Summary:
Main deposits
• In the Daisy complex, this includes the Daisy-Milano underground, Mirror/Magic, and Lorna Doone deposits.
• In the Mount Belches complex, this comprises the Maxwells, Cock-eyed Bob underground deposits, Santa and Flora Dora open pit deposits, Rumbles deposit.
• In the Aldiss Mining Centre, this comprises the Karonie, Tank/Atreides and French Kiss open pit deposits, Tank South underground deposit.
Daisy Complex
The Daisy Mining Centre lies along the Bulong Anticline within the Gindalbie Terrane of the Norseman-Wiluna Greenstone Belt. The core of the Bulong Anticline (the Yindarlgooda Dome) contains mineralised granitic intrusive rocks in a sequence of felsic to intermediate conglomeratic sedimentary rock, structurally overlain by a mafic-ultramafic succession. Quartz-feldspar porphyry dykes and sills intrude the sequence.
A northwest striking (50°), south dipping shear (the Godard Shear) separates the ultramafic sequence to the south from the felsic volcaniclastic rocks to the north. The structural setting of the Daisy Complex is analogous to that of the Kanowna gold mining centre, with is located on shears spaying away for the regional Kanowna Shear.
Mineralisation occurs along north–south trending, transpressional structures, commonly along or associate with the Main porphyry contacts and/or north–south shear structures. Haloes of sericitepyrite-fuchsite alteration around mineralised quartz carbonate veins typically highlight metal-rich fluid pathways. Galena and sphalerite are generally enriched in typical Daisy ore veins and compressional shear structures on vein margins are also excellent indicators of gold mineralisation.
Mineralised veins are typically subvertical, vary between 0.05 m and 1 m in width, generally trend north–south and exhibit significant wall rock alteration. Milky white (bucky), often wide (>2 m) nonmineralised veins trend east–west, have significant alteration halos and cross-cut ore mineralisation.
Mount Belches Centre
The Mount Belches area is underlain by the Mount Belches greywacke and lies between the Mulgabbie and Kurnalpi terranes to the east and the Gindalbie and Kalgoorlie terranes to the west. The Mount Belches greywacke is bound by the Mount Monger Fault, the Randalls Fault (locally known as the Bare Hill Shear) in the west and the Avoca Shear in the east.
The Mount Belches greywacke forms part of a turbidite sequence which comprises biotite-bearing siltstones, sandstones and greywackes and BIFs.
The known gold deposits at Mount Belches are hosted within several metamorphosed BIF units that form a minor component of the Mount Belches greywacke. Two BIF packages are recognised: a poorly outcropping lower or ‘Maxwells’ package and an outcropping upper or ‘Santa-Craze’ package. Two facies of BIF are recognised in the upper and lower sequences: a magnetite– grunerite-rich6 facies and a magnetite-rich facies.
The gold mineralisation is developed in shallow to moderate south plunging shoots in quartz veins preferentially hosted in both facies of the BIF. Minor amounts of mineralisation are also located in the chloritic siltstones adjacent to these BIF units. Sufidation is the key chemical mechanism controlling gold precipitation, and the iron-rich BIFs present an ideal host. Pyrrhotite is the dominant sulfide and is accompanied by pyrite and arsenopyrite. From petrological analysis, these minerals formed as part of the same metamorphic paragenetic assemblage. Arsenopyrite is commonly present as well-formed crystals up to 1 cm in size.
Aldiss Centre
The Aldiss area lies on the eastern margin of the Eastern Goldfields Greenstone Province (EGGP) where Archaean volcano-sedimentary sequences are juxtaposed against granitoid-gneissic terranes. The province is characterised by an interconnecting series of north-northwest trending greenstone belts surrounded by ovoid to elongate granite batholiths. The province differs from adjacent provinces in that it contains a higher proportion of komatiites and a lower proportion of BIF units. The overall stratigraphic succession in the greenstone of the EGGP is characterised by a lower sequence dominated by mafic to ultramafic volcanic and intrusive units, overlain by more felsic volcanics and an upper sequence dominated by felsic sedimentary units of clastic and chemical origin. Structurally the greenstone sequences record a protracted history of deformation that has produced steeply west dipping and north-northwest striking lithological contacts and a well-developed foliation that trends in the same direction.
Gold mineralisation occurs almost exclusively within the quartz amphibolites and occurs dominantly as native gold. The habit of the native gold is as coarse interstitial grains, located along hornblende and quartz grain boundaries or included within the hornblende grains along cleavage. Sulfides are present, with pyrite subordinate to pyrrhotite and lesser chalcopyrite and minor molybdenite. Gold mineralisation is not associated with the sulfides in any way, which is unusual. Mineralisation also occurs within the metasediments but appears to be related to a late-stage event. Visible gold within quartz veins, microscopic gold adjacent to quartz veins and gold associated with minute shear zones suggest that the mineralisation is structurally controlled.