独居石U-Th-Pb微区原位定年方法在微细浸染型金矿床成矿年代学中的应用进展

Advances in Micro-Beam Monazite U-Th-Pb Dating for the Metallogenic Geochronology of Sediment-Hosted Disseminated Gold Deposits

  • 摘要: 微细浸染型金矿床是全球金资源的重要来源之一,但成矿时代的精确厘定一直是该类型矿床研究的难点,限制了对该类型矿床成因及成矿过程的深入认识。独居石是微细浸染型金矿床普遍发育的副矿物,因其U、Th含量高,普通Pb含量极低,且具有较高的Pb封闭温度,是U-Th-Pb同位素定年的理想矿物。近年来,独居石U-Th-Pb同位素定年已被有效地用于限定微细浸染型金矿床的成矿时代,但独居石极易受后期流体交代作用影响发生溶解-再沉淀,复杂的成分变化与环带结构使得传统测试手段难以准确地获取该矿物的同位素地球化学数据。本文围绕独居石U-Th-Pb同位素体系,对比了四种定年方法,电子探针法(EPMA)适合识别微米级成分分带,但年龄精度和普通Pb校正能力有限;激光剥蚀电感耦合等离子体质谱法(LA-ICP-MS)分析效率高,适用于批量微区年龄的测试,但束斑较大且需同位素分馏和基体效应的校正;二次离子质谱法(SIMS)兼具较高空间分辨率和分析精度,但其受基体效应影响明显且成本较高;同位素稀释热电离质谱法(ID-TIMS)年龄精度最高,但无法区分复杂结构的单颗粒。综合已有研究,本文评述了独居石结构构造、化学成分对年龄结果的影响,指出独居石高空间分辨率(束斑<5 μm) SIMS U-Th-Pb法可有效地应用于厘定微细浸染型金矿床成矿时代的研究。在进行U-Th-Pb同位素定年之前,应通过独居石的矿物共生关系、ThO2含量、背散射电子(BSE)图像和元素面扫等成因矿物学研究,针对性地选择与成矿有关的热液独居石进行测试,同时通过匹配标样校正基体效应以获得更准确的年龄信息。

     

    Abstract: Sediment-hosted disseminated gold deposits are a crucial global source of gold. However, accurately determining the timing of hydrothermal mineralization has long been a challenge, which has hindered our understanding of ore genesis and ore-forming processes. Monazite is a common accessory mineral in sediment-hosted disseminated gold deposits. With high U and Th contents, extremely low common Pb, and a high closure temperature for Pb, monazite is an ideal mineral for U-Th-Pb isotopic dating. Recently, monazite U-Th-Pb geochronology has been successfully applied to constrain the timing of mineralization in sediment-hosted disseminated gold deposits. Nevertheless, monazite is highly susceptible to late-stage fluid metasomatism and frequently undergoes dissolution and reprecipitation. The complex compositional variations and zoning textures of monazite often prevent traditional analytical methods from obtaining accurate geochemical data. This paper focuses on the monazite U-Th-Pb isotopic system and compares four major dating methods. Electron probe microanalysis (EPMA) is suitable for identifying micrometer-scale compositional zoning, but its age precision and ability to correct common Pb are limited. Laser ablation inductively coupled plasma-mass spectrometry (LA-ICP-MS) has high analytical efficiency and is suitable for batch in situ micro-beam age analyses, but it has a relatively large spot size and requires correction for isotopic fractionation and matrix effects. Secondary ion mass spectrometry (SIMS) combines relatively high spatial resolution and analytical precision, although it is strongly affected by matrix effects and is costly. Isotope dilution-thermal ionization mass spectrometry (ID-TIMS) provides the highest age precision, but it cannot distinguish different domains within single grains with complex internal structures. Based on previous studies, this paper reviews the effects of monazite textures, structures, and chemical compositions on age results, and indicates that high-spatial-resolution SIMS U-Th-Pb dating with a spot size of <5 μm can be effectively applied to constrain the mineralization ages of sediment-hosted disseminated gold deposits. Before U-Th-Pb dating, genetic mineralogy investigations, including mineral assemblages, ThO2 contents, backscattered electron (BSE) images, and elemental mapping, should be conducted to selectively analyze hydrothermal monazite domains related to mineralization. Matrix-matched standards should also be used to correct matrix effects and obtain more reliable age information.

     

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