Fluid Inclusion Constraints on the Hydrothermal Evolution of the Dalucao Carbonatite-related REE deposit, Sichuan Province, China.
Fluid Inclusion Constraints on the Hydrothermal Evolution of the Dalucao Carbonatite-related REE deposit, Sichuan Province, China.
复制标题
流体包裹体对中国四川省大芦草碳酸岩相关稀土矿床热液演化的约束。
DOI：
10.1016/j.oregeorev.2019.02.014
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发表时间：
2019
影响因子：
3.3
通讯作者：
Liu Yan
中科院分区：
文献类型：
--
作者：
Shu Xiaochao;Liu Yan
与碳酸岩有关的稀土元素（REE）矿床是世界稀土资源最重要的来源。热液流体被认为在稀土元素的输送和沉淀中发挥着重要作用，但有关碳酸盐岩环境中热液过程的流体包裹体数据相对较少。大芦草矿床位于中国四川冕宁-德昌（MD）稀土矿带，是一个新生代碳酸岩相关的稀土矿床（约12Ma），为研究成矿流体的演化提供了绝佳的机会。该矿床中角砾岩和风化矿石很常见。前者的矿物组合为萤石+重晶石+天青石+方解石+石英+氟碳铈矿（1号矿体）或萤石+天青石+黄铁矿+白云母+方解石+石英+氟碳铈矿（3号矿体），后者则含有稀土矿物、粘土矿物和少量脉石矿物。我们对大路槽矿床的流体包裹体进行了全面的研究，以限制其热液演化。岩浆、伟晶岩、热液和表生阶段已经得到认可。伟晶岩阶段，形成的主要矿物为粗粒萤石、重晶石、天青石、方解石和石英，其中含有熔体包裹体、熔体流体包裹体和少量高盐度流体包裹体。熔体和熔体流体包裹体的存在表明成矿流体的岩浆起源。热液过程至少包括两个阶段，其特征是在碳酸岩-正长岩杂岩内的裂缝中发育热液脉：（1）前稀土阶段萤石-石英-重晶石脉形成过程中的流体包裹体在不混溶条件下被捕获，这一点可以通过与水相共存的含CO2包裹体的存在来证明。这些不混溶的含二氧化碳包裹体记录的压力范围为 1050 至 1600 巴。该阶段所有流体包裹体的均一温度在 278 至 442°C 之间变化，盐度在 3.2 至 45.1wt% NaCl 当量（当量）之间变化。 (2) REE 阶段流体以丰富的水包裹体为代表，其特征是均一温度范围为 147 至 323°C，盐度为 1.1 至 9.5wt% NaCl 当量。这些数据表明，形成大芦槽矿床的成矿流体从高温、高压、高盐度、富CO2演化到低温、低压、低盐度、贫CO2。气相色谱和离子色谱分析结合矿物学特征表明，初始流体富含稀土元素、(SO4)2−、Cl−、F−、Na+、K+、Ca2+ 和挥发性成分（例如 H2O、CO2、N2、CH4、Ar 和 C2H6）。石英的 H-O 同位素分析表明，热液流体主要具有岩浆特征，并逐渐被大气水稀释。热液稀土元素传输可能由 F−、(SO4)2− 和 Cl− 作为络合配体控制。我们认为，在热液活动减弱阶段，流体冷却和混合而不是不混溶导致了氟碳铈矿的沉淀。总而言之，包裹体数据以及蚀变、共生和矿化的观察为稀土矿化的发展和碳酸岩相关稀土资源的进一步勘探提供了见解。
Carbonatite-related rare-earth element (REE) deposits are the most important source of the world’s REE resources. Hydrothermal fluids have been proposed to play a significant role in the transport and precipitation of REEs, but fluid inclusion data on the hydrothermal processes in carbonatitic settings are relatively sparse. The Dalucao deposit, located in the Mianning–Dechang (MD) REE belt, Sichuan, China, is a Cenozoic carbonatite-related REE deposit (c. 12 Ma) that offers an excellent opportunity to investigate the evolution of ore-forming fluids. Brecciated and weathered ores are common in this deposit. The former are characterized by mineral assemblages comprising fluorite + barite + celestite + calcite + quartz + bastnäsite (No. 1 orebody) or fluorite + celestite + pyrite + muscovite + calcite + quartz + bastnäsite (No. 3 orebody), whereas the latter contain REE minerals, clay minerals, and minor gangue minerals. We present a comprehensive study of fluid inclusions from the Dalucao deposit to constrain its hydrothermal evolution.Magmatic, pegmatitic, hydrothermal, and supergene stages have been recognized. During the pegmatitic stage, the main minerals that formed were coarse-grained fluorite, barite, celestite, calcite, and quartz, which host melt inclusions, melt–fluid inclusions, and minor high-salinity fluid inclusions. The presence of melt and melt–fluid inclusions suggests a magmatic origin for the ore-forming fluids. Hydrothermal processes included at least two stages, characterized by hydrothermal veins that are developed in fractures within the carbonatite–syenite complex: (1) Fluid inclusions during the formation of the fluorite–quartz–barite veins in the pre-REE stage were trapped under immiscible conditions, as evidenced by the presence of CO2-bearing inclusions coexisting with aqueous ones. These immiscible CO2-bearing inclusions recorded a range of pressures from 1050 to 1600 bar. All of fluid inclusions in this stage exhibited homogenization temperatures varying from 278 to 442 °C, with salinities ranging from 3.2 to 45.1 wt% NaCl equivalent (equiv.). (2) The REE-stage fluids were represented by abundant aqueous inclusions, characterized by homogenization temperatures ranging from 147 to 323 °C and salinities between 1.1 and 9.5 wt% NaCl equiv. These data suggest that the ore-forming fluids forming the Dalucao deposit evolved from high-temperature, high-pressure, high-salinity, CO2-rich to low-temperature, low-pressure, low-salinity, CO2-poor. Gas- and ion-chromatographic analyses combined with mineralogical features indicate that the initial fluids were rich in REEs, (SO4)2−, Cl−, F−, Na+, K+, Ca2+, and volatile components (e.g., H2O, CO2, N2, CH4, Ar, and C2H6). H–O isotope analyses of quartz suggest that the hydrothermal fluids had a dominantly magmatic signature and were gradually diluted by meteoric waters. Hydrothermal REE transport was probably controlled by F−, (SO4)2−, and Cl−as complexing ligands. We propose that fluid cooling and mixing rather than immiscibility led to the precipitation of bastnäsite during the waning stage of hydrothermal activity. Taken together, the inclusion data and observations of alteration, paragenesis and mineralization have provided insights into the development of REE mineralization and the further exploration of carbonatite-related REE resources.