Origin of the disseminated magnetite pyroxenite in the Tieshanmiao-type iron deposits in the Wuyang region of Henan Province, China

• Tieshanmiao-type iron ores are characterized by disseminated magnetite pyroxenite.
• Submarine chemical precipitation with minor input from terrestrial or volcanic materials.
• Protolith of ores is constrained as quartz–carbonate iron formation.

The Tieshanmiao-type iron deposits in the southern North China Craton comprise two types of ores: banded pyroxene–magnetite quartzite (BMQ) and disseminated magnetite pyroxenite (DMP). Whether the quartz-poor DMP represents metamorphosed iron-bearing ultramafic rocks or chemical sedimentary rocks is still unclear. Pyroxene compositions in the DMP are low in Al2O3 and TiO2, which are similar to those from the BMQ and altered marble and pyroxenite. However, the compositions are different from those in the metamorphosed ultramafic rocks. The DMP and BMQ also show similar major element contents, with dominant SiO2, total Fe2O3, CaO, MgO but low contents of Al2O3, TiO2, MnO, Na2O, K2O, indicating a similar source through submarine chemical precipitation with little input from terrestrial or volcanic materials. The BMQ, DMP and magnetite separates from these rocks exhibit seawater-like signatures of REE patterns with LREE depletion, positive La, Gd and Y anomalies and high Y/Ho ratios, indicating that seawater participated in the formation of the iron ores. Combined with strong positive Eu anomalies, we infer that the deposition of the BMQ and DMP was mainly controlled by the mixing of seawater with hydrothermal fluids. The lack of negative Ce anomalies of the DMP, BMQ and magnetite separates indicate an anoxic marine environment. The DMP is rich in carbonate but relatively poor in silica and the BMQ is rich in silica but poor in carbonate. The protoliths of the DMP and BMQ in the Tieshanmiao-type iron deposits are inferred to be quartz–carbonate iron-bearing formations which underwent subsequent metamorphism.