The Boddington gold deposit is located approximately 130 km SSE of Perth in Western Australia and lies within the Saddleback greenstone belt, part of the Western Gneiss terrane, on the far south-western extremity of the Archaean Yilgarn Craton. The Saddleback greenstone belt is some 35 km long and 5 to 10 km wide.
The deposit was discovered in 1980 in the Darling Ranges Bauxite district and mining commenced in 1987. The original operation commenced on a resource of 60 Mt @ 1.6 g/t Au which occurred as very fine, generally <5 µm, gold grains spread through the 4 to 25 m thick lateritic weathering profile, sections of which are mined on a large scale for bauxite elsewhere in the district.
The lateritic gold mineralisation at Boddington occurred as a semi-continuous blanket over a strike length of more than 5 km and a width of around 1 km. Some 30% of the Some 30% of the gold was hosted by the same vertical zones as the bauxite, namely the upper lateritic gravels, the hardcap and the B-zone laterite. The remaining 70% was found in the underlying clay and saprolite zone, which was 10m, and locally up to 120 m in thickness.
The laterite resource was exhausted in 2001 and the mine placed on care and maintenance, after having produced 105 Mt @ 1.4 g/t recovered Au for 147 t Au and 6500 t Cu.
Beneath the lateritic ore, there are a variety of other ore types present in the greenstone belt bedrock, including rich lode deposits and the large low grade, hard-rock Wandoo body.
The Wandoo deposit occurs as disseminations and stockworks hosted by 2715 to 2690 Ma dyke and stock-like dioritic intrusives and andesitic volcanic rocks of the Saddleback Greenstone belt. The element chemistry of the host intrusions indicates an island arc tectonic setting. The host rocks underwent a ductile deformation event (D1-D2), followed by a second period of supracrustal deposition comprising intermediate volcaniclastic rocks and intrusion of coeval granodiorite-tonalite rocks dated at around 2675 Ma. The chemistry of the latter rocks is again consistent with formation in an arc setting. All of these rocks were then metamorphosed to upper-greenschist to lower-amphibolite facies at around 2640 Ma, and deformed by brittle-ductile faults (D3-D4). A late monzogranite intrudes the greenstone belt just east of the Boddington mine. This intrusion, dated at 2612Ma, is distinct in terms of its K feldspar-phyric texture, associated aplites and rare pegmatites, distinctive magnetic low signature, and its elevated U-Th and K radiometric character.
The core of the mineralised hydrothermal system is characterised by overlapping Cu, Au, Mo, Bi and W concentrations, with marked enrichments in Pb, Zn and Ag on the periphery.
Two stages of mineralisation are recognised at Boddington, namely:
i). An early widespread silica-biotite alteration, and complex quartz+albite+molybdenite ±clinozoisite ±chalcopyrite veins variably deformed by ductile shear zones. Re-Os ages from molybdenite in these veins indicates a formation age ofapproximately 2700 Ma.
ii). A second, main stage of mineralisation which cuts all of the above, and comprises,
a). complex quartz+albite+molybdenite ±muscovite ±biotite ±fluorite ±clinozoisite ±chalcopyrite veins (controlling the Mo distribution);
b). clinozoisite-sulphide-quartz-biotite veining (controlling the bulk of the lowgrade Au-Cu mineralisation);
c). actinolite ±sulphide ±quartz, carbonate-chlorite-sulphide, and sulphide veins (controlling high grade mineralisation). Re-Os dates from these mineral assemblages yields ages of around 2625 to 2615 Ma, broadly synchronous with the monzogranite to the east of the deposit.
Structurally, mineralisation is controlled by the following, in increasing order of importance: i). a late ductile WNW to NW striking, sub-vertical fault network with elevated mineralisation and alteration; ii). intersection of this fault network and competent lithologies; iii). intersection of these late faults and early ductile quartz-sericite shear zones; and iv). NE striking corridors which appear to compartmentalise the deposit by offsetting favourable hosts prior to mineralisation.
The Wandoo basement gold mineralisation is therefore interpreted to represent a structurally-controlled, intrusion-related Au-Cu deposit, paragenetically associated with both ~2700 and 2612 Ma events, with the main stage, higher grade mineralisation being apparently synchronous with the late, K-rich post-magmatic monzogranite suite.
The basement mineralisation in 2004 comprised (Newcrest Annual Report, 2005):
Proven + Probable Reserve - 395 Mt @ 0.87 g/t Au, 0.13% Cu; (reserves included in resources),
Measured + Indicated Resource - 505 Mt @ 0.86 g/t Au, 0.12% Cu, for 435 t Au;
Inferred Resource - 232 Mt @ 0.80 g/t Au, 0.09% Cu, for 185 t Au.
Following further drilling and development, the reserve and resources in 2010 (Newmont 2011) were:
Proven + Probable Reserve - 1089 Mt @ 0.58 g/t Au, 0.11% Cu, for 631 t Au, (reserves additional to resources)
Measured + Indicated Resource - 469 Mt @ 0.40 g/t Au, 0.08% Cu, for 187 t Au,
Inferred Resource - 163 Mt @ 0.43 g/t Au, 0.11% Cu, for 70 t Au.
(Source: Porter GeoConsultancy, www.portergeo.com.au, 2011)
