Search

The Vindhyan Basin in central India preserves a thick (~5 km) sequence of sedimentary and lesser volcanic rocks that provide a valuable archive of a part of the Proterozoic (~1800&ndash, 900 Ma) in India. Here, we present an analysis of key sedimentary pyrite textures and their trace element and sulfur isotope compositions in the Bijaigarh Shale (1210 &plusmn, 52 Ma) in the Vindhyan Supergroup, using reflected light microscopy, LA-ICP-MS and SHRIMP-SI, respectively. A variety of sedimentary pyrite textures (fine-grained disseminated to aggregates, framboids, lags, and possibly microbial pyrite textures) are observed reflecting quiet and strongly anoxic water column conditions punctuated by occasional high-energy events (storm incursions). Key redox sensitive or sensitive to oxidative weathering trace elements (Co, Ni, Zn, Mo, Se) and ratios of (Se/Co, Mo/Co, Zn/Co) measured in sedimentary pyrites from the Bijaigarh Shale are used to infer atmospheric redox conditions during its deposition. Most trace elements are depleted relative to Proterozoic mean values. Sulfur isotope compositions of pyrite, measured using SHRIMP-SI, show an increase in ծ34S as we move up stratigraphy with positive ծ34S values ranging from 5.9&permil, (lower) to 26.08&permil, (upper). We propose limited sulphate supply caused the pyrites to incorporate the heavier isotope. Overall, we interpret these low trace element signatures and heavy sulfur isotope compositions to indicate relatively suppressed oxidative weathering on land during the deposition of the Bijaigarh Shale.

Delhi, capital of the world’s largest democracy, is witnessing large-scale increase in population since the beginning of the twentieth century. Two prominent factors that have contributed to this include the shifting of capital of the British Raj from Calcutta (now Kolkata) to Delhi in 1911 and the partition of India that accompanied its independence in 1947. Delhi continued to witness high rate of migration in post-independent India due to uneven implementation of development policies. Rising population led to spatial expansion and the largest connotation of Delhi today (National Capital Region) is an area 36 times its size in 1947. Rising population has also had an adverse impact on Delhi’s natural resources. Consequently, clean air, water and land availability have become limited and Delhi today is undergoing a severe sustainability crisis. The latter requires urgent intervention for restoring Delhi’s urban ecosystem. Since urban areas are highly contested ecological spaces, urban ecological interventions are incomplete without political overtones. Thus, the success of urban ecological interventions lies in identifying politically correct boundaries which encompasses true ‘urban Delhi’ despite the political boundaries. This research contribution attempts to identify the geographical expanse of ‘urban Delhi’ amidst the various political terminologies that define Delhi. An understanding of various divisions and definitions of Delhi is also presented from the perspective of appreciating the challenges in urban planning. We conclude that urban ecology investigations in Delhi should be embedded within the ‘Delhi conurbation’, which represents a geographical area greater than the Delhi city-state but much smaller than Delhi NCR.

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.

Formation and transformation of mineral deposits are interactions of geospheres, one including the atmosphere, hydrosphere, biosphere, lithosphere, and asthenosphere and the other involving the mantle and the core of the earth. Complex chemical and thermal interactions between these two geospheres have led to distribution and concentration of elements and even, later modifications, producing the mineral or ore deposits of today. The essential processes involve magmatism, hydrothermal, and sedimentary processes with a strong impact of tectonism and in places, of weathering and erosion. The genetic processes vary in details. The principal ones are outlined below with the principal products in parentheses: (1) Essentially magmatic processes (Ni, Cu, PGE Cr, Fe–Ti); (2) Pegmatitic processes (rare metals, ceramic, and radioactive elements); (3) Essentially magmatic hydrothermal processes (Sn, W, U, Cu, Mo, REE); (4) Essentially amagmatic hydrothermal processes (Cu, Pb–Zn, Au, U); (5) Sedimentary (-diagenetic) processes (Fe, Mn, U, Sn, Ti, monazite, phosphorite, carbonate rocks, rock salt gypsum); (6) Lateritic and non-lateritic residual processes (Fe, Mn, Al, Ni, and clays); (7) Supergene oxidation and enrichment (Cu, Ag, Au, U); (8) Biogeochemical degradation of biomass (peat-lignite-coal, natural gas, and oil).

India is a country endowed with rich mineral resources, producing as many as 87 mineral commodities (including energy-producing minerals). The country is self-sufficient in bauxite, chromite, iron and manganese ores, ilmenite, rutile, coal (except coking coal) and lignite, and almost all industrial minerals. Along with barite and limestone, these are among the ten largest mineral reserves of the world. The structure of the Indian M & M sector has evolved considerably in the last two and half decades, since the liberalization of the economy and opening up of the sector to private domestic and foreign investment. The mineral commodities are thus extracted through large and small mines, which belong to either public or private sector enterprises. There is also a large informal sector of artisanal and small-scale mining (ASM), whose status remains rather ambiguous in the present regulatory regime. ASM operations need to be regularized by the government, acknowledging their livelihood and poverty alleviation potential. Mining, thus, is an important sector in Indian economy and contributed about 2.6% to the GDP in 2011. Since then the sector is, however, in a negative growth path with a substantial reduction in the number of operating mines. Various reasons have been ascribed to this decline. They include: regulatory issues, environmental activism and court cases, social unrest due to land acquisition problems and displacement, and Naxalite violence along the “Red corridor” in east-central India. The M & M sector is currently struggling to recover its earlier growth rate by streamlining regulatory and administrative procedures, developing the required infrastructure and taking care of the issues of sustainability. The framework of various regulatory provisions and policies governing this sector need to be understood to appreciate the issues in this regard. The 7th schedule of the Constitution includes Article 246 which categorizes the Union, State and Concurrent lists of responsibilities under which regulations of mines and mineral development fall. Thus, the proprietary control of onshore mineral resources is with the state government while the regulatory powers are with the central government. All mining activities in the country are controlled by the National Mineral Policy while the basic laws governing the mining sector come under the purview of the Mines & Minerals Development and Regulations (MMDR) Act. The other Acts concerned with mining are: Environment Protection Act, Environmental Impact Assessment Act, Forest Conservation Act, Land Acquisition, and related Acts and Panchayats Act (PESA). The regulatory regime is thus quite wide and complex, with duality of control exercised by the state and central governments. This consequently leads to divided accountability and poor implementation of the laws. The marine mineral resources are exploited keeping the “UN convention on the Law of the Sea” (UNCLOS) in view. It divides the sea into three parts: the territorial sea, the exclusive economic zone (EEZ), and the international area of the seabed. The coastal nations have exclusive rights to exploit marine resources up to their respective EEZs.

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.

Geodynamics is the dynamics of the Earth. Metallogeny is the genesis of metals, rather the metal-bearing mineral deposits. So, ‘Geodynamic context of metallogeny’ relates metallogeny to geodynamics. We generally refer to geodynamics in terms of plates and plumes, that ultimately control magmatism, sedimentation, and metamorphism and hence ore genesis in the crust-mantle system. The solid earth is made up of physically distinct layers (from surface downward) of lithosphere, asthenosphere, mesosphere, outer core, and an inner core. The lithosphere is presently divided in to seven large and a few small pieces (‘plates’). They underwent making, breaking, and remaking several times during much of the Earth’s history, giving rise to what we call supercontinents and supercycles today. Formation of mineral deposits, which are in fact products of petrogenesis of sorts, controlled by tectonics, therefore varied in time and space. Tectonic setting of mineralization are today accepted to be as follows: continental hot spot produce Cr, Ni–Pt–Cu deposits, diamonds, certain stratiform base metal deposits, and even granite-related mineralization of Sn. Constructive plate boundaries host Fe-, Mn-oxides along the rifted ridge, and also Alpine-type chromite deposits. Passive continental margins localize deposition of evaporites, phosphates, BIFs, carbonate-hosted Pb–Zn deposits. Subduction zones produce porphyry type Cu–Mo–(Au) deposits and may be also Cu + Au skarn deposits. Continental collision zones contain podiform chromite, VMS, U and porphyry Cu deposits. Greenstone belts (ancient volcano-plutonic arcs?) are known for being the major localizer of orogenic gold, komatiite-related Cu–Ni deposits and also Cu–Zn mineralizations. Intracratonic settings are credited with IOCG mineralization. Tectonically stable regions with plateau-like relief and located in tropical and sub-tropical regions are best suited for lateritization.

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.

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.

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.

The role of polyphase deformation in controlling the emplacement of gold-quartz lodes in dilational regimes is demonstrated from the Proterozoic Bhukia–Jagpura gold prospect in south Rajasthan. Earlier researchers deciphered the gold-sulphide mineralisation event as synchronous to the second phase of deformation (D2) without convincing microstructural or metamorphic evidences. In this contribution, we correlate the deformation and metamorphic imprints in the host rocks with those in the gold-sulphide mineralised zone, and present a new interpretation for the relative timing of gold emplacement vis-a-vis deformation. The ore-forming process first involved layer-parallel influx of ore-bearing hydrothermal fluids along S1 schistosity in the host rocks, synkinematic with respect to the first phase of deformation (D1). This initial ore concentration experienced metamorphism isofacially (~500°C at 5.3 kb) along with its host rocks during D1, and subsequently underwent extensive remobilisation, giving rise to gold-bearing silicified lodes along the hinges and axial surfaces of F2 folds during D2.

Proterozoic sedimentary successions of India are important archives of both the tectonic history of the Indian subcontinent and the geochemical evolution of Earth surface processes. However, the lack of firm age constraints on many of these stratigraphic units limits their current utility. Here, we present new detrital zircon age data from strata of the southern Aravalli–Delhi Orogenic Belt (ADOB) and the Rajasthan Vindhyan successions. The Alwar Group of the southern Delhi Supergroup yielded a large population of ∼1.2 Ga detrital zircon grains, which refutes the 1.9–1.7 Ga age assertion for this unit. Detrital zircon age distributions from the southern Alwar Group differ strongly from the Alwar Group of the “North Delhi Belt”, demonstrating miscorrelation between these two regions. The Jhamarkotra Formation of the Lower Aravalli Group contains a large population of 1.9–1.7 Ga detrital zircon grains. Therefore, the unit cannot be ∼2.1 Ga as traditionally assumed. Age distributions of the Aravalli and Delhi supergroups are similar to those of the lower and upper Vindhyan successions, and we postulate contiguous sediment sources for both regions, with strata of the tectonically deformed ADOB representing the distal margin equivalents of the Vindhyan successions. Additionally, a late Paleoproterozoic age for the Jhamarkotra Formation nullifies the hypothesis that the markedly positive carbonate δ13C values in this unit are linked to the 2.3–2.0 Ga Lomagundi–Jatuli positive isotope excursion. The potential of a large late Paleoproterozoic (ca. 1.7 Ga) positive δ13C excursion contrasts with the long-held view of a prolonged period of carbon isotope stasis during the so-called ‘boring billion’.

The Bhukia-Jagpura gold prospect is hosted by a Paleoproterozoic metasedimentary rock sequence that constitutes the lowermost part of the Aravalli Supergroup, lying in close proximity to the basement-cover contact. A combined ore petrographic, LA-ICPMS and EPMA studies of iron sulfides and sulfarsenides from the prospect has revealed four varieties of pyrite and two varieties of arsenopyrite with different trace element compositions. The data indicates a sedimentary-diagenetic origin for the earliest Pyrites (Py) I and II, which are devoid of any gold, in contrast to hydrothermal Py III and IV with a minor, but significant content of invisible gold (0.3–1.6 ppm). Associated loellingite (35–51 ppm) and Arsenopyrite (Aspy) I (0.28–10 ppm) contain the maximum concentration of invisible gold, while it is substantially lower in Aspy II (0.06–1.5 ppm). The compositions of the gold-bearing pyrites (low Ni:Co ratios), arsenopyrites and loellingite (rich in Co), and the presence of Au–Bi–Te mineral phases within arsenopyrite, support a magmatic hydrothermal model for the origin of gold–sulfide mineralization, which appears to have taken place during the peak of regional metamorphism, or at an earlier stage. The different stages of deformation and metamorphism released and remobilized gold in invisible and discrete forms from the sulfarsenides. During prograde metamorphism of Aspy I, gold was partially incorporated and relatively enriched within the structure of loellingite. Later, during retrograde metamorphism gold was exsolved as visible grains at the interface of loellingite and Aspy II and/or within loellingite.

Breithauptite (NiSb), a rare antimonide, is generally found associated with hydrothermal vein ores of Ni and Co. In India, it has so far been recorded only from the gold-quartz veins of the Kolar Gold Field. Occurrence of this antimonide in a sediment-hosted milieu however, has seldom been noted. We report here the identification and characterization of this rare phase in both the stratiform and the vein-type ores of the sediment-hosted Zn-Pb sulphide deposits and prospects within the Dariba-Rajpura-Bethumni metallogenic belt in Northwestern India. Metamorphic remobilization from the stratiform ores of the belt has been postulated as a possible mechanism for the conspicuous segregation of breithauptite as a discrete phase within these ores.

The very low-grade metamorphic sequence of volcano-sedimentary rocks, sandwiched between the platform sediments of the Vindhyan Supergroup to the east and the Banded Gneissic Complex (BGC) to the west, in the eastern fringe of the Aravalli-Delhi orogenic belt, has remained a stratigraphic enigma in the Precambrian geology of Rajasthan. This sequence known earlier as the Gwalior ‘series’ and in contemporary literature as the Hindoli Group, has been considered by several workers as a Proterozoic supracrustal unit and by some others, as an Archean secondary greenstone belt, based purely on geological considerations. U-Pb zircon geochronology was conducted to find an answer to this controversy on samples of felsic volcanics, conformably intercalated with the Hindoli sediments and hence, considered contemporaneous with them. Zircons from a sample of massive rhyodacite gave a concordia age of 1854k7 Ma though zircons from a sample of felsic tuff gave a wide range of ages between 3259-1877 Ma. Careful consideration of the nature of the samples and their constituent zircons suggests that the Hindoli Group rocks represent a low-grade Proterozoic supracrustal cover sequence in the eastern part of the Bhilwara belt, broadly synchronous to the Aravalli-Bhilwara sedimentation around 1.8 Ga.

Mineral Deposits in India

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.

Pyrite is an ubiquitous constituent of the Proterozoic massive sulphide deposit at Deri, in the South Delhi Fold Belt of southern Rajasthan. Preserved pyrite microfabrics in the Zn-Pb-Cu sulphide ores of Deri reveal a polyphase growth history of the iron sulphide and enable the tectono-thermal evolution of the deposit to be reconstructed.Primary sedimentary features in Deri pyrites are preserved as compositional banding. Regional metamorphism from mid-greenschist to low amphibolite facies is recorded by various microtextures of pyrite. Trails of fine grained pyrite inclusions within hornblende porphyroblasts define S1-schistosity. Pyrite boudins aligned parallel to S1 mark the brittle–ductile transformation of pyrite during the earliest deformation in the region. Isoclinal to tight folds (F1 and F2) in pyrite layers relate to a ductile deformation stage during progressive regional metamorphism. Peak metamorphic conditions around 550°C, an estimation supported by garnet–biotite thermometry, resulted in annealing of pyrite grains, while porphyroblastic growth of pyrite (up to 900 µm) took place along the retrogressive path. Brittle deformation of pyrite and growth of irregular pyritic mass around such fractured porphyroblasts characterize the waning phase of regional metamorphism. A subsequent phase of stress-free, thermal metamorphism is recorded in the decussate and rosette textures of arsenopyrite prisms replacing irregular pyritic mass. Annealing of such patchy pyrite provides information regarding the temperature conditions during this episode of thermal metamorphism which is consistent with the hornblendehornfels facies metamorphism interpreted from magnetite–ilmenite geothermometry (550°C) and sphalerite geobarometry (3.5 kbar). A mild cataclastic deformation during the penultimate phase produced microfaults in twinned arsenopyrite prisms.

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.

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.