Geochemistry of cassiterite and skarn minerals as indicators for formation mechanism and fluid evolution of the newly discovered Jinshui skarn tin deposit in the East Kunlun orogenic belt, NW China

Abstract

Discovery of the Jinshui skarn-type tin deposit marked the first instance of a tin deposit to be uncovered in the central segment of the East Kunlun Orogenic Belt (EKOB) in northwestern China. To determine the timing of tin mineralization and elucidate the progression of the ore-forming hydrothermal system, we conducted geochronology of cassiterite and trace element analysis on various minerals hosted in skarn ores. The U–Pb dating results of the cassiterite indicate that tin mineralization took place at 392.9 ± 5.8 Ma, which closely aligns with the syenogranite emplacement at 396.1 ± 2.1 Ma in the Jinshui deposit. This correlation suggests a temporal link between tin mineralization and granitic magmatism. The deposit is characterized by two generations of cassiterite, with trace element analyses consistently showing that the dark cathodoluminescent cores have relatively high W and U concentrations, whereas the bright rims are enriched in Sc, Ti, V, Zr, In, and Hf. Additionally, the Zr/Hf ratios confirm that the syenogranite was the source of the ore-forming fluids. During the prograde skarn stage, the ore-forming fluids maintained an equilibrium closed system. Initially, the ore-forming fluid experienced reducing conditions, characterized by a low water-to-rock (W/R) ratio and a neutral to slightly alkaline pH. This fluid subsequently evolved into an oxidizing fluid with an elevated W/R ratio and an acidic pH. During the retrograde skarn stage, the fluid continued to exhibit high levels of oxygen fugacity. As the mineralization process progressed from the oxide stage to the quartz-cassiterite-sulfide stage, the presence of cassiterite indicates that the ore-forming fluid experienced two increases in oxygen fugacity, and this fluctuation may be attributed to the mixing of external fluids. The post-collisional extensional environment provided the tectonic background for the formation of the Jinshui tin deposit. Magmas derived from mantle sources ascended, and partial melting of the felsic crustal materials led to the formation of the Jinshui syenogranite after differentiation. Skarn-type tin deposits formed at the favorable locations of the contact between the granite body and the Sn-rich Jinshuikou Group.