Insights into the petrogenesis of the Sudbury Igneous Complex and its giant Ni-Cu-PGE sulfide deposits from orthopyroxene mineral chemistry

Abstract

The world-class “contact” Ni-Cu-PGE sulfide deposits of the Sudbury Igneous Complex are associated with physical depressions termed “embayments” that occur at its base. Whereas some of the embayments are exceptionally well mineralized, others are poorly mineralized. The mineralization in the embayments in the northern part of the complex is associated with a fragment-rich unit referred to as the Sublayer which is overlain by a melanorite unit termed the Mafic Norite. This study aimed to establish whether the whole rock geochemistry of the Mafic Norite and the mineral chemistry of the orthopyroxenes in it could be used to discriminate between the well mineralized embayments and those that are not. The mineral chemistry of orthopyroxenes also helps to establish the origin of domains of melanorite that occur within the Sublayer.

The Mafic Norite samples from the well mineralized embayments have higher Ni and Cu grades than those of samples from the weakly mineralized embayments. The Ni tenors of the sulfides in the Main Mass Mafic Norite samples are also higher in the former than in the latter. The Ni contents of the orthopyroxene in Mafic Norite samples from the well mineralized embayments are higher than those of orthopyroxene in samples from the weakly mineralized embayments. Hence, whole rock and mineral chemistry data as well as the Ni contents of the sulfides all provide good discriminants between well mineralized and poorly mineralized environments in the Sudbury Ni-Cu-PGE sulfide camp. As orthopyroxenes are present in all of the rocks in the lower portion of the SIC, they provide a useful indicator of mineral potential.