胶东三山岛金矿床黄铁矿原位微区微量元素特征及对矿床成因的指示
In-situ Trace Element Analysis Characteristics of Pyrite in Sanshandao Gold Deposit in Jiaodong Peninsula: Implications for Ore Genesis
通讯作者: 赵新福(1982-),男,江苏镇江人,教授,主要从事经济地质研究. E-mail:xfzhao@cug.edu.cn
收稿日期: 2018-12-12 修回日期: 2019-02-10 网络出版日期: 2019-05-22
基金资助: |
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Corresponding authors: Zhao Xinfu(1982-), male, Zhenjiang City, Jiangsu Province, Professor. Research areas include economic geology.E-mail:xfzhao@cug.edu.cn
Received: 2018-12-12 Revised: 2019-02-10 Online: 2019-05-22
作者简介 About authors
胶东地区是我国最大金成矿聚集区,其金矿床的成因长期以来一直存在很大争议,三山岛金矿床是胶东地区最大的金矿床,通过采用LA-ICP-MS分析不同阶段黄铁矿中微量元素组成,可以探讨成矿流体演化及成矿物质来源。根据野外地质特征及岩相学观察,结合SEM结构分析将三山岛金矿床的黄铁矿分为3个阶段,6个亚类,即黄铁绢英岩化带(Py1)中包裹大量绢云母和石英的Py1-a和表面光滑的Py1-b,石英—黄铁矿±菱铁矿脉(Py2)中富含矿物包裹体的Py2-a和与菱铁矿共生且表面光滑的Py2-b,石英—多金属硫化物脉(Py3)中有很多细粒多金属硫化物包裹体的Py3-a和表面光滑的Py3-b。3个阶段黄铁矿晶格中金含量均很低,大部分小于1×10-6,金主要以可见金形式存在。从早阶段到晚阶段黄铁矿中Au与Ag,Cu,Pb,Sb有较好的正相关性,且含量有逐渐增加的趋势。最早阶段黄铁矿中Co+Ni的含量很高(最高为9 268×10-6),反映了早期黄铁矿可能来源于岩浆岩源区,后期Co/Ni值逐渐降低,暗示了成矿流体温度逐渐降低。结合地质特征和黄铁矿微量元素研究,表明三山岛金矿床成矿物质可能来源于深部岩浆热液储库,通过地震泵机制沿断裂构造多次侵位成矿。
关键词:
There are controversies about the genesis of the lode gold deposits in Jiaodong area. The Sanshandao gold deposit is the largest one in Jiaodong area. To better understand the genesis of the gold deposit, it is necessary to know the evolution of ore-forming fluid and the source of ore-forming material for the Sanshandao gold deposit. In this paper, the trace element composition of pyrite at different metallogenic stages in the Sanshandao gold deposit was analyzed by using LA-ICP-MS. Based on geological characteristics and petrography observation and SEM analysis, pyrite can be divided into three stages and six classes, including porous Py1-a and smooth Py1-b in pyrite-sericite-quartz alteration (Py1), many mineral inclusions in the Py2-a and smooth Py2-b associated with carbonate in the quartz-pyrite ± siderite veins (Py2), a lot of polymetallic sulfide inclusions hosted Py3-a and Py3-b with smooth and oscillation band in the quartz-polymetallic sulfide vein (Py3). The experimental results show that the gold content is very low in the lattice of pyrite, most of which is less than 1×10-6, and gold mainly occurs in the form of visible gold. The contents of Au, Ag, Cu, Pb and Sb of pyrite gradually increase from early stage to late stage. The high content of Co+Ni in the earliest stage (up to 9268×10-6) reflects that pyrite in the early stage is likely derived from magma, and the Co/Ni ratio gradually decreases, suggesting the decrease of ore forming fluid temperature. Combined with the geological observations and trace elements component of pyrite, the study shows that the genesis of the Sanshandao gold deposit is related to the evolution of magmatic hydrothermal reservoir. The ore-forming fluid is transported several times along the fault by seismic pumping.
Keywords:
本文引用格式
林祖苇, 赵新福, 熊乐, 朱照先.
Lin Zuwei, Zhao Xinfu, Xiong Le, Zhu Zhaoxian.
胶东地区金矿床黄金储量超过4 000 t,其黄金储量占中国黄金总储量的1/4以上[1]。关于胶东地区金矿床的成因类型划分,一直存在很大的争议[2,3]。一种观点认为成矿流体和成矿物质来源于古太平洋板块及上覆沉积物在俯冲过程中的脱挥发分,为一种特殊的造山型金矿[4,5,6,7];另一种观点认为华北克拉通周缘的金矿床形成于华北克拉通破坏的峰期,富集的岩石圈地幔熔融产生的熔/流体与地壳的相互作用造成金元素短期巨量聚集成矿,构造体制转变提供了动力学基础[8,9,10];还有一种观点认为在燕山期长期强烈构造—岩浆活动下,新太古代基底活化富集成矿[1,11];此外还有学者强调了胶东地区金矿床是一种与岩浆热液有关的中低温脉状金矿床,提出了胶东型金矿床[12,13]。因此,该类型金矿床成矿流体演化和成矿物质来源研究对解决胶东地区金矿床成因具有重要的理论意义。
黄铁矿作为中温热液脉状金矿床最丰富的硫化物,通常包含较多的亲铁亲铜元素,最丰富的元素是Co,Ni,Cu,As和Zn,最高可达10 000 ×10-6,其次是Se,Mo,Ag,Sb和Pb,含量可以达到1 000×10-6,最少的是Au,Bi和Sn,含量为10×10-6~100×10-6 [14]。如果黄铁矿发生了溶解再沉淀或者有外来流体的交代,上述元素的变化可以记录黄铁矿在不同地质条件下的形成和演化过程[15,16,17,18,19,20,21,22,23]。在金矿床中,由于无法直接限定金矿床中金的来源,考虑到黄铁矿与金的关系最紧密,因此可以通过对黄铁矿的研究来间接示踪金的来源。此外,在与不同类型的金矿床中黄铁矿的微量元素特征存在明显的差异,比如与还原性侵入岩有关的金矿床中黄铁矿的Au与As,Cu,Te,Sb,Pb,Ag和Bi有较好的正相关性,且含有较多的高温元素,如W和Sn[24,25,26,27];赋存在沉积岩中的金矿床中黄铁矿包含了大量的不可见金和较多的地层元素,比如Mo,Hg,Tl和Se[19,28];与变质作用有关的金矿床其黄铁矿中与金有关的元素有As,Bi,Pb和Ag[29]。对黄铁矿微量元素变化开展研究,可以反映成矿作用过程,对判定矿床类型有重要的指示作用。
1 区域地质背景
图1
胶东半岛位于华北克拉通的东缘,由北部的胶北隆起区和南部的苏鲁超高压造山带组成,两者以五莲—荣成断裂为界。金矿床几乎全部分布在胶北隆起区。胶北隆起区的地质单元主要由前寒武纪变质基底和中生代岩浆岩组成(图1b)。前寒武纪变质基底由太古宙花岗—绿岩带组成,主要包括新太古界胶东群、英云闪长岩—奥长花岗岩—花岗闪长岩(Tonalite-Trondhjemite-Grandiorite,TTG)片麻岩,古元古界粉子山群和荆山群高级变质泥岩[1]。中生代岩浆岩有晚侏罗世(165~150 Ma)玲珑、昆嵛山、鹊山和文登4个杂岩体[46,47]、早白垩世(132~123 Ma)郭家岭型似斑状花岗闪长岩[10,46,48]、130~110 Ma酸性至基性火山岩、125~90 Ma碱性花岗岩和中性—镁铁质脉岩[12,47,49]。
2 矿床地质
图2
断裂两侧围岩蚀变强烈且具有较明显的分带性,从内向外依次为黄铁绢云岩化带、钾化带和未蚀变花岗岩。根据野外地质穿插关系和矿物结构关系,矿化蚀变共分成5个阶段:
图3
图3
三山岛金矿矿脉切穿关系及典型蚀变特征
Fig.3
The crosscutting relationships of gold ore-bodies and typical hydrothermal alteration in the Sanshandao gold deposit
(a)矿体边缘的钾化蚀变带中被一条石英脉穿切;(b)黄铁绢云岩化带中发育石英角砾,且有少量碳酸岩脉沿裂隙充填;(c)石英—黄铁矿—菱铁矿脉穿切黄铁绢云岩化带,后被石英—多金属硫化物脉穿切;(d)乳白色石英脉中的黄铁矿呈粗粒自形;(e)石英—黄铁矿脉被晚期碳酸盐脉穿切;(f)石英—多金属硫化物脉被后期构造错断;Ccp:黄铜矿;Gn:方铅矿;Py:黄铁矿;Q:石英;Sd:菱铁矿;Ser:绢云母; Sp:闪锌矿
(a) Potassic alteration is cut by quartz vein at the edge of ore body;(b) Quarz phenocrysts associated with siderite in the pyrite-sericite-quartz alteration;(c) Quartz-pyrite- siderite vein cut pyrite-sericite-quartz alteration, then is cut by polymetallic sulfide vein; (d) A milk white quartz-pyrite vein cut pyrite-sericite-quartz alteration, pyrite occurs as euhedral granular; (e)Quartz-pyrite vein is cut by carbonate vein;(f) Polymetallic sulfide vein is displaced by latter structure. Ccp: Chalcopyrite; Gn: Galena; Py: Pyrite; Q: Quartz; Sd: Siderite; Ser: Sericite; Sp: Sphalerite
图4
图4
三山岛金矿床成矿期次与阶段划分
Fig.4
Paragenesis sequence of ore and gangue minerals of the Sanshandao gold deposit of the Sanshandao gold deposit
Py2产出于石英—黄铁矿±菱铁矿阶段中,黄铁矿主要与石英和菱铁矿共生呈脉状穿插黄铁绢云岩化带。Py2-a与石英共生,为该阶段主要类型的黄铁矿,其主要呈半自形—它形,中粗粒,大小为200~500 μm,可见少量黄铜矿、方铅矿、闪锌矿和自然金等矿物包裹体,黄铁矿脆性碎裂程度较高;在与菱铁矿共生的黄铁矿中,可见Py2-a被改造的特征,形成残余的核部Py2-b,其表面光滑,BSE图像上较Py2-a更亮(图5e)。
Py3产出于石英—多金属硫化物阶段中,半自形—它形,中粗粒,大小为400~600 μm,与黄铜矿、方铅矿和闪锌矿共生。BSE图像下可见明显核边结构,核部Py3-a在BSE图像下呈暗色,包裹了大量细粒黄铜矿、闪锌矿、方铅矿和金银矿等矿物,而边部Py3-b在BSE图像下呈亮色,表面平整光滑,偶尔包裹其他硫化物(图5h)。
图5
图5
不同世代黄铁矿镜下结构(a,d,f,g,i)和BSE图像(b,c,e,h)特征
Fig.5
Photomicrographs(a,d,f,g,i) and BSE images(b,c,e,h) of different pyrite generations
(a)黄铁绢云岩化阶段中浸染状黄铁矿Py1-a包裹了大量石英和绢云母,边部为光滑状Py1-b(反射光);(b)和(c)Py1-a与Py1-b的BSE图像不均一,反映其成分变化较大(BSE图像);(d)石英黄铁矿脉中有多金属硫化物细脉穿插(正交光);(e)石英—黄铁矿±菱铁矿阶段中Py2-a呈交代残余结构被Py2-b交代,Py2-a中包裹大量的方铅矿、黄铜矿、磁黄铁矿等与石英共生,Py2-b表面光滑,与菱铁矿共生(BSE图像);(f)石英—黄铁矿±菱铁矿阶段中自然金以裂隙金、包裹金的形式赋存在黄铁矿和石英中(反射光);(g)石英—多金属硫化物阶段中黄铁矿呈自形细粒零散分布于石英—多金属硫化物脉中(反射光);(h)石英—多金属硫化物阶段中可见早阶段包裹大量闪锌矿包裹体的Py3-a被与闪锌矿共生的Py3-b包裹,Py3-b表面光滑(BSE图像);(i)石英—多金属硫化物阶段中自然金与方铅矿、黄铜矿共生,沿裂隙充填黄铁矿中(反射光)
(a) Py1-a grains with irregular shape and numerous silicate inclusions,Py1-b with smooth surface in pyrite-sericite-quartz stage(reflected light);(b)and (c) The BSE images of Py1-a and Py1-b are not uniform, which reflects that the composition changes greatly(BSE image);(d) Polymetallic sulfide vein cut by quartz-pyrite vein(orthogonal light);(e) Py2-a as replacement remnant is metasomasised by Py2-b (BSE image);(f) Gold occurs in pyrite and quartz in the form of enclosure and fracture in the quartz-pyrite±siderite stage(reflected light);(g) Pyrite grains are cubic crystals in Sphalerite vein(reflected light);(h) Glossy Py3-b intergrown with sphalerite overgrowing the Py3-a with many sphalerite inclusions, the BSE image of Py3-a is more black than Py3-b(BSE image);(i) Gold, sphalerite, chalcopyrite, and galena assemblages in fracture of pyrite veins(reflected light)
3 样品采集与实验方法
实验样品采自三山岛矿区-346 m中段的129采场和钻孔编号为ZK78-2的岩芯,通过系统编录,从中选出最典型的矿石:浸染状黄铁绢英岩、石英黄铁矿脉和石英多金属硫化物脉,分别代表了第二、三、四主成矿阶段的黄铁矿。每个阶段选择1~2块典型样品,磨制厚度为0.7 mm的加厚探针片,结合光学显微镜图像与BSE图像(Back Scattered Electron Imaging)观察黄铁矿结构和共生组合,划分黄铁矿类型。
黄铁矿微量元素分析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。激光剥蚀系统标样采用NWR-213,实验分析仪器为Agilent 7700x型四级杆质谱仪。氦气(480 mL/min)运载剥蚀物离开剥蚀舱后与氩气(900 mL/min)均匀混合,之后进入ICP-MS进行元素含量测定。每次分析前有30 s的空白背景值测定,随后是60 s的激光剥蚀样品信号测定。激光斑束直径为50~70 μm,频率为10 Hz,激光能量约为5 J/cm2。分析方法为多外标—无内标法,GSE-1G,MASS和Py做外标。具体实验条件见参考文献[52]。测试元素包括49Ti,52Cr,55Mn,56Fe,59Co,61Ni,65Cu,66Zn,72Ge,75As,82Se,85Rb,88Sr,95Mo,109Ag,115In,118Sn,121Sb,125Te,197Au,202Hg,115Tl,208Pb和209Bi。对分析数据的离线处理(包括样品和空白信号的数据图谱选择等)依据Liu等[52]的标准方法采用ICPMSDataCal软件进行处理。
4 黄铁矿微观结构和世代划分
根据详细的野外地质调查和显微矿相学观察,可以将三山岛金矿床3个成矿阶段的黄铁矿,包括黄铁绢英岩阶段(Py1)、石英—黄铁矿±菱铁矿阶段(Py2)和石英—多金属硫化物阶段(Py3),进一步划分出6个亚类(表1)。
表1 三山岛金矿床不同亚类黄铁矿特征
Table 1
成矿阶段 | 黄铁矿世代 | 形态特征 | 矿物共生组合 | BSE特征 |
---|---|---|---|---|
黄铁绢云岩化阶段 | Py1-a | 半自形—它形,大小为50~200 μm,有大量绢云母和石英矿物包裹体 | 与绢云母和石英共生 | 不均一 |
Py1-b | 表面光滑 | 与绢云母和石英共生 | 不均一 | |
石英—黄铁矿±菱铁矿阶段 | Py2-a | 包裹少量方铅矿、黄铜矿等矿物 | 与自然金、方铅矿、黄铜矿和石英共生 | 暗色 |
Py2-b | 表面光滑 | 与菱铁矿共生 | 亮色 | |
石英—多金属硫化物阶段 | Py3-a | 包裹大量的方铅矿、黄铜矿等矿物 | 与自然金、方铅矿、黄铜矿和石英共生 | 暗色 |
Py3-b | 表面光滑 | 与自然金、方铅矿、黄铜矿和石英共生 | 亮色 |
Py1产于黄铁绢云岩化阶段中,该阶段中黄铁矿主要呈浸染状与绢云母和石英共生。根据产出状态,可以分为2个世代。Py1-a呈半自形—它形粒状,大小为50~ 200 μm,包裹了大量的绢云母和石英,部分Py1-a被Py1-b包裹,BSE图像显示黄铁矿成分不均一;Py1-b表面平整光滑,偶见少量黄铜矿和磁黄铁矿包裹体,常呈集合体形式产出,BSE图像显示变化较大(图5a~c)。
4 黄铁矿微区分析结果
4.1 黄铁矿激光剥蚀信号特征
由于一些黄铁矿中存在较多细小的矿物包裹体,在激光剥蚀过程中,部分情况下不可避免地会同时剥蚀到黄铁矿中的矿物包裹体,剥蚀曲线会呈现突然变高的情况,这在多孔状和与其他硫化物共生的黄铁矿中较多出现。如出现上述情况,在数据分析过程中将尽量避免选择这些信号。从图6中可以看出,As和Co信号平坦,说明这些元素以晶格替代的形式赋存在黄铁矿晶格中,而Au与Cu,Ag,Sb,Te,Pb和Bi具有相似的变化趋势,说明黄铁矿会包裹细小的金银矿、黄铜矿和Sb-Pb-Bi-Ag矿物包裹体。此外,Ag还与Pb,Bi有一致的信号变化规律。从不同元素激光剥蚀曲线的高低也可以观察出元素的相对含量,总体来看,Py3中金的信号较高,显示有较高的晶格金含量或者大量细小的金矿物包裹体,而Py1和Py2黄铁矿晶格中的金含量很低,接近检测限(图6)。
图6
图6
不同亚类典型黄铁矿激光剥蚀曲线
Fig.6
Representative time-resolved depth profiles for pyrite grains of different generations
4.2 黄铁矿微量元素特征
利用LA-ICP-MS对三山岛金矿床不同阶段和亚类的黄铁矿进行测试,共32种元素,其中Sc,Ti,V,Cr,Mn,Ga,Ge,Se,Rb,Sr,Y,Mo,In,Sn,Ba,W,Hg和Tl元素在黄铁矿中含量极低,且不同阶段变化不大,个别含量很高的分析点可能是受到了矿物包裹体的影响,主要元素分析结果如表2所示。
表2 不同亚类黄铁矿LA-ICP-MS微量元素分析结果(×10-6)
Table 2
黄铁矿世代 | 分析编号 | Co | Ni | Cu | Zn | As | Ag | Sb | Te | Au | Pb | Bi | Co/Ni | Pb/Bi |
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
检测限 | 0.01 | 0.14 | 0.41 | 1.05 | 0.41 | 0.05 | 0.02 | 0.23 | 0.02 | 0.06 | 0.01 | |||
Py1-a | SSD-2-1 | 374 | 210 | 0.33 | 0.96 | 627 | 0.10 | 0.05 | 0.48 | 0.02 | 1.34 | 1.05 | 1.78 | 1.28 |
SSD-2-2 | 456 | 137 | 5.04 | 1.26 | 34.53 | 3.15 | 0.38 | 5.14 | 0.08 | 21.51 | 49.32 | 3.32 | 0.44 | |
SSD-2-3 | 437 | 442 | 11.51 | 0.62 | 256 | 2.26 | 0.48 | 3.69 | 0.17 | 17.66 | 23.07 | 0.99 | 0.77 | |
SSD-2-4 | 431 | 198 | 5.65 | 0.97 | 208 | 2.77 | 0.83 | 5.00 | 0.16 | 45.24 | 47.07 | 2.18 | 0.96 | |
SSD-2-5 | 360 | 245 | 7.80 | 1.14 | 55.50 | 14.00 | 0.61 | 8.44 | 0.10 | 48.36 | 49.25 | 1.47 | 0.98 | |
SSD-17-1 | 9 074 | 194 | 7.07 | 0.39 | 36.45 | 0.05 | 0.01 | 1.59 | 0.00 | 1.33 | 3.70 | 46.69 | 0.36 | |
SSD-17-2 | 2 263 | 1 141 | 2.62 | 0.03 | 1 045 | 0.07 | 0.23 | 2.84 | 0.12 | 1.64 | 5.25 | 1.98 | 0.31 | |
Py1-b | SSD-2-6 | 21.66 | 212 | 1.75 | 0.67 | 1 914 | 1.57 | 0.83 | 1.85 | 0.22 | 13.53 | 6.74 | 0.10 | 2.01 |
SSD-2-7 | 110 | 355 | 1.20 | 0.49 | 1 366 | 0.35 | 0.20 | 0.49 | 0.05 | 4.26 | 2.07 | 0.31 | 2.06 | |
SSD-2-8 | 33.43 | 51.69 | 0.41 | 0.47 | 1 727 | 0.27 | 0.17 | 0.27 | 0.07 | 6.29 | 0.99 | 0.65 | 6.37 | |
SSD-9-1 | 109 | 28.48 | 1.68 | 0.63 | 1 116 | 0.74 | 0.44 | 0.58 | 0.15 | 10.55 | 6.92 | 3.82 | 1.52 | |
SSD-9-2 | 137 | 37.69 | 4.92 | 0.48 | 274 | 2.24 | 0.73 | 0.92 | 0.19 | 22.31 | 10.63 | 3.63 | 2.10 | |
SSD-9-3 | 13.93 | 33.40 | 6.38 | 0.61 | 447 | 11.86 | 2.97 | 1.71 | 0.35 | 3 902 | 38.07 | 0.42 | 102 | |
SSD-9-4 | 0.48 | 1.62 | 1.17 | 0.64 | 1 064 | 49.61 | 0.49 | 4.68 | 0.08 | 1 973 | 93.36 | 0.29 | 21.13 | |
SSD-9-5 | 14.13 | 14.71 | 3.92 | 0.72 | 208 | 2.79 | 0.86 | 0.76 | 0.26 | 32.70 | 9.82 | 0.96 | 3.33 | |
SSD-9-6 | 7.09 | 20.72 | 7.17 | 1.08 | 278 | 2.39 | 2.58 | 0.56 | 0.38 | 51.74 | 11.34 | 0.34 | 4.56 | |
SSD-9-7 | 5.70 | 23.25 | 1.52 | 0.55 | 497 | 48.77 | 0.90 | 5.19 | 0.06 | 430 | 240.00 | 0.25 | 1.79 | |
SSD-9-8 | 5.58 | 31.96 | 0.68 | 0.62 | 459 | 4.54 | 0.13 | 2.34 | 0.10 | 88.62 | 22.20 | 0.17 | 3.99 | |
Py2-a | SSD-7-1 | 0.10 | 3.87 | 19.55 | 2.64 | 202 | 13.64 | 8.32 | 0.46 | 0.38 | 65.52 | 0.95 | 0.02 | 68.95 |
SSD-7-2 | 0.16 | 92.55 | 77.91 | 2.73 | 185 | 2.92 | 1.27 | 0.47 | 0.12 | 17.35 | 0.24 | 0.00 | 71.79 | |
SSD-7-3 | 0.10 | 8.22 | 462 | 19.94 | 3.59 | 4.69 | 0.59 | 0.10 | 0.01 | 32.06 | 0.08 | 0.01 | 423 | |
SSD-7-4 | 0.34 | 0.32 | 9.89 | 0.34 | 1 288 | 10.23 | 3.81 | 0.21 | 0.20 | 15.71 | 0.12 | 1.06 | 127 | |
Py2-b | SSD-7-5 | 14.75 | 138 | 2.40 | 1.79 | 2 263 | 0.52 | 1.06 | 0.24 | 0.09 | 10.45 | 0.23 | 0.11 | 45.24 |
SSD-7-6 | 1.34 | 221 | 0.46 | 0.19 | 2 566 | 0.08 | 0.06 | 0.11 | 0.03 | 1.48 | 0.02 | 0.01 | 69.42 | |
SSD-7-7 | 2.87 | 938 | 0.46 | 0.53 | 3 211 | 0.01 | 0.18 | 0.05 | 0.01 | 2.07 | 0.03 | 0.00 | 75.96 | |
Py3-a | SSD-17-3 | 2.14 | 242 | 1 005 | 232 | 30.53 | 92.78 | 8.80 | 1.66 | 0.84 | 2 848 | 0.02 | 0.01 | 1.23×105 |
SSD-17-4 | 31.16 | 675 | 32.81 | 6.10 | 1 508 | 1.37 | 2.82 | 0.05 | 0.13 | 39.54 | 0.01 | 0.05 | 3 990 | |
SSD-17-5 | 8.82 | 211 | 8.56 | 1.88 | 175 | 4.92 | 4.49 | 0.05 | 0.22 | 76.18 | 0.02 | 0.04 | 4 441 | |
SSD-17-6 | 0.06 | 9.39 | 165 | 16 737 | 6.24 | 37.61 | 3.67 | 0.00 | 0.38 | 43.18 | 0.01 | 0.01 | 4 374 | |
SSD-17-7 | 1.43 | 50.59 | 241 | 18 904 | 69.97 | 37.14 | 5.74 | 0.07 | 0.68 | 124 | 0.03 | 0.03 | 3 597 | |
Py3-b | SSD-17-8 | 1.39 | 9.21 | 227 | 9 073 | 743 | 32.08 | 10.08 | 0.00 | 0.87 | 101 | 0.02 | 0.15 | 4 367 |
SSD-17-9 | 2.52 | 63.09 | 201 | 214 | 64.45 | 4.00 | 2.72 | 0.05 | 0.04 | 36.47 | 0.00 | 0.04 | — | |
SSD-17-10 | 46.55 | 27.83 | 28.07 | 3.28 | 8 320 | 19.68 | 1.65 | 0.19 | 7.80 | 408 | 0.00 | 1.67 | — | |
SSD-17-11 | 5.63 | 74.39 | 0.90 | 3.38 | 2.84 | 1.38 | 0.01 | 0.00 | 0.00 | 0.23 | 0.00 | 0.08 | — | |
SSD-17-12 | 0.16 | 22.96 | 367 | 14 262 | 2.54 | 120 | 3.96 | 0.07 | 3.10 | 68.50 | 0.02 | 0.01 | 4 424 | |
SSD-17-13 | 0.21 | 14.09 | 678 | 17 412 | 1.93 | 280 | 7.21 | 0.06 | 36.55 | 118 | 0.02 | 0.01 | 7 358 |
在黄铁绢英岩化阶段中的黄铁矿Py1-a与Py1-b中主要微量元素特征相似,仅Co和Ni等元素差异较明显。其中,Py1-a中的Co含量(中位数为1 913×10-6)比Py1-b(中位数为41.5×10-6)高2个数量级,Py1-a中Ni含量(中位数为367×10-6)比Py1-b(中位数为73.7×10-6)高1个数量级,Py1-a中的As含量(中位数为850×10-6)高于Py1-b(中位数为323×10-6),Py1-a中Te和Bi含量略高于Py1-b。Py1-b中Ag含量高于Py1-a。总体而言,Py1在各阶段黄铁矿中Co,Te和Bi含量较高,而Cu,Zn和Sb含量相对较低。
石英—黄铁矿±菱铁矿阶段中的黄铁矿Py2中As的含量较高,中位数为744×10-6,Ag和Co的含量较低。Py2-b比Py2-a含量增加的元素有Co,Ni和As。而亲铜元素均有所降低,如Cu,Zn,Ag,Sb,Au,Pb和Bi。其中Py2-a中有较多的Ag-Pb-Bi包体。
石英多金属硫化物阶段中Py3的Au含量中位数为0.68×10-6,最高值为36.55×10-6。这一阶段黄铁矿中Te和Bi含量最低,As含量变化很大(1.37×10-6~8 319×10-6)。黄铁矿中方铅矿、闪锌矿和黄铜矿等矿物包裹体较多。Py3-b较Py3-a微量元素Ni和As含量降低。
以Py1-a各元素的中位数为基准,将其他阶段黄铁矿微量元素与其比较,可以更好地反映各阶段黄铁矿微量元素特征(图7)。整体来看,各阶段黄铁矿微量元素蛛网图比较相似,反映了具有相似的来源。Py1-a最富集Co,Te和Bi;Py2-a最贫Co,Py2-b最富集As,最贫Ag,Sb,Au和Pb,Py3-a最富集Ag和Sb;Py3-b最富集Cu,Au和Pb,而最贫As和Bi。总体来看,随着成矿作用进行,成矿流体中Co,Te和Bi含量逐渐降低,而Cu,Ag,Sb,Au和Pb逐渐富集。
图7
图7
三山岛金矿各亚类黄铁矿微量元素含量特征蛛网图
Fig. 7
Spider diagram of trace element concentration in pyrites of main mineralization stages
4.3 黄铁矿微量元素相关性
Au与其他主要微量元素的协变关系如图8所示,根据激光剥蚀信号图谱,投图前已将部分剥蚀到矿物包裹体的数据排除在外,因为这些数据点会影响到元素的相关性。
图8
图8
黄铁矿微量元素相关图解
Fig.8
Binary plots of selected trace elements in pyrites of different generations
(a)Au vs Ag;(b)Au vs As,金饱和曲线公式为CAu = 0.02 ×CAs+ 4×10-5 [53];(c)Au vs Sb;(d)Cu+Pb vs Au+Ag;(e)Au vs Bi;(f)Au vs Te;(g)Te vs Bi;(h)Pb vs Ag;(i)Pb vs Bi;灰色区域为检测限
(a)Au vs Ag;(b)Au vs As,Au saturation line by CAu = 0.02 ×CAs+ 4×10-5 [53];(c)Au vs Sb;(d)Cu+Pb vs Au+Ag;(e)Au vs Bi;(f)Au vs Te;(g)Te vs Bi;(h)Pb vs Ag;(i)Pb vs Bi;The shadow areas show the average detection limit of the trace elements
从Py1至Py3,Au与Ag具有一定的正相关性。Py1分布趋势较稳定,主要沿着Au/Ag=0.1趋势线分布,Py2和Py3中Au/Ag值逐渐下降,主要沿着Au/Ag=0.01趋势线分布(图8a)。Au-As协变图中Au与As相关性不明显(图8b)。Au与Sb,Cu,Pb有很好的正相关关系(图8c和图8d)。Au与Te只在石英—黄铁矿±菱铁矿阶段显示一定的正相关性,且Au/Te约为0.5,说明在此阶段有碲金矿(AuTe2)沉淀(图8e)。Te与Bi有很好的正相关性,从早阶段到晚阶段Te和Bi含量逐渐降低(图8g)。Pb与Ag有较好的正相关关系,而Pb与Bi的相关图解中分成了3个区域,每一区域相关性很好,表明每个阶段流体Bi的含量有差别(图8h和图8i)。
5 讨 论
5.1 金的赋存状态
Au在黄铁矿中的赋存形式可以分为2种,一种是可见金,主要分布在黄铁矿裂隙中或者被黄铁矿包裹的包裹金中;另一种是不可见金,主要赋存在黄铁矿晶格中或者小于250 nm的显微金中[29]。在光学显微镜下可见大量自然金和银金矿,说明存在大量可见金(图5f和图5i)。LA-ICP-MS剥蚀曲线中Au的信号在检出限左右,但是也常见Au与Ag,Cu,Te,Sb,Pb等元素有一致的信号变化规律,显示有含金显微矿物包裹体存在,根据Reich等[53]模拟出的Au-As饱和曲线投图发现,除Py3-b个别数据高于Au饱和曲线,其余样品数据均落在曲线之下,显示在黄铁矿晶格中金不饱和。但是本次测试的三山岛金矿中黄铁矿晶格金含量大部分低于1×10-6,结合矿石中大量的可见金矿物颗粒,说明Au主要以可见金形式赋存。Py3-b中有个别测试点的金含量高于金饱和曲线,可能是局部空间震荡,使金发生大量卸载,以众多显微细小包裹体形式赋存在黄铁矿中[33,54]。
在卡林型金矿、浅成低温热液型金矿以及造山型金矿中,富As的黄铁矿通常有极高的Au含量,主要是由于As进入黄铁矿中引起寄主黄铁矿的矿物晶胞参数增大或产生晶格位错为Au的富集提供了空间[55]。三山岛金矿床中所有黄铁矿的As含量很低,中位数为274×10-6,且As与Au没有明显的正相关关系(图8b),说明成矿流体中As含量较低,并对Au的沉淀作用有限。Te的含量在黄铁绢云岩化阶段和石英多金属硫化物阶段与Au无相关性,而在石英—黄铁矿±菱铁矿阶段中,Te与Au有很好的相关性,但总体Te的含量很低,对Au的搬运和富集也比较有限(图6)。推测Au在成矿热液中主要以Au(HS)0和Au(HS)
5.2 黄铁矿结构及形成世代
Fe2++2Au(HS)0aq+2H2(g)=FeS2+Au2S0+H2S+2H+,
Fe2++Au(HS)
由于Au很少进入黄铁矿晶格中,而是以Au的显微矿物包裹体的形式存在于黄铁矿中,所以黄铁绢云岩化阶段在激光剥蚀信号中常见金的异常信号突起,而在光学显微镜下很少见到自然金[54]。Py1-a和Py1-b就是这个过程中早、晚2个亚类的黄铁矿。
石英—黄铁矿脉和石英—多金属硫化物脉通常沿着黄铁绢云岩化带中的裂隙充填,品位较高,有大量的可见金沉淀。流体包裹体研究显示原生的H2O-CO2和纯H2O包裹体共存,流体包裹体研究发现均一温度为160~325 ℃[31]。在张性环境下,流体快速上侵减压发生不混溶或者相分离作用,Au(HS)0分解,造成了金的快速沉淀和黄铁矿的形成,这个过程主要形成了Py2-a和Py3-a 2个亚类的黄铁矿。在石英黄铁矿脉中,与菱铁矿共生的黄铁矿在BSE下可见交代早世代黄铁矿的特征(图5e),表明晚期流体进一步改造黄铁矿Py2-a形成Py2-b。在石英多金属硫化物脉中,Py3-b在BSE下明显晚于Py3-a,其金含量变化较大,有3个测试点高于1×10-6,有2个测试点高于Au-As饱和曲线,其可能代表了石英—多金属硫化物脉上移充填的过程中,局部环境不稳定,导致了成矿物质大量卸载,金直接以纳米金的形式沉淀[53,60]。
5.3 黄铁矿微量元素对流体成分演化的制约
以往总结的大量经验表明不同成因类型的黄铁矿Co/Ni值存在较大差异,而且不同成矿阶段黄铁矿的Co,Ni含量和Co/Ni值也有所差异[22,61,62,63]。沉积岩中的黄铁矿有很低的Co和Ni值(Co+Ni小于100×10-6),且Co/Ni<1[19],而与岩浆岩有关的黄铁矿Co+Ni大于1 000×10-6,且Co/Ni>1[61]。围岩玲珑花岗岩中Co+Ni含量只有6×10-6左右[33],而主成矿期黄铁绢云岩阶段的黄铁矿中Co+Ni最高含量为9 268×10-6,反映了黄铁矿的形成可能与深源岩浆热液流体有关[63]。Co比Ni更易受温度控制,随着温度升高,Co往往比Ni更容易进入黄铁矿晶格中[55,64],Py1的Co/Ni测试数据主要落在0.1~10,Py1-a主要落在1~10,Py1-b主要落在0.1~1,而Py2和Py3的Co/Ni测试数据主要落在0.01~0.1,这2个阶段相差较小。从主成矿早阶段到晚阶段Co/Ni值的降低反映出成矿流体温度逐渐降低(图9)。罗栋[65]对三山岛不同阶段石英测温也发现,黄铁矿绢云岩阶段中石英包裹体完全均一温度主要在280~380 ℃,石英黄铁矿阶段中石英包裹体完全均一温度主要在220~320 ℃,石英多金属硫化物阶段中石英包裹体完全均一温度主要在200~340 ℃,反映了成矿早阶段温度最高,而石英黄铁矿与石英多金属硫化物阶段流体温度变化较小,与黄铁矿微量元素反应特征一致。
图9
图9
三山岛金矿不同阶段黄铁矿Co vs Ni 含量关系
Fig 9
Relationship between pyrite Co and Ni contents of different generations at Sanshandao gold deposit
6 结 论
(1)三山岛金矿床是典型蚀变岩型金矿床,通过对不同成矿阶段的黄铁矿矿相学研究发现,黄铁矿具有多个亚类,基于黄铁矿原位微量元素分析,发现黄铁矿晶格中Au含量极低,流体中As的含量较低限制了Au进入黄铁矿晶格的能力。
(2)从早阶段到晚阶段,黄铁矿中Au,Pb,Cu,Sb和Ag逐渐增加,Co/Ni值逐渐降低,反映了成矿流体逐渐富集Au,Pb,Cu,Sb和Ag,温度逐渐降低。
(3)黄铁绢云岩阶段的黄铁矿Co+Ni的含量很高(最高为9 268×10-6),反映了早阶段黄铁矿可能来源于岩浆岩源区,而随着岩浆热液演化进行,形成不同世代的黄铁矿。
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Tectonic evolution of the Qinling orogen, China: Review and synthesis
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Paleozoic-early Jurassic tectonic evolution of North China Craton and its adjacent orogenic belts
[J].
华北克拉通及相邻造山带古生代—侏罗纪早期大地构造演化
[J]. ,
The Heilongjiang Group: A Jurassic accretionary complex in the Jiamusi Massif at the western Pacific margin of northeastern China
[J]. ,
Timing, scale and mechanism of the destruction of the North China Craton
[J].
华北克拉通破坏的时间、范围与机制
[J]. ,
Destruction of the North China Craton
[J].
华北克拉通破坏
[J]. ,
40Ar/39Ar dating of the Jiehe gold deposit in the Jiaodong Peninsula, eastern North China Craton: Implications for regional gold metallogeny
[J]. ,
Constraints on crustal evolution and gold metallogeny in the Northwestern Jiaodong Peninsula, China, from SHRIMP U-Pb zircon studies of granitoids
[J]. ,
Evolution of syn-to post collisional magmaitsm from north Sulu UHP belt,eastern China:Zircon U-Pb geochronology
[J].
苏鲁超高压带北部中生代岩浆侵入活动与同碰撞—碰撞后构造过程:锆石U-Pb年代学
[J]. ,
Gold-hosting high Ba-Sr granitoids in the Xincheng gold deposit, Jiaodong Peninsula, East China: Petrogenesis and tectonic setting
[J]. ,
Geochronological sequence of Mesozoic intrusive magmatism in Jiaodong Peninsula and its tectonic constraints
[J].
胶东半岛中生代侵入岩浆活动序列及其构造制约
[J]. ,
Study on the multi-level controlling rule for tectonic system in Jiaodong gold-centralized area
[J].
胶东金矿集中区构造体系多层次控矿规律研究
[J]. ,
The golden transformation of the Cretaceous plate subduction in the west Pacific
[J]. ,
In situ analysis of major and trace elements of anhydrous minerals by LA-ICP-MS without applying an internal standard
[J]. ,
Solubility of gold in arsenian pyrite
[J]. ,
Implications of pyrite geochemistry for gold mineralisation and remobilisation in the Jiaodong gold district, northeast China
[J]. ,
Geology, Mineralization and Genesis of the Nibao, Shuiyindong and Yata Gold Deposits in SW Guizhou Province, China
[D].
黔西南泥堡、水银洞和丫他金矿床的成矿作用特征与矿床成因研究
[D].
Hydrothermal transport and depositional processes in Archean lode-gold systems: A review
[J]. ,
A comparison of Jiaojia- and Linglong-type gold deposit ore-forming fluids: Do they differ?
[J]. ,
High-angle reverse faults, fluid-pressure cycling, and mesothermal gold-quartz deposits
[J]. ,
Visible gold in arsenian pyrite at the Shuiyindong Carlin-type gold deposit, Guizhou, China: Implications for the environment and processes of ore formation
[J]. ,
Pyrite textures and compositions from the Zhuangzi Au deposit, southeastern North China Craton: Implication for ore-forming processes
[J]. ,
Element migration of pyrites during ductile deformation of the Yuleken porphyry Cu deposit (NW-China)
[J]. ,
Using integrated in-situ sulfide trace element geochemistry and sulfur isotopes to trace ore-forming fluids: Example from the Mina Justa IOCG deposit (southern Perú)
[J]. ,
LA-ICP-MS trace element analysis of pyrite from the Xiaoqinling gold district, China: Implications for ore genesis
[J]. ,
The standard characteristics and real meaning of pyrite
[J].
黄铁矿的标型特征及其实际意义
[J]. ,
Evolution of Ore-forming Fluids of Sanshandao Gold Deposit, Jiaodong Peninsula, Eastern China
[D].
胶东三山岛金矿床成矿流体演化
[D].
Textures, trace elements, and Pb isotopes of sulfides from the Haopinggou vein deposit, southern North China Craton: Implications for discrete Au and Ag-Pb-Zn mineralization
[J]. ,
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