University of Oulu

Hanski, E. et al. 2019 Zn-Pb-Cu sulfide-bearing glacial sandstone erratics near Raahe on the western coast of Finland as indicators of Paleozoic base metal mineralization at the bottom of the Bothnian Bay. Bulletin of the Geological Society of Finland, Vol. 91, 2019, pp 143–178,

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

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Author: Hanski, Eero1; Huhma, Hannu2; Lahaye, Yann2;
Organizations: 1Oulu Mining School, P.O. Box 3000, 90014 University of Oulu, Finland
2Geological Survey of Finland, P.O. Box 96, 02151 Espoo, Finland
3Atle Arctica, 463 Central Avenue, Mountain View, CA 94043, USA
4Näsilinnankatu 7 D 48, 33210 Tampere, Finland
Format: article
Version: published version
Access: open
Online Access: PDF Full Text (PDF, 15.2 MB)
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Language: English
Published: The Geological Society of Finland, 2019
Publish Date: 2020-02-11


Over the past ten 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 similat 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 metalvolcanite boulders whose carbon isotope and major and trace element compositions, respectively, indicate their derivations from the Kalix are, northern Sweden.

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Series: Bulletin of the Geological Society of Finland
ISSN: 0367-5211
ISSN-E: 1799-4632
ISSN-L: 0367-5211
Volume: 91
Pages: 143 - 178
DOI: 10.17741/bgsf/91.2.001
Type of Publication: A1 Journal article – refereed
Field of Science: 1171 Geosciences
Funding: The first author acknowledges support from the Academy of Finland (grant #281859).
Academy of Finland Grant Number: 281859
Detailed Information: 281859 (Academy of Finland Funding decision)
Copyright information: © 2019 The Authors. All published content of BGSF is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License.