Mineralogical study and geological significance of pyrite from the Donghuofang gold deposit in Inner Mongolia

The Wulashan–Jining Fe–Au–Mo–Cu–Pb–Zn–graphite–muscovite metallogenic belt is an important polymetallic gold province in the western segment of the northern margin of the North China Craton. The Donghuofang gold deposit in Inner Mongolia represents a typical gold mineralization example within this belt, yet its genesis remains debated. To constrain the genetic type and ore-forming mechanism, we conducted systematic in situ trace element and sulfur isotope analyses on hydrothermal pyrite from different mineralization stages. The results reveal that the Donghuofang deposit experienced multiple episodes of hydrothermal overprinting. Pyrite from the main mineralization stage (Stage II) shows notably higher cobalt (Co) contents and elevated S/Fe atomic ratios compared with Stages III and IV, suggesting relatively higher crystallization temperatures than the late-stage pyrites. However, Stage I pyrite exhibits the highest As contents, indicating the highest formation temperatures among all stages. The combined trace element and S/Fe systematics thus suggest a progressive cooling trend from Stage I to Stage IV, with the main gold mineralization (Stage II) occurring under intermediate-temperature conditions. Integrating pyrite trace element geochemistry and sulfur isotope compositions (δ³⁴S = + 4.30‰ to + 5.63‰) with the regional geological setting, we propose that the ore-forming fluids were derived from magmatic-hydrothermal activity, most likely related to the Hercynian alkaline magmatic system, with incorporation of crustal materials during ascent. The Donghuofang deposit is therefore classified as an orogenic gold deposit, with mineralization expressed as gold–quartz–carbonate veins and controlled by regional ductile shear zones.