Geochronology and textural and compositional complexity of apatite from the mineralization-related granites in the world-class Zhuxi W-Cu skarn deposit: A record of magma evolution and W enrichment in the magmatic system

Abstract

Apatite is a ubiquitous mineral in granites and crystallizes from the beginning of magma evolution to the latest magma-fluids transitional stage, and thus, is able to record petrogenetic-metallogenic information. In order to figure out the role of granites in the formation of the Zhuxi deposit, textures and compositions of apatites in the Zhuxi granites were investigated. Granites related to the Zhuxi W-Cu deposit include biotite granite, muscovite granite, and granite porphyry. Apatites in these granites are named Group A, Group B (core – Group B1; rim – Group B2), and Group C, respectively. Along this sequence, geochemical characteristics of apatites show gradual enrichment of total rare earth elements (ΣREE), high-field strength elements (HFSE; Th, U, Nb, Ta, Zr, and Hf), as well as ore-forming element W, and depletion of Sr. Meanwhile, Ga content in these apatites consecutively increased, indicating the reduction of oxygen fugacity in the Zhuxi magmatic system. Non-chondritic Y/Ho (mostly > 34) and Zr/Hf (broadly < 5) ratios in Group C apatite reflect the high volume of fluids interacted with the magma of the most evolved granite porphyry. Constant high F contents (3.4–4.4 wt%) in all apatite samples show abundant F in the Zhuxi magmas, which played an important role in the formation of the Zhuxi deposit in terms of facilitating W enrichment in magma and exsolution of W into hydrothermal fluids. The Zhuxi granites formed at ca. 153 Ma when South China Block was in an extensional tectonic setting, and the granite porphyry is the most potential intrusion to cause the mineralization of the Zhuxi deposit.