Geochemistry and origin of the Ore Chimney Formation, a transported paleoregolith in the Grenville Province of southeastern Ontario, Canada

Abstract

The Ore Chimney Formation is a mid-Proterozoic metapelitic rock unit that separates mafic to intermediate metavolcanic rocks of the Grenville Supergroup from an overlying clastic metasedimentary succession, the Flinton Group. It has been interpreted alternatively as a paleoregolith or a product of hydrothermal alteration of the volcanic rocks, in a major shear zone. The formation is thin (<10 m), massive to poorly layered, crops out over 40 km along strike and is typically composed of alternating layers of biotite schist, muscovite-biotite schist, hornblende-biotite schist and hornblende schist. It typically contains coarse porphyroblasts of garnet and less commonly of staurolite, kyanite, epidote and/or hornblende. Thirty-five samples of the Ore Chimney Formation, fifty samples of the underlying metavolcanics and fourteen samples of the overlying clastic metasediments were analysed for major and twenty-eight trace elements (including REE). Immobile trace-element ratios (Ti/Zr, Zr/Y, Nb/Y, La/Yb) indicate that the Ore Chimney Formation is locally derived from the underlying metavolcanics. Bulk chemistry, field relations and petrographic observations strongly suggest that the Ore Chimney Formation is a locally-reworked weathering product of the metavolcanic rocks.