Abstract
Abstract The isotopic composition of ore lead in galena from the volcanogenic sulfide ores at Garpenberg district is presented. Despite of textural, compositional, and mineralogical differences, as well as the variable extent of deformation of the various sulfide ore types, the ores are characterized by a narrow range in lead isotopic composition. Dolomite-hosted ores are commonly more radiogenic than those hosted by skarn or altered volcanic rocks. Compared with the average growth curve of Stacey & Kramers, the ore lead at Garpenberg is slightly enriched in radiogenic lead. The influence of metamorphism on the ore lead composition of galena samples is negligible. The observed trend in the standard diagrams may be explained in terms of a mixing line, where the ore lead was derived from two different crustal sources originally enriched in radiogenic lead (e.g. volcanic and sedimentary rocks). Alternatively, it could represent sulfide ores of slightly different ages derived from a common rock source. The composition of the ore lead in the Garpenberg district displays the typical signature of the volcanogenic sulfide ores in Bergslagen, which at a regional scale is relatively homogeneous and is interpreted as derived from an isotopically well mixed orogen-upper crust reservoir. The results are consistent with a common origin for all the sulfide ore types present in the Garpenberg and Garpenberg Norra deposits. However, at a local scale, minor variations in the ore lead composition can be detected within a particular district or single deposit. These differences may be interpreted as due to the influence of the local lithology on the ore lead or differences in mineralization age. Comparison of the ore lead composition at Garpenberg with that of other volcanogenic sulfide deposits in Bergslagen shows that the Garpenberg, Dammberg, Svärdsjö and Saxberget deposits have a very similar isotopic composition, whereas the Åmmeberg deposit is relatively more radiogenic, probably reflecting a large proportion of sediment-derived lead in the latter deposit. This difference reinforces the classical distinction of the volcanogenic sulfide deposits in Bergslagen into the Falun and Åmmeberg types.
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