The origin and evolution of V-rich, magnetite dominated Fe-Ti oxide mineralization; Northwest River Anorthosite, south-central Labrador, Canada

Abstract

Zones of massive to semi-massive Fe-Ti oxide mineralization are present in the Northwest River Anorthosite, a ca. 1625-Ma Paleoproterozoic massif-type anorthosite located in the Grenville Province of south-central Labrador. The oxide mineralization is predominantly composed of coarse to very-coarse grained V-rich magnetite with less abundant coarse granular ilmenite and pleonaste that formed by exsolution from the primary, chemically impure magnetite; all observed oxide minerals have complex and abundant subsolidus reequilibration and exsolution textures. Using petrography, oxide mineral chemistry, whole-rock geochemistry, and oxide-anorthosite field relationships, a three-process model is proposed for the formation of the oxide mineralization: (1) late-stage magmatic crystallization of impure magnetite; (2) concentration of Fe-Ti oxides via solid-state remobilization; and (3) reequilibration during subsequent post-emplacement cooling and Grenvillian metamorphism. These subsolidus processes significantly modified the primary magmatic composition of the magnetite through exsolution and reequilibration. Results from in-situ magnetite EPMA and LA-ICPMS analyses were plotted on magnetite mineral chemical discrimination diagrams. The data display a wide spread and typically do not plot exclusively in the expected anorthosite-hosted Fe-Ti-V magnetite fields. This is most likely due to modification of magnetite chemistry during subsolidus cooling and reequilibration. As such, care must be taken when using magnetite discrimination diagrams for Fe-Ti-V deposits given the complex petrogenetic histories that magnetite within these types of deposits experience.