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.
