Location: Perth in Western Australia.
Products: high-grade copper mines and secondary Gold .
Owner: Sandfire Resources.
The degrussa VHMS (volcanic-hosted massive sulphide) copper–gold deposit is located 900 kilometers north of perth and 150 kilometers north of meekatharra in the peak hill mineral field, on the peak hill 1:250 000 map sheet, sg 50-8. It lies on a major drainage divide between the gascoyne river catchment and the lake gregory to carnegie lake salinas internal drainage. The deposit lies beneath a mantle of scree and sheet wash deposits on gentle slopes mantling an upland area of exposed bedrock and relic duricrust.
The degrussa copper–gold massive sulphide deposit is hosted within the bryah basin, one of a number of separate palaeo-proterozoic depositional basins in the eastern part of the capricorn orogen, which is a major tectonic unit that lies between the archaean pilbara craton and the yilgarn craton. The bryah group is a succession of mafic rocks of mid-ocean ridge basalt to oceanic plateau affinity overlain by clastic and chemical sediments.
The age of the bryah group is poorly constrained between 2.0 ga and 1.8 ga. It is younger than 2.65 ga and older than 1785 ±11 ma (u-pb zircon age), the age of the uncomformably overlying mount leake formation.
The bryah basin has undergone two episodes of deformation. The 1.96 ga glenburgh orogeny (d1) accreted the narracoota oceanic plateau onto the yilgarn craton. Folding, faulting and shearing attributed to this orogeny have been largely overprinted by the 1.8 ga capricorn orogeny (d2) that was the result of the oblique collision between the pilbara and yilgarn cratons. During deformation the volcano-sedimentary succession was metamorphosed to greenschist facies.
The copper–gold rich-massive sulphide lenses are vhms-style based on the host rock package, mineralisation style, mineral composition and alteration.
The host rocks are submarine basalts, mafic volcaniclastic rocks and debris flows with sub-volcanic dolerite/gabbro sills of the de grussa formation.
Sulphide mineralisation consists of massive sulphide, semi-massive sulphide and stringer zone mineralisation. The transition from massive sulphide to an underlying stringer zone is not always present because of dolerite intrusion close to or at the base of the massive sulphide. Primary sulphide minerals present are pyrite, chalcopyrite, pyrrhotite, and sphalerite together with magnetite. The base of the massive sulphide is chalcopyrite rich with magnetite, passing upwards into iron sulphides with decreasing copper content and increasing zinc content higher up. Gold is associated with the chalcopyrite-rich zones and occurs as a high-silver electrum.
The oxide mineralisation is located vertically above conductor 1 and degrussa. The grade and width of the oxide mineralisation is highest proximal to the main lenses and then forms enriched plumes that transgress lithological boundaries as the mineralisation disperses and dissipates. Close to the main lenses there is significant native copper and elevated gold. As the plumes disperse away from the ore zones the grade dissipates and mineralisation transitions through chrysocolla, cuprite, azurite and malachite.
Alteration associated with the massive sulphide is chlorite + sericite + quartz + pyrite which is typical for vhms deposits. Stringers in the stringer zone are chalcopyrite rich.
The massive sulphide lenses are deformed and often exhibit a strong foliation. The harder pyrite and pyrrhotite tends to fracture, while the softer chalcopyrite and sphalerite are easily remobilized and recrystallised.
Beneath a hardpan cap there is about 80 metres of weathering over the sulphide lenses. Within the weathering profile is an upper, residual, gold-oxide zone overlying an oxide-copper zone. The oxide- copper zone contains the minerals malachite, chrysocolla, native copper and minor cuprite. A secondary supergene chalcocite blanket lies beneath the oxide-copper zone and immediately above fresh primary sulphides.
Four lenses of copper-rich massive sulphides have been discovered to date as shown in figure 3. Degrussa has a strike length of 180m, is some 20m thick on average and dips near vertically to the south. It has a vertical extent of 300m.
“Commencing with an initial 2-year open pit mining operation which was completed in April 2013, the DeGrussa Operation is based on a long-term underground mine delivering sulphide ore to an on-site 1.5Mtpa concentrator.”
The mine will produce up to 300,000 tonnes of high-grade copper concentrate annually.
Based on the figures published in the Western Australian Mineral and Petroleum Statistics Digest, this would make DeGrussa the largest copper producer in Western Australia.
In addition to the 1.5Mtpa DeGrussa Concentrator, other infrastructure and services on site include a Tailings Storage Facility (TSF), power station, paste plant for the underground mine, a sealed airstrip capable of accommodating small jets, a state-of-the-art 400-room mine village, a Next G mobile phone service and fibre optic communications, office buildings, assay laboratory and sealed access roads.