Regional-scale Metasomatism in the Fortescue Group Volcanics, Hamersley Basin, Western Australia: Implications for Hydrothermal Ore Systems

Abstract

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. These represent one of the oldest (2·8–2·7 Ga) known examples of a continental flood basalt sequence. A regional burial metamorphic gradient extends across the basin from prehnite–pumpellyite facies in the north to epidote–actinolite greenschist facies in the south and west. Superimposed on this metamorphic gradient, regional-scale metasomatism has affected extensive areas of the Fortescue Group. Metasomatized mafic lavas are characterized by well-developed assemblages dominated by pumpellyite–quartz or epidote–quartz associations. The mineral associations of metasomatic domain types broadly match the distribution of metamorphic isograd indicator minerals with a southward and westward increase in the proportion of epidote. A continuum exists between least altered rocks that preserve the regional metamorphic signature and the most intensely altered metasomatized rocks. Metasomatism is essentially continuous over a stratigraphic strike length of 100 km and across a strike width of 20 km. Regionally, metasomatically altered volcanic rocks occur widely across the Hamersley Basin and its outliers, over an area of some 450 km by 200 km. Metasomatic alteration is most conspicuous in the lower-grade metamorphic zones because pumpellyite- and epidote-rich rocks are green to yellow–green in outcrop. Whole-rock geochemical data indicate that metasomatism is associated with strong depletions in alkalis (Na, K, Li, Rb), alkali earths (Mg, Sr, Be, Ba) and heavy first transition series metals (Mn, Fe, Co, Ni, Cu, Zn), with a significant enrichment in Si. Calcium shows more variable mobility. Such geochemical trends, particularly depletions in Fe, Mn and base metals (Co, Ni, Cu, Zn), together with the metasomatic mineral assemblages, are comparable with those associated with the sub-sea-floor circulation of seawater, particularly in relation to the metal-depleted root zones of base metal deposits. Petrographic features indicate that the development of metasomatic mineral associations post-dates the formation of regional metamorphic assemblages. Consequently, it is interpreted that the hydrothermal fluid flowed through the buried pile after sufficient time to allow for the metamorphic mineral assemblages to approach equilibrium with ambient P–T conditions. The hydrothermal depletion in Fe observed across the Fortescue Group mafic lavas is intriguing given the abundance of iron ore in the overlying Hamersley Group, and a possible connection cannot be ignored. Furthermore, the scale of fluid flow observed in the Fortescue Group, which occurred through zones of inherent permeability, such as vesicular and brecciated lava flow tops, without the aid of major cross-cutting structures, has significant implications for the size of metasomatic systems in other mafic volcanic terranes, with potential consequences for the exploration for hydrothermal mineral deposits.