Trace element mobility in hydrothermal calcite during low temperature alteration – implications for radionuclide retention

The recent exploration for nuclear waste repositories includes the evaluation of safety mechanisms in case of a leakage of radiotoxic material. Possible retention mechanisms like the uptake of these elements from the conducting aquifer into ubiquitous fracture minerals like calcite have drawn much attention. In laboratory experiments, it has been shown that especially actinides are efficiently scavenged by (re)crystallization of calcite¹. To assess the long-term immobilization potential of calcite and the effect of low temperature alteration, element mobility in natural analogues needs to be studied.

A suitable study site is the Wenzel ore mine in the Black Forest (S-Germany), which was actively mined until 1823. Here, hydrothermal calcite veins are exposed at the tunnel wall. After it´s shut down the mine was naturally flooded and calcite remained in contact to groundwater for approx. 175 years until the mine was drained and reopened to the public in 1999.

Elemental distributions of La and Sr, which are considered as analogue elements for actinides and Ra are revealed by µXRF and LA-ICP-MS maps at high spatial resolution of 2 µm. First results show a complex pattern of Sr-enriched and depleted layers at the calcite-groundwater interface, which alternate at the 10 – 200 µm scale. In addition, localized enrichments of La appear, exceeding the pristine calcite concentrations by a factor of 4. Both features are interpreted as a result of secondary element mobility and discussed with respect of the retention potential of calcite during low temperature alteration.

¹Curti E. (1999) Applied Geochemistry 14: 433-445