Gold and aurostibite from the metaturbidite-hosted Au–Zn–Pb–Ag Hera deposit, southern Cobar Basin, central NSW, Australia: geochemical and textural evidence for gold remobilisation
Abstract The Devonian Hera metaturbidite-hosted polymetallic Au–Zn–Pb–Ag deposit of central NSW, Australia, contained a total undepleted resource of 3.6 Mt @ 3.3 g/t Au, 25 g/t Ag, 2.6% Pb and 3.8% Zn. The deposit comprises a number of distinctive lodes with each containing a distinctive ore and alteration/gangue mineralogy, though generally the sulfide ore comprises various mixtures of sphalerite, galena, chalcopyrite, pyrrhotite and relatively common visible gold–electrum. The North Pod and Far West lodes are distinctly Sb rich and contain a more diverse ore mineralogy with arsenopyrite, native silver, native antimony, gudmundite, tetrahedrite-(Fe), argentotetrahedrite-(Fe), acanthite, dyscrasite, nisbite and breithauptite. From analysis of 52,760 assays from across the deposit it was found that there was a very poor correlation between gold and each of Fe, Zn, S, Pb, Cu, As and Ag, whereas Ag correlated reasonably well with both Pb and Zn. Results from EPMA shows that gold varies widely in composition from host-rock associated gold (96 wt.% Au) through more intermediate compositions (88–73 wt.% Au) to electrum (46–27 wt.% Au), commonly associated with Sb-phases and containing significant Sb within the gold itself (1.05–2.58 wt.% Sb). From the Far West lense, aurostibite occurs as distinctive rims around gold. Although aurostibite associated with gold contains no silver, the gold itself contains constant moderate amounts (10.87–12.27 wt.% Ag). We suggest that the aurostibite and other Sb phases formed from a late-stage Sb-rich hydrothermal during low-temperature retrograde skarn alteration. There is abundant evidence for both chemical and physical remobilisation at Hera and this remobilisation is largely responsible for the spectrum of gold compositions observed. The source for these fluids may be an underlying magmatic body, evidence for which occurs as granite pegmatite dykes in various locations throughout the deposit. Furthermore, gold with a moderate to high Sb content may be indicative of a low temperature of formation.