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27. Application of Electron Probe Microanalyzer to Study the Textures and Compositions of Alteration Coronas of Monazite from the Longhuashan Granite, Northern Guangdong Province

Author

ZHANG Long, CHEN Zhenyu, WANG Fangyue, and ZHOU Taofa

Subjects
monazite, alteration coronas, electron probe microanalyzer, longhuashan granite, northern guangdong, Geology, QE1-996.5, Ecology, QH540-549.5
Abstract

BACKGROUND Monazite is a common uranium-bearing accessory mineral in granite-related uranium deposits in South China. The Longhuashan pluton is an important U-bearing granite in the Zhuguangshan batholith, Northern Guangdong. Distinct alteration coronas of monazite were observed in the pluton. However, textures and compositions of alteration coronas of monazite and their implications for uranium mineralization are still poorly understood. OBJECTIVES To investigate the detailed textural and compositional evolution of monazite in granites during alteration, and to provide insights into uranium mobilization and enrichment in granite-related uranium deposits. METHODS Textures and chemical compositions of alteration coronas of monazite were investigated using electron probe microanalyzer (EPMA). RESULTS Alteration coronas of monazite in the Longhuashan granite consisted of newly formed apatite, allanite, epidote, and Th-rich phases. The monazite alteration coronas area was a concentric zone composed of monazite, apatite (including thorium-rich minerals), and monazite-epidote from the inside to the outside. Mass balance calculations showed that during the alteration of monazite, light rare earth (LREE), Y, Th, and U elements were remobilized and migrated, and the fluid brought in elements such as Ca, Fe, Al, and F to form alteration coronas composed of apatite, epidote, and thorium-rich minerals. The EPMA mapping showed that monazite alteration led to the remobilization of uranium, but the uranium was mainly enriched in the altered coronas. In this pluton, 3.7% U was located in monazite, and more than 80% U was hosted by uraninite. CONCLUSIONS Monazite only contributes limited uranium to regional uranium mineralization, and uraninite is the most important host for uranium in the Longhuashan granite.

Published
2022
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38. The spatial distribution characteristics of Nb–Ta of mafic rocks in subduction zones

Author

Li Weiwei, Ge Can, Wang Fangyue, Sun He, and Gu Haiou

Subjects
mafic rocks, nb–ta, spatial distribution, geochemical characteristics, subduction zones, Geology, QE1-996.5
Abstract

High field-strength elements have been regarded as one of the most important discriminations in subduction zone magma. However, the spatial distribution of Nb and Ta in subduction zone-related rocks has been rarely studied; it is still unclear whether there is a quantitative relationship between the Nb–Ta concentrations and their subduction distance. In this paper, the Nb–Ta concentrations of mafic rocks in arc tectonic systems were calculated from a statistical perspective based on the combined geophysical model and geochemical database. The results showed a typical spatial distribution pattern. The threshold value of Nb (12.20 ppm) and Ta (0.796 ppm) in arc settings was estimated by a cumulative distribution function, which can be used to determine whether the rock is generated in arc tectonic environment. A probability density function of Nb–Ta contents and related subduction distance has been obtained using kernel function estimation. The Nb–Ta concentrations are exponentially correlated with the subduction distance (

Published
2021
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