The Archean Chalice gold deposit: a record of complex, multistage, high-temperature hydrothermal activity and gold mineralisation associated with granitic rocks in the Yilgarn Craton, Western Australia

Abstract

The Chalice gold deposit (∼20 t Au produced), in the southwestern portion of the Late-Archean Norseman–Wiluna greenstone belt of the Yilgarn Craton, Western Australia, is hosted both in a sequence of intercalated mafic and ultramafic amphibolites and a post-peak metamorphism monzogranite dike. The deposit is flanked on the western side by calc-alkaline plutonic rocks, and to the east by a predominantly monzogranitic pluton, with at least four generations of monzogranitic dikes that intrude the local mine stratigraphy. Locally, four deformation events (D 1–D 4) have affected this sequence of amphibolites, and two stages of gold mineralisation are recognised, controlled by the progressive D 2 event. Main Stage gold mineralisation (95% of the resource), which is demonstrably later than all granitic bodies except second generation and subsequent monzogranite dikes, is controlled by localised D 2 asymmetric folds developed in the mafic amphibolite. It is expressed as veins and wall-rock replacement, characterised by a prograde assemblage of quartz–diopside–plagioclase–K-feldspar–titanite–pyrrhotite–pyrite–magnetite±garnet, hornblende, scheelite and biotite. Second Stage gold mineralisation (5% of the resource) is associated with the intrusion of a monzogranite dike and is expressed as disseminated gold in the dike, as well as foliation-discordant veins of quartz–gold, quartz–diopside–gold, actinolite–gold and molybdenite–tellurobismuthite–gold. Detailed field mapping and petrography suggest that Main Stage gold mineralisation at Chalice is equivalent to hypozonal orogenic lode–gold mineralisation in other Archean greenstone belts of the Yilgarn Craton, whereas monzogranite-associated Second Stage gold mineralisation is atypical of such deposits. The Main Stage mineralisation cannot be correlated to any particular granitoid in the local environment, although temporally, it is bracketed by granitic magmatism. A deep magmatic or metamorphic source cannot be resolved from the field data. It is also uncertain as to whether Second Stage mineralisation represents a true magmatic-gold event, or is a product of the assimilation and remobilisation of Main Stage gold, during the intrusion of the monzogranite dike. However, its distinctive metal association of Au–Bi–Mo–Te–W suggests the former. Irrespective of this uncertainty, Chalice remains as a deposit which is more intimately related both spatially and temporally to granitic magmatism than other Yilgarn deposits.