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

Abstract

The Northwest River Anorthosite (NWRA) is a ca. 1625 Ma Paleoproterozoic massif anorthosite located in the Grenville Province of south-central Labrador. Abundant zones of coarse-grained massive to semi-massive Fe-Ti oxide mineralization and disseminated oxide mineralization, which occur in sharp irregular contact with the host anorthosite are present in the NWRA. The Fe-Ti oxide minerals in the NWRA are dominated by magnetite with less abundant ilmenite and pleonaste; all oxide minerals contain abundant and complex exsolution features. Whole-rock geochemistry, petrography and in situ analyses show that regardless of field texture, oxide mineralization is similar in all of the samples analyzed. Whole-rock analyses of massive to semi-massive Fe-Ti oxide mineralization in the NWRA show values of ~10 wt. % TiO₂ and 2700 ppm V (0.48 wt. % V₂O₅). In-situ magnetite analyses contain <0.5 wt. % TiO₂ and 0.6-0.7 wt. % V₂O₅. Composite ilmenite contains ~1 wt. % Mg, 5-7 mol% hematite component and negligible Cr and Al content. Based on oxide mineral chemistry, oxide-anorthosite field relationships, and whole-rock geochemistry, it can be inferred that the formation of the oxide mineralization occurred in a three step process: 1) late-stage magmatic crystallization of impure magnetite; 2) concentration by solid-state remobilization and/or crystal sorting; and 3) reequilibration during subsequent post-emplacement cooling and subsequent Grenvillian metamorphism. These subsolidus processes significantly modified the primary magmatic composition of the oxide minerals through exsolution and reequilibration.