ISSN 1009-6248CN 61-1149/P 双月刊

主管单位:中国地质调查局

主办单位:中国地质调查局西安地质调查中心
中国地质学会

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    西昆仑老并铁矿床磁铁矿矿物化学组成特征及其对成矿作用的约束

    Chemical Composition of Magnetite from the Laobing Iron Deposit in West Kunlun, and Its Constraints on Ore Formation

    • 摘要: 西昆仑塔什库尔干铁矿带是中国西部重要的铁矿富集区,老并铁矿床是该成矿带典型铁矿床之一,发育磁铁矿、石膏以及硫化物等矿物组合,是研究该区域铁成矿作用的理想对象。笔者在详细野外地质调查基础上,针对老并铁矿床3类不同产状磁铁矿开展了LA-ICP-MS原位微量元素分析,系统探讨了成矿物质来源、成矿物理化学条件及矿床成因。结果表明,3类磁铁矿微量元素蛛网图变化趋势基本一致,配分模式与高温热液矿床相当,指示其成矿物质主要来源于海底高温热液。磁铁矿(Al+Mn)-(Ti+V)温度计及Ga-MgO图解显示,Mag-1和Mag-2结晶温度约为300 ℃,而Mag-3受后期变质作用改造明显,记录了更高的再平衡温度。磁铁矿中Al、Ti、V的明显富集、较高的Ni/Cr值与较低的Ti/V值,共同指示其结晶于相对还原的成矿环境,且从Mag-1到Mag-3氧逸度逐渐降低。老并矿床磁铁矿的V+Ti及Al+Mn+Ca含量明显高于典型BIF型磁铁矿,而与热液型磁铁矿更为接近。结合矿区发育大量与海相沉积相关的膏盐层这一地质事实,认为老并矿床形成于早寒武世的海底高温热液系统,经历了早期海底热液铁质沉淀与晚期局限盆地膏盐析出两个演化阶段,并遭受了后期区域变质作用的改造。

       

      Abstract: The Taxkorgan area is an important iron metallogenic belt in western China. The Laobing iron deposit, one of the typical deposits in this belt, is characterized by the assemblage of magnetite, gypsum and sulfides, serving as an ideal proxy for investigating the iron mineralization processes in this region. Based on detailed field geological investigation, this study presents LA-ICP-MS in situ trace element analyses of three types of magnetite with different occurrences from the Laobing iron deposit, aiming to constrain the ore-forming material sources, physicochemical conditions and deposit genesis. The results show that the three types of magnetite display consistent trace element spider diagram patterns, which are comparable to those of high-temperature hydrothermal deposits, indicating that the ore-forming materials were derived from submarine high-temperature hydrothermal fluids. The (Al+Mn)-(Ti+V) geothermometer and Ga-MgO diagram indicate that Mag-1 and Mag-2 crystallized at approximately 300°C, whereas Mag-3 was modified by post-ore metamorphism, recording higher re-equilibration temperatures. The pronounced enrichment of Al, Ti and V, together with elevated Ni/Cr ratios and low Ti/V ratios in magnetite, jointly suggest crystallization under relatively reducing conditions, with oxygen fugacity exhibiting a systematic decrease from Mag-1 to Mag-3. The V+Ti and Al+Mn+Ca contents of magnetite from the Laobing deposit are significantly higher than those of BIF-type magnetite, while being more comparable to hydrothermal-type magnetite. Combined with the presence of abundant gypsum-bearing layers related to marine sedimentation in the mining area, this study proposes that the Laobing deposit formed in a submarine high-temperature hydrothermal system during the Early Cambrian, experiencing early submarine hydrothermal iron precipitation and late restricted basin gypsum-salt deposition, followed by post-ore regional metamorphic overprinting.

       

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