University of Oulu

Vasilopoulos, M., Molnár, F., O’Brien, H. et al. Geochemical signatures of mineralizing events in the Juomasuo Au–Co deposit, Kuusamo belt, northeastern Finland. Miner Deposita 56, 1195–1222 (2021).

Geochemical signatures of mineralizing events in the Juomasuo Au–Co deposit, Kuusamo belt, northeastern Finland

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Author: Vasilopoulos, Mikael1; Molnár, Ferenc2; O’Brien, Hugh2;
Organizations: 1Oulu Mining School, University of Oulu, P.O. Box 3000, 90014, Oulu, Finland
2Geological Survey of Finland, P.O. Box 96, FI-02151, Espoo, Finland
3Deutsches GeoForschungsZentrum, GFZ, Telegrafenberg, 14473, Potsdam, Germany
4Université de Lorraine, CNRS, GeoRessources, F-54000, Nancy, France
5Dragon Mining Oy, Sastamala, Finland
Format: article
Version: published version
Access: open
Online Access: PDF Full Text (PDF, 18.6 MB)
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Language: English
Published: Springer Nature, 2021
Publish Date: 2021-11-10


The Juomasuo Au–Co deposit, currently classified as an orogenic gold deposit with atypical metal association, is located in the Paleoproterozoic Kuusamo belt in northeastern Finland. The volcano-sedimentary sequence that hosts the deposit was intensely altered, deformed, and metamorphosed to greenschist facies during the 1.93–1.76 Ga Svecofennian orogeny. In this study, we investigate the temporal relationship between Co and Au deposition and the relationship of metal enrichment with protolith composition and alteration mineralogy by utilizing lithogeochemical data and petrographic observations. We also investigate the nature of fluids involved in deposit formation based on sulfide trace element and sulfur isotope LA-ICP-MS data together with tourmaline mineral chemistry and boron isotopes. Classification of original protoliths was made on the basis of geochemically immobile elements; recognized lithologies are metasedimentary rocks, mafic, intermediate-composition, and felsic metavolcanic rocks, and an ultramafic sill. The composition of the host rocks does not control the type or intensity of mineralization. Sulfur isotope values (δ³⁴S − 2.6 to + 7.1‰) and trace element data obtained for pyrite, chalcopyrite, and pyrrhotite indicate that the two geochemically distinct Au–Co and Co ore types formed from fluids of different compositions and origins. A reduced, metamorphic fluid was responsible for deposition of the pyrrhotite-dominant, Co-rich ore, whereas a relatively oxidized fluid deposited the pyrite-dominant Au–Co ore. The main alteration and mineralization stages at Juomasuo are as follows: (1) widespread albitization that predates both types of mineralization; (2) stage 1, Co-rich mineralization associated with chlorite (± biotite ± amphibole) alteration; (3) stage 2, Au–Co mineralization related to sericitization. Crystal-chemical compositions for tourmaline suggest the involvement of evaporite-related fluids in formation of the deposit; boron isotope data also allow for this conclusion. Results of our research indicate that the metal association in the Juomasuo Au–Co deposit was formed by spatially coincident and multiple hydrothermal processes.

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Series: Mineralium deposita
ISSN: 0026-4598
ISSN-E: 1432-1866
ISSN-L: 0026-4598
Volume: 56
Issue: 6
Pages: 1195 - 1222
DOI: 10.1007/s00126-020-01039-8
Type of Publication: A1 Journal article – refereed
Field of Science: 1171 Geosciences
Funding: Open Access funding provided by University of Oulu including Oulu University Hospital. This work was financially supported by grants from the Vilho, Yrjö, and Kalle Väisälä Foundation; the K.H. Renlund Foundation; the Finnish Cultural Foundation; the Horizon 2020 NEXT project; and the Academy of Finland #281670–MinSysPro project.
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