1. C. T. Barrie and Associates, Inc., 23 Euclid Avenue, Ottawa, ON K1S 2W2, Tubarrie@nrcan.gc.ca

2. Department of Geological Sciences, Cornell University, Ithaca, New York 14853, ahe2@cornell.edu

Recent geological studies in the Kidd-Munro Assemblage of the southwestern Abitibi Subprovince in Ontario have highlighted VMS-style Zn-Cu mineralization within komatiitic stratigraphic successions. In Dundonald township, the Terminus Zone Zn-Cu mineralization (tens of thousands of tonnes of near-ore grade Zn + Cu sulfide) is located within komatiitic basalt pillow breccia and graphitic argillite. This mineralization is 60-100m upsection from the Empire Flow, the basal, thickest, and most magnesian komatiite flow of the succession. In Munro township, the Potter Zn-Cu mine (~2 million tonnes Zn + Cu sulfide) is located within komatiitic basalt hyaloclastite lenses, 40-130m upsection from a komatiite lava lake with a 300m-thick dunite-peridotite basal section. Zn-Cu mineralization at bot Terminus and Potter is located above paleo-vertical walls at the edges of the komatiite flows. Geologically similar examples are known in two locations in the Rainy River and Red Lake areas of Northwest Ontario.

Simple heuristic calculations indicate that a komatiite flow core cooling from 1500oC to 350oC can drive ~4x its volume of hydrothermal fluid at 350oC, the typical maximum venting temperature of magma-driven seafloor hydrothermal venting. These calculations are tested with a series of relatively simple, two-dimensional, finite element heat and fluid flow models for the Terminus Zone mineralization where the geometry and geochemistry of th eflow are relatively well-constrained. Initial models have individual finite elements of 50m x 50m or smaller. Assumptions are: 1) a relatively permeable komatiitic basalt substrate at 10-15m2 ; 2) a less permeable komatiite flow (due to serpentinization) heat source at 10-16 m2; 3) a geothermal gradient of 25oC/km; 4) thermal conductivities of 7 x 10-3 cal/cm-secoC; 5) a liquidus temperature of 1440oC as estimated using the MgO geothermometer for komatiite liquids; and 6) thermal cracking, with permeability enhancement from 275-475oC, and permeability reduction at higher temperatures. The time steps are 10 years. The initial models show that relatively thin (<50m thick) "overbank" edges of the Empire Flow cool within to <200oC within 200 years, whereas the thicker (~200m-thick) peridotite-dunite core remains at >200oC for ~500 years. Venting is uniform and vigorous over the core of the flow for ~400 years, but in the absence of more permeable conduits to the sea floor, the venting temperature is always <200oC. The volume of fluid vented is in keeping with the heuristic calculations, and adequately explains the mass of Zn-Cu mineralization at Terminus.