Abstract

The sandstone-hosted lead deposits on the Baltic Shield occur in marine late Precambrian to Early Cambrian sedimentary rocks. A ground-water model in which the basement is postulated as the metal source has been proposed for these deposits. To test this model the Osen deposit was selected because it lies directly on a homogeneous granite basement.On the basis of U-Th-Pb whole-rock isotopic analyses for three samples of unweathered basement granite (Trysil granite), a basement age of 1,598 m.y. was established. The lead isotopic compositions of four out of five specimens of galena from the Osen deposit plot between isochrons calculated for the granite at 500 m.y. and 540 m.y. This is strong support for the hypothesis that the lead source was basement granite and suggests that mineralization took place soon after sedimentation. A basement source for the lead is further substantiated by the regional pattern of lead isotopic compositions of sandstone lead deposits, which reflects the character of adjacent basement terranes.

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