The role of fluid mixing in the formation of the large-scale Yanbei porphyry tin deposit, South China: Insights from fluid inclusion microthermometry and LA-ICP-MS analyses

• The hydrothermal process can be categorized into three stages, progressing from early to late. • The ore-forming fluid is primarily sourced from a single magmatic fluid origin. • Fluid mixing is likely the predominant mechanism responsible for the Sn mineralization. The Yanbei Sn deposit, located in Jiangxi Province, represents the largest porphyry-type Sn deposit in China. Tin mineralization in this deposit predominantly occurs as disseminated, veinlet, and breccia types, spatially associated with granite porphyry. Although systematic studies have been conducted on the geology, geochronology, and petrology of the granitic and volcanic rocks, detailed constraints on the ore-forming fluids remain poorly understood, significantly hindering our comprehension of the tin deposition mechanisms in such a large-scale porphyry tin deposit. This study presents, for the first time, a comprehensive investigation of fluid inclusions in pre-ore, syn -ore, and post-ore minerals from the Yanbei deposit. The mineralization process is divided into three main stages: (1) stage I: the topaz-cassiterite stage, (2) stage II: the sulfides-quartz stage, and (3) stage III: the siderite-fluorite stage. Our research focuses on fluid inclusion analyses in topaz (T), cassiterite (C) from stage I, quartz (Q) from stage II, and siderite (S) from stage III. Fluid inclusions hosted in topaz display a combination of halite-bearing three phase aqueous (H T ), vapor-dominated two-phase aqueous (V T ), and liquid-dominated two-phase aqueous fluid inclusions (L T ), while fluid inclusions in other minerals exclusively consist of liquid-dominated two-phase aqueous types (L C , L Q , and L S ). The microthermometry results show a synchronous descent in both homogenization temperature and salinity from the early to late stages of mineralization. The analysis of fluid composition reveals striking similarities in element ratios between H T and L T fluid inclusions within topaz, strongly suggesting that the liquid-dominated two-phase aqueous inclusions likely formed through the dilution of the halite-bearing three phase aqueous fluid. This similarity is also observed in cassiterite-hosted fluid inclusions, where the element ratios closely align with those in topaz. In contrast, quartz- and siderite-hosted fluid inclusions exhibit a significant decrease in the ratios of various metals. The Rb/Na, Cs/Na, and K/Na ratios in all fluid inclusions indicate a single, homogeneous magmatic-hydrothermal fluid source. Although fluid boiling occurred during the early topaz stage, the synchronous decrease in salinity and temperature, coupled with the consistent element ratios observed in fluid inclusions from topaz and cassiterite, indicating that fluid mixing with meteoric water is identified as the primary mechanism for cassiterite deposition. A comparison of ore element concentrations in pre-mineralization fluids from topaz at Yanbei with analogous data from diverse initial magmatic-hydrothermal fluids in global Sn-W mineralization and ore-barren systems, suggests that the mineralization potential of the Yanbei porphyry tin deposit is predominantly controlled by the relative concentrations of ore metals in the initial pre-mineralization fluid.