4D history of the Nimbus VHMS ore deposit in the Yilgarn Craton, Western Australia

Abstract

Due to its unique mineralogy in the Archaean Yilgarn Craton, the origin of the high-grade Nimbus Ag-Zn-(Au) deposit has been contentious for a number of years. Recent interpretations of the deposit as a shallow water and low temperature volcanic-hosted massive sulphide (VHMS) deposit with epithermal characteristics (i.e. a hybrid bimodal-felsic deposit), were based on detailed studies of its volcanology, mineralogy, hydrothermal alteration assemblages, geochemistry, multiple S isotopes and trace element content of sulphide. However, this model has been questioned in favour of a late, epithermal fault-hosted system. Effective greenfields exploration for similar deposits elsewhere requires a robust deposit model, and well understood timing of ore formation. We present a comprehensive multi-disciplinary study to further constrain the processes involved in the evolution of Nimbus and generate a 4D evolutionary model of the system.Host rock dacite formation is well constrained by SHRIMP U-Pb zircon geochronology to c. 2703 Ma. Re-Os ages for the first sulphide phase, gives an imprecise maximum age to the ore formation of 2682 ± 110 Ma. High precision Pb-isotope variations in ore-stage and late galenas track changes in the sulphides over time, complemented by new geochronology. The Pb-isotope data indicates that the polymetallic ore precipitation happened at the same time as the volcanism, with the second generation indicating minor, local remobilisation of Pb in late quartz-carbonate veins at c. 2630 Ma. 40Ar/39Ar dating of sericite-altered plagioclase from foliated dacite, and U-Pb SHRIMP dating of syn- to post-deformation monazites have yielded ages of ~2630 Ma. The late event at c. 2630 Ma is most likely related to widespread late low-Ca granite emplacement across the Eastern Goldfields.This study provides arguments that suggest that Nimbus is a syn-volcanic deposit. The ore timing is consistent with the model of replacement-type VHMS deposit, with the age of the main mineralisation similar to the age of the dacitic host-rocks at 2703 Ma. The late event modifies the pre-existing ore and overprints the previous ore-related alteration ages from the altered plagioclase. High precision, double spiked, Pb isotope constraints from late quartz-carbonate veins provides an opportunity to vector to hidden deposits.