岩石学报  2019, Vol. 35 Issue (7): 2026-2044, doi: 10.18654/1000-0569/2019.07.06   PDF    
冈底斯中段白容-白容西斑岩铜钼矿区的岩浆-热液-矿化中心厘定
邹心宇1,2,3, 秦克章1,2,3, 李光明1,2,3, 张西平4, 赵俊兴1,3, 张裴培5, 张夏楠1,3, 张金树6     
1. 中国科学院矿产资源研究重点实验室, 中国科学院地质与地球物理研究所, 北京 100029;
2. 中国科学院大学, 北京 100049;
3. 中国科学院地球科学研究院, 北京 100029;
4. 中国铜业有限公司, 北京 100082;
5. 云南铜业(集团)有限公司, 昆明 650000;
6. 西藏大学, 西藏 850000
摘要: 尼木矿集区位于中新世冈底斯斑岩成矿带中段,由白容矿区、白容西矿区、岗讲斑岩铜钼矿床、冲江斑岩铜钼矿床、厅宫斑岩铜钼矿床组成。前人对其内的岗讲、冲江、厅宫开展了较为系统的矿床学研究,然而对白容和白容西新区的研究还鲜有涉及。白容-白容西矿区为白容矿区和白容西矿区的统称,由于矿区内矿化分散、岩浆岩类型复杂,无明显中心式蚀变分带、无典型的成矿斑岩体,"白容-白容西矿区的岩浆-热液-矿化中心在哪里"成为了制约生产和科研工作进一步开展的最大问题。本文就这一关键问题展开了详细研究,基于详细的地表观察、钻孔岩芯编录,结合光学显微镜和成岩成矿年代学分析,取得了以下主要认识:(1)白容-白容西矿区属于同一个斑岩系统;(2)矿区岩浆侵位序列为黑云母二长花岗岩、二长花岗斑岩、花岗闪长斑岩、英安斑岩、煌斑岩,其中二长花岗斑岩、花岗闪长斑岩与成矿有关,锆石U-Pb年龄分别为13.9±0.2Ma和13.8±0.2Ma,英安斑岩和煌斑岩破坏矿体,锆石U-Pb年龄分别为12.6±0.4Ma和11.1±0.2Ma,矿体的单点辉钼矿Re-Os模式年龄在13.35±0.19Ma至13.82±0.20Ma之间;(3)岩浆-热液-矿化中心在白容-白容西矿区中部区域,尚未被钻孔控制;(4)当前的钻探工程仅揭露了斑岩系统的顶部,钻孔中主要揭露的是泥化带,但少量深部钻孔中揭示的高温脉系,暗示着蚀变和矿化"有根"并且往深部延伸。综上所述,白容-白容西是一个完整的斑岩系统的顶部,有着清晰的岩浆-热液-矿化中心,深部有着巨大潜力。从而为寻找岩浆-热液-矿化中心提供了重要参考,为进一步的矿床学研究提供了重要的宏观认识,为深部找矿提供了依据。
关键词: 白容-白容西斑岩铜钼矿区    锆石U-Pb年龄    辉钼矿Re-Os年龄    热液蚀变    岩浆侵位序列    冈底斯成矿带    
The magmatic-hydrothermal-mineralized center of Bairong-Bairongxi porphyry Cu-Mo district at the Gangdese belt, southern Tibet
ZOU XinYu1,2,3, QIN KeZhang1,2,3, LI GuangMing1,2,3, ZHANG XiPing4, ZHAO JunXing1,3, ZHANG PeiPei5, ZHANG XiaNan1,3, ZHANG JinShu6     
1. Key Laboratory of Mineral Resources, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China;
2. University of Chinese Academy of Sciences, Beijing 100049, China;
3. Institutions of Earth Science, Chinese Academy of Sciences, Beijing 100029, China;
4. China Copper Co., Ltd., Beijing 100082, China;
5. Yunnan Copper Co., Ltd., Kunming 650000, China;
6. Tibet University, Lassa 850000, China
Abstract: The Bairong-Bairongxi porphyry Cu-Mo district is located in the middle part of Gangdese porphyry copper belt in southern Tibet, China. It has sparse mineralization, complicated magmatic intrusions, no central alteration zonation and no typical ore-related porphyry. Therefore, 'where is the magmatic-hydrothermal-mineralized center of Bairong-Bairongxi' is becoming the key of further studies. To address this issue, based on detailed geological survey and drills records, combined with the observation of microscope, zircon U-Pb dating and molybdenite Re-Os dating analysis, we concluded the following points:(1) The Bairong-Bairongxi is not two separated ore districts but belongs to an united porphyry mineralization system; (2) The magmatic sequences include biotite monzonitic granite, monzonitic granite porphyry (13.9±0.2Ma), granodiorite porphyry (13.8±0.2Ma), dacite porphyry (12.6±0.4Ma) and lamprophyre (11.1±0.2Ma); (3) The Re-Os age of molybdenite is from 13.35±0.19Ma to 13.82±0.20Ma; (4) The magmatic-hydrothermal-mineralized center is in its central regions; (5) Most of drillings are located in the argillic alteration zone but deep drillings revealed medium-high temperature veinlets, indicating the alteration has its origin in the deep. As mentioned above, Bairong-Bairongxi is in the top of a Cu-Mo porphyry system with clear magmatic-hydrothermal-mineralized center, and has great potential in Cu-Mo mineralization in the deep. This work gives an overview picture of Bairong-Bairongxi porphyry Cu-Mo district and is helpful for further exploration and studies.
Key words: Brong-Bairongxi porphyry Cu-Mo district    Zircon U-Pb age    Molybdenite Re-Os age    Hydrothermal alteration    Magmatic intrusion series    Miocene Gangdese porphyry copper belt    

冈底斯斑岩铜钼成矿带位于南拉萨微地体的冈底斯岩基之中,是近20年发现的产出于碰撞造山带主碰撞、晚碰撞至后碰撞背景中的成矿带(Li et al., 2006; 侯增谦等, 2007; 杨志明和侯增谦, 2009; Qin, 2012; Mao et al., 2014; 秦克章等, 2014; Zhao et al., 2014; Chen et al., 2015, 2016)。冈底斯带中带发育了一系列串珠状分布的斑岩铜钼矿床,其中段尼木矿集区,集中产出有白容、岗讲、厅宫、冲江等斑岩铜钼矿床(图 1),具有极大的找矿潜力(王小春等, 2002; 徐德章, 2006)。前人对岗讲、冲江和厅宫开展了较为系统的矿床学研究工作,发现成矿与中酸性、埃达克质班状侵入体有关,成岩-成矿-蚀变年龄集中在17~12Ma(李光明和芮宗瑶,2004; 林武等,2004; 李金祥等, 2007; 冷成彪等, 2010; 杜等虎等, 2012; 周维德等, 2014),具有较深的剥蚀程度(冷成彪等,2010)。然而,岩浆岩类型复杂、岩枝众多(如岗讲, 冷成彪等, 2010; Leng et al., 2013)、难以区分含矿岩体与非含矿岩体(如冲江, 刘波等, 2004; 郑有业等, 2004)等问题制约着人们对尼木矿集区斑岩矿床的进一步理解。目前,前人对白容-白容西矿区尚未开展系统的研究工作,仅有李金祥等(2007)报道了白容矿区的热液蚀变年龄。不同于尼木矿集区内其它的斑岩矿床,白容-白容西矿区地表发育大量电气石脉、广泛发育低温硅化-粘土化-黄铁矿化蚀变、火山岩盖层中发育青磐岩化蚀变、矿化分散、未见中心式的蚀变分带、无典型的成矿斑岩体,这些现象多年来困惑着生产与科研工作。那么白容-白容西矿区是否具有成矿潜力?如果有,其岩浆-热液-矿化中心又在哪里?

图 1 拉萨地体冈底斯岩浆带地质简图及中新世主要斑岩铜钼矿床(据Wang et al., 2018修改 Fig. 1 Geology of the Gangdese magmatic belt in the Lhasa terrane, with major Miocene porphyry deposits Cu-Mo deposits (modified after Wang et al., 2018)

基于以上问题,我们对白容-白容西进行了为期4年的野外工作,进行了全面的地表追索和钻孔岩芯的详细观察与编录;结合光学显微镜和成岩成矿年代学分析,揭示了岩浆侵位序列、成岩成矿年代学、蚀变、矿化、脉系特征及其空间变化规律,结合化探与物探资料,推测白容-白容西矿区不是分立的独立矿化系统,而是统一斑岩成矿系统的两翼。厘定岩浆-热液-矿化中心在白容-白容西矿区的中部区域,主矿体埋深较大,并初步构建了矿床形成模式。

1 成矿地质背景

白容-白容西位于拉萨地体南部的冈底斯岩浆弧之中(图 1)。拉萨地体的北界为班公湖怒江缝合带,南界为雅鲁藏布江缝合带(Zhu et al., 2011, 2013),由前寒武纪的结晶基底、古生代-中生代的地层和古生代-新生代的岩浆岩组成(潘桂棠等, 2006; Zhang and Santosh, 2012; Lin et al., 2013; Hu et al., 2018)。在拉萨地体上,广泛发育了与新特提斯大洋岩石圈俯冲有关的大洋岩石圈俯冲阶段(220~65Ma)和大陆碰撞阶段(65~8Ma)的岩浆活动,大陆碰撞阶段又可以划分为印度-亚欧大陆碰撞同期(65~40Ma)和碰撞后汇聚过程(40~8Ma) (张泽明等, 2019)。在大陆碰撞阶段,冈底斯岩浆弧内广泛发育了古新世-始新世Ⅰ型侵入岩和林子宗火山岩(Mo et al., 2008; Zhu et al., 2015, 2017),他们的形成主要与新特斯板片在69~53Ma的回转和可能的53~50Ma板片断裂有关(Kapp et al., 2005; Wang et al., 2015; Zhu et al., 2015)。在55~45Ma和45~30Ma之间,青藏高原不断发生地壳加厚过程(Chung et al., 2009; Zhu et al., 2018),加厚的下地壳成为了冈底斯带广泛分布的埃达克质岩石的源区(Chung et al., 2003; Hou et al., 2004; Guo et al., 2007; Zeng et al., 2017)。随后,早-中中新世(18~14Ma)发育了大量埃达克质岩浆活动(Zhu et al., 2017),形成了著名的冈底斯斑岩铜钼矿带。带内各斑岩矿床的成矿年龄集中在17~13Ma (侯增谦等, 2003;李光明等, 2005;郑有业等, 2007;应丽娟等, 2009; Leng et al., 2013;秦克章等, 2014Li et al., 2017;杨震等, 2017)。此外,从24Ma开始,从西到东,冈底斯岩浆弧上开始发育伸展构造并伴随着钾玄岩和超钾质岩的形成,一直持续到约8Ma (Zhao et al., 2009)。值得注意的是,在斑岩Cu-Mo矿区,如冈底斯东段的驱龙(Yang et al., 2015)、中段的岗讲(Xu et al., 2017)、西段的朱诺(Sun et al., 2018),超钾质和钾玄质的岩浆活动,都晚于主成矿期,是矿区内最晚的一期岩浆活动。综上,冈底斯中新世斑岩铜钼成矿带形成于碰撞后的伸展背景(侯增谦等, 2006a)或碰撞挤压-伸展转换阶段(Qin et al., 2005;秦克章等,2014)。

2 矿区地质特征

矿区出露的地层主要有典中组(E1d)和设兴组(K2s)。典中组分布于矿区北西部,剖面厚843m,为一套中基性-中性火山熔岩夹火山碎屑岩,主要岩性为玄武岩、安山岩、英安岩、火山角砾岩夹英安质岩屑晶屑凝灰岩等,与上覆年波组及下伏设兴组呈不整合接触。设兴组分布于矿区北、北西、北东部,剖面厚1600m,为赋矿斑状二长花岗岩的直接围岩,二者呈侵入接触。在矿区一带主要出露为中酸性火山岩,岩性为安山岩、英安岩、凝灰岩夹火山角砾岩等,与上覆地层典中组呈不整合接触(图 2)。

图 2 白容-白容西地质简图(据拉萨天利矿业有限公司, 2014修改) Fig. 2 The sketch geological map of Bairong-Bairongxi

① 拉萨天利矿业有限公司. 2014.西藏拉萨市尼木县白容岗讲铜钼矿区详查报告

矿区内侵入岩发育,岩石类型较多,钻孔中揭示的主要岩性有黑云母二长花岗岩、二长花岗斑岩、花岗闪长斑岩、英安斑岩、闪长玢岩、煌斑岩、细晶斑岩、钾长细晶岩等(图 3)。二长花岗斑岩、黑云母二长花岗岩部分破碎形成角砾岩。岩体在钻孔中表现为岩枝的特征,地表呈现出复杂的侵位形态(图 4)。

图 3 矿区主要岩性手标本及其对应镜下照片 (a)凝灰岩; (b)黑云母二长花岗岩; (c)二长花岗斑岩,有硅化细脉; (d)二长花岗斑岩(细); (e)花岗闪长斑岩; (f)英安斑岩(细); (g)英安斑岩; (h)煌斑岩. Bio-黑云母; Kfs-钾长石; Qtz-石英; Hbl-角闪石; Pl-斜长石; Phl-金云母 Fig. 3 Hand specimen photographs and photomicrographs of rock types in the Bairong-Bairongxi district (a) tuff; (b) biotite monzogranite; (c) monzogranite porphyry; (d) monzogranite porphyry (fine); (e) granodiorite porphyry; (f) dacite porphyry (fine); (g) dacite porphyry; (h) lamprophyre. Abbreviations: Bio-biotite; Kfs-K feldspar; Qtz-quartz; Hbl-hornblende; Pl-plagioclase; Phl-phlogopite

图 4 矿区岩体侵位关系 (a-c、i)二长花岗斑岩侵入到黑云母二长花岗岩; (d-f)二长花岗斑岩侵入到凝灰岩;(g、h)二长花岗斑岩侵入到花岗闪长岩 Fig. 4 Field photographs of intrusions in the Bairong-Bairongxi district (a-c, i) biotite monzogranite batholith cut by monzogranite porphyry; (d-f) tuff cut by monzogranite porphyry; (g, h) granodiorite cut by monzogranite porphyry

矿区主要的矿石矿物为黄铁矿、黄铜矿、辉钼矿、孔雀石、斑铜矿、闪锌矿等,矿石构造为浸染状、细脉浸染状、细脉状和脉状。矿体主体赋存在黑云母二长花岗岩、二长花岗斑岩与花岗闪长斑岩中。目前,白容-白容西矿区尚未揭露到连续厚大工业矿体。

3 热液蚀变与热液脉系 3.1 热液蚀变特征

矿区内广泛发育电气石化、粘土化(高岭土化-伊利石化-水白云化)、绢云母化、硬石膏化、磁铁矿化、硅化、碳酸盐化、绿泥石化、绿帘石化、黑云母化、钾长石化等斑岩系统的典型蚀变(图 5)。

图 5 矿区热液蚀变显微照片 (a)黑云母二长花岗岩中的电气石脉(正交偏光); (b)黑云母二长花岗岩中的黑云母发生黑云母化(棕褐色)蚀变(单偏光); (c)英安斑岩中发育的粘土化蚀变(正交偏光); (d)英安斑岩中发育的绢云母化蚀变(正交偏光); (e)黑云母二长花岗岩中的硬石膏化蚀变(正交偏光); (f)磁铁矿化(反射光); (g)英安斑岩中发育的硅化、粘土化、黄铁矿化蚀变(正交偏光); (h)英安斑岩中发育的粘土化、碳酸盐化蚀变(正交偏光); (i)石英闪长玢岩的黑云母发生绿泥石化蚀变(单偏光). Anh-硬石膏; Mag-磁铁矿; Chl-绿泥石; Ser-绢云母; Tur-电气石; Cal-碳酸盐; Py-黄铁矿; Cpy-黄铜矿 Fig. 5 Photomicrographs of alteration minerals in the Bairong-Bairongxi district (a) tourmaline vein in biotite monzogranite (CPL); (b) biotite is altered to hydrothermal biotite in biotite monzogranite (PPL); (c) the argillic alteration of dacite porphyry (CPL); (d) the sericite alteration of dacite porphyry (CPL); (e) the anhydrite alteration in dacite porphyry (CPL); (f) magnetite (reflected light); (g) the silicate, argillic and pyrite alteration in dacite porphyry (CPL); (h) the argillic and calcite alteration in dacite porphyry (CPL); (i) chlorite alteration in dacite porphyry (PPL). Abbreviations: Anh-anhydrite; Mag-magnetite; Chl-chlorite; Ser-sericite; Tur-tourmaline; Cal-calcite; Py-pyrite; Cpy-chalcopyrite

(1) 电气石化:在矿区西侧高海拔区域,地表发育大量电气石脉、电气石角砾岩、石英-电气石脉。滚石中可见宽达1m以上的石英-电气石±黄铁矿脉。

(2) 黑云母化:黑云母化主要表现在热液黑云母交代暗色矿物、通常以浸染状形式产出在花岗闪长斑岩中,或是以黑云母脉的形式产出在较深部的黑云母二长花岗岩之中,发育范围较广。

(3) 粘土化:粘土化(伊利石化为主,水白云化、高岭土化次之)是矿区内发育最广泛的蚀变类型,大多数岩芯都发育不同程度的粘土化蚀变。

(4) 绢云母化:绢云母化的分布与石英-硫化物脉的分布关系密切,通常表现为石英-黄铁矿脉绢云母晕。中等的弥散状的绢云母化主要发育在矿区中心区域。

(5) 硬石膏化:硬石膏化主要发育在矿区中心区域的深部。

(6) 磁铁矿化:磁铁矿化主要以石英-磁铁矿-黄铜矿脉的形式产出,发育在矿区中心区域的深部。

(7) 硅化:硅化通常与脉系相伴,总体较弱。

(8) 碳酸盐化:碳酸盐化发育在矿区浅部及后期岩脉之中。

(9) 绿泥石化:绿泥石化发育在矿区浅部及后期岩脉之中。

(10) 绿帘石化:绿帘石化仅发育在矿区西部高海拔地区的火山岩之中。

(11) 钾长石化:钾长石化主要表现为石英-钾长石脉,石英脉-钾长石晕,石英-钾长石-黄铜矿±辉钼矿的形式,发育在矿区的深部。

3.2 脉系与蚀变的空间分布

矿区内具有丰富的热液脉系(图 6),具有明显的垂向变化(图 7)。

图 6 矿区脉系手标本照片 (a)碳酸盐脉; (b)电气石-石英脉被石膏脉切穿; (c)石英-黄铁矿-黄铜矿脉被石膏脉切穿; (d)硬石膏脉; (e)石英-黄铁矿脉(绢云母晕)被石膏脉切穿; (f)石英网脉被石膏脉切穿; (g)石英-黑云母脉被钾长石脉切穿后被石英-黄铁矿脉切穿; (h)石英-黄铁矿脉黑云母晕; (i)石英-黄铁矿-黄铜矿脉被石膏脉切穿; (j)硬石膏脉; (k)硬石膏脉被辉钼矿脉切穿; (l)石英-辉钼矿-钾长石脉; (m)石英-辉钼矿-黄铜矿脉; (n)石英-辉钼矿脉; (o)石英-辉钼矿-黄铁矿-黄铜矿脉. Gp-石膏; Mo-辉钼矿; Ep-绿帘石 Fig. 6 The photos of veins (hand specimen) (a) calcite vein; (b) tourmaline-quartz vein cut by gypsum veinlet; (c) quartz-pyrite-chalcopyrite vein with sericite selvage cut by gypsum veinlet; (d) anhydrite vein; (e) quartz-pyrite vein with sericite selvage cuts by gypsum veinlet; (f) quartz-pyrite vein cut by gypsum vein; (g) quartz-biotite veinlet cut by K feldspar veinlet and later by quartz-pyrite veinlet; (h) quartz-pyrite vein accompanied by brown alteration halo consisting of biotite; (i) quartz-pyrite-chalcopyrite vein cut by gypsum vein; (j) anhydrite vein; (k) molybdenite veinlet cuts anhydrite vein; (l) quartz-molybdenite-K feldspar vein; (m) quartz-molybdenite-chalcopyrite vein; (n) quartz-molybdenite vein; (o) quartz-molybdenite-pyrite-chalcopyrite vein. Abbreviations: Gp-Gypsum; Mo-molybdenite; Ep-epidote

图 7 矿区蚀变-脉系垂向变化图解s (a1-a5)黑云母的蚀变: (a1)未蚀变(5600, 单偏光); (a2)绿泥石化(5200, 正交偏光); (a3)绿泥石化+黑云母化(4900, 正交偏光); (a4)黑云母化+绿泥石化(4650, 正交偏光); (a5)黑云母化(4600, 单偏光). (b1-b5)长石分解蚀变: (b1)伊利石化(5600, 正交偏光); (b2)高岭土化(5200, 正交偏光); (b3)伊利石化+碳酸盐化(4900, 正交偏光); (b4)伊利石化+绢云母化(4700, 正交偏光); (b5)黑云母二长花岗岩(4600, 正交偏光). Kln-高岭石; Ill-伊利石 Fig. 7 The photos of vertical variations of alteration and veinlets (under microscope) (a1-a5) alteration of biotite: (a1) weak or no alteration (5600, PPL); (a2) biotite is replaced by chlorite (5200, CPL); (a3) biotite is replaced by chlorite and hydrothermal biotite (4900, CPL); (a4) Biotite is replaced by hydrothermal biotite and chlorite (4650, CPL); (a5) biotite is totally replaced by hydrothermal biotite (4600, PPL); (b1-b5) alteration of plagioclase: (b1) plagioclase is replaced by illite (5600, CPL); (b2) plagioclase is replaced by kaolinite (5200, CPL); (b3) plagioclase is replaced by illite and calcite (4900, CPL); (b4) plagioclase is replaced by illite and sericite (4700, CPL); (b5) biotite monzogranite porphyry (4650, CPL). Abbreviations: Kln-Kaolinite; Ill-illite

(1) 上部(图 7A):主要发育绿帘石脉、碳酸盐脉(图 6a)、电气石脉、电气石角砾岩脉、石英-电气石脉(图 6b)、石英-黄铁矿-黄铜矿脉、石膏脉(图 6c)、石英-黄铁矿脉(图 6e)、低温硅化脉和少量石英-黄铁矿-黄铜矿脉,石英-黄铜矿-绿泥石脉。

(2) 过渡区域(图 7B):电气石脉消失,钾长石脉和黑云母脉尚未出现,只发育硅化脉、石膏脉、硫化物脉和石英-硫化物脉。

(3) 下部(图 7C):主要发育黑云母脉、钾长石脉(图 6f)、石英-黄铁矿脉(黑云母晕)、硬石膏脉(图 6d)、石膏脉、石英-磁铁矿-黄铜矿脉、石英-黄铁矿-黄铜矿脉、辉钼矿脉(图 6k)、石英-辉钼矿-钾长石脉(图 6l),石英-辉钼矿-黄铜矿脉(图 6m)、石英-辉钼矿脉(图 6n)、石英-辉钼矿-黄铁矿-黄铜矿脉(图 6o)。

矿区内的黑云母和斜长石的蚀变也具有明显的垂向变化(图 7)。

(1) 黑云母:从上到下由未蚀变、至发育绿泥石化、绿泥石+次生黑云母化、次生黑云母化+绿泥石化、到深部只发育次生黑云母化(图 7a1-a5)。

(2) 斜长石:除了在脉系周围,斜长石可发育较为明显的绢云母化现象之外,从上到下,斜长石主要发生粘土化蚀变,且随着深度的不断加深,粘土化蚀变的程度不断减弱。在钾长石化出现的深度,斜长石的粘土化很弱(图 7b1-b5)。

3.3 蚀变-矿化阶段与矿物共生次序

结合典型蚀变与热液脉系的穿切关系,我们将白容-白容西矿区的热液成矿过程划分为4个阶段。

(1) 热液前锋阶段:在火山岩中发育青磐岩化组合,并形成绿帘石脉、电气石脉、石英-电气石脉、硬石膏脉、黑云母脉、石英-黑云母脉等早期脉系,并被后期脉系切穿,仅发育在黑云母二长花岗岩、二长花岗斑岩、花岗闪长斑岩之中。本阶段流体量巨大,在浅部形成了电气石角砾岩和绿帘石大脉,但没有发生铜钼矿化。

(2) 钾硅酸盐-钼矿化阶段:随后,钾长石-石英-辉钼矿脉、钾长石脉和石英-钾长石脉和辉钼矿脉形成,并穿切热液前锋阶段的脉系。

(3) 中级泥化-铜钼矿化阶段:发育伊利石、绿泥石、碳酸盐、绢云母、硬石膏和磁铁矿,叠加在前期蚀变组合之上。该阶段为矿区的主体铜钼矿化阶段,发育大量石英-黄铁矿脉、石英-黄铁矿-黄铜矿脉、硫化物脉(如黄铁矿脉、黄铜矿脉、辉钼矿脉)、石英-黄铁矿-黄铜矿±辉钼矿脉、石英-辉钼矿脉,石英-磁铁矿-黄铜矿脉±辉钼矿等。

(4) 热液尾声阶段:石膏脉为矿区内最晚的一期脉系,穿切了上述所有的脉系。在该阶段,不发生铜钼矿化,并形成了少量粘土化、绿泥石化、碳酸盐化等低温蚀变现象。石膏脉发育在黑云母二长花岗岩、二长花岗斑岩、花岗闪长斑岩、英安斑岩之中。

4 样品采集与年代学测试 4.1 样品采集

本次研究中所有样品采自白容-白容西矿区。经详细的岩相学观察之后,选取了矿区典型的火山岩围岩、二长花岗斑岩、花岗闪长斑岩、英安斑岩(多斑)、英安斑岩(少斑)、煌斑岩进行了锆石原位U-Pb年龄测试工作,选取了石英-辉钼矿-钾长石脉、石英-辉钼矿-黄铜矿脉、石英-辉钼矿脉、石英-辉钼矿-黄铜矿脉挑选辉钼矿进行Re-Os定年工作(图 6l-o)。

4.2 分析方法 4.2.1 锆石LA-ICP-MS原位分析

锆石U-Pb年龄测定在中国科学院地质与地球物理研究所Neptune多接收电感耦合等离子体质谱仪(MC-ICP-MS)和193nm激光采样系统上进行,分析时激光束直径为50μm,激光剥蚀时间约26s。分析中所用的激光脉冲速率为6~8Hz,激光束脉冲能量为100mJ。用美国国家标准技术研究院研制的人工合成硅酸盐玻璃标准参考物质NIST610进行仪器最优化。仪器的运行条件及详细的分析过程可参见(徐平等, 2004)。锆石LA-ICP-MS U-Pb年龄采用91500标准锆石外部校正法,年龄误差为1σ,数据结果处理采用ISOPLOT软件(Ludwig, 2003),测试结果见表 1

表 1 白容-白容西锆石U-Pb定年结果 Table 1 Zircon U-Pb age data of Bairong-Bairongxi district
4.2.2 辉钼矿Re-Os定年

辉钼矿单矿物挑选后送国家地质测试中心完成Re-Os化学分离及后续的质谱测定工作,采用Carius管溶样技术,通过蒸馏得到分离纯化的Re,通过丙酮萃取和阴离子交换得到分离纯化的Os,并采用电感耦合等离子体质谱仪TJA X-series ICP-MS测定Os同位素比值和Re同位素比值。详细的测试方法见杜安道等(1994, 2009),测试结果见表 2

表 2 白容-白容西辉钼矿Re-Os定年结果 Table 2 Molybdenite Re-Os age data of Bairong-Bairongxi district
4.3 分析结果

所测试的锆石颗粒大小为50~150μm,长宽比为1:1~3:1,内部结构简单,有明显的韵律环带,绝大多数测试锆石的Th/U大于0.5,为典型的岩浆锆石(Hoskin and Black, 2000)。

其中火山岩围岩中的锆石, U含量和Th含量较低(U:74×10-6~440×10-6;Th:49×10-6~367×10-6),U-Pb同位素年龄在在误差范围内谐和,得到的206U-238Pb加权平均年龄为52.6±1Ma (图 8a)。

图 8 矿区岩浆岩锆石U-Pb谐和图 Fig. 8 Zircon U-Pb diagrams of Bairong-Bairongxi

二长花岗斑岩、英安斑岩、英安斑岩(多斑)、花岗闪长斑岩中的锆石,有着接近的U和Th组成(U:259×10-6~2135×10-6;Th:124×10-6~2401×10-6),U-Pb同位素年龄在在误差范围内谐和,得到的206U-238Pb加权平均年龄分别为13.9±0.2Ma (图 8b)、12.6±0.4Ma和12.5±0.3Ma(图 8c, d)和13.8±0.2Ma(图 8e)。

煌斑岩中的锆石,U含量和Th含量极高(U: 658×10-6~5892×10-6; Th: 653×10-6~4215×10-6),U-Pb同位素年龄在在误差范围内谐和,得到的206U-238Pb加权平均年龄为11.1±0.2Ma (图 8f)。

4件辉钼矿样品的Re-Os模式年龄分别为13.82±0.20Ma、13.58±0.20Ma、13.73±0.19Ma和13.35±0.19Ma。

5 讨论 5.1 成岩成矿年龄、侵位序列与热液持续时间

结合各侵入岩与辉钼矿的成岩成矿年龄及其脉系-蚀变特征与相互穿切关系,厘定出矿区岩浆侵位序列从早到晚依次为黑云母二长花岗岩、二长花岗斑岩、花岗闪长斑岩、英安斑岩、煌斑岩。

结合前人的年代学工作(表 3李金祥等, 2007; Li et al., 2011; Xu et al., 2017)和蚀变、脉系发育情况,得到了白容-白容西矿区综合年龄图解(图 9)。年龄图解显示,早期的黑云母二长花岗岩侵位年龄和冷却年龄都在14Ma之前,花岗闪长斑岩与辉钼矿年龄一致。黑云母二长花岗岩、二长花岗斑岩、花岗闪长斑岩中都发育电气石脉及钾长石化现象,脉系较为丰富,矿化较好,且与辉钼矿Re-Os年龄相当或者稍早。英安斑岩、煌斑岩未发生钾化、电气石化现象,脉系较少,无矿化或矿化甚弱,且稍晚于成矿斑岩。综上认为黑云母二长花岗岩、二长花岗斑岩、花岗闪长斑岩是在早期脉系、高温脉系形成阶段即完成或发生就位的,成矿与花岗闪长斑岩、二长花岗斑岩有关,而英安斑岩和煌斑岩成矿后侵位,破坏矿体。

表 3 白容-白容西成岩-成矿-蚀变年龄汇总表 Table 3 Summary of geochronologic work in the Bairong-Bairongxi

图 9 矿区年龄综合图解 Fig. 9 Geochronologic diagram of Bairong-Bairongxi

值得注意的是,后侵位的英安斑岩都表现出了经历低温的蚀变特征,如粘土化、碳酸盐化等,这和黑云母、绢云母的Ar-Ar、K-Ar年龄都稍晚于英安斑岩是一致的。这种经历低温蚀变的现象暗示着,在英安斑岩侵位阶段,热液系统依然还在较低温度下运行。到了最晚期的煌斑岩,其年龄<12Ma,且不发育脉系,这说明煌斑岩形成的时候热液系统已经最终关闭了。因此,矿区的热液持续时间的上限为3Ma,即从黑云母二长花岗岩、花岗闪长斑岩、二长花岗斑岩侵位(约14Ma)开始,至煌斑岩(11.1Ma)的侵位结束。其热液持续时间的下限为2Ma,因为至12.6Ma时,英安斑岩依然存在石膏脉及明显的粘土化、碳酸盐化等低温蚀变,因而Ar-Ar、K-Ar年龄普遍约12Ma,可以视为是热液系统关闭的上限。综上认为,白容-白容西的岩浆热液时限约为持续了2~3Myr。

5.2 岩浆-热液-矿化中心的综合判定

图 10显示,代表斑岩系统浅部的青磐岩化、电气石化蚀变集中分布在矿区的西侧高海拔地区(图 10a);ZK002和BZK005钻孔中,首次出现钾化蚀变的标高最高(图 10b),而矿区的西侧和北侧,出现钾化蚀变的标高最低;化探结果(图 10c,拉萨天利矿业有限公司,2017)显示,指示斑岩系统远端的Zn异常、Pb异常集中在白容-白容西矿区的北部,指示斑岩系统近端的Cu异常、Mo异常分布在整个区域内,在中部和南部连续分布(图 10d)。结合脉系、矿化、物探等其他信息,我们推测白容-白容西矿区的中部之下为岩浆-热液-矿化中心。

① 拉萨天利矿业有限公司.2017. 2014-2016年度西藏拉萨市尼木县白容岗讲铜钼矿地质勘查工作总结

图 10 白容-白容西矿区岩浆-热液-矿化中心判别综合图 (a)矿区影像图:其中b1-b6为代表性地表/浅部样品的分布位置; 白框和红框分别对应(c)和(d)图对应位置; 浅绿色区域为发育电气石、绿帘石、绿泥石的区域. (b)为代表性地表样品: b1-石英-电气石脉; b2-电气石角砾岩; b3-产出于凝灰岩中的绿帘石脉; b4-凝灰岩,发育绿泥石化和星点状绿帘石化; b5-石英-钾长石细脉; b6-钾长石细脉. (c)钾长石化蚀变出现的最高位置; (d)化探异常图. d1为Zn化探异常; d2为Pb化探异常; d3为Mo化探异常; d4为Cu化探异常 Fig. 10 The graphs of magmatism-hydrothermal-mineralized center Photographic map of Bairong-Bairongxi (a), photograph of hand specimens (b), the map of highest altitude of K feldspar alteration (c) and graphs of geochemical anomaly (d). (b1) quartz-tourmaline veinlets; (b2) tourmaline breccia; (b3) tuff with epidote vein; (b4) tuff with chlorite and epidote alteration; (b5) quartz-K feldspar vein in monzonitic granite; (b6) K feldspar vein in monzonitic granite porphyry. (d1) Pb geochemical anomaly; (d2) Zn geochemical anomaly; (d3) Mo geochemical anomaly; (d4) Cu geochemical anomaly
5.3 剥蚀-埋深判定

综合矿区影像图与音频大地电磁(AMT)测深图(拉萨天利矿业有限公司,2017)及ZK006和BZK2512钻孔的深部样品,我们得到了白容-白容西矿区剥蚀-埋深综合图解(图 11)。图 11显示,ZK006的主体部分发育伊利石化-水白云化、绢云母化-硅化(图 11c3-c5),仅有最深部的样品,开始出现钾长石化现象(图 11c6),整个ZK006孔并未发现厚大矿体,石英-黄铁矿脉广泛发育; BZK2512的深部样品发育明显的粘土化和低温硅化现象,发育石英-黄铁矿脉,几乎未发现连续矿体。结合地表广泛发育的青磐岩化和电气石化现象,我们认为白容-白容西整体埋深很大。因此,AMT图中大面积的环状低阻异常指示的并不是斑岩矿床的主体铜矿化部分(钻孔中也没有观察到明显的铜矿化),而指示的是斑岩系统顶部的由大量石英-黄铁矿脉组成的“黄铁矿晕”。综上认为,目前矿区的钻探工程,都只打到了斑岩矿床的顶部,推测主矿体隐伏,在海拔4500m(甚至4000m)之下,埋深甚大。

图 11 矿区剥蚀-埋深综合图解 (a)矿区影像图,其中红蓝色条和星号对应于图(b)中的AMT测深剖面和ZK006与BZK2512; (b)AMT测深剖面图, 其中红色代表高阻,蓝色代表低阻; c1-c6为代表性钻孔深部样品的取样位置; (c)验证低阻异常的ZK006与BZK2512的深部样品: c1取自BZK2512的256m,为粘土化、低温硅化的细晶斑岩; c2取自BZK2512的300m,为强粘土化的角砾岩; c3取自ZK006的724m,为伊利石化-绢云母化的二长花岗斑岩; c4取自ZK006的826m,为伊利石化-绢云母化的黑云母二长花岗岩,发育石英-硬石膏-黄铜矿-黄铁矿脉; c5取自ZK006的861m,为伊利石化的黑云母二长花岗岩,被钾长细晶岩切穿; c6取自ZK006的900m,为钾化的黑云母二长花岗岩 Fig. 11 The graphs of the estimation of porphyry (a) photographic map of Bairong-Bairongxi. The red band, blub band and red star are corresponding to the map of AMT, ZK006 and BZK2512 in Fig. 11b. (b) map of AMT. Regions with low and high electricity resistance are shown in blue and red. The hand specimens shown in Fig. 11c are located in c1-c6. (c) typical hand specimens in the deep of drills. (c1) aplite with argillic and low temperature silicate alteration in 256m depth of drill BZK2512; (c2) strongly argillic altered breccia in 300m depth of drill BZK2512; (c3) monzonitic granite porphyry with illite and sericite alteration in 724m depth of drill ZK006; (c4) biotite monzonitic granite with quartz-anhydrite-chalcopyrite-pyrite veinlet in 826m depth of drill ZK006; (c5) K feldspar aplite cuts biotite monzonitic granite in 861m depth of drill ZK006; (c6) monzonitic granite with potassium alteration in 900m depth of drill ZK006
5.4 成矿模式

白容-白容西矿区成矿经历三个阶段。

(1) 第一阶段发生于约14Ma。与成矿有关的复式岩体黑云母二长花岗岩、花岗闪长斑岩(13.9±0.2Ma)、二长花岗斑岩(13.8±0.2Ma)侵位到火山岩中。岩体的侵位伴随着大量流体,使得火山岩围岩中形成了绿帘石大脉、绿泥石化、碳酸盐脉等青磐岩化组合,靠近岩浆-热液-矿化中心的黑云母二长花岗岩中,发育钾硅化蚀变并叠加了硅化绢云母化蚀变,远离岩浆-热液-矿化中心的黑云母二长花岗岩中,发育大量石英-黄铁矿脉、电气石脉、电气石角砾岩和粘土化蚀变(图 12a)。

图 12 矿区成矿模式图 Fig. 12 The graph of the ore forming model of Bairong-Bariongxi

(2) 第二阶段发生于约13~11Ma。后期岩脉英安斑岩(12.6±0.4Ma, 12.5±0.3Ma)、煌斑岩(11.1±0.2Ma)先后侵位破坏矿体。此时整个白容-白容西矿区的热液系统温度逐渐降低,在英安斑岩中仅发育粘土化蚀变,偶见石英-黄铁矿脉,不发育Cu-Mo矿化,而煌斑岩中几乎无脉系。在煌斑岩侵位之后,矿区内的岩浆-热液活动停止(图 12b)。

(3) 第三阶段发生于11Ma至今。白容-白容西矿区发生整体的抬升剥蚀。目前,矿区最高抬升至海拔约5800m的高度,在岩浆-热液-矿化中心附近,火山岩围岩已剥蚀殆尽,但是在岩浆-热液-矿化中心的外侧,依然保留有少量青磐岩化的火山岩围岩。目前,矿区的主体已经剥蚀到粘土化-黄铁矿化-电气石带,部分钻孔的深部揭示了钾硅化-硅化绢云母化带,但是主矿体依然隐伏在深部(图 12c)。

6 结论

(1) 白容-白容西矿区内黑云母二长花岗岩广泛发育的泥化蚀变(浅部)和钾硅化蚀变(深部)以及围岩火山岩中发育的青磐岩化蚀变,为典型的斑岩铜钼矿床蚀变类型。蚀变、脉系、物探、化探结果指示,白容矿区和白容西矿区为统一整体。

(2) 矿区岩浆侵位序列为黑云母二长花岗岩、二长花岗斑岩(13.8±0.2Ma)、花岗闪长斑岩(13.9±0.2Ma)、英安斑岩(12.6±0.4Ma, 12.5±0.3Ma)、煌斑岩(11.1±0.2Ma),其中二长花岗斑岩、花岗闪长斑岩与成矿关系密切,而英安斑岩与煌斑岩几乎不含矿且切穿矿体,为成矿后侵入体。辉钼矿单点Re-Os年龄显示,成矿主要发生在13.35±0.19Ma~13.82±0.2Ma之间。

(3) 矿区的岩浆-热液-矿化中心,位于矿区的中部区域。目前的工程仅控制了斑岩系统的顶部,主矿体依然隐伏,推测连续矿体发育在海拔4500m之下,深部铜钼矿化的潜力巨大。

致谢      野外工作得到了云铜集团拉萨天利矿业公司的王云凤总经理、曾红坤总经理、龙海滨工程师、王海涵工程师、云铜集团姜华高工等同仁的大力帮助;物探方面与有色地调中心陈卫教授级高工、四川冶金地质勘查院肖扬高工的讨论受益匪浅;曾庆栋研究员、徐兴旺研究员悉心审阅全文,提出了宝贵的意见与建议,对稿件的完善与提升大有裨益。在此一并致以衷心感谢!

通讯作者秦克章、合作者李光明在工作中承蒙叶大年先生多方面指点与教诲;作为地质与地球物理研究所研究生中的一员,第一作者邹心宇亦从叶先生的讲座中受益良多。谨以此文祝贺叶大年先生八十华诞!

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