湘西北海相磷块岩中纳米级钛铀矿的发现及地质意义

Discovery and Geological Significance of Nanoscale Brannerite from Marine Phosphorite in the Northwestern Hunan

在扬子陆块东南缘湘西北武陵地区下寒武统牛蹄塘组海相黑色岩系下部鲕粒磷块岩中发现超细小纳米级钛铀矿。通过扫描电镜能谱分析测定钛铀矿的化学成分,Ti质量分数为19.24%~20.37%,U为44.37%~46.86%,O为26.05%~31.01%,V和Cr元素以类质同象混入物形式存在于钛铀矿中。高分辨透射电子显微镜能谱曲线也表明其为U和Ti的复杂氧化物,化学式为（U）Ti2O6（OH）。钛铀矿产出于同沉积鲕粒磷块岩经强溶蚀重结晶作用形成的磷块岩微裂隙和重结晶磷灰石晶粒间微空隙中,与重结晶磷灰石、重结晶石英、锐钛矿、含铅黄铁矿、叶腊石、金云母、绢云母等共沉淀析出。通过钛铀矿空间占位和载体基质的微区解剖研究认为,湘西北海相磷块岩中钛铀矿的形成与镁铁质—超镁铁质海底喷流系统侵位于海相磷块岩发生热液脱碳作用有关。纳米级钛铀矿的发现对探讨钛铀矿在自然界的存在状态及其成因机理有重要意义。

It is firstly discovered that nanoscale brannerite from oolitic phosphorite in the lower part of Lower Cambrian Niutitang Formation marine black rock series in Wulin area of the northwestern Hunan,the southeastern Yangtze Plate.The chemical composition was analyzed by SEM and high-resolution transmission electron microscopy. The results show that the orthobrannerite occurs in the form of basaltiform platy prismatic joint,the mass fractions of Ti,U and O are 19.24%-20.37%,44.37%-46.86% and 26.05%-31.01%,and the V and Cr occur in brannerite in the form of isomorphic mixture;the calculated chemical equation with electron microprobe analysis is（U）Ti_2O_6（OH）;the orthobrannerite occurs in phosphorite microcrack and microvoid,and precipitates out together with recrystallization apatite,recrystallization quartz,anatase,lead pyrite,pyrophyllite,phlogopite and sericite;the phosphorite microcrack is formed by synsedimentary oolitic phosphorite,which is recrystallized and strongly eroded;according to the characteristics of space occupying and microarea of carrier matrix,the formation oforthobrannerite is in connection with the hydrothermal decarbonization process of maficultramafic submarine exhalation system emplaced in marine phosphate rock of the northwestern Hunan.The discovery of nanoscale brannerite is very important for discussing the natural existing state and genetic mechanism of orthobrannerite.