Mesoproterozoic Gold in the Central Gawler Craton, South Australia: Geology, Alteration, Fluids, and Timing

摘要

Similarities in mineralization and alteration style, host rocks, fluid compositions, and alteration-related ~(40)Ar/~(39)Ar ages from several recently discovered gold prospects support the existence of a significant Mesoproterozoic gold province spanning an arcuate region at least 300 km in length in the central Gawler craton, South Australia. At the recently discovered prospects of Tunkillia, Nuckulla Hill, Barns, and Weednanna, as well as at the historically mined Au deposit at Tarcoola, gold occurs as disseminated and veinlet-hosted mineralization focused in brittle to brittle-ductile faults and shear zones. Host rocks are principally approx 1715 to approx 1680 Ma granites but also include Hutchison Group (2000-1860 Ma) and Tarcoola Formation (1650 Ma) metasedimentary rocks. Hydrothermal alteration is characteristically zoned around gold mineralization, with intense sericite-pyrite alteration and quartz veining proximal to gold mineralization and chlorite +- epidote +- hematite alteration distal from mineralization. Alteration was either synchronous with or, in some cases, continued after deformation. Gold is associated with pyrite and minor to trace galena, sphalerite, and chalcopyrite. Iron oxides are low in abundance in mineralized zones, which correspond to demagnetized zones. Fluid inclusion analyses suggest that multiple fluids were present in some systems, but one key fluid type occurs in each of the gold prospects: a low- to moderate-salinity (up to 10 wt percent NaCl equiv) fluid with homogenization temperatures mostly in the range of approx 150 deg to 300 deg C and commonly containing CO_2 or associated with CO2-rich inclusions. Several features of the prospects are similar to those of orogenic- and intrusion-related gold deposits, but neither the prospect-scale geology nor the regional geologic setting in the central Gawler craton is sufficiently well understood to establish a genetic model or to confidently classify the mineralizing system. Hydrothermal white micas from several of the central Gawler craton gold prospects yield reproducible ~(40)Ar/~(39)Ar plateau ages of between 1567 and 1583 Ma. These ages, together with U-Pb zircon ages of granitic host rocks, constrain the timing of mineralization to the interval approx 1690 to 1570 Ma, although we interpret the~(40)Ar/~(39)Ar results to indicate a narrower mineralizing interval at approx 1580 +- 10 Ma. This age range overlaps with, and is indistinguishable from, the range of U-Pb zircon ages reported from regional Hiltaba Suite granites and Gawler Range Volcanics, although igneous rocks of this age have not been identified locally at each of the prospects.