The isotopic geochemistry of filling calcite in Jiujing and Xinchang granite in Beishan site for high-level radioactive waste disposal

In order to understand the origin, genesis, chemical evolution history and water/rock interaction of the paleo-groundwater in Beishan area, the pre-selected site for geological repository of high-level radioactive waste in China, the carbon, oxygen and strontium isotope composition of the filling calcite in Jiujing and Xinchang granite have been systematically analyzed. The results show that the δ¹³C values of calcite in Jiujing (-11.6‰∼-5.7‰) and Xinchang (-9.9‰∼-5.1‰) are both negative and relatively stable. The δ¹⁸O values of calcites in Jiujing (-0.7‰∼19.7‰) are wider in range than those in Xinchang (10.9‰∼21.9‰). The ⁸⁷Sr/⁸⁶Sr ratios are slightly different between Jiujing (0.708584∼0.718749) and Xinchang (0.708838∼0.732967), but both obviously decreased with depth increasing. Those results suggest that the origin and genesis of the paleo-groundwater in Beishan site are fairly complex, with strong influence of meteoric water in the shallow part, while affected by underground brine in the deep part. Most of calcites were formed due to low temperature fluid reaction. The groundwater environment of Beishan site was relatively stable with a less extent of the water/rock interaction. Compared with Jiujing sub-area, the geochemical environment of Xinchang sub-area is more stable for long-term disposal of high level radioactive waste. (authors)