光石沟花岗伟晶岩型铀矿床地质及流体包裹体特征

Characteristics of geology and fluid inclusion in Guangshigou granitic pegmatite uranium deposit

光石沟铀矿床大地构造位置处于秦岭造山系东部的北秦岭加里东褶皱带,丹凤三角铀成矿区的东南部,矿床赋铀主岩为黑云母花岗伟晶岩。铀矿体严格受伟晶岩脉控制,矿石矿物主要为晶质铀矿,硅钙铀矿及铀黑等次生铀矿物仅见于地表以下20 m范围内的氧化带中。光石沟矿床石英中发育大量流体包裹体和少量熔融包裹体。流体包裹体可划分为富液相水溶液包裹体(Ⅰ型)、CO2包裹体(Ⅱ型)、含CO2的三相水溶液包裹体(Ⅲ型)和含子晶包裹体(Ⅳ型)四类。包裹体测试结果表明,光石沟铀矿床形成于融体-流体过渡阶段,流体包裹体以富气盐水包裹体为主,含少量CO2包裹体和含子晶包裹体,矿床均一温度范围为152~308℃,平均为233℃,盐度范围为7.2%~26.5%NaCleqv,平均10.0%NaCleqv,指示光石沟铀矿床成矿流体为中-高温、中-低盐度的NaCl-H2O(-CO2)流体体系。岩矿石的结构构造表明光石沟花岗伟晶岩型铀矿是典型的岩浆型同生矿床,即岩浆结晶分异形成的。在岩浆冷凝结晶晚期形成高挥发分、富流体的伟晶岩浆流体,侵位形成黑云母花岗伟晶岩脉。伴随着流体中的黑云母和磷灰石等富氟矿物的大量结晶,导致熔浆中氟大量减少,引起氟铀酰络合物遭到破坏,最终铀以晶质铀矿的形式结晶成矿。

Guangshigou uranium deposit is located in the northern Qinling Caledonian fold belt of eastern Qinling Orogenic System and sets in the southeast of the Danfeng triangle metallogenic district.The host rock is biotite granitic pegmatite.The ore bodies are strictly controlled by pegmatite dykes.Ore minerals are mainly composed of uraninite,with some secondary uranium minerals(e.g.,uranophane,uranium black,etc.,)observed in the oxidized zone 20 m underground.Fluid inclusions from the Guangshigou deposit can be divided into liquid-rich aqueous inclusion(Ⅰ-type),CO2 inclusion(Ⅱ-type),CO2-bearing three-phase aqueous inclusion(Ⅲ-type),and daughter crystal inclusion(Ⅳ-type).Analyzed results reveal that Guangshigou deposit was formed in the transition stage from melt to fluid.The fluid inclusions mainly consist of gas-rich brine inclusions,with minor CO2 fluid inclusion and daughter crystal inclusion.The homogenization temperatures range from 152 to 308℃(average=233℃),and the salinities range from 7.2%to 26.5%NaCl eqv(average=10.0%NaCl eqv),implying the mineralization fluid was formed in a NaCl-H2O(-CO2)fluid system of medium to high temperature and low to medium salinity.The texture and structure of rocks and ores indicate the deposit is a typical syngeneic magma one formed by magmatic crystallization and differentiation.During the late period of magmatic cooling and crystallization,fluid-rich and high-volatile pegmatite magma and fluid were produced and emplaced into the pluton as the biotite granitic pegmatite dykes.With the great crystallization of F-rich minerals(biotite and apatite),the F content in the melts was sharply decreased.As a result,uranyl complex was deconstructed and uranium is concentrated as independent uranium mineral and uraninite.