Arc magmatic evolution of submarine volcanic-hosted iron deposits: Insights from an economic deposit class in the southern Central Asian orogenic belt, Northwest China

Submarine volcanic-hosted iron deposits represent an important group of iron ores. In recent years, significant progress has been achieved in the exploration of this iron ore type in the eastern Tianshan in China. However, the metallogenic epoch, material sources, and genetic mechanisms of this type of deposit remain controversial. Therefore, based on field investigations, microscopic observations, and integrated analytical data from representative submarine volcanic-hosted iron deposits in the region, this study establishes the following conclusions. The host rock constitutes a sequence of submarine volcanic formations, encompassing basalt, andesitic tuff, and dacitic tuff, predominantly comprising intermediate-basic volcanic rocks. New secondary ion mass spectrometry (SIMS) zircon U-Pb dating indicates that the basalt, andesitic tuff and dacitic tuff series were formed around 317.0 Ma, 313.9 Ma, and 313.2 Ma, respectively. Geochemically, these submarine volcanic rocks exhibit enrichment in large ion lithophile elements (LILEs) (e.g., Rb, Ba, and Pb) and light rare earth elements (LREEs), along with depletion in high field strength elements (HFSEs) (e.g., Nb, Ta, and Ti). All volcanic rocks exhibit similarly low δ¹⁸O values (4.7‰-6.2‰), positive ε_Hf(t) values (+8.2 to +17.0) and ε_Nd(t) values (+4.6 to +7.3). The trace element compositions of magnetite grains show relatively low Ga, Zn, V, Ti, Cr, as well as elevated Ni contents and Ni/Cr ratio. The oxygen isotopic composition of magnetite and sulfur-lead isotopic signatures of pyrite collectively indicate that the ore-forming materials were predominantly derived from deep-seated magmatic sources, with possible late-stage incorporation of seawater components. In conjunction with petrography and geochemical signatures, we propose that Carboniferous southward subduction of the Paleo-Tianshan Ocean triggered slab dehydration, metasomatizing the overlying mantle wedge and generating iron-rich basaltic magmas. Fractional crystallization segregated iron-enriched fluids, with residual melts erupting as volcanic rocks. Later tectonic activity released these fluids, inducing seawater convection and leaching additional iron from the volcanics, further enriching the hydrothermal system. Finally, magnetite precipitated from the hydrothermal fluids at favorable structural traps due to changes in temperature and pressure during fluid migration. Our study provides new insights into the characteristics and genesis of this important class of mineral systems, that could be useful for exploration.