Abstract

Great Australia is a small Cu-Co deposit in the Eastern succession of the Mount Isa inlier, northern Queensland. It may be viewed as a carbonate-rich member of the globally distributed, oxide-dominated, Fe-Cu-Au + or - rare earth element (REE) + or - U deposit group. It is also a gold-poor variant of a group of small- to medium-sized, carbonate-dominated vein-replacement deposits in the surrounding Cloncurry copper gold field, known as Mount Freda-style deposits. It lies within the Proterozoic Toole Creek Volcanics (TCV), adjacent to and within a major splay of the Cloncurry fault, which forms a regional tectonic contact with the metasedimentary Corella Formation and is consequently a fundamental structure in the inlier. The deposit has a gangue mineral assemblage of dolomite-calcite-quartz-pyrite (ore type 1) transitional to amphibole-quartz-pyrite (ore type 2), with mineralization largely restricted to thick, tabular veins and fault zones. Mineralization that is deeper in the explored deposit, within the sheared TCV/Corella Formation contact, or in magnetite-altered wall rocks is pre-dominantly ore type 2. The deposit occurs within a dilational jog that developed during D 3 dextral fault reactivation. Plunging ore shoots occur at the intersections of this contact with a north-south-striking fault (the Main lode), forming thick veins. Strong alteration halos exist around the ore zones within the TCV, and are subdivided into a regional, pre- to syn-D 2 albite-actinolite alteration (sodic-calcic I), and a local ore-related, albite-actinolite zone (sodic-calcic II); the latter has a fringing biotite alteration envelope. Alteration within the Corella Formation is geochemically similar to but more subdued than that of TCV alteration. Chalcopyrite-bearing secondary fluid inclusions from ore type 1 quartz contain high-temperature (450 degrees + or - 60 degrees C), high-salinity (45 + or - 10 wt % NaCl equiv), high-f (sub O 2 ) fluids that are interpreted to be syn-ore. Oxygen isotope values of delta 18 O quartz (11.3-13.4ppm) indicate that delta 18 O fluid = 8.3 to 10.3 per mil, consistent with a magmatic to metamorphic source. Calcite carbon isotope values require delta 13 C fluid = -0.3 to -3.7 per mil, best explained by a mixture of carbonate sedimentary carbon and magmatic carbon.The genetic model involves the emplacement of ore fluids into the previously altered Cloncurry fault at high fluid pressures and temperatures, initially forming zoned magnetite-albite-dominant alteration grading out to biotite-dominant alteration (sodic-calcic II) that was contemporaneous with carbonate-amphibole-magnetite-cobaltian pyrite-dominant veining and replacement within the dilational jog. Late in the history of carbonate precipitation, sulfate-bearing metalliferous fluids were introduced and were rapidly reduced by earlier magnetite, biotite, and pyrite-bearing alteration. Reduction of fluid sulfate and mineral sulfidation resulted in coprecipitation of chalcopyrite, chlorite, and minor gold, commonly replacing pyrite, magnetite, biotite, amphibole, and carbonate.

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