Electron Microprobe Analysis of Biotite with Reequilibration Texture in Altered Trachyte

Abstract

BACKGROUND The chemical composition of biotite contains important genetic information. The in-situ analyses of biotite with metasomatic texture can reflect the migration of the elements during the metasomatic alteration, and provide mineralogical evidence for the ore-forming mechanism. OBJECTIVES To investigate the migration of elements during trachyte alteration and reveal the Nb mineralization mechanism in altered trachyte. METHODS Biotite phenocryst with reequilibration texture in altered trachyte in the Pingli area of North Dabashan was studied, and the EMPA element mapping and line profile analyses were carried out. RESULTS The results show that the phenocryst was magmatic in the core and the reequilibrated origin on the rim, caused by diffusion-reaction process. Compared with the core, the TiO2 content decreased from an average of 5.78% to 2.22% in the rim, indicating that Ti was leached out into the fluid during the exchange alteration. The CaO content increased from 0.06% to 0.14% on average, and the F content increased from 0.60% to 0.80% on average, suggesting that the alteration fluid is rich in Ca and F. CONCLUSIONS The formation of Nb deposits in trachyte is closely related to the hydrothermal activity, and the components such as Ca and F in the ore-forming fluid play an important role in the migration and enrichment of Nb.

Keywords

• biotite
• electron microprobe analysis
• reequilibration textures
• nb deposit
• trachyte