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

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 10-15 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. BIF-hosted skarn contains 6-7 g/t gold, 2-3 g/t silver, and less than 0.1 wt. % 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 (≤ 0.5 %) pyrite, arsenopyrite-loellingite, and chalcopyrite. Native gold is enclosed in hedenbergite, actinolite, almandine and quartz, and occurs together with the alloy maldonite (Au2Bi) and a suite of bismuth tellurides. Skarns replacing amphibolite and metakomatiite adjacent to calcic skarn in BIF 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 BIF-horizons. The skarns are cut by barren pegmatite dikes, and are underlain by a massive pegmatite-granite pluton. These post-skarn 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 mid-crustal environment (10-15 km depth) 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 2-5 km.