Oxide mineralogy and trace element chemistry as an index to magma evolution and Marathon-type mineralization in the Eastern Gabbro of the alkaline Coldwell Complex, Canada

Abstract

The Eastern Gabbro of the alkaline Coldwell Complex, Canada, represents a Ni-poor conduit-type system that comprises two rock series, the Layered Series and Marathon Series, which intruded into a metabasalt package. Based on distinct variations in magnetite compatible (e.g., Ni, Cr) and incompatible (e.g., Sn, Nb) elements in Fe–Ti oxide intergrowths, the metabasalts, Layered Series, and Marathon Series must have crystallized from magmas that originated from compositionally distinct sources. Of these rock units, the metabasalts crystallized from a more primitive melt than the Layered Series as Fe–Ti oxides in the former have higher concentrations of magnetite-compatible elements. Unlike the metabasalts and Layered Series, the Marathon Series crystallized from multiple, compositionally distinct magmas as Fe–Ti oxides in this series exhibit large variations in both magnetite compatible and incompatible elements. Accordingly, the various rock types of the Marathon Series cannot be related by fractional crystallization of a single batch of magma. Rather, the magmas from which the rock types crystallized had to have interacted to variable degrees with a late input of more primitive melt. The degree of this magma interaction was likely controlled by the geometry of the conduit and the location of emplacement given that Fe–Ti oxides in the oxide-rich rocks occur in pod-like bodies and exhibit no compositional evidence for magma mixing. Mirrored variations in magnetite compatible and incompatible elements in Fe–Ti oxides in the Footwall Zone, Main Zone, and W Horizon of the Marathon Cu–PGE deposit indicate that these zones could not have formed from a single, evolving magma, but rather multiple batches of compositionally distinct magmas. Fe–Ti oxides exhibit no compositional difference between those hosted by barren and mineralized rock. This is likely because sulfide liquated at depth in all of the magmas from which the Marathon Series crystallized. The composition of Fe–Ti oxides in the Eastern Gabbro fall outside of the compositional fields for Ni–Cu mineralization defined by Dupuis and Beaudoin (Mineral Deposita 46:319–335, 2011) and Ward et al. (J Geochem Explor 188:172–184, 2018) demonstrating that their discrimination diagrams can distinguish between Ni-rich and Ni-poor systems that contain disseminated and massive sulfides.