Magnetite trace element characteristics and their use as a proximity indicator to the Avoca Tank Cu-Au prospect, Girilambone copper province, New South Wales, Australia

Abstract

The Avoca Tank orebody is one of a series of copper-rich orebodies occurring within the Girilambone Cu province of central New South Wales. Mineralisation at Avoca Tank is hosted within several narrow, chloritic, greenschist-facies shear zones which developed ~430 Ma (U-Pb titanite) within metasedimentary rocks around the margins of an Ordovician (ca. 470 Ma) mafic sill complex. Mineralisation at Avoca Tank preserves an early oxide phase (sulfide barren) as magnetite-rich shears that are overprinted by a pyrite-chalcopyrite-rich sulfide phase. The mineralogical and chemical footprint surrounding sulfide mineralisation is narrow (<50 m) offering limited ore vectoring using mineralogical and chemical change. However, magnetite-rich shears occur external to and within sulfide mineralised intervals, and magnetite within these shears displays distinctive trace element variation depending on proximity to Cu mineralisation. Changing magnetite trace element chemistry with increasing Cu abundance at Avoca Tank is best represented by two ternary systems. A ternary plot of Ni-V-Ti effectively separates magnetite from unmineralised zones via Ni abundance, while the ratio of Ti to V effectively separates magnetite from low-, moderate- and high-grade Cu zones. A ternary plot of Sn (100*Sn)-Zn-Ni effectively discriminates between unmineralised, low-grade and combined moderate- to high-grade zones. The greatest control here is the ratio of Zn to Sn, but the inclusion of Ni abundance provides a greater separation between low- versus combined high- and moderate-grade ore. Many of the trace element trends recorded in magnetite are mirrored in the overprinting sulfides. We propose a two-phase mineralising system, with initial development of chemically uniform, sulfide barren magnetite-chlorite-rich shear zones in proximity to the margins of older mafic sills. A subsequent, and potentially hotter (+60 °C), fluid harvested the early oxide for Fe, with the partial replacement, recrystallisation and re-equilibration magnetite within a sulfide-rich hydrothermal fluid under greenschist facies metamorphic conditions. As many known orebodies in the Girilambone Cu province are associated with magnetite-rich gangue, magnetic surveys are an effective exploration technique. Based on this pilot study, mapping trace element variation in magnetite within these bodies is an additional ore-vectoring technique in the search for economic Cu mineralisation in this province.

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