Low-temperature trace element mobility influenced by microbial activity - indications from fracture calcite and pyrite in crystalline basement

Stable isotopes and trace elements have been analysed in calcite and pyrite , from a fracture surface exposed at a depth of 200 m in the Precambrian ba sement of SE Sweden. The fracture filling calcite precipitated from low-tem perature solutions of at least four different hydrochemical regimes. In con trast to many fractures in the area, no hydrothermal activity was indicated in this fracture making the effects of low temperature processes easier to evaluate. Local production of HCO3- by anaerobic bacteria is indicated fro m low delta(13)C-values in the calcite. The findings of pyrite with partly low delta(34)S embedded in the calcite suggest that these bacteria may have been sulphate reducers. Inverse correlation of La/Yb ratio with delta(13)C in the calcite suggests release and incorporation of organic complexed Lig ht REEs in the calcite produced as a result of biogenic bicarbonate product ion. High Ba contents in the fracture filling may be related to the presenc e of organic material. A low but significant mobility of Th is indicated. ( C) 1999 Elsevier Science B.V. All rights reserved.