Interpretation of lead isotope data from the uraniferous Cu-Fe-sulfide mineralizations in the Proterozoic greenstone belt at Kopparåsen,northern Sweden |
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Authors: | R. L. Romer T. M. Boundy |
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Affiliation: | (1) Department of Economic Geology, University of Technology, Luleå, S-951 87 Luleå, Sweden;(2) Present address: Department of Geology and Geophysics, University of Wyoming, 82071 Laramie, Wyoming |
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Abstract: | The polymetamorphic Early Proterozoic basic metavolcanites of the Kopparåsen greenstone belt, northern Sweden, contain U mineralizations which are confined to Cu-Fe-sulfide mineralizations in mylonitized zones within basic metatuffs and graphite-bearing mica schists. Trace-lead isotope data from sulfides indicate contamination of the sulfide lead with two different lead components at ca. 430 Ma. One lead component was leached from rocks with a 232Th/238U ratio of 3.0–3.2, while the other lead component had evolved in a environment with a lower 232Th/238U ratio (0.0–0.05). The source for this latter lead component underwent a U-Th separation, probably in relation to the formation of the U mineralizations. If this lead component was leached from the U mineralizations, these mineralizations have a 207Pb/206Pb model age of ca. 1,780 Ma, which is less than the least radiogenic lead model age of the supracrustal belt (ca. 2,050 Ma). A possible genetic model for the uranium mineralizations includes the transport of U with an oxidized metamorphic fluid, which was channelled into the permeable zones, and the local reduction of the fluid by sulfides, which caused the precipitation of U. |
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