Mihir Deb

Age, source and stratigraphic implications of Pb isotope data for conformable, sediment-hosted, base metal deposits in the Proterozoic Aravalli-Delhi orogenic belt, northwestern India

Abstract Stratiform, sediment-hosted, base metal deposits in the Aravalli-Delhi orogenic belt of northwestern India are hosted by the sediment-dominated Bhilwara, Aravalli and Delhi sequences of Proterozoic age. Pb isotope data for 36 samples from five ore districts provide useful stratigraphic and approximate age information. Model Pb ages using a newly developed ‘Proterozoic model’ are ∼ 1800 Ma for the Rampura-Agucha, Rajpura-Dariba and Saladipura deposits, ∼ 1700 Ma for the Zawar deposits and ∼ 1100 Ma for the Ambaji and Deri deposits. Mineralization in this orogenic belt thus occurred at three stages in Middle to Upper Proterozoic times. The oldest of these, reflected by deposits in the Bhilwara sequence, was of broad regional extent. The Rampura-Agucha deposit is thus hosted by highly metamorphosed rocks that belong to the Bhilwara belt and not to an older supracrustal component of the Archean Banded Gneissic complex. The host rocks to the Saladipura deposit are Bhilwara-equivalent and are not part of the Delhi Supergroup, as previously mapped, and therefore stratigraphic assignments in the area must be re-examined. Model ages of ∼ 1100 Ma for the Ambaji-Deri deposits are the only evidence obtained in this study regarding the age of the Delhi Supergroup, and are in apparent conflict with much older ages indicated by previous studies. Rocks of such late Upper Proterozoic age may be restricted to the southwestern segment of the orogenic belt, or, alternatively, thrust stacking and/or interfolding of lithologically similar sequences of a wide range of ages throughout the belt may not yet be evident because of the limited age and isotopic data presently available. All the data can be readily explained by the extraction of Pb from sediments derived from an upper-crustal basement. The isotopic compositions of the Zawar deposits provide the clearest evidence for an ancient, U-enriched, upper-crustal source. The Ambaji-Deri Pb data suggest a mature status of the Delhi are when these deposits were formed.

Zircon U–Pb and galena Pb isotope evidence for an approximate 1.0 Ga terrane constituting the western margin of the Aravalli–Delhi orogenic belt, northwestern India

Abstract Zircon U–Pb ages of 987±6.4 and 986.3±2.4 Ma have been established for rhyolites in the southern and northern parts, respectively, of the Ambaji–Sendra arc terrane in the western fringe of the Aravalli–Delhi orogenic belt. Pb isotope data for volcanogenic massive sulphide deposits within this arc terrane define a linear trend, which is considered a useful reference palaeoisochron or mixing line at an age of about 990 Ma, and establish continuity of the terrane between Ambaji and Sendra. These results are in contrast to the traditional and continuing assignment of the Ambaji–Sendra terrane to the >1700 Ma Delhi Supergroup. The isotopic composition of galena from an apparently epigenetic occurrence at Punagarh Hill yields a model age of about 940 Ma, suggesting that the Punagarh Group could form part of the same arc sequence. The Phulad lineament, which has been considered by most workers to represent the western boundary of the Aravalli–Delhi orogenic belt, appears, in terms of this stratigraphic assignment, to represent an oblique structure, which has not greatly offset the Punagarh and Ambaji–Sendra domains within the arc terrane. The eastern boundary of the terrane is marked by the Sabarmati fault. A zircon U–Pb age of 836+7/−5 Ma for the Siwaya gneissic granite, in the southern part of the Ambaji–Sendra belt, is in accord with previous age data for felsic Erinpura plutons that have intruded the arc sequence. A monazite age of 826±5 Ma may reflect slow cooling of the Siwaya pluton or a younger thermal event. A model age of about 820 Ma for galena is obtained from the Tosham Sn–Cu mineralized zone, in Haryana state, about 280 km north–northeast of Ajmer, which is related to the felsic, anorogenic, Malani volcanism–plutonism. Hence, this widespread magmatism, found extensively west of the Ambaji–Sendra terrane, may have been coeval with Erinpura plutonism or followed it very closely. The present geochronological data warrant the recognition of the Ambaji–Sendra arc terrane, as defined here, as a distinct metallogenic province, which saw the formation of VMS-type deposits in the late Mesoproterozoic, around 1.0 Ga. The Pb-isotope compositions of the deposits however, do not clearly define the metal sources. The lead from Danva prospect is most primitive and must have the greatest component from a juvenile mantle source. The Birantiya Khurd deposit, on the other hand, contains a much greater component of lead from a significantly older crustal source.

Proterozoic tectonic evolution and metallogenesis in the Aravalli-Delhi orogenic complex, northwestern India

Abstract The Proterozoic Aravalli-Delhi orogenic complex hosts a large number of economically important stratabound base metal sulphide deposits. A few W-Sn deposits associated with granites are also known in the complex. Based on tectono-lithological features, the following domains are distinguished: the Archaean basement consisting of the Banded Gneissic Complex and granites, the Early Proterozoic Bhilwara, Aravalli, Jharol, and north Delhi belts and the Middle to Late Proterozoic south Delhi belt and the Vindhyan basin. Intra-cratonic rifting of the Archaean basement commencing ∼ 2.2 Ga ago resulted in the rock association of the Bhilwara belt, a thick pile of detritus derived largely from felsic sources and, to a minor extent, from tholeiitic sills with the geochemical characteristics of ocean-floor basalts. Stratiform Zn-Pb(-Cu) sulphide deposits at Rajpura-Dariba, Rampura-Agucha and also possibly those at Pur-Banera and Jahazpur formed 1.8 ± 0.04 Ga ago by convective seawater circulation in zones of crustal extension. The metal content of exhalative brines was precipitated in troughs where biologic activity was prolific. The shelf sediments of the Aravalli belt, characterized by dolomites with stromatolitic phosphorites, were deposited on a passive continental margin at the rifted western edge of the Archaean basement complex. The monotonous pelitic pile of the Jharol belt represents deep-pelagic continental-rise sediments. The Rakhabdev lineament delineates the shelf-rise boundary. The mafic-ultramafic bodies along this lineament represent Aravalli oceanic crust which subducted westward and eventually obducted as a consequence of an initial collision of the Aravalli continental margin with an incipient arc to the west around 1.5 Ga ago. In the Aravalli belt at Zawar, strongly radiogenic stratabound Pb-Zn deposits were formed close to the basement in second-order basins with biologic activity, by hydrothermal solutions convecting through a heterogeneous source. The north Delhi belt comprises three sedimentation domains: the Khetri sub-basin, the Alwar sub-basin and the Lalsot-Bayana sub-basin. Sedimentation in this belt commenced with shelf carbonates, followed by coarse clastic sediments and volcanites. Finally, pelites and semi-pelites were deposited in a multi-lagoonal, shallow-water, locally evaporitic environment. Synkinematic granitic intrusives in the north Delhi rocks are in the age range of 1.7-1.5 Ga. The stratabound Cu sulphide deposits of the Khetri belt and the pyrite-pyrrhotite deposits at Saladipura with a Pb-Pb model age of ∼ 1.8 Ga were formed from hydrothermal seawater and partly bacteriogenic sulphur in small, shallow, rift-related basins. The south Delhi belt, with extensive mafic volcanism, localized felsic volcanism, argillaceous-arenaceous-carbonate accumulations in successive, elongated basins and conspicuous felsic plutonism, is interpreted as an island arc. The geochemistry of pillowed and massive metavolcanites and an ophiolite assemblage indicate subduction in the western side of the arc. Part of the rocks of the Delhi belt may also have formed in rift-related back-arc basins as suggested by the ocean-floor characteristics of metavolcanic rocks. Subduction on the western side of the arc ultimately led to its terminal collision with the Aravalli continental margin around 1.0 Ga, involving intense progressive strain and oblique convergence. Small Cu-(Zn) deposits were formed within sedimentary intercalations in calc-alkaline basalts close to the hypothetical subduction zone. Extensional tectonics in the back-arc basins produced stratiform Zn-Pb-Cu deposits (Ambaji-Deri) around 1.1 Ga ago by high-temperature reduction of seawater convecting through multiple sources. Sn-W mineralizations are apparently related to the Malani felsic igneous phase of 735 Ma Rb-Sr age.

Artisanal and small scale mining in India: selected studies and an overview of the issues

In India, mining is one of the main economic activities since time immemorial, giving rise to a long historical tradition of artisanal mining. As modern mining rose during the colonial occupation, artisanal mining activities began to be overlooked and this great tradition became obscure. This invisibility, added with confusion with regard to legally accepted definitions has enhanced the negligence of the artisanal mining sector in India. This study draws attention to the contemporary artisanal mining practices in India – both traditional and non-traditional ones – with regard to four commodities, gold, tin, coal and lignite, and gemstones. It briefly discusses the occurrences of such mining, their salient features and concludes with four specific recommendations. Our recommendations primarily deal with the need for creation of a broader information base, delineating a responsible body to deal with this kind of mining, legal reforms leading to definitional changes and finally, the recognition of the poverty alleviation potential of this sector in view of the Millennium Development Goals.

Sustainable Development of Mineral Resources

The concept of Sustainable Development evolved through the efforts of the World Commission on Environment and Development (the Brundtland Commission) during the period 1982–1987 and rested on two basic premises: the issue of “need” for the deprived section of society and “limits” on the ability of the environment to satisfy the need of the present and future generations. Formally, Sustainable Development was defined as: “the development that meets the needs of the present without compromising the ability of the future generations to meet their own need.” The scope of this definition was widened in the World Summit of Johannesburg in 2002 to include “economic development, social development and environmental protection at local, regional and global levels.” Many interpretations of Sustainable Development were put forward in later years, but they were all based differently on the foundation of these “three pillars of sustainability.” More recently, Sustainable Development is understood in terms of complex systems which require nonlinear, organic approach and is not to be considered as a target to be achieved. The concept of sustainable development, however, has had an ambiguous relationship with the extractive industry because of its intrinsic nature, leading to negative consequences. But can our modern societies do without the earth materials for industrial use? Can we really do away with mining? The answer is obvious even to the layman. Hence, the challenge of “Sustainable Development in the Mines and Minerals sector” is to ensure “environmental responsibility” during mining and processing, in which the damage to the environment (including social environment) is to be balanced with the Earth’s capacity for accommodating it. Some well-accepted prescribed steps need to be followed in this regard to achieve sustainable development of mineral resources, keeping in mind the valuable guiding principles of reduce, replace, and recycle. “Free prior and informed consent” of all the stakeholders must be obtained by the mining industry if development is to be truly sustainable. Further, an alternative interpretation is required for a better understanding of the relationship between sustainable development and mining: a natural resource capital (a mineral or fuel resource) is converted by mining into an economic capital which can be reinvested to create or enhance other forms of capital, as exemplified by the “Alaska Fund.” Fast depletion of mineral resources and their long-term availability has been a serious concern for some time now, particularly since ecological footprint of humanity and limits to growth became the focal points of international debate. Though the proponents of “fixed stock paradigm” draw a gloomy picture, discovery of new deposits, advancement of technology and more use of low-grade material have extended the life expectancies of mineral resources. While social costs may limit the use of mineral commodities in future, per capita material throughputs of rich nations must be reduced if sustainable development should become a reality.

Deformation of Pyrite at Varying Metamorphic Grades in Sediment-Hosted Base Metal Sulphide Deposits of Rajasthan, India

The ubiquitous iron sulphide, pyrite, occurs in trace amounts in rocks or may form massive pyritic ore bodies with all perceivable gradations in between. Very often it shares the deformational and metamorphic history of its host rocks. Textural characteristics of pyrite, and its behavior in natural ores and in experimental conditions under varying temperature and pressure, have therefore been studied by different workers from time to time. An overview of these studies shows that there is a mismatch between the experimentally achieved deformation mechanisms at different temperature and pressure, and the observed brittle or ductile behaviour of pyrite in naturally deformed sulphide bodies. An attempt is made here to analyze the deformation behaviour of pyrite under different temperature-pressure conditions, by studying pyritic ores in three sediment-hosted Pb–Zn sulphide deposits of Rajasthan (Balaria-Zawar, Rajpura-Dariba and Rampura-Agucha), occurring in broadly similar geological settings, but deformed and metamorphosed at different grades (upper greenschist, middle amphibolite and upper amphibolite to granulite facies respectively). Observations of hand specimens and optical microscopy of pyritic ores from Rajasthan have shown that the mineral behaved in a macroscopically ductile manner—not only in the form of mesoscopic and microscopic folding of layers, but also by distortions, bending and stretching of individual grains. In general, pyrite plasticity increases with temperature as revealed by more definitive evidences of plastic deformation in higher metamorphic grade deposits (e.g. Rajpura-Dariba and Rampura-Agucha) than in lower grade Balaria ores from the Zawar ore district. However, the Balaria ore, characterized by the coexistence of framboidal (sedimentary-diagenetic) and idiomorphic (metamorphic) pyrite, is more intensely folded. Higher grade ores may, on the other hand, induce more grain growth and thereby are likely to lose the evidence of plastic deformation through polygonization and grain coarsening. This may be one reason behind the apparent scarcity of plastic deformation textures observed in pyrite from naturally deformed and metamorphosed sulphide ore deposits.

Soil: An Essential but Somewhat Neglected Natural Resource

Soil is an essential natural resource for the existence of life on Earth. The Science dealing with soil is Pedology. Soil is a complex aggregate of inorganic ± organic matter occurring at or close to the Earth’s surface. Soil is, however, defined differently for different purposes. Soil formed even in the Archean time, but was modified subsequently. The soil section starting from the surface layer to the protolith is called soil profile. Texturally, soil is referred to three end members: sand, soil, and clay. Major composition of the soil and their relative proportions are given as follows: O > Si > Al > Fe = C = Ca > K > Na > Mg > Ti > N > S. Soil essentially forms by the decomposition (with/without transport of the products) of the exposed rocks. Commonly organic matter is added to the upper horizons. The following factors play important roles in the formation of soil: climate, activity of organisms, nature of the protolith, topography, and duration of the exposure time. There are a number of systems of classification of soils, including one proposed by FAO. However, many countries prefer to use their own system(s), based on the ground reality. Soil erosion is the worst hazard in the case of soils. Erosion means removal of the upper fertile part of the soil, by natural or human interference. A grass cover is a natural protection for the soil. Despande and Sarkar (2009) estimated the different soil types of India as follows: alfisols 13.55%, inceptisols 39.74%, aridisols 4.3%, entisols, 28.1%, vertisols 8.5%, and ultisols 2.5%. Conditions of soil formation being different in different parts of the country, nature of soil in India varies widely from place to place.

Nonmetals, Industrial Minerals and Gemstones

Nonmetallic and industrial minerals are the essential raw materials for a number of industries. Gemstones have been in demand throughout much of human history. Nonmetallic and industrial minerals are divided into the following groups: refractory minerals, fertilizer minerals, minerals used in cement industry, chemical industry, electrical industry, glass and ceramic industry, abrasive industry, and those used as fillers and pigments, and as building stones. Refractory minerals are those that can, besides withstanding high temperatures (up to ~1500 ℃), endure thermal and mechanical shocks. They are mostly used in the internal lining of metallurgical furnaces. They are categorized into three sub-types: acid, neutral, and basic, depending on their relationship with various kinds of slags and furnace linings. The acidic variety includes quartz, fire clay, ball clay, kyanite, sillimanite; the neutral variety includes chromite, graphite and asbestos, while the basic variety comprises magnesite, dolomite, and bauxite. India has got a reasonably good reserve of refractory minerals, being the largest repository of alumino-silicate minerals. The principal fertilizer mineral is a phosphate (phosphorite, apatite), now used mostly for the manufacture of superphosphate using H2SO4. Bulk of phosphate mineral deposits are sedimentary (-diagenetic) in origin. A number of phosphate deposits are in Rajasthan, followed by Madhya Pradesh, Uttarakhand, and West Bengal. However, India imports phosphate ore from West Asia. K-bearing fertilizer minerals occur in large quantities in Nagaur-Ganganagar basin, Rajasthan. Portland cement requires limestone, clay, and some 5% gypsum for its manufacture. India has a satisfactory reserve of these raw materials. Plaster of Paris is made with gypsum. Minerals needed in chemical industry are native sulfur, Fe-sulfides, barite, and fluorite. In fact, >80% of barite mined is used in making “heavy mud” for the oil drilling industry while fluorite’s main use is as flux in metallurgical processes. Minerals used for thermo-electrical insulators comprise muscovite mica, asbestos, steatite, talc, vermiculite, and pyrophyllite. Li is extractable from lepidolite. Micas are available aplenty from the mica-pegmatites of Bihar, Rajasthan, and Andhra Pradesh. Glass and ceramic materials are in great demand for making quite a few things necessary for our everyday life, as well as in industries. We have necessary reserves of quartz, feldspar, and clays for this industry. Industrial abrasives vary in hardness and may be natural or manufactured. Hard abrasives are more manufactured these days. Natural hard mineral matter for abrasives includes diamond (industrial variety or bort) and corundum. Natural soft abrasive materials are many. Building stones comprise granite gneisses, granites, marble, limestone, sandstone and slate. Charnockites and khondalites also make good building stones. Quality of a building stone is determined by its beauty and durability. India’s annual business in this trade exceeds INR 10 billion. The most precious gemstones found in India are diamond, followed by ruby, sapphire and emerald. Diamonds are obtained from kimberlites and lamproites and related coarse sediments in central and south India. Ruby, sapphires, and emerald are found in the Kashmir Himalaya, Rajasthan, and Karnataka. Precious quality zircon (gomed) is found in the beach sands of peninsular India. The bulk of India’s need of gemstones is, however, satisfied by other countries including South Africa, Sri Lanka, and Myanmar.

Metallic Mineral Deposits

Iron, and hence iron ores, have played a very important role in the history of human civilization, because of which the latter has been named “Iron Age”. The principal types of iron ores are variously enriched hematite-dominated banded iron formation (Superior type) and variously modified magnetite-dominant banded iron formation (Algoma type). The Superior type deposits mainly formed during late Archean-early Proterozoic period. Algoma type is somewhat older. India’s iron ore deposits, with an estimated reserve of 28 Gt, are concentrated in South India, Central India, and Eastern India. Mn ores, principally oxides to hydroxides, formed during separate events from early Proterozoic to recent (Mn-nodules) in South Africa, Brazil, Ghana, Gabon, India, and countries of previous USSR. Genetically the deposits are hydrothermal, sedimentary (-diagenetic), and of supergene enrichment. Chromium, obtained from chromite, is essentially magmatitic (orthomagmatic) in origin and chromite deposits are commonly associated with mafic–ultramafic rocks. Out of India’s reserves of 139 Mt of chromite ores, >90% is located in eastern India and the rest in south India. Bulk of gold in nature forms in the “native state”, which does not mean 100% purity in reality. Alloying with Ag (and Hg) is common. Most ores form from hydrothermal fluids of primary and secondary origin and as placers (and nuggets). Gold is reported from different parts of India, but the largest yield came from the Kolar gold field in Karnataka amounting to 800 t Au. Copper mineralization was recorded from the Singhbhum copper belt, occurring in association with altered volcanic rocks, now interpreted by some to represent an IOCG situation. Khetri copper deposits are associated with metasediments. Malanjkhand Cu(–Mo) deposit is associated with calcalkaline granitic rocks. SEDEX-type Pb–Zn deposits of Rampura-Agucha, Rajpura-Dariba, and Zawar belt were deposited in Paleoproterozoic sediments of Rajasthan. All these deposits are regionally metamorphosed in grades varying from granulite to upper greenschist facies. Total reserves of Pb–Zn ores were estimated to be 685.59 Mt in 2010. Mineralization of Sn, W, Nb, Ta, Li, Be, F, and REE occur in the pegmatites of Bastar-Malkangiri belt, Bihar mica belt and Rajasthan-Gujarat areas. Monazite in the beach sands of South India is a good source of REE. The same sands are also endowed with large Ti resource in the form of ilmenite and rutile. India is one of the five major Al-ore containing countries of the world. Bauxite is of residual origin produced from Al-rich rocks. In India, the principal belts of bauxite are located along the East coast, Central India, and the West coast. Warm and wet climate with good drainage favor bauxitization.

Issues of Sustainable Development in the Mines and Minerals Sector in India

India, the third largest economy in the world, has shown phenomenal growth in the last decade or so, but is often criticized for its skewed development. This fact is glaring in the mineral-rich states of east-central India where the tribal population in the mining belts of Jharkhand, Odisha, Chhattisgarh, Madhya Pradesh, and Andhra Pradesh suffer from grinding poverty and malnutrition under a low Human Development Index. It raises a vital question: in rich land with poor people, is sustainable mining possible? We look into this complex socioeconomic and industrial challenge in this concluding chapter in terms of the vital things at stake: natural resources, people, culture and livelihoods, forests and wildlife, water resources, environmental and ecological integrity, and finally the issue of land acquisition for mining. The various issues have snow-balled and affected many large mining projects with very large FDI components. The problem is exacerbated by rampant and large-scale illegal mining of bulk minerals, including coal, which has forced courts to step in and ban iron ore mining in parts of Goa, Karnataka, and Odisha. Environmental degradation and inter-generational equity issues have also been raised by the judiciary to impose the ban. Natural resource utilization is an essential prerequisite for industrial development, and therefore, the three stakeholders, consumers, industry, and government, must join hands to pursue mining under a sustainable development framework which involves scientific mining, environmental protection and mitigation, community stakeholders’ engagement, local socioeconomic development along with a high degree of transparency, and accountability. A proper mine closure plan should also be an integral part of any mining proposal. Ecosystem management in a mining area should also be an equally important remedial measure when mining ceases. The other important aspects of sustainability in mining also need to be highlighted. Geogenic toxic elements which are released during mining into the ground and surface waters, like CrVI in the Sukinda valley in Odisha, have severe adverse effect on human health. Besides, acid mine drainage may be generated from waste rocks, tailing dumps, open pits, and abandoned underground mines of sulfidic ores and flow into rivers, streams, or lakes. Their negative effects need to be controlled and neutralized. Biological techniques are useful in this regard. Heap leaching of low-grade ores, of Cu for example, has the potentiality for acid water generation too. Cement plants are known to be the worst polluters, both in terms of cement dust and limestone mining. However, situation seems to be improving in modern plants. In the energy front, coal is a necessary evil generating a large part of thermal power but contributing to the global carbon emission and is thus responsible for global warming through greenhouse gas enhancement in the atmosphere. Oil and natural gas, absolutely essential commodities, are potent sources of environmental pollution right from exploration through production, refining, transportation, and use. Uranium ores also affect the environment when the radioactive metal is released into mine water, or is present in effluent of the ore processing plant and tailing dams and is finally passed on to the hydrologic system. Subsequently, it may enter the food chain and serve as a potential carcinogen. Nuclear energy production itself still has challenges of sustainable development as indicated by the recent disaster in Japan. Equally challenging is the issue of nuclear waste management.