四川呷村黑矿型矿床硅质岩的硅、氧同位素组成及其与现代海底硅质烟囱比较研究

SILICON AND OXYGEN ISOTOPIC COMPOSITION OF CHERTS FROM THE GACUN KUROKO-TYPE DEPOSIT,SICHUAN,AND COMPARISON WITH SILICA CHIMNEYS FROM THE MODERN SEAFLOOR

本文首次报道了川西呷村黑矿型矿床硅质岩的硅、氧同位素组成,其δ^(18)O为12.8‰-18.3‰纵,形成温度约99-120℃,δ^(30)Si为0.0‰-1.5‰,与Mariana和GalaPagos热液硅质烟囱的δ^(30)Si值范围基本相当,揭示两者具相似的形成机制。硅质岩和硅质烟囱δ^(30)Si最大值与相伴火山岩δ^(30)Si值一致,反映硅质来自被热液淋滤交代的火山岩系。根据SiO_2溶解度-温度关系提出,来自海底之下1-2km处的水-岩反应带、温度高于320℃的初始流体,通过绝热上升或传导冷凝,可在海底形成硫化物矿体,残余热液与海水大量混合及传导冷凝,可产生硅质岩和硅质烟囱。

The δ30Si and δ18O of the cherts from the Gacun Kuroko-type deposit were determined for the first time and have been compared with those of the silica chimneys from modern seafloor hy-drothermal fields. The δ18O values of sulfide chert and magnetite chert developed near massive orebodies in the Gacun deposit show a variation range from 12. 8‰ to 18. 3‰ with a mean of 15. 5‰ and it is estimated that they were formed at temperatures between 99℃ and 120℃. The δ30Si values of hydrothermal cherts show a variation range from 0. 0‰ to - 1. 5‰, which is similar to that of silica chimneys from the Mariana trough and Galapagos spreading center, suggesting that both chert and silica chimney were produced by a similar mechanism. The weak correlation between δ30Si and δ18O data of the hydrothermal siliceous precipitates with formation temperatures >40℃ reveals a temperature dependence of silicon isotopic fractionation of hydrothermal systems. The consistency of the initial δ30Si values of dissolved silicon in the hydrothermal solutions for silica (or chert) precipitation with those of possible host rocks strongly suggests that the silicon in the hydrothermal fluids was derived from the volcanic rocks leached by hydrothermal fluids. Based on SiO2 solubility-temperature relations for amorphous silica and quartz at various pressures, it is suggested that initial fluids with temperatures >320℃ from the water-rock interaction zones estimated to be about 1 - 2 km below the seafloor may form sulfide orebody or mound by adiabatic cooling and/or conductive cooling, while the conductive cooling and mixing of the initial fluid with seawater could result in formation of silica chimney or chert.