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On the Size and Spacing of Volcanogenic Massive Sulfide Deposits within a District with Application to the Matagami District, Quebec

Patrick M. Carr^,* and Lawrence M. Cathles, III

Department of Earth and Atmospheric Sciences, 2122 Snee Hall, Cornell University, Ithaca, New York 14850

C. Tucker Barrie

C.T. Barrie and Associates, 29 Euclid Avenue, Ottawa, Ontario, Canada K1S 2W2

Corresponding author: e-mail, pmc1{at}cornell.edu

Volcanogenic massive sulfide (VMS) districts are typically ~40 km in diameter and contain about a dozen regularly spaced Zn-Cu orebodies, one or two of which contain more than half of the district’s resources. We numerically investigate this deposit size and spatial distribution by calculating zinc transport across the sea floor, first above sills of simple geometry, and then above the Bell River Complex at Matagami. For sills with simple rectangular geometry (i.e., a constant thickness), convection is strongest at the edge. The edge convection induces a progression of convection cells above the sill, although the sill cools mainly by the retreat of its thermal edges toward the sill center. If the rock permeability is a function of temperature that is maximized at 375°C, the resulting vents occur at stable, discrete locations that proxy for ore deposits. The fraction of sill heat vented at or above 300°C and the potential metal resources of the districts are greatest if the host permeability is 10^–15 m² (1 millidarcy) and the sill top is at 1 km depth. The simulations suggest that, as the host permeability increases, a single large deposit will progressively dominate a host of smaller ones. The Matagami simulations are based on a sill that tapers from 6.5 to 0 km thickness over a distance of 30 km. In the model, convection occurs both above the sill and along its underside, and metal is extracted from both sides of the cooling sill. The spacing of the resulting discharge sites is similar to that observed in actual VMS districts and, where direct comparison is possible, the mass of metal deposited is similar for an accumulation efficiency of ~3 percent.