五台山地区韧性剪切带型金矿成因

GENETIC MODEL OF DUCTILE SHEAR TYPE GOLD DEPOSITS IN WUTAI MOUNTAIN AREA

五台山地区韧性剪切带型金矿分布于晚太古代碰撞造山带中，与世界许多太古宙绿岩带型金矿具有相似的特征。研究表明：成矿流体的δ１８ＯＨ２Ｏ＝－２．１‰～４．９‰，δＤ＝－６９‰～－１１２‰，蚀变碳酸盐矿物的δ１３ＣＰＤＢ＝－２．０‰～－１．４‰，δ１８ＯＳＭＯＷ＝１０．０‰～２０．０‰；载金黄铁矿的δ３４Ｓ＝－３．７‰～５．６‰。成矿前流体具有中—高盐度，成矿期流体以富含ＣＯ２、近似海水的δ１８Ｏ和海相碳酸盐的δ１３Ｃ值，成矿后流体具有较高的盐度及可变的δ１８ＯＨ２Ｏ、δ１３Ｃ值。这种金矿成矿流体很难与某单一流体源相对应，它反映的多是变质流体与海盆地铁镁质岩石在成岩过程中饱含的热卤水混合的结果。综合有关资料，提出本区韧性剪切带型金矿形成于太古宙末期汇聚型地球动力学背景下的弧－陆、陆－陆碰撞过程中，金矿化作用和分布受碰撞构造边界的大规模逆冲型韧性剪切带控制。

In this paper, regional geological setting, modes of occurrences, gold potential of host rock associations and their characteristics as well as alteration sequence of ductile shear type gold deposits are elaborated, with a detailed study of stable isotopes in these deposits. H, O, C, S isotopic analyses of quartz, carbonate minerals and wall rocks show that in ore forming fluids δ 18 O H 2O =-2 1‰～4 9‰, and δ D=-69‰～-112‰, in altered carbonates δ 13 C PDB =-2 0‰～-1 4‰, and δ 18 O SMOW =10‰～20‰, and in gold carrying pyrite δ 34 S=-3 7‰～5 6‰. The premineral fluids possess medium high salinity, the fluids of the ore forming stage are characterized by rich CO 2, seawater implying δ 18 O values and marine carbonate δ 13 C values. Studies demonstrate that instead of corresponding with a certain single fluid source, the ore forming fluids mainly reflect the mixture of metamorphic fluids and saturant thermal brine of mafic rocks formed during diagenesis in the sea basin. Synthesizing the relevant data, the authors have established a genetic model of ductile shear type gold deposits: the rock associations formed in different environments were tectonically juxtaposed during continent arc and continent continent collision in Late Archean, giving birth to forearc melange, island arc system and back arc melange. The above rock associations were subducted to different depths of the Earth′s crust and then metamorphosed and dewatered to produce a vast amount of fluids. A series of reversal ductile shear zones formed in the collision process changed the permeability of the crust and provided passageways for fluids which came from the depth of the crust. The crust was uplifted rapidly during the continuous collision, and the balance of hydrostatic and geostatic pressure in the depth of the crust was disrupted. The metallogenic fluids ascended rapidly along the ductile shear zones which occurred in margins of the central and southern tectonic slices and their neighboring areas. Metallogenic fluids precipitated as a result of the change of the physical and chemical environments, forming ductile shear type gold deposits.