A. S. Venkatesh

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

Abstract 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.

Banded Iron Formation to Blue Dust: mineralogical and geochemical constraints from the Precambrian Jilling-Langalata Deposits, Eastern Indian Craton

Abstract Numerous economic deposits of high-grade iron ores occur in the Singhbhum-Orissa Craton, in Eastern India. The deposits are mainly located in the Jilling-Langalata, Noamundi and Joda areas which are part of the eastern limb of the regional horseshoe shaped synclinorium, where millimetre to centimetre scale Archaean Banded Iron Formation units have been converted to steel grey, iron rich fine grained powder, known as Blue Dust. Field observations and subsequent laboratory investigations indicate that in this region, the Blue Dust deposits occur as pockets or lenses of varying dimensions and are randomly oriented. However, in most cases the Blue Dust deposits are found above the Fe-rich primary host rock known as Banded Haematite Jasper. Mineralogical observations indicate that the Blue Dust is mainly composed of haematite, martite and goethite while quartz and kaolinite are the gangue minerals. Silica removal is the primary control of iron enrichment. Geochemical and field observations indicate that the Blue Dust in these deposits is regarded to be of supergene-modified hydrothermal origin. In the first stage, the early hydrothermal process affects the primary unaltered Banded Iron Formation by simultaneously oxidising magnetite to martite and replacing quartz with hydrous iron oxides. In the second stage, the supergene processes upgrade the hydrous iron oxides to fine grain microplaty haematite. The supergene process causes the leaching of remnant silica from hydrothermally upgraded iron ore under a suitable Eh and pH condition and leads to the formation of Blue Dust.

‘Indicator’ carbonaceous phyllite/graphitic schist in the Archean Kundarkocha gold deposit, Singhbhum orogenic belt, eastern India: Implications for gold mineralization vis-a-vis organic matter

Carbonaceous rocks in the form of graphitic schist and carbonaceous phyllite are the major host rocks of the gold mineralization in Kundarkocha gold deposit of the Precambrian Singhbhum orogenic belt in eastern India. The detection of organic carbon, essentially in the carbonaceous phyllite and graphitized schist within the Precambrian terrain, is noted from this deposit. A very close relationship exists between gold mineralization and ubiquitous carbonaceous rocks containing organic carbon that seems to play a vital role in the deposition of gold in a Precambrian terrain in India and important metallogenetic implications for such type of deposits elsewhere. However, the role played by organic matter in a Precambrian gold deposit is debatable and the mechanism of precipitation of gold and other metals by organic carbon has been reported elsewhere. Fourier transform infrared spectroscopy (FTIR) results and total organic carbon (TOC) values suggest that at least part of the organic material acted as a possible source for the reduction that played a significant role in the precipitation of gold. Lithological, electron probe analysis (EPMA), fluid inclusions associated with gold mineralization, Total Carbon (TC), TOC and FTIR results suggest that the gold mineralization is spatially and genetically associated with graphitic schist, carbonaceous phyllite/shale that are constituted of immature organic carbon or kerogen. Nano-scale gold inclusions along with free milling gold are associated with sulfide mineral phases present within the carbonaceous host rocks as well as in mineralized quartz-carbonate veins. Deposition of gold could have been facilitated due to the organic redox reactions and the graphitic schist and carbonaceous phyllite zone may be considered as the indicator zone.

Relevance of geological aspects and ore mineralogy in selecting beneficiation methods for processing of eastern Indian iron ores

Abstract Geological aspects, particularly mineralogy and ore genesis of different iron ores have important roles to play in understanding their behaviour during processing and decide suitable beneficiation method for particular type of ore so to produce desired quality end products. The implications of different geological aspects and mineralogical characteristics specific to ore types in mineral beneficiation have been outlined in this paper. Experiments indicated that iron ores, such as massive hard ores, laminated hard ores, fine powdery ore and blue dust ore, do not require complicated processing, however at the other end; sub grade ores, such as flaky, friable, shaly, ochrous, powdery, lateritic and goethitic ores having low iron and high alumina content, require specific beneficiation treatments before their use as direct or as agglomerates in the blast furnace feed. While processing, different ore types need treatment differently for their up gradation. Hence, in addition to the conventional methods of processing such as crushing, screening and washing, advanced beneficiation techniques using concepts of gravity and magnetic separation have found their specific applications for beneficiation of different types of ores, based upon their physicochemical and mineralogical characteristics. Geological setup of the deposits and mode of occurrences of different ore types, physical, chemical and mineralogical characterisation and extensive beneficiation experiments conducted on Indian iron ores have established that quality of lumps can substantially be up graded through simple processing and log washing. On the other hand, for fines ores, Jigging has been found to be the best suitable method. Slime beneficiation requires combination of processes and treatment through hydrocyclones, multigravity separation, magnetic separation and sink float methods, etc. These applications would help not only to recover more values from the inferior grades of iron ores but also to conserve and preserve the natural resources too.