Oolitic Texture Research Articles

Unraveling Parent Rock and Mineral Influences in Tropical Weathering Profiles: REE, Nd and Sr Isotopic Geochemistry

This study aims to investigate the effects of parent rock and minerals on lateritic weathering. The study presents X-ray diffraction (XRD), whole-rock geochemistry, and Nd-Sr isotopic data for examining two profiles, 10 and 12 m thick, respectively, that illustrate the regional tropical weathering status in the Midwest of Brazil. The profiles, developed from metasedimentary and sedimentary rocks, are constituted by saprolite, mottled horizon, lateritic duricrust, and oxisol. Across the profiles, the minerals controlling the weathering geochemistry are muscovite, microcline, quartz, kaolinite, hematite, goethite, and gibbsite. Red and yellow zones in the saprolite and mottled horizon as well as the lateritic duricrust with breccia/fragmental, pisolitic, and oolitic textures make profile 1 more complex. In contrast, profile 2 has an oxisol that mantles the homogeneous vermiform lateritic duricrust. Fe2O3, accumulated during surface weathering, is a potent element in the geochemical profile control since it forms the harder goethite to hematite lateritic duricrust, bearing most of the trace elements (As, Cu, Cs, Pb, Sc, Sr, Th, U, V, and Zn) with similar ionic radii and electrovalence. The LREE have affinity for the elements of the Fe2O3 group of the lateritic duricrust. On the other hand, the K2O group together with Zr and TiO2 e in the phyllite, saprolite, and mottled horizon of profile 1, are associated with the HREE. Additionally, in profile 2, the HREE are mostly associated with the Al2O3 group and the residual minerals in the oxisol. The indication that REE is associated with phosphates, zircon, rutile/anatase, cereanite, and muscovite/illite, which have variable weathering behavior, caused the REE fractionation to occur across and between the profiles. Despite the REE fractionation, the ƐNd(0) values along the profiles consistently maintain the signature of the parent rock. Muscovite and microcline weathering, in profiles 1 and 2, respectively, control the decrease in 87Sr/86Sr signatures of both profiles and the distinct radiogenic ratios. The development of lateritic duricrust in both profiles indicates a similar weathering intensity, although the gibbsite–kaolinite predominance in the oxisol of profile 2 highlights a geochemical reorganization under humid conditions, as well as near-intense soluble silica leaching.

The mineralogy and geochemistry of the Pirashkaft bauxite, Zagros belt, Iran: New inferences on the paleo-climate at the time of formation and on the nature of the parent rock

The Pirashkaft karst type bauxite is located in north of the Paskuhak anticline, Pirashkaft region, in the Fars province (Iran). The deposit is composed of six bauxite horizons, occurring between upper Cretaceous Sarvak and Gurpy Formations, and formed in the late Turonian. Mineralogical composition includes boehmite, hematite, diaspore, kaolinite, goethite, anatase and minor minerals like, gibbsite, spinel, rutile, and kaolinite. The major oxides in the bauxite horizons mainly comprise Al2O3 (46.93–61.34 wt%), SiO2 (5.78–14.60 wt%), Fe2O3 (11.22–28.00 wt%), and TiO2 (1.66–3.03 wt%). The bauxite has a pisolitic–oolitic texture. Pisoids are mainly composed of large Al-bearing hematite cores, surrounded by layers of gibbsite or boehmite. This structure suggests that bauxite formed under a humid climatic regime, punctuated by periodic arid periods. In the Pirashkaft deposit, LREEs vary between 136.49 ppm and 928.56 ppm, whilst HREEs range from 16.48 ppm to 59.5 ppm. Positive anomalies of Ce (Ce/Ce* between 1.08 and 2.36) indicate dominant oxidizing conditions during the bauxitization process. The Eu/Eu* anomalies of the bauxite samples vary between 0.44 and 0.92, this suggesting that parent material likely derived from two geochemically distinct source rocks. Stratigraphical, mineralogical, and geochemical evidences, such as the REE distribution between the bauxitic horizons and the Sarvak marly limestone, suggest that the underlying marly carbonates were one of the sources of the Pirashkaft bauxite.

Multitarget Intelligent Recognition of Petrographic Thin Section Images Based on Faster RCNN

The optical features of mineral composition and texture in petrographic thin sections are an important basis for rock identification and rock evolution analysis. However, the efficiency and accuracy of human visual interpretation of petrographic thin section images have depended on the experience of experts for a long time. The application of image-based computer vision and deep-learning algorithms to the intelligent analysis of the optical properties of mineral composition and texture in petrographic thin section images (in plane polarizing light) has the potential to significantly improve the efficiency and accuracy of rock identification and classification. This study completed the transition from simple petrographic thin image classification to multitarget detection, to address more complex research tasks and more refined research scales that contain more abundant information, such as spatial, quantitative and category target information. Oolitic texture is an important paleoenvironmental indicator that widely exists in sedimentary records and is related to shallow water hydraulic conditions. We used transfer learning and image data augmentation in this paper to identify the oolitic texture of petrographic thin section images based on the faster region-based convolutional neural network (Faster RCNN) method. In this study, we evaluated the performance of Faster RCNN, a two-stage object detection algorithm, using VGG16 and ResNet50 as backbones for image feature extraction. Our findings indicate that ResNet50 outperformed VGG16 in this regard. Specifically, the Faster RCNN model with ResNet50 as the backbone achieved an average precision (AP) of 92.25% for the ooids test set, demonstrating the accuracy and reliability of this approach for detecting ooids. The experimental results also showed that the uneven distribution of training sample images and the complexity of images both significantly affect detection performance; however, the uneven distribution of training sample images has a greater impact. Our work is preliminary for intelligent recognition of multiple mineral texture targets in petrographic thin section images. We hope that it will inspire further research in this field.

Beneficiation of Oolitic Iron Ore Sourced from Gara Djebilet using Coal-Based Direct Reduction prior to Magnetic Separation

A process involving coal-based direct reduction followed by wet low-intensity magnetic separation is presented in this paper with the objective of promoting the Fe content and reducing the P assay of phosphorus-rich oolitic iron ore. A final direct reduced iron powder assaying 92.5 wt% Fe and 0.2wt % P at a recovery rate of 95.9% was obtained when a mixture of ore-coal-CaO was reduced at 1200°C during 60 min in the presence of 7.5% sodium sulfate. In addition, microscopic analyses reported that the oolitic texture was completely destroyed, and most of the iron occurred in the metallic state. These findings suggest the ease utilisation of the ore mined from Gara Djebilet in the steelmaking industry as a substitute to scrap steel in electric arc furnace.

Petrography and geochemistry of the iron-rich rocks in the banded iron formation of the Chilpi Group, Central India: Implications on the level of oxygen in the Paleoproterozoic atmosphere before the “Proterozoic iron ore gap”

The Chilpi Group (2050–1850 Ma) in the Bastar Craton contains Banded Iron Formation (BIF) deposited immediately after the Great Oxidation Event but before the “Proterozoic iron ore gap”. Baseline geochemical data generated from this crucial period of Earth's history representing the transition from an initial high productivity period followed by productivity collapse, thereby delaying the evolution of biota, are interpreted in terms of the redox state of the ocean and atmospheric oxygen content. Presence of chamosite, greenalite and siderite in the ironstones of the Chilpi Group, identified during present analysis, provide valuable information regarding the redox state of the shallow sea. Geochemical analyses reveal high concentrations of Fe2O3total and SiO2 (average ~ 87.85 wt%) in iron-rich bands. Trace elements commonly enriched in detrital phases (e.g. Sc, Hf, Nb, Th and Zr) show good correlation with total REE concentration, but these have concentration below the cut-off limit defined for detrital sediments (except 3 samples). Higher concentration of these elements in greenalite/chamosite-rich samples indicates accumulation of these elements in greenalite/chamosite during primary precipitation. Most of the samples have low concentrations of Al2O3 (<5 wt%) and TiO2 (<0.5 wt%), but some chamosite-bearing samples show enrichments of Al2O3 (up to ~20 wt%). Post-Archean Australian Shale normalised rare earth elements with Y (REEY) patterns, showing a superchondritic Y/Ho ratio (average 32.15) and positive La, Gd and Y anomalies, indicate the preservation of seawater like signatures. Though a low positive Eu-anomaly, and Al/(Al + Fe + Mn) versus Fe/Ti plot suggest seawater signature with possible mixing of hydrothermal fluids and/or <10 % detrital components, preservation of seawater signatures and no involvement of high-temperature hydrothermal fluids are deduced from Eu/Sm versus Sm/Yb and Eu/Sm versus Y/Ho patterns. The Eu/Sm and Y/Ho ratios lower than the seawater and a mixing trend away from oceanic hydrothermal solution indicate possible mixing of freshwater with seawater for the observed REEY patterns. Redox-sensitive trace element ratios indicate a dysoxic to suboxic-anoxic condition in the depositional basin. Presence of oolitic textures and occurrence of chamosite support the shallow water (<60 m water depth) and anoxic condition in the depositional basin. The oxygen content of 10−3–10−5 times the present atmospheric level in the atmosphere is inferred for the period during the deposition of the BIF.

Petrographic and geochemical features of the B3 bauxite horizon (Cenomanian-Turonian) in the Parnassos-Ghiona area: A contribution towards the genesis of the Greek karst bauxites

The karst bauxites of the Parnassos-Ghiona Unit (Greece) are hosted within Upper Jurassic to Late Cretaceous carbonate rocks. Three bauxite horizons are intercalated within shallow water limestones. They are named B1, B2 and B3 from bottom to top. In this study we present the characteristics of the B3 horizon (Cenomanian-Turonian) sampled in Agia Anna, Variani and Shila 1 underground mines. The bauxite ore has a prevailing oolitic texture. Diaspore, boehmite, hematite and goethite are the major mineral components of the analyzed samples. Kaolinite, rutile and anatase are common minor phases. Pyrite occurs in a small number of samples. Ilmenite, rutile, chromite, zircon, baddeleyite and serpentine are common detrital minerals. Authigenic minor minerals are mostly represented by REE-phosphates, barite, and gypsum. The presence of pyrite and diaspore suggests that after a first stage of additional in situ weathering and bauxitization, the deposits were affected by early diagenetic reducing conditions (allowing the formation of pyrite) and significant diagenesis, resulting in diaspore formation. This three-stages genesis is reflected in complex correlations between major, minor and trace elements. The occurrence of detrital chromite and serpentine in the bauxite reflects mafic parental rocks. The mafic component likely originated from the ophiolite suites exposed in the vicinity of the Parnassos-Ghiona bauxites.

Mineralogical Characterization of Manganese Oxide Minerals of the Devonian Xialei Manganese Deposit

The Guangxi Zhuang Autonomous Region is an important manganese ore district in Southwest China, with manganese ore resource reserves accounting for 23% of the total manganese ore resource reserves in China. The Xialei manganese deposit (Daxin County, Guangxi) is the first super-large manganese deposit discovered in China. The Mn oxide in the supergene oxidation zone of the Xialei deposit was characterized using scanning electron microscopy (SEM), energy spectrometer (EDS), transmission electron microscopy (TEM, HRTEM), and X-ray diffraction analysis (XRD). The Mn oxides have a gray-black/steel-gray color, a semi-metallic-earthy luster, and appear as oolitic, pisolitic, banded, massive, and cellular textures. Scanning electron microscopy images show that the manganese oxide minerals are present as fine-spherical particles with an earthy surface. TEM and HRTEM indicate the presence of oriented bundled and staggered nanorods, and nanopores between the crystals. The Mn oxide ore can be classified into two textural types: (1) oolitic and pisolitic (often with annuli) Mn oxide, and (2) massive Mn oxide. Pyrolusite, cryptomelane, and hollandite are the main Mn oxide minerals. The potassium contents of cryptomelane and pyrolusite are discussed. The unit cell parameters of pyrolusite are refined.

Nb-Ta-rare earth element mineralization in titaniferous laterite cappings over Sung Valley ultramafic rocks in Meghalaya, India

The present paper reports a significant amount of Nb-Ta-REE in titaniferous laterite (TL) cappings developed over ultramafic rocks of the Sung Valley ultramafic-alkaline-carbonatite complex, Meghalaya, India. These TL cappings were observed in the vicinity of Mawiong and Lumkynthang villages, East Khasi Hills district, Meghalaya, India. The TL cappings are highly ferruginous having ochre red and pink colours with patches of chocolate brown colour. TL cappings, having thickness of 1 m to 5 m, are matrix-supported and exhibit oolitic texture. The distribution pattern, normalized to chondrite exhibits a strong enrichment of light rare earth elements (LREE) with a negative Eu anomaly. The total REE (REE + Y + Sc) in the TL ranges from 0.28% to 1.10% with an average of 0.49%. XRD analysis identified the REE minerals florencite-(Ce), florencite-(La), florencite-(Nd), and kobeite-(Y). These minerals were further supported by the Ce, Ti, Fe, and P peaks obtained by EDX analysis. The oxide concentrations in TL are TiO2 (14.56–34.01 wt%), Al2O3 (11.71–29.82 wt%), and Fe2O3(t) (10.12–37.62 wt%). The other significant trace elements identified are (in ppm): Cr (166–1089), Ni (38–599), V (644–3329), Sc (422–747), Nb (267–1971), Ta (15–201), Zr (440–1954), and Hf (27–88). The positive correlation of Al2O3 with REE suggests an increase in REE concentration with increasing bauxitization in TL. This study is expected to influence the exploration of other alkaline-carbonatite complexes that have laterite cappings.

Genesis of karst bauxite-bearing sequences in Baofeng, Henan (China), and the distribution of critical metals

Bauxite-bearing sequences are not only the main economic sources of Aluminum, but also of many critical metals. The distribution of critical metals was determined from six bauxite profiles in Baofeng, and an evolution model was proposed for the formation of bauxite-bearing sequences. Following a terrestrial deposition of the lower pisolitic bauxite horizon and the underlying clayey layer, transportation and redeposition of bauxitic materials during submergence led to the formation of the upper bauxite horizon presenting finely-clastic and oolitic textures, as well as the intercalated clayey layers. Both bauxite ores and clay samples contain highly elevated contents of Li, B, Sc, V, Ga, Zr, Nb, W and total REEs. Li contents are extremely low in high-grade bauxite ores (10.92–44.77 ppm), compared to highly elevated values in low-grade bauxite ores (35.81–1601 ppm) and clay samples (38.54–1965 ppm), enlightening a new potential resource for Li, the bauxite-bearing sequences resting on karstified carbonate. The distribution pattern of Ga is similar to that of Li, and the correlation analysis indicates that Ga is primarily associated with silicates, i.e. clay minerals. The vertical distribution of Li, B, V, Cr and Ga shares the same pattern, similar to that of K2O and SiO2, being highly enriched in clayey layers relative to bauxite horizons. Both Sc and total REEs are also highly enriched in Baofeng bauxite profiles, highlighting a potential Sc and REEs resource.

D/H of late Miocene meteoric waters in Western Australia: Paleoenvironmental conditions inferred from the δD of (U-Th)/He-dated CID goethite

Nineteen (U-Th)/He ages were determined for eight samples from a core drilled in an ore-grade channel iron deposit (CID) of the Robe Pisolite (Robe Formation) of Mesa J in Western Australia. With one exception, uncorrected ages of the analyzed aliquots range from 6.7(±0.4) Ma to 30.2(±3.1) Ma, while molar ratios of Th/U range from 0.42 to 5.06. The exception is an aliquot with an apparent age of 2.7Ma and Th/U of 5.70. A three-component mixing model involving one generation of goethite and two generations of hematite suggests that the age of crystallization of the oolitic goethites is ∼7(±1) Ma. If so, the goethites have effectively been closed systems for ∼7 million years and should preserve a stable hydrogen isotope record of late Miocene rainfall in the vicinity of Mesa J. Cenozoic movement of the Australian continent had placed Mesa J and environs in the subtropics at a paleolatitude of about 29°S during the late Miocene.Al-adjusted δD values of oolitic goethite in the eight CID samples range from −153‰ to −146‰ and imply that the δD of the late Miocene meteoric waters ranged from −61‰ to −53‰, with an average of −56‰. These relatively negative δD values might indicate that near-coastal, late Miocene rain was derived primarily from summer-season tropical cyclones with storm tracks that extended into the subtropics of western Australia. The postulated late Miocene tropical cyclones would have occurred more often and/or exhibited greater intensity at a paleolatitude of 29°S than is the case for modern sites at approximately 30°S on the west coast of Australia (e.g., Perth).Higher fluxes of meteoric water in the Miocene summers would have facilitated dissolution and removal of BIF-sourced silica with concomitant enrichment in oxidized Fe. Moreover, wetter late Miocene summers could have promoted multiple cycles of microbially mediated dissolution and recrystallization of Fe(III) oxides in the aerobic systems. The oolitic textures may be indicative of such recycling. However, the oolitic goethites of Mesa J were closed systems after ∼7Ma. Therefore, the climate in the vicinity of Mesa J seems to have changed in the late Miocene to conditions that did not favor widespread recycling of Fe (III) oxides—perhaps changing from seasonally wet to the modern dry climate.

Mineralogy, geochemistry, and ore formation of Nuwaifa bauxite deposit, western desert of Iraq

Nuwaifa Formation is a part of sequence stratigraphy that belongs to the Jurassic system exposed in the western desert of Iraq. The Jurassic system consists of Ubaid, Hussainiyat, Amij, Muhaiwir, and Najmah formations. Each formation is composed of basal clastic unit overlain by upper carbonate unit. Nuwaifa karst bauxite was developed in fossil karsts within the Ubaid Formation in areas where maximum intersection of fractures and faults exist. This bauxitization process affected the upper surface of the Ubaid limestone formation, which directly underlies the Nuwaifa bauxite Formation. Nuwaifa Formation represents karst-filling deposit that consists of a mixture of allochthonous (sandstone, claystone, and mudstone) and autochthonous lithofacies (bauxite kaolinite, kaolinitic bauxite, iron-rich bauxite, and flint clay). Most bauxite bodies occur within the autochthonous lithofacies and are lenticular in shape with maximum thickness ranges from few meters to 35 m and in some place up to 100 m. Petrographically, the bauxite deposit exhibits collomorphic-fluidal, pisolitic, oolitic, nodular, brecciated, and skeletal textures indicative of authigenic origin. Mineralogy boehmite and gibbsite are the only bauxite minerals; the former is dominant in the upper parts of the bauxite profiles, whereas the latter is dominant throughout the lower and middle part of the bauxite. Kaolinite, hematite, goethite, calcite, and anatase occur to a lesser extent. The study bauxites are mainly composed of Al2O3 (33–69.6 wt.%), SiO2 (8.4–42 wt.%), Fe2O3 (0.5–15.9 wt.%), and TiO2 (0.7–6.1 wt.%) with LOI ranging from 13.5 to 19.1 wt.%. Geochemical investigations indicate that the immobile elements like Al2O3, TiO2, Cr, Zr, and Ni were obviously enriched, while SiO2, Fe2O3, CaO, MgO, Zn, Co, Ba, Mn, Cu, and Sr were depleted during bauxitization process. The results of this study strongly suggest that the bauxite deposits of the Nuwaifa Formation are derived from the kaolinite of the Lower Hussainiyat Formation.

Mineralogy, geochemistry and the origin of high-phosphorus oolitic iron ores of Aswan, Egypt

The Coniacian-Santonian high-phosphorus oolitic iron ore at Aswan area is one of the major iron ore deposits in Egypt. However, there are no reports on its geochemistry, which includes trace and rare earth elements evaluation. Texture, mineralogy and origin of phosphorus that represents the main impurity in these ore deposits have not been discussed in previous studies. In this investigation, iron ores from three localities were subjected to petrographic, mineralogical and geochemical analyses. The Aswan oolitic iron ores consist of uniform size ooids with snowball-like texture and tangentially arranged laminae of hematite and chamosite. The ores also possess detrital quartz, apatite and fine-grained ferruginous chamosite groundmass. In addition to Fe2O3, the studied iron ores show relatively high contents of SiO2 and Al2O3 due to the abundance of quartz and chamosite. P2O5 ranges from 0.3 to 3.4wt.% showing strong positive correlation with CaO and suggesting the occurrence of P mainly as apatite. X-ray diffraction analysis confirmed the occurrence of this apatite as hydroxyapatite. Under the optical microscope and scanning electron microscope, hydroxyapatite occurred as massive and structureless grains of undefined outlines and variable size (5–150μm) inside the ooids and/or in the ferruginous groundmass. Among trace elements, V, Ba, Sr, Co, Zr, Y, Ni, Zn, and Cu occurred in relatively high concentrations (62–240ppm) in comparison to other trace elements. Most of these trace elements exhibit positive correlations with SiO2, Al2O3, and TiO2 suggesting their occurrence in the detrital fraction which includes the clay minerals. ΣREE ranges between 129.5 and 617ppm with strong positive correlations with P2O5 indicating the occurrence of REE in the apatite. Chondrite-normalized REE patterns showed LREE enrichment over HREE ((La/Yb)N=2.3–5.4) and negative Eu anomalies (Eu/Eu*=0.75–0.89). The oolitic texture of the studied ores forms as direct precipitation of iron-rich minerals from sea water in open space near the sediment-water interface by accretion of FeO, SiO2, and Al2O3 around suspended solid particles such as quartz and parts of broken ooliths. The fairly uniform size of the ooids reflects sorting due to the current action. The geochemistry of major and trace elements in the ores reflects their hydrogenous origin. The oolitic iron ores of the Timsha Formation represent a transgressive phase of the Tethys into southern Egypt during the Coniacian-Santonian between the non-marine Turonian Abu Agag and Santonian-Campanian Um Barmil formations. The abundance of detrital quartz, positive correlations between trace elements and TiO2 and Al2O3, and the abundance mudstone intervals within the iron ores supports the detrital source of Fe. This prediction is due to the weathering of adjacent land masses from Cambrian to late Cretaceous. The texture of the apatite and the REE patterns, which occurs entirely in the apatite, exhibits a pattern similar to those in the granite, thus suggesting a detrital origin of the hydroxyapatite that was probably derived from the Precambrian igneous rocks. Determining the mode of occurrence and grain size of hydroxyapatite assists in the maximum utilization of both physical and biological separation of apatite from the Aswan iron ores, and hence encourages the use of these ores as raw materials in the iron making industry.

A mixed hydrogenous and hydrothermal origin of the Bahariya iron ores, Egypt: Evidences from the trace and rare earth element geochemistry

This study utilizes the trace and rare earth element geochemistry of bulk ores and in situ LA-ICP-MS analyses of mineral grains to clarify the controversy in the origin of the Bahariya sedimentary iron ores, Egypt. Different types of iron ores were identified in the Bahariya Oasis including high grade, high-Mn, high-Ba, oolitic, and ochreous iron ores. Hematite and goethite are the main constituents of the studied ores with some manganese oxide and hydroxide minerals (pyrolusite, bixbyite, cryptomelane, aurorite, romanechite, manjiroite, and pyrochroite). Barite is common in the high-Ba ore, while some quartz, calcite, and halite are detected in the oolitic iron ore. High-Mn iron ores are characterized by higher contents of SiO2, Al2O3, MgO, CaO, Na2O, and K2O, while high-Ba iron ore has higher TiO2 contents. Analyses of bulk ores and in situ mineral grains are characterized by substantially high contents of Ba (average of 9497ppm); also Zn and Sr occur in considerably high concentrations (averages of 4263 and 429ppm, respectively). Mn-rich ores and Mn-bearing minerals show relative enrichments of trace metals compared to the Mn-poor ores probably due to the ability of Mn-bearing minerals to fix such trace metals by adsorption, absorption, and/or replacement. The ΣREE ranges between 2.6 and 80ppm with an average of 23ppm. High-Mn ores show higher ΣREE (average of 61ppm) compared with the low Mn ore (average of 13ppm). The oolitic iron ore shows very low ΣREE content (7ppm). Red and yellow ochers from El Gedida mine have similar ΣREE values (38 and 39ppm, respectively), while red ocher from the Ghorabi area has relatively higher ΣREE (57ppm). Chondrite-normalized REE patterns of all types of iron ores, whether in the form of bulk ores or Fe- and Mn-bearing grains, have LREE enrichment relative to HREE as shown by (La/Yb)N ratios that vary from 1.7 to 29.4. Majority of the bulk samples and mineral grains have negative Eu anomalies with EuN/Eu* ranges from 0.68 to 0.8. However, bulk samples of one high grade and high-Ba ores as well as some of the Mn-bearing grains from El Gedida ores show positive Eu anomalies with Eu/Eu* ranges from 1.1 to 17.7. With few exceptions, all bulk ores and mineral grains show negative Ce anomalies with Ce/Ce* range from 0.28 to 0.96. A seawater precipitation (hydrogenous to hydrothermal exhalite) is proposed for the Bahariya iron ores. The hydrogenous origin is suggested based on the occurrence of high-Mn iron ores in the base of the iron ore succession, oolitic texture of some of these ores, Si–Al plot, low La/Ce ratios, high Y/Ho ratios, and LREE-enriched patterns with negative Eu anomalies of most of the bulk ores and mineral grains. The hydrothermal contribution to the source of these iron ores can be evident from the high contents of some trace elements such as Ba, Zn, and Mo, and plots the analyzed samples in the hydrothermal fields of the Fe–M–(Ni+Co+Cu)∗10 ternary diagram, (Co+Ni)–(As+Cu+Mo+Pb+V+Zn) and (Co+Ni+Cu–Co/Zn) binary plot, low ΣREE concentrations and positive Eu anomalies and high La/Ce ratios in some of the analyzed samples. The hydrothermal contribution looks for local effect and is restricted to El Gedida area.

Mechanism of phosphorus removal in beneficiation of high phosphorous oolitic hematite by direct reduction roasting with dephosphorization agent

High phosphorous oolitic hematite ore is one of typical intractable iron ores in China, and the conventional beneficiation methods are found to be impracticable to remove phosphorus from the ore effectively. Better beneficiation index were gotten by direct reduction roasting with dephosphorization agent followed by two stages of grinding and magnetic separation. P content decreases from 0.82% in the raw ore to 0.06% in the magnetic concentrate, and the total iron grade increases from 43.65% to 90.23%, the recovery of iron can reach 87%. Mechanisms of phosphorus removal in the beneficiation of high phosphorous oolitic hematite ore by direct reduction roasting with dephosphorization agent were studied using XRD, SEM and EPMA. The results showed that about 20% of the apatite in the raw ore transferred into phosphorus and volatilized with the gas in the process of reduction roasting, while the rest 80% apatite was not involved in the reaction of generation of phosphorus, and remained as apatite in the roasted products, which was removed to tailings by grinding and magnetic separation. A small amount of phosphorus existed in the magnetic concentrate as apatite. The oolitic texture of raw ore was partly changed during roasting, resulting in the formation of nepheline in the reaction between the dephosphorization agent, SiO2 and Al2O3 in the raw ore, which greatly improved the liberation degree of minerals in the roasted products, and it was beneficial to the subsequent grinding and magnetic separation.

Formation stages of the unique sangaredi bauxite-bearing group, Futa Jallon-Mandingo province, West Africa

The middle Miocene continental fluvial sediments of the Sangaredi Group in the western districts of the Futa Jallon-Mandingo bauxite-bearing province were initially more widespread. Their thickness in the axial zones of the main drainage systems (Paleocogon, Paleotomine, etc.) attained 150 m. The lower, early Miocene steps of topography formed during the first phase of regional neotectonic evolution were buried beneath these sediments by the end of middle Miocene. The sediments of the Sangaredi Group underwent substantial epigenetic alteration in the glei geochemical setting with removal of iron and bleaching. At the second stage of neotectonic uplifting and dissection, this low-Fe and high-Al protolith was again affected by lateritic weathering with formation of unique sedimentary-lateritic bauxites. The underlying bedrock was also affected by bleaching and lateritization with formation of high-quality infiltration-metasomatic bauxites with geliform cryptocrystalline structure and massive porcelainlike or oolitic texture. Both classes of bauxites are high-quality and of great economic importance.

Geology and petrography of the Masatdagi diasporic bauxites, Alanya, Antalya, Turkey

There are more than fifteen bauxite outcrops in the study area located in the southern part of the Central Taurides. The bauxites are developed in the Upper Permian Cebireis Formation (carbonate rocks interbedded with schist) and Lower Triassic Asmaca Formation (limestone, dolomite, quartzite and schist). The bauxite outcrops are observed as parts of two or three independent bauxite blocks that were broken apart by faulting and an erosional surface. Based on outcrop appearances, four ore types can be defined; black, earthy, clayey, and Fe- and Mn-bearing bauxites. Oolitic textures are common in all ore types, except the earthy ones. Bauxites are represented by a mixture of diaspore and clay in varying proportions. Diaspore, boehmite, gibbsite, corundum, hematite, goethite, amorphous iron hydroxide, magnetite, chamosite, pyrite, marcasite, psilomelane, rutile, kaolinite, illite, muscovite, hydro-muscovite, chlorite, chloritoid, rectorite, ephesite, albite, microcline, quartz, K–Mg–Al silicate, hydrated K–Al silicate, and dolomite are present in the ore paragenesis. Bauxites, which are observed at or near the contact between the Cebireis and Asmaca formations, are thought to be fillings of cavities within and/or on Late Triassic—early Cretaceous carbonate rocks.

Remanent magnetization of oolitic ironstone beds, Hazara area, Lesser Himalayan thrust zone, Northern Pakistan: Its acquisition, timing, and paleoenvironmental implications

Chamosite-hematite type oolitic ironstone is distributed in the Cretaceous-Tertiary (K-T) boundary zone of the Hazara area, Lesser Himalayan thrust zone, in the northwestern margin of the Indo-Pakistani subcontinent. A total of 52 oriented samples were collected from 6 beds in two open pit mines, Nathia Gali and Bagnotar. Ferromagnetic properties of the oolitic ironstone are dominated by fine-grained hematite that possesses a characteristic remanent magnetization (ChRM). The ChRM is a chemical remanent magnetization (CRM) acquired by the hematite which formed from Fe-rich initial materials (iron hydroxides) during early diagenesis of the ironstone bed. The direction of the remanent magnetization indicates that the magnetization was acquired in an equatorial region (average inclination = 2°, paleolatitude = 1°N), which is consistent with the paleoposition of the Indian subcontinent during early Paleocene time, as estimated by previous data. The initial Fe-rich sediments were deposited under anoxic conditions, probably in a non-marine or brackish environment. During early diagenesis, after development of oolitic textures, the paleoenvironment became arid, where the CRM was acquired through the conversion of amorphous hydroxides or goethite to fine-grained hematite. The paleomagnetic results reveal complicated local rotational movements of oolitic ironstone blocks about vertical axes, which might be a result of intense thrusting within the terrane after early Tertiary collision between India and Asia.

Very high 18o enrichment in Archean cherts from South India: implications for Archean ocean temperature

ABSTRACTStable carbon and oxygen isotopic compositions of essentially unmetamorphosed Archean (&gt; 2.6 Gyr old) cherts and carbonates of the Dharwar Sequence of southern India, from the northernmost part of the Dharwar‐Shimoga supracrustal belt (Kalche and Nagargali), have been determined. The cherts from the Nagargali area, which preserve oolitic texture and cryptocrystalline silica, show highly enriched δ18O values ranging from 28 to 31.4%o relative to SMOW. Such values are the highest yet reported from Archean nondetrital sediments, but are similar to those of modern marine cherts. On the assumption of a seawater δ18O of 0%0, calculation of temperature based on the maximum δ18O value of 31.4%0 yields a value of 40°C. This is significantly less than 70–80°C reported for the Archean oceans based on cherts and chert‐phosphate pairs. Diagenetically recrystallized microcrystalline chert‐dolomite pairs of Kalche area exhibit a range of oxygen isotopic ratios similar to those reported for Archean cherts and carbonates from other parts of the world. The temperature of diagenesis is estimated to be about 68°C.

An ancient manganese-iron deposit of freshwater origin, Islay, Argyllshire

Synopsis Veins composed of manganese and iron oxides occur within the Dalradian Jura Quartzite (Argyll Group) on the SW coast of Islay. The manganese oxides, which are composed principally of hollandite with minor pyrolusite and rare groutite, frequently display a colloform texture centred upon silicate grains. In a single section an oolitic texture was displayed by hollandite in a hematite groundmass. Hematite and goethite, in variable amounts, constitute the iron oxides, and usually enclose fragments of the host quartzite. The manganese and iron oxides frequently occur as separate, discrete patches within the vein material. The geochemistry of the oxides shows that, of the target and pathfinder elements, only Ba attains elevated levels (up to 4.9%). The absence of enrichments in the diagnostic hydrothermal assemblage (As–Ba–Cu–Mo–Pb–Sb–Sr–V–Zn) and in the marine assemblage (Na–K–Ca–Mg–Sr; Co–Cu–Ni) imply that this is a freshwater deposit. This conclusion was confirmed by discriminatory scatter plots of Na vs Mg and (Co + Ni) vs (As + Cu + Mo + Pb +V + Zn) which indicate a fresh-water, non-hydrothermal origin for the veins. The oxides were deposited from groundwater which penetrated the Jura Quartzite via a local fault system probably in Permo-Triassic time. The veins have no economic potential as manganese ore and the prospect of related base or precious metal mineralisation is remote.

Banded Iron-Formations through Much of Precambrian Time

Similarities between Proterozoic (~ 1.8-2.5 Gyr) and Archean (> 2.5 Gyr) banded iron-formations are probably more significant than their differences. The contrasts largely reflect differences in the tectonic settings of Proterozoic and Archean terrains. Archean banded iron-formations are not as thick nor laterally as extensive as the major Proterozoic iron-formations. Nevertheless, some Archean iron-formations have strike lengths of over 150-200 km and may have been quite extensive prior to the deformation that has affected most Archean terrains. Stratigraphic sequences in which iron-formations occur are highly variable and indicate that iron-formations formed in many depositional environments. Sedimentary textures in the iron-formations are dominated either by granules and oolites or laminations (including microbanding) reflecting differences in their physical conditions of deposition. Granular and oolitic textures are abundant in only three Proterozoic depositional basins and most Precambrian iron-formations are laminated. Despite differences in associated lithologies and sedimentary textures Precambrian iron-formations have similar bulk compositions and mineral assemblages, implying that the chemical conditions of iron-formation deposition were similar through much of the Precambrian. The formation of banded iron-formation appears not to have reached a maximum around 1.8-2.0 Gyr but to have been an important process over a long period in the Precambrian.