Uranium mineralization related to mantle fluid: A case study of the Xianshi deposit in the Xiazhuang uranium orefield

Abstract

In the east part of the Guidong composite granite body there are many basic dikes with NWW and NNE trends and some silicification zone with NNE trend. The Xianshi uranium deposit is situated in the intersection part between the diabase dikes of NWW trend and the silicification zone of NNE trend. Hydrothermal alterations are well developed, mainly including alkaline alteration, silicification, chloritization and hematite alteration. The latter two alteration types are closely related with the uranium mineralization. There were two periods of uranium mineralization. In the early mineralization, pitchblende occurred as broken grains distributed sporadically in calcitic veins, while in the later one pitchblende appeared as fine veins. Our U Pb isotope analyses for these different types of pitchblende yield apparent 238U 206Pb age of 125 Ma and 238U/204Pb 206Pb/204Pb isochron age of 81 Ma. The ore bodies are confined within the diabase dikes or their marginal parts. Pitchblende and pyrite are the main metallic minerals and calcite and quartz are the main gangue minerals of the ore. The hydrothermal fluid inclusions show high salinity in the liquid phase (18%～20%NaCl) and dominant CO2 in the gas phase, and high medium homogenization temperature (340～160 ℃). The hydrothermal fluids of pre mineralization of pitchblende and mineralization of pitchblende have δ18OH2O values between 1.4‰～6.6‰and δDH2O values between -65‰～-34‰, consistent with derivation from the mantle. The δ13C values of the calcite from the ore veins range from -8.5‰to -3.1‰, also suggesting derivation of ∑CO2 from the mantle. Hydrothermal fluid of post mineralization of uranium displays much lower δ18OH2O values (-5.3‰～0.6‰), reflecting dominance of meteoric water in the evolved later hydrothermal fluid. The Xianshi uranium deposit was formed in the period of lithosphere extension. During such a period, fluid derived from the mantle with high content of CO2 and thus with high capacity of leaching and carrying uranium was readily to emplace into high level in the crust where uranium could deposit in favorable geochemical environment. Therefore such fluids from the mantle may have played an important role in the mineralization of the Xianshi uranium deposit.