Early Proterozoic Alkaline Igneous Rocks, District of Keewatin, Canada: Petrogenesis and Mineralization

Abstract

Summary Extensive potassic alkaline magmatism accompanied early stages in the formation of intracratonic basins in central Keewatin. Subaerial trachyandesite and trachyte flows, minette, syenite and bostonite dykes and small pyroxenite-syenite stocks comprise the c. 1.8 Ga igneous suite. High Mg, Cr and Ni contents in mafic flows and mica-rich lamprophyres indicate crystallization from mantle-derived magmas. Enhanced LIL element content and LREE enriched noninflected REE patterns are consistent with an enriched upper mantle source containing high-Ti micas. Enrichment in Sr, K, Rb, Ba and Th is interpreted as due to mobilization by water-rich fluids. Polymodal zoned clinopyroxene and mica macrocrysts in lavas and dykes record combined effects of crystal fractionation, disaggregation of wall rock xenoliths and magma mixing during rapid magma ascent through extending continental lithosphere. High Fe-Ti-Ba xenocryst micas may have originated metasomatically in the upper mantle where they were incorporated as partly melted crystals near the site of subsequent magma generation. Inhomogeneous lithospheric reworking and cracking during ç 1.9–1.85 Ga collisions may be responsible for post-tectonic extension and the voluminous alkaline magmatism in central Keewatin. A wide spectrum of uranium mineralization and minor gold and base metal occurrences are found in rocks of the alkaline suite. Syngenetic mineralization is limited to disseminated REE+U+Th in bostonite dykes. Minor U±Cu±Ag±Pb mineralization results from thermal and hydrothermal effects of dyke and pluton intrusion. Epigenetic uranium mineralization is associated with localized propylitic alteration of the alkaline rocks and with crosscutting quartz veins, fracture systems and fault zones. Important metal associations are U+Cu+Pb+Ag±Mo±Zn, U+Cu+Se+Ag+Au and Pb+Cu±Zn±Cd±Ag±Bi.