Bulletin of the Geological Society of Finland (Dec 2019)
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
Abstract
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, detrital grains of quartz and minor feldspar are cemented by calcium carbonate, which is partly or wholly replaced by ore minerals, including sphalerite, galena, pyrite, marcasite, 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 pyrite. 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 Laisvall 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 pyrite showing mostly negative δ34S values from −15.6 to −7.6‰, indicating partly a strong bacteriogenic signature, which are distinctly different from the generally heavy sulfur isotope compositions (avg. δ34S +24‰) reported from the Laisvall-type deposits, pointing to a different source of sulfur. These results together with some mineralogical 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.
Keywords
