Genesis of submarine volcanic iron deposits in the Central Asian Orogenic Belt: Insights from the texture and chemical composition of magnetite from the Langwashan iron deposit in the Beishan orogenic belt, NW China

Abstract

Carboniferous submarine volcanic rocks hosting iron deposits are widely distributed in the Tianshan and Beishan orogenic belts. However, the genesis of these deposits remains unresolved, with contrasting magmatic and magmatic-hydrothermal models being proposed. The Langwashan iron deposit (83.18 Mt @ 37% Fe) is one of the largest iron deposits in the Beishan orogenic belt. In this paper, we present detailed textural and trace-elemental properties of magnetite from the Langwashan iron deposit to elucidate the genesis and evolution of iron mineralization hosted in Lower Carboniferous submarine volcanic rocks. The Langwashan iron deposit has various types of iron ore that are closely associated with skarn minerals, including garnet and epidote. Magnetite in these samples can be classified into two distinct types, early-stage mushketovite (Mag1) and late-stage granular magnetite (Mag2), both of which show oscillatory zoning, with low Ti (mean of 0.03–0.07 wt%) and V (mean of 0.03–0.04 wt%) contents, and variable Si contents (mean of 0.74–6.50 wt%). These textural and compositional data classify the Langwashan deposit as a typical Fe-skarn deposit. Results also reveal that iron oxides in the Langwashan deposit underwent a complex evolution comprising three episodes: (1) early-stage influx of low-oxygen-fugacity, calcium-rich hydrothermal fluids, causing the transformation of original hematite to mushketovite (Mag1-1) and the formation of scheelite inclusions; (2) subsequent dissolution–reprecipitation between high-temperature magmatic-hydrothermal fluids and Mag1-1, generating trace-element-rich mushketovite (Mag1-2) and the precipitation of trace-element-rich granular magnetite (Mag2-1) from magmatic-hydrothermal fluids; and (3) final multiple fluid metasomatism under the influence of trace-element-poor hydrothermal fluids, involving dissolution–reprecipitation that produced trace-element-deficient Mag1-3, Mag2-2, and Mag2-3. Results also highlight that the submarine volcanic iron deposits from both the Tianshan and the Beishan orogenic belts were probably formed in a similar geological setting, with similar timing of formation, mechanism of mineralization, and evolution, and they are likely Fe-skarn deposits.