Deformation-enhanced fluid evolution and skarn–IOCG transition in the Paleoproterozoic Elaine Dorothy Cu–Au system, Mount Isa Inlier, Australia

Abstract

Abstract The Elaine Dorothy Cu-Au deposit, located in the Mary Kathleen Domain of the Mount Isa Inlier, Australia, records the interaction of Paleoproterozoic skarn metasomatism, fluid evolution, and deformation. The deposit is hosted by carbonate- and calc-silicate-rich rocks of the Corella Formation and comprises diopside–garnet–scapolite–wollastonite ± amphibole assemblages associated with pyrrhotite–chalcopyrite–pyrite mineralisation. Field relationships and mineral textures indicate that sulphide deposition began during or shortly after prograde skarn formation and subsequently evolved from disseminated and vein-controlled to semi-massive, massive, and shear-zone-hosted mineralisation. LA-ICP-MS analyses of pyrite, pyrrhotite, chalcopyrite, and magnetite record systematic variation in trace-element compositions during this evolution. Early Py1 is enriched in Co and Se and displays Co/Sb–Se/As characteristics consistent with a magmatic-hydrothermal contribution. Se-in-pyrite thermometry yields formation temperatures of up to 544 ± 36 °C for the earliest stage, decreasing to approximately 323–417 °C in later Py1 and 342 ± 11 °C for Py2. The transition from Co-rich Py1 to Co-poor, Ni-enriched Py2, together with pyrrhotite-dominated assemblages and cross-cutting chalcopyrite–pyrrhotite textures, records progressive changes in fluid chemistry and conditions of sulphide stability. Variable magnetite trace-element compositions and brecciation further indicate repeated modification during continued fluid circulation. Structural relationships demonstrate that deformation enhanced permeability along the Mary Kathleen structural architecture, promoting fluid focusing, sulphide redistribution, and local concentration within retrograde alteration zones. We interpret Elaine Dorothy as a deformation-enhanced Paleoproterozoic calcic skarn that evolved toward a skarn–IOCG transitional system, rather than as a classic IOCG end member. The deposit demonstrates how deformation, fluid–rock interaction, and evolving redox–sulphur-fugacity conditions progressively modify an early calcic skarn framework, enhancing permeability and fluid focusing, and generating IOCG-affinity characteristics through continued hydrothermal reworking rather than a discrete IOCG-forming event. This provides a process-based framework for recognising structurally reworked skarn systems in Proterozoic mineral provinces.