The Savage Lode magnesian skarn in the Marvel Loch gold–silver mine, Southern Cross greenstone belt, Western Australia. Part 1: Structural setting, petrography, and geochemistry

Abstract

The Marvel Loch mine in the Archean Yilgarn Block. Western Australia, has produced 9.70 t gold and 1.92 t silver from 3.28 × 106 t of oxide–sulphide ore. The deposit is located in the Southern Cross greenstone belt, and occurs within the medium-grade metamorphic aureole of the Ghooli Dome granitoid batholith, about 1.5 km from the granitoid–greenstone contact, The deposit is controlled by a broad ductile shear zone and spatially associated with late syn- to post-mineralization pegmatite dykes. The major orebodies, represented by the Savage Lode, are hosted by a uniform sequence of metakomatiites, and form zoned replacement bodies oriented subparallel to the steeply dipping foliation of the shear zone. Lens-shaped domains of pillowed metakomatiiles (hornblende + cummingtonite + chlorite) are locally preserved between the orebodies.The gangue of the Savage Lode is laterally zoned, and shows distinct similarities to Phanerozoic magnesian skarns. Calcite–olivine rock and calcite–phlogopite–chlorite schist occur in the centre, and are enveloped by three types of calcite-poor ore, namely banded diopside–amphibole rock, quartz–diopside veins, and tremolite–phlogopite schist. The latter grades laterally into subeconomic, outer tremolite–chlorite schists. Local retrogression of prograde alteration minerals is evident in the partial replacement of olivine by iddingsite and serpentine, and in the occurrence of late muscovite, clinozoisite, and prehnite. The gangue in the lode reflects strong carbon dioxide, calcium, and potassium metasomatism.Hydrothermal oxides and sulphides (2–5 vol.%) occur disseminated throughout the Savage Lode. The oxide assemblage includes hercynite–spinel, magnetite, ilmenite, and scheelite, whereas the sulphide assemblage is dominated by pyrrhotite, loellingite, and arsenopyrite. Native gold occurs as discrete grains (0.001–3 mm) intergrown with sulphides or enclosed in gangue minerals. The average magnesian ore skarn is characterized by a MgO–FeO ratio of 2.0:1, a Au–Ag ratio of 1.7:1, and low base metal (< 500 ppm), anomalous tungsten (20–40 ppm), and high arsenic (2900 ppm) contents. The Savage and other lodes in the Marvel Loch mine may be classified as gold or gold–silver skarns by economic metal content. The magnesian rather than calcic nature of the Marvel Loch skarns is related to the high magnesium content (21 wt.% MgO) of the precursor metakomatiite rocks.