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

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 Fe²⁺ in the marine sediments and produced metastable FeS that turnt into pyrite through polysulfide pathway or H₂S pathway. This process resulted 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.