冀东下营坊金矿成矿年代学研究
2016-12-23王玉往王京彬张会琼赵路通解洪晶刘永振刘贵权
邹 滔,王玉往,王京彬,张会琼,赵路通,解洪晶,石 煜,刘永振,刘贵权
(1. 北京矿产地质研究院,北京 100012; 2. 有色金属矿产地质调查中心,北京 100012;3. 中色地科矿产勘查股份有限公司,北京 100012)
冀东下营坊金矿成矿年代学研究
邹 滔1,王玉往1,王京彬1,张会琼1,赵路通1,解洪晶1,石 煜1,刘永振2,刘贵权3
(1. 北京矿产地质研究院,北京 100012; 2. 有色金属矿产地质调查中心,北京 100012;3. 中色地科矿产勘查股份有限公司,北京 100012)
冀东下营坊金矿地处华北地台北缘燕山造山带东段,是该区一个重要的岩浆期后热液型金矿。该金矿产于大铜山杂岩体及外接触带中,有三种矿化类型,即斑岩型、角砾岩型、矽卡岩型,构成了典型的斑岩型金矿成矿系统。为精确厘定下营坊金矿的成矿年代,更好的理解该矿与区内其他金矿的关系,本文采用锆石U-Pb和辉钼矿Re-Os同位素定年,获得赋矿斑岩(花岗斑岩)的结晶年龄为163.32±0.90Ma,切穿矿体的煌斑岩脉结晶年龄为159.0±1.5Ma,由此限定成矿年龄可能在163.32Ma~159.0Ma之间;而由辉钼矿获得的Re-Os同位素模式年龄为164.2±2.3Ma。在误差范围内,Re-Os同位素模式年龄与U-Pb同位素年龄是一致的,表明下营坊金矿成矿年龄为164.2±2.3Ma,属于中侏罗世岩浆热液事件。结合前人研究结果,认为下营坊金矿以及冀东中侏罗世金矿的成岩成矿构造背景与区内该时期的岩浆事件一样,可能受古太平洋板块俯冲的影响而处于挤压的构造环境。
成矿年代学 U-Pb Re-Os同位素 下营坊金矿 冀东
Zou Tao,Wang Yu-wang,Wang Jing-bin,Zhang Hui-qiong,Zhao Lu-tong,Xie Hong-jing,Shi Yu,Liu Yong-zhe,Liu Gui-quan.Geochronology of the Xiayingfang Au deposit eastern Hebei province[J].Geology and Exploration,2016,52(1):0084-0097
0 引言
冀东地区处于华北板块、西伯利亚板块和太平洋板块复合作用带,是中国重要的金矿集中区,已发现有金厂峪(岩浆热液型/后韧性剪切带型)、峪耳崖(斑岩型/石英脉型)、下营坊(斑岩-角砾岩型)、牛心山(石英脉型)、唐杖子(角砾岩型)等大中型金矿床(杨廷栋,1983;赵海玲等,1997;陈斌,2009;宋扬等,2011;贾三石等,2012;李正远等,2014)。一直以来,金矿的年代学是该区金矿研究的热点和难点,目前已获得了峪耳崖金矿辉钼矿Re-Os年龄为169.84±0.95Ma(陈绍聪等,2014),唐杖子金矿辉钼矿Re-Os年龄为170±1Ma(李正远等,2014),金厂峪金矿蚀变岩绢云母的K-Ar年龄为169.8Ma(贾三石等,2011),牛心山花岗岩体锆石U-Pb年龄为172±2Ma(郭少丰等,2009)等等。然而金矿年代学研究的载体较难寻找,且以往不同研究方法获得金矿床的年代相差较大(罗振宽等,2001a;罗振宽等,2001b;宋扬等,2011;陈绍聪等,2014),甚至同一方法获得的年代时限范围较宽(袁万明等,2001;汤云晖等,2003),制约着区内金矿成矿规律的研究。
下营坊金矿位于平泉县城南30km,地处平泉与宽城两县交界处,已控制资源/储量达大型规模,是冀东地区重要的金矿之一。该金矿具有较长的勘探开发历史,自上世纪80年代以来,已有学者从矿床地质特征、成因矿物学、成矿流体等方面对下营坊金矿进行了研究(杨廷栋,1983;1987;栾文楼等,1995),认为该矿床具有斑岩型金矿的基本特征。前人所获得的锆石和磷灰石的裂变径迹年龄显示成矿年龄介于153.9Ma~103.3Ma之间,成矿作用跨越至少50.6Ma,可能由中侏罗世延续到晚白垩世(袁万明等,2001),显然该成矿年龄还不够精确。
本文在矿床地质学研究的基础上,选择金矿的赋矿斑岩体和切割矿体的侵入岩脉,以及成矿期的辉钼矿作为年代学研究对象,运用LA-ICP-MS锆石U-Pb和辉钼矿Re-Os同位素定年技术对矿床成岩成矿年龄进行精确厘定,并试图探讨其成岩成矿的动力学背景。
1 区域地质背景
冀东地区处于华北克拉通北缘,经历了漫长的地质历史演化,大致可以分为3个阶段:太古宙-古元古代为基底形成阶段;中元古代和早古生代为盖层发展阶段;晚古生代-中、新生代为强烈活动阶段(刘勇胜等,2004;潘建国等,2013)。其基底岩性为一套深变质地层,岩性为斜长角闪岩,斜长片麻岩、角闪斜长片麻岩等;盖层岩性为中元古代的长城系、蓟县系地层,岩性为石英砂岩、页岩、白云质灰岩和泥质白云岩等(谢玉玲等,1996)。冀东地区在古生代-中生代经历了一个复杂的构造演化过程,在古生代受到北侧古亚洲洋演化的影响(Li J Y,2006;潘建国等,2013),中生代又经历了强烈的陆内构造变形(王瑜,1996)。燕山运动导致华北地块北缘地壳缩短和抬升,造成元古代-古生代地层大面积剥蚀(张拴宏,2004;董树文等,2007)。区内构造形式十分复杂多样,以近EW向和NE向断裂构造为主,构成典型的菱形格状构造,制约了区域内的盆岭构造的形成和岩浆活动及其展布。区内岩浆活动强烈,以燕山期岩浆侵入活动为主,代表性岩体主要有都山、青山口、牛心山、贾家山、肖营子等,岩石类型主要有钾长花岗岩、黑云母花岗岩、花岗闪长岩、二长花岗岩等(孔德鑫等,2013)。区内矿产丰富,其中金矿床(点)约有197处,典型的金矿床有金厂峪、峪耳崖、牛心山及下营坊等(图1)(孔德鑫等,2013)。资料显示,冀东地区金矿与燕山期花岗质岩浆岩有着密切的关系,矿体多产于赋矿岩体或岩体附近围岩中;矿床类型以岩浆热液型为主,包括斑岩型、石英脉型、隐爆角砾岩型等(林尔为等,1985;赵海玲等,1997;王宝德等,2002;贾三石等,2011;罗镇宽等,2011a;李正远等,2014)。
2 矿区地质
2.1 矿床地质
下营坊金矿矿区地层主要为中元古界浅变质碎屑岩系;其次为太古代迁西群、侏罗系及新生界第四系。矿区构造十分发育,主要构造格架为NE向S状不对称褶曲(雷世和等,1995),断裂构造主要可分为:NE-NNE向、EW向、以及由大东山杂岩体侵位造成的环形构造及放射状构造;不同构造活动不仅控制矿区岩浆活动,而且控制着晚期矿化热液活动(图2)。
矿区岩浆活动十分强烈,既有侏罗系火山岩系,又有以大铜山杂岩体为中心的中酸性杂岩体。该杂岩体呈不规则椭圆状,侵入太古宙迁西群及中元古界地层中,其地表出露分为两部分,即大铜山杂岩体和下金宝岩体。大铜山杂岩体出露面积约2.27km2,呈舌状向300°方向延长,杂岩体岩性多样,按侵位次序依次为闪长玢岩(似斑状花岗岩、花岗斑岩、石英斑岩、流纹斑岩),安山角砾岩,以及一系列后期脉岩,如煌斑岩脉、流纹斑岩脉、闪长玢岩脉、霏细岩等等。
2.2 矿体地质
下营坊金矿产于大铜山杂岩体及外接触带中,空间上分为四个矿段,即下金宝沟矿段、东梁矿段、大东山矿段、南梁矿段(图2)。其中前两者已进入开采阶段,大东山矿段已进入详查,南梁矿段正在勘探中。该矿四个矿段矿化类型不尽相同,构成了一个典型的斑岩金矿矿化系统。主要矿段的矿体特征具体如下:
下金宝沟矿段以斑岩型矿化为主,共圈定20条金矿体,矿体呈平行脉状产于下金宝沟岩体及蚀变斜长片麻岩中(图3),矿体总体走向NE30°~40°,倾向NW,倾角33°~82°,多数倾角为45°~65°之间。沿走向及倾向形态近平行产出,具有膨胀收缩、分枝复合、尖灭再现等现象。矿石构造以脉状、网脉状为主(图4A、B、C),金属矿物主要有黄铁矿、闪锌矿、方铅矿、黄铜矿,其次为磁铁矿、赤铁矿等,少量自然金,局部见有辉钼矿、车轮矿、针镍矿。大东山矿段处于大铜山杂岩体北部,以矽卡岩型矿化为主,圈定金矿体1条,矿体主要赋存于长城系高于庄组含砂白云岩中,矿体形态呈向斜顺层产出,矿体长530m,宽320m。矿石构造有条带状、胶结状及蜂窝状(图4D、E)。金属矿物主要有黄铁矿、闪锌矿、方铅矿、黄铜矿、褐铁矿,少量银金矿、磁铁矿等。东梁矿段处于杂岩体东部,北与大东山矿段接壤,矿化类型以角砾岩型为主,部分矽卡岩化及斑岩型矿化。圈定两个矿体,其中主矿体分布于角砾岩及蚀变岩中,并下穿至高于庄组、大红峪组地层及花岗斑岩、闪长玢岩中。矿体呈弯月形分布,长约1.7km,矿体从北向南呈现分枝复合特征,倾角变化较大,在20°~60°之间。
图1 冀东地区区域地质简图(据1∶20万区域地质图①,②,③,④和范正国等,2012 改编)
矿石构造有块状、浸染状及脉状(图4F、G)。主要金属矿物为黄铁矿,其次为黄铜矿、方铅矿、闪锌矿、磁铁矿,少量辉钼矿、赤铁矿等。
3 样品采集及分析方法
3.1 样品采集及特征
下营坊金矿岩浆岩类型多样,本次研究主要采集与金矿化关系密切的下金宝花岗斑岩体(Z4102)及成矿后侵位的煌斑岩岩脉(Z4104),进行锆石U-Pb定年研究,采样位置分别为下金宝沟露天采矿场及ZK0715钻孔105m处。其中赋矿斑岩(下金宝岩体:花岗斑岩)用于限定金矿化下限年龄,而穿切金矿体的煌斑岩岩脉用于限定金矿化的上限年龄。
下金宝岩体位于大铜山杂岩体西部,属于杂岩体的一部分,呈NE-SW向分布,出露面积约0.36km2。岩体呈岩株状与中元古及太古代地层呈侵位接触关系,为下金宝矿段金矿赋矿岩体。岩体主要由花岗斑岩组成,肉红色,斑状-似斑状结构,块状构造;因有绢云母化、绿帘石化、碳酸盐化、萤石化等蚀变,而呈现灰白-灰绿色。岩石斑晶约25%~60%,粒度0.2mm~4mm,主要矿物组成为石英(8%~15%)、斜长石(5%~20%)、钾长石(5%~25%)、黑云母(5%±)。基质为显微花岗结构,粒度在0.05mm~0.2mm,矿物组成主要为石英、斜长石和钾长石,石英略多。副矿物主要磷灰石、榍石、锆石等(图5A、B)。
图2 下营坊金矿矿区地质简图⑤
煌斑岩岩脉,岩脉侵位于花岗斑岩,并切穿金矿体及金矿化蚀变(图5C),深灰色-灰绿色,自形细粒结构,暗色矿物主要为自形的角闪石(约20%~30%),且多已绿泥石化,但保留针状、长条状的结构。浅色矿物主要为斜长石(约60%~70%),自形程度低,并且多数已绢云母化和碳酸盐化(图5D)。
矿区辉钼矿比较常见,以辉钼矿石英或裂隙面状辉钼矿形式出现(图4H),但样品中辉钼矿含量极少且难以挑选,仅成功挑选一件(PD09),采样位置为下金宝矿段北侧平硐中。
3.2 分析方法
本文挑选新鲜无蚀变的样品进行锆石U-Pb同位素年龄测试,挑选辉钼矿进行Re-Os同位素测年。锆石及辉钼矿的分选在北京天元永泰地质技术服务有限公司完成,在双目镜下将分选的锆石和标样粘贴在环氧树脂靶上,并对其进行磨制抛光,使锆石核部暴露。完成锆石制靶后,对锆石靶进行透反射照相及阴极发光(CL)照相,检查锆石内部结构,最后挑选出最佳的锆石测试部位。锆石的制靶和阴极发光(CL)照相均在中国地质科学院北京离子探测中心所完成,LA-ICP-MS U-Pb法测年在北京大学造山带与地壳演化教育部重点实验室完成,锆石LA-ICP-MS测年的精度和分析误差见文献(Blacketal.,2003;袁洪林等,2003;Zhaoetal.,2007)。辉钼矿样品Re、Os同位素测试工作在国家地质实验测试中心Re-Os同位素实验室完成,分析方法及程序详见有关文献(Duetal.,2004)。
图3 下营坊金矿下金宝沟矿段07勘探线剖面图⑤
4 分析结果
4.1 锆石年代学
本次挑选的锆石多为浅黄色-无色透明,以自形晶为主。CL图像显示(图6),锆石韵律环带发育,晶形基本完好,极少量锆石具有熔蚀凹陷;锆石颗粒较大,粒径在50um×80 um~150 um×280 um,长宽比介于1∶1~5∶1之间。而煌斑岩脉(Z4104)锆石内部均匀遍布密集环带特征,显示岩浆结晶时的环境温度是由高到低不断变化的过程(Rubattoetal.,2000)。锆石微量元素特征显示,Th/U比值均大于0.4,处于0.5~2.4之间,符合岩浆成因锆石特征(Corfuetal.,2003)。
本次测试数据点均分布在谐和线附近,且有效数据成群分布(图7)。其中花岗斑岩(Z4102)共测定19颗锆石,测点的206Pb/238U表观年龄值介于161Ma~166Ma之间(表2),加权平均年龄为163.32±0.90Ma(MSWD=0.58),代表了花岗斑岩的结晶年龄。煌斑岩脉(Z4104)共测定13颗锆石,测点的206Pb/238U表观年龄值介于159Ma~162Ma之间(表1),加权平均年龄为159.0±1.2Ma(MSWD=1.15)。
图4 下营坊金矿典型矿石类型
图5 下营坊金矿花岗斑岩及煌斑岩脉手标本及显微照片
4.2 辉钼矿Re-Os年代学
辉钼矿样品Re-Os同位素测试结果见表2。模式年龄t计算公式:t=1/λ[ln(1+187Os/187Re)],λ(187Re衰变常数)=1.666×10-11/a(Smoliaretal.,1996)。样品PD09模式年龄为164.2±2.3Ma。
5 讨论
5.1 成矿年代
本次分别对寄主岩石(花岗斑岩)和成矿后岩脉(煌斑岩)进行了锆石LA-ICP-MS U-Pb定年,其中花岗斑岩结晶年龄为163.32±0.90Ma,煌斑岩脉结晶年龄为159.0±1.5Ma,由此可限定成岩成矿年龄在163.32 Ma ~159.0 Ma之间;而由辉钼矿获得的Re-Os同位素模式年龄为164.2±2.3Ma,在误差范围内这与锆石限定的年龄是一致的,表明此次辉钼矿的Re-Os同位素模式年龄是可靠的,显示下营坊金矿成矿时代为中侏罗世,成矿时间为164.2±2.3Ma。区域上,在矿区外围存在大面积的侏罗系髫髻山组火山岩和火山碎屑岩,其年龄为162.7±1.4Ma(未发表数据),显示该区在164Ma左右岩浆热液活动是十分强烈的;而下营坊金矿与寄主岩石(花岗斑岩)成矿成岩年龄的一致性,表明下营坊金矿成矿作用与岩浆活动(大铜山杂岩体)的关系是十分密切的,为同一岩浆-热液活动的产物,这也进一步说明区内强烈岩浆活动是形成金矿的必要条件。煌斑岩脉的岩相学特征显示其具有碳酸盐化和绿泥石化,并未表现出与成矿热液有关的褪色蚀变现象,而在矿区外围煌斑岩脉的广泛分布,说明它仅可能是区域性的岩浆活动,它的侵位时间指示着下营坊金矿有关的岩浆热液活动的结束。
图6 下营坊金矿花岗斑岩及煌斑岩脉锆石的CL图像(图中数据分别表示206Pb/238U年龄)
图7 下营坊金矿花岗斑岩及煌斑岩脉锆石U-Pb年龄谐和曲线及加权平均年龄图
5.2 成矿动力学背景
多年来,冀东地区金矿年代学研究一直是众多研究者重点关注的,已取得了大量的年代学数据;尤其近年来,测年技术的发展,已获得一批高精度的年代学数据,详细数据见表3。从这些年代学数据看,冀东金矿不论在时间上还是空间上与区内燕山期岩浆岩存在显著的成因联系(周传雷等,2014)。且冀东金矿的成矿相对集中(成矿时间在164.2Ma~176.8Ma),表明该区很可能存在一个短时间的金矿成矿爆发期。如此短时间且巨量的金金属量的堆积暗示该地区必然存在一个特殊的成岩成矿动力学背景。
表1 下营坊金矿花岗斑岩及煌斑岩脉锆石U-Pb分析结果
Table 1 Zircon U-Pb isotope analysis results of granite porphyry and lamprophyre vein in Xiayingfang gold deposit
点号Pb(ppm)U(ppm)Th(ppm)232Th/238U207Pb/206Pb±1σ207Pb/235U±1σ206Pb/238U±1σ208Pb/232Th±1σ207Pb/235U±1σ206Pb/238U±1σZ4102-0110.60962330.60289.970.8771020.049170.001500.173900.005190.025660.000310.007900.0002216341632Z4102-025.673477194.8797.150.4985370.050520.002210.176600.007540.025360.000360.008540.0003916571612Z4102-0314.57522464.63386.810.8325120.047370.001150.165470.003910.025340.000280.007890.0001915531612Z4102-0413.56953414.51358.390.8646110.048080.001260.170420.004360.025720.000290.008720.0002116041642Z4102-056.286629207.29128.090.6179270.047780.001710.170630.006000.025910.000310.008080.0002816051652Z4102-065.329687175.7398.150.5585270.049430.002230.176710.007800.025940.000360.008810.0003916571652Z4102-077.338561239.98153.610.6400950.049610.001680.174060.005760.025450.000320.008770.0002816351622Z4102-088.211222271.39164.200.6050330.048450.001570.171840.005440.025730.000310.008370.0002816151642Z4102-098.057684278.50143.260.5143990.049470.001520.174170.005200.025540.000300.007600.0002816341632Z4102-1010.55171348.54223.310.6407010.052040.001400.183350.004790.025560.000300.008030.0002417141632Z4102-119.647440313.73229.660.7320310.051590.001570.181080.005350.025460.000310.007790.0002416951622Z4102-126.318702206.28149.530.7248890.050520.001930.179060.006680.025710.000330.007410.0002816761642Z4102-136.714340222.87123.270.5531030.049330.001900.176360.006620.025930.000340.008490.0003316561652Z4102-146.631723227.78117.210.5145750.050780.001730.178250.005910.025470.000320.008020.0003116751622Z4102-154.633057153.9485.920.5581400.050700.002270.179940.007910.025750.000350.008600.0004016871642Z4102-164.062574133.0278.310.5887080.049520.002550.176810.008940.02590.000370.008700.0004316581652Z4102-177.636516250.97158.040.6297170.050340.001680.177700.005800.025610.000310.008350.0002816651632Z4102-185.630594185.45106.500.5742790.048640.001950.174970.006860.026090.000330.008320.0003316461662Z4102-1915.51954488.86427.380.8742380.050600.001180.176760.004020.025340.000280.007780.0002016531612Z4104-017.95030180.75440.102.4348550.051280.002110.176950.007100.025030.000330.007770.0002216561592Z4104-024.625524132.94163.261.2280730.047240.002650.164260.009050.025220.000380.008050.0003115481612Z4104-031.64991047.8259.951.253660.049410.008080.165280.026750.024260.000650.008340.00061155231554Z4104-0410.64585366.72188.080.5128710.049050.001340.171730.004560.025400.000300.007910.0002816141622Z4104-055.589575186.32119.890.6434630.053320.002010.186990.006860.025440.000340.007540.0003317461622Z4104-068.234000276.79176.440.6374510.049180.001570.169760.005270.025040.000310.007980.0002915951592Z4104-074.626243159.9190.440.5655680.047080.002100.162660.007120.025060.000320.007660.0003415361602Z4104-084.046827106.70180.881.6952200.047120.002690.160150.008990.024650.000350.007930.0002715181572Z4104-092.37672066.2995.811.4453160.047250.005710.163770.019400.025140.000700.007600.00054154171604Z4104-102.44543273.7889.461.2125240.052020.003840.176300.012760.024580.000440.007230.00039165111573Z4104-116.300750210.70128.160.6082580.054370.002200.187820.007390.025060.000350.008340.0003817561602Z4104-124.530007116.76211.171.8085820.053230.002850.182380.009540.024850.000380.007670.0002717081582Z4104-139.974520260.63480.401.8432260.052570.001800.175720.005850.024250.000310.007580.0002316451542
注:表中数据由北京大学造山带与地壳演化教育部重点实验室完成,测试时间为2014年。
表2 下营坊金矿辉钼矿Re-Os同位素分析结果
Table 1 Re-Os isotopic analytic results of molybdenite in Xiayingfang gold deposit
原样名样重(g)Reng/gC普Osng/gRe187ng/gOs187ng/g模式年龄Ma测定值不确定度测定值不确定度测定值不确定度测定值不确定度测定值不确定度PD090.60083957.47.30.01700.0008601.74.61.6480.010164.22.3
注:本表数据在国家地质实验测试中心Re-Os同位素实验室完成,测试时间为2015年。
表3 冀东地区主要金矿床年代学统计
Table 3 Chronology statistics of major gold deposits in eastern Hebei province
矿床名称矿床类型赋矿地层岩浆岩测试对象测试方法成矿时代(Ma)资料来源金厂峪后韧性剪切带型/岩浆热液型太古界迁西群变质杂岩带青山口岩体蚀变岩绢云母K-Ar169.8Ma贾三石等,2011辉钼矿Re-Os242.6±6.8Ma宋扬等,2011青山口岩体U-Pb199.1±2Ma罗振宽等,2001a峪耳崖斑岩型/中低温热液型/石英脉型长城系高于庄组峪耳崖岩体灰白色花岗岩和肉红色花岗岩U-Pb175±1Ma/174±3Ma罗振宽等,2001b辉钼矿Re-Os169.84±0.95Ma陈绍聪等,2014含金石英脉Rb-Sr189.0Ma、163.8Ma贾三石等,2011牛心山石英脉型太古界迁西群变质岩牛心山花岗岩体花岗岩体U-Pb173±2Ma/172±2Ma罗振宽等,2001b;郭少丰等,2009含金石英脉Ar-Ar175.8Ma白洪生等,1992;李颖等,1996矿石中鳞片状白云母K-Ar187Ma陈斌,2009唐杖子角砾岩型金矿床长城系地层花岗斑岩花岗斑岩U-Pb173±2Ma郭少丰等,2009;辉钼矿Re-Os170.1±1.6Ma李正远等,2014山家湾子似层状金矿蚀变矿物Rb-Sr176.8Ma吴珍汉等,1998毛家店石英脉型太古界八道河群王厂组变质岩黑云母花岗岩黑云母花岗岩K-Ar168Ma敖颖锋等,2008下营坊斑岩型-角砾岩型长城系地层大东山杂岩体花岗斑岩、煌斑岩U-Pb163.32±0.90Ma、159.0±1.5Ma本文辉钼矿Re-Os164.2±2.3Ma本文
冀东乃至华北地台北缘的构造演化一直以来是学者的研究重点。晚古生代以来,该地区先后受到了古亚洲闭合、蒙古-鄂霍茨克洋关闭和古太平洋俯冲等周边板片俯冲-碰撞事件的影响(董树文等,2008)。尤其自中生代以来,本区进入了广泛的板内造山阶段(即燕山造山带),强烈的板内挤压变形形成了一系列的推覆构造和伸展构造。而这种陆内造山似乎存在阶段性,表现出230Ma~210Ma和180(170)Ma~160(150)Ma两期挤压构造(翟明国等,2006),同时伴随多期次的火山喷发和岩浆侵入活动(代军治等,2006)。而这种多期次的火山岩浆活动所受控的区域构造背景显然是不相同的,早侏罗世南大岭期(兴隆沟期)火山岩展布与源区明显受到古亚洲洋俯冲的影响(李晓勇等,2004)。中-晚侏罗世髫髻山期(即166Ma~153Ma)火山岩的形成很可能与古太平洋向亚洲大陆快速、斜向俯冲有关(李伍平等,2007;马强等,2013)。随着古太平洋板块俯冲的影响,华北地块北缘的构造体系发生了转换,表现为形成一系列NE向褶皱-断裂系统(陈云峰等,2005;2007),而这种构造转换时间很可能发在160Ma前后(赵越等,2004;赵丽君,2010)。岩石地球化学研究显示,中-晚侏罗世火山岩主体是由幔源岩浆的底侵导致的古老下地壳物质的部分熔融,以及与幔源岩浆的物质交换的产物,而这种幔源岩浆的底侵很可能受古太平洋板块的俯冲的影响(Guoetal.,2007;Wangetal.,2007;马强等,2013)。
下营坊金矿以及冀东中侏罗世的金矿均集中分布于冀东辽西地区(图1),时间上与冀东辽西地区中侏罗世火山岩(海房沟组、下花园组、郭家庄组)及中晚侏罗世的髫髻山组(蓝旗组)火山岩形成时间一致。根据上述前人对区域成矿背景研究,冀东中侏罗世金矿的成岩成矿构造背景与区内该时期的岩浆事件一样,可能受古太平洋板块俯冲的影响而处于挤压的构造环境。在板块的俯冲影响下使燕山造山带下部的地幔发生部分熔融,导致地幔岩浆的底侵,诱发了古老下地壳物质的部分熔融,从而在该地区集中形成一系列岩浆-热液活动。
6 结论
(1) 下营坊金矿赋矿斑岩及成矿期后岩脉的成岩年龄分别为163.32±0.90Ma、159.0±1.5Ma,辉钼矿Re-Os同位素成矿年龄为164.2±2.3Ma,三者误差范围内一致,显示其成矿属于中侏罗世岩浆热液事件。
(2) 区内金矿年代学统计显示,冀东金矿的成矿相对集中(成矿时间在164.2Ma~176.8Ma),表明该区很可能存在一个短时间的金矿爆发期。通过区内地质事件对比研究认为,冀东中侏罗世金矿的成岩成矿构造环境与区内该时期的岩浆事件一样,可能受古太平洋板块俯冲的影响而处于挤压的构造环境。
致谢:野外工作获得了金宝矿业赵西波经理及中色地科矿产勘查股份有限公司项目经理王冠龙的大力支持,锆石年龄测试得到了北京大学造山带与地壳演化教育部重点实验室马芳老师的协助,辉钼矿Re-Os同位素年龄测试得到了中国地质科学院国家地质实验测试中心李超博士的热心帮助,岩矿鉴定过程中获得了姜福芝老师的指导,在此表示衷心感谢!同时,也由衷地感谢审稿专家的评阅与建议!
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Geochronology of the Xiayingfang Au Deposit in Eastern Hebei Province
ZOU Tao1,WANG Yu-wang1,WANG Jing-bin1,ZHANG Hui-qiong1,ZHAO Lu-tong1,XIE Hong-jing1,SHI Yu1,LIU Yong-zhen2,LIU Gui-quan3
(1.BeijingInstituteofGeologyforMineralResources,Beijing1000122.ChinaNon-ferrousMetalResourcesGeologicalSurvey,Beijing100012;3.SinotechMineralsExplorationCO.,LTD.,Beijing100012)
The Xiayingfang Au deposit,an important Au deposit in eastern Heibei Province,is located in the eastern side of the Yanshan orogenic belt,North China Ctraton.Its Au ore bodies can be classified into porphyry,breccia and skarn types,which are hosted in the Datongshan complex and its external contact zone,both of which are typical features of a porphyry gold metallogenic system.In order to unraveling the mineralization age of this deposit and its relation with other Au deposits in eastern Heibei Province,this work adopted zircon U-Pb and molybdenite Re-Os dating technology for chronology.The results show that zircon U-Pb ages of granite porphyry hosting Au ore bodies and lamprophyric vein cutting the ore bodies are 163.32±0.90 Ma and 159.0±1.5Ma,respectively.The mineralization age is determined as between 163.32Ma to 159.0Ma.Re-Os model age of molybdenite is 164.2±2.3Ma,which is consistent with the age gap defined by magmatic rocks,and suggests that mineralization age of the Xiayingfang Au deposit is 164.2±2.3Ma,belonging to Middle Jurassic.Combined with previous research results,this work suggests that the Xiayingfang Au deposit formed in a compressive environment influenced by the subduction of the paleo-Pacific plate,just like magma events and the Au deposits of middle Jurassic in eastern Hebei province.
geochronology,U-Pb,Re-Os isotope,Xiayingfang Au deposit,Eastern Hebei
2015-05-17;
2015-12-25;[责任编辑]陈伟军。
国家自然科学基金(41372102)、国家“973”计划 (2014CB440803)、中国地质调查局河北承德下营坊地区矿产地质调查项目(12120113059800)和老矿山典型矿床成矿规律总结研究(12120113086300)的联合资助。
邹 滔(1983年-),男,工程师,主要从事矿床学及岩石地球化学的研究。E-mail:13436751833@163.com。
P618.51
A
0495-5331(2016)01-0084-14