哀牢山长安金矿成因机制及动力学背景初探:来自LA-ICP-MS锆石U-Pb定年和黄铁矿原位微量元素测定的证据**
2014-03-14田广张长青彭惠娟周云满李建荣张星培胡明月TIANGuangZHANGChangQingPENGHuiJuanZHOUYunManLIJianRongZHANGXingPeiandHUMingYue
田广 张长青 彭惠娟 周云满 李建荣 张星培 胡明月TIAN Guang, ZHANG ChangQing*, PENG HuiJuan, ZHOU YunMan, LI JianRong, ZHANG XingPei and HU MingYue
1. 中国地质大学地球科学与资源学院,北京 1000832. 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 1000373. 云南黄金矿业集团股份有限公司,昆明 6502244. 国家地质实验测试中心,北京 1000371. School of Earth Science and Mineral Resources, China University of Geosciences, Beijing 100083, China2. MLR Key Laboratory of Metallogeny and Mineral Assessment, Institute of Mineral Resources, CAGS, Beijing 100037 China3. Yunnan Gold Group Co. Ltd, Kunming 650000 China4. National Research Center for Geoanalysis, Beijing 100037 China2013-09-01 收稿, 2013-12-09 改回.
印度与欧亚大陆碰撞引发西南三江地区在新生代时期发生了强烈的陆内变形,形成了一系列NW-NNW走向的深大断裂带,如哀牢山-红河断裂带(Wangetal., 2001);与之相伴出露大面积的富碱侵入岩体,形成了长达千余千米,宽50~80km的富碱侵入岩带(张玉泉等,1997;王登红等, 2004; Zhangetal., 2009)。该富碱侵入岩带内发育有一系列大中型金、铜、钼等金属矿床,如北衙金多金属矿、马厂菁铜钼金矿、大坪金矿、镇沅金矿、铜厂铜钼金矿、长安冲铜钼金矿、长安金矿等,成为中国重要的斑岩铜-钼-金成矿省和成矿远景区之一。该成矿省中,金矿床与喜马拉雅山期富碱斑岩在时空和成因上的密切关系及其独具特色的新生代富碱斑岩型金多金属成矿系统引起了大量学者的关注(毛景文等,2005;Geetal., 2009; 邓军等,2010b;Dengetal., 2010;Maoetal., 2013; Tranetal., 2013)。
随着地质科学的不断发展,矿物的微量元素组成、特征对于提供成矿流体、成矿物质来源、成矿物理化学条件及矿床成因等科学信息的报道越来越多。激光剥蚀电感耦合等离子质谱(LA-ICP-MS)作为新的测试技术,以其抗干扰能力强、检出限低、灵敏度高等优势,受到学者们的广泛关注(Cooketal., 2009; Koglinetal., 2010; Nadoll and Koening, 2011; 胡明月等, 2008;周涛发等, 2010;段超等, 2012)。黄铁矿作为地壳中分布最广的硫化物,它不仅能在沉积岩、岩浆岩和变质岩中形成,而且是重要的载金矿物;其地球化学成分的变化特征是一个重要的矿物成因指示(Braliaetal., 1979; Henkelman, 2004),通过黄铁矿微量元素含量和比值,可以推断黄铁矿的类型与成因,提供成矿物质来源、成矿流体来源、矿床成因等信息。黄铁矿微区微量元素研究虽然仍处于起步阶段,但目前已获得的一些重要的科研成果,表明它为金矿成矿作用研究提供了一个新的途径(Largeetal., 2009; Gregoryetal., 2013)。
长安金矿是云南省地质调查院第二地质矿产调查所于2001年间进行金铜矿资源调查时发现的具有经济意义的矿床。该矿位于东经103°02′00″,北纬22°48′30″,隶属于金平县铜厂乡,金属量达30t,是哀牢山-红河断裂南段的一个大型金矿床,且与铜厂铜钼金矿床,长安冲铜钼金矿床相邻产出。作为哀牢山成矿带的重要组成部分之一,长安金矿的成因类型受到了广泛的关注,矿区内脉岩广布,金成矿与新生代富碱斑岩之间的关系成为众多学者研究方向之一。前人的研究表明,长安金矿的形成受到了深部幔源物质的影响,成矿流体具有岩浆水与地层变质水混合的特征(应汉龙等,2006;和中华等,2008;郭春影等,2009;张静等,2010;Chenetal., 2010; 李士辉等,2011;Tranetal., 2013)。哀牢山成矿带内大多数斑岩型矿床,岩浆岩呈多期次(17~55Ma)活动的特点,并且观察到有多期成矿事件的存在(曾普胜等,2006)。因此长安金矿区中脉岩的成岩年代以及金成矿与新生代富碱斑岩之间的关系都有待进一步的研究。为此本文以与长安金矿矿体密切共生的脉岩年龄及黄铁矿微量元素的化学组成为切入点,来探讨长安金矿的成矿机制过程及动力学背景,为进一步的完善富碱斑岩金多金属成矿系统提供新的信息。
1 区域及矿床地质特征
1.1 区域地质
长安金矿为与哀牢山逆冲推覆构造带金平推覆体的中南部,金平推覆体呈楔形夹持于绿春推覆体和哀牢山基底推覆体之间。构造带内发育了三条主要断裂,它们沿走向向北西收敛。带内出露的地层由这三条深大断裂所挟持(图1)。以中部的哀牢山深大断裂为界,东部位深变质带(古元古界哀牢山群),变质程度达角闪岩相;西部为浅变质带(局部为中生界未变质地层所覆盖),地层是低绿片岩相古生界及上三叠统,哀牢山群沿金沙江-哀牢山断裂向南西推覆到上三叠统之上;两条变质带共同组成“双变质带”,变质带向北随3条断裂带合并而消失。在九甲-安定深大断裂以西出露的地层则主要为中生界未变质地层。断裂带内岩浆岩特别发育,岩浆据有多旋回、多样性的特点,沿断裂形成三个主要岩浆带,即哀牢山断裂两侧的超基性岩带,九甲-安定断裂东侧的基性岩带和哀牢山断裂两侧的富碱侵入岩带。
图1 哀牢山推覆构造分带图(据李定谋等,1998修改)Fig.1 Zoning map for nappe structure of the Ailaoshan region (modified after Li et al., 1998)
1.2 矿床地质
长安金矿床位于金平推覆体中的NW向推覆构造滑脱面内的脆性破碎带中,与铜厂、长安冲铜钼金矿床相伴产出(图1),矿区范围内出露的地层由北至南呈现从老至新分布,依次为下奥陶统向阳组粉砂岩(O1x)、中上志留统康廊组白云岩(S2-3k)、下泥盆统青山组白云质灰岩(D1q)、中泥盆统烂泥箐组灰岩(D2ln)、上泥盆统干沟组灰岩(D3gg)及石炭系尖山营组灰岩与白云质灰岩(Cj)(图2),其中向阳组粉砂岩为主要含矿地层与康廊组白云岩呈假整合接触。
图2 矿区地质简图(据和中华等,2008修改)Fig.2 Geological sketch map of the Chang’an gold deposit (modified after He et al., 2008)
区域构造线的主导方向受NW向的哀牢山深断裂和藤条河断裂控制,使矿区内的多数次级断裂呈NW向展布。甘河断裂(F5):走向北西,贯穿矿区。总体倾向南,地表局部倾向北,倾角80°。破碎带宽度100~200m,内含10.8m厚的糜棱岩。岩性为碎裂白云岩和灰质糜棱岩,部分地段具黄铁矿化和金矿化,断层泥内含金品位大于3g/t,为主要的含金断裂。F6断裂走向北西,与F5平行,在铜厂街附近与S2-3/O1界面耦合,F6早期为压扭性断裂,后期为张性断裂。对长安金矿成矿起着重要作用,长安金矿最主要的V5矿体就产于F6断裂与不整合面共同控制的构造破碎带中。F7断裂走向北东。以西的S2-3地层走向北西,以东的地层走向北东,分布岩株式正长斑岩,错切S2-3/O1界线。F8断裂走向北东,倾向南东,倾角40°~60°,下盘地层S2-3,上盘D1q,大部分地段缺D1底部的石英砾岩层。部分地段的破碎带宽10~50m,构造岩由碎裂白云岩、糜棱岩和角砾岩组成,断层性质为压扭性。
矿区内发育多种类型的岩浆岩,主要包括基性岩类、中-基性碱性脉岩类。基性岩类主要为辉绿岩和辉长岩,呈脉状产出,侵位于上述沉积地层中,走向与地层走向基本一致,矿物成分为斜长石、单斜辉石、橄榄石、石英等,发育有黏土化、绢云母化,碳酸盐化,蛇纹石化。碱性脉岩类主要包括煌斑岩和正长岩类。正长岩类包括正长斑岩、正长岩、细晶正长岩,呈脉状产出,宽约2~3m(图3)。由钾长石、微斜长石、环带斜长石、黑云母、角闪石、少量石英组成,副矿物为磷灰石、檐石、独居石、钻石、磷忆矿。
图3 脉岩产出及镜下照片(a)-侵入到V5矿体中的细晶正长岩脉;(b)-细晶正长岩脉和煌斑岩脉(V5矿体剖面);(c)-细晶正长岩手标本;(d)-正长斑岩手标本;(e)-细晶正长岩(CA034,正交偏光);(f)-正长斑岩(Zk805,正交偏光)Fig.3 Petrography of the magmatites of Chang’an gold deposit
煌斑岩在矿区普遍产出,呈脉状。多数煌斑岩脉体边缘与围岩接触带风化后常形成残积型金矿。少量脉体外接触带发现有斑点浸染状雄黄。岩石由斑晶和基质组成,基质具显微粉晶结构,斑晶为云母、钾长石和少量石英。基质以微粒正长石、石英为主,并有少量云母和铁质星点分布。(云南地矿资源股份有限公司,2002*云南地矿资源股份有限公司. 2002. 云南省金平县长安金矿详查地质报告)。
1.3 矿体地质特征
长安金矿床由V1-V9矿体组成,其中仅V5矿体达到详查程度,其估算的资源量占矿区资源总量的95%以上。V5矿体产于F6断裂与志留系、奥陶系之间发育的不整合面重合部位,受构造破碎带的控制,矿体整体产状与F6断层一致。矿体总长度约1800m,矿体厚度为0.86~39.49m,整体为较大透镜状,走向340°,倾向北东东,倾角20°~90°,矿体上缓下陡、1600m标高以上倾角20°~60°,1600m标高以下矿体近于直立、局部出现反倾(倾向南西西)、呈倒“S”型,沿倾向延伸最大达510m。矿体在走向及倾向上具有膨大缩小及舒缓波状变化特征,矿体浅部品位高、厚度大、连续性好,深部呈品位变低、局部不连续。含矿岩石主要为下奥陶统向阳组炭质泥岩、破碎的粉砂岩、石英细砂岩、石英砾岩等。矿石矿物主要有黄铁矿、毒砂,少量黄铜矿、闪锌矿、方铅矿、辰砂、自然金、银金矿、自然银。脉石矿物主要为石英、长石、白云石、绢云母等,次为方解石、白云石、白云母、伊利水云母、蒙脱石、高岭石等,少量绿泥石、黝帘石、沸石、葡萄石等。矿石具自形-半自形晶结构、它形晶结构、镶嵌结构、包含结构等;矿石构造主要有碎裂状构造、角砾状构造、浸染状构造、细网脉状构造、团块状构造等。成矿阶段可分为:黄铁矿-石英阶段、黄铁矿-石英-绢云母阶段、黄铁矿-方解石-石英阶段及后期硅化阶段。与金矿化有关的蚀变有:硅化、黄铁矿化、碳酸盐化、石英细脉化、毒砂化、绢英岩化、局部有闪锌矿、方铅矿化。
2 实验样品和测试方法
2.1 实验样品
锆石U-Pb定年的样品采自长安金矿V5矿体中与金矿体密切共生的细晶正长岩及矿区8勘探线钻孔内的正长斑岩。
黄铁矿微量元素分析的样品选自长安金矿区矿石、围岩及细晶正长岩中的黄铁矿和铜厂铜钼金矿斑岩体、长安冲铜钼金矿斑岩体中的黄铁矿。
图4 不同样品中黄铁矿的镜下照片Fig.4 Photomicrographs of pyrite from different samples
铜厂铜钼金矿斑岩中的黄铁矿(简称PyⅠ)和长安冲铜钼金矿矿石中的黄铁矿(简称PyⅡ),均呈浅黄色,晶体形态为自形-半自形,粒状结构,自形程度较好,与黄铜矿紧密共生;长安金矿细晶正长岩中的黄铁矿(简称PyⅢ),呈黄色,晶体形态半自形-自形,粒状结构。长安金矿围岩-O1x粉砂岩中的黄铁矿(简称PyⅣ),这类黄铁矿周围分布着大量微小球状黄铁矿,从黄铁矿的形态和分布来看,受到了后期热液叠加作用而成。角砾岩型矿石中的黄体矿(简称PyⅤ)呈浅黄色,自形程度差,胶状构造,为低温热液的产物,与毒砂共生(图4)。
2.2 实验方法
锆石U-Pb定年在中国地质科学院矿产资源研究所激光多接收等离子质谱LA-MC-ICP-MS实验室完成。所用测试仪器为Finnigan Neptune型MC-ICP-MS及与之配套的New Wave UP-213 激光剥蚀系统。测试时激光束斑直径为30μm,剥蚀深度20~40μm,激光脉冲10Hz,能量34~40mJ;电感耦合等离子体质谱(ICP-MS)系统为Agilent 7500a。锆石的同位素组成以锆石91500为外标进行校正,微量元素组成以玻璃标样NIST610做外标,SiO2含量为内标进行校正(Yuanetal., 2004)。锆石微量和同位素数据采用GLITTER程序,普通铅校正采用Andersen(2002)的方法,年龄计算使用Isoplot(ver3.00)完成(Ludwig, 2003)。详细实验测试过程可参见侯可军等(2009)。
黄铁矿激光剥蚀等离子质谱实验在国家地质实验测试中心完成。使用仪器为Thermo ElementⅡ等离子质谱仪,激光剥蚀系统为New Wave UP-213。实验采用He作为剥蚀物质的载气,激光波长213nm、束斑40μm、脉冲频率10Hz、能量0.176mJ、密度23~25J/m2,测试过程中首先遮挡激光束进行空白背景采集15s,然后进行样品连续剥蚀采集45s,停止剥蚀后继续吹扫15s清洗进样系统,单点测试分析时间75s。等离子质谱测试参数为冷却气流速(Ar)15.55L/min; 辅助气流速(Ar)0.67L/min; 载气流速(He)0.58L/min; 样品气流速0.819L/min,射频发生器功率1205W。测试数据采外标法,标样使用NIST-612,MASS-1。
3 测试结果
3.1 成岩年代
细晶正长岩(CA005)、正长斑岩(zk805)测试样品中锆石均为半自形-自形晶,无色透明,粒径一般为100~200μm,呈短柱状,长宽比约2:1,锆石阴极发光(CL)图像(图5)中振荡环带发育,应为典型的岩浆结晶锆石。选取其中裂纹不发育的20颗锆石进行U-Pb年龄分析,LA-ICP-MS锆石测年数据见表1,谐和图见图5。由表1可以看出所选锆石的Th/U均大于0.1,显示典型的岩浆锆石Th/U值特征(Rowleyetal., 1997; Crofuetal., 2003)。其中细晶正长岩样品(CA005)中17个数据点的206Pb/238U年龄测定得出加权平均年龄:32.5±0.1Ma, MSWD=0.11。正长斑岩ZK805的17个数据点的206Pb/238U 年龄测定得出加权平均年龄:33.0±0.1Ma, MSWD=1.18(图5)。上述年龄可以代表这两脉岩锆石的结晶年龄。
3.2 黄铁矿微量元素
由于Co、Ni、As、Sb、Se是黄铁矿中最常见的元素,Co、Ni可以类质同象的形式替换黄铁矿中的Fe;As、Sb、Se可以取代S。因此选取Co、Ni、As、Sb、Se及成矿元素进行分析(表2、图6)。
PyⅠ以富Co、Se,贫Ni、As等元素为特征。其中Co的含量最高,为215×10-6~30542×10-6,平均为13713×10-6,Se含量为19.7×10-6~85.78×10-6,平均为43.7×10-6,Ni含量为18.59×10-6~241.5×10-6,平均为65.6×10-6,As含量为0.028×10-6~30.42×10-6,平均为8.08×10-6,Au含量值较低0×10-6~0.878×10-6,平均为0.473×10-6。
相对PyI黄铁矿,PyⅡ黄铁矿的Co含量有所减少,Ni、As、Se的含量相当,仍是以富Co贫Ni、As等元素为特征。其中Co的含量为100.2×10-6~1208×10-6,平均为513.8×10-6,Se含量为19.86×10-6~66.91×10-6,平均为48.2×10-6,Ni含量为5.694×10-6~52.79×10-6,平均为24.32×10-6,As含量为1.663×10-6~25.12×10-6,平均为9.15×10-6,Au含量值较低,为0×10-6~0.86×10-6,平均为0.228×10-6。
PyⅢ与PyⅠ、PyⅡ比较,Co含量略有减少,Se含量明显降低,As、Ni含量则显著增加,Cu、Pb、Au、Ag等成矿元素的含量也有所增加。其中Co含量为55.82×10-6~942.7×10-6,平均为421.8×10-6, Se含量为0.037×10-6~34.31×10-6,平均为4.96×10-6,Ni的含量为15.49×10-6~382.7×10-6,平均为173.7×10-6,As的含量为2147×10-6~15144×10-6,平均为8722.5×10-6,Au含量为0.771×10-6~20.45×10-6,平均为11.27×10-6;Cu平均为12.5×10-6;Pb平均为32.86×10-6;Ag平均为1.89×10-6。
表1长安金矿锆石LA-ICP-MS测年结果
Table 1 Zircon LA- ICP-MS U-Pb dating results of the Chang’an gold deposit
测点号232Th238UTh/U207Pb/206Pb207Pb/235U206Pb/238U207Pb/206Pb207Pb/235U206Pb/238U(×10-6)RatioRatio1sigmaRatio1sigmaRatio1sigmaAge(Ma)1sigmaAge(Ma)1sigmaAge(Ma)1sigmaCA005-1238.3599.20.400.04820.00060.03370.00060.00510.0000109.431.533.70.632.50.2CA005-278.5242.60.320.04750.00100.03300.00070.00510.000072.351.833.00.732.50.2CA005-3174.0446.50.390.04880.00070.03400.00050.00510.0000139.033.334.00.532.50.2CA005-4310.4592.70.520.04720.00050.03300.00040.00510.000057.525.932.90.432.60.2CA005-5223.2354.70.630.04690.00080.03270.00050.00510.000055.738.932.70.532.50.2CA005-7236.7534.20.440.04830.00170.03360.00100.00510.0001122.381.533.61.032.50.4CA005-887.7172.00.510.04890.00220.03390.00150.00510.0001142.7110.233.91.532.50.4CA005-960.5254.80.240.04650.00110.03210.00080.00500.000020.559.332.10.832.40.3CA005-10181.9359.90.510.04780.00070.03330.00050.00510.000087.133.333.30.532.50.2CA005-12107.9304.40.350.04820.00090.03350.00060.00510.0000109.444.433.50.632.50.3CA005-13215.9401.50.540.04720.00060.03300.00040.00510.000057.529.633.00.432.70.2CA005-14108.2324.60.330.04790.00090.03330.00070.00500.000094.541.733.30.732.50.3CA005-15250.4540.50.460.04730.00060.03290.00040.00510.000064.932.432.90.432.50.2CA005-16327.8756.40.430.04850.00060.03370.00040.00500.0000124.227.833.60.432.40.3CA005-18188.0597.00.310.04830.00050.03360.00040.00500.0000122.325.933.50.432.40.2CA005-19430.9634.60.680.04790.00050.03340.00040.00510.000094.521.333.30.432.50.2CA005-20102.9275.70.370.04810.00290.03360.00220.00510.0001101.9137.033.52.232.50.6zk805-1123.2116.21.060.05450.00180.03840.00130.00510.0000394.574.138.21.232.90.2zk805-298.8114.30.860.05300.00160.03700.00110.00510.0000327.868.536.91.132.80.2zk805-388.698.20.900.05320.00220.03680.00150.00500.0000344.594.436.71.532.40.2zk805-447.464.10.740.05780.00270.04070.00190.00510.0000524.1100.040.51.832.90.3zk805-5121.2140.40.860.05130.00130.03650.00090.00520.0000253.863.936.40.933.30.2zk805-699.7104.80.950.05210.00200.03650.00140.00510.0000287.188.936.41.432.80.2zk805-788.1105.80.830.05270.00200.03710.00140.00510.0000316.7119.437.01.433.00.3zk805-8106.799.41.070.04920.00200.03480.00140.00510.0000166.892.634.71.333.00.3zk805-9102.7111.90.920.05160.00180.03690.00130.00520.0000333.479.636.71.333.20.3zk805-10101.8111.40.910.05160.00170.03680.00120.00520.0000333.475.936.71.233.30.3zk805-11136.799.51.370.05220.00190.03680.00140.00510.0000294.583.336.71.333.00.3zk805-1297.0112.10.860.05310.00190.03780.00140.00520.0000331.578.737.71.433.30.2zk805-13154.1136.01.130.04760.00150.03410.00110.00520.000079.7-123.134.11.033.50.3zk805-16190.8179.31.060.05040.00120.03570.00090.00510.0000213.083.335.60.933.10.3zk805-17146.5155.20.940.05050.00130.03590.00090.00520.0000216.754.635.80.933.20.3zk805-19297.9328.00.910.05800.00180.04140.00160.00520.0001527.870.441.21.533.10.3zk805-20194.3240.70.810.04740.00100.03320.00080.00510.000177.950.033.20.732.80.3
图5 锆石背散射照片及LA-ICP-MS U- Pb谐和图Fig.5 Back scattering electron images and LA-ICP-MS U-Pb concordia diagram of zircons
PyⅣ中Co含量与PyⅡ、PyⅢ相当,低于PyⅠ;Se含量低于PyⅠ、PyⅡ,高于PyⅢ;Au、As含量高于PyⅠ、PyⅡ,低于PyⅢ;Ni、Cu、Zn、Pb、Ag等含量较PyⅠ、PyⅡ、PyⅢ显著增加;以含Tl为特征。Co为17.37×10-6~649.6×10-6,平均为410.0×10-6,Se含量为0.075×10-6~21.55×10-6,平均为9.1×10-6,Ni含量为286.6×10-6~1877×10-6,平均为1203×10-6;As含量为558.4×10-6~5619×10-6,平均为2323×10-6,Au含量为0×10-6~7.665×10-6,平均为2.73×10-6;Cu平均为355.2×10-6;Pb平均为620.7×10-6;Zn平均为48.99×10-6;Ag平均为30.2×10-6。
PyⅤ与PyⅣ中Co、Ni、Cu、Pb、Zn、Ag含量均有所降低,As、Au含量显著升高,Co含量为9.044×10-6~63×10-6,平均为38×10-6,Se含量为0×10-6~33.41×10-6,平均为5.8×10-6,Ni含量为122.7×10-6~651.4×10-6,平均为344.8×10-6,As含量为7121×10-6~46492×10-6,平均为22075×10-6,Au含量为45.71×10-6~375.2×10-6;平均为156.1×10-6,Cu平均为166.2×10-6;Pb平均为588.7×10-6;Zn平均为20.4×10-6;Ag平均为7.12×10-6。
4 讨论
4.1 成岩与成矿的时限
长安金矿及相邻矿区已有的年代学资料显示,长安金矿区煌斑岩中黑云母39Ar-40Ar年龄坪年龄为35.62±0.16Ma,等时线年龄为35.27±0.74Ma(王勇,2008)。金平铜厂铜钼金矿床赋矿岩体二长花岗斑岩的形成年代为35.1±0.3Ma(黄波等,2009);铜厂辉钼矿Re-Os等时线年龄34.4±0.5Ma(王登红等,2004);长安冲辉钼矿等时线年龄为34.5±0.7Ma(胥磊落等,2010)。
矿床地球化学研究表明,长安金矿矿石206Pb/204Pb、207Pb/204Pb、208Pb/204Pb比值分别为:18.998~19.575、15.711~15.784、39.494~40.200,与富碱斑岩的同位素比值基本相同(Tranetal., 2013),流体包裹体中的δ13C、δD、δ18O分别为:-3.5‰~8.749‰、-118‰~-78.383‰、10.527‰~13.565‰,暗示矿区内岩浆活动带来的岩浆热液参与了成矿作用(李士辉等,2011)。铜厂铜钼金矿石英正长斑岩中硫化物的δ34S为-1.1‰~0.9‰,矽卡岩中硫化物的δ34S为-0.1‰~1.3‰,长安金矿中的细晶正长岩δ34S为-3.5‰~-1.6‰,矿石δ34S为-13‰~3.6‰,主要集中在-0.1‰~1.3‰ (表3、图7)。幔源或者源于深部与岩浆作用有关硫化物的硫同位素组成特征δ34S约为0‰(Ohmoto,1972),斑岩型矿床中硫化物δ34S一般介于-3.5‰~1.0‰之间(Rollinson,1993),上述岩石、矿石中的黄铁矿δ34S值均符合这一特征,可以认为两个矿床中的成矿热液基本来源于已基本均一化的深源硫,而两个矿床又受同一构造-热事件的控制,表明矿石和围岩中的黄铁矿形成的硫源主要为岩浆热液,且与铜厂、长安冲铜钼金矿中硫化物的形成是一个连续的岩浆热过程。
表2长安金矿黄铁矿微量元素(×10-6)
Table 2 Trace element analysis for pyrite from Tongchang Chang’anchong and Chang’an deposit (×10-6)
样品测点号CoNiCo/NiCuZnAsSeAgAuTlPbBiSbPyⅠTC007_13054277.25395.371.534.4712.51564.60.075000.0460.0520TC007_210020241.541.491.6255.0966.10165.680.03200.01800.1160.079TC007_31353023.14584.701.4716.1625.74361.260.3660.8780.0520.130.0370.186TC007_41153829.24394.601407.85213.0219.71.23300.0653.29800.325TC007_51754686.57202.680.9983.4450.02885.780.012000.0360.0080.019TC007_61494626.49564.214.1772.2077.73224.70.25100.066000.074TC007_71115418.59600.00167317.3830.4221.53.40200.05626.160.710.739TC007_81393331.64440.361.5657.826.17324.980.0150.0690000TC007_9215.456.143.84226.0121.00425.020.106000.3481.0780.018PyⅡCAC006_1557.711.6447.912.7022.8381.97221.740.0350.6860.0278.5716.1780.089CAC006_2868.747.7118.21161.721.2612.1157.6900.1680.03712.74.220.001CAC006_3902.852.7917.100.7783.7877.32458.560000.6660.3070.028CAC006_4120844.926.900.8826.18125.1255.910.08200.0195.7070.6740CAC006_5644.443.5714.790.6477.32612.8463.250.026000.3030.1560CAC006_645116.3527.580.9364.2019.91851.430.11600.0170.1010.0220.103CAC006_7138.55.71424.240.0056.3256.63834.720.0050.56100.030.0040CAC006_8109.75.81418.871.0536.7368.93266.910.09100.0071.45700.037CAC006_9157.15.69427.594.9579.7791.66319.860.67200.02215.1629.50.322CAC006_10100.29.01211.120.2164.9034.96752.320.1210.860.0283.6560.0070.165PyⅢCA9-3_495.9842.672.257.7476.08579730.3970.43714.020.26716.885.0884.181CA9-3_6389.5176.42.2129.929.97328521.9685.6961.6741.10896.1177.8816.73CA9-3_81465334.24.3815.137.30559800.7382.7895.4740.39352.826.089.748CA9-2_4161.755.042.946.4076.06195462.7690.46314.440.3910.752.7473.657CA9-2_5186.281.692.284.9584.816117002.0360.51715.240.0627.4950.7671.44CA9-2_8225.2109.62.054.8085.473129113.8470.75520.450.09410.151.4741.943CA9-2_9942.7323.32.9231.035.39957772.1894.9945.4130.2467.9218.0516.78CA032_2286.2215.41.3315.144.5791319534.310.52219.920.0515.80.0322.201CA032_3409.9382.71.076.3915.678151441.3721.11315.30.00325.860.0451.538CA032_755.8215.493.603.5094.25421470.0371.0760.7710.12324.820.0341.851PyⅣCA8-5_1589.412920.46317.210.9619198.81359.062.938188.7594.32.099352.1CA8-5_2362286.61.26164.160.7558.413.096.80805.735265.70.24726.57CA8-5_376413930.55301.4234.121988.80954.773.698187.1605.32.292304.8CA8-5_4634.318770.34140.324.24172414.3123.51.39997853.10.656349.4CA8-5_541.07949.50.04951.613.2445152.0755.3537.6655.256490.49.324196.1CA8-5_617.3717210.01999.915.2756190.0757.8074.8517.4025997.194219.9CA8-5_7649.614100.4640.85105.7170921.557.7080.01480.4616340.211393.6CA8-5_8255.9975.60.26175.57.171174913.740.392.521702940.53353.5CA8-5_9470.69790.48205.49.64715672.0634.221.746126.9505.40.587290.4CA8-5_10315.411460.28256.48.92316686.1462.372.437202.5365.91.2363.9PyⅤCA1-5_114.16259.30.05351.445.314649212.617.27317.96.51740702.964198.4CA1-5_29.0441580.0612013.66359313.8766.219145.26.9724320.6772.47CA1-5_325.65428.90.06131.68.931734104.31864.447.16279.550.42671.21CA1-5_434.54617.60.06165.410.12425033.416.29145.719.003203.40.897143.3CA1-5_543.19651.40.07166.812.3424674011.0363.95.312269.83.15782.44CA1-5_643.09555.70.08318.918.28268692.93210.58375.28.791283.42.785108.3CA1-2_148.84303.20.16188.216.572194004.313119.127159.52.94547.13CA1-2_257.951940.3091.5762.4376363.0835.47486.626.06198.14.946135.1CA1-2_346.15122.70.3860.349.58971211.6312.99969.518.7293.411.711106.1CA1-2_463.01156.90.4067.56.756850502.73481.4113.4597.571.34391.14
图6 黄铁矿LA-ICP-MS 测试过程中典型电感耦合等离子质谱输出信号图谱Fig.6 Typical ICP-MS counts output for pyrite analysis by laser ablation
图7 长安金矿、铜厂铜钼金矿硫化物硫同位素分布直方图Fig.7 δ34S histogram of metal sulfides in the Chang’an and Tongchang deposit
前人对哀牢山成矿带富碱斑岩金多金属矿床的成岩成矿年龄的统计显示,与成矿相关的富碱斑岩成岩年龄范围为37.9~31.3Ma(李勇等,2011),成矿年龄集中分布在32~36Ma,34±2Ma为主要的成岩成矿期(邓军等,2010b)。由于金矿化缺乏高精度测年数据,只能通过侵入岩的成岩年龄来推测成矿年龄,与金矿成因密切相关的细晶正长岩的结晶年龄为32.5±0.1Ma,王登红等(2004)认为岩浆期后热液矿床的成矿时代晚于成岩时代0.5~3Myr左右。通过对比发现,其成岩作用与成矿作用与哀牢山的新生代富碱斑岩金多金属矿床的成岩成矿高峰期一致。表明长安金矿、铜厂铜钼金矿与长安冲铜钼金矿的形成受同一构造-热事件的控制。因此长安金矿床大规模的成矿作用应该发生在32Ma左右,确切的成矿时代仍有待证实。
表3长安金矿及铜厂铜钼金矿硫化物硫同位素测试结果
Table 3 Sulfur isotope values of the sulfide in the Tongchang and Chang’an deposit
样品号岩性矿物δ34S(‰)资料来源长安金矿T06平硐矿石黄铁矿2.3T16平硐矿石黄铁矿0.5T15平硐矿石黄铁矿0.3T13平硐矿石黄铁矿3.1T17平硐矿石黄铁矿2.8T14岩芯矿石黄铁矿1.0T1岩芯矿石黄铁矿2.7应汉龙,2006CA-6含黄铁矿细脉糜棱岩黄铁矿2.0CA-14浸染状原生矿石黄铁矿3.0CA-15粉砂岩原生矿石黄铁矿1.1李士辉,2011CA01-3角砾岩型矿石黄铁矿-1.7CD201-2角砾岩型矿石黄铁矿1.7CA01-4硅化矿石黄铁矿1.42-1含石英细脉矿石黄铁矿1.93-4含石英细脉矿石黄铁矿2.47-2含石英细脉矿石黄铁矿-7.4AL08154-1含石英细脉矿石黄铁矿0.9CD001-8含石英细脉矿石黄铁矿2.5AL08154-10含石英细脉矿石黄铁矿3.6CD201-5含石英细脉矿石黄铁矿2.8CD201-6含石英细脉矿石黄铁矿2.71633CD305-3含石英细脉矿石黄铁矿0.51633CD101-3含石英细脉矿石黄铁矿2.3CD301-1硅化矿石黄铁矿-7.6CD301-2硅化矿石黄铁矿-12.0CD301-3硅化矿石黄铁矿-13.03-5硅化矿石黄铁矿-2.15-1硅化矿石黄铁矿2.91-4正长岩黄铁矿-3.52-5正长岩黄铁矿-1.6Chenetal.,2010铜厂铜钼金矿T02石英正长斑岩黄铁矿-1.1T12石英正长斑岩黄铁矿-0.2应汉龙,2006TM-21矽卡岩黄铁矿-0.1TM-17矽卡岩黄铁矿0.3TM-19矽卡岩黄铁矿0.4ch-7矽卡岩黄铁矿1.0TM-17矽卡岩辉钼矿1.3ch-7矽卡岩辉钼矿0.4TM-21矽卡岩黄铜矿1.3TM-7石英正长斑岩黄铁矿0.9TM-12石英正长斑岩黄铁矿-0.1TM-16石英正长斑岩黄铁矿0.4TM-27-3石英正长斑岩黄铁矿0.5TM27-1石英正长斑岩黄铁矿0.6TM-16石英正长斑岩辉钼矿-0.1TM-27-1石英正长斑岩辉钼矿-0.7TM-27-2石英正长斑岩辉钼矿-0.2TM-27-3石英正长斑岩辉钼矿-0.8TM-16石英正长斑岩黄铜矿0.5Tranetal.,2013
4.2 黄铁矿微量元素的指示意义
岩浆作用对于金成矿是提供了成矿流体还是仅作为热源存在,金是来自循环流体对围岩的汲取还是来自于深源富碱斑岩岩浆经演化分异作用形成的流体?随着岩浆热液矿床研究的不断深入,成矿物质源自岩浆的证据逐渐增加,成矿作用经历了早期的岩浆热液阶段与晚期的大气降水阶段(Rombach and Newberry, 2001;Heinrich, 2005),Cu、Au主要来自深部幔源的认识得到了广泛的重视(Sillitoe, 1997;Mungall, 2002;Munteanetal., 2011)。在热液金矿床中,由于各种矿化元素地球化学活动性的不同,会导致矿化空间上出现元素分带的现象。矿化元素的分带性往往反映到相关黄铁矿的微量元素之中。对黄铁矿中的微量元素进行空间分析更能显示出成矿的地球化学空间演化规律(Коробейников等,1986)。因此,对三个矿床中黄铁矿的微量元素进行对比研究,有利于正确认识矿床的成因类型及其成矿物质来源,对进一步的找矿勘查工作具有指示意义。
从黄铁矿微量元素测试分析数据(表2)中可以看出这五类黄铁矿中所含的微量元素特征彼此间即存在着一定的相似性、规律性,又有一些显著的差异性。黄铁矿的Co/Ni值是黄铁矿微量元素中研究最多的课题之一,国内外学者通过对大量的矿床中的黄铁矿微量元素进行总结分析,认为Co、Ni和Co/Ni值对具有一定的指示意义,不同成因类型的黄铁矿其Co/Ni值存在变化趋势,但不能仅用黄铁矿的Co/Ni值大于1或是小于1为分界(陈光远等,1987;Braliaetal.,1979)。盛继福(1999)认为一般情况下黄铁矿的Co/Ni比值大说明黄铁矿的形成温度较高。从测试结果可得,PyⅠ的Co/Ni介于3.84~600之间, PyⅡ的Co/Ni介于11.12~47.91之间,PyⅢ的Co/Ni介于1.07~4.38之间, PyⅣ的Co/Ni介于0.01~1.26之间, PyⅤ的Co/Ni介于0.05~0.4之间;从岩浆岩至沉积地层中的黄铁矿其Co/Ni值具有一个下降的趋势,与前人总结的规律相同。
实验证明,平衡状态下硫化物中Au的含量大小顺序依次为:斑铜矿、黄铜矿、磁黄铁矿、黄铁矿(朱永峰和安芳,2010)。Jugo(1999)对磁黄铁矿-次生固熔体Iss-花岗岩熔体-Au体系在850℃、100MPa下开展的实验表明:从熔体中分离出来Au进入Iss的量远大于进入磁黄铁矿中的量。花岗质岩浆演化过程中,硅酸盐矿物的结晶或溶解并不影响金的状态,金在硫游离的岩浆体系中可能以不相容元素存在,趋向于在残余熔体中富集(朱永峰和安芳,2010)。细晶岩为残余岩浆沿岩体及附近围岩中的裂隙充填形成(路凤香和桑隆康,2006),因此PyⅢ的金含量要高于PyⅠ、PyⅡ的金含量。
热液体系中的Au沉淀是由流体地球化学、温度、压力及氧逸度等诸多因素导致的。在高温体系中金主要存在及运移方式是以氯化物-AuCl2-形式(>500℃),导致金沉淀的主要机制是温度的下降。当温度的降低(<350℃),金在热液中的存在及运移方式则以Au(HS)2-为主(Gammons and Williams-Jones, 1997),二者之间的转化取决于初始流体的pH和HS/Cl比值,及不混溶性,温度约为350~460℃之间。金的运移方式的变化为斑岩型铜矿外围远接触带中金的富集提供了一个合理的解释(Rowins, 2000)。
在斑岩型铜矿中,金与铜的硫化物,如斑铜矿、黄铜矿在高温钾质蚀变带同时沉淀下来,导致含铜硫化物成为主要的载金矿物 (Cygan and Candela, 1995; Simonetal., 2000; Kesleretal., 2002)。随着热液系统的演化,流体化学条件及蚀变矿物的变化,形成了新的蚀变带与早期蚀变带叠加(Sillitoe,1994;Seedorffetal., 2005)。研究表明这一过程使金重新活化、富集,而早期岩浆侵位活动使得围岩形成大规模、低品位的金矿化(Gammons and Williams-Jones, 1997; Kesleretal., 2002)。从地层中的黄铁矿的PyⅣ、微量元素特征我们可以发现,PyⅣ的Au、As略低于PyⅤ,而Se、Sb含量及Co/Ni值高于PyⅤ,认为其形成的温度略高于PyⅤ,推测PyⅣ可能为铜厂斑岩体侵位时形成的低品位的金矿化;后期细晶正长岩的侵入使金重新活化富集,形成Au含量最高的PyⅤ。PyⅢ中Au、As的含量高于PyⅣ,则暗示了金可能来自岩浆。
由黄铁矿元素的相关性图解(图8)中可以看出:Cu、Pb、Zn、Ag、Au、As等元素的含量随Co/Ni比值降低而增加,表现出一定的负相关性。由于岩浆热液系统的逐步演化,形成了PyⅠ、PyⅡ、PyⅢ、PyⅣ、PyⅤ等具不同微量元素特征的黄铁矿。综上所述,铜厂铜钼金矿、长安冲铜钼金矿和长安金矿应为同一岩浆热液系统作用的结果。长安金矿应为与富碱斑岩有关的热液矿床。
图8 长安金矿床黄铁矿的部分微量元素相关性图解Fig.8 Correlation of selected trace elements in pyrite from Chang’an gold deposit
4.3 成矿背景与成矿机制浅析
哀牢山成矿带构造上属于适应和调整印度-欧亚大陆强烈的碰撞和变形的过渡带,也是中国重要的斑岩Cu-Au成矿带。新生代以来,印度-欧亚大陆发生碰撞,使得该地区先后经历了板块汇聚(65~41Ma),构造转换(40~26Ma)、地壳伸展(<25Ma)三个阶段(邓军等,2010b)。从时间上看,富碱斑岩应该是形成于区域构造动力体制的转换阶段。此时板块汇聚速率迅速降低,运动方向发生显著变化,应力的相对松弛导致富碱斑岩及剪切走滑断裂系统的大量发育,为成矿流体活动提供了通道(杨立强等,2010)。扬子板块与印支板块边界之间的哀牢山-红河断裂的切割深度可能深达岩石圈地幔,富Au、Cu等成矿物质的地幔流体受构造活动的激发,并沿深大断裂上升至岩浆源区,使源区岩石部分熔融形成了组分复杂的富碱岩浆流体,地幔流体与岩浆源区通过水-岩相互作用,汲取了岩浆源区内的成矿物质,并随岩浆一起演化上升(邓军等,2010a)。流体上升至浅部逐渐冷却后,发生了气液相分离,当流体冷却到400℃时,铜达到过饱和状态,且SO2和H2S的不均衡,导致了硫化物的沉淀(Williams-Jones and Heinrich, 2005);尽管金、银等金属矿物没有在热液中达到饱和状态,但有可能和黄铜矿同时沉淀(Gammons and Williams-Jones, 1997; Simonetal., 2000; Kesleretal., 2002);形成了铜厂、长安冲铜钼金矿床,并于围岩中的裂隙或层间破碎带中形成了早期低品位的金矿化。富碱岩浆流体演化分异形成的细晶正长岩随后侵位,成矿热液与变质流体混合,形成富H2S流体,金在其中可以溶解度较高的硫氢配合物形式存在(Gammons and Williams Jones, 1997)。在热源的驱动下,成矿流体与在围岩的裂隙中和层间破碎带中循环,萃取Au等成矿物质。使得早期沉淀的Au再次的活化、富集,并随着流体运移。在温度、压力的下降和水岩反应等因素的作用下,成矿流体中的Au再次在裂隙及层间破碎带中沉淀,并富集形成了长安金矿。
根据以上地质现象及数据分析,认为长安金矿、铜厂铜钼金矿、长安冲铜钼金矿成岩与成矿作用是一个连续的岩浆热液过程,受同一地质事件制约,长安金矿为铜厂斑岩体外围的浅成低温热液型金矿。这一成矿机制与哀牢山金矿带中的金多金属矿床类似(和文言等, 2012; 熊德信等, 2007),代表了哀牢山富碱斑岩金多金属矿床的成矿特点。
5 结论
(1)通过对长安金矿床中与其成因及矿体密切共生的脉岩锆石的LA-ICP-MS U-Pb测年,获得了细晶正长岩的成岩年龄为32.5±0.1Ma,正长斑岩的成岩年龄为33.0±0.1Ma,成岩时代与哀牢山的新生代富碱斑岩金多金属矿床的成岩成矿高峰期一致(34±2Ma),是碰撞造山走滑构造系统深部壳幔相互作用的产物。
(2)通过对长安金矿床、铜厂铜钼金矿床、长安冲铜钼金矿床中黄铁矿的LA-ICP-MS微量元素原位组成测定,黄铁矿中成矿元素随Co/Ni比值的下降而上升,其中金的含量为PyⅠ、PyⅡ< PyⅣ< PyⅢ< PyⅤ,可能主要来源于富碱岩浆流体,喜山期大规模的富碱岩浆上侵不仅为含矿流体的上升提供了动力和热能,而且还是成矿物质和成矿流体的主要来源,这可能是本区得以形成金矿床的重要原因之一。
致谢野外工作得到了云南黄金矿业集团股份有限公司长安项目部的支持与帮助;锆石测年得到了中国地质科学院矿产资源研究所激光多接收等离子质谱实验室的帮助;陈懋弘副研究员和叶会寿研究员对本文提出的宝贵的修改意见;在此一并表示真诚的感谢。
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