Seafloor deposit models, geochemistry, and petrology of the mafic-ultramafic hosted Big Lake VMS occurrence, Marathon, Ontario

The Big Lake volcanogenic massive sulphide (VMS) occurrence, located in the Schreiber-Hemlo belt of the Superior Province, was discovered in March 2006 near Marathon, Ontario. It is hosted in a mafic-ultramafic metavolcanic sequence lacking felsic volcanic or volcaniclastic rock, and consists of a thin, locally anastomosing sheet of veined pyrrhotite, chalcopyrite, and sphalerite (+/- galena, cobaltite) currently defined over a plan area of approximately 0.5x0.5 km, along the base of a series of peridotite and pyroxenite cumulates termed the Big Lake Ultramafic Complex (BLUC). Hydrothermal alteration at Big Lake is restricted to within a few metres above visible sulphide mineralisation. The geometry of the occurrence may be in part a function of limited permeability in a flow-dominated setting.;The metavolcanic rocks that host the occurrence are basalts to komatiitic basalts, with ∼10% of samples having >18 wt. % MgO. Those stratigraphically below the VMS mineralisation are transitional to alkaline basalts, with convex-upward LREE and fractionated HREE. Their NEB-like characteristics can be explained by mixing of depleted and enriched (OIB-like) mantle plume melt components. Basalts stratigraphically overlying the VMS mineralisation have the flat REE patterns (La/Ybcn=1-2) typical of Wawa tholeiitic plateau basalts. As a whole, the lithostratigraphic assemblage at Big Lake is consistent with an oceanic plateau setting, as in the mafic category of VMS deposits, and likely formed by eruption of variably enriched plateau basalts and thick ultramafic flows from a heterogeneous mantle plume.;Depending on the exact roles or effects of shear deformation, different possibilities for heat sources, and other aspects that could not be clearly resolved in this study, four models were developed to describe the genesis of the Big Lake VMS occurrence. The favoured model involves a genetic relationship between the ultramafic cumulates and the stratigraphically overlying VMS occurrence, whereby cooling of the BLUC flow(s) drove the hydrothermal circulation responsible for VMS mineralisation near its upper margins. If this model is accurate, the Big Lake VMS occurrence is the first documented example of a VMS system related to ponded ultramafic flows outside of the Abitibi subprovince, and demonstrates that similar styles of shallow intrusion or komatiite flow-driven VMS convection cells may have developed elsewhere in the Superior Province or in other Archean cratons.;Several lines of evidence were used to conclude that the host lithostratigraphy to the Big Lake VMS occurrence is overturned. Moving down hole and up stratigraphy, the sequence consists of cumulate peridotite and pyroxenite, adjacent flows, VMS-hydrothermal alteration and sulphides, siltstone, and flows with interbedded, locally pelitic siltstones and iron formation. The ultramafic cumulates and adjacent underlying volcanic rock are likely a series of ponded ultramafic flows.