DeGrussa Copper Mine (DeGrussa Gold-Copper Mine; DeGrussa Mine), Doolgunna Station, Peak Hill Mining District, Meekatharra Shire, Western Australia, Australiai

Regional Level Types
DeGrussa Copper Mine (DeGrussa Gold-Copper Mine; DeGrussa Mine)	Mine
Doolgunna Station	- not defined -
Peak Hill Mining District	Mining District
Meekatharra Shire	Shire
Western Australia	State
Australia	Country

The DeGrussa VHMS (volcanic-hosted massive sulphide) copper–gold deposit is located 900 kilometres north of Perth and 150 kilometres 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. 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.

Operates as underground workings and an open pit by Sandfire Resources. According to the company, it was cash strapped, and on the verge of collapse, when drilling un-covered the deposit. It is one of the few deposits found through exploration over the past 20 years in Western Australia, which has progressed to a major new mine. Sandfire Resources is also one of the few mining companies in Western Australia, who have actively supported the local mineral specimen collecting community. Native copper, chrysocolla, malachite, cuprite and the rare species mcguinnessite have been provided to the public via a shop in central Perth, and donated by the company to the Western Australian Museum, and as fund raising for MINSOCWA. Further, at great expense, it has flown members of the club to inspect the mine. [per Kim McDonald]

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