Influence of contamination on banded iron formations in the Isua supracrustal belt, West Greenland: Reevaluation of the Eoarchean seawater compositions

Banded Iron Formations（BIFs） are chemical sediments, ubiquitously distributed in the Precambrian supracrustal belts; thus their trace element compositions are helpful for deciphering geochemical evolution on the Earth through time. However, it is necessary to elucidate factors controlling the whole-rock compositions in order to decode the ancient seawater compositions because their compositions are highly variable. We analyzed major and trace element contents of the BIFs in the 3.8-3.7 Ga Isua supracrustal belt（ISB）, southern West Greenland. The BIFs are petrographically classified into four types：Black-,Gray-, Green-and White-types, respectively. The Green-type BIFs contain more amphiboles, and are significantly enriched in Co, Ni, Cu, Zn, Y, heavy rare earth element（HREE） and U contents. However,their bulk compositions are not suitable for estimate of seawater composition because the enrichment was caused by secondary mobility of metamorphic Mg, Ca and Si-rich fluid, involvement of carbonate minerals and silicate minerals of olivine and pyroxene and/or later silicification or contamination of volcanic and clastic materials. The White-type BIFs are predominant in quartz, and have lower transition element and REE contents. The Gray-type BIFs contain both quartz and magnetite. The Black-type BIFs are dominated by magnetite, and contain moderate to high transition element and REE contents. But,positive correlations of V, Ni, Zn and U contents with Zr contents suggest that involvement of detrital,volcanic and exhalative materials influences on their contents. The evidence for significant influence of the materials on the transition element contents such as Ni in the BIFs indicates the transition element contents in the Archean ocean were much lower than previously estimated. We reconstructed secular variations of V,Co, Zn and U contents of BIFs through time, which show Ni and Co contents decreased whereas V, Zn and U contents increased through time. Especially, the Ni and Co contents drastically decreased in the Mesoarchean rather than around the Great Oxidation Event. On the other hand, the V,Zn and U contents progressively increased from the Mesoarchean to the Proterozoic. Stratigraphical trends of the BIFs show increase in Y/Ho ratios and decrease in positive Eu anomaly upwards, respectively. The stratigraphic changes indicate that a ratio of hydrothermal fluid to seawater component gradually decrease through the deposition, and support the Eoarchean plate tectonics, analogous to the their stratigraphic variations of seafloor metalliferous sediments at present and in the Mesoarchean.

Depositional age and tectonic environment of the Gouap banded iron formations from the Nyong group,SW Cameroon:Insights from isotopic,geochemical and geochronological studies of drillcore samples

The discovery of the Gouap banded iron formations(BIFs)-hosted iron mineralization in the northwestern of the Nyong Group(Ntem Complex)in southwestern Cameroon provides unique insights into the geology of this region.In this contribution,we firstly report detailed study of geochemistry,isotopic and geochronology of well preserved samples of the Gouap BIFs collected from diamond drillcores.The Gouap BIFs consist mainly of amphibole BIFs and amphibole-pyrite BIFs characterized by dominant Fe_(2)O_(3)+SiO_(2)contents and variable contents of CaO,MgO and SO_(3),consistent with the presence of amphibole,chlorite,epidote and pyrite,formed during amphibolite facies metamorphism and overprinted hydrothermal event.The amphibole–pyrite BIFs are typically enriched in trace and rare earth elements(REE)compared to the amphibole BIFs,suggesting the influence of detrital materials as well as secondary hydrothermal alteration.The Post Archean Australian Shale(PAAS)-normalized REE–Y profiles of the Gouap BIFs display positive La,Eu anomalies,weak negative Ce anomalies,indicating a mixture of low-temperature hydrothermal fluids and relatively oxic conditions probably under relative shallow seawater.We present here the first isotopic data of BIFs within the Ntem Complex.Theδ^(30)Si_(NBS28)values of the quartz from the Gouap BIFs vary from-1.5‰to-0.3‰and from-0.8‰to-0.9‰for the amphibole BIFs and amphibole–pyrite BIFs,respectively.The quartz hasδ^(18)OV-SMOW values of 6.8‰–9.5‰(amphibole BIFs)and 9.2‰–10.6‰(amphibole–pyrite BIFs).The magnetite from the Gouap BIFs showsδ^(18)O values ranging from-3.5‰to-1.8‰and from-3‰to-1.7‰for the amphibole BIFs and amphibole–pyrite BIFs,respectively.Moreover,the pyrite grains in the amphibole–pyrite BIFs displayδ34S values of 1.1‰–1.8‰.All isotopic data of the Gouap BIFs confirm that they might have precipitated from low-temperature hydrothermal fluids with detrital input distant from the volcanic activity.According to their geochemical and isotopic characteristics,we propose that the Gouap BIFs belong to the Superior type.In situ U–Pb zircon dating of BIFs was conducted to assess the BIF depositional age based on strong evidence of zircon in thin section.The Gouap BIFs were probably deposited at 2422±50 Ma in a region where sediments extended from continental shelf to deep-water environments along craton margins like the Caue Formation of the Minas Supergroup,Brazil.The studied BIFs have experienced regional hydrothermal activity and metamorphism at 2089±8.3 Ma during the Eburnean–Transamazonian orogeny.These findings suggest a physical continuity between the protocratonic masses of both Sao Francisco and Congo continents in the Rhyacian Period.

Neoproterozoic Banded Iron Formation in the Central Tianshan, NW China: The Shalong Example

Objective Banded Iron Formations （BIFs） are ferruginous chemical sedimentary rocks that precipitated throughout the Precambrain, which constitute the most important iron resources in the world. The majority of BIFs were developed in the Neoarchean and early Paleoproterozoic periods （3.2-1.8 Ga）, which are well known and have been mined for centuries. Another type of the BIFs which was formed in the Neoproterozoic period （0.85-0.7 Ga） is much smaller in scale but widespread on the Earth and record important information of the evolution of the Earth. The Neoarchean and Paleoproterozoic BIFs have been well studied and understood, while few detailed studies on Neoproterozoic BIFs have been made, and only a few modem geochronology studies were carried out on Neoproterozoic BIFs.

Seeing through the magnetite:Reassessing Eoarchean atmosphere composition from Isua(Greenland) ≥3.7 Ga banded iron formations

Estimates of early atmosphere compositions from metamorphosed banded iron formations(BIFs)including the well-studied ≥3.7 BIFs of the Isua supracrustal belt(Greenland)are dependent on knowledge of primary versus secondary Fe-mineralogical assemblages.Using new observations from locally well preserved domains,we interpret that a previously assumed primary redox indicator mineral,magnetite,is secondary after sedimentary Fe-clays(probably greenalite)±carbonates.Within ~3.7 Ga Isua BIF,pre-tectonic nodules of quartz+Fe-rich amphibole±calcite reside in a finegrained(≤100 μm)quartz+magnetite matrix.We interpret the Isua nodule amphibole as the metamorphosed equivalent of primary Fe-rich clays,armoured from diagenetic oxidative reactions by early silica concretion.Additionally,in another low strain lacunae,~3.76 Ga BIF layering is not solid magnetite but instead fine-grained magnetite+quartz aggregates.These magnetite+quartz aggregates are interpreted as the metamorphosed equivalent of Fe-clay-rich layers that were oxidised during diagenesis,because they were not armoured by early silicification.In almost all Isua BIF exposures,this evidence has been destroyed by strong ductile deformation.The Fe-clays likely formed by abiotic reactions between aqueous Fe^(2+)and silica.These clays along with silica±carbonate were deposited below an oceanic Fe-chemocline as the sedimentary precursors of BIF.Breakdown of the clays on the sea floor may have been by anaerobic oxidation of Fe^(2+),a mechanism compatible with iron isotopic data previously published on these rocks.The new determinations of the primary redoxsensitive Fe-mineralogy of BIF significantly revise estimates of early Earth atmospheric oxygen and CO_2 content,with formation of protolith Fe-rich clays and carbonates compatible with an anoxic Eoarchean atmosphere with much higher CO_2 levels than previously estimated for Isua and in the present-day atmosphere.

Origin of the Neoproterozoic Baijianshan Banded Iron Formation at the Southeastern Margin of the Tarim Block in NW China:Implication for an Extremely Reducing Ocean

The Neoproterozoic banded iron formations(BIFs)were closely associated with the“Snowball Earth”during the breakup of the Rodinia,thus they played an important role in our understanding of the atmospheric and oceanic oxygen levels during this period.In this contribution,the Neoproterozoic(ca.737 Ma)Baijianshan BIF at Southeast Tarim,northwestern China was identified.Magnetite is the dominated iron-species,which occurs as the lamina interbedded with chert.The BIF contains low concentrations of trace elements,and is depleted in light rare earth elements(LREEs)based on comparison with the Post-Archean Australian Shale(PAAS).In addition,the BIF exhibits slightly positive La-Eu anomalies,negligible Ce anomalies,insignificant Y anomalies,chondritic Y/Ho ratios(23-32),and slightly chondritic initial ε_(Nd)(t=737 Ma)values(−0.45 to 1.46,averaging 0.37).All these features indicate that the precipitation of Baijianshan BIF was closely related to the submarine low-T hydrothermal fluids with little detrital contribution.Moreover,the Baijianshan BIF is characterized by the significant enrichment of heavy Fe isotopes,with δ^(57)Fe_(IRMM-014) values ranging from 1.78‰ to 3.05‰,revealing the partial oxidation of Fe^(2+) into Fe^(3+) during the precipitation of this BIF.Our data suggest that the formation of Baijianshan BIF was closely associated with a significantly reducing ocean,which most likely was isolated from the oxidized atmosphere by a local ice sheet.This Neoproterozoic Baijianshan ocean has the initial oxygen levels as low as,or even lower than that of Archean and Paleoproterozoic oceans.

Modelling Textural Anisotropy of Magnetic Susceptibility of Banded Iron Formations

Anisotropy of magnetic susceptibility (AMS) of banded iron formations (BIFs) is characterized by high anisotropy and well-developed bedding-parallel magnetic foliation. Since most previous studies were focused on palaeomagneism of BIFs and BIF-derived iron ores, little effort has been made to further understand this special type of AMS for BIFs. A detailed theoretical analysis, incorporating with the previous experimental data, is made to understand the formative mechanism of this special anisotropy for BIFs. The good consistence between the theoretical and experimental results demonstrates that this type of anisotropy is likely caused by the layered structure of BIFs, and thus verifies the term of textural anisotropy for BIFs. Theoretical analysis also shows that in the negligence of the inter-layer magnetic action BIF’s apparent anisotropy increases with an increase in intrinsic susceptibility of magnetic layers, but decreases with an increase in length-to- diameter ratio of the magnetic layer.

Life on Earth Originated Where Later Microbial Oxygenic Photosynthesis Precipitated Banded Iron Formation, Suppressing Life Diversification for 1.4 Ga

The earliest Precambrian microbial structures appear in successions with banded iron formations (BIF) suggesting genetic relationships. The hypothesis of the deep ocean origin of BIFs associated with Mid-Ocean Ridge (MOR) like features seems to have been recently supported by the discovery of peculiar microbial ecosystems with unique faunal assemblages restricted to these volcanic vents. However, new sedimentological evidence points to the accumulation of varved BIF in huge, very shallow lakes of hydrothermal-water situated on continental plates while passing through thePolar Regions, where UV radiation is minimal. The mineral-rich solutions seeped from numerous fumaroles, providing suitable conditions for chemical reactions between inorganic components, incidentally creating organic-like self-multiplying molecules long before the biologically-initiated BIF deposition. Some of these early chemoautotrophic prokaryotes developed oxygenic photosynthesis during half a year of solar illumination. The released oxygen formed iron oxides and carbonates deposited with amorphous silica (geyserite) in laminae as BIF during 3.8 - 1.9 Ga. BIF deposition consumed most of the photosynthetic oxygen for 1.4 billion years. Intensified cyanobacteria oxygenic photosynthesis during 2.4 - 2.2 Ga raised the atmospheric oxygen content (Great Oxidation Event) over the Polar Regions, forming an oxygen-ozone shield against UV radiation. It gradually extended to lower latitudes, enabling prokaryotes to leave their ecologically stable habitat and acclimatize in new ecosystems, where they diversified, leading to eukaryote evolution. The 231/2° inclination of Earth’s rotation axis differentiated the solar effect on the Polar Regions, which controlled life evolution on Earth, as well as on planet Mars (25° inclination), where life probably did not evolve beyond early prokaryotes.

Experimental data for high-temperature decomposition of natural celadonite from banded iron formation

Three experiments were set up to evaluate conditions for the high-temperature decomposition of celadonite from a banded iron formation in an alumina-free system and identify its decomposition products. It was estimated that at 650 and 750 °C, with a Ni Ni O buffer and pressure of 3 kbar, celadonite completely decomposes and the decomposition products were tetraferribiotite, magnetite and quartz. Under more oxidizing conditions(hematite-magnetite buffer instead of Ni Ni O), ferrous potassium feldspar sanidine forms instead of magnetite.During the celadonite decomposition in oxidizing conditions more magnesian and aluminous tetraferribiotite,along with ferrous sanidine, are formed than at reducing conditions.

Spectral characteristics of banded iron formations in Singhbhum craton,eastern India:Implications for hematite deposits on Mars

Banded iron formations （BIFs） are major rock units having hematite layers intermittent with silica rich layers and formed by sedimentary processes during late Archean to mid Proterozoic time. In terrestrial environment, hematite deposits are mainly found associated with banded iron formations. The BIFs in Lake Superior （Canada） and Carajas （Brazil） have been studied by planetary scientists to trace the evolution of hematite deposits on Mars. Hematite deposits are extensively identified in Meridiani region on Mars. Many hypotheses have been proposed to decipher the mechanism for the formation of these deposits. On the basis of geomorphological and mineralogical studies, aqueous environment of deposition is found to be the most supportive mechanism for its secondary iron rich deposits. In the present study, we examined the spectral characteristics of banded iron formations of Joda and Daitari located in Singhbhum craton in eastern India to check its potentiality as an analog to the aqueous/marine environment on Mars. The prominent banding feature of banded iron formations is in the range of few millimeters to few centimeters in thickness. Fe rich bands are darker （gray） in color compared to the light reddish jaspilitic chert bands. Thin quartz veins （〈4 mm） are occasionally observed in the handspecimens of banded iron formations. Spectral investigations have been conducted in VIS/NIR region of electromagnetic spectrum in the laboratory conditions. Optimum absorption bands identified include 0.65, 0.86, 1.4 and 1.9 μm, in which 0.56 and 0.86 μm absorption bands are due to ferric iron and 1.4 and 1,9 μm bands are due to OH/H2O. To validate the mineralogical results obtained from VlS/NIR spectral radiometry, laser Raman and Fourier transform infrared spectroscopic techniques were utilized and the results were found to be similar. Goethite-hematite association in banded iron formation in Singhbhum craton suggests dehydration activity, which has altered the primary iron oxide phases into the secondary iron oxide phases. The optimum bands identified for the minerals using various spectroscopic techniques can be used as reference for similar mineral deposits on any remote area on Earth or on other hydrated planetary surfaces like Mars.

Geochemical Characteristics of Wuyang Siliceous Rocks in the Southern Margin of North China Craton and its Constraint on the Formation Environment of BIF of Tieshanmiao Formation

Precambrian banded iron formation(BIF)is one of the most important mineral resources in China,mostly abundant in the North China Craton(NCC)with relatively less common in South China.Since the BIF and siliceous rocks both originated from chemical deposition,the syngenetic BIF and Siliceous rocks can help evaluate their environment of formation.We examine here the mineralogy and geochemistry of siliceous rocks associated with the Tieshanmiao Formation BIF,aiming to decipher the conditions of formation of both BIF and Siliceous rocks in the Wuyang area in the NCC.Analysis of the geochemical characteristics of whole rock shows that the Si O2 content of the siliceous rock ranges from 90.11%to 94.85%and is relatively high overall.Trace element contents of Ba and U are also high,the Ba/Sr ratio ranges from 3.89 to 25.28 and the U/Th ratio ranges from 0.09 to 0.20.Finally,theΣREE value of rare earth elements ranges from 57.03 ppm to 152.59 ppm,and these indexes all indicate that siliceous rock resulted from hydrothermal deposition.Plots of Al2 O3-Si O2,Si O2/(K2 O+Na2 O)-Mn O2/Ti O2 and Mn-10×(Cu+Co+Ni)-Fe in discrimination diagrams also verify this interpretation.However,both the Mg O content,ranging from 0.16 to 0.32,and the Fe/Ti ratio,ranging from 2.50 to 9.72,suggest that terrigenous material was added during the depositional process.Major and trace element parameters of siliceous rock,such as the Al/(A1+Fe+Mn)ratio(from 0.81 to 0.93),Mn O/Ti O2(from 0.00 to 0.17),Al/(Al+Fe)(from 0.82 to 0.93),Sc/Th ratio(from 0.21 to 0.50),U/Th(from 0.09 to 0.20),(La/Yb)N(from 0.83 to 3.04),and the(La/Ce)N(from 0.01 to 0.02)all imply that the siliceous rock formed in a continental margin.In addition,the Sr/Ba ratio from 0.08 to 0.26,theδCe value from 0.31 to 0.90,and theδEu value from 0.14 to 0.58,all indicate that the siliceous rock was formed at a relatively deeper water depth and under weak hydrodynamic conditions.Siliceous rock and BIF formed in the same geological setting,with the Si O2/(K2 O+Na2 O)ratio of siliceous rock ranging from 28.61 to 47.43,the Si O2/Al2 O3 ratio from 16.53 to 32.37,and the Si O2/Mg O ratio from 287.28 to 592.81,which are all in agreement with chemical deposition associated with volcanic eruptions.The Al2 O3/Ti O2 ratio from 37.82 to 50.30 indicates that the magma source of siliceous rock was of slightly intermediate composition.During the Late Archean in the Wuyang area,the high concentration and high purity Si O2 quickly precipitated from hydrothermal fluids to finally result in the accumulation of siliceous rock in a marginal sea,while the input corresponding to iron formation components was deposited to form iron formation layers,and limestone was only the product formed during the deposition intervals of siliceous rock and iron formations.In this study,the synsedimentary siliceous rocks of BIF act as a new way to provide direct evidence to understand the formation environment of BIF due to its high geochemical stability.

Insight into the Origin of Iron Ore Based on Elemental Contents of Magnetite and Whole-Rock Geochemistry:A Case of the Bipindi Banded Iron Formations,Nyong Complex,SW Cameroon

The Bipindi iron ore district is located in the central section of the Nyong Complex at the northwestern margin of the Congo Craton in Southwest Cameroon.This iron district contains numerous iron mineralization hosted by the Mewongo,Bibole,Kouambo,and Zambi banded iron formations(BIFs).These BIFs contain magnetite as the main iron ore mineral associated with pyrite,and gangue minerals are quartz with minor chlorite and amphibole.The origin of iron ore from these BIFs was investigated using a combination of in-situ magnetite and whole-rock chemistry.The studied BIF ore samples have a narrow range of TFe between 30.90 wt.%and 43.20 wt.%,indicating a low-grade ore.The geochemical signatures of magnetite such as low contents of base metals(e.g.,Cu,Co,V,and Zn)and low Co/Zn ratios<0.85 indicate a hydrothermal origin.Combined with the geochemical features of these BIFs,e.g.,high Fe/Ti and Fe/Al ratios(mean>600 and>75,respectively),we suggest that magnetite was derived from a mixture of seawater and~0.1%low-temperature hydrothermal fluids in an oxidizing environment.Collectively,low-temperature hydrothermal and later metamorphic fluids were necessary for the transformation of the protolith Nyong Complex BIFs to iron ore.

鲁西地区BIF型铁矿床控矿规律及找矿前景

鲁西地区是中国东部重要的铁矿集区,其中BIF型铁矿成矿地质条件优越。本文通过梳理研究区成矿地质背景、地球物理特征及该类型铁矿床时空分布特征,并综合研究其赋矿地层特征的基础上,对该类型铁矿床地层赋矿规律、构造控矿规律、岩浆岩控矿规律和物探异常示矿规律进行深入剖析。研究表明该区BIF型铁矿成矿时代为新太古代,且具有多个铁矿赋矿层位,其中泰山岩群雁翎关组、山草峪组、柳杭组和济宁岩群颜店组均为主要赋矿层位;该类型铁矿构造控矿特征明显,尤以向形构造处铁矿富集现象最为显著;岩浆岩活动对该类型铁矿具有早期提供成矿物质来源和晚期进行矿体改造的作用;物探异常(如重磁异常)对于该类型铁矿勘查具有明显的指示作用。并在区内圈定了沂源县韩旺、苍峄铁矿带、东平—汶上、济宁市北等20处铁矿找矿靶区,表明鲁西地区BIF型铁矿勘查潜力巨大;研究表明该类型铁矿勘查方向在其已有矿床深部和边部进行勘查开发仍是首选地段,在隐伏区和低缓异常区利用深部钻孔进行勘查并开辟新的找矿空间是鲁西地区今后BIF型铁矿勘查工作的重要方向。

Comment on “Integrated Mineralogical Characterization of Banded Iron Ores of Orissa and Its Implications on Beneficiation” by Mohanty <i>et al</i>. (2012), Journal of Minerals and Materials Characterization and Engineering, 11, 1133-1142

Mohanty et al. (2012) have wrongly analyzed M?ssbauer spectra obtained from various iron ore samples. We criticized their approach suggesting suitable methodology, and suggested to refer earlier work carried out by Nayak et al. (2001, 2004) on representative samples from the same geological belt.

BIFs的形成及其铁氧化机制研究进展与展望

条带状铁建造(banded iron formations, BIFs)是早前寒武纪(3.8~1.8 Ga)缺氧古海洋中大规模Fe(Ⅱ)氧化形成的一类富铁沉积岩,具有典型的硅质层与铁质层互层的条带状或层状结构,形成的铁矿物主要以磁铁矿和赤铁矿为主。BIFs构成了全球规模最大、储量最多的铁矿类型,具有重大的经济价值。BIFs也是地球环境与生命协同演化的产物,是研究早期地球许多重大演化事件(如大氧化事件)独特的载体。然而,由于早前寒武古海洋环境和沉积条件的独特性,尽管经过了近百年的研究,BIFs成因仍存在许多未解之谜。其中,最关键的科学问题莫过于溶解态Fe(Ⅱ)如何在缺氧古海洋中大量氧化形成特定的铁矿物组合。本文首先概述了BIFs类型、物质组成及来源,以及沉积条件等基本信息;然后从BIFs特殊的沉积环境入手,综述了前人提出的Fe(Ⅱ)氧化机理及存在的一些问题,讨论了微生物驱动下的氮元素生物地球化学循环对Fe(Ⅱ)氧化及BIFs形成的贡献及相关研究进展,最后对BIFs成因研究提出了展望。

Evidences by Multiscale Analysis and 2D3/4 Models from the Magnetic Anomalies and Iron Mineralization (BIF) over the DJADOM-ETA Area, Southeastern Cameroon

The Magnetic method is one of the best geophysical techniques used to delineate subsurface structures. This study was conducted to investigate the basement faulting and ore mineralization into the overlying geological cover. The GEOSOFT v8.4 software was used to process the data. Upward continuation of the residual magnetic intensity map at various altitudes and the maxima of their horizontal gradient magnetic were used to highlight faults from shallow to deep, as well as, their dips and mineralization zones. The faults with the directions E-W, ESE-WNW and ENE-WSW are identified confirming the result of [1]. This study also reveals that, the layer is affected by faults propagating from the basement upwards into the cover. Our results added additional information to the knowledge of the geological structure and the mineral resources potential in the study area. Based on the 2D3/4 modeling, the Dja Fault (DF) is revealed and highlighted sub-area marked by a magnetite/or hematite dolerite, schist and sandstone blocks, which show strong magnetization. Specifically, in this area, models are made of BIF (bounded iron formation) and BIQ (bounded iron quartzite) as dominant minerals.

条带状铁建造的原始矿物组成:进展与问题

原生矿物信息作为前寒武纪条带状铁建造(BIF)研究的最基本命题,是研究BIF成矿规律及其早期地球环境效应的基础。长期以来,将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的原始矿物组成与后期演变过程一直是科学家们关心的关键问题。目前,关于BIF原始矿物组成主要有三种认识:三价铁的氢氧化物、铁硅酸盐(成分类似于铁蛇纹石和黑硬绿泥石)和绿锈。其中,绿锈为含二价和三价铁的过渡态化合物,仅可见于局部缺氧环境。由于绿锈的亚稳态性质,对于其在类似于前寒武纪的富硅海洋条件下转变成何种矿物及相关转变的具体途径仍不清楚。因而,当前关于原始铁质矿物的争议主要存在于三价铁的氢氧化物和铁硅酸盐之间,由此引发的对BIF成矿理论及其对古环境指示意义的理解也存在较大分歧。本文系统回顾和评述了近年来BIF原始矿物组成研究的重要进展;在此基础上,认为三价铁的氢氧化物应是BIF最为主要的原始矿物,其次为铁硅酸盐矿物,二者可能在地质历史时间和空间分布上具有差异性。然而,支持铁硅酸盐原始成因的现有证据主要为经典的岩相学特征,急需更多元素和同位素地球化学等方面的佐证。BIF中现有铁质矿物组合应是原始沉淀物在成岩-变质作用条件下发生转变的产物,相较于成岩早期的菱铁矿,磁铁矿可能形成时间偏晚,究竟是生物-有机质成因抑或是菱铁矿的热化学分解成因亟待深入探讨。在此背景下,同时考虑到华北作为国际上BIF的重要产区,且发育不同变质级别的太古宙末期BIF,可能完整记录了BIF的全部历程。因此,可立足于华北,在已有的初步工作基础上,深度挖掘,力图为BIF矿物成因演变规律提供进一步约束,极大深化和推动我国BIF的成因研究。

Silicon Isotopic Compositions of Archean Banded Si-Fe Formation (BIF)in the Gongchangling Ore Deposit, Liaoning Province, China

The results of silicon isotope analyses of Precambrian BIF are reported for the first time. The δ30Si values of magnetite-quartzite in the Gongchangling ore deposit are between —0.9 and —2.2‰, which are obviously different from those in host rocks （0.2——0.6‰）. It is suggested that the BIF may be a product of chemical precipitation in Archean submarine hydrothermal environment. There is a possibility of using silicon isotopes to distinguish the protolith of metamorphic rocks and trace the origin of silicon of altered minerals.

河南舞阳铁山庙式BIF铁矿的矿物学与地球化学特征及对矿床成因的指示

河南舞阳铁矿位于华北克拉通南缘。铁山庙式铁矿是舞阳铁矿的一部分,赋存于新太古界太华杂岩铁山庙组表壳岩中。本文根据铁山庙式铁矿中三种不同类型矿石(条带状石英-辉石-磁铁矿、块状辉石-磁铁矿、块状石英-磁铁矿)中磁铁矿的矿物成分、全岩/矿的主量元素及微量元素特征,探讨铁山庙式铁矿床的成因。磁铁矿单矿物成分分析表明,条带状石英-辉石-磁铁矿矿石中磁铁矿的FeOT含量90.6%～93.1%,平均91.8%;块状辉石-磁铁矿矿石中磁铁矿的FeOT含量90.7%～91.2%,平均91.0%;块状石英-磁铁矿矿石中磁铁矿的FeOT含量92.0%～93.0%,平均92.4%。上述平均值均与磁铁矿FeOT的理论值(93.1%)接近。三种类型矿石的其它元素如TiO2、MgO、MnO、CaO、Al2O3、Cr2O3、NiO等含量均<0.1%,无明显区别,表明该区磁铁矿为含杂质极少的纯磁铁矿,表现出沉积变质成因磁铁矿的特征。矿石中斜方辉石-单斜辉石及近矿围岩紫苏辉石-长石-石英矿物组合,表明铁山庙式矿床经受了高级变质作用,石英、磁铁矿等矿物普遍发生变质重结晶,颗粒粗大,但仍保存原有的地球化学组成。元素地球化学分析显示,三种类型矿石中SiO2、TiO2、Al2O3、P2O5的含量相近;块状辉石-磁铁矿较其它二者相对贫铁、富钙、镁,这是由于块状辉石-磁铁矿石中富含铁普通辉石和铁次透辉石所致;矿石中TiO2、Al2O3含量都极低,说明该区成岩成矿过程中未受到碎屑物质的混染。三种不同类型矿石的主量元素含量总体上都与世界典型BIF的相近。对于稀土元素,三种类型矿石均具有轻稀土亏损、重稀土富集((La/Yb)PAAS=0.29～0.995<1),La、Eu、Y的正异常(La/La*=1.10～1.89;Eu/Eu*=1.30～2.23;Y/Y*=1.47～1.84),较高的Y/Ho比值(39.7～51.3),具有现代海水及高温热液混合特征。因此,我们认为铁山庙式铁矿三种不同类型的矿石是极少受到陆源碎屑混染的化学沉积成因,虽遭受后期变质作用,但仍属BIF型铁矿。

广西新元古代BIF的铁同位素特征及其地质意义

通过分析广西三江地区新元古代条带状含铁建造的Fe同位素和主量元素组成,对海水的氧化还原状态提供了制约,为富禄期的地球处于间冰期提供了证据。相对于标准物质IRMM-014,新元古代含铁建造不同条带全岩样品的δ57Fe值变化范围1.60‰～2.20‰,平均值为1.85‰,表明BIF样品富集铁的重同位素。条带状含铁建造主要由Fe2O3和SiO2组成,但却具有较高的Al2O3含量。这表明条带状含铁建造样品不是纯净的化学沉积物,而是具有一定的碎屑物质输入。碎屑输入量的不同引起深色和浅色条带之间铁同位素组成存在着0.4‰的差别。剔除碎屑的影响,新元古代BIF从海水中沉淀的赤铁矿δ57Fe的平均值在2‰左右,略高于太古代条带状铁建造的Fe同位素组成,这表明当时海水的氧逸度可能比太古代还低。这说明在富禄期绝大部分海洋仍旧被冰盖覆盖,只在局部出现融化。因此,富禄期的地球可能出于冰期的相对温暖阶段,而不是间冰期。

鞍山陈台沟BIF铁矿与太古代地壳增生:锆石U-Pb年龄与Hf同位素约束

新发现的陈台沟隐伏铁矿床位于辽宁鞍山附近,矿石类型以条带状铁矿石为主,铁矿层顶板围岩为绿泥石英片岩,底板围岩为黑云石英片岩。元素地球化学分析表明,铁矿石及磁铁矿单矿物均富集重稀土,具La、Eu及Y正异常,无明显Ce负异常,反映成矿物质来源于海底高温热液(约占0.1%)与海水的混合溶液,且BIF沉积时海水处于缺氧环境。标型组分分析显示铁矿石及磁铁矿属于沉积变质型或BIF型。原岩恢复表明,绿泥石英片岩原岩为酸性火山岩,黑云石英片岩原岩为泥砂质岩石,二者皆富集Rb、Th、U、LREE,亏损Nb、Ta、Ti。构造背景分析表明两类片岩的原岩均形成于岛弧背景,反映了陈台沟BIF沉积时的构造环境。LA-ICP-MS锆石U-Pb定年显示铁矿体夹层绿泥石英片岩中岩浆锆石形成于2551±10Ma,代表陈台沟BIF形成时代;变质锆石形成于2469±23Ma,代表后期变质作用时代。Hf同位素分析显示大多数锆石具有正的εHf(t)值(-2.23～7.54),表明岩浆源区以亏损地幔物质为主,但明显受到古老地壳物质的混染;二阶段模式年龄(tDM2)主要介于3133～2580Ma之间。结合其他矿区Hf同位素资料,指示鞍本地区可能存在新太古代(～2.55Ga)地壳增生事件。综合分析认为,陈台沟铁矿属Algoma型BIF,是新太古代末华北克拉通大规模BIF成矿事件的代表之一。