Constraining the origin of the Mesozoic Xishimen Fe deposit in the Taihang Mountain Range, North China using geochemistry of magnetite

Abstract

The Xishimen (XSM) iron (Fe) deposit located in the Taihang Mountain Range in North China is one of the largest known Mesozoic skarn Fe deposits in China. Recent research suggests that the Fe deposit exhibits some magmatic features. Here, we report new major and trace element, mineral assemblage and Fe isotope data of magnetite from the XSM Fe deposit in the Han-Xing Fe metallogenic belt, which allows for the identification of 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 consists of minerals diopside (Di), tremolite (Tr), ±garnet (Gt) and/or augite (Aug), spreading over diopside, tremolite, phlogopite and/or Aug as an interstitial mineral, which can dissolve or melt pre-existing minerals (such as Di and Tr). Magnetite type I also contains 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 is marked by euhedral shape, consists of Cc, Ap and/or Py, enriched in SiO2, CaO, Sr, Ba, Rb, Nb, and total REE, and depleted in TiO2. These properties 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 value also differs for the four types of magnetite: δ56Fe is the highest for type I magnetite (average 0.085‰); and the lowest for magnetite type IV (average 0.016‰). We also find that δ56Fe value tends to be higher under high temperature environment and in lower part of the main orebody. Statistical analysis shows that δ56Fe and altitudes are negatively correlated.In order to constrain the origin of the XSM Fe deposit, we present a new conceptual model based on our new geochemistry data. This model suggests a stepwise pattern of Fe ore genesis, including (1) “melt-fluid bearing Fe” forms in the deep magma chamber; (2) “melt-fluid bearing Fe” rises along the magmatic conduit due to fluid overpressure in the deep magma chamber; (3) changes in ambient temperature, pressure, and oxygen fugacity cause the different composition of magnetite along the magmatic conduit; and (4) Fe isotope fractionation occurs during the rising and settlement of “melt-fluid bearing Fe” in response to changes in temperature and oxygen fugacity. This model explains the magmatic characteristics of magnetite formed under high temperature in the lower part of the conduit, and the hydrothermal characteristics of magnetite occurred under low temperature in the upper part of the conduit.