Mineralogy and chemistry of banded iron formations (BIF) of Tiruvannamalai area,Tamil Nadu

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.     

Ancient graphite in the Eoarchean quartz-pyroxene rocks from Akilia in southern West Greenland II: Isotopic and chemical compositions and comparison with Paleoproterozoic banded iron formations

We present detailed petrographic surveys of apatite grains in association with carbonaceous material (CM) in two banded iron formations (BIFs) from the Paleoproterozoic of Uruguay and Michigan for comparison with similar mineral associations in the highly debated Akilia Quartz-pyroxene (Qp) rock. Petrographic and Raman spectroscopic surveys of these Paleoproterozoic BIFs show that apatite grains typically occur in bands parallel to bedding and are more often associated with CM when concentrations of organic matter are high. Carbonaceous material in the Vichadero BIF from Uruguay is generally well-crystallized graphite and occurs in concentrations around 0.01 wt% with an average δ¹³C_gra value of −28.6 ± 4.4‰ (1σ). In this BIF, only about 5% of apatite grains are associated with graphite. In comparison, CM in the Bijiki BIF from Michigan is also graphitic, but occurs in concentrations around 2.4 wt% with δ¹³C_gra values around −24.0 ± 0.3‰ (1σ). In the Bijiki BIF, more than 78% of apatite grains are associated with CM. Given the geologic context and high levels of CM in the Bijiki BIF, the significantly higher proportion of apatite grains associated with CM in this rock is interpreted to represent diagenetically altered biomass and shows that such diagenetic mineral associations can survive metamorphism up to the amphibolite facies.Isotope compositions of CM in muffled acidified whole-rock powders from the Akilia Qp rock have average δ¹³C_gra values of −17.5 ± 2.5‰ (1σ), while δ¹³C_carb values in whole-rock powders average −4.0 ± 1.0‰ (1σ). Carbon isotope compositions of graphite associated with apatite and other minerals in the Akilia Qp rock were also measured with the NanoSIMS to have similar ranges of δ¹³C_gra values averaging −13.8 ± 5.6‰ (1σ). The NanoSIMS was also used to semi-quantitatively map the distributions of H, N, O, P, and S in graphite from the Akilia Qp rock, and relative abundances were found to be similar for graphite associated with apatite or with hornblende, calcite, and sulfides. These analyses revealed generally lower abundances of trace elements in the Akilia graphite compared to graphite associated with apatite from Paleoproterozoic BIFs.Graphite associated with hornblende, calcite, and sulfides in the Akilia Qp rock was fluid-deposited at high-temperature from carbon-bearing fluids, and since this graphite has similar ranges of δ¹³C_gra values and of trace elements compared to graphite associated with apatite, we conclude that the Akilia graphite in different mineral associations formed from the same source(s) of CM. Collectively our results do not exclude a biogenic origin of the carbon in the Akilia graphite, but because some observations can not exclude graphitization of abiogenic carbon from CO₂- and CH₄-bearing mantle fluids, there remain ambiguities with respect to the exact origin of carbon in this ancient metasedimentary rock. Accordingly, there may have been several generations of graphite formation along with possibly varying mixtures of CO₂- and CH₄-bearing fluids that may have resulted in large ranges of δ¹³C_gra values. The possibility of fluid-deposited graphite associated with apatite should be a focus of future investigations as this may prove to be an alternative pathway of graphitization from phosphate-bearing fluids. Correlated micro-analytical approaches tested on terrestrial rocks in this work provide insights into the origin of carbon in ancient graphite and will pave the way for the search for life on other ancient planetary surfaces.     

Petrology and geochemistry of the banded iron-formations from Ntem complex greenstones belt,Elom area,Southern Cameroon: Implications for the origin and depositional environment

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 La_CN/Yb_CN = 8.01) and HREE depletion (Mean Tb_CN/Yb_CN = 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.     

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 Fe₂O₃+SiO₂contents and variable contents of CaO,MgO and SO₃,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.Theδ³⁰Si_NBS28values 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 hasδ¹⁸OV-SMOW values of 6.8‰–9.5‰(amphibole BIFs)and 9.2‰–10.6‰(amphibole–pyrite BIFs).The magnetite from the Gouap BIFs showsδ¹⁸O 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.     

Highly depleted Hadean mantle reservoirs in the sources of early Archean arc-like rocks, Isua supracrustal belt, southern West Greenland

Growing evidence from the accessible geological record reveals that crust-mantle differentiation on Earth started as early as 4.4 Ga. In order to assess the extent of early Archean mantle depletion, we obtained ¹⁷⁶Lu-¹⁷⁶Hf, ¹⁴⁷Sm-¹⁴³Nd, and high field strength element (HFSE) concentration data for the least altered, well characterized boninite-like metabasalts and associated metasedimentary rocks from the Isua supracrustal belt (southern West Greenland). The metasediments exhibit initial εHf₍₃₇₂₀₎ values from −0.7 to +1.5 and initial εNd₍₃₇₂₀₎ values from +1.6 to +2.1. Initial εHf₍₃₇₂₀₎ values of the least altered boninite-like metabasalts span a range from +3.5 to +12.9 and initial εNd₍₃₇₂₀₎ values from −0.3 to +3.2. These initial Hf-isotope ratios display coherent trends with SiO₂, Al₂O₃/TiO₂ and other relatively immobile elements, indicating contamination via assimilation of enriched components, most likely sediments derived from the earliest crust in the region. This model is also consistent with previously reported initial γOs₍₃₇₂₀₎ values for some of the samples. In addition to the positive εHf₍₃₇₂₀₎ values, the least disturbed samples exhibit positive εNd₍₃₇₂₀₎ values and a co-variation of εHf₍₃₇₂₀₎ and εΝd₍₃₇₂₀₎ values. Based on these observations, it is argued, that the most depleted samples with initial εHf₍₃₇₂₀₎ values of up to +12.9 and high ¹⁷⁶Lu/¹⁷⁷Hf of ∼0.05 to ∼0.09 tap a highly depleted mantle source with a long term depletion history in the garnet stability field. High precision high field strength element (HFSE) data obtained for the Isua samples confirm the contamination trend. Even the most primitive samples display negative Nb-Ta anomalies and elevated Nb/Ta, indicating a subduction zone setting and overprint of the depleted mantle sources by felsic melts generated by partial melting of eclogite. Collectively, the data for boninite-like metabasalts support the presence of strongly depleted mantle reservoirs as previously inferred from Hf isotope data for Hadean zircons and combined ¹⁴²Nd-¹⁴³Nd isotope data for early Archean rocks.     

鲁西杨庄条带状铁建造特征及锆石年代学研究

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。     

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.     

Fractal properties of multi-order folding as a tool for exploration of low-grade banded iron ores in the Krivoy Rog basin (Ukraine)

The low-grade Palaeoproterozoic stratabound banded iron ores of the Krivoy Rog basin (Ukraine) underwent strong tectonometamorphic deformation into superimposed folds of several orders, with amplitudes from centimetres to hundreds of metres. The across-strike sections of bed surfaces defining the low-grade ore bodies resemble self-similar fractal curves; hence, a fractal geometrical model was developed in order to quantify the complexity and sinuosity of bed contours. Two different methods of measurement (polygonal approximation and two-dimensional grid cell counting) were used for 5–8 different scales. Factual similarity dimension D and other model parameters have been estimated by means of linear regression and compared for both measurement methods. From the fractal model a sinuosity coefficient of contours of the folded bed surfaces K _s and a coefficient of degree of exploration of iron ore bodies K _e were constructed. It is pointed out that parameters of the model can be used for determination of the optimal exploration length scales.     

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。     

Hydrothermal-metasomatic and tectono-metamorphic processes in the Isua supracrustal belt (West Greenland): a multi-isotopic investigation of their effects on the earth’s oldest oceanic crustal sequence

Despite superimposed metamorphic overprinting and metasomatic alterations, primary volcanic features remain preserved in low-strain domains of mafic volcanic sequences in the western Isua supracrustal belt (ISB, West Greenland). These basaltic successions represent the hitherto oldest known fragments of oceanic crust on Earth. Early Archean metasomatic fluids, rich in light rare earth elements (LREE), Th, U, Pb, Ba, and alkalies, invaded the supracrustal package and distinctively altered the basaltic sequences. Field relationships, source characteristics traced by Pb isotopes, and geochronological results provide indications that these fluids were genetically related to the emplacement of tonalite sheets into the ISB between 3.81 and 3.74 Ga ago. Subsequent early Archean metamorphism homogenized the mixed primary and metasomatic mineral parageneses of these metavolcanic rocks. Allanite occurs as the most characteristic and critical secondary metasomatic-metamorphic phase and is developed in macroscopically discernible zones of increased metsomatic alteration, even in domains of low strain. Because of its high concentration of LREE, Th, and U, this secondary mineral accounts for much of the disturbances recorded by the Sm-Nd and Th-U-Pb isotope systematics of the pillowed metabasalts.The supracrustal sequences were tectono-metamorphically affected to varying degrees during a late Archean, ∼2.6- to 2.8-Ga-old event, also recognized in the adjacent gneiss terranes of the Isuakasia area. The degree to which bulk rocks were isotopically reequilibrated is directly dependent on the different relative contributions of allanite-hosted parent-daughter elements to the overall whole-rock mass budget of the respective isotope systems. Although low-strained (initially only weakly metasomatized) pillow basalts remained more or less closed with respect to the U-Pb and Rb-Sr systems since ∼3.74 Ga, the Sm-Nd system appears to have been partially opened on a whole-rock scale during the late Archean event. This diversified behavior of the whole-rock isotope systems with respect to late Archean overprinting is explained by the combination of mass budget contributions of the respective elements added during metasomatism and the partial opening of metasomatic macroenvironments during late Archean recrystallization processes with associated renewed fluid flow. In reactivated zones of high strain, where primary metasomatic alteration is most prominently developed, late Archean partial resetting also of the U-Pb isotope system on a whole-rock scale occurred. This is consistent with an apparent late Archean age of kyanite, which initially crystallized during the early Archean metamorphism. Its age is controlled by the U-Pb systematics of allanite inclusions, which have exchanged their isotopic properties during the tectono-metamorphic event that overprinted the oceanic crustal sequence at Isua more than 1000 myr later.These results underline the need for care in the interpretation of whole-rock geochemical data from polymetamorphic rocks in general, and from the Isua oceanic crustal sequences in particular, to constrain isotopic models of early Earth’s evolution. Likewise, this study cautions against the indiscriminate use of geochemical data of metavolcanic rocks from Isua to infer models for geotectonic settings relevant for their formation.     

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。     

Meta-igneous (non-gneissic) tonalites and quartz-diorites from an extensive ca. 3800 Ma terrain south of the Isua supracrustal belt, southern West Greenland: constraints on early crust formation

In the Itsaq Gneiss Complex south of the Isua supracrustal belt (West Greenland) some areas of early Archaean tonalite and quartz-diorite are non-gneissic, free of pegmatite veins, and in rarer cases are undeformed with relict igneous textures and hence were little modified by heterogeneous ductile deformation under amphibolite facies conditions in several Archaean events. Such well-preserved early Archaean rocks are extremely rare. Tonalites are high Al, and have bulk compositions close to experimental liquids. Trace element abundances and modelling suggest that they probably originated as melts derived from basaltic compositions at sufficiently high pressures to require residual garnet + amphibolites ± clinopyroxene in the source. The major element characteristics of the quartz-diorites suggest these were derived from more mafic magmas than the tonalites, and underwent either igneous differentiation or mixing with crustal material. As in modern arc magmas, high relative abundances of Sr, Ba, Pb, and alkali elements cannot be generated simply from a basaltic source formed by large degrees of melting of a depleted mantle. This may indicate an important role for fluids interacting with mafic rocks in generating the earliest preserved continental crust. The high Ba/Th, Ba/Nb, La/Nb and low Nb/Th, Ce/Pb, and Rb/Cs ratios of these tonalites are also observed in modern arc magmas. SHRIMP U-Pb zircon geochronology was undertaken on seven tonalites, one quartz-diorite, a thin pegmatitic vein and a thin diorite dyke. Cathodoluminescence images show the zircon populations of the quartz-diorite and tonalites are dominated by single-component oscillatory-zoned prismatic grains, which gave ages of 3806 ± 5 to 3818 ± 8 Ma (2σ) (quartz-diorite and 5 tonalites) and 3795 ± 3 Ma (1 tonalite). Dating of recrystallised domains cutting oscillatory-zoned zircon indicates disturbance as early as 3800–3780 Ma. There are rare ca. 3600 Ma and 3800–3780 Ma (very high U and low Th/U) ≤ 20 μm wide partial overgrowths on the prismatic grains. Given likely Zr-undersaturation of precursor melts and evidence of zircon recrystallisation and metamorphic regrowth as early as 3800–3780 Ma, the age determinations on the prismatic oscillatory-zoned zircon populations give the igneous crystallisation age of the tonalite and quartz-diorite protoliths. When the coherency of the geochemistry is considered, these samples represent the best preserved suites of ca. 3800 Ma felsic igneous rocks yet documented. Received: 1 December 1998 / Accepted: 23 July 1999     

Geochemistry and origin of the Neoproterozoic Dahongliutan banded iron formation (BIF) in the Western Kunlun orogenic belt,Xinjiang (NW China)

The Neoproterozoic (593–532 Ma) Dahongliutan banded iron formation (BIF), located in the Tianshuihai terrane (Western Kunlun orogenic belt), is hosted in the Tianshuihai Group, a dominantly submarine siliciclastic and carbonate sedimentary succession that generally has been metamorphosed to greenschist facies. Iron oxide (hematite), carbonate (siderite, ankerite, dolomite and calcite) and silicate (muscovite) facies are all present within the iron-rich layers. There are three distinctive sedimentary facies BIFs, the oxide, silicate–carbonate–oxide and carbonate (being subdivided into ankerite and siderite facies BIFs) in the Dahongliutan BIF. They demonstrate lateral and vertical zonation from south to north and from bottom to top: the carbonate facies BIF through a majority of the oxide facies BIF into the silicate–carbonate–oxide facies BIF and a small proportion of the oxide facies BIF.The positive correlations between Al₂O₃ and TiO₂, Sc, V, Cr, Rb, Cs, Th and ∑REE (total rare earth element) for various facies of BIFs indicate these chemical sediments incorporate terrigenous detrital components. Low contents of Al₂O₃ (<3 wt%), TiO₂ (<0.15 wt%), ∑REE (5.06–39.6 ppm) and incompatible HFSEs (high field strength elements, e.g., Zr, Hf, Th and Sc) (<10 ppm), and high Fe/Ti ratios (254–4115) for a majority of the oxide and carbonate facies BIFs suggest a small clastic input (<20% clastic materials) admixtured with their original chemical precipitates. The higher abundances of Al₂O₃ (>3 wt%), TiO₂, Zr, Th, Cs, Sc, Cr and ∑REE (31.2–62.9 ppm), and low Fe/Ti ratios (95.2–236) of the silicate–carbonate–oxide facies BIF are consistent with incorporation of higher amounts of clastic components (20%–40% clastic materials). The HREE (heavy rare earth element) enrichment pattern in PAAS-normalized REE diagrams exhibited by a majority of the oxide and carbonate facies BIFs shows a modern seawater REE signature overprinted by high-T (temperature) hydrothermal fluids marked by strong positive Eu anomalies (Eu/Eu¹_PAAS = 2.37–5.23). The low Eu/Sm ratios, small positive Eu anomaly (Eu/Eu¹_PAAS = 1.10–1.58) and slightly MREE (middle rare earth element) enrichment relative to HREE in the silicate–carbonate–oxide facies BIF and some oxide and carbonate facies BIFs indicate higher contributions from low-T hydrothermal sources. The absence of negative Ce anomalies and the high Fe³⁺/(Fe³⁺/Fe²⁺) ratios (0.98–1.00) for the oxide and silicate–carbonate–oxide BIFs do not support ocean anoxia. The δ¹³C_V-PDB (−4.0‰ to −6.6‰) and δ¹⁸O_V-PDB (−14.0‰ to −11.5‰) values for siderite and ankerite in the carbonate facies BIF are, on average, ∼6‰ and ∼5‰ lower than those (δ¹³C_V-PDB = −0.8‰ to + 3.1‰ and δ¹⁸O_V-PDB = −8.2‰ to −6.3‰) of Ca–Mg carbonates from the silicate–carbonate–oxide facies BIF. This feature, coupled with the negative correlations between FeO, Eu/Eu¹_PAAS and δ¹³C_V-PDB, imply that a water column stratified with regard to the isotopic omposition of total dissolved CO₂, with the deeper water, from which the carbonate facies BIF formed, depleted in δ¹³C that may have been derive from hydrothermal activity.Integration of petrographic, geochemical, and isotopic data indicates that the silicate–carbonate–oxide facies BIF and part of the oxide facies BIF precipitated in a near-shore, oxic and shallow water environment, whereas a majority of the oxide and carbonate facies BIFs deposited in anoxic but Fe²⁺-rich deeper waters, closer to submarine hydrothermal vents. High-T hydrothermal solutions, with infusions of some low-T hydrothermal fluids, brought Fe and Si onto a shallow marine, variably mixed with detrital components from seawaters and fresh waters carrying continental landmass and finally led to the alternating deposition of the Dahongliutan BIF during regression–transgression cycles.The Dahongliutan BIF is more akin to Superior-type rather than Algoma-type and Rapitan-type BIF, and constitutes an additional line of evidence for the widespread return of BIFs in the Cryogenian and Ediacaran reflecting the recurrence of anoxic ferruginous deep sea and anoxia/reoxygenation cycles in the Neoproterozoic. In combination with previous studies on other Fe deposits in the Tianshuihai terrane, we propose that a Fe²⁺-rich anoxic basin or deep sea probably existed from the Neoproterozoic to the Early Cambrian in this area.     

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。     

塔尔塔格铁矿床——新疆阿吾拉勒铁成矿带内首例IOA型铁矿床的厘定及意义

塔尔塔格铁矿是新疆西天山阿吾拉勒海相火山岩型铁成矿带内的一处中型铁矿床,其铁矿体呈透镜状产于粗安玢岩内顶部,在揭示海相火山岩中铁矿床与侵入岩关系方面具有重要意义。该铁矿床发育典型的磁铁矿 磷灰石组合,基本不含硫化物,矿石以似斑状、浸染状、球粒状和晶洞等构造类型为特色,磁铁矿内发育由条带状或叶片状钛铁矿和钛氧化物构成的出溶结构,并有少量的榍石、钍石、萤石等副矿物与之共生,矿床地质特征表明该矿床为典型的与富碱、中性侵入岩有关的IOA（iron oxide apatite）型铁矿床。该矿床围岩英安岩、赋矿粗安玢岩与成矿后正长花岗岩的锆石SHRIMP U Pb同位素加权平均年龄分别为311. 3±1. 4 Ma、304. 7±2. 9 Ma和301. 1±2. 8 Ma,铁矿石样品中与磁铁矿共生的榍石的LA ICP MS U Pb同位素加权平均年龄为302. 3±4. 0 Ma和303. 7±4. 2 Ma,成岩成矿年龄高度吻合,进一步确定其在晚石炭世晚期成矿。本文根据地质特征认为该矿床矿石是由粗安玢岩顶部富铁挥发分聚集而形成的,并初步提出“超浅成侵入岩顶部富挥发分囊体”成矿模型,即富碱、中性岩体超浅成侵位导致挥发分在岩体顶部迅速聚集形成多处富铁和挥发分的囊体,岩体冷凝固结的同时磁铁矿快速结晶成矿。塔尔塔格“IOA型”型铁矿的成因厘定,不仅表明海相火山岩型铁矿与IOA型铁矿可能存在成因联系,还暗示了海相火山岩型铁成矿带内具有寻找IOA型铁矿的潜力。     

山西吕梁袁家村条带状铁建造沉积相与沉积环境分析

山西吕梁作为华北克拉通上条带状铁建造(BIF)的重要产区之一,位于华北中央构造带中。袁家村BIF分布于吕梁岚县袁家村一带,极有可能是华北克拉通内最为典型的Superior型BIF。与华北克拉通其他大多数BIF相比,袁家村BIF具有明显的差异性,其中包括它的形成时代(2.3~2.1Ga)、铁建造类型和低级变质程度(低绿片岩相)等。因此,研究袁家村BIF具有特殊的研究意义,可为探讨大氧化事件之后古海洋氧化还原状态以及国内Superior型BIF的成因提供研究基础。袁家村BIF产于吕梁群袁家村组变沉积岩系的下部,前人根据上覆和下伏含火山岩地层的时代,推测袁家村组的形成时代为2.3~2.1Ga。BIF整体产状陡倾,沿北北东-北东东向呈L形带状分布。依据原生矿物的共生组合及产出特征,可将BIF沉积相划分为氧化物相(60%)、硅酸盐相(30%)和碳酸盐相(10%)。氧化物相是本区BIF最主要的沉积相,主要矿物为赤铁矿、磁铁矿和石英,从而可进一步划分为赤铁矿(24%)和磁铁矿(36%)亚相;硅酸盐相BIF以大量硅酸盐矿物出现为特征,散布于研究区,主要矿物组成除了石英和磁铁矿之外,还有铁黑硬绿泥石、绿泥石、铁滑石、镁铁闪石和阳起石等。在与碳酸盐相BIF构成过渡相的BIF中,还可发现大量的铁白云石。而碳酸盐相主要矿物为菱铁矿、铁白云石和石英等,主要发育于研究区的南部。依据含铁岩系构造格局特点复原获得了原始沉积相分布略图,沉积相主要呈南北向延展,自东向西显示出相变规律,西边为碳酸盐相,东边为氧化物相,其间是过渡的硅酸盐相。通过袁家村BIF的岩相学和含铁矿物化学成分的研究,可大致推测原始沉积的矿物组成为无定形硅胶、水铁矿、与铁蛇纹石和黑硬绿泥石组成类似的铁硅酸盐凝胶、富Al的粘土碎屑和含铁、镁、钙的碳酸盐软泥。这些沉积物在随后的成岩期和绿片岩相的区域变质作用下发生矿物之间的相互转变。BIF中主要含铁矿物的PO-P-Eh 2CO2和pH相关图解说明除了赤铁矿之外,其他矿物均是在较低氧逸度环境中形成的,且所有矿物共存的水体系为中性到弱碱性。袁家村BIF氧化物相中发育豆粒、内碎屑结构和板状交错层理等原始沉积构造,指示氧化相部分是在相对高能的浅水环境下沉积的。但BIF大部分应该形成于浪基面以下(200m)较为深水的环境中,沉淀可能同时发生于上部氧化和下部还原的水体之中,由于还原弱酸性的深部富铁海水在海侵的过程中上升到浅部相对氧化和弱碱性的浅水环境中,因为Eh、pH及氧逸度等物化条件的骤然变化,最终导致铁质的沉淀和沉积相自上而下的变化。     

The origin and compositions of Mesoarchean oceanic crust: Evidence from the 3075 Ma Ivisaartoq greenstone belt, SW Greenland

The Mesoarchean (ca. 3075 Ma) Ivisaartoq greenstone belt contains well-preserved primary magmatic structures, such as pillow lavas, volcanic breccias, and clinopyroxene cumulate layers (picrites), despite the isoclinal folding and amphibolite facies metamorphism. The belt also includes variably deformed gabbroic to dioritic dykes and sills, actinolite schists, and serpentinites. The Ivisaartoq rocks underwent at least two stages of post-magmatic metamorphic alteration, including seafloor hydrothermal alteration and syn- to post-tectonic calc-silicate metasomatism, between 3075 and 2961 Ma. These alteration processes resulted in the mobilization of many major and trace elements. The trace element characteristics of the least altered rocks are consistent with a supra-subduction zone geodynamic setting and shallow mantle sources. On the basis of geological similarities between the Ivisaartoq greenstone belt and Phanerozoic forearc ophiolites, and intra-oceanic island arcs, we suggest that the Ivisaartoq greenstone belt represents a relic of dismembered Mesoarchean supra-subduction zone oceanic crust. This crust might originally have been composed of a lower layer of leucogabbros and anorthosites, and an upper layer of pillow lavas, picritic flows, gabbroic to dioritic dykes and sills, and dunitic to wehrlitic sills.

The Sm–Nd and U–Pb isotope systems have been disturbed in strongly altered actinolite schists. In addition, the U–Pb isotope system in pillow basalts appears to have been partially open during seafloor hydrothermal alteration. Gabbros and diorites have the least disturbed Pb isotopic compositions. In contrast, the Sm–Nd isotope system appears to have remained relatively undisturbed in picrites, pillow lavas, gabbros, and diorites. As a group, picrites have more depleted initial Nd isotopic signatures (ε_Nd = + 4.23 to + 4.97) than pillow lavas, gabbros, and diorites (ε_Nd = + 0.30 to + 3.04), consistent with a variably depleted, heterogeneous mantle source.

In some areas gabbros include up to 15 cm long white inclusions (xenoliths). These inclusions are composed primarily (> 90%) of Ca-rich plagioclase and are interpreted as anorthositic cumulates brought to the surface by upwelling gabbroic magmas. The anorthositic cumulates have significantly higher initial ε_Nd (+ 4.8 to + 6.0) values than the surrounding gabbroic matrix (+ 2.3 to + 2.8), consistent with different mantle sources for the two rock types.     

内蒙古固阳绿岩带三合明BIF型铁矿的形成时代、地球化学特征及地质意义

三合明BIF型铁矿位于华北克拉通西部陆块北缘,产出于固阳绿岩带。矿体赋存于新太古界色尔腾山群斜长角闪岩中。矿石主要呈粒状变晶结构、粒状-针柱状变晶结构,条带-条纹状构造;组成矿石的金属矿物主要为磁铁矿,非金属矿物主要为石英,次为角闪石等。对选自斜长角闪岩中的锆石进行SIMS U-Pb定年,具有核边结构、Th/U比大于0.4的锆石其核部给出了2562±14Ma的上交点年龄,可大致作为三合明BIF的形成时代。原岩恢复显示斜长角闪岩为正变质岩,Zr/Ti-Nb/Y图解显示为亚碱性玄武岩系列,Fe₂O₃^T+TiO₂-Al₂O₃-MgO图解落入高铁拉斑玄武岩区;斜长角闪岩主量元素特征与MORB相近,REE配分曲线和蛛网图都近于平坦,且介于N-MORB和E-MORB之间,LREE略微富集,Th、U相对亏损,Nb、Ta、Zr和Hf无明显异常;(La/Sm)_N和Nb/U分别为0.76和50,由此推断原岩可能为T-MORB。Ti-V、Th-Hf-Ta构造环境判别图解中,分别落入MORB和弧后盆地的重叠区、N-MORB区。结合T-MORB形成的构造环境以及前人提出的岛弧叠加地幔柱模式,本文认为三合明BIF形成于弧后盆地并有地幔柱叠加的构造环境。铁矿石化学组分主要为SiO₂、Fe₂O₃和FeO,较低的Al₂O₃(0.68%)、极低的TiO₂(0.04%)和HFSE表明只有极少量陆源碎屑物质的加入。铁矿石的球粒陨石标准化REE配分模式与固阳绿岩带底部的科马提岩极为相似,PAAS标准化的铁矿石REY配分模式与高温热液海水混合物相似,即LREE亏损,HREE富集((La/Yb)_SN=0.34),具有明显的正Eu异常(δEu=2.33)和微弱的正Y异常(δY=1.13),Y/Ho重量比29,摩尔比53。根据铁矿石兼具有与科马提岩和高温热液海水混合物相似的地球化学特征,本文推断海底高温热液淋滤科马提岩为三合明BIF型铁矿提供了大量的Fe和Si。     

山西新太古界柏芝岩组条带状铁建造中的菱铁矿:成因机制与古环境意义

菱铁矿是前寒武纪条带状铁建造(BIF)中重要的矿物组分和古海洋信息载体,但它可能具有原生、早期成岩和晚期成岩多种成因,这在一定程度上限制了其在古海洋条件分析中的应用。虽然前人对菱铁矿开展了广泛的地球化学分析,但在岩相学研究方面相对薄弱。为进一步揭示BIF中菱铁矿的成因机制,以山西代县羊角沟矿区新太古界柏芝岩组的BIF为研究对象,开展了系统的岩相学工作。研究表明,该BIF主要由厘米级交互的富铁与富硅条带构成,其中普遍缺少水体扰动沉积构造,偶见交错层理和风暴碎屑,表明主要沉积于风暴浪基面之下。菱铁矿的主要产出形式有3种:(1)亚毫米级条带,其内"悬浮"有风暴成因碎屑颗粒,具有水柱或沉积物/水界面的原生成因特征;(2)在富铁条带中的晶体间隙充填,可能为早期成岩成因;(3)富硅条带中存在绿泥石层间脉状充填或截切石英和铁白云石的脉体,具有晚期成岩成因。原生菱铁矿的产出,表明新太古代在风暴浪基面之下的海水强烈缺氧、富铁并具有低硫酸盐浓度的特征。尽管原生菱铁矿条带的产出表明菱铁矿具有反映海洋化学条件的潜力,但多种成因菱铁矿的同时产出,也要求在应用菱铁矿分析古海洋条件时应当分组构进行。     

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

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。