ABSTRACT

The geochemical characteristics of Archean unusual siliceous rocks at Mt. Goldsworthy region in the Pilbara Craton were studied. The siliceous rocks have been assigned to the uppermost Warrawoona Group mafic volcanic rocks, and are overlain by quartz-rich sandstone units of the Strelley Pool Chert that probably represent continental margin sedimentation. The Warrwoona rocks have been heavily altered and are now composed dominantly of microcrystalline quartz, with subordinate mica, Fe-Ti oxides and unidentified silicates; original magmatic textures are only locally preserved. The complex alteration is assumed to result from multiple events including weathering during subaerial exposure, circulation of hydrothermal fluids, and metasomatic silicification. During alteration, Al, Ti, Zr, Th, Cr and Sc remained immobile, although their concentrations were lowered by a substantial increase in silica. Mutual ratios of these immobile elements such as Al₂O₃/TiO₂ Cr/Th, Th/Sc, Cr/Al₂O₃ and Zr/TiO₂ and comparison with the least-altered contemporaneous mafic-ultramafic rocks in the Pilbara Craton show that the altered rocks originated from high-MgO rocks such as komatiite and high-MgO basalt, possibly of Al-depleted type. The Mt. Goldsworthy rocks have significantly higher Th/Sc (0.024-0.1) values compared with primitive mantle values (0.005) and komatiite (0.01). This feature is interpreted as a result of crustal contamination, which is consistent with the early evolution of continental crust.