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Ghost Crab, Mount Marion |
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Western Australia, WA, Australia |
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Au
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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All papers now Open Access.
Available as Full Text for direct download or on request. |
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The Ghost Crab gold deposit, mined as the Ghost Crab open pit and underground Mount Marion mine, is located some 35 km south of Kalgoorlie and 20 km SE of Coolgardie. It was exploited as part of the greater New Celebration operation. Ghost Crab is approximately 26 km to the west of the New Celebration treatment plant.
Development of the shallow oxide resource at Ghost Crab was initially commenced in mid 1997 to exploit high grade oxide ore and reduce production costs of the New Celebration operation. However, this work established the potential for a larger body of ore and an underground hard rock resource was delineated, extending beyond a depth of 400 m. Underground mining, known as the Mt Marion Mine, began in 1999.
The ore deposit at Ghost Crab lies on the western margin of the NW to NNW striking Archaean Saddle Hills Greenstone Belt, to the NE of the syn-tectonic Depot Granodiorite. The deposit lies within a flexure in the NNW to NW trending Karramindie Shear Zone, on the eastern side of the Coolgardie Domain, within the Kalgoorlie Terrane of the Archaean Yilgarn Craton. The Karramindie Shear Zone follows a regional contact between mafic-ultramafic rocks (mainly talc-tremolite) and the Archaean Black Flag Group sediments (quartz-veined biotitic greywackes).
The deposit is located towards the SW margin of a 100 to 500 m wide, WNW-ESE trending, tear shaped 'boudin' of greenstones within the broad Karramindie Shear Zone, juxtaposed obliquely to the ESE with the main mass of the 12 x 4 km, lens-shaped, NNW-SSE trending Saddle Hills Greenstone Belt that is to the NE. The 'boudin' is separated from the main greenstone mass by a wedge of biorite schist.
The deposit is overlain by a thin veneer of ferruginous soils and minor sheetwash above a superficial succession of transported laterite, clays and palaeochannel sands that are up to 50 m thick, increasing in depth from east to west. This cover is underlain by a deeply weathered saprolite profile that persists to a depth of ~8O m. Supergene mineralisation is restricted to this saprolite zone, with grades diminishing rapidly away from the mineralised shear (Miller et al., 1999).
The basement geology of the Saddle Hills greenstone belt is a sequence of ultramafic units comprising komatiites, high magnesium basalts, with thin shale and chert inter-beds, dolerites, basalts and minor felsic sequences. The ~400 m wide interval between the Saddle Hills greenstone belt and the Depot Granodiorite, is occupied by a strongly foliated sequence in which biotite-garnet gneiss after greywacke and tuffaceous felsic volcanics predominate. All of the lithologies in the area have been subjected to Mid to Upper Amphibolite facies metamorphism (Miller et al., 1999).
Whilst the sequence generally dips at 60 to 70°NE, the mineralised shoot plunges at ~48°NW. The immediate host sequence to the gold mineralisation comprises:
• Hangingwall Ultramafic, which is generally 20 to 60 m thick, and consists of variably serpentinised komatiites with strong chlorite ±talc and biotite alteration. This unit is the lower section of an overlying up to100 m thick ultramafic, which in turn, passes up into ~60 m of basalts, and then further ultramafics. Within this unit, as the lode gneiss is approached, biotite and silica increase, accompanied by a strong foliation. Gold mineralisation occurs in the ultramafic, but is generally restricted to a 5 m zone immediately above the Lode Gneiss (Miller et al., 1999).
• Lode Gneiss, the host for the bulk of the Ghost Crab mineralisation. Within the upper parts of the open cut, this gneiss is generally <1 m thick, with the bulk of the gold mineralisation within the enclosing ultramafic units. However, at depth it expands to ialmost 40 m wide. It has a similar composition to the footwall and hangingwall gneiss units and consists of a sequence of metasedimentary rocks varying from pelite and greywacke to volcanoclastic derived sedimentary rocks. It may be either a thrusted wedge of the footwall or hangingwall gneisses, or a separate unit of the stratigraphic package. A number of narrow 0.1 to 2 m thick pegmatites which intrude the lode gneiss, are broadly concordant with the foliation and stratigraphy and are weakly mineralised (Miller et al., 1999).
• Footwall Ultramafic, which has a maximum thickness of 25 m and appears to pinch and swell down dip and down plunge, and is underlain by a thick garnet-biotite gneiss to schist. It appears to be similar to have a similar composition to the the hanging wall ultramafic sequence, although gold mineralisation is usually restricted to the top one metre. The thickness and presence of this unit are believed to strongly influence localisation of the gold mineralisation (Miller et al., 1999).
The alteration assemblages developed within the deposit in the Lode Gneiss and marginal enclosing units, comprise sodic plagioclase at depths of <700 m, silica, occurring as quartz veining and silica flooding, cummingtonite, anthophyllite, biotite and chlorite. The Lode Gneiss characteristically contains disseminated pyrrhotite and pyrite which are aligned parallel to the dominant foliation direction as well as boudinaged quartz-feldspar veins that are developed sub-parallel to the foliation. These veins occasionally enclose visible coarse gold whilst fine gold (2 to l2 µm) occurs along the boundaries of quartz and plagioclase grains and in the groundmass of the gneiss. All of the gold occurs as native metal. At depths of >600m, the mineralisation bifurcates, with high grade zones confined to the footwall and to a lesser degree the hanging wall sections of the Lode Gneiss unit. The contacts between the Lode Gneiss and the enclosing ultramafic units are interpreted to have provided the fluid channel pathways for the ingress gold mineralisation (Miller et al., 1999).
Resource dimensions were 1100 m down plunge, 150 to 350 m across plunge with an average thickness of 11 m. The resource extends from the base of the Ghost Crab open pit to 850 m below surface. Drilling below this level indicated mineralisation was open to 1200 m below surface.
Reserves in December 1997 totalled 3.5 Mt @ 5.1 g/t Au = 17.8 t of gold.
Measured + indicated + inferred resources in June 1999 totalled 7.8 Mt @ 4.7 g/t Au at a 2 g/t Au cut-off = 37 t of gold (Miller et al., 1999).
The most recent source geological information used to prepare this decription was dated: 1999.
Record last updated: 18/6/2016
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
Ghost Crab
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Miller, M., McLeod, R., Devlin, S. and Vinar, J., 1999 - Discovery of the Ghost Crab Deposit, Kalgoorlie, Western Australia: in New Generation Gold Mines 99, Case Histories of Discovery, Perth, Nov, 1999, Australian Mineral Foundation, Adelaide, Conference Proceedings, pp. 127-141.
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