滇东南老寨湾金矿床地质及同位素特征
Characteristics of geology and isotopic geochemistry of the Laozhaiwan gold deposit in southeastern Yunnan Province, China
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摘要: 滇黔桂地区位于扬子板块西南缘，是我国重要的卡林型（微细浸染型）金矿集中区之一，区内弥勒-师宗、南盘江、富宁等深大断裂等控制了区域构造变形的发生和发展以及矿床的分布。该金三角内的老寨湾金矿床是目前在云南境内发现的唯一一处大型卡林型金矿床，已探明储量31.40t，具有矿石物质组成简单、金微细浸染分布、品位低、储量大的特点。该矿床目前划分为袁家坪矿段、椿树湾矿段和老鹰山矿段三个矿段，矿体的产出主要受不整合面和构造的双重控制。赋矿地层主要是加里东不整合面之上的下泥盆统坡松冲组的灰色、灰白色及褐黄色厚层块状细粒石英砂岩。最大的V3矿体分布在椿树湾矿段，后期辉绿岩脉沿着北西向的F7断层侵入，紧邻脉岩的矿体金品位明显增高。热液成矿过程可以划分为早、中、晚三个阶段，分别以石英-黄铁矿、石英-黄铁矿-绢云母、辉锑矿-方解石为典型矿物组合特征。论文对脉石矿物和流体包裹体开展了H-O同位素研究，对不同阶段的矿石矿物开展了S-Pb同位素研究。成矿流体的δDH2O值介于为-109‰~-93‰，而不同成矿阶段流体的δ18OH2O值略有变化，早阶段3个硅化的石英砂岩中的δ18OH2O值变化于7.8‰~9.2‰，中阶段流体的δ18O值=5.9‰~7.0‰，晚阶段δ18OH2O值为2.70‰，在投影图中位于不同的区域内，表明早阶段成矿热液来源于区域变质水和/或部分地层建造水，主成矿阶段岩浆流体的参与对局部的金矿化富集起到了重要作用，在成矿晚期有大气降水的参与。不同阶段、不同类型矿石或蚀变岩中硫化物δ34S值变化范围较大（2.096‰~32.289‰），早阶段蚀变岩中的2个黄铁矿样品的δ34S值为6.115‰和6.412‰；晚阶段的5个辉锑矿的δ34S值集中在2.096‰~4.691‰；而中阶段的不同矿石中黄铁矿的硫同位素变化较大；暗示了硫的多来源特征。但总体上矿石硫同位素组成以正值为主，硫化物δ34S峰值集中在2‰~8‰。矿石铅同位素组成分别为：206Pb/204Pb为18.178~18.992，207Pb/204Pb为15.635~15.774，208Pb/204Pb变化于38.456~39.051，相对富集放射成因铅。硫铅同位素综合分析表明老寨湾金矿床的成矿物质具有双重来源的特点：区内的沉积碎屑岩及岩浆活动共同提供了必要的成矿物质。辉绿岩中的石英的热活化ESR测年结果为64.8±6.5Ma，结合矿体产出特征，认为老寨湾金矿的主成矿作用发生在燕山晚期-喜马拉雅期；矿床的形成经历了早泥盆世的初始富集、燕山晚期-喜山期变质/岩浆热液对成矿物质的萃取、迁移和富集成矿以及成矿后的氧化富集等过程。Abstract: The Dian-Qian-Gui area, located in the southwestern Yangtze craton, is one of the most important Carlin-type (disseminated) gold concentration areas in China. The regional Mile-Shizong fault, Nanpanjiang fault and Fu'ning fault etc. controlled the tectonic deformation and the deposit location. In the Dian-Qian-Gui gold triangle area, the Laozhaiwan gold deposit, owning 31.40t proven gold reserves at present, is the only large Carlin-type gold deposit discovered in Yunnan Province and possesses the features of simple ore minerals, disseminated gold, low gold grade and large tonnage. This gold deposit includes three ore sections, i.e., the Yuanjiaping, Chunshuwan and Laoyingshan sections. The occurrence of ore bodies are mainly controlled by the unconformity surface and fold or fault structures in the ore district. The ore bodies are dominantly hosted in the Lower Devonian Posongchong Formation which is above the Caledonian unconformity and mainly consists of gray, brown and yellow thick fine sandstone. The largest orebody V3 occurs in the Chunshuwan section; and the gold grade obviously increases where the later diabase dyke invaded along the NW-trending F7 fault. The hydrothermal ore-forming process can be divided into early, middle and late three stages, which are characterized by the quartz-pyrite, quartz-pyrite-sericite and stibnite-calcite assemblages respectively. In this paper, we analyzed the H-O isotopic composition of the quartz and ore-forming fluid and S-Pb isotopic composition of the sulfides from different ores at Laozhaiwan. The δDH2O values ranges between -109‰ and -93‰, while the δ18OH2O values of the early, middle and late stage are 7.8‰~9.2‰, 5.9‰~7.0‰ and 2.70‰ respectively and plotted in the different origin area. The H-O isotopic study indicates that the ore-forming fluid in early stage was initially sourced from metamorphic devolatilization or partial formation water from the sedimentary strata, in middle stage the magmatic water played an important role for the some gold enrichment, and in late stage gradually mixed with meteoric water. The δ34S values of different ores and altered rocks range from 2.096‰ to 32.289‰, with peaks ranging from 2‰ to 8‰. The δ34S values of two pyrite samples from early-stage altered rocks are 6.115‰ and 6.412‰, the δ34S values of five stibnite samples from late-stage Sb-ores are 2.096‰~4.691‰, at the same time the pyrite samples from middle stage ore show a wide δ34S value range, which suggesting that the multi-sources of sulfur in metallogenesis. The lead isotopic compositions of ore sample are 206Pb/204Pb=18.178~18.992, 207Pb/204Pb=15.635~15.774 and 208Pb/204Pb=38.456~39.051, indicating the ores are rich in U-radiogenic Pb. The studies on S-Pb isotope systematics show that the ore-forming elements were mainly derived from the ore-hosted sedimentary strata as well as magmatic activity. The available ESR dating of the quartz from diabase is 64.8±6.5Ma indicating that the ore-forming process mainly took place in the Late Yanshanian to Early Himalayan period. The formation of the Laozhaiwan gold deposit experienced the initial enrichment in Early Devonian period, the extraction, migration and enrichment by metamorphic/magmatic hydrothermal system in Late Yanshanian to Himalayan period; and finally the secondary oxidation and enrichment after hydrothermal mineralization.
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