The genesis of Eocene granite‐related Lailishan tin deposit in western Yunnan, China: Constraints from geochronology, geochemistry, and S–Pb–H–O isotopes

Abstract

The Tengchong‐Lianghe tin metallogenic district in western Yunnan is an important tin mineralization area in the Sanjiang Tethyan Metallogenic Domain from southwestern China. The Lailishan deposit is one of the largest tin deposits in this district and spatially and temporally associated with Eocene granites. LA‐MC‐ICP‐MS cassiterite U–Pb dating (49.8 ± 2.6 Ma) suggests that the tin mineralization is coeval with the Lailishan granites. The contact between the monzogranite and the syenogranite is gradual and both are weakly peraluminous, with high SiO2, Al2O3, and total alkali contents. They are characterized by negative Eu anomalies and the depletion in high‐field‐strength elements and enrichment in large‐ion lithophile elements. Compared with the monzogranite, the syenogranite is more evolved and exhibit ‘tetrad effect’. The Lailishan monzogranite was probably derived from the partial melt of the ancient crustal basement from the Tengchong Block under syn‐collisional tectonic setting. The syenogranite was cogenetic with monzogranite and experienced the process of fractional crystallization. The mineralization process is divided into three successive mineralization stages (I–III) in this study. These stages are characterized by the coarse‐grained cassiterite and high temperatures (216–337°C), light brown or colourless cassiterite and moderate temperatures (160–243°C), and dark brown cassiterite and low temperatures (116–199°C) of ore‐forming fluids, respectively. From the Stage I to later stages, both the δ18OH2O and δDSMOW values of fluids are decreasing and shift from magmatic water towards meteoric water, indicating that the ore‐forming fluids derived from granitic melts and mixed over time with meteoric water with the decrease of temperature. The δ34S values of pyrites and pyrrhotites (2.4–5.9‰) and Pb isotopic compositions for pyrites from Lailishan deposit (206Pb/204Pb = 18.687–19.402) are similar with those of Lailishan granites. We propose that the Lailishant in deposit is derived from the Lailishan granites.