冈底斯成矿带位于拉萨地体中南部,夹于雅鲁藏布江缝合带与狮泉河—纳木错蛇绿混杂岩带之间,是我国西部重点成矿区带之一(黄瀚霄等,2019)。近些年随着勘查和研究的不断深入,在该带已成功发现驱龙、甲玛、冲江、邦铺、亚贵拉、蒙亚阿、纳如松多、龙玛拉等大型—超大型矿床(图1),显示冈底斯是一条资源潜力巨大的铜—多金属矿带(李光明等,2006;孟祥金等,2007;唐菊兴等,2014)。关于该带的成岩—成矿年龄主要集中于62~41 Ma、30~23 Ma和18~12 Ma 3个阶段(费光春等,2010;张松等,2012;赵晓燕等,2013;纪现华等,2014;黄勇等,2015;马旺等,2020),分别对应于印度—亚洲大陆主碰撞造山成矿、晚碰撞转换成矿和后碰撞伸展成矿时段(侯增谦等,2006,2012)。在该带东段发育的铅—锌—银矿化以矽卡岩型为主,而西段发育的铅—锌—银矿化多以热液脉型矿床为主,并有少量隐爆角砾岩型,其中热液脉型矿体以脉状产于古生代或新生代地层中的构造破碎带内,多与新生代侵入的花岗斑岩小岩体有关(臧文栓等,2007;李光明等,2011;唐菊兴等,2016)。
西藏谢通门县则不吓铅锌矿床是冈底斯西段新发现的一受构造—岩浆活动控制的热液脉状铅—锌—多金属矿床(杜保峰等,2019),受限于矿床较低的勘查程度,区内广泛侵位的钾长花岗斑岩脉侵入时代及形成环境、与铅—锌成矿之关系等方面的研究较为薄弱。因此,笔者等通过对矿区钾长花岗斑岩开展岩石学、地球化学和锆石U-Pb年代学等研究,探讨其成岩时代、形成环境及与铅—锌成矿的关系,以资区域地质研究和矿产勘查参考。
则不吓铅锌矿床大地构造位置位于拉萨地体的南冈底斯火山—岩浆弧带北部,北临隆格尔—工布江达断隆带(潘桂棠等,2004)。区域出露地层主要为石炭系—二叠系及古近系,其中古近系林子宗群大面积分布,与下伏地层呈角度不整合接触,由中酸性火山岩、火山碎屑岩夹沉积碎屑岩组成;区域出露侵入岩主要为燕山期和喜马拉雅期花岗岩类,并有较多小型斑岩体分布。区域构造线总体呈近东西向,以线性复式褶皱、压扭性逆冲推覆构造为主;北东向及近南北向构造形成较晚,以发育张性构造为主要特征(臧文栓等,2007;赵晓燕等,2013)。
图1 西藏冈底斯成矿带地质矿产简图(底图据臧文栓等,2007修改)
Fig. 1 Sketch showing geological and mineral resources of Gangdise metallogenic belt
(modified after Zang Wenshuan et al.,2007&)
JSS—金沙江缝合带; BNS—班公湖—怒江缝合带; SNMZ—狮泉河—纳木错蛇绿混杂岩带;LMF—洛巴堆—米拉山断裂带;YTS—雅鲁
藏布江缝合带;N—新近系;E—古近系; Mz—中生界;Pz2—上古生界;γ6—喜马拉雅期花岗岩;γ52-3—燕山期花岗岩
JSS—Jinsha River suture zone;BNS—Bangong Lake—Nujiang River suture zone;SNMZ—Shiquanhe—Nam Lake ophiolite mélange zone;LMF—Luobadui—Mila Mountain fault zone;YTS—Yarlung River suture zone;N—Neogene;E—Paleogene;Mz—Mesozoic;Pz2—Upper Paleozoic;γ6—Himalayan granite;γ52-3—Yanshanian granite
则不吓铅锌矿区出露地层主要为下二叠统昂杰组(P1a)、古近系林子宗群典中组(E1d)及第四系(Q)(图2)。昂杰组出露较多,主要岩性为石英砂岩、粉砂质板岩、泥质板岩;典中组广泛分布于矿区,主要岩性为安山质凝灰岩、英安质晶屑凝灰岩、流纹质(岩屑)晶屑凝灰岩和含角砾凝灰岩组成;第四系主要沿沟谷及河流两侧发育,以砂砾石堆积为主,为含泥砾石层、含砂砾石层。侵入岩主要为始新世似斑状黑云母二长花岗岩及钾长花岗斑岩出露;其中钾长花岗斑岩数量众多,广泛发育,其以岩脉、岩枝状呈NE—NNE向展布,少量呈近SN向(图3a)。矿区发育NNE—NE、近SN和NWW向三组脆性断裂,其中以NNE—NE向断裂较为发育,与成矿关系最为密切,其与NW向断层交汇部位严格控制了铅锌矿(化)体的展布。
图2 冈底斯成矿带西段则不吓铅锌矿床地质简图
Fig. 2 Simplified geological map of Zebuxia Pb—Zn deposit in
western Gangdese metallgenic belt
图3 冈底斯成矿带西段则不吓矿床钾长花岗斑岩特征
Fig. 3 Characteristics of K-feldspar granite porphyries from the Zebuxia Pb—Zn deposit in western Gangdese metallgenic belt
(a) 钾长花岗斑岩野外产出特征;(b) 断裂带内产出的钾长花岗斑岩和铅锌矿体;(c) 钾长花岗斑岩近照;(d) 钾长花岗斑岩显微照片及绢云母化;(e) 钾长花岗斑岩内副矿物磷灰石;(f) 钾长花岗斑岩内零星分布的黄铁矿和方铅矿;Kf—钾长石;Qz—石英;Ser—绢云母;Ap—磷灰石;Py—黄铁矿;Gn—方铅矿
(a) Field characteristics of K-feldspar granite porphyry;(b) K-feldspar granite porphyry and Pb—Zn orebody in fault zone;(c) close-up of K-feldspar granite porphyry;(d) micrograph of K-feldspar granite porphyry and its sericitization;(e)apatite of K-feldspar granite porphyry;(f) scattered pyrite and galena of K-feldspar granite porphyry. Kf—K-feldspar;Qz—quartz;Ap—apatite;Ser—sericite;Py— pyrite;Gn—galena
矿区内现已发现的7条铅锌矿(化)体呈不规则扁透镜状和脉状产出,均赋存于典中组NE—近SN向展布的断层破碎带内,且基本与相邻产出的钾长花岗斑岩走向一致,个别矿体与钾长花岗斑岩一起产于NE向断裂带内(图3b),反映其可能受构造—岩浆活动的双重控制。各矿体Pb品位变化于0.24%~19.42%,Zn品位为0.32%~5.46%,伴生Ag品位为2.7×10-6~125×10-6。矿石中主要金属矿物为方铅矿、黄铁矿和闪锌矿,局部见黄铜矿,表面和裂隙发育氧化矿物孔雀石和褐铁矿;方铅矿、黄铁矿、闪锌矿等主要以集合体形式呈浸染状、细脉状分布于碎裂凝灰岩中,少量呈致密块状。矿石具自形—半自形粒状结构、他形填隙结构、交代残余结构、脉状充填结构等,构造类型发育角砾状、细脉状和块状构造(杜保峰等,2019)。矿体围岩为典中组流纹质晶屑凝灰岩、含角砾凝灰岩及钾长花岗斑岩,靠近矿体的围岩中亦可见不同程度的黄铁矿化,局部可见零星方铅矿化。围岩蚀变发育硅化、绢云母化、高岭石化、碳酸盐化,铅锌矿化主要与硅化和绢云母化密切相关。
本次研究的钾长花岗斑岩均位于铅锌矿体周围,岩石呈灰红色,斑状结构(图3c),基质呈微隐晶质结构,块状构造。斑晶矿物主要由钾长石(12%~18%)、石英(3%~6%)和黑云母(1%~3%)等组成(图3d)。其中钾长石斑晶,半自形—自形板状,粒径0.6~4.5 mm,多数大于1 mm,可见卡式双晶和微条纹;石英斑晶呈不规则状,部分呈浑圆状,个别发育溶蚀孔洞,粒径0.3~1 mm;黑云母呈片状,发育绿泥石化。基质矿物主要由微隐晶长英质矿物(75%~82%)组成,副矿物主要为磁铁矿、锆石和磷灰石等(图3e)。岩石多蚀变较强,表现为斑晶钾长石发育绢云母化、硅化和高岭土化,局部可见黄铁矿化和零星方铅矿化沿长石边缘分布(图3f)。
用于U-Pb测年的1件样品(ZB/2)采自矿区Pb5矿体边部的钾长花岗斑岩内。锆石的样品破碎及挑选由河北廊坊区域地质矿产研究所实验室完成。室内将样品粉碎至120目以下,后用磁法和重力方法挑选,再在双目镜下挑选用于测年的锆石。将待测试的锆石颗粒采用环氧树脂固定,之后抛磨至锆石核部露出,最后对待测锆石进行镜下透射光、反射光和阴极发光(CL)照相,锆石制靶和照相均在北京锆年领航科技有限公司完成。样品测年工作在天津地质矿产研究所同位素实验室完成,采用LA-MC-ICP-MS进行锆石U-Pb定年测试,ICP-MS为Agilent 7500a,分析采用直径为35 μm的激光束斑,剥蚀物质的载气为氦气,分析流程详见耿建珍等(2012),采用Glitter4.0软件对同位素比值等数据处理,普通铅校正则使用Anderson(2002)的方法,并通过Isoplot3.0程序进行锆石谐和图绘制。
钾长花岗斑岩样品的主量-稀土-微量元素的配套分析由西南冶金地质测试中心完成,选择其中较弱蚀变的5件样品进行测试。对样品清洗烘干,在保证无污染后粉碎至200目。采用X射线荧光熔片法(XRF)测定主量元素,分析相对误差小于1%,而微量元素和稀土元素分别采用电感耦合等离子质谱仪(ICP-MS)和电感耦合等离子体原子发射光谱法(ICP-AES)完成,分析相对误差小于5%。
3.1.1 主量元素特征
5件钾长花岗斑岩样品的SiO2含量为69.86%~73.62%,Al2O3为 13.82%~14.14%,K2O+ Na2O为7.32%~8.69%(表1),其中K2O的含量6.22%~6.78%,明显大于Na2O(含量为1.0%~2.41%),且K2O/Na2O值为2.61~6.32;TFeO(1.28%~1.87%),CaO(0.76%~1.78%),MgO(0.23%~0.29%)和TiO2(0.22%~0.31%)含量较低,钾长花岗斑岩总体具高硅、富钾而贫镁特征。岩石里特曼指数σ值介于1.73~2.68之间,均小于3.3,显示出钙碱性岩浆岩的特征,在SiO2—K2O图 (图4a)中样品点多落在钾玄岩系列岩石区域。A/CNK值介于1.08~1.38之间(平均值1.22),CIPW标准中均出现了刚玉分子(1.27%~3.94%),无透辉石,基本属强过铝质,且在A/CNK—A/NK图解(图4b)中均落入过铝质花岗岩区域。岩石分异指数(DI)为86.41~90.23,显示岩浆分异程度较高。
图4 冈底斯成矿带西段则不吓矿床钾长花岗斑岩SiO2—K2O(a)和A/CNK—A/NK(b)判别图解
Fig. 4 SiO2—K2O diagram (a) and A/CNK—A/NK diagram(b) of K-feldspar granite porphyries from
the Zebuxia deposit in western Gangdese metallgenic belt
表1 冈底斯成矿带西段则不吓铅锌矿床钾长花岗斑岩主量元素(%)、微量和稀土元素(×10-6)分析结果
Table1 Analysis results of major elements (%),Trace elements and REE (×10-6) of K-feldspar granite
porphyries in Zebuxia Pb—Zn deposit, western Gangdese metallogenic belt
样品号ZB2-01ZB2-02ZB2-03ZB2-04ZB2-05SiO273.6272.1969.8672.1070.81Al2O313.8214.0514.4214.3314.47Fe2O31.021.441.771.461.77FeO0.370.260.280.310.24CaO0.851.021.780.761.14MgO0.260.250.290.240.23K2O6.326.786.226.536.28Na2O1.001.192.081.812.41TiO20.220.270.320.280.31P2O50.040.070.110.070.10MnO0.040.040.060.040.05烧失量2.262.232.621.901.97总量99.8199.8099.8199.8399.78A/NK1.631.511.421.431.34A/CNK1.381.261.081.251.13DI89.1888.9286.4190.2389.33σ1.732.152.522.372.68La7453.672.374.879Ce125100134133142Pr14.411.815.415.416.1Nd47.640.252.851.553.3Sm6.425.757.527.157.46Eu0.941.011.331.181.24Gd4.974.295.95.555.76Tb0.640.550.760.690.71Dy2.72.333.312.922.97样品号ZB2-01ZB2-02ZB2-03ZB2-04ZB2-05Ho0.520.430.580.530.53Er1.621.331.841.611.62Tm0.270.210.30.260.26Yb1.651.291.921.641.66Lu0.290.230.30.270.27Y16.412.618.616.115.8ΣREE281.02223.02298.26296.50312.88LREE268.36212.36283.35283.03299.10HREE12.6610.6614.9113.4713.78LREE/HREE21.2019.9219.0021.0121.71(La/Yb)N32.1729.8027.0132.7234.14δEu0.490.600.590.550.56δCe0.880.930.940.910.92Rb392402390398383Ba355582610522628Th120112108114115U202121.62019.8Ta3.013.092.762.932.95Nb20201919.420.6Sr81.5116173150183Zr214238243234252Hf7.707.637.807.827.96V41.641.646.842.245.3Sc4.987.034.303.225.70Pb28.150.641.849.542.8
3.1.2 稀土和微量元素特征
则不吓钾长花岗斑岩稀土元素总含量在223.0×10-6~312.9×10-6之间(表1),轻、重稀土元素比值(LREE/HREE)为19.9~21.7,(La/Yb)N值为27.0~34.1,反映轻、重稀土元素发生较显著的分异;在稀土元素球粒陨石标准化配分模式图中呈现向右倾斜显著的趋势 (图5a),表明轻稀土富集而重稀土亏损,且均具有较明显的中等负Eu异常(δEu=0.49~0.60),暗示岩浆形成过程中可能存在钾长石的分离结晶作用或者源区部分熔融时有斜长石的残留。钾长花岗斑岩富集Rb、K、U、Th、Pb等大离子亲石元素,而Ba、Sr和Nb、Ta、Ti、P等高场强元素则显示相对亏损;在微量元素原始地幔标准化蛛网图(图5b)中,呈现出显著的Rb、U、Th等元素正异常和Ba、Sr、Nb、Ti、P等元素的负异常。
图5 冈底斯成矿带西段则不吓矿床钾长花岗斑岩稀土配分模式图(a)和微量元素蛛网图(b)
(球粒陨石及原始地幔标准化值据Sun and McDonough,1989)
Fig. 5 Chondrite-normalized REE distribution patterns (a) and primitive mantle-normalized trace element spider patterns (b) for K-feldspar granite porphyries from the Zebuxia deposit in western Gangdese metallgenic belt (The chondrite data and primitive mantle data for normalization after Sun and McDonough,1989)
则不吓钾长花岗斑岩样品ZB/2的锆石多数为浅黄色,次为无色,呈自形短柱状或长柱状、粒状,粒径长度在70~200 mm,长宽比大致为1∶1~3∶1,阴极发光(CL)图像显示锆石多具清晰且均一的岩浆振荡环带(图6),其边部或晶体内部常见港湾状溶蚀,可能为浅成—超浅成侵位造成的溶蚀。本次选择对24颗韵律环带明显的岩浆锆石进行了U—Th—Pb同位素分析。分析结果显示,锆石的Th与U含量变化较大,分别为383×10-6~ 1586×10-6和394×10-6~1864×10-6(表2),且二者呈正相关关系,对应的Th/U值在0.49~1.76之间,与岩浆锆石Th/U值(大于0.4)一致(Hoskin and Black,2000;Griffin et al.,2004)。锆石CL图像显示具有清晰岩浆生长的韵律环带,这些特征均显示钾长花岗斑岩中锆石为典型的岩浆成因锆石(Hoskin and Schaltegger,2003;Belousova et al.,2002;Moeller et al.,2003;吴元保等,2004;薛传东等,2010)。
图6 冈底斯成矿带西段则不吓矿床钾长花岗斑岩锆石阴极发光图
Fig. 6 Cathodoluminescence images from zircon grains of K-feldspar granite porphyry from the Zebuxia deposit in
western Gangdese metallgenic belt
图7 冈底斯成矿带西段则不吓矿床钾长花岗
斑岩锆石U-Pb年龄谐和图
Fig. 7 U-Pb age concordia plots from zircon grains of K-feldspar granite porphyry from the Zebuxia deposit in western Gangdese metallgenic belt
锆石U-Pb测年结果显示(表2),除去异常稍偏高的2个测点(12、22号)年龄值,在U-Pb年龄谐和图中22个分析点均分布于谐和线上(图7),表现出良好的谐和性,说明锆石形成之后的U-Pb同位素体系是封闭的,基本无U或Pb同位素的丢失或加入。22个锆石测点的n(206Pb)/n(238U)年龄范围在13.69~14.67 Ma之间,其加权平均年龄值为14.18±0.15 Ma(95%可信度,MSWD=2.2,n=22),代表钾长花岗斑岩的冷却结晶年龄,表明其形成于中新世。
目前花岗岩成因类型通过特征矿物和微量元素来判定已有大量文献论述。通常将含铝指数用来区分I型和S型花岗岩,I型花岗岩的A/CNK通常小于1.1,而S型花岗岩的A/CNK则往往大于1.1(Chappell,1992),则不吓钾长花岗斑岩属富硅过铝质花岗岩,A/CNK为1.08~1.38,平均值1.22,且刚玉分子含量>1%(1.27%~3.94%),具有S型花岗岩特征。在K2O—Na2O图解中(图8a),所有样品均位于S型花岗岩范围之内;微量元素Rb—P2O5相关性趋势图解显示(图8b),则不吓钾长花岗斑岩明显具有S型花岗岩的特征;另外其Rb/Sr比值为2.1~4.8,远大于0.9,亦符合S型花岗岩特征(董旭舟等,2014)。
图8 冈底斯成矿带西段则不吓矿床钾长花岗斑岩成因类型判别图解
Fig. 8 Geochemical classification diagrams of K-feldspar granite porphyries from the Zebuxia deposit in
western Gangdese metallgenic belt
OGT—未分异的I、S、M型花岗岩;FG—分异的长英质花岗岩
OGT—unfractionated I-, S- and M-type granite;FG—fractionated felsic granite
则不吓钾长花岗斑岩富集Rb、K、U、Th、Pb等大离子亲石元素,而Ba、Sr和Nb、Ta、Ti、P等高场强元素呈现相对亏损,这些特征反映其形成过程中应存在大量地壳物质的混染。赵振华等(2008)研究表明C1型球粒陨石Nb/Ta值为17.3~17.6,大陆地壳的Nb/Ta值却相对偏低(10~14),则不吓钾长花岗斑岩Nb/Ta值为8.4~10.1,比较接近大陆地壳,而Zr/Hf值27.8~31.6(平均30.3)亦接近大陆地壳平均值(33),反映以壳源为主;在(La/Yb)N—Eu/Eu*图解上(图8c),投点主要位于靠近壳幔型的壳型范围内,指示其主体具有地壳物质源区的特征,可能有地幔物质的少量加入。岩石CaO/Na2O值(0.42~0.86,平均0.69)>0.3,与地壳的变砂岩源区相近,在A/MF—C/MF源岩判别图解(Alther et al.,2000)上(图8d),样品投点主要落入变质砂岩部分熔融区域内,也反映其成因主要为地壳部分熔融。
图9 冈底斯成矿带西段则不吓矿床钾长花岗斑岩(Y+Nb)—Rb(a) (据Pearce,1996)和
(Rb/30)—Hf—(Ta×3)(b) (据 Hairs et al.,1986)判别图解
Fig. 9 (Y+ Nb)—Rb diagram (a) (after Pearce et al.,1996) and (Rb/30)—Hf—(Ta×3) diagram(b)(after Hairs
et al.,1986) of K-feldspar granite porphyries from the Zebuxia deposit in western Gangdese metallgenic belt
钾长花岗斑岩样品在TFeO/MgO—(Zr+Y+Ce+Nb)图解(whalen et al.,1987)上显示其为分异的花岗岩(图8e);在Bouseily and Sokkary(1975)提出的用于判别普通花岗岩和高分异花岗岩的Rb—Ba—Sr图中(图8f),钾长花岗斑岩样品全部落入高分异型的区间。岩石中副矿物含有磷灰石,锆石中U、Th含量较高,全岩Zr/Hf值大于25而小于55,属中等分异花岗岩(吴福元等,2017)。另外,钾长花岗斑岩本身分异程度较高(分异指数DI为86.41~90.23),且稀土配分曲线具较明显的中等负Eu异常,同样指示其发生了相对较强的结晶分异作用。上述综合判别指示则不吓钾长花岗斑岩应为地壳物质发生部分熔融形成的岩浆,期间可能有少量幔源物质加入,后经结晶分异演化形成的S型花岗岩。
中新世,随着俯冲的印度大陆地壳边缘的岩石圈板片断离(Miller et al.,1999;Maheo et al.,2002),深部软流圈物质沿断离板片窗上涌,诱发了亚欧大陆岩石圈地幔熔融。之后形成的幔源岩浆上侵并加热增厚的下地壳物质而发生壳—幔岩浆混合(Hou Zengqian et al.,2009),在东西向伸展构造背景下形成了冈底斯带一系列钾质钙碱性熔岩、超钾质—钾质岩浆事件,以及数量众多的含矿斑岩体及中新世大规模成矿事件(曲晓明等,2002;Hou Zengqian et al.,2004;赵志丹等,2006)。
本次钾长花岗斑岩的侵位年龄为14.2±0.2 Ma,指示则不吓矿区广泛分布的钾长花岗斑岩应属中新世构造—岩浆活动的产物。在(Y+Nb)—Rb构造环境判别图解(图9a)上,则不吓钾长花岗斑岩数据点均投于后碰撞花岗岩区域(Pearce,1996;Forster et al.,1997);(Rb/30)—Hf—(Ta×3)判别图解(图9b)进一步确定钾长花岗斑岩投影于同碰撞花岗岩与碰撞晚期—碰撞后花岗岩交界处(Harris et al.,1986),但主体偏向后者,具有后碰撞花岗岩的特征。Sylvester(1998)认为绝大多数与碰撞有关的强过铝质花岗岩都是“碰撞后”的,而则不吓钾长花岗斑岩为钾玄岩系列的强过铝质花岗岩,且钾玄质花岗岩可以形成于板块碰撞汇集后的松弛或局部伸展阶段(Barbarin,1999),表明其形成于碰撞后的张性构造环境。因此,则不吓钾长花岗斑岩应与冈底斯带同时代含矿斑岩体构造环境相似,可能与印度—亚洲大陆碰撞后伸展背景下的引张构造有关。
则不吓钾长花岗斑岩在各铅—锌矿体附近均有侵位,少量钾长花岗斑岩赋存于含矿构造带内,走向与矿体大体一致,其内局部有星点状黄铁矿化和方铅矿化沿长石边缘分布,且与铅锌矿体紧邻的钾长花岗斑岩也发育不同程度的绢云母化和硅化,反映铅—锌矿体的形成应与钾长花岗斑岩的侵位存在较密切的成因联系,其应属受构造—岩浆活动控制的热液矿床。近年来的研究表明,与金属成矿有关的花岗岩多具高K、高的氧逸度和富含挥发分特征(Sillitoe,1997;Kelley and Ludington,2002;赵振华等,2002);则不吓钾长花岗斑岩为钾玄岩系列,成岩较浅且存在表征高氧逸度的磁铁矿和富含挥发分的磷灰石(图3e),符合上述条件。另外,则不吓矿区物化探特征反映其深部可能存在隐伏岩体,地表出露的断裂构造很可能是岩浆侵入在顶部引爆形成的裂隙,而各类斑岩脉为深部岩体向上延伸的分支(杜保峰等,2019)。因此,则不吓钾长花岗斑岩很有可能与铅锌矿体为一对同源分体(汪相等,2022),其侵位年龄可间接代表铅—锌矿化的发生时间,但其是否为成矿母岩还是仅仅提供成矿热量或部分成矿物质仍有待进一步研究确认。
冈底斯成矿带中新世成矿以斑岩型铜(—钼)矿化为主,部分伴生同时期的矽卡岩型、热液脉型铅—锌矿化(李光明等,2011;赵晓燕等,2013),这些矿床在成岩—成矿年龄上具有高度的一致性,集中形成于18~12 Ma,与前述大规模岩浆事件时代一致,均为青藏高原经历了强烈碰撞挤压以及剪切走滑之后而进入地壳伸展阶段的产物(芮宗瑶等,2004;王立强等,2014)。则不吓钾长花岗斑岩侵位年龄为14 Ma左右,反映钾长花岗斑岩及相关铅—锌矿化系中新世岩浆活动的产物,这与冈底斯带中新世大规模斑岩侵位时代和相关斑岩型铜(—钼)多金属矿化时代亦较一致(李光明等,2011;侯增谦等,2012),尤其相邻的朱诺铜矿区含矿花岗斑岩形成年龄为15.6 Ma(郑有业等,2007),因此则不吓铅锌矿床与冈底斯成矿带其它矿床很可能具有相同的成岩成矿环境,应属于同一构造演化阶段产物。
(1) 则不吓钾长花岗斑岩属钾玄岩系列,岩石总体具高硅、富钾而贫镁特征,A/CNK值介于1.08~1.38之间,属强过铝质花岗岩;REE具有较明显的中等负Eu异常,总体呈现出向右倾斜的轻稀土富集模式;微量元素富集Rb、K、U、Th、Pb等大离子亲石元素,而Ba、Sr和Nb、Ta、Ti、P等高场强元素相对亏损。根据矿物组成和岩石地球化学特征,表明其应属分异的S型花岗岩,可能以壳源成分为主。
(2)锆石U-Pb同位素年代学显示,则不吓铅锌矿床钾长花岗斑岩形成年龄为 14.18±0.15 Ma,系中新世岩浆活动的产物,与印度—亚洲大陆碰撞后伸展背景下的引张构造有关,且与冈底斯带中新世大规模斑岩侵位时代和相关斑岩型铜(—钼)矿化时代一致,可能具有相同的成岩成矿环境,应属于同一构造演化阶段产物。
(3)则不吓铅—锌矿床位于西藏冈底斯中部驱龙—邦铺—朱诺斑岩铜—钼—铅—锌成矿亚带的西段,且存在与铅锌矿化有关的岩浆活动,这为在该成矿带中西段寻找到有工业价值的与斑岩有关的铅—锌矿床提供了理论及实际依据,因此在后续找矿工作中,应该按照成矿系统的思想,注意在热液脉型铅锌矿体深部开展隐爆角砾岩型铅—锌银矿和斑岩型铜—钼—多金属矿的寻找。
致谢:野外地质调查及成文过程中得到河南省地质调查院的张彦启教授级高级工程师、董海敏工程师等的大力支持和指导,中国地质调查局天津地质调查中心耿建珍在实验过程中给予了帮助,同时审稿专家对论文提出了建设性意见,在此一并表示感谢!
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