甘肃厂坝—李家沟超大型铅锌矿床成因与成矿背景——矿床地质、矿物元素地球化学和^(40)Ar-^(39)Ar同位素年代学证据

Genesis and Metallogenic Background of Changba-Lijiagou Giant Pb-Zn Deposit in Gansu Province:Evidence from Geology,Geochemistry and ^(40)Ar-^(39)Ar Geochronology

厂坝—李家沟铅锌矿床位于甘肃西成多金属矿集区北部、吴家山背斜北翼,矿体主要赋存于中泥盆统安家岔组海相碳酸盐-碎屑岩建造体系中。矿床地质特征研究表明,厂坝—李家沟铅锌矿床成矿可划分为沉积和接触交代热液两期,后者又分为干矽卡岩、湿矽卡岩、石英-硫化物和碳酸盐岩4个阶段。矿石矿物元素分析和蚀变矿物绢云母的^(40)Ar-^(39)Ar测年实验结果表明:沉积期黄铁矿(Py_(1))As质量分数和S/Fe(原子比)值出现完全相反的分布趋势,说明As主要以类质同象取代S,富集在沉积的黄铁矿晶格内;根据黄铁矿(Py_(1))的S/Fe值(2.009~2.059,平均为2.037)、闪锌矿(Sp_(1))的S/Zn值(1.059~1.101,平均为1.079)与Fe的质量分数(1.56%~3.64%,平均为2.56%),说明沉积成矿过程富硫(S^(2-))、锌(Zn^(2+))而贫铁(Fe^(2+))。总体表明沉积成矿期成矿环境为低还原硫浓度,细菌通过还原硫酸盐而成矿。接触交代热液早期浸染状黄铁矿(Py_(2))的S/Fe值(1.982~1.997,平均为1.993)与脉状成矿阶段黄铁矿(Py_(3))的S/Fe值(1.951~2.012,平均为1.989)相比,以及脉状闪锌矿(Sp_(2))的S/Zn值(1.118~1.215,平均为1.160)、Fe的质量分数(4.28%~8.44%,平均为6.43%)与浸染状闪锌矿(Sp_(3))的S/Zn值(1.128~1.243,平均为1.179)、Fe的质量分数(5.39%~7.57%,平均为6.40%)对比相结合,推断接触交代热液期为高硫逸度的高温流体,伴生流体混合作用经过降温过程而成矿。结合沉积期赋矿地层时代和本次获得热液蚀变绢云母^(40)Ar-^(39)Ar坪年龄((190.5±2.2)Ma),进一步揭示晚古生代中泥盆统海相沉积周期性变化可能是导致该矿床韵律型层状矿体发育的主要因素,而中生代早侏罗世岩浆作用是导致块状、脉状矿体发育的关键因素。

The Changba-Lijiagou Pb-Zn deposit is located in the north of the Xicheng polymetallic ore cluster,and developed on the north side of the Wujiashan anticline.The orebodies are mainly hosted in the carbonate-clastic formation system of the Middle Devonian Anjiacha Formation.Through the study of the geological characteristics of the orebodies,the mineralization of the Changba-Lijiagou lead-zinc deposit can be divided into sedimentary period,and contact-metasomatic hydrothermal period in which four stages of prograde skarn,retrograde skarn,quartz-sulfide and carbonatite were recognized.The ore minerals in the sedimentary and hydrothermal periods were analyzed by using electron-probe micro-analysis(EPMA)for the purpose of finding detailed ore forming information.According to the EPMA results,the distribution trends of As contents and S/Fe atomic ratios in the Py_(1)depositional stage are completely opposite,which illustrates that As mainly replaced the position of S in pyrite lattice as isomorphism during precipitation.The S/Fe atomic ratios of Py_(1)range from 2.009 to 2.059,with an average of 2.037,the S/Zn atomic ratios in Sp_(1)range from 1.059 to 1.101,with an average of 1.079,and the Fe contents range from 1.56%to 3.64%,with an average of 2.56%,showing that Fe content was lower and Sand Zn content was relatively high in the sedimentary metallogenic process with the reduction of sulfate Bacteria.The S/Fe atomic ratios of Py_(2)contact-metasomatic hydrothermal period range from 1.982 to 1.997,with an average of 1.993,the S/Zn atomic ratios of Sp_(2)range from 1.118 to 1.215,with an average of 1.160,the contents of Fe range from 4.28%to 8.44%,with an average of 6.43%,the S/Fe atomic ratios of the veined Py_(3)range from 1.951 to 2.012,with an average of 1.989,the Fe contents of the disseminated Sp_(3)range from 5.39%to 7.57%,with an average of 6.40%,and the S/Zn atomic ratios range from 1.128 to 1.243,with an average of 1.179,which suggests that the temperature of the contact-metasomatic hydrothermal fluid was high with high sulfur fugacity.The mineralization was caused by fluids mixing.The^(40)Ar-^(39)Ar dating method was used to study the sericite formed in the hydrothermal alteration stage,and the plateau age of sericite is(190.5±2.2)Ma.Combined with the ore-bearing strata in the sedimentary period,the Devonian marine sedimentary periodicity in Late Paleozoic is the reason for the rhythmic layered ore bodies.The Mesozoic Early Jurassic magmatism formed the massive and vein-like ore bodies,and the mineralization of Changba Pb-Zn deposit occurred in Jurassic.The long-term repeated magmatic intrusions formed the rhythmic layered ore bodies.