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

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.

Keywords

• monazite
• alteration coronas
• electron probe microanalyzer
• longhuashan granite
• northern guangdong