Low temperature alteration of massive sulphides below the weathering front

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Alteration of magmatic sulphides within the regolith, or soil profile, is well documented, but alteration at depth, where meteoric fluids may infiltrate fresh rock adjacent to faults, is less studied and poorly understood. Yet such alteration may affect the mineralogy of the magmatic sulphides, and the characteristics of this alteration provide insights into the distribution and timescales of fluid flow and redox processes within the upper crust.Pentlandite [(Fe,Ni)9S8] is a Ni-rich sulphide mineral, found in magmatic sulphide assemblages with chalcopyrite (CuFeS2) and pyrrhotite (Fe1−xS). Pentlandite associated with massive sulphides from apparently fresh rock at the Mawson Deposit, Western Australia, shows in-situ partial replacement by violarite (nominally FeNi2S4), whereas pentlandite associated with adjacent disseminated sulphides is unaltered. Petrographic, geochemical, and in-situ sulphur isotope analysis reveals that violarite has variable Fe:Ni ratios. Constant volume and constant sulphur mass balance calculations indicate an overall loss of Ni and other metal cations during violarite formation. In addition, newly formed violarite has a 34S-depleted isotopic composition compared to precursor pentlandite. Altered pentlandite (δ34S = 1.02 ‰) is isotopically fractionated relative to co-existing pyrrhotite (3.44 ‰) and chalcopyrite (3.15 ‰), in contrast to the unfractionated sulphur isotope ratios expected for co-crystallising magmatic sulphides. These features are interpreted as a consequence of electrochemical alteration within electrically connected massive sulphides and addition of isotopically light sulphur derived by bacterial sulphate reduction. Based on previous evidence of deep-seated fluid flow along long-lived regional faults, the alteration is plausibly associated with Cenozoic neotectonic deformation and short-lived pulses of fluid flow with durations of hundreds to thousands of years. Similar petrographic evidence of deep alteration in apparently fresh sulphides from samples across the regional study area indicate this could be an under-reported feature in magmatic sulphides globally.