铂族元素和锇同位素在西南印度洋脊龙旂热液区硫化物烟囱体中的组成及物源示踪

Seafloor hydrothermal sulfides which will likely be mined as important mineral resources soon, are the dominant form of the metallic substances carried by the hydrothermal fluid from the deep earth to the seabed and the ubiquitous geological phenomena during the long earth's evolution. This is a natural metallogenic laboratory to study the formation process, mineralization mechanism and geological environment of the seafloor hydrothermal sulfide deposits. The hydrothermal fluids occur along seafloor are capable of transporting a great amount of platinum group elements (PGE) from deep interior of the earth where it is highly enriched to the ocean floor, this process plays a significant role in the global cycling of PGE and will have a great impact on the seawater PGE budget.The compositions and provenience of PGE and Osmium (Os) isotopes in the hydrothermal sulfides hold some of the keys to an understanding of the formation process and control factors of the seafloor massvie sulfide deposits. This project takes the sulfide chimneys from the Dragon Flag hydrothermal field on the Southwest Indian Ridge, which was the first active high temperature hydrothermal field found on any ultraslow mid-ocean ridge worldwide as the research object, analyzes the distribution of PGE and Os isotopes in the different circle layers, different parts, and different mineral assemblages of the chimneys, establishs the three-dimensional models and variation law of the distribution of PGE and Os isotopes in the chimneys, calculates the partitioning coefficient in the major mineral phases of the chimneys. In this study, we research the paragenetic or associated relationship in PGE, between PGE and other metallogenic elements, and the response relationship between PGE and Os isotopes. After comparing to the distribution of PGE and Os isotopes from other different ocean region and different tectonic environment，and considering the large contrasts among different sources of PGE and Os isotopes, we combine the normalized PGE patterns, and the ratios of the characteristic elements and Os isotopes, reveal the main material sources and the relative contribution of PGE and Os isotopes in the seafloor hydrothermal sulfide chimneys. This study has a general relevance to research on many volcanic-associated massive sulfide deposits found on land, and may deepen our understanding of the evolution law and the genetic mechanism of its historical analogue on Earth.

海底热液硫化物是地球深处流体喷发到海底的金属物质的集中体现，是贯穿漫长地球演化历史而普遍存在的地质现象和重要的矿产资源。海底热液活动是深部来源铂族元素（PGE）运移到地表系统的重要媒介，是地球PGE循环的重要组成部分，并会对海洋的PGE收支产生极大的影响。因此，理解PGE和锇（Os）同位素在热液硫化物中的分布和来源具有重要的科学意义。本项目以西南印度洋龙旂热液区的硫化物烟囱体为研究对象，通过分析烟囱体不同部位、不同矿物中PGE和Os同位素的组成和变化特征，建立它们的三维分布模式和在主要矿物中的配分系数；分析PGE与其他成矿元素的共生或伴生特征，探讨它们在烟囱体不同部位的响应关系；利用PGE和Os同位素在各物源端元中的巨大差异进行物源限定，结合标准化配分模式、特征元素比值及图解，最终阐明它们在烟囱体中的主要物源端元和相对贡献，并为陆上类似矿体的物质演化规律和成因机制提供现代视角上的有益思路。

本项目通过分析西南印度洋龙旂热液区内硫化物不同层位中的PGE和Os同位素组成及变化特征，探讨了PGE与Fe、Cu、Zn等主要成矿元素的相关关系，并与不同海区不同构造环境下的热液硫化物进行了对比研究，同时尝试系统性揭示了Os、Au等贵金属在洋中脊热液系统内的分布和演化特征。结果表明龙旂热液区硫化物样品内各层位的PGE含量较低且与Os同位素组成一样均呈现出外部高、内部低的特征，这可能是外部低温热液成因物质的吸附富集和内部硫化物重结晶亏损共同作用的结果，这也得到了全球各热液区内硫化物的PGE与ΣFe+Cu+Zn具有较好负相关关系的佐证。龙旂热液区硫化物内的PGE可能主要源自热液流体对区域内下覆玄武质围岩的淋滤，海水贡献比例较低。此外，研究还发现相比于镁铁质围岩海底热液系统，超镁铁质围岩热液系统普遍具有的围岩物理化学属性优势、热液流体中的溶解态非生物有机质和气态物质含量高、硫化物堆积体所处区域裂隙发育、构造稳定等因素是造成这类硫化物内显著富集Au的主要原因。每年喷出海底面的热液流体中的溶解态Au仅有约3～4%滞留在了近喷口硫化物体内，约6～7%随远端热液羽状扩散沉降在了含金属沉积物中，剩余90%的部分约292 kg都溶入了海洋水体内，这严重限制了海底热液矿床内Au的富集成矿。本项研究工作为未来通过恰当的海底人工干预，提升海底贵金属富集进入硫化物中的程度和规模，从而获得更好的经济效益积累了有益信息，同时也为人类更为精准地认识海底热液系统及其物质循环过程提供了参考和借鉴。