西藏林子宗群火山岩中首次发现低硫化型浅成低温热液型矿床——以斯弄多银多金属矿为例

唐菊兴, 丁 帅, 孟 展, 胡古月, 高一鸣, 谢富伟, 李 壮,袁 梅, 杨宗耀, 陈国荣, 李于海, 杨洪钰, 付燕刚

1)中国地质科学院矿产资源研究所, 国土资源部成矿作用与资源评价重点实验室, 北京 100037;

2)成都理工大学地球科学学院, 四川成都 610059;

3)西藏中瑞矿业发展有限责任公司, 西藏拉萨 850000; 4)中国地质大学(北京), 北京 100083

西藏林子宗群火山岩中首次发现低硫化型浅成低温热液型矿床
——以斯弄多银多金属矿为例

唐菊兴1), 丁 帅2), 孟 展2), 胡古月1), 高一鸣1), 谢富伟2), 李 壮2),袁 梅2), 杨宗耀2), 陈国荣3), 李于海3), 杨洪钰3), 付燕刚4)

1)中国地质科学院矿产资源研究所, 国土资源部成矿作用与资源评价重点实验室, 北京 100037;

2)成都理工大学地球科学学院, 四川成都 610059;

3)西藏中瑞矿业发展有限责任公司, 西藏拉萨 850000; 4)中国地质大学(北京), 北京 100083

西藏冈底斯成矿带分布着大面积古近纪(70–40 Ma)林子宗群火山岩。但如此强烈的火山-岩浆作用,与安第斯成矿带相比(如马力昆带(Franja de Maricunga)、印地—帕斯瓜带(Franja El Indio-Pascua)), 除了碰撞伸展阶段形成的驱龙、甲玛等斑岩-矽卡岩型铜多金属矿床外(23–13 Ma), “缺位”资源规模大、经济价值高的浅成低温热液型矿床成矿亚系列。该类矿床是剥蚀了, 还是没有发现？本文在前人工作基础上, 通过详细的地质勘探、地质填图、地质编录、镜下鉴定、能谱和电子探针分析, 在南木林盆地斯弄多地区林子宗群陆相火山岩中识别出低硫化浅成低温热液型银铅锌(铟镉金)矿床。矿体由产于流纹斑岩中的隐爆角砾岩型银铅锌矿体、火山机构旁侧的热液脉型银铅锌矿体及断裂上盘的银(铅锌)矿体组成, 目前控制Pb+Zn资源量超过30万吨(331+332为主)@Pb+Zn＞5％, Ag资源量超过400吨@Ag＞50 g/t。主要金属矿物为方铅矿、闪锌矿、辉银矿、硫砷铜银矿、黄铁矿, 微量黄铜矿, 主要蚀变组合为: 石英-玉髓-碧玉, 重晶石-萤石, 冰长石-伊利石-绢云母, 菱铁矿-菱锰矿; 矿石构造以脉状、角砾状、网脉状、条带状、层纹状、皮壳状、块状、浸染状等, 发育结晶作用和交代作用形成的矿石结构; 近地表发育古热泉喷口, 堆积条带状、层纹状硅质沉积物。综合上述地质信息, 确定该矿床为典型低硫化浅成低温热液型银多金属矿床。这一重要矿床类型的发现和确定在冈底斯成矿带乃至西藏特提斯成矿省尚属首例, 对冈底斯成矿带广泛发育形成于70～40 Ma的林子宗群火山岩地区区域找矿具有积极和重要的指导意义, 其重要性不容低估。

斯弄多; 林子宗群火山岩; 冰长石-绢云母-伊利石; 低硫化型; 浅成低温热液银多金属; 冈底斯

“浅成低温热液”这一术语最初由Lindgren (1922)对热液矿床按其形成的温度和深度进行分类研究时首次提出。其后, 许多研究者不断对浅成低温热液型矿床含义及分类进行着补充和完善, 目前定义为含矿热液上升至浅地表(＜2 km)、在中低压(＜100 Pa)、中低温度(200～300℃)条件下形成的一类矿床(Lindgren, 1922, 1933; Hendenquist, 1987; White and Hendenquist, 1990; Hendenquist et al., 2000; Corbett, 2002; Simmons et al., 2005)。并根据流体中硫的氧化还原态与蚀变矿物组合划分为高硫化(明矾石-高岭石型)和低硫化(冰长石-绢云母型)两种端元类型(Hendenquist, 1987; Heald et al., 1987; Einaudi et al., 2003; Sillitoe and Hedenquist, 2003)。同时,随着全球范围内大量浅成低温热液型矿床相继被发现, 这类矿床逐渐成为全球金、银、铜、铅、锌等有色资源重要矿床类型之一(Corbett, 2002; 江思宏等, 2004; Sidorov et al., 2015; Páez et al., 2016), 地质学家们逐渐认识到这类矿床主要产于板块俯冲带边缘的岛弧、陆缘弧中(Hendenquist et al., 2000; Corbett, 2002; Richards, 2013), 与同期陆相火山-次火山活动在时空上有着密切联系, 并强调火山作用对成矿的贡献(Sidorov et al., 2015; Nadeau et al., 2016)。

西藏冈底斯带发育的多条不同时期、线性分布的火山-岩浆岩带是形成大-超大型矿床的有利条件,现已发现多个具有重要经济价值的斑岩-矽卡岩型铜多金属矿(如: 甲玛、驱龙、雄村、帮浦等)、矽卡岩型铅锌矿(龙玛拉、洞中拉—洞中松多、亚贵拉等)及热液脉型银铅锌矿床, 使得该区成为我国一条重要铜多金属成矿带(Hou et al., 2009; 郎兴海等, 2012; 唐菊兴等, 2012, 2014a, b; Tang et al., 2015; Zheng et al., 2016)。作为冈底斯构造-岩浆-成矿带中规模最大的林子宗群火山岩, 其东西展布大于1 200 km, 分布范围占冈底斯岩浆带面积的一半以上(图1a)(Mo et al., 2008), 与冈底斯大岩基一起构成冈底斯带最重要的岩浆岩组合, 代表着白垩纪晚期—早新生代(70—40 Ma)青藏高原南部的一次大规模的构造岩浆事件(Ding et al., 2003, 2005; 侯增谦等, 2006)。然而, 如此强烈的火山-岩浆活动是否伴有重要的成矿作用？与安第斯成矿带相比(如马力昆带(La Franja de Maricunga)、印地—帕斯瓜带(Franja El Indio-Pascua)), 是否形成与之类似的浅成低温热液型贵金属矿床？尽管近年来南木林盆地相继发现了诸如纳如松多、则学等多个中-大型矿床,但产于林子宗群中的矿床至今尚未引起足够的重视。赋存于林子宗群火山岩中的斯弄多银多金属矿床的发现, 为深入研究及剖析该套火山岩的形成与成矿关系提供了良好的契机。为此, 本文以新发现的斯弄多银多金属矿为研究对象, 通过详细的地质勘探、地质填图、地质编录、镜下鉴定、能谱和电子探针分析, 确定矿床类型, 对冈底斯成矿带广泛发育林子宗群火山岩地区的区域找矿具有积极和重要的指导意义。

图1 西藏地区构造分区图(a) (根据Hou et al., 2004修改)和冈底斯北带区域地质及矿床分布图(b) (根据Zheng et al., 2016修改)Fig. 1 Sketch tectonic map (a) (modified after Hou et al., 2004) and simplified regional geological map of northeastern Gangdese belt with ore deposits (b) (modified after Zheng et al., 2016)

图2 斯弄多矿区地质图(a)和斯弄多矿区剖面(b)Fig. 2 Geological map of the Sinongduo deposit (a) and section A-A’ of the Sinongduo deposit (b)

1 矿床地质

1.1 成矿地质背景

斯弄多矿区位于西藏自治区谢通门县境内, 大地构造位置处于拉萨地体隆格尔—工布江达弧背断隆带上, 属于冈底斯北缘Pb-Zn-Ag成矿带中段(图1a)。区域地层主要以火山-沉积建造为主, 研究区出露石炭—二叠系碳酸盐-碎屑岩建造, 中生界(J3-K1)浅海相至海陆交互相碎屑岩、碳酸盐岩建造, 新生界林子宗群(E1-2)陆相火山岩(图1b), 该套火山岩自下而上划分为典中组、年波组和帕那组, 成岩年龄集中在64.43～61.45 Ma、54.07 Ma、48.72～43.93 Ma(董国臣等, 2005), 其中典中组表现为弧火山岩特征, 年波组显示为陆缘弧、碰撞和板内环境特征, 帕那组显示为大陆碰撞、板内环境特征, 由南向北岩石碱度增高, 由东向西由偏基性过渡为偏酸性, 反应了由南向北大洋向大陆转换, 岩浆源区具有不均一性的特征(Lee et al., 2009 Chen et al., 2014)。斯弄多矿区含矿围岩为古新统典中组(E1d),其形成时代在60～62 Ma(丁帅等, 未刊数据), 岩性主要包括流纹斑岩、晶屑凝灰岩、火山角砾岩等(图2a)。邻区纳如松多隐爆角砾岩型银铅锌、则学热液脉型铅锌银矿则产于典中组火山岩中(纪现华等, 2012), 拉宗热液脉型银铅锌矿产于帕那组火山岩。区内发现多个火山机构, 具有典型火山角砾岩-流纹斑岩-凝灰岩岩相分带, 火山机构旁侧发育多条张性断裂, 是热液脉型矿体主要赋存空间。矿区岩浆岩主要为黑云母花岗斑岩, 呈岩脉、岩枝分布在13–15号勘探线附近(图2a), 与典中组火山岩接触带多形成热液隐爆角砾岩。

1.2 矿体特征

新发现银铅锌矿体产于典中组火山岩中(前人认为是年波组火山岩), 2013—2014年西藏地质二队开展了卓有成效的地质工作, 初步圈定了矿体, 但在矿床类型确定、矿体特征等方面研究程度较低。通过对钻孔和探矿坑道详细地质编录, 根据赋矿围岩及成矿元素共划分出三种类型矿体, 即: 产于流纹斑岩中隐爆角砾岩型银铅锌矿体、火山机构旁侧受断层破碎带控制的热液脉型铅锌银矿体及断裂上盘的独立银矿体。

隐爆角砾岩型矿体: 位于矿区西侧, 近直立筒状, 目前控制长60 m, 宽约30 m, 厚度约50 m(图2b)。角砾成分主要为流纹斑岩及火山碎屑岩, 呈三角状、板状、椭圆状及不规则状, 大小0.5～10 cm之间, 占整个角砾岩体积约30％～50％, 胶结物主要为岩粉、石英、长石、绢云母、伊利石、菱铁矿、菱锰矿及黄铁矿、方铅矿、闪锌矿等硫化物(图3a)。除了筒状角砾岩型矿体以外, 还在脉状矿体中发育大量脉状、板状隐爆角砾岩型矿石, 与块状、网脉状矿石一同构成脉状矿体。

脉状矿体: 近南北向位于矿区中部, 是矿区规模最大、品位高的银铅锌矿体, 主要产于火山机构旁侧张性断裂中, 呈板状陡倾斜, 矿体走向长度超过300 m, 倾向延伸200 m, 厚度在2～30 m之间(图2b)。方铅矿、闪锌矿等硫化物多呈块状、网脉状、细脉状集合体, 系含矿热液沿构造裂隙充填交代作用形成(图3b)。

独立银(铅锌)矿体: 分布在脉型矿体上盘, 多与红色碧玉及含铁锰碳酸盐矿物密切共生(图3c), Ag平均品位可达400～1 000 g/t。

截止目前, 斯弄多银多金属矿共探明Pb+Zn金属量大于30万吨, Ag金属量超过400吨(331+332类别), 远景规模可达大型。

1.3 矿石特征

矿区矿石呈典型热液矿床的构造, 以块状、角砾状、网脉状为主, 局部发育脉状-网脉状、浸染状矿石, 金属矿物由方铅矿、闪锌矿、黄铁矿、黄铜矿、辉银矿、硫砷铜银矿、黄钾铁矾、赤铁矿和菱铁矿、菱锰矿等组成(据丁帅等, 未刊电子探针数据)。其中方铅矿多呈中细粒自形-半自形与黄铁矿、闪锌矿等矿物产出(图3d); 闪锌矿中多发育固溶体分离结构的黄铜矿(图3e); 银矿物主要为辉银矿、硫砷铜银矿, 除部分以类质同象形式产于方铅矿中外(图3f), 多数辉银矿、硫砷铜银矿等独立银矿物赋存于碧玉及铁锰碳酸盐裂隙中或呈粒间银分布于早期硫化物晶隙间(图3g, h), 与我国江西冷水坑及环太平洋地区浅成低温热液型Ag矿床极为类似(卢燃等, 2012; Chinchilla et al., 2016)。黄铁矿呈浸染状、脉状, 在隐爆角砾岩中呈团斑状, 粒径以1～2 mm为主, 具有多阶段形成特征, 早期黄铁矿被方铅矿、闪锌矿等硫化物交代, 晚期黄铁矿交代闪锌矿。非金属矿物可见石英、斜长石、绢云母、伊利石、玉髓、冰长石、方解石、重晶石, 萤石等(据丁帅等, 未刊电子探针数据)。根据钻孔编录、高光谱测量、光薄片鉴定和矿物能谱分析, 蚀变矿物组合有石英-玉髓-碧玉(图3i, j), 重晶石-萤石, 冰长石-伊利石-绢云母(图3k, l), 碳酸盐矿物组合(包括铁锰碳酸盐岩(图3m)及叶片状方解石(图3n))及表生硅华(图3o)。其中石英-玉髓化分布在1号勘探线附近, 冰长石-伊利石-绢云母化叠加在石英-玉髓化之上, 铁锰碳酸盐分布在5–13号勘探线, 空间上与独立银矿物密切共生。

2 讨论

2.1 低硫化浅成低温热液矿床的确定

Hendenquist(1987)依据流体中硫的氧化还原状态将浅成低温热液型矿床划分出高硫化和低硫化两种类型(Hendenquist, 1987)。前者主要与安山质和流纹质岩浆活动有关, 由酸性、氧化的热液流体形成(White and Hendenquist, 1990; Corbett, 2002; 张德全等, 2005), 以高含量的金铜硫化物及高价硫的明矾石+高岭土等硫酸盐矿物组合为主(Heald, 1987; Hendenquist, 1987; Sillitoe and Hedenquist, 2003; 唐菊兴等, 2014b), 常见硅帽(由块状石英和多孔状石英组合)(Corbett, 2002; 张元厚等, 2009; Elizabeth, 2012); 后者与碱性和偏碱性玄武质-流纹质岩浆活动有关, 由近中性、还原的热流体形成(Corbett,2002; Taylor, 2007), 发育热泉及隐爆角砾岩(Kouhestani et al., 2012), 常见刃片状/叶片状/板状碳酸盐及条带状、层纹状、梳状石英+玉髓+冰长石+绢云母+伊利石组合的蚀变, 以 金、银、铅、锌矿化为主, 伴生铜、锑、硒等元素(Hendenquist, 1987; Simmons et al., 2000; Sillitoe and Hedenquist, 2003;Moncada et al., 2012)。

图3 斯弄多矿区矿石、矿物及蚀变特征Fig. 3 The characteristics of ores, minerals and alterations in the Sinongduo deposita-角砾岩型矿体, 胶结物为黄铁矿、方铅矿及闪锌矿等硫化物; b-热液脉型矿体; c-高品位银矿石; d-半自形粒状方铅矿; e-固溶体分离结构的闪锌矿与黄铜矿; f-辉银矿, 与方铅矿共生体; g, h-碧玉等脉石矿物裂隙中的辉银矿及硫砷铜银矿; i-石英及玉髓脉; j-红色碧玉,发育大量赤铁矿; k-绢云母交代冰长石; l-鳞片状伊利石; m-环状铁菱锰矿; n-叶片状方解石; o-条带状热泉喷口硅华; Py-黄铁矿; Gn-方铅矿; Sph-闪锌矿; Ccp-黄铜矿; Arn-辉银矿; Pe-硫砷铜银矿; Hem-赤铁矿; Q-石英; Cha-玉髓; Jas-碧玉; Ser-绢云母; Aul-冰长石; Ili-伊利石; Sd-菱锰矿; Cal-方解石; Si-硅质矿物a-breccia-type ores with the cement of pyrite, galena, sphalerite and other sulfides; b-hydrothermal vein-type orebody; c-high-grade silver ore; d-subhedral granular galena; e-exsolution texture of sphalerite and chalcopyrite; f-association of argentite and galena; g, h-argentite and proustite in the fissures of jasper and other gangue minerals; i-quartz and chalcedony veins; j-lots of hematites in red jasper; k-adularia replaced by sericite; l-flake illite; m-annular iron-rhodochrosite; n-bladed calcite; o-banded supergene geyserite; Py-pyrite; Gn-galena; Sph-sphalerite; Ccp-chalcopyrite; Arn-argentite; Pe-proustite; Hem-hematite; Q-quartz; Cha-chalcedony; Jas-jasper; Ser-sericite; Aul-adularia; Ili-illite; Sd-rhodochrosite; Cal-calcite; Si-siliceous minerals

斯弄多银多金属矿体赋存于林子宗群火山岩中, 包括产于流纹斑岩中隐爆角砾岩型银铅锌矿体、火山机构旁侧的热液脉型铅锌银矿体及断裂上盘的独立银矿体。与世界上低硫化浅成低温热液型矿床有相似的矿化特征, 发育典型浅成低温热液系统中硅化-冰长石-碳酸盐(铁锰碳酸盐+方解石)蚀变。硅化表现为石英+硅华+玉髓+碧玉等矿物组合(图3i, j), 且表现出自上而下垂直分带特征: 由浅部热泉喷口附近的硅华→中部微细粒石英+玉髓脉→深部梳状、脉状石英。其中这些非晶质硅质矿物大多形成于相对高pH和中低温(＜300℃)环境中(James, 1994), 反映了沸腾热液在浅地表环境下快速冷却沉淀条件(张元厚等, 2009)。冰长石形成温度为180～320℃(Pirajno, 1992), 在近中性-碱性条件下大量钾质蚀变而成(张元厚等, 2009), 后期多受绢云母等含水硅酸盐矿物所交代(图3k)。斯弄多矿区碳酸盐化表现两种形式: 铁锰碳酸盐和方解石(图3m, n), 其中铁锰碳酸盐矿物在空间上与银矿体相伴产出, 与我国江西冷水坑和浙东地区浅成低温热液型银矿床极为相似(魏元柏和赵宇, 1996; 卢燃等, 2012)。一般认为这种铁锰碳酸盐形成于低温(200℃左右), 中性环境中(pH在6.7左右)(Wei and Chen, 1993), 其形成加速了银的沉淀速度, 促进了银矿物(主要是自然银)的析出(魏元柏和赵宇, 1996)。此外,矿区常见叶片状方解石(图3n), 这种方解石多因富挥发分流体快速沸腾, CO2较其他挥发分溶解度偏低, 而从液相中优先强烈分离进入气水相, 促使方解石快速结晶并按扁平习性生长形成(Canet et al., 2011), 是低硫化浅成低温热液型矿床形成过程中存在流体沸腾的有利证据(Simon et al., 1999; Etoh et al., 2002)。以上这些矿物均反映了斯弄多矿区成矿流体具有浅成、低温、中-还原性特征, 具有典型低硫化浅成低温热液型矿床成矿特征(Hendenquist et al., 2000; Corbett, 2002; Sillitoe and Hedenquist, 2003), 这在冈底斯成矿带乃至西藏特提斯成矿省尚属首例。

2.2 区域成矿前景

浅成低温热液型矿床的形成与深部岩浆作用形成的热液体系密切相关, 其深部常发育斑岩型铜多金属矿, 它们共同组成一个完整的火山-岩浆成矿系统(Sillitoe, 2010)。如在我国西藏多龙矿集区,铁格隆南(荣那)矿段由浅部高硫化浅成低温热液与深部斑岩型铜(金、银)型矿体组成, 这是我国目前已发现的规模最大的高硫型浅成低温热液-斑岩型矿床(唐菊兴等, 2014b; 杨超等, 2014; 方向等, 2015;李光明等, 2015)。此外, 唐菊兴等(2014a)按照斑岩矿床成矿系列的“缺位理论”, 提出雄村矿集区具有低硫化浅成低温热液型矿床找矿潜力, 并根据雄村外围洞嘎金矿矿物组合认为其具有浅成低温热液型矿床特征。然而, 近年来在广泛分布的陆相火山岩地区大量浅成低温热液型Au、Ag、Pb、Zn矿床(低硫型)相继被发现, 地质学家们逐渐重视火山作用对成矿的贡献, 同时强调火山作用中火山热液体系是成矿关键因素(Sidorov et al., 2015; Nadeau et al., 2016)。

林子宗群火山岩是冈底斯成矿带乃至青藏高原发育的最大规模的火山岩带, 该套火山岩东西展布大于1 200 km, 分布范围占冈底斯岩浆带面积的一半以上(Mo et al., 2008), 代表着白垩纪晚期—早新生代(70～40 Ma)青藏高原南部的一次大规模的构造岩浆事件(Ding et al., 2003, 2005; 侯增谦等, 2006)。同时也是冈底斯Ag-Pb-Zn矿床重要赋矿层位, 包括纳如松多、斯弄多、则学、扎扎龙等多个中-大型矿床均产于该套火山岩中。斯弄多银多金属矿是林子宗群火山岩中首次确定的低硫化浅成低温热液型矿床, 尽管这套火山岩对成矿作用影响尚未展开深入研究, 但其成矿环境可媲美安第斯成矿带(如马力昆成矿带(La Franja de Maricunga)(John et al., 2001; Richards et al., 2013)、印地—帕斯瓜成矿带(Franja El Indio-Pascua) (Deyella et al., 2005; Bissiga et al., 2015)等火山岩地区, 此外, 斯弄多矿区外围还发现有多个热液隐爆角砾岩筒及古热泉喷口,是斑岩-浅成低温热液型银、金矿床形成的有利条件,有望在该地区实现浅部未被剥蚀的独立金矿体找矿突破。另一方面, 就整个冈底斯成矿带上林子宗群火山岩而言, 其出露区域均与地球化学异常套合较好, Au、Ag等成矿元素均具有明显的浓度分带, 较高的峰值, 指示了林子宗群火山岩具有良好的找矿前景(李光明等, 2004; 谭钢等, 2011), 是寻找斑岩-浅成低温热液型、隐爆角烁岩型、热液脉型金、银、铅、锌多金属矿的有利地段。

3 结论及地质意义

1)斯弄多银多金属矿产于冈底斯成矿带中段南木林火山盆地, 是在林子宗群陆相火山岩中新发现的中大型银多金属矿床, 由产于流纹斑岩中隐爆角砾岩型银多金属矿体、火山机构旁侧的热液脉型铅锌银矿体及断裂上盘的银(铅锌)矿体组成。

2)矿石发育典型的条带状、层纹状、皮壳状、角砾状、网脉状矿石构造, 以石英-玉髓-碧玉-伊利石-绢云母-碳酸盐等蚀变矿物组合为主, 显示该矿床具有典型的低硫化浅成低温热液型矿床典型的矿石组构和蚀变矿物组合。

3)产于林子宗群火山岩中的低硫化浅成低温热液型矿床在冈底斯成矿带上尚属首次识别, 不仅丰富和完善了该区矿床类型, 同时对冈底斯成矿带广泛发育林子宗群火山岩地区的区域找矿具有积极和重要的指导意义, 对于1 200 km带状分布的林子宗群出露区的银(金)多金属矿找矿勘查其意义非同小可。

致谢:感谢西藏中瑞矿业发展有限责任公司黄若朝董事长、李祥总经理为笔者的野外工作和室内工作提供的资助。同时对编辑老师以及审稿专家为本文提出的宝贵意见, 在此深表谢意!

Acknowledgements:

This study was supported by China Geological Survey (No. 12120114068401), and Zhongrui Mining Co., Ltd. (No. XZZR-2015).

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The First Discovery of the Low Sulfidation Epithermal Deposit in Linzizong Volcanics, Tibet: A Case Study of the Sinongduo Ag Polymetallic Deposit

TANG Ju-xing1), DING Shuai2), MENG Zhan2), HU Gu-yue1), GAO Yi-ming1), XIE Fu-wei2), LI Zhuang2), YUAN Mei2), YANG Zong-yao2), CHEN Guo-rong3), LI Yu-hai3), YANG Hong-yu3), FU Yan-gang4)
1) MLR Key Laboratory of Metallogeny and Mineral Resource Assessment, Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing 100037; 2) College of Earth Sciences, Chengdu University of Technology, Chengdu, Sichuan 610059; 3) Zhongrui Mining Co., Ltd., Lhasa, Tibet 850000; 4) China University of Geosciences(Beijing), Beijing 100083

Sinongduo; Linzizong group volcanic rock; valencianite-illite-sericite; low sulfidation; epithermal Ag polymetallic deposit; Gangdise

P317.3; P618.4

A

10.3975/cagsb.2016.04.08

本文由中国地质调查局地质调查项目“西藏雄村-普桑果斑岩-矽卡岩型铜多金属矿成矿地质背景与找矿潜力调查”(编号: 12120114068401)和西藏中瑞矿业发展有限责任公司项目(编号: XZZR-2015)联合资助。

2016-02-27; 改回日期: 2016-04-02。责任编辑: 魏乐军。

唐菊兴, 男, 1964年生。博士, 研究员。主要从事矿床学和固体矿产勘查与评价研究工作。通讯地址: 100037, 北京市西城区百万庄大街26号。E-mail: tangjuxing@126.com。

Abstract:Since the Paleogene (70–40 Ma), volcanic rocks of Linzizong Group have been distributed in a large area in Gangdise metallogenic belt, Tibet; nevertheless, there is an “absence” of mass resource and high economic value epithermal deposit except for the Qulong and Jiama porphyry-skarn copper polymetallic deposit (23–13 Ma) formed in the collision-stretch phase, in contrast with the Andean metallogenic belt (such as La Franja de Maricunga, Franja El Indio-Pascua) formed under the condition of extensive volcanic -magmatism. Are these deposits denudated or even not found yet? The authors have recognized a type of low sulfidation epithermal lead-zinc (In-Cd-Au) deposit in Linzizong continental volcanic rock group in Sinongduo of Namling basin, based on the detailed exploration, geological mapping, geological logging, microscopic examination, energy spectrometer analysis, electron microprobe analysis (EPMA) and the result of previous researchers. The orebodies consist of cryptoexplosion breccia type Ag-Pb-Zn orebody hosted in liparophyre, hydrothermal vein type Ag-Pb-Zn orebody beside volcanic edifice, and Ag (Pb-Zn) orebody on the hanging side of the fault. The amount of the controlled resource of Pb+Zn is over 0.3 million tons (331+332)@Pb+zinc＞5％, and resource amount of Ag@Ag＞50g/t is more than 400 tons. The major metallic minerals are galena, sphalerite, and argentite, pearceite, and pyrite together with rare chalcopyrite. Quartz-chalcedonite-jasper, siderite-rhodochrosite, barite-fluorite, and valencianite-illite-sericite are main associations of alteration. The main structures include veined sturcture, brecciated, mesh-veined, banded and laminated, crustified, massive and disseminated structures, the ore textures of this deposit are developed on the basis of crystallization and metasomatism. The authors found a vent of ancient hot spring near the surface, around which there are some banded and laminated siliceous sediments. These geological findings led the authors to believe that this is a typical low-sulfidation epithermal Ag polymetallic deposit. This is for the first time a deposit of this significant type was found and verified in Gangdise belt and even in Tethys metallogenic province. The essentiality for guiding exploration in the area where Linzizong Group volcanic rock was developed in 70–40 Ma in Gangdise belt would be very obvious.