LA−ICP−MS trace element analysis of pyrite from Jiaodingshan cobalt deposits in Sichuan，and its constraints on the ore genesis

The Jiaodingshan cobalt deposit is situated in the southwestern of Sichuan basin, and the source of the ore materials and fluids is still under debated. Cobalt is enriched in vulcanized species because of its double properties of iron and sulfur affinity. In order to analyse the genesis of the deposit, this study investigates the pyrite microstructure and the geochemical compositions of pyrite based on LA−ICP−MS. The results show that three generations of pyrite were identified according to its microstructure characteristics in Jiaodingshan cobalt deposit. The first generation (pyrite Ⅰ)is identified as original sedimentary pyrite, including colloform pyrite, subhedral pyrite and framboidal pyrite. The pyrite Ⅰ is characterized by low Co/ Ni ratio (<1 and the average ratio is 0.53) and low Co content (99.8 × 10⁻⁶). Due to the influence of tectonic deformation and hydrothermal superimposition, the pyrite Ⅱ often presents the structure with metasomatic relict texture(pyrite Ⅱ−a) and recrystallization(pyrite Ⅱ−b), which is characterized by high Co/ Ni ratios (Co/ Ni >1, the average ratios is 1.31) and middle Co content (1060.8 × 10⁻⁶). Pyrite Ⅲ exists in the form of fine sand−like and fine colloidal aggregates, along with strong metal luster. Such pyrite is often associated with linnaeite, siegenite and chalcopyrite, with the ratios of Co/ Ni higher than 1(the average ratios is 2.05) and the Co content being 10453.5×10⁻⁶. The Co and Ni contents in Py Ⅲ perhaps represented by siegenite inclusions enclosed in pyrite. Overall, the early sedimentary pyrite I with high ore−forming element background value may be affected by tectonic deformation−hydrothermal superposition transformation in the later period, resulting in further enrichment of ore−forming elements (Co) in pyrite Ⅱ. While the main ore−forming period of Py Ⅲ was hydrothermal superimposed on Py Ⅱ, and finally formed the existing cobalt deposit.