滇西镇康水头山Pb-Zn矿床成矿流体及矿质来源探讨--H、O、S、Pb同位素地球化学证据

Discussion on sources of metallogenic fluids and materials of the Shuitoushan Pb-Zn deposit in Zhenkang,western Yunnan: Evidence from H,O,S and Pb isotopes

滇西镇康水头山Pb-Zn矿床是保山地块镇康Pb-Zn-Fe-Cu多金属矿集区内又一重要找矿成果。矿体呈似层状、透镜状产于上寒武统保山组大理岩化灰岩中,呈NEE向顺层产出,矿石矿物主要为闪锌矿和方铅矿,偶见黄铜矿和黄铁矿等;脉石矿物主要有白云石、绿泥石、方解石、石英和绢云母等。本文基于对矿床地质特征的详细研究,结合矿床H、O、S、Pb同位素组成,对其成矿流体和矿质来源进行了探讨,同时与毗邻的芦子园超大型Pb-Zn-Fe-Cu多金属矿床进行了对比。研究表明:该矿床石英的δD值介于-101.1‰^-93.3‰之间,均值为-96.85‰(n=4),δ^(18)O_(H_2O)值为3.37‰~3.77‰之间,均值为3.57‰(n=4),表明成矿流体早期以原生岩浆水为主,有大气降水的混入。矿床金属硫化物的δ^(34)S值均为正值,介于4.1‰~12.2‰,均值为8.23‰(n=10),与旁侧的芦子园矿床δ^(34)S值(8.9‰~12‰)较为接近。该矿床可划分出三个成矿阶段,阶段Ⅱ为以闪锌矿和方铅矿为主的主要成矿阶段(δ^(34)S主要集中在4.1‰~6.2‰之间),其δ^(34)S均值可近似代表成矿热液中的δ^(34)S∑S值,即δ^(34)S∑S≈δ^(34)S均值=6.56‰(n=7),闪锌矿和方铅矿δ^(34)S值有部分重叠,但总体上具有δ^(34)S闪锌矿>δ^(34)S方铅矿以及不同颜色闪锌矿之间δ^(34)S深棕色闪锌矿>δ^(34)S棕褐色闪锌矿>δ^(34)S浅棕色闪锌矿的分布特征,暗示硫同位素在硫化物间的分馏达到平衡,表明S同位素组成较为稳定,显示水头山矿床具有深部壳源岩浆成因的特征。矿床金属硫化物的Pb同位素分析显示,Pb同位素组成非常集中(^(206)Pb/^(204)Pb=18.3408~18.4483,均值为18.3815,^(207)Pb/^(204)Pb=15.8337~15.9440,均值为15.8745,^(208)Pb/^(204)Pb=38.8224~39.4391,均值为38.9941,n=10),投点主要分布在上地壳演化线上方,表明其Pb主要来自于以岩浆作用为主的上地壳物质。本文认为矿区深部壳源岩浆热液是水头山矿床最重要的成矿流体与矿质来源,流体的混合作用是矿床金属元素沉淀和富集的重要机制,矿床具有低温、后生成矿特征,推测矿床的形成与燕山晚期的岩浆热液作用有关。

Shuitoushan Pb-Zn deposit is a low temperature type deposit,which was one of the significant results from the Zhenkang Pb-Zn-Fe-Cu polymetallic ore concentration area in Baoshan block,western Yunnan. The ore bodies of this deposit occurred as either stratiform-like or lentiform within the marbleization-limestone of the Upper Cambrian Baoshan Formation,and was controlled by NEE trending fault,with sphalerite and galena as main and chalcopyrite and pyrite as occasionally ore minerals; dolomite,chlorite,calcite,quartz and sericite as dominant gangue minerals. Based on the detailed study of the geological features of the deposit,combined with H,O,S and Pb isotope compositions,this paper focused on the sources of metallogenic fluids and materials,and compared with the adjacent Luziyuan super-large Pb-Zn-Fe-Cu polymetallic deposit. This research indicates that the values of δD and δ-（18）OH_2O of quartz in the deposit range from-101. 1‰ to-93. 3‰ with average of-96. 85‰（ n = 4） and from 3. 37‰ to 3. 77‰ with average of 3. 57‰（ n = 4） respectively,implying magmatic as dominant early metallogenic fluids,while increasing gradually mixed with meteoric water later. δ^34 values of the sulfides are all positive,varying from 4. 1‰ to 12. 2‰ with average of 8. 23‰（ n = 10）,and approaching δ^34 values（ 8. 9‰ to 12‰） of the adjacent Luziyuan deposit. This deposit can be divided into three metallogenic phases,phase II is the main metallogenic stage of the deposit related to sphalerite and galena（ δ^34 values are mainly concentrated between 4. 1‰ to6. 2‰）. The δ^34Averagecan be used to represent approximately the δ^34∑Sof metallogenic hydrotherm i. e. δ^34∑S≈ δ^34Average= 6. 56‰（ n = 7）. δ^34 values of sphalerite and galena are partially overlapped,but have the distribution characteristics of δ^34Sphaleriteδ^34Galena,and δ^34Dark-brown δ^34Brown δ^34Light-brownbetween the different colors of sphalerite on the whole,showing a sulfur isotope equilibrium fractionation,that S isotopic compositions are relatively stable,that the Shuitoushan deposit has the characteristics of the deep crustal magma in origin. Pb isotope analysis of metal sulfides in this deposit is made,and the results show that it is very concentrated（ ranges from 18. 3408 to 18. 4483 with average of 18. 3815,and from 15. 8337 to 15. 9440 with average of 15. 8745,and from 38. 8224 to 39. 4391 with average of 38. 9941 respectively for206Pb/204 Pb,207Pb/204 Pb and208Pb/204 Pb,n = 10）. The points are mainly distributed above the upper crust evolution line,indicative of primarily the upper crust materials related to magmatism. This paper argues that the deep crustal magmatic hydrotherm in the mining area is the most important sources of metallogenic fluids and materials in the Shuitoushan deposit,that the mixing of fluids is an important mechanism for the precipitation and enrichment of metal elements,that the deposit is characterized by low temperature and epigenetic mineralization,speculating that the formation of the deposit is related to magmatic hydrothermalism of Late Yanshan epoch.