Volcanic, structural, and hydrothermal controls on coincident Archean Low and High Sulfidation VMS Systems within the Onaman Assemblage, Onaman-Tashota Greenstone Belt, Northern Ontario, Canada

Abstract

High sulfidation volcanogenic massive sulfide (VMS) deposits show characteristics of VMS and high sulfidation epithermal deposits and are suggested to have formed through magmatic fluid contribution to an evolved seawater hydrothermal system. Two superimposed systems within the Neoarchean Onaman assemblage in Northwestern Ontario present an opportunity to assess the association between high sulfidation and traditional “low sulfidation” VMS systems. The stratigraphy of the Onaman assemblage consists of pillowed mafic volcanic flows, volcaniclastic rocks, and felsic flows and domes. The assemblage hosts a metamorphosed alteration system defined by kyanite and chloritoid-bearing mineral associations that overprint more traditional VMS associations of calcite, chlorite, and sericite. The sequence is consistent with subaqueous volcanism followed by uplift and erosion, which subsequently subsides to marine conditions. We postulate that the reactivation of synvolcanic structures, during uplift and subsidence, allowed for a direct magmatic volatile input into an evolved seawater dominated, VMS hydrothermal system.