岩石学报  2017, Vol. 33 Issue (10): 3161-3182   PDF    
赣东北朱溪钨(铜)矿区高分异花岗岩的成因及与钨矿的关系
刘经纬1 , 陈斌1,2 , 陈军胜3 , 李壮1 , 孙克克1     
1. 北京大学地球与空间科学学院, 造山带与地壳演化教育部重点实验室, 北京 100871;
2. 合肥工业大学资源与环境学院, 合肥 230009;
3. 国土资源部中央地质勘查基金管理中心, 北京 100812
摘要:朱溪钨(铜)矿是江西东北部近年来发现的超大型钨矿,本文报道了与成矿相关岩体的主量元素、微量元素、Nd同位素成分和黑云母、斜长石电子探针数据,以探讨岩体成因以及F在岩浆演化及成矿过程中的作用。朱溪矿区花岗岩具有富硅铝、贫钙镁的特征,属于典型的S型花岗岩,而不是A型花岗岩。Nd同位素(εNdt)=-10.1~-7.9)、岩石学、地球化学特征表明朱溪矿区花岗岩是在超高温麻粒岩相条件下形成于下地壳含富Mg/Ti黑云母的麻粒岩相残留体的再次部分熔融,并非双桥山群变质沉积岩的直接部分熔融以及九岭(和杨草尖)黑云母花岗岩的重融形成。三类岩石可能代表同一个母岩浆的分离结晶的产物,结晶的先后次序为二云母花岗岩-白云母花岗岩-白云母花岗斑岩。这些花岗岩具有明显的富F特征,F的加入降低了花岗岩岩浆的固相线温度(低锆饱和温度,621~779℃)和粘度,延长了岩浆演化过程,导致了强烈的分离结晶作用和熔体-流体作用,并形成偏离正常岩浆体系的微量元素比值(低K/Rb、Zr/Hf、La/Nb、La/Ta比值)。朱溪矿区花岗岩较长的岩浆演化时间和较低的结晶温度使其可以长时间萃取双桥山群等围岩中的W元素。F与W具有明显的正相关性,W与F络合随流体迁移,巨量的W在合适的温压条件下沉淀。朱溪矿区高分异花岗岩对于钨矿的形成起了重要作用。
关键词: 朱溪钨矿     富F花岗岩     钦杭成矿带     燕山期    
Highly differentiated granite from the Zhuxi tungsten (copper) deposit in northeastern Jiangxi Province:Petrogenesis and their relationship with W-mineralization
LIU JingWei1, CHEN Bin1,2, CHEN JunSheng3, LI Zhuang1, SUN KeKe1     
1. MOE Key Laboratory of Orogenic Belts and Crustal Evolution, School of Earth and Space Sciences, Peking University, Beijing 100871, China;
2. School of Resources and Environmental Engineering, Hefei University of Technology, Hefei 230009, China;
3. Central Geology and Mineral Exploration Foundation Management Center, Ministry of Land and Resources, Beijing 100812, China
Abstract: The Zhuxi tungsten (copper) deposit is a super-large tungsten found in the northeastern Jiangxi Province in recent years. In this study, we report major elements, trace elements, Nd isotopic data, plagioclase and biotite microprobe data for mineralized granite to explain its petrogenesis and the role of fluorine in magmatic evolution and ore-forming process. Zhuxi granite is rich in alumina and silica, poor in calcium and magnesium, indicating they belong to typical S-type granite rather than A-type granite. Evidence of Nd isotopic data (εNd(t)=-10.1~-7.9), petrology and geochemistry indicate that Zhuxi granite is the product of partial melting of residue containing Mg/Fe-rich bitotite in granulite-facies, the residue is equilibrated with the melt generating Jiuling and Yangcaojian biotite granite. Three kinds of granite belong to comagmatic evolution with order two-mica granite, muscovite granite and granite porphyry. Peculiarly, Zhuxi granite is enriched in fluorine which reduced the solidus temperature (low zirconium saturation temperature, 621~779℃) and viscosity of the granitic magma, prolonged the magmatic evolution. As a consequence, fractional crystallization and melt-fluid interaction were strengthened; some elements' ratios were changed relative to normal igneous system (low K/Rb, Zr/Hf, La/Nb, La/Ta ratios). Longer magma evolution time and lower crystallization temperature of granite from the Zhuxi ore deposits have long extraction time of W element from Shuangqiaoshan Group. Meanwhile, F and W have apparently positive correlation, they form complex compound to migrate in the fluid. Massive tungsten will precipitate in appropriate temperature-pressure condition. Therefore, we conclude that Zhuxi highly differentiated granite has a decisive effect on tungsten ore formation.
Key words: Zhuxi tungsten deposit     F-rich granite     Qinzhou-Hangzhou metallogenic belt     Yanshanian    
1 引言

高分异花岗岩是花岗岩类中特殊的岩石类型,以富含F、B等挥发分为特征。挥发分F等可大幅降低熔体的粘度(Dingwell and Mysen, 1985; Mysen et al., 2004)、密度和固相线温度(至450~550℃, F=4%~5%; Webster and Rebbert, 1998),导致富F花岗岩发生极端的分离结晶作用而成为高分异花岗岩。其特征是高硅富碱,含白云母、铁锂云母、萤石、黄玉、电气石、钛铁矿等矿物,稀土元素有不同程度四分组效应和强Eu负异常(Bau and Dulski, 1995; Chen et al., 2014; Wang et al., 2014a)。高分异花岗岩与钨锡铜钼等金属成矿有密切关系,因此在最近十年引起国内外高度关注。如千里山柿竹园钨锡钼铋多金属矿(毛景文等, 2008; Chen et al., 2014)、西华山锡矿(Guo et al., 2012)和内蒙岔路口钼矿(Li et al., 2014)等。富水、高氧逸度的中酸性岩体及相关的典型斑铜钼矿在国内外已经有大量研究(Kerrich et al., 2000; Hou et al., 2003; Cooke et al., 2005),但贫水、低氧逸度的高分异花岗岩的成因及与相关的钨锡铜多金属矿的成因联系却并不清楚。高分异花岗岩的源区是什么?F等挥发分的来源及对岩浆房演化、矿化有何意义?我们的初步研究认为,高分异花岗岩富集Li、F的性质能大大延长岩浆的结晶温度区间,促使岩浆长时间呈岩浆状态,并充分演化;由于其长时间加热围岩,使得围岩中孔隙水被加热至300~500℃,高温孔隙水在岩体和围岩之间充分循环,不断萃取围岩中钨锡等成矿物质,并最终在流体条件发生变化之时在岩体中或岩体与围岩接触带中富集成矿。而正常的花岗岩由于与围岩中流体作用相对较弱,不利于成矿,因此高分异花岗岩有利于钨锡铜多金属矿床的形成(Chen et al., 2014; Wang et al., 2014a)。

华南地区中生代与壳幔岩浆活动有关的成矿作用十分强烈,长江中下游、南岭、钦杭结合带是其中三个重要的构造岩浆成矿区(陈毓川和王登红, 2012)。长江中下游成矿带主要与燕山期中基性-中酸性岩浆岩有关,并且受长江断裂带控制,幔源物质对成矿起主导作用(Li et al., 2011; Wang et al., 2013a, 2015a; Xie et al., 2012; Yan et al., 2015; Yang et al., 2014; 陈毓川等, 2014)。南岭成矿区的构造环境为华南加里东期褶皱带在中生代的构造活化,其形成与壳源花岗岩成矿作用有关,也有幔源物质参与(Li et al., 2009a; Zhou and Li, 2000; 陈毓川和王登红, 2012; 华仁民等, 2010; 毛景文等, 2008; 朱金初等, 1990)。作为华南重要成矿区的钦杭成矿带是一条古海洋喷流热水沉积矿床密集分布带,有人提出其属于两期复式成矿:元古宙或古生代出现的喷流热水沉积矿床叠加了燕山期的岩浆-热液成矿作用(蒋少涌等, 2015; 周永章等, 2012, 2015)。近年来皖南东源、赣北大湖塘、赣东北朱溪等大型钨矿的相继发现改变了钨矿的地域分布特征,南钨北扩已成为目前和今后钨矿勘查和钨工业布局的新方向(王登红等, 2012)。东源钨钼矿是受晚期岩浆热液作用形成的中高温热液型矿床(傅建真等, 2011),钨钼矿流体经历了不混溶或沸腾作用,随后与被加热的大气降水混合,引起体系含W和Mo络合物的不稳定而造成大量成矿物质析出、沉淀,富集成矿(杜玉雕等, 2011)。大湖塘钨矿区的成矿流体特征与此类似,为中高温、低盐度浅成热液型矿床(阮昆等, 2013),相关的似斑状白云母花岗岩的高F和低氧逸度的环境有利于W的迁移和富集(Huang and Jiang, 2014; 黄兰椿和蒋少涌, 2012, 2013)。关于朱溪钨矿的成因,陈毓川等(2014)认为中生代太平洋板块对欧亚板块俯冲波及古构造,使江绍缝合带重新活动,形成了中生代成岩成矿期内的断裂构造、推覆构造(赣东北大断裂、景德镇大断裂),赣北地区由北向南的推覆构造导致了朱溪矽卡岩型钨铜矿床的形成。华南燕山期花岗岩分布广泛,复式花岗质岩基中常有晚期小岩体(补体)出现(主要为细粒二云母花岗岩和白云母花岗岩),与W-Sn-U等稀有金属矿床有关的主要是这些高度分异演化的晚期小岩体(高硅,富含F、B等挥发组分)(Chen et al., 2014; Wang et al., 2014a)。朱溪地区发育一系列北东向断裂,沿断裂有许多岩株侵入到新元古界双桥山群浅变质岩和石炭系-二叠系灰岩中,这些岩株具有富F高分异花岗岩的特征。F对于岩浆演化以及成矿过程有重要影响(Chang and Meinert, 2008; Gunow et al., 1980; Idrus et al., 2007; Sotnikov et al., 2003, 2006; Xiong et al., 2002; Zamana and Bukaty, 2004; Zaraisky, 1995),探究F在这些花岗岩演化过程中的作用以及对晚期熔体与流体相互作用的影响对于解释朱溪钨矿成因具有重要的意义。

前人对于本区的花岗岩研究程度较低,多集中于年代学研究。通过LA-ICP-MS锆石U-Pb定年分析得到与成矿关系最为密切的白云母花岗岩年龄为147±1Ma (王先广等, 2015),白云母花岗斑岩年龄为151±2Ma (李岩等, 2014)。朱溪矿区的煌斑岩脉锆石U-Pb年龄为160.3±2.1Ma,所捕获围岩锆石的U-Pb年龄为856±10Ma (刘善宝等, 2014)。本文对朱溪矿区与成矿关系密切的岩体进行了岩石学、地球化学研究,讨论了其成因,特别是F在熔体-流体相互作用过程中对于岩浆演化和成矿的影响,这为解释朱溪矿区高分异花岗岩与成矿关系提供了重要线索。同时,通过富F的高分异花岗岩与钨矿的紧密共生关系以及成因上的联系,可以建立特定岩石类型的成矿专属性。

2 区域地质背景

朱溪钨(铜)多金属矿床位于钦杭成矿带的东段,钦杭成矿带是一条新元古代碰撞对接带(图 1),在中晚侏罗世再次活化,并显示从印支期碰撞挤压的构造背景转向燕山期伸展拉张的构造背景(Li, 2000; Li et al., 2009a; Li and Li, 2007; Zhou and Li, 2000)。中生代太平洋板块对欧亚板块俯冲的影响不仅波及新元古代的古构造使其重新活动,也生成了一系列新构造,尤以中生代成岩成矿期内的断裂构造、推覆构造最为重要,如赣东北大断裂、景德镇大断裂(陈毓川和王登红, 2012; 华仁民等, 2013; 舒良树, 2012),这些深大断裂对朱溪矿区岩体的形成具有重要的控制作用。与朱溪矿区有关的次一级构造单元是塔前-赋春推覆构造带,推覆构造带内部褶皱密集发育,呈倾向NW的单斜构造;断裂带构造发育,以NE向为主(苏晓云等, 2015)。扬子板块最老的地层是崆岭群(2.87~3.0Ga),但仅在其北缘岀露(Gao et al., 1999),在崆岭群附近发现一系列古元古代杂岩体(Wang et al., 2015b)。在扬子板块西缘存有古元古代河口群(Chen et al., 2013),但在扬子板块东部并未发现早于新元古代的地层(Li et al., 2010, 2009b; Wang et al., 2010a, b; Zhao et al., 2011, 2013)。研究区的基底主要是新元古代双桥山群,普遍发生低绿片岩相变质作用,以千枚岩为主夹绢云板岩、变质粉砂-细砂岩。最新的年代学研究表明双桥山群形成于新元古代(Wang et al., 2014b, 2013b; Wang and Zhou, 2013; 王孝磊等, 2013)。研究区的盖层主要是石炭系砂砾岩、中厚层状灰岩,二叠-三叠系灰岩夹炭质页岩和砂岩等,侏罗-白垩系砂砾岩、沙泥岩等(陈国华等, 2012),以狭长带状少量分布在朱溪矿区附近(图 2)。推覆作用使双桥山群叠覆于盖层晚三叠世安源群之上,只保存了由石炭-二叠系组成的向斜南翼,其北翼被掩盖,现在构造上反映的只是向NW倾斜的单斜构造。

图 1 中国华南地区燕山期花岗岩及火山岩分布图(据Deng et al., 2014修改) Fig. 1 Distribution of Yanshanian granitic and volcanic rocks in South China (modified after Deng et al., 2014)

图 2 朱溪矿区区域地质简图(据刘善宝等, 2014修改) Fig. 2 The regional geological sketch of the Zhuxi ore deposits (modified after Liu et al., 2014)

朱溪矿区花岗岩并未在地表出露,样品均采自岩心。二云母花岗岩采自侵位于双桥山群的一个岩株中,深度集中在2000~2200m,岩株上部为钨矿体并穿插多条铜矿带(图 3a),钨矿体上部为由石炭纪灰岩交代变质而成的矽卡岩带。白云母花岗岩采自另一个岩株中,赋存深度为700~1100m (图 3b),岩株上部为已发生大理岩化的二叠纪灰岩,下部为石炭纪灰岩,部分发生较强的石榴石-透辉石化,矿体主要赋存在岩株和下伏的石炭纪灰岩中,呈透镜状和脉状产出(受地层接触界面控制),岩株的分布限制了钨矿体的展布范围。白云母花岗斑岩采自42号勘探线剖面图上的小岩脉(图 3b),深度为1400~1700m,这些岩脉都分布在钨(铜)矿体中。

图 3 朱溪矿区54号(a)与42号(b)勘探线剖面图(据陈国华等,2012修改) Fig. 3 No.54 (a) and No.42 (b) prospecting section of the Zhuxi ore deposits (modified after Chen et al., 2012)
3 样品特征和分析方法 3.1 样品特征

样品采自朱溪矿区钻孔ZK4210、ZK4211和ZK5406(采样位置见图 2)。二云母花岗岩为中细粒花岗结构,部分样品发生轻微绿泥石化(图 4a)。矿物组成为钾长石30%,斜长石20%,石英20%,黑云母15%,白云母10%。斜长石多具环带结构,大部分发生了微弱绢云母化。钾长石有不同程度高岭土化,有的见自形石英包体(图 4d)。黑云母较自形,部分发生微弱绿泥石化;个别黑云母蚀变为白云母,释放的铁与硫生成黄铁矿,包裹在次生白云母中。白云母大部分分布在其他矿物粒间,说明结晶较晚,但个别边界平直的白云母被斜长石包裹。原生白云母粒径较大,晶形较好;次生白云母较细小,集合体保持黑云母晶形。发生蚀变的二云母花岗岩中出现较自形白钨矿(粒径100~600μm),与发生弱绢云母化的斜长石共生,个别颗粒被他形石英包裹。磷灰石大部分被黑云母包裹。萤石分布在矿物粒间,部分包裹于自形石英中。其他副矿物为锆石(棱柱状)、钛铁矿(较自形)、黄铁矿等。

图 4 二云母花岗岩代表性样品手标本和显微照片 (a)手标本照片;(b)显微照片,见较自形黑云母;(c)显微照片,斜长石中包裹多颗白云母,早期白云母与晚期白云母共存;(d)显微照片,自形石英被钾长石包裹.Qtz-石英;Pl-斜长石;Kfs-钾长石;Bi-黑云母;Mus-白云母 Fig. 4 Photographs and micrographs of representative samples of two-mica granite (a) photograph of hand specimen; (b) microphotograph show euhedral biotite; (c) microphotograph show multiple muscovites is enclosed by plagioclases, early-stage and late-stage muscovite coexist; (d) microphotograph show euhedral quartz packaged in potassium feldspar. Qtz-quartz; Pl-plagioclase; Kfs-potassium feldspar; Bi-biotite; Mus-muscovite

白云母花岗岩为细粒花岗结构,块状构造(图 5a),钻孔的部分层位见方解石脉穿插。石英40%~50%,部分呈自形被长石包裹(图 5b);长石30%~40%,大部分发生弱绢云母化,保留斜长石和钾长石晶形。粒间偶见方解石充填,与石英颗粒呈共生接触关系(图 5c),并非后期脉体侵入。萤石与石英边界接触平直,部分萤石的接触关系显示结晶早于石英(图 5d)。白钨矿大部分分布在发生绢云母化的长石中。其他副矿物包括磷灰石、锆石、钛铁矿、闪锌矿、黄铁矿、黄铜矿等。

图 5 白云母花岗岩代表性样品手标本和显微照片 (a)手标本照片;(b)显微照片,弱绢云母化的斜长石中包裹自形石英;(c)显微照片,弱绢云母化的斜长石颗粒中包裹粒状方解石;(d)显微照片,萤石分布在石英颗粒之间,具有明显的菱形解理,萤石与石英呈共生关系.Ser-绢云母;Cal-方解石;Fl-萤石 Fig. 5 Photographs and micrographs of representative samples of muscovite granite (a) photograph of hand specimen; (b) microphotograph show euhedral quartz packaged in slightly sericitizated plagioclase; (c) microphotograph show granulous calcite packaged in slightly sericitizated plagioclase; (d) microphotograph show fluorite has rhombus cleavage and show coexisting relationship with quartz. Ser-sericite; Cal-calcite; Fl-fluorite

白云母花岗斑岩具有典型的斑状构造,斑晶主要为:石英、斜长石、钾长石。其中,石英斑晶(平均1.2mm)较自形(图 6a),部分自形石英颗粒包裹在白云母和斜长石中。长石颗粒大部分已发生绢云母化和高岭土化。次生白云母呈半自形结构,部分颗粒残留绿泥石化的黑云母假象。副矿物含量和粒径大小均较上述两类岩石降低,偶见石英与长石颗粒中夹磷灰石、锆石(不规则粒状)等。三类花岗岩在野外和勘探线剖面图上没有看到直接接触,但二云母花岗岩的侵位深度最大,白云母花岗岩深度最小,白云母花岗斑岩作为岩脉处于两者之间(图 3),三类花岗岩的斑晶大小依次降低(图 4图 5图 6),白云母花岗斑岩的锆石U-Pb年龄(151±2Ma; 李岩等, 2014)略大于白云母花岗岩(147±1Ma; 王先广等, 2015)。

图 6 白云母花岗斑岩代表性样品显微照片 (a)石英颗粒周围分布次生白云母;(b)斜长石颗粒中包裹自形石英颗粒 Fig. 6 Microphotographs of representative samples of muscovite granite porphyry (a) quartz is surrounded by secondary muscovite; (b) euhedral quartz packaged in plagioclase
3.2 分析方法及步骤

主量元素和微量元素均在国家地质实验测试中心完成。主量元素:用无水四硼酸锂和硝酸铵为氧化剂,于1200℃左右熔融制成玻璃片,使用X-射线荧光光谱仪(XRF)测定SiO2、Al2O3、Fe2O3T、Na2O、K2O、CaO、MgO、TiO2、MnO、P2O5等元素。然后将样品置于聚四氟坩埚中,加入氢氟酸和硫酸分解样品,重铬酸钾标准溶液滴定FeO含量。用离子选择性电极法测定F含量:将样品放入石墨坩锅中,待到Na2O2全熔,然后用高纯水提取。微量元素:称取试样于高压消解罐的Teflon内罐中,加入HF、HNO3装入钢套中,于190℃保温48h,取出冷却后,在电热板上蒸干至赶尽HF,加入HNO3再次封闭溶样3h,溶液转入洁净塑料瓶中,使用ICP-MS测定。

矿物主量元素测试在北京大学电子探针实验室JXA-8100型电子探针上进行。测试过程中,加速电压为15kV,电流为10nA,束斑直径为1μm,标样为美国SPI公司的53种矿物,基体效应是用PRZ方法修正的,测试精度优于1%~5%。

元素Sm、Nd的分离和纯化是在北京大学造山带与地壳演化教育部重点实验室通过传统的阳离子交换柱法实现。同位素质谱分析在天津地质矿产研究所完成,通过负热电离质谱法(N-TIMS)在TRITON上进行测定。143Nd/144Nd的原始测量值对146Nd/144Nd=0.7219进行校正。样品测试过程中,Jndi Nd标样给出的测试值143Nd/144Nd=0.512111±4(2σ)。以同样化学流程处理的BCR-2标样给出如下测试值:Sm=6.547×10-6,Nd=28.799×10-6147Sm/144Nd=0.1376,143Nd/144Nd=0.512624±3(2σ)。

4 分析结果 4.1 常量元素

主量元素结果见表 1。二云母花岗岩具有较高的SiO2(70.2%~74.2%)、Al2O3(14.0%~15.2%)、K2O (4.3%~5.6%)、F (0.09%~0.23%),较低的CaO (0.62%~1.69%)、FeOT(1.5%~2.5%)、Na2O (0.1%~3.7%)。TiO2在三类岩石中含量最高(0.20%~0.27%),这与矿物组成中含较多黑云母是一致的。白云母花岗岩具有变化较大的SiO2(71.2%~76.6%)、CaO (2.0%~6.6%)、F (0.13%~0.90%),较低的Al2O3(10.3%~14.6%)、K2O (0.6%~4.8%)、Na2O (0.4%~3.9%)、FeOT(0.1%~1.7%),LOI在三类岩石中最高(2.0%~5.0%)(图 7)。白云母花岗斑岩的SiO2=74.4%~76.8%,K2O=3.9%~5.0%,F=0.13%~0.32%,CaO=0.77%~2.36%,Na2O=0.10%~0.17%,FeOT=0.3%~3.4%,TiO2含量最低(0.05%~0.05%)(图 7)。三类花岗岩具有明显富F特征(0.09%~0.90%,平均0.29%),属于富F长英质体系(F>0.2%; Christiansen et al., 1986),但岩浆演化晚期F的可能溢出会导致很多富F花岗岩的全岩F含量低于1%(Price et al., 1999)。CIA指数(Al2O3/(Al2O3+CaO+Na2O+K2O),分子比)能够评价岩石的蚀变程度,相对于未发生蚀变的花岗岩样品(CIA=45~55; Nesbitt and Young, 1982),部分样品的CIA指数(41~70) 显示蚀变特征,但这是晚期岩浆与流体相互作用的结果,并非固相岩石蚀变导致。朱溪花岗岩部分样品的全碱含量较低,这有可能是因为富F流体在岩浆演化晚期与熔体强烈相互作用的结果,因为Na和K在富F流体中的溶解度很高,流体可能带走了部分Na和K (Badanina et al., 2004),白云母花岗岩的F含量在三类岩石中最高,熔流体作用最强,相应的K2O也较低。

表 1 朱溪矿区花岗岩的主量(wt%)和微量(×10-6)元素成分 Table 1 Major (wt%) and trace (×10-6) elements compositions of granite from the Zhuxi ore deposits

图 7 朱溪矿区花岗岩哈克图解 Fig. 7 Haker diagram for granite from the Zhuxi ore deposits
4.2 微量元素

二云母花岗岩在三类岩石中具有相对较高的稀土总量(60.3×10-6~90.3×10-6)和(La/Yb)N(9.2~13.6) (图 8a),Eu异常中等(δEu=0.52~0.65)。在原始地幔标准化微量元素蛛网图上(图 8b),二云母花岗岩富集大离子亲石元素,高场强元素Nb、Ti明显亏损,Sr呈现强烈的负异常,Rb、Ta、U表现为明显的正异常。

图 8 朱溪矿区花岗岩球粒陨石标准化稀土元素配分图(a)和原始地幔标准化微量元素蛛网图(b)(标准化值据Sun and McDonough, 1989) Fig. 8 Chondrite-normalized REE patterns (a) and primitive mantle-normalized trace element spidergram (b) of granite from the Zhuxi ore deposits (normalization values after Sun and McDonough, 1989)

白云母花岗岩的稀土总量较低(21.9×10-6~35.1×10-6), (La/Yb)N也相对减小(4.6~8.8),具有不明显的右倾式稀土元素配分模式(图 8a),Eu负异常加强δEu=0.24~0.88,其中5个样品δEu=0.72~0.88,其余9个样品δEu=0.24~0.51,平均值0.37)。在原始地幔标准化微量元素蛛网图上(图 8b),该花岗岩与二云母花岗岩类似,但Ba的亏损程度增大;Sr表现为正异常,可能与粒间方解石带入部分Sr有关。

白云母花岗斑岩在三类岩石中具有最低的稀土总量(12.5×10-6~16.2×10-6)和(La/Yb)N(1.6~3.3),表现为较平坦的稀土元素配分模式(图 8a),Eu负异常在三类花岗岩中最显著(δEu=0.16~0.25,平均值0.21)。在原始地幔标准化微量元素蛛网图上(图 8b),Sr、Ba、Nb、Ti具有明显的负异常,其中Ti的负异常在三类岩石中最明显,富集U、Ta。

4.3 矿物化学特征

黑云母电子探针数据见表 2,朱溪矿区二云母花岗岩的黑云母为富铁黑云母,Fe/(Fe+Mg)介于0.79~0.82之间,Mg#介于29~33。斜长石电子探针数据见表 3。斜长石以钠长石-奥长石为主,An值变化范围为4~22,平均为13,核部和边部的An值变化不具有明显的规律性。

表 2 二云母花岗岩代表性黑云母电子探针数据(wt%) Table 2 Microprobe analysis for biotite of two-mica granite (wt%)

表 3 二云母花岗岩代表性斜长石电子探针数据(wt%) Table 3 Microprobe analysis for plagioclase of two-mica granite (wt%)
4.4 Nd同位素

朱溪矿区花岗岩全岩Nd同位素成分见表 4图 9。二云母花岗岩147Sm/144Nd和143Nd/144Nd比值分别为0.1248~0.2399和0.512145~0.512224,两阶段亏损地幔模式年龄tDM2为1.55~1.70Ga,εNd(t)为-8.9~-7.6。白云母花岗岩147Sm/144Nd和143Nd/144Nd比值分别为0.1438~0.1682和0.512108~0.512179,两阶段亏损地幔模式年龄tDM2为1.60~1.72Ga,εNd(t)为-9.8~-8.2。白云母花岗斑岩147Sm/144Nd和143Nd/144Nd比值分别为0.1528~0.2184和0.512120~0.512232,两阶段亏损地幔模式年龄tDM2为1.55~1.93Ga,εNd(t)为-9.3~-7.8。

图 9 朱溪矿区花岗岩Nd同位素演化图解 九岭花岗岩和杨草尖黑云母花岗岩数据均为未发表数据; 双桥山群数据引自Chen and Jahn (1998), Huang and Jiang (2014), Mao et al. (2015), 李献华等(1994), 马长信(1991), 马长信和项新葵(1993, 1999), 王勇等(2002), 张海祥等(2000), 赵建新等(1995) Fig. 9 Nd isotopic evolution plot of granite from the Zhuxi ore deposits Data of Jiuling and Yangcaojian biotite granite are unpublished; Shuangqiaoshan Group data from Chen and Jahn (1998), Huang and Jiang (2014), Li et al. (1994), Ma et al. (1991), Ma and Xiang(1993, 1999), Mao et al. (2015), Wang et al. (2002), Zhang et al. (2000), Zhao et al. (1995)

表 4 朱溪矿区花岗岩Nd同位素组成 Table 4 Whole-rock Nd isotopic analyses of granite from the Zhuxi ore deposits
5 讨论 5.1 岩石成因 5.1.1 朱溪矿区花岗岩的类型

华南燕山期有许多二云母花岗岩小岩株出现,并常与钨锡成矿有关(毛景文等, 2008; Chen et al., 2014; Wang et al., 2014a),这些花岗岩由于高硅的特点(部分有REE四分组效应; Chen et al., 2014)被许多人认为是铝质A型花岗岩(King et al., 2001; Li et al., 2007)。朱溪矿区花岗岩的特征是含有丰富的白云母(非常富Al的矿物)而黑云母相对较少、石英含量较高、含有萤石,而缺乏富钙斜长石(本区斜长石An=4~22,平均13) 和榍石等,这些特征表明朱溪矿区花岗岩具有富硅铝、贫钙镁的特征,属于典型的S型花岗岩,而不是A型花岗岩(虽然有高硅特点)。这得到以下证据支持:(1) 在A型花岗岩判别图上,朱溪矿区花岗岩大部分落入非A型花岗岩区域,部分样品具有较高的Ga/Al比值而接近A型花岗岩(图 10),可能与体系富F有关,这与F-Al负相关性一致(图 11);(2) A型花岗岩通常侵位较晚,相对富集HFSE (如Zr、Nb、Y、REE和Ga)(Whalen et al., 1987),但朱溪矿区花岗岩的这些元素表现为相对亏损;(3) A型花岗岩通常具有较高的岩浆温度(830℃; Whalen et al., 1987),而朱溪矿区花岗岩的锆饱和温度平均为690℃(二云母花岗岩=730~779℃,白云母花岗岩=621~719℃,白云母花岗斑岩=673~694℃),具有相对低温的特征;(4) 朱溪矿区花岗岩的碱(平均4.93%)显著低于典型A型花岗岩(8.72%; Whalen et al., 1987),而且朱溪矿区花岗岩相对富集Al2O3(平均13.4%,部分样品的Al2O3由于高F偏低,而典型A型花岗岩是12.4%;Whalen et al., 1987)。因此,我们认为朱溪二云母-白云母花岗岩并不是A型花岗岩,而是高硅S型花岗岩(SiO2=70.2%~76.8%;成因讨论见后)。

图 10 朱溪矿区花岗岩A型花岗岩判别图解(据Whalen et al., 1987) FG-分异的M、I和S型花岗岩; OGT-未分异的M、I和S型花岗岩 Fig. 10 Chemical discrimination diagrams of A-type granite for granite from the Zhuxi ore deposits (after Whalen et al., 1987) FG-fractionated M, I and S-type felsic granites; OGT-unfractionated M, I and S-type granites

图 11 朱溪矿区花岗岩Al2O3-F图解 Fig. 11 Al2O3 vs. F plot of granite from the Zhuxi ore deposits
5.1.2 朱溪矿区花岗岩的源区特征和熔融条件

如前所述,朱溪矿区二云母花岗岩、白云母花岗岩和白云母花岗斑岩的εNd(t)分别为-8.9~-7.6,-9.8~-8.2和-9.3~-7.8,基本一致。在Nd同位素图解(图 9)中,朱溪矿区花岗岩的εNd(t)值与双桥山群陆源碎屑岩(εNd(t)=-12~-7.2; Chen and Jahn, 1998; 马长信, 1991; 马长信和项新葵, 1993, 1999)类似,而明显低于双桥山群变质火山岩(εNd(t)=-7.6~-2.2; Chen and Jahn, 1998; 王勇等, 2002; 张海祥等, 2000; 赵建新等, 1995)和燕山期煌斑岩(εNd(t)=-4.9;未发表数据)。朱溪矿区花岗岩Nd同位素成分与江南造山带新元古代九岭黑云母花岗岩(εNd(t)=-8.2~-6.2;未发表数据)和朱溪矿区南部杨草尖黑云母花岗岩(εNd(t)=-8.0~-6.9;未发表数据)有很大重叠。因此,朱溪矿区二云母-白云母花岗岩的可能的源区包括:双桥山群陆源碎屑岩、九岭(和杨草尖)黑云母花岗岩和下地壳抽取黑云母花岗岩之后的麻粒岩相残留体。九岭黑云母花岗岩已经被大量岩石学、地球化学数据证明是典型S型花岗岩,形成于双桥山群变质沉积岩的部分熔融(Li et al., 2003),证据包括:常见堇青石、偶见尖晶石,具有与双桥山群陆源碎屑岩一致的Nd同位素成分。根据泥质岩变质-深熔的相平衡关系(Douce and Harris, 1998; Stevens et al., 1997; Harley, 2008; 魏春景和朱文萍, 2016),九岭黑云母花岗岩的形成主要与麻粒岩相条件下黑云母脱水深熔反应有关:斜长石+黑云母+石英=斜方辉石/石榴石+钾长石+富Ti/Mg黑云母+熔体,该反应发生的温压条件是T=800~900℃,P=0.5~1.0GPa (压力大则形成石榴石),形成强过铝花岗岩熔体(熔体量30%~40%, Stevens et al., 1997; Harley, 2008; 魏春景和朱文萍, 2016),残留体是:斜方辉石+钾长石+富Ti/Mg黑云母±石英。源区残留富Ti/Mg黑云母是因为黑云母在脱水熔融反应过程中进行Fe-Mg固溶体成分调整(滑动反应),富Mg (和Ti, F; Stevens et al., 1997)黑云母具有非常高的稳定性而可能稳定到900~930℃以上。九岭黑云母花岗岩中堇青石(也许有尖晶石)可能形成于花岗质熔体侵位冷却过程中的变质反应或是低压下泥质岩的部分熔融(魏春景和朱文萍, 2016)。从以上反应可知,双桥山群变质沉积岩在深熔形成S型花岗岩(如新元古代九岭黑云母花岗岩和燕山期杨草尖黑云母花岗岩)过程中,源区斜长石可能消失殆尽,但部分钾长石(和石英,如果源岩有很高石英含量)可残留下来。我们认为朱溪矿区花岗岩可能形成于下地壳这种含富Mg/Ti黑云母的麻粒岩相残留体的在超高温麻粒岩相条件下再次部分熔融(还是通过黑云母脱水熔融, 只是在更高温度条件下; Stevens et al., 1997; 魏春景和朱文萍, 2016),主要证据如下:(1) 朱溪矿区花岗岩的侵位年龄为150Ma (刘善宝等, 2014),而矿区南部杨草尖黑云母花岗岩侵位年龄为170Ma (未发表数据),后者形成之后上侵,离开下地壳源区而无法成为朱溪矿区花岗岩的源区(虽然两者具有类似的Nd同位素成分),而新元古代九岭黑云母花岗岩由于形成更早并抬升至更浅地壳层位也不太可能是朱溪矿区花岗岩的源区;根据泥质岩变质-深熔反应相平衡模拟,双桥山群沉积岩部分熔融应该形成黑云母花岗岩(如九岭黑云母花岗岩和杨草尖黑云母花岗岩),而不是富含白云母的二云母-白云母花岗岩,所以双桥山群沉积岩也不太可能是朱溪矿区花岗岩的源区;(2) 上述麻粒岩相残留体再次部分熔融可形成过铝质花岗岩(通过黑云母脱水熔融; Harley, 2008),这与朱溪矿区花岗岩的小体积吻合(图 3),因为残留体再熔需要很高温度,而且残留富Mg/Ti黑云母含量(相对于沉积岩)较少,无法形成大规模花岗岩体,也与其中黑云母具有较高的TiO2一致(平均近3.0%, 表 3; 而九岭黑云母花岗岩中黑云母为2.5%; 孙克克等, 2017)。残留体再熔的高温可能来自同期幔源岩浆的底侵,如区内出现煌斑岩;(3) 由于残留体富Mg、Ti,这解释了朱溪花岗岩的贫Mg、Ti;残留体有较高的钾长石与朱溪矿区花岗岩低Ba一致,而源区缺乏斜长石解释了朱溪花岗岩的低Sr特征;有些样品具有较高的Sr和CaO与熔体跟围岩相互作用形成粒间分布的碳酸盐矿物有关(图 5c),如白云母花岗岩与围岩(碳酸盐岩地层)相互作用程度最高,可见Sr-Ca明显的正相关关系(图 12);(4) 朱溪花岗岩相对高硅(SiO2=70.15%~76.75%)与这种难熔残留体只能发生低程度部分熔融有关,也与源区可能有较高比例石英和该岩浆体系富F导致充分分离结晶(见下节讨论)有关。所以,朱溪矿区花岗岩的母岩浆可能是江南造山带双桥山群变质沉积岩在抽取黑云母花岗岩之后的残留体再在幔源岩浆底侵条件下,经历超高温麻粒岩相的部分熔融而形成。

图 12 朱溪矿区花岗岩Sr-CaO图解 Fig. 12 Sr vs. CaO plot of granite from the Zhuxi ore deposits
5.1.3 朱溪矿区花岗岩的分离结晶和演化

上述麻粒岩相残留体再熔形成的母岩浆可能是富F体系,这与朱溪花岗岩中见到萤石一致。富F体系岩浆由于粘度低,可能发生比较充分的分离结晶。朱溪矿区二云母花岗岩、白云母花岗岩和白云母花岗斑岩的εNd(t)差别不大,表明三类岩石可能代表同一个母岩浆的分离结晶的产物,结晶的先后次序为二云母花岗岩-白云母花岗岩-白云母花岗斑岩,主要证据是:(1) 在勘探线剖面图上二云母花岗岩的侵位深度最大(图 3);(2) 三类花岗岩的斑晶大小依次降低(图 4图 5图 6);(3) 三类花岗岩的Eu负异常依次增大;(4) 通过分离结晶模拟计算可以看出(图 13),通过以黑云母为主、独居石为辅的分离结晶过程能够很好地解释三类岩石的稀土元素含量关系,因为独居石在富F体系下更容易发生分离结晶作用(Keppler, 1993)。可见,朱溪矿区花岗岩主要发生黑云母为主的分离结晶,可能还有锆石、磷灰石等,因为花岗岩的Zr、Ti、P和REE含量很低(双桥山群陆源碎屑岩的Zr平均含量为250×10-6; 张彦杰等, 2012, 而朱溪矿区花岗岩为45×10-6),但长石分离结晶并不明显。因此,朱溪铝质高硅花岗岩的低Sr、Ca、Ba不是长石分离结晶所致,而是源区亏损斜长石(在抽取黑云母花岗岩熔体过程中被熔融消耗)但比较富集钾长石(富Ba)所致。

图 13 朱溪矿区花岗岩微量元素模拟图解 可能的分离结晶组合:15%左右的黑云母+0.1%左右的独居石等副矿物 Fig. 13 Trace element modeling for the petrogenesis of granite from the Zhuxi ore deposits Possible fractionated assemblage: 15% biotite + 0.1% monazite

岩浆体系的相对富F主要有两个来源:(1) 地幔岩浆底侵加热下地壳时导入F等挥发分,因为软流圈地幔岩浆通常富集F、Cl、CO2、B等挥发分,如黄石公园和贝加尔湖玄武岩中橄榄石和辉石的包裹体中F和Cl含量分别达到2%和0.9%(Black et al., 2012),其交代下地壳岩石时使之富集F (Beyer et al., 2012; Martin, 2006);(2) 由于残留体中的黑云母相对富F (富F黑云母才能稳定到900℃以上; Stevens et al., 1997),其分解熔融可释放F而进入熔体体系(Collins et al., 1982),这得到双桥山群陆源碎屑岩的沉积物中云母本来具有较高的F含量支持(地层F的平均含量高达1600×10-6; 刘英俊等, 1982),而岩浆上侵过程中与围岩相互作用也会获得额外的F。

5.2 F对朱溪矿区花岗岩地球化学特征的影响

朱溪矿区花岗岩具有明显的高F特征(0.09%~0.90%,平均0.29%),特别是白云母花岗岩(0.13%~0.90%,平均0.36%),这得到三种类型花岗岩中常见自形石英的支持(图 4图 5图 6),因为富F花岗岩体系使得石英稳定域显著扩大,导致石英先于长石结晶(Manning, 1981)。挥发分F的介入使得花岗岩体系物理和化学性质发生重要改变。当F进入熔体中,可以弱化Si-O和Al-O键,破坏四面体结构,使硅酸盐熔体产生解聚作用(Dingwell et al., 1985; Manning, 1981),导致熔体粘度和固相线温度显著下降(Agangi et al., 2012)。高F含量还显著提高流体在熔体和硅酸盐矿物中的溶解度(Kubis and Broska, 2005)。例如,在较高的温度(300~600℃)下,石英中HF的溶解度会显著的提高(>2×10-2mol/L),这有可能是因为形成SiF2(OH)2这样的络合物有关(Haselton et al., 1988)。在2.75kb以及水饱和的条件下“花岗岩-H2O-HF”体系实验表明,F的加入能降低花岗岩的初熔温度,从不含F时的665℃下降至F含量为5%时的603℃(Wyllie and Tuttle, 1961);犹他州斯波尔山的玻基斑岩中存在1.2%的F,这导致固相线温度降到525℃(Webster et al., 1987)。所以,在花岗质岩浆中F可以降低岩浆的粘度、密度、固液相线温度、改变熔体结构(Agangi et al., 2012)。虽然利用锆饱和温度恢复岩浆温度存在诸多问题,但对于岩浆温度有一定的指示意义(Miller et al., 2003; Watson, 1996; Watson and Harrison, 1983)。朱溪矿区花岗岩具有明显的“低温”岩浆特征,并且随着F含量的增加,锆石饱和温度具有轻微的降低趋势,这种趋势在白云母花岗岩中表现尤为明显(图 14)。

图 14 朱溪矿区花岗岩锆饱和温度-F图解 Fig. 14 Zircon saturation temperature vs. F plot of granite from the Zhuxi ore deposits

朱溪矿区花岗岩具有较低的Al2O3含量,并且与F含量具有明显的负相关性(图 11)。与Cl等其他卤素相比,F能与Al形成更加稳定的络合物(Tagirov et al., 2002)。在含F系统中Al以Al (OH)3F-和Al (OH)2F2-作为主要形式存在于流体中,这将使Al在富F流体中的含量提高将近100倍,这也就大大加强了Al的活动性(Mysen et al., 2004; Sharova et al., 2012)。实验岩石学研究表明,F能够显著的提高流体中Al的溶解度,在400℃和500Pa的条件下,当流体中F的含量达到10-4mol/L时,来自铝硅酸盐溶解的Al的浓度比在纯水中高出1.5倍;当F的含量达到10-3mol/L,Al的浓度高出6倍,主要的溶解形式是Al (OH)2F2-(Chang and Meinert, 2008)。

高分异花岗岩常出现某些微量元素比值(如,Zr/Hf、La/Ta、Y/Ho、La/Nb、K/Rb)的异常,被称为non-CHARAC性质(即偏离价态和离子半径控制的地球化学性质; Irber, 1999)。朱溪矿区大部分花岗岩表现出比正常花岗岩低的K/Rb、La/Ta、La/Nb和Zr/Hf比值,但部分样品的K/Ba、Y/Ho和La/Nb比值与正常花岗岩类似(图 15a)。Zr与Hf (Y与Ho)两个元素具有相同的电价和相似的离子半径,它们具有相似的地球化学行为,一般地质作用过程中Zr/Hf (Y/Ho)不产生明显分异,其比值类似于C1球粒陨石比值(Y/Ho=28、Zr/Hf=38) (Anders and Grevesse, 1989)。在大部分岩浆岩中Zr/Hf比值在36~40范围内,包括球粒陨石和地壳(Jochum et al., 1986)。朱溪矿区花岗岩Zr/Hf平均值为25(双桥山群陆源碎屑岩Zr/Hf=37) (图 15b),其中二云母花岗岩为28~33,白云母花岗岩为20~28,白云母花岗斑岩为15~17,明显低于岩浆岩的平均值38,而锆石结晶并不能改变残留熔体的Zr/Hf比值(Dostal and Chatterjee, 2000),这可能与富F花岗岩体系特殊的性质有关。朱溪矿区花岗岩的Y/Ho平均值为31(双桥山群Y/Ho=29,二云母花岗岩Y/Ho=28~31,白云母花岗岩Y/Ho=29~35,白云母花岗斑岩Y/Ho=28~33) (图 15c),Y/Ho>28与F的络合物有很大关系(Bau and Dulski, 1995)。实验岩石学研究表明,从氟化物-硅酸盐不混溶的熔体中可以结晶出冰晶石(Na3AlF6),冰晶石的微量数据表现出REE总量较低并有强烈的四分组效应和Y/Ho异常,这表明氟铝络合物与Y/Ho异常有很大关系(Veksler et al., 2005)。同样,La与Ta (La与Nb)具有相似的地球化学行为。在正常岩浆岩中,La/Ta与La/Nb比值分别为16~20与0.96~1.20(Jahn et al., 2001)。朱溪矿区花岗岩La/Ta比值均小于8(双桥山群陆源碎屑岩La/Ta=34),其中二云母花岗岩为4.7~7.3,白云母花岗岩为0.6~3.0,白云母花岗斑岩为0.2~0.5(图 15b)。朱溪矿区花岗岩La/Nb平均值为0.68(双桥山群陆源碎屑岩La/Nb=2.3) (图 15c),其中二云母花岗岩为0.88~1.31,白云母花岗岩为0.29~0.97,白云母花岗斑岩为0.15~0.30。高分异花岗岩中这些异常的微量元素比值的原因并不清楚,可能与富F导致岩浆强烈分离结晶、熔-流体相互作用有关(Irber, 1999; Jahn et al., 2001),也可能是F与这些元素选择性络合所致(Cerny et al., 1986; Chen et al., 2014)。总之,富F可能是导致La、Ta、Nb等强烈分异的主要因素(Cerny et al., 1986),实验岩石学也表明F浓度对Ta在分配系数有较大的影响(王玉荣等, 1992),说明富F体系可以造成La/Ta、La/Nb比值的异常(Dupuy et al., 1992)。

图 15 朱溪矿区花岗岩K/Rb-K/Ba (a)、Zr/Hf-La/Ta (b)和Y/Ho-La/Nb (c)图解 Fig. 15 K/Rb vs. K/Ba (a), Zr/Hf vs. La/Ta (b) and Y/Ho vs. La/Nb (c) of granite from the Zhuxi ore deposits

图 16是反映朱溪花岗岩分异程度(SiO2)和F含量与REE四分组效应(TE1, 3) 和Eu异常的关系。从图中可见,朱溪矿区花岗岩虽然SiO2和F含量有较大变化,但只显示轻微REE四分组效应(TE1, 3=0.97~1.15; 表 1),而且与SiO2和F含量没有明显的相关性。而与朱溪花岗岩不远的珍珠山花岗岩具有明显的REE四分组效应,虽然后者的SiO2和F含量并不比前者高(图 16)。类似地,朱溪花岗岩的Eu负异常也远不如珍珠山花岗岩明显,但朱溪花岗岩的Eu负异常大致地随着SiO2和F含量的增加而更加明显(图 16)。高分异花岗岩REE四分组和强烈负Eu异常的成因一直是个争论的问题,有的学者认为REE四分组效应与岩浆体系强烈分离结晶有关(Pan and Breaks, 1997; Tin and Keppler, 2009),另外一些学者认为与熔-流体相互作用有关(Bau, 1996, 1997),后一种模式得到越来越多的实验岩石学(Irber, 1999)和质量平衡计算(Liu and Zhang, 2005)的支持,并得到Li同位素数据的支持(Teng et al., 2006; Romer et al., 2014)。虽然封闭体系长石的强烈分离结晶会导致形成显著Eu负异常,但模拟计算表明巨大的Eu负异常可能不是仅仅通过分离结晶而形成,熔-流体相互作用也对Eu负异常有重要贡献(Jahn et al., 2001)。朱溪矿区花岗岩虽然具有高硅和富F的特点,但并没有显著的REE四分组效应和巨大的负Eu异常,可能与这些花岗岩固结之前没有经历强烈的熔-流体相互作用有关。据Liu and Zhang (2005)质量平衡计算结果,这种流体通常不是岩浆水热流体,而是外部(围岩)流体。所以,朱溪矿区花岗岩的围岩缺乏粒(层)间流体可能是熔-流体作用微弱的原因。朱溪矿区花岗岩作为含F花岗岩体系,花岗质熔体具有很长的岩浆演化过程(固相线温度降低),与富钙围岩相互作用增强,使得岩体获得额外的Ca、Sr (和Eu),这会导致花岗岩的球粒陨石标准化REE图中Eu负异常减弱。另外,副矿物独居石、磷灰石、锆石等的分离结晶使得花岗岩的稀土含量降得很低,Sm和Gd含量的下降也导致了Eu的负异常程度相对减弱。

图 16 朱溪矿区花岗岩四分组效应指数(TE1, 3)、Eu负异常(δEu)与SiO2、F协变图解 Fig. 16 TE1, 3 vs. SiO2, δEu vs. SiO2 and TE1, 3 vs. F, δEu vs. F plots of granite from the Zhuxi ore deposits
5.3 朱溪矿区花岗岩与钨矿的关系

在朱溪矿区勘探线剖面图上(图 3),钨(铜)矿体与二(白)云母花岗岩具有密切时空关系,钨矿体赋存状态分为两类:第一种矿体赋存于晚古生代碳酸盐岩地层与新元古代变质基底的不整合面之上,与二云母花岗岩和白云母花岗岩呈接触关系,矿体中有白云母花岗斑岩岩脉;第二种矿体与白云母花岗岩共生。在岩相学上,白钨矿分布在弱绢云母化的长石中,这些特点说明岩体对于成矿具有重要意义。朱溪钨矿的源区主要是赣北双桥山群碎屑沉积岩,因为有的双桥山群样品含WO3高达0.18%(刘英俊等, 1982)。双桥山群中W的平均含量为7.4×10-6,石炭系为7.2×10-6,二叠系为4.5×10-6,三叠系为3.6×10-6。在双桥山群中上部夹有大量的凝灰岩、沉积凝灰岩、凝灰质砂砾岩和凝灰质板岩层,W的平均含量为10.5×10-6(李兆麟等, 1986)。从钻孔的剖面图(图 3)上可以看出,朱溪花岗岩的围岩主要为双桥山群地层,其次为石炭系、二叠系、三叠系的碳酸盐岩地层,由于熔体富F等特点使其具有较低的粘度和结晶温度,岩浆从下地壳源区沿着深大断裂进行长距离运移到地壳浅部,运移的过程中不断与围岩相互作用,并萃取围岩中的W而成矿,或导致围岩粒间流体增温活化而萃取成矿物质成矿。

F对成矿的作用表现在:(1) 在热液中与成矿元素形成稳定络配合物形式迁移(Aksyuk, 2000);(2) 充当氧化剂,改变W的价态(从+4到+6) 进而促进白钨矿的沉淀(Aksyuk, 2000)。南岭地区的钨锡矿体大多与富F花岗岩有关(Chen et al., 2014; Wang et al., 2014)。在Hendrson辉钼矿的云英岩中发现高含量F,随着F含量的增加,具有高硅特征的岩浆作用时间延长,岩体中的钾化、交代作用加强(Sotnikov et al., 2006),这一点与朱溪矿区花岗岩特征类似。朱溪矿区花岗岩中W-F明显的正相关性进一步说明了F的重要性(图 17)。

图 17 朱溪矿区花岗岩W-F图解 Fig. 17 W vs. F plot of granite from the Zhuxi ore deposits

岩浆作用晚期流体开始出溶,形成高F的岩浆热液。通过形成Si-F-OH和Al-F-OH络合物,流体开始在裂缝、矿物晶格间分解早期结晶的岩浆成因长石、石英,发生矿物的自交代现象,生成绢云母等蚀变矿物。由此产生的自由活动的Si和Al可以为矽卡岩的形成提供物质基础。当与矿体紧密接触的晚古生代碳酸盐岩发生部分溶解时,Ca通过淋滤作用进入热液流体中,导致热液流体中Ca的浓度增加,使得一些典型的矽卡岩矿物(硅灰石、富钙斜长石、石榴石、透辉石)和白钨矿的溶解度低于石英、钾长石、钠长石开始结晶沉淀而成矿(图 3)。

6 结论

(1) 朱溪矿区花岗岩具有富硅铝、贫钙镁的特征,属于典型的S型花岗岩,而不是A型花岗岩,在超高温麻粒岩相条件下形成于下地壳含富Mg/Ti黑云母的麻粒岩相残留体的再次部分熔融,并非双桥山群变质沉积岩的直接部分熔融以及九岭(和杨草尖)黑云母花岗岩的重融形成。

(2) 朱溪矿区花岗岩可能代表同一个母岩浆的分离结晶的产物,结晶的先后次序为二云母花岗岩-白云母花岗岩-白云母花岗斑岩,主要发生黑云母为主的分离结晶,可能还有锆石、磷灰石等,但长石分离结晶并不明显,低Sr、Ca、Ba不是长石分离结晶所致,而是源区亏损斜长石但比较富集钾长石所致。

(3) 朱溪矿区花岗岩具有明显的富F特征,F的加入降低了花岗岩岩浆的固相线温度和粘度,延长了岩浆演化过程,增强了Al的活动性,出现了偏离正常岩浆体系的元素比值(低K/Rb、Zr/Hf、La/Ta、La/Nb比值)。

(4) 朱溪矿区花岗岩较长的岩浆演化时间和较低的结晶温度使其可以长时间萃取双桥山群等围岩中的W元素。F与W形成稳定络离子运移,与围岩贡献的Ca在适宜的温压条件沉淀形成白钨矿。因此,朱溪矿区高分异花岗岩促进了朱溪超大型钨(铜)矿的形成。

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