The transition from barren Sudbury Breccia to Sudbury Breccia zones hosting footwall Cu-Ni-PGE sulfide ore at the Fraser mine, Ontario, is accompanied by an increase in the concentration of bulk-rock Cl and Br and water-soluble (at 20°C) Cl and Br and in the Cl:Br ratio in the matrix (pseudotachylite) of the breccia. Anomalous bulk-rock concentrations of Cl (1.5-10 times background) and Br (1.5-2 times background) are detectable for distances of at least 150 m away from the first visible signs of footwall sulfide mineralization. The Cl:Br ratio in the bulk rock increases with proximity to ore, reflecting the increasing abundance of Cl-bearing and Br-excluding silicate minerals in the matrix of the breccias. Up to ∼70% of the Cl (on average, 35-40%) and ∼80% of the Br (on average, 25-35%) in Sudbury Breccia matrix are water-soluble at room temperature, and reside in (i) primary fluid inclusions along recrystallized grain-boundaries, which increase in abundance with proximity to ore, and (ii) halite crystals in pore spaces and vugs in the matrix. Aqueous solutions prepared by leaching powdered samples of the matrix of the Sudbury Breccia exhibit variable Ca:Na and Cl:Br ratios, which may be explained partly by the precipitation of small amounts of halite (<10-5 wt% of the bulk rock) in pore spaces and vugs within the matrix of the breccia by parental groundwaters. As an alternative, the range in leachate compositions may be explained by the presence of trapped mixtures (in fluid inclusions) of Cadominant groundwater and a Na- and K-dominant fluid that had elevated and variable Cl:Br ratios, possibly exsolved from the SIC (Sudbury Igneous Complex). In general, elevated bulk Cl and Br, water-soluble Cl and Br, and bulk Cl:Br ratio are reliable indicators of proximity to footwall Cu-Ni-PGE sulfide mineralization, particularly in samples devoid of disseminated sulfide or where sulfide minerals are sparse.

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Canadian Mineralogist
Department of Earth Sciences

Hanley, J.J. (Jacob J.), Mungall, J.E, Bray, C.J. (Colin J.), & Gorton, M.P. (Michael P.). (2004). The origin of bulk and water-soluble Cl and Br enrichments in ore-hosting Sudbury Breccia in the Fraser copper zone, strathcona embayment, Sudbury, Ontario, Canada. Canadian Mineralogist, 42(6), 1777–1798. doi:10.2113/gscanmin.42.6.1777