The Archaean Mount Gibson gold deposits, Yilgarn Craton, Western Australia: Products of combined synvolcanic and syntectonic alteration and mineralisation

Abstract

The Mount Gibson gold deposits are sited at the southern tip of the Archaean Yalgoo–Singleton greenstone belt, in the southern portion of the Murchison Province, Yilgarn Craton, Western Australia. Primary gold mineralisation is hosted within an eastward facing, dominantly tholeiitic metabasalt sequence which has been metamorphosed to amphibolite facies conditions. Alteration and mineralisation occurs within the north to north-northeast striking, steeply east-dipping, Mount Gibson shear zone, paralleling primary layering within the tholeiitic mine sequence. Two main styles of wallrock alteration and associated sulphide mineralisation are present. The first of these is represented by garnet (spessartine–almandine)–gahnite and cordierite–muscovite-bearing schists. These lithologies are unusual for lode-gold mineralisation, and, despite their apparently pelitic nature, have mafic precursors and reflect pre-metamorphic alteration of tholeiitic basalts. The garnet-bearing schists form a coherent stratiform horizon which overlies the main ore zone at the Orion Two deposit and are associated with recrystallised, metamorphosed, polymetallic sphalerite–galena–pyrite–(pyrrhotite–chalcopyrite–tetrahedrite) mineralisation. A more typical (for Archaean lode-gold), pervasive, syn-peak metamorphic, syn-shearing quartz–biotite±sulphide alteration envelope is associated with pyrrhotite–pyrite±chalcopyrite mineralisation at Mount Gibson. Although free gold is rare, there is a strong empirical relationship between enhanced gold grades and the pyrrhotite–pyrite±chalcopyrite assemblage, suggesting that gold was introduced as part of this event. Mass balance calculations at Orion Two show enrichment of Au, Ag, K, Rb, Ba and W and depletion of Na, Ca and Sr over mineralised zones, which is typical of lode-gold mineralisation. However, the deposit also exhibits strong base-metal enrichment, particularly within the garnet-bearing schists, and zonation from the Au–Cu enrichment of the main ore zone to the overlying, stratiform Pb–Zn–Ag–(Mn) enrichment, which is dominant in the garnet-bearing schist horizon. These features are typical of VHMS mineralisation. Hornet exhibits similar overall elemental enrichment and depletion to Orion Two. Lead-isotope data for sulphides from Orion Two and Hornet provide evidence of two discrete mineralising events at Mount Gibson, separated by approximately 300 m.y. A two-stage model best explains the features of the Mount Gibson gold deposits. Metamorphism of the aluminous alteration and weak mineralisation associated with an early synvolcanic event resulted in the formation of cordierite–muscovite and garnet (spessartine–almandine)–gahnite bearing assemblages, respectively, which were less competent than the surrounding tholeiitic metabasalts and consequently tended to localise shearing. Shear-hosted lode-gold mineralisation was synchronous with peak metamorphism and shearing. Like the shearing, hydrothermal alteration was mostly concentrated in and around the less competent rocks which were produced by metamorphism of synvolcanically altered precursors, resulting in a fundamental stratigraphic control on gold mineralisation at Mount Gibson.