辽东砖庙矿区硼矿床的海相蒸发成因——来自硼、硫、碳同位素的证据

胡古月, 范昌福*, 李延河, 侯可军, 刘 燚, 陈 贤

1)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 100037; 2)金玛宽甸矿业有限公司, 栾家沟硼矿, 辽宁丹东 118200; 3)中国地质大学(北京)地球科学与资源学院, 地质过程与成矿作用国家重点实验室, 北京 100083

辽东砖庙矿区硼矿床的海相蒸发成因
——来自硼、硫、碳同位素的证据

胡古月1), 范昌福1)*, 李延河1), 侯可军1), 刘 燚2), 陈 贤3)

1)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 100037; 2)金玛宽甸矿业有限公司, 栾家沟硼矿, 辽宁丹东 118200; 3)中国地质大学(北京)地球科学与资源学院, 地质过程与成矿作用国家重点实验室, 北京 100083

辽宁砖庙硼矿区的硼矿体呈层状或透镜状赋存于辽河群里尔峪组火山-沉积建造下部的蛇纹石化大理岩中, 内部含有大量镁橄榄岩包裹体。本研究利用LA-MC-ICP-MS技术对砖庙硼矿区内的硼矿石硼同位素进行了微区原位分析, 对矿石及大理岩围岩的硫、碳稳定同位素进行了系统研究。硼矿石的δ11BNISTSRM-951为12.6‰~13.9‰, 具海相蒸发沉积特征; 硼矿石和大理岩的δ34SV-CDT为11.6‰~24.3‰, 具海相沉积成因特征; 矿体上下层位中蛇纹石化大理岩的 δ13CV-PDB为–5.0‰ ~ –0.5‰, 部分未蛇纹石化大理岩的δ13CV-PDB为4.1‰~4.6‰, 具有古元古代海相碳酸盐岩特有的碳同位素正异常现象。据此提出, 砖庙矿区的硼矿床可能形成于海相蒸发沉积和火山喷发旋回交替的滨海环境, 随后同期喷发的超基性火山岩覆盖于海相蒸发沉积成因硼矿体之上, 保护了易溶的硼酸盐矿物, 经后期变质和热液改造, 形成目前独特的硼酸盐矿物, 碳酸盐岩与超基性岩岩石组合。

辽河群; 里尔峪组; 砖庙硼矿区; 硼同位素; 硫同位素; 碳同位素

华北克拉通东部基底在古元古代时打开, 形成辽吉裂谷(Zhang et al., 1988; Zhai et al., 2011; Zhao et al., 2012), 在辽东地区沉积了一套火山-沉积建造, 被命名为辽河群, 其中赋存有大规模的层控硼酸盐类矿床(张秋生, 1984; Zhang et al., 1988; Peng et al., 1995, 1998, 2002; Jiang et al., 1997; 刘敬党等, 2007)。相对于广为人知的土耳其西安拉托尼亚地区和美国加利福尼亚地区的第三系(或更年轻)硼酸盐矿床, 辽东地区的硼酸盐矿床以其特殊的矿物组合(遂安石-硼镁石型和硼镁铁矿-硼镁石型)和古老的成矿时代(古元古代)而倍受关注。辽东硼矿形成于火山活动(李守义, 1994; 孙敏等, 1996; 肖荣阁等, 2007)与蒸发沉积作用(Peng et al., 1995, 2002; Jiang et al., 1997)旋回交替的环境, 后期经历了达绿片岩相-角闪岩相的变质作用(张秋生, 1984; 刘敬党等, 1993; 孙敏等, 1996),属沉积-变质型矿床, 主矿区包括后仙峪、翁泉沟和砖庙—杨木杆。由于辽东硼矿后期经历强烈的混合岩化作用和蛇纹石化蚀变作用(张秋生, 1984),原始沉积特征遗失殆尽, 导致对硼来源的认识存在两种迥异的观点: (1)火山热泉供硼-非海相蒸发沉积(Peng et al., 1995, 1998, 2002; Jiang et al., 1997; Wang et al., 1998; Xu et al., 2004); (2)海底火山喷发形成矿源层, 后期混合岩化成矿(冯本智等, 1998; 肖荣阁等, 2007; 刘敬党等, 2005, 2007; 王翠芝等, 2008a)。本研究试图通过分析辽东砖庙矿区硼矿床的矿石和蛇纹石化大理岩围岩中的硫、硼、碳同位素地球化学特征和变化规律, 探讨硼矿的成矿环境。

1 地质背景

1.1 区域地质

以绿片岩和花岗片麻岩为主的鞍山群于古元古代时期打开而形成辽吉裂谷, 并接受了一套火山沉积建造, 形成辽河群。辽河群中赋存有硼矿、菱镁矿、滑石、岫岩玉等一系列大型-特大型非金属矿床。辽吉古元古代裂谷呈近东西向展布, 西窄东宽呈楔形横贯辽宁东部与吉林南部。西以复县华铜、盖州、大石桥一带为界, 向东经岫岩、凤城、宽甸、桓仁延伸进入吉林长白山、朝鲜, 南北宽约100 km, 东西长约300 km(翟裕生等, 2008)。根据岩相建造与构造特征, 裂谷带横向上被划分为北缘斜坡(鞍山—桓仁北缘滨海斜坡), 中央凹陷(大石桥—宽甸轴部浅海凹陷), 南缘浅台(岫岩—丹东南缘滨海浅台)三个构造岩相区(陈荣度, 1990)(图1)。沉积地层辽河群底部石英岩不整合于鞍山群变质岩之上, 自下而上划分为五个组: 浪子山组, 里尔峪组, 高家峪组, 大石桥组和盖县组, 属一套火山-沉积建造, 普遍遭受绿片岩相至角闪岩相的变质作用(姜春潮, 1987)。其中里尔峪组历来有南里尔峪组(属辽河群中央凹陷区和南缘浅台区)和北里尔峪组(属辽河群北缘斜坡区)之分。南里尔峪组含有大量酸性火山岩和硼矿, 硼含量较高而被称为“含硼岩系”(张秋生, 1984), 而北里尔峪组则不含硼矿。总的看来, 里尔峪组主要为层状分布的条痕状花岗岩、变粒岩、浅粒岩和大理岩, 最大厚度达 1400余m。

1.2 矿区地质及成矿时代

砖庙硼矿区位于辽宁省宽甸县硼海镇以东20 km处, 自西向东可细分为大阳沟、二人沟、老人沟、花园沟、砖庙沟、栾家沟和小汤石等7个小型矿床和矿点, 本次工作重点研究了其中的二人沟、花园沟、砖庙沟和栾家沟四个小型硼矿床(图2)。矿区内含硼岩系整体走向近东西(90°~110°), 倾角较陡(65°~80°)。含矿地层普遍经角闪岩相变质和强烈褶皱变形作用, 因受到多条北北东和北北西向断裂切割(图2), 有较大错移(Lu et al., 2005)。矿区含硼岩系的岩石类型主要为一套变粒岩和浅粒岩(图 2), 代表性的矿物组合为黑云母-角闪石-钠长石-石英-钾长石-电气石(Sun et al., 1993), 在变粒岩层位中发育一套夹杂有镁橄榄石, 普通辉石和透辉石等基性矿物(图 3a, b)的硼矿石和(蛇纹石化)大理岩(图3c, d), 构成矿床的主体。后期有时代不明的煌斑岩、伟晶岩(图3d)和闪长岩等脉岩穿切砖庙矿区的含硼岩系和硼矿体。矿体总体呈层状和似层状产出, 明显受层位控制。主要的矿石矿物有纤维硼镁石、遂安石(图 3a), 其次有极少量的柱状硼镁石、板状硼镁石和硼镁铁矿。脉石矿物主要是白云石、蛇纹石, 其次有镁橄榄石、金云母、透辉石、粒硅镁石、斜硅镁石、水镁石和透闪石等(刘敬党等, 2007)。

图1 辽东裂谷中硼矿床的分布简图(据陈荣度, 1990; 郝德峰等, 2004; Li et al., 2006修改)Fig. 1 Simplified geological map showing locations of boron deposits (modified after CHEN, 1990; HAO et al., 2004; Li et al., 2006)

砖庙矿区外围分布有大量条痕状花岗岩, 属含硼岩系的底层岩石(姜春潮, 1987), 前人得到的条痕状花岗岩的锆石TIMS, LA-MC-ICP-MS和SHRIMP年龄数据集中分布在2.17~2.24 Ga之间(Sun et al., 1993; Lu et al., 2006; Li et al., 2007), 原岩可能为酸性火山-沉积岩(张秋生, 1984; 姜春潮, 1987; 赵凤顺等, 1989; 刘敬党等, 2007)。由于硼矿属于沉积变质型矿床, 因此, 由酸性火山岩原地重熔而成的条痕状花岗岩的岩浆锆石年龄直接将辽东地区硼矿的沉积成矿时代下限限定在2.2 Ga左右。

图2 辽东砖庙硼矿区含硼岩系中段的地质简图(据Lu et al., 2005修改)Fig. 2 Geological sketch map of the middle parts of boron-bearing sequence in the Zhuanmiao borate ore district, eastern Liaoning Province (modified after Lu et al., 2005)

2 分析方法

2.1 LA-MC-ICP-MS微区原位硼同位素测试

图3 辽东砖庙矿区的硼矿石和围岩照片Fig. 3 Photographs of Mg-borate ores and wall rocks in the Zhuanmiao borate ore district, eastern Liaoning Province

LA-MC-ICP-MS微区原位硼同位素分析在中国地质科学院矿产资源研究所 MC-ICP-MS实验室完成, 所用标准为美国国家标准技术研究所 NIST SRM951硼酸样品(11B/10BNISTSRM951=4.05003), 所用仪器为Neptune型MC-ICP-MS及与之配套的New Wave UP 213激光剥蚀系统。激光剥蚀所用斑束直径为 25 μm, 频率为 10 Hz, 能量密度约为 8 J/cm2, 以 He气为载气(0.8 L/min)。10B和11B 分别用法拉第杯 L3、H4静态同时接收, LA-MC-ICP-MS激光剥蚀采样采用单点剥蚀的方式,数 据 分 析 前 用 电 气 石 IAEA B4(δ11B为(–8.36±0.58)‰)调试仪器, 使之达到最优状态, 以电气石IMR RB1(δ11B为–(12.97±0.97)‰)为内标, 以电气石IAEA B4(δ11B为(–8.36±0.58)‰)为外标进行校正。测试过程中每测定 3个样品前后重复测定两次IMR RB1(δ11B为(–12.97±0.97)‰)对样品进行校正,精度(2σ)均为1‰左右。详细过程见侯可军等(2010)和Hou等(2010)。在花园沟和二人沟采集的4件硼矿石的微区原位测试结果列于表1。

2.2 硫同位素

砖庙硼矿区硼矿石和围岩大理岩的硫同位素分析在国土资源部成矿作用与资源评价重点实验室完成。先用艾氏卡试剂(Eschka)将硼酸盐岩和碳酸盐岩中的硫酸根转化为硫酸钠, 然后用BaCl2溶液将硫酸盐类转化为BaSO4沉淀, 沉淀经过滤、清洗、烘干后,用V2O5氧化剂制备SO2, 使用气体质谱仪MAT-253进行硫同位素测试, 分析精度为±0.2‰, 结果以相对国际标准为 V-CDT的 δ34SV-CDT值表示, 在花园沟,二人沟和砖庙沟采集的4件硼矿石和12件大理岩样品的硫同位素测试结果见表1。

2.3 碳同位素

砖庙硼矿区蛇纹石化大理岩和大理岩的碳同位素分析在国土资源部成矿作用与资源评价重点实验室MAT-253型质谱计上完成。碳同位素测试采用100%磷酸法, 结果以相对国际标准为 V-PDB的δ13CV-PDB值表示, 精度优于±0.2‰。在花园沟, 栾家沟, 二人沟和砖庙沟采集的 10件大理岩样品的碳同位素测试结果见表1。

3 结果与讨论

3.1 硼同位素

在砖庙矿区的花园沟和二人沟硼矿床采集的4个硼矿石(12HYG-1, 12HYG-6, 12ERG-6和12ERG-7)的δ11BNISTSRM951值为12.2‰~13.9‰(表1), 数据分布集中而稳定, 与前人得到的砖庙矿区硼矿石的δ11BNISTSRM951值(8.8‰~12.6‰)基本一致(Peng et al., 2002)。硼在地壳中储库主要包括碎屑沉积岩, 海相蒸发岩和陆相蒸发岩(Peng et al., 1995)。全球已探明的两个最大硼矿区, 土耳其西安拉托尼亚地区和美国加利福尼亚地区的硼酸盐矿床均属于陆相蒸发盐型硼矿床; 而哈萨克斯坦印德硼矿区的硼酸盐矿床则属于海相蒸发成因硼矿(刘敬党等, 2007)。陆相硼酸盐矿物主要分布在陆相咸化湖泊, 硼主要来源于与火山活动有关的热泉和热液(Alonso et al., 1988);海相硼酸盐矿物则主要分布在海相蒸发沉积地层,来自于海水硼的蒸发富集(Swihart et al., 1986; Kloppmann et al., 2001; 肖应凯等, 2005; Tan et al., 2010)。陆相咸化湖泊沉积碳酸盐岩的 δ11B值与海洋碳酸盐岩的δ11B值具有明显的差异, 两者的δ11B值分别为(–7±10)‰和(25±4)‰(Swihart et al., 1986), B同位素组成可有效判别海相或非海相蒸发盐(Vengosh et al., 1992)。辽东地区古元古代砖庙矿区硼矿石的δ11BNISTSRM951值为8.8‰~13.9‰, 处于非海相蒸发岩和海相蒸发岩之间(图4)。

表1 砖庙矿区的硼矿石和大理岩中硫、硼、碳同位素测试结果Table 1 Sulfur, boron and carbon isotopic compositions of borate ores and marbles in the Zhuanmiao borate ore district

含硼矿物的变质和脱水作用可导致矿物的δ11B值降低(Peacock et al., 1999), 由于几乎无法避免后期变质和流体作用, 前寒武纪海相碳酸盐岩的 δ11B值均处于零值附近(δ11BNISTSRM951为–6.2‰~4.4‰) (Barth, 1993; Kasemann et al., 2005)。辽东硼矿普遍遭受了绿片岩相-角闪岩相的变质作用, 因此古元古代的辽东砖庙矿区的硼矿床初始沉积时的δ11BNISTSRM951值可能比测定值高, 暗示硼矿床可能形成于海相蒸发沉积环境。另外, 根据地球上海水中硼同位素的一般演化规律以及目前得到的早前寒武纪海相碳酸盐和海相地层中电气石矿物的δ11B值普遍较低的地球化学现象, Chaussidon等(1992)估算得到太古代海水的δ11BNISTSRM951值为(27±11)‰, 明显低于现代海水的值。因此, 在古元古代海水蒸发沉积形成的硼酸盐类矿物的δ11B值也比现代海相蒸发沉积硼酸盐的值低。

图4 不同地质体中δ11B值的分布范围(数据来自Swihart et al., 1986; Chaussidon et al., 1992, 1995; Peng et al., 2002; Tan et al., 2010)Fig. 4 Range of δ11B values from different boron sources (except for data from the Zhuanmiao Mg-borate ore district of eastern Liaoning province, all data from Swihart et al., 1986; Chaussidon et al., 1992, 1995; Peng et al., 2002; Tan et al., 2010)

3.2 硫同位素

砖庙矿区的 4个硼矿石 δ34SV-CDT分布在16.1‰~23.7‰, 平均值为18.8‰。12个围岩大理岩的 δ34SV-CDT分布在 11.6‰~24.3‰, 平均值为19.1‰。与硼矿石共生的硬石膏 δ34SV-CDT分布在20.7‰~24.9‰, 平均值为22.7‰(黄作良等, 1996)。碳酸盐岩和硼酸盐矿物的δ34SV-CDT值相对于围岩中石膏单矿物值偏低 3‰左右。在整体上, 花园沟硼矿床的蛇纹石化大理岩的δ34SV-CDT值相对于蚀变较轻的二人沟硼矿床大理岩的值偏低(表1)。变质和热液作用通常使岩石中的硫元素发生逸失和再分配,使变质岩的硫同位素组成均一化(Andreae, 1974),而在硫同位素体系中硫酸盐的 δ34SV-CDT值最高, 在变质过程中其δ34SV-CDT值降低幅度可能更大。因此,围岩大理岩和石膏中 δ34SV-CDT的最高值(24‰)可能大致代表了其沉积时海水硫酸盐的硫同位素组成。太古代海水中的硫主要来自火山, 以还原硫的形式存在(Ono et al., 2009), 具有接近地幔的S同位素组成(图5), 古元古代早期(2.4~2.0 Ga)“大氧化事件”使得陆壳中大量硫化物氧化而进入海洋, 形成了古元古代海水硫酸盐储库(Canfield, 2005; Schroder et al., 2008), 海水 δ34SV-CDT值升高(Reuschel et al., 2012), 太古代缺氧环境中光化学反应形成的非质量硫同位素分馏现象结束(Pavlov et al., 2002)。显生宙海相蒸发岩 δ34S值维持在 11‰~36‰的范围(Strauss et al., 1997), 而现代海相蒸发硫酸盐 δ34S值比较稳定, 为20‰左右(郑永飞等, 2000)。由图5可看出, 砖庙硼矿区同沉积海洋硫酸盐的 δ34SV-CDT值高于世界其他地区同期海洋硫酸盐的值, 也暗示砖庙矿区的硼矿石和含硫酸盐大理岩可能形成于封闭的蒸发海洋沉积环境, 这与根据硼同位素得到的结果一致。

图5 辽东砖庙矿区的硼矿石和大理岩的硫同位素组成(背景数据来自Bottomley et al., 1992及其中的参考文献,为前寒武纪海相沉积碳酸盐岩, 硫酸盐岩的硫同位素组成)Fig. 5 Sulfur isotopic composition of disseminated sulfur in borate ores and marbles from the Zhuanmiao Mg-borate ore district, eastern Liaoning Province (background data from Bottomley et al., 1992 and the references therein, representing sulfur isotopic composition of Precambrian marine carbonate and sulfate)

3.3 碳同位素

辽东砖庙矿区硼矿体的直接容矿围岩——蛇纹石化大理岩的δ13CV-PDB为–5.0‰ ~ –0.5‰; 少量未发生蛇纹石化大理岩的δ13CV-PDB为 4.1‰~4.6‰(表 1)。在碳同位素体系中, 碳酸盐的δ13CV-PDB值最高, 而海相碳酸盐的 δ13CV-PDB值又明显高于非海相, 正常海相碳酸盐的 δ13CV-PDB值多分布在0值左右(郑永飞等, 2000)。变质作用和后期热液改造常使海相碳酸盐的δ13CV-PDB值降低(Veizer et al., 1999; Jacobsen et al., 1999; Melezhik et al., 2005)。砖庙矿区硼矿体的直接容矿围岩——蛇纹石化大理岩的δ13CV-PDB值较外围未蚀变大理岩的值明显降低(图6), 可能为大理岩遭受强烈蚀变的结果。因此, 砖庙矿区初始沉积碳酸盐的 δ13CV-PDB值可能较大理岩的值要高, 表明其可能形成于海相蒸发环境, 这与根据硼、硫同位素得到的认识是一致的。

前人获得辽东硼矿下盘普遍出露的条痕状花岗岩的锆石TIMS, LA-MC-ICP-MS和SHRIMP年龄数据分布在2.14~2.24 Ga左右(Sun et al., 1993; Lu et al., 2006; Li et al., 2007), 可作为辽东硼矿的沉积成矿时代下限。同一时期, 在2350~2000 Ma(Shields et al., 2002)或2220~2060 Ma(Karhu et al., 1996)期间海相碳酸盐发生碳同位素正异常的Lomagundi事件。古元古代蒸发环境在全球多有分布, 可能是造成部分局域封闭环境海相碳酸盐碳同位素正异常的原因(Melezhik et al., 2005), 因此砖庙地区海相大理岩的碳同位素正异常也有可能是古元古代Lomagundi事件引起的。

图6 地质历史时期海相碳酸盐岩沉积(数据来自Shields et al., 2002)与辽东砖庙硼矿区的大理岩和蛇纹石化大理岩的碳同位素组成Fig. 6 Carbon isotopic evolution of marine carbonate (after Shields et al., 2002), and carbon isotopic compositions of marbles and serpentinized marbles in the Zhuanmiao Mg-borate ore district of eastern Liaoning Province

超基性喷发岩在辽东硼矿各个矿区普遍以矿体直接容矿围岩的形式存在(王翠芝等, 2006a, b, 2008b; 王翠芝, 2007; 肖荣阁等, 2007), 在砖庙矿区则主要是以镁橄榄石和普通辉石的形式与矿石交错伴生(图3a, b)(Peng et al., 2002; 王翠芝等, 2007)。超基性火山岩可能在海相蒸发沉积成矿过程中喷发并覆盖在硼矿体之上, 在后期的角闪岩相变质过程中起到了保护硼矿体的作用。因此, 超基性火山喷发岩及相关热液活动可能是导致蒸发沉积型硼矿体附近镁质碳酸盐发生蚀变, 形成蛇纹石化大理岩,碳同位素值降低的主要因素。

4 结论

辽东砖庙矿区硼矿石的δ11BNISTSRM951为8.8‰~13.9‰, 硼矿石及围岩的 δ34SV-CDT分布在11.6‰~24.3‰, 未蚀变的大理岩围岩的 δ13CV-PDB为4.1‰~4.6‰, 三者均显示明显正异常, 表明砖庙矿区的硼矿可能形成于古元古代海相蒸发环境; 同期的超基性火山岩喷发覆盖对硼酸盐在变质和后期改造中免遭破坏及硼矿的最终形成起到了很好的保护作用。

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Marine Evaporative Genesis of Mg-borate Deposits in the Zhuanmiao Ore District, Eastern Liaoning Province: Evidence from B, S, C Isotopes

HU Gu-yue1), FAN Chang-fu1)*, LI Yan-he1), HOU Ke-jun1), LIU Yi2), CHEN Xian3)
1) MRL Key Laboratory of Metallogeny and Mineral Resource Assessment, Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing 100037; 2) Luanjiagou Boron Mine, Jinma (Kuandian) Boron Co., Ltd., Dandong, Liaoning 118200; 3) State Key Laboratory of Geological Processes and Mineral Resources, School of Earth Science and Resources, China University of Geosciences(Beijing), Beijing 100083

Zhuanmiao Mg-borate ore bodies exhibit layered or lenticular structure in the wall rocks of serpentinized marbles, which are located in the lower strata of volcanic-sedimentary Lieryu Formation in Kuandian County of eastern Liaoning Province. The authors applied LA-MC-ICP-MS in-situ technology to analyze the B isotopic composition of borate ores in the Zhuangmiao Mg-borate ore district, and systematically studied the S and C stable isotopic compositions of the ores and marbles. The δ11BNISTSRM-951values of Mg-borate ores vary from 12.6‰ to 13.9‰, showing marine evaporative characteristics; the δ34SV-CDTvalues of Mg-borate ores and serpentinized marble vary from 11.6‰ to 24.3‰, suggesting marine sedimentary characteristics; (3) the δ13CV-PDBvalues of serpentinized marble vary from –4.6‰ to –0.5‰, while those of marble vary from 4.1‰ to 4.6‰, exhibiting positive carbon isotopic anomaly of Paleoproterozoic marine carbonates. Therefore, the ore-forming environment of the Mg-borate deposit in Zhuanmiao area was probably an alternate cycle of marine sedimentation and volcanism of the littoral facies, while a suite of homochronous Mg-rich ultrabasic volcanicrocks covered the borate deposits and preserved the borate ores during hydrothermalism and metamorphism at late stages, and formed a unique rock unit of borate minerals, carbonates and ultrabasic minerals.

Liaohe Group; Lieryu Formation; Zhuanmiao Mg-borate ore district; boron isotope; sulfur isotope; carbon isotope

P578.93; P571; P597.2

A

10.3975/cagsb.2014.04.06

本文由国土资源部公益性行业科研专项(编号: 201211074-2; 200911043-20)资助。

2013-09-23; 改回日期: 2014-01-10。责任编辑: 闫立娟。

胡古月, 男, 1985年生。博士研究生。主要从事同位素地球化学和分析化学研究。E-mail: wanghuguyue@126.com。

*通讯作者: 范昌福, 男, 1979年生。副研究员。长期从事地质环境研究。E-mail: fancf@cags.ac.cn。