Bulletin of the Geological Society of Finland (Jun 1999)
Radiometric ages and other isotopic data bearing on the evolution of Archaean crust and ores in the Kuhmo-Suomussalmi area, eastern Finland
Abstract
The Archaean greenstone-gneiss terrain in the Kuhmo-Suomussalmi district in eastern Finland has been isotopically studied in connection with regional bedrock mapping and local mineral exploration projects. The studies have aimed at testing correlations of lithologic units in partly poorly exposed areas, determining times of ore formation and obtaining ore genetic information in order to better understand the general evolution of the Archaean formations within the Fennoscandian Shield. Isotopic results on 63 zircon and titanite fractions from 13 samples, common lead analyses of 14 sulphide separates from two mineral prospects and 33 whole rock Pb-Pb analyses warrant the following conclusions: 1) Although some dates in excess of 3 Ga have been determined from the Finnish Archaean, most of the granite gneiss terrain was formed between 2.85 and 2.65 Ga with a major period of rock formation from about 2.75 to 2.69 Ga. 2) The majority of the metavolcanic rocks within the Kuhmo-Suomussalmi greenstone belt are 2.79 Ga old, but the meta-andesites of the Luoma Group are distinctly older at 2.97 Ga. 3) The Taivaljärvi Ag-Zn-Pb deposit appears to be syngenetic with the local metavolcanic rocks and is thus 2.79 Ga old. Its lead probably represents the initial lead of both mafic and felsic metavolcanic rocks in the area. 4) When compared to the Abitibi region in Canada, the available Pb-Pb data may suggest a heterogeneous Archaean mantle, but as even this study shows, common lead data is inconclusive evidence, as it may be easily influenced by later hydrothermal processes.
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