Paleohydrological inferences from fracture calcite analyses: an example from the Stripa Project, Sweden

Abstract

Three samples of calcite fracture coatings (all less than 100 mg), recovered from the N1 borehole drill-core at the Stripa Project, Sweden, have been analyzed for their uranium, thorium and radium contents and isotopic ratios. The data indicate significant differences in uranium concentrations and 234U/ 238U activity ratios between the precipitating solutions and the groundwaters recently pumped from nearby fractures in this borehole. Higher concentrations and low activity ratios would be expected in these groundwaters at the onset of weathering in the Stripa granite; subsequent large increases in activity ratios are probably the result of near surface microfracturing caused by post glacial uplift and more recent extensive mining operations. Mean times of calcite deposition as short as 95 500 a, estimated from 230Th/ 234U ratios, support the contention that these changes are of fairly recent origin.