| With the rapid development of nuclear energy, treatment and disposal of high-levelradioactive waste (HLW) will become a major safety and environmental issue. Currently,the deep geological disposal is considered as the only safe and technically feasible way todisposal of HLW all over the world. As one of the important factors in repository siting, thenear-field geochemical environment greatly affects the release and migration ofradionuclides outward. In order to understand the origin, genesis, chemical evolutionhistory and water/rock interaction of the paleo-groundwater in the Beishan area,geochemical characteristics of fracture minerals, including the carbon, oxygen, strontiumisotope and REE compositions of calcite, as well as the oxygen isotope composition ofquartz, have been systematically analyzed, based on the identification of the type,distribution, occurrence, formation ages and mineralogical characteristics of fractureminerals in crystalline rock. With the comparative analysis of geochemical characteristicsof fracture fillings in Jiujing, Xinchang and Jijicao granite, the following results have beenobtained:Typical fracture minerals found at Beishan site include quartz, calcite, clay minerals(montmorillonite, chlorite, etc.) and a small amount of pyrite, limonite, etc. As the mainobject of study, calcite is widely distributed. According to crystal forms, occurrence and itsassociated minerals, the calcite can be classified into three groups, namely calcite occurredin closed fractures, calcite occurred in open fractures and calcite occurred in the fracturedzone. Several tests have been used to further characterize fracture fillings. The results oftests show that:1) The calcite formed from the underground fluid activities of differenttimes that span an age range from114.4ka to463.2ka. Compared with calcites occurred inJiujing and Jijicao sub-area, calcites occurred in Xinchang formed later with the relativelyconcentrated age range (162.9ka~286.9ka);2) The13C values of calcite in Jiujing(-11.6‰~-5.7‰), Xinchang (-9.9‰~-5.1‰) and Jijicao (-10.4‰~-5.5‰) are all negativeand relatively stable; the18O values of calcite are more wide-ranging (-0.7‰~19.7‰,10.9‰~21.9‰and11.5‰~16.1‰in Jiujing, Xinchang and Jijicao respectively), whichhave larger differences in the fractured zone calcite of Jiujing sub-area. Meanwhile, the variation of13C and18O values with depth show a character of diversity;3) The18Ovalues of quartz have a stable composition (8.4‰~12.3‰), indicative of similar origin andstable geochemical environment of fluid;4) The87Sr/86Sr ratios of calcite have a wide range(0.708584~0.732967), which are smaller in Jiujing (0.708584~0.718749) and Xinchang(0.708838~0.732967) and larger in Jijicao (0.714716~0.729015). These ratios decreasedstrongly with depth increasing in Jiujing and Xinchang, but no obvious trend in Jijicao;5)All of calcites show strongly fractionated REE-patterns with enrichment of the LREEs.However, some differences in REE contents and patterns among calcites indicate formationfrom fluids of different compositions and varying degrees of water/rock interaction. Incomparison, REE contents of calcite in Jiujing are higher; calcites in Xinchang show morefractionated REE-patterns with more obvious behavior of negative Eu anomaly and positiveCe anomaly.Those results reveal that the origin and genesis of the paleo-groundwater in Beishansite were relatively complex，with strongly influence of meteoric water in the shallowportion of the rock，while affected by the mixture of underground brine, meteoric water andother fluids in the deep portion. The fluid environment of the Beishan site was relativelystable with a less degree of the water/rock interaction. Compared with the Jiujing andJijicao sub-area，the geochemical environment of the Xinchang sub-area is more stable forlong-term disposal of high level radioactive waste. |
PDF Full Text Request