湘西花垣铅锌矿田方解石REE元素和Sr同位素地球化学

摘要

湘西花垣铅锌矿田随着杨家寨、大脑坡及清水塘等一系列大型–超大型铅锌矿床的发现,已经成为世界千万吨级铅锌矿田.通过矿脉切穿关系和不同的矿物共生组合,将花垣矿田成矿过程分为3个阶段.对矿田内主成矿期方解石的REE及Sr同位素进行了研究,并与含矿围岩及不同时代地层的REE和Sr同位素进行了对比.结果表明:方解石稀土元素特征与地层具有相似性,其中 Ⅰ 阶段方解石REE含量较低,ΣREE为1.73×10-6~10.94×10-6,平均为4.77×10-6;LREE/HREE为6.21~10.19,平均为7.96;87Sr/86Sr比值0.70935~0.71001,平均为0.709643.Ⅱ 阶段方解石REE含量比 Ⅰ 阶段高,ΣREE为24.53×10-6~67.88×10-6,平均为46.71×10-6;LREE/HREE为6.21~10.19,平均为7.96;87Sr/86Sr比值为0.70923~0.70944,平均为0.709301,比 Ⅰ 阶段的低,说明 Ⅰ、Ⅱ 阶段成矿流体存在差异.结合围岩及不同时代地层地球化学特征认为,Ⅰ 阶段成矿流体主要为含矿层建造水,混有深循环热卤水;Ⅱ 阶段成矿流体主要为深循环热卤水,混有大气降水,从 Ⅰ 阶段至 Ⅱ 阶段成矿流体还原性增强.%With the discovery of the large to super large Yangjiazhai, Danaopo and Qingshuitang Pb-Zn deposits, the Huayuan orefield becomes a world-class super large Pb-Zn orefield. According to the crosscutting relationship of ore veins and coexisting mineral assemblage, the mineralization process was subdivided into three stages. REE and strontium isotope of calcites were analyzed and compared with those of ore-hosting limestone and strata of different geological times. The results show that the REE patterns of calcites are similar to those of strata. Stage Ⅰ calcites display low total contents of REE (ΣREE=1.73×10-6-10.94×10-6,average 4.77×10-6), 87Sr/86Sr ratios ranging from 0.70935 to 0.71001 (average 0.709643) and LREE/HREE ratios range from 6.21 to 10.19 (average 7.96). Stage Ⅱ calcites display higher contents of REE (ΣREE=24.53×10-6-67.88×10-6,average 46.71×10-6) and lower 87Sr/86Sr ratios ranging from 0.70923 to 0.70944 (average 0.709301) than those of stage Ⅰ calcite, and similar LREE/HREE ratios range from 6.21 to 10.19 (average 7.96). These indicate that the ore-forming fluids of stages Ⅰ and Ⅱ are derived from different origins. According to geochemical characteristics of wall rock and strata of different geological times, it is concluded that the ore-forming fluids of stage Ⅰ are mainly connate water in ore-hosting strata, mixed by deep-source circular brine; the ore-forming fluids of stage Ⅱ are mainly deep-source circular brine, mixed by meteoric water, and the reducibility increases from stage Ⅰ to Ⅱ.