Hydrothermal alteration of middle Proterozoic basalts, Stuart Shelf, South Australia; a possible source for Cu mineralization

Abstract

Geochemical studies on core material from middle Proterozoic rocks in the vicinity of Mount Gunson, South Australia, have identified the causes of their secondary alteration and revealed a possible relationship between hydrothermal alteration of volcanic rocks and base metal mineralization in the Stuart shelf-Adelaide geosyncline copper province. This province includes the sedimentary stratiform deposit at Mount Gunson and the giant granite breccia-and hematite breccia-hosted Olympic Dam deposit.Drill holes have intersected the Pandurra Formation and an unconformably underlying sequence of tuffs, siltstones, volcanic breccias, and felsic, intermediate, and mafic igneous rocks. Metasediments and a basal brecciated granite were also intersected in one drill hole. The chemistry of the mafic and intermediate volcanics is similar to ultrapotassic and potassic rocks associated with continental tensional regimes (e.g., high K and Fe 2 O 3 /FeO ratios, and relatively high Mg, Ba, Cr, Ni, Rb, Zr, Ce, La, and Y contents). Chemical, mineralogical, and textural features indicate that Si, Ca, Fe, Na, K, and Cu were mobile and Mg, Al, P, Pb, and Zn relatively inert. Copper values (mostly <20 ppm) are lower than values typical of basaltic rocks, suggesting that Cu has been removed from the system during the circulation of hydrothermal fluids and alteration of primary Fe-Ti oxide, Cu sulfide, and ferromagnesian silicate minerals.Fluid inclusion studies show the presence of both NaCl-CO 2 -rich and CaCl 2 -rich fluids. The former are in early replacement segregations of quartz, fluorite, and orthoclase, whereas the Ca-rich fluids are associated with a later, but possibly overlapping, hydrothermal event resulting from the interaction of magmatic fluids and fluids released during decarbonation reactions.The delta 13 C values of vein and amygdule-filling carbonate in the igneous rocks indicate a dominance of magmatic fluids, with variable addition and mixing of carbon from either solution and/or reaction of sedimentary carbon. Sulfide mineral delta 34 S values suggest a magmatic sulfur source. The delta 34 S values of pyrite from drill hole PY3 (1.9 + or - 0.7ppm) are within the range for sulfide minerals from mafic and ultramafic igneous rocks.Geochemical trends associated with Cu (U, Pb, Zn) mineralization in South Australian Proterozoic rocks indicate it was derived from a mafic to intermediate igneous source. Hydro-thermally altered Gawler Range volcanic and subvolcanic rocks in the region are likely source rocks for these metals.