The ore-forming process of the Maoping MVT Pb-Zn deposit in northeastern Yunnan, SW China: Constraints from trace element compositions of pyrite

Pyrite—which forms predominantly during the early ore-forming stages of Mississippi Valley-type (MVT) Pb-Zn deposits, can provide critical constraints on early ore-forming processes. This study utilizes an integrated approach including field geology, microstructural analysis, in situ geochemistry (LA-ICP-MS trace element and sulfur isotope analysis), and multivariate statistics. These methods are applied to investigate the different types of pyrite in the super-large Maoping MVT Pb-Zn deposit in northeastern Yunnan, SW China, focusing on their microstructural characteristics, elemental occurrence states, fluid properties, and sources of reduced sulfur. The ore formation processes of the Maoping deposit mainly comprise the pre-ore stage, the syn -ore stage, and the post-ore stage. Pyrite is mainly distributed in the pre-ore stage (Py0), ore stage I (Py1), ore stage II (Py2), and ore stage III (Py3). Py2 and Py3 occur in association with sphalerite and galena. The indicative significance of pyrite formation for the Pb-Zn ore-forming process is explored. The results show that: (1) In pre-ore stage, Ni, Sb, and Pb occur mainly via isomorphous substitution. In Stage I, Sb exists mainly through isomorphous substitution. In Stages II and III, Ag, Sb, and Pb show dominant isomorphous substitution. (2) From Py0, Py1 and Py2 to Py3, the content of Co and Ni gradually decrease, the Co/Ni ratio gradually increases, and the δ 34 S value shifts from lower to higher value. These phenomena reveal a trend of gradually decreasing fluid temperature and gradually strengthening reducing conditions during ore mineralization. The reduced sulfur in Py0 originates from the bacterial sulfate reduction (BSR) process, while that in Py1, Py2, and Py3 stems from the thermochemical sulfate reduction (TSR) process, with Py1, Py2, and Py3 also showing the presence of BSR contribution. (3) In this deposit, pyrite precipitated during the early ore-forming stages, and the subsequent precipitation of pyrite, followed by sphalerite and galena, is primarily controlled by increasing reduced sulfur concentrations. As metal sulfides continue to precipitate, the concentration of reduced sulfur within the fluid begins to decrease until it is completely depleted, and oxygen fugacity gradually increases. A detailed systematic study of pyrite in the MVT Pb-Zn deposits can provide effective constraints on the early-stage fluid evolution process during ore mineralization.