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

摘要

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 basement of SE Sweden. The fracture filling calcite precipitated from low-temperature solutions of at least four different hydrochemical regimes. In contrast 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 HCO_3~- by anaerobic bacteria is indicated from 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) in the calcite suggests release and incorporation of organic complexed light REEs in the calcite produced as a result of biogenic bicarbonate production. High Ba contents in the fracture filling may be related to the presence of organic material. A low but significant mobility of Th is indicated.