江苏栖霞山铅锌多金属矿床成因探讨:流体包裹体及氢-氧-硫-铅同位素证据

Genetic study on the Qixiashan Pb-Zn polymetallic deposit in Jiangsu Province: Evidence from fluid inclusions and H-O-S-Pb isotopes

江苏栖霞山铅锌多金属矿床位于长江中下游成矿带宁镇地区,是华东地区最大的铅锌矿之一,近年来在其深部取得重大找矿突破,但该矿床的成因仍存在争议。本文在详细的野外调查基础上,重点对深部样品进行了系统的观察和采集。通过对其各成矿阶段流体包裹体及H、O、S、Pb同位素系统测定及分析,并结合地质事实,最终确定其矿床成因,为该矿床及区域下一步找矿提供方向。流体包裹体研究表明,栖霞山矿床流体包裹体类型以纯液相包裹体(L)和气液两相包裹体(V+L)为主。显微测温结果显示:主成矿期(热液成矿期)第一阶段磁铁矿-石英阶段(Ⅰ)的包裹体均一温度集中变化于280~380℃,盐度变化于4.24%~9.86%Na Cleqv;第二阶段石英-硫化物阶段(Ⅱ)的包裹体均一温度集中变化于180~320℃,盐度变化于1.74%~8.00%Na Cleqv;第三阶段石英-碳酸盐岩阶段(Ⅲ)的包裹体均一温度变化于80~160℃,盐度变化于0.53%~6.74%Na Cleqv。从第一阶段到第三阶段,均一温度和盐度均有降低的趋势,显示流体混合的特征,可能是其矿质沉淀的重要机制。H-O同位素分析(δ^(18)OH2O值为-1.9‰~5.5‰,δD值为-80.3‰^-69.9‰)显示成矿流体主要为岩浆流体,后期有大气降水的加入。硫化物S同位素研究显示,δ^(34)S值总体变化范围-4.6‰~3.8‰,呈塔式分布,位于零值附近,暗示着栖霞山矿床硫化物的S主要来源于岩浆,且可能有部分赋矿地层S的混入。矿石硫化物的(206)~Pb/(204)~Pb为17.616~17.817,207Pb/(204)~Pb为15.513~15.718,(208)~Pb/(204)~Pb为37.907~38.585,说明Pb主要来源于岩浆,可能有部分震旦系基底地层Pb的加入。综合矿床地质、流体包裹体及H、O、S、Pb同位素特征可知,栖霞山矿床属于主要受石炭系黄龙组灰岩与高丽山组砂岩之间"硅钙面"控制的岩浆热液矿床,与本区早白垩世晚期岩浆活动密切相关。

The Qixiashan Pb-Zn polymetallic deposit is located in the Nanjing-Zhenjiang area in Jiangsu Province of the Middle and Lower Yangtze River metallogenic belt, which is one of the largest lead-zinc deposits in the eastern China. Although much progress in ore-prospecting at the depth of Qixiashan deposit has been achieved in the last recent years, the genesis of this deposit is still controversial. Based on detailed field geological survey, we collect systematically samples at the depth of this deposit. Through the analyses and interpretations of fluid inclusions from different ore-forming stages and H-O-S-Pb isotopic systems, we constrain the origin of this deposit to lay the foundation of ore-prospecting at this deposit and region. Based on petrographic observations, primary inclusions of the Qixiashan deposit primarily belong to two types：two-phase liquid-vapor and liquid-only fluid inclusions. Fluid inclusions occurred in the quartz-magnetite phase （Ⅰ） of main ore-forming stage show homogenization temperatures of 280~380℃ and salinities of 4.24%~9.86% NaCleqv. Fluid inclusions occurred in the quartz-sulfide phase （Ⅱ） show homogenization temperatures of 180~320℃ and salinities of 1.74%~8.00% NaCleqv. Fluid inclusions occurred in the quartz-carbonate phase （Ⅲ） of main ore-forming stage show homogenization temperatures of 80~160℃ and salinities of 0.53%~6.74% NaCleqv. From the first （Ⅰ） to the third （Ⅲ） phase, the homogenization temperatures and salinities of the fluid inclusions all had a reduced tendency, showing the characteristics of fluid mixing, which may be the mechanism of the precipitation of ore-forming metals. Hydrogen and oxygen isotopic results （δ18OH2O：-1.9‰~5.5‰, δD：-80.3‰^-69.9‰） indicate that ore-forming fluids are magmatic fluids with meteoric water mixing at the later period. The δ34S values of sulfides vary from -4.6‰ to 3.8‰, close to zero in average, which indicate that sulfur of the sulfide stem from the magmatic source with parts of contribution from the ore-bearing strata sulfide. The 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb values of sulfide ores range from 17.616 to 17.817, 15.513 to 15.718 and 15.513 to 15.718, respectively. The Pb isotope results indicate that Pb mainly stems from magmatic source with parts of contribution from Sinian system Pb. Based on ore-forming geological characteristics, fluid inclusions, and H-O-S-Pb isotope, the mineralization of Qixiashan deposit is closely related to the Early Cretaceous Yanshanian magmatism, it is interpreted as a magmatic hydrothermal deposit which is mainly determined by the Calcium-Silicon Interface between Carboniferous limestone of Huanglong Formation and sandstone of Gaolishan Formation.