Ore Group Research Articles

A 3.51 Ga bimodal volcanics‐BIF‐ultramafic succession from Singhbhum Craton: implications for Palaeoarchaean geodynamic processes from the oldest greenstone succession of the Indian subcontinent

The Palaeoarchaean greenstone belt of the southern Iron Ore Group (SIOG) (3.51 Ga) in the Singhbhum Craton, eastern India, includes low‐strained and low‐greenstone grade bimodal volcanics, Banded Iron Formation (BIF) and chromiferous ultramafics as enclaves within tonalite–trondjhemite–granodiorite (TTG) granitoids, collectively referred to as the Singhbhum Granite (3.4 Ga to 3.1 Ga). The succession comprises, from base to top, a lower unit of massive and pillowed basalt conformably overlain by dacitic lava and pyroclastics which in turn is overlain by a major BIF unit. The ultramafics are juxtaposed with the volcanics‐BIF succession along a thrust fault. The lithological association of pillow lava, subaqueous dacitic lava and pyroclastic rocks and BIF collectively, suggests that the entire succession was deposited in a deep‐marine depositional setting. The ash‐poor dacitic volcanic rock succession with evidences of a transition from suppressed‐volatile deep‐water lava flow and pyroclastics to more evolved mass‐flow deposits with increasing trend of subaqueous flow transformation, records a transition from a deep‐water low‐height volcanic chain to a shallower subaqueous eruption in an aggradational volcanic chain. Geochemical proxies from the bimodal volcanics and ultramafics showing enrichment of La/Nb, Th/Nb, Th/La, Ba/La, Pb/Ce, depletion in Nb–Ta relative to neighbouring REE, together with tectonic discrimination criteria using Nb, Y, Zr, Ti compositions, suggest an extending oceanic arc–forearc geodynamic setting similar to many of the Phanerozoic supra‐subduction zone ophiolites where ophiolite development in the extending upper plate in a relatively short time span is facilitated by slab rollback processes. The positive Eu‐anomaly together with high Y/Ho values from the BIFs also suggests their deposition in close proximity to spreading centres that might have developed over a rifted arc. The bimodal volcanic rock‐BIF‐ultramafic succession of the SIOG with evidence of a convergent margin geodynamic setting is an important example for Palaeoarchaean plate tectonic processes operating on Earth. Copyright © 2011 John Wiley & Sons, Ltd.

Variation in U-238 and Th-232 enrichment in U-mineralized zone and geological controls on their spatial distribution, Singhbhum Shear Zone of India

The determination of the natural radioactivity has been carried out, by means of gamma-ray spectrometry, in exposed rock samples collected from various geological formations across the Singhbhum Shear Zone of India. The spatial variation in the concentration of primordial radionuclides reflects, in general, the geochemical variance among the analyzed rocks and affirms the earlier views on genesis of uranium mineralization. This study further speculates new possibilities of thorium depletion during shearing and metamorphism. All the obtained U-values were more than the worldwide average U-concentration (which is about 40 Bq/kg). Interestingly, U-concentration in mica schists from Singhbhum group of rocks and in slates and argillaceous quartzites from Iron Ore group of rocks was five to seven times higher than the global values. In contrast, the granites in study area showed comparatively lower U-concentration. The overall results suggest the control of complex metamorphism and shearing on U-distribution in the terrane. The contribution of 238U, 232Th and 40K to the total radiation dose in the study area was found to be significant ranging from 110 to 310 nGy/h in comparison with the lower global, as well as Indian average values.

Detrital type manganese ore bodies in the iron ore group of rocks, Orissa, Eastern India

Detrital type of manganese ore bodies in the Precambrian Iron Ore Group of rocks occur in the Bonai-Keonjhar belt, Orissa besides stratiform (bedded type) and stratabound-replacement types of deposits. These ores appear in form of large boulders within lateritised aprons at various depths, often reaching beyond 30 m from the surface. Overprinting of primary structures, presence of mixed Fe-clasts and Mnooliths/pisoliths, mineral species of different generations and wide chemical variation amongst morphological varieties and from boulder to boulder are the characteristic hallmarks of such ore bodies. Features associated with ores occurring in different morphologies, namely: spongy, platy, recemented, and massive varieties from a typical profile of Orahari Mn-ore body in Keonjhar district are described. Recemented variety may be further classified into sub-varieties such as canga, agglomerate, and mangcrete. Common primary Fe-minerals are hematite, martite with relict magnetite. The secondary Fe-Mn phases are goethite, specularite, cryptomelane, lithiophorite, chalcophanite, manganite, and pyrolusite.These are ore bodies of allochthonous nature developed through a number of stages during terrain evolution and lateritisation. Secondary processes such as reworking of pre-existing crust through remobilisation, solution, precipitation, cementation, transport, etc. are responsible for the development of such detrital ore bodies in the Bonai-Keonjhar belt of Eastern India.

Mineralogical Characteristics of Iron Ores in Joda and Khondbond Areas in Eastern India with Implications on Beneficiation

Precambrian iron ores of the Singhbhum-North Orissa region occur in eastern India as part of the Iron Ore Group (IOG) within the broad horse-shoe shaped synclinorium. More than 50% of Indian iron ore reserves occur in this region. Massive-hard, flaky-friable, blue dust and lateritic varieties of iron ores are the major ore types, associated with banded hematite, jasper and shales. These ores could have formed as a result of supergene enrichment through gradual but extensive removal of silica, alumina and phosphorus from banded iron formations and ferruginous shale. Attempts for optimal utilization of these resources led to various ore characterization studies using chemical analysis, ore and mineral petrography, XRD analysis, SEM and electron probe micro analysis (EPMA). The ore chemistry indicates that the massive hard ores and blue dust have high iron, low alumina and phosphorus contents. Because of high quality, these ores do not require any specialized beneficiation technique for up-gradation. However, flaky-friable, lateritised and goethitic ores are low in iron, high in alumina and phosphorus contents, requiring specific beneficiation techniques for up-gradation in quality. XRD, SEM and ore microscopic studies of massive hard ores indicate the presence of hematite and goethite, while flaky and lateritic ores show a higher concentration of goethite, kaolinite, gibbsite and hematite. EPMA studies show the presence of adsorbed phosphorous as fine dust in the hard ores. Sink and float studies reveal that most of the gangue minerals are not completely liberated in the case of goethitic and lateritic ores, even at finer fractions.

New U-Pb zircon ages from Paleo-Mesoarchean TTG gneisses of the Singhbhum Craton, Eastern India

Two new U–Pb ages of 3448 ± 19 Ma and 3527 ± 17 Ma were obtained on zircons from the Older Metamorphic Tonalite Gneisses (OMTG), Singhbhum North Orissa Craton, Eastern India. These dated TTG-type OMTG gneisses are from the least studied northern area of the Singhbhum Craton. They occur as enclaves in the Singhbhum granite batholith (SBG), emplaced during two distinct periods: from 3440 to 3330 Ma (SBG-A) and from 3200 to 3050 Ma (SBG-B). The U–Pb age of 3448 ± 19 Ma age is close to the older SBG-A phase. The U–Pb age of 3527 ± 17 Ma could corresponds to an older intrusive or inherited from a xenocryst population. Dacitic lava from the southern BIF-bearing Iron Ore Group (IOG) greenstone belt has been recently dated at 3506 ± 2.3 Ma suggesting that it is older than both phases of the SBG. It is thus possible that the metasedimentary rocks of the Older Metamorphic Group (OMG), occurring as enclaves in both OMTG and SBG suites, might be partly derived from the IOG rocks.

Superposed deformation fabrics in the Precambrian metabasic rocks of the Iron Ore Group, Singhbhum craton, Eastern India: Evidences from anisotropy of magnetic susceptibility studies

Anisotropy of magnetic susceptibility (AMS) data have been utilized to build up the multiple deformation sequences and strain variation from an apparently undeformed metabasic lava sequence of the basal part of the Paleo-Mesoarchean greenstone succession of the Iron Ore Group rocks in the western Iron Ore basin, Singhbhum craton, eastern India. The basal lava crops out bordering the regional horseshoe shaped outcrop (the horseshoe synclinorium structure) of the overlying BIF rocks. AMS data of 892 cylindrical specimens drilled from 83 samples collected from different structural domains within the synclinorium structure are presented in the analysis. AMS orientation data have been analyzed in the light of superposed fold (Type-1 interference) fabrics developed within BIF as a result of successive deformation episodes (D 1 and D 2). It is found that the lava samples ubiquitously present two consistent magnetic fabrics homogeneous over smaller sub-areas, which have been successfully correlated with the D 1 and D 2 deformation fabrics from the BIF rocks. Moreover, a regional scale variation in intensity and nature of strain is noticed from the AMS data, which have been utilized to decipher folding mechanism as well as the changing deformation pattern during successive folding events.

Mineralogy and geochemistry of banded iron formation and iron ores from eastern India with implications on their genesis

The geological complexities of banded iron formation (BIF) and associated iron ores of Jilling-Langalata iron ore deposits, Singhbhum-North Orissa Craton, belonging to Iron Ore Group (IOG) eastern India have been studied in detail along with the geochemical evaluation of different iron ores. The geochemical and mineralogical characterization suggests that the massive, hard laminated, soft laminated ore and blue dust had a genetic lineage from BIFs aided with certain input from hydrothermal activity. The PAAS normalized REE pattern of Jilling BIF striking positive Eu anomaly, resembling those of modern hydrothermal solutions from mid-oceanic ridge (MOR). Major part of the iron could have been added to the bottom sea water by hydrothermal solutions derived from hydrothermally active anoxic marine environments. The ubiquitous presence of intercalated tuffaceous shales indicates the volcanic signature in BIF. Mineralogical studies reveal that magnetite was the principal iron oxide mineral, whose depositional history is preserved in BHJ, where it remains in the form of martite and the platy hematite is mainly the product of martite. The different types of iron ores are intricately related with the BHJ. Removal of silica from BIF and successive precipitation of iron by hydrothermal fluids of possible meteoric origin resulted in the formation of martite-goethite ore. The hard laminated ore has been formed in the second phase of supergene processes, where the deep burial upgrades the hydrous iron oxides to hematite. The massive ore is syngenetic in origin with BHJ. Soft laminated ores and biscuity ores were formed where further precipitation of iron was partial or absent.

Manganese ore deposits in Koira-Noamundi province of Iron Ore Group, north Orissa, India: In the light of geochemical signature

Several small Mn–Fe oxide and Mn-oxide ore bodies associated with Precambrian Iron Ore Group of rocks are located within Koira-Noamundi province of north Orissa, India. These deposits are classified into in situ (stratiform), remobilized (stratabound) and reworked categories based on their field disposition. Volcaniclastic/terrigenous shale in large geographic extension is associated with these ore bodies. The in situ ore bodies are characterised by cryptomelane-, romanechite- and hematite-dominating minerals, low Mn/Fe ratio (1.1) and relatively lower abundance of trace (1500–2500 ppm) constituents. In such type of deposits the stratigraphic conformity of oxides with the tuffaceous shale suggests precipitation of Mn and Fe at a time of decreased volcaniclastic/terrigenous contribution. The minor and trace elements were removed from solution by adsorption rather than by precipitation. Both Mn and Fe oxides when precipitated adsorb trace elements strongly but the partitioning of elements takes place during diagenesis. The inter-elemental relationship reveals that Cu, Co, Ni, Pb and Zn were adsorbed on precipitating hydrous Mn oxides and form manganates. Some of these elements probably get desorbed from Fe oxide because of their inability to substitute for Fe 3+ in the lattice of its oxide. However, P, V, As and Mo were less partitioned and retained in Fe-oxide phase. Positive correlation between Al 2O 3 and SiO 2, MgO, Na 2O, TiO 2 and some traces like Li, Nb, Sc, Y, Zr, Th and U points to their contribution through volcaniclastic/terrigenous detritus of both mafic and acidic composition. The remobilized ore bodies are developed in a later stage through dissolution, remobilization and reprecipitation of Mn oxides in favorable structural weak planes under supergene environment. Increase in average Mn/Fe ratio (8) and trace content (5000–8500 ppm) by 5–2.5 orders of magnitude, respectively, or more above its abundance in adjoining/underlying protore is characteristic of these deposits. The newly formed Mn ores constituting lithiophorite, cryptomelane/romanechite and goethite get quantitatively enriched in traces like Cu, Co, Ni, Pb and Zn. Positive correlation between Mn, Li, Co and Zn is due to the formation of mineral of lithiophorite–chalcophanite group during redistribution and reconcentration of Mn oxide. P and V, which were present in Fe oxide, also get dissolved and reprecipitate with Fe oxyhydroxide in these ores. Some other elements like Y, Th and U show positive relation with Fe. This is probably due to leaching of these elements during chemical weathering of associated shale and getting re-adsorbed in Fe-oxyhydroxide phase. However, under oxidizing environment selective cations like Ba, K, etc. resorb from Mn-structure, resulting in the development of pyrolusite (Mn/Fe>20). In such transformation, trace metals from pyrolusitic structure expels out, resulting thereby in a considerable reduction in total trace value (<3000 ppm). The reworked ore bodies are allochthonous in nature and developed through a number of stages during terrain evolution and lateritisation. Secondary processes such as reworking of pre-existing crust; solution and remobilization; precipitation and cementation and transport, etc. are responsible for their development. Such deposits are usually very low in Mn/Fe ratio (3) and trace content (<2000 ppm).

Radioactive quartz-pebble conglomerates from western margin of Bonai granite pluton, Sundargarh district, Orissa — A new find

Several radioactive quartz-pebble conglomerate (QPC) occurrences at the western margin of Archaean Bonai granite and overlying Iron Ore Group (IOG) rocks have recently been located over a total strike length of 8-10 km intermittently in a NE-SW to E-W trend with steep dips due north- west to north in parts of Sundargarh district of Orissa. The QPC samples have analysed up to 0.039% U 3 O 8 and 0.035% ThO 2 with high concentration of Y (74 to 518 ppm), La (<100 to 880 ppm), Cr (126 to 633 ppm), Zr (137 to 1250 ppm) and Pb (31 to 581 ppm). Cellulose Nitrate (CN) film studies of few QPC samples indicated adsorbed uranium over goethite and infiltrated ferruginous material (limonite), secondary uranium as encrustation and fracture filling and discrete sub-rounded grains of monazite, zircon, allanite and rare xenotime in the matrix of QPC as radioactive phases. Higher content of Th over U, elevated concentration of Y and La in QPC eliminates the possibility of its low temperature product by epigenetic processes. Poor correlation of U with elements like Pb, Y, Zr, La and Cr can be explained due to surficial leaching of uranium from QPC after its deposition as reflected by adsorbed U over iron- oxides and low U/Th ratio in QPC in the area.

Chromite and PGE deposits of mesoarchaean ultramafic-mafic suites within the greenstone belts of the Singhbhum craton, India: Implications for mantle heterogeneity and tectonic setting

The Archaean cratonic nuclei of the continents are important as they contain the most significant evidences for the evolution of Earth e.g. the greenstone sequences. In the Indian Shield, one of the important cratons is the Singhbhum craton, where nearly 95% of the Indian chromite deposits and only PGE deposits are located which are hosted within Mesoarchaean ultramafic-mafic rock sequences. The ultramafic units occur as sill like intrusions within the Iron Ore Group (IOG) greenstone belts and often associated with gabbroic intrusions. In the Nuasahi and Sukinda mining districts of these occurrences, detailed petrological, geochemical and isotopic studies have been carried out in the last decades. Petrological and geochemical studies indicate a supra-subduction zone (SSZ) tectonic settings in Archaean for the origin of these ultramafic-mafic sequences. The Os isotopic and platinum group element (PGE) geochemical studies of chromites from the two mining districts indicate presence of a subchondritic source mantle domain beneath and within the Singhbhum craton similar to the Zimbabwean craton of southern African continent. The Os model age calculation indicates melt extraction from a subcontinental lithospheric mantle (SCLM) before 3.7 Ga which is similar to the other ancient cratons. As a whole the study supports the premise that India was part of the African continent in pre-Gondwana times and even in early Archaean and suggest possible amalgamation and building up of a supercontinent during late Archaean. However, in comparison with other occurrences, the Singhbhum craton of the Indian Shield and the Zimbabwean craton in southern Africa are characterized by the presence of subchondritic lithospheric mantle domains within the SCLM, which were developed prior to 3.7 Ga.

Detrital iron-ore deposits in the Iron Ore Group of rocks, northern Orissa, eastern India

Detrital iron deposits (DID) are located adjacent to the Precambrian bedded iron deposit (BID) of Joda near the eastern limb of the horseshoe-shaped synclinorium, in the Bonai–Keonjhar belt of Orissa. The detrital ores overlie the Dhanjori Group sandstone as two isolated orebodies (Chamakpur and Inganjharan) near the eastern and western banks of the Baitarani River, respectively. The DID occur as pebble/cobble conglomerates containing iron-rich clasts cemented by goethite. Mineralogy, chemistry and lamination of these clasts are similar to that found in the nearby BID ores. Enrichment of trace and rare-earth elements in the DID relative to the BID is attributed to their concentration during the precipitation of cementing material. The detrital iron orebodies formed when Proterozoic weathering processes eroded pre-existing BID outcrops located on the Joda Ranges, and the resulting detritus accumulated in the paleochannels. In situ dissolution in association with abundant organic material produced Fe-saturated groundwater, which re-precipitated as goethite within the aggraded channel to cement the detritals. Growth of microplaty hematite in the goethite matrix suggests some level of subsequent burial metamorphism.

Stratabound magnetite deposits from the eastern outcrop belt of the Archaean Iron Ore Group, Singhbhum craton, India

Most high-grade banded iron formation (BIF)-hosted iron ores are composed almost exclusively of microplaty haematite and martite-textured haematite, i.e. pseudomorphs of haematite after magnetite. Goethite abounds only where haematite-rich iron ores are affected by geologically recent lateritic weathering. Magnetite and kenomagnetite occur usually only as small, irregular remnants enclosed in martite. Although low-grade magnetite–carbonate ores have been discovered in spatial association with high-grade iron ore bodies, high-grade BIF-hosted iron ores composed predominantly of magnetite have as yet only been reported from the Iron Quadrangle in Minas Gerais (Brazil). In this contribution, the authors describe such magnetite-rich high-grade BIF-hosted iron ores from the Gorumahishani deposit in the state of Orissa, India. The high-grade magnetite ores occur as stratabound bodies hosted by a single prominent BIF unit of the Mesoarchaean Iron Ore Group (IOG). The ores are composed of an interlocking xenotopic mosaic of fine-grained magnetite, accompanied by trace amounts of Fe-rich talc, sulphides and carbonaceous matter. The occurrence of the magnetite ores is very similar to that of economically significant microplaty haematite–martite ore bodies hosted by the same BIF from other parts of the IOG; both are regarded as being of hydrothermal–metasomatic origin, formed at the expense of the iron formation protolith. The abundance of magnetite, combined with the presence of sulphides, carbonaceous matter, and a marked enrichment of Ni, Co and Zn are attributes that differentiate the magnetite-rich ores from haematite–martite counterparts. These differences are tentatively attributed to differences in hydrothermal fluid composition. Based on the available evidence, it is suggested that magmatic–hydrothermal fluids derived from closely associated mafic intrusives were instrumental in the formation of the magnetite orebodies of the Gorumahishani deposit. The abundance of magnetite and the presence of sulphides and carbonaceous matter indicate that the hydrothermal fluid was mildly reducing, rather than oxidising as proposed for haematite-rich assemblages. The abundance of martite in the coexisting haematite–martite orebodies of other parts of the IOG may be used to speculate that the formation of massive magnetite bodies may have been an essential intermediate stage during the hydrothermal formation of high-grade iron ore deposits of the IOG.

Rare Earth Element Enrichment in Late Archean Manganese Deposits from the Iron Ore Group, East India

AbstractThe major, trace and rare earth element (REE) composition of Late Archean manganese, ferromanganese and iron ores from the Iron Ore Group (IOG) in Orissa, east India, was examined. Manganese deposits, occurring above the iron formations of the IOG, display massive, rhythmically laminated or botryoidal textures. The ores are composed primarily of iron and manganese, and are low in other major and trace elements such as SiO2, Al2O3, P2O5 and Zr. The total REE concentration is as high as 975 ppm in manganese ores, whereas concentrations as high as 345 ppm and 211 ppm are found in ferromanganese and iron ores, respectively. Heavy REE (HREE) enrichments, negative Ce anomalies and positive Eu anomalies were observed in post‐Archean average shale (PAAS)‐normalized REE patterns of the IOG manganese and ferromanganese ores. The stratiform or stratabound shapes of ore bodies within the shale horizon, and REE geochemistry, suggest that the manganese and ferromanganese ores of the IOG were formed by iron and/or manganese precipitation from a submarine, hydrothermal solution under oxic conditions that occurred as a result of mixing with oxic seawater. While HREE concentrations in the Late Archean manganese and ferromanganese ores in the IOG are slightly less than those of the Phanerozoic ferromanganese ores in Japan, HREE resources in the IOG manganese deposits appear to be two orders of magnitude higher because of the large size of the deposits. Although a reliable, economic concentration technique for HREE from manganese and ferromanganese ores has not yet been developed, those ores could be an important future source of HREE.

Dating the Oldest Greenstone in India: A 3.51-Ga Precise U-Pb SHRIMP Zircon Age for Dacitic Lava of the Southern Iron Ore Group, Singhbhum Craton

Abstract This article reports a precise 3506.8 ± 2.3-Ma U-Pb SHRIMP zircon age for dacitic lava in a well-preserved low-grade metamorphic and low-strained greenstone belt succession of the southern Iron Ore Group, Singhbhum craton, India. This age makes the succession the oldest-known greenstone belt succession in India and one of the oldest low-strain greenstone successions in the world after the 3.51-Ga Coonterunah Group of the Pilbara craton, Western Australia, and the moderately deformed 3.54-Ga Theespruit Formation of the Barberton Greenstone Belt, Kaapvaal craton, South Africa. The geochemical composition of the dacitic lava and related volcanic rocks suggests that they formed in a volcanic arc setting. The succession also contains a major ∼120-m-thick oxide facies banded iron formation that distinguishes it from the slightly older successions of the Pilbara and Kaapvaal cratons. This banded iron formation may well be one of the oldest and most well preserved, and together with associated volcanics,...

A comparative mineralogical and geochemical characterisation of iron ores from two Indian Precambrian deposits and Krivoy rog deposit, Ukraine: implications for the upgrading of lean grade ore

Iron ores from two important Precambrian belts in India are studied in detail. The first of these is the Jilling-Langalota deposit, hosted by banded iron formations along with generations of shales, tuffs belonging to Iron Ore Group of Eastern India and is hosted in the Singhbhum-North Orissa Craton. The second group of ores is from the Chitradurga basin in Eastern Dharwar Craton, Southern India. These form part of the Archaean greenstone belts and show a typical oxide–carbonate–sulphide association. The Jilling-Langalota deposit contains considerable amounts of blue dust that is absent in the Chitradurga deposit. Comparisons are made between the Indian iron ores and those of the Krivoy Rog province of the Central Ukrainian Shield. The Indian iron ores are relatively richer in Fe and contain higher amounts of alumina and phosphorous compared with those of the Krivoy Rog deposit. The Indian iron ore samples contain porous and friable oxides and hydroxides of iron with kaolinite, gibbsite and quartz. In contrast, the ores from Krivoy Rog are massive with negligible clay and a higher quartz content leading to very low alumina and very high silica contents in the ores and slime. The Indian ores and slimes are manganiferous in nature with high alumina, which is deleterious to processing and is due to the presence of intercalated tuffaceous shales and clay. The Eastern Indian iron ore deposits could have been formed due to enrichment of the primary ore by gradual removal of silica. It is believed that the massive ores result from direct precipitation while powdery blue dust is formed owing to circulating fluids, which leach away the silica from the protore. The host rock is exhalatic banded iron formation and the ubiquitous presence of intercalated tuffaceous shales point towards a genesis that could have involved Fe leaching from sea floor volcanogenic rocks. The nature of these ores along with the parting shale is responsible for production of large amounts of alumina rich slime during mining and handling. The detailed mineralogical characterisation studies aided by X-ray diffraction, scanning electron microscopy—energy dispersive spectroscopy, physical parameters and chemical characteristics have indicated the presence of various mineral phases and established the nature of iron-bearing and gangue assemblages of the bulk ores and slime samples from the three iron ore deposits. These in turn are useful in understanding the amenability of the ores and slimes for beneficiation and waste utilisation.

Genesis of High-Grade Iron Ores of the Archean Iron Ore Group around Noamundi, India

High-grade hematite ores of the Iron Ore Group in the Noamundi area, Jharkhand state, India, are hosted by a laterally extensive, 220-m-thick banded iron formation (BIF) in a folded greenstone belt succession of Paleoarchean age. Single orebodies, which are up to 3 km long along strike and several hundred meters wide, depending on dip of the beds, are strata bound and composed of two major ore types—namely, hard hematite ore that is of ancient geologic origin and ores related to recent weathering along a lateritized Cretaceous-Cenozoic land surface. The ancient hard hematite orebodies comprise laminated hematite ores in which microplaty hematite is dominant, and massive ores composed almost entirely of martite. Supergene ores, in contrast, are comprised of goethite-rich duricrust and soft saprolitic hematite ores representing the leached zone of the Cretaceous-Cenozoic laterite profile. Hard hematite-martite ores formed by hydrothermal replacement of BIF protolith, not only through leaching of silica but possibly also through introduction of iron by hydrothermal fluids of meteoric origin. Hematite iron ore pebbles in a conglomerate at the base of the Meso- to Neoproterozoic Kolhan Group near Noamundi attest to the antiquity of the hard hematite orebodies. Soft saprolitic iron ores formed as a product of lateritic weathering processes in Cretaceous-Cenozoic times. Supergene alteration took place under the influence of reducing and acidic meteoric water, which is typical for lateritic soil profiles developed under humid tropical climatic conditions with lush plant cover. Goethitic duri-crusts capping the laterite profile suggest alternating wet and dry seasons in this environment.

Contrasting REE Signatures on Manganese Ores of Iron Ore Group in North Orissa, India

The distribution pattern of Rare Earth Elements (REE) in three categories of manganese ores viz. stratiform, stratabound-replacement, and detrital of Precambrian Iron Ore Group from north Orissa, India was reported. These categories of Mn-ore differed in their major and trace chemistry and exhibited contrasting REE signature. The stratiform ores were relatively enriched in Σ REE content (697 μg · g −1) and their normalized pattern showed both positive Ce and Eu anomalies, whereas the stratabound-replacement types were comparatively depleted in Σ REE content (211 μg · g −1) and showed negative Ce and flat Eu signatures. The detrital categories showed mixed REE pattern. The data plotted in different discrimination diagrams revealed a mixed volcaniclastic and chemogenic source of material for stratiform categories, and LREE (Light Rare Earth Elements) and HREE (Heavy Rare Earth Elements) are contributed by such sources, respectively. In contrast, the stratabound ore bodies were developed during the remobilization of stratiform ores, and associated Mncontaining rocks under supergcne condition followed by the redeposition of circulating mineralized colloidal solutions in structurally favorable zones. During this process, some of the constituents were found only in very low concentration within stratabound ores, and this is attributed to their poor leachability/mobility. The detrital ores did not exhibit any significant characteristic in respect of REE as their development was via a complex combination of processes involving weathering, fragmentation, recementation, and burial under soil cover.

Reappraisal of the structure of the Western Iron Ore Group, Singhbhum craton, eastern India: Implications for the exploration of BIF-hosted iron ore deposits

The middle to late Archean Iron Ore Group rocks occurring along the western margin (the Western Iron Ore basin) of the Singhbhum Granite massif in the Singhbhum craton were deformed during Iron Ore orogeny and are disposed in a horseshoe-shaped synclinal structure in the eastern part of the Indian shield. The Western Iron Ore basin hosts almost all the major high-grade iron ore deposits of eastern India. Contrary to the established view, present analysis emphasizes that the horseshoe fold in reality is a synclinorium consisting of a syncline–anticline fold pair which were later cross-folded along an east–west axis. Structural analysis in the eastern anticline of the ‘horseshoe synclinorium’ suggests that the BIF hosting the high-grade iron ore bodies are disposed in three linear NNE–SSW trending belts, each showing an open synclinal geometry. Later cross folding produced development of widespread dome and basin pattern at the sub-horizontal hinge zones of these synclinal fold belts. The major iron ore deposits in the eastern anticline at the present level of erosion are preferentially localized within shallow elongated basinal structures only. The axis of the adjoining western syncline was similarly uplifted as partial culminations where cross-folded against E–W anticlinal axes. But here, the BIF-iron ore bodies are preferentially localized within elongated domal structures in contrast to the basinal sites in the adjacent eastern anticline. Such an inference based on structural analysis could probably be utilized as a potential tool for all future explorations, reserve estimation and recovery of the iron ore deposits in the terrain.

Utilization of optical image analysis and automatic texture classification for iron ore particle characterisation

Optical image analysis is a very convenient tool for obtaining comprehensive information about fine iron ore size fractions. Data can be obtained on mineral abundances, porosity, particle shape and ore textures with a high level of accuracy. A range of techniques has been used to characterise iron ore samples on a particle-by-particle basis. Automatic textural classification of iron ore particles was used to establish classes containing particles with very similar mineral composition and texture. Image analysis coupled with probe analysis and mineral density measurements provided information about the chemical composition and density of each particle class. The combination of these results enabled a “virtual feed” to be created, which can be a key input into a beneficiation unit model for predicting its performance. Identification and classification of the textural type of each particle was performed according to the CSIRO-Hamersley Iron Ore Group Classification Scheme. If more detailed classification is needed, further classification can be performed based on dimensional, chemical or mineral criteria, such as the presence of certain minerals in particles or total iron content. Some deficiencies of the current image analysis procedures and their further improvement and automation are also discussed.

The genesis of Archaean chromitites from the Nuasahi and Sukinda massifs in the Singhbhum Craton, India

The chromitite deposits of the Nuasahi and Sukinda massifs are part of layered ultramafic bodies which occur within Archaean low-grade metamorphic rocks of the Iron Ore Group (IOG) in the Singhbhum Craton of the Indian Shield. The chromitite seams are interlayered with dunite, and associated with orthopyroxenite. Detailed electron microprobe study reveals very high Mg-numbers (0.66–0.82) and Cr-numbers (0.75–0.87) for chromite in massive chromitite from the seams, high Fo content (Fo 92–95) for olivine in dunite and high En content (En 89–94) for orthopyroxene in orthopyroxenite. Our study suggests that the original igneous compositions for these minerals are preserved in the respective monomineralic rocks and can be used to evaluate the primary magmatic petrogenesis of the rocks. The primitive compositions of the minerals in monomineralic rocks suggest high degree of partial melting of the source mantle during the Mesoarchaean. The parental magma from which the massive chromitites crystallized was of siliceous high-Mg basaltic or boninitic compositions, similar to the compositions of spatially associated chromite-bearing siliceous high-Mg basalts of the Iron Ore Group. The parental magma may have generated due to the interaction of a depleted mantle with a fluid-enriched melt possibly derived in response to the dehydration of a subducting slab. Monomineralic chromitite layers formed either in response to mixing of magmas or due to the suppression of silicate mineral crystallization in response to elevated H 2O concentrations in the parent magmas. The parental magmas that produced chromitites may have intruded into the volcano-sedimentary greenstone belts of a supra-subduction zone setting.