安徽东溪浅成低温热液型金矿床成矿流体特征和形成时代——流体包裹体和赋矿安山岩U-Pb年代学约束

摘要

The Dongxi gold deposit is an important epithermal gold deposit in the Tongbai-Dabie orogenic belt,central China,with reserve of ＞5 tons(t) Au.It belongs to a suite of low-sulfidation epithermal systems associated with the volcanism.Gold mineralization is hosted in the Early Cretaceous andesite,and consists primarily of auriferous calcite-quartz veins that are confined to the NW-trending faults.The mineralization process is composed of two stages,i.e.,coarse grained calcite veins (Stage Ⅰ) and quartz + calcite veins (Stage Ⅱ).Primary fluid inclusions in calcite include three types:monophase vapor,monophase liquid,and two-phase liquid-rich inclusions.The early-stage fluid inclusions have low homogenization temperature (128～172℃) and low salinity (0.35 ％ to 0.92 ％),while the late-stage fluid inclusions have homogenization temperatures of 105～ 160℃ and salinities of 0.18 ％ ～0.92 ％.The temperatures and salinities of ore-forming fluids tend to decrease progressively.The ore-forming fluid seems to be the mixture of the magmatic and meteoric components.The crystallization age of the ore-bearing andesite has been fixed at 126.7 ± 1.4 Ma (1σ,MSWD =0.95) according to U-Pb analysis of zircons by LA-ICP-MS.The Dongxi deposit might have been formed in a lithospheric extension and mantle upwelling setting,which provided abnormally high heat and fluid fluxes necessary for gold mineralization.%安徽东溪金矿床位于桐柏-大别造山带北淮阳构造带的中生代晓天火山盆地中,是桐柏-大别成矿带具有代表性的浅成低温热液金矿床.金矿体以含金方解石脉和方解石-石英脉形式产出,受NW向断裂控制,赋矿围岩为毛坦厂组安山质火山岩.该矿床热液成矿过程由早到晚可分为粗晶方解石阶段和方解石-石英阶段.早、晚阶段方解石中流体包裹体十分发育,主要类型有纯液相、纯气相和富液二相流体包裹体.早阶段流体包裹体均一温度范围为128～172℃,冰点温度介于-0.7～ +23.4℃之间,盐度为0.35％ ～0.92％;晚阶段流体包裹体均一温度范围为105～160℃,冰点温度介于-0.5～+9.1℃之间,盐度为0.18％ ～0.52％.成矿流体从早阶段演化到晚阶段,温度和盐度具有小幅度的降低趋势.岩浆热液和加热循环的大气降水的混合可能是引起金属元素富集、沉淀的主要机制,该矿床是地表浅部热液对流系统的产物.LA-ICP-MS锆石U-Pb同位素研究表明,赋矿安山岩的锆石206 pb/238U加权平均年龄为126.7±1.4 Ma(1σ,MSWD=0.95),代表了其火山作用时限.结合矿床地质特征认为,东溪金矿床成矿时间可能与安山岩喷发时限基本一致.该矿床形成于早白垩世陆壳强烈伸展和岩石圈地幔上涌的动力学背景.