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首页> 外文期刊>Economic geology and the bulletin of the Society of Economic Geologists >CARBONATE-HOSTED ZINC-LEAD DEPOSITS IN THE LOWER CAMBRIAN OF HUNAN, SOUTH CHINA: A RADIOGENIG (Pb, Sr) ISOTOPE STUDY
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CARBONATE-HOSTED ZINC-LEAD DEPOSITS IN THE LOWER CAMBRIAN OF HUNAN, SOUTH CHINA: A RADIOGENIG (Pb, Sr) ISOTOPE STUDY

机译：中国南方湖南寒武纪的碳酸盐化锌铅矿床：放射性同位素（Pb，Sr）同位素研究

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摘要

Zinc-lead deposits hosted by Lower Cambrian carbonate rocks near Huayuan in the Hunan-Guizhon orogenic belt, southern China, have been studied by means of Pb and Sr isotopes. The deposits are characterized by two mineralization stages: (1) a main economic stage comprising cockade-style sphalerite, galena, pyrite-marcasite, fluorite, barite, and calcite, and (2) a subsequent noneconomic stage with nodular (rosette) sphalerite, saddle dolomite, and bitumen. Both ore stages are postdated by a generation of late calcite. Main-stage sulfides (~(206)Pb/~(204)Pb - 18.162-18.225) display a steep linear trend on conventional Pb-Pb diagrams which indicates binary mixing of lead components bearing the imprint of ancient, ~(207)Pb-dominated crustal rocks and reservoirs with elevated Th/U ratios, possibly located in the lower crust. Nodular rosette sphalerites fall outside this line but reflect a second mixing trend between mineralizing fluids characterized by ~(206)Pb/~(204)Pb> 18.3 and lead isotopically similar to specific main-stage cockade sphalerites, suggesting either remobilization of Pb from main-stage ores or contemporaneous mixing of three distinct lead components in the course of an evolving hydrothermal event. The ~(87)Sr/~(86)Sr ratios of diagenetic and hydrothermal carbonate cements and barites range from 0.70905 to 0.71006 and increase systematically with the paragenetic sequence, indicating progressive mixing of marine Sr derived from the Lower Cambrian carbonate host rocks with radiogenic Sr supplied by external mineralizing fluids. In order to identify possible local metal sources, U-Pb and Rb-Sr analyses were performed on six whole- rock samples from the Lower Cambrian sequence and their time-integrated Pb and Sr isotope evolution was compared to the isotopic signature of the mineralization. Whereas shales and black shales of the Cambrian Balang and Niutitang Formations underlying the ore-bearing Qingxudong carbonate unit appear to be possible external sources for the radiogenic Sr found in hydrothermal carbonates, the U-Pb data do not permit these shales to be considered as primary source rocks for the ore lead. Both the Pb and the Sr isotopes indicate mineralizing fluids that had interacted with various external metal sources, some of them most probably being located in sedimentary or crystalline basement rocks. This refutes prevailing genetic models favoring the hydrocarbon- and metal-rich Lower Cambrian Niutitang black shales as the most likely source reservoir for the Zn and Pb in the deposits of the Huayuan district. The Pb and Sr isotope mixing trends observed for the ore and gangue preciptates reflect small-scale heterogeneous fluid systems that were not capable of producing large deposits. This may indicate that the Huayuan district is not a favorable target for future exploration of economic ore deposits. It also limits the prospects of successful radiometric dating of the mineralization, since conventional genchronologic models require initial isotopic homogeneity.
机译：利用Pb和Sr同位素研究了华南-桂庄造山带华远附近下寒武统碳酸盐岩中的铅锌矿床。矿床的特征是有两个矿化阶段：（1）主要经济阶段，包括帽形闪锌矿，方铅矿，黄铁矿-镁铁矿，萤石，重晶石和方解石，以及（2）随后的非经济阶段，呈球状（菱形）闪锌矿，鞍状白云石和沥青。这两个矿石阶段都晚于方解石的产生。主阶段硫化物（〜（206）Pb /〜（204）Pb-18.162-18.225）在常规Pb-Pb图上显示出陡峭的线性趋势，这表明铅成分的二元混合形成了古老的〜（207）Pb烙印Th / U比升高的地壳岩石和储层，可能位于下地壳中。结节状的玫瑰晶闪锌矿落在该线外，但反映了以〜（206）Pb /〜（204）Pb> 18.3为特征的矿化流体与铅的同位素混合，类似于特定的主要阶段帽徽闪锌矿，这表明Pb可能从主要矿体中迁移出来。不断发展的水热事件过程中的阶段矿石或三种不同铅组分的同时混合。成岩和热液碳酸盐胶结物和重晶石的〜（87）Sr /〜（86）Sr比值在0.70905至0.71006范围内，并随着共生序列的增加而系统地增加，表明下寒武统碳酸盐岩宿主岩中的海洋Sr与放射源逐渐混合Sr由外部矿化液供应。为了确定可能的局部金属来源，对来自下寒武统的6个全岩样品进行了U-Pb和Rb-Sr分析，并将其时间积分的Pb和Sr同位素演化与矿化的同位素特征进行了比较。含矿清徐洞碳酸盐岩单元下面的寒武纪巴朗和牛塘塘组的页岩和黑色页岩似乎是热液碳酸盐中放射性Sr的可能外部来源，但U-Pb数据不允许将这些页岩视为主要页岩。铅矿石的源岩。 Pb和Sr同位素都表明与各种外部金属源相互作用的矿化流体，其中一些很可能位于沉积或结晶基底岩石中。这驳斥了普遍的遗传模型，这些遗传模型倾向于将富含烃和金属的下寒武统牛塘塘黑色页岩作为华源地区矿床中最可能的锌和铅源库。矿石和脉石沉淀物中观察到的Pb和Sr同位素混合趋势反映了不能产生大量矿床的小规模非均质流体系统。这可能表明，华远地区不是未来勘探经济矿床的有利目标。由于传统的年代学模型要求初始同位素均一性，这也限制了成矿作用的放射性测年的成功前景。

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