Nickel, Cobalt, and Manganese Distributions in Olivine and Orthopyroxene as Indicators of Sulfide-Rich Mafic-Ultramafic Systems: A Case Study From the Nova-Bollinger Ni-Cu-Co Deposit in Western Australia

Abstract

Compositional variations in olivine and orthopyroxene from the Nova-Bollinger Ni-Cu-Co deposit (Western Australia) were examined to assess how mineral chemistry relates to orebody proximity and to expand their utility as indicators of magmatic sulfide prospectivity. The Nova-Bollinger deposit comprises a mineralized lower intrusion and a comparatively barren upper intrusion, each containing mafic to ultramafic mesocumulates and orthocumulates. In the upper intrusion and sulfide-poor intervals of the lower intrusion, olivine and orthopyroxene compositions are consistent with fractional crystallization and variable degrees of sulfide-absent and sulfide-present trapped liquid shift. Positive intra-sample Ni-Co correlations persist in ferromagnesian silicates not significantly associated with sulfides, which can arise from trapped liquid shift and should be examined with respect to an appropriate baseline. By contrast, olivine and orthopyroxene in sulfide-rich intervals of the lower intrusion display anomalously low Co and high Mn concentrations for given molar Mg contents, producing elevated Ni/Co ratios that cannot be explained by fractional crystallization or trapped liquid shift. Trace element mapping reveals local Co, and to a lesser extent Ni, depletion in olivine adjacent to sulfides. Reexamination of pentlandite chemistry at Nova-Bollinger shows that disseminated pentlandite is enriched in Co, and less so in Ni, relative to pentlandite in massive sulfides. It is proposed that ferromagnesian minerals are preferentially stripped of Co during reequilibration with high-temperature pentlandite, leading to slight postcumulus upgrading of Co tenors in net-textured ores. New discrimination diagrams and trapped liquid shift models are presented to help interpret ferromagnesian silicate chemistry by distinguishing the effects of fractional crystallization, trapped liquid shift, and sulfide interaction. Although olivine and orthopyroxene compositions show broad systematic variation with orebody proximity, these trends are modified by postcumulus processes that must be considered when establishing chemical baselines. Integration of mineral chemistry with petrographic context is therefore critical for utilizing ferromagnesian silicates as prospectivity indicators or exploration vectors.