岩石学报  2018, Vol. 34 Issue (4): 1119-1138   PDF    
鞍本地区大孤山条带状铁建造含铁矿物和相分带特征及形成环境分析
佟小雪1,2,3 , 张连昌1,2,3 , 王长乐1,2 , 彭自栋1,2,3 , 南景博3,4     
1. 中国科学院地质与地球物理研究所, 中国科学院矿产资源研究重点实验室, 北京 100029;
2. 中国科学院地球科学研究院, 北京 100029;
3. 中国科学院大学, 北京 100049;
4. 中国科学院深海科学与工程研究所, 三亚 572000
摘要:鞍山-本溪条带状铁建造(Banded Iron Formation,简称BIF)位于华北克拉通东北缘,是世界上典型BIF之一,也是我国最重要的铁矿资源基地。大孤山位于鞍山地区南部矿带,为新太古代典型的Algoma型BIF,与华北克拉通其它大多数BIF相比,具有较低变质程度(绿片岩相-低角闪岩相)和较完整的沉积相分布特征。因此,通过大孤山BIF的研究有利于追踪Algoma型BIF的原生矿物组成及其后期成岩-变质过程,进而通过分析原生矿物形成的物理化学条件探讨古海洋环境。依据原生矿物共生组合及产出特征,可将大孤山BIF沉积相划分为氧化物相(30%)、硅酸盐相(50%)和碳酸盐相(20%)。氧化物相主要分布于主矿体南部,主要矿物组成为磁铁矿和石英;硅酸盐相分布于主矿体中部,主要矿物组成除了石英和磁铁矿之外,还有黑硬绿泥石、绿泥石、镁铁闪石等;碳酸盐相分布于矿体北部,主要矿物组成为菱铁矿、磁铁矿和石英等。本文通过大孤山BIF岩相学观察和含铁矿物化学成分研究,推测原生沉积物的组成为无定形硅胶、三价铁氢氧化物和富铝粘土碎屑,在经历了成岩和低级变质作用后转变为具不同相带的条带状铁建造。通过分析磁铁矿、菱铁矿和黑硬绿泥石等矿物在不同PO2-PCO2和pH-Eh条件下的共生相图可知,这些矿物均是在较低氧逸度、中到弱碱性环境下形成。综合考虑矿物成分、共生组合及受变质作用较弱等信息,本文推测制约原生矿物形成的控制因素主要是古海水氧化还原状态、酸碱度、CO2含量和硫逸度。
关键词: 条带状铁建造     矿物成因     原生矿物组成     沉积相与形成环境     鞍山-本溪地区    
The characteristics of iron-bearing minerals and the depositional facies of BIF in Dagushan iron deposit, Anshan area: Indications of formation environment
TONG XiaoXue1,2,3, ZHANG LianChang1,2,3, WANG ChangLe1,2, PENG ZiDong1,2,3, NAN JingBo3,4     
1. Key Laboratory of Mineral Resources, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China;
2. Institutions of Earth Science, Chinese Academy of Sciences, Beijing 100029, China;
3. University of Chinese Academy of Sciences, Beijing 100049, China;
4. Institute of Deep Sea Science and Engineering, Chinese Academy of Sciences, Sanya 572000, China
Abstract: Anshan-Benxi area is located in the northeast margin of North China Craton, which is the most important resource base of the banded iron formation (BIF) type iron ores in China. As a typical Algoma-type BIF, Dagushan BIF locates in the south ore belt in Anshan area, and comparing with characteristics of other BIFs in China, the Dagushan BIF is obviously distinct mainly in mild metamorphism (greenschist-to lower amphibolite-facies) and integrated depositional facies. Thus, the Dagushan BIF can be used as a good probe to pursue the precursor materials, to invert the primary minerals and their evolutions as well as to discuss the environment of ancient oceans. Hosted in the metasedimentary sequence of Yingtaoyuan Formation in Anshan Group, the Dagushan BIF strikes integrally north-west and dips steeply north-west at 30° to 60°. Oxide (30%), silicate (50%) and carbonate (20%) facies iron formations are recognized based on predominant iron minerals within the iron-rich layers. The oxide facies is composed of magnetite and quartz; the silicate facies consists mainly of stilpnomelane, chlorite and cummingtonite other than quartz and magnetite; while the most prominent carbonate minerals in the carbonate facies is siderite. Integration of petrographic evidence and mineral chemistry indicates that the most likely precursor materials were probably amorphous silica gels, Al-rich detrital clay, which has converted to the minerals today due to subsequent processes, such as diagenesis and metamorphism. The PO2-PCO2 and pH-Eh fields of these minerals (and/or their precursors) indicate anoxic and near-neutral to slightly alkaline conditions for the original depositional environment, from which we can infer that the significant factors affecting the formation of minerals are probably redox statement of ancient ocean, pH value, the content of CO2 and sulphur fugacity.
Key words: Banded iron formation     Mineral genesis     Primary mineral     Depositional facies and environment     Anshan-Benxi area    

条带状铁建造(Banded Iron Formation, 简称BIF)是前寒武纪古老克拉通内特有的全铁含量大于15%的化学沉积岩,主要由富铁和富硅条带组成(James, 1954, 1983)。它不仅是全球铁矿资源的重要来源,也是记录古海洋和大气环境及原始生命活动的关键载体(Trendall, 2002; Huston and Logan, 2004; Konhauser et al., 2017)。长期以来,科学家多通过研究BIF的矿物学、岩石学和地球化学特征来追溯其形成、演化以及古海洋环境(Bekker et al., 2010),但是由于BIF沉积后普遍会经历成岩及不同程度变质作用的改造(Klein, 2005),导致其矿物组成和原始沉积结构发生不同程度的变化,对了解BIF的原生矿物组成并推测其形成环境造成困难。因此,寻求经历低级变质且保存部分原始矿物及沉积结构的BIF作为研究对象,进而追踪BIF的原始矿物组成并探讨BIF沉积的古海洋环境一直以来是科学家们关心的关键问题。

为便于研究BIF中矿物的形成与沉积环境的关系,James (1954)根据含铁矿物组合将BIF沉积相划分为氧化物相(磁铁矿和赤铁矿)、硅酸盐相(铁蛇纹石、黑硬绿泥石、角闪石和辉石等)、碳酸盐相(菱铁矿和铁白云石)和硫化物相(黄铁矿)。但由于黄铁矿的沉积成因仍存在较大争议(Groves et al., 1987; Simonson, 2003),目前大部分学者认为硫化物相(包括黄铁矿化页岩和含硫化物燧石等)尚不能当做一类BIF沉积相(Beukes and Gutzmer, 2008; Bekker et al., 2010; Bekker and Holland, 2012)。Gross (1980)根据BIF的构造-沉积环境,将其划分为阿尔戈玛型(Algoma型)和苏必利尔湖型(Superior型)。其中,Algoma型BIF主要产于活动大陆边缘,如岛弧与弧后盆地,多经历较强烈的构造-热事件,一般变质程度较高(常为高绿片岩相-角闪岩相,部分可至麻粒岩相),难以保存原生矿物和原始沉积结构等特征;且沉积相类型较少,多发育氧化物相和硅酸盐相,主要矿物组成为石英、磁铁矿、角闪石等,如西格陵兰Isua地区BIF、加拿大Abitibi地区BIF及中国五台地区BIF等(Li, 2014; Wang et al., 2014; Sun and Li, 2017)。而Superior型BIF主要产于被动大陆边缘,所处构造背景稳定,一般变质程度较低(常为绿片岩相),常发育多种沉积相,主要矿物组成为燧石(石英)、磁铁矿、赤铁矿、菱铁矿、铁蛇纹石和黑硬绿泥石等,如西澳哈默斯利Dales Gorge BIF,南非德兰士瓦BIF和中国山西袁家村BIF等(Klein and Beukes, 1989; Smith et al., 2012; Wang et al., 2015a)。当前关于BIF矿物成因、原生矿物组成以及沉积相的研究多集中于Superior型BIF,初步认为三价铁氢氧化物或铁硅酸盐微粒(主要成分为铁蛇纹石或黑硬绿泥石)可能是BIF原生矿物的主要成分,在后期成岩或变质作用过程中转变为赤铁矿、磁铁矿、菱铁矿等矿物(Konhauser et al., 2002; Beukes and Gutzmer, 2008; Rasmussen et al., 2013, 2016)。但对变质程度相对较高的Algoma型BIF研究较少,其原生矿物是什么,与Superior型BIF是否一致,都是亟待解决的问题。

我国BIF多分布于华北克拉通前寒武纪变质岩区(图 1a),主要为Algoma型,在鞍山-本溪(以下简称鞍本)、冀东、鲁西、固阳、五台、河南舞阳等地区集中产出(张连昌等, 2012; 沈保丰, 2012; Zhai and Santosh, 2013);变质程度一般较高,多为角闪岩相,局部变质程度较低,为高绿片岩相,且沉积相主要为氧化物相和硅酸盐相,矿物种类较少,以磁铁矿、石英及角闪石为主。大孤山BIF位于华北克拉通东北缘,分布于鞍山地区南部,属于Algoma型BIF,与鞍本地区乃至华北克拉通其他Algoma型BIF相比,其特殊性体现在以下两点:(1)变质程度较低:大孤山BIF赋存于鞍山群樱桃园组中,为绿片岩相-低角闪岩相变质(周世泰, 1994; 沈其韩, 1998);(2)矿物组合即沉积相类型多样:具有氧化物相、硅酸盐相和碳酸盐相三种沉积相,存在菱铁矿、黑硬绿泥石、绿泥石、镁铁闪石等多种矿物,有助于探索原生矿物及其后期演化,揭示其形成环境。因此,本文选取大孤山BIF作为研究对象,着重分析各沉积相的矿物之间的交代关系,探索矿物成因、演化及其形成条件,为研究Algoma型BIF的原生矿物组成和探讨古海洋环境提供依据。

图 1 华北克拉通主要BIF分布简图(a, 据Zhao et al., 2005; 张连昌等, 2012修改)和鞍本地区地质简图(b, 据Wan et al., 2015; Wang et al., 2016修改) 图示BIF赋存围岩的锆石U-Pb年龄来源: a-杨秀清(2013); b-代堰锫等(2013); c-王守伦和张瑞华(1995); d-万渝生等(2012); e-Wang et al. (2016); f-Zhu et al. (2015); g-代堰锫等(2012); h-崔培龙(2014) Fig. 1 Distribution of major BIFs in the North China Craton (NCC) (a, modified after Zhao et al., 2005; Zhang et al., 2012) and geological sketch map of the An-Ben area (b, modified after Wan et al., 2015) Numbers show previous zircon U-Pb ages of supracrustal rocks associated with BIFs. Age data sources: a-Yang (2013); b-Dai et al. (2013); c-Wang and Zhang (1995); d-Wan et al. (2012); e-Wang et al. (2016); f-Zhu et al. (2015); g-Dai et al. (2012); h-Cui (2014)
1 区域与矿区地质特征 1.1 区域地质特征

华北克拉通北临中亚造山带,南邻中央造山带,整体呈倒三角型(图 1a),是全球最古老的克拉通之一(Liu et al., 1992; Sengör et al., 1993; Song et al., 1996; Wan et al., 2005; Kusky et al., 2007),其前寒武纪岩石主要由太古宙-古元古代变质基底及上覆古元古代末期-新元古代早期盖层组成(Lu et al., 2008)。其中,基底岩石主要为TTG片麻岩、钙碱性花岗岩、紫苏花岗岩、混合岩、角闪-绿片岩相变基性岩、变泥砂质片岩、孔兹岩、BIF及大理岩等(Wilde, 2014);盖层包括长城系(1.8~1.6Ga)、蓟县系(1.6~1.0Ga)及青白口系(1.0~0.8Ga)(Lu et al., 2008)。

鞍山-本溪地区位于华北克拉通东北缘,整体呈弧形分布,区内广泛出露太古代表壳岩和花岗杂岩体(图 1b),包括:(1)始太古代白家坟、东山和深沟寺片麻岩(3.77~3.81Ga)(Liu et al., 1992, 2008; Song et al., 1996; Wan et al., 2005, 2012; Wu et al., 2008; Wang et al., 2015b);(2)古太古代陈台沟表壳岩(~3.62Ga)及花岗杂岩(3.45~3.3Ga)(主要由片麻状奥长花岗岩、混合岩、斑岩和细粒花岗岩组成)(3.45~3.3Ga)(Song et al., 1996; 周红英等, 2008; Wan et al., 2012; Wang et al., 2015b);(3)中太古代立山、东鞍山和铁架山花岗杂岩体(3.1~2.9Ga)(Song et al., 1996; Wang et al., 2015b);(4)新太古代表壳岩(鞍山群变火山-沉积岩)(2.55~2.53Ga)及齐大山花岗岩(~2.5Ga)(Song et al., 1996; Wan et al., 2012; Zhu et al., 2015)。此外,区域上还发育太古宙后岩浆岩以及古元古代辽河群、震旦系、古生界、中生界和新生界地层,其中岩浆岩主要有古元古代超基性岩、基性岩,以及燕山期中酸性岩浆岩(周世泰, 1994)。

鞍本地区的主要赋矿岩层为新太古代鞍山群表壳岩,但鞍山和本溪地区的岩石组合、变质及变形程度有所差异。鞍山地区发育鞍山群上部的樱桃园组,该组分布于鞍山地区的齐大山、东西鞍山、大孤山、眼前山等地,岩层厚度最大可达800m以上,主要岩石组合为条带状铁建造、绢云绿泥石英千枚岩、黑云变粒岩、云母石英片岩、绿泥石英片岩等,原岩为硅铁质沉积岩、成熟度较高的泥质-粉砂质碎屑沉积岩及中酸性火山凝灰岩;变质程度为绿片岩相-低角闪岩相,变质温度约为500~600℃,压力小于5×108Pa(周世泰, 1994)。本溪地区发育鞍山群中部的大峪沟组和茨沟组,分布于歪头山、弓长岭和南芬地区,岩层厚度最大可达2000m以上,岩石组合以黑云变粒岩、斜长角闪岩及BIF为主,局部夹云母片岩和石英岩等,在歪头山、弓长岭等地还有少量大理岩;原岩主要为基性-中酸性火山岩和泥质-粉砂质碎屑沉积岩,变质程度一般达角闪岩相,变质温度约为550~650℃,压力为3×108~6×108Pa(周世泰, 1994)。

鞍本地区的条带状铁矿主要形成于新太古代晚期(2.55~2.52Ga),如齐大山BIF(2533±53Ma)(王守伦和张瑞华, 1995)、东鞍山BIF 2544±8Ma(杨秀清, 2013)、弓长岭地区BIF(2528±10Ma)(万渝生等, 2012)、大孤山BIF(~2548±7.9Ma)(崔培龙, 2014)等。然而,Dai et al. (2014)测得BIF夹层变中酸性火山岩(绿泥石英片岩)的锆石U-Pb年龄为3110±32Ma,因此,我们对大孤山BIF夹层变杂砂岩(黑云变粒岩)中的碎屑锆石定年(待发表),其平均年龄为2526.2±4.9Ma,代表沉积时代的上限,并应用区域上BIF围岩的最大变质年龄2512±21Ma(大台沟变砂岩的变质年龄)(Wang et al., 2016)作为沉积下限,最终确定大孤山BIF形成时间为2512~2526Ma。

鞍山地区的铁矿储量占鞍本地区铁矿总储量的60%以上,占全国铁矿总储量的15%,为鞍本地区乃至全国最大的铁矿集中区(周世泰, 1994)。根据铁矿床分布特征,可将鞍山地区铁矿分为南、北两个主矿带,其中北矿带主要为齐大山-王家堡子-胡家庙子-关门山-眼前山铁矿,北西-南东向延长14.5km,主矿层厚100~300m;矿石以磁铁石英岩、阳起磁铁石英岩和绿泥磁铁石英岩为主,其上部多被氧化为假象赤铁石英岩,此矿带的围岩主要是黑云绿泥石英片岩及黑云变粒岩等(王守伦和张瑞华, 1995; 杨秀清, 2013)。南矿带自西向东依次为西鞍山-东鞍山-黑石砬子-大孤山铁矿,东西延长12.5km,基本连续分布,主矿层多为单层,厚90~300m;矿石以磁铁石英岩、菱铁磁铁石英岩、角闪磁铁石英岩及少量绿泥磁铁石英岩为主,此矿带的围岩主要是绿泥石英片岩及少量黑云变粒岩(周世泰, 1994; 辽宁省冶金地质勘查局地质勘查研究院资料, 2006)。

①  辽宁省冶金地质勘查局地质勘查研究院. 2006.辽宁省鞍山市大孤山铁矿床补充勘探报告

1.2 矿区地质特征

大孤山铁矿是鞍山地区南矿带的典型矿床,为辽宁省重要铁矿开采基地(姚培慧, 1993)。矿区内出露的岩系主要为新太古代鞍山群樱桃园组变沉积岩、古元古代辽河群千枚岩以及震旦系钓鱼台组石英岩(图 2a, b)。大孤山BIF赋存于樱桃园组变沉积岩中,该组主要由绿泥石英片岩、黑云变粒岩、磁铁石英岩和云母石英片岩组成,整体走向北西-南东,倾向北东,倾角约40°~70°,其东南端与白垩纪千山花岗岩、西南端和西北端与太古代花岗岩均以断层接触(图 3a)。BIF下盘为厚度 < 2m的薄层绿泥石英片岩,与下部太古代花岗岩为断层接触(图 3c, d),相对于其它绿泥石英片岩来说,下盘岩石整体破碎且绿泥石含量升高。BIF夹层主要为绿泥石英片岩、黑云变粒岩及少量云母石英片岩,一般呈薄层状,与矿体呈整合接触(图 3b),靠近BIF的岩石片理化程度加强,柔皱等变形较发育。此外,矿区内还有一层较厚的绿泥石英片岩夹层,厚度约200~300m,其将矿体分为中部主矿体和北部矿体两部分。其中部主矿体整体产状为陡倾的单斜,层厚约200~400m,主要由氧化物相-硅酸盐相BIF组成。北部矿体呈薄层状,层厚约20~40m,分布范围较小,主要由碳酸盐相BIF组成,但其受后期构造作用影响较大,发育褶皱构造,局部产状改变,倾向南西,倾角变化范围较大,为30°~70°。另外,在后期构造作用下,矿石柔皱变形较发育,在剪切应力影响下裂隙发育,局部见左行的雁列节理。

图 2 大孤山铁矿地质图(a, 据辽宁省冶金地质勘查局地质勘查研究院, 2011修改)和大孤山矿区野外实测剖面图(b) Fig. 2 Geological map of the Dagushan iron deposit (a) and measured section of the Dagushan iron deposit (b)

图 3 大孤山铁矿体与围岩的野外关系 (a)矿区整体图,可见矿体与其千山花岗岩、太古代花岗岩均呈断层接触;(b)绿泥石英片岩夹层与矿体整合接触;(c)下盘薄层绿泥石英片岩与太古代花岗岩断层接触;(d)碳酸盐相BIF与太古代花岗岩呈断层接触.其中红色实线代表断层,黄色虚线代表岩性界线 Fig. 3 Field relationships of rocks and iron orebodies in the Dagushan iron deposit (a) the overview of Dagushan pit showing that the faults between orebodies and Qianshan granite as well as Archean granite; (b) the boundaries between the BIF and surround rocks are gradually changed and conformable; (c) the footwall of BIF is chlorite-quartz schist which presents fault-unconformity on the Archean granite; (d) carbonate-facies BIF presents fault-unconformity on the Archean granite. Red lines represent for faults while yellow dash lines represent for lithologic boundary

①  辽宁省冶金地质勘查局地质勘查研究院. 2011.辽宁省鞍山市大孤山铁矿地形地质图

矿区内发育大量断裂构造,根据其方向和性质,可划分为EW走向的斜交逆断层和NE-NNE向的直交逆断层(姚培慧, 1993; 刘云, 2010; Dai et al., 2014)。前者主要分布在矿体西部,为太古代花岗岩与含铁岩系的分界断裂(如断层F14,F15),倾向NW-N,倾角50°~60°,断距达400m以上;后者主要分布在矿体东南部,与矿体近直交,为千山花岗岩(断层F1)、闪长玢岩与含铁岩系的交界断裂(断层F8),倾向SE,倾角由地表(43°)向深部逐渐变陡(85°)(姚培慧, 1993)。区内岩浆岩较为发育,主要为太古代东鞍山花岗岩(万渝生等, 1998)、白垩纪千山花岗岩(Wu et al., 2005)及花岗斑岩、闪长玢岩和辉绿岩脉等(姚培慧, 1993; 刘云, 2010)。

2 大孤山BIF沉积相分带特征

根据矿物共生组合将大孤山BIF划分为三个沉积相:氧化物相、硅酸盐相和碳酸盐相。氧化物相主要含铁矿物为磁铁矿,硅酸盐相主要含铁矿物由磁铁矿和铁硅酸盐矿物组成,如黑硬绿泥石和镁铁闪石等,而碳酸盐相主要含铁矿物是菱铁矿和磁铁矿。

2.1 氧化物相

大孤山氧化物相分布于主矿体南部,约占矿体体积的30%,与硅酸盐相渐变过渡(图 2图 3),主要岩性为磁铁石英岩,局部氧化成假象赤铁石英岩。该磁铁石英岩呈细条带-条纹状构造,条带较平直且连续,单条带一般宽约0.2~1mm(图 4a),由不同比例的磁铁矿和石英组成,可分为富铁条带和富硅条带(图 4b),条带界线较明显。根据磁铁矿单矿物的形态特征将其划分为两类,一类呈半自形粒状,粒径为0.01~0.1mm,多赋存于富硅条带中,浸染状于石英颗粒之间(图 4c),或形成细粒磁铁矿条带(图 4d);第二类磁铁矿结晶粗大,粒度在0.1~0.5mm,多赋存于富铁条带中,其内部裂隙较发育(图 4e);也可呈分散颗粒状分布于石英颗粒间(图 4d)。与磁铁矿伴生的石英常呈自形-半自形粒状,粒径为0.02~2mm,其粒度大小与重结晶程度相关,且由于受构造应力影响而常存在波状消光(图 4f)。

图 4 大孤山氧化物相BIF手标本和镜下特征 (a)磁铁石英岩手标本,条带较平直,宽约0.2~1mm;(b)富硅条带和富铁条带由不同比例的磁铁矿(Mt)和石英(Q)组成(单偏光);(c)半自形细粒状磁铁矿散布于石英颗粒之间(背散射);(d)细粒磁铁矿形成纹层条带,且可见结晶粗大的磁铁矿穿切石英颗粒(单偏光);(e)粗粒磁铁矿中发育裂隙(背散射),D1-D7为电子探针位置,实验结果见表 1;(f)经历不同程度重结晶作用而形成不同粒度的石英和磁铁矿,其中石英常见波状消光(正交偏光).Chl-绿泥石 Fig. 4 Hand specimen and micro-photographs of Dagushan oxide-facies BIF (a) the hand specimen of BIF showing bands are straight and 0.2 to 1mm in width; (b) black magnetite (Mt)-rich and white quartz (Q)-rich microbands are composed of different proportion of magnetite and quartz (plane-polarized); (c) magnetite crystals occur as small dispersed grains between quartz (backscattered electron); (d) fine-grain magnetites occur as discontinuous laminations and coarse grain magnetite crystals truncate the quartz (plane-polarized); (e) fractures in coarse-grain magnetites (backscattered electron) and D1 to D7 represent for positions of EPMA (Table 1); (f) magnetite and quartz indifferent sizes are formed through recrystallization in varying degrees, and there are some wavy extinction in quartzs (crossed polarizer). Chl-chlorite

表 1 大孤山条带状铁建造磁铁矿电子探针分析结果(wt%) Table 1 Electron probe microanalyses of magnetite from the Dagushan BIF (wt%)
2.2 硅酸盐相

硅酸盐相以大量含铁硅酸盐矿物(>10%)出现为特征,是本矿区最主要的沉积相,约占矿体体积的50%。该相主要集中于矿区中部,与氧化物相、碳酸盐相均成渐变过渡,且可见有薄层绿泥石英片岩及黑云变粒岩夹层。矿石整体呈细条带-条纹状构造,条带较平直连续,宽度约0.1~1mm,局部受后期构造作用的影响发生变形(图 5a)。矿石主要由磁铁矿(~35%)、石英(~35%)和黑硬绿泥石、镁铁闪石、绿泥石等硅酸盐矿物(~25%)组成,局部可见少量菱铁矿、铁白云石和方解石等碳酸盐矿物(~5%)。根据磁铁矿相对含量的多少可将BIF分为富硅条带和富铁条带,其中富硅条带主要由石英和硅酸盐矿物(如绿泥石和镁铁闪石)组成,且常见细粒磁铁矿浸染分布(图 5b);富铁条带主要由磁铁矿、硅酸盐及石英组成(图 5c),局部富硅条带和富铁条带之间界线不明显(图 5d)。

图 5 大孤山硅酸盐相铁建造野外和镜下特征 (a)硅酸盐相矿石手标本,条带弯曲变形;(b)不同比例的磁铁矿、石英和含铁硅酸盐组成条带,条带边界不明显(单偏光);(c)富硅条带由硅酸盐和石英组成,且常见由细小磁铁矿微条带(正交偏光);(d)强烈变质的硅酸盐相BIF的富硅条带中仅含镁铁闪石(Cum)(单偏光);(e)细柱状黑硬绿泥石(Stp)结晶方向多与原始纹层斜交(单偏光);(f)片状黑硬绿泥石与磁铁矿、石英共生(单偏光).Sid-菱铁矿 Fig. 5 Hand specimen and micro-photographs of Dagushan silicate-facies BIF (a) the hand specimen of silicate-facies ores showing bands are bend; (b) bands are composed of different proportion of magnetite, quartz and iron-bearing silicates (plane-polarized); (c) quartz-rich bands are composed of quartz and silicates and there are some magnetite-rich microbands (crossed polarizer); (d) quartz-rich bands in large deformed BIF only contain cummingtonite (Cum) (plane-polarized); (e) fine columer stilpnomelane (Stp) cuts cross the primary microbands (plane-polarized); (f) stilpnomelane flakes coexist with magnetite and quartz (plane-polarized). Sid-siderite

黑硬绿泥石为本区常见的低级变质硅酸盐矿物,主要有两种产出形式:一种是呈细柱状或束状,长约0.1~0.4mm,多呈棕褐色,穿插于菱铁矿之上,且其结晶方向多与原始纹层斜交(图 5e),说明其为变质期的产物;另一种是呈片状,长宽约0.05~0.1mm,浅黄-绿色,多色性明显,多分布于富铁条带中,与磁铁矿和石英共生,常被绿泥石交代(图 5f)。

镁铁闪石常与磁铁矿、石英和方解石共生,常被绿泥石交代,多呈纤维状(图 6a)或针柱状(图 6b),长约0.1~0.4mm,或呈集合体状交代穿插于黑硬绿泥石之上(图 6c),或呈近菱形片状,粒度约0.05~0.2mm,常见包裹石英或磁铁矿细粒的现象(图 6d)。

图 6 大孤山BIF镁铁闪石、绿泥石与铁白云石镜下特征 (a)纤维状镁铁闪石与石英和磁铁矿共生(单偏光);(b)针柱状镁铁闪石与石英、磁铁矿和方解石(Cal)共生(正交偏光);(c)镁铁闪石集合体交代黑硬绿泥石(单偏光);(d)近菱形片状镁铁闪石,局部包裹石英或磁铁矿细粒(反射光);(e)绿泥石交代镁铁闪石(正交偏光);(f)近菱形片状铁白云石(Ank)与磁铁矿、石英和绿泥石等矿物(正交偏光) Fig. 6 Micro-photographs of Dagushan cummingtonite, chlorite and ankerite (a) fibrous cummingtonite intergrown with magnetite and quartz (plane-polarized); (b) columnar cummingtonite intergrown with magnetite, quartz and calcite (Cal) (crossed polarizer); (c) cummingtonite replaced early stilpnomelane (plane-polarized); (d) rhombus cummingtonite contain magnetite and quartz grains (reflected); (e) fine columer cummingtonite replaced by chlorite (crossed polarizer); (f) ankerite (Ank) are rhombus and intergrown with magnetite, quartz and chlorite (crossed polarizer)

绿泥石在硅酸盐相和氧化物相中均有分布,浅绿色-浅黄色,多色性明显,根据其形态及矿物共生组合可将其分为三类:(1)常呈不规则片状孤立分布于富硅条带中(图 5e),与石英、磁铁矿等共生,粒度约为0.05~0.1mm;(2)交代镁铁闪石、黑硬绿泥石等硅酸盐矿物,呈片状-针柱状假象(图 5f图 6e, f),在揉皱发育的矿石中可见绿泥石与磁铁矿构成富铁条带(图 5c);(3)呈片状-板状,与菱铁矿、磁铁矿和石英共生,主要分布于富铁条带中。

铁白云石主要分布于硅酸盐相中,常呈较自形的菱型粒状,粒度小于0.1mm,多与磁铁矿、石英和绿泥石等矿物共生(图 6f)。

2.3 碳酸盐相

不同于James (1954)定义的碳酸盐相BIF(主要矿物为菱铁矿和石英),研究区碳酸盐相BIF除以上两种矿物外,还有大量磁铁矿,更接近于氧化物-碳酸盐过渡相,约占矿体体积的20%。该相主要发育于研究区北部,其中见厚度小于5m的薄层绿泥石英片岩夹层。矿石整体呈条带-条纹状构造,条带较平直且连续,宽度约0.2~1.5mm(图 7a);局部因压实作用而保留有间断分布的扁豆状石英,后经构造应力显现为右行旋转碎斑(图 7b),且该相中断裂构造发育,常见条带错断现象;碳酸盐脉发育,多穿切条带或充填断裂(图 7c)。根据条带内的主要矿物含量,可将其分为富硅条带和富铁条带(图 7d),其中前者主要由石英(70%)和菱铁矿(20%)组成,还有少量细粒磁铁矿不均匀散布或组成微条带(图 7e);后者主要由磁铁矿(60%)、菱铁矿(25%)和石英(10%)组成,还有少量绿泥石等矿物(图 7f)。菱铁矿为该相主要的含铁碳酸盐矿物,有两种产出形式,一种是呈半自形菱形粒状,粒径约0.01~0.05mm,较均匀地分布于富硅条带中(图 7d);另一种呈不规则集合体,与磁铁矿和少量石英构成富铁条带(图 7f)。

图 7 大孤山碳酸盐相BIF野外及镜下特征 (a)碳酸盐相BIF条带较平直且连续;(b)由石英组成的右行旋转碎斑;(c)富硅条带和富铁条带界线明显,条带宽度约0.2~1mm(单偏光);(d)富硅条带中常见磁铁矿细粒组成微条带(单偏光);(e)富铁条带中菱铁矿集合体与绿泥石、磁铁矿和少量石英共生(单偏光);(f)富铁条带主要由磁铁矿和菱铁矿及少量石英组成(单偏光) Fig. 7 Hand specimen and micro-photographs of Dagushan carbonate-facies BIF (a) the bands of carbonate-facies BIF are straight and continuous; (b) rotational porphyroclasts composed of quartz display right shearing characteristics; (c) the quartz-rich bands and magnetite-rich microbands are 0.2 to 1mm in width and the boundary lines between them are obvious (plane-polarized); (d) there are some fine-magnetite microbands in quartz-rich bands (plane-polarized); (e) siderite aggregation intergrown with magnetite, quartz and chlorite (plane-polarized); (f) magnetite-rich microbands are composed of magnetite, quartz and siderite (plane-polarized)
3 大孤山BIF主要矿物化学成分

铁建造中矿物种类丰富,我们对所有含铁矿物的主量成分进行了电子探针分析测试。实验在中国科学院地质与地球物理研究所岩石圈演化国家重点实验室电子探针实验室的JXA-8100型电子探针上完成。定量分析实验条件为加速电压15kV,束流10nA,空间分辨率达7nm,束斑直径为5μm,局部硅酸盐矿物颗粒较细,束斑直径为1μm,以下按照种类分别介绍矿物成分。

3.1 磁铁矿

大孤山BIF中的磁铁矿不是纯的Fe3O4,常含有少量Si、Mg、Ca等其他元素,而不同形态大小的磁铁矿主量成分在误差范围内较为一致(表 1)。半自形细粒状磁铁矿(粒径 < 0.1mm)成分较为均一,以Fe含量最高,变化范围为87.50%~90.63%,平均含量为89.54%,SiO2含量次之,变化范围为0.16%~3.74%,平均含量为1.49%,TiO2、Al2O3、MgO含量均小于0.30%,平均值在0.03%~0.07%之间,其余元素含量均低于0.10%,如部分磁铁矿MnO、CaO、K2O、NiO含量低于检出限(≤0.01%)。粗粒磁铁矿(粒径>0.1mm)的FeO含量稍高于前者,在89.40%~91.41%之间,平均为90.63%,但其SiO2含量稍低于细粒磁铁矿,变化范围为0.03%~1.56%,平均值为0.37%,其余元素含量也较低,平均值均低于0.05%。上述两种磁铁矿在误差范围内较为一致,推测不同粒径的磁铁矿成因较为相似。两种磁铁矿的FeO与SiO2含量大致呈负相关(图 8a),而FeO与Al2O3、MgO等含量之间相关性较弱(图 8b, c),推测Al2O3可能与陆源碎屑物质的加入相关,而铁质可能与海底热液有关。此外,沿粗粒磁铁矿对角线进行电子探针分析(图 4),发现其中心及边缘位置的FeO、SiO2等含量没有明显差异(表 1)。

图 8 大孤山粗粒和细粒磁铁矿FeO含量与SiO2、Al2O3、MgO含量之间的相关性图 Fig. 8 The correlation of SiO2, Al2O3, MgO as well as FeO in Dagushan coarse-and fine-grain magnetite
3.2 黑硬绿泥石

不同形态的黑硬绿泥石具有相似的主量成分(表 2),其中SiO2含量最高(43.90%~45.44%),平均值为44.63%,且FeO含量较高(29.12%~31.03%),平均值为30.03%,Mg2+以类质同象替换Fe2+,因而黑硬绿泥石中常含有一定量MgO(平均含量为5.32%),其与Al2O3(平均含量为5.23%)含量相近。此外,还有少量K2O(0.60%~1.34%,平均为0.97%)、Na2O(0.06%~0.47%,平均为0.22%)、CaO(0.29%~0.39%,平均为0.34%)。

表 2 大孤山条带状铁建造中黑硬绿泥石电子探针分析结果(wt%) Table 2 Electron probe microanalyses of stilpnomelane from the Dagushan BIF (wt%)
3.3 绿泥石

绿泥石电子探针化学成分分析结果及以14个氧原子为标准计算的结构式和特征值见表 3。由于矿物的包裹体、混层结构和复杂的共生关系都能使电子探针在分析绿泥石成分时产生误差,因此采用(Na2O+K2O+CaO)(质量分数)判断绿泥石数据能否应用,若所测绿泥石的(Na2O+K2O+CaO)(质量分数)>0.5%,说明其成分存在混染,需要剔除(Foster, 1962; Zang and Fyfe, 1995)。据此判断,本次绿泥石数据均可应用。绿泥石的主要成分为SiO2、Al2O3、MgO、FeO,其含量分别为21.48%~37.13%(平均为26.48%)、9.71%~20.30%(平均为17.52%)、2.31%~19.44%(平均为10.51%)和18.67%~42.90%(平均为32.07%)。总体来看,FeO、MgO含量变化较大,且此消彼长,反映二者相互置换比较普遍。

表 3 大孤山条带状铁建造中绿泥石电子探针分析结果(wt%) Table 3 Electron probe microanalyses of chlorite from the Dagushan BIF (wt%)

此外,根据绿泥石Fe-Si图解(图 9),Fe、Si原子数以28个氧原子为标准计算,第一类呈孤立片状的绿泥石主要为铁镁绿泥石,少量蠕绿泥石,其Fe/(Fe+Mg)比值为0.42~0.71,平均值为0.56,镁铁含量相近。第二类交代其他硅酸盐矿物的绿泥石主要为铁镁绿泥石及少量铁绿泥石,其Fe/(Fe+Mg)比值变化较大(0.35~0.86之间),而第三类与菱铁矿共生的主要为铁绿泥石,Fe/(Fe+Mg)值较高,可达0.91,指示其形成于富铁环境(Laird, 1988; Hillier and Velde, 1991; Zang and Fyfe, 1995)。此外,前人研究表明绿泥石Fe/(Fe+Mg)值的变化与系统的氧逸度有关,系统越还原,形成绿泥石的Fe/(Fe+Mg)值越大(Bryndzia and Scott, 1987a, b),说明后两种绿泥石的形成环境较为还原。

图 9 大孤山BIF中绿泥石分类图解(底图据Foster, 1962) 第一类绿泥石呈孤立片状,第二类绿泥石交代其他硅酸盐矿物,第三类绿泥石与菱铁矿共生 Fig. 9 Classification of chlorites from the Dagushan BIF (base map after Foster, 1962) The first type of chlorite presents isolated flake, and the second one replaces other silicates, while the third type has intergrowth relationships with the siderite
3.4 镁铁闪石

镁铁闪石是铁建造中重要的硅酸盐矿物,其电子探针分析结果如表 4,Mg/(Mg+Fe2+)在0.37~0.61之间,主要化学成分为SiO2、FeO和MgO,其中SiO2含量最高(51.22%~54.18%),FeO(24.27%~33.83%)和MgO(12.15%~18.44%)含量次之,K2O、Na2O、Al2O3、NiO的含量均低于0.1%。此外,有少量MnO(0.17%~0.56%)和CaO(0.13%~0.39%),说明仅存在少量Mn2+和Ca2+置换Fe2+和Mg2+

表 4 大孤山条带状铁建造中镁铁闪石电子探针分析结果(wt%) Table 4 Electron probe microanalyses of cummingtonite from the Dagushan BIF (wt%)
3.5 菱铁矿与铁白云石

菱铁矿是组成碳酸盐相BIF的主要矿物之一,与菱锰矿、菱镁矿成完全的固溶体系列,其矿物化学分析见表 5。菱铁矿主要成分为FeO(47.39%~51.92%),由于Fe2+可被Mn2+、Mg2+、Ca2+等替代而含有少量的MnO、MgO和CaO。此外,菱铁矿中TiO2、Al2O3、K2O、Na2O、Cr2O3、NiO含量均低于0.10%。在碳酸盐相BIF中,富硅条带中的菱形-半自形粒状菱铁矿和富铁条带中的菱铁矿集合体具有相似的FeO含量(平均值分别为49.87%和50.99%)、MnO含量(平均值分别为0.92%和0.99%)、MgO含量(平均值分别为3.78%和4.03%)和CaO含量(平均值分别为0.49%和0.58%),但是富硅条带中的菱铁矿中的SiO2含量较高于富铁条带中的菱铁矿(平均值分别为1.48%和0.03%)。

表 5 大孤山铁建造中菱铁矿电子探针分析结果(wt%) Table 5 Electron probe microanalyses of siderite and calcite from the Dagushan BIF (wt%)

铁白云石相比于菱铁矿而言,FeO含量较低(7.95%~15.21%),而CaO含量(26.93%~27.92%)、MgO含量(8.18%~15.54%)则相应升高,同时,其具有较高的MnO含量(0.34%~3.91%,平均值1.82%)(表 6)。

表 6 大孤山铁建造中铁白云石电子探针分析结果(wt%) Table 6 Electron probe microanalyses of ankerite from the Dagushan BIF (wt%)
4 原生矿物组成及其演化分析

探索原生矿物信息是BIF研究最基本的内容之一,不仅有助于理解BIF的沉淀机制和条带成因(Posth et al., 2008; Rasmussen et al., 2013),而且可约束BIF各沉积相的内在联系(Klein and Beukes, 1989)。本文在大孤山BIF岩相学和矿物化学成分研究的基础上,推测原生矿物组成,探讨成岩和变质作用条件下矿物的演化过程。

磁铁矿是研究区最主要的含铁矿物,主要呈两种形式,一是呈粗晶粒状分散分布(图 4g图 10a),或组成不规则状顺层延伸的铁条带(图 10b),此类磁铁矿可能是细粒磁铁矿重结晶作用的产物。二是呈半自形粒状浸染于石英颗粒之间或形成细粒磁铁矿条带(图 4d, f, 10b),目前大部分学者认为BIF中磁铁矿是三价铁氧化物、氢氧化物或菱铁矿在成岩和变质过程中发生转变的产物(Ayres, 1972; Ohmoto, 2003; Klein, 2005; Li et al., 2011, 2013; Sun et al., 2015; Sun and Li, 2017),主要由以下3种形成机制:

图 10 大孤山BIF磁铁矿和菱铁矿镜下特征 (a)粗粒状磁铁矿分散穿插切断石英(正交偏光);(b)半自形粒状磁铁矿浸染于石英颗粒之间或形成细粒条带(背散射);(c)磁铁矿交代菱铁矿现象(背散射);(d)菱铁矿集合体中包裹有细粒磁铁矿(背散射);(e)半自形粒状菱铁矿内包裹细粒磁铁矿(背散射);(f)菱铁矿集合体穿原始纹层(反射光) Fig. 10 Micro-photographs of Dagushan magnetite and siderite (a) coarse-grain magnetite truncate the quartz (crossed polarizer); (b) subhedral magnetite grains intersperse among quartz or form as microbands (backscattered electron); (c) siderite grains are replaced by magnetite grains (backscattered electron); (d, e) siderite present as aggregation and rhombus grain, both of which contain fine-grain magnetite (backscattered electron); (f) siderite aggregation truncate the primary lamination (crossed polarizers)

(1) 三价铁氢氧化物在微生物异化还原作用(Dissimilatory Iron Reduction, 简称DIR)下或与Fe2+反应形成磁铁矿(Lovley, 1993; Ohmoto, 2003; Frost et al., 2007; Johnson et al., 2008; Li et al., 2011, 2013);

(2) 赤铁矿和菱铁矿的变质反应过程(Koziol, 2004; Mloszewska et al., 2012)

3FeCO3(菱铁矿)+Fe2O3(赤铁矿)→Fe3O4(磁铁矿)+CO2(.=480~650℃,.=5~12kbar);

(3) 早期菱铁矿的氧化过程,反应方程式如下(French, 1971):

12FeCO3(菱铁矿)+2H2O→4Fe3O4(磁铁矿)+11CO2+CH4(.=450℃, .=2kbar)。

在矿区内并未见磁铁矿同时交代赤铁矿和菱铁矿的现象,可排除第二个形成机制。值得注意的是,在碳酸盐相BIF中局部可见磁铁矿交代菱铁矿现象,如图 10c方框中所示,磁铁矿沿菱铁矿边缘生长,推测此类磁铁矿是由菱铁矿氧化形成。由于这种现象仅较少存在且无法大规模形成磁铁矿,说明菱铁矿氧化作用可能为磁铁矿形成的次要机制。当前大多研究者认为三价铁氢氧化物的后期转化形成磁铁矿是主要途径,通过实验模拟微生物异化还原反应过程、分析磁铁矿的晶体化学等证据可以证明(Ohmoto, 2003; Johnson et al., 2008; Li et al., 2011, 2013; Raye et al., 2015),推测此机制可能同样适用于大孤山矿区。

然而,也有学者通过实验模拟认为磁铁矿是原生矿物,可由富Fe2+的热液与沉淀的三价铁氢氧化物在水体中反应直接形成(Li et al., 2017),但此反应也可在成岩过程中发生,且实验未考虑前寒武纪海洋高Si浓度和高温热液温度的影响(>250℃)(Siever, 1992; Bau and Dulski, 1996),不足以证明原生磁铁矿的存在。

关于菱铁矿的成因,目前主要有两种观点,早期学者认为菱铁矿类似于海相碳酸盐岩,当水中存在溶解的二氧化碳时,可能与Fe2+结合会形成菱铁矿(Ayres, 1972; Klein and Beukes, 1989; Kaufman et al., 1990; Bolhar et al., 2004)。随后,有学者发现菱铁矿中存在燧石和赤铁矿的包裹体(Klein, 2005; Pecoits et al., 2009),且菱铁矿的碳同位素值(.13C)明显低于上覆碳酸盐矿物的碳同位素值(0‰)(Fischer et al., 2009),说明菱铁矿并非直接沉积成因,可能是由于早期形成的三价铁氢氧化物在有机碳存在的情况下经微生物DIR作用而生成。在大孤山矿区,菱铁矿有两种分布形式,或呈自形菱形-半自形粒状弥散分布于富硅条带中(图 7c),或呈粒状集合体与磁铁矿、石英共生(图 7e),总体看来二者成分较为一致,推测成因类似,且在两种菱铁矿中均可见包裹有细粒磁铁矿的现象(图 10d, e)和穿插原始纹层现象(图 10f),说明菱铁矿非原生矿物,可能为三价铁氢氧化物在成岩-变质过程中经微生物DIR作用转变而成,且在成岩或变质作用过程中由于Ca2+、Mg2+的加入而转化为铁白云石和方解石等(Klein, 2005; Heimann et al., 2010)。

黑硬绿泥石(K2x(H2O)2{Fe2-xAl[Si4O10](OH)3})是BIF中常见的硅酸盐矿物,关于黑硬绿泥石的成因问题仍存在较大争议,目前主要有以下两种观点:(1)变质成因:黑硬绿泥石常赋存于低级变质的BIF中(Klein, 1983),其富铁贫铝,化学成分与现代海洋中常见的绿脱石(Nax(H2O)4{Fe23+[AlxSi4-xO10](OH)2})等沉积矿物相似,且绿脱石可由三价铁氢氧化物和溶解硅反应生成(Dekov et al., 2007),因此推测黑硬绿泥石是由绿脱石等铁硅酸盐矿物在成岩-变质作用下形成(Klein, 2005)。(2)原生成因:部分学者发现燧石结核中存在大量黑硬绿泥石球形微粒(5~20μm)或纳米级铁硅酸盐微粒(主要成分为黑硬绿泥石和铁蛇纹石),推测黑硬绿泥石的形成应早于硅质胶结,可能为原生沉积矿物(Rasmussen et al., 2013, 2016)。大孤山地区的黑硬绿泥石或呈细柱状或束状穿插于菱铁矿之上(图 5e),或呈片状与磁铁矿和石英共生(图 5f),前者结晶方向多与原始纹层斜交(图 5e),说明其为变质期的产物;后者内部局部可包裹细粒磁铁矿,为成岩-变质期的产物(图 11a),由此可判定大孤山BIF中的黑硬绿泥石主要为绿脱石等硅酸盐矿物变质而成。

图 11 大孤山含铁硅酸盐矿物镜下特征 (a)片状黑硬绿泥石内部可包裹细粒磁铁矿(背散射);(b)不规则片状绿泥石零星分散与矿物之间(单偏光);(c)绿泥石与菱铁矿、磁铁矿和石英共生,其内可见包裹细粒磁铁矿(背散射);(d)绿泥石交代黑硬绿泥石(单偏光) Fig. 11 Micro-photographs of Dagushan iron-bearing silicates (a) stilpnomelane flakes contain fine-grain magnetite; (b) irregular chlorite disperse among magnetite and quartz; (c) chlorite intergrown with siderite, magnetite and quartz, which wrap some fine-grain magnetite; (d) stilpnomelane replaced by chlorite

镁铁闪石主要有两种分布形式,或交代早期形成片状的黑硬绿泥石(图 6c),可能为黑硬绿泥石进变质的产物;或与磁铁矿、石英和方解石等矿物共生(图 6b),前人研究表明镁铁闪石也有可能是由铁白云石与石英反应形成,反应方程式为:7Ca(Fe, Mg)(CO3)2+8SiO2+H2O→(Fe, Mg)7Si8O22(OH)2+7CaCO3+7CO2(Klein, 2005),但在大孤山矿区内仅见镁铁闪石与方解石共生,未见明显的闪石类矿物交代铁白云石等矿物的现象,因此,此类闪石矿物的成因还有待深入研究。

绿泥石为一种富铝的层状硅酸盐矿物,其最有利的形成条件是富铁或镁的碱性水介质环境,一般出现于含有碎屑物质的富铁岩石中(James, 1954)。在大孤山BIF中的绿泥石主要有三种产出形式,一是绿泥石呈孤立片状(图 11b),零星分散与矿物之间,多为铁镁绿泥石,可能是由富铝的粘土碎屑在成岩-变质期,孔隙水中富铁、镁且pH为碱性环境时转变而成(Wilson, 1982; Bloch et al., 2002)。第二种绿泥石与菱铁矿、磁铁矿和石英共生(图 7e),多出现于富铁条带中,属于铁绿泥石,且其内可见包裹细粒磁铁矿现象(图 11c),说明其形成于成岩-变质过程。铁绿泥石是角闪岩相变质的铁建造中的常见矿物,如西格陵兰Isua地区BIF(Dymek and Klein, 1988),其可能是高温平衡的产物,或者其他矿物退变质的产物。第三种交代黑硬绿泥石(图 11d)或闪石类矿物(图 6e, f),可能为硅酸盐矿物进变质或退变质的产物,此类绿泥石多为铁绿泥石。

石英和燧石常被认为是早期沉淀的硅质在成岩或变质过程中重结晶的产物(Klein, 2005)。由于显生宙之前海洋中普遍缺乏能消耗硅的生物,因而前寒武纪海水硅浓度异常高(~2毫摩),大约为现代海洋(~0.1毫摩)的数十倍,接近无定形硅的饱和浓度(Siever, 1992; Konhauser et al., 2007)。因此,硅可因蒸发作用或温度变化导致过饱和而沉淀(Garrels, 1987; Posth et al., 2008),或者与固相含铁矿物(如三价铁氢氧化物)同沉淀(Ewers, 1983; Konhauser et al., 2007; Fischer and Knoll, 2009)。

综合以上矿物结构、成分及可能的成因,我们推测,大孤山BIF的原生矿物主要为三价铁的氢氧化物、无定型硅胶和富铝粘土碎屑,并在后期成岩-变质作用下转变为现存矿物(图 12)。其中三价铁氢氧化物为最主要的原生含铁矿物,其与硅质、碳质、Fe2+等反应可形成不同沉积相矿物,是沉积相形成的“原材料”。

图 12 大孤山铁建造原生矿物组成及后期演化图 Fig. 12 Primary mineral composition and its evolutionary histories for the Dagushan BIF
5 物理化学条件及对BIF形成环境的指示

在沉积-成岩-低级变质过程中,BIF中铁氧化物的稀土元素几乎不分馏,因而许多学者通过稀土元素特征来研究其沉淀时的海洋环境(Bau and Dulski, 1996; Bolhar et al., 2004; Alexander et al., 2008; Planavsky et al., 2010; Viehmann et al., 2015; Wang et al., 2015a; 张连昌等, 2012)。大孤山BIF稀土元素特征与典型的Algoma型BIF一致,其中大多数样品CePAAS异常不明显,说明BIF在缺氧的环境下发生沉淀;LREEPAAS相对HREEPAAS亏损;正LaPAAS、YPAAS异常以及高Y/Ho比值等特点指示出BIF与海水的关系密切;样品中正EuPAAS异常明显,说明BIF沉淀的海水中有高温(>250℃)海底热液的加入(Dai et al., 2014; 李志红等, 2008; 杨秀清, 2013; 姚通等, 2014)。

BIF中现存矿物均经历了一定程度的成岩-变质作用,因此矿物共生相图所反映的形成条件多用于指示成岩-变质阶段的环境条件。根据BIF中黑硬绿泥石、铁蛇纹石和菱铁矿等矿物共生组合,利用相平衡图解,可得出不同矿物共生的温度、压力、氧逸度、酸碱度、氧化还原电位等物理化学条件(Klein, 2005; 王长乐等, 2015)。大孤山BIF变质程度在绿片岩相-低角闪岩相,未发现铁蛇纹石、铁滑石等矿物,目前识别的主要矿物组合为磁铁矿-黑硬绿泥石和磁铁矿-菱铁矿。如图 12为铁蛇纹石、磁铁矿和菱铁矿稳定的PO2-PCO2图解,可以看出磁铁矿的形成需要相对较低的氧气分压(10-63~10-10),说明其形成于低氧逸度环境,且磁铁矿在研究区范围内分布最广,大部分矿物均与磁铁矿共生,考虑到其稳定时较低的PO2,说明铁建造大部分矿物是在缺氧环境中形成的;而菱铁矿稳定区间相对较大,氧气分压小于0,但需一定量的CO2;从菱铁矿与磁铁矿共生线可看出,二氧化碳分压波动范围较小,且当氧气分压升高时,只有二氧化碳分压相应升高才能维持共生组合稳定,否则会导致菱铁矿氧化形成磁铁矿。

图 13显示了富铁水体系中磁铁矿、黑硬绿泥石、菱铁矿等矿物平衡时的Eh和pH范围(Klein, 2005),此体系硅浓度接近饱和(A[H4SiO4]=10-2.7),且受氧分压和溶解的碳质活度的制约,可以看出不同矿物稳定存在于不同的物理化学条件。磁铁矿可在较高氧逸度条件下形成,虽然其稳定的酸碱度范围较大,但整体更倾向于中到碱性环境,且pH值越大,形成磁铁矿、黑硬绿泥石等矿物所需的氧逸度就越低;黑硬绿泥石的存在范围大于铁蛇纹石、铁滑石等硅酸盐矿物,其可在氧化还原电位小于0.3且pH接近中性或碱性条件下稳定存在;菱铁矿稳定存在需要溶解碳质的热力学活度大于0.1且氧化还原电位较低(-0.4~0)等条件。值得注意的是,Fe2+可在低氧(Eh < 0.8)和酸性(pH < 6)条件下稳定存在、累积和迁移,而Fe(OH)3多在低氧、中到碱性环境下形成,说明从Fe2+向Fe(OH)3转化过程中,海水中pH可能发生变化,其可能是导致铁质沉淀的因素之一。此外,大孤山铁建造中黄铁矿含量极低,说明体系中硫逸度低。

图 13 含铁矿物共生相图 (a) 25℃下铁蛇纹石、Fe(OH)3、磁铁矿和菱铁矿的PO2-PCO2图解,其中硅浓度接近饱和(Klein and Bricker, 1977);(b、c) 25℃富铁水体系中磁铁矿、菱铁矿、铁蛇纹石和黑硬绿泥石的pH-Eh图解,氧气分压用虚线表示(Klein, 2005) Fig. 13 Coexisting phases of iron-bearing minerals (a) stability relations among greenalite-Fe(OH)3-magnetite-siderite as a function of PO2 and PCO2 at saturation with respect to amorphous silica at 25℃ (after Klein and Bricker, 1977); (b, c) Eh-pH diagrams depicting the stability fields of magnetite, siderite, greenalite and stilpnomelane in the system Fe-H2O-O2-SiO2 dissolved C at 25℃ and 1atm total pressure. Dotted lines represent for the pressure of O2 (Klein, 2005)

综上所述,大孤山BIF在沉积阶段主要为缺氧还原环境,而在成岩-变质阶段为缺氧、中到碱性和低硫的环境,推测大孤山BIF可能沉积于还原性水体中,且影响矿物形成及演化的因素主要为古海水氧化还原状态、酸碱度、CO2含量和硫逸度等条件。

6 结论

大孤山BIF为变质程度较低的Algoma型BIF,矿物种类较多,且矿物间发生复杂的反应,导致矿物成因多样性。根据矿物特征及矿物化学信息可初步推测大孤山BIF的原生组成为无定型硅胶、三价铁氢氧化物及富铝的粘土碎屑等,其中三价铁氢氧化物为最主要的含铁原生矿物,其在后期的成岩和变质作用过程中可转变为磁铁矿、菱铁矿、黑硬绿泥石、镁铁闪石等矿物。矿物间的反应条件和共生组合反映除BIF成岩-变质期为低氧逸度(或低氧化还原电位)、中到碱性和低硫的环境,推测大孤山BIF可能沉积于还原性水体中。通过对大孤山BIF的含铁矿物成因、原生矿物组合及形成环境等方面的研究,可大体了解Algoma型BIF的沉积矿物相特征及其原生矿物组成。

致谢 感谢辽宁省冶金地质勘查局地质勘查研究院刘明军工程师在野外考察中的全力帮助,感谢中国科学院地质与地球物理研究所电子探针实验室毛骞老师和张迪老师在实验中的悉心指导。感谢万渝生老师的邀稿!
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