Magnetite compositions of the iron-rich agglomerates of the Heijianshan iron deposit in Eastern Tianshan Mountains and magmatic-hydrothermal evolution processes

The Heijianshan iron deposit represents a typical submarine volcanic rock-hosted deposit of the Aqishan-Yamansu ore belt in Eastern Tianshan Mountains. Abundant irregular iron-rich agglomerates are developed in the brecciated andesite lava (wall rock), and they can be subdivided into five types, i.e., albite-magnetite type, albite-K-feldspar magnetite type, K-feldsparmagnetite type, epidote-magnetite type and quartz-magnetite type, likely representing evolving products of the magmatic-hydrothermal ore-forming process, which can constrain the ore-forming process and metallogenic environment of the Heijianshan iron deposit. Magnetite compositions of the five types of agglomerates were analyzed using electron microprobe analysis. For the purpose of obtaining precise Fe content, the content of undetermined O was added by difference method and the ZAF matrix correction was conducted. The Ti values of the five types of agglomerates display a positive relationship with the Fe values. Magnetite of the albite-magnetite type has highest Ti content, the albite-K-feldspar magnetite and the K-feldspar-magnetite types show medium Ti content, whereas the epidote-magnetite and quartz-magnetite types are characterized by the lowest Ti content. Also, the Fe content of the epidote-magnetite and the quartz-magnetite types is similar to that of the ores. These features indicate that the albite-magnetite type seems to have been the earliest crystallization product from a residual iron-rich melt, the albite-K-feldspar-magnetite and K-feldspar-magnetite types display features of magmatic-hydrothermal transition, whereas the epidote-magnetite and quartz-magnetite types represent products of hydrothermal alteration. The Fe content of magnetite of each type of agglomerate is higher than its content of the corresponding alteration zone, suggesting a simultaneous relationship between the crystallization of iron-rich agglomerates and hydrothermal activities.