Citation: | ZHANG Long, CHEN Zhenyu, WANG Fangyue, ZHOU Taofa. Application of Electron Probe Microanalyzer to Study the Textures and Compositions of Alteration Coronas of Monazite from the Longhuashan Granite, Northern Guangdong Province[J]. Rock and Mineral Analysis, 2022, 41(2): 174-184. doi: 10.15898/j.cnki.11-2131/td.202109070118 |
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.
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.
Textures and chemical compositions of alteration coronas of monazite were investigated using electron probe microanalyzer (EPMA).
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.
Monazite only contributes limited uranium to regional uranium mineralization, and uraninite is the most important host for uranium in the Longhuashan granite.
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Simplified geological map of the Zhuguangshan batholith, Northern Guangdong (Modified after References [21-22])
Photographs of hand specimen samples and alteration coronas of monazite from the Longhuashan granite. a—Hand specimen photographs of samples collected from drill cores within the Longhuashan granite; b, c, d—Backscattered electron images of monazite (Mnz) alteration coronas consisting of apatite (Ap), Th-rich minerals, allanite (Aln), and epidote (Ep); e—Monazite alteration coronas showing no alteration coronas near zircons (Zrn); f—Monazite partly enclosed in apatite
Elemental maps of alteration coronas of monazite from the Longhuashan granite. a—Backscattered electron image of alteration coronas of monazite; b-i—Elemental maps of alteration coronas of monazite showing distributions of La, Ce, Ca, P, Th, U, F and Si
(a) Backscattered electron image and (b) weighted mean chemical age of uraninite from the Longhuashan granite