Tracking dynamic hydrothermal processes: Textures, in-situ Sr-Nd isotopes, and trace-element analysis of scheelite from the Yangjiashan vein-type W deposit, South China
Tracking dynamic hydrothermal processes: Textures, in-situ Sr-Nd isotopes, and trace-element analysis of scheelite from the Yangjiashan vein-type W deposit, South China
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DOI：
10.2138/am-2021-7677
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发表时间：
2021-12-01
影响因子：
3.1
通讯作者：
Zhang, Zhiyuan
中科院分区：
文献类型：
--
作者：
Li, Wei;Xie, Guiqing;Zhang, Zhiyuan
结构复杂的矿物可提供关于动态热液过程的重要信息。本研究结合阴极发光（CL）、激光烧蚀电感耦合等离子体质谱（LA-ICP-MS）和高分辨率飞秒激光烧蚀多接收电感耦合等离子体质谱（fs-LA-MC-ICP-MS）分析，对中国南方杨家山钨存款结构复杂的白钨矿进行了结构、原位Sr-Nd同位素系统学和微量元素组成的研究。这一贡献的主要动机是揭示CL响应，纹理和微量元素浓度之间的相关性;文件的各种稀土分馏模式的起源;和表征粒度尺度的Sr-Nd同位素的原位变异性的白钨矿。五个子类型的白钨矿从两个阶段，包括Sch 1和Sch 2从第一阶段，Sch 3到Sch 5从第二阶段，被确定。CL图像具有复杂的振荡，斑片状，并耦合溶解-再沉淀反应的证据。这些白钨矿从还原流体中沉淀出来，其组成接近端元，钼浓度低于46 ppm。其他元素的浓度不同，例如，Sr（36-1025 ppm）、Nd（8-351 ppm）和Na（7-300 ppm）。LA-ICP-MS元素图显示，稀土元素浓度的振荡带之间的变化很大，没有一致的行为之间的稀土元素，Sr和Mo的浓度，和CL强度。稀土元素分馏模式分为轻稀土富集型、中稀土富集型、重稀土富集型和平坦型。复杂的Eu异常（Δ Eu = 0.2至20.7）之间的五个子类型中被确认，通常在单个颗粒内观察到。流体组合物、不同的取代机制（即，Ca ~（2+）+W ~（6+）= RE ~（3+）+Nb ~（5+），2Ca（2+）= RE ~（3+）+ Na ~+，3Ca（2+）= 2 REE（3+）+方Ca，方Ca为Ca位空位），原生-次生过程（即，振荡和溶解-再沉淀），都对稀土元素分馏模式的变化有贡献。复杂的Eu异常是由流体pH的局部波动引起的。五个亚类白钨矿的Sr和Nd同位素特征显示出相对较大的变化范围，即，原始Sr-87/Sr-86比值为0.71336 ~ 0.72617，原始Nd值为-24.9 ~-7.7，表明源区为岩浆热液和新元古代板岩的混合物。从Sch 2到Sch 5，Sr-87/Sr-86比值的降低记录了流体-岩石相互作用强度的降低。振荡环带结构白钨矿的Sm Nd（t）值变化较大（-24.9 ~-7.7），可能与其Sm/Nd比值的变化和围岩Nd同位素组成不均匀的混染有关。这项研究强调了进行耦合LA-ICP-MS映射和原位Sr-Nd同位素分析的样品材料，其特征在于在微米尺度上的重要性。
Texturally complex minerals can provide critical information on dynamic hydrothermal processes. This study combines cathodoluminescence (CL), laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS), and high-resolution femtosecond laser ablation multi-collector inductively coupled plasma mass spectrometry (fs-LA-MC-ICP-MS) analyses to document textures, in situ Sr-Nd isotope systematics, and trace element compositions of texturally complex scheelite from the Yangjiashan W deposit, South China. The major motivation for this contribution was to reveal the correlation between CL response, textures, and trace element concentrations; document the origin of various REE fractionation patterns; and characterize grain-scale in situ variability of Sr-Nd isotopes of scheelite.Five sub-types of scheelite from both stages, including Sch1 and Sch2 from Stage 1, and Sch3 to Sch5 from Stage 2, are identified. CL images feature complex oscillatory, patchy, and evidence for coupled dissolution-reprecipitation reaction. These scheelites precipitated from reduced fluids and are close to end-member in composition, with Mo concentrations below 46 ppm. Concentrations of other elements vary, e.g., Sr (36-1025 ppm), Nd (8-351 ppm), and Na (7-300 ppm). LA-ICP-MS element maps reveal a large variability in REE concentrations among oscillatory zones and no consistent behavior between REE, Sr and Mo concentration, and CL intensity. Four distinct chondrite-normalized REE fractionation patterns are recognized: LREE-enriched, MREE-enriched, HREE-enriched, and flat patterns. Complex Eu anomalies (delta Eu = 0.2 to 20.7) are recognized among the five sub-types and are commonly observed within individual grains. Fluid compositions, different substitution mechanisms (i.e., Ca2+ + W6+ = REE3+ + Nb5+, and 2Ca(2+) = REE3+ + Na+, 3Ca(2+) = 2REE(3+)+ square Ca, where square Ca is a Ca-site vacancy), primary-secondary processes (i.e., oscillatory and dissolution-reprecipitation, respectively), all contribute to the variation in REE fractionation patterns. Local fluctuation in fluid pH is responsible for the complex Eu anomalies. In situ Sr and Nd isotope signatures for the five sub-types of scheelite show relatively large ranges, i.e., the initial Sr-87/Sr-86 ratios range from 0.71336 to 0.72617, and the initial epsilon Nd values ranging from -24.9 to -7.7, suggesting a source derived from a mixture of magmatic-hydrothermal fluids and the Neoproterozoic slate. Decreasing Sr-87/Sr-86 ratios from Sch2 to Sch5 record decreasing fluid-rock interaction intensity. Large variation of epsilon Nd(t) values (-24.9 to -7.7) of scheelite with oscillatory zoning textures may relate to changes of Sm/Nd ratio of scheelite and contamination from wall rock with inhomogeneous Nd isotope composition. This study highlights the importance of performing coupled LA-ICP-MS mapping and in situ Sr-Nd isotope analyses on sample material that has been characterized in detail at the micrometer scale.