Platinum-group mineralogy of the Fazenda Mirabela intrusion, Brazil: the role of high temperature liquids and sulphur loss

Abstract

Two distinct platinum-group mineral (PGM) assemblages have been identified in the Fazenda Mirabela intrusion that hosts the Ni–Cu–platinum-group element (PGE) Santa Rita ore zone in Bahia State, north-eastern Brazil: (i) Pt–Pd–Ni tellurides accompanied by Ag–Te, with minor electrum and native Au, and (ii) Pd alloys accompanied by minor PGE arsenides and sulphides. Assemblage (i) is present in the Santa Rita ore zone and underlying S-poor footwall dunite whereas assemblage (ii) is observed in the dunite only. The assemblage (i) PGE tellurides crystallised from a late stage semimetal-rich PGE-bearing melt produced by sulphide fractionation and/or via exsolution from sulphides during subsolidus cooling. In assemblage (i) in the Santa Rita ore zone, PGM are also commonly found within base metal sulphide (BMS) veinlets which have either formed as a result of post-magmatic hydrothermal remobilisation or by the simultaneous crystallisation of PGM and BMS from a late stage volatile-rich melt. In the S-poor footwall dunite that hosts assemblage (ii), Pd alloys have formed through the interaction of sulphides with a late stage melt or high temperature hydrothermal fluid. This liquid had a high oxygen fugacity () that caused S loss evidenced by the formation of micro-scale textures that resemble symplectites or intergrowths of sulphides with olivine and occasionally orthopyroxene, and the formation of magnetite. During sulphur loss, semimetals (particularly Te) were also stripped from the dunite while PGE were expelled from the symplectite-like sulphides forming Pd alloys. PGE tellurides are present in the dunite where S and semimetals have not been completely stripped from the rock suggesting that this late stage melt or high temperature hydrothermal fluid was not pervasive throughout the dunite.