The Basil Cu–Co deposit, Eastern Arunta Region, Northern Territory, Australia: A metamorphosed volcanic-hosted massive sulphide deposit

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The Basil Cu–Co deposit, Harts Range, central Australia, is hosted by the Riddock Amphibolite, a sequence that has been metamorphosed at upper-amphibolite- to granulite-facies conditions at 480–460Ma (Larapinta Event), and subsequently reworked at amphibolite-facies conditions (450–300Ma). As a result, many of the primary mineralization textures and other features that could characterise ore genesis have been obliterated. However, preserved textures and mineral relationships in the mineralized zone, allow some constraints to be placed on the genetic history of the deposit using mineralogical, petrographic and geochemical studies of host rocks and sulphides.Results of this study permit at least two genetic models to be ruled out. Firstly, whole rock geochemistry and garnet compositions suggest that the deposit is not a skarn system. Secondly, the lack of any significant Ni-signature, and the presence of abundant zircons in the host amphibolite (indicating that not all host rocks are mafic in composition and/or magmatic in character), make an orthomagmatic Ni–Cu–(PGE) system unlikely. Alternatively, Basil is assigned to a volcanic-hosted massive sulphide (VHMS)-style of mineralization, formed on the seafloor, within basaltic and sedimentary host rocks, typical of deposits occurring in such settings. The lack of a recognisable hydrothermal alteration zone is consistent with either destruction of the alteration zone during metamorphism or detachment of the ore from alteration during later deformation.The occurrence of sulphide inclusions within garnet and amphibole indicates that the sulphides must be syn-metamorphic or earlier. Partitioning of trace elements between pyrite and co-existing pyrrhotite suggests that (re)crystallization occurred under equilibrium conditions. The composition of sphalerite coexisting with pyrite and pyrrhotite indicates crystallization at pressures of at least 10kbar, consistent with peak metamorphism during the Early Ordovician Larapinta Event. Zr-in-titanite geothermometry indicates peak temperatures of 730–745°C.