The komatiite-associated Boa Vista Ni-Cu sulfide deposit, Crixás greenstone belt, Central Brazil: Insights into komatiite flow facies and exploration

Abstract

The Boa Vista Ni-Cu deposit is an uncommon example of Neoarchean mineralization associated with aluminum-depleted komatiite (ADK). New data for the Boa Vista deposit, including geological and petrographic descriptions, as well as geochemical and isotopic results for komatiites and sulfide ore, are integrated into this study to improve the understanding of the processes associated with its origin. Although the deposit and host rocks were affected by tectonism and greenschist to amphibolite facies metamorphism, primary magmatic textures and compositions are variably preserved throughout the greenstone belt. Our results show that the Boa Vista deposit is hosted by an undifferentiated olivine cumulate channelized/conduit flow facies unit overlying flows of basalts and basaltic komatiites interlayered with sulfide-bearing graphite schist. The Boa Vista deposit is a Type I (basal/contact) of the komatiite-associated Ni-Cu-(PGE) deposit that was variably disrupted along shear zones. The main orebody consists predominantly of matrix ore breccia consisting of pentlandite, pyrrhotite, and minor chalcopyrite and has high Zn contents. Lithogeochemistry, AFC model, ore geochemistry, and sulfur isotopic data support crustal contamination of the komatiitic magma by graphite schist. Strongly positive δ³⁴S isotopic values for the Ni-Cu ore, ranging from +16 to +19, are modeled as resulting from contamination of the komatiitic magma with host graphite schist that contains sulfides with δ³⁴S values ranging from +30 to +34. The relatively low grades of the ores are attributable to relatively low metal tenors, consistent with a magma: sulfide ratio (R factor) calculated between 300 and 400. High Ni-MgO-Zn content can be a geochemical exploration criteria guide to finding new targets for komatiite-associated Ni-Cu-(PGE) sulfide deposits in central Brazil.