南太行邯邢地区白涧矽卡岩型铁矿中磁铁矿与赤铁矿交代现象对矿床成因的启示

An metallogenetic insight of the metasomatic relationship between magnetite and hematite in the Baijian skarn type iron deposit in the Handan-Xingtai area of the southern Taihang Mountains

对邯邢地区白涧铁矿中磁铁矿与赤铁矿进行成因矿物学研究,精细刻画了铁的成矿过程。在岩相学观察过程中作者发现该矿床中存在2个期次的赤铁矿,其分别交代磁铁矿或被磁铁矿交代。根据磁铁矿和赤铁矿的交代关系,我们将赤铁矿划分为早、晚2个期次,并根据成矿流体的演化,将铁矿化过程划分为4个阶段。成矿流体从岩浆中分离并交代碳酸盐围岩形成矽卡岩,同时形成接触带矿(第1成矿期),随着接触带磁铁矿的形成,具有更高氧逸度的演化的流体沿着断裂带充填和交代碳酸盐地层形成早期赤铁矿(第2成矿期)。随着赤铁矿形成,氧逸度降低至磁铁矿-赤铁矿缓冲线之下,形成层间的磁铁矿(第3成矿期),SO42-转化成HS-,同时形成大量自由氧,导致成矿流体的氧逸度进一步升高,从而形成晚期赤铁矿(第4成矿期),伴随温度和氧逸度的进一步降低,成矿作用进入硫化物形成时期。整个成矿过程中氧逸度控制了成矿过程及其产物,对矽卡岩型铁矿的形成起到了主导作用。

This study has mainly focused on the genetic mineralogical research of magnetite and hematite in the Baijian iron deposit in the Handan-Xingtai area for detailedly describing the iron mineralization process of the deposit. Two stages of hematites, which replaced magnetite or were replaced by magnetite respectively, had been identified through the petrographic observation. According to the metasomatic relationship between magnetite and hematite, the formation of hematite can be divided into the early and late two stages. Based on the study of the evolution of ore-forming fluid, four stages of the iron mineralization process have been identified. In the first ore-forming stage, the ore-forming fluids which were separated from magma had interacted with carbonate wall rocks to have formed skarn and iron ore in the contact zone between intrusive body and wallrock at the same time. In the second ore-forming stage, with the formation of magnetite in the contact zone, the evolved ore-forming fluids with high oxygen fugacity had filled into the fault zone and then metasomated with carbonate stratum to have resulted in the formation of the early-stage hematite. In the third mineralization stage, with the formation of hematite, oxygen fugacity of the for-forming fluids was decreased into the area below the magnetite-hematite buffer line. This resulted in the formation of interlayer magnetite. In the last ore-forming stage, the transformation of HS-from SO42-and the simultaneous formation of large amounts of free oxygen had resulted in the further increase of oxygen fugacity of the ore-forming fluids, and then the formation of the late-stage hematite. With the further decrease of temperature and oxygen fugacity of the evolved ore-forming fluids, sulfides were formed in the last stage of sulfide mineralization. It is believed that oxygen fugacity had controlled the whole mineralization process and its products, and played a leading role in the formation of skarn type iron ore.