南沙海槽表层沉积自生石膏-黄铁矿组合的成因及其对天然气渗漏的指示意义

FORMATION OF AUTHIGENIC GYPSUM AND PYRITE ASSEMBLAGE AND ITS SIGNIFICANCE TO GAS VENTINGS IN NANSHA TROUGH,SOUTH CHINA SEA

南海海槽是潜在的天然气水合物发育区,在表层沉积物中分布有石膏-黄铁矿组合。通过对石膏、黄铁矿的形貌特征、矿物组成、化学元素、硫同位素等的分析,讨论和揭示了石膏-黄铁矿组合的成岩环境、形成机理及其与天然气渗漏的关系。石膏集合体为土块状和多孔状、玫瑰状,表面分布有孔洞或微孔,黄铁矿为莓球状、球粒结块状和虫管状。不同形状的石膏、黄铁矿的化学成分没有明显差别。石膏-黄铁矿的形成机理是海底渗漏的天然气与硫酸盐发生缺氧甲烷氧化反应,产物HS-与沉积物中的Fe2+反应产生FeS并转变为黄铁矿,在形成FeS的过程中产生的H+促进碳酸盐溶解,Ca2+与SO42-达到过饱和沉淀出重34S的石膏。因此,石膏-黄铁矿组合是海底存在天然气渗漏的证据,这一发现对开展南沙海槽潜在天然气水合物的调查以及对天然气渗漏事件的研究具有一定的科学意义。

Nansha Trough located in the southwestern South China Sea （SCS） is a promising area of gas hydrate resources in China, in which authigenic gypsum and pyrite assemblages in surface sediments are widely distributed. Gypsum and pyrite were picked out from surface sediment collected from 1987 to 2004 and very useful to structural, mineralogical, geochemical and isotopic studies. Gypsum often occurs as muddy aggregates with milk-white micrite grains or intergrowth rosettes with large pores, and surface of the aggregates has pores or micro-pores, and pyrite often shows framboids, granular masses or worm tubes. However, two types of recognized gypsum are not distinctly different from each other in chemical composition, and types of pyrite are not obviously different in chemical composition, either. The results demonstrate that diagenesis environment of gypsum and pyrite assemblages were associated with gas venting in the seafloor where seepage gas came in contact with downward-diffusing seawater sulfate, and then anaerobic oxidation of methane took place. HS^- reagent reacted with Fe^2＋ in the marine sediments and produced metastable FeS that turnt into pyrite through polysulfide pathway or H2 S pathway. This process resuited in a lowering of pH,which promoted carbonate dissolution. The elevated levels of both calcium and sulfate led to a supersaturation and precipitation of gypsum with heavy^34S. It shows that gypsum and pyrite assemblage is an evidence of gas venting in the seafloor of Nansha Trough, where gas hydrates most likely occur. Therefore, gypsum and pyrite assemblage here would provide new implications for further exploration of potential gas hydrates in modern submarine sediments and for detailed researches of ancient methane seeps in the Nansha Trough.