The Nevoria gold skarn deposit in Archean iron-formation, Southern Cross greenstone belt, Western Australia; I, Tectonic setting, petrography, and classification

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.