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1.
The mineralogy and chemistry of banded iron formations (BIF) of Archaean high grade granulite gneiss belt of Tiruvannamalai
area are presented here. The BIF of this area is chemically different from those around the world. The iron formations and
associated granulites are of different origin namely metasedimentary and metavolcanic respectively. 相似文献
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冀东地区柞栏杖子BIF出露于绿片岩相—低角闪岩相朱杖子岩群变质岩中。铁矿石主要由石英和磁铁矿组成,还含有少量透闪石和黑云母。主量元素主要为Si O2、Fe2O3和Fe O,其次为少量的Ca O和Mg O。较低的Al2O3含量、极低的Ti O2含量和高场强元素(HFSE)暗示,铁矿石中陆源碎屑物质含量很低。铁矿石的稀土元素含量较低,稀土元素配分模式特征为轻稀土元素相对亏损、重稀土元素相对富集。较明显的Eu正异常、轻微的Y正异常及较高的Y/Ho值的稀土元素特征,类似于高温热液和海水的混合热液,暗示成矿物质主要来自于海水和高温热液。对柞栏杖子BIF矿体夹层黑云斜长变粒岩进行SHRIMP锆石U-Pb定年,207Pb/206Pb年龄加权平均值为2572±8Ma(MSWD=5.8),可代表柞栏杖子BIF的形成年龄。综合前人研究,认为冀东地区变质级别不同的BIF物质来源相同、形成年代相近,BIF的变质可能和2500Ma左右华北克拉通东部陆块发生的地幔岩浆底侵事件有关。 相似文献
4.
A typical Algoma-type banded iron formation (BIF) occurs in Orvilliers, Montgolfier, and Aloigny townships in the Abitibi Greenstone belt, Quebec, Canada. The BIF is composed of millimeter to decimeter thick beds of alternating fine-grained, dark gray to black, well laminated, magnetite-rich (and/or hematite) beds and quartz–feldspar metasedimentary (graywacke) beds. The BIF is well defined by magnetic anomalies. These BIF layers are commonly associated with decimeter to meter thick horizons of metasedimentary rocks and mafic to intermediate volcanic rocks, which are locally crosscut by dikes of felsic or mafic intrusive rocks and, as well, narrow dikes of lamprophyre. The upper and lower contacts of the BIF are gradational with the adjacent graywacke. All geological units in the area are metamorphosed to the greenschist facies of regional metamorphism. Magnetite is mainly associated with subordinate amounts of hematite, quartz, Na-rich plagioclase, and muscovite. The fine-grained magnetite content is composed of 77% to 89% of the principal iron oxide minerals present. The magnetite occurs as disseminated idiomorphic to sub-idiomorphic small crystals, which average 20 μm ± 5 μm in size. Hematite is the second most abundant iron oxide mineral. Although less abundant, red jasper occurs in cherty horizons with strongly folded fragments and within fault zones. This particular Algoma-type iron formation stratigraphically extends more than 36 km along strike. It dips sub-vertically with a true width from 120 m to 600 m. The origin of the BIF is closely linked to regionally extensive submarine hydrothermal activity associated with the emplacement of volcanic and related subvolcanic rocks in an Archean greenstone belt. 相似文献
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ASTER多光谱遥感数据目前可以用于岩石矿物资源信息的识别和提取。本研究尝试利用ASTER 可见光近红外（VNIR）和短波红外（SWIR）的多光谱遥感数据提取干旱地区的岩石与矿物信息。基于新疆天山西南缘柯坪隆起东部不同地层单元岩石的化学组成和矿物成份以及VNIR SWIR谱域光谱吸收特征的分析,我们采用相关吸收波段深度（RBD）和波段比值（BR）方法对研究区的多光谱遥感数据进行图像处理,有效区分和识别了白云岩、石灰岩、砂岩以及阿克苏群的蓝片岩—绿片岩和砂质片岩。白云岩的CO2-3吸收谱带中心波长位于232〖KG*3〗μm,与灰岩的CO2-3 吸收谱带中心波长位置235 μm相比,具有向短波长方向移动的特点,据此可以利用RBD7、RBD8分别有效的识别白云岩和灰岩; 长英质岩石显示Al OH和Fe3+ VNIR SWIR吸收特征,而基性 超基性岩石显示Fe2+ 和Fe、Mg OH特征,利用不同的铁价态和次要矿物可以区分它们：ASTER band2/band1代表了含Fe3+ 矿物分布信息、ASTER band5/band4代表了含Fe2+ 矿物分布信息、RBD6可以估计Al OH矿物的丰度; 砂质/泥质片岩含较多的多硅白云母、绿泥石、黑硬绿泥石以及风化后表面覆盖的其它粘土矿物,在221 μm（band 6）存在有特征的吸收谱带,并且在165 μm（band 4）具有较高的反射率,而蓝片/绿片岩在221 μm（band 6）反射率较高,不具有明显特征吸收谱带,同时其在165 μm（band 4）反射率较低,因此蓝片/绿片岩ASTER band4/band6 比值低。应用ASTER band4/band6波段比值可以有效的区分开砂质/泥质片岩与蓝片岩/绿片岩。 相似文献
6.
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 总被引：1，自引：1，他引：0
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P_(O_2)-P_(CO_2)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、CO_2含量和硫逸度。 相似文献
7.
P.V. Sunder Raju 《Journal of Asian Earth Sciences》2009,34(5):663-673
The ～1.2 km long and ～250 m wide Chikkasiddavanahalli (C.S. Halli) hill range consists of mixed sulphidic-oxide banded iron formations (BIFs) and Fe-rich phyllites (±carbonaceous), which overlie carbonated schistose and massive meta volcanics. In stratigraphic succession, the lithologies represent the Ingaldhal Formation and are an integral part of the Chitradurga schist belt in the Western Dharwar Craton. The general strike at C.S. Halli varies from N–S to 340° with vertical to steep dips towards east and west. The sulphides, oxides and silicates exhibit intergrowth replacement textures developed during regional greenschist- and amphibolites- facies metamorphism. The BIFs show mesobands of recrystallised cherts and iron sulphides such as pyrite, arsenopyrite, and silicates such as subordinate grunerite, hornblende, chlorite, muscovite, actinolite and minor carbonates such as ankerite, calcite and magnesian siderite. Chemical data indicate depletion in Ti, Mn, Co, Cu, Cr and Ni in these iron formations. Most chondrite normalized REE patterns of the iron formation show moderate LREE and HREE enrichment coupled with strong positive Eu anomaly; the mineralized portions exhibit characteristic negative Ce and Eu anomalies (Eu/Eu1 0.21 to 3.00). The total REE abundance varies, correlates well with the iron contents of the BIFs, and similar to those exhibited by hydrothermal plumes [Chown, E.H., Dah, E.N., Mueller, W.G., 2000. The relation between iron formation and low temperature alteration in a Archean volcanic environment. Precambrian Research 101, 263–275]. Trace and REE data suggest that primary mantle-derived hydrothermal solutions were added to the pore fluids of sediments of the Chitradurga basin and supplied chemical constituents such as FeO, SiO2 and REE. Oxidation of FeO to Fe2O3 was caused by the photosynthesis of primitive stromatolite-building cyanobacteria. Geochemical data suggest a model involving epigenetic gold mineralisation in close association with shear zone deformation, quartz-calcite vein activity and sulphidation in the mixed sulphide oxide facies BIF and associated iron phyllites in the C.S. Halli area, Western Dharwar Craton, India. 相似文献
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《Resource Geology》2018,68(3):287-302
Banded iron formations (BIFs) are the most significant source of iron in the world. In this study, we report petrographic and geochemical data of the BIF from the Meyomessi area in the Ntem Complex, southern Cameroon, and discuss their genesis and the iron enrichment process. Field investigations and petrography have revealed that the studied BIF samples are hard; compact; weakly weathered; and composed of magnetite, subordinate quartz, and geothite. The geochemical composition of the whole rock reveals that iron and silica represent more than 98 wt% of the average composition, whereas Al2O3, TiO2, and high‐field strength elements (HFSE) contents are very low, similar to detritus‐free marine chemical precipitates. The total iron (TFe) contents range from 48.71 to 65.32 wt % (average of 53.29 wt %) and, together with the low concentrations of deleterious elements (0.19 wt % P on average), are consistent with medium‐grade iron ores by global standards. This interpretation is confirmed by the SiO2/Fe2O3total versus (MgO + CaO + MnO)/Fe2O3total discrimination plot in which most of the Meyomessi BIF samples fall in the field of medium‐grade siliceous ore. Only one sample (MGT94) plots in the high‐grade magnetite–geothite ore domain. The high Fe/Ti (376.36), Fe/Al (99.90), and Si/Al (29.26) ratios of the sample are consistent with significant hydrothermal components. The rare earth elements (REE) contents of the studied BIF samples are very low (∑REE: 0.81–1.47 ppm), and the Post‐Archaean Australian Shale (PAAS)‐normalized patterns display weak positive Eu anomalies (Eu/Eu*: 1.15–1.33), suggesting a syngenetic low‐temperature hydrothermal solutions, similar to other BIF worldwide. However, the Meyomessi BIFs show high Fe contents when compared to the other BIFs. This indicates an epigenetic mineralization process affected the Meyomessi BIF. From the above results and based on the field and analytical data, we propose that the genetic model of iron ores at the Meyomessi area involves two stages of the enrichment process, hypogene enrichment of BIF protore by metamorphic and magmatic fluids followed by supergene alteration as indicated by the presence of goethite in the rocks. 相似文献
9.
BIF微量稀土元素分析方法及其在冀东司家营铁矿中的应用 总被引：3，自引：2，他引：1
以硅铁条带交替出现为特征的条带状铁建造(BIF)是世界上最重要的铁矿资源类型,精确分析磁铁矿的化学组成具有重要意义。本文开展了磁铁矿样品不同溶样方法分析结果的比对,并详细分析和讨论了冀东司家营铁矿磁铁矿与燧石单条带的微量及REE元素分析的地球化学特征。分析结果表明,对于磁铁矿样品,常规HF+HNO3溶样法与HBr+HNO3组合溶样法具有一致的溶样效果;司家营BIF的Zr,Sc,Th含量极低,表明未受陆源碎屑的污染;铁质与硅质具有低LREE、高HREE、La和Y正异常的海水REE特征,同时具有Eu正异常的热液REE特征;Ce负异常的缺乏,说明当时的古海洋是一个缺氧的环境。研究发现富铁条带的稀土总量大于富硅条带的稀土总量,这可能与硅、铁沉积物的地球化学习性相关,铁质沉积物更易吸收稀土元素。富矿和普通矿石具有原生的热液与海水的混合来源,部分富矿受到后期流体的强烈扰动,甚至表现出热液流体的特征。 相似文献
10.
冀东、五台和吕梁地区条带状铁矿的稀土元素特征及其地质意义 总被引：8，自引：0，他引：8
详细报道了冀东、五台和吕梁地区条带状铁矿全岩样品的稀土元素分析结果。结果表明,研究区BIF具有非常相似的特征：稀土总量均较低;经页岩标准化的稀土元素配分模式均呈现轻稀土亏损、重稀土富集的特征;Y/Ho比值较高;具有明显的Eu、Y、La的正异常,且这些特征表明研究区BIF的稀土元素来源于火山热液和海水的混合溶液。虽然BIF均显示Eu正异常,但不同类型、不同沉积年龄BIF的铕异常程度不同：与吕梁地区Superior型铁矿相比,冀东和五台地区的Algoma型铁矿显示了更大的Eu正异常;并且自中太古代-新太古代-古元古代,BIF的铕正异常逐渐减小,这可能反映了随着BIF沉积年龄的减小,进入到该地区海水中的高温热液流体逐渐减少;同时,研究区BIF缺乏明显的Ce负异常,可能暗示在BIF沉积时海水的氧化还原状态为缺氧环境。 相似文献
11.
鲁西杨庄条带状铁建造特征及锆石年代学研究 总被引：3，自引：0，他引：3
泰山群主要分布于鲁西地区中部,是鲁西花岗-绿岩带的一个重要组成部分。近年来,在沂水县杨庄发现了一定规模的沉积变质铁矿,铁矿层位赋存于柳杭岩组上段的斜长角闪岩段内。本文对杨庄铁矿BIF及侵入地层的岩浆岩进行锆石年代学测定,测定的含磁铁矿斜长角闪岩中锆石U-Pb年龄数据主要集中在2.6Ga附近,确定斜长角闪岩的形成年龄为2615±61Ma;铁矿顶板黑云母石英片岩的形成年龄小于2527±66Ma,该岩段又被晚期混合花岗岩穿插,其锆石年龄测定结果为2469±34Ma,所以推测黑云母石英片岩的形成年龄在2.5Ga附近。据此我们认为鲁西地区的"柳杭岩组"似可近一步解体为新太古的斜长角闪岩段(含磁铁矿建造)和古元古的表壳岩段(以片岩系为主)。混合花岗岩的形成年龄属于古元古代早期,似乎可以填补全球地质演化的静寂期 (2.3~2.5Ga)。以泰山群为代表的变质岩系在地下较深部位出现或者被中晚元古代盖层覆盖,为探讨华北克拉通早期演化和开展华北克拉通BIF型铁矿研究具有重要意义。 相似文献
12.
黑龙江佳木斯地区羊鼻山BIF型铁矿床的形成时代及地质意义 总被引：2，自引：0，他引：2
羊鼻山铁矿位于佳木斯地块中部,铁建造主要产于兴东群大盘道组第一岩段,含铁建造为一套由石榴石云母石英片岩、矽线石黑云母石英片岩和片麻岩等组成的孔兹岩系,其原岩形成于浅海陆棚沉积环境。矿石中金属矿物主要为磁铁矿,并被少量后期黄铁矿交代,非金属矿物主要为石英与矽线石,二者均呈定向排列,矿石构造以条带状构造为主,少量呈块状构造。据成矿地质条件和矿床特征,结合区域矿床对比研究,认为羊鼻山铁矿成因属BIF型,初步划归Superior型。含铁建造中矽线石黑云母石英片岩中碎屑锆石U-Pb年龄可分为4组,分别为1 006~1 212 Ma、1 238~1 480 Ma、1 521~1 742 Ma以及1 800 Ma,据最小年龄可推断羊鼻山铁矿含铁建造的沉积上限年龄为1 006Ma;1 238~1 480 Ma年龄组丰度最大,反映铁建造与围岩的剥蚀区岩石以中元古界岩浆岩为主;而最大年龄组中的2 672 Ma与2 711 Ma样品为佳木斯地块最古老的锆石之一,说明佳木斯地块存在太古代结晶基底。结合同样被认为是佳木斯地块结晶基底的麻山群近期年代学研究结果,认为兴东群应为麻山群的下伏地层。 相似文献
13.
东南极南查尔斯王子山条带状含铁建造(BIF)产于鲁克山古元古代鲁克群的底部,总厚400 m,矿体厚度30~70 m,铁矿平均品位33.5%。该条带状含铁建造形成过程可能与变质火山岩有联系,在成因分类上属于苏必利尔湖型含铁建造和阿尔戈马型含铁建造之间的过渡类型。高精度航磁测量在鲁克山圈定出宽约10 km的北、南两条磁异常条带,延长分别约为50 km和60 km。据此初步建立该地区沉积变质型铁矿预测模型,圈定了含铁建造的资源分布范围,最终估算出铁矿石可开采的资源量大于百亿吨。 相似文献
14.
华北克拉通前寒武纪BIF铁矿研究:进展与问题 总被引：29，自引：18，他引：11
研究表明,BIF铁矿在华北克拉通的分布具有一定规律性.大规模BIF铁矿主要发育在绿岩带分布区的鞍山-本溪、冀东、霍邱-舞阳、五台、鲁西和固阳等地;华北克拉通时代最古老的BIF形成于古太古代,最年轻BIF形成于古元古代早期,但BIF铁矿的峰期为新太古代晚期(2.52 ～2.56Ga);BIF铁矿类型可划分为阿尔戈马型和苏比利尔湖型两类,但华北以晚太古代绿岩带中的阿尔戈马型为主,仅吕梁的古元古代袁家村铁矿具典型苏比利尔湖型铁矿特征.根据BIF在绿岩带序列中的产出部位和岩石组合关系,可将华北BIF划分为:1)斜长角闪岩(夹角闪斜长片麻岩)-磁铁石英岩组合;2)斜长角闪岩-黑云变粒岩-云母石英片岩-磁铁石英岩组合;3)黑云变粒岩(夹黑云石英片岩)-磁铁石英岩组合;4)黑云变粒岩-绢云绿泥片岩-黑云石英片岩-磁铁石英岩组合;5)斜长角闪岩(片麻岩)-大理岩-磁铁石英岩组合等5种类型.华北克拉通BIF形成时代与早前寒武纪岩浆活动的时间基本一致(2.5～2.6Ga),但与华北克拉通陆壳增生的峰期(2.7～2.9Ga)有一定偏差,其原因可能与新太古代晚期华北克拉通构造-热事件十分强烈有关.华北克拉通新太古代BIF大多形成于岛弧环境,但局部地区(如固阳)BIF铁矿可能形成于深部有地幔柱叠加的岛弧环境.华北克拉通BIF富矿主要有三种类型:原始沉积、受后期构造-热液叠加改造和古风化壳等,但总体不发育富铁矿,国外发育的风化壳型富铁在我国甚为少见.本文认为在探讨BIF铁矿类型时,需要从绿岩带发育序列进行综合判别.阿尔戈马型铁矿一般产于克拉通基底(绿岩带)环境,苏比利尔湖型铁矿一般形成于稳定克拉通上的海相沉积盆地或被动大陆边缘.华北克拉通BIF铁矿地球化学研究结果表明,BIF铁矿无Ce负异常且Fe同位素为正值,从而暗示铁矿沉淀的环境为低氧或缺氧环境,而铕正异常可能指示BIFs为热水沉积成因,其机制可能为海水对流循环从新生镁铁质-超镁铁质洋壳中淋滤出F(e)和Si等元素,在海底排泄沉淀成矿,而条带状构造的形成可能归咎于成矿流体的脉动式喷溢.但对于BIF铁矿的物质来源、成矿条件和机制、富铁矿成因、华北克拉通不发育苏比利尔湖型铁矿的原因等方面,仍需深入研究. 相似文献
15.
The Huoqiu iron ore field in northwest Anhui Province is located in the North China Craton (NCC). As a large banded iron formation (BIF) iron ore field, ore bodies occur in a middle-high grade of Neoarchean metamorphic formation, forming a banded silicon–iron series from north to south. The main ore bodies can be divided into two sub-belts from bottom to upper layers, i.e. the A + B ore belt consisting of leptynite–schist–magnetite–quartz formation, and the D ore belt consisting of schist–marble–hematite–quartz formation. Based on a dataset from geological settings, geophysical and geochemical exploration, ore-forming conditions and structural analysis of the iron deposit, we discuss structural types, sedimentary environments, deep tectonic and ore-controlling factors as well as characteristics and distribution of this colossal BIF ore field in the Huoqiu region.Using LA-ICP-MS techniques, we obtained the oldest U–Pb age of ca. 2.7 Ga for plagioclase amphibolite as its original rock, and 1.8 Ga for magmatic granite in the Huoqiu Group. The Hf isotopes of zircon were also determined, resulting in the oldest Hf model age of 3.5 Ga.Geochemical data indicate that the protolithes of amphibolites belong to a series of subalkaline rocks with enrichments of large ion lithophile elements and depletions of high field strength elements, which are typical volcanic arc rocks. The amphibolites have low K2O concentrations with low ratios of Ti/V (22.7 to 25.9 averaging 24.5), similar to island arc tholeiite. This suggests that the iron deposit and BIF are of the Superior type in the Huoqiu region. 相似文献
16.
17.
内蒙古朱拉扎嘎金矿ETM+数据提取蚀变异常方法研究 总被引：2，自引：2，他引：0
中国西北部大部分地区属于荒漠景观区,自然条件恶劣,常规的地质、物探、化探找矿工作难度大,如何借助遥感技术进行矿产资源勘查,是地质工作者积极探索的方向之一。本次工作选择内蒙古阿拉善盟朱拉扎嘎金矿作为实验区,利用卫星遥感ETM+数据进行蚀变异常提取方法研究。从蚀变矿物的波谱特征出发,对Crosta法则进行改造,建立了适合本区的异常提取模型：铁化蚀变异常提取以Band 2、 Band 3 、Band 4 、Band 5/Band 1主成分分析为核心技术;含羟基和碳酸盐化蚀变异常提取以Band 2、Band 4、 Band 5 、Band 7主成分分析为核心技术。运用该模型完成了矿区及其外围蚀变异常提取工作,经过地面光谱验证,检测到铁化、含羟基和碳酸盐化蚀变矿物,认为提取方法合理,提取结果可信。本次研究工作可以为矿区外围找矿提供新方法,为推广和应用遥感技术在荒漠景观区找矿提供参考。 相似文献
18.
西嘎山(Siga Hills)地区位于维多利亚湖南部绿岩带上,该地区金矿床大多属绿岩带条带状铁建造(BIF)型金矿,金矿床赋存于条带状铁建造(BIF)中,矿体受"岩性+构造"双重控制。磁法测量显示马黑加矿区M1、M2、M3三条磁异常带,异常由条带状含铁建造和金矿体共同引起;土壤地球化学测量显示Au与As、Sb具有显著相关性。综合对比区域金矿信息,提出马黑加金矿化带南北延长线及其东部与花岗岩接触带、西嘎山南北走向带、西嘎山与马黑加之间等3个区域为今后西嘎山地区的找矿方向。 相似文献
19.
The ~2,752-Ma Weld Range greenstone belt in the Yilgarn Craton of Western Australia hosts several Fe ore deposits that provide insights into the role of early hypogene fluids in the formation of high-grade (>55 wt% Fe) magnetite-rich ore in banded iron formation (BIF). The 1.5-km-long Beebyn orebody comprises a series of steeply dipping, discontinuous, <50-m-thick lenses of magnetite–(martite)-rich ore zones in BIF that extend from surface to vertical depths of at least 250 m. The ore zones are enveloped by a 3-km-long, 150-m-wide outer halo of hypogene siderite and ferroan dolomite in BIF and mafic igneous country rocks. Ferroan chlorite characterises 20-m-wide proximal alteration zones in mafic country rocks. The magnetite-rich Beebyn orebody is primarily the product of hypogene fluids that circulated through reverse shear zones during the formation of an Archean isoclinal fold-and-thrust belt. Two discrete stages of hypogene fluid flow caused the pseudomorphic replacement of silica-rich bands in BIF by Stage 1 siderite and magnetite and later by Stage 2 ferroan dolomite. The resulting carbonate-altered BIF is markedly depleted in SiO2 and enriched in CaO, MgO, LOI, P2O5 and Fe2O3(total) compared with the least-altered BIF. Subsequent reactivation of these shear zones and circulation of hypogene fluids resulted in the leaching of existing hypogene carbonate minerals and the concentration of residual magnetite-rich bands. These Stage 3 magnetite-rich ore zones are depleted in SiO2 and enriched in K2O, CaO, MgO, P2O5 and Fe2O3(total) relative to the least-altered BIF. Proximal wall rock hypogene alteration zones in mafic igneous country rocks (up to 20 m from the BIF contact) are depleted in SiO2, CaO, Na2O, and K2O and are enriched in Fe2O3(total), MgO and P2O5 compared with distal zones. Recent supergene alteration affects all rocks within about 100 m below the present surface, disturbing hypogene mineral and the geochemical zonation patterns associated with magnetite-rich ore zones. The key vectors for identifying hypogene magnetite-rich Fe ore in weathered outcrop include textural changes in BIF (from thickly to thinly banded), crenulated bands and collapse breccias that indicate volume reduction. Useful indicators of hypogene ore in less weathered rocks include an outer carbonate–magnetite alteration halo in BIF and ferroan chlorite in mafic country rocks. 相似文献
20.
《Chemie der Erde / Geochemistry》2015,75(3):375-387
Banded iron-formations (BIFs) form an important part of the Archaean to Proterozoic greenstone belts in the Southern Cameroon. In this study, major, trace and REE chemistry of the banded iron-formation are utilized to explore the source of metals and to constraint the origin and depositional environment of these BIFs. The studied BIF belongs to the oxide facies iron formations composed mainly of iron oxide (mainly magnetite) mesobands alternating with quartz mesobands. The mineralogy of the BIF sample consists of magnetite and quartz with lesser amount of secondary martite, goethite and trace of gibbsite and smectite. The major element chemistry of these iron-formations is remarkably simple with the main constituents being SiO2 and Fe2O3 which constitute 95.6–99.5% of the bulk rock. Low Al2O3, TiO2, and HFSE concentrations show that they are relatively detritus-free chemical sediments. The Pearson’s correlation matrix of major element reveals that there is a strong positive correlation (r = 0.99) of Al with Ti and no to weak negative correlation of Ti with Mn, Ca and weak positive correlation of Si with Ca, suggesting the null to very minor contribution of detrital material to chemical sediment. The trace elements with minor enrichments are transition metals such as Zn, Cr, Sr, V and Pb. This is an indicator of direct volcanogenic hydrothermal input in chemical precipitates. The studied BIF have a low ΣREE content, ranging between 0.41 and 3.22 ppm with an average of 0.87 ppm, similar to that of pure chemical sediments. The shale-normalized patterns show depletion in light REE, slightly enrichment in heavy REE and exhibit weak positive europium anomalies. These geochemical characteristics indicate that the source of Fe and Si was the result of deep ocean hydrothermal activity admixed with sea water. The absence of a large positive Eu anomaly in the studied BIF indicates an important role of low-temperature hydrothermal solutions. The chondrite-normalized REE patterns are characterized by LREE-enriched (Mean LaCN/YbCN = 8.01) and HREE depletion (Mean TbCN/YbCN = 1.61) patterns and show positive Ce anomalies. With the exception of one sample (LBR133), all of the BIF samples analyzed during this study have positive Ce anomalies on both chondrite- and PASS-normalized plots. This may indicate that the BIFs within the Elom area were formed within a redox stratified ocean. The positive Ce anomalies in the studied samples likely suggest that the basin in which Fe formations were deposited was reducing with respect to Ce, probably in the suboxic or anoxic seawaters. 相似文献