The Nimbus Ag-Zn-Pb-Au deposit is located approximately 15km east of Kalgoorlie-Boulder in Western Australia within the Kalgoorlie Terrane, near the margin with the Kurnalpi Terrane. Nimbus is bounded by northwest trending shear zones, namely the Kanowna Shear Zone to the east and the Boorara Shear Zone to the west.
Nimbus is a shallow-water and low-temperature volcanic hosted massive sulphide (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 northwest-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 drillhole (Hollis 2015).
Nimbus primary sulfide resources occur as a series of stacked plunging lenses, overlying mined supergene and oxide mineralisation. Silver production from the Discovery and East pit produced 3.6 million ounces of silver.
In the primary sulfide zone, early well-developed massive pyrite is underlain by:
- semi-massive, stringer and breccia-type Ag-Zn±Pb (Cu-Au) sulfides associated with the autoclastic facies of thick units of dacite; and
- stringer and disseminated sulfides (dominated by pyrite and sphalerite) in coherent pseudo-brecciated dacite at depth. Hydrothermal alteration is characterised by intense and pervasive quartz-sericite-carbonate±Cr-V mica, with chlorite predominantly associated with mafic units (Hollis, 2015).
The current Mineral Resource Estimate of Nimbus (JORC 2012) is:
12.1 million tonnes at 52 g/t silver, 0.2 g/t gold and 0.9% zinc containing 20.2 million oz of silver, 78,000 oz of gold and 104,000 tonnes of zinc
using cut-off grades of 12 ppm for Ag, 0.5% for Zn and 0.3 g/t for Au over a 2 metre down hole composite.
Within this global resource, the Nimbus project has a high grade silver zinc resource of:
255,898 tonnes @ 773 g/t Ag and 13% Zn containing 33kt of zinc metal and 6.4Moz of silver.
using a cut-off grade of 500 g/t Ag and a top cut grade of 2,800 g/t Ag.
A historical Technical review and concept study confirmed the potential of mining the higher-grade lenses to generate a silver and zinc concentrate with interest being received from potential offtakers. In addition, the concept study also included an assessment of the refractory gold ore that can be concentrated from various zones such as the from Teal deeps. Additional drilling is required to increase the overall tonnage to feed the proposed conceptual plant.
An Exploration target has been defined at depth for 550-700kt Ag-Pb-Zn grading 3.4-3.6% Zn, 140-210g/t Ag and minor Pb.