Abstract

Average lead isotope growth curves for shale-hosted zinc-lead deposits have been constructed empirically for a large but well-defined tectonic segment of the Canadian Cordillera. The model defined is applicable to the dating of galena-lead from shale-hosted deposits in the Selwyn basin and in the Ogilvie, Richardson, Wernecke, and Mackenzie Mountains, in the Eastern fold belt, and in much of the Omineca crystalline belt. This area of the cordillera generally had a coherent geochemical evolution. Lead for deposits was extracted from sediments which had been deposited along the western margin of the North American craton and which were derived largely from upper crustal crystalline rocks that formed the western part of the Canadian Shield.The average shale growth curves involve three stages of evolution. The third (most recent) stage of the three-stage model, starts at 1.89 m.y. and evolves to 0 m.y. with mu of 12.16 and omega of 49.09. The first two stages follow the two-stage model of Stacey and Kramers (1975). The model allows precise model age estimates, with an error generally less than 0.05 m.y., to be made for deposits of known age. This is particularly useful in property evaluation during exploration because in many cases the certainty of a syngenetic or epigenetic origin can control future philosophy about exploration and/or evolution of a deposit or an area. Model ages, in concert with stratigraphic information, commonly indicate the likelihood of epigenetic or syngenetic origins.

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