Summary:
Nimbus is a shallow-water and low-temperature VHMS deposit with epithermal characteristics (i.e. a hybrid bimodal felsic deposit), which is consistent with its position near the margin of the Kalgoorlie Terrane.
The local stratigraphy comprises a NW-trending and steeply dipping bimodal-felsic package of volcanic rocks (i.e. quartz-feldspar porphyritic dacite and lesser basalt, plus their autoclastic equivalents) with subordinate carbonaceous mudstone, tuff, polymict conglomerates and volcanic breccias. Komatiite flows, volcanic sandstones/siltstones, carbonaceous mudstone, basalt and dolerite were intersected in a distal drill hole (Hollis 2016).
Economic VHMS mineralisation in the Archaean Yilgarn Craton of Western Australia is largely restricted to two main zones of juvenile crust as revealed through regional (Nd, Pb) isotope variations. One of these zones runs north- south through the central Eastern Goldfields Superterrane and is associated with the high-grade Teutonic Bore, Jaguar and Bentley deposits, plus subeconomic VHMS mineralisation further south at Anaconda and Erayinia (Hollis 2016).
Mineralisation
Nimbus primary sulfide resources occur as a series of stacked plunging lenses, overlying mined supergene and oxide mineralisation.
In the primary sulfide zone, early well-developed massive pyrite is underlain by:
1. Semi-massive, stringer and breccia-type Ag-Zn±Pb(Cu-Au) sulfides (including: pyrite, lowand high-iron sphalerite, galena, pyrargyrite, marrite, boulangerite, arsenopyrite, chalcopyrite, Ag-bearing tetrahedrite) associated with the autoclastic facies of thick units of dacite;
2. Stringer and disseminated sulfides (dominated by pyrite and sphalerite) in coherent pseudobrecciated dacite at depth. Hydrothermal alteration is characterised by intense and pervasive quartz-sericite-carbonate±Cr-V mica, with chlorite predominantly associated with mafic units.
Hydrothermal alteration is characterised by extensive and pervasive quartz-sericite-carbonate which becomes more intense towards mineralisation. Compared to other VHMS occurrences in the Yilgarn Craton, the Nimbus deposit is unusual in terms of its tectono-stratigraphic position, the geochemistry of its host sequence (i.e. FI affinity felsic rocks, ocean-plateau like low-Th basalts), mineralogy (e.g. low Cu-Au through most of the deposit, abundance of Ag and Sb sulphosalts) and alteration assemblages (e.g. lack of chlorite, presence of kaolinite at depth, fuchsite).
Nimbus primary sulfide resources occur as a series of stacked plunging lenses, overlying mined supergene and oxide mineralisation. The mineralogy is complex and includes the presence of mercury in the oxide and transition zones.
Three main zones of mineralisation have been defined, Western, Discovery and Eastern. Only the Discovery zone (oxide and transition mineralisation) and Eastern zone (oxide mineralisation) have been mined.
Other zones of mineralisation have been intersected but the data available makes interpretation of these somewhat difficult. The intersections themselves are significant nevertheless with thin zones of semi-massive sphalerite occurring along with high silver grades in excess of 500g/t.