Mineralogy and PTX relations of the Archean Hannan South Au-Cu (Co-Bi) deposit, Kalgoorlie, Western Australia: Thermodynamic constraints on the formation of a zoned intrusion-related skarn

The Hannan South Au-Cu skarn deposit is located 12 km southeast of Kalgoorlie in the 2.7 Ga Eastern Goldfields orogen of the Archean Yilgarn craton, Western Australia. Two epidote-quartz-calcite-magnetite ore bodies, controlled by the same reverse fault but separated by 450 m, occur in granodiorite porphyry: a Au-Cu skarn in the southeast (30 vol % calcite) and a Au-Co-Bi skarn in the northwest (5-10 vol % calcite). Both are interpreted as remnants of a single eroded body. Regional and skarn pressure estimates of 400±150 MPa indicate formation at 14±5 km paleo-depth. Thermodynamic analysis of the silicate-calcite and oxide-sulfide assemblages constrains the temperature of the H2O-CO2 fluid to 480-530°C in the proximal SE-skarn (0.5 wt % Cu, 0.35 g/t Au, 6 g/t Ag), and to 417±32°C in the more distal NW-skarn (<0.1 wt % Cu, 3.5 g/t Au, 1-2 g/t Ag). The oxygen fugacity of the fluid declined with temperature from log fO2 = -18 to -19 (at 500°C) in the southeast, defined by magnetite + rutile ± hematite and by iron-rich epidote (pistacite = 0.31), to log fO2 = -24 to -26 (at 415°C) in the northwest, defined by magnetite + rutile ± ilmenite and by low-iron epidote (ps = 0.25). The high oxygen fugacity stabilized phengitic muscovite + magnetite and restricted the occurrence of biotite to the northwest skarn front. The fugacity of total dissolved sulfur decreased with temperature from log fS2 = -3 to about log fS2 = -6.5 buffered by pyrite ± pyrrhotite. In the SE-skarn, copper precipitated mainly as intermediate solid solution (ISS, Cu=Fe), which exsolved inclusions of sphalerite, galena and Ag-Bi alloy but retained elevated bismuth (≤1600 ppm) and silver (≤1800 ppm) during cooling and inversion to chalcopyrite. Minor gold and electrum (≤60 wt % Ag) precipitated together with chalcopyrite and pyrite. In the more distal NW-skarn, fluid cooling and reduction caused the deposition of pyrite, minor cobaltite, native gold and bismuth minerals leading to the large-scale enrichment of gold (3.5 ppm) and bismuth (130-240 ppm). While the SE-skarn contains trace hessite, tetradymite and aikinite, all stable to at least 540°C, the NW-skarn contains abundant bismuth sulfosalts (krupkaite, pekoite, junoite, felbertalite, nuffieldite, cosalite) and tellurosulfides (joseite, aleksite). Cosalite is not stable above 425±25°C. The zoned Hannan South skarn is part of a cluster of intrusion-related Au-Cu and distal Ag-Au-Pb-Zn deposits located within 60 km east to southeast of the giant Golden Mile (>1500 t Au) at Kalgoorlie, and part of a larger genetic group including the diorite-hosted Boddington porphyry-skarn deposit (968 Mt at 0.11% Cu and 0.65 g/t Au).