云南哀牢山金矿带成矿时序和矿质来源的Re-Os同位素约束

The Ailaoshan gold belt is one of the most importantr gold production bases in China. In this region, several typical orogenic type gold deposits including Zhenyuan gold deposit ( super large scale ), Mojiang gold deposit( large scale), Chang'an gold deposit (large scale ) and Daping gold deposit ( super large scale ) are distributed in the NW to SE direction respectively. The genesis of this deposits has been debated for a long time focusing on the metallogenic epoch and metallogenic material sources of the gold deposits. Re - Os isotope has a particular role in the study of metal ore accurate dating and tracing of metallogenic material sources.Ths technique become increasingly mature in recent years. Recently, the preliminary research indicates that the Re - Os dating results of the gold-bearing pyrite differs significantly, from the previous obtained metallogenic age by using the indirect method,in Zhenyuan gold deposit. It is known that the Ailaoshan composite orogen experienced complicated evolutional processes, including formation of Precambrian-Early Paleozoic basement, subduction and orogenesis of Late Paleozoic era, collisional orogenesis of Late Hercynian-Indosinain epoch, and extensional or striking orogenesis of Yanshainan-Cenozoic epoch. The polycycle tectonic evolution and magmatism in this area caused multi-stage and superposition metallization characteristics of Ailaoshan gold belt. In this project, we intends to use Re - Os isotope system as the core technique, to systematically study the metallogenic series and the ore-forming materials source of the all kinds of main gold ore types from the 4 gold deposits of Ailaoshan gold belt, and the coupling relationship between the gold metallogenic events and the tecono-magmatic events revealed in the region. Therefore, these research results can provide important evidence for setting up deposits genetic model.By launching this project, it not only can contribute to the the perfection of theory of orogenic gold deposit, but open the possibility to create collision orogenic type gold deposit in the new theory, effectively guide the exploration of gold deposits.

云南哀牢山金矿带是我国目前最重要的黄金生产基地。其中，有镇沅金矿、墨江金矿、长安金矿和大坪金矿依秩由北西向至南东分布，是典型的造山型金矿。其矿床成因长期以来一直是个激烈争论的问题,争论焦点集中在成矿时代和矿质来源。Re-Os同位素在金属矿床精确定年与成矿物质来源示踪研究中具有独特的作用，其技术近年来已日趋成熟。近期初步研究表明，镇沅金矿的载金黄铁矿Re-Os定年结果与前人用间接法获得的年龄相差甚远。构造-岩浆演化历史表明，哀牢山复合造山作用经历了前寒武纪-早古生代基底的形成、晚古生代俯冲造山作用、海西期末-印支期强烈碰撞造山、燕山期-喜马拉雅期伸展（走滑）造山等复杂的演化过程。本项目拟以Re-Os同位素体系为核心，系统研究哀牢山金矿带4个主要矿床之各种矿石类型，对成矿时序及物源问题进行精细刻划，深入揭示本区金成矿事件与构造-岩浆作用的耦合关系,为哀牢山金矿带的成因模式建立提供重要依据.

哀牢山金矿带位于“三江”特提斯成矿域的东缘，是哀牢山造山作用的产物, 它是中国目前重要的黄金生产基地，同时也是研究古特提斯地质演化历史的重要窗口。长期以来，由于缺乏精确的同位素年龄数据，哀牢山金矿带一直被认为是喜马拉雅期陆-陆碰撞造山型金矿。本项目采用负离子热表面电离质谱对哀牢山金矿带最具代表性的镇沅金矿和大坪金矿主要矿石进行Re-Os同位素定年，结果显示：镇沅金矿田全碳酸盐化蚀变超基性岩型金矿形成时代为268±34Ma，初始Os187/Os188为0.745±0.057。表明成矿作用发生在晚二叠世-早三叠世，成矿物质具有壳-幔混合来源的特征，但以幔源为主。矿区内出露的花岗岩和石英斑岩锆石U-Pb LA-ICP-MS测定年龄为252±2.6Ma和245.8±2.8Ma。元素地球化学显示，花岗岩和石英斑岩形成于岛弧环境向陆陆碰撞转变的构造背景。大坪金矿主要载金矿物黄铜矿Re-Os等时线年龄为253±7.7Ma，具有成矿物质来源示踪信息的初始Os187/Os188也具有壳幔混合来源特征。综合最新研究成果，古特提斯洋自晚泥盆世开始打开，形成三个主支(修沟)玛沁洋、金沙江)哀牢山洋、澜沧江)昌宁孟连洋),至早二叠世扩张到最大规模后开始俯冲消减,逐渐缩小,至晚三叠世末)早侏罗世初洋盆闭合。哀牢山增生造山的时间限定在P1-T1 ，提出哀牢山金矿带为P2-T1增生造山作用的产物，增生造山过程具有强烈壳幔相互作用，在此背景下,造山型金矿成矿物质来源表现出以幔源为主的壳-幔混合来源特征。哀牢山复合构造带所经历的最末一次造山作用为喜山期碰撞造山运动，其主要表现为水平方向上的走滑剪切作用，强度远低于P2-T1增生造山作用，伴随的岩浆事件主要表现为在哀牢山金矿带中出露的~30Ma的富碱斑岩。这些碱性斑岩规模较为零星，不可能提供哀牢山金矿带巨量的金堆积，其与哀牢山金矿带的关系为“兄弟关系”，而不是“父子关系”。因此，本期构造-岩浆事件仅可能对早期形成的哀牢山金矿带产生改造。项目研究成果在成矿物质来源方面丰富了造山型金矿理论，对研究区深部找矿指明了方向。