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A lead isotope study of mineral deposits in the Kootenay Arc Sinclair, Alastair James

Abstract

Twenty-one galena specimens from 16 mineral deposits in the Kootenay district analysed isotopically in replicate have a linear relationship of isotopic compositions on a Pb²º⁶/Pb²º⁴ versus Pb²º⁷/Pb²º⁴ graph. Twelve of the galenas, including two from Sullivan mine (East Kootenay district), were analysed by an intercomparison technique; the first application of the intercomparison method to an anomalous lead suite. Intercomparison results verify the linear compositional pattern (slope = 0.1084 ± 0.0033). These results indicate that: 1. Kootenay arc leads are multi-stage leads. 2. Sullivan-type lead is the parent common lead from which the anomalous suite developed. 3. Kootenay arc mineral deposits formed during one major mineralizing episode. Time of anomalous lead mineralization, tm, is probably Jurassic as deduced from potassium-argon dates and crosscutting relations of ores and igneous rocks. Assuming tm = 180 m.y. time of emplacement of uranium and thorium that produced the radiogenic component of the anomalous leads is approximately 1700 m.y., possibly the age of Lower Purcell strata of the East Kootenay district. Lead isotopic compositions show no obvious correlations with age of wallrock, type of wallrock, geological nature of ore deposits, or minor element content of galena. Post-ore thermal metamorphism and hydrothermal alteration do not appear to have changed lead isotopic compositions. The history of evolution of Kootenay arc anomalous leads is interpreted as follows: 1. Formation of source rocks containing uranium and thorium about 1700 m.y. ago. 2. Introduction of Sullivan-type lead into source rocks about 1340 m.y. ago. 3. Mixing of Sullivan-type lead with radiogenic lead formed by decay of uranium and thorium, and transportation and deposition of these "lead mixtures” to form anomalous lead deposits during Coast Range orogeny. Geologic and isotopic data from Sullivan mine are reconciled most easily with an epigenetic origin of the Sullivan orebody, and probably with a genetic relation of Sullivan ore fluids with the source magma of Moyie Intrusions. Holmes-Houtermans model age for Sullivan ore, based on intercomparison data, is 1340 m.y. Sullivan lead evolved in a source with U²³⁸/Pb²º⁴ = 9.02, Th²³²/Pb²º⁴ = 36.71, and Th/U = 4.07 (in terms of present day abundances). A method of estimating volume of source rock of anomalous lead deposits is outlined. Calculations for Jersey, Reeves Macdonald and Bluebell mines indicate that the radiogenic lead component of these deposits could have been derived from 1 to 10 cubic kilometers of source rock containing 3 ppm uranium (and approximately 12 ppm Th) if only one-third to one-half of the radiogenic lead in the source were extracted during a period of concentration and mineralization. Kootenay arc deposits can be divided into two classes on the basis of minor element contents of galenas. This division closely corresponds to the following geological types of deposits: 1. replacement deposits with no evidence of open space filling, and 2. deposits with evidence of open space filling and variable amounts of wallrock replacement. Minor element contents of galenas from Salmo-type replacement deposits suggest but do not prove a fairly low temperature of mineral deposition.

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