Facies analysis and palaeoclimatic significance of ironstones formed during the Eocene greenhouse

Lower and middle Eocene ironstone sequences of the Naqb and Qazzun formations from the north‐east Bahariya Depression, Western Desert, Egypt, represent a proxy for early Palaeogene climate and sea‐level changes. These sequences represent the only Palaeogene economic ooidal ironstone record of the Southern Tethys. These ironstone sequences rest unconformably on three structurally controlled Cenomanian palaeohighs (for example, the Gedida, Harra and Ghorabi mines) and formed on the inner ramp of a carbonate platform. These palaeohighs were exposed and subjected to subaerial lateritic weathering from the Cenomanian to early Eocene. The lower and middle Eocene ironstone sequences consist of quiet water ironstone facies overlain by higher energy ironstone facies. The distribution of low‐energy ironstone facies is controlled by depositional relief. These deposits consist of lagoonal, burrow‐mottled mud‐ironstone and laterally equivalent tidal flat, stromatolitic ironstones. The agitated water ironstone facies consist of shallow subtidal–intertidal nummulitic–ooidal–oncoidal and back‐barrier storm‐generated fossiliferous ironstones. The formation of these marginal marine sequences was associated with major marine transgressive–regressive megacycles that separated by subaerial exposure and lateritic weathering. The formation of lateritic palaeosols with their characteristic dissolution and reprecipitation features, such as colloform texture and alveolar voids, implies periods of humid and warm climate followed major marine regressions. The formation of the lower to middle Eocene ironstone succession and the associated lateritic palaeosols can be linked to the early Palaeogene global warming and eustatic sea‐level changes. The reworking of the middle Eocene palaeosol and the deposition of the upper Eocene phosphate‐rich glauconitic sandstones of the overlying Hamra Formation may record the initial stages of the palaeoclimatic transition from greenhouse to icehouse conditions.     

冀西北铁岩类型与沉积环境

冀西北铁岩的结构和构造类似于碳酸盐岩,因此,借鉴于现代碳酸盐相模式与沉积环境,有可能恢复古代铁岩的沉积环境。本文对铁岩分类及其沉积环境进行了探讨。     

The geology of the giant Nyankanga gold deposit,Geita Greenstone Belt,Tanzania

Nyankanga is the largest gold deposit in the Geita Greenstone Belt of the northern Tanzania Craton. The deposit is hosted within an Archean volcano-sedimentary package dominated by ironstones and intruded by a large diorite complex, the Nyankanga Intrusive Complex. The supracrustal package is now included within the intrusive complex as roof pendants. The ironstone fragments contain evidence of multiple folding events that occurred prior to intrusion. The supracrustal package and Nyankanga Intrusive Complex are cut by a series of NE–SW trending, moderately NW dipping fault zones with a dominant reverse component of movement but showing multiple reactivation events with both oblique and normal movement components. The deposit is cut by a series of NW trending strike slip faults and ~ E–W trending late normal faults. The Nyankanga Fault Zone is a major NW dipping deformation zone developed mainly along the ironstone–diorite contacts that is mineralised over its entire length. The gold mineralization is hosted within the damage zone associated with Nyankanga Fault Zone by both diorite and ironstone with higher grades typically occurring in ironstone. Ore shoots dip more steeply than the Nyankanga Fault Zone. The mineralization is associated with sulfidation fronts and replacement textures in ironstones and is mostly contained as disseminated sulphides in diorite. The close spatial relationship between gold mineralization and the ironstone/diorite contact suggests that the reaction between the mineralising fluid and iron rich lithotypes played an important role in precipitating gold. Intense brecciation and veining, mainly in the footwall of Nyankanga Fault Zone, indicates that the fault zone increased permeability and allowed the access of mineralising fluids. The steeper dip of the ore shoots is consistent with mineralization during normal reactivation of the Nyankanga Fault Zone.     

川东及邻近地区早侏罗世铁岩的沉积环境及形成过程探讨

一、区域地质概况 川东及邻近地区早侏罗世铁岩(即綦江式铁矿)及其赋存的綦江段地层,分布在四川东南部及黔北、鄂西一带。共分两片:重庆、永川、綦江、遵义片称西区,区内綦江段发育齐全,工业矿床多,研究程度亦高;石柱、利川片称东区,綦江段发育较差,研究程度亦低(图1)。     

Debate about ironstone: has solute supply been surficial weathering,hydrothermal convection,or exhalation of deep fluids?

Ironstone is any chemical sedimentary rock with > 15% Fe. An iron formation is a stratigraphic unit which is composed largely of ironstone. The solutes which have precipitated to become ironstone have dissolved from the Earth's surface, from the upper crust, e.g. the basaltic layer of oceanic crust, or from deeper within the Earth. Genetic modellers generally choose between surficial weathering, e.g. soil formation, and hydrothermal fluids which have convected through the upper kilometre of oceanic crust. Most genetic modellers attribute cherty laminated iron formations to hydrothermal convection and noncherty oolitic iron formations to surficial weathering. However, both types of iron formations are attributable to the exhalation of fluids from a source region too deep for convection of seawater. Evidence for a deep source of ferriferous fluids comes from a comparison of ancient ironstone with modern ferriferous sediment in coastal Venezuela. A deep-source origin for ironstone has wide-ranging implications for the origins of other chemical sedimentary ores, e.g. phosphorite, manganostone, bedded magnesite, sedimentary uranium ore, various karst-filling ores, and even petroleum. Preliminary study of a modern oolitic iron deposit described herein suggests that the source of iron and silica to iron formations may have been even deeper than envisioned within most hydrothermal convection models.     

Ironstone-Gossan discrimination: Pitfalls of a simple geochemical approach — A case study from northeast Scotland

Located in northeast Scotland, the Lecht manganiferous ironstone occurs as several minor and one principal outcrop within deeply weathered Dalradian meta-sediments. The distribution of these shows is controlled primarily by an underlying porous breccia pipe and not by Dalradian stratigraphy or faulting, as previously suggested. The deposit is composed principally of goethite and cryptomelane, with minor hematite, ramsdellite, pyrolusite, lithiophorite, chalcophanite and woodruffite. The ironstone is enriched in several target and pathfinder elements, particularly Zn and Ba which are primarily concentrated in the manganese oxides. Detailed examination of the geochemistry demonstrates that the enrichments are actually more typical of non-economic ironstones (particularly bog-ore) than gossans (a conclusion supported by field, textural and mineralogical evidence), illustrating the danger of relying upon simple geochemical surveys alone for ironstone-gossan discrimination. No relict sulphides, secondary ore minerals, native metals, gangue minerals or “boxwork” textures were observed in either hand specimen or polished section. The morphology and textures of the Lecht ironstone are typical of those observed in bog-iron ores and in weathered profiles.The Lecht ironstone is considered to have been derived from prolonged weathering of the local Dalradian meta-sediments. These are enriched in target and pathfinder elements and are regarded as a prospective sequence. Cementation of the subsequent regolith by solutions rich in iron, manganese and other elements, combined with bog-ore formation and penetration of the breccia pipe by these solutions, produced the complex and varied morphology and geochemistry seen in the deposit today. The Lecht deposit may represent the distal manganiferous expression of a goldrich zinc-lead exhalative deposit hosted by the Dalradian meta-sediments of the region.     

中国泥盆纪鲕铁石沉积环境、成因

中国泥盆纪鲕铁石分布在华南的云南、四川、贵州、广西、湖南、江西、湖北等7个省(区)。鲕铁石结构有鲕粒、假鲕粒、凝胶团粒。鲕粒一般有环带、有核心,也有仅有环带、无核心的鲕环2-3层,少数数十层。鲕粒环带、团粒一般为自形晶微粒赤铁矿、绿泥石、菱铁矿,含少量粘土矿物及其他自形品矿物。核心为石英碎屑,或铁矿物碎屑。鲕粒间基质自形晶矿物、粘土矿物及其碎屑矿物。以接近封闭的浅水泻湖环境沉积最好(占鲕铁石总储量60%),以河口三角洲砂坝沉积最差(占4.2%)。鲕铁石是海水中胶体沉积的,当海水有一定能量时,铁质围绕碎屑物或铁矿物碎屑转动沉积鲕粒环带,铁质也可围绕先凝聚的铁矿物转动沉积无核心的环带。当海水能量低时形成凝胶团粒。     

Genesis of the Hussainiyat ironstone deposit, Western Desert, Iraq

The Hussainiyat ironstone deposit (Jurassic) is mainly pisolitic, intraclastic and concretionary in texture, associated with kaolinite mudstones and/or with quartzose sandstone. The ironstone consists mainly of goethite, hematite, kaolinite and quartz. The deposits were derived from a variety of parent rocks that included low- and medium-rank metamorphics, intermediate igneous rocks and pre-existing sediments of the Nubio-Arabian Shield. The source rocks suffered deep chemical weathering in the hinterland, and the products (Fe-oxyhydroxides, kaolinite and quartz) were later transported by rivers to the depositional site. Iron was mostly carried in association with the clay fraction and organic matter. Several genetic processes were involved in the ironstone formation. Iron concretions were mostly formed by bacterial build-up in swamps and marshes, and were subsequently embedded in kaolinitic mud. Large-scale development of groundwater laterite blanket (ferricrete) occurred later in the overbanks and floors of wadis, under oxidizing pedogenic conditions. During this stage iron was mobilized from the kaolinitic deposits and migrated upward in dry seasons and, to a lesser extent, downward in wet seasons. Pisolites and oolites grew in situ in the kaolinitic soil at the upper limit of the fluctuating water table. This ferricrete blanket had a wide and continuous extension within an elongated paleodepression. Seasonal heavy rain periods resulted in the flow of ephemeral streams and rivers, where major parts of this ferricrete was reworked and deposited with quartz sand and mud clasts as channel deposits. The original pisolitic-colloform ironstone was reworked continuously to form a semi-continuous sheet. In such wet seasons, additional Fe-enrichment took place as cementing materials or overgrowths. Received: 28 April 1995 / Accepted: 10 July 1997     

Tectonic and eustatic control on deposition and preservation of Upper Cretaceous ooidal ironstone and associated facies: Peace River Arch area, NW Alberta, Canada

The Late Coniacian, shallow-marine Bad Heart Formation of the Western Canada foreland basin is very unusual in that it contains economically significant ooidal ironstone. Deposition of shallow-water and iron-rich facies appears to have been localized over the crest and flanks of a subtle intrabasinal arch, in part interpreted as a forebulge and partly attributed to reactivation of the long-lived Peace River Arch. The formation comprises two upward-shoaling allomembers, typically 5–10 m thick, that are bounded by regionally mappable ravinement surfaces. The lower unit, allomember 1, grades up from laminated mudstone to bioturbated silty sandstone, which is abruptly overlain by bioturbated ooidal silty sandstone grading into an almost clastic-free ooidal ironstone up to 7 m thick. Ooidal ironstone was concentrated into NW- to SE-trending ridges, kilometres wide and tens of kilometres long. Ironstone formation appears to have been promoted by: (a) drowning of the arch, which progressively curtailed sediment supply; and (b) enhanced reworking over the shallowly submerged arch and over a fault-bounded block that underwent episodic vertical movement of 10–20 m during Bad Heart deposition. Allomember 2 also shoals upwards from mudstone to bioturbated and laminated silty sandstone but lacks ooids, apparently reflecting a rejuvenated supply of detrital sediment from the arch. The marine ravinement surface above allomember 2 is a Skolithos firmground, above which is developed a regional blanket of ooidal sediment. In the east, ooids are dispersed in a bioturbated silty sandstone with abundant evidence of repeated reworking and early siderite and phosphate cements. Westwards, this facies grades, over about 40 km, into almost clastic-free ooidal ironstone about 5 m thick; the lateral facies change may reflect progressive clastic starvation distal to a low-relief source area. The two allomembers are interpreted to reflect eustatic oscillations of about 10 m, superimposed on episodic tectonic warping and block-faulting events. The development of ooidal ironstone immediately above initial marine flooding surfaces indicates a close relationship to marine transgression, reflecting sediment-starved conditions. Ironstone does not appear to be related to either sequence boundaries or maximum flooding surfaces. The Bad Heart Formation is blanketed by marine mudstone deposited in response to major flexural subsidence and rejuvenation of clastic sources in the Cordillera to the SW.     

华北古元古代末鲕铁岩:Columbia超大陆 裂解初期的沉积响应

铁建造和鲕铁岩是地史上两类主要的富铁沉积,不仅记录了地球大气与海洋氧化还原状态和化学条件演变,而且也反 应了构造运动、岩浆活动和生物的相互作用过程.过去对铁建造已有深入研究,而有关前寒武纪铁岩成因与古海洋和构造背景 研究甚少.运用扫描电镜(scanningelectronicmicroscopy,简称SEM)、X射线衍射(X-raydiffraction,简称XRD)、能谱(energy dispersive spectroscopy,简称EDS)技术分析铁鲕的微组构、矿物成分和化学组成,讨论华北串岭沟组(1.65~1.64Ga)鲕铁岩 的成因环境及其与Columbia超大陆裂解的关系.研究表明,铁鲕主要由赤铁矿和少量高岭石组成,贫陆源碎屑和Al2O3;鲕包 壳由微片状赤铁矿构成的致密和疏松纹层交互组成;Fe-Al呈明显的负相关性,表明铁主要源于缺氧富铁深海水体而非陆源 风化.鲕铁岩集中在快速海进和低陆源输入引起的沉积饥饿期,发育于氧化还原界面附近的潮下贫氧环境.与超大陆裂解伴生 的岩浆活动、基底沉降和快速海侵是促进鲕铁岩形成的重要因素.串岭沟组底部铁岩是华北地台响应Columbia超大陆裂解而 发生构造与环境转化的重要沉积记录.      

Iron ore workings near Horsham,Sussex, and the sedimentology of Wealden clay ironstone

The distribution of old workings, or ‘minepits’, for clay ironstone in an area between Horsham and Crawley is shown on geological sketch-maps. The geological structure of the area is described in outline. The old workings, except for those in two anomalous patches, are restricted to two argillaceous units in the Upper Tunbridge Wells Sand and to an ironstone horizon some 2 m. below the Horsham Stone in the Weald Clay. Slags from two bloomery sites are described. The amount of ore dug from the minepits is estimated to correspond roughly to the requirements of local blast furnaces, of sixteenth- and seventeenth-century date. Depletion of iron ore reserves is suggested as a factor deciding against resumption of the iron industry in the area after the Civil War.     

Origin of ferricretes in fluvial-marine deposits of the Lower Cenomanian Bahariya Formation,Bahariya Oasis,Western Desert,Egypt

The type section of the Lower Cenomanian Bahariya Formation at Gebel El-Dist (Bahariya Oasis, Western Desert), Egypt, comprises claystones, mudstones, siltstones and sandstones deposited in fluvial-deltaic coastal plain, lagoonal, estuarine and shallow marine environments. The formation is characterized by an abundance of ferruginous sandstones that locally weather to form prominent iron crusts. These centimeter to decimeter-scale ferruginous horizons display a continuum of features ranging from unaltered sandstone with a pervasive ferruginous matrix to distinct ironstone beds with massive, nodular, vesicular and pisolitic textures. Ferruginous sandstone typically occurs at the tops of sandstone beds, or bracketing the base and top of beds, in the fining-upward cycles of deltaic plain deposits in the lower part of the formation and on a low-energy fluvial floodplain in the middle of the formation. Indurated ironstone beds occur mainly as the caps of coarsening-upward cycles of prograding shoreface sediments through much of the formation. We interpret the ironstone crusts as ferricretes, formed by iron accumulation that resulted from the oxidation and precipitation of soluble iron or colloids transported in the sediment load or by groundwater. This accumulation possibly took place at the water table or possibly below the water table at the fresh water/saline water interface. However, base-level fall and subsequent subaerial exposure of the sediments resulted in reworking and pedogenic modification of some of the iron-impregnated horizons.     

藏南聂拉木北部喜马拉雅山主脊带侏罗系重解

通过野外调查和地层系统校正,认为前人在该区划分的侏罗纪地层系统存在偏差。下侏罗统在喜马拉雅山主脊带南北两侧岩相变化较大,普普嘎组可能穿时;中上侏罗统岩相变化较小;侏罗系总厚度不超过1400m。中侏罗统鲕粒铁质岩所对应的岩石地层单元按命名先后原则应为定结组,形成于外陆棚环境,可能与全球侏罗纪海泛时期洋流涌入有关,代表特提斯喜马拉雅地区中侏罗世晚巴通期一次沉积地质事件;晚侏罗世基末利期在特提斯喜马拉雅可能存在与欧洲同期的缺氧事件。     

The end for the Frodingham Ironstone?

In 1988 the last Frodingham Ironstone was extracted for steelmaking. Is Britain's last-but-one Jurassic ironstone field destined to disappear as almost all the others have before it, or can some part of it be preserved for geologists?     

A metasomatic origin for the iron-oxide Au-Cu Starra orebodies, Eastern Fold Belt, Mount Isa Inlier

The Starra ironstone-hosted Au-Cu deposit in northwest Queensland, Australia, displays evidence of a polyphase history of late-post metamorphic, structurally controlled metasomatism. The mineralisation is hosted within several massive ironstone units which occur at a lithological contact of metamorphosed sandstone and siltstone sequences and foliated chlorite-biotite-magnetite schists. The ironstones strike north-south and occur as discontinuous lenses that cross cut the host rocks in places. Magnetite ironstone textures are generally massive in appearance, but relict deformed breccia and foliation textures are locally preserved. Sulphide ore textures are more brittle and consist of fine, pervasive brecciation of the ironstones and host rocks. Petrological and textural observations support a hydrothermal origin for both ironstone and mineralisation. Three dominant post-peak metamorphic paragenetic stages of alteration and mineralisation are recognised: (1) early widespread Na-Ca metasomatism consisting of albite-quartz-actinolite-scapolite, (2) localised K-Fe metasomatism consisting of biotite-magnetite-hematite- quartz-pyrite and (3) the mineralising stage consisting of anhydrite-calcite-hematisation of magnetite-pyrite-chalcopyrite-gold and extreme chloritisation of biotite. The Na dominant alteration occurs throughout the Eastern Fold Belt and for at least 50 km to the east and 150 km to the north of Starra. The ironstones were products of localised Fe-metasomatism associated with shearing and brecciation of previously Na-metasomatised host rocks. Gold and copper mineralisation resulted from the interaction of magnetite, a late oxidising fluid and an increase in pH. This produced variable hematisation of magnetite and caused the solubility of chloride-complexed gold to decrease dramatically. High gold:copper ratios can be explained in terms of the solubility differences of gold and copper in the thermochemical conditions that existed at Starra. Received: 15 May 1996 / Accepted: 17 January 1997     

The geochemistry of siderite in relation to ironstone in the Paradise Creek Formation, northwest Queensland

Siderite containing up to 43 mole % MgCO₃ has been discovered in the Lower Proterozoic Paradise Creek Formation of northwest Queensland. Field observations together with mineralogical and geochemical data indicate that the weathering of siderite-rich strata has given rise to many of the stratigraphically controlled ironstone outcrops.     

ASD field hyperspectral measurements for discrimination of the ferruginous rocks and the iron ore types at El Gedida-Ghorabi area,Bahariya Oasis,Western Desert,Egypt

The ironstone succession at El Gedida-Ghorabi-Naser area of El Bahariya depression is subdivided into lagoonal manganiferous mud and fossiliferous ironstone consisting mainly of hematite and goethite-hydrogoethite. The application of the ASD field spectroradiometer measurements (spectral range) in the ASTER data led to the interpretation of the presence of ferruginous units as quartzitic sandstone, gluconitic sandy clay, and pink marly limestone. The existing iron ore minerals in the iron ore localities were also classified into high Mn hematite, low Mn hematite, goethite, hydrogoethite as well as low- and high-grade Hematite and Barite. Quartz, feldspars, rutile, and clay minerals (e.g., kaolinite and illite) are mainly associated with the iron ore. Accessory minerals of manganese, e.g., psilomelane and pyrolusite, were also present. The Barite mineral is recorded as a common mineral association with the iron ore deposits at El Gedida and Ghorabi localities. The stratigraphical units investigated in the study area include the oldest gravely clayey sandstones of the Bahariya Formation overlain by the fossiliferous and oolitic limestones of the El-Hamra, Qazzun, and Naqb Formations. Quartztic sandstones and clayey sandstones of the Radwan Formation and youngest Quaternary sediments of sandy-clayey materials were often found as intermittent cover and overburden in unconformity surfaces over the iron ore bands.     

华北下古生界碳酸盐岩中一些结构的形成

碳酸盐岩的结构是在特定的沉积环境或成岩环境里形成,因此,能反映岩石的成因与发展史。某些典型的结构不仅是有指“相”意义,而且能作为找矿标志。本文对华北下古生界碳酸盐岩中的正延性玉髓、石膏型方解石假晶和具有多圈层氧化铁薄膜结构的高能鲕粒等三类结构的形成与环境意义和找矿意义依次进行了论述。     

应用重磁方法勘查铁矿的效果——以辽宁建昌县马道铁矿为例

文章运用重磁方法以辽宁建昌县马道铁矿找矿为例,说明了重磁法的找矿效果,特别指出重力、磁法的科学组合是勘查评价铁矿、区分磁异常性质、寻找富铁矿的有效综合找矿方法。通过对一磁异常进行重新解释和评价,最终验证该异常为一大型鞍山式沉积变质磁铁矿引起的。同时说明GIS、MRAS计算技术模拟、判别分析等现代先进勘探技术的综合运用,是在老矿山的深部和外围铁矿勘查过程中的重要手段。因此,在对危机矿山的研究中,对如何加深地球物理异常的解释和评价等问题,应予以高度重视。     

The first fossil mayfly nymph (Insecta: Ephemeroptera) from the British Isles

Mayflies are very rare in the British fossil record. The first nymph to be found, Schistonotorum wallisi gen. et sp. nov., is described from the non-marine Lower Cretaceous of southeast England. This Early Barremian find is from the Upper Weald Clay Formation at Smokejacks brickworks, Surrey. It is preserved as an adpression in concretionary sideritic ironstone from the upper insect bed exposed in the northeast face of the pit. The palaeoecological significance of this record is discussed.