Abstract

The Nevoria gold skarn deposit is located in the Southern Cross belt of the Archean Yilgarn craton, Western Australia, and occurs in amphibolite facies greenstones between two dome-shaped granitoid batholiths. Regional pressure estimates suggest that the terrane is deeply eroded and that the skarn formed at a 10- to 15-km depth. The orebodies are confined to the folded contacts of three beds of silicate facies iron-formation and extend over a strike length of more than 1 km. Iron-formation-hosted skarn contains 6 to 7 g/t gold, 2 to 3 g/t silver, and less than 0.1 wt percent base metals. The gangue is calcic, highly reduced, and composed of two distinct mineralogic types: hedenbergite-actinolite and almandine-hornblende skarn. The calc-silicates are intergrown with abundant pyrrhotite (10 vol %) and with accessory (< or =0.5%) pyrite, arsenopyrite-loellingite, and chalcopyrite. Native gold is enclosed in hedenbergite, actinolite, almandine, and quartz and occurs together with the alloy maldonite (Au 2 Bi) and a suite of bismuth tellurides. Skarns replacing amphibolite and metakomatiite adjacent to calcic skarn in banded iron-formation form broad (up to 17 m) contact zones of outer biotite-rich alteration. These zones are composed of numerous alternating bands of reduced diopside-hornblende, almandine-hornblende, and almandine-cummingtonite-plagioclase skarn. Veins of oxidized grossular-diopside skarn occur in the amphibolites between and adjacent to the banded iron-formations. The skarns are cut by barren pegmatite dikes and are underlain by a massive pegmatite-granite pluton. These postskarn intrusions, in turn, are displaced by brittle-ductile faults, which control zones of mesothermal muscovite-chlorite-calcite alteration replacing amphibolite. These zones lack pyrite and gold mineralization. The Nevoria gold skarn is part of a larger group of reduced skarn deposits, which formed in a deep midcrustal environment (10-15 km) during late Archean magmatism, possibly in broad continental arcs at convergent plate margins. The Archean skarns are distinct from the copper-gold skarns in Cenozoic continental margins and from gold-rich iron-copper skarns in Cenozoic to Mesozoic island-arc terranes, which all formed at much shallower depths of 2 to 5 km.

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