Galena mineralization in the Orcadian Basin,Scotland: Geological and isotopic evidence for sources of lead |
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| Authors: | John Parnell Ian Swainbank |
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| Institution: | (1) Department of Geology, Queen's University, BT7 1NN Belfast, UK;(2) British Geological Survey, WC1 London, UK |
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| Abstract: | Galena and other sulphides are widespread in the Old Red Sandstone (Devonian) Orcadian Basin. Syndiagenetic occurrences are associated with organic-rich sedimentary rocks, especially algal limestones. Epigenetic galenas are often spatially related to these syndiagenetic occurrences, particularly in the vicinity of Permo-Carboniferous igneous intrusions. Mineralization is concentrated about the granite-gneiss basement inlier in southwest Orkney.Orcadian Basin galenas have lead isotope compositions which show a linear trend related to Lewisian granulite facies metamorphism at about 2,700 Ma. The most radiogenic leads must include a component of average orogene or upper crustal lead. The isotopic compositions are notably similar to those of Caledonian granite feldspar leads measured by Blaxland et al. (1979). Granite was probably a source for lead in galena deposits about the basement in south-west Orkney, but away from the basement an average orogene component would have become increasingly important. There was no major input of magmatic lead to the basin during Permo-Carboniferous times, only a remobilization of existing lead. |
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