内蒙古黄岗梁铁锡矿床中红色萤石的地球化学特征及成因探讨

Geochemical characteristics and genesis of red fluorite in the Huanggangliang Fe-Sn deposit,Inner Mongolia

内蒙古黄岗梁萤石呈现稀有的红色特征,该颜色萤石全球产量稀少,本文利用LA-ICP-MS原位测试技术对其微量元素地球化学特征进行了系统分析,探讨黄岗梁铁锡矿床中红色萤石的成因。微量元素分析结果显示黄岗梁萤石样品中总稀土元素含量较低,并表现出左倾的富集重稀土特征,其中红色和粉色萤石的稀土元素配分曲线显示出Dy-Tm拱顶式凸起的配分模式,表明其结晶作用发生在成矿流体演化的最后阶段;Y/Ho比值104~197,与热液成因萤石一致,且La/Ho比值为0.027~0.126,具强的负异常,表明了黄岗梁萤石为热液成因;Ce/Ce^(*)比值0.57~0.98,Eu/Eu^(*)比值0.22~0.63,均表现出负异常,表明形成黄岗梁萤石的热液流体是在相对还原环境下形成且温度高于200℃。综合分析萤石的地球化学特征和其与围岩的接触关系,认为黄岗梁萤石主要分为两阶段结晶,一阶段为与石英同期结晶形成红色、粉色部分,二阶段为在石英形成后结晶形成的由阳起石他色致色的绿色部分,而黄岗梁萤石罕见的红色是由于Y元素较为富集且含量远高于其他稀土元素,从而形成YO_(2)色心并在萤石中作用而显色。

The fluorites from Huanggangliang,Inner Mongolia have a rare red characteristic,and the global production of this kind of fluorite is low.In this contribution,we systematically analyzed the trace element characteristics of the Huanggangliang fluorites by LA-ICP-MS,and discussed its genesis.The results show that rare earth element(REE)contents of the Huanggangliang fluorites are generally low but characterized by relative enrichment of HREE,and the REE distribution patterns of red and pink fluorites show a convex-arc shape at Dy-Tm,suggesting that the crystallization took place at the last stage of the evolution of the ore-forming fluid.The Y/Ho ratio of 104~197 is consistent with that of hydrothermal fluorite,and the La/Ho ratio of 0.027~0.126 has strong negative anomaly,both demonstrating that the Huanggangliang fluorite deposit is hydrothermal.The Ce/Ce^(*) ratio of 0.57~0.98 and Eu/Eu^(*) ratio of 0.22~0.63 are less than 1.0,indicating that the ore-forming fluid of the Huanggangliang fluorites was formed in a relatively reductive environment and the temperature was higher than 200℃.The geochemical characteristics of fluorite and its contact relationship with surrounding rocks show that the formation of the Huanggangliang fluorites can be mainly divided into two stages of crystallization,one stage occurring at the same time with quartz to form the red and pink part,and the other stage occurring after the formation of quartz to form the green part.The green part is allochromatic color caused by actinolite,while the rare red color of Huanggangliang fluorite is due to the enrichment of Y element and its content is much higher than other rare earth elements,thus forming YO_(2) color center and interacting with fluorite to develop color.