Ore-lead isotope pattern from the Iglesiente-Sulcis Area (SW Sardinia) and the problem of remobilization of metals |
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Authors: | M Boni V Koeppel |
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Institution: | (1) Dipartimento di Scienze della Terra dell'Università degli Studi di Napoli, L. San Marcellino 10, I-80138 Napoli;(2) Institute for Crystallography and Petrography, Swiss Federal Institute of Technology, CH-8092 Zuerich |
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Abstract: | In SW Sardinia syngenetic to syndiagenetic Pb-Zn ores occur in Cambrian carbonates, along the unconformity between the Cambrian and Ordovician, in Permo-Triassic karsts and in vein-type deposits related to late Hercynian granites, which also contact-metamorphosed some Cambrian deposits. In all types of deposits the leadisotope ratios show similarly high (=238U/204Pb) and high W (=232Th/204Pb) values indicating a crustal origin for the lead. Most of the Cambrian ores contain isotopically similar or identical leads, whereas in the younger deposits the isotope ratios vary and suggest that especially the lead of Permo-Triassic ores may consist to a large extent of remobilized Cambrian, possibly also Ordovician, ore lead plus a Hercynian component.The lead of three feldspar samples from Hercynian granites of the area also shows high and W values. Two of them are similar to the ore leads from a vein-type deposit and from contact-metamorphosed deposits. The third sample from the Capo Pecora granite contains a very unradiogenic lead that closely resembles many of the presumed Cambrian-Hercynian mixtures. Therefore, the possibility cannot be dismissed that at the time the Permo-Triassic ores were formed lead sources other than the already existing ores were present with the appropriate isotopic compositions. |
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