archaean gold ores in Western Australia can be divided into oxide and sulphide ores (Fig. 2). 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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Below the weathered zone, almost all of the larger gold deposits in the Yilgarn Craton are sulphide-associated to varying degrees. Typically, sulphides constitute ~1–10 wt.% of the mined ore. One or more of pyrite, arsenopyrite and pyrrhotite dominate most deposits. Pyrite is almost always present, arsenopyrite is common and significant amounts of pyrrhotite are present in some deposits. Other sulphides are usually present in only minor amounts. Of particular note is that these deposits are Cu-poor, in contrast to some Proterozoic and younger deposits that contain high levels of copper sulphide minerals. A few ores contain magnetite, with which some gold may be associated.
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:
- Pyritic pyrite ± pyrrhotite + native gold
pyrite ± pyrrhotite + gold tellurides + native gold
- Arsenical
pyrite + arsenopyrite ± pyrrhotite + native gold pyrite + arsenopyrite + gersdorffite ± pyrrhotite + native gold
pyrrhotite + arsenopyrite + loellingite + native gold
- Antimonial
pyrite + stibnite + aurostibite + native gold (±other Sb sulphides)
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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Arsenical ores are common in the Yilgarn Craton, and in terms of processing they vary from free-milling to highly refractory. Their sulphide mineralogy is dominated by both arsenopyrite and pyrite (± pyrrhotite ± loellingite). One deposit investigated, Mickey Doolan, also contains abundant gersdorffite, which is the result of reaction between the As-bearing ore fluid and Ni-rich host rocks. Figure 3 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.
Sulphide assemblages in arsenical ores
The simplest sulphide assemblage in arsenical ores is pyrite + arsenopyrite, e.g. the Wiluna ore. Sulphides here are relatively fine-grained (arsenopyrite 5–50 µm and pyrite 10–250 µm) and are disseminated throughout the host metabasalt. Euhedral arsenopyrite crystals occur intergrown with pyrite, and peripheral inclusions of arsenopyrite in pyrite are common, as are inclusions of pyrite in arsenopyrite. Kyin (1995) concludes that on the basis of textural relationships, arsenopyrite, pyrite, minor chalcopyrite and gold were deposited contemporaneously during the main ore stage at Wiluna. This was followed by a second stage, lower-temperature event that is characterized by quartz-stibnite-native gold veining.
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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FIG. 4. Bands of fine-grained rhomb-shaped arsenopyrite (asp) cutting coarser-grained pyrite (py), Lancefield ore. Reflected light (x100).
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The Paddington and Coolgardie deposits are different from the Lancefield one in that arsenopyrite is much coarser grained (mm size) and there is no banding in the mafic host rocks. The sulphide assemblage at Paddington is pyrite + arsenopyrite + pyrrhotite, although in ore from near surface to ~180 m depth, pyrrhotite is replaced by secondary (porous) pyrite (Hancock et al., 1990). Arsenopyrite occurs as medium to coarse-grained euhedral rhombs (0.2 to 4 mm), either as single crystals or small aggregates of a few grains (Bacigalupo-Rose, 1992). Primary pyrite is medium- to coarse-grained (up to 2 mm) and intergrown with arsenopyrite. Most of the pyrrhotite above 180 m has been converted to secondary pyrite, although some pyrrhotite survives as rounded inclusions in arsenopyrite (Bacigalupo-Rose, 1992). Native gold is strongly associated with arsenopyrite and quartz (Hancock et al., 1990). It occurs as inclusions in arsenopyrite, or in microfractures and along grain boundaries in arsenopyrite (Fig. 5).
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FIG. 5. Native gold in arsenopyrite microfracture, Paddington ore. Width of gold is ~1 µm. Reflected light (x500).
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At Coolgardie, paragenetic relationships between the sulphides are more equivocal. Textural evidence indicates that pyrite, arsenopyrite and pyrrhotite are in equilibrium (Fig. 6). 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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FIG. 6. Co-existing pyrite (py), arsenopyrite (asp) and pyrrhotite (po), Coolgardie ore. Reflected light (x100).
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FIG. 7. Native gold in arsenopyrite (asp) microfractures, Coolgardie ore. Adjacent pyrrhotite (po) microfractures contain no gold. Reflected light (x200).
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The sulphide assemblage at the Exhibition deposit is arsenopyrite + pyrrhotite + pyrite + marcasite. However, there is textural evidence that pyrite + marcasite is a later, lower-temperature, hydrothermal replacement of earlier pyrrhotite (Kyin, 1995). This evidence includes the presence of rounded inclusions of pyrite + marcasite aggregates in large arsenopyrite crystals, very similar to the rounded shape of pyrrhotite grains that are included in the same arsenopyrites (Fig. 8). 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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FIG. 8. Large arsenopyrite crystal containing rounded inclusions of pyrrhotite (po) and pyrite plus marcasite aggregates (py + mc). Exhibition deposit. Reflected light (x50).
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FIG. 9. Arsenopyrite crystal with deformed, poikiloblastic core zone and clear rim overgrowth. Exhibition deposit. Reflected light (x50).
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FIG. 10. Detail of arsenopyrite crystal in Fig. 9, showing inclusions of idioblastic metamorphic silicates and chalcopyrite (cp), and rounded gold grains (Au). Exhibition deposit. Reflected light (x200).
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The assemblage pyrrhotite+loellingite+arsenopyrite is not dealt with here, but has been the subject of several recent publications (Barnicoat et al., 1991; Neumayr et al., 1993; Lapointe and Chown, 1993; Hagemann et al., 1998; Tomkins and Mavrogenes, 2001). It appears that loellingite incorporates gold into its crystal lattice at moderate to high temperatures, which is later released as particulate gold during the replacement of loellingite by arsenopyrite at lower temperatures.
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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FIG. 11. SIMS images of pyrite crystal, Wiluna gold deposit, Western Australia. (a) As distribution, concentrated in rim zone. (b) Submicroscopic Au distribution, concentrated in similar, but not exactly the same, rim zone as arsenic.
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FIG. 12. SIMS image of submicroscopic Au distribution, arsenopyrite crystal, Wiluna deposit, Western Australia. Gold is concentrated in rim zone.
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The Paddington, Coolgardie and Exhibition deposits are free-milling, and both arsenopyrite and pyrite in these ores contain only small amounts (<1 to 4 ppm) of submicroscopic gold. The other four deposits in Table 1a 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.
In summary, the concentration of submicroscopic gold in both arsenopyrite and pyrite in free-milling arsenical ores is very low. In refractory arsenical ores, arsenopyrite carries high concentrations of submicroscopic gold, and in addition, the accompanying pyrite mostly carries moderate concentrations of submicroscopic gold. Where co-existing arsenopyrite and pyrite both carry submicroscopic gold, arsenopyrite contains higher concentrations than pyrite, a relationship that has been previously noted in other gold ores (e.g. Tarnocai et al., 1997).
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.
Sulphide grain sizes
Visual inspection of free-milling and refractory arsenical ores both in hand specimen and in polished sections indicates that arsenopyrite in free-milling ores is generally coarse-grained (mm to cm size), while arsenopyrite in refractory ores is fine-grained (sub mm size). Kyin (1995) quantified this by measuring grain sizes (maximum dimension) of sulphides in several arsenical ores (Table 2). 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.
Particulate gold
In addition to submicroscopic gold in sulphides, all of the ores under investigation contain particulate native gold (or electrum). The location of this gold was determined optically for several ores (Table 3, 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.
It is apparent from Table 3 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.
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Pyritic ores
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Pyritic ores are the most common gold ore type in the Yilgarn Craton. A pyritic ore is defined here as one which contains no arsenopyrite (or only traces at most). Their sulphide mineralogy is dominated by pyrite (± pyrrhotite). With few exceptions they are free-milling or mildly refractory and easier to treat metallurgically than arsenical gold ores. The Golden Mile pyritic ores near Kalgoorlie are important exceptions in that they are moderately refractory. However, the refractory nature of Golden Mile ores is caused by abundant gold-bearing tellurides in the ores, rather than submicroscopic gold (Clout et al., 1990; Vaughan et al., 1997). The Golden Mile deposits are the only significant telluride-bearing ores in the deposits under consideration. Therefore, the section that follows is concerned with pyritic ores except those of the Golden Mile.
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.
Pyrite composition
Pyrite is an essential component of both arsenical and pyritic gold ores. The composition of pyrite from several arsenical and pyritic ores was determined by electron microprobe. The relationship between As and submicroscopic gold in pyrite for five deposits (New Celebration, Lancefield, Ora Banda, Mickey Doolan and Wiluna) is shown in Fig. 14. 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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For arsenical ores, this investigation leads to the conclusion that there are two end-member types of arsenopyrite present in the ores under consideration, with some overlap between the two:
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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TABLE 5. Temperature ranges for deposition of gold ores, calculated from arsenopyrite compositions (Kretchmar and Scott, 1976).
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Cathelineau et al. (1989) rejected the use of the Kretschmar and Scott (1976) geothermometer because of large temperature ranges calculated from a single arsenopyrite crystal, which they suggested might be due to disequilibrium between arsenopyrite and other sulphides. Sharp et al. (1985) concluded that the arsenopyrite geothermometer could be applied to ore deposits at metamorphic conditions of greenschist to lower amphibolite facies, but that some lower-temperature hydrothermal deposits gave anomalously high formation temperatures. It is therefore possible that the lower temperature range determined for refractory arsenopyrites (Type 1) is too high (<300 to ~425°C), and may be closer to that estimated by Cathelineau et al. (1989).
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 <
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ABSTRACT
Introduction
