青海尕林格矽卡岩铁多金属矿床Fe-Ti氧化物及其热动力学意义

摘要

尕林格大型矽卡岩型铁多金属矿床地处东昆仑祁漫塔格山与柴达木盆地结合部位靠近盆地一侧。本文主要针对尕林格矿床Ⅱ矿群内发现的热液性质的 Fe-Ti 氧化物共生组合(HYM)和新鲜镁铁质玄武安山岩中火成性质的Fe-Ti氧化物共生组合(IGM)的化学成分及其热动力学平衡进行了详细研究。镜下观察发现, HYM与镁铁尖晶石共生,钛铁矿呈叶片状平行于磁铁矿八面体的(111)面出溶,根据氧化反应的平衡公式计算得到出溶反应的平衡温度集中在581.8~688.9℃之间,氧逸度fO2介于10-14.74~10-17.94之间； IGM主要与岩浆中的镁铁质矿物相平衡,其出溶反应的平衡温度范围为690.7~740.3℃,氧逸度集中在10-15.44~10-17.30之间。计算结果表明,尕林格矽卡岩型铁矿的最初成矿温度可达700℃,接近于水饱和岩浆结晶时的温度,因此判断 HYM 形成于岩浆演化早期高温高盐度流体的最初冷却过程。由于早期高温高盐度流体与围岩地层中的镁铁质火山岩发生交代反应,淋滤出大量的Fe、Ti、Al、Mg、Cu等金属物质。在高温条件下, Ti和 Al 进入磁铁矿晶格,导致含钛磁铁矿在降温过程中经历了氧化出溶反应形成钛铁矿-磁铁矿共生组合。此外,由于热液受到镁铁质火山岩的缓冲作用,导致HYM和IGM各自平衡时的氧逸度很接近。%TheGalinge skarn iron deposit is located at the junction between the Qimantag metallogenic belt in the East Kunlun Mountains and the Qaidam Basin. There are two types of Fe-Ti oxides coexisting assemblages in the Galinge skarn deposit: one is the hydrothermal magnetite-ilmenite coexisting assemblage (HYM) which was discovered in No. 2Ⅱ ore group, whereas the other one is the igneous magnetite-ilmenite coexisting assemblage (IGM) which is developed in the fresh mafic basaltic andesite. This research aimed to analyze the distinctive composition and significant thermodynamic features of the HYM and IGM. The HYM shows an intergrowth with ferromagnesium spinel, and the crystallographic orientation of growing ilmenite lamellae are parallel to the (111) planes of the magnetite octahedron. Based on the equilibrium of oxidation, the equilibrated temperatures range between 581.8C and 688.9℃, and the oxygen fugacities vary between 10-14.74 and 10-17.94. The IGM is mainly in equilibrium with silicate minerals in the mafic magma. The equilibrated temperatures of the IGM range between 690.73℃ and 740.26℃, and the range of oxygen fugacities is 10-15.44-10-17.30. It is reasonably inferred that the primary metallogenic temperature might have reached 700℃, approaching the crystallographic temperature of water saturated magma. Therefore, the HYM was formed after the primary cooling procedure of the early high-temperature and high-salinity hydrothermal fluid which evolved from the magma. Because of the infiltration metasomatism between the hydrothermal fluid and the mafic igneous rocks, large amounts of metal elements such as Fe, Ti, Al, Mg and Cu were leached out. In the early high temperature setting, Ti and Al were incorporated into the magnetite, and consequently the oxyexsolution of titaniferous magnetite resulted in the intergrowth of magnetite and ilmenite. As the oxyexsolution did not rapidly re-equilibrate under new conditions, the iron-titanium oxide relationship could be preserved and hence could indicate the equilibrium temperature and the oxygen fugacity of the oxidation.