Metamorphism and heterogeneously distributed metasomatism in the Fortescue Group, Hamersley Basin, Western Australia

Mafic to intermediate volcanic rocks of the Fortescue Group form the lowermost stratigraphic unit of the 100,000 km2 Hamersley Basin on the southern margin of the Archaean Pilbara craton, Western Australia. A regional burial metamorphic gradient extends across the basin from prehnite-pumpellyite facies in the north through to epidote-chlorite-actinolite lower greenschist facies in the south and west. Superimposed on this metamorphic gradient is a heterogeneous distribution of pervasive hydrothermal alteration dominated by quartz-pumpellyite and quartz-epidote assemblages, with additional zones of intense silicification or albitization. Alteration assemblages are particularly well developed and most widely distributed in lower grade zones where they are associated with identifiable flow tops and extend along strike for several kilometres. In contrast, occurrences of distinct alteration assemblages become increasingly rare towards higher regional metamorphic grades. Whole-rock geochemical data indicates that zones of hydrothermal alteration are associated with additions of Si and Ca, and depletions in Fe and Mn. This same geochemical signature is, however, regularly observed in higher grade samples that do not contain distinct alteration assemblages. These data, combined with mineral chemical data and phase equilibria modelling suggest that the extent of metasomatism is significantly more widespread than is the distribution of observable mineralogical alteration. The age, size and compositional class of the Fortescue Group makes for an interesting comparison to the higher grade, more extensively deformed and highly mineralized greenstones of the eastern Yilgarn craton. Specifically, the size of the metasomatic system observed in the Fortescue Group has important implications for the extent of mineralized systems in the eastern goldfields.