Age and lead isotope systematics of uranium-enriched cobalt mineralization in the Modum complex, South Norway Implications for Precambrian crustal evolution in the SW part of the Baltic Shield

In the Modum complex in the Precambrian Kongsberg sector of South Norway, cobalt sulphides and arsenides, chalcopyrite and pyrite occur in stratiform bands and lenses. The cobalt ore minerals are intimately associated with thorian uraninite both on a regional and on a microscopic scale. Due to the presence of coeval inclusions and intergrowths of uraninite in the cobalt minerals, hand-picked separates of ore minerals show a large range of U/Pb ratios. This has led to a very large range of present-day lead isotopic composition (²⁰⁶Pb/²⁰⁴Pb= 18.1 to 277, ²⁰⁷pb/²⁰⁴pb=15.6 to 36 ²⁰⁸pb/²⁰⁴pb = 37 to 57). Two groups of samples can be discerned: Group 1 defines an imperfectly fitted Pb-Pb lead isochron with an age of 1434 ± 29 Ma. Group 2 defines a Pb-Pb correlation line with an age of 1146 ± 66 Ma. These results indicate that the uranium-bearing cobalt ore was deposited during the 'orogenic interlude' commonly thought to separate the Gothian/Kongsbergian (ca. 1.5 Ga) and Sveconorwegian (1.2-0.9 Ga) orogenies in the Baltic Shield. Multi-stage lead modelling indicates that the initial isotopic composition of the lead in the ores is indistinguishable from that of lead in the country rock metasediments, suggesting that it was extracted from such a source. The Group 2 leads were affected by a strongly localized hydrothermal reworking process during the Sveconorwegian orogeny, which led to small-scale partial Pb isotopic homogenization and to strong fractionation of uranium, thorium and lead. The 1.43 Ga age for primary mineralization defines a definite lower age limit for the onset of clastic sedimentation in the Kongsberg sector. The crustal precursor of the sediments must have retained a high LILE 'upper continental crust' character for several hundred million years prior to sedimentation.