Copper-gold endoskarns and high-Mg monzodiorite-tonalite intrusions at Mt. Shea, Kalgoorlie, Australia: implications for the origin of gold-pyrite-tennantite mineralization in the Golden Mile

摘要

Five Cu-Au epidote skarns are associated with the Mt. Shea intrusive complex, located in the 2.7-2.6 Ga Eastern Goldfields Province of the Archean Yilgarn craton, in greenstones bounded by the Boulder Lefroy and Golden Mile strike-slip faults, which control the Golden Mile (1,435 t Au) at Kalgoorlie and smaller "orogenic" gold deposits at Kambalda. The Cu-Au deposits studied are oxidized endoskarns replacing faulted and fractured quartz monzodiorite-granodiorite. The orebodies are up to 140 m long and 40 m thick. Typical grades are 0.5 percent Cu and 0.3 g/t Au although parts are richer in gold (1.5-4.5 g/t). At the Hannan South mine, the skarns consist of epidote, calcite, chlorite, magnetite (5-15 percent), and minor quartz, muscovite, and microcline. Gangue and magnetite are in equilibrium contact with pyrite and chalcopyrite. The As-Co-Ni-bearing pyrite contains inclusions of hematite, gold, and electrum and is intergrown with cobaltite and Cu-Pb-Bi sulfides. At the Shea prospect, massive, net-textured, and breccia skarns are composed of multistage epidote, actino-lite, albite, magnetite (5 percent), and minor biotite, calcite, and quartz. Gangue and magnetite are in equilibrium with Co-Ni pyrite and chalcopyrite. Mineral-pair thermometry, mass-balance calculations, and stable-isotope data (pyrite delta~(34)S_(CDT) velence 2.5 per thousand, calcite delta~(13)C_(PDB) velence -5.3 per thousand, and delta~(18)O_(SMOW) velence 12.9 per thousand) indicate that the Cu-Au skarns formed at 500 + - 50 deg C by intense Ca-Fe-CO_2-S metasomatism from fluids marked by an igneous isotope signature. The Mt. Shea stock-dike-sill complex postdates the regional D1 folding and metamorphism and the main phase of D2 strike-slip faulting. The suite is calc-akaline and comprises horn-blende-plagioclase monzodiorite, quartz monzodiorite, granodiorite, and quartz-plagioclase tonalite porphyry. The intrusions display a wide range in silica content (53-73 wt percent SiO_2), in Mg/(Mg + Fe_(TOTAL)) ratio (0.37-0.89), and in K/(K + Na) ratio (0.02-0.31). Chromium (62-345 ppm), Ni (23-158), Sr (311-1361 ppm), and Ba (250-2,581 ppm) contents are high, Sr/Y ratios are high (24-278, mostly >50), and the rare earth element patterns are fractionated (Ce_N/Yb_N velence 17 - 41). These features and a negative niobium anomaly relative to the normal mid-ocean ridge basalt indicate that the suite formed by hornblende fractionation from a subduction-related monzodiorite magma sourced from metasomatized peridotite in the upper mantle. The magnesian composition of many intrusions was enhanced due to hornblende crystallization under oxidizing hydrous conditions and during the subsequent destruction of igneous magnetite by subsolidus actinolite-albite alteration. At the Shea prospect, main-stage Cu-Au epidote skarn is cut by biotite-albite-dolomite schist and by red biotite-albite replacement bands. Post-skarn alteration includes 20-m-tbick zones of sericite-chlorite-ankerite schist confined to two D3 reverse faults. The schists are mineralized with magnetite + pyrite + chalcopyrite (up to 0.62 percent Cu, 1.6 g/t Au) and are linked to skarn formation by shared Ca-Fe-CO_2 metasomatism. Red sericitic alteration, marked by magnetite + hematite + pyrite, occurs in fractured porphyry. The biotite/sericite alteration and oxidized ore assemblages at the Shea prospect are mineral-ogically identical to magnetite-hematite-bearing gold lodes at Kambalda and in the Golden Mile. Published fluid inclusion data suggest that a "high-pressure", oxidized magmatic fluid (2-9 wt percent NaCl equivalent, X_(CO_2) velence 0.1 - 0.2, 200-400 Mpa) was responsible for gold mineralization in structural sites of the Boulder Lefroy and Golden Mile faults. The sericite-alkerite lodes in the Golden Mile share the assemblages pyrite + tennantite + chalcopyrite and bornite + pyrite, and accessory high-sulfidation enargite with late-stage sericitic alteration zones developed above porphy copper de