Identifying superimposed W–Sn mineralization events using cassiterite microtextures, trace-element chemistry, and geochronology

Abstract

Numerous large deposits are formed by multiple mineralization events, however, how to identify superimposed mineralization events is poorly understood. The Xiangdong deposit is a large quartz vein-hosted W–Sn deposit within the Dengfuxian composite granite in South China. Previous studies have suggested that there are two discrete W–Sn mineralization events in the deposit, whereas the ore-forming conditions and processes of the two mineralization events are still unclear. To address these ambiguities, this contribution characterized the microtextures, trace-element chemistry, and geochronology of cassiterite assemblages within W–Sn-rich quartz veins. Based on cathodoluminescence (CL) images, cassiterite crystals were classified into two types with textural differences — Cst 1 and Cst 2. Cassiterite 1 is subhedral to irregular and exhibits complex zonation, whereas Cst 2 is euhedral and exhibits oscillatory zonation. Trace-element concentrations and maps of cassiterite obtained via LA–ICP–MS indicate that Cst 1 has high Fe, W, U, and Sb, but low Nb, Ta, and Zr concentrations, whereas Cst 2 has high Nb, Ta, and Zr, but low Fe, W, U, and Sb concentrations. This indicates that Cst 2 formed from higher temperature and more oxidizing hydrothermal fluids than Cst 1. Laser ablation ICP–MS U–Pb geochronological results of Cst 1 and Cst 2 demonstrate they precipitated at 150.0 ± 2.6 Ma and 136.3 ± 5.5 Ma, respectively. This contribution demonstrates that integrated textural and compositional studies of cassiterite assemblages are critical to identifying hydrothermal events that superimposed W–Sn mineralization, and to constrain ore-forming mechanisms and the physicochemical conditions of the environment.