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内蒙古常福龙沟金矿床地质特征和硫、铅同位素地球化学

2015-03-14李志丹俞礽安王佳营文思博陈军强

地质与勘探 2015年3期
关键词:龙沟黄铁矿泰山

李志丹,俞礽安,王佳营,文思博,陈军强,汤 超

(天津地质矿产研究所,天津 300170)



内蒙古常福龙沟金矿床地质特征和硫、铅同位素地球化学

李志丹,俞礽安,王佳营,文思博,陈军强,汤 超

(天津地质矿产研究所,天津 300170)

常福龙沟金矿位于华北陆块北缘隆起带,是中元古界渣尔泰群阿古鲁沟组在内蒙古中部金矿找矿的新突破。常福龙沟金矿体呈中脉状-薄脉状产出于渣尔泰山群阿古鲁沟组浅变质碎屑岩中,矿石矿物为黄铁矿及少量方铅矿、黄铜矿、自然金和银金矿;脉石矿物有石英、方解石、绢云母等。发育自形-半自形粒状结构、它形粒状结构、包含结构、交代残余结构、显微鳞片粒状变晶结构和压碎结构等,以及稀疏或稠密浸染状、网脉状、碎裂状、角砾状等构造。围岩蚀变主要为硅化、黄铁绢英岩化、碳酸岩化,其中硅化强度与金矿化呈正相关关系。载金矿物黄铁矿硫同位素组成介于-0.6‰~2.9‰之间,极差较小,暗示硫源比较单一且成矿环境和物理化学条件较为稳定,硫可能来自于岩浆。黄铁矿铅同位素206Pb/204Pb= 16.849~17.035,207Pb/204Pb=15.343 ~15.411,208Pb/204Pb=37.310 ~37.547,与渣尔泰山群全岩铅同位素范围相近,结合铅同位素模式年龄,推测中元古界渣尔泰山群可能是成矿物质的提供者。

金矿床 地质特征 S-Pb同位素 常福龙沟 内蒙古

Li Zhi-dan, Yu Reng-an, Wang Jia-ying, Wen Si-bo, Chen Jun-qiang, Tang Chao. Geological characteristics and sulfur and lead isotopic geochemistry of the Changfulonggou gold deposit in Inner Mongolia[J]. Geology and Exploration, 2015, 51(3):0414-0421.

1 引言

内蒙古中部乌拉山-大青山地区金矿床(点)星罗棋布,构成华北陆块北缘重要的金成矿带(Hartetal., 2002; Nieetal., 2002)。成矿带西段哈达门沟金矿田近年新增金金属量达50余吨,累计查明资源储量超过100t(章永梅,2012);中段固阳县下湿壕地区发现长胜渠金矿储量达23.5t;东段呼和浩特市北部武川县一带发育有常福龙沟、摩天岭、大石槽、鹿场、鹿场西、种地窑子、卯独庆、补换沟及大营子等一系列金矿床(点)(钟长汀等,2005)。这些金矿床(点)大多与新太古界乌拉山岩群、古元古界二道凹群等基底建造有关,而常福龙沟金矿床赋存于中元古界渣尔泰山群阿古鲁沟组中,与朱拉扎嘎金矿容矿层位一致(陈志勇等,2004;李俊建等,2010),是渣尔泰山群阿古鲁沟组层位在内蒙古中部金矿找矿的新突破。

常福龙沟金矿位于呼和浩特市以北37 km处,该矿最初在20世纪70年代末由1∶20万化探扫面发现金异常,2002年底异常检查发现3条金矿化蚀变带(赵维宽等,2008),2003年~2005年内蒙古第一地质矿产开发院、天津地质矿产研究所开展了预查工作,2006年天津地质矿产研究所进行了普查、详查工作,探获金4.2t(平均品位5.02g/t)。作为乌拉山-大青山金矿带的一例重要金矿床,赵维宽等(2008)最先报道了其地质特征并探讨了区域找矿方向;俞礽安等(2009)探讨了控矿因素;高帮飞等(2012)研究了Au品位分布特征;陈志广等(2012)初步总结了成矿模式;汪劲草等(2012)探讨了正花状节理系对金矿床的控制。由此可见,常福龙沟金矿床自发现以来备受学者关注,但有关成矿物质来源一直缺乏同位素约束,基于此,本文开展了常福龙沟金矿床载金黄铁矿硫、铅同位素组成研究,以期对成矿物质来源有所认识。

2 地质背景

常福龙沟金矿位于呼和浩特市北部的大青山地区,大地构造位置属华北陆块北缘隆起带(俗称内蒙地轴)(图1A)。区内构造演化大致包括4个阶段:早前寒武纪基底形成阶段、中元古代裂陷阶段、晚古生代-早中生代活动陆缘演化阶段和中新生代断陷隆升阶段(王惠初等,2012)。早前寒武纪岩石地层广泛发育,包括新太古界麻粒岩相变质的兴和岩群、麻粒岩相-角闪岩相变质的乌拉山岩群、古元古界高绿片岩相-低角闪岩相变质的二道凹群等,这些地层与早元古代侵入体等一起构成研究区早前寒武纪结晶基底(图1B)。中元古代裂陷环境形成低绿片岩相变质的渣尔泰山群,同期还发育有中元古代闪长岩-石英闪长岩、黑云母花岗岩等。中生界为一套粗碎屑陆缘沉积和火山沉积建造,晚中生代发育有大型推覆构造和变质核杂岩(郑亚东等,1998;Darbyetal., 2001;Davisetal., 2002;戚国伟等,2007)。显生宙花岗岩包括古生代黑云母花岗岩、花岗闪长岩和碱长花岗杂岩体,中生代为钾长花岗岩和花岗斑岩(聂凤军等,2013)。区域金成矿作用显著,发育有常福龙沟、大石槽、补换沟、卯独庆、摩天岭、哈拉沁等金矿床(图1B),矿区北部还发育有与晚古生代岩浆活动相关的赵井沟大型铌钽矿和马房子钨矿等(聂凤军等,2013)。

图1 内蒙古常福龙沟金矿区域地质简图(A-Zhao et al.,2012; B-俞礽安等,2009)Fig. 1 Sketch showing regional geology of the Changfulonggou gold deposit in Inner Mongolia (A-after Zhao et al., 2012; B-after Yu et al., 2009) 1-第四系;2-新近系;3-中生界;4-中元古界渣尔泰山群;5-古元古界二道凹群;6-太古宙基底;7-显生宙花岗岩;8-中 元古代闪长岩;9-中元古代石英闪长岩;10-中元古代黑云母花岗岩;11-古元古代片麻状花岗岩;12-金矿床 1-Quaternary; 2-Neogene; 3-Mesozoic; 4-Mesoproterozoic Zhaertaishan group; 5-Paleoproterozoic Erdaowa group; 6-Archean basement; 7-Phanerozoic granite; 8-Mesoproterozoic diorite; 9-Mesoproterozoic quartz diorite; 10-Mesoproterozoic biotitegranite; 11-Paleoproterozoic gneissic granite; 12-gold deposit

3 矿床地质

常福龙沟金矿区主要出露新太古界乌拉山岩群和中元古界渣尔泰山群。乌拉山岩群分布于矿区西南(图2),呈捕掳体产出,岩性组合以蛇纹石化大理岩、蛇纹石化橄榄石大理岩、金云透辉大理岩为主,夹厚度不等的矽线石榴片麻岩、黑云石榴斜长(钾长)片麻岩、石榴紫苏斜长片麻岩条带。矿区北部出露渣尔泰山群阿古鲁沟组,总体呈SW-NE走向,该组下部为灰黑色黑云母千枚岩夹浅灰色糜棱岩化长石石英砂岩、砂质千枚岩;上部为灰黑色含红柱石黑云母千枚岩、黄褐色含砂绢云母千枚状板岩夹变质长石石英砂岩、硅化大理岩(俞礽安等,2009),局部可见顺层脆-韧性与韧性剪切带,带内发育顺层掩卧褶皱、构造透镜体、雁列石英脉、构造片岩或构造千枚岩等(汪劲草等,2012)。阿古鲁沟组浅变质碎屑岩是常福龙沟金矿赋矿围岩。矿区东部发育晚二叠世中细粒角闪二长花岗岩,其与中元古界呈逆断层接触,断层呈弧形,接触断层具双断面,面上可见几厘米厚的断层泥,断层面所夹破裂带宽约3~5m,带内残存花岗岩构造透镜体,靠近断层面有几十厘米厚的面理化碎裂岩(汪劲草等,2012)。矿区南部出露石英闪长岩-闪长岩,岩石呈灰绿色,变余花岗结构,片麻状构造,主要矿物为斜长石、石英、角闪石和斜方辉石(钟长汀等,2014)。矿区南部尚发育有早侏罗世细粒钾长花岗岩(图2)。

图2 内蒙古常福龙沟金矿区地质简图(据俞礽安等,2009)Fig.2 Generalized geological map of the Changfulonggou gold deposit, Inner Mongolia (after Yu et al., 2009) 1-第四系; 2-渣尔泰山群变质砂岩、板岩、千枚状板岩; 3-乌拉山岩群大理岩; 4-细粒钾长花岗岩; 5-似斑状角闪二长花岗岩; 6 -石英闪长岩; 7-闪长岩脉; 8-含金蚀变带位置; 9-金矿体位置及编号; 10-断层 1-Quaternary; 2-Zhaertaishan group metasandstone, slate and phyllitic slate; 3-Wulashan group marble; 4-fine grained K-feldspar granite; 5-porphyritic hornblende monzogranite; 6-quartz diorite; 7-diorite dyke; 8-Au-bearing alteration zone; 9-gold orebody; 10-fault

矿区共圈出7个矿体,其中主要矿体为Au-1号、Au-2号、 Au-3号、 Au-4号。主矿体走向约290°~300°,在平面上和剖面上都呈平行斜列特征(高帮飞等,2012;汪劲草等,2012)。其中,Au-1号为最主要矿体,呈不规则中脉状,倾向SW,倾角49°~80°,矿体连续,厚度稳定,平均品位3.74 g/t,矿体由黄铁矿化硅化构造角砾岩组成。Au-2号呈薄脉状产出,矿体倾向SW,倾角50°~60°,矿体厚度稳定,平均品位1.49 g/t,主要由硅化黄铁矿化蚀变岩组成。Au-3号矿体呈薄层脉状,倾向SW,倾角50°~60°,矿体厚度稳定,矿体由黄铁矿化硅化角砾岩、黄铁矿化变质砂岩组成。Au-4号呈薄层脉状产出,倾向SW,倾角50°~60°,厚度稳定,金品位1.94 g/t,矿体由硅化蚀变岩组成。

矿石中金属矿物主要为黄铁矿,少量方铅矿、黄铜矿、自然金和银金矿;脉石矿物有石英、方解石、绢云母等(俞礽安等,2009)。其中,黄铁矿发育至少2期,早期粗粒黄铁矿为立方体晶形,呈浸染状分布于蚀变岩中;成矿期黄铁矿呈粉末状-细粒状结构,呈细脉状或团块状构造分布于脉石中,是最主要的载金黄铁矿。自然金多呈它形粒状、不规则粒状集合体产于脉石矿物裂隙和粒间,少量产于黄铁矿和褐铁矿裂隙中;银金矿分布于脉石英、黄铁矿、褐铁矿裂隙中,形态多为它形粒状、不规则状,少量叶片状。

原生矿石结构有自形-半自形粒状结构、它形粒状结构、包含结构、交代残余结构、显微鳞片粒状变晶结构和压碎结构等,发育稀疏或稠密浸染状、网脉状、碎裂状、角砾状等构造,氧化矿石发育土状-蜂窝状等构造。

矿体围岩蚀变较强,主要有硅化、黄铁绢英岩化、碳酸岩化,局部见绿泥石化、黑云母化、钾化、粘土化等,其中硅化强度与金矿化呈正相关关系。

4 样品及实验测试

本文对常福龙沟金矿主要载金矿物黄铁矿进行了硫、铅同位素组成测定,采样位置见表1。选取具有代表性的样品,经手工逐级破碎、过筛至40~60目,然后在双目镜下挑选纯度大于99%的黄铁矿单矿物2g以上。将挑纯后的黄铁矿单矿物研磨至200目以下,供硫、铅同位素组成分析,分析测试工作均在核工业北京地质研究院分析测试研究中心完成。

硫同位素组成分析时,首先将黄铁矿单矿物与氧化亚铜按一定比例研磨、混合均匀后进行氧化反应,使矿物中硫全部转换成SO2并用冷冻法收集,然后用Finnigan MAT251气体同位素质谱仪分析硫同位素组成,相对标准采用V-CDT,分析精度为±0.2‰,分析结果见表1。铅同位素组成分析时,首先将黄铁矿样品放入聚四氟乙烯坩埚中,用混合酸(HF+HClO4)溶样,然后用树脂交换法分离出铅,蒸干后用热表面电离质谱法进行铅同位素测量,仪器型号为ISOPROBE-T,1μg铅的206Pb/204Pb测量精度<0.05%,208Pb/206Pb测量精度≤0.005%。铅同位素H-H单阶段演化模式计算时,运用Faure(1986)给出的公式进行,分析结果及计算结果见表2。

5 分析结果及讨论

5.1 硫同位素组成

由表1可知,常福龙沟金矿5件黄铁矿样品硫同位素组成介于-0.6‰~2.9‰之间,平均值为1.34‰,极差仅为3.5‰。常福龙沟金矿黄铁矿硫同位素特征明显不同于渣尔泰山地区层控硫多金属矿床的硫同位素组成(丁悌平等,1992;付超等,2010)。将常福龙沟金矿与内蒙古中西部重要金矿床硫同位素组成进行对比(图3),其与大多数金矿硫同位素组成一致,靠近0‰值线。与同样赋存于渣尔泰山群阿古鲁沟组层位中的朱拉扎嘎金矿硫化物硫同位素特征较为相似(江思宏等,2001,图3),而明显不同于近些年有重大突破的哈达门沟金矿(富集32S)和浩尧尔忽洞金矿(富集δ34S)硫同位素组成。

表1 内蒙古常福龙沟金矿床黄铁矿硫同位素组成Table 1 Sulfur isotopic composition of pyrite from the Changfulonggou gold deposit

表2 内蒙古常福龙沟金矿床黄铁矿铅同位素组成及特征参数Table 2 Lead isotopic composition and characteristic parameters of pyrite from the Changfulonggou gold deposit

图3 常福龙沟金矿与内蒙古中西部金矿床硫 同位素组成对比图 (朱拉扎嘎金矿数据引自江思宏等,2001;浩尧尔忽洞金矿数据引自Wang et al., 2014; Liu et al., 2014; 其它数据引自Nie et al., 2002)Fig.3 Comparison of sulfide sulfur isotopes between the Changfulonggou deposit and major gold deposits in the Midwest Inner Mongolia (data of Zhulazhaga after Jiang et al., 2001; Haoyaoerhudong after Wang et al., 2014 and Liu et al., 2014; others after Nie et al., 2002)

在矿物组合简单且缺乏硫酸盐矿物的情况下,硫化物δ34S值的平均值可大致代表热液的总硫同位素组成(Ohmoto, 1986)。常福龙沟金矿中硫化物主要为黄铁矿,少量方铅矿、黄铜矿,未见硫酸盐,因此黄铁矿的δ34S值可代表热液的δ34S∑S。常福龙沟金矿黄铁矿硫同位素极差较小,平均值为1.34‰,明显不同于炭窑口矿区渣尔泰山群中硫酸盐(重晶石平均值32.0‰,石膏平均值32.4‰,胆矾34.6‰,丁悌平等,1992)和甲生盘矿区硫酸盐(石膏平均值20.9‰,丁悌平等,1992)。常福龙沟黄铁矿硫同位素组成集中,暗示硫源比较单一且成矿环境和成矿物理化学条件较为稳定。已有研究表明,原始地幔δ34S值接近0‰(Chaussidonetal., 1989),花岗岩的δ34S值一般为-10‰~10‰(Rollinson, 1993),沉积岩的硫同位素组成变化极大,达-40‰~50‰(韩吟文等,2003)。常福龙沟金矿黄铁矿硫同位素组成反映成矿所需的硫可能来自于岩浆。

5.2 铅同位素组成

常福龙沟金矿黄铁矿206Pb/204Pb比值范围为16.849~17.035(平均为16.936),207Pb/204Pb比值范围为15.343 ~15.411(平均为15.381),208Pb/204Pb比值范围为37.310 ~37.547(平均为37.453)。经H-H单阶段铅演化模式计算(Faure,1986),常福龙沟金矿黄铁矿铅同位素模式年龄介于903Ma~1019Ma之间,平均值为975Ma,μ值变化范围为9.18~9.31,平均值为9.25,ω值变化于38.46~39.78之间,平均值为39.11,κ值变化于4.04~4.14之间,平均值为4.09,Δα值变化于49.48~55.84之间,平均值为52.96,Δβ值变化于6.78~11.32之间,平均值为9.16,Δγ值变化于39.79~46.78之间,平均值为43.21。

将常福龙沟金矿黄铁矿铅同位素投点于铅构造模式图(图4),在207Pb/204Pb-206Pb/204Pb图上(图4左),所有样品铅同位素投点于地幔演化线与下地壳之间,在208Pb/204Pb-206Pb/204Pb图上(图4右),投点与下地壳和造山带演化线之间。将常福龙沟金矿黄铁矿铅同位素数据投点于△β-△γ图解(图5),所有样品均投点于造山带铅范围内。本文收集了前人发表的朱拉扎嘎金矿铅同位素数据(江思宏等,2001)、渣尔泰山群火山岩及沉积岩全岩(丁悌平等,1992,彭润民等,1993,江思宏等,2001)以及太古宙变质岩全岩(Nieetal., 1994; Nieetal., 2002)铅同位素数据进行对比分析。与朱拉扎嘎金矿相比,朱拉扎嘎金矿更富集放射成因铅,而常福龙沟金矿黄铁矿铅同位素可能更富集普通铅。在铅构造模式图(图4)和△β-△γ图解(图5)中,常福龙沟金矿均靠近渣尔泰山群铅同位素范围,考虑到铅同位素模式年龄为中元古代晚期-新元古代早期,与容矿层位时代相近,因此推测中元古界渣尔泰山群可能是成矿物质的提供者。

图4 常福龙沟金矿石铅同位素构造模式图 (底图据Zartman, 1981;渣尔泰山群火山岩及沉积岩数据来自丁悌平等,1992,彭润民等,1993,江思宏等,2001;太古 宙变质岩数据来自Nie et al., 1994; Nie et al., 2002; 朱拉扎嘎金矿数据来自江思宏等,2001)Fig.4 Tectonic models of 208Pb/204Pb versus 206Pb/204Pb and 207Pb/204Pb versus 206Pb/204Pb from the Changfulonggou gold deposit (base map after Zartman, 1981; data of Zhaertaishan Group after Ding et al., 1992, Peng et al., 1993 and Jiang et al., 2001; Archean metamorphic rock after Nie et al., 1994 and Nie et al., 2002; Zhulazhaga after Jiang et al., 2001) A-地幔;B-造山带;C-上地壳;D-下地壳 A-mantle; B-orogene; C-upper crust; D-lower crust

图5 常福龙沟金矿石黄铁矿铅成因的△β-△γ图解 (底图据朱炳泉等,1998;数据来源同图4)Fig.5 △β-△γ diagram of genetic classification by pyrite lead isotopes of Changfulonggou gold ore (base map after Zhu et al., 1998; the data sources are same with Fig.4) 1-地幔源铅;2-上地壳铅;3-上地壳与地幔混合的俯冲带铅(3a-岩浆作用;3b-沉积作用);4-化学沉积型铅;5-海底热水作用铅;6-中深变质作用铅;7-深变质下地壳铅;8-造山带铅; 9-古老页岩上地壳铅;10-退变质铅 1-mantle-derived lead; 2- upper crust lead; 3-mixed lead of the upper crust and mantle subduction zones(3a-magmatism, 3b-sedimentation); 4-chemical sedimentary lead; 5-submarine Hydrothermal lead; 6-medium-high grade metamorphism lead; 7-lower crust lead of high grade metamorphism; 8-orogenic belt lead; 9-upper crust lead of ancient shale; 10-retrograde metamorphism lead

6 结论

常福龙沟金矿位于华北陆块北缘隆起带,该矿床赋存于渣尔泰山群阿古鲁沟组中,是该层位在内蒙古中部金矿找矿的新突破。

矿体呈中脉状-薄脉状分布于渣尔泰山群浅变质碎屑岩中。金属矿物为黄铁矿及少量方铅矿、黄铜矿、自然金和银金矿;脉石矿物有石英、方解石、绢云母等。矿区发育硅化、黄铁绢英岩化、碳酸岩化等蚀变,硅化强度与金矿化呈正相关关系。

常福龙沟金矿载金矿物黄铁矿硫同位素组成介于-0.6‰~2.9‰之间,反映成矿所需硫可能来自于岩浆。铅同位素206Pb/204Pb= 16.849~17.035,207Pb/204Pb=15.343~15.411,208Pb/204Pb=37.310 ~37.547,推测中元古界渣尔泰山群可能是成矿物质的提供者。

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Geological Characteristics and Sulfur and Lead Isotopic Geochemistry of the Changfulonggou Gold Deposit in Inner Mongolia

LI Zhi-dan, YU Reng-an, WANG Jia-ying, WEN Si-bo, CHEN Jun-qiang, TANG Chao

(TianjinInstituteofGeologyandMineralResources,Tianjin300170)

The Changfulonggou gold deposit, located in the uplift belt of the northern margin of the North China block, is a new prospecting breakthrough in the Agulugou Formation of the Zhaertaishan Group in central Inner Mongolia. The vein gold orebodies are hosted in the low-grade metamorphic clastic rocks of the Agulugou Formation. The main metallic minerals are pyrite and minor galena, chalcopyrite, native gold and electrum, and gangue minerals are mainly quartz and calcite. The ores have textures of euhedral-subhedral granular, anhedral granular, poikilitic, metasomatic relict, lepidoblast and crush, and structures of dissemination, net vein, cataclastic and brecciate. Wall-rock alteration contains silication, pyrite-sericitization and carbonation, of which the silication is closely related with gold mineralization. The pyrite δ34SV-CDTvalues vary from -27.9‰ to 2.9‰, indicating that the sulfur was sourced from magma. The206Pb/204Pb,207Pb/204Pb and208Pb/204Pb ratios for pyrite have homogenous compositions, varying in 16.849~17.035, 15.343 ~15.411 and 37.310 ~37.547, respectively. The Pb-isotopic compositions of pyrite are very close to the field of the Zhaertaishan Group and the model ages are Late Mesoproterozoic to Early Neoproterozoic time. The Pb-isotopic compositions indicate that the ore-forming materials likely emanated from the Mesoproterozoic Zhaertaishan Group.

gold deposit, geological characteristics, S and Pb isotopic geochemistry, Changfulonggou, Inner Mongolia

2015-01-31;

2015-03-12;[责任编辑]陈伟军。

中国地质调查项目(12120113057300)资助。

李志丹(1986年-),男,2011年毕业于中国地质大学(北京),获硕士学位,工程师,从事矿产勘查与研究工作。

俞礽安(1980年-),男,工程师,从事矿产勘查与研究工作。E-mail:cugcug@qq.com。

P61

A

0495-5331(2015)03-0414-08

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