论湖南前寒武系金矿床的成矿物质来源

ON THE SOURCE OF ORE-FORMING SUBSTANCES OF PRECAMBRIAN GOLD DEPOSITS IN HUNAN PROVINCE

湖南前寒武系不同地区金的背景含量变化在1.8～9.2ppb之间,一般为4—7ppb,而赋矿层位则高达17.4～74ppb。地层中的黄铁矿与变质热液改造型金矿床中的黄铁矿Co/Ni均小于1,含As,Sb,Zn,Cu,W较高,含Ag低微。大部分样品的铅同位素组成均落在岛弧铅平均演化曲线b之上,K为4.1左右。δ^(34)S值与矿床成因类型关系密切:变质热液改造型金矿δ^(34)S值一般大于5‰,变化大,而火山沉积-变质热液型及岩浆期后热液叠加改造型金矿则接近陨石值。氧、氢同位素组成及包体成分测定证实,成矿溶液以变质水为主,有少量大气循环水加入。主要金矿床石英包裹体水属Na-Ca-Cl型,含盐度(NaCl)不高(4wt％)。上述情况表明,湖南前寒武系金矿床的成矿物质主要来自壳源(上地壳和下地壳),部分来自幔源或幔源为主的混合源。

Results of research show that background contents of gold in variousregions where occurred Precambrian formations in Hunan vary from 1.8 to9.2 ppb, generally range from 4 to 7 ppb, and are as high as 17.4-74 ppbwithin the ore-hosting strata. The Co/Ni ratios of pyrites both in fore-going strata and in the hydrothermal metamorphic transformation type golddeposit are less than 1, which are characterized by relatevely high As,Sb, Zn, Cu and low Ag. It is indicated that Pb isotope compositions inmost of samples fell on average evolution curve (b) of island-arc lead. K(^(232)Th/^(238)U)≈4.1. The δ^(34)S value is closely related to the genetic typeof the ore deposit. The δ^(34)S value of the hydrothermal metamorphic trans-formation type gold deposit is generally more than 5‰, whereas the δ^(34)Svalues of the volcanogene sedimentary-hydrothermal metamorphic gold depo-sit and the postmagmatic hydrothermal superimposed transformation typegold deposit are close to meteorite abundance. On the basis of the oxygenand hydrogen isotope compositions and the component determination of in-clusions, it is confirmed that ore-forming hydrothermal solutions consistlargely of the formation water coupled with a small quantity from the me-teoric circulating water. Quartz inclusion water of the major gold depositbelongs to thd Na-Ca-Cl type with a lower salinity (4 NaCl wt%). Fore-going data show that ore-forming substances are derived largely from crust-source magma (the upper crust and the lower crust) coupled with a partfrom the mantle-source magma or from the mixed source magma dominant-ly with materials of mantle sources in large amount.