Variations in Large Synvolcanic Alteration Zones at Snow Lake, Manitoba, Canada, with Proximity to Associated Volcanogenic Massive Sulfide Deposits

Abstract

Juvenile 1.89 Ga oceanic arc volcanic rocks of the Flin Flon volcanic belt at Snow Lake are characterized by extensive zones with anomalous 1.82 Ga metamorphic mineral assemblages, including porphyroblasts of garnet, staurolite, amphibole, biotite, gahnite, and/or kyanite. They were produced from altered rocks created during premetamorphic, 1.89 Ga, synvolcanic, hydrothermal fluid-rock interaction. Three separate episodes of hydrothermal alteration are recognized that span evolution of the host volcanic rocks from a primitive (nascent arc?) to mature arc geotectonic setting. The geologic, geochemical, mineralogical, and isotopic attributes of the zones indicate that they include volcanogenic massive sulfide (VMS)-related alteration, and they were produced at high and low temperatures and formed in seafloor/near seafloor and subseafloor (intrastratal) environments. Large-scale alteration zones at Snow Lake are up to 20 km in strike length and 0.8 km wide. Their large exploration “footprint,” which is 36 times greater in areal extent compared to associated “pipe-like” alteration zones, means that such zones provide a useful target to “vector-in” exploration to VMS depositional settings within volcanic belts. The VMS-related large-scale alteration zones at Snow Lake display diagnostic variations in intensity and style of alteration along strike toward VMS deposits, are stratigraphically underlain by altered portions of synvolcanic intrusions, are crossed by discordant zones of more intensely altered rocks, and can be demonstrated to have formed by interaction with high-temperature(>350°C) hydrothermal fluids. Cu-Zn–rich VMS deposits at Snow Lake formed in flow-dominated sequences, are hosted by large rhyolite flow complexes, and comprise lensoid orebodies. In contrast, Zn-Cu–rich VMS deposits, although also rhyolite associated, formed in volcaniclastic-dominated sequences, are spatially related to a district aquifer, and comprise stratiform, laterally continuous orebodies. This suggests that exploration for Cu-Zn VMS deposits could selectively target flow-dominated sequences and focus on rhyolite flow complexes that display significant alteration. Similarly, exploration for Zn-Cu VMS deposits could focus on volcaniclastic-dominated sequences in which a potential aquifer has been identified.