湘、黔地区前寒武——寒武纪过渡时期硅质岩生成环境研究

A study of the fluid environment of silicalite of transitional Precambrian-Cambrian age in Hunan and Guizhou provinces

湘、黔地区张家界、遵义一带分布有黑色岩系型Ni-Mo-PGE矿床,硅质岩作为矿化体标志之一。对黑色岩系中硅岩的地质产状、标型特征、成因意义的研究均反映了热水成因,其生成时的热水环境温度变化为80 ~98 ℃。硅的稳定同位素δ30Si值( -0·5‰^-0·3‰)清楚地反映湖南张家界三岔硅质岩形成环境属于特殊的较深水海槽类型,并总体与武陵期基性、超基性岩类在黑色岩系物质供应方面所可能起的积极作用有关。而湖南三岔硅质岩的氧同位素不仅与美国阿拉斯加De Long山海底喷流沉积的氧同位素组成比较接近,与湘西北慈利层状硅岩的变化区间(18·12‰~22·55‰)以及中国秦岭凤太硅岩的变化区间(18·74‰~22·05‰)吻合,应为热水喷流沉积型。热水矿化也为Ni、Mo矿床形成的主要机制。经过投点研究证实,本区硅质岩主要是热水沉积物区的硅质岩;;是富硫化物层多种成矿元素沉淀高峰期前后出现的富硅质热卤水活动的遗迹。而通过依据自生矿物组合对黑色岩系型镍钼铂族多金属矿化作用介质的酸碱度和氧化还原电位进行了恢复;;证明包括后生阶段的硫化物-方解石细脉;;成岩成矿均对应着一定的介质条件;;本区域硅质岩生成于微碱性的介质环境;;硅质岩的介质pH≈7·0 ~7·8 ;; Eh≈0·0 ~0·1 V。

Ni-Mo-PGE deposits of black-rock-series-type are located in the Zhangjiajie of Hunan Province and the Zunyi of Guizhou Province. The silicalite rock mainly occurs as an indicator of the mineralized body. Its geological occurrence, typomarphic features and composition indicate that the silicalite rock was of hydrothemkal genesis, and that temperatures of the hydrothemaal water varied from 80～98℃. The δ^30Si（-0. 5‰～-0. 3‰） clearly reveals that the environment of the formation of the silicalite at Sancha of Zhangjiajie, Hun-an, was a moderately deep water marine trough, with the material supply partially derived from the mafic and ultramafic rocks typical of the Wuling Stage. The δ^18 O at Sancha is not only close to the value of a seamount exhalative-sedimentary vein deposit at De Long of Alaska, USA, but is also coincident with the range of （18. 12‰～22.55‰） for layer silicalite rock at Cili of NW Hun-an and the range of （18. 74‰～22.05‰） at Fengtai in the Qinling Mountain area;the latter serves as an index to the exhalative-sedimentary system. The Ni-Mo orebodies were also mainly formed by hydrothermal mineralization. The silicalite rock is the remnant of silica-rich thermal brine water activity slightly before and after the peak age of sedmentation of polymetallic elements. Based on our research of the authigenic mineral assemblage, we have determined the acidity-alkalinity and the redox electric state of the silicalite. The fluid medium of the silicalite rock was slightly alkaline, with a pH≈7. 0～7. 8, and an Eh≈0. 0～0. 1 V.