南大西洋中脊“德音1号”热液区硫化物成矿机理研究：黄铁矿中S、Fe同位素的制约

Submarine hydrothermal sulfide is an important deep-sea mineral resource, and its issues such as genesis and ore-forming material source are still controversial. Large scale seafloor hydrothermal sulfide deposites are usually developed at slow spreading ridge that is focal region for researching on seafloor hydrothermal activity and mineralization. Deyin-1 hydrothermal field is a seafloor hydrothermal field near 15°S Mid-Atlantic Ridge discovered in 2011 during the 22nd cruise carried out by the China Ocean Mineral Resources Research & Development Association (COMRA). The applicant team was fortunate enough to obtain a large amount of sulfide samples derived from Deyin-1 hydrothermal field. Pyrite is one of the dominant sulfide minerals in most sulfide samples and can crystallize during high-, intermediate- and low-temperature environment. Thus, the trace element as well as S- and Fe-isotope composition of pyrite record the information of fluid composition and physical-chemical conditions during pyrite formation. In this program, on the basis of mineralogical and geochemical characteristics of the sulfide chimney samples collected from the Deyin-1 hydrothermal field, in situ trace elemental and S-isotopic microanalysis of pyrite from different sulfide zones of chimney samples will be carried out systematically in order to trace the source of sulfur and other ore-forming elements and calculate the contribution ratio of each end-member. Moreover, Fe-isotope of individual minerals from different sulfide zones is also going to be analyzed to indicate the redox process. Thus, it will reveal the sources of ore-forming materials and element migration and enrichment mechanism of hydrothermal sulfides in this area, illuminate the growth process of sulfide chimneys, and develop the hydrothermal sulfide mineralization mechanism.

海底热液硫化物是一种重要的深海矿产资源，其成因和物源等科学问题仍存在争议。慢速扩张洋脊通常发育有大型海底热液硫化物矿床，是研究海底热液活动及其成矿作用的重点区域。“德音1号”热液区是我国科学家于2011年在慢速扩张的大西洋中脊15°S附近发现的海底热液活动区，申请人团队有幸获得了大量硫化物样品。其中，黄铁矿是热液硫化物的主要组成矿物之一，在高、中和低温条件下均可形成，记录了各个成矿阶段的信息。本项目选取“德音1号”热液区保存完整且分带明显的硫化物烟囱体，在前期矿物学和地球化学系统研究的基础上，拟利用原位微区分析方法对不同环带黄铁矿中的微量元素和S同位素进行分析，示踪硫等成矿元素的物质来源，计算各端元的贡献比例；利用微区取样技术测定不同环带的Fe同位素组成，示踪氧化还原过程；进而揭示该区热液硫化物的成矿物质来源和元素迁移、富集机制，确定硫化物烟囱体生长过程，完善热液硫化物成矿机理。

海底热液烟囱体是重要的金属矿产资源，对海洋化学平衡有着重要的影响。其中，黄铁矿是最常见的硫化物矿物，在低温到高温条件下均可沉淀。本课题通过分析德音1号热液区烟囱体黄铁矿的微量元素含量、S同位素和Fe同位素组成特征对烟囱生长过程中的元素迁移和富集机制、同位素分馏作用及其控制因素做了详细研究。结果表明，As、Ag、Tl、Mn等指示低温条件的元素其含量从烟囱体的外侧向内部逐渐降低，而Se、Co、Ni等指示高温条件的元素其含量总体上呈逐渐增大的趋势，说明在烟囱体的生长过程中存在一个明显的温度梯度，外侧温度低，内侧温度高。黄铁矿的δ34S和δ56Fe值从烟囱内壁向外升高，这可能与温度梯度导致的流体与黄铁矿之间的S和Fe同位素分馏作用增强有关。此外，与其他热液区类似，黄铁矿的δ34S和δ56Fe值呈良好的线性正相关关系。热液黄铁矿的沉淀模型表明，在热液流体保持不变的条件下，不同的温度和相似的黄铁矿-流体交换程度可导致δ34S和δ56Fe值之间的线性关系。烟囱壁内Fe、S同位素的规律性和耦合关系有效地限制了热液硫化物的形成过程。