西藏雄村斑岩型铜金矿集区I号矿体的硫、铅同位素特征及其对成矿物质来源的指示

郎兴海,唐菊兴,李志军,黄 勇,丁 枫,王成辉,张 丽,周 云

1)成都理工大学地球科学学院,四川成都 610059;

2)中国地质科学院矿产资源研究所,北京 100037;

3)中国地质调查局成都地质调查中心,四川成都 610081

西藏雄村斑岩型铜金矿集区I号矿体的硫、铅同位素特征及其对成矿物质来源的指示

郎兴海1),唐菊兴2),李志军1),黄 勇1),丁 枫1),王成辉2),张 丽3),周 云1)

1)成都理工大学地球科学学院,四川成都 610059;

2)中国地质科学院矿产资源研究所,北京 100037;

3)中国地质调查局成都地质调查中心,四川成都 610081

西藏雄村斑岩型铜金矿集区是近年来西藏冈底斯斑岩铜矿带内发现的一处超大型铜金矿集区,其形成于与新特提斯洋向北的洋内俯冲作用有关的岛弧环境,成矿时代为中侏罗世。该矿集区位于冈底斯火山-岩浆弧的中段南缘,其南侧紧邻日喀则弧前盆地,目前探明I(原命名为雄村铜矿床)、II、III号铜金矿体规模达大型-超大型,同时还存在多个矿化异常带。本文以雄村I号矿体为研究对象,对雄村I号矿体含矿斑岩、赋矿凝灰岩和主要硫化物的硫、铅同位素开展研究,结果表明: ①含矿斑岩、赋矿凝灰岩和主要硫化物具有较为一致的硫同位素组成,δ34SCDT变化范围为–3.5‰～2.7‰,平均–1.07‰,十分接近于零,塔式分布效应显著,硫可能主要来自地幔; ②含矿斑岩、赋矿凝灰岩和主要硫化物具有相对一致的铅同位素组成,均以放射性成因铅含量低为特征,206Pb/204Pb、207Pb/204Pb和208Pb/204Pb变化范围分别为18.369～18.752、15.473～15.589和 38.389～39.1531,位于地幔与造山带铅演化线之间,并且相对靠近地幔铅演化线,显示出铅主要来源于地幔,可能有少量地壳物质的混染。通过西藏冈底斯斑岩铜矿带碰撞造山环境和岛弧环境(以雄村 I号矿体为代表)斑岩型铜矿床的硫、铅同位素组成特征对比,认为两者的成矿物质来源是相似的,碰撞造山环境的地壳物质混染较强烈,而岛弧环境的地壳物质混染较弱。

硫同位素; 铅同位素; 雄村; 斑岩铜矿床; 西藏冈底斯带

西藏冈底斯斑岩铜矿带产于冈底斯火山-岩浆弧内,近年来该带的资源评价工作取得了重要进展,已发现的矿床(点)主要位于东起工布江达县、西到昂仁县的区域,大致分布于雅鲁藏布江北岸20～60 km范围内(以25～35 km 最为集中),发现有雄村、驱龙、甲玛、沙让、冲江、厅宫、白容、朱诺、程巴、冲木达、拉抗俄、达布、吹败子、吉如等斑岩型矿床,总体具有东西成带、北东成群分布特征(芮宗瑶等,2003a,b; 李光明等,2005,2006; 曲晓明等,2001;王全海等,2002; 侯增谦等,2003; 郑有业等,2007;黄志英等,2004; 张洪涛等,2004; 唐菊兴等,2009b,2010a,b,2012; 胡正华,2011; 郎兴海等2010c,2012a)。

雄村斑岩型铜金矿集区位于西藏冈底斯斑岩铜矿带西段,是近年来西藏冈底斯斑岩铜矿带上发现的以铜为主伴生金、银的超大型铜金矿集区,属西藏日喀则地区谢通门县荣玛乡管辖。雄村斑岩型铜金矿集区大地构造位置属冈底斯火山-岩浆弧的中段南缘,其南侧紧邻日喀则弧前盆地(郎兴海等,2010c),研究表明其形成于与新特提斯洋向北的洋内俯冲作用有关的岛弧环境,成矿时代为中侏罗世(唐菊兴等,2009b; 郎兴海等,2010c)。在雄村斑岩型铜金矿集区,目前探明I(原命名为雄村铜矿床)、II、III号铜金矿体规模达大型-超大型,同时还存在多个矿化异常带。三个主矿体呈北西-南东向展布,II号矿体位于I号矿体(原雄村铜矿床)北西向约3.4 km处,III号矿体位于II号矿体北西向约2.1 km处(图1)。雄村斑岩型铜金矿集区的地质研究工作起步较晚,其中以雄村I号矿体(原命名为雄村铜矿床)研究最为详尽(Qin et al.,2005; 徐文艺等,2005,2006a,b;丁枫等,2006; 张丽,2007; 唐菊兴,2007,2009a,c,2010; 曲晓明等,2007a,b; 郎兴海等,2010a,b,c,2011,2012b; 黄勇等,2011a,b; 黄勇,2012; 丁枫,2012),但其成矿物质来源尚未得到明确的认识。尽管丁枫等(2006)、徐文艺等(2006b)、曲晓明等(2007b)、黄勇等(2011a)对I号矿体的成矿物质来源有过研究,但由于测试数据较少且不够全面和系统,缺乏对成矿物质来源的深入探讨,因此,本文将在前人研究基础之上,通过系统研究雄村 I号矿体的含矿斑岩、赋矿凝灰岩和主要硫化物的硫、铅同位素组成,分析其成矿物质来源,并与区域上的斑岩型铜矿床进行对比,初步探讨新特提斯洋俯冲阶段岛弧环境与印度-亚洲大陆碰撞造山环境形成的斑岩铜矿床成矿物质来源的异同,为深入研究雄村斑岩型铜金矿集区的成矿作用乃至整个冈底斯成矿带构造-岩浆演化与成矿作用奠定基础。

1 雄村I号矿区地质概况

雄村 I号矿区出露地层(图 1)主要为中-下侏罗统雄村组(J1-2x)和全新统崩积物-冲积物(唐菊兴等,2006)。雄村组分为酸性凝灰岩、安山质凝灰岩、石英玄武质砂岩、玄武质砂岩夹粉砂岩、粉砂岩夹泥质岩等岩性段,其中酸性凝灰岩和安山质凝灰岩是雄村 I号矿体的主要赋矿岩石之一,形成时间为176±5 Ma(唐菊兴等,2010a)。

雄村 I号矿区发育成矿前、成矿期和成矿后岩浆岩(图1)。成矿前岩浆岩主要为角闪石英闪长玢岩,侵入时间为173±3 Ma(唐菊兴等,2010a)。成矿期岩浆岩为含眼球状石英斑晶的角闪石英闪长玢岩,侵入时间为164.3±1.9 Ma(Tafti et al.,2006),该岩体是雄村 I号矿体的含矿斑岩体,斑晶主要为石英和角闪石,石英斑晶呈方形或浑圆状(图 2a,b),含量为10%～15%(有时大于 15%),粒径为 1～1.5 cm,角闪石斑晶呈长条状自形斑晶,含量 10%,粒径1～2 cm,基质主要由细粒的石英、长石、角闪石及少量的黑云母组成。成矿后岩浆岩主要为始新世侵位的黑云母花岗闪长岩、斜长闪长玢岩、安山岩脉、花岗细晶岩脉和煌斑岩脉,其中黑云母花岗闪长岩侵位时间为 46.96±0.42 Ma(唐菊兴等,2009c,2010a); 煌斑岩脉侵位时间为49.59±0.58 Ma(唐菊兴等,2009c,2010a); 安山岩脉早于黑云母花岗闪长岩,花岗细晶岩脉晚于黑云母花岗闪长岩; 斜长闪长玢岩早于安山岩脉,其侵位的准确时间还未确定。

雄村I号矿区断裂构造发育(图1),主要呈北西、北西西或北北西向展布,其中 F1主断层展布于矿区南部,F2主断层展布于矿区中部,两断层均具有多期次活动的特点,为成矿后断层,呈 265°～280°走向,倾向北,倾角 40°～75°不等,表现出先压后张和先韧性后脆性的特征,沿两断层普遍分布有断层角砾岩和构造蚀变岩,断裂破碎带内及上盘近侧矿石破碎; 次级断层大多具有陡倾特点,呈北西、北东或北北西走向,倾向北东或东,均为成矿后断层,切穿矿体和F1、F2主断层。

雄村 I号矿体产于含眼球状石英斑晶的角闪石英闪长玢岩及其接触带的凝灰岩中,属斑岩型铜金矿床,已经完成勘探工作,进入矿山开采阶段。矿体(按50 m×50 m勘探网度控制)形态平面上呈大透镜状,宽度(南东向)约 300～600 m,长度(东西向)约1000～1400 m,走向北西-南东(图1)。在南北向剖面上呈似层状－层状,倾向北东,倾角40°～53°。该矿体总体特征是规模大(含矿斑岩及接触带凝灰岩全岩矿化)、富金而贫钼(Au 0.56×10-6,Mo 19.7×10-6)、蚀变强烈,已探明铜资源量 100多万吨、伴生金资源量100多吨、伴生银资源量900多吨(Cu 0.41%,Au 0.56×10-6,Ag 19.7×10-6)。

由于含矿斑岩体的侵位,雄村 I号矿体的围岩蚀变作用极为强烈,主要蚀变类型有钾硅酸盐化蚀变、强硅化蚀变、黄铁绢英岩化蚀变和青磐岩化蚀变,其中重要赋矿蚀变为钾硅酸盐化蚀变和强硅化蚀变。除上述热液成矿期的蚀变类型外,在成矿后侵位的黑云母花岗闪长岩岩基的接触带发育一个强烈的角岩化带,该期蚀变基本无矿化; 在成矿前侵位的角闪石英闪长玢岩中发育强烈的钠化-钙化蚀变。矿石构造主要为细脉—浸染状或网脉状,主成矿元素以 Cu 为主,伴生有用组分为 Au、Ag、Zn和Pb等,伴生有害元素(如As、Sb、Bi等)含量极少;主要的脉体类型有磁铁矿-硫化物脉、黑云母硫化物脉、石英硫化物脉、黄铁矿-磁黄铁矿-黄铜矿脉、黄铁矿脉、多金属硫化物脉,其中主要赋矿脉体为石英硫化物脉和黑云母硫化物脉。矿石中主要金属矿物为黄铜矿、黄铁矿和磁黄铁矿,次要金属矿物为闪锌矿、方铅矿、辉钼矿、辉铜矿、毒砂、辉砷铜矿和蓝辉铜矿等; 主要非金属矿物为石英、红柱石和绢云母,次要非金属矿物为长石、黑云母、绿泥石和绿帘石等。

雄村 I号矿体的矿化具有多期多阶段性。根据对矿石结构、构造、有用组分及相关关系的研究,结合野外观察的情况,将成矿过程分为岩浆期、矽卡岩期、热液成矿期、表生期。热液成矿期是主要的成矿期,分为早期石英硫化物阶段(硫化物大量沉淀,形成矿体)、晚期石英多金属硫化物阶段(主要形成多金属硫化物脉,出现方铅矿、闪锌矿以及有 Au和Ag等的局部富集)。在表生期,雄村I号矿体经历了发育较弱的表生(次生)作用。岩浆期和矽卡岩期的矿化作用极弱。

2 样品采集及测试

本文采集了雄村I号矿体含矿斑岩(含眼球状石英斑晶的角闪石英闪长玢岩)、赋矿凝灰岩以及主要硫化物(黄铁矿、磁黄铁矿、黄铜矿、闪锌矿)进行硫、铅同位素测定,样品共计15件,均采自钻孔岩芯。样品由核工业北京地质研究院同位素室完成测试,硫同位素测试仪器采用MAT-251质谱计,检测方法和依据为DZ/ T0184.14-1997《硫化物中硫同位素组成的测定》,测试结果见表1; 铅同位素测试仪器采用 ISOPROBE-T热电离质谱仪,检测方法和依据为GB/ T17672-1999《岩石中铅锶钕同位素测定方法》,测试结果见表1。

3 硫、铅同位素特征及成矿物质来源

3.1 硫同位素特征

雄村I号矿体硫化物的δ34SCDT较为均一,变化范围为–3.5‰～2.7‰,平均–1.06‰,其中黄铁矿的δ34SCDT为–2.92‰～2.7‰(平均–0.61‰),黄铜矿为–1.1‰～–1.7‰(平 均 -1.4‰), 闪 锌 矿 为 –0.8‰～–3.5‰(平 均 –1.84‰), 磁 黄 铁 矿 为 –1.1‰ ～–3.1‰(平均-2.3‰)。

雄村I号矿体含矿斑岩和赋矿凝灰岩的δ34SCDT也较为均一,变化范围为–1.7‰ ～ –0.4‰,平均–0.775‰,其中含矿斑岩的δ34SCDT为–1.7‰ ～–0.4‰(平 均 –1.05‰),凝 灰 岩 的 为 –0.6‰ ～–0.4‰(平均–1.05‰)。

雄村 I号矿体含矿斑岩、赋矿凝灰岩以及主要硫化物的δ34SCDT与典型斑岩铜矿床的大体相似(图3)。

3.2 铅同位素特征

雄村 I号矿体主要硫化物的铅同位素组成较为均一,放射性成因铅含量较低,206Pb/204Pb、207P b/204P b 和208P b/204P b 变化范围为18.034～ 8.425、15.473～15.589 和 37.918～38.593,其中黄铁矿分别为 18.104～18.422、15.473～15.589和37.923～ 38.593,黄铜矿分别为 18.384～18.425、15.78～15.581和 38.491～38.584,闪锌矿分别为18.034～18.408、15.479～15.581 和 37.918～38.562,磁黄铁矿分别为 18.145～18.15、15.511～15.516 和38.065～38.074。

雄村 I号矿体含矿斑岩和赋矿凝灰岩的铅同位素组成也较为均一,放射性成因铅含量较低,与硫化物的铅同位素组成相近,206Pb/204Pb、207Pb/204Pb和208Pb/204Pb 变化范围为 18.17～18.247、15.517～15.525和 38.087～38.196,其中含矿斑岩分别为 18.17～18.183 、 15.517～15.525 和 38.087～38.196,凝灰岩分别为 18.183～18.247、15.527～15.549 和38.125～ 38.178。

3.3 成矿物质来源

硫、铅同位素示踪是一种研究成矿物质来源的有效手段(Zartman et al.,1981; Rollinson,1993; Zhu et al.,2011)。

在207Pb/204Pb-206Pb/204Pb和208Pb/204Pb-206Pb/204Pb 图解(图 4),含矿斑岩和硫化物投点于地幔与造山带铅演化线之间,并且相对靠近地幔铅演化线,这说明Pb主要来源于地幔,可能有少量地壳物质的混染。含矿斑岩和硫化物的铅同位素组成特征相似,说明硫化物中的Pb主要来自斑岩体,也说明成矿流体从围岩中萃取成矿物质的作用不强,主要成矿物质还是来自斑岩本身。

雄村 I号矿体缺乏硫酸盐,说明成矿流体中主要为还原性硫,硫化物的平均δ34S值可以代表成矿流体的δ34S值。矿石的主要硫化物是黄铁矿、磁黄铁矿、黄铜矿和闪锌矿,它们硫同位素组成特征相似,变化范围较窄(–3.5‰～2.7‰),平均值为–1.06‰,十分接近于零(图 3),塔式分布效应显著,与陨石硫的同位素组成特征相似,说明硫可能主要来自地幔,与 Pb来源相似。雄村 I号矿体的含矿斑岩体(含眼球状石英斑晶的角闪石英闪长玢岩)的δ34S值变化范围为–0.4‰至–1.7‰,平均–1.05‰,与硫化物的硫同位素组成特征相似,说明硫化物的硫主要来自斑岩体,地壳物质的混染较弱。

综上所述,雄村 I号矿体的成矿物质是由含矿斑岩带来,主要来自地幔,地壳物质的混染作用不强。

4 讨论

已有的研究资料表明,西藏冈底斯斑岩铜矿带存在三个构造演化阶段的斑岩成矿作用(唐菊兴等,2009b; 郎兴海等,2010c): ①新特提斯洋俯冲阶段的斑岩成矿作用(65～200 Ma),仅发现有雄村斑岩型铜金矿集区,形成于与新特提斯洋洋内俯冲作用有关的岛弧环境; ②印度—亚洲大陆主碰撞汇聚阶段的斑岩成矿作用(65～40 Ma),发现有吉如斑岩型铜(钼)矿、沙让斑岩型钼矿和冲木达斑岩-矽卡岩型铜金矿; ③印度—亚洲大陆后碰撞伸展阶段的斑岩成矿作用(25～0 Ma),是西藏冈底斯斑岩铜矿带的主要成矿期,发现有众多的斑岩型矿床,如甲玛、冲江、吹败子、厅宫、达布、拉抗俄、南木、驱龙、朱诺、程巴、白容等。曲晓明等(2002)、孟祥金等(2006)和 Qu等(2007)对西藏冈底斯斑岩铜矿带印度-亚洲大陆碰撞造山环境的斑岩型铜矿床S、Pb同位素组成及成矿物质来源进行了较为系统的研究,将他们的研究结果与本文研究成果进行对比可以得出一些初步的认识: ①碰撞造山环境和岛弧环境斑岩型铜矿床的含矿斑岩和矿石硫化物的δ34SCDT变化范围分别为–6.3‰～1.2‰(平均–1.27‰)和–3.5～2.7‰(平均–1.07‰)(图 3),两者的δ34SCDT塔式分布效应显著,硫可能主要来自地幔,但碰撞造山环境的含矿斑岩和矿石硫化物的δ34SCDT更偏负值,这可能说明有更多的地壳物质混染; ②碰撞造山环境和岛弧环境斑岩型铜矿床的含矿斑岩和矿石硫化物的206Pb/204Pb、207Pb/204Pb、208Pb/204Pb 变化范围分别为18.369～ 18.752、15.473～15.589、38.389～39.1531 和18.034～ 18.425、15.502～15.7329、37.918～38.593,前者相较于后者更富放射性成因铅,表明有更多地壳物质的混染。在207Pb/204Pb-206Pb/204Pb 和208Pb/204Pb-206Pb/204Pb 图解上(图 4),碰撞造山环境的位于地壳、造山带与地幔铅演化线之间,更趋向于造山带铅演化线,而岛弧环境的位于地幔与造山带铅演化线之间,更趋向于地幔铅演化线,两者构成一条线性关系很好的由地壳到造山带至地幔铅的演化线,这种铅同位素组成特征可能暗示两者具有相似的铅源,但地壳物质的混染程度不同,碰撞造山环境的有更多的地壳物质混染,而岛弧环境的地壳物质混染较弱。根据碰撞造山环境和岛弧环境的斑岩型铜矿床的硫、铅同位素组成特征对比,两者成矿物质来源是相似的,但由于碰撞造山环境地壳很厚(约70 km),含矿斑岩岩浆上侵过程中地壳物质混染作用较为强烈,而岛弧环境地壳较薄或缺失,含矿斑岩岩浆上侵过程中地壳物质混染较弱。

5 结论

1)雄村I号矿体含矿斑岩、赋矿凝灰岩和主要硫化物具有较为一致的硫同位素组成,δ34SCDT变化较小,变化范围为–3.5‰～2.7‰,平均–1.07‰,十分接近于零,塔式分布效应显著,与陨石硫的同位组成特征相似,硫可能主要来自地幔,地壳物质混染较弱。

2)雄村I号矿体含矿斑岩、赋矿凝灰岩和主要硫化物具有相对一致的铅同位素组成,均以放射性成因铅含量低为特征,206Pb/204Pb、207Pb/204Pb和208Pb/204Pb 变 化 范 围 为 18.369～18.752、15.473～ 15.589 和 38.389～39.1531,位于地幔与造山带铅演化线之间,并且相对靠近地幔铅演化线,显示出铅主要来源于地幔,可能有少量地壳物质的混染。

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Sulfur and Lead Isotope Characteristics of No.I Ore Body in the Xiongcun Porphyry Copper-Gold Ore Concentration Area of Tibet:Implications for the Source of Metals in the Ore Deposit

LANG Xing-hai1),TANG Ju-xing2),LI Zhi-jun1),HUANG Yong1),DING Feng1),WANG Cheng-hui2),ZHANG Li3),ZHOU yun1)
1)College of Earth Science,Chengdu University of Technology,Chengdu,Sichuan610059;
2)Institute of Mineral Resources,Chinese Academy of Geological Sciences,Beijing10037;
3)Chengdu Center of China Geological Survey,Chengdu,Sichuan610081

The Xiongcun porphyry copper-gold ore concentration area,one of the large-size metallogenic areas discovered along the Gangdise porphyry copper belt,occurs in an island arc environment associated with northward intra-oceanic subduction of Neo-Tethys oceanic crust.The ore-forming age of the Xiongcun porphyry copper-gold ore concentration area is middle Jurassic.It is located in the middle of the southern margin of theGangdise orogenic belt,and its south margin is Shigatse forearc basin.The latest exploration data indicate that there are several potential mineralization-alteration areas and three large-size copper-gold ore bodies (No.I,II and III) in the Xiongcun porphyry copper-gold ore concentration area.In this paper,Xiongcun No.I ore body was chosen as the research object.According to sulfur and lead isotope composition of ore-baring porphyry,tuff and main sulfides of the ore,the authors have reached the following two conclusions: 1) Sulfur isotopic compositions vary in a narrow range of δ34S values from −3.5‰ to +2.7‰ (−1.07‰ on average) and are close to zero,indicating a mantle sulfur signature.2) They have uniform Pb isotope compositions with low content of radiogenic Pb.The206Pb/204Pb,207Pb/204Pb,and208Pb/204Pb ratios vary in the ranges of 18.104-18.432,15.473-15.533 and 37.918-38.3072,respectively.They are located in the transition zone of mantle Pb and orogen Pb but are closer to the former,reflecting a mixing between the mantle material and minor crust material.Through a comparative study of sulfur and lead isotopes of porphyry copper deposits of the island arc environment (e.g.,Xiongcun No.I ore body) and those of the collisional orogenic environment in the Gangdise porphyry copper belt,the authors found that they have similar metal sources,but porphyry copper deposits of the island arc environment have experienced weak contamination of crust materials,whereas porphyry copper deposits of the collisional orogenic environment have experienced strong contamination of crust materials.

sulfur isotope; lead isotope; Xiongcun; porphyry copper-gold deposit; Gangdise belt in Tibet

P588.13; P597 文献标志码: A doi: 10.3975/cagsb.2012.04.07

本文由国家自然科学基金(编号: 41172077)、国家973项目(编号: 2011CB403103)、西藏天圆矿业资源开发有限公司项目(编号: 2006G-07)和中国地质调查局地调项目“西藏大型矿床成矿专属性研究”(编号: 资[2012]03-002-055)联合资助。

2012-06-03; 改回日期: 2012-06-21。责任编辑: 魏乐军。

郎兴海,男,1982年生。博士。从事矿床学、矿产普查与勘探的教学和研究工作。E-mail: langxinghai@126.com。