Mineralogy and mineral chemistry of carbonates from the Zhulingou Ge-Zn deposit in Guizhou Province and its geological significance

Abstract

以碳酸盐岩为容矿岩石的后生热液铅锌矿床（Mississippi Valley-type，MVT矿床）是世界上一种重要的铅锌矿床类型，也是锗的重要工业来源之一。热液碳酸盐矿物是MVT矿床中最主要的脉石矿物，其形成贯穿整个MVT矿床成矿过程。因此，碳酸盐矿物携带丰富的成矿信息，是认识MVT矿床成因的重要补充。位于贵州省贵定县境内的竹林沟锗锌矿床，是近年来新发现的富锗锌矿床（平均品位97.9×10^-6 Ge，6.54% Zn），赋存于泥盆系碳酸盐岩中。本次工作发现该矿床不同期次热液白云石的矿物学和微量元素地球化学特征存在明显差异：成矿期前白云石（Dol1）主要呈细脉状穿插围岩，被成矿期白云石和硫化物脉穿插，部分呈细粒状被后期白云石包裹；成矿早期白云石（Dol2）主要呈粗粒状，与闪锌矿共生；主成矿期白云石（Dol3）主要呈脉状、团块状，与闪锌矿和黄铁矿共生；成矿晚期白云石（Dol4）呈团块状充填于闪锌矿矿石或者围岩中；成矿期后白云石（Dol5）呈脉状穿插或包裹其它期次白云石/闪锌矿-黄铁矿条带。C-O同位素研究表明，成矿期白云石主要来源于碳酸盐围岩的溶解，成矿流体中的C来源于围岩，而较低的δ¹⁸O值可能是亏损¹⁸O的成矿流体和围岩间水/岩反应过程中O同位素发生交换的结果。激光剥蚀电感耦合等离子体质谱（LA-ICPMS）白云石原位微量元素分析结果表明，Dol2-Dol4的Y/Ho比值相对稳定（30.5~47.9），结合区域成矿特征，认为形成成矿期白云石的流体主要为盆地卤水，与该矿床属于MVT矿床事实吻合。Dol1和Dol5具有相对较低的稀土总量（∑REE=3.97×10^-6~29.7×10^-6），很可能与白云岩围岩（∑REE=25.2×10^-6~61.3×10^-6）溶解作用有关，直接继承围岩的稀土元素组成特征；成矿期Dol2-Dol4的∑REE较高（∑REE=39.2×10^-6~117×10^-6），暗示成矿流体本身也携带了部分稀土元素。成矿各期次白云石均具有明显的Eu负异常（δEu=0.38~0.72）特征，指示成矿温度可能较高（>200℃），Ce正异常相对稳定（δCe=1.10~1.28），暗示其成矿流体pH值保持弱酸性。综上，本文认为竹林沟锗锌矿床的形成经历如下过程：在伸展背景下，深循环盆地卤水萃取下伏地层和基底岩石中的Zn、Ge和REE等元素，形成富金属流体，受构造作用驱动沿区域性断裂不断向上运移，在赋矿层位与富硫流体发生混合作用，导致闪锌矿等硫化物沉淀；在整个成矿过程，成矿环境经历了还原→氧化的转变，成矿元素发生了共生分异，在竹林沟形成富锗锌矿体，在牛角塘形成富镉锌矿体，而pH始终保持弱酸性，直至被围岩碳酸盐岩中和。;Carbonate-hosted epigenetic hydrothermal Pb-Zn deposits, i.e. Mississippi Valley-type (MVT) deposits, are important lead-zinc resources in the world and a major industrial source of germanium (Ge). Wherein hydrothermal carbonates are the most important gangue minerals and their formation runs through the whole metallogenic process of MVT deposits. Therefore, hydrothermal carbonates contain rich mineralization information and are an important supplement for understanding the genesis of MVT deposits. The Devonian carbonate-hosted Zhulingou Ge-Zn deposit (97.9×10^-6 Ge, 6.54% Zn) located in Guiding County, Guizhou Province, is newly discovered in recent years. The hydrothermal dolomite of the Zhulingou deposit have various mineralogical features:the pre-ore dolomite (Dol1) occurs mainly as veinlets or interspersed in host rocks and cross-cut by ore-stage dolomite and sulfides, or occurs as fine crystals enclosed by the late dolomite; the early ore-stage dolomite (Dol2) is mainly coarse-grained and coexists with sphalerite; the main ore-stage dolomite (Dol3) is mainly vein-like or clumpy, coexisting with sphalerite and pyrite; the late ore-stage dolomite (Dol4) fills into sphalerite or host rocks in the form of clumps; the post-ore dolomite (Dol5) crosscuts/encloses early stages of dolomite/sphalerite-pyrite bands. The C and O isotopic data show that the formation of Dol2-Dol4 is likely related to the dissolution of the carbonate host rocks, and the C in the ore-forming fluid derives from the country rocks, the lower δ¹⁸O may be contributed to the isotope exchange between the host rock and the ore-forming fluid with ¹⁸O depletion. The laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) in situ analytical results show that the Y/Ho ratios of Dol2-Dol4 are relatively constant (30.5~47.9). This indicates that the fluids formed metallogenic dolomite are basin brine, which is supported by the MVT genesis of the Zhulingou deposit. Dol1 and Dol5 with lower ∑REE values (3.97×10^-6~29.7×10^-6) may be related to the dissolution of the host rock dolostone (∑REE=25.2×10^-6~61.3×10^-6), and directly inherit the REE characteristics of the latter. The relatively high ∑REE values (39.2×10^-6~117×10^-6) of Dol2-Dol4 suggest an REE source from the ore-forming fluids. Hydrothermal dolomite at Zhulingou is characterized by significantly negative Eu anomalies (δEu=0.38~0.72), implying a relatively high ore-forming temperatures (>200℃). The positive Ce anomalies (δCe=1.10~1.28) suggest weakly acidic ore-forming fluid. In conclusion, this study proposed that the formation of the Zhulingou Ge-Zn deposit experienced the following process:under the regional extensional stress, the deep circulated brine extracted the ore-forming elements (e.g. Zn, Ge and REE) from underlying strata and basement rocks, and formed the metal-rich fluids, which continued to transport upward along the regional faults driven by tectonic action. The metal-rich fluids were mixed with the sulfur-rich fluids in the ore-bearing strata to form the sulfide such as sphalerite. During the ore-forming process, the metallogenic environment went through the transition from reduction to oxidation accompanied by symbiosis and differentiation of ore-forming elements, forming Ge-rich zinc ore body at Zhulingou, and Cd-rich zinc ore body at Niujiaotang, while the fluids remain weakly acidic until neutralized by the host rock carbonates.