岩石学报  2016, Vol. 32 Issue (8): 2433-2450   PDF    
胶西北新城金矿床热液蚀变作用
张潮1,2, 黄涛2,3, 刘向东2,4, 刘育2, 赵海5, 王旭东5     
1. 中国地质调查局发展研究中心, 北京 100037 ;
2. 中国地质大学地质过程与矿产资源国家重点实验室, 北京 10008 ;
3. 江苏长江地质勘查院, 南京 210046 ;
4. 山东省第六地质矿产勘查院, 威海 264209 ;
5. 山东黄金矿业股份有限公司新城金矿, 莱州 261438
摘要: 新城金矿床是胶西北金矿集区中典型的破碎带蚀变岩型金矿床,其矿体受控于NE-NNE向焦家断裂及其次级断裂系统,主要赋存于断裂下盘(黄铁)绢英岩与红化花岗岩体中。焦家断裂下盘分带性明显,自主断裂面向外依此发育主断裂面和断层泥、挤压片理带、构造透镜体带、密集节理带和稀疏节理带。其中,主断裂面和断层泥发育粘土化蚀变;挤压片理带发育面状黄铁绢英岩化蚀变,其内赋存黄铁绢英岩型矿体;构造透镜体带发育脉型(黄铁)绢英岩化、面状绢英岩化和硅化蚀变,其次级断裂内赋存石英硫化物脉型矿体;节理带(包括密集节理带和稀疏节理带)主要发育成红化和细脉型(黄铁)绢英岩化,沿节理面赋存细脉型矿体。碳酸盐化蚀变叠加于上述热液蚀变之上。焦家断裂带表现为以水平为主的蚀变-矿化特征,(黄铁)绢英岩化蚀变与金成矿关系最为密切。论文在厘定断裂构造分带与蚀变-矿化分带空间关系的基础上,通过对各类蚀变岩与新鲜新城花岗岩体元素地球化学分析,剖析了热液蚀变作用过程及其机制。选取TiO2作为不活动组分,质量平衡计算表明,成矿前新城花岗岩体发生红化作用时,带入组分有Fe2O3、K2O、Al2O3以及少量Au、Ag、Cu、Pb、Zn、Sb和Bi等,而被带出组分有SiO2、CaO和Na2O等;成矿期红化花岗岩体蚀变为(黄铁)绢英岩过程中,明显带入组分有SiO2、Fe2O3、FeO、Al2O3、MgO、K2O、Au、Ag、As、Cu、Pb、Zn、Sb和Bi等,而被带出组分为Na2O。稀土元素地球化学特征与REE球粒陨石标准化配分模式曲线表明,红化和(黄铁)绢英岩化热液蚀变作用影响REE迁移。REE分别在红化和(黄铁)绢英岩化蚀变中带入和带出;Eu在红化过程中呈显著带入,表现为显著Eu正异常(δEu=1.34),而在(黄铁)绢英岩化蚀变中活化带出,表现出Eu负异常(0.89~0.95)。成矿期发生(黄铁)绢英岩化蚀变时,Eu从氧化态Eu3+转变为Eu2+,进入流体被带走,造成Eu负异常。金主要以Au(HS)2-形式在变质流体中运移。成矿流体沿片理面运移时,在挤压片理带发生黄铁绢英岩化蚀变,硫化作用使得流体还原硫活度降低,导致Au(HS)2-络合物失稳沉淀并赋存于黄铁矿和石英等矿物裂隙或晶格中,形成黄铁绢英岩型矿化;在构造透镜体带,成矿流体沿次级断裂面和碎裂岩裂隙发生蚀变形成脉型(黄铁)绢英岩,成矿元素在次级断裂面/裂隙内沉淀并形成石英硫化物脉型矿化;在节理带,成矿流体压力瞬时降低导致流体发生不混溶现象,使得Au(HS)2-络合物失稳沉淀并充填节理中形成细脉型矿化。
关键词: 热液蚀变     质量平衡     断裂带结构     新城金矿床     胶西北    
Hydrothermal alteration of the Xincheng gold deposit, northwestern Jiaodong, China
ZHANG Chao1,2, HUANG Tao2,3, LIU XiangDong2,4, LIU Yu2, ZHAO Hai5, WANG XuDong5     
1. Development and Research Center, China Geological Survey, Beijing 100037, China ;
2. State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing 10008 ;
3. Jiangsu ChangJiang Geological Survey Institute, Nanjing 210046, China ;
4. 6th Geology & Mineral Resources Survey Institute, Weihai 264209, China ;
5. Xincheng Gold Company, Shandong Gold Mining Stock Co., LTD., Laizhou 261438, China
Abstract: The Xincheng gold deposit, located at the Zhaoyuan-Laizhou goldfield in the northwestern Jiaodong, is one of the most typical altered-type gold deposits. The gold mineralization is controlled by Jiaojia fault with NE-NNE strikes and its secondary faults and facture system. The ores are mainly hosted in (pyrite)-sericite-quartz and red colouration granite of Xincheng in the footwall. Distinct structural zonation is recognized vertically with the fault plane and the fault gouge zone, extruded schistosity zone, tectonic lens zone, dense joints zone and sparse joints zone. The fault plane and the fault gouge zone develop clay alteration. The types of hydrothermal alteration in the extruded schistosity zone mainly are sericitization and sulfuration, with the pyrite-sericite-quartz type ore bodies. The hydrothermal alterations in the tectonic lens zone are signified by the veins of (pyrite)-sericite-quartz, sericite-quartz and silicification, and being as the quartz-sulfide vein type ore bodies. The alterations in the joint zone are the veins of (pyrite)-sericite-quartz and red colouration, and changed into the fine ore veins ore bodies along the joint surface. Carbonatization mostly overlaps all of the alterations. Horizontal zonation of mineralization-alteration is the main signature for the ore-controlling mechanism in Xincheng gold deposit, and the (pyrite)-sericite-quartz alteration is critical for gold mineralization. The alteration process and mechanism have been studied based on the spatial relationship between the distinct structural zonation, the alterations and mineralizations, and along with the geochemical analysis of the altered rocks and the unaltered Xincheng granite. TiO2 is selected as an inactive component to perform the mass balance calculations. Based on the results, Fe2O3, K2O, Al2O3, Au, Ag, Cu, Pb, Zn, Sb and Bi were brought in, contrary with SiO2, CaO and Na2O during red colouration process, While during (pyrite)-sericite-quartz alteration, SiO2, Fe2O3, FeO, Al2O3, MgO, K2O, Au, Ag, As, Cu, Pb, Zn, Sb and Bi were brought in, however, Na2O is out. The REE geochemical characteristics and the chondrite-standardized plotting show that red colouration and (pyrite)-sericite-quartz alteration have influence on the migration of REE. The REE were brought in by the former alteration with obvious Eu positive anomaly (δEu=1.34). While, they were carried out by the latter alteration with Eu negative anomaly (0.89~0.95). Eu changeed from the oxidation state of Eu3+ to the reduction state of Eu2+, and then entered into the ore-forming fluids, leading to negative Eu anomaly during the (pyrite)-sericite-quartz alteration. The gold is transported as Au(HS)2- complex in the metamorphic ore-foming fluids. When the ore-forming fluid migrates along the foliation in the extruded schistosity zone, the interaction between fluid and red colouration granite of Xincheng caused the deposition of the pyrite, sericite, quartz and other sulfide minerals. The sulfurication reduces the activity with decreasing of sulfur content, and then lead to the disintegratation of Au(HS)2- complex and precipitation of gold in the crystal fractures or lattices of pyrite/quartz. In the tectonic lens zone, the interaction of fluid and red colouration granite of Xincheng caused the alteration of vein of (pyrite)-sericite-quartz when the fluid migrates along the secondary faults. The quartz-sulfide vein can be observed along the secondary fault plane. The fluid immiscibility due to instantaneous reduction of fluid pressure caused the disintegratation of Au(HS)2- complex. The gold-bearing pyrite and other sulfide minerals precipitated along the joint planes with the fine veins.
Key words: Hydrothermal alteration     Mass balance     Fault zone structure     Xincheng gold deposit     Northwestern Jiaodong    
1 引言

流体在热液金矿床热液蚀变作用和金成矿过程中起着重要作用(Yang et al.,2016b; Meier et al.,2015),通过对热液蚀变作用的研究能够分析其蚀变机制,厘定蚀变与金矿化的关系(Meier et al.,2015; Laurich et al.,2014; 邱昆峰等,2015; 张志超等,2015),这对进一步探讨矿床成因和勘查找矿至关重要。同时,前人通过对断裂带结构不同部位流体输运与流体-岩石反应研究,很好地探讨了中温热液矿床热液蚀变作用与矿化的关系(Dugdale and Hagemann,2001; Kolb et al.,2004)。元素地球化学可用于研究热液蚀变过程中矿物组合变化与元素迁移规律,对进一步探讨热液蚀变机制以及成矿机制具有重要意义(Laurich et al.,2014; 张炳林等,2014; 张志超等,2015)。

胶西北是我国最重要的金矿集区,以发育燕山期“胶东型”金矿床闻名于世(Deng and Wang,2016; 杨立强等,2014),按矿化样式的不同,该区金矿床又可划分为“焦家式”、“玲珑式”和“蓬家夼式”金矿床(Deng et al.,2015a)。新城金矿床即为胶东金成矿省中典型“焦家式”金矿床(Yang et al.,2016c),自被发现以来,随着新的成矿理论和勘查技术的应用(Yang and Badal,2013),已累计探明金资源量>200t,平均品位6.8g/t(Wang et al.,2015a)。新城金矿床热液蚀变发育,主要有(黄铁)绢英岩化、硅化、红化、碳酸盐化和粘土化等蚀变类型。其中,(黄铁)绢英岩化蚀变与金矿化时空关系密切,是研究热液蚀变在金成矿过程中作用的理想选区。前人对新城金矿床蚀变类型、矿化样式,以及蚀变-矿化水平分带特征等(马学东,2011; 王中亮,2012)、断裂构造与控矿机理(马学东,2011; 王中亮,2012; 杨立强等,2014)、流体演化与金沉淀机制(陆丽娜等,2011; 王中亮,2012; Wang et al.,2015a)等进行了研究并取得了诸多重要进展。但前人较少考虑断裂带结构对不同热液蚀变类型和金矿化样式空间分布的影响,对其蚀变、矿化机制缺乏系统探讨,这在一定程度上制约了矿床成因研究和找矿勘探部署。为此,本文在详实的野外地质工作基础上,精细研究了焦家断裂带结构,刻画了断裂构造分带及显微构造变形特征,厘定了其内成矿流体输运通道,阐述了蚀变-矿化特征,对其岩(矿)石进行了元素地球化学研究,讨论了蚀变机制与元素迁移规律,初步探讨了新城金矿床热液蚀变作用与金矿化的关系,以及断裂构造分带对蚀变和矿化的控制作用。

2 区域与矿床地质

新城金矿床位于胶东半岛内胶北隆起的西北部。胶北隆起变质基底主要由太古宇胶东群TTG、古元古界粉子山群和荆山群、以及新元古界蓬莱群变质沉积岩组成(图 1)(Deng et al.,2015b2011; Zhai and Santosh,2011)。区内出露大面积晚侏罗世-白垩纪岩浆岩,主要为玲珑型岩体、郭家岭型岩体和艾山型岩体等(Yang et al.,2016a; Deng et al.,2015b; Wang et al.,2015b; Song et al.,2014; Gong et al.,20132015)(图 1)。区内构造自西向东依次为三山岛断裂带、焦家断裂带、招平断裂带、栖霞断裂带、牟平-即墨断裂带和牟平-乳山断裂带(Wang et al.,2010ab),呈NE-NNE向展布,其与次级断裂共同控制了金矿床的分布(图 1)(Goldfarb and Santosh,2014; Yang et al.,2006; Deng et al.,20062008; 杨立强等,2014)。其中,焦家断裂带走向为NE-NNE,长约60km,宽约50~2000m,控制了带内20多个金矿床(Yang et al.,2016c),为新城金矿床的主要控矿断裂(图 1图 2)。

图 1 胶西北区域地质简图(据Wang et al.,2015a Fig. 1 Simplified geological map of the northwestern Jiaodong (modified after Wang et al.,2015a
图 2 新城金矿床地质简图(据Wang et al.,2015a修编) Fig. 2 Sketch geological map of the Xincheng gold deposit(modified after Wang et al.,2015a)

新城金矿床位于山东省莱州市北东方向35km处,是焦家金矿带内最大的金矿床之一。主断裂面沿新城花岗岩体内展布(刘跃等,2014),倾向NW,倾角一般为30°~50°,在成矿期表现为正断层活动(Deng et al.,2015b)。矿区新城花岗岩体主要为胶东群变质基底经部分熔融形成,侵位年龄为123~132Ma(刘跃等,2014; Wang et al.,2014)。矿区亦发育胶东群变粒岩、少量长英质脉岩等。变粒岩主要呈残留体分布在玲珑型花岗岩内,与岩体呈渐变接触,主要赋存在焦家断裂上盘(图 2)。长英质脉岩是矿区内少量发育的一种浅色脉岩,一般呈宽约5~20cm的脉状侵入到新城花岗岩体内。长英质脉岩为似伟晶结构,块状构造,主要矿物为钾长石(50%~60%),石英(40%~50%),矿物颗粒较大。长英质脉岩常被石英黄铁矿脉切穿,表明其形成应早于脉状黄铁矿化。

新城金矿床发育红化、(黄铁)绢英岩化、硅化、硫化、碳酸盐化、粘土化和绿泥石化等多种类型热液蚀变,严格受焦家断裂及其次级断裂控制。其中,红化作用主要表现为节理带新城花岗岩体被染红,部分学者认为是钾长石化(曹晖等,2013),而刘跃(2015)通过研究认为,其与斜长石中赋存细粒赤铁矿和鳞片状绢云母有关,可能是斜长石热液蚀变产物,在此过程中伴随热少量热液钾长石的形成。因而,红化作用可能代表了多种热液蚀变类型。其早于与金矿化有关的热液蚀变(王中亮,2012)。(黄铁)绢英岩化蚀变表现为新城花岗岩体中斜长石、钾长石和黑云母等矿物的部分或全部绢云母化,并伴随着细粒石英化(硅化)和黄铁矿化。硫化作用常叠加于绢英岩化蚀变之上,以细脉状-网脉状石英-黄铁矿脉和石英-硫化物脉形式产出。碳酸盐化蚀变以石英-碳酸盐脉的形式产出,并穿切早期硫化作用形成的各类矿脉。粘土化蚀变主要发育焦家断裂面及其下盘次级断裂面中。

新城金矿床主要发育两种类型的矿化样式,分别为微细浸染状矿化和细脉-网脉状矿化,均严格受焦家断裂及其次级断裂控制。主矿体均为微细浸染状和网脉状矿化的破碎带蚀变岩型矿体,赋存于焦家断裂下盘(黄铁)绢英岩蚀变带中,以强烈的黄铁绢英岩化蚀变为特征(图 2图 3),蚀变围岩与矿体呈渐变过渡。矿体主要赋存于焦家断裂带产状变化处或膨大部位。次要矿体主要为细脉-网脉状矿化的含金石英脉型矿体,赋存于焦家断裂下盘绢英岩中走向NE-NNE次级断层中,以及红化花岗岩体中走向NE-NNE呈雁列式展布的一系列次级构造内(图 3)。前人依据产状不同将其进一步划分为石英硫化物脉型矿体和细脉型矿体(王中亮,2012)。

图 3 新城金矿床断裂构造分带不同蚀变矿物的体积百分含量变化图据(张潮,2015修编) Fig. 3 Diagram of volume percentage changes of different altered minerals from fault zone structure in Xincheng gold deposit(modified after Zhang,2015)
3 断裂构造分带与蚀变-矿化特征

岩石所处温度和压力条件是影响岩石和矿物变形的重要因素。矿物在不同温-压条件下所受变形机制不同,一般会表现出不同的显微构造变形现象(Vernon,2004; 胡玲等,2009)。前人研究表明,焦家断裂下盘结构分带性明显,野外坑道观察可见,自焦家断裂面向外,在下盘依次发育断裂面和断层泥、挤压片理带、构造透镜体带、密集节理带和稀疏节理带(图 3)(邓军等,1996)。

3.1 断裂构造分带 3.1.1 主断裂面和断层泥

主断裂面内发育黑、白色断层泥(图 4a),其内赋存次棱角状-次圆状浸染状黄铁绢英岩角砾。断层泥发育挤压片理(图 3图 4b),可能是由碎裂流动沿正向或反向吕德尔剪切滑动形成的(胡玲等,2009)。断层泥X射线粉晶衍射物相测试表明,其成分主要为石英、伊利石、高岭石、白云石、方解石和黄铁矿等粘土化蚀变矿物;断层泥粒度分布曲线和分维值指示热液蚀变作用、构造运动强烈,有利于成矿流体输运和矿质沉淀(张潮,2015)。

图 4 新城金矿床断裂构造分带断层岩特征 (a)黑色断层泥;(b)主断裂面附近片理面;(c)露头尺度σ型碎斑;(d)长石书斜构造与石英消光带(+);(e、f)黄铁绢英岩质碎裂岩(图f+);(g、h)构造透镜体带绢英岩质碎裂岩(图 h+);(i、j)密集节理带碎裂红化花岗岩(图j+);(k、l)稀疏节理带碎裂红化花岗岩(图l+). Py-黄铁矿;Ser-绢云母;Q-石英;Kfs-钾长石;Pl-斜长石;Cal-方解石 (a)black faultgouge;(b)foliation near the main fault palne;(c)σ type porphyroclast in the outcrop;(d)bookshelf structure of potash feldspar;(e,f)cataclasite of pyrite-sericite-quartz alteration rock(Fig. 4f+ );(g,h)cataclasite of sericite-quartz alteration rock in the tectoniclens zone(Fig. 4h+);(i,j)cataclastic rock of red colouration granite in the dense joints zone(Fig. 4j+) ;(k,l)cataclastic rock of red colouration granite in the sparse joints zone(Fig. 4l+ ). Py-pyrite; Ser-sericite; Q-quartz; Kfs-potash feldspar; Pl-plagioclase; Cal-calcite Fig. 4 The characteristics of fault rocks from fault zone structure in Xincheng gold deposit
3.1.2 挤压片理带

紧邻焦家断裂面下盘发育挤压片理化带(图 3),片理产状344°∠41°,主要位于黄铁绢英岩内。其宽度一般在十米到几十米不等,岩石破碎强烈,主要为碎裂岩,发育黄铁绢英岩化蚀变,与金矿化密切相关。碎裂岩分布不连续,叠加于初糜棱岩上。前人研究表明,碎裂岩是成矿期脆性变形产物,叠加于成矿前韧性变形产物初糜棱岩之上,因而其错断了初糜棱岩的连续性(邓军等,1996)。靠近焦家断裂面主要为初糜棱岩,具有较为典型的宏观和微观韧性变形特征,宏观韧性变形特征表现为露头尺度发育σ型碎斑(图 4c)。矿物主要为石英、绢云母、钾长石和绿泥石等。初糜棱岩具有较为明显的糜棱结构,长石残斑粒径较大,多呈不规则状,常为眼球状或透镜体状,趋于定向排列;初糜棱岩基质含量较少,约占30%,石英动态重结晶新晶粒较多,形成了重结晶多晶条带;残斑中可见脆性破裂发育,如长石机械双晶及扭折、云母扭折、石英波状消光及消光带等显微构造(图 4d);石英亦常发育核幔构造。挤压片理带成矿前岩石矿物的变形现象组合指示其形成环境为高绿片岩相条件,温度约为400~500℃(胡玲等,2009)。成矿期碎裂岩是由原岩遭受较为强烈破碎后形成的一类动力变质岩石(胡玲等,2009),无明显定向构造(图 4e,f)。岩石普遍具有碎裂结构,碎斑一般较少且较小,部分岩石已破碎成为碎粒或碎粉,碎基含量较多,一般>50%,碎粒或碎粉粒径趋于均一,花岗结构基本难以辨认。矿物主要为重结晶石英,绢云母、黄铁矿等。碎裂岩内发育大量黄铁矿,黄铁矿亦呈破碎结构,金品位较高,构成了新城金矿床主矿体。

3.1.3 构造透镜体带

构造透镜体带受焦家断裂下盘SN、NNE和NE向次级断裂控制,受次级断裂产状影响,其发育规模亦有差异。构造透镜体带碎裂岩与挤压片理带呈断层接触(图 3)。其发育宽度一般在十几米到几十米不等,岩性主要为碎裂岩和初碎裂岩,可能指示该带变形环境介于高绿片岩相和低绿片岩相之间。碎裂岩具碎裂结构,呈透镜体状、角砾状,碎斑一般较少且较小,碎基含量较多,一般>50%,花岗结构基本难以辨认。其普遍发生绢英岩化,主要矿物为石英、绢云母和少量长石。沿次级断层面蚀变强度增大,常发育脉型黄铁绢英岩化蚀变,断层面内发育石英硫化物脉型矿化(图 4g)。初碎裂岩位于碎裂岩外侧,与其无明显界线,一般呈碎粒结构和碎斑结构,碎块呈残留碎斑状,且发生位移或有较大的转动,被周围碎基包围。碎基含量较碎裂岩内显著减少,占到20%左右,少于碎斑,碎斑粒径显著变小,较大者也仅0.2mm(图 4h)。碎斑中常发育碎裂和边缘化现象(图 4h)。初碎裂岩在一定程度上保留了花岗岩结构构造。该带岩石脆性变形常发育张性断层、裂隙,常充填石英脉等,并伴随绢云母化蚀变(图 4g,h),指示构造透镜体带内次级断层、裂隙体系为成矿流体提供输运通道。

3.1.4 密集节理带

密集节理带发育在构造透镜体带外侧,亦受焦家断裂下盘SN、NNE和NE向次级断裂控制。其宽度一般在二十米到几十米不等,如在-680m中段169线坑道,密集节理带宽约30m,发育三组节理,产状分别为125°∠55°、335°∠80°和320°∠48°,其中,产状为335°∠80°的这组近直立节理为主要控矿节理,节理面发育细脉状石英黄铁矿脉(图 4i)。部分区域上述节理密集发育,密度为5~10厘米/条。可见次级断层面发育脉型绢英岩化蚀变。密集节理带岩石主要为碎裂红化花岗岩,主要矿物为长石,石英和少量绢云母,基本保留了花岗岩结构构造。碎块主要为钾长石,呈残留碎斑状,发育晶内和晶间裂隙,且有较大位移,被黄铁矿、石英和绢云母等矿物充填(图 4j)。碎斑含量较多,粒径一般小于2mm,碎基含量较少。石英一般为单晶丝带构造,可见重结晶多晶条带,呈现一定的韧性变形特征(图 4j)。带内矿物的变形现象组合表明,其形成环境可能为低绿片岩相条件,温度约为300~400℃。上述野外和镜下显现表明,密集节理带次级断层、节理和显微张性裂隙组成了成矿流体输运的通道。

3.1.5 稀疏节理带

稀疏节理带发育在密集节理带外侧,和密集节理带相似,也受到焦家断裂下盘SN、NNE和NE向次级断裂控制。其宽度一般为几十米,如在-680m中段169线坑道,稀疏节理带宽度约为45m,主要控矿节理产状为316°∠76°,亦为近直立节理。整体特征与密集节理带相似,仅节理密度发育有差异,密度为15~20厘米/条(图 4k,l)。其形成环境可能亦为低绿片岩相条件,温度约为300~400℃。

3.2 蚀变-矿化特征

综上所述,新城金矿床焦家断裂构造分带呈现不同的热液蚀变类型和金矿化样式(表 1),表现为以水平为主的蚀变-矿化特征,其内不同蚀变矿物的蚀变强度和金品位详见图 3。主断裂面和断层泥热液蚀变、金矿化较为复杂,发育粘土化蚀变。黑色断层泥和白色断层泥金品位分别为0.15g/t和0.01g/t,不构成矿体(表 2)。

表 1 新城金矿床断裂构造分带与蚀变-矿化特征据(邓军等,2005修编) Table 1 he characteristics of fault zone structures,alterations and mineralizations in the Xincheng gold deposit(modified from Deng et al.,2005)

挤压片理带热液蚀变类型主要为绢英岩化和以黄铁矿为主的硫化作用(图 5a,b),(黄铁)绢英岩化蚀变宽度一般在十米至数十米不等(图 3)。下盘靠近主断裂面处发育两种蚀变的叠加,构成黄铁绢英岩型矿化,金品位较高,一般>1g/t(表 2),构成新城金矿床主矿体。黄铁绢英岩为新城金矿床最为重要的矿石类型,其呈灰绿色,发育压碎结构,浸染状-细脉状浸染状、网脉状构造(图 5b)。矿石矿物主要为黄铁矿,其次为黄铜矿、闪锌矿和方铅矿等(图 5c),黄铁矿为主要载金矿物(张潮等,2014),金矿物有银金矿、自然金和金银矿,以裂隙金、晶隙金和包体金等形式赋存(Yang et al.,2016c)。脉石矿物主要为石英、绢云母和少量长石等(图 5b)。

图 5 新城金矿床热液蚀变-矿化特征 (a)黄铁绢英岩型矿化;(b)黄铁绢英岩发育黄铁矿化、绢云母化和石英(+);(c)黄铁绢英岩中发育黄铁矿、黄铜矿、闪锌矿和方铅矿(反射光);(d)绢英岩及其中红化花岗岩碎块;(e)脉型绢英岩穿切红化花岗岩;(f)绢英岩中叠加方解石脉(+);(g)红化花岗岩中沿节理发育石英硫化物细脉型矿化;(h)花岗岩中沿节理发育红化;(i)方解石脉穿切石英硫化物脉(+);(j、k)红化花岗岩中斜长石发生钾长石化蚀变(+);(l)斜长石发生钾长石化化蚀变(BSE图像)(刘跃,2015). Ccp-黄铜矿;Sp-闪锌矿;Gn-方铅矿;Ab-钠长石;Mus-白云母;Chl-绿泥石 (a)pyrite-sericite-quartz mineralization;(b)pyrite,sericitization and quartz in the pyrite-sericite-quartz alteration rock(+);(c)pyrite,chalcopyrite,sphalerite,galena in the pyrite-sericite-quartz altered rock(reflected light);(d)sericite-quartz altered rock and red colouration granite breccia;(e)vein of sericite-quartz altered rock cut across the red colouration granite;(f)calcite veins overlop the sericite-quartz alteration rock(+);(g)vein of quartz-sulfide mineralization precipitated along the joint planes in the red colouration granite;(h)red colouration along the joint planes in the granite;(i)calcite vein cuts across quartz-sulfide vein(+);(j,k)K-feldspar alteration from plagioclase in the red colouration granite;(l)K-feldspar alteration from plagioclase(BSE image)(Liu,2015). Ccp-chalcopyrite; Sp-sphalerite; Gn-galena; Ab-albite; Mus-muscovite; Chl-chlorite Fig. 5 The characteristics of alterations and mineralizations in Xincheng gold deposit

构造透镜体带发育有(黄铁)绢英岩化、硅化、碳酸盐化等热液蚀变,残留有红化花岗岩碎块(图 5d,e)。黄铁绢英岩化蚀变紧靠次级断裂面发育,其蚀变特征及矿石矿物组合与上述挤压片理带黄铁绢英岩相似,仅热液蚀变宽度较小,一般在几十厘米至数米不等,金品位一般较高,与次级断层面内石英硫化物脉共同组成工业矿体。远离次级断裂面则发育绢英岩化和硅化蚀变,矿石矿物减少,脉石矿物中绢云母含量亦减少,金品位较低,约为0.056~0.78g/t,不构成工业矿体(表 2)。碳酸盐化蚀变以方解石脉形式产出,切穿成矿期绢英岩化蚀变(图 5f),指示其为成矿晚期或成矿后产物。绢英岩呈灰绿色,细粒变晶结构,致密块状构造,矿物组合为绢云母、石英和少量蚀变残余长石,可见黄铁矿发育,其含量少于5%(图 5f)。

表 2 新城金矿床不同蚀变岩主量(wt%)、微量元素和稀土元素(×10-6)组成 Table 2 Major(wt%),trace and rare earth(×10-6)elements of altered rocks in the Xincheng gold deposit

野外与镜下观察表明,密集节理带和稀疏节理带主要发育大面积面状和带状-网脉状红化(图 5g,h),其上叠加石英硫化物脉和碳酸盐脉等(图 5g,i)。前已述及,红化可能与金矿化并无直接成因关系。石英黄铁矿脉沿红化花岗岩张性裂隙充填(图 5g),并被石英-方解石脉穿切(图 5i)。节理带部分地段发育次级断层,沿断层面发育脉型(黄铁)绢英岩化蚀变等,一般规模较小,宽度一般仅数十厘米。节理带矿化样式主要为节理面中细脉型矿化,石英黄铁矿脉较窄,一般为厘米级(图 5g),金品位很高,一般为2~5g/t,构成工业矿体。而红化花岗岩中金含量很低,与新鲜新城花岗岩金含量相当(表 2),两者呈逐渐过渡的接触关系。红化花岗岩呈肉红色,花岗结构,块状构造,矿物组合为钾长石、斜长石和石英等,可见长石已部分蚀变形成绢云母和细粒石英(图 5i)。

4 样品采集与分析方法

基于新城金矿床断裂构造分带与蚀变-矿化特征,在详细的野外调研基础上,系统采集了具有代表性的19件岩(矿)石样品。样品采自不同中段不同巷道,包括1件黑色断层泥样品、1件白色断层泥样品、5件黄铁绢英岩样品、6件绢英岩样品和4件红化花岗岩样品。在远离蚀变、矿化区域采集2件新鲜花岗岩样品。

全岩粉末样在河北省廊坊市地源矿物测试分选技术服务有限公司进行处理,粉碎至200目。全岩主量、微量元素分析均在核工业北京地质研究院分析测试中心完成。其中,主量元素分析在飞利浦PW2404 X射线荧光光谱仪上完成,分析精度优于1%;稀土与微量元素分析在高分辨率等离子体质谱仪(HR-ICP-MS,Finnigan MAT制造,ElementⅠ)上完成,分析精度优于5%;Au、Ag、As和Ag在ASF2202原子荧光光度计和Z-2000石墨炉原子吸收分析仪上完成。电感耦合等离子体质谱分析及其元素浓度标定见刘颖等(1996)。测试结果见表 2-表 4图 6-图 8

表 3 新城金矿床蚀变过程中主量(wt%)、微量元素(×10-6)平均得失量 Table 3 Average gain or loss of major(wt%)and trace(×10-6)elements during different alteration processes in the Xincheng gold deposit
表 4 新城金矿床与不同蚀变岩稀土、微量元素分析结果(×10-6)及部分参数值 Table 4 The data analysis(×10-6)and related calculated parameters of rare earth and trace elements of altered rocks in the Xincheng gold deposit
图 6 新城金矿床岩(矿)石主量元素组成变化图 Fig. 6 The diagrams showing the changes of major elements of gold ores and altered rocks in Xincheng gold deposit
图 7 不同蚀变岩与新城花岗岩球粒陨石标准化稀土元素配分图(a)和原始地幔标准化蛛微量元素网图(b)(标准化值据Sun and McDonough,1989) Fig. 7 Chondrite-normalized REE patterns(a)and primitive mantle-normalized trace element spider diagrams(b)of altered rocks and granodiorite in Xincheng gold deposit(normalization values after Sun and McDonough,1989)
图 8 新城金矿床岩/矿石成矿元素含量变化图 Fig. 8 The diagrams showing the changes of ore-forming elements of gold ores and altered rocks in Xincheng gold deposit
5 热液蚀变过程的地球化学特征

流体-岩石反应将引起介质物化条件改变,使得岩石原来的矿物组合失稳,为与环境达到新的平衡,早期矿物组合将趋于形成新的相对稳定矿物组合,并伴随元素迁移(Verma et al.,2005)。目前主要通过质量平衡法研究元素迁移规律,该方法基于Gresens方程,后经过简化推导出以不活动元素为基准的质量变化表达式(张炳林等,2014),前人研究表明主量元素Al和Ti通常表现为不活动元素(Condie and Sinha,1996)。Gong et al.(2016)通过坪台法进一步研究表明Al和Ti在高温高压模拟实验中亦表现为不活动元素。而Al在变形作用(唐红峰等,2000)和长石绢云母化(O’Hara and Blackburn,1989)过程中会表现出一定的活动性。前已述及,新城金矿床挤压片理带和构造透镜体带变形作用较强,且长石绢云母化亦很强烈。因而,Al不适于作为研究新城金矿床质量迁移的不活动元素。Ti在岩石变质变形中活动性极小,在流体-岩石反应中亦相当稳定(张可清和杨勇,2002),因而,本文选择TiO2作为不活动组分,通过运用ΔCi=CiA/k-Cia(Cia、CiA分别为原岩、蚀变岩中第i中元素含量;k=CA/Ca,两者分别为原岩和蚀变岩中不活动元素的质量)计算新城金矿床红化热液蚀变和(黄铁)绢英岩化热液蚀变过程中主量元素和成矿元素得失变化情况(图 9图 10)。碳酸盐化蚀变为成矿晚期或成矿后产物,与金成矿无关,且常叠加于其它热液蚀变之上;硅化蚀变虽与金矿化有关,但其常伴随绢云母化蚀变,形成绢英岩化蚀变。因而,无法挑选出纯净蚀变岩样品。本文将不对碳酸盐化、硅化蚀变进行质量平衡计算。

图 9 新城金矿床不同蚀变过程中主量元素得失图 Fig. 9 Gain-loss diagram for major oxides in the different alteration processes in Xincheng gold deposit
图 10 新城金矿床不同蚀变过程中成矿元素得失图 Au、Ag、As单位为×10-9;由新城花岗岩蚀变为红化花岗岩,As得失量为-0.64 Fig. 10 Gain-loss diagram for ore-forming elements in the different alteration processes in Xincheng gold deposit

新城金矿床红化蚀变、绢英岩化蚀变和黄铁绢英岩化蚀变过程元素迁移计算及对比研究(表 3表 4)表明:从新鲜新城花岗岩体→红化花岗岩蚀变过程中,SiO2、CaO、Na2O明显迁出,K2O、Al2O3、Fe2O3迁入,Au、Ag、Cu、Pb、Zn、Sb、Bi等成矿元素少量富集。由红化花岗岩→绢英岩蚀变过程中,SiO2、Fe2O3、Al2O3、CaO、FeO、MgO、K2O迁入,Na2O迁出,Au、Ag、As、Cu、Zn、Sb、Pb、Bi等微量元素迁入。而由红化花岗岩→黄铁绢英岩蚀变过程中,SiO2、Fe2O3、FeO、MgO、Al2O3、K2O迁入,Na2O迁出,Au、Ag、As、Cu、Pb、Zn、Sb、Bi等微量元素迁入富集。图 6-图 8表明,由红化花岗岩→黃铁绢英岩蚀变过程中SiO2、Fe2O3、Al2O3、Au、Ag、As、Cu、Pb、Zn、Sb、Bi迁入量显著大于由红化花岗岩→绢英岩蚀变过程。

6 讨论 6.1 热液蚀变作用机制

新城金矿床成矿前红化蚀变普遍发育,可分为两种类型:明显受焦家主断裂控制,呈面状分布,表现为大面积岩石被染红;受次级断裂控制,沿次级断层、节理或岩石微裂隙呈带状或网脉状分布(刘跃,2015)。其可能由新城花岗岩体中钾长石、斜长石等发生绢云母化蚀变,生成了一些细粒赤铁矿等(Yang et al.,2016待刊),此外,花岗岩中钾长石和斜长石呈现弱绢云母化,也使得岩石呈现红色,可能不单单是前人所认为的钾长石化蚀变(曹晖等,2013)。在此过程中,流体处于碱性、氧化环境(王中亮,2012),有少量热液钾长石的形成(刘跃,2015),钾长石化可能是邓军等(2010)提出的斜长石的钾长石化(图 5j-l)和钾长石加大现象,这已被王玉荣和胡受奚(2000)钾交代成矿模拟实验所证实(方程式1、2)。前人研究表明,新城金矿床成矿前发生韧-脆性左行剪切变形(李瑞红等,2014),该变形作用使得早先应变较弱时分散颗粒尺度的流体不断彼此相连形成与主剪切面近乎平行的流体运移网络(高帮飞,2008),可能为成矿前焦家断裂下盘新城花岗岩体发生大范围红化热液蚀变流体提供运移通道。

$\text{Na}{{\left[ \text{AlS}{{\text{i}}_{3}}{{\text{O}}_{8}} \right]}_{\left( 钾长石 \right)}}\text{+KCl}\to {{\left[ KAIS{{\text{i}}_{3}}{{\text{O}}_{8}} \right]}_{\left( 钾长石 \right)}}+NaCl$ (1)
$\begin{align} &\text{3Ca }\!\![\!\!\text{ A}{{\text{l}}_{2}}\text{Si}{{\text{O}}_{8}}{{\text{ }\!\!]\!\!\text{ }}_{\left( 钙长石 \right)}}+2KCl\to 2K{{\left[ \text{A}{{\text{l}}_{3}}\text{Si}{{\text{O}}_{8}} \right]}_{\left( 钙长石 \right)}} \\ &+2\text{A}{{\text{l}}_{2}}{{\text{O}}_{3}}+2CaO+CaC{{l}_{2}} \\ \end{align}$ (2)

新城金矿床成矿期(黄铁)绢英岩化热液蚀变是对早期红化花岗岩的改造,在此过程中,花岗岩发生压剪脆性变形(李瑞红等,2014)。流体包裹体均一法显微测温结果为171~304℃(Wang et al.,2015a),与成矿期热液蚀变矿物组合指示的温度相一致,表明新城金矿床为中低温热液金矿床。其成矿热液处于酸性环境(Wang et al.,2015a),沿挤压片理带片理面、构造透镜体带次级断裂、节理带共轭节理等进行运移(张潮,2015)。红化花岗岩中斜长石、钾长石和黑云母等矿物在含H+、HS-溶液中失稳(Parsapoor et al.,2009),从而发生黄铁绢英岩化交代作用形成黄铁矿、绢云母和石英等新生矿物(胡受奚等,2004)。其发生流体-岩石反应(方程式3、4、5和6);而局部常见的方解石呈脉状或块状叠加于(黄铁)绢英岩之上,则可能是发生了反应(方程式7)(祁冬梅等,2015),从而形成碳酸盐化热液蚀变。这与上文野外调查与镜下观察相一致,如岩石结构变为碎裂结构,绢云母交代长石而呈鳞片状结构等。斜长石和钾长石斑晶最终被黄铁矿、绢云母和石英等矿物替代,形成了新城金矿床典型矿石矿物组合特征。从而形成了挤压片理带大范围黄铁绢英岩化蚀变、构造透镜体带脉型(黄铁)绢英岩化蚀变和节理带沿断层面发育的细脉型(黄铁)绢英岩化蚀变。在酸性条件下,反应产生的SiO2,达到饱和程度后则沉淀出新城金矿床常见的石英脉或与围岩反应形成硅酸盐矿物。

$\begin{align} &Na\text{ }\!\![\!\!\text{ A}{{\text{l}}_{2}}\text{Si}{{\text{O}}_{8}}\text{ }\!\!]\!\!\text{ }\cdot \text{Ca }\!\![\!\!\text{ A}{{\text{l}}_{2}}\text{S}{{\text{i}}_{2}}{{\text{O}}_{8}}{{\text{ }\!\!]\!\!\text{ }}_{\left( 斜长石 \right)}}+HCl+KCl\to \\ &K\text{A}{{\text{l}}_{2}}\text{ }\!\![\!\!\text{ A}{{\text{l}}_{2}}\text{Si}{{\text{O}}_{10}}\text{ }\!\!]\!\!\text{ }{{\left( OH \right)}_{2\left( 绢云母 \right)}}2\text{Si}{{\text{O}}_{2}}+NaC{{l}_{2}} \\ \end{align}$ (3)
$\begin{align} &\text{3K }\!\![\!\!\text{ A}{{\text{l}}_{2}}\text{S}{{\text{i}}_{3}}{{\text{O}}_{8}}{{\text{ }\!\!]\!\!\text{ }}_{\left( 钾长石 \right)}}+2HCl\to 2K\text{A}{{\text{l}}_{2}}\text{ }\!\![\!\!\text{ AlS}{{\text{i}}_{3}}{{\text{O}}_{10}}\text{ }\!\!]\!\!\text{ }{{\left( OH \right)}_{2\left( 绢云母 \right)}} \\ &+6\text{Si}{{\text{O}}_{2}}+2KCl \\ \end{align}$ (4)
$\begin{align} &3K{{\left( Mg,Fe \right)}_{3}}\left( \text{AlS}{{\text{i}}_{3}}{{\text{O}}_{10}} \right){{\left( OH \right)}_{2\left( 黑云母 \right)}}+8{{H}_{2}}O\to \\ &K\text{A}{{\text{l}}_{3}}\text{AlS}{{\text{i}}_{3}}{{\text{O}}_{10}}{{\left( OH \right)}_{2\left( 绢云母 \right)}}+9\left( M{{g}^{2+}},F{{e}^{2+}} \right) \\ &+2{{K}^{2+}}+6\text{Si}{{\text{O}}_{2}}+2O{{H}^{-}} \\ \end{align}$ (5)
$F{{e}^{2+}}+{{\left( {{\text{S}}_{2}} \right)}^{2-}}\to Fe{{\left[ {{\text{S}}_{2}} \right]}_{\left( 黄铁矿 \right)}}$ (6)
$\begin{align} &{{\left( 6Na\left[ \text{AlS}{{\text{i}}_{3}}{{\text{O}}_{8}} \right]+\text{3Ca }\!\![\!\!\text{ A}{{\text{l}}_{2}}\text{S}{{\text{i}}_{2}}{{\text{O}}_{8}}\text{ }\!\!]\!\!\text{ } \right)}_{\left( AAAAA \right)}}2+{{K}_{2}}C{{O}_{3}}+ \\ &4{{H}_{2}}O+4C{{O}_{2}}\to 4K\text{A}{{\text{l}}_{2}}\text{ }\!\![\!\!\text{ A}{{\text{l}}_{2}}\text{S}{{\text{i}}_{3}}{{\text{O}}_{10}}\text{ }\!\!]\!\!\text{ }{{\left( OH \right)}_{2\left( AAAAA \right)}}+ \\ &\text{3Na}C{{O}_{3}}+\text{3Ca}{{\left[ C{{O}_{3}} \right]}_{\left( AAAAA \right)}}+12Si{{O}_{2}} \\ \end{align}$ (7)
6.2 热液蚀变过程中的元素迁移

稀土元素通常被认为是稳定的、性质相似的元素组合(Yang et al.,2015),而目前已有较多研究表明其在花岗质岩石热液蚀变过程中是活动的(Parsapoor et al.,2009; Nishimoto and Yoshida,2010),且在不同类型热液蚀变中REE表现特征有所差异(Taylor and Fryer,1983; Parsapoor et al.,2009)。REE活动习性受控于原岩类型、蚀变矿物和热液流体REE含量(Michard,1989),以及热液流体性质等(Haas et al.,1995)。对新城金矿床不同蚀变岩和新鲜岩体样品投稀土元素分布型式图(图 7a),并计算了一些稀土元素参数(表 4),可见蚀变岩与新鲜岩体REE元素含量、稀土元素配分模式图拟合度有一定差异,表明热液蚀变对REE迁移有影响。红化过程中,REE含量增加表明该过程伴随着稀土元素的加入;而其在(黄铁)绢英岩化蚀变中则伴随着流失(表 4)。红化花岗岩和原岩均表现为Eu正异常(δEu>1),且前者Eu正异常高于后者;而黄铁绢英岩和绢英岩则表现为一定的Eu负异常(δEu:0.89~0.95)(表 4)。这表明Eu在红化热液蚀变中显著迁入,而在(黄铁)绢英岩化蚀变中活化迁出。红化花岗岩Eu正异常明显,与大量出现的蚀变矿物钾长石有密切关系(Alderton et al.,1980)。除钾长石外,斜长石也为Eu的重要来源(Budzinski and Tischendorf,1989),(黄铁)绢英岩Eu负异常可能与蚀变过程中钾长石、斜长石的蚀变有关(Alderton et al.,1980)。同时,REE均显示稳定的+3价态,而Eu为+2价态。Eu以何种价态稳定地存在于体系中,则受到多方面的影响,如体系的温度、fO2和pH等(如Sverjensky,1984; Michard,1989)。如绢云母形成时成矿流体温度<250℃(Genna et al.,2014),而Eu2+在熔体中稳定存在需要温度>250℃(Sverjensky,1984),此时新城金矿床成矿环境处于还原条件(张潮等,2014),Eu易从氧化态Eu3+转变为Eu2+,并以Eu2+形式进入流体被带走,从而造成Eu负异常。

图 6可见,节理带红化花岗岩K2O、Al2O3含量高于新鲜新城花岗岩体,而SiO2、CaO、Na2O等含量低于后者,这表明成矿前焦家断裂带红化热液蚀变过程中,发生了流体-岩石反应方程式1和2。这也与对新城花岗岩体发生红化热液蚀变进行的质量平衡计算结果相一致(表 3图 9)。在此过程中,K交代了Na被大量带入,Na、CaO被带出。而测试结果和质量平衡计算结果均显示红化花岗岩中Al含量增加而被带入,这指示钾长石可能也来源于热液,而不全是交代作用的结果(高帮飞,2008)。SiO2的带出可能指示碱性氧化流体将硅活化带出,形成了富硅流体。红化花岗岩中Fe含量略微升高可能指示角闪石或黑云母少量分解。Rb含量相对原岩含量增加,Ba、Sr含量降低(表 4图 7b)。这可能是由于Rb的载体矿物主要是钾长石(高帮飞,2008),K多与Ba发生类质同象(刘英俊和曹励明,1987),而Sr可能被活化出来进入到溶液里。成矿期挤压片理带和构造透镜体带红化花岗岩发生(黄铁)绢英岩化热液蚀变(方程式3、4、5和6)。表 3图 6图 9表明,在此过程中Fe2O3、FeO、SiO2、MgO被带入,方程式5、6指示Fe的带入与暗色矿物分解和黄铁矿化有关;方程式3、4、5、7表明,长石等矿物在蚀变中发生分解,形成绢云母和SiO2,但SiO2并没有被流体带走,其可能以石英形式产出或与围岩反应形成硅酸盐矿物,造成SiO2富集。MgO的带入则可能与黄铁绢英岩化蚀变过程中形成含Mg绿泥石有关(祁冬梅等,2015)。Na的持续带出应与蚀变过程中方程式3、7发生有关;方程式3、4、5、7表明,K既可能带入、也可能带出,(黄铁)绢英岩化蚀变过程中其带入指示方程式3居于主导地位,而成矿后期碳酸盐化蚀变可能也带入了部分K(方程式7)。野外调研和镜下观察亦表明,绢云母化和硅化是伴生的。

6.3 热液蚀变与金矿化关系

新城金矿床矿化类型为黄铁绢英岩型矿化、石英硫化物脉型矿化和细脉型矿化三种,矿石矿物组合主要为黄铁矿、黄铜矿、闪锌矿和方铅矿等(Wang et al.,2015a; 张潮等,2014)。成矿流体主体为中低温、低盐度的含CO2-H2O-NaCl变质流体(Wang et al.,2015a),考虑到Cr、Co、Ni为幔源元素,Mo为指示深源的元素,Th、V、U、W和Mn等为亲石元素(韩吟文和马振东,2003)。同时,前人通过对金矿床黄铁矿La-ICP-MS微区原位微量元素分析研究表明,与Au矿化密切的微量元素主要为Ag、As、Cu、Pb、Zn、Sb、Bi等元素(Zhang et al.,2014; Yang et al.,2016c)。因此,选取Au、Ag、As、Cu、Pb、Zn、Sb和Bi元素作为研究对象,以分析热液蚀变对成矿元素迁移的影响,进而探讨热液蚀变与金矿化的关系。由图 8表明,由黄铁绢英岩到新城花岗岩体Au、Ag、As、Cu、Pb、Zn、Sb、Bi元素含量变化曲线形态基本一致,整体呈下降趋势。红化花岗岩中Ag、Cu、Pb、Sb含量升高,而其含量在新城花岗岩体中又降低。相对新鲜花岗岩,红化花岗岩中Au、Ag、Sb和Bi元素含量与之基本相当,仅表现为极少量带入,Cu、Pb、Zn元素表现出一定量带入(表 3图 10)。这也与野外和镜下观察一致,表明红化热液蚀变与金成矿无直接关系。新城金矿床矿石矿物组合特征表明,其成矿流体为含硫量较高的酸性流体(Parsapoor et al.,2009),成矿期为中低温、偏酸性、较高氧逸度和硫逸度条件(张潮等,2014),使得(黄铁)绢英岩化蚀变(方程式3、4、5、7)产生的SiO2以石英或硅酸盐矿物形式沉淀,产生的Fe2+则(方程式6)则以黄铁矿形式沉淀,导致流体还原硫活度降低(李楠,2013),从而使以Au(HS)2-络合物形式存在的Au沉淀(Wang et al.,2015a)。同时,随着成矿流体与围岩反应,流体中K+和H+逐渐减少,CO2逐渐增加也进一步降低了Au的溶解度(Gao and Kwak,1997),导致其沉淀。因此,(黄铁)绢英岩化蚀变使得Au、Ag等成矿元素大量带入而富集成矿。通过对比不同蚀变岩蚀变强度发现,蚀变强度高,Fe含量和金含量也高(图 4f,h图 8图 10)。这可能因为Au、Ag不仅形成银金矿,以包体形式赋存在黄铁矿内;Au还以固溶体或黄铁矿晶格金的形式存在(Li et al.,2014; Zhang et al.,2014; Yang et al.,2016c)。在(黄铁)绢英岩化蚀变过程中发生硫化作用,Cu、Pb、Zn等元素以硫化物矿物形式沉淀(王中亮,2012);As以类质同象形式进入黄铁矿内,Bi可能以固溶体形式存在于方铅矿包体中,Sb以辉锑银矿(AgSbS2)形式存在(Yang et al.,2016c)。

6.4 断裂构造分带对蚀变-矿化的控制作用

金成矿作用过程包括金从源区活化、在成矿流体携带下沿输运通道运移到合适位置,在成矿物理化学条件发生改变时,从流体中沉淀并富集成工业矿体。新城金矿床硫同位素研究,矿石硫可能主要源于新城花岗岩等中生代岩体,最初主要来源于胶东群变质基底(邓军等,1996; 杨立强等,2014; 王中亮,2012; 张潮等,2014)。当玲珑型黑云母花岗岩和新城花岗岩先后定位之后(图 11a),在NNW-SSE区域构造应力作用下,沿着玲珑型黑云母花岗岩与新城花岗岩接触带形成了NE向断裂带,下盘新城花岗岩体遭受挤压变形形成挤压片理带,发育初糜棱岩化,并遭受了大规模红化热液蚀变,少量带入Au、Ag、Cu、Pb、Zn等元素(图 11b)。在此过程中,深部温度-压力条件高、使金的化学位升高,脱离其原来在花岗岩岩石(矿物)中的赋存位置,从而使金发生活化。金活化后,被成矿期中温、富CO2和低盐度的变质成矿流体体系携带(Yang et al.,2016c),并以硫氢络合物的形式迁移(Wang et al.,2015a)。

图 11 新城金矿床断裂带构造控矿模式示意图(据张潮,2015修编) (a)玲珑型母花岗岩侵位时间160~150Ma(郭敬辉等,2005),新城花岗岩侵位时间123~132Ma(刘跃等,2014; Wang et al.,2014);(b)NE向断裂带、挤压片理带和红化作用发生时间可能介于119.9~123Ma;(c)成矿事件发生时间可能介于120.9~119.9Ma(Yang et al.,2016c) (a)emplacement time of Linglong granite and Xincheng granite are 160~150Ma(Guo et al.,2005)and 123~132Ma(Liu et al.,2014; Wang et al.,2014);(b)occurrence time of NE fault zone,fault gouge zone and red colouration process may be between 119.9~123Ma;(c)occurrence time of gold metallogenic event may be between 120.9~119.9Ma(Yang et al.,2016c) Fig. 11 Sketch map of ore-controlling model of structures in fault zone in Xincheng gold deposit(modified after Zhang,2015)

深成挤压片理带形成后随胶北隆起而被抬升并不断遭受剥蚀(Yang et al.,2016d),而其深部挤压变形作用仍在继续,先期形成的较深层次初糜棱岩在上升过程中开始叠加脆性变形,并且导致温度和压力条件的改变,挤压片理带先期形成的片理和微裂隙开始扩容。此时在深部形成的含金变质流体在构造压力驱动等因素驱动下开始迁移。前人研究表明,断裂构造分带及其内部发育的各类热液蚀变岩、脉体、金矿体等均为热液活动的直接或间接证据(高帮飞,2008; 王中亮,2012; 张闯,2013)。在断裂构造分带内,成矿流体垂向上沿焦家断裂带输运,水平上垂直于焦家断裂面沿下盘挤压片理带片理面、构造透镜体带和节理带断裂/裂隙系统侧向运移(张潮,2015)。挤压片理带由于成矿前构造差应力作用,岩石片理面和微裂隙异常发育,矿物颗粒更加细粒化,出现位错滑移、晶界滑移和颗粒边界滑移等韧性变形(李瑞红等,2014),有利于成矿热液的运移并在已发生脆-韧性和脆性变形的片理面和微裂隙内发生全岩交代蚀变形成了黄铁绢英岩。硫化作用使溶液中Au(HS)2-络合物失去稳定性(李楠,2013),导致Au元素沉淀并赋存于黄铁矿和石英等矿物裂隙或晶格中。表现为红化花岗岩蚀变为黄铁绢英岩时,Au、Ag、As、Cu、Pb、Zn、Sb、Bi等元素大量带入。在构造透镜体带,成矿流体沿次级断裂进入碎裂岩时,沿着次级断裂面和碎裂岩裂隙发生蚀变形成脉型(黄铁)绢英岩和碎裂绢英岩,在次级断裂面/裂隙内沉淀并充填形成石英硫化物脉型矿化。在节理带,成矿流体沿脆性断裂/裂隙系统运移并沉淀,断裂/裂隙系统会伴随先期沉淀的石英硫化物脉沉淀而闭合,随着流体聚集其压力进一步增大从而诱使围岩发生破裂而发生水力致裂现象,流体压力瞬时降低导致流体发生不混溶现象(Wang et al.,2015a),Au(HS)2-络合物失去稳定性,Au等成矿元素随着硫化物矿物沉淀充填到新形成的裂隙中,形成节理带细脉型矿化。综上所述,成矿流体在断裂构造分带内发生的物理化学作用使得成矿流体物理化学条件(成矿压力、成矿温度、氧逸度等)发生改变(张潮,2015),热液流体中携带金的络合物随即遭受破坏,导致包括金在内的成矿元素从成矿流体中析出,并在挤压片理带,构造透镜体和节理带合适部位分别形成黄铁绢英岩型矿化,石英硫化物脉型矿化和细脉型矿化(图 11c)。

7 结论

(1) 新城金矿床焦家断裂下盘结构分带性明显,自焦家断裂向外依次发育主断裂面和断层泥、挤压片理带、构造透镜体带、密集节理带和稀疏节理带。其分别发育粘土化蚀变;成矿期黄铁绢英岩化蚀变,赋存黄铁绢英岩型矿体;成矿期脉型(黄铁)绢英岩化,面状绢英岩化和硅化蚀变,次级断裂内赋存石英硫化物脉型矿体;成矿前面状、带状-网脉状红化和成矿期细脉型(黄铁)绢英岩化,沿节理面赋存细脉型矿体,表现为以水平为主的蚀变-矿化分带。成矿晚期或成矿后碳酸盐化蚀变常叠加其上。(黄铁)绢英岩化蚀变与金成矿最为密切。

(2) 新城花岗岩体发生红化蚀变过程中,带入组分有Fe2O3、K2O、Al2O3以及少量Au、Ag、Cu、Pb、Zn、Sb和Bi等,带出组分有SiO2、CaO和Na2O等;红化花岗岩蚀变为(黄铁)绢英岩过程中,明显带入组分有SiO2、Fe2O3、FeO、Al2O3、MgO、K2O、Au、Ag、As、Cu、Pb、Zn、Sb和Bi等,被带出组分为Na2O。

(3) REE分别在红化和(黄铁)绢英岩化蚀变中带入和带出;Eu在红化热液蚀变中表现为Eu正异常(δEu=1.34),而在(黄铁)绢英岩化蚀变中表现为Eu负异常(0.89~0.95)。(黄铁)绢英岩化蚀变过程中,Eu从氧化态Eu3+转变为Eu2+,进入流体被带走,造成Eu负异常。

(4) 金主要以Au(HS)2-形式在变质流体中运移。在构造应力等因素驱动下,成矿流体在垂向上沿焦家断裂带输运,水平上垂直于焦家断裂面沿下盘挤压片理带片理面、构造透镜体带和节理带断裂/裂隙系统侧向运移。成矿流体沿挤压片理带片理面输运时发生黄铁绢英岩化蚀变,硫化作用导致Au(HS)2-络合物失稳沉淀形成黄铁绢英岩型矿化;成矿流体沿构造透镜体带次级断裂面和碎裂岩裂隙发生蚀变形成脉型(黄铁)绢英岩,矿质在次级断裂面/裂隙内沉淀并形成石英硫化物脉型矿化;在节理带,成矿流体压力瞬时降低导致流体发生不混溶现象,使得Au(HS)2-络合物失稳沉淀并充填节理中形成细脉型矿化。

致谢 研究工作得到了中国地质大学(北京)邓军教授和杨立强教授的指导和帮助;王中亮讲师为论文的完善提出了宝贵的意见;野外工作得到山东黄金矿业股份有限公司新城金矿有关领导与相关技术人员的大力支持及帮助;全岩主微量测试工作得到了核工业北京地质研究院分析测试研究中心支持;博士生李瑞红、张良和郭林楠参与了部分工作;两位审稿人提出了宝贵的修改建议;在此致以诚挚的感谢!
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