Rhyolitic volcanism and the geochemical evolution of an Archaean central ring complex: the Blake River Group volcanics of the southern Abitibi belt, Superior province

The stratigraphic and geochemical relations of the Blake River Group volcanics of the southern Abitibi metavolcanic belt are explained within the context of an Archaean ring complex. Tholeiitic eruptions were limited to the periphery of the complex, whilst the central complex was characterized by voluminous eruptions of calc-alkaline andesites and rhyolites. Relative to the tholeiitic lavas, these andesites are enriched in LIL-elements and are interpreted to represent contaminated tholeiitic magmas. The rhyolites associated with the central complex are high-SiO₂ (75-80% SiO₂) and show variable trace element enrichments. The development of such SiO₂-rich rhyolite liquids is interpreted as taking place within a high-level, zoned magma chamber. The presence of the ore bodies in the Blake River Group is directly related to the rhyolitic volcanics and it is suggested that these ore bodies may, in part, represent magmatic hydrothermal fluids.