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黄金科学技术, 2019, 27(5): 659-677 doi: 10.11872/j.issn.1005-2518.2019.05.659

矿产勘查与资源评价

滇西北烂泥塘斑岩铜金矿床铁氧化物LA-ICP-MS微量元素特征及其地质意义

郭剑衡,1,2, 冷成彪,1,3, 张兴春1, 张伟1, 尹崇军4, 张陆佳4, 田振东1,2

1. 中国科学院地球化学研究所矿床地球化学国家重点实验室,贵州 贵阳 550081

2. 中国科学院大学,北京 100039

3. 东华理工大学核资源与环境国家重点实验室,江西 南昌 330013

4. 云南华西矿产资源有限公司,云南 昆明 650200

Trace Elemental Compositions of Iron Oxides from the Lannitang Porphyry Cu-Au Deposit in the Zhongdian Region (Northwest) and the Geological Significances:A LA-ICP-MS Study

GUO Jianheng,1,2, LENG Chengbiao,1,3, ZHANG Xingchun1, ZHANG Wei1, YIN Chongjun4, ZHANG Lujia4, TIAN Zhendong1,2

1. State Key Laboratory of Ore Deposit Geochemistry,Institute of Geochemistry,Chinese Academy of Sciences,Guiyang 550081,Guizhou,China

2. Chinese Academy of Science University,Beijing 100039,China

3. State Key Laboratory of Nuclear Resources and Environment,East China University of Technology,Nanchang 330013,Jiangxi, China

4. Yunnan Huaxi Mineral Resources Co. ,Ltd. ,Kunming 650200,Yunnan,China

通讯作者: 冷成彪(1982-),男,山东临沂人,教授,从事矿床地质与地球化学研究工作。lcb8207@163.com

收稿日期: 2019-06-28   修回日期: 2019-08-03   网络出版日期: 2019-10-29

基金资助: 国家重点研发计划项目“青藏高原大陆碰撞斑岩铜—钼—金矿系统结构与形成机制”.  2016YFC0600305
国家自然科学基金项目“滇西北中甸岛弧印支期斑岩铜矿床的保存与剥蚀程度研究:低温年代学制约”.  41373051

Received: 2019-06-28   Revised: 2019-08-03   Online: 2019-10-29

作者简介 About authors

郭剑衡(1992-),女,甘肃张掖人,博士研究生,从事矿物学、岩石学和矿床学专业研究工作124322611@qq.com , E-mail:124322611@qq.com

摘要

烂泥塘斑岩铜金矿床位于云南省西北部的中甸地区,矿体主要以细脉—浸染状、网脉状产于石英二长斑岩和石英闪长玢岩之中。矿区热液蚀变作用发育,围绕矿体由深部至浅部依次发育钾化带、绿泥石—绢云母化带、绢云母化带和泥化带。钾化带中发育3种不同产状的磁铁矿,根据磁铁矿产出状态与脉体之间的相互穿插关系,将其划分为浸染状分布的磁铁矿(Ⅰ类)、单一脉状磁铁矿(Ⅱ类)和石英—硫化物脉中的磁铁矿(Ⅲ类)。此外,矿区常见产于成矿期后白云石—石英大脉中的镜铁矿。采用激光剥蚀电感耦合等离子体质谱(LA-ICP-MS)对上述铁氧化物进行了原位微区成分测试。结果表明:3类磁铁矿均富集Ti、V、Cr、Ni、Co、Al、Mg、Mn、Ga和Zn等微量元素。早期Ⅰ类磁铁矿含有钛铁矿出溶体,与Ⅱ、Ⅲ类磁铁矿相比,相对富集Mg、Ni和V等元素,属于岩浆磁铁矿;Ⅱ类磁铁矿相对富集Mn、Zn、Sn和Sc等元素,属于热液磁铁矿。岩浆磁铁矿(Ⅰ类磁铁矿)与后期脉状磁铁矿(Ⅱ类和Ⅲ类)相比,Ti、Al和Cr等元素含量相差不大。这可能是由于后期热液蚀变对Ⅰ类磁铁矿的强烈改造,导致其中Ti、Al和Cr等元素含量降低(通常岩浆磁铁矿比热液磁铁矿更富集Ti、Al和Cr)。Ⅱ、Ⅲ类脉状磁铁矿属于热液磁铁矿且二者微量元素含量差别不大,说明它们属于同一期流体中沉淀的产物。与磁铁矿相比,镜铁矿中的Ti、Al和V元素含量相差不大,而Cr、Ga、Ni和Co等元素含量比磁铁矿低一个数量级。结合前人资料,认为Al、Mn、Mg和Sc元素在磁铁矿中主要以类质同象形式存在,而Ca、S、Cu、Ba、Sr和Zr等元素主要以显微包裹体形式存在。钾化带中广泛发育的磁铁矿—赤铁矿共生组合、镜铁矿以及磁铁矿中异常低的Mn含量表明,烂泥塘矿区成矿流体的氧逸度高达赤铁矿—磁铁矿缓冲线。

关键词: 斑岩铜金矿床 ; 磁铁矿 ; LA-ICP-MS ; 氧逸度 ; 热液蚀变 ; 烂泥塘 ; 滇西北地区

Abstract

The Zhongdian area, located in northwestern Yunnan, is an important porphyry belt in China. It hosts a large number of Triassic intermediate-felsic porphyritic intrusions and porphyry deposits such as Pulang porphyry Cu-Au, Xuejiping porphyry Cu, Chundu porphyry Cu, Langdu Cu skarn and Lannitang porphyry Cu-Au deposit. The Lannitang porphyry Cu-Au deposit is located in west belt of the Zhongdian area. The magnetite in Lannitang porphyry Cu-Au deposit is widespread and it occurred as disseminated and vein types in potassic and chlorite-sericite alteration zone.Specularite is also observed frequently in the post-mineralization dolomite-quartz coarse veins.We conducted the petrography and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) to determine the texture and composition of iron oxides (magnetite and specularite). In this study, we identified three types of magnetite. Type-Ⅰ magnetite is disseminated in potassic alteration of deposit. It is generally contains ilmenite lamellas. Type-Ⅱ and Type-Ⅲ magnetite are occurred in magnetite single vein and magnetite-bearing quartz stockwork vein separately. Type-Ⅱ and Type-Ⅲ are distributed in potassic and chlorite-sericite alteration zone. The LA-ICP-MS analyses show that Type-Ⅰ magnetite is relatively rich in V, Ni and Mg than other two types of magnetite. Type-Ⅱ and Type-Ⅲ magnetite are more enriched in Mn, Zn, Sn, Sc and high-Ni/Cr ratio than Type-Ⅰ magnetite.Type-Ⅱ and Type-Ⅲ magnetite has similar content of many trace elements. The concentration of Cr,Ga,Ni and Co in specularite is obviously lower than those of magnetite. The ilmenite lamellae and low-Ni/Cr(Ni/Cr<1) ratio revealed that Type-Ⅰ magnetite belongs to igneous magnetite. Type-Ⅱ and Type-Ⅲ are distributed in veinlets and displayed high-Ni/Cr ratio (Ni/Cr>1). We suggested that they are hydrothermal magnetite. Type-Ⅰ magnetite (igneous) is intergrown with hydrothermal minerals including chlorite and sericite and it has quiet similar contents of Ti, Al and Cr with the other two hydrothermal magnetite.We suggest that Type-Ⅰ magnetite (igneous) experienced late-stage fluid alteration, which induced the loss of Ti, Al and Cr.The similar content of trace element between Type-Ⅱ and Type-Ⅲ magnetite indicated that they may precipitate from same period of fluid.In combination with previous studies, we propose that the presence of elements such as Al, Mn, Mg and Sc are in solid solution within magnetite (and/or specularite),but the Ca, S, Cu, Ba, Sr and Zr may be present in micro-/nano-scale mineral inclusions.The widespread presence of magnetite-hematite and specularite in the potassic alteration zone and low Mn concentration of magnetite indicates a high oxygen fugacity of the Lannitang porphyry Cu-Au deposit (magnetite-hematite buffer).

Keywords: porphyry Cu-Au deposit ; magnetite ; LA-ICP-MS ; fugacity ; hydrothermal alteration ; Lannitang ; Northwest Yunnan

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郭剑衡, 冷成彪, 张兴春, 张伟, 尹崇军, 张陆佳, 田振东. 滇西北烂泥塘斑岩铜金矿床铁氧化物LA-ICP-MS微量元素特征及其地质意义[J]. 黄金科学技术, 2019, 27(5): 659-677 doi:10.11872/j.issn.1005-2518.2019.05.659

GUO Jianheng, LENG Chengbiao, ZHANG Xingchun, ZHANG Wei, YIN Chongjun, ZHANG Lujia, TIAN Zhendong. Trace Elemental Compositions of Iron Oxides from the Lannitang Porphyry Cu-Au Deposit in the Zhongdian Region (Northwest) and the Geological Significances:A LA-ICP-MS Study[J]. Gold Science and Technology, 2019, 27(5): 659-677 doi:10.11872/j.issn.1005-2518.2019.05.659

磁铁矿是自然界分布极为广泛的矿物之一,既可以出现在沉积岩、火成岩和变质岩等各类不同岩性地质体中,也能作为矿石矿物分布在多种类型的铁矿床中(如钒钛磁铁矿、条带型铁矿、Kiruna型铁矿和铁氧化物铜金矿床等)[1]。磁铁矿中含有大量的微量元素(Mg、Al、V、Cr、Mn、Co、Ni、Zn、Ga、Ge、Pb、Sn、Pb、Sc、Cu),这些微量元素的含量与磁铁矿形成时岩浆和流体的组成、温度、氧逸度及硫逸度等多种因素密切相关[1,2,3,4],这使得磁铁矿成为岩石成因和矿床勘查应用的重要指示剂[2,3,4]。近年来,随着激光剥蚀电感耦合等离子体质谱(LA-ICP-MS)方法在单矿物微区微量元素分析中的应用,许多学者研究了不同类型矿床中磁铁矿的标型特征,并取得了若干重要成果[5,6,7,8,9,10]。Pisiaka等[11]提出磁铁矿的微量元素可以有效指示底部隐伏的斑岩体。Dupuis等[9]提出的(Al+Ca+Mn)-(Ti+V)判别图和Nadoll等[10]提出的Ni/(Cr+Mn)-(Ti+V)判别图,可以区分不同成矿环境下产生的磁铁矿(如钒钛磁铁矿矿床、斑岩—矽卡岩矿床、Kiruna型铁矿床、铁氧化物铜金矿床和条带型铁矿等)。然而,后期流体蚀变作用的叠加以及氧逸度、温度等外界因素的改变,会使原生磁铁矿结构和微量元素含量发生变化,从而导致上述判别图解不够准确。胡浩等[12,13]和Wen等[14]对磁铁矿岩相学和微量元素进行研究发现,岩浆磁铁矿在流体交代作用下会发生溶解—再沉淀现象,使得磁铁矿结构和微量元素含量发生改变(Ti、Al、Mg、Cr、Mn和Zn等元素亏损,Fe元素富集)。同时,原生磁铁矿氧化成为磁赤铁矿或镜铁矿的过程中,由于原子价态与半径不一致会降低二价阳离子在磁铁矿中的含量[15]。因此,对磁铁矿结构和微量元素进行深入研究,不仅可以明确磁铁矿经历的后期改造作用,而且能够进一步鉴定上述判别图解的适用性。

在斑岩型和矽卡岩型矿床中,磁铁矿既作为岩体中普遍存在的副矿物,也常常以热液蚀变矿物的形式出现[1]。前人对斑岩矿床中产出的磁铁矿进行了大量研究,发现斑岩型矿床中分布着岩浆磁铁矿和热液磁铁矿,且二者具有不同的岩相学和微量元素组成特征[5,6,7,8,9,10]。岩浆磁铁矿以浸染状产出在岩体或靠近岩体中心的钾化带中,通常含有较多的Ti、V、Al、Cr和Ga等元素。这与岩浆磁铁矿形成于较高的温度以及继承了较多岩浆成分有关,而热液磁铁矿以不同类型的细脉状和浸染状产出于岩体的钾化带和绿泥石—绢云母蚀变带中。热液磁铁矿含有相对较高的Ni、Mn、Co、Sn和Zn等元素,这与热液磁铁矿从流体中沉淀且继承更多的流体成分有关[2,3,10]。矽卡岩型矿床中块状磁铁矿属于富Fe流体交代而成的热液磁铁矿,其含有相对较高的Mn、Mg,这与矽卡岩矿化过程中广泛发育的水岩相互作用密切相关[16,17,18]。斑岩型和矽卡岩型矿床中广泛发育的热液对原生磁铁矿的改造,有助于进一步探讨不同类型磁铁矿的微量元素差异以及热液的成分、温度和氧逸度等特征。

烂泥塘斑岩铜金矿床位于滇西北迪庆藏族自治州东部,矿床中发育多期热液蚀变带,含有浸染状和脉状2种产状的磁铁矿,主要分布在钾化蚀变带和绿泥石—绢云母化蚀变带中。同时,成矿期后的白云石—石英大脉中发育有镜铁矿。本文在详细的野外地质调查和室内鉴定工作的基础上,利用LA-ICP-MS方法对烂泥塘矿床中不同产状的磁铁矿和镜铁矿进行了原位微区成分测试,据此对该斑岩型矿床中铁氧化物的微量元素特征及其地质意义进行了探讨。

1 区域地质背景

中甸岩浆弧位于义敦弧的南端,主体为近SN向延伸,其东部和南部以甘孜—理塘缝合带为界,西部以SSE向延伸的乡城—格咱深大断裂为界[19,20,21,22,23]。区域内断裂和褶皱发育,一系列NW向紧密线性褶皱和同向断裂是控制变质作用和岩浆活动的主要构造。晚期发育规模较小的NE向断层,切断了早期断裂及褶皱(图1)。中甸地区的沉积岩系主要为上三叠统,从下往上依次为曲嘎寺组、图姆沟组和喇嘛亚组。中甸地区的岩浆活动主要经历了印支期俯冲造山作用(237~206 Ma)、燕山期碰撞造山作用(135~73 Ma)和喜山期陆内汇聚作用(65~15 Ma)[22,24]。俯冲造山过程中的中酸性岩浆侵入活动,形成了该区的雪鸡坪、普朗和烂泥塘等斑岩型铜多金属矿床,以及浪都矽卡岩型铜多金属矿床;碰撞造山阶段的花岗岩侵入和期后热液活动,形成了一些蚀变花岗岩—热液石英脉型为主的钨—钼矿化,以休瓦促、红山和热林等为代表;陆内造山过程中的二长(斑)岩侵入活动,形成了诺东和东炉房等斑岩型铜矿化点[25,26,27,28,29,30,31,32,33]

图1

图1   中甸弧区域地质简图(底图据文献[30,31]修改)

1.第四系;2.喇嘛亚组;3.图姆沟组;4.曲嘎土寺组;5.燕山期花岗岩;6.印支期斑岩体;7.断裂;8.斑岩型矿床;9.矽卡岩型矿床

Fig.1   Regional geological map of the Zhongdian arc(modified after references [30,31])


2 矿区地质特征

烂泥塘斑岩铜金矿床地处中甸岩浆弧的西带,格咱断裂东侧约5 km处。地理坐标为99°80′E~99°49′E,28°06′N~28°08′N。自20世纪60年代起,云南地矿局在该地区开展了大量物化探工作。从2006年开始,云南华西矿产资源有限公司在该地区开展了大量地质矿产调查和钻孔勘探工作。截至2013年底,该矿床探明的铜金属量为40×104 t,平均品位为0.38%;伴生金金属量超过30 t,平均品位为0.24×10-6 [34]

矿区出露地层为上三叠统图姆沟组二段一亚段,岩性主要为蚀变安山岩和蚀变安山质凝灰岩,总体上属于火山岩系—碎屑岩建造。矿区内断裂发育,NW向断裂规模较大,与主构造线方向一致,还发育有近EW向次级张裂隙。NW向断裂切穿矿体,构造活动使区内岩体出现强烈的片理化(图2)。

图2

图2   烂泥塘斑岩铜金矿区地质简图(底图据文献[34]修改)

1.蚀变安山岩;2.闪长玢岩;3.石英闪长玢岩;4.石英二长斑岩;5.地质界线;6.工业矿体;7.低品位矿体;8.实测断层;9.取样钻孔

Fig.2   Geological sketch map of the Lannitang porphyry Cu-Au deposit (modified after reference [34])


区内印支期岩浆岩发育,岩性以中—中酸性浅成—超浅成侵入岩为主,从早到晚依次为闪长玢岩、石英闪长玢岩和石英二长斑岩。闪长玢岩分布于矿体的北东侧,岩石呈灰绿色,斑晶主要由斜长石、角闪石和少量石英组成,基质主要由长英质及暗色矿物组成。角闪石多被绿泥石和绿帘石交代,斜长石呈现轻微的绢云母化,岩体发生弱蚀变作用。石英闪长玢岩呈NW向分布,出露于闪长玢岩的南西侧,岩石呈灰白色,岩体发育片理化,斑晶主要由斜长石、角闪石和石英组成,偶见钾长石和黑云母,基质由斜长石、石英和少量钾长石组成,岩体发生黄铁绢英岩化蚀变。石英二长斑岩出露于矿体中部,呈岩枝状侵位于石英闪长玢岩中,岩石呈灰绿—浅灰色,中粒斑状结构,片理化不发育,斑晶主要由斜长石、钾长石和石英组成,基质主要由钾长石、斜长石、石英和少量云母组成。金属矿物主要由磁铁矿、黄铜矿和黄铁矿组成。

矿体主要分布在石英二长斑岩中,呈NNW向展布(图2图3),根据矿体的空间位置和赋矿围岩性质,圈出2条矿体,分别为KT1和KT2。KT1矿体的顶底板岩石均为石英二长斑岩,矿体呈似层状和透镜状,连续性较好。矿体产状与斑岩产状基本一致,走向NW,倾向NE,倾角平均为65°。KT2矿体位于KT1矿体的下部,位于整个矿体的底部,主要产于石英闪长玢岩中,矿体呈透镜状,不连续分布。矿体产状与斑岩产状基本一致,总体走向NW-SE,倾向NE,平均倾角为65°。

图3

图3   烂泥塘斑岩铜金矿床1号勘探线剖面图(位置见图2;底图据文献[34]修改)

1.第四系;2.闪长岩;3.石英闪长玢岩;4.石英二长斑岩;5.矿体;6.低品位矿体;7.地质界线

Fig.3   Cross section of No.1 exploration line in the Lannitang porphyry Cu-Au deposit (location in Fig.2modified after reference [34])


矿石结构主要为细脉—浸染状和网脉状。矿石矿物简单,金属矿物主要有黄铜矿、黄铁矿、磁铁矿和镜铁矿,含有少量闪锌矿、方铅矿、孔雀石、褐铁矿和辉钼矿。脉石矿物主要有石英、伊利石、铁白云石、方解石、硬石膏、石膏和绿泥石等。矿化从深部至浅部具有较明显的分带特征。在绿泥石—绢云母化蚀变带中,黄铜矿以团块状与石英、白云石及石膏共生。分析结果显示,Au与Cu元素之间具有明显的正相关关系[34]。闪锌矿和方铅矿主要产于地表出露的石英脉—硫化物大脉中,辉钼矿以薄层状产于构造裂隙中。

区内围岩蚀变强烈,由深部至浅部大致可划分为钾化带、绿泥石—绢云母化带、绢云母化带和泥化带。矿体底部为钾化带,岩石发生钾长石化,呈浅肉红色[图4(a)],赋存有浸染状磁铁矿和大量脉状磁铁矿,矿物组合为钾长石、磁铁矿、石英、黄铜矿和黄铁矿。向上过渡为绿泥石—绢云母化带,岩石呈灰绿色,黑云母和角闪石等暗色矿物发生绿泥石化蚀变,斜长石发生绢云母化蚀变。此带中磁铁矿脉逐渐减少,出现石英+硫化物脉,矿物组合为绿泥石、绢云母、磁铁矿、石英、石膏、黄铜矿和黄铁矿。绿泥石—绢云母化带之上为绢云母化带,岩石呈灰白色,主要发生斜长石的绢云母化,同时伴有石英±碳酸盐±石膏脉的大量产出,矿物组合为绢云母、石英、方解石、白云石、黄铁矿和少量镜铁矿。顶部为泥化带,矿物组合为高岭石、伊利石等黏土矿物和石英、黄铁矿。钾化带普遍遭受绿泥石—绢云母化的叠加,而绢云母化蚀变又常常叠加在先前的蚀变带之上。

图4

图4   烂泥塘斑岩铜金矿床不同矿化阶段代表性样品照片

(a)硫化物脉切穿单一磁铁矿脉,磁铁矿—石英脉切穿磁铁矿和硫化物脉;(b)磁铁矿—石英硫化物脉切穿磁铁矿脉;(c)后期白云石石英脉切穿磁铁矿脉;(d)白云石—石英—黄铜矿粗脉;(e)黄铜矿—石膏角砾被后期白云石—石英脉所包裹;(f)镜铁矿存在于晚期白云石—石英—石膏脉之中

Fig.4   Photos of some representative ore samples at different mineralization stages from the Lannitang porphyry Cu-Au deposit


烂泥塘矿床的钾化带和绿泥石—绢云母化带中发育有大量磁铁矿,根据其产状及脉体穿插关系大致划分为3类(磁铁矿手标本及镜下照片见图4图5)。Ⅰ类磁铁矿:他形,呈浸染状产于钾化带中,内部常含有微细钛铁矿出溶体(Fe-Ti氧化物;Fe/Ti原子比为1∶1),由于钾化蚀变阶段形成的Ⅰ类磁铁矿普遍遭受了后期的绿泥石—绢云母化蚀变,因此这类磁铁矿常与暗色矿物(角闪石、云母)蚀变产生的绿泥石伴生[图5(a)、5(b)]。Ⅱ类磁铁矿:半自形—他形,呈单一的磁铁矿脉体产于钾化带中[图4(a)~4(c)、图5(c)],常常被后期的磁铁矿+烟灰色石英脉、白云石+乳白色石英脉体截穿。Ⅲ类磁铁矿:他形细粒集合体,产于烟灰色石英+磁铁矿脉中[图4(a)、4(b),图5(d)、5(e)],这种脉体主要分布在钾化带中,普遍经历了后期的绿泥石—绢云母化蚀变。常见Ⅲ类磁铁矿—石英脉切穿Ⅱ类单一磁铁矿脉,浸染状磁铁矿又被后期脉状磁铁矿叠加[图4(a)~4(c)]。因此,在时间序列上,Ⅰ类磁铁矿形成时间最早,Ⅱ类磁铁矿形成时间又早于Ⅲ类磁铁矿。Ⅱ类与Ⅲ类磁铁矿颗粒间隙中常填充有黄铜矿和黄铁矿等硫化物[图5(e)、5(f)]。

图5

图5   烂泥塘斑岩铜金矿床典型矿石样品的镜下显微照片

(a)反射光下的Ⅰ类磁铁矿,黑云母蚀变形成的绿泥石与Ⅰ类磁铁矿密切共生;(b)含有钛铁矿出溶体的Ⅰ类磁铁矿;(c)反射光下的Ⅱ类磁铁矿单一脉;(d)正交偏光镜下Ⅲ类磁铁矿+石英脉;(e)反射光下的磁铁矿+石英脉;(f)石英磁铁矿脉,磁铁矿裂隙中填充后期黄铜矿;(g)赤铁矿—磁铁矿共生现象;(h)后期石英—白云石脉中的叶片状镜铁矿

Fig.5   Microphotos of typical ore samples from Lannitang porphyry Cu-Au deposit


烂泥塘矿床中还见有叶片状镜铁矿[图4(f)、图5(f)],其结晶较好,在正交偏光镜下呈灰白色略带蓝白色,未见镜铁矿与磁铁矿交代共生现象。镜铁矿产于成矿晚期的石英+白云石±石膏脉体中,这种脉体主要产于外围的绢云母化带中。

3 采样与测试分析

本研究选择矿区3个代表性钻孔(ZK1-2、ZK3-7和ZK4-7)和1个平硐进行采样,共采集17件含有磁铁矿的岩(矿)石标本,其中Ⅰ类磁铁矿样品3件,Ⅱ类磁铁矿样品3件,Ⅲ类磁铁矿16件,含镜铁矿的样品2件。样品采集位置见表1

表1   样品编号、采样位置及其矿石标本描述

Table 1  Ore sample number,location and its description

样品编号钻孔编号采样位置/m磁铁矿分类样品描述
ZK1-2-6ZK1-2180Ⅰ类+Ⅲ类绿泥石—绢云母化,石英+磁铁矿脉体
ZK1-2-7ZK1-2251Ⅰ类+Ⅲ类绿泥石—绢云母化,石英+磁铁矿脉体
ZK1-2-9ZK1-2279.5Ⅲ类绿泥石—绢云母化,石英+磁铁矿脉
ZK1-2-11ZK1-2326Ⅱ类钾化叠加绿泥石—绢云母化,单一磁铁矿脉
ZK1-2-16ZK1-2360Ⅲ类钾化蚀变,石英+磁铁矿脉
ZK1-2-30ZK1-2720Ⅰ类钾化蚀变,单一磁铁矿脉与石英+磁铁矿脉体
ZK3-7-2ZK3-7196.7Ⅲ类钾化蚀变叠加绿泥石—绢云母化蚀变石英+磁铁矿脉
ZK3-7-4ZK3-7198Ⅲ类钾化蚀变叠加绿泥石—绢云母化石英+磁铁矿脉
ZK3-7-5ZK3-7198.7Ⅱ类钾化蚀变叠加绿泥石—绢云母化蚀变单一磁铁矿脉与石英+磁铁矿脉
ZK3-7-9ZK3-7228Ⅱ类+Ⅲ类钾化蚀变,石英+磁铁矿脉
ZK3-7-10ZK3-7312镜铁矿绿泥石—绢云母化蚀变,石英+白云石+黄铁矿+镜铁矿
ZK3-7-11ZK3-7314Ⅲ类钾化蚀变,石英+磁铁矿呈网脉状
ZK4-7-9ZK4-7895Ⅰ类+Ⅲ类绿泥石—绢云母化蚀变,石英+磁铁矿脉
LNT14-2平硐矿石堆Ⅰ类+Ⅲ类钾化蚀变叠加绢云母化蚀变,石英+磁铁矿脉

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3.1 测试方法

本次实验工作在中国科学院地球化学研究所矿床地球化学国家重点实验室完成。首先将采集的样品制成光薄片,在显微镜下鉴定后圈定不同产状的磁铁矿,然后进行激光剥蚀电感耦合等离子质谱测试分析。激光剥蚀系统由美国Coherent公司生产的193 nm准分子激光系统和Agilent公司生产的7700X型电感耦合等离子质谱仪组成。氦气为载气,激光剥蚀出的气溶胶与氦气混合后通过Y型接头与氩气共同进入ICP-MS中。测试采用的能量密度为6 J/cm2,剥蚀频率为5 Hz,束斑大小为32 μm,氦气流量为620 mL/min。进行点剥蚀时,采集仪器背景值为15 s,信号采集60 s。测试过程中以GSE-1G、BHVO-2G、BCR-2G和BIR-1G作为外标,Fe元素含量作为内标校正仪器质量歧视与元素分馏,GSE-1g用于校正仪器漂移。天然磁铁矿BC-28 (Bushveld岩体,实验室内标)作为测试中的质控样品,监控微量元素的数据质量。原始数据的计算利用ICPMSDataCal软件进行离线处理[35]

3.2 测试结果

本次工作对烂泥塘矿区17件样品中的磁铁矿(93个点)进行分析,分析结果列于图6图7表2中。磁铁矿中全铁含量介于94.7%~99.2%之间,除Fe元素之外,还含有Ti、Al、Mg和V等微量元素,以及Si、Ca、Cr、Zn、Ni、Co、Pb、Mn、Cu、Sc、Sn和Ga多种可以检测到的痕量元素,其他元素如Sb、Rb、Sr、Hf、Ta、Th和U等均低于其检出限。

图6

图6   烂泥塘斑岩铜金矿床铁氧化物微量元素蛛网图(缺失的点表示低于检出限)

(a)Ⅰ类磁铁矿;(b)Ⅱ类磁铁矿;(c)Ⅲ类磁铁矿;(d)镜铁矿

Fig.6   Multi-element spider diagrams of iron oxides from the Lannitang porphyry Cu-Au deposit


图7

图7   烂泥塘斑岩铜金矿床铁氧化物中主要微量元素箱线图

注:箱体的上线和下线表示数据95%的置信区间的上限和下限;黑色的横线代表中值;黑色实心圆代表平均值

Fig.7   Box and whisker plots for important magnetite minor and trace element of iron oxides from the Lannitang porphyry Cu-Au deposit


表2   烂泥塘铜金矿铁氧化物LA-ICP-MS微量元素测试结果

Table 2  LA-ICP-MS trace element analysis data of iron oxides from the Lannitang porphyry Cu-Au deposit(×10-6

样品Ⅰ类:浸染状磁铁矿(17个)
w(FeO)/%MgAlScSiCaTiVCrMnCoNiZnCuGaSnPb
1-2-7-0197.39802 0444.914 3214455 6756233.327.123.224.075.70.24.158.61.1
LNT14-2-0198.618590.581 59958.35 364663131.01.220.321.147.20.33.217.15.6
LNT14-2-0298.52912260.501 457<LOD5 4755663.820.820.228.837.82.93.286.25.8
LNT14-2-0398.8341710.581 5389.94 3026348.01.323.121.041.00.23.655.83.3
LNT14-2-0498.2612420.461 4031 1787 039782207.04.413.613.749.90.63.667.43.4
LNT14-2-0598.3372150.741 34582.96 903680158.01.316.019.636.00.83.337.95.1
LNT14-2-0697.31343 2870.551 4558387 3357801486.622.313.368.90.113.47.72.4
1-2-6-1697.42711 9241.423 8404786 6456291922.314.321.428.91.25.08.311.8
1-2-7-0398.41154411.062 0101 3225 2469722062.612.521.374.7<LOD4.076.84.6
1-2-7-0598.23771 7751.561 15545.22 3811 3351977.828.745.711.212.01.674.27.3
1-2-30-1-1097.86321 5161.469 1321 7373 9222 2332093.810.057.421.13.43.295.43.1
1-2-30-1-1296.83 5324 1030.845 59334.31 2441 34732010.57.472.931.22.85.821.48.5
1-2-30-1-1495.41 1025 1682.725 6711 1598 7671 13512620.610.022.939.236.04.187.748.9
4-7-9-1-0196.27823 7033.413 074<LOD8 8971 64621626.210.628.536.640.95.4610.147.3
4-7-9-1-0296.41 1153 7742.593 76240.66 6561 49923630.111.024.747.531.55.146.736.0
4-7-9-1-0395.61 5084 37815.47 2071 2827 4101 6796.527.510.028.722.11.63.966.22.7
4-7-9-1-0495.68 0627001.251 9652683 4341 36368.56.87.910.966.40.74.192.321.9
平均值97.31 1211 9842.353 3255615 6881 09214311.815.428.043.37.964.566.4612.8
标准差1.151 9871 7463.582 3725952 11848795.310.86.416.219.013.412.482.1415.4
样品Ⅱ类:单一磁铁矿脉(15个)
w(FeO)/%MgAlScSiCaTiVCrMnCoNiZnCuGaSnPb
3-7-5-0898.4863831.231 79348.45 3959736.93.527.928.868.83.43.836.710.5
3-7-5-1095.31 6764 68616.581 25641.27 127561119.018.326.937.547.41.17.295.22.1
3-7-5-1199.02104901.551 3241431 8098954.75.828.623.237.92.62.866.52.7
3-7-5-1297.11 4472 4999.086 24113623 3788936.615.627.324.463.81.04.631.50.1
3-7-9-0197.01 7932 25811.2394621.93 18470811.917.127.824.959.10.94.232.90.5
3-7-9-0299.220740.941 8623482 26788711.24.254.045.627.83.02.292.81.0
3-7-9-0397.08311 4167.071 3211759 5421 08320.69.355.639.3<LOD0.23.092.10.1
3-7-9-0798.01202852.092 9158007 9431 5984.33.453.334.533.91.12.188.61.2
1-2-11-0198.24867641.201 4281 0853 64979662.75.914.211.917.71.62.927.82.1
1-2-11-0197.96706801.782 3161 3945 65688627.56.913.612.315.81.32.4110.20.2
1-2-11-0496.87004 3222.702 8831538 94388912.77.216.09.325.81.89.85.017.9
1-2-11-0498.03788661.861 6836235 2782 2432459.015.718.133.81.53.7510.20.3
1-2-11-0598.46822971.8316 1327005 3157818.37.313.716.713.70.92.7210.110.0
1-2-11-0698.24659590.853 6421724 37370724.82.814.815.310.20.82.553.518.3
1-2-11-0797.41 4392 4413.624 5591 3033 43887111.28.015.017.317.42.12.844.523.0
平均值97.77341 4954.203 3535585 15398538.58.327.023.933.80.83.833.518.3
标准差0.995901 4574.703 8205182 36141764.54.915.411.119.422.62.13.519.6
样品Ⅲ类:石英+磁铁矿脉
w(FeO)/%MgAlScSiCaTiVCrMnCoNiZnCuGaSnPb
1-2-7-0695.82 8315 49522.971 70859.97 3384273.95.64.513.2147.22.17.434.523.0
1-2-7-09971 2323 8012.012 1062664 2291 03415.41.63.5321392.847.325.957.1
1-2-7-1097.38603 8324.175 2211565 8821 234528.73.36.377.24.9118.583.178.1
3-7-11-0198.81481 5121.435 9891112 2481 2101.19.52.88.617.2<LOD9.97<LOD<LOD
3-7-11-0297.11 1873 35013.429 9298066 0841 1464.721.42.212.963.71.336.5110.3
样品w(FeO)/%MgAlScSiCaTiVCrMnCoNiZnCuGaSnPb
3-7-11-0497.77054 22112.383 5102304 0498423.444.24.712.9144.4<LOD58.418.510.1
3-7-11-0598.5763020.41 3841595 9494331592.716.523101.62.44.411.90.5
3-7-11-0796.31 5492 8138.24 8431143 98740232.514.812.927.995.40.36.23.00.6
3-7-11-0996.28161960.782 97140.718 4518958.4107.11.21.518.80.53.54.23.9
1-2-9-0198.5272500.693 01146.56 20568710.52.218.417.676.81.24.084.86.8
1-2-9-0398.21229781.262 16226 6587063.81.617.417.383.80.24.449.25.6
1-2-9-0598.845800.321 2363794 1044523.51.317.820.740.10.64.1810.33.1
1-2-9-0796.01 0393 82212.061 061504 4243833.46.718.922.786.80.27.5915.03.4
1-2-16-0196.92683 0851.274 4434324 80037110.82.112.426.666.00.36.819.51.2
1-2-16-0298.5111020.766 1471065 9095336.30.618.015.7101.40.14.0210.48.4
1-2-16-0398.9321730.73 50730.23 3744914.31.916.320.3227.3<LOD3.278.85.3
1-2-16-0598.4835230.635 04398.54 7274645.61.314.220.791.10.83.788.826.8
LNT14-2-0996.71 1223 0186.43 7781278 4641 0883.713.327.023.273.72.67.435.711.3
LNT14-2-1196.02 2333 79517.322 85139.16 3981 04116.924.826.326.482.60.28.432.61.0
LNT14-2-1395.81 0975 87910.547 8261516 7051 1745.24.829.416.0426.14.511.619.413.0
4-7-5-0399.010691.092 845562 3191 4211924.453.227.434.80.22.386.6<LOD
4-7-5-0496.11 3052 0405.973 03225.29 2571 2563.110.854.934.475.31.12.958.04.1
4-7-5-0594.75 1577 3281.597 3917103 5602799.310.112.175.041.01.76.396.81.0
4-7-5-0696.91 7703 6902.151 868372 0661857.25.210.966.817.41.34.7416.14.3
4-7-5-0797.91 6262 8313.242 4401978491413.65.913.875.419.01.05.138.40.1
4-7-5-0898.46862 2501.175 0175572 76722791.35.212.137.413.31.83.668.4<LOD
4-7-5-0997.25913 1662.605 9083984 79067029.611.313.728.215.03.92.124.04.1
4-7-5-1096.77634 3592.461 88810.63 74868137.79.213.829.917.21.22.194.43.2
1-2-6-0197.07563 3952.754 9633355 44662612214.116.328.516.75.81.625.87.9
1-2-6-0297.73743 1031.231 07938.93 7337313412.813.137.013.43.43.556.36.0
样品w(FeO)/%MgAlScSiCaTiVCrMnCoNiZnCuGaSnPb
1-2-6-0297.73743 1031.231 07938.93 73373134.012.813.137.013.43.43.556.36.0
1-2-6-0398.14231 7421.741 03789.73 54673842.59.614.031.513.41.71.864.316.6
1-2-6-0497.68082 6592.081 4991394 33469035.517.411.930.917.57.62.076.67.1
1-2-6-0596.125614 95115.722 6494284 37613814.842.217.327.333.60.12.675.01.2
1-2-6-0695.327735 65013.942 1467057 5442774.259.916.925.746.41.93.033.14.0
1-2-6-0895.031416 43018.682 2691543 6297612.349.917.628.542.5<LOD3.855.62.1
1-2-6-0996.621033 79517.011 29358.52 519144.244.717.72342.51.43.496.96.7
1-2-6-1096.19273 3276.021 3691228 470163715.616.18.126.523.40.65.364.32.0
1-2-6-1195.88523 6903.542 20982.211 094129917.914.591935.70.74.238.01.4
1-2-6-1298.46581 1443.171 84321.34 1477530.115.111.933.169.80.74.926.32.3
1-2-6-1397.99351 8953.021 8861625 12572511.36.912.633.386.61.85.446.30.4
1-2-6-1495.53 8304 6924.92 2071305 63271820.873.312.330.4176.10.28.242.90.3
1-2-6-1598.31583302.553 6114166 0188281.44.612.531.3152.92.34.6210.31.3
3-7-9-0896.42 4232 6946.721 93345.98 21280811.411811.731.5112.87.76.324.10.8
3-7-4-0598.82515982.21 5602763 99873012.51.912.525.886.125.74.655.40.7
3-7-4-0698.36861 7392.731 7553054 1337341.27.012.525.2113.5<LOD4.983.80.3
3-7-2-0998.03702 1624.353 7415236 43570910.84.912.919.4219.3<LOD6.276.20.3
3-7-2-1096.91 0242 36411.625 8539469 0429672.827.814.125.039.79.114.445.46.8
3-7-2-1197.71833249.391 6211449 1169306.313.113.215.522.810.312.574.98.6
4-7-9-3-0198.43687662.622 2366644 53591112.539.58.519.365.510.617.425.25.7
4-7-9-3-0298.27641 6424.241 65785.72 13392316.820.314.227.126.415.83.826.06.4
4-7-9-3-0395.61 1415 19010.514 6501488 2348862.26.229.115.61 13910.37.724.69.7
4-7-9-3-0497.55592 9513.141 4581103 7957474.84.734.322.1138.313.46.996.28.5
4-7-9-3-0599.2292860.151 76370575511846.90.40.1<LOD1.51.32.0810.90.5
平均值97.3104726515.513158231538472920.918.615.226.4973.617.356.85.28
标准差1.18106418535.641966235288736537.624.810.413.81635.049.062.965.56
样品镜铁矿(8个)
w(FeO)/%MgAlScSiCaTiVCrMnCoNiZnCuGaSnPb
3-7-10-0197.72 8937080.142 0681 1834 2041 6368.161.95.30124.60.63.8<LOD21.3
3-7-10-0298.61 1234190.331 5544583 1999351.7251.60.739.10.62.440.18.9
3-7-10-0398.7158420.271 84983.43 1281 20412.500.32.410.00.72.74<LOD0.7
3-7-10-0498.4331 335<LOD1 24932.34 2191 8148.32.50.3<LOD10.0<LOD2.960.11.5
3-7-10-0599.0101 1050.091 17463.71 9657599.93.40.30.26.30.32.6<LOD24.6
3-7-10-0697.9517261.889945029 0403181.71.31.41.710.30.53.5319.53.2
4-7-8-0199.172300.351 35943.33 154181<LOD00.50.32.8<LOD2.328.80.8
4-7-8-0297.4128760.980540212 0284826.90.71.11.912.10.72.9710.22.0
平均值98.45187800.51 3813465 1179166.1411.91.350.926.90.432.924.847.87
标准差0.621 0343520.624253923 5095994.4821.91.680.9641.00.30.527.329.71

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Ⅰ类磁铁矿具有相对较高的w(Cr)[(143±95.3)×10-6](均值±标准偏差,下同)、w(Ti)[(5 687±2 117)×10-6]、w(V)[(1 092±487)×10-6]、w(Ni)[(18±16.2)×10-6]和w(Mg)[(1 121±1 987)×10-6],而w(Sc)[(2.35±3.58)×10-6]含量相对较低。

Ⅱ类磁铁矿w(Co)含量相对较高[(27±15.4)×10-6],而w(Mn)[(8.3±4.9)×10-6]、w(Sn)[(5.95±3.27)×10-6]、w(Zn)[(33.8±19.4)×10-6]和w(Ga)[(3.85±2.05)×10-6]含量相对较低。

Ⅲ类磁铁矿与其他两类磁铁矿相比,w(Al)[(2 651±1 853)×10-6]、w(Zn)[(96.8±163)×10-6]、w(Sc)[(5.51±5.64)×10-6]、w(Mn)[(18.6±24.8)×10-6]和w(Ga)[(9.06±7.53)×10-6]含量相对较高,而w(Cr)[(20.9±37.6)×10-6]、w(V)[(729±365)×10-6]和w(Pb)[(5.28 ± 5.56)×10-6]含量相对较低。

镜铁矿除w(Al)[(916 ± 561)×10-6]、w (V)[(916 ± 600)×10-6]和w(Ti)[(5 117±3 509)×10-6]含量无明显变化之外,其他元素如Si、Mg、Zn、Sn、Ga和Cr等含量均比磁铁矿低,其中w(Ni)[(0.9±0.89)×10-6]、w(Co)[(1.35±1.57)×10-6]、w(Sc)[(0.5±0.58)×10-6]和w(Cu)[(0.43±0.3)×10-6]含量比磁铁矿至少低一个数量级。

4 讨论

4.1 磁铁矿的微量元素赋存形式

磁铁矿中常见有包裹体,前人曾开展过许多磁铁矿包裹体的研究工作。Ilton等[36]认为斑岩铜矿中热液磁铁矿异常高的铜含量是由于包裹体导致的。Ray等[37]曾报道过矽卡岩铜金矿中较高含量的P和REE(约为1 000×10-6)。Kamvong等[38]发现磁铁矿中高Ba、Sr含量包裹体的存在。

通过分析烂泥塘磁铁矿中各微量元素LA-ICP-MS信号值随时间变化的趋势,发现在不含包裹体的磁铁矿测试点中,Fe、Al、Mg、V、Ti、Co、Sc和Mn等元素的信号呈现相对平缓的曲线[图8(a)];而在含有包裹体的磁铁矿中,部分元素的信号呈现出急剧上升和下降的变化趋势,如图8(b)~8(d)分别反映了磁铁矿中的S、Cu、Ba、Sr、Zr、Y、Hf和U元素的信号异常变化趋势。这可能是由硫化物[Cu-S元素的异常,图8(b)]、天青石[Sr-Ba元素的异常,图8(c)]和锆石[Zr-Hf-Th-U的异常,图8(d)]等矿物包裹体引起的。其中Cu和S元素异常可能是由磁铁矿中硫化物包裹体引起的,这与磁铁矿颗粒之间充填后期的微细黄铜矿颗粒的现象一致(图5)。异常的Zr、Y、U和Pb元素的信号值则反映了锆石包裹体存在于磁铁矿中,并且早期形成的锆石被晚期结晶的磁铁矿所包裹[39]。综上所述,Cu、S、Ba、Sr、Zr和Y等元素很可能以包裹体形式赋存于磁铁矿之中,而其他元素如Mg、Al、Mn、Sc和Co等均呈现相对平缓的直线,表明它们是以类质同象形式赋存在磁铁矿中。通过相关系数分析和二元散点图分析得到,烂泥塘矿区磁铁矿中Al、Mg、Sc、Mn与FeO呈明显的负相关,且随着Mg替代的增强,Al、Sc、Mn的替代也逐渐增强,这可能是由于这些元素(Al、Mg、Sc)之间的成对替代(A3++B4+=2Fe2++Fe3+,A3++B2+=Fe2++Fe3+)造成的(图9),与前人的研究结果相一致[9,40,41]

图8

图8   4个样品的LA-ICP-MS时间信号

(a)稳定平直信号,显示不含包裹体;(b)硫化物包裹体,S、Cu信号同时增加;(c)天青石包裹体,Ba、Sr信号同时增加;(d)锆石包裹体,Zr、Y、Hf、U、Th信号同时增加

Fig.8   Time resolved signals for LA-ICP-MS analyses of four samples


图9

图9   烂泥塘斑岩铜金矿床铁氧化物部分元素之间的协变图

Fig.9   Correlation plots of some elements for iron oxides from the Lannitang porphyry Cu-Au deposit


4.2 磁铁矿的成因以及微量元素特征

在烂泥塘斑岩铜金矿床中,磁铁矿以3种不同产状存在。Ⅰ类磁铁矿以浸染状分布在钾化带和绿泥石—绢云母化带中,磁铁矿内部分布着细小钛铁矿出溶体[图5(b)]。在温度高于600 ℃条件下,岩浆磁铁矿会从硅酸盐熔体中结晶出来。岩浆磁铁矿富Ti,且随着温度的降低,富钛相(钛铁尖晶石(Fe2TiO4))从磁铁矿中析出形成钛铁矿出溶体[42]。因此钛铁矿的出溶显示了Ⅰ类磁铁矿具有岩浆成因[43,44]。这类磁铁矿与绿泥石、绢云母伴生产出的现象,说明其经历了广泛的流体改造。早期与Ⅰ类磁铁矿共生的长石和黑云母(岩浆成因)在流体作用过程中蚀变成为绿泥石和绢云母[45,46],反应式[43]如下:

2K(Mg,Fe)3AlSiO10(OH)2+4H+=Al(Mg,Fe)5AlSi3O10(OH)8绿+ (Mg,Fe)2++2K++3SiO2
3KAlSi3O8+2H+=KAl2AlSi3O10(OH)2+2K++6SiO2

Ⅱ类与Ⅲ类脉状磁铁矿主要分布在钾化蚀变带中。这两类脉体是斑岩铜金矿床中早期钾化蚀变的特征脉体。两类脉体不含硫化物,脉体边缘平直且无蚀变晕[图4(b),4(c)],具有斑岩矿床早期脉体的典型特征。Ⅲ类磁铁矿+石英脉体常切穿Ⅱ类单一磁铁矿脉体[图4(a),4(b)],说明Ⅱ类单一磁铁矿脉体的产出早于Ⅲ类石英+磁铁矿脉。Ⅱ、Ⅲ类磁铁矿是钾化蚀变过程中从流体中沉淀出的热液磁铁矿。

Dare等[3]提出,Ti-Ni/Cr图解可以有效区分岩浆和热液磁铁矿。通常,岩浆中含有较高的Ti,且Ti含量也受温度的控制,Ti含量会随着温度的升高而增加。因此,Ti在岩浆磁铁矿中的含量大于热液磁铁矿。Ni和Cr元素的行为在岩浆和热液体系中具有差异性。在硅酸盐熔体中,Ni与Cr元素的行为是耦合的,二者比值Ni/Cr≤1;而在热液系统中,Cr相比Ni具有更低的溶解度,因此二者比值Ni/Cr>1。将烂泥塘斑岩铜金矿床的3类磁铁矿投到Ti-Ni/Cr图解[图9(a)]中发现,Ⅰ类磁铁矿微量元素值主要分布在岩浆磁铁矿区域,而Ⅱ、Ⅲ类磁铁矿微量元素值主要分布在热液磁铁矿区域。岩浆磁铁矿(Ⅰ类)与热液磁铁矿(Ⅱ、Ⅲ类磁铁矿)的Ti含量并没有大的差异,这可能是由于Ⅰ类磁铁矿在后期流体蚀变过程中Ti元素被溶解出去而造成的[12,13,14]。胡浩等[12]和Hu等[13]在研究矽卡岩中的磁铁矿时发现,原生磁铁矿经历流体交代作用后发生溶解—再沉淀作用,Ti、Si、Al、Mg、Sn、Ca、Si、Ga和Mn等元素含量降低。Wen等[14]在研究华北克拉通邯郸—邢台地区花岗质岩石中的磁铁矿时发现,流体的蚀变作用使得原生磁铁矿Fe含量升高和Ti、Al、Mg、Zn及Cr元素含量降低。通过对比日本西南部未蚀变花岗岩中的岩浆磁铁矿(15个样品)[47,48]与Ⅰ类磁铁矿的微量元素时发现,未蚀变花岗岩中的岩浆磁铁矿相对富集V、Ni、Mn、Zn和Ga等元素,这可能也归因于热液流体的蚀变作用带走了早期Ⅰ类岩浆磁铁矿中的上述元素所造成的[12,13,14]

Nadoll等[10]研究了美国西部斑岩铜矿中的热液磁铁矿,得出斑岩矿床中不同脉体的热液磁铁矿可由Ti、V、Mn和Zn元素含量加以区分。然而,在研究烂泥塘斑岩铜金矿床的脉状磁铁矿时发现,Ⅱ类和Ⅲ类脉状磁铁矿的微量元素含量差别并不大,说明这两类脉状磁铁矿形成时的温度、氧逸度和流体组成等条件并没有较大的差异,属于同一期流体中沉淀的产物。

4.3 氧逸度

烂泥塘钾化蚀变带中存在着磁铁矿与赤铁矿共生的现象[图5(i),5(j)],磁铁矿常被赤铁矿沿其周边或裂缝氧化[图5(j)]。对于赤铁矿的形成过程曾有一定的争论,有2种可逆的化学反应[49]解释这种现象,分别为

2Fe3O4+0.5O2aq=3Fe2O3
Fe3O4+2H+aq=Fe2O3+Fe2+aq+H2O

其中,式(3)是氧化还原反应,式(4)是在酸性溶液中的非氧化还原反应[49]。由于式(4)的逆过程是体积增大的过程,且缺少Fe2+和其他相关离子的迁移证据[50],因此在烂泥塘斑岩铜金矿床中赤铁矿很可能是由式(3)即磁铁矿的氧化作用形成的,因此磁铁矿—赤铁矿矿物对可以反映体系的氧化还原状态。

前人研究发现斑岩铜矿与高氧逸度岩浆密切相关[45,51],高氧逸度使硫以SO2或硫酸盐的形式存在,有利于消除岩浆源区残余相中的硫化物,从而大幅提高岩浆中铜、金的初始含量。但是,铜矿化最终取决于还原态的硫(S2-[52,53],这就需要岩浆过程中的硫酸盐还原成矿阶段的还原硫(H2S/HS-/S2-)或多硫化物(S3-/S22-)。

我国许多大型斑岩矿床中均存在着磁铁矿—赤铁矿共生矿物对[54,55],如德兴、玉龙、驱龙和雄村等斑岩铜矿床。Sun等[54]提出磁铁矿—赤铁矿矿物共生组合的存在反映了热液流体中发生了Fe2+的氧化和硫酸盐的还原作用。同时,磁铁矿在结晶过程中会降低体系的pH值[54][式(5)],氧逸度也会随着pH值的降低而升高,使得体系的氧逸度升高至赤铁矿—磁铁矿缓冲线[54]图10箭头方向)。Richards[56]也提出磁铁矿是由于FeCl2被SO2氧化而成的[式(6)],并且若赤铁矿是高温蚀变矿物组合中普遍存在的矿物,则其氧逸度将会被控制在磁铁矿—赤铁矿缓冲线之间(FMQ+5)。

图10

图10   斑岩型铜金矿成矿过程中氧逸度与硫的价态(据文献[54,57]修改)

FMQ-铁橄榄石—磁铁矿—石英;NNO-镍—镍氧化物;HM-赤铁矿—磁铁矿

Fig.10   Oxygen fugacity and oxidation potential of sulfate at the stage of porphyry mineralization (modified after references [54,57])


6SO42-aq+52H2O+57Fe2+aq=2S3-aq+19Fe3O4+104 H+aq
9FeCl2aq+SO2aq+10H2O=3Fe3O4+H2Saq+18HClaq

此外,镜铁矿作为高氧逸度指示矿物也出现在成矿晚期。成矿晚期的镜铁矿产于白云石+石英+石膏脉体中,与磁铁矿并不共生。与3类磁铁矿相比,镜铁矿中的二价元素Co、Ni和Cr含量显著降低,发生了数量级变化。Mn和Mg元素虽然在平均含量上与磁铁矿相差不大,但是在镜铁矿中部分样品的Mn和Mg元素含量低于检测限(表2)。而在镜铁矿中以三价离子存在的Al、Ti和V元素含量与磁铁矿并没有差别。这种现象可能是由于二价离子和镜铁矿所需离子的原子价态与半径不一致所导致的[15]

4.4 磁铁矿微量元素的找矿意义

在过去10多年,许多学者将磁铁矿的微量元素与矿床成因类型联系起来[9,10,58],提出了一系列判别图解,如(Ni+Cr)-(Si+Mg)、(Al+Ti)-(Ti+V)和(Ni/Cr)-(Ti+V)等。在这些判别图解中,斑岩矿床以较高Ti、V和Al元素含量区别于其他类型矿床[9,10]。斑岩矿床中的岩浆磁铁矿可以通过高Ti、低Ni/Cr比值与热液磁铁矿进行区分[3]。然而,将烂泥塘斑岩铜金矿床的磁铁矿微量元素投在(Al+Mn)-(Ti+V)与(Ni/Cr+Mn)-(Ti+V)判别图,发现大部分样品数据落在斑岩区域之外(图11),这可能与烂泥塘斑岩矿床中异常低的Mn含量有关。在烂泥塘斑岩铜金矿床中,Mn元素含量在0.4×10-6~118.0×10-6之间,平均含量为15.4×10-6,明显低于Nadoll等[10]所统计的美国西南部和印度尼西亚斑岩铜矿床中Mn元素含量(热液磁铁矿77.2×10-6~19 300×10-6,岩浆磁铁矿115×10-6~10 500×10-6)。通常,在低氧逸度条件下Mn2+更易进入磁铁矿的晶格中[59]。因此,烂泥塘斑岩铜金矿床中磁铁矿的较低Mn含量可能是由于较高的氧逸度(磁铁矿—赤铁矿缓冲线)引起的。

图11

图11   烂泥塘斑岩铜金矿床磁铁矿成因判别图解[9,10]

Fig.11   Discrimination diagram of magnetite genesis in Lannitang porphyry Cu-Au deposit[9,10]

(a)(Ti+V)-Ni/(Cr+Mn);(b)(Ti+V)-(Al+Mn)


斑岩铜金矿床的显著特征是广泛发育热液磁铁矿。对比斑岩铜钼矿床,斑岩铜金矿床富含磁铁矿±透闪石和磁铁矿—石英脉体[45]。在烂泥塘斑岩铜金矿床中,Cu、Au共同发育在钾化蚀变带中且Cu、Au含量是同步变化的[34,60],而黄铜矿稍晚于热液磁铁矿沉淀,因此大量磁铁矿脉体和磁铁矿石英脉体通常可作为斑岩铜金矿的找矿标志[45,61]

5 结论

(1)烂泥塘斑岩铜金矿床钾化蚀变带中存在3种不同产状的磁铁矿,分别为浸染状磁铁矿(Ⅰ类)、单一脉状磁铁矿(Ⅱ类)和分布于石英—硫化物脉的磁铁矿(Ⅲ类)。常见Ⅲ类磁铁矿—石英脉切穿Ⅱ类单一磁铁矿脉,浸染状磁铁矿又被后期脉状磁铁矿叠加。因此,Ⅰ类磁铁矿形成时间最早,Ⅱ类磁铁矿的形成时间又早于Ⅲ类磁铁矿。

(2)在微量元素上,Ⅰ类磁铁矿更加富集V、Ni和Mg等元素,而Ⅱ类与Ⅲ类磁铁矿则相对富集Mn、Zn、Sn和Sc等元素。Ⅰ类磁铁矿由于遭受了后期流体的蚀变作用,使得这类磁铁矿的Ti、Al和Cr等微量元素与Ⅱ类和Ⅲ类磁铁矿相差不大。Ⅱ类与Ⅲ类磁铁矿在微量元素上没有特别大的差异,因此认为它们属于同一期流体中沉淀的产物。Ⅰ类磁铁矿中常含有钛铁矿出溶体,属于岩浆磁铁矿,Ⅱ类与Ⅲ类磁铁矿存在于脉体中,属于热液磁铁矿。Ti-Ni/Cr图解可以很好地区分岩浆磁铁矿(Ⅰ类)和热液磁铁矿(Ⅱ和Ⅲ类)。

(3)Al、Mn、Mg和Sc等元素在烂泥塘磁铁矿中以类质同象形式存在,而Ca、S、Cu、Ba、Sr和Zr等元素主要以显微包裹体形式存在。

(4)烂泥塘斑岩铜金矿床钾化蚀变带中存在的磁铁矿—赤铁矿矿物共生现象、大量的石膏脉体和晚期镜铁矿均表明该矿床具有较高的氧逸度,且氧逸度达到磁铁矿—赤铁矿缓冲线。高氧逸度使得烂泥塘斑岩铜金矿床中具有异常低的Mn值,这也使得烂泥塘矿床中部分磁铁矿投在(Al+Mn)-(Ti+V)和(Ni/Cr+Mn)-(Ti+V)判别图的斑岩区域之外。

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