桂北沙子江铀矿床流体包裹体初步研究

A tentative discussion on fluid inclusions in quartz veins of Shazijiang uranium ore deposit,northern Guangxi

对沙子江矿床内与成矿关系密切的各成矿阶段石英中流体包裹体进行了研究,显示成矿早阶段部分高温流体对铀的迁移可能发挥了一定作用,而整个成矿过程中主要成矿流体为中-低温、低盐度及中等密度热液。含子矿物包裹体可能捕获于流体沸腾作用过程中。沸腾作用降低了成矿流体中碳酸铀酰络离子的浓度,有利于铀的沉淀。成矿晚阶段,源自赋矿的富烃花岗岩体中烃类组分的加入为营造铀沉淀的还原环境贡献了力量,同时,这类富烃的花岗岩体对铀矿体形成以后的保存也到起了很大的作用。

The Miaoershan uranium orefield,one of the five large-size uranium orefields in central South China,hosts the Chanziping uranium ore deposit,which is the largest carbonaceous-siliceous-pelitic rock type uranium ore deposit in China,and some granite uranium ore deposits.A series of petrographic,microthermetric and Laser Raman microspectroscopic studies of fluid inclusions（FI）in the quartz veins of the Shazijiang uranium ore deposit,one of the important granite-hosted uranium ore deposits in the orefield,were carried out by the authors.Five types of FI were observed or identified,namely liquid FI,daughter mineral-bearing FI,pure gas FI,pure liquid FI and hydrocarbon FI.As the main fluid inclusion type of the whole ore-forming process,liquid FI have characteristics of the ore-forming fluid.The homogenization temperatures,salinities and densities of liquid FI are in the range of 109.0～342.6℃,0.53%～7.73%,and 0.71～1.02 g/cm3 respectively.It is concluded that part of high-temperature（〉300℃）fluids carried significant amounts of uranium at the early ore-forming stage.Of course,dominant ore-forming fluids are characterized by low-moderate temperature,low salinity and moderate density.Daughter mineral-bearing FI could be captured under the condition of ore-forming fluid boiling.Daughter minerals are halite and sylvite.The homogenization temperatures,salinities〔w（NaCleq）〕 and densities of daughter mineral-bearing FI are in the range of 211.8～322.0℃,32.39%～39.76%,and 1.07～1.15 g/cm3,which should not be considered to represent the characteristics of original ore-forming fluid.Boiling reduced the activity of uranyl-carbonate complexes and resulted in uranium precipitation.At the last ore-forming stage,the addition of hydrocarbon probably from host granite into the ore-forming fluid contributed to the reduction and precipitation of hexavalent uranium.Meanwhile,hydrocarbon-rich host granite created favorable conditions for uranium ore body preservation.The hydrocarbon is composed of lots of saturated hydrocarbon,benzene,olefin and bitumen.