地幔流体与铀成矿作用:以下庄矿田仙石铀矿床为例

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

仙石大型铀矿床位于粤北贵东复式岩体东部,矿区内NWW和NNE向基性岩脉十分发育。矿床位于NNE向硅化带和NWW向基性岩脉(辉绿岩)交接部位,矿体严格赋存于基性岩脉内部或其边缘。金属矿物以沥青铀矿和黄铁矿为主,脉石矿物以方解石和(微晶)石英为主,蚀变作用发育,主要类型包括碱交代、硅化和赤铁矿化。沥青铀矿U-Pb年龄测定结果表明,矿床中存在81Ma(主要)和125Ma(次要)两期铀矿化。H、O同位素研究表明,矿前期和成矿期成矿流体的δ18OH2O=1.4‰～6.6‰,δDH2O=-65‰～-34‰,反映成矿流体主要由地幔流体组成。矿脉中方解石的δ13C=-8.5‰～-3.1‰,反映矿化剂∑CO2来自地幔。上述特征表明,地幔流体在仙石铀矿床的形成中具有十分重要的意义。

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.