A carbonate-dominated copper-cobalt Breccia-Vein system at the great Australia deposit, Mount Isa Eastern succession

Great Australia is a small Cu-Co deposit in the Eastern succession of the M ount Isa inlier, northern Queensland. It may be viewed as a carbonate-rich member of the globally distributed, oxide-dominated, Fe-Cu-Au +/- rare eart h element (REE) +/- U deposit group. It is also a gold-poor variant of a gr oup of small- to medium-sized, carbonate-dominated vein-replacement deposit s in the surrounding Cloncurry copper gold field, known as Mount Freda-styl e deposits. It lies within the Proterozoic Toole Creek Volcanics (TCV), adj acent to and within a major splay of the Cloncurry fault, which forms a reg ional tectonic contact with the metasedimentary Corella Formation and is co nsequently a fundamental structure in the inlier. The deposit has a gangue mineral assemblage of dolomite-calcite-quariz-pyrite (ore type 1) transitio nal to amphibole-quartz-pyrite (ore type 2), with mineralization largely re stricted to thick, tabular veins and fault zones. Mineralization that is de eper in the explored deposit, within the sheared TCV/Corella Formation cont act, or in magnetite-altered wall rocks is pre dominantly ore type 2. The d eposit occurs within a dilational jog that developed during D-3 dextral fau lt reactivation. Plunging ore shoots occur at the intersections of this con tact with a north-south-striking fault (the Main lode), forming thick veins . Strong alteration halos exist around the ore zones within the TCV, and ar e subdivided into a regional, pre- to syn-D-2 albite-actinolite alteration (sodic-calcic I), and a local ore-related, albite-actinolite zone (sodic-ca lcic 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 inclusion s from ore type 1 quartz contain high-temperature (450 degrees +/- 60 degre es C), high-salinity (45 +/- 10 wt % NaCl equiv), high-fo(2) fluids that ar e interpreted to be syn-ore. Oxygen isotope values of delta(18)O(quartz) (1 1.3-13.4 parts per thousand) indicate that delta(18)O(fluid) = 8.3 to 10.3 per mil, consistent with a magmatic to metamorphic source. Calcite carbon i sotope values require delta(13)C(fluid) = -0.3 to 3.7 per mil, best explain ed by a mixture of carbonate sedimentary carbon and magmatic carbon. The genetic model involves the emplacement of ore fluids into the previousl y altered Cloncurry fault at high fluid pressures and temperatures, initial ly forming zoned magnetite-albite-dominant alteration grading out to biotit e-dominant alteration (sodic-calcic II) that was contemporaneous with carbo nate-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 redu ced by earlier magnetite, biotite, and pyrite-bearing alteration. Reduction of fluid sulfate and mineral sulfidation resulted in coprecipitation of ch alcopyrite, chlorite, and minor gold, commonly replacing pyrite, magnetite, biotite, amphibole, and carbonate.