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Journal of the Geological Society; 1993; v. 150; issue.1; p. 77-82;
DOI: 10.1144/gsjgs.150.1.0076
© 1993 Geological Society of London

Papers

Metallogenic evolution in Wales: constraints from lead isotope modelling

C. J. N. FLETCHER1, I. G. SWAINBANK2 & T. B. COLMAN3

1 British Geological Survey, Bryn Eithyn Hall, Llanfarian, Aberystwyth, Dyfed SY23 4BY, UK
2 NERC Isotope Geosciences Laboratory, c/o British Geological Survey, Keyworth, Nottingham NG12 5GG, UK
3 British Geological Survey, Keyworth, Nottingham NG12 5GG, UK

Galena has been collected from different styles of mineralization throughout Wales. Galenas from the Ordovician volcanogenic mineralization at Parys Mountain, north Wales, give highly variable lead isotope ratios, which could have been produced either by a two-stage history or by the mineralizing fluids interacting and mixing with radiogenic lead derived from uranium-rich upper crust. The variation in the lead isotopic ratios of galena from veins cutting lower Palaeozoic sediments is considered to be due to separate periods of mineralization during the early Devonian and early Carboniferous. The most likely source for this mineralization is the thick sequence of lower Palaeozoic basinal mudstones. Both the disseminated galena in Mesozoic sedimentary rocks and galena from veins within the underlying Carboniferous Limestone give Triassic and Jurassic model lead ages, which agree with mineralization ages deduced from geological considerations. It is suggested that the source for this lead mineralization is the underlying Carboniferous shales. These and other galenas from Wales have significantly different isotope ratios to those from the Irish carbonate-hosted ore deposits, which would indicate that the mineralizing fluids had either different sources or evolutionary histories.





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