哈萨克斯坦萨亚克铜矿田代表性矿区矽卡岩矿物的组成及其意义研究

Skarn mineral assemblage in representative ore districts of Sayak copper orefield,Kazakhstan,and its genetic implications

哈萨克斯坦萨亚克铜矿田产于晚石炭世闪长玢岩、石英闪长玢岩或花岗闪长玢岩与中石炭统灰岩的接触带上,铜矿体呈透镜状、脉状产于矽卡岩中。其成矿期可以划分为4个阶段:石榴子石矽卡岩阶段(Ⅰ)、绿帘石-石榴子石矽卡岩阶段(Ⅱ)、磁铁矿阶段(Ⅲ)和石英-硫化物阶段(Ⅳ)。铜矿化主要发生在石英-硫化物阶段,形成石英、黄铁矿、黄铜矿、磁黄铁矿,呈浸染状或脉状产于不同类型的矽卡岩或块状磁铁矿中。矽卡岩中的石榴子石有3种类型:石榴子石矽卡岩中的钙铁榴石(Grt-a)、交代钙铁榴石的含Al钙铁榴石(Grt-b)和绿帘石-石榴子石矽卡岩中具有环带结构的石榴子石(Grt-c)。从第一类到第三类石榴子石,平均w(Al2O3)从<1%逐渐升高至～5%;分子式中平均Fe3+原子数从2.15逐渐降低至1.57,显示成矿体系中Al的摩尔浓度逐渐升高、氧逸度逐渐降低。绿帘石-石榴子石矽卡岩中发育少量辉石,属于钙铁辉石-透辉石系列,在辉石分类图中落于普通辉石范围内。矽卡岩的地质特征、矿物组合和矿物化学特征表明,萨亚克矽卡岩是与花岗岩类侵入岩有关的岩浆热液与灰岩通过接触交代反应形成的钙质矽卡岩,随着矽卡岩化和成矿作用的进行,成矿体系的温度和氧逸度逐渐降低、pH值升高,导致磁铁矿和黄铁矿-磁黄铁矿-黄铜矿矿物组合依次发生沉淀。

The skarn copper orefield in Sayak is hosted in the contact zone of Late Carboniferous granitoid intrusions and Middle Carboniferous limestone. Orebodies occur in lenticular and veinlet forms in skarn. The hydrothermal process can be divided into four stages： garnet skarn stage （Ⅰ）, epidote-garnet skarn stage （Ⅱ）, magnetite stage （Ⅲ） and quartz-sulfide stage （Ⅳ）. The copper mineralization is hosted in the stage Ⅳ quartz-sulfide veins and shows a mineral assemblage of quartz-pyrite-chalcopyrite-pyrrhotite. Three types of garnet was recognized in skarn, i.e., yellow green garnet in garnet skarn, colorless garnet replacing the first type, and colorless garnet with zonation in epidote-garnet skarn. The compositions of garnets of different types are also distinct. With the evolution of the hydrothermal system, the average w（Al2O3） content in garnet increased from 〈1% to -5%, while the Fe^3＋ aupf in garnets decreased from 2.15 to 1.57, indicating the decrease of oxygen fugacity and increase of Al content in the ore-forming system. The compositions of garnet and pyroxene in skarn are very similar to those from world-class skarn Mo-Cu-Au deposits. Based on geological characteristics, mineral assemblages and chemistry of skarn minerals, the authors have reached the conclusion that the mineralization of the Sayak skarn copper ore district was generated by the reaction between magmatic water and limestone. The increase of pH, decrease of temperature and oxygen fugacity induced the deposition of magnetite, pyrite, chalcopyrite and pyrrhotite.