The Savage Lode magnesian skarn in the Marvel Loch gold–silver mine, Southern Cross greenstone belt, Western Australia. Part 2: Pressure–temperature estimates and constraints on fluid sources

Abstract

The Savage Lode gold skarn orebody in the Marvel Loch mine, Southern Cross greenstone belt, Western Australia, replaces foliated metakomatiites in a subvertical ductile shear zone, and is located within the broad metamorphic aureole of the Ghooli Dome granodiorite–granite batholith. Pressure estimates based on metamorphic as well as alteration mineral parageneses indicate that skarn formation took place at P = 4 ± 1 kbar (1 kbar = 100 MPa), corresponding to a burial depth of about 13 km. Metamorphism reached temperatures of 550–630 °C in the mine area, and preceded skarn formation.Pressure–temperature calculations constrain the maximum temperature of the hydrothermal fluid in the Savage Lode to 640 ± 20 °C during the formation of early olivine–calcite and diopside–amphibole rocks, and to 500–600 °C during the formation of phlogopite–chlorite–calcite schists and quartz–diopside veins. Lower fluid temperatures (500 to ~ 400 °C) are recorded by retrograde serpentine (after olivine), talc (after tremolite), and petrographically late aggregates of muscovite + clinozoisite + prehnite. The oxygen fugacity of the fluid is estimated at 10−20–10−24 bar (1 bar = 100 kPa), based on the assemblage magnetite + ilmenite + olivine in calcite-rich prograde skarn. The sulphur fugacity of the fluid is estimated at 10−7–10−9 bar, based on the assemblages pyrrhotite + loellingite, pyrrhotite + arsenopyrite and pyrite + marcasite, which were deposited in a retrograde régime, when fluid temperatures fell from 550 to < 400 °C.The isotopic composition of the fluid (206Pb/204Pb = 13.77, 87Sr/86Sr = 0.7024, δ13C = −4.8‰), as inferred from hydrothermal galena, scheelite, and calcite, indicates equilibration at high temperatures with rocks of granodioritic or granitic composition. Late syn- to post-mineralization pegmatite dykes, exposed in the present mine workings, provide evidence for magmatic activity at depth. The Marvel Loch deposit represents the first well-described example of an Archean gold skarn system.