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

Author(s):: QI Fu-cheng; LI Zhi-xing; WANG Wen-quan; YANG Zhi-qiang; PENG Bo; WANG Zhen-yun; DONG Yan; CNNC Key Laboratory of Uranium Resource Exploration and Evaluation Technology, Beijing Research Institute of Uranium Geology, Beijing 100029, China

Keywords:: brannerite; marine phosphorite; SEM; microarea; submarine exhalation; hydrothermal decarbonization; genetic mechanism; Hunan

Abstract:: 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)Ti₂O₆(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 of orthobrannerite is in connection with the hydrothermal decarbonization process of maficultramafic 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.