THE ROLE OF SUBVOLCANIC SILLS AND PEPERITES IN FACILITATING THE ERUPTION OF ARCHEAN KOMATIITIC FLOWS AT THE DUNDONALD BEACH NI-CU-(PGE) DEPOSIT, NORTHERN ONTARIO

The volcanic-sedimentary succession at Dundonald Beach comprises (oldest to youngest): bedded rhyolitic heterolithic volcaniclastic rocks, graphitic argillaceous metasediments, komatiitic basalt sills, differentiated komatiitic basalt flows, and poorly-differentiated komatiitic peridotitic flows. The argillites are intruded by at least 5 generations of cm to m-thick basaltic komatiitic sills, as established by multiphase peperite generation and cross-cutting relationships between sills. The sequence of extrusive and intrusive events appears to have been: 1) Initial emplacement of komatiitic basalt sills into wet, unconsolidated argillite, resulting in passive fragmentation and generation of blocky peperite ranging from sediment-dominated to in situ komatiitic peperite. 2) Continued emplacement of komatiitic basalt sills that cross-cut and engulf earlier peperite. These sills have minimal peperite along their margins, but are characterized by local finger-like injections into peperite and argillite, and flow-banded, locally spherulitic chilled margins. 3) Continued emplacement of komatiitic basalt sills into argillites and earlier-formed sills. 4) Eruption of pyroxenite and peridotite spinifex-textured komatiitic flows without intercalated argillite. The initial emplacement of sills is interpreted to have dewatered and indurated the argillaceous sediments. Peperite formation appears to have not only indurated the sediments, such that subsequent sills show minimal peperite formation and are intruded into earlier sills, sediments, and peperite, but is also interpreted to have increased the bulk density of the sediment-sill package, such that subsequent komatiitic magmas were erupted onto the seafloor. Importantly, emplacement of thin basaltic komatiitic sills did not result in assimilation of sedimentary strata or achievement of sulfide saturation. However, subsequently-erupted thick channelized pyroxenitic and peridotitic komatiitic lavas appear to have thermomechanically-eroded S-rich sedimentary rocks, achieved sulfur saturation, and segregated Ni-Cu-(PGE) sulfide mineralization. The volcanic architecture of the Dundonald Beach area and abundance of high-level subvolcanic komatiitic sills suggests a proximal to near-proximal volcanic environment.