粤北棉花坑铀矿床深部矿体矿化剂CO_2来源探讨

棉花坑铀矿床是我国华南重要的大型花岗岩型铀矿床,该矿床碳酸盐化与铀矿化关系密切,本研究对棉花坑铀矿床深部矿体不同期次的方解石进行碳、氧同位素分析。该矿床矿前期、成矿期、矿后期都发育方解石,从早到晚,方解石的δ~(13)C_(PDB)值有明显降低的趋势,而δ~(18)O_(SMOW)值矿前期最低,成矿期与矿后期方解石的δ~(18)O_(SMOW)值变化范围与平均值相似。棉花坑矿床矿前期方解石δ~(13)C_(PDB)为-4.9‰~-6.0‰,平均值为-5.5‰,与幔源的CO_2的碳同位素变化范围和平均值相似,而成矿期与矿后期由于CO_2发生脱气作用引起同位素分馏而降低,表明该矿床成矿流体中的碳来自地幔去气作用,方解石的δ~(13)C_(PDB)-δ~(18)O_(SMOW)组成呈负相关关系,进一步证实棉花坑铀矿床成矿流体中CO_2矿化剂由地幔去气作用形成。

Discuss on Origin of Mineralizer CO2 from the Deep Seated Orebodies of Mianhuakeng Uranium Deposit in Northern Guangdong,China

The Mianhuakeng uranium deposit is a large granite-type uranium deposit in Southern China .The carbonatization is closely correlated with the uranium mineraliza-tion in this deposit .The calcite of different stages of deep seated orebodies in the Mian-huakeng uranium deposit were sampled and the carbon and oxygen isotopes were investi-gated .The calcite veins in the Mianhuakeng uranium deposit were divided into three stages ,namely pre-mineralization stage , the principal mineralization stage and post-mineralization stage .The values of δ13 CPDB of calcites decreased from the pre-mineraliza-tion to the principal mineralization to the post-mineralization stages . The value ofδ18 OSMOW was the smallest in the pre-mineralization stage ,and was similar with the aver-age value in the principal and post-mineralization stages . The δ13 CPDB of calcites was from -4.9‰ to -6.0‰ with -5.5‰ in average in the pre-mineralization stage ,which were consistent with CO2 in the mantle .The δ13CPDB decreased in the principal minerali-zation stage and post-mineralization stage ,because of the isotope fractionation during the degassing of CO2 ,indicating that the carbon of ore-forming fluid of Mianhuakeng uranium deposits was mainly from mantle .Theδ13CPDB performed the negative correla-tion with the δ18 OSMOW ,suggesting that the mineralizer in ore-forming fluids of Mian-huakeng uranium deposit was formed by the mantle degassing .