Zn-Pb-Cu sulfide-bearing glacial sandstone erratics near Raahe on the western coast of Finland: Indicators of Paleozoic base metal mineralization at the bottom of the Bothnian Bay.

Over the past tens of years, glacial sandstone erratics variably enriched In Zn, Pb and Cu have been collected from the coast south of Raahe, but their source has remained ambiguous. In these non-metamorphosed and non-deformed boulders, detrltal grains of quartz and minor feldspar are cemented by calcium carbonate, which is partly or wholly replaced by ore minerals, Including sphalerite, galena, pyrlte, marcaslte, and chalcopyrite. Analyzed mineralized boulders have yielded total base metals contents between 1 and 19 wt.%. The FeS content in sphalerite Is low and variable, ranging commonly between 0.5 and 15 mol.%, which is in harmony with its coexistence with pyrlte. Galena shows very radiogenic Pb isotope compositions, with ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁷Pb/²⁰⁴Pb falling In the ranges of 20.55-21.06 and 15.90-15.94, respectively. These compositions are similar to those measured for the Lalsvall MVT Pb-Zn deposit in the Swedish Caledonides, being consistent with a similar Ordovician age of ore formation. However, S isotope analyses yielded heterogeneous compositions, with pyrlte showing mostly negative δ³⁴S values from -15.6 to -7.6%o, Indicating partly a strong bacteriogenic signature, which are distinctly different from the generally heavy sulfur isotope compositions (avg. δ³⁴S +24%O) reported from the Lalsvall-type deposits, pointing to a different source of sulfur. These results together with some mlneralogical differences (relatively abundant chalcopyrite, scarcity of fluorite and barite) suggest that the boulders were not derived from the eastern front of the Caledonian orogen but their provenance occurs much closer, potentially In a so-far-undiscovered occurrence of stratabound base metal mineralization in Cambrian sediments under the Bothnia Bay. The direction of transportation of the sandstone boulders can be estimated using associated carbonate and mafic metavolcanite boulders whose carbon isotope and major and trace element compositions, respectively, Indicate their derivations from the Kalix area, northern Sweden. [ABSTRACT FROM AUTHOR]