粤北棉花坑铀矿床萤石矿物标型特征及其地质意义

Typomorphic characteristics and geological significance of fluorite in Mianhuakeng uranium deposit,north Guangdong

粤北棉花坑铀矿床铀成矿与萤石关系密切,为探讨该矿床成矿流体演化特征,采用X射线粉末衍射分析、化学分析等方法对该矿床不同深度及不同成矿阶段矿石中的萤石开展标型特征研究。结果显示:成矿期紫黑色萤石相较于矿前期绿色及成矿晚期紫色萤石的Eu亏损更明显,紫色萤石Eu亏损则稍强于绿色萤石;各阶段萤石的Ce多数表现出微弱负异常,少部分表现出弱正异常;Y/Ho-La/Ho图解中各阶段萤石样品投点大致呈水平直线分布,Y/Ho较为均一;相同阶段萤石a0值随深度增加表现出先增大,后减小再增大现象,相同中段早期形成的萤石a0值比晚期的大。综合研究认为形成萤石的成矿流体具有从相对还原过渡至氧化、相对封闭的热液系统转换至开放系统的环境演化;成矿流体具有同源性,可能为同一成矿流体不同阶段形成的产物,紫色萤石阶段可能来源于前两个阶段萤石的再迁移重结晶;REE含量与a0值指示本区成矿流体具有酸碱溶液交替的特征。深部萤石晶胞参数仍较大,推测矿体剥蚀不深,深部具有较好的成矿前景。

Uranium mineralization of Mianhuakeng uranium deposit in northern Guangdong is closely related to fluorite.To explore the evolution characteristics of the ore-forming fluid in the deposit,X-ray powder diffraction analysis,chemical analysis,and other methods were used to study the typomorphic characteristics of fluorite in ores at different depths and stages of mineralization.The results showed that from green fluorite in the early stage of mineralization to purple black fluorite in the later stage of mineralization,the Eu loss of purple black fluorite in the early stage of mineralization was more significant than the green fluorite in the early stage and purple fluorite in the late stage of mineralization,but the Eu loss of purple fluorite was slightly stronger than that of green fluorite;fluorite at each stage mostly shows weak negative Ce anomalies,a small portion shows weak positive Ce anomalies;in the Y/Ho-La/Ho diagram,fluorite samples in each stage are roughly projected in a horizontal straight line and Y/Ho is relatively uniform;at the same stage,the a0 value of fluorite shows a phenomenon of first increasing,then decreasing and then increasing with depth.The a0 value of fluorite formed in the early stage of the same middle stage is larger than that in the late stage.After comprehensive research,it was believed that the ore-forming fluid with fluorite formed under an environmental evolution from relative reduction to oxidation,and from a relatively closed hydrothermal system to an open system;the ore-forming fluids were isogenous.The purple fluorite stage may originate from the re migration and reformation of fluorite in the first two stages;The REE content and a0 value indicate that the ore-forming fluid in this area has the characteristic of alternating acid-base solutions.The cells parameters of deep fluorite crystal are still relatively large,indicating that the ore body is not deeply eroded and the depth has good mineralization prospects.