A re-appraisal of the Epoch nickel sulphide deposit, Filabusi Greenstone Belt, Zimbabwe: A hydrothermal nickel mineral system?

Abstract

The Neoarchaean Epoch nickel sulphide (NiS) deposit, discovered in 1970 and subsequently mined from 1972 to 1998, has been previously interpreted to be a magmatic ore system. However, on the basis of field, petrographic and analytical data obtained during the initial exploration and evaluation stages, integrated with limited new analytical data (SEM and ore microscopy), we propose that the Epoch NiS may be of hydrothermal origin. Key features that have prompted a re-appraisal of the genesis of the Epoch mineralisation, include: a) ore lenses are irregularly distributed along foliation fabrics of the host talc-carbonate rocks and as such no relationship to basal ultramafic/mafic units; b) the thickness of the ore lenses are highly variable from tens of centimetres to several metres; c) ore mineralogy is characterised by dominant millerite–pyrite–chalcopyrite assemblages; and d) high Pd/Pt ratios. Although our hydrothermal model is somewhat speculative, we suggest that on present evidence there is little doubt that the Epoch NiS lacks the features of typical magmatic sulphide deposits. More geochemical and isotopic data are needed to test our model.