NEOARCHEAN PEPERITES NEAR FIVEMILE LAKE, VERMILION DISTRICT, NORTHEASTERN MINNESOTA

Extremely detailed volcanic facies mapping (1:50 to 1:100 scale), petrographic studies, and lithogeochemical analyses have identified two distinctive deposits of peperites along the southern and northern shorelines of Fivemile Lake in the steeply north-dipping and north-facing Neoarchean Lower Member of the Ely Greenstone Formation in the Vermilion District of northeastern Minnesota. On the south shore of Fivemile Lake, matrix-supported blocky peperites occur in northeast-trending zones where massive basaltic andesite to andesite tuffs and lapilli tuffs have intermingled with northeast-trending, commonly pillowed, basaltic andesite to andesite dikes. Clasts in these deposits have curviplanar to angular and jigsaw puzzle-fit shapes, have extremely fine-grained margins, are moderately- to highly vesicular (10-50%), and compositionally similar to, more common adjacent to, and have their long axes aligned with, the northeast-trending basaltic andesite to andesite dikes. On the north shore of Fivemile Lake, basaltic andesite to andesite tuffs and lapilli tuffs that comprise the base of a 700 meter wide, 200 meter thick submarine tuff cone are intruded by northeast-trending, synvolcanic, amoeboid, pillow-like, and lobe-like coherent-facies basaltic andesite and andesite dikes that have locally undergone in-situ fragmentation to produce lapilli tuffs and tuff-breccias containing moderately to highly vesicular, curviplanar, ameoboid, and blocky jigsaw puzzle-fit lapilli and blocks. Locally, numerous thin (up to 1cm wide), parallel, vesicle-rich zones that more or less mimic the orientations of the margins of amoeboid basaltic andesite to andesite dikes, or the margins of basaltic andesite to andesite blocks, occur within the volcaniclastic component of these peperites. Blocky peperites south of Fivemile Lake appear to have formed in lava flow-confined submarine aquifers located several hundred meters below the paleoseafloor, whereas more amoeboid peperites found on the north shore of Fivemile Lake may have formed nearer to the seafloor within water-saturated submarine vent-fill deposits. Discordant zones of peperites can be used to identify synvolcanic fault zones that may be important in the exploration for volcanic-hosted massive sulfide deposits.