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.
Bamboo
Creek
Wallaby
NorsemanWiluna
Greenstone
Belt
FIG. 1. Simplified geological map of Western Australia
showing greenstone belts and gold deposits
Stratigraphy
The upper greenschist lower amphibolite facies rocks of the
Golden Mile deposit are hosted by the Archean granitegreenstone 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 pyritepyrrhotite- 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 KalgoorlieBoulder 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 magnetitecalcite 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 goldbearing 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 granitoidgreenstone 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. Goldrelated 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 fuchsitecarbonate 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
NorsemanWiluna
Greenstone
Belt
FIG. 1. Simplified geological map of Western Australia
showing greenstone belts and gold deposits
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 magnetitehematite-siderite-pyrite-ankerite-albite-muscovite-ilmenite ± rutilequartz ± 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 fluidrock reaction with decreasing temperature can drive pyrrhotitemagnetite 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 fluidderived 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.