Archean gold skarns in iron formation, Southern Cross greenstone belt, Yilgarn Craton, Western Australia: Mid-crustal granite-related replacement deposits

The Bounty (41 t Au), Nevoria (12 t Au) and Southern Star (2.2 t Au) deposits in the 3.05-2.95 Ga amphibolite-facies Southern Cross greenstone belt, Yilgarn Craton, Western Australia, are stratabound to quartz-grunerite iron formation (BIF). The pyrrhotite-rich ore (5-8 g/t) is characterized by a hedenbergite-actinolite and almandine-hornblende ± quartz gangue classified as “skarn” in the sense of Törnebohm (1875). Amphibolite intercalated with BIF is replaced in contact zones by diopside-hornblende-biotite, cummingtonite-plagioclase-biotite, and biotite-cordierite-quartz skarn, the latter two containing minor almandine and gedrite. Grossular-diopside ± prehnite ± scheelite veins are part of the proximal skarn at Nevoria, where the TIMS garnet age (2635.7±1.2 Ma) of the ore and the SHRIMP zircon age (2634±4 Ma) of the granite below agree within error. In combination with Au-Bi enrichment in the roof pegmatite, the U-Pb ages identify the granite as the source of the skarn-forming fluid. In the distal Copperhead deposit (29 t Au), actinolite-carbonate ± pyrrhotite ore replaces BIF and forms thick veins in adjacent hornblende-biotite skarn and talc-chlorite schist. Granite porphyry sills are altered to quartz + chlorite + cordierite. The mineral assemblages indicate that the gold skarns formed at 550-600°C fluid temperature in a mid-crustal environment of 330-400°C ambient temperature.