Geochemistry, Tectonic Setting, and Mineralization of High-Potassium Middle Proterozoic Rocks in Central Labrador, Canada

Abstract

The Sylvia Lake Formation of the Bruce River Group is one of several middle Proterozoic volcanic sequences along the northern margin of the Grenville structural province. It comprises a variety of basaltic to rhyolitic lava flows and pyroclastic rocks, of which the majority are felsic welded ash-flow tuff. The Sylvia Lake Formation is a bimodal sequence, in which low silica (basaltic) andesite and rhyolite are the predominant compositions. The rocks are enriched in potassium, and exhibit other characteristics of the high-K calcalkaline and shoshonite association. The volume of rhyolitic ash-flows in relation to the amount of mafic material, the presence of trachyandesites, and the overall chemistry are indicative of substantial influence of continental crust on the development of the volcanic pile. The volcanics are host to numerous base metal and uranium showings, and have potential for precious metal concentrations. The volcanism of the Sylvia Lake Formation represents the last expression of a long-lived middle Proterozoic plutonic episode in this area, but the tectonic framework at the time of generation of these magmas is poorly constrained.