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1.
Banded iron formations of the Iron Ore Group (Archean greenstone belts) of Jharkhand-Orissa region, India host a good number of large iron ore deposits (Fe wt %> 62). Iron ore mineralization of Gandhamardan hill is one of them where iron ores occur in two stratigraphic horizons. One is strictly confined within banded iron formation (stratabound mineralization) with irregular geometry, and show fracture filling and replacement vein-type mineralization along the fringes of hard massive ores of the core. This type of mineralization is exposed along the western slope of the hill. Hard massive and laminated ores dominate this mineralization. The other type occurs as low dipping sheet like body above banded iron formation and covered by laterites forming the top of the hill. Flaky ores dominate this mineralization with formation of hard goethitic crust near the top. Both the mineralizations contain mineralized banded iron formation corestones surrounded by hard massive or flaky iron ores. Hard massive ores are entirely represented by martite-microplaty hematite mineralogy. Hard laminated ores contain microplaty hematite and few martite grains representing early magnetites of the banded iron formation. Flaky ores are high porosity ores produced by leaching of silica, martite and microplaty hematite. Hard goethitic ores are developed due to replacement of martite and microplaty hematite or precipitation of goethite in the pore spaces.  相似文献   

2.
Enrichment iron ore of the Hamersley Province, currently estimated at a resource of over 40 billion tonnes (Gt), mainly consists of BIF (banded iron-formation)-hosted bedded iron deposits (BID) and channel iron deposits (CID), with only minor detrital iron deposits (DID). The Hamersley BID comprises two major ore types: the dominant supergene martite–goethite (M-G) ores (Mesozoic–Paleocene) and the premium martite–microplaty hematite ores (M-mplH; ca 2.0 Ga) with their various subtypes. The supergene M-G ores are not common outside Australia, whereas the M-mplH ores are the principal worldwide resource. There are two current dominant genetic models for the Hamersley BID. In the earlier 1980–1985 model, supergene M-G ores formed in the Paleoproterozoic well below normal atmospheric access, driven by seasonal oxidising electrochemical reactions in the vadose zone of the parent BIF (cathode) linked through conducting magnetite horizons to the deep reacting zone (anode). Proterozoic regional metamorphism/diagenesis at ～80–100°C of these M-G ores formed mplH from the matrix goethite in the local hydrothermal environment of its own exhaled water to produce M-mplH ores with residual goethite. Following general exposure by erosion in the Cretaceous–Paleocene when a major second phase of M-G ores formed, ground water leaching of residual goethite from the metamorphosed Proterozoic ores resulted in the mainly goethite-free M-mplH ores of Mt Whaleback and Mt Tom Price. Residual goethite is common in the Paraburdoo M-mplH-goethite ores where erratic remnants of Paleoproterozoic cover indicate more recent exposure.

Deep unweathered BIF alteration residuals in two small areas of the Mt Tom Price M-mplH deposits have been used since 1999 for new hypogene–supergene modelling of the M-mplH ores. These models involve a major Paleoproterozoic hydrothermal stage in which alkaline solutions from the underlying Wittenoom Formation dolomite traversed the Southern Batter Fault to leach matrix silica from the BIF, adding siderite and apatite to produce a magnetite–siderite–apatite ‘protore.’ A later heated meteoric solution stage oxidised siderite to mplH + ankerite and magnetite to martite. Weathering finally removed residual carbonates and apatite leaving the high-grade porous M-mplH ore. Further concepts for the Mt Tom Price North and the Southern Ridge Deposits involving acid solutions followed, but these have been modified to return essentially to the earlier hypogene–supergene model. Textural data from erratic ‘metasomatic BIF’ zones associated with the above deposits are unlike those of the typical martite–microplaty hematite ore bodies. The destiny of the massive volumes of dissolved silica gangue and the absence of massive silica aureoles has not been explained. Petrographic and other evidence indicate the Mt Tom Price metasomatism is a localised post-ore phenomenon. Exothermic oxidation reactions in the associated pyrite-rich black shales during post-ore removal by groundwater of remnant goethite in the ores may have resulted in this very localised and erratic hydrothermal alteration of BIF and its immediately associated pre-existing ore.  相似文献   

3.
The BIF-hosted iron ore system represents the world's largest and highest grade iron ore districts and deposits. BIF, the precursor to low- and high-grade BIF hosted iron ore, consists of Archean and Paleoproterozoic Algoma-type BIF (e.g., Serra Norte iron ore district in the Carajás Mineral Province), Proterozoic Lake Superior-type BIF (e.g., deposits in the Hamersley Province and craton), and Neoproterozoic Rapitan-type BIF (e.g., the Urucum iron ore district).The BIF-hosted iron ore system is structurally controlled, mostly via km-scale normal and strike-slips fault systems, which allow large volumes of ascending and descending hydrothermal fluids to circulate during Archean or Proterozoic deformation or early extensional events. Structures are also (passively) accessed via downward flowing supergene fluids during Cenozoic times.At the depositional site the transformation of BIF to low- and high-grade iron ore is controlled by: (1) structural permeability, (2) hypogene alteration caused by ascending deep fluids (largely magmatic or basinal brines), and descending ancient meteoric water, and (3) supergene enrichment via weathering processes. Hematite- and magnetite-based iron ores include a combination of microplaty hematite–martite, microplaty hematite with little or no goethite, martite–goethite, granoblastic hematite, specular hematite and magnetite, magnetite–martite, magnetite-specular hematite and magnetite–amphibole, respectively. Goethite ores with variable amounts of hematite and magnetite are mainly encountered in the weathering zone.In most large deposits, three major hypogene and one supergene ore stages are observed: (1) silica leaching and formation of magnetite and locally carbonate, (2) oxidation of magnetite to hematite (martitisation), further dissolution of quartz and formation of carbonate, (3) further martitisation, replacement of Fe silicates by hematite, new microplaty hematite and specular hematite formation and dissolution of carbonates, and (4) replacement of magnetite and any remaining carbonate by goethite and magnetite and formation of fibrous quartz and clay minerals.Hypogene alteration of BIF and surrounding country rocks is characterised by: (1) changes in the oxide mineralogy and textures, (2) development of distinct vertical and lateral distal, intermediate and proximal alteration zones defined by distinct oxide–silicate–carbonate assemblages, and (3) mass negative reactions such as de-silicification and de-carbonatisation, which significantly increase the porosity of high-grade iron ore, or lead to volume reduction by textural collapse or layer-compaction. Supergene alteration, up to depths of 200 m, is characterised by leaching of hypogene silica and carbonates, and dissolution precipitation of the iron oxyhydroxides.Carbonates in ore stages 2 and 3 are sourced from external fluids with respect to BIF. In the case of basin-related deposits, carbon is interpreted to be derived from deposits underlying carbonate sequences, whereas in the case of greenstone belt deposits carbonate is interpreted to be of magmatic origin. There is only limited mass balance analyses conducted, but those provide evidence for variable mobilization of Fe and depletion of SiO2. In the high-grade ore zone a volume reduction of up to 25% is observed.Mass balance calculations for proximal alteration zones in mafic wall rocks relative to least altered examples at Beebyn display enrichment in LOI, F, MgO, Ni, Fe2O3total, C, Zn, Cr and P2O5 and depletions of CaO, S, K2O, Rb, Ba, Sr and Na2O. The Y/Ho and Sm/Yb ratios of mineralised BIF at Windarling and Koolyanobbing reflect distinct carbonate generations derived from substantial fluid–rock reactions between hydrothermal fluids and igneous country rocks, and a chemical carbonate-inheritance preserved in supergene goethite.Hypogene and supergene fluids are paramount for the formation of high-grade BIF-hosted iron ore because of the enormous amount of: (1) warm (100–200 °C) silica-undersaturated alkaline fluids necessary to dissolve quartz in BIF, (2) oxidized fluids that cause the oxidation of magnetite to hematite, (3) weakly acid (with moderate CO2 content) to alkaline fluids that are necessary to form widespread metasomatic carbonate, (4) carbonate-undersaturated fluids that dissolve the diagenetic and metasomatic carbonates, and (5) oxidized fluids to form hematite species in the hypogene- and supergene-enriched zone and hydroxides in the supergene zone.Four discrete end-member models for Archean and Proterozoic hypogene and supergene-only BIF hosted iron ore are proposed: (1) granite–greenstone belt hosted, strike-slip fault zone controlled Carajás-type model, sourced by early magmatic (± metamorphic) fluids and ancient “warm” meteoric water; (2) sedimentary basin, normal fault zone controlled Hamersley-type model, sourced by early basinal (± evaporitic) brines and ancient “warm” meteoric water. A variation of the latter is the metamorphosed basin model, where BIF (ore) is significantly metamorphosed and deformed during distinct orogenic events (e.g., deposits in the Quadrilátero Ferrífero and Simandou Range). It is during the orogenic event that the upgrade of BIF to medium- and high-grade hypogene iron took place; (3) sedimentary basin hosted, early graben structure controlled Urucum-type model, where glaciomarine BIF and subsequent diagenesis to very low-grade metamorphism is responsible for variable gangue leaching and hematite mineralisation. All of these hypogene iron ore models do not preclude a stage of supergene modification, including iron hydroxide mineralisation, phosphorous, and additional gangue leaching during substantial weathering in ancient or Recent times; and (4) supergene enriched BIF Capanema-type model, which comprises goethitic iron ore deposits with no evidence for deep hypogene roots. A variation of this model is ancient supergene iron ores of the Sishen-type, where blocks of BIF slumped into underlying karstic carbonate units and subsequently experienced Fe upgrade during deep lateritic weathering.  相似文献   

4.
Several major iron deposits occur in the Quadrilátero Ferrífero (QF), southeastern region of Brazil, where metamorphosed and heterogeneously deformed banded iron formation (BIF) of the Cauê Formation, regionally called itabirite, was transformed into high- (Fe >64%) and low-grade (30%?2O3, with a higher amount of detrimental impurities, especially MnO, in the soft ore. Both hard and soft ores are depleted in trace elements. The high-grade ores at the Águas Claras Mine have at least a dual origin, involving hypogene and supergene processes. The occurrence of the hard, massive high-grade ore within “fresh” dolomitic itabirite is evidence of its hypogene origin. Despite the contention about the origin of the dolomitic itabirite (if this rock is a carbonate-rich facies of the Cauê Formation or a hematite–carbonate precursor of the soft high-grade ore), mineralogical and geochemical features of the soft high-grade ore indicate that it was formed by leaching of dolomite from the dolomitic itabirite by meteoric water. The comparison of the Águas Claras, Capão Xavier and Tamanduá orebodies shows that the original composition of the itabiritic protore plays a major role in the genesis of high- and low-grade soft ores in the QF. Under the same weathering and structural conditions, the dolomitic itabirite is the more favorable to form high-grade deposits than siliceous itabirite. Field relations at the Águas Claras and Capão Xavier deposits suggest that it is not possible to form huge soft high-grade supergene deposits from siliceous itabirite, unless another control, such as impermeable barriers, had played an important role. The occurrence in the Tamanduá Mine of a large, soft, high-grade orebody formed from siliceous itabirite and closely associated with hypogene hard ore suggests that large, soft, high-grade orebodies of the Quadrilátero Ferrífero, which occur within siliceous itabirite, have a hypogene contribution in their formation.  相似文献   

5.
西澳大利亚州铁矿分布规律及矿床成因分析   总被引：2，自引：0，他引：2  
西澳大利亚州铁矿资源主要分布在北部皮尔巴拉和南部的伊尔岗两个太古宙克拉通。皮尔巴拉克拉通BIF型铁矿在汤姆普赖斯山、恰那和布鲁克曼的矿石矿物组合为假象赤铁矿一微板状赤铁矿,马拉曼巴的为赤铁矿一针铁矿,CID型铁矿在罗布河和杨迪矿石类型主要为褐铁矿;伊尔岗克拉通BIF型铁矿在库里阿诺的矿石矿物组合为针铁矿一假象赤铁,比温和曼迪尕的为磁铁矿±假象赤铁矿和针铁矿±赤铁矿。BIF型铁矿为浅生一变质成矿,而CID型铁矿则是先前形成的BIF经侵蚀、搬运、沉积和埋藏作用形成。  相似文献   

6.
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 δ18Oav = ?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.  相似文献   

7.
Banded iron formation (BIF)-hosted iron ore deposits in the Windarling Range are located in the lower greenstone succession of the Marda–Diemals greenstone belt, Southern Cross domain, Yilgarn Craton and constitute a total hematite–martite–goethite ore resource of minimum 52 Mt at 60 wt.% Fe (0.07 P). Banded iron formation is interlayered with high-Mg basalts at Windarling and precipitated during episodes of volcanic quiescence. Trace element content and the rare earth element (REE) ratios Y/Ho (42 to 45), Sm/Yb (1.5), together with positive La and Gd anomalies in ‘least-altered’ hematite–magnetite–metachert–BIF indicate the precipitation from Archean seawater that was fertilised by hydrothermal vent fluids with a basaltic HREE-Y signature. Hypogene iron ore in sub-greenschist facies metamorphosed BIF formed during three distinct stages: ore stage 1 was a syn- to post-metamorphic, syn-D1, Fe–Ca–Mg–Ni–Co–P–REE metasomatism that produced local Ni–REE-rich Fe–dolomite–magnetite alteration in BIF. Hydrothermal alteration was induced by hot fluid flow controlled by brittle–ductile reactivation of BIF-basalt margins and crosscutting D1 faults. The Ni–Co-rich content of dolomite and a shift in REE ratios in carbonate-altered BIF towards Archean mafic rock signature (Y/Ho to 31 to 40, Sm/Yb to 1 to 2 and Gd/Gd* to 1.2 to 1.4) suggest that high-Mg basalts in the Windarling Range were the primary source of introduced metals. During ore stage 2, a syn-deformational and likely acidic and oxidised fluid flow along BIF-basalt margins and within D1 faults leached carbonate and precipitated lepidoblastic and anhedral/granoblastic hematite. High-grade magnetite–hematite ore is formed during this stage. Ore stage 3 hydrothermal specular hematite (spcH)–Fe–dolomite–quartz alteration was controlled by a late-orogenic, brittle, compressional/transpressional stage (D4; the regional-scale shear-zone-related D3 is not preserved in Windarling). This minor event remobilised iron oxides, carbonate and quartz to form veins and breccia but did not generate significant volumes of iron ore. Ore stage 4 involved Mesozoic(?) to recent supergene oxidation and hydration in a weathering environment reaching down to depths of ～100 to maximum 200 m below surface. Supergene ore formation involved goethite replacement of dolomite and quartz as well as martitisation. Important ‘ground preparation’ for supergene modification and upgrade were mainly the formation of steep D1 to D4 structures, steep BIF/basalt margins and particularly the syn-D1 to syn-D2 carbonate alteration of BIF that is most susceptible to supergene dissolution. The Windarling deposits are structurally controlled, supergene-modified hydrothermal iron ore systems that share comparable physical, chemical and ore-forming characteristics to other iron ore deposits in the Yilgarn Craton (e.g. Koolyanobbing, Beebyn in the Weld Range, Mt. Gibson). However, the remarkable variety in pre-, syn- and post-deformational ore textures (relative to D1 and D2) has not been described elsewhere in the Yilgarn and are similar to the ore deposits in high-strain zones, such as of Brazil (Quadrilátero Ferrífero or Iron Quadrangle) and Nigeria. The overall similarity of alteration stages, i.e. the sequence of hydrothermal carbonate introduction and hypogene leaching, with other greenstone belt-hosted iron ore deposits supports the interpretation that syn-orogenic BIF alteration and upgrade was crucial in the formation of hypogene–supergene iron ore deposits in the Yilgarn Craton and possibly in other Archean/Paleoproterozoic greenstone belt settings worldwide.  相似文献   

8.
The Wiluna West small (~ 130 Mt) high-grade bedded hematite ore deposits, consisting of anhedral hematite mesobands interbedded with porous layers of acicular hematite, show similar textural and mineralogical properties to the premium high-grade low-phosphorous direct-shipping ore from Pilbara sites such as Mt Tom Price, Mt Whaleback, etc., in the Hamersley Province and Goldsworthy, Shay Gap and Yarrie on the northern margin of the Pilbara craton. Both margins of the Pilbara Craton and the northern margin of the Yilgarn craton were subjected to sub-aerial erosion in the Paleoproterozoic era followed by marine transgressions but unlike the Hamersley Basin, the JFGB was covered by comparatively thin epeirogenic sediments and not subjected to Proterozoic deformation or burial metamorphism. The Joyner's Find greenstone belt (JFGB) in the Yilgarn region of Western Australia was exhumed by middle to late Cenozoic erosion of a cover of unmetamorphosed and relatively undeformed Paleoproterozoic epeirogenic sedimentary rocks that preserved the JFGB unaltered for nearly 2 Ga; thus providing a unique snapshot of the early Proterozoic environment.Acicular hematite, pseudomorphous after acicular iron silicate, is only found in iron ore and BIF that was exposed to subaerial deep-weathering in early Paleoproterozoic times (pre 2.2 Ga) and in the overlying unconformable Paleoproterozoic conglomerate derived from these rocks and is absent from unweathered rocks (Lascelles, 2002). High-grade ore and BIF weathered during later subaerial erosion cycles contain anhedral hematite and acicular pseudomorphous goethite. The acicular hematite was formed from goethite pseudomorphs of silicate minerals by dehydration in the vadose zone under extreme aridity during early Paleoproterozoic subaerial weathering.The principal high-grade hematite deposits at Wiluna West are interpreted as bedded ore bodies that formed from BIF by loss of chert bands during diagenesis and have been locally enriched to massive hematite by the introduction of hydrothermal specular hematite. No trace of chert bands are present in the deep saprolitic hematite and hematite–goethite ore in direct contrast to shallow supergene ore in which the trace of chert bands is clearly defined by goethite replacement, voids and detrital fill. Abundant hydrothermal microplaty hematite at Wiluna West is readily distinguished by its crystallinity.The genesis of the premium ore from the Pilbara Region has been much discussed in the literature and the discovery at Wiluna West provides a unique opportunity to compare the features that are common to both districts and to test genetic models.  相似文献   

9.
条带状铁建造:特征、成因及其对地球环境的制约   总被引：1，自引：1，他引：0  
条带状铁建造(BIF)是由硅质和铁质组成的化学沉积岩,通常具有典型的薄层或薄板状构造。BIF不仅记录了丰富的岩石圈、大气圈、水圈和生物圈状态及演化的信息,而且与其相关的铁矿也是世界上最重要的铁矿资源类型。虽然对BIF的研究已取得了丰富的成果,近年来若干新颖的观点和重要的发现对进一步深入研究BIF成因提供了重要思考途径和研究线索。文章总结了BIF类型、矿物学和地球化学特征,分析了BIF形成条件及大规模缺失机理,阐述了BIF的形成和大气圈两次重大氧化事件(古元古代大氧化事件,2.4～2.1 Ga,新元古代氧化事件,0.8～0.55 Ga?)之间的关系,认为BIF与海底岩浆-热液活动具有密切的成因联系。最后,提出BIF的沉积和消亡与重大地质事件密切相关,这些重大地质事件也相互作用,共同制约着大气海洋环境演化、地球生物学和地球动力学过程。关于BIF精细成矿时代、条带/条纹形成的机制、重大地质事件对BIF形成的制约和BIF微生物成因等方面仍存在较多问题。随着大数据、精细成矿年代学、模拟实验、非传统稳定同位素(Fe、Cr、U、Mo和Cu同位素等)新技术方法的进步,及对现代洋底热液系统的深入认识,将有助于揭开BIF成因的神秘面纱。  相似文献   

10.
Precambrian iron ores of the Singhbhum-North Orissa region occur in eastern India as part of the Iron Ore Group (IOG) within the broad horse-shoe shaped synclinorium. More than 50% of Indian iron ore reserves occur in this region. Massive-hard, flaky-friable, blue dust and lateritic varieties of iron ores are the major ore types, associated with banded hematite, jasper and shales. These ores could have formed as a result of supergene enrichment through gradual but extensive removal of silica, alumina and phosphorus from banded iron formations and ferruginous shale. Attempts for optimal utilization of these resources led to various ore characterization studies using chemical analysis, ore and mineral petrography, XRD analysis, SEM and electron probe micro analysis (EPMA). The ore chemistry indicates that the massive hard ores and blue dust have high iron, low alumina and phosphorus contents. Because of high quality, these ores do not require any specialized beneficiation technique for up-gradation. However, flaky-friable, lateritised and goethitic ores are low in iron, high in alumina and phosphorus contents, requiring specific beneficiation techniques for up-gradation in quality. XRD, SEM and ore microscopic studies of massive hard ores indicate the presence of hematite and goethite, while flaky and lateritic ores show a higher concentration of goethite, kaolinite, gibbsite and hematite. EPMA studies show the presence of adsorbed phosphorous as fine dust in the hard ores. Sink and float studies reveal that most of the gangue minerals are not completely liberated in the case of goethitic and lateritic ores, even at finer fractions.  相似文献   

11.
沈其韩  宋会侠 《岩石学报》2015,31(10):2795-2815
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。  相似文献   

12.
李旭平  陈妍蓉 《岩石学报》2021,37(1):253-268
条带状铁建造(BIF)是3.5～1.8Ga前陆架和洋盆的常见沉积物.前寒武纪条带状铁建造构成了世界上重要的铁矿资源.虽然它们成矿过程及其演化的许多方面的问题仍未解决,但人们普遍认为,它们沉积方式的长期变化与地球的环境和地球化学演化有关.条带状铁建造记录了前寒武纪古海洋、古环境、大气条件和细菌代谢条件以及铁的来源和沉积过...  相似文献   

13.
Several iron-ore deposits hosted within Mesoarchean banded iron formations (BIFs) are mined throughout the North Pilbara Craton, Western Australia. Among these, significant goethite±martite deposits (total resources >50 Mt at 55.8 wt% Fe) are distributed in the Wodgina district within 2 km of the world-class pegmatite-hosted, tantalum Wodgina deposits. In this study, we investigate the dominant controls on iron mineralisation at Wodgina and test the potential role of felsic magma-derived fluids in early alteration and upgrade of nearby BIF units. Camp-scale distribution and geochemistry of iron ore at Wodgina argue against any significant influence of identified felsic intrusions in the upgrade of BIF. Whereas, the formation of BIF-hosted goethite±martite iron ore at Wodgina involves: (i) early (ca 2950 Ma) metamorphism of BIF causing camp-scale recrystallisation of pre-existing iron oxides to form euhedral magnetite, with local enrichment to sub-economic grades (～40 wt% Fe) within or proximal to metre-wide, bedding-parallel shear zones, and (ii) later supergene lateritic enrichment of the magnetite-bearing BIF and shear zones, forming near-surface goethite±martite ore. The supergene alteration sequence includes: (i) downward progression of the oxidation front and replacement of magnetite by martite, (ii) local development of silcrete at ～40 m below the modern surface caused by the lowering of the water-table, (iii) intensive replacement of quartz by goethite, resulting in the goethite±martite ore bodies at Wodgina, and (iv) late formation of ferricrete and ochreous goethite. Goethitisation most likely took place within the hot and very wet climate that prevailed from the Paleocene to the mid-Eocene. Goethite precipitation was accompanied by the incorporation of trace elements P, Zn, As, Ni and Co, which were likely derived from supergene fluid interaction with nearby shales. Enrichment of these elements in goethite-rich ore indicates that they are potentially useful pathfinder elements for concealed ore bodies covered by trace element-depleted pedogenic silcrete and siliciclastic rocks located throughout the Wodgina mine.  相似文献   

14.
The Águas Claras and Pico Mines are two world-class iron-ore mines hosted within the Lower- Proterozoic banded iron-formations (locally known as itabirites) of the Minas Supergroup located in the Quadrilátero Ferrífero district, Minas Gerais, Brazil. The Águas Claras orebody consists of a 2,500-m-long roughly tabular-shaped lens hosted within the dolomitic itabirite of the Cauê Formation. Dolomitic itabirite is the protore of the soft high-grade iron ore, which is the main ore type of the Águas Claras orebody, representing about 85% of the 284 Mt mined since 1973, with the remaining 15% comprising hard high-grade ore. Hematite is the main constituent of the iron ores. It occurs as martite, granular hematite and locally as specularite. Magnetite appears subordinately as relicts within martite and hematite crystals. Gangue minerals are very rare. These consist of dolomite, chlorite, talc, and apatite, and are especially common in contact with the protore. This virtual absence of gangue minerals is reflected in the chemistry of ores that are characterized by very high Fe contents (an average of 68.2% Fe).The Pico orebody is a continuous ~3,000-m-long body of a lenticular shape hosted within siliceous itabirite, which is the protore of the soft high- and low-grade ores at the Pico Mine. The soft high-grade ores, together with the low-grade ores, called iron-rich itabirite, are the main types of ore, and respectively represent approximately 51 and 29% of the reserves. The remaining 20% consists of hard high-grade ore. The iron oxide mineralogy is the same as that of the Águas Claras Mine, but in different proportions. Gangue minerals are very rare in the high-grade ores, but are slightly more common in the iron-rich itabirite. Quartz is the dominant gangue mineral, and is found with minor quantities of chlorite. The chemistry of the high-grade ores is characterized by high Fe contents (an average of 67.0%) and low P, Al2O3, and SiO2, which are concentrated in the fines. Iron-rich itabirites average 58.6% Fe and 13.5% SiO2.The genesis of the soft high-grade ores and iron-rich itabirites is related to supergene processes. Leaching of the gangue minerals by groundwater promoted the residual iron enrichment of the itabirites. This process was favored by the tropical climate and topographic situation. The original composition of the itabirites and the presence of structures controlling the circulation of the groundwater have influenced the degree of iron enrichment. The hard high-grade ores are of a hypogene origin. Their genesis is attributed to hydrothermal solutions that leached the gangue minerals and filled the spaces with hematite. This process remains a source of debate and is not yet fully understood.Editorial handling: S.G. Hagemann  相似文献   

15.
庐枞盆地龙桥铁矿床中菱铁矿的地质特征和成因意义   总被引：6，自引：0，他引：6  
龙桥铁矿床是庐枞火山岩盆地中的一个大型的铁矿床,多年来对其矿床成因的认识存在较大的争论.文章在野外地质研究工作的基础上,通过对矿床中菱铁矿的岩矿分析鉴定和电子探针测试,确定了矿床纹层状矿石中的菱铁矿为沉积成因.通过对菱铁矿的产出特征分析,并结合龙桥铁矿床的部分地质地球化学研究成果,认为在该矿床形成过程中,早期沉积形成了纹层状的菱铁矿层,在燕山期的岩浆热事件中,部分沉积菱铁矿被交代形成了磁铁矿和具有残余骸晶结构等一系列矿石交代组构特征的矿物.纹层状矿石既具有沉积特征,也具有热液改造特征,证实了矿床的形成存在早期(三叠纪)的沉积成矿(菱铁矿)作用和晚期(燕山期)的热液成矿(磁铁矿)作用.菱铁矿的研究为进一步确定龙桥铁矿床的成因提供了新的佐证.  相似文献   

16.
The Nkout deposit is part of an emerging iron ore province in West and Central Africa. The deposit is an oxide facies iron formation comprising fresh magnetite banded iron formation (BIF) at depth, which weathers and oxidises towards the surface forming caps of high grade hematite/martite–goethite ores. The mineral species, compositions, mineral associations, and liberation have been studied using automated mineralogy (QEMSCAN®) combined with whole rock geochemistry, mineral chemistry and mineralogical techniques. Drill cores (saprolitic, lateritic, BIF), grab and outcrop samples were studied and divided into 4 main groups based on whole rock Fe content and a weathering index. The groups are; enriched material (EM), weathered magnetite itabirite (WMI), transitional magnetite itabirite (TMI) and magnetite itabirite (MI). The main iron minerals are the iron oxides (magnetite, hematite, and goethite) and chamosite. The iron oxides are closely associated in the high grade cap and liberation of them individually is poor. Liberation increases when they are grouped together as iron oxides. Chamosite significantly lowers the liberation of the iron oxides. Automated mineralogy by QEMSCAN® (or other similar techniques) can distinguish between Fe oxides if set up and calibrated carefully using the backscattered electron signal. Electron beam techniques have the advantage over other quantitative mineralogy techniques of being able to determine mineral chemical variants of ore and gangue minerals, although reflected light optical microscopy remains the most sensitive method of distinguishing closely related iron oxide minerals. Both optical and electron beam automated mineralogical methods have distinct advantages over quantitative XRD in that they can determine mineral associations, liberation, amorphous phases and trace phases.  相似文献   

17.
弓长岭铁矿床是鞍山本溪地区最典型的BIF型铁矿床之一,而且是该地区最大的富铁矿产区。从野外产出关系来看,弓长岭矿区的富铁矿与蚀变岩密切相关,蚀变岩与富铁矿基本上是形影相随。蚀变岩具有分带性,由富铁矿向外依次为镁铁闪石岩石榴石岩绿泥石岩弱蚀变斜长角闪岩斜长角闪岩。弱蚀变岩保留了蚀变原岩的岩貌特征,矿物的蚀变并不完全,可见残余的原生矿物。强蚀变岩的蚀变较彻底,基本无原生矿物残留。将蚀变岩与斜长角闪岩、磁铁石英岩的地球化学特征进行对比可以发现弱蚀变岩、石榴石岩、绿泥石岩与斜长角闪岩的痕量元素特征基本一致,而镁铁闪石岩的痕量元素特征更接近磁铁石英岩。再结合镜下特征、野外接触关系、主量元素特征等证据,认为除了镁铁闪石岩是由磁铁石英岩蚀变形成,其余蚀变岩都是由斜长角闪岩蚀变形成。根据各类蚀变岩中主要矿物的(Fe+Mg)/Si值以及蚀变岩的SiO2和Fe2OT3含量变化规律可以发现,在蚀变岩和富矿形成过程中发生了Mg、Fe以及Si的迁移。对本次取样的样品进行原岩恢复和构造环境判别投图,投图结果表明,绿泥石岩和弱蚀变岩的最初原岩都是形成于弧后盆地的玄武岩。  相似文献   

18.
中国叠生型铁矿床成矿特征探讨   总被引：3，自引：1，他引：2  
沈保丰  张阔 《矿床地质》2016,35(2):213-224
叠生成矿作用主要是指早期成矿作用被晚期成矿作用叠加、复合和改造。晚期成矿作用的性质常与早期成矿作用不同,也就是说,在早先己有矿床(或矿体、矿源层)的基础上,叠加复合了晚期成矿作用,即成矿时间上有先后、空间产出上有重叠、并对早先形成的矿床进行复杂的复合、叠加和改造,使成矿作用具有多样性、复杂性,并可形成大矿、富矿。铁矿床中叠生成矿作用广泛发育。按矿床或矿体产出和形成的地质特征,中国叠生型铁矿床可分为风化淋滤型、热液叠加改造型和热液叠加复合型3个亚类。风化淋滤型铁矿床在中国分布有限,规模不大,工业利用价值不大,因而中国的叠生型铁矿床主要是指热液叠加改造型和热液叠加复合型两个亚类。热液叠加改造型主要是指早期的铁矿床(或矿体、矿源层)经后期热液叠加改造,使早期的较贫铁矿床(或矿体、矿源层)成为较富铁矿床(或矿体),这是中国BIF型铁矿床中最重要的富铁矿类型,以鞍本地区弓长岭二矿区为典型代表。弓长岭二矿区铁矿,早期在新太古代形成条带状磁铁石英岩(2528 Ma,贫矿石),后期在古元古代,含矿热液交代改造贫铁矿形成富铁矿(1840 Ma)。热液叠加复合型主要是指后期脉型铁(或稀土元素等)矿床叠加在早期(沉积或其他成因)铁等矿床上而形成的矿床,如白云鄂博铁-铌-稀土元素矿床和黔西菱铁矿矿床。白云鄂博铁-铌-稀土元素矿床的形成与火成碳酸岩有关,在中元古代(1.3 Ga)左右,区内火成碳酸岩的侵位,在早期主要形成以岩浆熔离作用为主的铁-铌-稀土元素矿,晚期叠加了加里东期稀土-铌矿化热液脉。古陆边缘构造带或陆内活化带是形成叠生型铁矿床的有利构造空间,较大的地球化学块体,为形成多期、多成因的矿床提供物质来源,叠生型铁矿床的形成明显受构造的控制。叠生成矿是复杂地质过程的一种具体表现。热液叠加改造型和热液叠加复合型的叠生型铁矿床的形成是因中国独特的大地构造环境决定的。叠生成矿作用的研究,尚处在初步阶段。加强对叠生成矿作用的研究,了解其形成的地质背景、成矿机制、作用过程、控矿因素等,对发展矿床学研究,认识区域成矿特征和指导地质找矿具有重要的理论和实际意义。  相似文献   

19.
论安徽省姑山铁矿床的热液成矿特征   总被引：4，自引：3，他引：4       下载免费PDF全文
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。  相似文献   

20.
This paper models the physico-chemical conditions of a Neoarchæan to Palæoproterozoic marine basin in which the sedimentary sequence of BIF, Fe and Mn ores of the Lake Superior-type formed. The model is based on Eh-pH diagram stability fields for Fe, silica and Mn solubilities (taken from the literature) and on field observations of the lithological sequences. BIF formation took place in epicontinental marine basins with free access to the ocean. The main Fe source for BIF formation was ocean enriched with about 6–10 ppm ferrous Fe of hydrothermal geochemical affinity. Land-derived Fe influxes into the BIF-forming basins certainly contributed, but the lack of clastic sedimentation precludes estimation of element budgets. The main silica source for formation of chert layers is sea water. If silica was precipitated by evaporation, the silica concentration of the BIF-forming sea must have been close to saturation (15–20 ppm). Biogenic silica concentration from a possible silica undersaturated sea may not be excluded. These inferred BIF-forming conditions fit the global occurrence of Lake Superior-type BIF in general, whereas special sedimentary environments were probably responsible for the formation of highly enriched laminated Fe ore at the Maremane Dome and in the Sishen-Kathu mining district in Griqualand West, and for the FeMn ores in the Kalahari field. Formation of laminated Fe ore in the Maremane Dome and in the Sishen-Kathu areas were restricted to local deeps within the BIF basins, caused by karst collapse in the underlying Campbellrand dolomites. In such deeps, increased pH values relative to the normal BIF-forming sea caused sufficiently increased silica solubility, resulting in the almost exclusive sedimentation of colloidal Fe precipitates.In the Kalahari field, the BIF sedimentation pile became silica-depleted when approaching the Mn layers. This was genetically controlled by the increased pH of sea water and increased silica solubility. Under such increased pH conditions, Mn oxides become stable for precipitation if minimum Mn activity is achieved in the sedimentary basin. The sedimentation sequence of low silica BIF - kutnahoritic BIF - jacobsitic BIF - braunitic Mn ore can be explained, using combined Eh-pH diagrams, as reflecting a precipitation path of increasing redox potential in a pH environment slightly above 9. These conditions were achieved by closing the access of the basin to the open ocean, resulting in the reduction of water level by evaporation and thereby increasing salinity and pH. Precipitation of low silica BIF followed and, in the presence of sufficient Mn activity with increasing Eh in the precipitating water stratum, deposition of the Mn mineral associations occurred.  相似文献   

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