Structural setting and geochemistry of Archaean I-type granites in the Bardoc-Coolgardie area of the Norseman-Wiluna belt, Western Australia

Abstract

Abstract Regional structures in the Bardoc-Coolgardie area of the Archaean Norseman-Wiluna greenstone belt in Western Australia developed during early, large-scale thrusting followed by a transpressional regime characterized by ENEWSW shortening and large-scale sinistral strike-slip faults. Structural relationships between granites and greenstone successions indicate emplacement of granitic rocks throughout this deformation history. Emplacement mechanisms include solid- or liquid-state diapirism, forceful dilation of country rocks, and passive stoping. I-type granites are common, and include early- and late-tectonic intrusions. The widespread occurrence of I-type granodiorite and monzogranite, probably derived by partial melting of lower continental crust or subducted oceanic crust, suggests a continental margin setting for the belt. Alkalic quartz-diorite occurs as rounded enclaves in granodiorite, and is enriched in Ba, Rb, Sr, Ce, La, P and F, which suggests a mantle-derived volatile component. These quartz-diorite melts may have formed by partial melting of metasomatized mantle or, alternatively, mantle degassing may have lowered solidus temperatures in the source region, and thus induced melting. Emplacement of hot, volatilerich quartz-diorite melts into the lower crust may have lowered solidus temperatures in the source rocks for granodiorite and monzogranite. Recent models for Archaean gold mineralization suggest that auriferous fluids contained CO 2 derived from lower crustal or mantle sources. In the Bardoc-Coolgardie area, CO 2 may have been flushed out during partial melting of the lower continental crust. However, the origin of the gold in the mineralizing fluids requires some other explanation because gold mineralization is generally a late-stage event, and took place over a considerably shorter time interval than the formation and emplacement of I-type granites.