Ore Group Research Articles

Mode of Occurrence and Characteristics of Mn‐ore Bodies in Iron Ore Group of Rocks, North Orissa, India and Its Significance in Resource Evaluation

Abstract. Several meso‐scale manganese ore bodies, scattered within Jone's horse‐shoe shaped synclinorium, in Bonai‐Keonjhar region of north Orissa are well known in the mineral map of India. Different grades of manganese ores are being exploited from this region by various agencies over a few decades. However, deceptive nature of ore bodies and complexity in control of mineralisation greatly confuse the exploration geologists for evaluation of these resources.In a recent study, the authors have classified Mn‐ore bodies of this region into three broad categories such as stratiform, stratabound (‐replacement) and lateritoid types based on mode of occurrence and their other chemical characteristics. Mn‐ore bands occur in close association with BIF and iron ores. Volcaniclastic shale in large geographic extension encloses these ore bodies.In the stratiform category of ore bodies (BMnF, analogous of BIF), manganese and shale bands, in variables thickness, alternate with each other and extend to a great depth. Such ore bodies generally constitute marginal to low‐grade ores, are characterised by low Mn/Fe ratio (∼2) and have relatively lower abundance of trace (1500 to 2500 ppm) and relatively higher REE constituents. The stratabound‐replacement types of ore bodies are of intra‐stratal nature, occurring within tuffaceous shale. These are mostly shear‐controlled ore bodies extending along a zone of certain width. Increase in average Mn/Fe ratio (∼6) and trace content (5000 to 8500 ppm) by 5 to 2.5 order of magnitude respectively or more above stratiform category are characteristic of these deposits. The lateritoid ore bodies have limited depth persistency. Such deposits are usually very low in Mn/Fe ratio (<1), trace (<2000 ppm) and REE contents.Different methods of exploration techniques are suggested for various categories of Mn‐ore bodies. In this context, the above findings would be the database for the exploration geologists to evaluate the potential of newer/existing Mn‐ore resources in this part of north Orissa.

The Archaean Nucleus of Singhbhum: The Present State of Knowledge

The supracrustal rocks of the Older Metamorphic Group (OMG), consisting of metasediments and ortho-amphibolite, constitute the oldest unit in the Archaean nucleus of Singhbhum. However, there are indications that still older (3.4–3.8 Ga) crust of both sialic and mafic composition existed in this region. The OMG ortho-amphibolites were formed by partial melting of mantle with near chondritic composition ca. 3.3 Ga ago, probably as a result of plume activity. Shortly afterwards, partial melting of the underplated mafic material produced a tonalitic melt (Older Metamorphic Tonalitic Gneiss — OMTG), which intruded the OMG supracrustals and the entire suite was deformed and metamorphosed to upper amphibolite facies. Subsequent to this, melting of the OMG ortho-amphibolites and the lower crustal material of probable andesitic composition produced melts varying in composition from tonalite to granite and these intruded in different phases to produce plutons of Singhbhum Granite, Bonai Granite and Kaptipada Granite, which together form volumetrically the major part of the Archaean nucleus. The older OMG and OMTG occur as enclaves within these younger granitoids. The time difference between the emplacements of the OMTG and the early phases of younger granitic intrusion was of the order of 100–200 Ma. Thus, serial additions of juvenile material led to the formation of a stable microcontinent by 3.2 Ga. Thermally triggered stretching in this microcontinent produced basins peripheral to the present day Singhbhum Granite pluton, and in these basins the younger supracrustal rocks of the Iron Ore Group (IOG), consisting of BIF, associated argillaceous and subordinate arenaceous rocks, and mafic lavas were laid down. There is inadequate field or geochronological evidence to resolve the issue of whether the different iron ore basins were coeval or not. Meagre geochronological data suggest that some of the BIFs are older than ca. 3.1 Ga. Post-IOG activity is confined to the intrusion of mafic dyke swarms and formation of intracratonic basins, the ages of both being uncertain.

Ore Group Classification - A Case Study from Ingaldhal Sulphide Deposit, Karnataka Craton, India

All varieties of base metal sulphide deposits may be classified using base metal ratios viz. Copper Ratio (CR=100Cu/{Cu+Zn}), Zinc Ratio (ZR=100Zn/{Zn+Pb}) and Lead Ratio (PR=100Pb/{Pb+Cu}). Based on this ore group classification an attempt has been made to identify ore groups present in the late Archaean volcanic associated Ingaldhal Sulphide Deposit (ISD) of Karnataka Craton. Using base metal ratios (CR, ZR and PR) five distinct fields are suggested: Cu, Cu-Zn, Cu-Zn-Pb, Zn-Cu and Zn-Cu-Pb/Zn-Pb-Cu with their associated geological and geochemical characteristics. A transitional change in the ore groups is observed from the North Block (Cu rich groups; CR>50) to South Block (Zn and Pb dominated groups; CR<50) and the Main Block is represented by all the five ore groups. This classification may be useful as a pointer in the exploration of polymetallic base metal deposits. Copper rich groups (Cu, Cu-Zn and Cu-Zn-Pb) exhibit high concentration of Bi and Co, and low CaO and Cd whereas the Zn rich groups (Zn-Cu and Zn-Cu-Pb/Zn-Pb-Cu) show high CaO and Cd, and low Bi and Co. The CR of most of the ore groups exhibit a sympathetic relation with Co and Ag. A preliminary attempt has been made for better understanding of ore group genesis of ISD by integrating ore group characteristics with deduced geothermometry.

Application of factor and discriminant analyses to the iron ore geochemistry of Daitari, Jajang, Joda East and Kalta deposits, Iron Ore Group, Orissa, India.

Major and trace elements data generated on 4 iron ore deposits (Daitari, Jajang, Joda East and Kalta) of Iron Ore Group of Orissa have been processed by using R-mode factor analysis. Total 10 elemental variables are expressed in terms of 3 common rotated factors to understand the mineralogical signatures and processes of metal formation and distribution in the ore. Three distinct processes have been identified: (1) formation of rich iron ores by weathering and erosion with simultaneous removal of silica and development of aluminium bearing minerals, (2) development of nickeliferous ferruginous laterites and (3) accumulation of residual Pb. Bivariate discriminant analysis has been applied to distinguish any pair of deposits of the 4 areas under investigation which are geochemically alike. A combination of 10 elements Fe2O3-SiO2-Al2O3-Ni-Zn-Cu-Co-Cr-Mn-Pb can establish linear discriminant function for any pair without misclassification error.

REE distribution in ferromanganese oxide ores from Iron Ore Group, Western Koira Valley, Orissa, India.

The REE distribution in ferromanganese oxide ores from Iron Ore Group, Western Koira valley India, are studied. Mn-ores in the region generally occur as stratabound and lateritoid: bodies, showing remarkable similarity in their mineral assemblages and having contrasting REE signature. The stratabound ores show higher concentration of both LREE (208 to 640 ppm) and HREE (121 to 285 ppm) as compared to that of the lateritoid types. Similar Chondrite and Shale normalized REE profiles in different deposits suggest a common mode of Mn-mineralization in the area. However variable La/Lu, La/Yb and Nd/Sm ratios attest to significant LREE fractionation in lateritoid deposits. Intense lateritization resulting in oxidation and weathering, mobilization and fractionation, has caused two distinct REE domains in the Koira valley region, India.

Universal Theory of Certain Solvable Groups and Bounded Ore Group Rings

Universal Theory of Certain Solvable Groups and Bounded Ore Group Rings

Geochemistry and tectonic significance of the Ongarbira metavolcanic rocks, Singhbhum District, India

The volcanic-sedimentary sequence of Ongarbira occurs to the south of the Singhbhum Shear Zone and rests uncomformably on a basement topographic-low between the Chakradharpur Granite and the Singhbhum Granite. Structural analysis reveals that the Ongarbira synform and other adjoining structures are kilometre-scale first-generation folds on E-W striking axial planes. These structures are in marked contrast with the corresponding kilometre-scale first-generation folds typical of the Iron Ore Group synclinorium towards the south of the Ongarbira belt. The Ongarbira metavolcanic rocks exhibit a limited range of tholeitic basalt compositions. The basalts are grouped according to their Mg# and the details of their REE patterns. Groups I and II are relatively unevolved and very similar to LREE- and LIL-depleted ocean ridge tholeiites. The most evolved Group III with LREE enrichment is most similar to LIL-enriched ocean floor basalts, but may be contaminated with crustal materials. In general, the Ongarbira volcanics evolve upward stratigraphically. Petrogenetic major element modelling of the Ongarbira basalts suggests that the primary melt was generated by batch melting (30%) of lherzolite at pressures of at least 1.5 GPa. Sequential fractionation of Ol, O1+Cpx+Pl and Cpx+Pl followed. Comparison of incompatible and compatible element distributions indicates a very similar batch m melting-fractional crystallization sequence for Ongarbira Groups I and II basalts. Group III basalts may have been derived from a batch melt initially more enriched in incompatible elements. Trace element discrimination of the Ongarbira basalts suggests that they were generated in an extensional environment. Primordial mantle normalized element distributions indicate that the basalts are similar to continental rift basalts but with oceanic affinities. The chemical data along with the associated sediments indicate that the Ongarbira basalts were probably emplaced in a mature continental rift, although a back-arc origin cannot be completely disregarded. It is concluded that the Ongarbira volcanics were emplaced as part of the same rifting event that resulted in outpourings of the Proterozoic Dalma and Dhanjori volcanics, implying that the Ongarbira suite is Proterozoic and not part of the Archaean Iron Ore Group. The structural geometry of the Ongarbira rocks is more conformable to that of the Singhbhum Mobile Belt than to the Iron Ore Group synclinorium in the Singhbhum Craton. It is likely that the southern limit of the Singhbhum Mobile Belt is defined by the interface of its E-W structures with the NNE structures of the Iron Ore Group rocks. Thus, the Singhbhum Shear Zone does not mark the interface between the Archaean Singhbhum Craton and the Proterozoic Singhbhum Mobile Belt in the eastern Indian Shield.

Gravity field over Singhbhum, its relationship to geology and tectonic history

The gravity field over Singhbhum and adjoining areas lying between latitude 21° to 23°15′N and longitude 84° to 87°30′E has been analysed. The region has a very complex Precambrian history dating as far back as 3200 m.y. and extending up to 850 m.y., during which time it experienced a number of orogenic cycles. The activity has left an imprint on the gravity field which is marked by prominent gravity highs and lows. The highs are invariably surrounding the Singhbhum granite batholith, whereas the lows are conspicuous over the granites and their margins. The regional gravity field has been removed using a 4th order polynomial surface. The residual anomalies over prominent highs and lows have been interpreted using two-dimensional and three-dimensional models for underlying rock formations. It is inferred that the gravity highs are generally related to synclinal structures filled with sedimentary or metasedimentary formations and volcanics, such as the Iron Ore Group, the Singhbhum group, Dhanjori and Simlipal basins. The gravity lows can be attributed to a considerable thickness of granites of varying density, which could have been formed during different metamorphic/orogenic cycles by melting of a preexisting sialic crust.

One orogenic belt or two? A structural reinterpretation supported by Landsat data products of the Precambrian metamorphics of Singhbhum, eastern India

Recent structural studies, supported by an analysis of digitally enhanced Landsat data products, attempt at bringing in certain significant changes in the two contrasting concepts about the stratigraphic and tectonic classification of the highly deformed and metamorphosed Precambrian rocks of Singhbhum, eastern India. Both the concepts introduced large-scale horizontal displacements of the rocks in a N-S direction along a prominent E-W trending shear zone. An earlier concept professed that the rocks in the north and south of the shear zone, in spite of their apparent structural and metamorphic contrasts, belong to one broad stratigraphic division (“Iron Ore Series”). A later concept grouped the rocks into two different stratigraphic and orogenic belts: the belt north of the shear zone with an E-W strike (“Singhbhum Group”) being younger than the belt in the south striking NNE-SSW (“Iron Ore Group”). The present structural and remote sensing studies reveal that: (1) the shear zone does not demarcate two different stratigraphic and orogenic belts; and (2) it does not involve any large-scale N-S displacements; rathe, it indicates severe ENE-WSW subhorizontal sinistral shearing in western Singhbhum that brought about marked structural contrasts within the same stratigraphic belt.

Primary depositional and diagenetic features in the banded iron formation and associated iron-ore deposits of Noamundi, Singhbhum district, Bihar, India

The Banded Hematite Jasper (BHJ) Formation of Noamundi region in Bihar, belonging to the lower part of the Iron Ore Group of early Precambrian age (c. 2900–3200 Ma), exhibits numerous primary depositional and diagenetic features, both in BHJ as well as the associated iron ore deposits. Observed primary depositional features include banding and bedding of different geometric-types, surface-markings including interference ripple-marks, current crescents, linear markings, scour-and-fill structures, etc. and within-mass microstructures such as spherulites, granules, discs and maculose cylindrical bodies. Diagenetic features, such as fabric changes, micro-dessication structures, gravity-density features, etc. and motion-and-disruption features of various kinds are also seen. The significance of various features in terms of probable mode and environments of deposition of BHJ and the iron ore beds has been considered. In general, shallow water environment of deposition in a region proximal to the shoreline with a rather steep paleoslope of the shelf has been surmised.

An analysis of the gravity field over north Singhbhum, Bihar, India

About seven hundred gravity stations were established 2–3 miles apart over the Precambrian terrain of Singhbhum that lies between latitude 22° 15′ to 23°°15′N and longitude 85° to 87°E. Bouguer anomalies ranging from +4 to −62 mGal are found in the area. The observed Bouguer anomaly map was analyzed into regional and residual components. The residual anomaly map shows an excellent correlation with geology. The Singhbhum granite batholith is associated with several gravity lows. The residual anomaly map outlines nine plutonic granitic masses within the Singhbhum batholith. Negative residuals are also observed over some intrusive granites outside the batholith. Residual gravity highs are noted over the Dalma hill as well as over the Dhanjori lava complex on the eastern part of the Singhbhum batholith. Two-dimensional models suggestive of subsurface configuration of several major geologic units in the area are presented. These indicate that some of the plutonic granites within the Singhbhum batholith are of relatively large dimensions. The basin containing the Iron Ore Group of rocks to the west of the batholith, as well as the basin containing Singhbhum Group of rocks outside the Copper Belt thrust, may have sedimentary thicknesses of the order of 6–7 km. The Dalma lavas attain their maximum thickness of about 2.5 km in the form of a syncline, underneath which the Singhbhum Group of rocks is also found to be the thickest. The Copper Belt thrust, a major Precambrian fracture around the Singhbhum batholith, is moderately north-dipping near the surface but possibly attains a steeper slope at depth. The thrust appears to be quite deep seated. A threedimensional computer-based model for the Dhanjori lava—gabbro complex on the eastern part of the Singhbhum batholith has been deduced. Maximum thickness of these basic rocks is found to exist underneath a thin cap of granophyre. The geological implication of these results is discussed. Variation in the regional anomalies seems to be attributable to a mass deficiency under the Singhbhum batholith. The batholith may extend subsurfacially towards the north across the Copper Belt thrust. The northern tip of the batholith probably became dissected along the line of intersection of the two orogenic trends in the area and subsided. Over this subsided part, the Singhbhum Group of rocks was deposited at a later stage. Gravity data suggest a fairly large amount of subsidence in the area.

Silica diagenesis in the banded hematite jasper and bedded chert associated with the iron ore group of Jamda-Koira Valley, Orissa, India

The banded hematite jasper and bedded chert associated with the Precambrian Iron Ore Group of Orissa still retain their diagenetic features almost intact and are thus amenable to sedimentological analysis. Application of fabric criteria, as applied to carbonates, to these rocks demonstrates their potential value in the understanding of the diagenesis of bedded chert. Recrystallization, i.e., change in fabric,appears to be a long-continued process in these rocks and has affected them in variable degrees. The process, in some instances, is guided by structural discontinuities in the fabric, whereas in other instances it has advanced without any regard to the inhomogeneities in the initial fabric. Recrystallization led to the formation of microcrystalline quartz and then to megaquartz mosaic rather than chalcedony. Growth of chalcedony was possibly impeded due to simultaneous initiation of recrystallization at closely spaced nuclei distributed throughout the rock. Replacement of early diagenetic rhombic carbonates by silica preceded inception of recrystallization and also a generation of the fracture-filling quartz veins. Evidence of fracture-filling and recrystallization within the penecontemporaneous breccia fragments implies that both the processes started immediately after the deposition of silica. Fractures constitute the commonest sites for accumulation of open-space mosaics. Cross-cutting relationship among the fracture-filling veins and also differences in their fabrics suggest the emplacement of secondary silica, not only in different generations but also under varying physico-chemical conditions. Paucity of chalcedony in the fractures is possibly linked with the planar nature of their bounding surfaces and also with the close spacing of the centers of crystallization of silica.

Geochronology of the Pre-Cambrian rocks of Singhbhum and adjacent regions, Eastern India

SUMMARYTectonic implications of 32 new potassium-argon age determinations of minerals and rocks of Singhbhum and adjacent regions are discussed along with those of recent structural, stratigraphic and petrologic studies, and some pre-existing geochronologic data. Three distinct orogenic cycles with closing dates at c. 3200, 2700 and 850 m.y. respectively have been recognized. The earliest cycle is represented by the Older Metamorphic Group of metasediments with associated basic intrusives and biotite granodiorite gneiss; they occur mainly as relics within the huge Singhbhum granitic complex. Emplacement of the latter (c. 2700 m.y.) marks the culmination of the Iron Ore orogenic cycle which is characterized by the low-grade metamorphic Iron Ore Group sediments and volcanics and associated basic intrusives. Along the northern and eastern edges of the stabilized Iron Ore orogenic belt, thick sedimentation of the Singhbhum and Gangpur Groups (?2000–1700 m.y.) was followed by large scale basic volcanic activity (1700–1600 m.y.); these rocks were folded and regionally metamorphosed in several phases between c. 1550 and c. 850 m.y. (Singhbhum orogenic cycle); extensive thrusting and granitization occurred in the later part of the cycle.