Geochemistry of metabasalts and hydrothermal alteration zones associated with c. 3.45 Ga chert and barite deposits: implications for the geological setting of the Warrawoona Group, Pilbara Craton, Australia

Abstract

Relatively unaltered metabasalts of the Archaean Coonterunah and Warrawoona Groups, Pilbara Craton are compared with altered metabasalts from immediately beneath bedded cherts of these groups to provide evidence for the depositional environment and hydrothermal alteration processes of crust formation. The geochemistry of relatively unaltered basalt, stratigraphy, and inherited zircon data indicate that the lower Warrawoona Group (3.53–3.43 Ga) formed as an oceanic plateau complex built on a sialic basement to 3.724 Ga, following an analogue with the Phanerozoic Kerguelen oceanic plateau, and not as a mid-ocean ridge or convergent volcanic-arc complex as previously proposed. Advanced argillic, argillic, phyllic, and propylitic alteration zones in footwall basalts of this succession are products of repeated episodes of seafloor hydrothermal circulation, syngenetic with bedded chert deposition, in the distal parts of high-sulphidation epithermal systems. The upper part of the Warrawoona Group (3.350–3.315 Ga Euro Basalt) represents a continental flood basalt event, up to 8 km thick, that erupted onto the older succession across a regional unconformity on which the Strelley Pool Chert was previously deposited. Widespread silica–alunite alteration of dolomitic chert protoliths and phyllic and propylitic alteration of footwall basalts are interpreted as products of fluid circulation driven by heat from the overlying, newly erupted lavas.