The granite-pegmatite-associated Marvel Loch gold deposit, Yilgarn Craton, Western Australia: magnesian and calcic gold skarns in a 1.5 km long contact zone

Marvel Loch (63 t Au) represents a group of high-P (300–400 MPa) gold deposits spatially associated with 2.65–2.62 Ga, garnet-muscovite and spodumene pegmatites in the 3.0 Ga Southern Cross greenstone belt, Yilgarn Craton. The ore bodies terminate at 650–800 m depth at pegmatitic granite. Structural relations, mass-balance calculations, 87Sr/86Sr and 206Pb/204Pb in scheelite and galena, and the δ13CPDB of CO2 (-4.8‰) in the calcite-depositing fluid implicate the I-type granite below as the source. Magnesian olivine-calcite-chlorite-phlogopite, quartz-diopside, and calcic diopside-hornblende skarn (4–15 g/t Au), and low-grade biotite-anorthite-microcline gneiss occur in a 1.5 km long foliated zone bound to the contact meta-komatiite / meta-gabbro. Garnet-biotite and Ti-in-biotite thermometry indicate a peak temperature of 650 –550 °C during calcic skarn and biotite-anorthite-microcline replacement in meta-gabbro, and cooling to about 400 °C during retrograde alteration of anorthite to phengite + prehnite + clinozoisite, confirming estimates for magnesian skarn. The fluid (log fO2 = -18 bar at 650 °C) became reduced during cooling and progressive replacement, as suggested by Fe3+/Fetotal ratios in whole rocks, CH4 in fluid inclusions, and positive Eu-anomalies in scheelite. At a sulfur fugacity of 10− 7-10− 9 bar, cooling is reflected in pyrrhotite + Ni-loellingite (600 –500 °C) in calcite-rich magnesian skarn, arsenopyrite + pyrrhotite + base-metal sulfides ± bismuth (550 –400 °C) in quartz-diopside skarn, and retrograde pyrrhotite + pyrite ± marcasite (450 –350 °C) in metasomatic gneiss. Marvel Loch is a gold skarn deposit formed at 11–15 km depth in a batholith environment. Reduced tungsten skarns (7–10 km) share the W-Au-As-Bi signature, and the reduction of a magmatic H2O-CO2 fluid during cooling and replacement of ferrous or graphitic host rocks.