贵州普定纳雍枝铅锌矿矿床成因：S和原位Pb同位素证据

Ore genesis of the Nayongzhi Pb-Zn deposit,Puding City,Guizhou Province,China:Evidences from S and in situ Pb isotopes

通过近五年(2011~2015)勘查实现找矿重大突破的贵州普定纳雍枝铅锌矿床,位于扬子陆块西南缘,五指山背斜南东翼北中部,是黔西北铅锌成矿区的重要组成部分。矿区内已发现20余个铅锌矿体,探获铅锌金属资源储量超135万吨,是川滇黔接壤铅锌矿集区贵州境内目前已发现和探明规模最大的铅锌矿床。主矿体多呈层状、似层状、透镜状和陡倾斜脉状产出,除了陡倾斜脉状矿体产于F7断层破碎带,其余(似)层状矿体均产于下寒武统清虚洞组和上震旦统灯影组白云岩中,与围岩产状一致,层控特征明显。其矿石类型主要有块状、角砾状、细脉状和浸染状等,矿石矿物以闪锌矿为主,其次为方铅矿和黄铁矿,脉石矿物以方解石、白云石为主,含少量石英,偶见重晶石。本次研究表明,该矿床硫化物δ^(34)S_(CDT)值介于15.94‰~25.49‰之间,均值为22.41‰(n=21),其中黄铁矿δ^(34)S_(CDT)值为22.06‰,闪锌矿δ^(34)S_(CDT)值为19.37‰~25.49‰,均值为23.17‰(n=17),方铅矿δ^(34)S_(CDT)值为15.94‰~19.70‰(n=3),均值为18.23‰。各类硫化物δ^(34)S值部分重叠,总体上不具有δ^(34)S黄铁矿>δ^(34)S闪锌矿>δ^(34)S方铅矿的特征,暗示硫同位素在硫化物矿物间的分馏未达到平衡。此外,矿石存有少量硫酸盐矿物(重晶石),暗示成矿流体的δ^(34)S3∑S值应高于硫化物的平均δ4S值(22.41‰),接近赋矿地层中海相硫酸盐岩的δ^(34)S值(22‰~28‰)。因此,成矿流体中的还原硫最可能为海相硫酸盐岩热化学还原的产物,来源于赋矿地层中的蒸发岩。应用飞秒激光剥蚀多接收器等离子体质谱法首次获得了纳雍枝铅锌矿中方铅矿原位Pb同位素数据,结果显示Pb同位素组成非常集中(^(206)Pb/204Pb=17.828~17.860,均值17.841,^(207)Pb/204Pb=15.648~15.666,均值15.659,^(208)Pb/204Pb=37.922~37.979,均值37.960,n=32),位于上地壳平均Pb演化曲线上,表明其成矿物质具壳源特征,可能来源于基底岩石。综合矿床地质、矿物学、S和原位Pb同位素数据,本文认为纳雍枝铅锌矿床S主要来源于其赋矿地层,Pb等金属元素主要来源于基底岩石,这两组流体的混合是导致其金属硫化物沉淀成矿的重要机制,成矿流体具后生、低温热液等特征,属于密西西比河谷型(MVT)矿床,很可能形成于燕山期,与右江盆地演化有关。

The Nayongzhi Pb-Zn deposit discovered during 2011 to 2015, in the noah-central part of the SE wing Wuzhishan anticline, is located in the western Yangtze Block, Southwest China and is an important component of the NW Guizhou Pb-Zn metallogenic district. In the Nayongzhi mining area, more than twenty Pb-Zn ore bodies have been controlled until now, with more than 1.35 million tones of Zn and Pb metal reserves at grades range from 1.11% to 15.65% Zn and 0. 59% to 0. 97% Pb, respectively, indicating that the Nayongzhi deposit is the largest Pb-Zn deposit in the Guizhou part of the Sichuan-Yunnan-Guizhou （SYG） Pb-Zn metallogenic province. Ore bodies in this deposit mainly occur as stratiform, stratiform-like, lentoid （scrotiform） and steep veined. Besides the steep veined ore bodies occur along F7 thrust fault, the other types of ore bodies were hosted by dolostone of Lower Cambrian Qingxudong Formation and Upper Sinian Dengying Formation and had the similar occurrence to that of the ore-hosting strata. Ore bodies are characterized by massive, brecciated, veinlet and disseminated. Ore minerals are given priority to with sphalerite, followed by galena and pyrite. Gangue minerals are mainly calcite and dolomite, with a small amount of quartz and barite, δ34SCDT values of sulfide minerals range from 15.94‰ to 25.49‰ （ mean 22.41‰, n = 21 ） , of which pyrite has δ34SCDT value of 22. 06‰, sphalerite has δ34SCDT values ranging from 19. 37‰ to 25.49‰ （mean 23. 17‰, n = 17） and galena has δ34SCDT values ranging from 15.94‰ to 19. 70‰ （mean 18. 23‰, n = 3 ）. The δ 34S values of pyrite, sphalerite and galena do not present the feature of δ34 S pyrite 〉 δ34Spyite 〉δ34Sgalena, suggesting that the sulfur isotope fraetionation in hydrothermal system did not reach balance. Together with a small amount of barite in sulfide ores, we believe that the δ4S∑s value of hydrothermal fluid was higher than those of sulfide minerals. Therefore, reduced sulfur in the ore-forming fluid was likely the produce of marine sulfate in the ore-hosting Cambrian and Sinian strata by thermal-chemical reduction. In situ Pb isotopes of galena analyzed by fs LA-MC-ICPMS demonstrate that 206Pb/204apb = 17. 828 - 17. 860 （ average 17. 841 ）, 207 pb/204 Pb = 15. 648 - 15.666 （ average 15. 659 ）, 208 Pb/204 Pb = 37. 922 - 37. 979 （ average 37. 960, n = 32）. These Pb isotopic data plotted into the average evolution curve of upper ernst, indicating a clearly crustal source of Pb and the basement rocks was likely the most sources. Studies on ore deposit geology, mineralogy, S isotopes and in situ Pb isotopes reveal that the Nayongzhi Pb-Zn deposit is a MVT deposit with mixed fluids from basement and wall rocks, respectively, which formed sulfide ores likely during Yanshanian and related to the evolution of the Youjiang basin.