张照伟,李文渊,高永宝,谢燮,王亚磊,张江伟,郭周平,李侃
(1.西安地质矿产研究所,陕西西安710054;2.长安大学地球科学与国土资源学院,陕西西安710054)
青海省拉水峡基性杂岩体地球化学特征及其对矿床成因的约束
张照伟1,2,李文渊1,高永宝1,2,谢燮1,王亚磊1,张江伟1,郭周平1,李侃1
(1.西安地质矿产研究所,陕西西安710054;2.长安大学地球科学与国土资源学院,陕西西安710054)
对与青海省拉水峡铜镍硫化物矿床密切相关的基性杂岩体的地球化学研究表明,该岩体以低Ti、亏损Nb和Ta、富集轻稀土元素和大离子亲石元素为特征。结合其同位素组成特征,提出该岩体的形成与祁连山及其邻区460~440 Ma时期俯冲向碰撞转换的作用有关,岩浆起源于富集的岩石圈地幔,并在高位岩浆房中遭受了地壳的混染作用,从而导致岩浆中Si的增加,并引起富硫化物矿浆与富硅酸盐岩浆的不混熔作用。
基性杂岩体地球化学矿床成因铜镍矿拉水峡青海
Zhang Zhao-wei,Li Wen-yuan,Gao Yong-bao,Xie Xie,Wang Ya-lei,Zhang Jiang-wei,Guo Zhou-ping,Li Kan.Geochemical characteristics of the Lashuixia basic complex in Qinghai Province and its constraints on genesis of the deposit[J].Geology and Exploration,2012,48(5):0959-0968.
青海省化隆县拉水峡铜镍矿床是20世纪70年代发现的一个赋存于基性杂岩体中的中小型富铜镍矿床,对青海省化隆县的经济发展起到了重要作用。就青海化隆地区,在西起青海湖东南的裕龙沟而东经贵德县的阿什贡,东到化隆县的塔加,在长160 km,宽20 km地带,已产出有32个大小不同的基性-超基性杂岩体,这些岩体的含矿性目前尚不清楚。要解决这个问题,关键要查清这些岩体的成因及其与拉水峡岩体的关系,那么首先要解决拉水峡岩体的成因。前人对该矿床已进行了一些简单研究(矿石的结构构造、矿石地球化学以及稳定同位素等),尽管对其具体的形成过程还存在不同的看法(汤中立等,1987;李文渊,1996;刘应汉,2003;李文渊,2006;赵恒川等,2007;樊光明等,2007;申勇胜等,2009;许长坤,2011),但归结起来主要认为铜镍硫化物的形成与岩浆的熔离作用以及结晶分异作用有关,很少从岩体的岩石地球化学角度探讨该岩体的成因进而研究其矿床成因。另外,高质量的地球化学数据也鲜见报道。本文试图通过对拉水峡岩体地球化学特征的研究来探讨岩浆形成和演化过程,并为矿床成因研究提供线索。
青海省有八条基性-超基性岩带,且多数岩体与成矿关系密切。具体到化隆基性-超基性岩带,不仅是青海省八条基性-超基性岩带之一,更重要的是多数基性-超基性岩体与铜镍成矿关系密切。拉水峡基性杂岩体及其铜镍矿床就位于化隆基性-超基性岩带内(图1)。该区在大地构造位置上属祁连褶皱系与松潘甘孜褶皱系之间的化隆晚元古代隆起带,其南邻秦岭褶皱带西延部分,北部为拉脊山加里东褶皱带。区内岩浆活动频繁,侵入岩广泛分布,构造运动强烈,矿化现象不甚普遍。构造岩带主要地层是元古界化隆群,其次是少量的中、新生界。化隆群分布于整个岩带,北西端起自青海湖北的刚察县,经日月山、拉脊山以南至贵德县的阿什贡,到尖扎、循化黄河沿岸以北地带,它是化隆岩带基性-超基性岩侵入的主要围岩。岩性为中深变质的片岩、片麻岩夹大理岩及石英岩,混合岩化显著。
图1 青海省化隆一带区域地质图Fig.1Regional geological map of Hualong county in Qinghai Province
拉水峡岩体属于基性杂岩体,主要为角闪石化辉石岩,岩石化学恢复原岩为二辉橄榄岩。其m/f为0.5~1.9,属于铁质系列,对铜镍成矿有利。拉水峡岩体岩石在SiO2-Na2O+K2O图上,样品均投影在亚碱性系列区。在AFM图解上,均投影于钙碱性系列区。拉水峡岩体大部分被铜、镍硫化物矿化,含矿率近90%,岩体即矿体,侵入于化隆群关藏沟岩组下岩段,平面上呈透镜状、板柱状,顺层分布,处于拉水峡背斜外倾转折端。岩体呈南东东-北西西展布,地表出露53 m,平均宽10 m;深部岩体向南东侧伏,控制延伸207 m,长61~68 m,厚5.7~8.4 m,岩体上缓下陡,深部变薄,呈板柱状,近地表倾向5°,倾角40°~60°,深部倾向40°~50°,倾角70°~80°(图2)。
拉水峡含矿岩体已遭受强烈蚀变,以角闪岩为主,矿物成分以普通角闪石、辉石为主,含少量黑云母、斜长石、石英等。其中橄榄石极少见,呈圆粒状晶体,为不规则状晶体,多数晶体被蛇纹石全部交代;辉石多为粒状或不规则粒状,从辉石解理可见,有单斜辉石和斜方辉石两种,斜方辉石为顽火辉石,单斜辉石为普通辉石,辉石常被角闪石交代;斜长石晶体呈他形粒状,常见斜长石双晶,普遍具有次生蚀变,主要为绢云母化;角闪石晶体多为纤维柱状,常呈集合体出现,有微羽多色性,闪石式解理,为普通角闪石;黑云母呈片状晶体,常被绿泥石交代。
从拉水峡矿床不同类型矿石100%硫化物中Cu、Ni含量数据(表1)中可以看出,浸染状矿石分布在低Ni边缘附近区域,反映了典型的岩浆熔离成矿特征;而块状矿石则偏离其上分布范围,Cu含量较高,可能与热液作用使金属硫化物矿物组合发生改变,铜矿物的明显增多有关。
图2 拉水峡岩体地质平面(左)及剖面图(右)Fig.2Geological map(left)and cross section(right)of the Lashuixia intrusive rock bodies
表1 拉水峡矿床不同类型矿石中主量元素及铂族元素含量Table1 Content of PGE and major elements in different types of ores of the Lashuixia deposit
选择拉水峡矿区8个不同类型岩矿石样品进行稀土元素分析,以期对其岩体特征进行研究,结果见表1。其不同类型矿石中稀土元素总量差异不大,块状矿石的ΣREE为40.39×10-6~136.40×10-6,平均81.75×10-6;浸染状矿石的ΣREE为33.69×10-6~137.72×10-6,平均70.25×10-6,略小于块状矿石。所有样品的稀土元素均属右倾轻稀土富集型(图3),块状矿石的LREE/HREE平均为10.34,略高于浸染状矿石的9.62。各类矿石均具有明显负δEu异常,块状矿石的δEu为0.41~0.77,平均0.60,浸染状矿石的δEu为0.47~0.71,平均0.64,说明岩浆演化过程中发生了大量斜长石等的分离结晶作用;块状矿石的δCe为0.95~0.99,平均为0.98,浸染状矿石的δCe为0.96~0.99,平均为0.98。可见拉水峡矿区各类矿石的稀土元素特征相似,说明它们来自同一源区,是同一原始岩浆分异演化的产物。微量元素特征基本一致,总体上显示出相容元素(如Ni,Cr,Co,V)含量高,不相容元素(如Rb,Ba,Th,U等)含量低的特点。富集大离子亲石元素(Sr、Rb、Ba)和相对亏损高场强元素(Nb、Ta、Hf、Zr)。
图3 拉水峡矿床矿石REE原始地幔标准化分布型式图(标准地幔数据据Sun and McDonough,1989)Fig.3REE primitive mantle normalized distribution pattern of ores from the Lashuixia deposit (Base diagram from Sun and McDonough,1989)
表2 拉水峡矿床不同类型矿石稀土元素含量(×10-6)Table2 REE element content in different types of ores in the Lashuixia deposit
表3 拉水峡矿床不同类型矿石中δ34S值Table3 Values of δ34S in different type of ores of the Lashuixia deposit
从拉水峡矿区不同类型矿石样品的主量元素分析可知,拉水峡矿床矿石中主要含Ni、Cu、Co、S等元素,其Ni平均含量10.18%,Cu平均含量4.26%,Co平均含量0.17%。块状矿石Ni平均含量为9.55%,略低于浸染状矿石(平均10.80%);但块状矿石中Cu含量为3.90%~9.46%,平均6.53%,明显高于浸染状矿石(平均0.47),可能与热液作用对金属硫化物矿物组合的改变,铜矿物的明显增多有关;Co含量基本相当。就稳定同位素特征而言,在分析研究的基础上,获得了拉水峡矿床的S同位素数据(表3)。从硫同位素组成直方图来看(图4),拉水峡矿床金属硫化物的硫同位素变化范围很小,说明成矿物质均一化程度很高;δ34S分布范围在0.84‰~4.5‰之间,平均为2.24‰,与正常幔源硫的范围(0 ±3‰)基本一致,反映其硫化物中的硫以地幔为主,有少量地壳硫的加入。
图4 拉水峡矿床硫同位素组成直方图Fig.4Histogram of S isotopic composition in the Lashuixia deposit
从拉水峡Re-Os同位素数据中,可见其岩体遭受了不同程度的地壳混染,其矿石γOs值为68.3~84.6,平均74.6,由此可见,拉水峡矿床中的Os以幔源为主(芬兰Keivinsta矿床的γOs(t)值(+130~+ 170,壳源Os占>28%);加拿大Sudbury(+430~+ 814,壳源Os占>80%);澳大利亚Kimberley岩体(+950~+1300,壳源Os>70%))(McDonough et al.,1995;Ebel et al.,1996;Barnes et al.,1997;钱壮志等,2009)。同样,在Nb/Ce-Nb/U、Nb/Th-Ce/ Pb和Th/Yb-Ta/Yb协变图上均显示了明显的正相关,表明在岩浆演化过程中发生了一定程度的同化混染作用。
拉水峡矿床Pd/Ir比值0.91~8.77,平均3.96,而Ir的含量为214.14×10-9~332.82×10-9,平均264.85×10-9,显示具有较高的Ir含量和低的Pd/Ir比值,具有岩浆硫化物矿床特征(Keays,1995)。块状矿石Pd/Ir比值为0.91~8.77,平均4.85;浸染状矿石Pd/Ir比值为1.67~3.19,平均2.48,均大于原始地幔Pd/Ir比值1.22,说明均经历了一定程度的演化,但块状矿石受热液作用影响,Pd/Ir比值略高于浸染状矿石。
从不同类型矿石100%硫化物铂族元素原始地幔标准化分布模式图(图5)可以明显看出,块状矿石Pt、Pd含量最高,但Os、Ir、Ru、Rh含量却显的最低,浸染状矿石则Pt、Pd含量较低,Os、Ir、Ru、Rh含量又较高。拉水峡矿床不同类型矿石Pt/Pd比值均小于原始地幔,但块状矿石(Pt+Pd)/(Os+Ir+Ru)比值明显高于原生浸染状矿石,这显然代表了其完全不同的成因特点,同时也表明,Pt、Pd的局部富集。
图5 拉水峡矿石100%硫化物铂族元素原始地幔标准化分布模式图Fig.5PGE primitive mantle normalized distribution pattern in 100%sulfide of the Lashuixia deposit
一般认为,层状岩体代表了岩浆房中分离结晶作用的产物。就拉水峡岩体产状而言,尽管是顺层产出,但由于岩体规模较小,且呈板柱状,无法理解为层状岩体。SiO2与MgO、FeOT、CaO、Al2O3等相关性表明岩浆演化过程中发生了斜方辉石和斜长石的分离结晶作用。推测进入岩浆房中母岩浆不是地幔直接熔融形成的,而可能是经历了深部岩浆房分离结晶作用的产物。
由图3及特征可知,岩浆是富集轻稀土元素和LILE的。由于La、Ba、Th、Zr和Nb具有相近的分配系数,分离结晶作用不会影响岩浆中La/Nb、La/ Ba、Ba/Nb比值,这些元素对比值的显著不同指示了源区特征。Nb/Yb-Th/Yb、Th/Yb-Ba/La以及La/Nb-La/Ba之间的关系表明岩浆源区存在沉积物交代改造的富集岩石圈地幔。拉水峡基性杂岩体的TiO2含量较低,一般在1%以下,具有岛弧岩浆的特点(DePaolo et al.,1981;Barnes et al.,1985;Mckenzie et al.,1988;汤中立等,1995;张招崇等,2003)。区域地质研究表明,祁连山及邻区在460~440Ma时恰是由俯冲向碰撞的转换期,所以拉水峡岩体形成时不可能为单纯的岛弧环境,其地球化学特征之所以具有岛弧岩浆的某些特征可能是源区保留有早期俯冲的洋壳。拉水峡岩体中含有大量的角闪石和黑云母等含水矿物,由此推测母岩浆是富含水的。而软流圈地幔则通常是干的体系(Rad’ko et al.,1991;Naldrett et al.,1993),并且在一般条件下也难以熔融。可能正是由于早期俯冲的洋壳物质在热的作用下释放出的水降低了液相线温度而使软流圈地幔发生部分熔融,因此其部分熔融形成的岩浆保留了岛弧岩浆的某些特点:富水及亏损Nb和Ta、Ti等。
从目前的资料分析,岩浆从源区上升到就位至少经历了两次岩浆房的分离结晶过程。如前所述,现在所见的拉水峡岩体代表了高位岩浆房,而进入高位岩浆房中的母岩浆不是上地幔直接熔融形成的,而是在深部发生过一定程度的分离结晶的产物。根据母岩浆镁含量较低的特点,推测其深部岩浆房的分离结晶相主要为斜长石。前已述及,有些岩石中具有不均一的矿物分布,地球化学特征具有高的La/Sm和Th/Ta值,这些均指示了地壳的混染作用。所以现在所见的拉水峡岩体应是母岩浆在高位岩浆房中分离结晶和同化混染联合作用的产物,即经历了AFC过程(Lightfood et al.,1993;Brugmann et al.,2001;吕林素等,2007;王团华等,2009)。
传统观点认为,岩浆铜镍硫化物矿床是岩浆在岩浆管道(column)或深部岩浆房中由于物理化学条件(包括温度、压力、氧逸度和硫逸度)变化最终导致富硫化物的液相和岩浆失去平衡而产生不混熔作用,并以重力下沉的方式富集成矿(Irvine,1975; Wendlandt,1982;Keays,1995;Lightfood et al.,1997;李文渊,2004;徐德明等,2006)。众所周知,引起液态不混熔作用的原因是硫在岩浆中的溶解度,如果处于过饱和状态,则硫化物就会下沉。然而研究表明,如果在岩浆演化的早期硫就处于过饱和状态而发生下沉,则会由于Cu、Ni含量太低,而不可能形成有经济价值的矿床(陈隽璐等,2006;何世平等,2008)。一般认为,硫在岩浆中的溶解度是温度、压力、氧逸度和硫逸度的函数(汤中立等,2006),这些参数的改变可能会引起硫达到饱和状态。后来一些学者发现赋矿基性岩体普遍存在地壳的混染作用,所以提出地壳硫的加入是引起岩浆硫达到过饱和的重要原因(Wang et al.,2006a;Li et al.,2008)。拉水峡岩体也发生过地壳的混染作用,但是大量的硫同位素研究表明,δ34S变化很小,基本上接近于0,并呈塔式分布,显示出岩浆硫的特点,由此看来,地壳硫的加入不是引起拉水峡矿床产生硫过饱和而引起液态分异的原因。
Irvine的实验研究显示(Irvine et al.,1975),硫在镁质岩浆中的溶解度还取决于Si的含量,当富Si的地壳物质加入到镁质岩浆中时,就会改变成分点的位置,从而引起硫的溶解度的变化。当富SiO2的地壳物质加入到均一的富硫化物的液相时,则成分点由A变为B,这就意味着,岩浆分裂为两部分,一为富硅酸盐的相,另一为富硫化物的相。有关实验表明,基性岩浆通常是硫不饱和的(Wang et al.,2006b;惠卫东等,2011),并且原始岩浆中Fe含量越低,硫的溶解度越高(宋谢炎等,2009;聂江涛等,2012)。因为拉水峡岩体Fe的含量较低,所以推测原始岩浆中可能溶解了大量的硫。由于拉水峡岩体的围岩(片麻岩、混合岩)SiO2含量较高,所以拉水峡矿床可能由于本身具有高含量的硫,并且由于地壳物质的混染而提高了岩浆中Si的含量,从而引发不混熔作用,形成富硫化物的矿浆。后在构造应力的作用下,岩浆(矿浆)侵位到现存部位,成为现在所看到的拉水峡铜镍硫化物矿床。
(1)拉水峡岩体以低Ti、亏损Nb、Ta和强烈富集轻稀土元素和大离子亲石元素为特征,大多数岩石属于钙碱性玄武岩系列,只有少部分岩石属于拉斑玄武岩系列。
(2)岩浆可能经历了两次岩浆房的演化过程,拉水峡岩体的平均成分可能代表了进入高位岩浆房的原始岩浆,岩浆在上升到高位岩浆房之前,在深部曾经历了斜长石的分离结晶作用。
(3)造成硫化物矿浆不混熔作用的原因主要为地壳物质混染作用导致Si的加入,在混染作用前,岩浆本身可能溶解了大量的硫,但没有明显的外来硫的加入。
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Geochemical Characteristics of the Lashuixia Basic Complex in Qinhai Province and Its Constraints on Genesis of the Deposit
ZHANG Zhao-wei1,2,LI Wen-yuan1,GAO Yong-bao1,2,XIE Xie1,WANG Ya-lei1,
ZHANG Jiang-wei1,GUO Zhou-ping1,LI Kan1(1.Xi’an Institute of Geology and Mineral Resources,Xi’an,Shaanxi 710054; 2.College of Earth Science and Resources,Chang’an University,Xi’an,Shaanxi 710054)
The Lashuixia Ni-Cu sulfide deposit is an important Cu-Ni-rich deposit in Qinhai Province.Geochemical studies of the basic complex closely associated with the deposit show that it is characterized by low Ti,depletion of Nb or Ta,and enrichment of LREE and large lithosphile elements. Combined with their isotope geochemistry,it is concluded that the formation of the basic complex is related to the conversion from subduction to collision between the Qilian Mountains and their adjacent areas at 460~440 Ma.The magma was derived from the enriched lithospheric mantle,contaminated by crustal materials within the high-level magma chamber.This process led to the increasing amount of Si,and triggered the immiscibility between the sulfidesrich liquid and the silicate-rich liquid.
basic complex,geochemistry,genesis of deposit,Ni-Cu deposit,Lashuixia,Qinghai Province
book=9,ebook=493
P318.42
A
0495-5331(2012)05-0959-10
2012-03-05;
2012-07-08;[责任编辑]郝情情。
国家自然科学基金项目(编号40772062)和中国地调局地质大调查项目(编码1212010911032和1212010918024)联合资助。
张照伟(1976年-),男,2004年毕业于昆明理工大学,获硕士学位,助理研究员,在读博士,长期从事岩浆作用矿床及区域成矿研究工作。E-mail:zhaoweiz@126.com。