A preliminary study on microtextures, structures and mineralizing processes of hydrothermal chimneys in Mariana Trough

Microtextures and structures of 9 hydrothermal chimneys sampled on the small spreading ridge of the Mariana Trough during SONNE Cruise 57 (5057) and 69 (5069) in 1988 and 1990 were studied by means of scanning electron microscope. The results reveal that the two types of chimneys from the Mariana Trough, the 0silicic chimneys" containing Fe sulfide and the pure "silica chimneys" without sulfide, vary distinctly not only in mineral and chemical compositions, but also in microtextures and structures. The silicic chimneys generally show three major types of microtextures. The inside of the chimneys is dominated by holocrystalline or semicrystalline idiomorphic and hypidiomorphic granular textures and the outwall by amorphous colloidal texture. In some of the silicic chimneys relict texture is observed. However, the pure silica chimneys show chiefly amorphous colloidal texture from the inside to the outwall. Microstructurally, the inside of the silicic chimneys is dominated by a "silica-covering" structure and a "voidfilling" structure, and the outwall by a chain-like or three-dimentional chain-network structure built up by individual opal globules, whereas the pure silica chimneys are mainly three-dimentiottal chain-network and the dense silica bands in structure. These variations suggest that not only mineralizing conditions but also processes differ distinctly during the formation of the two types of chimneys, and that hydrothermal mineralization in the Mariana Trough shows a multistage or multieplsodical nature.