滇西芦子园远程矽卡岩Pb-Zn-Fe(Cu)多金属矿床流体包裹体初探及矿床成因探讨

Fluid inclusion and ore genesis of the Luziyuan distal skarn Pb-Zn-Fe(-Cu) poly-metallic deposit,West Yunnan,SW China

滇西镇康芦子园是"三江"成矿带保山地块内迄今发现的唯一超大型Pb-Zn-Fe(Cu)多金属矿床,是区内系列同类层控热液铅锌矿床的典型代表。矿体呈似层状、脉状及透镜状产于寒武系碳酸盐岩建造的矽卡岩和大理岩层间破碎带,矿石构造以条带状、浸染状和脉状-网脉状为主要特征;围岩蚀变复杂、分带明显,由下至上依次为石榴子石-透辉石-透闪石-阳起石化带→绿泥石-绿帘石-阳起石-蔷薇辉石化带→碳酸盐-大理岩化带。矿床成矿流体从早期到晚期经历了多个矿化阶段。本文选取了该矿床早矽卡岩阶段、晚矽卡岩阶段、石英硫化物阶段和石英碳酸盐阶段的多种脉石矿物及闪锌矿进行了系统的流体包裹体研究。结果显示,早矽卡岩阶段发育微溶CO_2富液相和纯液相水溶液包裹体,并含大量K^+、Na^+、Ca^(2+)、F^-和Cl^-和少量SO_4^(2-),气相成分主要为H_2O、CO_2及少量CH_4和N_2,包裹体均一温度为233.6~315.6℃,盐度为10.6%~17.6%NaCleqv;晚矽卡岩阶段发育含CO_2和子矿物三相包裹体,均一温度214.9~388.0℃,盐度5.9%~16.4%NaCleqv;石英硫化物阶段发育含CO_2的水溶液包裹体,气相成分为CH_4、H_2O和少量CO_2,均一温度150.0~285.0℃,盐度为2.5%~13.8%NaCleqv;石英碳酸盐阶段为单一成分的水溶液包裹体,均一温度为105.0~187.5℃,盐度为0.5%~12.3%NaCleqv。结合H-O同位素研究表明,成矿流体最初来源于具中高温、中高盐度、高K、Na,富CO_2、Cl、F等特征的深部岩浆热液,在石英硫化物阶段开始有大气降水混入,演化为中阶段中低温、低盐度、贫CO_2的热液流体,至成矿晚阶段转化为以大气降水占主导。该矿床成矿环境的改变、流体混合以及流体的沸腾作用可能是导致成矿物质富集沉淀的重要机制。综合矿床地质特征、成矿流体包裹体和HO同位素研究认为,该矿床为与陆陆碰撞造山和深部隐伏岩体有关的远程矽卡岩成矿系统。

The Luziyuan Pb-Zn-Fe（-Cu） poly-metallic deposit in Zhenkang County,West Yunnan,is the only gigantic deposit recently discovered in the Baoshan block of ＂Sanjiang metallogenic belt＂,and has unique characteristics for correlating with other similar strata-bound hydrothermal Pb-Zn deposits of the block. Ore bodies in the Luziyuan deposit occurred as stratiform-like,veined and lenticular in the interlayer fracture zone of skarn and marble in the Late Cambrian carbonate formation. The ores are mainly characterized by banded,disseminated and veined-stockwork structures. The wallrocks are marked by intense hydrothermal alteration with a complexed and prominent zonation pattern in the sequence from bottom to top： garnet-diopside-tremolite-actinolite to chloriteepidote-actinolite-rhodonite,and to carbonate-marbleization. The ore-forming fluids have undergone a significant evolution from early to late stages. The present paper concentrates on a systematic study of fluid inclusions involving various gangue minerals and sphalerite from the early and late skarn stages,quartz-sulfide mineral and quartz-carbonate stages. CO2-bearing liquid-rich fluid inclusions and liquid inclusions are common in the early skarn stage. These inclusions contain a large amount of K^＋,Na^＋,Ca^2＋,F^-,Cl^-,and a little SO4^2- in the liquid phase,and H2 O-and CO2-dominant and a small amount of CH4 and N2 in the vapor phase. Their homogenization temperatures range from 233. 6 to 315. 6℃ and salinities from 10. 6% to 17. 6% NaCleqv. The late skarn stage is characterized by CO2-rich and NaCl^-bearing fluid inclusions,with homogenization temperature ranging from 214. 9 to 388. 0℃ and salinity from 5. 9% to 16. 4% NaCleqv. The quartz-sulfide stage is dominated by CO2-bearing fluid inclusions,whose vapor phase mainly consists of H2 O,CH4,and a small amount of CO2. The homogenization temperature ranges from 150. 0 to 285. 0℃ and salinity from 2. 5% to 13. 8% NaCleqv. In the quartz-carbonate stage the inclusion fluid with a single composition has homogenization temperatures of 105. 0 - 187. 5℃ and salinity of 0. 5% to 12. 3% NaCleqv. Fluid inclusion characteristics combined with the H-O isotope results suggest that the Luziyuan ore-forming fluid is sourced from deep magmatic hydrothermal solution with medium-high temperature and salinity,high K and Na contents,and rich in CO2,Cl,F in the early stage,but in the quartz-sulfide stage,is characterized by medium-low temperature,low salinity,and poor in CO2 due to being mixed with meteoric water that becomes dominant in the late stage. Therefore,the change in ore-forming environment,mixing of different fluids and possible fluid boiling may be the important mechanisms for precipitation and enrichment of metallic elements in the Luziyuan deposit. In the light of ore geology,fluid inclusions and H-O isotopes,we propose that the Luziyuan Pb-Zn-Fe（-Cu） poly-metallic deposit is a distal skarn type mineralization system which is related to the deep-hidden granite and the continental collision orogeny.