Geochemical and Fe-isotope characteristics of the largest Mesozoic skarn deposit in China: Implications for the mechanism of Fe skarn formation

Skarn deposits are major repositories of Fe and are the principal sources for iron mining in China. However, their mechanism of formation remains debated. Here we investigate the Xishimen (XSM) Fe deposit, located in Wu'an, China. This deposit is a well-known Mesozoic large skarn Fe deposits in the Han-Xing Fe metallogenic belt. Here we report the various features of this Fe deposit that suggest a clear magmatic affinity. We present data on the mineral assemblages, major and trace element data and Fe isotope analysis of magnetite from the XSM Fe deposit. Our results identify four distinct types of magnetite (Type I, II, III and IV). Magnetite type I is characterized by anhedral shape with vesicles in the ores, and is associated with diopside (Di), tremolite (Tr), ±garnet (Gt) and/or augite (Aug), together with phlogopite. Magnetite type I shows high content of TiO₂ (0.25–1.21 wt%) and Al₂O₃ + MnO (0.24–0.72 wt%). These features suggest that magnetite type I is of magmatic origin. Magnetite type IV displays euhedral shape, and is associated with calcite, apatite and pyrite, and is enriched in SiO₂, CaO, Sr, Ba, Rb, Nb, and total REE, and depleted in TiO₂. These features suggest that magnetite type IV is of hydrothermal origin. The features of magnetite types II and III fall in between magnetite I and magnetite IV. The δ⁵⁶Fe values are also distinct for the four types of magnetites: δ⁵⁶Fe is the highest for type I magnetite (average 0.085‰); and the lowest for magnetite type IV (average 0.016‰). Our results also show that the δ⁵⁶Fe value tends to be higher under high temperature environment and in lower part of the main orebody. Statistical analysis shows a negative correlation between the δ⁵⁶Fe values and depth. With a view to constrain the origin of the skarn type Fe deposit, we present a conceptual model based on the geochemical and isotope data. We envisage distinct phases during the Fe ore genesis, including: (a) the formation of an iron-rich magma of dioritic composition which reacted with the carbonate wall rocks; (b) the iron-rich magma sinks into the bottom of the magma chamber with high density; (c) volatiles from deep magmatic chamber evolve and interact with the surrounding carbonate rocks; and (d) due to fluid overpressure in the chamber, the iron bearing melt and fluids migrate along structural conduits and precipitate the Fe ore. Because of the difference in temperature, pressure, and oxygen fugacity, the composition of magnetite varied along the fluid/melt conduits. Thus, magnetite type I (high Ti Mt) was mainly distributed in the lower part of the conduit, and magnetite IV (high Si Mt) was mainly concentrated in the upper part during progressive cooling in the conduit. Fe isotope fractionation occurred during the evolution of the Fe-bearing melt/fluid in response to changes in temperature and oxygen fugacity.