黔西北天桥铅锌矿床热液方解石C、O同位素和REE地球化学

利用连续流动质谱和电感耦合等离子体质谱(ICP-MS)对黔西北天桥铅锌矿床原生矿石中脉石矿物热液方解石C、O同位素组成和稀土元素含量进行了分析,结果表明热液方解石C、O同位素组成相对均一,不同标高方解石C、O同位素组成不具明显差别,其δ13CPDB和δ18OSMOW分别为-3.4‰～-5.3‰和14.7‰～19.5‰,在δ13CPDB-δ18OSMOW图上介于原始碳酸岩与海相碳酸盐岩之间。热液方解石总稀土元素含量较低(ΣREE=6.80×10-6～49.1×10-6),表现为轻稀土富集、Eu负异常的"M"型,其Eu/Eu*变化范围为0.30～0.55,与硫化物具有相似的稀土配分模式。根据热液方解石与蚀变围岩、远矿围岩及不同时代地层碳酸盐岩的C、O同位素组成和REE含量特征对比结果,结合前人研究成果,认为该矿床成矿流体具"多来源混合"特征,其中围岩碳酸盐岩为成矿流体提供了主要的C和REE来源,地层中膏岩海相硫酸盐岩为成矿流体提供了主要的S来源,而成矿流体中的水则主要为变质基底昆阳群等提供的变质水,并受到大气降水的影响。

C, O Isotope and REE Geochemistry of the Hydrothermal Calcites from the Tianqiao Pb-Zn Ore Deposit in NW Guizhou Province, China

Hydrothermal calcites from the Tianqiao Pb-Zn ore deposit, NW Guizhou Province, China, were analyzed using Continuous Flow Mass Spectrometry and Inductively Coupled Plasma Mass Spectrometry (ICP-MS) for their carbon and oxygen isotope and rare earth elements (REE) compositions. The results show that the C and O isotopic compositions of the hydrothermal calcites are relatively homogeneous, and samples from different elevations show similar C and O isotopic compositions. The δ13CPDB and δ18OSMOW range from -5.3 per mil to -3.4 per mil and from 14.7 per mil to 19.5 per mil, respectively. In the δ13C vs. δ18O diagram, the hydrothermal calcite samples are all fall into the zone between igneous carbonatite and marine carbonate rocks. The ΣREE contents of the hydrothermal calcites are low (6.80×10-6 to 49.1×10-6), exhibiting enriched light rare earth elements, negative Eu anomalies (the Eu/Eu* ratios range from 0.30 to 0.55), and M-shape REE patterns, which are similar to those of the sulfides. Comparing the host rocks, unaltered rocks, and altered rocks with the hydrothermal calcites regarding C and O isotopic compositions and REE patterns, we believe that the ore-forming fluids were mixture of multiple source fluids, within which the C and REE were mainly sourced from carbonate wall-rocks, the sulfur from the thermo-chemical sulfate reduction of sulfates in the sedimentary strata, and the metamorphic water from underlying basement-rocks such as Kunyang Group, and affected by meteoric water.