Abstract
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.
Highlights
Finland is one of the few countries in the European Union that has economic reserves of “battery metals” and the only one producing Co at present
Results of electron probe micro-analyzer (EPMA) analyses of tourmaline are presented in Online Resource 6
Type 4 tourmaline has Al contents between 4.0 and 6.2 with an average of 5.4 apfu; the X-site is dominated by Na, with an average of 0.9 apfu and a maximum of 1.0 apfu
Summary
Finland is one of the few countries in the European Union that has economic reserves of “battery metals” and the only one producing Co at present. Several epigenetic hydrothermal Au deposits in northern Finland, hosted by the Kuusamo belt (KB), the Central Lapland Greenstone Belt (CLGB), and the Peräpohja Belt (PB), are enriched in Co (± Cu ± Ni ± LREE ± U) (Fig. 1a) These types of deposits have been classified by some workers as orogenic Au deposits with an atypical metal association (e.g., Eilu 2015). The added knowledge concerning these Co-rich deposits can have implications for exploration on a global scale, as Au deposits with similar metal associations can be found in several terranes outside of Finland (e.g., Slack 2013) Prominent examples of such deposits are those in the Idaho cobalt belt (Slack 2012), the Skuterud deposit in Norway (Grorud 1997), and the Werner Lake (Pan and Therens 2000) and NICO (Goad et al 2000) deposits in Canada
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