The Golden Mile Deposit in Kalgoorlie, W Australia The Kalgoorlie gold field contains structurally controlled, epigenetic gold deposits hosted by mafic rocks in the Archean Yilgarn craton of Western Australia. Its giant size has prompted much interest in the processes that led to its formation, particularly of the Golden Mile mineralization, which hosts over 70 percent of the gold in the Kalgoorlie gold field.

Geology Adjacent to the city of Kalgoorlie, the giant Golden Mile deposit is located approximately 595km east of Perth, Western Australia. Gold was first discovered in Kalgoorlie by Patrick Hannan in 1893, with underground mining commencing shortly after. Current gold production is managed by Kalgoorlie Consolidated Gold Mines, a joint venture between Normandy Mining Limited and Homestake Gold of Australia Limited. For the financial year ending June 30 1999, total gold production from the Golden Mile "Superpit" was 586,918 ounces, with a head grade of 2.33 g/t. The Golden Mile is Australia’s largest gold producer, and has a proved and probable reserve of 11.65 million Au ounces.

Wallaby

BambooCreek

FIG. 1. Simplified geological map of Western Australia showing greenstone belts and gold deposits

Norseman-WilunaGreenstoneBelt

Stratigraphy

The upper greenschist lower amphibolite facies rocks of the Golden Mile deposit are hosted by the Archean granite-greenstone Yilgarn craton.

Stratigraphy at the Golden Mile consists of the Paringa Basalt (2690±5 Ma), the Golden Mile Dolerite (2675±2 Ma), and the felsic volcaniclastic and sedimentary Black Flag Beds (2680±5 Ma). Multiple porphyries and lamprophyres have intruded the greenstones and sediments at the Golden Mile. 

Gold mineralisation is dominantly hosted by the Golden Mile Dolerite, a internally differentiated tholeiitic sill, and the high MgO, Paringa Basalt. Some porphyries are cut by mineralised zones, however gold tenor is significantly lower than the greenstones.

Nth wall Horseshoe pit.  Golden Mile dolerite and graphitic shale in the Black Flag Beds (BFB).  The Golden Mile fault (GMF) is located on the eastern contact of the Black Flag Beds and Golden Mile dolerite.  An albite porphyry is present to the east of the fault.  Oxidised stope backfill is visible in the pit wall to the west of the Black Flag Beds, indicating the location of number Three Western lode.  Further west New lode is cut by the Drysdale stockwork.

Au Mineralisation

Three styles of gold mineralisation are present at the Golden Mile, representing at least two gold mineralisation events.

Fimiston style shear zone hosted lode deposits are economically the most important source of gold at the Golden Mile. Gold occurs in solid solution within fine disseminated pyrite, as coarse "free" gold, and as telluride species with Ag, Pb, Sb and Hg.

Alteration minerals associated with Fimiston lodes include, quartz-pyrite-ankerite-sericite-albite-tourmaline. 

Highly sheared Fimiston style mineralisation with abundant "free" gold, No 3 Western lode. Five cent coin for scale.

"Greenleader" mineralisation is characterised by the presence of green vanadium muscovite, and contains abundant "free" gold and tellurides. The Oroya shoot is the best example of greenleader mineralisation, producing 62t of gold.

Greenleader mineralisation, Oroya cutback -130m level. Pen lid for scale.

Quartz stockwork gold deposits are present within and around the Golden mile, mostly confined to unit 8 of the Golden Mile Dolerite due to rheological and chemical controls. Stockwork mineralisation clearly cuts Fimiston style lodes, indicating that the stockwork deposits were formed by a younger gold mineralisation event.

Stockwork vein mineral assemblages contain quartz-albite and carbonates including scheelite. Wall rock alteration includes pyrite-pyrrhotite- ankerite-sericite-siderite. The Mt Charlotte underground mine adjacent to the Golden Mile is the only stockwork deposit currently in operation.

"Free" gold in a stockwork quartz vein.  Fe-carbonate wall rock alternation.  Drysdale stockwork, -160m level.  Pen for scale.

Mining

In 1903, 10 years after the discovery of gold at Kalgoorlie peak annual production was reached with 1 225 700 Oz (38.124 t) of gold produced. Over 30 shafts were operating at this time. In May 1989 Kalgoorlie Consolidated Gold Mines was formed with the intention of operating a large single "super" pit at the Golden Mile. Mining contractors Roche Bros were hired for earth moving and drill and blast operations. By mid 1991 underground mining at the Golden Mile had ceased due to the expanding Fimiston open pit. At the time of closure, some of the shafts had reached depths greater than 1,300m below surface, and over 1181 t of gold had been recovered. Chaffers shaft is still used as an exploration platform for diamond drill hole delineation of the ore body.

TELLURIDE MINERALOGY OF THE GOLDEN MILE DEPOSIT, KALGOORLIE, WESTERN AUSTRALIA

The Golden Mile is a giant orogenic lode gold deposit located within the Norseman-Wiluna Belt in the Yilgarn Craton, Western Australia. It is a unique deposit because of its size (having produced greater than 1200 tonnes of Au) and in that telluride mineralization is responsible for 15 to 20% of gold production. Gold mineralization is hosted primarily by Archean-aged dolerites and basalts that have been metamorphosed to the greenschist facies. This mineralization occurs within hundreds of shear zones as auriferous and telluride-bearing lodes, which have been classified into three structural types based on orientation (main (trending NNW), caunter (trending NW) and cross lodes (trending NE)). Three distinct styles of mineralization are present within the area. The older Fimiston (characterized by main, caunter and cross lodes) and Oroya (vanadium and telluride rich cross lodes) styles are overprinted by younger Mount Charlotte (quartz vein stockwork) style mineralization.

Native tellurium and sixteen tellurides (calaverite (AuTe2), krennerite ((Au,Ag)Te2), sylvanite ((Ag,Au)Te4)), montbrayite ((Au,Sb)2Te3), petzite (Ag3AuTe2), coloradoite (HgTe), hessite (Ag2Te), stuetzite (Ag5-xTe3), altaite (PbTe), tellurantimony (Sb2Te3), nagyagite (Au(Pb,Sb,Fe)8(S,Te)11), melonite (NiTe2), weissite (Cu2-xTe), tetradymite (Bi2Te2S), rickardite (Cu4Te3), and one poorly identified gold-arsenic telluride occur in the Golden Mile. Calaverite, petzite, coloradoite, altaite, and native gold are commonly found throughout the Golden Mile. Tellurantimony, melonite, hessite, stuetzite, krennerite and sylvanite are rarely found in the main and caunter lodes. Additionally, hessite is found in trace amounts in the cross lodes and montbrayite is found in trace amounts in the main lodes. Krennerite is restricted to the central part of the deposit whereas montbrayite has been found only along the margins of the Golden Mile. First generation tellurides in the Fimiston and green leader ores formed at logfTe2= -11.4 and logfS2= -12.6, approximately (assuming a T of ore formation of 300oC). Ultimately the results obtained from this study will be useful as potential guides to ore on mine, local, and regional scales.

The Fimiston Open Pit, colloquially known as the Super Pit, is Australia's largest open cut gold mine. The Super Pit is located off the Goldfields Highway on the south-east edge of Kalgoorlie-Boulder, W Australia.

Most of the gold mined in the Super Pit occurs within ore lodes formed by ancient shears in a rock unit called the Golden Mile Dolerite. As a result, the area is known as the Golden Mile even though the lodes occur in an area over 2 km in width and 1 km in depth. This renowned Kalgoorlie-Boulder land mark will eventually stretch 3.8 km long, 1.4 km wide and reach a depth over 500 m. Since 1893, when Irishman Paddy Hannan first made his famous discovery, more than 50 million ounces (1,550 t) of gold have been harvested from the Golden Mile.

Super Pit gold mine

Originally consisting of a number of small underground mines, consolidation into a single open pit mine was attempted by Alan Bond, but he was unable to complete the takeover. The Super Pit was eventually created in 1989 by Kalgoorlie Consolidated Gold Mines Pty Ltd, a joint venture between Normandy Australia and Homestake Gold of Australia Limited.

As of 2005 Kalgoorlie Consolidate Gold Mines Pty Ltd is owned by the Australian subsidiaries of the Barrick Gold Corporation and Newmont Mining Corporation and produces up to 900,000 ounces (28 t) of gold every year.

The Age of the Giant Golden Mile Deposit, Kalgoorlie

Open-pit and underground mines in the Golden Mile at Kalgoorlie, Western Australia, have produced more than 1,475 metric tons (t) of gold since 1893. Despite the economic importance of the deposit, the age of the mineralized shear zone array and its temporal relationship to other structures and to porphyry intrusions in the host greenstone terrane are poorly constrained.

The SHRIMP ion microprobe has been used to date zircons and monazites recovered from a chlorite-carbonate–altered, synmineralization lamprophyre dike intruded into sericite-ankerite–altered Paringa basalt below the high-grade Oroya hanging-wall shear zone. Analyses of magmatic-hydrothermal zircons define a weighted mean 207Pb/206Pb age of 2642 ± 6 Ma (n = 37, MSWD = 0.88). Analyses of cogenetic hydrothermal monazites yield a concordant 207Pb/206Pb age of 2637 ± 20 Ma (n = 9, MSWD = 1.8).

The world-class Wallaby gold deposit, Laverton, Western Australia: An orogenic-style overprint on a magmatic-hydrothermal magnetite-calcite alteration pipe

Gold mineralisation at the Wallaby gold deposit is hosted by a 1,200 m thick mafic conglomerate. The conglomerate is intruded by an apparently comagmatic alkaline dyke suite displaying increasing fractionation through mafic-monzonite, monzonite, syenite, syenite porphyry to late-stage carbonatite. In the mine area, a pipe-shaped zone of actinolite-magnetite-epidote-calcite (AMEC) alteration overprints the conglomerate. Gold mineralisation, associated with dolomite-albite-quartz-pyrite alteration, is hosted in a series of sub-horizontal, structurally controlled zones that are largely confined within the magnetite-rich pipe. The deposit has a current ore reserve of 2.0 Moz Au, and a total resource of 7.1 Moz Au.

TIMS U–Pb analysis of magmatic titanite and SHRIMP U–Pb analysis of gold-related phosphate minerals are used to constrain the timing of magmatism and gold mineralisation at Wallaby.

Monzogranite and carbonatite dykes of the Wallaby syenite intruded at 2,664±3 Ma, at least 5 m.y. and probably 14 m.y. before gold mineralisation at 2,650±6 Ma. The significant hiatus between proximal magmatism and gold mineralisation suggests that gold-bearing fluids were not derived from magmas associated with the Wallaby syenite, particularly since intrusive events are unlikely to drive hydrothermal systems for more than 1 m.y.

Analysis of the C and O isotopic compositions of carbonates from regional pre-syenite alteration and AMEC alteration at the Wallaby gold deposit suggests that AMEC alteration formed via interaction between magmatic fluids and the pre-syenite wallrock carbonate. The C and O isotopic composition of gold-bearing fluids, as inferred from ore-carbonate, are isotopically distinct from proximal magmatic fluids, as inferred from magmatic carbonate in carbonatite dykes.

Thus, detailed isotopic and geochronological studies negate any direct genetic link between proximal magmatic activity related to the Wallaby syenite and gold mineralisation at Wallaby.

The gold endowment of the Wallaby gold deposit, combined with the relatively low solubility of gold as thiosulfide complexes in low-salinity ore fluids at temperatures of about 300°C, implicates the influx of very large volumes of auriferous hydrothermal fluids. No large-scale shear-zones nor faults through which such large fluid-volumes could pass have been identified within the immediate ore environment, so fluid influx most probably occurred largely in a unit-confined, brittle-ductile fracture system. This was the ~500-m diameter AMEC alteration pipe, which was a brittle, iron-enriched zone in an otherwise massive conglomerate. During compressional deformation, the competency contrast between unaltered and AMEC-altered conglomerate created a zone of increased fracture permeability, and geochemically favourable conditions (high Fe/Fe+Mg ratio), for gold mineralisation from a distal source.

Oldest Gold: Deformation and Hydrothermal Alteration in the Early Archean Shear Zone-Hosted Bamboo Creek Deposit, Pilbara, Western Australia

The Early Archean Bamboo Creek gold deposit contrasts with most other orogenic deposits because of its relatively early timing in the tectonic evolution of the Pilbara granitoid-greenstone terrane.

Lead-lead model ages for galena, together with the relationships between the Bamboo Creek shear zone and dated granites, indicate a relatively early age of gold deposition of ca. 3400 Ma. Correlation of structures associated with gold deposition and regional structural phases shows that gold deposition was most likely related to an extensional tectonic phase. The early timing and association with extension is unlike the tectonic setting of other Archean gold deposits, which tend to form during the final, compressional or strike-slip stages of orogenesis. The Bamboo Creek gold mineralization may have been related to an Early Archean lower crustal delamination event. This may explain the anomalous timing and the low gold endowment of the Pilbara relative to Late Archean greenstones.

The Bamboo Creek deposit is situated in a bedding-parallel, brittle-ductile shear zone (the Bamboo Creek shear zone) within a komatiite sequence. The laminated quartz-carbonate gold lodes occur in carbonate-altered boudins within the Bamboo Creek shear zone and are associated with early sinistral, northeast-up deformation in the shear zone, whereas dextral reactivation of the zone postdates gold deposition. Gold-related alteration zones reflect an increase in XCO2 toward the mineralized zone. Variations in original host-rock composition give rise to asymmetric alteration zoning, with a fuchsite-carbonate zone in the more Mg- and Cr-rich cumulate-textured footwall and a chlorite-quartz zone in the more aluminous spinifex-textured hanging wall.

The alteration envelope is enriched in Na2O, K2O, Rb, Pb, As, and Sb. Whereas pyrite and minor chalcopyrite occur in all alteration zones, tetrahedrite, galena, and sphalerite are strongly associated with gold in the lodes. The alteration and metal enrichment of the Bamboo Creek gold deposit are indistinguishable from those of other orogenic (mesothermal) lode gold deposits in Archean terranes. Carbonate 13C(PDB) and 18O(SMOW) isotope signatures are consistent throughout the alteration envelope at 0.2 ± 0.6 and 14.6 ± 0.6 per mil, respectively. The 13C value, in particular, is higher than typical values for orogenic gold deposits, implying interaction of auriferous fluids with preexisting marine carbonates that formed during an early sea-floor alteration event. The temperature of deposition, estimated from chlorite thermometry and alteration assemblages, is about 250°C, which is within the lower part of the range for orogenic gold deposits.

Supplement

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, Western Australia

FIG. 1. Simplified geological map of Western Australia showing greenstone belts and gold deposits

Norseman-WilunaGreenstoneBelt

FIG. 2. Processing subdivision of Archaean gold ores

FIG. 3. As content of gold ores plotted against gold recovery.

FIG. 4. Bands of fine-grained rhomb-shaped arsenopyrite (asp) cutting coarser-grained pyrite (py), Lancefield ore. Reflected light (x100).

FIG. 5. Native gold in arsenopyrite microfracture, Paddington ore. Width of gold is ~1 µm. Reflected light (x500).

FIG. 6. Co-existing pyrite (py), arsenopyrite (asp) and pyrrhotite (po), Coolgardie ore. Reflected light (x100).

FIG. 7. Native gold in arsenopyrite (asp) microfractures, Coolgardie ore. Adjacent pyrrhotite (po) microfractures contain no gold. Reflected light (x200).

FIG. 8. Large arsenopyrite crystal containing rounded inclusions of pyrrhotite (po) and pyrite plus marcasite aggregates (py + mc). Exhibition deposit. Reflected light (x50).

FIG. 9. Arsenopyrite crystal with deformed, poikiloblastic core zone and clear rim overgrowth. Exhibition deposit. Reflected light (x50).

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).

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.

FIG. 12. SIMS image of submicroscopic Au distribution, arsenopyrite crystal, Wiluna deposit, Western Australia. Gold is concentrated in rim zone.

FIG. 13. Arsenopyrite compositions.

Rock-Buffering of Auriferous Fluids in Altered Rocks Associated with the Golden Mile-Style Mineralization, Kalgoorlie Gold Field, Western Australia

It is generally agreed that the widespread presence of hematite and the moderately negative sulfur isotope composition of some of the pyrite (34S of –10 to –2) in the Golden Mile lodes and associated alteration indicate the presence of a relatively oxidizing ([SO2 –4 ] ~ [HS–] + [H2S], with hematite stable) fluid during gold deposition and wall-rock alteration, but the origin and evolution of this fluid are not well constrained. A piece of evidence that has not been fully integrated into interpretations is the low variance of mineral assemblages in the alteration haloes of the Golden Mile lodes (e.g., coexisting magnetite-hematite-siderite-pyrite-ankerite-albite-muscovite-ilmenite ± rutile-quartz ± chlorite).

Thermodynamic modeling, using HCh and a purpose-built code that facilitates investigation of systems that involve complex mineral solid solutions, CO2-rich fluids, and open-system chemical behavior was used to investigate the constraints that the low variance assemblages place on the source and evolution of mineralizing fluids. Results of the modeling show that fluid-rock reaction with decreasing temperature can drive pyrrhotite-magnetite assemblages, in equilibrium with a fluid that contains aqueous sulfide, to hematite-pyrite-magnetite assemblages in equilibrium with a fluid that contains aqueous sulfate. This modeled shift arises from cooling-driven oxidation of sulfides and reduced sulfur-bearing aqueous species by ferric iron in magnetite and formation of hematite and siderite from magnetite and CO2; there is no requirement for electron acceptors other than those provided by the rock.

The implication of the model results for the Golden Mile mineralization is that hematite growth and sulfate-bearing fluids could have resulted from fluid-wall rock interaction without involvement of an externally derived oxidizing fluid. The change from aqueous-sulfide dominated to aqueous-sulfate–rich solutions would destabilize gold sulfide complexes in solution and lead to gold precipitation. Formation of aqueous sulfate species would also result in the precipitation of pyrite with negative 34S. Mass balance calculations show that production of hematite in the carbonate zone of the Golden Mile mineralization could have occurred without any requirement for addition of fluid-derived electron acceptors although open-system behavior is not precluded. Overall, the characteristics of the carbonate zone alteration are consistent with electron redistribution caused by interaction between a reduced auriferous fluid and the host dolerite.