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 |
| |
Authors: | Andreas G Mueller |
| |
Institution: | (1) 12a Belgrave Street, Maylands, Western Australia, 6051, Australia |
| |
Abstract: | 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% 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%), 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, actinolite, albite, magnetite (5%), 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 δ34SCDT = 2.5‰, calcite δ13CPDB = −5.3‰, and δ18OSMOW = 12.9‰) indicate that the Cu–Au skarns formed at 500 ± 50°C by intense Ca–Fe–CO2–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 hornblende–plagioclase
monzodiorite, quartz monzodiorite, granodiorite, and quartz–plagioclase tonalite porphyry. The intrusions display a wide range
in silica content (53–73 wt% SiO2), in ratio (0.37–0.89), and in 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 . 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-thick zones of sericite–chlorite–ankerite schist confined to two D3 reverse faults. The
schists are mineralized with magnetite + pyrite + chalcopyrite (up to 0.62% Cu, 1.6 g/t Au) and are linked to skarn formation
by shared Ca–Fe–CO2 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 mineralogically 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% NaCl equivalent, , 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 porphyry copper deposits. |
| |
Keywords: | Archean Skarn Epidote Copper Gold Mt Shea Kalgoorlie Australia |
本文献已被 SpringerLink 等数据库收录! |
|