Metal sources in the Early Proterozoic Svecofennian terrain of central Sweden: Pb isotope evidence |
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| Authors: | A Hallberg |
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| Institution: | (1) Institute of Geology, Uppsala University, Box 555, S-751 22 Uppsala |
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| Abstract: | The lead isotope composition of three Cu-Zn-Pb sulfide deposits and one Au deposit in the Early Proterozoic Svecofennian of central Sweden reveals the existence of two distinct lead sources in the region. Rockliden and Enstern contain lead of upper crustal affinity and are isotopically homogeneous. Lead in these deposits probably originates from Archean-derived detritus which was well homogenized by sedimentary processes and deposited over large areas as a component in Early Proterozoic sedimentary and tuffaceous rocks. Lead isotopic homogeneity within the Tjärnberget and Enåsen deposits is less pronounced, and the less radiogenic character of the lead suggests a source of mantle affinity. In areas where metasedimentary rocks are absent or contain very little Archean-derived material the isotopic signature of mantle-derived volcanic rocks is reflected in the ore lead. This approach is also applicable to other Svecofennian deposits. Together with deposits in west-central Finland the Rockliden deposit constitutes the northern boundary for lead of upper-crustal affinity, i.e., Archean-derived lead, in Svecofennian sulfide deposits.Despite these differences the isotope ratios of the four deposits indicate that they formed roughly contemporaneously with other Svecofennian deposits in Sweden and Finland. |
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