Formation Mechanisms of Fe-dominated Versus Fe Polymetallic Deposits in the West Tianshan, NW China: A Magnetite Perspective

The West Tianshan in the Central Asian Orogenic Belt is one of important Fe belts in China, where many submarine volcanogenic iron-oxide deposits constitute the Awulale metallogenic belt. The Dunde deposit is a Fe -ZnAu deposit, whereas Chagangnuoer, Zhibo, and Beizhan are Fe-dominated deposits. To examine the factors controlling the formation of Fe polymetallic deposits, the texture and chemical composition of magnetite from the Dunde deposit were studied in detail and compared with those of other deposits in the same belt. The Dunde deposit magnetite can be divided into Mag -1, Mag -2, and Mag -3, which correspond to stages of retrograde, sulfide, and carbonate stages, respectively. They have experienced oxy-exsolution and coupled dissolution and reprecipitation (CDR), forming multiple generations of magnetite. The exsolution of gahnite in Mag -1 and 10 times higher Zr, Hf, Sc, Ta, Nb, Mn, and Zn contents in the all stages of magnetite than those of Fe-dominated deposits, indicating the Dunde magnetite was crystallized from high-temperature, high-salinity, Fe-Zn-rich fluids. The similar chalcophile element contents in host volcanic rocks between Dunde and other deposits indicate host volcanic rocks are not the reason for Zn enrichment in Dunde. The higher Mg + Mn and Si + Al contents in Dunde magnetite than magnetite of other deposits indicate stronger fluid-rock interaction. Extensive CDR texture in Mag -1 and Mag -2 and lower Zn in secondary magnetite indicate that Zn leached from Zn-rich magnetite may have also contributed to the formation of sphalerite in sulfide stage. Our study has demonstrated that high-temperature, high-salinity, Fe- and Zn-rich primary fluids, a higher degree of fluid-rock interaction, and repeated fluid infiltration are key factors controlling the formation of Fe -Zn polymetallic deposits, such as the Dunde deposit in West Tianshan.