Textural and geochemical analysis of chalcopyrite, galena and sphalerite across the Mount Isa Cu to Pb-Zn transition: Implications for a zoned Cu-Pb-Zn system

Abstract

The genesis of the Mount Isa Cu-Pb-Zn system, northern Australia, is highly debated. Three metallogenic models have previously been proposed to explain the close spatial relationship between a world-class Cu and Pb-Zn deposit: (1) ca. 1650 Ma syngenetic Pb-Zn mineralisation that has been re-mobilised and overprinted by ca. 1520 Ma epigenetic Cu mineralisation; (2) ca. 1650 Ma syn-sedimentary Cu-Pb-Zn mineralisation; (3) ca. 1520 Ma epigenetic Cu-Pb-Zn mineralisation. To assess the viability of these models, detailed petrography and core logging was completed on numerous samples across eight sections of drill core that intersect both Cu and Pb-Zn mineralisation. This was accompanied by the analysis of trace elements in chalcopyrite, galena and sphalerite from various samples across the lateral transition from Cu- to Pb-Zn-rich mineralisation.Across the Cu to Pb-Zn transition there is a primary change in host lithology from Cu-rich silica-dolomite breccia to highly-stratiform Pb-Zn-rich, carbonaceous, carbonate-rich, and fine-grained pyrite-rich shale. Petrography suggests that sphalerite, galena and chalcopyrite co-crystallised, consistently enveloping and/or replacing pre-mineralisation silica-dolomite, siderite, calcite, arsenopyrite, fine- and coarse-grained pyrite. From Cu to Pb-Zn-rich mineralisation, sphalerite and galena transition from micro-inclusions primarily located within coarse-grained chalcopyrite to dominant fine- to coarse-grained components that infill around deformed carbonaceous shale and selectively replace carbonate-rich and fine-grained pyrite-rich shale. Chalcopyrite transitions from a coarse-grained component that replaces silica-dolomite alteration to an uncommon medium- to fine-grained component residing within stratabound Pb-Zn mineralisation.Trace element trends demonstrate that as Pb-Zn mineralisation is approached, the concentrations of Bi in chalcopyrite, Cd, Se, Te, Bi in galena, and Bi and Co in sphalerite decrease, or are relatively depleted. Conversely, the concentrations of Pb in chalcopyrite, Sb in galena, and Pb, Sb and Ag in sphalerite increase, or are relatively enriched. These trends are consistent with a temperature gradient between Cu and Pb-Zn mineralisation during ore emplacement. The sphalerite GGIMFis geothermometer yields a median homogenisation temperature of 343 ± 44 °C, consistent with previous temperature estimates of Cu mineralisation, silica-dolomite formation and chlorite alteration. An epigenetic and coeval Cu-Pb-Zn model is favoured where a slight temperature gradient (<50 °C) resulted in the deposit scale zonation of Cu → Pb → Zn-rich mineralisation outwards from the Paroo fault. This allowed Pb and Zn-bearing fluids to migrate further into the host shale, resulting in different styles of mineralisation relative to Cu.