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扬子地块西南缘铅锌成矿作用与分散元素镉镓锗富集规律
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摘要
扬子地块西南缘位于特提斯构造域与环太平洋构造域的结合部位,晚太古代以来经历了复杂的地质构造演化历史,形成了丰富多样的矿产资源,富含分散元素镉镓锗的铅锌矿床即为其中富有特色的重要矿种。论文以扬子地块西南缘若干典型的富含分散元素的铅锌矿床(云南会泽、富乐,四川天宝山、大梁子,贵州杉树林、牛角塘)为重点解剖对象,在系统野外工作的基础上,运用化学分析、等离子质谱分析、电子探针等手段对铅锌矿石、单矿物和容矿岩石等开展了元素地球化学、同位素地球化学以及赋存状态等方面的综合研究,对区域铅锌成矿的地质构造和地球化学条件及分散元素镉镓锗的赋存状态和富集规律进行了较为系统的总结。
     扬子地块西南缘铅锌矿床的容矿岩石及区域地层中Zn、Pb和Cd等的高背景值是铅锌成矿和分散元素富集的物质基础。元素地球化学研究表明,区域地层和容矿岩石中主成矿元素Zn和Pb及分散元素Cd的含量普遍高于地壳同类岩石丰度,其中Zn和Cd的含量通常高出十倍以上。
     矿床中的硫主要来自容矿地层。赋存于不同时代地层中的典型矿床的硫化物硫同位素组成,总体上显示出与容矿地层同期形成的海相硫酸盐硫同位素组成相一致的特征。铅同位素模式年龄(约140~470Ma)显示矿床的形成时代晚于或近于容矿地层的形成时代。矿床的矿石铅主要来自上地壳。
     综合矿石铅同位素模式年龄和矿床产出的地质特征,初步厘定出扬子地块西南缘存在着两种不同成因类型的铅锌矿床,即MVT型和SEDEX型。MVT型矿床的成矿时代显著小于容矿地层时代,时差约为200~300 Ma,此类矿床的矿体多以脉状和网脉状产出;而SEDEX型矿床的形成时代与容矿地层时代相近,矿体主要以层状、似层状产出。
     在原生硫化物闪锌矿、方铅矿、黄铁矿和黄铜矿中,分散元素镉镓锗主要以类质同象的形式存在,少量可能以超显微独立矿物的形式存在。在表生氧化条件下,局部发育分散元素镉的独立矿物—硫镉矿,但尚未发现镓和锗的独立矿物。
     扬子地块西南缘铅锌矿床均以富集镉(约330~8200×10~(-6))为特征,而镓(约2~30×10~(-6))和锗的富集程度较低。闪锌矿为镉和镓的主要载体矿物。MVT型矿床中镉和镓的含量(Cd约1700~8200×10~(-6),Ga约5~30×10~(-6))显著高于SEDEX型矿床(Cd约300~400×10~(-6),Ga约2~4×10~(-6))。
     初步总结了铅锌矿床形成的区域地质构造背景、成矿机制以及铅锌成矿对分散元素富集的制约作用。扬子地块西南缘铅锌矿床主要形成于古生代拉张构造体制下,而中生代挤压体制下仅在局部区域成矿。
The southwestern Yangtze block, located in the combined zone of Tethys and circum-Pacific tectonic domains, has undergone a complex geologic-tectonic evolutionary history since late Archean. Among various kinds of mineral resources in this area, Pb-Zn deposits are characterized by enrichment in dispersed elements such as cadmium (Cd), gallium (Ga) and germanium (Ge). A case study on some typical Pb-Zn deposits enriched in Cd, Ga, and Ge from SW Yangtze block (Huize and Fule in Yunnan, Tanbaoshan and Daliangzi in Sichuan, and Shanshulin and Niujiaotang in Guizhou) was undertaken during this study. Based on systematic field investigations, multiple methods including chemical analysis, ICP-MS and electron microprobe (EMP) are used to study ore deposit geochemistry and occurrence state of dispersed elements. In addition, the geologic, tectonic and geochemical conditions of Pb-Zn mineralization, the occurrence state and enrichment regularity of Cd, Ga, and Ge in SW Yangtze block are summarized in this dissertation.
    Regional strata and host rocks of the Pb-Zn deposits in SW Yangtze block have high contents of Zn, Pb, and Cd, which is fundamental for the Pb-Zn mineralization and the enrichment of dispersed elements. Element geochemical study shows that the contents of Zn, Pb, and Cd in the regional strata and host rocks are typically higher than those of similar lithologies of the earth's crust.
    Sulfur in the deposits was chiefly derived from host rocks. Sulfur isotopic compositions of sulfides in the typical ore deposits hosted in different strata are on the whole consistent with those of marine sulfates in corresponding host strata. Lead isotopic model ages suggest that the mineralization ages of the studied Pb-Zn deposits are later than or close to the ages of the host strata. The ore lead in the deposits mainly came from the upper crust.
    According to the lead isotopic model ages and geologic characteristics of the ore deposits, two main types of Pb-Zn deposits can be classified in SW Yangtze block, i.e., the Mississippi valley type (MVT) and the sedimentary exhalative (SEDEX) type. Metallogenetic ages of the MVT deposits, in which the orebodies occur as veins or stockworks, are ca. 200-300 Ma younger than the ages of corresponding host strata. By contrast, the metallogenetic ages of SEDEX deposits, in which the orebodies are stratiform or layer-like, are slightly younger than or close to the ages of corresponding host rocks.
    In primary ore minerals like sphalerite, galena, pyrite and chalcopyrite, Cd, Ga and Ge occur mainly as isomorphic forms. Trace amounts of independent minerals of cadmium, such as greenockite, were locally observed in the oxidized zones of the deposits.
    Cd contents in the studied Pb-Zn deposits are abnormally high, whereas Ga and Ge contents are relatively low. Cd and Ga are mainly enriched in sphalerite. Concentrations of Cd and Ga in the MVT deposits are much higher than those in SEDEX deposits
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