In situ LA-ICP-MS Determination of Trace Elements in Magnetite from a Gypsum-Salt Bearing Iron Deposit and Geochemical Characteristics
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摘要:
膏盐建造会影响成矿流体的氧逸度和成矿流体成分，表现在磁铁矿元素组成会发生变化，从而对铁矿床的形成具有重要的指示作用，因此可以应用磁铁矿元素组成变化进行矿床类型划分和成因的厘定。膏盐建造广泛发育在新疆“帕米尔式”铁矿床和长江中下游宁芜铁矿床中，但膏盐建造的控矿机制尚不清楚。本文以新疆“帕米尔式”铁矿床和长江中下游宁芜玢岩铁矿床中的磁铁矿为研究对象，应用激光剥蚀电感耦合等离子体质谱法(LA-ICP-MS)测定其元素组成，分析磁铁矿中微量元素种类、含量及其地球化学特征，进而反演两种类型磁铁矿的成矿过程与形成环境，探讨膏盐建造在磁铁矿床形成过程中的控制作用。结果表明：①宁芜地区磁铁矿主要具有高Ti(平均含量16401μg/g)、V(平均含量2256μg/g)特征，说明其与岩浆作用密切相关。②新疆塔什库尔干地区磁铁矿床中的磁铁矿中Nb、Ta、Zr、Hf等高场强元素(HFSE)含量明显偏低，结合磁铁矿类型判别图将该地区磁铁矿床主要划分为两种成因类型，即与海相火山活动相关的岩浆热液型磁铁矿特征和热液交代矽卡岩型。分析表明两地区膏盐建造控矿作用明显不同：在塔什库尔干地区磁铁矿床形成过程中改变了氧逸度，而在宁芜地区玢岩铁矿形成过程中，为成矿提供了重要成矿物质来源。
Abstract:BACKGROUND Gypsum-salt formation affects the oxygen fugacity of ore-forming fluid and changes the fluid composition, and then affects the trace element composition, which plays an important indicator role in the formation of iron ore deposits. Therefore, changes in the elemental composition of magnetite can be used to classify the types of deposits and determine their genesis. Gypsum-salt formation is widely developed in "Pamir-type" iron deposits in Xinjiang and Ningwu iron deposits in the middle and lower part of the Yangtze River. However, the controlling mechanism of gypsum-salt formation is still unclear.
OBJECTIVES In order to investigate the metallogenic process and formation environment of the two types of magnetite, and to discuss the role of gypsum-salt formation in the formation of magnetite deposits.
METHODS In situ LA-ICP-MS were employed to determine trace elements in magnetite. RESULTS: LA-ICP-MS results showed that magnetite in the Ningwu area were mainly rich in Ti and V, indicating that it was closely related to magmatism, while the contents of Nb, Ta, Zr, Hf and other high field strength elements (HFSE) in magnetite in the Tashkurgan magnetite deposit in Xinjiang were depleted. Combined with the discrimination diagram of magnetite types, it was mainly divided into two genetic types: magmatic hydrothermal magnetite and hydrothermal metasomatic skarn magnetite related to marine volcanic activity.
CONCLUSIONS The results show that the gypsum-salt formation changes the oxygen fugacity during the formation of magnetite deposits in the Tashkurgan area, and provides an important source of ore-forming material for mineralization during the formation of porphyrite-type iron ore in the Ningwu area.
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Key words:
- LA-ICP-MS /
- gypsum-salt formation /
- magnetite /
- trace elements /
- "Pamir-type" iron deposit /
- Ningwu iron deposit
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图 2 (a) 新疆塔什库尔干地区铁矿野外照片；(b)塔什库尔干地区铁矿石石膏柱状结合体；(c)塔什库尔干地区铁矿石镜下照片；(d)凹山铁矿床中辉石闪长玢岩中的铁矿石；(e)宁芜地区凹山铁矿中铁矿露头；(f)凹山铁矿床镜下照片显示磁铁矿和石英共生
Figure 2.
表 1 磁铁矿中微量元素测定时LA-ICP-MS仪器设定参数
Table 1. Machine conditions for LA-ICP-MS trace element analysis of magnetite
激光参数 实验条件 ICP-MS参数 实验条件 激光源 Telydyne Cetac HE Photon Machines Excimer ICP-MS系统 Analytik Jena Plasma Quant MS Elite 波长 193nm 功率 1400W 脉冲宽度 20ns 等离子冷却气(Ar)流速 13.5L/min 激光束 均值化平顶光束 辅助气(He)流速 0.850L/min 脉冲能量 0.01~0.1mJ/pulse 样品传输气(He)流速 0.250L/min 能量密度 2.5J/cm2 样品传输气(Ar)流速 0.90L/min 焦点 表面 扫描模式 峰跳跃模式，1点/峰 光栅扫描速度 5Hz 获取模式 时间分辨率分析 激光束直径 35μm (仪器配置1~180μm) 分析持续时间 70s(20s背景，30s信号，20s冲洗) 
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