Problems of Siderite Formation and Iron Ore Epochs: Communication 1. Types of Siderite-Bearing Iron Ore Deposits

It is shown that siderite is unstable during sedimentation, diagenesis, and metamorphism of sedimentary and volcanosedimentary rocks. Regularities in the distribution of siderite in Precambrian jaspilites (iron formations), metasomatic ores of the Bakal type, continental–marine coaliferous formations, and oolitic iron ores are discussed. The genesis of the Precambrian iron formations and Riphean–Lower Paleozoic elisional–hydrothermal deposits is considered. The genetic relation of nodular siderites from coaliferous formations and oolitic iron ores with lowmoor coal-forming peat deposits is noted.     

Epochs of Diamond Placer Formation in the Precambrian and Phanerozoic

Diamond paleoplacers in conglomerates intermittently formed during almost the entire geological history. Several epochs of their formation are distinguished in the Early Precambrian, Late Precambrian, and Phanerozoic. Such paleoplacers, as well as REE-bearing ones, are a peculiar indicator of epochs of alkaline ultramafic (including kimberlitic) magmatism related to continental rifting. Diamond paleoplacers are also an important indicator of epochs of ancient peneplanation and development of chemical weathering crusts, because most of them are confined to highly mature terrigenous formations.     

Problems of Iron and Phosphorus Geochemistry in the Precambrian

The localization of economic sedimentary iron ore and phosphorite resources is discussed in comparative aspects. It is shown that the major economic resources of iron ore are hosted in Precambrian rocks, whereas the phosphorites are related to Upper Phanerozoic. High-temperature hydrothermal solutions served as an important source of iron for jaspilite ores. The low P₂O₅content therein indicates that the phosphorus deposition was only weakly related to the hydrothermal activity. Thus, the hydrothermal origin of phosphorite is denied from the geochemical standpoint.     

前寒武纪条带状硅铁建造的形成机制与地球早期的大气和海洋

前寒武纪条带状硅铁建造(BIFs)是地球早期特有的化学沉积建造类型,记录了当时大气和海洋的化学成分、氧化还原状态及演化。本文系统测定了华北地台条带状硅铁建造的硫硅氧同位素组成。不同时代和不同类型条带状硅铁建造中石英的硅同位素组成非常相似,强烈亏损30Si,δ30SiNBS-28大部分位于-2.0‰～-0.3‰之间,平均-0.8‰;硅铁建造中石英的δ18OV-SMOW相对较高,8.1‰～21.5‰,平均13.9‰;二者均与现代海底黑烟囱、泉华及热水沉积硅质岩的硅氧同位素组成相似。在同一样品中,磁铁矿条带中石英的δ30SiNBS-28普遍低于相邻硅质条带中石英的值,而δ18OV-SMOW刚好相反,反映了硅铁建造沉积时的初始特征。BIFs中硫化物的δ34SV-CDT变化范围很大,-22.0‰～+11.8‰,但大部分集中分布在0值附近。Δ33S=-0.89‰～+1.2‰,显示出了明显的硫同位素非质量分馏特征,说明当时大气氧浓度很低。与火山活动关系密切的Algoma型硅铁建造的Δ33S多为负值,而远离火山活动中心的Superior型硅铁建造的Δ33S多为正值。提出无论是Algoma型,还是Superior型BIFs都是由海底热液喷气作用形成的。富含溶解硅和铁的热水溶液喷发到在海底以后,由于温度突然下降,硅酸H4SiO4在海水中达到过饱和状态,导致SiO2首先沉淀,形成硅质层;随着热水溶液与海水的不断混合,温度不断降低,Eh值不断升高,Fe2+逐渐被氧化生成Fe3+随后沉淀,形成富铁层。一套硅铁韵律层代表了一次大的海底喷气活动;海底热液喷气的周期性活动形成了规律性的硅铁韵律层。BIFs的广泛分布和硫同位素非质量分馏效应的普遍存在,表明当时大气氧水平很低,可能不足现在氧水平的1‰;火山和海底喷气活动非常强烈,海水温度较高,呈酸性,pH值在3.0～5.5之间;海洋中可溶解硅H4SiO4和Fe2+的浓度很高;而可溶硫酸盐的浓度极低,1mM。早元古代(1.8Ga)以后海洋硫酸盐浓度升高,由富铁海洋转化为富硫酸盐的海洋,是造成BIFs消失的根本原因。大规模火山喷发和海底喷气活动对海洋的成分和氧化还原状态影响很大,使海洋的氧化时间较大气至少推迟了6亿年。     

花状构造与成藏成矿的聚集关系

花状构造(Flower Structure)是走滑-拉分盆地的一种特有构造,因在剖面上呈近直立的基底扭断裂自下向上成花状撒开而得名。花状构造分为正花状构造、负花状构造和复合型花状构造,并总结了花状构造的鉴别标志。通过对花状构造的动力学机制、控制因素和成藏成矿的聚集关系,指出在盆地和造山带中具有巨大的勘探潜力和广阔的勘探前景,为油气和固体矿产勘探提供新的思路。     

Flow of ultra-hot orogens: A view from the Precambrian, clues for the Phanerozoic

This contribution emphasizes first-order structural and metamorphic characters of Precambrian accretionary orogens to understand the kinematics and thermomechanical state of the continental lithosphere in convergent settings involving massive juvenile magmatism. We define a new class of orogens, called ultra-hot orogens (UHO), in which the weakest type of lithosphere on Earth is deformed. UHO are characterized by (1) distributed shortening and orogen-scale flow combining vertical and horizontal longitudinal advection, under long-lasting convergence, (2) homogeneous thickening by combined downward movements of supracrustal units and three-dimensional mass redistribution in the viscous lower crust, and (3) steady-state, negligible topography and relief leveled by syn-shortening erosion and near-field sedimentation. The flow analysis of UHO provides clues to understanding crustal kinematics beneath high plateaus and suggests that the seismic reflectivity pattern of hot orogens is an image of the layering produced by lateral flow of the lower crust and associated syn-kinematic plutonism.In between the UHO and the modern cold orogens (CO), developed by shortening of lithosphere bearing a stiff upper mantle, two classes of orogens are defined. Hot orogens (HO, representative of Cordilleran and wide mature collisional belts) share flow pattern characteristics with UHO, but involve a less intense magmatic activity and develop high topographies driving their collapse. Mixed-hot orogens (MHO, representative of magmatic arcs and Proterozoic collisional belts) are orogens made of UHO-type juvenile crust and display CO-like structure and kinematics. This classification points to the fundamental link between the presence of a stiff lithospheric mantle and strain localization along major thrusts in convergent settings. A high Moho temperature (> 900 °C), implying thinning of the lithospheric mantle, enhances three-dimensional flow of the lithosphere in response to convergence. Overall, this classification of orogens emphasizes the space and time variability of uppermost mantle temperature in controlling plate interactions and continental growth.     

Book reviews: Precambrian geology and petrology. General and regional problems

A symposium of Precambrian papers compiled in a volume dedicated to N.V. Frolova. According to the reviewers many of the articles are excellent. The articles cover diverse and comprehensive subjects concerning the Precambrian. — M.A. Klugman.     

云南大红山地区铁、铜矿床类型及形成机理

大红山地区是云南省重要矿集区之一,已发现大红山大型铁铜矿床及东么、底巴都、底戛母、河口、坝达等多个中小型铁铜矿床（点）,区内分布多个磁异常及化探异常,成矿地质条件优越。近年云南省三年地质找矿行动计划项目“云南省新平县大红山外围铁、铜矿资源普查”经过勘查在大红山外围东么、底巴都矿区圈定多个工业铁、铜矿体及超贫磁铁矿体,取得一定找矿新进展。本文根据近年工作中观察到的新现象、取得的新资料,结合前人研究成果,对大红山地区铁、铜矿床成因类型及形成机理进行探讨。     

Magnetite Dyke in Precambrian Banded Iron Formation,Singhbhum Craton,India: Mineralogical and Chemical Characteristics

Banded iron formation (BIF) of the Gorumahisani–Sulaipat–Badampahar (GSB) belt in Singhbhum Craton, India, consists predominantly of magnetite. This BIF is intruded by a magnetite dyke. The magnetite dyke is massive and compact with minor sulphide minerals while the host banded magnetite ore, a component of the BIF, shows thin lamination. The magnetite ore of the dyke is fine to medium grained and exhibits interlocking texture with sharp grain boundaries, which is different from the banded magnetite that is medium to coarse grained and show irregular martitised and goethitised grain boundaries. Relics of Fe–Ca–Mn–Mg‐carbonate and iron silicates (grunerite and cummingtonite) are observed in the banded magnetite. The intrusive magnetite is distinctly different in minor, trace and REE geochemistry from the banded magnetite. The banded magnetite contains higher amounts of Si, Al, Mn, Ca, Mg, Sc, Ga, Nb, Zr, Hf, Co, Rb and Cu. In contrast, the massive magnetite is enriched in Cr, Zn, V, Ni, Sr, Pb, Y, Ta, Cs and U with higher abundance of HREE. In the chondrite normalized plot, the massive magnetite shows a slight positive Eu anomaly while the banded ore does not show any Eu anomaly. Field disposition, morphology, mineralogy and chemistry show that the intrusive magnetite dyke is of igneous origin, while magnetite in BIF formed from a carbonate protolith through the process of sedimentation.     

Phenanthrene biomarkers in the organic matter of Precambrian and Phanerozoic deposits and in the oils of the Siberian Platform

The composition and distribution of phenanthrenes (polyaromatic compounds) have been studied in chloroform extracts from dispersed organic matter (OM) of clayey, siliceous, carbonate, and terrigenous rocks of different ages and facies and from some oils of the Siberian Platform. Phenanthrenes have been analyzed by gas chromatography-mass spectrometry. High contents of 1,7,8-trimethylphenanthrene and 1,1,7,8-tetramethyl-1,2,3,4-tetrahydrophenanthrene are present in the OM of Vendian and Cambrian carbonate-shale deposits and in ancient oils of the Nepa-Botuobiya and Anabar anteclises. The OM of Permian continental deposits and oils of the Vilyui syneclise is dominated by 1-methyl-7-isopropylphenanthrene (retene). A triangular diagram for identification of the types of original OM of rocks and classification of genetically related oils has been constructed based on the assessment of phenanthrene biomarker distribution. Putative pathways of the formation of phenanthrene biomarkers are discussed.     

河北省邯邢式铁矿找矿方法回顾与展望

邯邢式铁矿是河北省重要的铁矿成因类型,已经探明资源储量达到8.7亿吨.以往在该地区经过了由高磁异常验证-复杂异常查证-低缓异常查证的过程,实践证明磁法找矿为邯邢式铁矿找矿的基本方法.随着找矿难度的逐步加大,对地质控矿条件的分析越来越重要.目前,该地区浅部铁矿资源基本探明,正在该地区开展邯邢式铁矿的深部找矿工作.回顾原来的找矿方法,对今后开展工作是十分有益的.本文以邯邢武铁矿的岩矿石物性(磁化率、密度、电阻率等)特点为主要依据,在前人实验的基础上,提出重磁电综合物探方法是今后邯邢式铁矿的找矿方法的发展方向,加强物探数据的综合处理和地质成矿条件相结合是该地区进行深部找矿的前提和基础.     

Elisional processes and salt tectonics: Communication 2. Formation mechanism of salt diapirs

The possible role of endogenic factors in the formation of salt deposits and salt diapirs is discussed with emphasis on the sedimentary origin of salts and elisional nature of epigenetic salt bodies. It is shown that dehydration and high solubility of chlorides, density inversion, and high coefficient of linear expansion under heating are the main factors responsible for the growth of salt domes. Elisional processes in clays of subsalt formations exert a considerable impact upon their growth in rock basins. Tectonic processes play an important, although secondary, role in these transformations.     

Black shales and other sediments with high organic matter contents in Phanerozoic climatic cycles: Communication 2. Black shales during the Pangea existence

According to recent concepts, the Earth surface was permanently transformed during its geological history. Some stages of its evolution were marked by the convergence of separate continental blocks to result in the formation of supercontinents, which resisted successfully centrifugal processes. Other stages were characterized by the opposite tendency: after their long existence, the supercontinents became disintegrated into several large and small blacks, the motion of which was accompanied by opening of new sea basins and closure of former basins with the oceanic crust. The second half of the Paleozoic was marked by amalgamation of large continental blocks. In the Devonian, collision between Laurentia and Baltica culminated in the formation of the Euroamerica continent. After the closure of the Ural paleocean in the terminal Carboniferous–initial Permian, it was united with the Siberian and Kazakhstan continental blocks. These events provided the prerequisites for the formation of a new supercontinent (Pangea), which acquired its final configuration at the end of the Permian. One of its segments located mainly south of the equator included Gondwana. Another segment located northward included Euroamerica, Kazakhstan, Siberian, and two China continental blocks. During its geological history, Pangea suffered many dramatic events including several extinctions of organisms. The most significant event took place in the terminal Permian–initial Triassic and at the transition between the Triassic and Jurassic periods.     

华北克拉通南部早前寒武纪基底形成与演化

简要总结了华北克拉通南部鲁山地区、小秦岭地区、登封及中条山地区的早前寒武纪地质事件序列及其地质意义,并对各地区地质特征和变质演化特点进行对比。结合前人研究工作,初步探讨了华北克拉通南部早前寒武纪基底的演化特点、陆壳形成的主要时期和华北南部基底的构造区划等问题,提出几点认识:1)华北克拉通南部鲁山、中条山、小秦岭等地区均有2.7~2.9Ga岩石记录,以英云闪长质-奥长花岗质-花岗闪长质(TTG)岩石为主,它们共同构成华北南缘的古老结晶基底,并经历了新太古代晚期~2.5Ga构造-热事件,标志着华北克拉通南部在新太古代末期可能已经形成统一基底;2)华北克拉通南部主要的陆壳形成时期为中太古代晚期-新太古代,与全球其他主要克拉通一致,而古元古代早-中期则以地壳再循环为主;3)综合地质、地球化学等特点,将华北南部鲁山-小秦岭地区和中条山等地区划归为"南部古陆块",并提出该陆块呈现为一个大型的倾伏向斜构造,可能在新太古代晚期已经形成,其枢纽向南东倾斜。"南部古陆块"在新太古代末期与其它微陆块拼合,并发生了变质作用和陆壳的活化与再循环,共同指示新太古代晚期华北克拉通统一基底的形成。     

海南岛前寒武纪基底组成及演化

海南岛位于欧亚板块、印度板块和太平洋板块的交接部位, 区内构造复杂.根据沉积地层学、岩石学、岩石地球化学、同位素年代学等研究成果综合分析, 海南岛存在元古代的古老变质基底, 以九所-陵水断裂为界, 可进一步划分为南部的三亚块体和中北部的五指山块体.二者的基底组成及特征明显不同, 三亚块体的基底属南海-印支地块的一部分.五指山块体在早元古代末期地壳初步固结, 形成古陆的雏形; 中元古代大规模构造热事件(1.63Ga、1.4Ga) 形成了海南岛中北部地区成型的、稳定的结晶基底; 晚元古代(0.95Ga) 构造活动相对和缓, 已固结成型的古陆局部增生.此间深大断裂即开始活动并控制前寒纪基底构造格局和演化, 五指山块体的空间范围几乎遍及整个海南岛中北部地区.      

Principal stages in evolution of Precambrian organic world: Communication 1. Archean and Early Proterozoic

Expounded in this work are the results of critical consideration of published and original data on biologic nature and appearance chronology of different groups of Archean and Lower Proterozoic (3.5–1.65 Ga) paleontological remains known from geological record. Conclusions are substantiated by morphological analysis of structurally preserved microfossils, their facies distribution, and by inferable genesis and principal evolutionary trends of Archean stromatolites. A special attention is paid to variations of organic and carbonate carbon isotope composition in sedimentary successions with paleontological remains and to recent information about discovered, most ancient biomarkers of large groups of organic world. As a result of this approach, a detailed model of Precambrian organic world evolution is suggested.     

Compositional heterogeneity of the continental lithospheric mantle beneath the Early Precambrian and Phanerozoic structures: Evidence from mantle xenoliths in kimberlites and alkaline basalts

Data on the petro- and geochemical characteristics of mantle xenoliths in kimberlites, which sampled the mantle beneath Early Precambrian tectonic structures (Archean cratons: the basement of the Eastern Siberian Platform, Karelian, Kaapvaal, Wyoming, Western Dharvar; Early and Middle Proterozoic foldbelts: Western Olenek, Natal, and Halls Creek), and xenoliths in alkaline basalts, which sampled the mantle benath Late Proterozoic-Phanerozoic structures (foldbelts: Central Asian, Mozambique, southern tip of South America, and Central German) indicate the following: (1) The major and trace element and REE composition of the mantle is different beneath Early Precambrian structures and Late Proterozoic-Phanerozoic foldbelts and reflects the degree of partial melting of the primitive mantle and its depletion in magmaphile components beneath ancient structures compared to young ones. (2) The original composition of the mantle was different beneath the Early Precambrian and Late Proterozoic structures in terms of both major oxides and incompatible trace elements and REE and their ratios; the composition of the mantle beneath the Eastern Siberian Platform, Wyoming, and Karelian cratons is different in terms of Zr/Y, La/Sm, Ce/Sm, Gd/Yb, and Lu/Hf. (3) The degree of melting of the primitive mantle decreases with depth, as follows from the negative correlation between the MgO/SiO₂ ratio and pressure (i.e., depth) and the positive correlation between the Al₂O₃/MgO ratio and pressure in the xenoliths. (4) The Y, Zr, Ti, Sm, Gd, and Yb conncentrations and the sum of HREE in the mantle decrease with increasing degree of melting; correspondingly, the material most strongly depleted in these incompatible trace elements and REE composes the upper levels of the lithospheric continental mantle.     

北京市铁矿资源形势及找矿方向

目前,我国年钢铁产量快速增长,造成市场上铁矿石供小于求,价格快速上扬。铁矿为北京市允许开采矿种,保有储量仅2亿吨,可供现有铁矿山开采35年,因此应加大铁矿资源的勘查力度,增添新的矿产地,满足钢铁生产的需求。北京密云、怀柔北部山区的变质岩中鞍山式铁矿石,为主要找矿方向。     

Timing of Precambrian melt depletion and Phanerozoic refertilization events in the lithospheric mantle of the Wyoming Craton and adjacent Central Plains Orogen

Garnet peridotite xenoliths from the Sloan kimberlite (Colorado) are variably depleted in their major magmaphile (Ca, Al) element compositions with whole rock Re-depletion model ages generally consistent with this depletion occurring in the mid-Proterozoic. Unlike many lithospheric peridotites, the Sloan samples are also depleted in incompatible trace elements, as shown by the composition of separated garnet and clinopyroxene. Most of the Sloan peridotites have intermineral Sm–Nd and Lu–Hf isotope systematics consistent with this depletion occurring in the mid-Proterozoic, though the precise age of this event is poorly defined. Thus, when sampled by the Devonian Sloan kimberlite, the compositional characteristics of the lithospheric mantle in this area primarily reflected the initial melt extraction event that presumably is associated with crust formation in the Proterozoic—a relatively simple history that may also explain the cold geotherm measured for the Sloan xenoliths.

The Williams and Homestead kimberlites erupted through the Wyoming Craton in the Eocene, near the end of the Laramide Orogeny, the major tectonomagmatic event responsible for the formation of the Rocky Mountains in the late Cretaceous–early Tertiary. Rhenium-depletion model ages for the Homestead peridotites are mostly Archean, consistent with their origin in the Archean lithospheric mantle of the Wyoming Craton. Both the Williams and Homestead peridotites, however, clearly show the consequences of metasomatism by incompatible-element-rich melts. Intermineral isotope systematics in both the Homestead and Williams peridotites are highly disturbed with the Sr and Nd isotopic compositions of the minerals being dominated by the metasomatic component. Some Homestead samples preserve an incompatible element depleted signature in their radiogenic Hf isotopic compositions. Sm–Nd tie lines for garnet and clinopyroxene separates from most Homestead samples provide Mesozoic or younger “ages” suggesting that the metasomatism occurred during the Laramide. Highly variable Rb–Sr and Lu–Hf mineral “ages” for these same samples suggest that the Homestead peridotites did not achieve intermineral equilibrium during this metasomatism. This indicates that the metasomatic overprint likely was introduced shortly before kimberlite eruption through interaction of the peridotites with the host kimberlite, or petrogenetically similar magmas, in the Wyoming Craton lithosphere.     

Morphological and microstructural characteristics of bauxite developed over a part of Precambrian Iron Ore Group of rocks,Sundergarh District,Eastern India

Bauxite occurring as a blanket over volcanics of Precambrian Iron Ore Group in Kusumdih and Jaldih of Sundergarh district, Odisha, Eastern India exhibits four types of morphology viz. pisolitic, disseminated, spotted and massive. The volcanics/tuffs are altered to shale and is predominantly constituted of kaolinite. Microstructures documented in different morphologies of bauxite are collomorphous, framework, chain, stalactitic, reticulate, vesicle filled and foliated-platy types. Gibbsite, diaspore and kaolinite exhibit the abovementioned morphological and microstructural features. The morphological and microstructural characteristics are interpreted in terms of genetic evolution of aluminous minerals of bauxite in such a setup. Gibbsite and diaspore are formed through solution and precipitation/recrystallisation from kaolinite booklets present in parent tuffaceous shales. Some diaspores were formed at the expense of gibbsites. Small well-ordered kaolinites are of authigenic nature. Kaolinite formed inside a localized closed system containing supersaturated solution exhibits growth spirals. The textural and morphological diversities of the bauxite, abundance of well-ordered bauxite minerals and the absence of any iron/silica phase along with limited occurrence of primary kaolinite together suggest that the deposit has attained a high degree of bauxitisation in weathering environment at surface temperature.