Platinum-group element mineralization in Nipissing Gabbro intrusions and the River Valley intrusion, Sudbury Region, Ontario

The ∼2.5 Ga River Valley intrusion is one of several Early Proterozoic layered mafic intrusions that, along with the ∼2.2 Ga Nipissing Gabbro suite of intrusions, form the dominant mafic intrusive bodies within the Huronian Magmatic Belt, in the Sudbury Region, Ontario. These intrusive suites present excellent exploration targets for PGE-Cu-Ni sulphide mineralization.; In the River Valley intrusion, the Marginal Series rocks are host to magmatic, low-sulphide, Cu-PGE-rich mineralization that is associated with heterogeneous fragment-bearing rocks (Breccia Unit). The Breccia Unit is up to 100 m wide and occurs within a few metres of the intrusive contact. It is characterized by pyroxenitic fragments hosted by a gabbroic matrix. The geochemistry of the fragments indicates that they are xenoliths, entrained in the magmas that now constitute the matrix and PGE-rich sulphide mineralization. PGE-rich sulphide, which is predominant in the matrix but also occurs in the fragments, was introduced to the intrusion as suspended droplets in a second-stage magma.; The Nipissing Gabbro represents the intrusive portion of an eroded Continental Flood Basalt. Low-sulphide, Cu-PGE-rich mineralization is best represented by stratabound sulphides that occur in the lowermost orthopyroxene gabbro unit. Chalcophile element variations through mineralized and poorly differentiated intrusions indicate in-situ sulphide fractionation from the base upwards, whereas other intrusions exhibit simultaneous inward-directed sulphide precipitation from the base and top of the sill.; Evidence for significant crustal contamination of the mantle-derived parental magmas (high (Th/Nb)N), the high PGE tenor of the sulphides, and the lack of PGE-depletion in the rock units that overly the Breccia Unit at River Valley, suggests that the sulphides formed in a deep seated "staging chamber". Here, primitive mantle-derived magma assimilated crustal rocks and became crustally contaminated, which induced S-saturation and co-precipitation of PGE-rich sulphides and olivine-orthopyroxene cumulates. When new primitive magma flowed into the staging chamber it displaced the initial magma and the early-formed cumulates (fragments) and PGE-rich sulphides were carried upwards in pregnant magmas and emplaced into the River Valley chamber.; Adiabatic decompression in the ascending magma permitted some of the S in the sulphide melts to dissolve into the magma; PGE would have also remained in the residual sulphides leading to an increase in the PGE tenor. Evidence for PGE depletion in rocks overlying PGE-rich mineralized zones in Nipissing Gabbro suggests that the sulphides went into solution rather than being entrained as droplets in the ascending magmas.