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滇西北衙金多金属矿床辉钼矿Re-Os同位素年龄及其地质意义

2015-03-07牛浩斌胡文懿宁括步董黎阳

地质与勘探 2015年1期
关键词:金属矿床辉钼矿斑岩

牛浩斌,胡文懿, 丁 俊, 李 俊, 宁括步,王 鹏,任 飞, 董黎阳

(1.成都地质矿产研究所,四川成都 610081; 2.四川省科源工程技术测试中心,四川成都 610091)



滇西北衙金多金属矿床辉钼矿Re-Os同位素年龄及其地质意义

牛浩斌1,胡文懿2, 丁 俊1, 李 俊1, 宁括步1,王 鹏1,任 飞1, 董黎阳1

(1.成都地质矿产研究所,四川成都 610081; 2.四川省科源工程技术测试中心,四川成都 610091)

北衙金多金属矿床产出于滇西北富碱斑岩-多金属成矿带内。矿床严格受富碱斑岩及其成生的深大断裂构造控制,并发育热液交代现象,且在围岩内以产出充填型矿脉(囊)、交代型条带矿(似层状)为主,构成斑岩-矽卡岩成矿系统,具超大型金矿床规模。本文利用Re-Os同位素定年方法首次对北衙铁金多金属矿床全矿段金属硫化物成矿年龄进行精确测定,获得5件辉钼矿样品的模式年龄为37.46~39.44 Ma,其加权平均年龄为(38.48±0.54)Ma,等时线年龄为(37.9±2.5)Ma,两者在误差范围内一致,与区域及区内富碱斑岩锆石U-Pb年龄基本保持一致;辉钼矿中Re含量主体26.84×10-6~43.15×10-6之间,指示其成矿物质来源于上地幔和下地壳,为壳幔混合源;两者均指示成矿作用与该时期具幔壳混合源性质的富碱岩浆浅成侵入关系密切。

辉钼矿Re-Os同位素年龄 成矿年龄 动力学背景 北衙 滇西

Niu Hao-bin,Hu Wen-yi,Ding Jun,Li Jun,Ning Kuo-bu,Wang Peng,Ren Fei,Dong Li-yang. Re-Os isotope age of molybdenite in the Beiya Au-polymetallic deposit, western Yunnan Province and its geological implications[J]. Geology and Exploration, 2015, 51(1):0001-0012.

滇西北衙金多金属矿床为西南三江富碱斑岩多金属成矿带重要、典型的富碱斑岩-热液型、接触交代型多金属矿床;截止2010年底,金矿累积探明共伴生金属量达160t以上,铁矿资源量超过3000万吨,已达超大型金矿及大型铁矿规模,并伴有大量银、铅、锌、铜等①;矿区外围及整个矿集区内具较好的找矿远景。目前已圈定南大坪-干海子-马头湾、响水河、观音山、黄草坡、马鞍山等重点找矿远景区,并在黄草坡一带已有新的矿化显示②。前人对此矿床及矿集区控矿地质特征、成矿作用及模式和成矿斑岩岩石及地球化学特征、成岩时代、演化过程及其成矿控矿作用等进行了大量深入的研究(蔡新平等,1991a,1991b,1993;甫为民,1994;胡朝平等,1994;葛良胜等,2002;刘建云,2004;吴开兴,2005;徐受民等, 2007;徐兴旺,2006,2007;Xuetal.,2007a,2007b,2007c;薛传东等,2008;Dengetal.,2010;肖晓牛等,2009,2011;杨金永,2010;和文言等,2012,2013;王宏伟等,2013;和中华等,2013),已获得一批与成矿作用相关的成岩成矿时限,为区域成岩成矿系统研究和矿集区成矿找矿模式构建提供基础数据,但本区金多金属矿床主矿体至今未获得精确成矿年龄数据。

本文基于“北衙地区三维地质调查”项目地质、物化、遥感及岩芯编录等三维建模工作(孙岳等,2013),首次在北衙万硐山采场及其深部钻孔岩芯发现辉钼矿,并对其进行了Re-Os同位素年龄测定, 准确地厘定了矿区内万硐山、红泥塘矿段主矿体的金属硫化物成矿年龄,结合前人已获得的年龄数据探讨该矿床及矿集区成矿物质来源及其成矿地球动力学背景。

1 区域地质背景

滇西北衙多金属矿集区位于扬子准地台西缘之丽江-盐源台缘西缘中生代坳陷,与德格-中甸陆块、兰坪-思茅陆块呈弧形构造结合部东侧,为金沙江-红河、宾川-程海、丽江-木里断裂所挟持范围西侧(图1);整体处于西南三江构造带“蜂腰”地段和众多深部、浅部构造交汇、集中发育部位(李文昌等,2001;潘桂堂等,2003),为深源的富碱斑岩成群分布和岩浆热液活动提供条件,形成新生代金沙江-红河富碱斑岩带(涂光炽,1984)及其多金属成矿带(侯增谦等,2004),并统一于晚二叠世以来的金沙江洋盆消减闭合-弧陆碰撞-陆内碰撞造山-陆内汇聚的多岛弧盆碰撞造山过程(潘桂堂等,2003)。

区内地层主要以上二叠统峨眉山玄武岩(P2β)为“基底”④,发育的一套三叠纪地层及小范围的新生代地层;整体呈中新生代地层为玄武岩所围绕的“四周老,中部新”的样式分布(图1)。玄武岩之上晚三叠纪地层为弧陆碰撞过程中弧后盆地转变弧后前陆盆地内沉积(潘桂堂等,2003),包括主要赋矿层北衙组(T2b)碳酸盐岩;新生代地层多为陆内造山过程中的走滑拉伸作用、拉分盆地沉积,局部发育较大规模的“红色粘土型”铁金矿⑥或现代残坡积铁金矿、河湖相沉积型(砂砾粘土型)及喀斯特堆积砾砂矿③(蔡新平等,1991a,1991b,1993)。

受金沙江-红河、宾川-程海、丽江-木里断裂控制,断裂构造主要有SN向弧形展布的天坪断裂④、近SN向后本箐断裂④、马鞍山断裂断裂⑤、两条近EW向隐伏基底断裂(蔡新平等,1991a;葛良胜等,1999),以及近EW向构造断裂。

图1 西南三江大地构造分区图(据潘桂堂等,2003整理)及北衙铁金多金属矿集区地质图(据②,④修改)Fig.1 Tectonic zone map of the Sanjiang in southwestern China ( modified from Pan Guitang et al.,2003 ) and geological map of the Beiya Au-polymetallic mineralization concentrated area Ⅰ-扬子陆块;Ⅱ-德格-中咱陆块;Ⅲ-芒康-思茅陆块;Ⅳ-保山-掸邦陆块;Ⅴ-冈底斯-拉萨-腾冲陆块;Ⅵ-印度陆块;1-第四 系;2-三营组;3-宝相寺组;4-松桂组;5-中窝组;6-北衙组;7-青天堡组;8-峨眉山玄武岩;9-石英正长斑岩;10-采样位置Ⅰ-Yangtze block; Ⅱ-Dege-Zhongdian block; Ⅲ-Mangkang-Simao block; Ⅳ-Baoshan-Shanbang block; Ⅴ-Gangdisi-Lasa-Tengchong block; Ⅵ-India block;1-Quaternary; 2-Sanying group; 3-Baoxiangsi group; 4-Songgui group; 5-Zhongwo Group; 6-Beiya group; 7-Qingtianbao group; 8-Emeishan basalt; 9-quartz orthophyre; 10-sampling location

区内富碱斑岩主要沿近EW向隐伏基底断裂、马鞍山断裂及其次级断裂分布;前者影响着南大坪、干海子、小干海、马头湾、桅杆坡、白沙井等富碱斑岩体(脉),后者控制着SN向展布的铺台山、狮子山、万硐山、红泥塘、焦石硐、老马涧等富碱斑岩体(脉)⑥。

2 矿床地质特征

北衙金多金属矿床位于区内中部偏南,与万硐山、红泥塘(石英)正长斑岩一起分布于松桂-北衙复式向斜南部翘起端的北衙向斜核部及两翼部位,受斜切北衙向斜西翼的马鞍山断裂及其次级断裂控制,并与两条近EW向隐伏基底断裂交汇的万硐山、红泥塘处成岩成矿,且产于斑岩体边部及围岩裂隙、层间破碎带内。受区域上构造影响,区内主要发育近SN向弧形控矿断裂和近EW向的破矿断裂,并多沿EW向断裂穿插充填有黑云正长斑岩脉。

围岩主要有青天堡组(T1q)黄褐、灰绿色岩屑石英砂岩夹黄绿、紫红色页岩层,北衙组中下部(T2b1+2)灰色、浅灰色,中厚层瘤状、蠕虫状(生物碎屑)灰岩及泥质灰岩,局部夹砂泥质碎屑岩薄层,以及向上渐变为互层产出的灰色、深灰色粒屑白云岩和微晶白云岩(T2b3);北衙组上部(T2b4)则为灰白色微晶白云岩与浅灰色含粒屑白云岩。其中,白云岩(T2b3、T2b4)普遍存在强烈的碳酸盐化蚀变、褐铁矿化特征,而灰岩(T2b1+2)局部发育矽卡岩蚀变(图3)及磁铁矿化现象;部分碎屑岩(T1q)存在热液接触、交代成矿作用,呈角岩或角岩化产出,伴随着黄铁矿、黄铜矿等硫化矿物发育,并首次发现团状、条带状磁铁矿化现象(图4),具有较大的成矿潜力。

图2 北衙金多金属矿床地质及采样位置图(据①,③修改)Fig.2 Geological map and sampling position map of the Beiya Au-polymetallic deposit①,③ 1-第四系;2-北衙组四段;3-北衙组三段;4-北衙组一、二段;5-青天堡组;6-峨眉山玄武岩;7-石英正长斑岩;8-煌斑岩脉;9-产状;10-实测及推测界线;11-实测及推测断层;12-矿体及其编号;13-采样位置;14-隐伏基底断 裂大致位置1-Quaternary ; 2-4th section of the Beiya group; 3-3th section of the Beiya group; 4-1th and 2th sections of the Beiya group; 5-Qingtianbao group; 6-Emeishan Basalt; 7-quarta orthophyre; 8-lamprophyre vein; 9-attitude; 10-measured and inferred geological bounderies; 11-measured and inferred faults; 12-ore bodies and their number; 13-sampling location; 14-location of concealed basement faults

与成矿关系密切的富碱斑岩主要为(石英)正长斑岩;岩性表现为角斑结构,斑晶(>50%)主要为钾长石,少见石英、斜长石斑晶;基质(<50%)主要为微粒状长英质矿物,以钾长石为主,石英次之,常发育弱绢云母化蚀变。此外,沿顺层及切层断裂多穿插发育有云母煌斑岩脉及黑云母正长斑岩脉,时代上与(石英)正长斑岩相似具有多期次活动特征(Xuetal.,2007a, 2007b, 2007c ),并且早期云煌岩脉多具有褪色蚀变和较高的金含量,显示可能与成矿作用有着内在的联系(杨金永,2010),或指示着多类型岩脉产出区具有较好的成矿前景③。

从矿体(原生)产出特征上看,区内矿化类型可分为3类:Ⅰ.产于斑岩与北衙组灰岩段接触带围岩内的条带状(似层状)交代型铁金矿体(脉),似接触交代作用的矽卡岩型矿床(和文言等,2012);其中,青天堡组(T1q)粉砂质泥(板)岩中普遍存在的团状、条带状磁铁矿化(图3)亦属此类型;Ⅱ.产于斑岩、围岩内及其间并沿节理、裂隙及层间破碎带、溶蚀坑洞穿插充填的脉状及囊状铁金矿体(脉)(图3),属热液成矿作用的热液矿床类型(和文言等,2012);Ⅲ.产于斑岩及围岩之间受后期压扭性逆冲构造作用的铁金角砾岩型矿体,包括主要的磁-赤铁矿及部分碳酸盐岩角砾,后经热液作用,发生强烈的褐铁矿化及绿帘-绿泥石化蚀变及矿质进一步富集作用,属热液叠加成矿类型(图3)。其中,产于斑岩体与围岩之间,且围绕万硐山石英正长斑岩体呈弧形不闭合环带状展布的K52矿体是北衙铁金矿床最主要的矿体,发育Ⅱ、Ⅲ两种成矿类型。

图3 北衙金多金属矿床矿化与蚀变特征Fig.3 Mineral alteration characters of the Beiya Au-polymetallic deposit a-砂岩内的磁铁矿脉(采自55ZK30-B3,T1q);b-矽卡岩内共生的磁铁矿、黄铁矿(采自60ZK7_304 m,T2b1+2);c-磁铁矿交代石榴子石形成残余结构(单偏光,采自60ZK7_293.20 m);d-黄铁矿交代磁铁矿形成残余结构(反光,采自55ZK30-B3);e-压扭性逆冲构造及热液叠加改造的铁金矿体;f-绿帘石化及硅孔雀石铜矿化现象;矿物代号(Kretz R,1983):Mag-磁铁矿;Py-黄铁矿; Ccl-硅孔雀石;Grt-石榴子石;Ep-绿帘石;Chl-绿泥石;Qtz-石英a-magnet Vein in sandstone(from the 55ZK30-B3, T1q); b-paragenetic magnets and pyrites in the skarn(from the 60ZK7_304 m, T2b1+2 ); c-relic structures by magnets replacing garnets(polarizing microscope, from the 60ZK7_293.20 m ); d-relic structures by pyrites replacing magnets(reflecting microscope, from the55ZK30-B3); e-Fe and Au ore body of compresso-shear overthrust tectonic and hydrothermal superimposition and reformation; mineral abbreviations(from Kretz R,1983): Mag-magnet; Py-pyrite; Ccl-chrysocolla; Grt -garnet; Ep-epidote; Chl-chlorite; Qtz-quartz

图4 北衙金多金属矿床辉钼矿赋存特征Fig.4 Occurrence characters of molybdenite from the Beiya Au-polymetallic deposit a-石榴子石矽卡岩内团斑状、浸染状辉钼矿(采自万硐山矿段,T2b1+2); b-矽卡岩内与黄铁矿、黄铜矿共生的辉钼矿(采自万硐山矿段,T2b1+2);c-磁铁矿化粉砂质泥岩内脉状辉钼矿(采自55ZK30_640 m,T1q); d-菱铁矿内的黄铁矿、黄铜矿及辉钼矿(采自79ZK6_240 m);矿物代号(Kretz R,1983):Mag-磁铁矿;Py-黄铁矿;Gn-方铅矿;Ccp-黄铜矿;Mo-辉钼矿;Sd-菱铁矿;Grt -石榴子石;Qtz-石英a-porphyritic and disseminated molybdenites in garnet skar(from Wangdongshan ore block, T2b1+2); b-molybdenites with pyrites and chalcopyrites(from Wangdongshan ore block, T2b1+2); c-veined molybdenites in silty mudstone with magnetites(from 55ZK30_640 m, T1q); d-pyrites, chalcopyrites and molybdenites within siderites(from 79ZK6_240 m);Mineral abbreviations(from Kretz R,1983): Mag -magnet; Py-pyrite; Gn-gelenite; Ccp-chalcopyrite; Mo-molybdenite; Sd-siderite; Grt-garnet; Qtz-quartz

围岩蚀变及矿化大体上可划分两个阶段;Ⅰ.矽卡岩、角岩(化)及铁成矿阶段:伴随着石榴子石、透辉石、阳起石、硅灰石等矽卡岩矿物和角岩(化)形成过程,以形成条带状、团状交代型磁铁矿、磁-赤铁矿和脉状、囊状充填型磁铁矿为主(图3);Ⅱ.热液叠加及金属硫化物阶段:以石英、方解石等脉石矿物和黄铁矿、黄铜矿、方铅矿、辉钼矿等金属硫化物形成为主,呈脉状、团斑状、浸染状等形式叠加于早期阶段矿化之上(图3)。磁铁矿多于后一阶段氧化成褐铁矿,而金主要以自然金状态以裂隙、包裹形式赋存于褐铁矿石内①。

3 样品采集与测试结果

为进行精确确定和对比研究矿床成矿年龄及富碱斑岩成岩年龄,本次工作采集北衙矿区2件万硐山矿段及3件红泥塘矿段的含辉钼矿接触交代型的硫化物矿石及脉状矿石。其中,辉钼矿呈团斑状、浸染状产于石榴子石矽卡岩内,与黄铁矿、黄铜矿等硫化物矿物紧密共生(图4),或呈细脉状产于浸染状或团状磁铁矿化的粉砂质泥(板)岩,与石英细脉紧密共生(图4);该产出及共生特征指示辉钼矿与区内金属硫化物成矿同期(张世铭等,2013),基本可代表万硐山、红泥塘全矿段金属硫化物成矿年龄。

辉钼矿的分选工作在河北省廊坊区域地质矿产调查研究所实验室完成。用常规方法将岩石样品粉碎至300 μm左右,经淘洗、重选富集,再经磁选和密度分选后,在双目镜下进一步分离、挑纯锆石和辉钼矿单矿物。辉钼矿质纯,无包体,无污染,纯度达98%以上。

本次辉钼矿Re-Os测试是在国家地质测试中心Re-Os同位素实验室进行。化学分离过程和分析方法见文献(Du Andaoetal., 2004;杜安道等,2001;屈文俊等2003;Shireyetal.,1995)。辉钼矿Re-Os同位素模式年龄t计算公式如下:

187Os——现在矿物中187Os含量(mol/g),分析中采用质量浓度(ng/g);

187Re——现在矿物中187Re含量(mol/g),分析中采用质量浓度(μg/g);

λ——187Re衰变常数1.666×10-11a-1(1.02%);

t矿物形成后的年龄,在公式中单位为百万年(Ma)表示(Smolar M Ietal.,1996;Shen J Jetal.,1996)。

北衙金多金属矿床5件辉钼矿样品经上述方法测定,得到模式年龄为(37.46±0.57)~(39.44±0.79)Ma。采用Isoplot软件作等时线和加权平均值(图5),得到等时线年龄为(37.9±2.5)Ma,加权平均年龄为(38.48±0.54)Ma,两者在误差范围内一致。

图5 北衙金多金属矿床辉钼矿Re-Os同位素 等时线及模式年龄加权平均值Fig.5 Re-Os isochron age and weighted average age for molybdenite from the Beiya Au-polymetallic deposit

4 讨论

4.1 成矿年龄限定

西南三江富碱斑岩带是新生代以来印度-亚欧板块碰撞过程中的重要响应(张玉泉等,1998a,1998b),并伴随着与富碱斑岩活动有关的Cu、Mo、Au、Ag、Pb等多大规模金属成矿作用(侯增谦等,2004;曾普胜等,2006;薛传东等,2008;邓军等,2010;邓军等,2011),时代集中于38~33 Ma(邓军等,2011)。

北衙金多金属矿床是此富碱斑岩成矿带中南段的典型矿床之一(薛传东等,2008),与形成于37.93~31.3 Ma的富碱斑岩密切相关(徐受民等,2006;李勇等,2011;和文言等,2012)。斑岩成矿通常出现在最晚期次含矿斑岩侵入前1~3 Ma(侯增谦等,2003),或晚于主成矿斑岩0.5~3 Ma(侯增谦等,2004;王登红等,2004)。和文言等(2013)在北衙金多金属矿床红泥塘矿段取得1件辉钼矿Re-Os年龄36.87±0.76 Ma和斑岩锆石LA-ICP-MS U-Pb年龄36.48±0.26 Ma,与万硐山岩体及区域上新生代富碱斑岩带成岩作用峰期一致(和文言等,2013)。

为更全面精确研究北衙金多金属矿床全矿段成矿时代,本次就整个北衙金多金属矿床采得5件含辉钼矿岩石样品,辉钼矿呈团斑状、浸染状与硫化物矿物紧密共生产于交代蚀变岩或矽卡岩内,或呈细脉状与石英细脉紧密共生产于浸染状或团状磁铁矿化的粉砂质泥(板)岩内,指示着其为同期成矿作用产物;由于辉钼矿较云母、长石等蚀变矿物K-Ar及40Ar-39Ar具有封闭温度高(~500℃)的优点,可以更直接准确测得硫化物成矿年龄(Minsteretal.,1979;Ishiharaetal.,1989;Suzukietal.,1996;Steinetal.,1997,2001;Selbyetal.,2002);本次测得Re-Os同位素等时线年龄37.9±2.5Ma,加权平均年龄为38.48±0.54Ma,与已有万硐山、红泥塘含矿斑岩锆石LA-ICP-MS U-Pb同位素年龄大致在同一范围,可以代表北衙金多金属矿床大规模成矿年龄。

4.2 成矿动力学背景

本区位于西南三江的“蜂腰”地段,处于多岛弧盆与扬子陆块碰撞结合处(潘桂堂等,2003)。自始新世以来,该区经历了主碰撞陆陆汇聚阶段(65~41 Ma)、晚碰撞构造转换阶段(40~26 Ma)及后碰撞地壳伸展阶段(25~0 Ma)(侯增谦等,2006;邓军等,2011),并伴随近SN向的褶皱推覆、压扭性及张扭性走滑、近EW向伸展等构造作用和相应期次的富碱岩浆活动(侯增谦等,2004)。西南三江富碱斑岩带中的江达-鹤庆-大理含矿斑岩亚带(侯增谦等,2004)主要形成于37.93~31.3Ma(徐受民等,2006;李勇等,2011;和文言等,2012),与构造转换时期一致,受SN走滑断裂及其次级断裂构造控制;同时该斑岩带内的玉龙(辉钼矿Re-Os:41.2±0.3 Ma)、北衙(辉钼矿Re-Os:37.9±2.5 Ma)、马厂箐(辉钼矿Re-Os:35.8±1.6 Ma)等超大型多金属矿床(曾普胜等,2006),均与其各斑岩岩浆活动同期,表现出与其存在着密切成生关系,具有相似的成矿动力学背景及条件。也由此,金属矿物和含矿斑岩的相关示踪、对比研究可直接反映成矿源区及动力学背景等方面的信息。

表1 北衙金多金属矿床Re-Os同位素分析测试数据

前人研究显示,Re-Os同位素体系内Re含量可以示踪金属硫化物源区,衡量地壳物质混然程度(Foster, J Getal.,1996),且从幔源-幔壳混源-壳源,辉钼矿内Re含量呈n×10-4-n×10-5-n×10-6数量级降低(Steinetal.,1997;Mao Jinwenetal.,1999)。如表1,区内Re含量在2.719~43.15×10-6之间变化,仅有1件样品Re含量为2.719×10-6,显示地壳物质混染程度较高,其余为2.684~4.315×10-5区间,与和文言等(2013)所测的1件辉钼矿Re含量(2.027±0.16)×10-5保持一致,均指示着金属硫化物成矿物质主要为幔壳混合来源。这也与已有的含矿斑岩及矿物质示踪、对比研究认识相一致。

区域上,与成矿紧密相关的富碱斑岩存在如下特征:SiO2饱和或过饱和、高K2O、低N2O,143Nd/144Nd值0.512317~0.512597,εNd(t)值-6.75-~1.28,εSr值21.1~42.3,而208Pb/204Pb、207Pb/204Pb、206Pb/204Pb分别为38.442~38.676、15.541~15.615、18.49~18.603;这些特征显示幔源特征,为其经地壳不同程度混染的产物(曾普胜等,2002)。另一方面,富碱斑岩中常见有包体岩石,其富铁玻璃组成和富铁熔浆独立包体中存在着碳硅石、含铬自然铁、钛铁矿及含钛镜铁矿等地幔标志型矿物,且富铁熔浆包体硅同位素呈现出未经动力分馏低负值(34Si=-0.8‰),也均表明富碱斑岩来源于地幔(刘显凡等,2010)。本区富碱斑岩微量分析也证实源于加厚的下地壳底部或“壳幔混合层”(钟大赉等,2000),具有低Cr、低Ni,富集LILE、LREE,亏损HFSE、HREE的特征,特别是富集Ba、Rb、Th、K和亏损Nb、Ta、P、Ti等元素(徐受民等,2006);而代表着同期成矿流体的含矿斑岩内石英斑晶流体包裹体,其与斑岩全岩具有一致的稀土配分模式,与矿体内石英、方解石脉内流体包裹体温度、盐度测定结果基本相同(肖晓牛等,2011),表明含矿质流体与富碱斑岩同源,是富碱斑岩岩浆深部结晶分异产物或结晶分异后期残余岩浆出溶产物(肖晓牛,2009),具有深部幔源特征。矿石内黄铁矿、黄铜矿、方铅矿的34S值在-2.4~4.5‰,均值1.18‰,接近幔源硫的(34S值(0‰);而208Pb/204Pb、207Pb/204Pb、206Pb/204Pb分别变化在38.256~39.0394、15.457~15.703、18.380~18.587之间,与区域上富碱斑岩Pb同位素组成保持一致(肖晓牛等,2011)。再者,刘显凡等通过对富碱斑岩、硅化石英、蚀变矿化斑岩、围岩等进行的34Si研究显示,非矿化斑岩与次生石英34Si为0.0~0.4‰,而蚀变矿化斑岩及代表成矿流体的硅化石英(34Si为-0.1‰~-2.4‰,存在着明显的原始地幔低负值特征,为成矿流体作用的结果(刘显凡,2006;赵普峰等,2011)。以上均与本次辉钼矿Re-Os同位素研究认识相同,均指示成矿物质为幔壳混合来源,以富硅的、不混溶含矿流体形式受一定程度的地壳混染或交代蚀变作用而于适当部位成矿。

综上,北衙金多金属矿床及矿集区成矿是在构造转换时期(40~26Ma)出现的幕式应力松弛、下地壳减薄的背景下,形成了大规模SN向马鞍山深大断裂及其次级断裂构造,以及近EW向隐伏基底断裂;同时,幔源区或下地壳发生减压熔融,富硅、含矿质流体与富碱岩浆于深部发生成份分异作用,呈不混溶状沿同生断裂上涌,并受地壳物质混染或交代蚀变作用至浅表层次合适部位就位成矿;循环往复,造就了始新世-渐新世(38~33Ma)时期大规模成矿作用的发生(邓军等,2011)。因此,北衙金多金属成矿受构造体制转换阶段的深部幔壳相互作用和浅表构造、蚀变过程控制,并伴随着富碱斑岩的就位,形成具代表性的超大型金多金属矿床。

5 结论

(1) 通过对北衙金多金属矿床万硐山及红泥塘矿段地表及钻孔内的5件辉钼矿Re-Os同位素精确测年,得到模式年龄(37.46±0.57)~(39.44±0.79)Ma,加权平均年龄为38.48±0.54 Ma,等时线年龄为37.9±2.5 Ma,精确限定了本区金属硫化物主成矿年龄,与富碱岩浆活动高峰期一致。

(2) 北衙金多金属矿床内辉钼矿Re含量主体在26.84~43.15×10-6之间,与壳幔混合源岩浆矿床的辉钼矿Re含量相当;结合已有岩石、矿床地球化学研究成果,进一步证明了该矿床成矿物质来源为壳幔混合源。

(3) 北衙金多金属矿床辉钼矿Re-Os同位素研究也进一步证实,在构造转换时期,随着下地壳减薄、深大断裂及其次级断裂的形成,幔源区或下地壳发生减压熔融,且幔源流体或含矿质流体与幔壳混合源岩浆发生成份分异作用,而呈不混溶状沿同生断裂上涌,并受地壳物质混染或交代蚀变作用至浅表层次合适部位就位成矿。

致谢: 野外期间得到了云南黄金矿业集团北衙项目部各位领导、同仁的大力支持和热心帮助;辉钼矿样品测试得到了国家地质测试中心屈文俊研究员和李超博士的帮助,在此表示由衷感谢!

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Re-Os Isotope Age of Molybdenite in the Beiya Au-Polymetallic Deposit, Western Yunnan Province and its Geological Implications

NIU Hao-bin1,HU Wen-yi2,DING Jun1,LI Jun1,NING Kuo-bu1,WANG Peng1,REN Fei1,DONG Li-yang1

(1.ChengduInstituteofGeologyandMineralResources,Chengdu,Sichuan610081; 2.Keyuan,SichuanEngineeringTechnicalTest,Chengdu,Sichuan610091)

The Beiya Au-polymetallic deposit lies in the alkali-rich porphyry polymetallic metallogenic belt of northwestern Yunnan Province. The deposit is well controlled by the alkali-rich porphyry and deep genetic faults, simultaneously developing thermal contact and contact metasomatic-hydrothermal alteration, where exist ore types of metasomatite and filling veins in the host rocks, forming a mineralization system of porphyry and skarn that is a super large-size Au-polymetallic deposit. This work firstly applied Re-Os isotopic dating technique to all ore sections. Five model ages of molybdenite obtained are from 37.46 Ma to 39.44 Ma, with the weighted average age 38.48±0.54 Ma and the isochron age 37.9±2.5 Ma, which are within the error range and consistent with the Zircon U-Pb ages of the alkali-rich porphyries in the area. The Re content of the molybdenite is mostly in the range 26.84×10-6to 43.15×10-6, suggesting that the metallogenic material originated from the upper mantle or lower crust, and should be a mixed source of the mantle and crust. Both of the results show that the gold polymetallic metallogeny was related to alkali-rich porphyry that was derived from the mixed source of the mantle and crust.

geochemistry, Re-Os isotopic dating, metallogenic age, geodynamic background, Beiya, western Yunnan

2014-06-25;

2014-08-21;[责任编辑]郝情情。

中国地质调查局工作项目(编号:1212011220249)、12120113094500和12120114013501资助。

牛浩斌(1984年-),男,主要从事矿物学、岩石学、矿床学研究。E-mail:aibornew@126.com。

P597.3, P618.51

A

0495-5331(2015)01-0001-12

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