Microbiological processes in banded iron formation deposition

Banded iron formations have been studied for decades, particularly regarding their potential as archives of the Precambrian environment. In spite of this effort, the mechanism of their deposition and, specifically, the role that microbes played in the precipitation of banded iron formation minerals, remains unresolved. Evidence of an anoxic Earth with only localized oxic areas until the Great Oxidation Event ca 2·45 to 2·32 Ga makes the investigation of O₂‐independent mechanisms for banded iron formation deposition relevant. Recent studies have explored the long‐standing proposition that Archean banded iron formations may have been formed, and diagenetically modified, by anaerobic microbial metabolisms. These efforts encompass a wide array of approaches including isotope, ecophysiological and phylogeny studies, molecular and mineral marker analysis, and sedimentological reconstructions. Herein, the current theories of microbial processes in banded iron formation mineral deposition with particular regard to the mechanisms of chemical sedimentation and post‐depositional alteration are described. The main findings of recent years are summarized and compared here, and suggestions are made regarding cross‐disciplinary information still required to constrain the role of the biosphere in banded iron formation deposition.     

Iron isotopes constrain biologic and abiologic processes in banded iron formation genesis

The voluminous 2.5 Ga banded iron formations (BIFs) from the Hamersley Basin (Australia) and Transvaal Craton (South Africa) record an extensive period of Fe redox cycling. The major Fe-bearing minerals in the Hamersley-Transvaal BIFs, magnetite and siderite, did not form in Fe isotope equilibrium, but instead reflect distinct formation pathways. The near-zero average δ⁵⁶Fe values for magnetite record a strong inheritance from Fe³⁺ oxide/hydroxide precursors that formed in the upper water column through complete or near-complete oxidation. Transformation of the Fe³⁺ oxide/hydroxide precursors to magnetite occurred through several diagenetic processes that produced a range of δ⁵⁶Fe values: (1) addition of marine hydrothermal , (2) complete reduction by bacterial dissimilatory iron reduction (DIR), and (3) interaction with excess that had low δ⁵⁶Fe values and was produced by DIR. Most siderite has slightly negative δ⁵⁶Fe values of ∼ −0.5‰ that indicate equilibrium with Late Archean seawater, although some very negative δ⁵⁶Fe values may record DIR. Support for an important role of DIR in siderite formation in BIFs comes from previously published C isotope data on siderite, which may be explained as a mixture of C from bacterial and seawater sources.Several factors likely contributed to the important role that DIR played in BIF formation, including high rates of ferric oxide/hydroxide formation in the upper water column, delivery of organic carbon produced by photosynthesis, and low clastic input. We infer that DIR-driven Fe redox cycling was much more important at this time than in modern marine systems. The low pyrite contents of magnetite- and siderite-facies BIFs suggests that bacterial sulfate reduction was minor, at least in the environments of BIF formation, and the absence of sulfide was important in preserving magnetite and siderite in the BIFs, minerals that are poorly preserved in the modern marine record. The paucity of negative δ⁵⁶Fe values in older (Early Archean) and younger (Early Proterozoic) BIFs suggests that the extensive 2.5 Ga Hamersley-Transvaal BIFs may record a period of maximum expansion of DIR in Earth’s history.     

Physicochemical formation conditions of banded iron formations and high-grade iron ores in the region of the Kursk Magnetic Anomaly: Evidence from isotopic data

The oxygen and carbon isotopic compositions of minerals from banded iron formations (BIFs) and high-grade ore in the region of the Kursk Magnetic Anomaly (KMA) were determined in order to estimate the temperature of regional metamorphism and the nature of rock-and ore-forming solutions. Magnetite and hematite of primary sedimentary or diagenetic origin have δ¹⁸O within the range from +2 to 6‰. During metamorphism, primary iron oxides, silicates, and carbonates were involved in thermal dissociation and other reactions to form magnetite with δ¹⁸O = +6 to +11‰. As follows from a low δ¹⁸O_av = ?3.5‰ of mushketovite (magnetite pseudomorphs after hematite) in high-grade ore, this mineral was formed as a product of hematite reduction by organic matter. The comparison of δ¹⁸O of iron oxides, siderite, and quartz from BIFs formed at different stages of the evolution of the Kursk protogeosyncline revealed specific sedimentation (diagenesis) conditions and metamorphism of the BIFs belonging to the Kursk and Oskol groups. BIF of the Oskol Group is distinguished by a high δ¹⁸O of magnetite compared to other Proterozoic BIFs. Martite ore differs from host BIF by a low δ¹⁸O = ?0.2 to ?5.9‰. This implies that oxygen from infiltration water was incorporated into the magnetite lattice during the martite formation. Surface water penetrated to a significant depth through tectonic faults and fractures.     

西昆仑赞坎铁矿床地质特征、形成时代及高品位矿石的成因

新疆赞坎铁矿床位于西昆仑塔什库尔干地块西段,是近年新发现的一个大型沉积变质型磁铁矿床。赋矿岩系布伦阔勒群主要由黑云母石英片岩、斜长角闪片岩、变粒岩、硅质岩及磁铁石英岩等组成。目前探明工业矿体4条,单个矿体长度大于2.5km,矿体厚10~70m;局部见高品位铁矿段(mFe50%),长度达900m,厚度40m左右。矿石类型主要为2种,一种为原生的条纹-条带状磁铁矿(为主);另一种为热液改造形成的块状(高品位铁矿石)及浸染状磁铁矿。矿石稀土元素配分(PAAS)表明,原生条纹-条带状铁矿石Ce和Y元素异常不明显(~1.15、~0.94),Eu具正异常(~1.69),Y/Ho平均值为25,稀土配分模式与沉积变质型铁矿相似。而受改造的矿石中,浸染状矿石具有较高的稀土总量,明显富集轻稀土,La和Ce显示正异常(~1.46、~1.17),Y显示负异常(=0.66~0.72),Eu表现为强烈的正异常(~4.37),稀土配分模式明显不同于原生条纹-条带状铁矿石。矿体围岩斜长角闪片岩(变沉积岩)中的碎屑锆石U-Pb年龄为591±1Ma,结合前人对矿区内侵入体的年代学研究(霏细斑岩,533Ma),大致反映沉积铁矿的形成时代为新元古代至早寒武世。电子探针显示,条带状磁铁矿中的TiO_2、AL_2O_3、MgO、MnO含量较低,标型组分含量与沉积变质型磁铁矿颇为接近,在磁铁矿单矿物成因图解中,条带状磁铁矿整体显示磁铁矿为沉积变质型铁矿;浸染状矿石和块状矿石的组成与典型沉积变质型铁矿的偏离反映了后期岩浆-构造热事件对条带状铁矿石的改造;上述结果显示赞坎铁矿整体属于沉积变质型铁矿(BIF)。调查发现赞坎高品位铁矿体与早寒武世侵入的霏细斑岩联系密切,高品位矿石及其围岩发育一定程度的矽卡岩化,如阳起石化、碳酸盐化和黄铁矿化。本文推测高品位铁矿石的成因可能为霏细斑岩的岩浆热液溶解并运移早期沉积变质铁矿中的含铁物质,在构造发育处充填交代形成块状磁铁富矿石。在早寒武世侵入到矿区中部的霏细斑岩体中,同时发育有角砾状磁铁矿和脉状磁铁矿,因此,岩浆热液改造原生条带状铁矿石形成高品位铁矿石的时代应为早寒武世。     

鲁东昌邑古元古代BIF铁矿矿床地球化学特征及矿床成因讨论

昌邑铁矿位于华北克拉通东部的胶北地体,为赋存于古元古代粉子山群变质岩中的条带状铁建造(BIF)铁矿。矿体主要呈透镜状、似层状,以(含)角闪石英磁铁岩为主要矿石,经历了温度高达636℃的角闪岩相变质作用。铁矿石富SiO₂和Fe₂O₃^T(SiO₂+Fe₂O₃^T=82.5%~97.7%),含少量Al₂O₃、MgO和CaO等,显示主要为化学沉积但有少量碎屑或泥质加入的特征。与PAAS相比轻稀土元素亏损、高的Y/Ho比值以及La和Y正异常表明铁矿沉淀于海相环境,而高的Ti/V比值、高Cr、Co和Ni含量以及Eu的正异常表明火山热液的参与,成矿物质来源于火山活动。无明显的Ce负异常表明当时可能存在一个缺氧的大气环境。昌邑铁矿与华北克拉通太古宙BIF相比,总体上没有显著差别,但Al₂O₃、CaO、MgO和K₂O含量相对较高,Eu正异常相对较弱,表明其可能形成于具有更多碎屑物质和更少热液参与的浅水环境。     

A mixed seawater and hydrothermal origin of superior-type banded iron formation (BIF)-hosted Kouambo iron deposit,Palaeoproterozoic Nyong series,Southwestern Cameroon: Constraints from petrography and geochemistry

The Kouambo iron deposit contains banded iron formations (BIFs) and is located in the northwestern margin of the Congo craton. The BIFs are hosted in Palaeoproterozoic Nyong series, a dominantly metasedimentary formations, which were metamorphosed into greenschist to granulite facies. The Kouambo BIFs are medium- to coarse-grained banded rocks consisting of alternation of Si-rich and Fe-rich mesobands, and belong to oxide facies iron formations. Geochemistry analyses reveal that these iron formations are composed of > 96 wt% Fe₂O₃ and SiO₂ and have low concentrations of Al₂O₃, TiO₂ and trace HFSE, suggesting chemical precipitates of silica and iron. Moreover, these BIFs have low concentrations of Al₂O₃, TiO₂ and trace HFSEs (high field strength elements, e.g., Zr, Hf, Ta, Pb and Th), suggesting that terrigenous detrital materials contributed insignificantly to the sedimentation. The Post-Archean Australian Shale (PAAS)-normalized REE-Y patterns display seawater-like profile: minor LREE depletion and HREE enrichment, positive Y anomalies. However, they display positive Eu and negative Ce anomalies, and low Y/Ho ratio (average 29), which suggest the influence of the hydrothermal fluids. The weak positive Eu/Eu*(_PAAS) ratio (average 1.5), associated with the low V (17.5 ppm), Co (6.1 ppm) and Ni (27.5 ppm) contents similar to other Superior-type BIFs worldwide, are consistent with the deposition of the Kouambo BIFs in continental marginal sea or back-arc basin environment. In summary, the Kouambo BIFs show a seawater-like REE + Y signature, however, the positive Eu anomalies and reduced Y/Ho ratios relative to seawater indicates a possible mixing with hydrothermal fluids (∼ 0.5%).     

Geochemistry of banded iron formation of Orissa,India

Major and trace element analyses of representative samples of various types of banded iron-formation and its various minerals, associated sediments, iron ores and volcanic tuff from different localities of Orissa, India, are presented in this paper. The Orissa banded iron-formation is classified as Precambrian banded iron formation and is similar to the oxide facies iron formation of Lake Superior type. The Orissa iron formation consists only of iron oxide and silica with total absence of iron silicate, sulfide and carbonate minerals, and is devoid of terrigenous material. The trace element content suggests the source of the underlying quartzite to be a continental igneous rock mass, while the interbedded tuff are of undoubted volcanic origin. The overlying iron formation were chemically precipitated as oxidate sediments in which the principal iron mineral — magnetite — was formed at low temperature in a shallow marine environment. From the overwhelming similarity of major and trace element contents of all the samples from the different localities, it is postulated that these detached outcrops originated in the same continous basin.     

Archaean lode gold mineralisation in banded iron formation at the Kalahari Goldridge deposit, Kraaipan Greenstone Belt, South Africa

The Kalahari Goldridge Mine is located within the Archaean Kraaipan Greenstone Belt, about 60 km southwest of Mafikeng in the North West Province, South Africa. The ore body thickness varies from 15 to 45 m along a strike length of about 1.5 km within approximately N–S striking banded iron formation (BIF). The stratabound ore body is hosted primarily by BIF, which consists of alternating chert and magnetite–chlorite–stilpnomelane–sulphide–carbonate bands of millimetre- to centimetre scale. A footwall of sericite–carbonate–chlorite schist underlain by mafic amphibolite occurs to the west and carbonaceous metapelites in the hanging wall to the east. Overlying the hanging wall, carbonaceous metapelites, units of coarse-grained metagreywackes fining upwards, become increasingly conglomeratic up the stratigraphy. Small-scale isoclinal folds, brecciation, extension fractures and boudinage of cherty BIF units reflect brittle-ductile deformation. Fold axial planes have foliation, with subvertical plunges parallel to prominent rodding and mineral lineation in the footwall rocks. Gold mineralisation is associated with two generations of quartz–carbonate veins, dipping approximately 20° to 40° W. The first generation consists of ladder-vein sets (group IIA) preferentially developed in centimetre-scale Fe-rich mesobands, whereas the second generation consists of large quartz–carbonate veins (group IIB), which locally crosscut the entire ore body and extend into the footwall and hanging wall. The ore body is controlled by mesoscale isoclinal folds approximately 67° E, orthogonal to the plane of mineralised, gently dipping veins, defining the principal stretching direction and development of fluid-focussing conduits. The intersections of the mineralised veins and foliation planes of the host rock plunges approximately 08° to the north. Pervasive hydrothermal alteration is characterised by chloritisation, carbonatisation, sulphidation and K-metasomatism. Gold is closely associated with sulphides, mainly pyrite and pyrrhotite, and to a lesser extent, with bismuth tellurides and carbonate minerals. Mass balance transfer calculations indicate that hydrothermal alteration of BIF involved enrichment of Au, Ag, Bi, Te, S and CO₂ (LOI), MgO, Ba, K and Rb, but significant depletion of SiO₂ and, to a lesser extent, Fe₂O₃. Extensive replacement of magnetite and chlorite in BIF and other pelitic sedimentary rocks by sulphide and carbonate minerals, both on mesoscopic and microscopic scales, is evidence of interaction of CO₂- and H₂S-bearing fluids with the Fe-rich host rocks. The fineness of gold grains ranges from 823 to 921, similar to that of other epigenetic Archaean BIF-hosted gold deposits, worldwide.     

Native elements in rocks of the banded iron formation,Kola Peninsula

Eleven native minerals and intermetallic alloys were identified in rocks of the banded iron formation (BIF) in the Kola Peninsula: copper, silver, gold, electrum, auricupride, cuproauride, tetraauricupride, bismuth, sulfur, tellurium, and graphite. Graphite is a common mineral of sulfide-bearing BIF and gneiss. Sulfur occurs in supergene-altered sulfide-bearing BIFs together with Fe- and Ca-sulfates. Gold of low fineness (electrum) in association with electrum, acanthite, auricupride, volynskite, hessite, cervelleite, pavonite, petzite, and bismuth is related to the areas of hydrothermally altered skarnoids with greenalite, chamosite, aegirine, and Na-Ca amphibole. Redeposited gold of high fineness associated with auricupride, hessite, silver, electrum, kostovite, cuproauride, tetraauricupride, and sperrylite occurs in low-temperature zonal hydrothermal segregations hosted in aluminous gneiss and which formed under the effect of alkalized, highly siliceous solutions at the regressive stage of BIF metamorphism.     

华北克拉通条带状铁建造中富铁矿成因类型的研究进展、远景和存在的科学问题

本文在查阅前人大量资料的基础上,对华北克拉通条带状铁建造中富铁矿的研究历史进行了回顾和总结,将研究历史分为1949年以前,1950~1965年期间,1978~1986年期间,1987~1994年期间和2009年以来5个阶段。重点介绍了鞍本地区、冀东-吕梁地区和河南舞阳地区富铁矿的基本地质特征以及典型富铁矿的研究概况,针对鞍本地区弓长岭二矿区磁铁富矿成因的复杂性,对不同成因观点以及目前已取得的共识进行了详细阐述。目前大多数学者不支持接触交代假说和菱铁矿经变质转化为富铁矿成矿假说,近半数学者支持变质热液成矿假说,半数学者支持混合岩化热液成矿假说。作者在综合分析前人大量资料后,认为变质热液成矿说依据不足,理由有四点:(1)磁铁富矿中往往见有磁铁贫矿的残体;(2)磁铁富矿与蚀变岩紧密伴生,蚀变矿物石榴子石、部分角闪石(透闪石)和部分绿泥石均属非变质热液成因;(3)研究区遭受区域高绿片岩相至低角闪岩相变质作用的时间为2500~2450Ma,而与蚀变矿物石榴石紧密伴生的热液锆石SHRIMP U-Pb定年结果为1840±7Ma,明显小于区域变质作用年龄,据此可将热液作用时间限定于古元古代晚期,相当于大陆地壳伸展阶段;(4)部分热液成因富铁矿利用Re-Os方法定年,除一种属原生沉积成矿外,年龄范围也在古元古代晚期,可作为参考。此种热液是否为混合岩化热液尚缺乏足够证据,故本文暂将其作为古元古代晚期热液。此外,本文对华北克拉通条带状铁建造中富铁矿成因类型及其远景进行了初步总结,认为古元古代晚期形成的磁铁富矿规模属大型矿床,有较好远景;原生较富贫铁矿因褶皱构造产生磁铁矿流变而形成的富铁矿(可能尚有热液叠加)规模较大,具有一定远景;其他类型均为小型规模,不具工业意义。最后,本文指出富铁矿成因研究中尚存在的主要问题,包括早元古代晚期热液的来源;热液的形成是一期还是多期;铁建造遭受区域变质达高绿片岩相时,贫铁矿的围岩变质演化机理等,尚需进一步探讨。     

Petrography and geochemistry of the Mesoarchean Bikoula banded iron formation in the Ntem complex (Congo craton), Southern Cameroon: Implications for its origin

Precambrian banded iron formations (BIFs) represent an important source of mineable iron, as well as an archive recording secular changes in the chemistry of the Earth’s early oceans. Here we report petrographic and geochemical characteristics of unweathered drill core samples from the Bikoula BIF, a virtually uncharacterized oxide facies iron formation, hosted in the Mesoarchean Ntem complex, southern Cameroon. The BIF is cross-cut with syenitic veins. The entire succession is highly deformed and metamorphosed under granulite facies conditions. The BIF is characterized by alternating micro-bands of magnetite, quartz and pyroxene. Sulfides (pyrite, pyrrhotite, and chalcopyrite), oligoclase, ferro-pargasite, biotite and ilmenite occur as minor phases. The presence of pyroxene, ferro-pargasite and oligoclase, relatively high contents of major elements such as Al₂O₃ (0.76–7.52 wt.%), CaO (1.95–4.90 wt.%), MgO (3.78–5.59 wt.%), as well as positive correlations among Al₂O₃, TiO₂, HFSEs, LILEs and transition metals (V, Cr, Ni, Cu and Zn), suggest that the BIF protolith included a significant amount of clastic material. Several samples have preserved seawater-like PAAS-normalized REE-Y patterns, including LREE depletion, and positive La and Y anomalies. Positive Eu anomalies observed in some of the analyzed samples indicate influx of hydrothermal fluids (possibly including Fe and Si) within the basin where the BIF precipitated. However, few samples show unusual negative Eu anomalies that likely result from a large proportion of clastic contamination. The lack of Ce anomalies suggests that the Bikoula BIF was deposited in a basin that was (at least partly) anoxic or suboxic, where it was possible to transport and concentrate dissolved Fe²⁺.     

庐枞盆地与A型花岗岩有关的磁铁矿-阳起石-磷灰石矿床——以马口铁矿床为例

庐枞中生代火山岩盆地位于长江中下游断陷带内,地处扬子板块的北缘.庐枞盆地内的火山岩和侵入岩分布广泛,包括龙门院、砖桥、双庙和浮山四组火山岩以及34个侵入岩体.最近在庐枞盆地南部正长岩中发现一种新类型铁矿床——马口铁矿床.本次工作通过详细野外地质和室内研究,系统开展了马口铁矿床矿床地质特征、成矿年代学和成矿流体特征研究.马口铁矿床的矿体沿石英正长斑岩体中构造破碎带产出,产状严格受构造破碎带控制,矿床围岩蚀变晕范围较为局限.矿床的成矿作用可分为碱性长石阶段、磁铁矿阶段、石英-硫化物阶段和碳酸盐阶段,其中磁铁矿阶段是主要成矿阶段,形成与宁芜玢岩型铁矿床相类似磁铁矿-磷灰石-阳起石三矿物组合.矿床中与磁铁矿共生的磷灰石流体包裹体均一温度范围为252.2 ～ 322.6℃,反映其成矿温度略低于典型的玢岩铁矿床.通过赋矿岩体锆石LA-ICP-MS U-Pb定年和金云母Ar-Ar方法,确定马口铁矿床的成矿时代为127.3 ±0.8Ma,赋矿岩体的形成时代为129.4±1.4Ma,结合矿床地质特征,可以得出矿区内石英正长斑岩只是提供了赋矿空间,矿床的形成与其无直接成因联系,而可能与黄梅尖A型花岗岩体有关.马口铁矿床是庐枞盆地成矿作用最后阶段的产物,也可能是长江中下游成矿带中最晚一次成矿事件的代表,具有显著的特色.     

南非巴伯顿绿岩带条带状铁建造岩石磁学及磁性矿物的组成特征

条带状铁建造(BIFs)中含有大量的亚铁磁性矿物,其组成及来源是认识BIF成因的重要依据。本文研究了南非巴伯顿绿岩带无花果树群(距今约32亿年)恩圭尼亚组的BIFs样品的磁学和矿物学特征。通过测量富铁层与富硅层的磁滞回线、等温剩磁获得曲线与退磁曲线、矫顽力谱分析、一阶反转曲线(FORC)、低温(20～300K)有场/无场冷却曲线以及k-T曲线、Lowrie三轴热退磁曲线,结合扫描电镜观测,揭示出研究样品中磁性矿物主要为赤铁矿和磁铁矿。基于矫顽力谱分析,富铁层中磁铁矿主要是多畴及假单畴颗粒,相对含量平均为2. 1%;赤铁矿的相对含量平均为97. 9%。富硅层中磁铁矿主要为假单畴及超顺磁性颗粒,相对含量平均为4. 6%;赤铁矿相对含量平均为95. 4%。测试样品具有Morin转变特征,转变温度介于250～260K,说明BIFs中主要为赤铁矿(0. 5～6mm)。富硅层样品出现～107K、～125K两个Verwey转变温度,表明其中可能存在生物成因和非生物成因两种类型磁铁矿。     

Examples and genetic significance of the formation of iron oxides in the Nigerian banded iron-formations

Ore microscopic studies reveal two main parageneses in the banded iron-formations of Nigeria. In the low-grade metamorphic schist belts of northern Nigeria, a magnetitic paragenesis comprising magnetite, silicates (grunerite and garnet), and quartz is developed. Magnetite which sometimes contains carbonate inclusions is markedly martitized. In contrast, the higher-grade metamorphic terrains of central Nigeria exhibit a different paragenesis consisting of hematite (including specularite) and quartz. Here, minerals of the magnetitic paragenesis only occur as relics. The protolith of these banded iron-formation occurrences envisioned as carbonate-containing sediments, with high concentrations of Fe and Si, and lower contents of Ca, Mg, Al (and also Mn where they are associated with gondite) underwent both submarine weathering and metamorphic changes in their evolution. During submarine weathering, sheet silicates and porphyroblasts of Fe-Mn-(Mg-Ca)-carbonate solid solutions, were formed. At the outset of a regional metamorphic episode, grunerite, garnet and porphyroblastic magnetite were developed. Magnetite formed at the expense of carbonate and sheetsilicates but was later martitized under post-metamorphic conditions. In the course of a later heterogeneous tectono-metamorphic event, martitized magnetite was transformed as follows: under low-grade metamorphism, as observed in the northern Nigerian schist belts, recrystallization into coarse-grained martite occurred, while at the higher grades of metamorphism in central Nigeria, recrystallization into hematite and, ultimately, specularite, took place. This relationship between magnetite and hematite has also been observed in many other banded iron-formations from different parts of the world, thus underscoring its widespread significance. Magnetite crystallizes first at the expense of carbonate and silicate minerals and hematite is subsequently derived from it directly or generally through martitization. This metamorphic phenomenon contradicts the common assumption that magnetite and hematite in banded iron-formations are invariably the products of direct precipitation from solution, in response to changes in environmental Eh/pH or different (reducing/oxidizing) diagenetic alterations of precipitated ferric hydroxide.     

Origin of graphite in early precambrian banded iron formation in Anshan,China

Graphite which occurs in the early Precambrian banded iron formation (BIF) (3.1x10⁹yr) at Gongchangling, Anshan, China, can be divided into two genetic types on the basis of its modes of occurrence: biogenic and inorganic; the former occurs in garnet-mica-quartz schist and the latter in rich magnetite ore. The garnet-mica-quartz schist is located at the bottom of the formation. Its original rock is a volcanic tuff-bearing clayey siltstone. Graphite is fairly uniformly disseminated in the schist Chemical analysis of 20 samples of graphite yields an average content of 0.29±0.22%. The average δ¹³C value of 4 samples is -26.6 ±0.6‰ (PDB). Rich magnetite ore bodies occur in the form of lenses and layers within the banded magnetite quartzite, and wallrock alteration is also noticed. Graphitebearing rich magnetite ore is composed of magnetite, maghemite and minor graphite. Late chlorite and siderite are recognized locally. Disseminated graphite is generally distributed in scaly aggregates interstitial to the grains of magnetite, occasionally found within the grains of magnetite. It is non-uniformly distributed in the horizon of rich ore, mainly in the core. No graphite is found in the outer part of the rich ore, poor ore in the same horizon, wallrock near the rich ore and altered rock, indicating that graphite has a great bearing on the rich ore. Chemical analysis of 15 samples gives an average graphite content of 0.89±0.51%. The average δ¹³C value of 18 samples is-4.7 ±2.1%.(PDB). This kind of graphite seems to have been formed by the following reaction: 6 FeCO₃=2Fe₃O₄ + 5CO₂+C in the primary sedimentary siderite under the condition of amphibole-facies regional metamorphism.     

New data on mineral composition of noble metal mineralization of the stoilo iron ore deposit of KMA,Central Russia

Doklady Earth Sciences -     

论安徽省姑山铁矿床的热液成矿特征

姑山铁矿床产在燕山期辉石闪长岩与中三叠统黄马青组砂页岩的接触带上。矿石具有典型的斑状、球颗状和骨架状结构。镜下研究表明,这些结构不是矿浆结晶的产物,而是热液成因微晶赤铁矿在后期地质过程中发生变晶生长所致。矿床中的蚀变和矿化自下而上呈现规律性的分带。碳酸盐化是特征性的蚀变类型,其形成与成矿密切有关,并可作为一种近矿找矿标志。在热液成矿过程中,辉石闪长岩中的铁很可能以羰基络合物的形式发生活化转移。在热液演化过程中由于氧逸度的升高和碱度、压力的降低,羰基络合物发生分解而使铁在接触带附近沉淀成矿。     

Geologic and metallogenic aspects concerning the Nahuelbuta mountains banded iron formation,Chile

Paleozoic banded-iron-formation (BIF) deposits occur within the Nahuelbuta-Queule Complex (south central Chile) which hosts the following stratigraphic units: Cabo Tirúa (green schists, mica schists, and metacherts), Lleu-Lleu (iron-bearing metacherts, mica schists, and serpentinites), and Colcura (metagraywackes and metapelites). The lithological, structural, and geochemical characteristics of the Lleu-Lleu and Cabo Tirúa units indicate that they were part of a tectonic mélange accreted to the South American paleocontinent during the Paleozoic. BIF ores are restricted to the Lleu-Lleu metacherts and are characterized by oxide-silicate-sulfide BIF facies. The iron-bearing metacherts present mineralogical and geochemical characteristics close to the volcanogenic BIF types and are thought to have been formed by submarine volcanic exhalative activity.