Setting and Style of VMS Mineralization in the Eastern Goldfields Superterrane, Western Australia: Insights from the Ag-Zn-(Au) Nimbus Deposit

English Abstract:

Economic VMS mineralization in the Archean Yilgarn Craton of Western Australia is largely restricted to two main zones of juvenile crust as revealed through regional (Nd, Pb) isotope variations (Huston et al. 2014). Interpreted as Archean paleo-rift zones, one of these runs N-S through the central Eastern Goldfields Superterrane and is associated with the high-grade Teutonic Bore, Jaguar and Bentley deposits, plus sub-economic VMS mineralization further south at Anaconda and Erayinia. The Nimbus Ag-Zn deposit (4.9 Mt at 149 g/t Ag-eq) is located approximately 265 km south of Teutonic Bore, near the margin of this paleo-rift zone and 17km ESE of Kalgoorlie. The local stratigraphy comprises a NW-trending and steeply-dipping bimodal-felsic package of volcanic rocks (i.e. FI affinity quartz-feldspar porphyritic dacite and lesser basalt, plus their autoclastic equivalents) with subordinate carbonaceous black shale, chert and polymict conglomerates. Primary sulfide mineralization occurs as a series of stacked plunging lenses which parallel a mineral elongation lineation. Early well-developed massive pyrite is underlain by 1) semi-massive, stringer and breccia-type Ag-Zn±Pb(Cu-Au) sulfides (including: pyrite, sphalerite, galena, pyrargyrite, marrite, boulangerite, arsenopyrite, chalcopyrite and tetrahedrite) associated with the autoclastic top of a thick unit of dacite; and 2) stringer and disseminated sulfides (dominated by pyrite and sphalerite) in largely coherent pseudo-brecciated dacite at depth. Hydrothermal alteration is characterized by extensive and pervasive quartz-sericite-carbonate which becomes more intense towards mineralization. Compared to other VMS 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 volcanic rocks, ocean-plateau like low-Th basalts), mineralogy (e.g. low Cu-Au through most of the deposit, abundance of Ag & Sb sulfosalts) and alteration assemblages (e.g. lack of chlorite, presence of kaolinite at depth). Classification of Nimbus as a shallow-water and low-temperature VMS deposit with epithermal characteristics (i.e. a hybrid bimodal-felsic deposit), is consistent with its position near the margin of this paleo-rift zone.