Oolitic Ironstones Research Articles

Microbe‐mediated, marine authigenic formation of ooidal chamosite: Insights from upper Ordovician carbonates of the South‐Western Yangtze platform (China)

AbstractChamosite is a common marine authigenic mineral and microbial involvement has been often assumed during its formation. However, the fossil record of such microbial activity is ambiguous. Chamosite oolitic ironstones widely occur in the Upper Ordovician of the Yangtze Platform, but their origin remains unclear. In this study, detailed sedimentological and mineralogical investigations on ironstones from the Haima section (south‐western Yangtze Platform) helped unravel the critical role of cyanobacterial activity during marine chamosite authigenesis. Stratigraphically, deposition of the Haima oolitic ironstones corresponded with an early Katian transgression, temporally equivalent to many other early Palaeozoic counterparts worldwide. Petrographic observations revealed that different chamosite forms (ooids, peloids and matrix) all developed in close association with calcified cyanobacteria, suggesting that cyanobacteria‐involved chamosite formation existed across a wide range of hydrodynamic conditions from above fair‐weather wave base to below storm wave base. Further mineralogical characterization (backscatter imaging and electron‐microprobe analyses) at single‐grain scale revealed clear mineral zonation from the matrix (dispersed iron oxide and microquartz) through the calcified cyanobacterial sheaths (dispersed chamosite) to the ooid (chamosite). This zonation is best explained by cyanobacterial microbial mats that created benthic reducing and high‐pH conditions. These conditions drove near‐seafloor, dispersed iron oxide and microquartz (primary precipitates formed in oxic seawater) to form chamosite. The authigenic chamosite, originally dispersed within and surrounding the microbial mats, could be reworked to develop other chamositic grains. It is proposed here that the early Katian transgression was responsible for the chamosite–oolitic–ironstone development. The marine transgression submerged earlier‐exposed lands and likely supplied Fe‐Al‐rich colloidal materials to an open marine setting for the chamosite authigenesis. Moreover, transgression likely triggered upwelling, delivered ferruginous deeper water as an extra Fe source and provided nutrients for the cyanobacterial microbial‐mat development, which was essential for the widespread chamosite authigenesis across the south‐western Yangtze Platform.

Geoheritage in the Minett UNESCO Biosphere (Southern Luxembourg): Inventory, Evaluation, and Conservation Aspects of Representative Geosites

The southwestern part of Luxembourg, known as Minett in the local language use, exposes an exceptionally high diversity of marine near-shore sediment rocks from the Early to Middle Jurassic, owing to its proximal position at the north-eastern margins of the Paris Basin. The iconic Minette ironstone formation is known as the worldwide largest oolitic ironstone deposit from the last 500 my and the abandoned open cast mines are nowadays protected sites with a high biodiversity, intrinsically linked to geodiversity. The schistes bitumineux unit, a lateral equivalent of the well-known Posidonienschiefer of the Holzmaden region (South Germany), is frequently studied for its exceptionally preserved fossils of marine vertebrates, cephalopods, and insect remains, deserving its status as a Fossil-Lagerstätte of international relevance. Of regional importance are the Rumelange limestones, with coral patch-reef paleo-environments. The geomorphological main feature of the region is the Cuesta of the Middle Jurassic (‘Dogger-Schichtstufe’), with several outliers (‘Zeugenberge’) that represent widely visible landmarks in the landscape. In this paper, 16 geosites, representative of the geological, palaeontological, geomorphological, and hydrogeological heritage in the Minett UNESCO biosphere, are presented and evaluated according to scientific, educational, and geotouristic criteria.

Petrography, Diagenesis and Paragenesis of Wadi Halfa Oolitic Ironstone Formation, North Sudan

Utilizing petrography, diagenesis and paragenesis are the major step to study the sediments of Wadi Halfa Oolitic Ironstone Formation for their origin and economic potential. Petrographical analysis of Wadi Halfa Oolitic Ironstone Formation were carried out depending on the investigation of six vertical lithofacies profiles from the region of Wadi Halfa. The petrographic classification of sandstone samples of Wadi Halfa Oolitic Ironstone Formation according to mineral composition (quartz, lithic fragments, and feldspars) are classified as quartzarenite 66.7%, sub lithic arenite 20.9% and subarkose 12.5%. The provenance of the sandstone revealed that all sample were derived mainly from interior stable craton. The stable craton offered good environments for detrital constitution to be recycled and transported for relatively long-distance. The present study adopted an intergranular pressure solution (IGV) diagram for IGV values more than 50 percent for all studied samples indicating that more of their original porosity has been destroyed cementation rather than by compaction processes for most samples.

The iron oolitic deposits of the lower Devonian Yangmaba formation in the Longmenshan area, Sichuan Basin

Oolitic ironstones are widely distributed in South China, particularly in Middle and Upper Devonian strata, and are referred to as Ningxiang-type iron deposits. The hybrid siliciclastic-carbonate deposits of the Lower Devonian Yangmaba Formation iron-enriched interval (dominated by hematite ooids and chamosite ooids) are recognized in the Shigouli section of the Longmenshan area, Sichuan Basin. The Yangmaba Formation ironstones differ from other Devonian Ningxiang-type oolitic ironstones: The Ninxiang-type ironstone deposits are commonly interpreted to have been deposited in a narrow coastal zone or inland restricted bays and lagoons with clastic and carbonate sedimentary sequences; by contrast, the studied oolitic ironstones are interpreted to have been deposited in a shoreface zone to inner shelf based upon field and petrographic investigations. Our study shows that this oolitic ironstone consists of dominantly hematite ooids and chamosite ooids. Major element and trace element discrimination diagrams, PAAS (Post-Archaean average Australian sedimentary rock) normalized REE (Rear earth element) patterns, and positive Ce/Ce* anomalies show evidence of a hydrogenous origin. The weathering of adjacent terrestrial rocks from the formation of the palaeo-land surface is interpreted to be the main Fe source for the oolitic ironstones.Environmental energy and redox conditions are the important factors controlling ironstone formation and distribution of the hybrid deposits of the Yangmaba Formation in the Shigouli section of the Longmenshan area. Oolitic ironstones formed in oxidizing conditions of the upper shoreface zone include the carbonate concentric ooids with slightly hematite ooids occurring in higher energy upper oxidizing shoreface subzone, moderately hematite concentric ooids deposited in middle oxidizing shoreface subzone of fluctuant environmental energy, and hematite ooids present in lower oxidizing shoreface subzone of lower-energy conditions or weakly agitated environment. The chamosite concentric ooids and superficial ooids formed in reducing conditions of the lower shoreface and upper offshore transition. The former occurred in upper reducing shoreface subzone near the fair weather wave base, and the chamosite superficial ooids were deposited in lower reducing shoreface subzone and upper offshore transition that was affected by episodic high energy events.

Palaeoenvironmental reconstruction and oolitic ironstone mapping of the Agbaja Ironstone Formation in the Nupe Basin, North-central Nigeria: Insights from sedimentological and aeromagnetic analyses

This study focusses on the sedimentological and aeromagnetic characteristics within the Agbaja Ironstone Formation to establish the sedimentary depositional history and map oolitic ironstone zones in the Lokoja district of the Nupe Basin. Forty sandstone samples were collected for granulometric, sedimentary facies and petrographic analyses to reconstruct depositional processes of the Agbaja Ironstone Formation. Aeromagnetic data analysis was engaged to map the oolitic ironstone rich zones. The iron-rich sandstones in the Agbaja Ironstone Formation occur as oolitic ironstone, ironstone concretion and laterites. The bivariate grain size plots and multivariate discriminant analyses suggest that 92% of the sandstone in the formation were deposited in beach and coastal dunes under fluvial-river actions, while 8% are of the marine environment deposited under wave actions. The quartz-feldspar lithic (QFL) ternary diagram and palaeocurrent analyses of the samples revealed that the matured, sub-arkosic sandstones originated from the cratonic interior (Abakaliki uplifts). The bioturbation and reactivation by Ophiomorpha burrows and herringbone on the fine-medium grained sandstones in the formation contributed to the observed high magnetic intensity values. High magnetic signatures (−94.784 to 1.191 nT), analytic signal peaks and depth levels from the 2-D source parameter images are the basis for the produced oolitic ironstones prospectivity map in this study.

Petrology of oolitic ironstones of Ashumaysi Formation of Wadi Fatima, western Arabian Shield, Saudi Arabia

The present study concerns Oligo–Miocene oolitic ironstones of the Ashumaysi Formation between Jeddah and Makkah in the western part of the Arabian Shield, western Saudi Arabia. The Ashumaysi Formation contains oolitic iron ore as 2–3 m thick beds. The formation is exposed along the western margin of Wadi Ashumaysi. It dips toward the east and unconformably overlies Precambrian rock and conformably underlies a Miocene basalt flow of the Sita Formation.The Ashumaysi ironstones are mainly hematitic sandstone and oolitic ironstone. The oolitic ironstones vary in color (reddish, brownish, yellowish, or violet). Petrographic analysis indicated that the ironstones are composed of different shapes of ooids (ooliths), pelloids, and rarely pisoids (pisoliths) within ferruginous cement and matrix, which consists of clay minerals with fine to medium grained quartz XRD with EDX analysis of representative samples indicated that hematite, goethite, chamosite, and quartz are the main components of the oolitic ironstone.The main processes responsible for the formation of these oolitic ironstones are progressive diagenetic dehydration and recrystallization of the amorphous ferrous hydroxides and the formation of oolitic and pisolitic goethite and hematite within different fabrics.

Barite Concretions in Wadi Halfa Oolitic Ironstone Formation, North Sudan

During our examination of the outcrops of the sedimentary formations in northern Sudan, we found discoidal-shape grains of the heavy mineral, barite in a sandstone of the Wadi Halfa Oolitic Ironstone Formation, which was recorded by all the earlier workers as a reworked sandstone. Petrography-wise, the framework of the sandstone consists of very angular to angular quartz grains, in which monocrystalline grains dominate over polycrystalline grains. Barite is the main cementing material of this sandstone, which occurs as concretions. Barite concretions indicate that more of the original porosity has been destroyed by cementation rather than by compaction processes with the inter-granular porosity being reduced mainly due to cementation. The origin of these concretions, as a cementing material in the sandstone, is ascribed to the reaction of Ba with some soluble sulfate to form the extremely insoluble heavy barite that appears as rounded concretions. The sulfur of the sulfate may be from the hydrothermal fluids related to submarine volcanism and/or biogeochemical processes. The deposition of these concretions might have taken place not long after the formation of the sandstone. The source of the barium, however, remains an unsolved problem. Further work is needed to interpret the origin and occurrence of these concretions along the region of Wadi Halfa.

Polygenic chamosite from a hydrothermalized oolitic ironstone (Saint-Aubin-des-Châteaux, Armorican Massif, France): crystal chemistry, visible–near-infrared spectroscopy (red variety) and geochemical significance

AbstractSeveral generations of chamosite, including a red variety, occur in the Ordovician hydrothermalized oolitic ironstone from Saint-Aubin-des-Châteaux (Armorican Massif, France). Their chemical re-examination indicates a low Mg content (0.925 < Fe/(Fe + Mg) < 0.954), but a significant variation inIVAl. Minor vanadium is present at up to 1.1 wt.% oxide. Variations inIVAl, the vanadium content and the colour of chamosite are related to the hydrothermal reworking of the ironstone. Taking into account other published data, the ideal composition of chamosite is (Fe5–xAl1+x)(Si3–xAl1+x)O10(OH)8, with 0.2 <x< 0.8 (0.2: equilibrium with quartz; 0.8: SiO2deficit). The red chamosite (IIbpolytype) has a mean composition of (Fe3.87Mg0.23Mn0.01□0.07Al1.74V0.07)(Si2.33Al1.67)O10(OH)8. This chamosite is strongly pleochroic, from pale yellow (E|| (001)) to deep orange red (E⊥ (001)). Visible–near-infrared absorbance spectra show a specific absorption band centred at ~550 nm forE⊥ (001), due to a proposed new variety of Fe/V intervalence charge-transfer mechanism in the octahedral sheet, possibly Fe2+– V4+→ Fe3+– V3+. While the formation of green chamosite varieties is controlled by reducing conditions due to the presence of organic matter as a buffer, that of red chamosite would indicate locally a weak increase offO2related to oxidizing hydrothermal solutions.

English

Wadi Halfa Oolitic Ironstone Formation (WHOIF) covers large areas around Wadi Halfa border town in Northern Sudan. The detrital framework grains of sandstone of WHOIF are composed of sub-angular to sub-rounded quartz grains followed by feldspars, lithic fragments, micas and heavy minerals. The dominant cementing materials are iron oxides, carbonate cement, and some clay minerals. Twelve block samples were carefully selected and collected from six sites to represent different parts of vertical lithofacies sections of the studied formation. The petrographical characteristics of the specimens were first studied and the specimens were then subjected to laboratory tests to determine some of their basic physical and engineering properties. The physical tests constituted unit weight, specific gravity and water absorption. The measured engineering properties included uniaxial compressive strength (UCS) and ultra-sonic pulse velocity (UPV). The UCS and UPV were measured for specimens parallel and perpendicular to the bedding planes of the studied samples. The geotechnical study shows that the UCS values of WHOIF generally range from strong 83.7 MPa to very strong 153.7 MPa for perpendicular direction to bedding planes. Very good statistical correlation coefficient was developed between UCS and UPV. Good correlation was obtained between water absorption, porosity and UPV. These relations indicated that a decrease in void ratio and porosity resulted in an increase in UCS and UPV. An increase in water absorption of sandstone yielded a decrease in ultrasonic pulse velocity and uniaxial compressive strength. Key words: Wadi Halfa Oolitic ironstone formation, lithofacies, uniaxial compressive strength, ultrasonic pulse velocity.

Long term reactivity of CO2 in a low salinity reservoir-seal complex

An understanding of the long-term reactivity of different rock types to injected CO2 is needed for sequestration site assessment. Relative to saline aquifer studies, the long term reactivity of CO2 in low salinity aquifers has received little attention. Currently in Australia, the Surat Basin is being appraised for its large-scale CO2 storage potential within low salinity aquifers. Sixteen core samples from the Precipice Sandstone and Evergreen Formation – the notional target reservoir and seal complex – were characterized for mineral content; helium, mercury-injection and micro CT porosities; air permeability; and, imaged with SEM-EDS. Samples consisted of quartz rich reservoir sandstones, feldspar and clay rich or calcite cemented sandstones (secondary reservoir), mudstones (sealing complex), and oolitic ironstones (sealing complex) derived from braided river, fluvial-deltaic, and restricted marine shoal depositional environments, respectively. The reservoir sandstone samples characterized here had measured total porosity that ranged from 11 to 23% with pore throats mainly between 90 and 100 μm, and core air permeability from 558 to 3397 mD. In the Precipice Sandstone reservoir sample μCT plugs, 98% of the pore space was connected with calculated vertical permeability 145–4611 mD and horizontal 4291–8200 mD. Feldspar and clay rich sandstone and mudstone samples from the overlying Evergreen Formation had porosity that ranged between 0.2 and 22.9%, with a wide range of pore throat sizes from ~0.005 to 30 μm, and permeability from 0.2 to 28.1 mD, respectively. Ironstone and mudstone samples from the Westgrove Ironstone Member (Evergreen Formation) had porosity from 0.7 to 9.7% and a low permeability of 0.04 mD.Kinetic geochemical CO2 reactivity models made predictions over two time-scales: 30 or 1000 years. Selected models also accounted for the potential presence of 10 ppm SO2 gas. The Precipice Sandstone quartz-rich reservoir sandstones had consistently small amounts of reactive minerals and mineral trapping or scaling of the reservoir was not predicted over 30 years, with the pH approximately 4.5 after 30 years. Samples from the Evergreen Formation included feldspar and clay rich sandstones and mudstones, several contained variable amounts of carbonate cement. Their response to CO2 was more influenced by mineral content than rock type. Plagioclase feldspars and Fe-rich chlorite were the main silicate minerals that reacted to produce siderite and ankerite mineral trapping up to 2.57 kg/m3 CO2. In the very unlikely event that CO2 rich fluids migrated upwards as far as the Westgrove Ironstone Member, chlorite is predicted to alter to siderite. This study indicates that the Precipice Sandstone reservoir in the study region has a low likelihood of mineral scaling which is favorable to avoid CO2 injectivity issues. Mineral trapping as ankerite and siderite could be expected to trap CO2 in the chlorite and plagioclase rich Evergreen Formation seal lithologies. Further work is suggested on validating long term predictions with observation data from natural analogue studies.

Facies architecture and depositional evolution of Palaeocene–Eocene, Subathu Formation, Garhwal Himalaya, Uttarakhand, India

The Late Palaeocene–Middle Eocene marine sedimentary sequence of the Himalayan foreland basin, represented by the rocks of Subathu Formation, is enriched with the presence of foraminifera, bryozoa, corals, gastropods, bivalves and so on within both carbonate and clastic dominant lithologies. The carbonate and the shale consist mainly of larger benthic foraminifera (LBF) and the assemblage is composed of Nummulites and Assilina along with the other skeletal and non-skeletal components. The presence of a palaeosol at the base of Subathu indicates a gap in sedimentation. The lithological and biotic assemblages along with the bed form, bed geometry and primary sedimentary features helped us to establish four facies associations, A, B, C and D, and have been corroborated with the shoreline migration (transgression/regression) history. The facies association-A, representing the basal horizons of Subathu Formation, indicates the onset of transgression and deposition in lagoonal condition with carbonaceous shale and oolitic ironstone, followed by the facies association-B and -C deposited in shallow marine shoreface. The uppermost unit, i.e., the medium- to coarse-grained sandstone (facies association-D) of Subathu Formation, represents a fall in relative sea level (progradational stacking pattern), whereas the underlying contact between the facies association-B and -C is represented by an aggradational stacking pattern between the siltstone and the shale, but certainly without exposing the shelf. Petrographic studies based on characteristic features such as framework constituent, the percentage of matrix and grains, nature of cementing material, textural features and fossil content help to deduce a distinct change in depositional setting from an open marine (shelf) to shoal, a lagoon that gradually grades to foreshore/beach environment. The study reveals that the basin has gone through a transgressive (facies association-A and -B), regressive (facies association-C) as well as distinct forced regressive (facies association-D) phase of shoreline migration history.

Sequence stratigraphic analysis of the Lower Jurassic Precipice Sandstone and Evergreen Formation in the Surat Basin, Australia: Implications for the architecture of reservoirs and seals for CO2 storage

Abstract The Lower Jurassic Precipice Sandstone and Evergreen Formation in the Surat Basin, Australia are a highly prospective interval for carbon capture and storage (CCS). However, their stratigraphic nomenclature is inconsistently applied, and a basin-wide stratigraphic framework that is ubiquitously agreed upon remains elusive. In particular, a sandy transitional sandstone interval that occurs above the main Precipice Sandstone reservoir unit has caused confusion about how the two units should be subdivided. To address this issue, we integrate core observations with seismic interpretation and wireline log correlation to construct a sequence stratigraphic framework for the Lower Jurassic System that goes beyond lithostratigraphy. The main findings of our work include: (1) the identification of three 3rd-order sequences, SQ1-SQ3, from base to top. Two main sandstone intervals, the Precipice Sandstone and the Boxvale Sandstone Member (Evergreen Formation), represent the lowstand systems tracts (LSTs) of SQ1 and SQ3, respectively. The Westgrove Ironstone Member in the upper Evergreen Formation – a distinctive oolitic ironstone unit – represents the transgressive system tract (TST) of SQ3; (2) the transitional interval above the main Precipice Sandstone reservoir corresponds to the lower TST in SQ1 (lower Evergreen Formation). It is manifest as an upward-fining succession with a retrogradational facies stacking pattern, and is significantly different from the main Precipice Sandstone reservoir which shows an aggradational stacking pattern; (3) the Precipice Sandstone and lower Evergreen Formation progressively onlap the western and the eastern margins of the Surat Basin, and this confines the Precipice Sandstone to the central portion of the basin; (4) the three sequences (SQ1-SQ3) approximately coincide with global sea-level cycles documented from the early Jurassic. Two episodes of accelerated tectonic subsidence were responsible for the thinning and onlap of SQ1 and SQ3 onto the basin margins. In contrast, weaker tectonic subsidence was responsible for the overall finer grained sediment, the uniformity in thickness, and widespread distribution of SQ2. The results of our stratigraphic analysis indicate that the architecture of the reservoir and seal significantly differs from previous thinking, such that: (1) the Precipice Sandstone, the main reservoir unit, has a distribution that is limited to the basin centre; (2) the transitional interval above the main Precipice Sandstone reservoir forms poorly-connected, localized reservoirs that onlap and backstep towards the basin margins; and, (3) SQ2 and SQ3 are sealing units because they are very widespread and mud-dominated. Viewing the Precipice Sandstone and Evergreen Formation from a sequence stratigraphic standpoint provides important information about how to subdivide them and helps clarify the stratal relationships that were previously unclear from a mixture of terminology. Furthermore, this study gives insights into the spatial-temporal evolution of this reservoir and sealing interval that is an important CCS target in Australia.

U-Pb Ages and Hf Isotopes of Detrital Zircon Grains from the Mesoproterozoic Chuanlinggou Formation in North China Craton: Implications for the Geochronology of Sedimentary Iron Deposits and Crustal Evolution

The Chuanlinggou Formation is the lower formation of the Changchengian System, and hosts sedimentary iron deposits (marine oolitic ironstones) of the North China Craton (NCC). To determine the age of the iron deposits, and provide insight into the crustal growth of the craton, laser ablation multiple collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS)U-Pb and in situ Hf isotope analysis were performed on detrital zircon grains. Samples were taken from the roof sand-shale of the sedimentary iron deposits at Jiangjiazhai and Pangjiapbu. Overall, 186 detrital zircon grain U-Pb ages yield three major age populations, with weighted average ages of 2450 Ma, 1848 Ma, and 1765 Ma, respectively. Four younger ages from magmatic zircon grains were obtained, ranging from 1694 to 1657 Ma. Combined with observations from published studies, the results define the lower limit for the age of the Chuanlinggou Formation, and constrain the age of the sedimentary iron deposits (marine oolitic ironstone) close to 1650 Ma. The peak ages of 1848 Ma and 2450 Ma define the major collisional events of the NCC. The age of 1765 Ma can be linked to the age range of the widespread mafic dyke swarms that represent the rifting of the NCC within the Columbia supercontinent. Detrital zircon grains from the Chuanlinggou Formation form two obvious groups, with different εHf (t) values ranging from −1 to −8 and from +1 to +8, which correspond to the U-Pb age ranges of 1.7–1.9 Ga and 2.3–2.6 Ga, respectively. They have a similar two-stage Hf model age peak at 2.65–2.85 Ga, suggesting that the source rocks for each of these events were derived from the recycling of ancient crust. The source rocks of the older group of zircon grains might be derived from juvenile crust with a short reworking period. The critical crust–mantle differentiation event might happen during the period of 2.65–2.85 Ga, marking the most significant stage of the crustal growth in the NCC.

Lithostratigraphic characterization of the Upper Campanian – Maastrichtian succession in the Afikpo Sub-basin, southern Anambra Basin, Nigeria

Lithostratigraphic mapping of the Afikpo Sub-basin of southeastern Nigeria was undertaken to reappraise and characterize lithostratigraphic package of the Upper Campanian – Maastrichtian in the sub-basin. The study involved descriptive lithostratigraphic and sedimentological methods supported by palynological biofacies. Five component lithostratigraphic units were identified: the Afikpo Sandstone, Nkporo, Mamu, Ajali and Nsukka formations. The Afikpo Sandstone is the oldest lithostratigraphic unit in the sub-basin. The sandstone unit overlies an angular unconformity above the Turonian Eze-Aku Group facies around Amasiri in the southern Benue Trough. It consists of fine to very coarse and occasionally pebbly sandstone interbedded with fossiliferous siltstones/sandstones, mudstones and coals. The Nkporo Formation overlies the Afikpo Sandstone and consists of thick succession of fossiliferous dark – grey to black shales, micritic limestones, oolitic ironstone and a distinct sandstone body enclosed within the dominant shale facies. The Nkporo Formation in the sub-basin comprises of three lithologically homogenous subunits: the Asaga Amangwu Shale, Owutu Sandstone and Amaiyi Edda Shale. The type locality is retained in Ndagbo Nkporo. The Owutu Sandstone is intervening between the Asaga Amangwu Shale and Amaiyi Edda Shale subunits. The Mamu Formation overlies the Nkporo Formation and is conformably overlain by the Ajali Formation. The Nsukka Formation is the topmost lithostratigraphic unit mapped in the sub-basin. The type localities for the Mamu, Ajali and Nsukka formations are retained in the Central Anambra Basin. This study has de-emphasized usage of the term “Nkporo Group” in addressing the Afikpo Sandstone and Nkporo Formation because the two lithostratigraphic units are distinct formations.

Mineralogical and Geochemical Features of Oolitic Ironstones from the Sinara–Techa Deposit, Kurgan District, Russia

The paper discusses the mineralogical and geochemical features of oolitic ironstones from the Sinara–Techa deposit, Transural region, Kurgan district. The ore unit is localized in the lower part of a thick Mesozoic–Cenozoic sequence of sedimentary rocks that fill the West Siberian Basin beneath calcareous clay and overlying beds enriched in glauconite and clinoptilolite. The ironstone consists of goethite ooids in smectite–opal cement. Accessory minerals are pyrite, galena, sphalerite, and monazite. The texture and structure make it possible to suggest the formation of sediments enriched in iron as a result of colloid coagulation. The most probable source of iron is related to inland drift. Deposition of iron took place in the estuaries of subtropical rivers due to mixing of colloidal solution of river water with seawater electrolyte. The chemical features of rocks are controlled by the composition of the adsorbed iron oxi/hydroxide complex.

Origin and mechanism of formation of the Oligo-Miocene ironstones of Umm Himar Formation, Turabah area, southwestern Arabian Shield, Saudi Arabia

Turabah area is located in the southwestern Arabian Shield of Saudi Arabia. The present study aims to shed light on the origin and mechanism of formation of the ironstones of the Tertiary sedimentary succession of the Umm Himar Formation in Turabah area. These ironstones are enclosed within mudstones and carbonates with less frequent sandstone. These lithologies show lateral and vertical facies changes and thickness variations and are stacked in cyclic nature within seven sedimentary units. The studied ironstones are present in two main horizons within the lower part of the succession as follows. (1) Lower massive and oolitic hematitic ironstones which are present as thin continuous and discontinuous bands and laminae within the varicolored tuffaceous mudstone unit of the lower part of the succession. The oolitic ironstones are present in successive small cycles. The lower parts of these cycles are composed of ferruginous friable mudstones. The middle parts are composed of hard ferruginous mudstones with abundant vertical and inclined burrows. The upper parts of these cycles are composed of indurated oolitic hematitic ironstones. The iron ooids of these ironstones were formed during successive and subsequent stages of diagenetic ferruginization of the precursor tuffaceous materials. (2) Upper bedded peloidal hematitic-magnetitic ironstones which are present in the hematitic-magnetitic ironstone unit no. 6 just overlying the bedded basalt unit no. 5. These ironstones were formed during subsequent stages of diagenetic ferruginization of precursor tuffaceous materials. These stages include the formation of green clays, devitrification, and finally ferruginization of the tuffaceous materials. This ironstone type is laterally changed into the penecontemporaneous bedded tuffaceous basalt.

Compositional variation of glauconites in Upper Cretaceous-Paleogene sedimentary iron-ore deposits in South-eastern Western Siberia

Glauconite occurs either as unaltered greenish or as altered brownish variety in Upper Cretaceous-Palaeocene sediments in the southeastern corner of Western Siberia. Studied section within the Bakchar iron-ore deposit includes Ipatovo, Slavgorod, Gan'kino and Lyulinvor formations, which are represented by sandstones, siltstones, claystones and oolitic ironstones of coastal-marine facies. The origin of unaltered glauconite is explained by the “verdissement theory”. Transgressions during Lower Coniacian, Santonian and Campanian favored the formation of unaltered glauconites in dysoxic to anoxic conditions. Subaerial exposure of glauconite resulted in leaching of potassium, oxidation of iron and formation of iron hydroxides in Upper Coniacian, Maastrichtian and Palaeocene. Glauconite ultimately converts to leptochlorite and hydrogoethite by this alteration. Abundant microscopic gold inclusions, besides sulphides, sulphates, oxides and silicates characterize this glauconite. Mineral inclusions include precious, rare metals and non-ferrous metals. The concentration of gold in glauconite may be as high as 42.9ppb. Abundant inclusions of various compositions in glauconites indicate enrichment of marine sediments in precious and non-precious metals. While major element composition of glauconites is affected by subaerial exposure, the broadly similar micro-inclusions in both altered and unaltered varieties are possibly related to the comparatively immobile nature of REE and trace elements.

Sedimentology of an early Cambrian tide-dominated embayment: Quyuk formation, Victoria Island, Arctic Canada

The early Cambrian (series 2, stage 4) Quyuk formation is exposed in the Minto Inlier of western Victoria Island, Canadian Arctic Islands, and forms the base of the Phanerozoic succession. Coeval with other sandstones of this age in Laurentia, it was deposited in a shallow-marine embayment on a passive margin during the initial phase of the early Paleozoic transgression. Four facies associations are recognized: (1) offshore muds consisting dominantly of dark gray laminated mudstone with discontinuous laminae of medium- to coarse sand; (2) offshore sand dune fields characterized by laterally continuous, planar cross-stratified beds up to 1.4m thick of medium- to coarse-grained sandstone; (3) distal nearshore consisting dominantly of fine- to medium-grained bioturbated sandstone and fine- to medium-grained sandstone interbedded with laminated mudstone; and (4) proximal nearshore characterized by laterally continuous fine- to medium-grained bioturbated sandstone and medium-grained oolitic ironstone. Large scale dunes of facies association 2 record areas where tidal currents were amplified and had available sediment supply in contrast to facies association 1, which was sediment starved. Dunes are, for the most part, non-bioturbated or contain just a few individual burrows belonging to Skolithos. In nearshore settings, bioturbation in the form of a typical early Cambrian suite of shallow-subtidal ichnofossils predominated, representing a low-diversity Cruziana ichnofacies. Oolitic ironstone horizons in the proximal nearshore mark periods of low sedimentation rates when iron became concentrated and calcite was the primary cementing agent. The coastline is envisaged as a complex of bays and lagoons.

An approach to paleoclimatic conditions for Devonian (upper Lochkovian and middle Givetian) ironstone formation, NW Anatolian carbonate platform

Lower-middle Devonian iron-bearing successions were studied along 2 measured stratigraphic sections in the Camdag region of NW Anatolia. Ironstones in the upper part of the Findikli Formation in Kabalakdere are characterized by alternating red and green mudstones and sandstones at the bottom, followed by a series of dolomite, dolomitic limestone with oolitic ironstones, and chamositic mudstones at the top. Conodonts from these carbonates indicate the delta-pesavis zones of the late Lochkovian. The 12- to 45-m-thick Ferizli Formation unconformably overlies the Findikli Formation with a quartz-arenite succession at the bottom. The formation comprises alternating red, iron-rich limestones and dolomitic limestones, where iron-rich bioclastic grainstones are more dominant than iron-rich oolitic grainstones. The dolomitic limestones in this succession mark the ensensis and hemiansatus zones of the middle Givetian age. Mineralogically, the carbonates are dominated by goethitized and chamositized fossil fragments and chamositic oolites. In the oolitic facies, the oolites are made up of iron-bearing carbonates/iron, the bioclast of micritized/ironized brachiopods, and crinoids, whereas the matrix includes goethite, brown iron-silicates, chamosite, sideritic oolites, quartz clasts, and brachiopods. Partial iron precipitation within microborings or precipitation along the spine holes on echinoid grains is observed in the bioclastic grainstone/biosparite facies. Iron peloids are also recognized in the grainstone facies. Iron precipitation could be explained as precipitation of transported and dissolved iron from a terrestrial environment under wet/subtropical climate conditions within oxidizing and increased pH conditions, or as dissolved iron transported by upwelling currents over the shelves and precipitated under an oxidizing environment. The cyclic occurrence of primary iron in a marine carbonate environment and its extensive distribution over large areas indicates that a controlling mechanism for iron-rich carbonates and mudstones could be related to the cooperation of climate, sea level, and oceanographic changes in the middle Givetian. During the late Lochkovian, the same or very similar controlling factors might have operated, where the alternation of red mudstones can be explained by lateral facies changes or changes in terrestrial/nutrient influx.

First African diamonds discovered in Algeria by the ancient Arabo-Berbers: History and insight into the source rocks

It is generally believed that the first diamonds ever found in Africa were discovered in South Africa in 1867. Actually, three diamonds had already been found in 1833 near Constantine (Algeria). One of these, still preserved, shows radiohalos that suggest an old age. It could be a Sahara diamond reworked in more recent sediments, possibly the Oligo-Miocene Numidian Flysch; however, this occurrence remains uncertain. The ancient Arabs or Berbers also knew of diamonds in the Reggane region (Algerian Sahara), at Bilād al-mās (▪: “country of the diamond”). Since 1975, some 1500 diamonds have been collected from the alluvial deposits of this area. A manuscript written in Arabic in 1851 mentions diamonds in this region and describes their source rock, looked for in vain by modern geologists. The description is unclear, but might refer to Devonian oolitic ironstones. Modern investigations would rather suggest a kimberlitic primary source with intermediate Early Cretaceous palaeoplacers.