Earth Processes and Resources

Metallogeny

Prof. Mihir Deb Department of Geology

University of Delhi Delhi-110007

& Dr. Gurmeet Kaur

Centre for Advanced study in Geology Panjab University, Chandigarh

CONTENTS Factors controlling mineral availability Global mineral reserves and resources Metals

Iron Manganese Chromium Copper Lead-Zinc Gold Aluminium

Non-metals Refractory minerals Acid refractories Silica Fire clay Ball clays Kyanite Sillimanite Neutral refractories Chromite Graphite Asbestos Basic refractories Magnesite

Dolomite Minerals of Fertilizer industry Rock phosphates Minerals of Cement industry Limestone Gypsum Minerals of Chemical industry

Sulphur Pyrite, Pyrrhotite and Marcasite Barite

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Building stones Gemstones

Diamond Other common gemstones

Ruby, Sapphire and Emerald Zircon Jade and Nephrite

Global tectonics and metallogeny through geological times Methods of mineral exploration, exploitation and processing Environmental implications References Factors controlling mineral availability

Mineral concentrations in the crust of the Earth are produced by the interplay of various geological processes. However, even if such a concentration is located and explored, it can not be exploited till several critical factors are satisfied. The main factors controlling the availability of mineral deposits in this sense are: geological, engineering, environmental and economic (cf. Kesler, 1994). All the metals commonly exploited in ore deposits are found in all crustal rocks, but in very minute quantities (such crustal abundance of metals being expressed in ‘Clarke’ values, which represent a ratio of a particular element in a rock to the average amount of the element in the Earth’s crust). Various crustal processes like magmatism, metamorphism, sedimentation etc. bring about concentrations of such dispersed metals in crustal rocks in specific geological settings. For example a copper skarn deposit is found only where a felsic pluton has intruded into an impure limestone country or a concentration of refractory kyanite is found only where aluminous sediments have undergone regional metamorphism. Thus geological factors play a paramount role in the availability of mineral deposits. However, even if a mineral deposit is identified, it may not be workable unless certain engineering constraints are satisfied. For example, present level of mining engineering in advanced countries does not allow deposits to be worked at depths greater than about 4 km, while the deepest oil wells are about 8 km long. Any resource beyond these limits is unavailable for exploitation. Some of these engineering constraints are also closely linked to the economics of exploitation. For example, the Kolar gold mine (Champion Reef) closed down because, although possible, it was not economically viable to mine at depths of 11,000 feet where a huge expense was borne for air-conditioning of the mines. At times, mineral technology also does not allow the exploitation of a known resource. For example, the large Pb-Zn sulfide deposit at McArthur River in Australia could not be mined for several years because the fine grained ore was not amenable to beneficiation and hence an ore concentrate for smelting could not be produced. Environmental factors play an equally vital role in the exploitation of a known resource. A large deposit of uranium has been located and explored in recent years in the Lambhapur area of Srisailam district of Andhra Pradesh. But will this deposit be given environmental clearance for mining by the Ministry of Environment and Forests since it is located within the Rajiv Gandhi Tiger Reserve? We will need to wait and see. If environmental degradation is assessed to be high a good known resource may not be allowed to be exploited by mining.

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Factors of economics play the most crucial role in the exploitation of a resource. Profit remains the major reason for mining a particular resource, except in the case of strategic minerals which can be mined without considering economic factors at a time of emergency, such as, war. In a free economy, the cost of mineral production will regulate the price of the commodity. Even for a good deposit with high grade and reserve, if the cost of extraction, beneficiation and environmental protection is too high, it may not be amenable to mining at a particular point in time, when it might be cheaper for a country to import that metal from abroad. The other important factor controlling the price is demand and supply. Demand for a metal may change over a short period of time because of stock piling, recycling as well as substitution and availability of new technology. Global mineral reserves and resources The global demand for minerals has increased steadily since the industrial revolution and exponentially in the latter half of the 20th century. In the last few decades the demand is not only for higher tonnage but also for a range of mineral commodities, such as the high tech metals like In, Ga, Ge etc. which are required for specific applications due to their physical and chemical characters, and electrical conductivity. The increasing global population and affluence of societies is bound to keep this demand rising. The global economy, at least a significant part of it, is clearly dependant on mineral production which is the backbone of modern industrialization. This is more than obvious from the large value and amount of global mineral production (Fig. 1A & B).

Fig. 1: (A): The value of world’s mineral production in the 1990s (modified from Kesler, 1994). (B): Quantity of world’s mineral production in 2004, based on British Geological Survey data. However, several complex issues are involved in the understanding of mineral resources in the context of global economy. Only some major issues are highlighted here. There is ample statistical data available to show the remarkable correlation between economic activity of a country, industrial production and consumption of basic mineral commodities. A good indication of this correlation should be obtained by comparing the value of mineral production in a country with the Gross Domestic Product (GDP) (Kesler, 1994). We however, find that mineral production in USA and Japan makes up less than 2 % of the GDP while it is about 25% for Saudi Arabia and 35% for Kuwait, the two major oil producing countries in the Middle East. This is in spite of the fact that USA is the leading producer of 19 different mineral commodities in the world.

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Another aspect is that the pattern of consumption of mineral commodities in a country and their availability in it shows no relationship in the modern world. Countries with large and important mineral resources, such as South Africa and Australia are no doubt economically strong, but Japan with very limited mineral resource is also equally strong, if not more. As it is mineral resources are neither uniformly distributed in different parts of the world (see a later section for more details), nor are they consumed equitably by the different countries. For example, 95% of world resources of chromium are found in South Africa alone. This has led to conflicts and even war in different periods of history. Even today the control of strategic world resources like petroleum continues to play a major role in world politics. Besides, in recent years, stringent environmental laws have made mining economically unviable in many developed countries though their requirements of mineral raw material have not diminished at all. Many of the remaining high grade resources occur in the developing world where the pressure and impact of mining is continuously increasing. While many of the developed countries like USA, Canada, Australia, South Africa, Sweden, Finland and a few developing countries like Chile and Botswana have achieved considerable economic success through reliance on metals whereas many developing world nations with long history of mining have failed to get the direct benefit from the exploitation of their resources. In general, in the initial stages of development of a country the raw materials are exported to earn much needed foreign exchange. With time and with the earnings from these exports the industrial infrastructure of the country is put in place and manufactured goods and products are then exported which fetch much more revenue. India is probably going through this phase now. But in the context of the developed countries with limited mining activities and that of Japan in particular, import reliance is the key factor. To safeguard continuous supply of raw materials most of these developed countries, including Japan, are investing in the promotion of mining projects in foreign countries. For example the railway infrastructure to transport the Bailadila iron ore from Bastar to the port of Vishakhapatnam was fully aided by Japan in early 1970s. India earns substantial foreign exchange by selling a part of its vast resource of iron ore. It is almost self sufficient in aluminium, manganese (though chemical grade of Mn has to be imported) and lead-zinc but has to import copper, nickel and tin. Thus the concept that the security of a country depends on its mineral supply does not hold in the contemporary world in the era of free trade. A developed country can buy whatever mineral it needs in the world mineral market which however, is controlled by large multinational groups or companies. A better idea of the distribution of important mineral commodities in the world will be obtained in a later section. Geological setting, mineralogical characteristics and distribution of important mineral deposits in India Metals Iron Iron is one of the most abundant metals and has the third highest crustal abundance (5.6 %), next to aluminium (8.2%) and silicon (28.2%). Iron accounts for more than 95% of all metals used by the modern society. In fact, the industrial growth of a country is measured, amongst other criteria, by the amount of iron consumption and steel production. The ore minerals from which iron is extracted are hematite (Fe2O3), magnetite (Fe3O4) and goethite (FeOOH). Iron smelting is carried out by reducing iron oxides to iron metal by reaction with carbon monoxide gas, usually derived from coke (Craig et al., 1996). Iron ores of magmatic, sedimentary and metamorphic origin are found in different geological settings. Magnetite occurs associated with layered mafic-ultramafic intrusions as magmatic

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segregations. Volcanic exhalations are also thought to be responsible for magnetite mineralization, as in Kiruna type deposits of Sweden. Magnetite and micaceous hematite (specularite) are produced by contact metamorphism, as in Iron springs, Utah, USA. Iron ores, initially of sedimentary origin, are the ones which account for the largest resource of the metal and are exploited extensively the world over. They are discussed in detail in the next section. Lateritic iron ores are prevalent in tropical humid regions over ferruginous bed rocks. Two major groups of iron-rich sedimentary rocks are commonly recognized (James, 1966): Banded Iron Formations (BIF): These are represented by extensive thick sequences of Precambrian (Proterozoic) age. Typically laminated with fine grained hematitic layers interbanded with chert/jasper/quartzite. The hematite is generally non-oolitic. Different terminologies are used for referring to BIFs in different countries: taconite in Lake Superior region of U.S.A, Itabirite in Brazil; Calico rock in S. Africa; Jaspilite in Western Australia and Banded Hematite Quartzite (BHQ)/Banded Magnetite Quartzite (BMQ) in India. BIFs are often loosely termed as ‘iron ores’, although they are the protoliths of most large iron ore deposits. Iron stones: These are poorly banded, non-cherty and oolitic ores, represented by hematite and goethite. They are Phanerozoic in age. Gross (1966) distinguished two main types of BIFs from the Precambrian. These are the Archean Algoma type and the Proterozoic Superior type of Iron Formations. The former is characterized by thin banding and absence of oolitic and granular texture, limited lateral extent and close association with volcanic rocks, greywackes and pyritic black shales. The latter is characterized by thick bandings, large lateral extent and close association with sediments like quartzites, dolomites and pelites with no direct affiliation with volcanic rocks. This two-fold classification of the BIFs is however, problematic. For example, in India the BIFs possess the characteristics of both Algoma and Superior types. The same is true for the West Australian deposits, particularly those in the Hammersley basin. In terms of environment of deposition, most Proterozoic BIFs are typical of platform association while their Archean counterparts show characteristics of deep water environment. The BIFs, particularly in the Archean, often show the development of four different facies (James, 1954). These are: oxide facies with magnetite and hematite subfacies; silicate facies; carbonate facies and sulphide facies (Fig. 2). The relationship of these four facies, which rarely occur together at the same place, has been described to be gradational, one type passing into the other from oxide facies in the shallow waters to the sulfide facies in the deep waters.

Fig. 2. Cartoon depicting the four facies of iron formations developed in a shallow sea.

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Indian distribution The iron ore deposits of India can be divided into four groups according to their mode of formation. The most important group includes the banded iron ores of Precambrian age. These deposits are the back bone of iron and steel industry in India and their export to countries like Japan fetch a huge amount of foreign exchange for the country. The total reserves are estimated at over 17,000 million tons, of which 14,000 million tons represent hematitic ores and the rest are magnetitic ores. These iron-ore deposits can be considered under two main groupings (Radhakrishna et al., 1986): (a) those occurring within complexly folded BIFs in high grade terrain in parts of Andhra Pradesh, southern Karnataka, Kerala and Tamil Nadu, and (b) those confined to the Archean schist (greenstone) belts in Jharkhand, Orissa, Madhya Pradesh, Maharashtra, Goa, and Karnataka, accounting for the predominant Iron resource of the country. The first group deposits are considered to be > 3000 Ma old whereas the deposits of the second group formed during the period 2900 to 2600 Ma.

Continuous bands of Iron Formations are exposed in three principal belts around the Singhbhum granite massif in south Jharkhand - northern Orissa: 1. Gorumahisani and Badampahar hills of Mayurbhanj district of Orissa in the east; 2. Jamda-Koira valley deposits (including the well known Noamundi deposit of TISCO) to the west and 3. The E-W trending formations of the Sukinda valley including the Tomaka-Daiteri and Malayagiri deposits in the south. The deposits in the region provide the iron ore to the SAIL steel plants at Rourkella, Bokaro, Durgapur and Kulti, besides TISCO.

Prominent iron ore deposits occur extensively in the Bailadila range and Rowghat hills of Bastar district of Chhattisgarh The Dhalli-Rajhara deposits in Durg district of Chhattisgarh serve as the captive mines to the Bhillai steel plant.

Extensive deposits of BMQ and BHQ occur in the hilly tracts of Goa and Karnataka. In the latter state, prominent occurrences are found in the Bababudan hills, at Kudremukh, in Bellary and Sandur. Proved magnetite deposits are confined to the Chikmagalur district of Karnataka (Bababudan and Kudremukh) and also in the high grade terrains of Salem and North Arcot in Tamil Nadu. The distribution of the important iron ore occurrences in India is shown in Figure 3. The second group comprises the apatite-magnetite ores of the Singhbhum shear zone, in Jharkhand and the titaniferous, vanadiferous magnetite deposits associated with intrusive mafic plutons in Mayurbhanj district of Orissa. The former is concentrated as small lensoid bodies all along the 150 km long shear zone, to the foot wall side of the Cu and U orebodies. The third group consists of sedimentary iron-ores of limonitic or sideritic composition. In Raniganj and Jharia coalfields of lower Gondwana age, ironstone shale formations are encountered in the Barren Measures overlying the lower coal-measures. It is middle Permian in age and its thickness is about 650 metres in the Jharia coalfield. In Raniganj coalfields, the thickness of ironstone shale is about 500 metres. These low grade iron concretions were used as raw material in the Iron works at Kulti, West Bengal, before the advent of the first blast furnace at Tatanagar, now in Jharkhand. Lastly, the lateritic iron-ores found extensively on the Eastern and Western Ghats, are derived from the sub-aerial alterations of iron-bearing minerals in igneous, metamorphic and sedimentary rocks. The basic lavas of the Deccan traps and Rajmahal traps are altered, under humid and tropical climatic conditions, resulting in the formation of hydrated oxides of iron, along with aluminium and manganese. These lateritic iron-ores are low-grade commodities, containing only

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25% to 35% of Fe, so they are not considered now as a source of iron, but in future when the high-grade iron ore deposits will be fully exploited, they may provide the main resource of iron.

Manganese In comparison to iron and aluminium, manganese is much less abundant (0.095%) in the earth’s crust. Hence, manganese deposits are not as common or abundant as the iron ore deposits. However, manganese a metal vital to steel production occurs in deposits of diverse genetic types in many countries and in a large span of the terrestrial geological record. These were produced by direct hydrothermal activity, sedimentary processes, continental weathering (Roy, 1981). They also occur as ferromanganese nodules on many parts of the deep ocean floor. Most of the existing demand for manganese is met from the sedimentary and residual deposits. The deep sea nodules form future resource of manganese and some other important metals such as cobalt, nickel, copper etc. Indian distribution Manganese deposits of Archean age are found to occur in parts of Orissa, Andhra Pradesh and Karnataka (Fig. 3). Mn oxide ores interstratified with shale occur in the Iron Ore Group rocks at Joda, Kalimati, Gurda, Phagua and Mahulsukha areas in Orissa. These deposits are considered to have formed in cratonic shelf environment at ca. 3.0 Ga (Roy, 1981). The Eastern Ghat sequence in Kodur, Garividi and Garbham in Andhra Pradesh, metamorphosed to granulite facies, host Mn oxide ores in a conjectured shallow water shelf environment. The oxide ores are located in these Archean (>2.6 Ga) high grade, pelitic and calc-silicate granulites while Mn silicate-carbonate rocks occur in calc-silicate granulites and garnetiferous quartzites (Roy, 1981). These ores and their host rocks were described as Kodurites by Fermor in 1909. The Kodurites of Andhra Pradesh are considered to be hybrid in character, due to granitic assimilation of the manganese bearing sediments. Similar deposits are described from the Khondalites of Kalahandi and Koraput districts of Orissa. Similar koduritic manganese-ore occurrences have also been reported from Goldongri

Fig. 3. Distribution of iron, manganese and chromium deposits in India.

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hill, north of Jothvad, Panch Mahal district, Gujarat. Mn oxide ores, interstratified with chert and phyllite and closely associated with stromatolitic limestone, occur in Chitradurga-Tumkur, Kumsi-Hornhalli areas of Karnataka. These Chitradurga Group strata are believed to have developed on the shallow platform margins about 2.6 Ga ago (Roy, 1981). Similar association of Mn oxide ores is also found extensively in the Sandur and Shimoga schist belts in Karnataka, spatially adjacent to the Banded Iron Formations. Large scale deposition of Mn ores started from the Early Proterozoic. The Proterozoic (ca. 2.0 Ga) Saussar Group, spreading from Maharastra to Madhya Pradesh in central India (Figs. 3, 4) hosts the largest concentration of Mn ores in India, mostly in gonditic rocks. These regionally metamorphosed manganiferous sediments from central India were first described and named Gondite by L. L. Fermor in 1909. The gondites are quartzose rocks containing spessertite and rhodonite, usually associated with Mn minerals like braunite, along with bixbyite, hausmanite and jacobsite. These Mn silicate-oxide rocks are complexly deformed and interstratified with metapelites and orthoquartzites in the Mansar Formation and less commonly occur as conformable lenses in carbonate rocks of the older Lohangi Formation. The litho-sequence is metamorphosed to grades ranging from low greenschist facies in the east to upper amphibolite facies in the west. It represents metamorphosed equivalents of a limestone-shale-orthoquartzite assemblage that formed in a cratonic shelf environment without any volcanic rock association (Roy, 1966; 1981). The well known deposits are in Ukwa and Bharweli in Balaghat district in the east, Chikla, Tirodi and Mansar in the central part of the belt and Gowari Wadhona in Chindwara district in the west. Manganese oxide ores interbedded with chert and enclosed in limestone also occur in the Late Proterozoic (ca. 800 Ma) sedimentary sequence of the Penganga Group in Andhra Pradesh, developed in the Godavari valley, a major continental rift in the Indian peninsula (Roy, 1981). Lateritoid manganese ores include both in situ residual ores (lateritic) and the replacement deposits formed by enrichment of manganese in meteoric water from other rocks and subsequent precipitation from solution. Thus, these ores are clearly epigenetic with respect to their host rocks. Such epigenetic ore deposits are found in fairly large quantities in Dharwar Supergroup rocks in Belgaum, Karnataka, Maharashtra, Goa, parts of Madhya Pradesh and in Iron ore Group rocks of

Fig. 4. Location of Mn deposits in central Indian Sausar Belt in Maharashtra and Madhya Pradesh (after, Roy, 1966).

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Singhbhum district, Jharkhand, and Keonjhar district, Orissa (Fig.3). In many such lateritic deposits, Al and Fe are characteristically concentrated in the upper zone and Mn in the lower zone of a weathering profile. In the Sausar Group a large supergene deposit with a strike length of 1.5 km and a thickness exceeding 130 m occurs at Dongri Buzurg, Nagpur district, Maharashtra. The deposit was formed by oxidation of pre-existing metamorphosed Mn oxide and Mn silicate rocks (Roy, 1981). The total reserves of manganese in India is estimated to be 93,000,000 ( 93 Mt ) metric tons (USGS, Mineral commodities summary, 2005). Chromium Chromium is extracted from the spinel group mineral chromite (FeCr2O4). The chromite ore is used for three principal industrial purposes: (1) Metallurgical, (2) Chemical and (3) Refractory. The metal is required for alloying with steel, for chrome plating and for production of chemicals like potassium dichromate (K2Cr2O7). For metallurgical use the chromite ore must have a minimum of 48% Cr2O3, with a Cr/Fe ratio of 3:1. Chromium is concentrated in basic to ultrabasic magma and thus it commonly gets concentrated as chromite ore in gabbros, peridotites, dunites and anorthositic plutonic rocks. The chromite deposits display two distinct modes of occurrences: as stratiform, layered deposits in large igneous complexes, known as the Bushveld type, or as podiform or sackform deposits in orogenic belts, known as the Alpine type. Sometimes the characteristics of both types are found in some occurrences. Indian distribution The major chromite occurrences in India are restricted to the states of Orissa, Jharkhand, Karnataka, Goa and Tamil Nadu (Fig. 3). Minor chromite occurrences mainly of academic interest are also found in Ladakh in the Indus suture zone. The igneous complex around Boula-Nausahi, in the Keonjhar district of Orissa, is intruded into the Precambrian metasedimentaries of the Iron Ore Group. The 2000 Ma-old ultramafic suite of rocks, extending in a NW-SE direction as a lense with a length of 3 km and a width of 0.5 km, is represented mostly by dunite-pyroxenite and subordinate amounts of pyroxenite. Four prominent sub-parallel chromite lodes are present and are named as Durga lode, Laxmi lode, Sankar lode and Ganga lode. The stratiform nature of the mineralization is obvious though later deformation has affected the layerings. This igneous complex is also an important repository of Ti-V bearing magnetite and Platinum Group Elements (PGE + Au) mineralization. The latter is associated with base metal sulfides in a breccia zone located in the junction of ultramafic and mafic rocks (Baidya et al., 1999).

Chromitites of the Sukinda valley (Chakraborty and Chakraborty, 1984) can be traced along the strike for about 8 miles in the NE-SW direction, from Saruabil in the east to Katpal in the west in the Cuttack and Dhenkanol districts of Orissa, respectively. The dunite-peridotite body hosting the chromite ores is intrusive into the quartzites and Banded Iron Formation of Precambrian age. The chromitite layers show evidence of gravitative settling during magmatic crystallization. Four chromitite layers occur near Kalrangi, in the Cuttack district at the southwestern end of the Sukinda ultrabasic belt. Sackform chromite deposits also occur at the intersection of olivine gabbro dykes near Moulabhanja hills in the Dhenkanal district. These dykes intrude Precambrian granite gneiss of the Eastern Ghat orogenic belt.

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Well known chromite deposits, now nearly exhausted, occur in ultrabasic rocks at Jojohatu in the Singbhum district of South Jharkhand. The serpentinised ultrabasic rocks ranging from dunite to enstatitite occur as laccolithic intrusions within slates and phyllites of the Iron Ore Group. In Karnataka, chromite deposits and occurrences are located in the Archean Nuggahalli, Shimoga and Sargur schist belts (Radhakrishna, 1996). The best known deposits occur at Byrapur and Aladahalli in Nuggihalli belt within serpentinised peridotite. Other known chromite occurrences in ultramafic rocks are at Anthargange in Shimoga belt and Sindhuvalli in Sargur schist belt. Stratiform chromite ores are found associated with metamorphosed anorthositic rocks of the Sittampundi complex in Tamil Nadu. These are of sub-economic grade. The total reserves of chromium reserves in India is estimated to be about 25,000,000 (25 Mt) metric tons (USGS Mineral commodities summary, 2004). Copper Copper is the most useful base metal. Due to its electrical conductivity and ductility, it is used widely in the manufacture of wires, plates and rods for use in electrical industry and domestic utility. Mineralogically, copper ores are divided into four groups: native metal, sulphides, oxides and complexes. The native copper (occurring as an individual mineral with 100% Cu) is commonly found in oxidised zones. The sulphide ores are the most valuable and are commonly associated with intrusions of quartz monzonite and related calc-alkaline plutonic rocks, and also with mafic volcanic rocks. They also occur in clastic sedimentary rocks. The complex ores containing copper may also be associated with zinc, lead, gold and silver minerals. Copper deposits have originated by diverse processes, but most of them are either the direct result of hydrothermal activity, submarine exhalations, bacteriogenic precipitation and oxidation and supergene enrichment. Porphyry copper deposits, the main present-day resource of copper, are large, epigenetic, low grade (0.5 to 1 % Cu), disseminated, hypogene mineralisation that can be exploited by bulk-mining techniques. Such ores are closely associated with intrusions of monzonite, quartz monzonite, or quartz porphyry. Contact metasomatism also accounts for some deposits of copper in carbonate rocks. Most copper deposits in unglaciated regions have undergone oxidation and some supergene enrichment with rich grades at the upper levels just below the water table. Indian distribution The major copper deposits in the country are located in the states of Jharkhand, Rajasthan, Chhattisgarh and Karnataka. Minor occurrences of copper in polymetallic association are found in Sikkim, Maharashtra and Andhra Pradesh. Their distribution is shown in Fig. 5.

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The Singhbhum Thrust Belt, a 160-km long arcuate structural zone in the southern part of Jharkhand state is host to several mineral occurrences of economic importance. This belt hosts several copper, uranium and apatite-magnetite deposits (Fig. 6). Besides nickel, gold, molybdenum, silver, tellurium and selenium are being extracted as by-products from the copper and uranium ores. The copper sulfide mineralization is found along the entire shear zone, right from Baharagora in Mayurbhanj district of Orissa in the southeast to Galudih-Duarpuram in the west in Jharkhand. However, only certain sections are mineralized richly to be of economic or sub-economic importance. These sections are: Baharagora, Badia-Mosabani, Pathargarah-Surda, Kendadih-Chapri, Roam-Rakha Mines-Tamapahar, Ramchandra Pahar-Nandup-Turamdih.

Fig. 5. Distribution of copper deposits in India.

Fig. 6. Distribution of mineral deposits along the Singhbhum shear zone (after Sarkar, 1984).

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Out of these, the Badia-Mosabani sector is the richest. Here the mineralization is localized in the soda granite close to the contact with the underlying Dhanjori metabasalts. The Mosabani mine first went into production in 1928 and was continuously being mined up to depths of around 1100 m but was closed a few years ago by Hindusthan Copper Ltd. This mine had two sub-parallel lode structures dipping generally at 20 degrees to the NE, one designated as the Main Lode and the other more productive one as the West Lode. The host rock of mineralization for other deposits in the belt is biotite-chlorite-quartz schist grading at places to chlorite schist. Though opinions vary widely regarding ore genesis in the Singhbhum Copper Belt, considering all aspects a model proposing volcanic hydrothermal activity along a syn-volcanic thrust zone seems to be most satisfactory (Sarkar, 1984). Total estimated reserves for mines under HCL’s lease in the entire belt were 173 Mt at 1.38 % Cu, out of which Mosabani contributed 19.77 Mt at 1.70 % Cu (Mining Magazine, November, 1983).

Hindusthan Copper Ltd’s Malanjkhand mine is the biggest open pit base metal mine in India. It is located in the Balaghat district of Chhattisgarh state, 90 km NE of the town of Balaghat. Lode-type copper (-molybdenum) mineralization occurs within calc-alkaline tonalite-granodiorite plutonic rocks of early Proterozoic age (Sarkar et al., 1996). The mineralized host rock is about 2 km in strike, has a maximum thickness of 200m, dips 65 to 75 degrees along which low grade mineralization is traced upto a depth of 1 km. A conservative estimate of the ore reserve is 92 million tonnes with an average Cu grade of 1.3 %. At 0.83 % Cu, the reserves escalate to 789 million tonnes (Sikka, 1989). Supergene oxidation with limited enrichment is recorded upto a depth of 100m. The bulk of the mineralization occurs in sheeted quartz-sulfide veins and K-silicate alteration zones. The main primary minerals are chalcopyrite and molybdenite. In terms of several geological aspects this deposit is comparable to Phanerozoic (and reported Precambrian) porphyry copper systems in other parts of the world (Sikka, 1989; Sarkar et al., 1996).

The 100 km long Khetri Copper Belt (KCB) in Jhunjhunu district of Rajasthan contains copper mineralization (from north to south) at Banwas, Madan Kudan, Kolihan, Chandmari, Usri, Akwali, Sathkui, Dhanaota and Charana. Of these, larger concentrations have been exploited by HCL at Madan Kudan, Kolihan and Chandmari. A total reserve of 83 Mt of ores with 0.88 to 1.5 wt. % Cu was estimated at the KCB (Sarkar, 2000). The orebodies in KCB are in the form of single or compound lenses hosted by garnetiferrous chlorite schist and banded amphibolite-quartzite in Madan Kudan and Kolihan, and only garnetiferous chlorite schist at Chandmari. In the south, the mineralization is hosted by carbonaceous phyllite. The mineralization is concentrated at the interface between the Alwar and Ajabgarh Groups of the Delhi Supergroup. Chalcopyrite, pyrite and pyrrhotite are the principal sulfide phases. Opinions about ore genesis along the KCB ranges from epigenetic hydrothermal to sedimentary diagenetic with later metamorphism.

In Karnataka, Chitradurga Copper Company mined a small pyritic copper deposit within Chitradurga Group volcanic rocks at Ingaldhal, a few km away from the town of Chitradurga. This deposit attracted special attention in recent years when gold was detected in the footwall pyritic zone.

It is presently estimated that India holds about 5.3 million tonnes of copper reserves (HCL News, 2005). Lead-Zinc

The metal zinc generally occurs in combination with other elements, most commonly with lead. The important minerals of zinc include: sphalerite or zinc blende (ZnS), smithsonite (ZnCO3),

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zincite (ZnO) and hemimorphite (2ZnO.SiO2.H2O). Similarly, the most common lead mineral is galena (PbS). Other minerals containing lead are cerussite (PbCO3) and anglesite (PbSO4). Zinc is used extensively for coating and galvanizing iron and steel products, in the manufacture of pigments, as a component in alloys like brass, bronze and German silver. Zinc dust and plates are used to precipitate gold from cyanide solutions in the treatment of gold ores. Lead is widely used in the manufacture of electric storage batteries and in various electric appliances. It also finds use in water pipes, chemical plants, ammunitions, solders, pewter, nuclear shield in atomic plants and in certain lead chemicals. Majority of the lead ore deposits of the world are also zinc producers and most zinc ore deposits carry lead. Both lead and zinc bodies usually occur as veins and massive or tabular lodes, and as disseminations and patches, commonly in limestone or dolomites, but also in shales. Majority of lead-zinc ores occur as cavity-filling and replacement bodies formed by low-temperature hydrothermal solutions of diverse origin. Indian distribution The lead-zinc ore deposits and prospects are distributed widely in India, with the predominant part of the resources being confined to the state of Rajasthan. Other states with some lead-zinc resource are Andhra Pradesh, Orissa, Gujarat, Sikkim, Uttaranchal, Maharashtra and Jammu & Kashmir (Fig. 7).

The most important zinc-lead deposits of economic value in India are the Rampura-Agucha, Rajpura-Dariba and Zawar deposits in Bhilwara, Rajsamand and Udaipur districts of Rajasthan, respectively. The Rampura-Agucha deposit is the most important Zn-Pb-(Ag) deposit in India producing 9 x 105 tonnes per annum with a total reserve of 63.7 Mt with 13.6% Zn, 1.9% Pb and 45 ppm Ag (cf. Sarkar, 2000). The rock types around the single ore lens are: garnet-biotite-

Fig. 7. Distribution of Pb- Zn deposits in India.

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sillimanite gneiss (GBSG) with bands of calc silicate rocks and amphibolites intruded by pegmatite/aplite veins with graphite-mica-sillimanite schist hosting the ore. The major mineralogy of the ore is simple: sphalerite, pyrrhotite, pyrite, galena and graphite are the main phases. The available geological information indicates that the deposit is sediment-hosted and the mineralization localized by the anoxic environment. The Dariba-Rajpura-Sindeswar Kalan-Bethumni belt is a 17 km long mineralized zone with a 16.85 Mt of ore reserve in the richest Dariba-Rajpura sector, with a grade of 8% Zn and 2.26% Pb (Sarkar, 2000). Cu and Ag are obtained as by-products and Cd, Hg and Tl are important trace metals in the ore. Metamorphosed siliceous dolostone and graphite-mica schists are the main host rocks of the mineralization which at Dariba, show zoning from Cu in the footwall through Pb-Zn in the middle to Fe in the hanging wall. The ores here have the characteristics of a sedimentary exhalative deposit. In Zawar area, the Mochia Magra, Balaria, Zawar Mala and Baroi Magra Hills contain extensive deposits. The main mine is located in the Mochia Magra hill (reserve 19.3 Mt of 3.8% Zn + 1.7% Pb), with smaller mines in Balaria (reserve 16 Mt of 5.66% Zn + 1.44% Pb), Zawarmala (reserve 16 Mt of 3.72% Zn + 2.16% Pb) and at Baroi Magra (reserve 11 Mt with 1.33% Zn + 4.29% Pb). The principal rock type of Zawar area consists of phyllites, slates, mica schists, dolostones and quartzites of the Aravalli Supergroup. But sulphide mineralization is solely confined to the dolomites, whereas adjoining phyllites are almost barren. The localization of the ore is structurally controlled along shears. The initial mineralization is believed to have taken place during sedimentation and early diagenesis with the bulk of the mineralization being translocated along extensional fractures and shears during later deformation (Sarkar, 2000). Zinc and lead ores have been located in different parts of Andhra Pradesh. The most important deposit is found in Agnigundala belt in Guntur district in the Nallamalai hill range. The most important deposit, till recently mined by Hindusthan Zinc Ltd., is at Bandalamatto. The mineralization is in the form of veins and stringers of galena associated with sphalerite, chalcopyrite and pyrite. The host rock is brecciated dolomite, dolomitic limestone and coarse grained calcareous quartzites belonging to the Cuddapah Supergroup. A Pb-Zn deposit was being mined by Hindusthan Zinc Ltd. around Sargipalli in Sundergarh district of Orissa till a few years ago. This early Proterozoic Pb-Zn deposit is hosted by graphite-sillimanite schist of the Gangpur Group (Sarkar, 1974). Polymetallic Zn-Pb-Cu ore lenses are found around Ambaji in the Banaskantha district of Gujarat and 8 km away at Deri in the Sirohi district of Rajasthan. These ores occur in metamorphosed basalt-rhyolite bimodal volcanic suite, now represented by cordierite-anthophyllite-chlorite rocks belonging to the South Delhi fold belt. The Ambaji ore zone contains 8.29 Mt of ores with 5.52% Zn, 4.91% Pb and 1.75% Cu (Deb, 2000). Polymetallic Cu-Pb-Zn mineralization is also found at Rangpo in Sikkim within phyllites, quartzites and metabasic rocks of the Daling Group (GSI, 2001). It is estimated that India has 176.8 million tonnes recoverable reserves of lead and zinc ore as on April 2000. Gold Gold is a noble metal which has the yellow radiant colour and high reflectance. It is also highly malleable and ductile and has high specific gravity. Its main use is for monetary purposes,

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followed by use in jewelry. Gold is also used for therapeutic purposes, in dentistry and specialized equipments. Both primary and secondary processes produce gold concentrations in nature. Fluids play a key role in concentrating gold in both these environments. In early Archean times, Mg-Fe-rich or ultramafic lavas reacted with sea water creating primary greenstones and concentrating gold along with nickel, copper and iron. With subsidence and tectonism, the primary greenstones underwent partial melting and differentiation and gave rise to silica-rich plutonic rocks which had more gold abundance than their precursors. Subsequent hydrothermal activity leached the metal and concentrated it in lodes to produce the gold-quartz veins along structural locales. While primary mineralization served as the principal source of gold for several centuries, of late, gold concentrations associated with low temperature processes in supergene environment have been located in laterites in South America, South Africa, Western Australia, Madagascar and southern India. Gold eroded from primary ore deposits also commonly accumulates as detrital particles in streams and are in many ways younger, smaller versions of the ancient gold-uranium-bearing conglomerates of the Witwatersrand type. The origin of this large gold resource however, is debated in recent years with two strong opposing views: sedimentary (diagenetic) and hydrothermal. Beginning of gold use has been traced back to more than six thousand years ago. During the ‘pre-historic and ancient times’ a total of more than 10 thousand tones of gold is estimated to have been produced (Bache, 1987). At best a small part of it could be primary. Same must be true for the gold produced during the ‘middle ages’. Indian distribution Most known primary auriferous zones in India are the vein type deposits located in the eroded remnants of ancient volcano-sedimentary rocks, known as schist belts or greenstone belts of late Archaean age in the Dharwar geological province in South India (Fig. 8). Thus the largest cluster of gold occurrences is located in the states of Karnataka and Kerala. Gold is also known to have high potential in south Jharkhand, parts of Madhya Pradesh and in south Rajasthan. Gold occurs in a variety of geological settings. The following modes of occurrences are recorded in India (Radhakrishna and Curtis, 1999): Lode Gold (quartz–carbonate vein type) deposits: Usually confined to metamorphosed volcanic rocks forming linear schist belts of late Archaean age (Greenstone belt) and invariably occupying fissure and shear zone and commonly persisting to great depths. This type of deposit still remains as the chief source of gold in India. The best examples are Kolar gold field in Karnataka, passing into the junction of Andhra Pradesh and Tamil Nadu; Hutti gold field in Karnataka. The most typical of this type of occurrence is the Champion lode of the Kolar Gold Field which is the richest gold-bearing quartz lode so far encountered in India. Gold in banded iron formations: Generally occurs in association with schistose amphibolite and banded iron formation of late Archaean age. Best examples are Ajjanahalli and Sandur deposits in Karnataka and Sonadehi in Madhya Pradesh.

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Gold in granulite terrain: This type is similar to the vein and stratiform types described above under 1 & 2. Example is the Wynad gold field in Kerala. Disseminated gold: Gold commonly occurring in disseminated form throughout an intrusive body or volcanic rock. Such deposits are of low grade but often amenable to open cast mining. Example is Malanjkhand copper deposit in Chhattisgarh where gold is detected in the ore and not in the host rocks. Gold associated with early Proterozoic volcanic or sediment-hosted polymetallic sulphide deposits: In this category are included deposits of copper, lead and zinc containing values of gold, silver and other metals which can be extracted as by-products. Examples are Rajpura-Dariba deposit in Rajsamand district; Danva prospect in Sirohi district, and Khetri copper belt of Jhunjhunu district, Rajasthan. Gold in quartz pebble conglomerates and quartzites (ancient placers): Detrital gold commonly occur in quartz-pebble conglomerates resting unconformably on older gneisses, schists at the Archaean-Proterozoic boundary. Some of the world’s largest concentrations are found in this deposit type known commonly as QPC or Witwatersrand type. Examples are found in Bababudan hills in Karnataka and at the base of the Dhanjori basin in Jharkhand. Greywacke or turbidite-hosted deposits: These deposits occur in late Archaean sedimentary successions along with volcanic intercalations and may form part of greenstone belts. Examples are Gadag in Karnataka.

Fig. 8. Distribution of primary gold deposits in India.

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Carbonate-hosted deposits: Gold in this environment may be present as invisible type in carbonate rocks in close spatial association with granitic rocks. Example is Bhukia gold in Banswara district of Rajasthan. Gold in coal: Possible in lamproite dyke rocks intruding Gondwana coal-bearing succession. Epithermal bonanza type deposits of Tertiary age: The younger granites of fold mountain chains and altered volcanic rocks are likely to show such concentration of gold. Yet to be identified in Central Himalayan Range or in Andaman islands. Placer and alluvial gold: Alluvium of rivers draining auriferous tracts and showing concentrations of detrital gold. Nilambur valley, Kerala; Subarnarekha River, Jharkhand. Gold in laterite, soil and weathering profiles: This is a newly recognized mode of occurrence of economically exploitable gold. Example is in Nilambur valley in Kerala. Aluminium Aluminium is extracted from bauxite ore which may contain one or more of the aluminous minerals, gibbsite (Al2O3.3H2O), boehmite (Al2O3.H2O) or diaspore (Beta Al2O3.H2O). It also invariably contains various iron hydroxides and titanium oxides. Bauxite is used not only for the extraction of Al metal but also in chemical industry, as refractory material and as an abrasive. It is also used to produce ‘cement fondu’, aluminous cement characterized by rapid hardening qualities. Metallic aluminium is light weight and has 60% of the electrical conductivity of copper. Even then in countries not having enough copper it is used for making electrical wires, as in India. The process of bauxitisation is primarily related to processes of mechanical disintegration, chemical weathering, and leaching under favourable hot and humid climate where heavy rainfall and good drainage pattern accelerates the process. The bauxite deposits may be produced by chemical sedimentation, by solution and redeposition, chemical replacement of pre-existing rocks, sub-aerial weathering in situ or detrital deposition, from high altitude to low lying areas. Usually occurs as blanket deposits. Any igneous (excepting ultramafic), sedimentary and metamorphic rock can act as the protolith of bauxite deposit provided it contains tangible amount of Al2O3. Indian distribution Large resources of low grade bauxite ores have been explored by Government agencies on known aluminous laterite occurrences (Krishnan, 1935) capping flat-topped plateaux in a 300 km stretch of the Eastern Ghat belt from East Godavari district of Andhra Pradesh to Sambalpur and Bolangir districts of Orissa covering an area of almost 25,000 sq.km (Fig. 9). These are underlain by Precambrian granulite facies metamorphites comprising an interlayered sequence of khondalites and leptynites, the former being the bed rock of blanket-type deposits with large aerial extent (~ 0.5 km), appreciable thickness (max. ~50 m) and usually good profile differentiation with little or no overburden (Deb and Joshi, 1984). The well known aluminous laterite occurrences in this East Coast Bauxite Province are at Anantagiri area in Vishakhapatnam district of Andhra Pradesh where bauxite cappings at altitudes of 1090 to 1445 m are found at Galikonda, Raktakonda, Katuki and Chittamgondi (Raman, 1976). In Orissa, aluminous laterite deposits occur at Pottangi, Panchpatmalli and Baphlimalli hills in the Koraput district, around Kashipur in Kalahandi district and on the Gandhamardan plateau of Bolangir district. A rough estimate of the bauxite resources in the East Coast bauxite province is of the order of one billion tonnes of aluminous laterites with about 40 % Al2O3.

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Fig. 9: Distribution aluminium deposits in India. The bauxite belt of Central India, some 400 km long and 50 km wide, trends in ENE-WSW direction and comprises a string of mesas with group of bauxite deposits at Balaghat, Amarkantak, Putkapahar and Mainpat, Jamirpat, Bagru-Manduapat-Neturhat. This belt is sub-parallel and south of the Son-Narmada lineament, extending from from Madhya Pradesh through Chhattisgarh to Jharkhand. All these deposits occur above the critical contour of 2,500 ft. at the contact of contrasting litho-units. A large area of about 4000 sq. km between latitudes 23o00’ and 23o30’ and longitudes 84o00’ and 84o45’ in Ranchi and Palamau districts of Jharkhand contain valuable deposits of bauxite underlying lateritic capping. The bed rocks in the area include Chotanagpur granite gneiss, older metamorphics of the Iron Ore Supergroup and some basaltic trap rocks in the western part of the area in the plateaus of Netarhat, Jamira Pat and Luchutpat. Segregations of bauxite are present in the laterites as small pockets, continuous bands or beds, 1 to 1.5 m in thickness, seen on the scarp face. The bauxite deposits of Khamar Pat and Mandua Pat in Ranchi district are close to the town of Lohardaga. They have high grade bauxites with Al2O3 content varying between 49.7 and 60 %. Most important occurrence of bauxite in the Lohardaga region is Bagru hills where rich bauxite with upto 51.6 % Al2O3 is exploited for production of alumina in the Muri plant of the Indian Aluminium Company. Isolated pockets and bands of high grade bauxite also occur in Pokhra Pat and Dudhia Pahar areas on the Ranchi plateau. Another promising area nearby is the Serangdag plateau, skirted by Koel river, containg more than 3.5 Mt of bauxite. In Palamau district bauxite occurs on the Netarhat plateau. The best deposits of high grade pisolitic bauxite, with upto 58 % Al2O3, occur on the western edge of the Burha river valley. High level laterites with thickness varying from 9 m to 61 m, containing irregular patches of bauxite, also occur in the Kharagpur hills in the Monghyr district of Bihar at Khapra, Maira-Maruk and Maira areas.

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There are three main bauxite-producing areas in Madhya Pradesh and Chhattisgarh. These are in the Sarguja-Raigarh-Bilaspur districts, continuing from the laterites of Ranchi and Palamau; in the Maikala range in the districts of Durg, Mandla and Balaghat; and Katni-Newer-Jabalpur areas of Jabalpur district. In the first area, lateritic cappings is found on Deccan traps which at places overly the Lametas. In Bilaspur district the principal occurrences of bauxite with 50 to 55 % Al2O3 are in Korba, Churi and Uprora areas. Although there is an alumina plant in Korba, much of these bauxites are well suited for refractory use. The Amarkantak plateau bauxites are the principal source of raw material used by HINDALCO at Renukoot, Mirzapur district of U.P, close to the border with M.P. The Amarkantak plateau with an extensive blanket of laterite is situated at the source of the river Narmada and has the best development of bauxite in the southern end of the plateau. Massive bauxite deposits, about 3 m in thickness, are found at several places under the pisolitic laterite. The Al2O3 content varies between 55 and 60 %. The total reserve of such high grade bauxite in this plateau is at least 5-6 Mt. In Jabalpur district, the aluminous laterites and bauxites are generally found associated with lower Vindhyan limestones. The bouldery bauxites near Katni are very high grade with about 60 % Al2O3 and low silica and occur at times in the alluvium in the valleys. In Karnataka, high alumina, high silica and ferruginous bauxites occur in different regions. Rich bauxite deposits occur on bed rocks of Deccan traps in Belgaum district of northwestern part of the state. The estimated reserves in the district are about 7 Mt with good deposits, with more than 50 % Al2O3, found at Kasarpada and in the Mogalgad areas. The total reserve of aluminium ore in India is estimated to be 2650 Mt, located mainly in the states of Andhra Pradesh, Orissa, Madya Pradesh, Maharashtra and Jharkhand.

NON-METALS

Non-metallic minerals, including industrial rocks and building stones, form the major part of natural resources used by modern societies in terms of their total output and value. These minerals form the back bone of several industries such as chemical, ceramic, fertilizer, refractories etc. and India is endowed with some of the largest deposits of these industrial minerals (cf. Deb, 1980). Unfortunately, much less R & D is carried out on these important economic minerals compared to their metallic counterparts. As a result there is a dearth of scientific literature on them as also of reference text books in the field. The brief account of the important industrial minerals in India (see Fig. 10) that follows is based primarily on the above reference. Refractory minerals Refractory minerals are those which can withstand high temperatures as well as sudden temperature changes, abrasion and shock, and have good resistance to different chemicals and changing pressures under extreme conditions. They are used for various purposes, the most important use being in the linings of furnaces for smelting and refining metals. They are also used for lining incinerators, kilns in ceramic industry and in glass and cement manufacture, for coke ovens/boilers used in gas or electric plants. They also find use in spark plugs for automobiles. With the tremendous growth in metallurgical plants in India in recent years the use and geology of refractories has acquired special significance.

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The refractory minerals are divided into three categories, based on their reaction with various kinds of slags:

Acidic Neutral Basic Silica: Quartzite etc Fire clay, Ball clay

Chromite

Magnesite

Kyanite Graphite Dolomite Sillimanite Asbestos Bauxite

Acid refractories: Silica Silica for refractory purposes is derived from quartzites, sandstones, vein quartz and sands. Such quartzites are of metamorphic origin, sandstones and sand of sedimentary derivation and vein

Fig.10. Distribution of selected non-metallic mineral deposits in India.

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quartz from igneous (hydrothermal) source. These above mentioned rock types occur in almost all the geological formations, from the Precambrian to Recent.

Because of their heat resistant property, these refractories are widely used in the arches and crowns of furnaces. Silica bricks are used in open hearth furnaces, in acid Bessemer converter and in electric furnaces. Indian distribution Quartzites Quartzites occur commonly in the geological formations belonging to the Dharwar, Aravalli, Kurnool, Cuddapah and Vindhyan Supergroups. State-wise, the major exploitable occurrences are as follows: Bihar: In the Kharagpur hills, fine grained, massive quartzite occurs, composed predominantly of quartz, with garnet, magnetite, biotite, and muscovite as accessories. Silica content varies between 97.5% and 98%. Jharkhand: Quartzites occur in Singhbhum district, near Chandil and Chaibasa. Large quantites are also available in Gangpur Group of rocks. Karnataka: Quartzites consisting almost entirely of pure quartz occur in the Dharwar schist belts of Karnataka. Near Bhadravati alone, in the Shimonga schist belt, about 2 Mt of quartzites are found. Quartzites also occur near Bangalore, Krishnarajasagar, Holalkere, Holenarsipur and in the neighbouring areas of Mysore city. Andhra Pradesh: In Cuddapah and Kurnool formations there are ferruginous silica beds in the Chaiyar Group of rocks found on the Oopalpad plateau and in the vicinity of Yadakee. Fire clay Fire clays are refractory sedimentary clays characterized by very low alkali content. They are commonly found in the coal measures of the Gondwana coal fields. They may be plastic or non-plastic in character. When very finely ground the plasticity increases. In most of the fire clays, kaolinitic and bauxitic materials occur together in different proportions along with free quartz and other impurities. Mineralogically the fire clays comprise the aluminium hydroxides, diaspore or gibbsite. The SiO2 content varies between 45-55% and Al2O3 content between 30-40%. The plastic fire clays are similar to ball clays, and used as bond clays for the manufacture of saggers, glass pots, crucibles, mortars, and refractory cement. Non-plastic fire clays usually contain a high proportion of silica and are low in clays and aluminous minerals. The fire clays generally have fusion point usually above 1600oC. Indian distribution Jharkhand produces the highest quantity of fire clays from the coal fields of Karanpura and Jharia. West Bengal comes next in order of importance, producing fire clays from the Raniganj coal fileds. Chhattisgarh also contributes to fire clay production much of which comes from Korba coal field. Orissa also produces good quality fire clay from Sambalpur and Denekanal districts. In some other states such as Karnataka and Tamil Nadu refractory kaolin occurring in Precambrian strata are regarded as substitutes for fire clays or refractory clays. Ball clays These are greyish-white to light cream in colour, fine-grained, sometimes carbonaceous and are highly plastic in nature. They have high bonding capacity and tensile strength. They are extracted

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from sedimentary formations in lumps or in ball-shaped forms and are marketed in the raw stage, without any beneficiation. The chemical composition of ball clay is almost similar to kaolin or china clay, except that it is high in silica and poor in alumina. Ball clays cannot be easily shaped or used in casting or dewatered easily by filter pressing. High shrinkage causes fine cracks or hair-like lines on the body of the fired wares. On firing, the ball clays produce a vitreous substance at a much lower temperature than Kaolin. These clays are extensively used in bonding furnace sands and refractory materials. Indian distribution Ball clays are distributed mainly in states of Maharastra, Rajasthan, Gujarat, Kerala and Tripura. Several localities of ball clays are found near Bombay and surrounding areas in Maharashtra and near Barmer in Rajasthan. Kyanite (Al2SiO5) The mineral is characterised by its bluish colour, bladed form, good cleavage and varying hardness in different cleavage directions. It usually occurs in crystalline schistose rocks formed under high pressures at great depths and associated with minerals, such as, corundum, staurolite and andalusite. The transparent crystals are used as semiprecious gemstones. Indian distribution India has the largest resource of kyanite in the world. The state-wise distribution of this important refractory mineral is as follows: Jharkhand: Kyanite deposits at Lapsa Buru in Singhbhum district contain massive development of high grade kyanite of great economic importance. The Lapsa hill forms the central high portion of a long ridge about 3 km in length and rises about 300 m above the plains, where there are kyanite-producing quarries of TISCO. The kyanite-bearing rocks form segregations in the mica schists. Along the extension of the Lapsa Buru hills, towards east and south-east in Seraikela near Kharswan, massive kyanite rocks occur in association with aluminous mica schists and kyanite-quartz rocks. In and around Sini, in Singhbhum district, there are also several exposures of coarse bladed kyanite within mica schists, appearing as seggregated veins. Andhra Pradesh: Kyanite-quartz rocks have been reported from Nellore district. The deposits occur in mica schists of Nellore mica-belt, northwest of Saidapuram. Kyanite forms in pockets and lenticular bands having lengths upto 150 m. Kyanite also occurs intercalated with quartz schists and quartzites. In Khamman district, there are several exploitable deposits of kyanite and garnet. The productive area is about 15 sq. km containing garnetiferous kyanite mica rocks. Rajasthan: Pockets and lenticles of quartz and kyanite, as big as 0.3 to 1m in size, occur randomly along the contact of pegmatites and quartz veins intruded into biotite-garnet-kyanite schists in Dungarpur district. All these highly metamorphosed rock formations are overlying the Banded Gneissic Complex basement. Sillimanite (Al2SiO5) The mineral occurs in compact radiating masses and fibrous aggregates in high-grade metamorphic rocks. Colour varies from grey to light brown or pale green; the lusture is often vitreous. It is distinguished easily by its acicular needle shaped crystal habit. Sillimanite is a product of high-grade metamorphism of aluminous rocks often occurring directly at the contact with igneous rocks. Sillimanite also occurs in crystalline schists in association with metamorphic minerals such as cordierite, corundum, andalusite and spinel.

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Indian distribution Sillimanite deposits are widely distributed in India, particularly in the Precambrian crystalline complexes in Meghalaya, Rajasthan, Madhya Pradesh, Bihar, Orissa, Kerala, Andhra Pradesh and Tamil Nadu. Meghalaya: This state alone possesses nearly 70% of the total reserves in India. India’s richest and unique deposit of sillimanite is situated at Sonapahar in Khasi hills. The deposit has been traced in an area of 78 sq. km. It is associated with highly aluminous rocks, such as cordierite-biotite-quartz-microcline gneiss and sillimanite-quartz schist enveloped in granite. The main exposures are at twelve different places. The sillimanite is of massive variety and occurs in huge boulder form. Madhya Pradesh: Sillimanite occurs in Pipra in Waidhan tehsil of Sidhi district. Both boulder and vein deposits are reported. Maharashtra: Corundum, kyanite and sillimanite bearing rocks are found for a length of 5 km between Dahegaon and Pipalgaon in Bhandara district. A small quantity of sillimanite is reported from Tannilai mine in Tiruchirapalli district, Tamil Nadu and Madar mine, Udaipur district, Rajasthan. India also possesses extensive deposits of sillimanite associated with the beach sands of Kerala, Tamil Nadu and Orissa. Neutral refractories Chromite (FeCr2O4) High grade chromite ore, which is hard and lumpy, is used in the manufacture of chrome bricks, chrome-magnesite bricks and allied refractory products. The refractory grade chromite ore should have moderate to high (30-48%) Cr2O3 and Al2O3 content between 12-30%. They should have low Fe2O3 (<15%) and SiO2 (<5%) as well. In industrial practice, an excess of MgO is added to the chrome ore in order to combine all the excess silica. The mode of occurrence and Indian distribution of chromite deposits (cf. Fig.3) has been covered in the foregoing section on metallic deposits. Graphite (C) Graphite occurs in two forms: Natural graphite which includes (a) crystalline and (b) amorphous varieties, and artificial/manufactured graphite. The inherent qualities of graphite, for which it is so much in demand in the manufacturing industries, are its high lubricity, refractoriness or ability to withstand high temperature, good electrical and heat conductivity, and resistance to reaction with ordinary chemical reagents. Thus, flaky graphite is used in the manufacture of crucibles for melting metals. It is also used in the manufacture of lead pencil, batteries, lubricants and brushes. It is also used in atomic reactors. The commercial graphite is graded mainly on its carbon content. Graphite can develop by four different geological processes: regional metamorphism and contact metamorphism; crystallisation in igneous rocks, such as in basalts and nepheline syenites, and through hydrothermal solutions from deep-seated magma, such as vein graphite in pegmatites. Majority of graphite deposits form by the metamorphism (both contact and regional) of sedimentary carbonaceous matter, such as, those present in black shales. Graphite is also found in iron meteorites. The best known graphite deposits in the world are found in Sri Lanka and Madagascar.

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Indian distribution The major share of graphite production in India comes from the states of Orissa, Jharkhand, Karnataka and Andhra Pradesh. Orissa: Graphite in Orissa is reported from khondalitic rocks of the Eastern Ghats. The deposits occur in form of veins, lenses and pockets. Two varieties are reported, namely, flaky and amorphous. The graphite from Orissa has fixed carbon content between 55% and 60%. The rich deposits of graphite are found in Patna, Sonpur, Atmallik, Koraput, and Kalahandi districts of Orissa. Jharkhand: Graphite occurs in khondalites, schists and gneisses, pegmatites, limestones, calc-granulites and quartzites in Daltonganj and Palamau districts. Rich graphite concentrations are known in the Sokra, Khamdih and Rajhara areas. The carbon content of graphite ore in these occurrences is around 50%. Andhra Pradesh: Graphite occurs in khondalites of East Godavari district of the Eastern Ghat belt. Graphite is reported in the form of irregular lenses and pockets and rarely as veins of varying thickness. In Vishakhapatnam district graphite occurs in the form of irregular veins and as highly disseminated material in an area called Marupalli. Krishna and west Godavari districts also have graphite occurrences. Karnataka: Fine grained, amorphous variety of graphite is reported from Kolar schist belt in Bangarpet taluk and flaky variety is found in crystalline schists near Mavinhalli and Tonavalli areas of Mysore district. Graphite is also found in areas of Chitradurga schist belt and in the Bababudan hills. Asbestos Asbestos is a commercial name for a group of minerals characterized by fibrous habit and wide variety of compositions. Depending on the strength and flexibility of fibres they are used for various purposes. The fibrous nature and its high resistance to fire makes it commercially so important. Asbestos has two distinct groups, namely, the serpentine group and the amphibole group. The former includes the elastic and the silky chrysotile variety and the latter comprises short and brittle-fibred anthophyllite, tremolite, actinolite etc. Asbestos is mixed with magnesite in the proportion of 85% MgCO3 and 15% asbestos powder to produce a quality refractory material. Three main factors control the formation of asbestos in different kinds of rocks, particularly the ultramafic rocks. The process of serpentinisation plays an important role in the development of chrysotile asbestos. Another important consideration is the transformation of non-fibrous serpentine into the fibrous mineral, and lastly the gradual change of chrysotile into tremolite asbestos. Generally accepted view about the genesis of asbestos is that the hydrothermal residual solution left after the consolidation of ultrabasic magmas was responsible for the transformation of peridotite and dunite into serpentine and later to asbestos. Indian distribution The best quality chrysotile asbestos is found in the state of Andhra Pradesh while the amphibole asbestos is widely distributed in Jharkhand, Rajasthan, Tamil Nadu and Karnataka. Andhra Pradesh: The most important occurrences are in the districts of Kurnool and Cuddapah and the less important ones in Anantpur district. In the Pulivendla taluk of Cuddapah district, between Brahmanapalle and Lopalanutola, asbestos was formed over a distance of 15 km by

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contact metasomatism of the Vempalli limestones and shales by a dolerite body. The asbestos is cross fibred chrysotile variety with an average thickness of 0.9m. Numerous veins of chrysotile asbestos also occur in Rajupalam area of Cuddapah district. In Kurnool district asbestos occurs in Dhone taluk in Vempalli limestones associated with trap rocks. Jharkhand: Amphibole asbestos is found extensively in Singhbhum district of southern Jharkhand. Crysotile asbestos is however rare. The former occurs in actinolite-tremolite talc chlorite rock north east of Chaibasa and also around Manpur. Some amphibole asbestos is also found in the chromite quarries west of Chaibasa where the country rock is dunite and peridotite. Asbestos also exists in the Saraikela area. Rajasthan: Chrysotile variety of asbestos is found in six localities of Udaipur district and two in Ajmer district. All other remaining occurrences in Udaipur, Dungarpur, Bhilwara, Ajmer, Jodhpur and Pali districts are of amphibole variety. Basic refractories Magnesite (MgCO3) This carbonate of magnesium is found to occur in ultrabasic igneous rocks, formed by alteration of Mg-rich silicates, and in dolomitic limestones. Its main use is in refractories. Dead burnt magnesite, MgO, which is calcined at high temperature between 1400 oC and 1500 oC and converts to crystalline periclase, is used in the manufacture of bricks for furnace linings. Magnesite is also used as an ore of metallic Mg, but at present the entire production of Mg comes from brines and seawater. Magnesite also finds use in chemicals and fertilizer industries. Magnesite commonly occurs in veins and irregular masses derived from the alteration of Mg-rich metamorphic and igneous rocks (serpentinites and peridotites) through the action of waters containing carbonic acid. Such magnesites are compact, cryptocrystalline and often contain opaline silica. Beds of cleavable magnesite are (i) of metamorphic origin associated with talc schists, chlorite schists, and mica schists or (ii) of sedimentary origin, formed as a primary precipitate or as a replacement of limestones by Mg-bearing solutions, dolomite forming as an intermediate product. Mainly three varieties of magnesite are recognized: i) cryptocrystalline, ii) crystalline, iii) amorphous. Indian distribution The major producers of magnesite in the country are the states of Tamil Nadu, Karnataka and Uttaranchal. Tamil Nadu: Magnesite occurs in biotite gneisses and charnockites which are intruded by dunites of Chalk Hills in Salem district. The magnesite is found as irregular veins in ultrabasic intrusive masses over an area of 11 sq. km. It has an average magnesium carbonate content of 95-97%. The magnesite is of very good quality, white to grayish in colour, compact and massive. Estimated reserves are more than 100,000 tonnes upto a depth of 15 m. Karnataka: Magnesite in this state occurs as a decomposition product of ultrabasic rocks. It forms as a network of veins, of various shapes and sizes, in the serpentinised rocks. Magnesite of massive amorphous type occurs, particularly in Hassan, Mysore, and Coorg districts. Uttaranchal: Magnesite deposits are associated mainly with dolomite and also at places with talc in Almora and Someshwar districts. It occurs in the form of veins, stringers and as massive

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crystalline deposits. Valdiya (1968) reported stromatolite-bearing dolomitic rocks with lentiform deposits of coarsely crystalline magnesite in the Gangolighat formations of the calc-zone of Pithoragarh district. The Magnesite deposits are very extensive, originating from the Kali river valley towards east and continuing to the Alaknanda valley towards west. Dolomite [Ca Mg (CO3)2] Dolomite and calcite are common rock forming minerals. The dolostone, loosely called dolomite in most literature, is considered as a principal raw material in the iron and steel industries. It is also used as a refractory material, for which calcined products are preferred. Dolomite is used as a basic lining in open hearth furnaces and in Bessemer converters, for which dead-burnt material is required. Dolomite is also used in the manufacture of high magnesia lime, basic magnesium carbonate, Epsom salts, and for the manufacture of metallic magnesium. Dolomite also finds use in chemical industry, in manufacture of paper, leather, glass etc. and as building material, as terrazzo stucco and also as crushed stones. Dolomite is associated with sedimentary carbonate facies and occurs in all geological ages but the economically important deposits are mostly confined to the Precambrian and Palaeozoic eras. Usually the dolomites are associated with limestones and sometimes they occur as irregular beds, lenticles, and pockets and rarely do they occur as hydrothermal vein deposits. Dolomite is commonly found associated with gypsum, anhydrite and alkali salts in saline evaporative basins. Dolomite also results from partial or complete dolomitization of the marine calcium carbonate including marine shells and organic remains. Indian distribution Dolomite (Dolostone) and dolomitic marble occur extensively in several states in different stratigraphic horizons. West Bengal: Large deposits of dolomite which extend over an area of about 13 sq. km occur in Buxa Duars area in the north-west of Jalpaiguri district. There are several varieties of dolomite in this region out of which two are important: one is massive, compact, and light grey in colour, and the other is dark brecciated type, possessing distinct bedding planes. Chemically both are pure dolomites. A part of this dolomite band enters into Bhutan. Rajasthan: This state has a large resource of dolomite, mostly occurring in the Delhi Supergroup of rocks. The high grade dolomite deposits occur in Ajmer district in the areas known as Kesarpura, Hatondi and Akhri. They are usually high magnesium crystalline dolomites, low in silica, alumina and iron-oxides. Karnataka: The dolomite deposits constitute part of the Dharwar Supergroup, occurring chiefly, in Dharwar-Shimoga and Gadag-Chitradurga schist belts. Dolomite deposits are also found in the lower Kaladgi Group of Cuddapah age. In Belgaum district large reserves of dolomite also occur. Jharkhand: The dolomite occurs in Palamau district with most of the occurrences located near the town of Daltonganj. The dolomite occurs as bands or as thin lenticular patches. The bands are mostly associated with calc-silicate rocks, containing serpentine and diopside, but some of the bands are also associated with magnetite-tremolite schist. Some outcrops of crystalline magnesium limestones are also found in the iron ore group of rocks in Singhbhum district.

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Orissa: Enormous deposits of dolomitic marble belonging to the Gangpur Group of rocks occur in the Birmitrapur and Sundergarh areas of the state. Minerals of Fertilizer industry Three principal elements are necessary for plant growth and high crop yield. These are nitrogen, phosphorous and potassium. Natural nitrates are hardly used now as they have been widely replaced by nitrogenous fertilizers made from atmospheric nitrogen. Phosphatic fertilizers, earlier produced from bones of dead animals, are now processed from phosphate rocks using sulfuric acid to produce soluble super-phosphate. Potassium fertilizers are at places extracted from evaporate deposits. Other minerals used as fertilizers, include gypsum, sulphur and borax. Rock phosphates (Phosphorites) Phosphorous is present in most rocks in minor to trace amounts. However, only in the phosphate rocks or phosphorites the values can be as high as 40% P2O5. More than 180 mineral species are known to contain 1% or more of P2O5. However, most of the phosphorous in the earth’s crust occurs in the mineral apatite, which is a phosphate of calcium, with fluorine and chlorine. More than 90% of rock phosphates are consumed in the manufacture of super-phosphates of different strengths for increasing the soil fertility. A small part of the phosphate rock is used for the recovery of elemental phosphorous, for the manufacture of phosphate chemicals such as disodium-phosphate, monocalcium phosphate, used in different industries. Elemental phosphorous is used in match industry as well as for the manufacture of incendiary bombs and fireworks.

Fig. 11. Cartoon depicting various modes of formation of different kinds of phosphate concentrations (after, Craig et al., 1996)

Sedimentary phosphate deposits are known as phosphorites, which form beds, a few cm to tens of meters thick, composed of cryptocrystalline fluorapatite, referred to loosely as collophane. Marine phosphorites, which constitute the principal reserves of this material, account for roughly 80% of world’s phosphate rock production. Major accumulations of this

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resource appear to have developed only where upwelling cool phosphate-saturated sea waters moved across near-shore continental margins. Here the phosphate precipitated by complex microbiological processes, into phosphatic mud, nodules and crust (Fig. 11). Indian distribution Rajasthan: Large phosphorite deposits occur in the rocks of the Aravalli Supergroup in the vicinity of Udaipur (Banerjee, 1971) and in Banswara districts. Phosphorite in the vicinity of Udaipur city occurs in two zones. Jhamarkotra, Matoon, Kanpur, and Kharbaria-ka-Gurha deposits occur close to the base of the Aravalli stratigraphy to the southeast of Udaipur. Bargaon, Nimachmata, Sisarma and Dakankotra, occur to the west of Udaipur. The Jhamarkotra phosphorite deposit, located about 24 km from Udaipur, represents the largest rock-phosphate deposit in India. The Jhamarkotra deposit, like other deposits in the Aravalli Supergroup of the Udaipur area, is associated with dolomitic limestone beds which have been metamorphosed to low grade and are silicified and brecciated at places. They are also stromatolitic at many places. In Jhamarkotra, the phosphorite-bearing horizon extends for more than 16 km in length with thickness varying from 1 to 25 m. The phosphorite horizon forms a broad arcuate belt, which overlies a very thick orthoquartzite and cherty sequence. The phosphorite occurrences at Jhamarkotra comprise three different types: i) columnar stromatolitic (algal) phosphorite; ii) laminar algal phosphorite; iii) reworked, silicified and brecciated phosphorite showing fragments of stromatolites set in a cherty and quartzose matrix. The P2O5 content of different varieties of phosphorites varies between 12 and 38 % at Jhamarkotra. The Siriska and Kushalgarh formations of the Delhi Supergroup have minor occurrences of phosphorites. A more or less regular bed of phosphorite, 2.5 to 3.9 m thick, occurs at Achraul, near Siriska. The phosphorite contains 12.5 to 31.3% P2O5. Sedimentary phosphate deposits, both stromatolitic and bedded varieties, of considerable economic importance have been found in Birmania area of Jaisalmer district. The deposits are intimately associated with dolomite, chert, carbonaceous shale, and sandstone. The rock sequence is correlated with the Marwar Supergroup, considered equivalent to the Vindhyans. The phosphorite deposit here, 1 to 9 m thick, and having 8 to 13% P2O5, extends as a single horizon for 6.8 km. Uttaranchal: Low-grade phosphorites occur in the basal chert member of the lower Tal Formation in the Mussoorie hills of the Lower Himalayas. The phosphorites usually occur in granular, fine grained, nodular and stromatolitic forms closely associated with black shale and chert, overlying the Krol dolomitic limestone of the Mussoorie syncline. Himachal Pradesh: Phosphorite occurrences have been reported from Sirmaur district, where phosphate rocks are found along the Krol-Tal contact rocks and are confined mostly to the lower Tal formation in close association with the chert. In an area known as Nigali-Dhar, the synclinal structure contains fairly large amounts of phosphorites for a distance of 56 km. Minerals of Cement industry Limestone The most important use of limestone is in the manufacture of cements. Cement grade limestone contains four essential chemical elements: calcium, silicon, iron, and aluminium. It is also used extensively as flux for smelting of various metallic ores. It is an important raw material for chemical industry and also used in lithography. Finely crushed limestone is used as a soil conditioner, for whitening and whitewashing. It is used as an aggregate in concrete, and as road

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material. As a dimension stone for both construction purposes and for decorative exterior facings limestone finds extensive use. Limestones are sedimentary rocks, deposited in shallow or deep water marine environment. They are often associated with silica, clay, pyrite and organic matters. Enormous accumulations of calcareous materials are at first formed as calcareous silts containing dead marine plants and invertebrate animal shells. In course of time, all these materials are progressively converted into limestones which are composed of calcite crystals, with varying percentage of magnesium carbonate and mechanically admixed impurities. Sometimes limestones are formed at the sea bottom by the accumulation and lithification of particles of calcareous materials, originally secreted in the sea water by living marine organisms. Usually oolitic limestones are formed in littoral zones due to the coagulation of colloidal solutions of calcium carbonate around minute sand grains. At first the oolites are composed of aragonite which is later converted into calcite. However, calcite is relatively unstable in the weathering atmosphere due to its high solubility in acidic waters. Indian distribution Calcareous rocks occur in all the principal geological formations of India, right from the Precambrian to Recent. The most important economic concentrations are found in the Vindhyan sequences of Bihar, Madhya Pradesh, Rajasthan, and Uttar Pradesh. The deposits in Bihar are mostly in the Rohtas Formation of the Vindhyan Supergroup in Sahabad district. In Madhya Pradesh, large deposits occur in the Semri Group rocks of the Jabalpur district. In Rajasthan, dolomitic limestone in the Raialo Group occurs around Alwar, Nagaur, Kishengarh and Udaipur. Tertiary limestones are distributed near Jaisalmer. In Uttar Pradesh, extensive deposits are found in Mirzapur and Dehra Dun districts. The former are in Vindhyan sequence while the latter are in Krol rocks. In Orissa, limestone is found in the Birmitrapur Formation of Gangpur Group in the vicinity of Sundergarh. Reserves estimated are in the range of 250 Mt. Important occurrences are also known from Sambalpur and Koraput districts. Andhra Pradesh also contains workable limestone deposits in the Cuddapah basin. Narji limestone in the Kurnool Group and Vempalle limestone in Anantapur district are exploited extensively. Extensive deposits of limestone are also found in the Saurashtra region of Gujarat. Gypsum (CaSO4.2H2O): Pure gypsum is colourless to white. Satin spar is a fibrous variety of gypsum with silky lusture; alabaster is the fine-grained massive variety and selenite is a variety that yields broad colorless and transparent cleavage folia. Gypsum is mainly used for production of Plaster of Paris. Satin spar and alabaster are cut and polished for various ornamental purposes but are restricted in their use because of their softness. Gypsum also serves as a soil conditioner. Gypsum is a common mineral widely distributed in sedimentary rocks, often as thick beds. It frequently occurs interstratified with limestones and shales and is usually found as a layer underlying beds of rock salt, having been deposited there as one of the first minerals to have crystallized on the evaporation of salt waters. More rarely it may crystallize in veins, forming satin spar. It is also found as lenticular bodies or scattered crystals in clays and shales. Found in volcanic regions, especially where limestones have been acted upon by sulfur vapors. Also found commonly as a gangue mineral in metallic veins. Usually the deposits have very little or no overburden and the material being very soft and friable are very easy to mine. Indian distribution

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Rajasthan: This state is the biggest producer of gypsum in the country. The deposits are confined to the Tertiary rock formations of Jodhpur region at Bhadwasi and Nagaur and in Bikaner region at Jamser, Lunkaranswar etc. Barmer district also has potential deposits of dessert gypsum. Tamil Nadu: This state accounts for the largest resource of gypsum in south India. Usually the deposits are found in highly fossiliferrous rocks of Uttatur and Trichinopoly Formations of Cretaceous age. They are intimately mixed up with the black cotton soil and estuarine clays of Pleistocene period or even the recent sediments. The three main gypsum-producing areas are in Coimbatore district, Ramanathapuram district and Tiruchirapalli district. Karnataka: Some amount of gypsum has been reported from alkaline earth regions in Chamarajnagar taluk of Gulbarga district. Small occurrences are also known in Bellary district. Himachal Pradesh: Gypsum has been reported from Chamba, Mahasu and Sirmaur districts, mostly as lumps, veins and bands, associated with Krol limestones and dolomites and also with Subathu Formation. Jammu & Kashmir: In the districts of Baramula and Doda, rich gypsum occurs as lenticular bands or as regular bedded deposits in the Precambrian Salkhala schists or associated with nummulitic limestones of Eocene age. Gujarat: In districts of Bhavnagar, Jamnagar, Junagarh and Kutch rich deposits of Gypsum have been reported from several areas. The richest deposits are found to occur in Rann associated with Gaj Formation. Minerals of Chemical industry: Sulphur (S) Sulphur is a non-metallic mineral, occurring as native sulphur, sulphides of base metals, and sulphates of calcium, magnesium and rarely potassium. The native sulphur and sulphides are the principal sources of sulphur. A good amount of sulphur is recovered from gases from smelters treating sulphide minerals. Sulphur is sometimes found as stalactites in caves and caverns and also as earthy masses. Native sulphur in India is reported as sulphur emanations in the bore-holes sunk by GSI, in Puga valley of Jammu and Kashmir which is associated with borax and a small amount of arsenic. Sulphur was/is recovered as by product from sulphurous fumes of copper smelters of HCL at Moubhandar in Jharkhand and Khetri in Rajasthan. Pyrite (FeS2), Pyrrhotite (Fe1-xS) and Marcasite All these three iron sulphides are used for manufacture of sulphuric acid for various industrial purposes. Indian distribution Bihar: Sedimentary pyrite deposits are reported from Amjhor, near Dehri-on-Son in Shahabad district. The pyritic lenses and beds are hosted by carbonaceous Bijaigarh shales (Guha, 1971) overlying the Kaimur Group rocks of the Vindhyan sequence.

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Rajasthan: Pyrite-pyrrhotite deposits occur as concordant, stratiform bodies co-folded with the host amphibolites (Sarkar et al., 1980) belonging to the Delhi Supergroup rocks at Saladipura, south of Khetri Copper Belt. Himachal Pradesh: Pyrite deposit occurs at Taradevi, near Chotashimla within Simla slates. Karnataka: The copper sulphide deposit in mafic volcanic rocks at Ingaldhal, in the Chitradurga schist belt, show good pyrite concentration in the footwall. Barite Pure barite is white, opaque to transparent and referred to as heavy spar. It is very heavy and its specific gravity varies from 4.3 to 4.6., which helps to distinguish it from most other non-metallic minerals. This non-metallic mineral is used in paint and varnish industry, as an extender and filler in paper, linoleum and rubber. It is also used extensively as an ingredient in the heavy drilling muds of oil drilling operations. Workable barite deposits are chiefly of three different types: a) veins replacing limestones and dolomites; b) residual deposits in argillaceous formations derived from the weathering of barium-bearing rocks; c) bedded deposits of barite in volcano-sedimentary successions. Barites occur also as common gangue mineral of many non-ferrous ore deposits. Indian distribution The most important barite occurrences are in Kurnool, Cuddapah, and Anantpur districts of Andhra Pradesh. Smaller deposits are also found in Rajasthan, Jharkhand, Orissa, Madhya Pradesh and Karnataka. Most of these deposits are of the vein type. Rare bedded deposits have been recorded from the Archean Sargur succession at Ghattihosahalli in Karnataka and Proterozoic volcano-sedimentary successions in Cuddapah district of Andhra Pradesh and Udaipur district of Rajasthan. Andhra Pradesh: In Cuddapah district, barite veins are reported from the neighbourhood of Mittamidapalle, Uppalapalle and Rajupalem. Most of the veins appear to be related to the Cuddapah traps and are found as replacement in the Vempalle limestone. One of the largest barite deposits in the world occurs at Mangampet with a resource of 37 Mt. (Neelakantam, 1989). The two lensoid bodies of barite occur interlayered with tuffs, carbonaceous shales and dolomites. The barium in this bedded barite deposit is considered to have been contributed by volcanic sources and the sulfate from the sea water. The northern lense is presently being mined by the Andhra Pradesh State Mining Corporation. In the Kurnool district, high grade deposits of barite occur at Ippatla, Midipenta, Nadipalle, Kottapalle and Balapalapalle. In Kottapalle area the barite veins are about a metre wide and extend for 2.5 km. They are fissure-filling type veins in dolomitic country rocks. In Anantapur district, principal deposits of barite occur at Nerijumapalle, Mutsokota, and Chandana areas. Barite occurrences are also known near Khammam. Rajasthan: Barite lenses occur in a linear zone within mafic metavolcanics of the Delwara Group underlying the Aravalli Supergroup sediments near Udaipur. A large lensoid deposit occurs at Jagat near Udaipur in a mafic volcanic inlier within the Banded Gneissic Complex (Deb et al., 1991). Veins of barite are also widespread within Alwar quartzites in Alwar district.

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Building stones Stones have always been used in a variety of ways in the building industry. In recent years, spurt in building construction, structural works and road and pavement making have created a huge demand for the minerals and rock-based materials of inorganic origin. Usually all types of stones which are hard, tough, and can withstand weathering and abrasion, that is, high durability, are preferred. The workability of building materials depend on their hardness. Colours of building stones and their directional properties are considered to be other essential criteria. All types of rocks, particularly granite-gneisses, crystalline schists, massive and compact sandstones and limestones, dolomites, marble, slates, khondalites, and compact laterites, are used extensively as building and roofing material and also as substitutes for bricks. These rocks are used directly with some amount of trimming of the surface and sometimes by processing them in measured dimensions after polishing of uneven surfaces. Such building stones which are cut and dressed after quarrying are called dimension stones. The dimension stones are also used for other structural purposes such as construction of bridge pillars, abutments, fences, retaining walls, monuments, paving stones, switch-boards, etc. When the materials are broken into pieces, they are called crushed stones. These are used in concrete materials with cement and lime. Indian building stones The gneisses, granites, charnockites, slates, crystalline limestones, marbles and quartzites of Precambrian age are considered as excellent building materials in India. Granite gneisses and granitic rocks are abundant in Peninsular India. They also occur in different localities of extra-Peninsular India. The banded gneissose rocks, the Bundelkhand gneisses of Rajasthan and Madhya Pradesh, Erinpura granites of Rajasthan and similar granite-gneissic materials of Bihar, Madhya Pradesh, Andhra Pradesh, Karnataka and Tamil Nadu have provided superb building materials for the construction of temples, palaces, monuments and tomb stones, etc. in almost all the states in India. In Rajasthan, pink, white and grayish white granites of Precambrian age are used in building royal palaces in Bikaner, Jodhpur, Mewar, and other areas. Granite-gneisses of Archaean complex are used as size stones, slabs, pillars, pedestals and for similar other constructional purposes. Some of these are exported to the foreign countries to serve as kerb-stones and tomb stones. These stones are found in Chitradurga district and other districts of Karnataka. In Bihar, Orissa, Madhya Pradesh and U. P. granitic rocks are extensively used as building stones. In Koderma mica-belt of Jharkhand, most of the buildings of the mica-mines are built of hard and compact mica-schists and granite-gneiss. The charnockites of Tamil Nadu, Karnataka and Andhra Pradesh are considered as the most durable stones in the world. The tomb of Job Charnock, earlier thought to be the founder of the city of Calcutta, is made of charnockite from the St. Mary’s hill in the vicinity of Chennai. The temples and monuments of Mahabalipuram, south of Chennai, have been carved out of solid and compact charnockitic rocks. Khondalites of Orissa and Andhra Pradesh are not as durable as the granites but still they are extensively used as building stones in these two states. In Andhra Pradesh, most of the buildings are made of khondalites and compact laterites. In Konarak and Puri temples of Orissa, most of the stones are either khondalites or hard and compact laterites. The statues and figures are invariably carved out of khondalites, particularly the gigantic wheels of the chariot in the Sun temple of Konarak. Laterites of Western Ghats are used as building stones in different parts of Maharashtra, Karnataka and Madhya Pradesh. Laterite can be cut and shaped very easily into required sizes and it hardens considerably on exposure.

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Crystalline limestones and marbles of Rajasthan, particularly the Makrana marbles, are being used for many centuries as building and ornamental stones. Taj Mahal of Agra and Victorial memorial of Calcutta are built from Makrana marble. Marbles of Raialo Group of Rajasthan are extensively quarried in Raialo, Alwar, and Jaipur. In Mewar, marbles are exploited in Rajnagar, Kankroli and Nathdwara. The white marble of Betul and multicolored marbles of Chindwara, Nagpur and Narsinghpur are also used as building stones. Motipura marbles from Baroda district, Gujarat are serpentinous marbles, mottled with pink and rose striations, which are used extensively in the construction of temples. In Koraput district and Gangpur region of Orissa, there are several varieties of crystalline limestones which are used as building materials. Vindhyan limestones of lower Bhander Formation contain spherulitic structures, in which the semi-circular shells display different colours. The deposits occur near Gwalior, in Sabalgarh area. The limestone and sandstone deposits of Vindhyan Supergroup are quarried in Son valley in Bihar and Uttar Pradesh, in Rewa and Jabalpur in Madhya Pradesh, in Guntur and Bhima area, Andhra Pradesh. Vindhyan sandstones of Khatu area of Jodhpur district, Rajasthan, yield very good flagstones particularly suitable for fine carvings and are considered good for fabricating perforated and ventilating windows and screens, usually found in big palaces. Vindhyan sandstones of Bhander Group of uppermost Vindhyan age are known as excellent building stones, due to their regular bedded formation, uniform grain-size, soothing colours, high durability and easy workability. The stones are cream and light grey in colour with crimson and pinky tints. The famous Sanchi Stupa and stupas of Sarnath and Barhut are built of Vindhyan sandstones. The famous Fatehpur Sikri, built by Emperor Akbar, is entirely of pink Vindhyan sandstones. The Delhi secretariat and Rashtrapati Bhawan of New Delhi are made of red sandstones of Bhander Group. A major part of the sandstones are quarried in Rajasthan, particularly in Bundi, Kota, Dholpur, Jaipur, Bharatpur and Bikaner and also in Mirzapur district of U. P. The Aravalli slates of Rajasthan, which can be cleaved, are used as roofing materials. The slates occur in the vicinity of Ajmer and Jharol. The Alwar quartzites of Moundla and the micaceous gritty rocks of Ajmer and Nasirabad produce thick and durable building slabs and blocks. The Cretaceous Deccan Traps contain compact, hard, and durable building materials. In Maharashtra, in the vicinity of Bombay, however, the light buff and cream coloured trachytes are very much in use locally and they are preferred more than the dark coloured basalts. The ‘Gateway of India’ in Bombay is entirely made of trachyte. The trachytes occur extensively in Salsette Island near Bombay. The trachytic rocks are also quarried in Malad and Kharodi in the neighbourhood of Bombay. Gemstones Diamond (C) Diamond is the hardest substance known. When properly faceted, light falling on the stone undergoes total internal reflection giving it the dazzling brilliance. Major part of diamond recovered from the rocks is of the industrial variety, known as bort, carbonado etc. based on their physical attributes. Only a minor part of diamond produced is of the gemstone variety. Primary sources of diamonds are kimberlite pipes and vents, and lamproite, or peridotite dykes. Secondary source is in conglomerate beds, alluvial gravels and sand. The kimberlites are dense, and dark-coloured ultrabasic rocks, rich in magnesium, containing olivine, enstatite-bronzite, chrome-diopside, phlogopite, and pyrope garnet with minor amount of ilmenite and perovskite. Diamonds require very high pressures for generation and growth which is not realized in the normal crust of

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the earth. It is therefore believed that diamonds originate in the upper mantle or in the root zone of thickened continental crust and are brought to the crust as inclusions in kimberlite pipes. Indian distribution Madhya Pradesh: A belt of upper Vindhyan sandstones extend in ENE to WSW direction through Panna in central India, on the south eastern side of Bundelkhand granite massif. This diamondiferous belt covers an area of 1,000 sq. km stretching between Jhanda in the east and Majhgawan in the west. The workings for diamonds are mostly confined to the alluvium and gravel but there are also workings in conglomerates at the base of Rewa and Bhander sandstones of upper Vindhyan age. Shahidan is the best known centre for diamond mining in Panna district. The kimberlite pipes have been discovered in Majhgawan and some other localities, near Panna. The pipes are represented by a circular depression containing calcareous tuffs mixed up with serpentinous materials. The highly brecciated rock consists of highly altered pseudomorphs of olivine along with phlogopite and leucoxene. Xenoliths of country rocks are also found in the pipe rocks. In the Majhgawan and other pipes of Panna diamond field, diamond crystals occur as coarse to fine, mostly imperfect to perfect crystals, sometimes even as fragments of crystals. Some of the primary inclusions in diamonds are euhedral, green olivine, diopside, garnet, and spinel. Andhra Pradesh: The districts of Cuddapah, Anantapur, Kurnool, Krishna and Godavari famous for diamond production in south India. In all these areas, loose diamond crystals occasionally picked up from the alluvium. Sometimes diamonds are recovered not only from the alluvium but also from the conglomerates and sandstones of Banganapalli stage of the Kurnool Group underlying the Palnad limestones. The region from where many valuable diamond crystals have been recovered is Wajrakarur in Anantpur district. Other common gemstones Gemstones usually occur in gravels and in mineral veins of igneous origin. Sapphire and pyrope occur in some of the diamondiferous kimberlites. Usually potash-rich or soda-lithium-rich pegmatites are the host rocks of many beautiful gemstones, such as, topaz, sapphire, ruby and zircon. Gems are also found in basic and andesitic lava flows, and granite intrusives. Although metamorphic rocks are generally barren of gem stones, some contact-metamorphic limestones may contain lapis lazuli and ruby. Opals are deposited from volcanic waters while amethyst develops in vein deposits. Turquoise is a gem stone of supergene origin. Almost all gem stones are found in stream gravels, due to their highly resistant and chemically inert character. Distribution of these gemstones in the Indian sub-continent is shown in Fig. 12.

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Fig. 12: Distribution of gemstones (other than diamond) in India and in some adjoining countries.

Ruby, Sapphire and Emerald Both ruby and sapphire are the gem varieties of crystallized alumina called corundum. Sapphire of different colours occurs in nature except red and pink, which are usually called ruby. In Kashmir Himalayas, in and around Nanga Parbat, sapphire deposits are known to occur. The outcrops of the sapphire-bearing formations lie hidden in the remote areas of the lofty mountain range. The formations comprise granites and other igneous intrusions particularly pegmatites penetrating the crystalline metamorphic schists. Intimately intermixed with the sapphire deposits are aquamarine, rubicelle green tourmaline. Al2O3, which is the main constituent of ruby and sapphire, forms spinel instead of corundum in the presence of magnesia. Spinel and corundum of gem quality are found in several places in Karnataka, particularly in Kadmane and Kelkoppa. They have deep colors and are harder than the Burma rubies. Translucent dark coloured rubies are found at Adihali near Bageshpura and Hardur district. Karnataka also produces rock crystals, opal, garnets, aquamarine and some emeralds. The coloured and transparent varieties of beryl are sold in the market as emerald and aquamarine. The deep green coloured mineral is known as emerald and the light green coloured stones are known as aquamarine. Because of its deep green colour and rarity of occurrence, emerald is the most expensive gem stone, other than diamond. The green colour is due to the presence of

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chromium in the mineral. In India, the famous emerald deposits are found in Mewar region of Rajasthan. The area is Kaliguman where emerald mines are present near the village of Amet in the neighbourhood of the old fortress of Kumbhalgarh. The precious mineral occurs within bands of biotite schists, very much like garnets and andalusite. Emeralds are also found at Banas in Ajmer district, Rajasthan. The best quality of emerald is found in Burma and Sri Lanka. Zircon (ZrSiO4) Zircons are the common accessory minerals, occurring in granites and gneisses. The Indian name of zircon is Gomed. Zircon is classified in four categories and it comes next to diamond in brilliance and internal glow. Zircon crystallises in tetragonal system and is prismatic in habit with pyramidal terminations. Hardness is high around 7.5, the colour being yellowish brown. Zircon is transparent to translucent and usually contains monazite as small inclusions which render it radioactive. Much of the colourless or transparent zircons are used as gemstones. When the crystals are heated, they become colourless. Zircon occurs in the beach sands of south-west India as placer deposits. In India, gem variety of zircon is found in small quantities in nepheline syenite rock, in the neighbourhood of Kangayam, in Coimbatore district of Tamil Nadu. Gem variety is also found in Kerala, near Travancore, in pegmatite veins, associated with charnockite complexes and also in the pegmatite veins of Kadavur, Tiruchirapalli district of Tamil Nadu. The Seitur graphite mines of Ramnad taluk of Tamil Nadu produce considerable quantity of gem variety of zircon. The pegmatite veins in Hazaribagh and Gaya districts, produce some zircon in Jharkhand. Jade and Nephrite The mineral includes two varieties: jadeite and nephrite. The latter is the more common form of jade. This mineral is a monoclinic amphibole, very hard (between 6 and 6.5) and compact, with a splintery fracture. The colour is usually leaf green to grass green, due to the presence of ferrous iron. The pyroxene group jadeite is rarer than nephrite, and is very tough, compact and splintery in character. The hardness varies from 6.5 to 7, with greenish white to emerald green colour. It is available in ample quantity in the Darjeeling Himalayas and various types of jade-bearing articles are sold in the markets of Darjeeling.

DISTRIBUTION OF MINERAL DEPOSITS IN SPACE AND TIME The quest for minerals through the ages led to the observation that mineral deposits are distributed non-uniformly on the crust of the Earth. For example, the largest concentrations of gold, chromite and platinum are found in South Africa, nickel in Ontario province of Canada, tin in the Malaya peninsula in Southeast Asia. In India, we have very large reserves of iron ores in Jharkhand and Orissa, manganese ores in the contiguous parts of Maharashtra and Madhya Pradesh in central India, and of mica in Bihar and Jharkhand. Such uneven distribution, globally as well as regionally, has given rise to the concept of Metallogenic provinces. These are regions of the crust generally more enriched with a single metal, several metals or metal associations than are the adjacent regions (Wright, 1992). Obviously, these provinces are characterized by some specific attributes of geology and tectonics, such as, having the right kind of sedimentary basin or the right type of lineaments at different periods of time to have been able to produce the exceptional concentration of one or more metals or minerals. Several important metallogenic provinces for certain metals are depicted in Fig.13 and some of the most prominent ones are identified in its caption. The Lake

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Fig.13. World map showing metallogenic provinces of selected metals. Important concentrations mentioned in text are: Iron: 1 = Lake Superior region, Canada-USA; 2 = Quadrilatero Ferrifero, Brazil; 3 = Krivoi Rog, Ukraine; 4 = Kiruna, Sweden; 5 = Jharkhand-Orissa, India; 6 = Hamersley Basin, Western Australia. Manganese: 1 = Minas Gerais, Brazil; 2 = Damara sequence, Namibia; 3 = Nsuta, Ghana; 4 = Nikopol, Ukraine; 5 = Sausar belt, India. Chromium: 1 = Bushveld Complex, South Africa. Gold: 1 = Superior province, Canada; 2 = Mother Lode, California, USA; 3 = Carlin deposits, Nevada, USA; 4 = Witwatersrand, South Africa; 5 = Murantao, Uzbekistan; 6 = Eastern Dharwar goldfields, India; 7 = Yilgarn craton, Western Australia. Copper: 1 = Zambian copper belt, Africa. Lead-Zinc: 1 = Broken Hill; 2 = Mt. Isa belt, Australia; 3 = Mississippi valley type deposits, Missouri, USA; 4 = Sullivan, British Columbia; 5 = Red Dog, Alaska. Aluminium = 1 = Jamaica, West Indies. Superior region of Canada and USA, Quadrilatero Ferrifero in Brazil, Krivoi Rog in Ukraine, Kiruna in Sweden and the Hammersley basin in Western Australia have some of the world’s largest concentration of iron. Similarly, some of the best resources of manganese are found in Minais Gerais, Brazil, Damara sequence in Namibia, Nsuta, Ghana, Nikopol in Ukraine and Sausar sequence in central India. A major part of world’s total resource of chromium is found in the Bushveld Complex in South Africa. Some of the largest concentration of gold is found in the Superior province in Canada, Mother Lode, California, Carlin deposits, Nevada in western USA, Witwatersrand in South Africa, Murantao in Uzbekistan, and Yilgarn craton in Western Australia. For more than last three decades the porphyry deposits are the most important resource of copper in the world. The largest concentration of porphyry deposits are found along the South and North American Cordillera, from Chile in the south to British Columbia, Canada in the north. The copper

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deposits in islands rimming the circum-Pacific region to the west are also of this type. Prior to the advent of technology to mine the low grade-large tonnage porphyry deposits in the early 70s, much of the world’s copper came from the Zambian copper belt in Africa. Rich Pb-Zn deposits are located in the Broken Hill area and Mt. Isa belt of Australia, the type area for Mississippi valley type deposits in Southern Appalachians in Missouri, USA, Sullivan deposits in British Columbia and Red Dog deposit in Alaska. All the aluminium deposits in the world are found along the equatorial belt, with some of the best deposits found in Jamaica, West Indies. Equally interesting is the concept of Metallogenic epochs, periods in Earth’s history marked by the development of exceptional concentration of a particular metal or metal association in a particular metallogenic province. The concept of metallogenic province and epoch are well illustrated by the somewhat rare metal tin (cf. Evans, 1997). Tin is found in a large metallogenic province, which got separated into several parts, in the different continents around the Atlantic Ocean when it opened up (Fig. 14, depicts the separation of S. American and African tin deposits with the opening of Atlantic ocean). It is also conspicuous in the tin belts of south-east Asia and of eastern Australia. All these concentrations took place in post-Precambrian metallogenic epochs in post-tectonic granites.

Fig. 14. Matching of tin belts across the Atlantic

Ocean (Modified from Evans, 1997) The temporal distribution of various deposits, such as those of iron, nickel, gold and base metals, record distinct patterns of deposition/concentration for a particular genetic type of metal deposit at a particular period of Earth history (cf. Hutchinson, 1993). Thus, most hematitic Banded Iron Formations in the world formed in the time window of 2.4 to 2.0 Ga while the Algoma-type magnetite-rich iron formations formed in the middle to late Archean; Greenstone-hosted gold deposits formed in the late Archean time between 2.8 and 2.6 Ga; nickel deposits in mafic-ultramafic flows were generated in the late Archean around 2.8 Ga while those in layered mafic intrusions formed in the early Proterozoic around 1.8 Ga; volcanogenic polymetallic massive sulfides (Pb-poor, Zn-Cu deposits) formed in the Archean (> 2.5 Ga) whereas similar Pb-rich, Zn-Pb-Cu deposits formed in the early Proterozoic as well

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Fig. 15. Schematic representation of secular variation of selected metal deposits. The vertical bar signifies periods of high concentration. as in the time span between late Proterozoic (1.0 Ga) to recent. The spatial and temporal distribution of mineral deposits have far-reaching implications in exploration programmes in different parts of the world and can be linked to crustal, mainly tectonic, and atmospheric evolution of the Earth. Low partial pressure of oxygen in the reducing paleo-atmosphere before 2.4 Ga ago was responsible for detrital concentration of uraninite and pyrite around 2.6 Ga in Au-bearing paleoplacer deposits like the Witwatersrand, whereas enhanced oxygen levels after 2.4 Ga was almost certainly a factor in the formation of super-large and extensive Banded Iron Formations, sedimentary manganese deposits, red bed-hosted stratiform copper deposits and unconformity-type uranium deposits. On the other hand, the porphyry copper deposits which proliferated in the last one third period of the Phanerozoic, are the result of subduction of oceanic plates and calc-alkaline magmatism along convergent plate margins. The porphyry copper deposits being generally emplaced near the earth’s surface (< 4 km depth), it is possible that many of these

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deposits were weathered and eroded away leaving only some here and there in the convergent margin setting. Examples of some of the metal types and their secular distribution are shown schematically in Fig. 15. GLOBAL TECTONICS AND METALLOGENY THROUGH GEOLOGICAL TIMES Tectonics involves the study of earth structures on a macroscopic scale. This branch of Geology considers megastructures vis-à-vis the dynamics of their generation. Such mega-structures in the continental or oceanic crusts are commonly produced during attainment of thermo-mechanical equilibrium in the crust-mantle system. The relationship between tectonics and ore genesis has been recognized since long when the ‘Geosynclinal concept’ was introduced in the later half of 19th century to explain mountain-building or orogenesis. Ore geologists have tried to identify specific types of mineralization in particular tectonic domains, such as, `shields’, ‘orogenic belts’, ‘stable platforms’ etc. Ore mineralization was also linked to different units of the geosynclinal model, such as, with the ‘miogeosyncline’ or the ‘eugeosyncline’. With the advent of the ‘plate tectonics theory’ in the 1970s, which superceded the ‘continental drift theory’, a more direct correlation of specific types of mineralization with tectonic setting was available (Mitchell and Garson, 1981; Condie, 1982, 1997; Sawkins, 1984; Sarkar, 1985). Some basic ideas of plate tectonics need to be highlighted to comprehend the relationship of ore mineralization with tectonic setting of a particular type. According to plate tectonics, the earth’s outer shell, the lithosphere, is divided into eight large and some smaller segments called ‘plates’. They are mechanically rigid and are in continuous motion relative to each other and with the axis of earth’s rotation. Such plate movements are primarily the result of the basic requirement of the mantle to dissipate heat. The convective transfer of this heat to the crust through various kinds of magmatism not only makes the plates move and interact with each other, but also sets in motion different potential ore-generating processes. This forms the basis of the relationship between plate tectonics and ore genesis. Three distinct types of plate margins are distinguished: the constructive boundary occurring at ocean ridges where new oceanic crust is generated; the destructive boundary at oceanic trenches where the oceanic crust sinks and is consumed; the transform faults along which the lithosphere moves but is conserved. The plates may comprise only oceanic crust or more commonly, both oceanic and continental crust. But plate generation (accretion) or destruction (subduction) takes place only in the oceanic crust. The orogens produced by plate interactions are of three types also: The Andean-type, where the oceanic crust subducts directly underneath the continent; the (Japanese) island arc type, where the subduction takes place away from the continent and there is a marginal sea between the continent and the arc; and lastly the Himalayan-type, characterized by continent-continent collision when the intervening oceanic crust is totally consumed, or virtually so.

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Fig. 16: Mineralisation in terms of certain plate tectonic scenarios, discussed in the text (after, Mitchell and Garson, 1981). Two types of arcs are also recognized: the compressional and the extensional. The first, represented by the Andean type, develops thickened crust, more differentiated volcanism (andesite-dacite-rhyolite) and more acidic plutonic rocks. The second, represented by the Japanese-arc type, is characterized by basalt-andesite volcanism and equivalent plutonism, limited topography and consequently, restricted volcaniclastic sedimentary fans. Such plate tectonic scenarios can easily be identified all through the Phanerozoic geological record and can also be

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extended into the Proterozoic, but the application of plate tectonic model to the Archean remains a controversial issue. Ore mineralization at divergent or constructive boundary setting: Zones of initial divergence are restricted to inter-continental rift zones, intra-continental hot spots, (Fig. 16 A, B) or oceanic spreading centers (Fig. 16 C). The intrusive rocks in the continental situations, such as, along the East African rift are peralkaline granites, alkaline rocks and carbonatites. Instances of mineralization, such as those of tin, niobium, fluorite, are found in such continental environments, e.g., in Nigeria, western Africa. Numerous metals, non-metals and elements are commonly concentrated in carbonatites. These include Nb, Fe, Ti, Cu, REE, apatite, fluorite and vermiculite. Aborted continental rift zones have a larger array of ore deposits. Besides the deposits found around hot spots, apatite-magnetite mineralization, as well as hydrothermal copper, exhalative Pb-Zn-Ag mineralization (Sullivan type), or carbonate-hosted Pb-Zn sulfide deposits (Mississippi valley type) characterize such rifts. Where continental separation has taken place with the accretion of an incipient oceanic crust, such as along the Red sea, base metal mineralized sediments form around vents on the sea floor (Fig. 16 A). Deposition of massive Cu, Zn sulfides is known from several oceanic spreading centres in the Pacific ocean, such as, the East Pacific Rise, Galapagos rift (Cu), Juan de Fuca rift (Zn) etc. Cuprous pyrite deposits also form in the oceanic crust at spreading centers (Fig. 16 C), to be eventually obducted on the land surface upon collision (see section on collisional settings below). In the Atlantic Ocean however, the slow spreading Atlantic ridge is characterized by the precipitation of Fe and Mn-oxides. Ore mineralization at convergent or destructive boundary setting: Three main types of such boundaries are recognized: island arc type, continental margin type and collisional type. In the first, the most important process operative in the convergent plate margins is the subduction of the oceanic lithosphere. Mineralization takes place in the principal arcs, as well as in the inner side of principal arcs. The arcs are linear belts of volcano-plutonic igneous rocks that are found above a subducting lithosphere slab. Mineralisation of Zn, Pb, Cu, Fe, Mo, Au and Ag are closely associated with calc-alkaline magmatism in principal arcs. The major part of the world’s production of copper comes from the ‘porphyry copper deposits’ in such tectonic settings. These deposits are more common in the compressional type arcs where the subduction takes place below the thickened crust of the continent (Fig. 16 C), such as, along the American cordillera. In the tensional arcs, such as the Japanese islands, the Kuroko type Zn-Pb-Cu sulfide deposits (Fig. 16 C) are more common than the porphyry deposits. On the inner side of the principal arc, common types of mineralization include contact metasomatic (skarn) deposits of Zn, Pb, Ag as well as W-Sn vein, greisen and replacement type deposits. While the former group is conspicuous in Mexico to Peru, the latter is found in Bolivia and Korea. Ore mineralization at passive continental margin setting: This kind of setting does not show any relative movement between the continental and oceanic lithospheres and occurs at the margin of opening ocean basins commonly related to continental rifting. Such margins are characterized by minimal tectonic and magmatic activity and are constituted by mature clastic sediments and shallow water carbonates. Mineralization in the present day environment, such as the Atlantic margin, is represented by evaporites and phosphorite deposits mainly. Pb-Zn mineralization in both clastics and carbonates are common in older sequences (cf. Fig.16 C, left end). Ore mineralization at collisional setting: This type of boundary is formed during and following the final stage of subduction of the ocean floor between two continents, between two island arcs or between a continent and an island arc. The important tectonic zones in this kind of setting are the hinterland margins, the suture zone, foreland thrust belts and the foreland basins. The last three

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zones are important from the point of mineralization. The suture zone often contains stratiform exhalative cuprous pyrite mineralization of the Cyprus type (Fig. 16E), such as in the type locality at Cyprus, and in New Foundland. Podiform chromite deposits are also found in the suture zone, such as at Oman and along the Indus Suture zone in the Himalayas (Fig. 16 E). In the foreland thrust belt, Sn-W mineralization occurs in S-type granites, best example being the deposits in Cornwall-Devonshire in the SW of England. In the foreland mollase basin uranium mineralization is found in the Siwalik Hills of Himachal Pradesh and Uttaranchal (Fig. 16D). Ore mineralization at transform faults: Transform faults are plate boundaries along which plates slide past each other. Normally the transform faults are not expected to be mineralized. Some possible exceptions are the stibnite mineralization along the Cenozoic Chaman fault in Pakistan and some late Cenozoic gold deposits in California. The Salton Sea geothermal system is a good example of mineralization in short segments of actively rifting crust in a leaky transform fault. The foregoing discussion on the distribution of metal deposits in space and time and their close linkage with tectonic evolution of the crust allows us to relate metallogenesis with crustal evolution through geological times. Five different stages of crustal evolution of the Earth can be identified with their characteristic mineralization (cf. Radhakrishna, 1984): The greenstone belt style of mineralization is characteristic of the Archean and early Proterozoic. Typical mineral deposits formed during this period include Algoma type iron formations in volcano-sedimentary successions; hydrothermal lode gold deposits and the massive Cu-Zn sulfide mineralization in volcanogenic host rocks. The cratonisation stage begins in late Archean and persists mainly in the early Proterozoic and thus co-exists with and finally superseeds the earlier tectonic regime. This period is characterized by the detrital-sedimentary gold-uranium paleoplacers (Witwatersrand type), the chemical sedimentary banded iron formations (Superior type) and sedimentary manganese (Kalahari and gondite type). The rifting stage around 1.8 Ga in the middle Proterozoic, which affected the previously stabilized continental crust and produced extensive mafic-ultramafic magmatism. The typical deposits are of the sedimentary exhalative types of base metal sulfides, intrusive-related nickel deposits and hydrothermal unconformity-type uranium deposits. The stable craton phase in the middle to late Proterozoic with alkalic volcanism and plutonism. The significant deposits during this period are of exogenic type confined to the cratonic sedimentary cover. Examples are of unconformity-related uranium, manganese and copper deposits. The Phanerozoic stage, characterized by abundant and varied mineralisations, particularly of the hydrothermal type, often related to granitic plutonism in the Paleozoic orogenic belts. The gamut of deposits range from vein deposits of gold, silver, tin, tungsten etc, porphyry Cu-Mo-Au deposits , epithermal deposits of noble metals, Mississippi valley-type Pb-Zn deposits, the Kuroko and Cyprus-type base metal sulfide deposits and ophiolitic chromite deposits. It is obvious from the above outline that from early stages of crustal evolution to the more recent ones, there is a proliferation of metallogenic processes. While the early part of earth history in the Precambrian saw only a few distinct types of mineralization, the Phanerozoic was marked by diverse types of mineralization.

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METHODS OF MINERAL EXPLORATION, EXPLOITATION AND PROCESSING The metals that we use in our everyday life, in some form or the other, go through several stages of handling before they are available to us. The mineral deposit must first be located by geological, geochemical and geophysical techniques and then explored by drilling and exploratory mining to estimate its reserve and grade and workability in general. Only then the mineral is extracted from the deposit either by underground mining or by surface or open-cast mining. The ore (a combination of ore mineral/s of interest and useless gangue minerals) so extracted is then processed to produce an ore concentrate by removing the gangue minerals which go into the tailing. This process of producing the concentrate is called ore beneficiation. The ore concentrate is next sent to the smelting plant where different metallurgical processes are used to extract the metal/s. The metal/s so produced commonly undergoes further refining before it/they can be marketed. Exploration is the first and most important phase of the mineral supply. Several different methods are available for exploration at different stages and in different environments. A potential area can be targeted by a survey of existing literature, maps and documents. Most commonly, areas are targeted in the vicinity of known deposits. Also possible is a systematic approach using all applicable techniques, starting from remote sensing, stream and soil geochemistry, geobotany, airborne and ground geophysics. A modern approach in regional exploration is a concept-oriented or model oriented programme of intensive investigation of mineral occurrences in specific geological environments. In India, till recently, exploration was limited to areas with old workings or to test the strike and depth continuity of known deposits. Also, unlike in developed countries, in India adequate consideration was seldom given to the cost and economics of exploration, as most of the agencies involved belonged to the Government. With the liberalization of mineral policies in the country, we now find comprehensive multi-technique exploration being carried out mostly by multinational companies in different potential regions. In all these approaches the basic requirement is of geological maps at various scales, starting from a small scale for a large coverage, e.g 1:50,000 topo-sheet scale, to successively larger scales, such as 1:25,000, 1:10,000, 1:5000 or even 1:1000 in which every outcrop in the ore zone can be shown. Features, other than lithology, shown on the maps could be structural data, trace of gossans, limits of old workings etc. Geochemical techniques include measurement of concentration and dispersion of trace elements either in secondary stream sediments, in soil samples or in bed rock chips which can then be linked with possible hidden mineral deposits. The concentration in the bed rocks is called primary dispersion while that in stream sediments and soil (Fig. 17) is referred to as secondary dispersion. The stream samples are commonly chosen from different orders of streams while soil samples are commonly collected in a grid pattern and contours of anomalies are drawn to locate the target area. Geobotanical techniques are based on the possible relationship of vegetation and mineral deposits. Specific plants thrive when the concentration of a particular metal in the soil is anomalous.

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Only when a proper target is obtained using these techniques, drilling is resorted to. It is always at a later stage because it is far more expensive. Initially the drilling is done in such a way that it intersects the orebody at a shallow level and its strike continuity is established. These are called first-order drill holes. In successive stages, second and third order drilling is done mainly to establish the morphology and depth continuity of the orebody. In India, almost all drilling carried out is of diamond drilling - coring type where bore hole cores are continuously recovered, laid out sequentially and logged megascopically for rock characteristics and ore zone identification. While this process is very good for reserve estimation and for easy future reference to any section of the hole, it is a very slow, laborious and expensive process. Also maintenance of the cores is another costly proposition. Only when possible reserve estimation has been carried out based on drilling, a small scale exploratory mine is opened up either by open pitting or by underground methods. This provides direct access to the ore body for various investigations and sampling. This stage also helps in correlating what has been envisaged about the reserves by drilling with what is actually in the ground. A successful positive correlation at this stage paves the way for actual mining and exploitation of the mineral deposit. There are two primary methods of extracting solid mineral resources from the crust of the Earth. These are underground mining and surface or open cast mining (Fig. 18). The former is more dangerous, expensive and slower process than surface mining because of the possibility of rock fall, water inflow and gas build up in the workings. Depending on the deposit size, shape, depth below the surface and grade (percentage of valuable mineral/metal) any of the two mining methods is chosen. In fact, many mines which start as open cast end up as underground mines when it is no more possible to make the pit deeper. Open pit or open cast mining is an economical method of extraction which involves large reserves and high rates of production. The waste material overlying the ore body must be thin enough to be removed easily. An open pit mine from

Fig. 17. Concentrations of copper determined in a regional geochemical survey in southern Rajasthan from (A) stream sediments and (B) soils (after Joshi and Singh, 2000)

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which building stones or gravel is extracted is called a quarry. The largest open pit mine in the world is at Bingham, Utah in SW USA. Another type of open pit mining is called strip mining which is used for flat sub horizontal beds, like that of coal. Underground mining is carried out for resources found at considerable depth from the surface. Such mines have one or more means of access to the ore body through a vertical shaft, or a horizontal adit or slanting roadway called incline. The mining in this case is done along horizontal tunnels parallel to the trend of the ore body called drives and also cross cutting tunnels exposing the width of the ore body at a particular depth (level), called cross cuts. Between successive levels of drives the block of ore is removed by mining methods called stoping. Ventillation, roof support and dewatering are important aspects of underground mining. Ore beneficiation involves crushing and grinding of the ore as a first step, which is generally carried out at the mine site. Magnetic minerals like magnetite or pyrrhotite are removed with the aid of electromagnets. Density differences are also used for separation of the ore minerals from the gangue since the former is always heavier than the latter. Most commonly the ground ore is immersed in a heavy medium of organic liquid like xanthate or pine oil which attach to specific minerals which float with the froth or bubble and is thus separated from minerals which do not attach and sink. This process of beneficiation is called froth floatation and is widely used for base metals. Separation of the metals from the concentrate produced by beneficiation takes place in smelters through pyrometallurgy. Here the concentrate melts into two immiscible liquids; metal-bearing liquid being heavier sinks to the bottom of the furnace and is removed from the slag above.

Fig. 18. Cartoon depicting the basic elements of open-pit and underground mining (modified after Kesler, 1994)

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ENVIRONMENTAL IMPLICATIONS During the utilization of non-renewable natural resources, environmental impacts are created in different stages: (i) during extraction (mining) of resources, (ii) during processing of resources, and (iii) during use and disposal of various resource products. Underground mines commonly have less impact on the surface unless there is a collapse in the mined out area or unless mining requires lowering of the ground water table to prevent mine flooding. Surface mining generally creates more obvious environmental damage than underground mining because there is a larger volume of rocks excavated and moved and a large open pit with a large pile of waste rock is created. The overburden removed during surface mining produces large, ever-increasing dumps which can cover and damage a lot of useful land around the mine. Acid mine drainage affecting the ground water quality in the neighborhood of the mine is a common problem in base metal sulfide mining. Oxidation of sulfides, particularly of the common pyrite (FeS2) produces sulfuric acid which enhances the capacity of the mine water to leach metals. Particular concern here is the leaching of toxic metals. Other problems associated with mining include the generation of dust, enhancement of noise level due to blasting and deployment of heavy machinery (Sarkar, 2002). Processing of resources or beneficiation consumes a large quantity of water and unless carefully monitored and checked, affects the water quality around the mining and plant area. The water consumption problem is addressed by recycling the water as much as possible. The escape of mine and beneficiation plant water into the ground water aquifer in the area enhances its toxic metal content due to the leaching of metals from the ore and host rocks in the mine, overburden dumps, tailing dumps and leaching pads. The metals which are highly toxic even in small quantities include Cd, Hg, Sb, As, and Pb. This problem is generally controlled by continuous monitoring of the ground water chemistry and by creating and putting impermeable barriers against downward movement of the waters. Special efforts and techniques are used where cynide solutions are used for heap leaching of gold from mined ore material kept in heaps on the surface. Where careless mining and processing are carried out, not only are the ground and surface waters polluted, but the soil and sediments are affected as well. Smelting of metals in metallurgical plants also brings about pollution in a different way. Harmful gases and dust are produced in many places. SO2 is the main gas of concern because in humid regions in particular, it can produce acid rain in the region around the smelter causing devastation of vegetation and agriculture. Some toxic metals present in the ore concentrate, which can vaporize during smelting operations can get dispersed over wide areas by emission from the plant chimneys. The processing of fossil fuels produces different kinds of environmental pollution, related to the escape of various hydrocarbons. Atmospheric pollution data in an highly developed country like USA (Kesler, 1994) shows that although there is an overall trend in the decrease of air pollution due to mineral production, it still accounts for about 30 % of man-made Pb emission, 25% of particulate emission, 18% of SOx emission, 13% of volatile organic compound (VOCs) emission, 3 % of CO emission and 2% of NOx emissions. Once the metal is turned into a product of any sort, the product itself will have a finite life. After some years of use it will have to be scraped or thrown into the garbage dump. In countries where collection and recycling are well controlled, much less quantity of metals end up in the land fills outside urban centres. But in many less developed countries where adequate legislations and collection mechanisms are lacking, the unusable product (for example, batteries), ends up in a land fill where it is further degraded with time and with water percolation, particularly in humid climates, will eventually pollute the ground water system.

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All these environmental impacts caused by mining, utilization of raw materials and finally the mine closure are now controlled in most countries through legislations and use of appropriate technologies. But all these efforts to curb the environmental impacts carry substantial costs, thereby raising the price of the final commodity. Therefore, in recent years, the start of a mining project is always preceded by a cost-benefit analysis. REFERENCES CITED Baidya, T.K., Mandal, S.K., Balaram, V., Parthasarathy, R., Verma,R., Mathur, P.K., 1999. PGE-Ag-Au mineralisationin a Cu-Ni-Fe-sulfide rich breccia zoneof the Precambrian Nausahi ultramafic-mafic complex, Orissa, India. Jour. Geol. Soc. India, v.54(5), p.473-482. Banerjee, D. M., 1971. Precambrian stromatolitic phosphorites of Udaipur, Rajasthan, India, Bull. Geol. Soc. America, v.82, p. 2319-2330. Chakraborty, K.L. and Chakraborty, T.L., 1984. Geological features and origin of the chromite deposits of the Sukinda valley, Orissa, India. Mineral. Deposita, v.19. p.256-265. Condie, K.C. 1982, Plate tectonics and crustal evolution. 2nd ed. Pergamon Press, New York, 310p. Condie, K.C., 1997, Plate tectonics and crustal evolution. 4th ed. Butterworth & Heinemann, Oxford, 282p. Craig et al., J.R., Vaughan, D.J. and Skinner, B.J., 1996. Resources of the Earth: origin, use and environmental impact. Prentice Hall, New Jersey, 2nd Ed. 472p. Deb, S., 1980. Industrial minerals and rocks of India. Allied Publishers Pvt. Ltd., New Delhi, 603p. Deb, M., Hoefs, J. and Baumann, A., 1991. Isotopic compositions of two Precambrian barite deposits from the Indian shield. Geochimica Cosmochimica Acta, v.55, p.303-308. Deb, M., 2000. VMS deposits: geological characteristics, genetic models and a review of their metallogenesis in the Aravalli range, NW India. In Deb, M. (ed.) Crustal evolution and metallogeny in the northwestern Indian shield. p.329-362. Narosa Publ. House, Delhi. Evans, A.M., 1997. An introduction Economic Geology and its environmental impact. Blackwell Science Ltd., Oxford, U.K., 364p. Fermor, L.L., 1909. The Manganese ore deposits of India. Mem. Geol. Surv. India, v. 37, 610p. Gross, G.A., 1966. Principal types of iron formations and derived ores. Canadian Min. Met. Bull., v.59 (646), p. 150-153. Guha, J., 1971. Sulfur isotope studies of the pyrite deposits of Amjhor, Sahabad district, Bihar, India. Econ. Geol. V. 66(2), p.320-330. James, H.L., 1954. Sedimentary facies of iron-formation. Econ. Geol. V.49, p.235-293.

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James, H.L., 1966.Chemistry of iron-rich sedimentary rocks. U.S.G.S Prof. Paper 440W, 60p. Joshi, A. and Singh, A., Geochemical mapping in southern Rajasthan: implications for delineation of anomalous zones of base metal concentration. In Deb, M. (ed.) Crustal evolution and metallogeny in the northwestern Indian shield. p.483-508. Narosa Publ. House, Delhi. Kesler, Stephen E., 1994. Mineral Resources, Economics and the Environment. Macmillan College Publishing Company, Inc., New York, 391 p. Krishnan, M.S., 1935. Lateritisation of khondalites. Rec. Geol. Surv. India, v.68, Pt.4, p.392-399. Mitchell, A.H.G. and Garson, M.S., 1981. Mineral deposits and global tectonic settings. Academic Press, London, 405 p. Neelakantam, S. 1989. Mangampet barite deposit, Cuddapah district, Andhra Pradesh. Geol. Soc. India, Mem. No.5, p. 429-458. Radhakrishna, B.P., 1984. Crustal evolution and metallogeny: evidence from the Indian shield. Jour. Geol. Soc. India, v. 25, p.617-640. Radhakrishna, B.P., Devaraju, T.C., Mahabaleswar, B., 1986. Banded Iron-Formation in India. Jour. Geol. Soc, India, v.28, p.71-91. Radhakrishna, B.P., 1996. Mineral deposits of Karnataka. Geol. Soc. India, Bangalore, 417p. Radhakrishna, B.P. and Curtis, L.C., 1999. Gold in India. Geol. Society of India, p.307. Ramam, P.K., 1976. Bauxite deposits of Anantagiri, Vishakhapatnam district, A.P. Jour. Geol. Soc. India, v.17 (2), p.236-244. Roy, S., 1966. Syngenetic Manganese Formations of India. Jadavpur University Press, Calcutta. 219p. Roy, S., 1981. Manganese deposits. Academic Press, London. 458p. Sarkar, S.C., 1974. Sulphide mineralization at Sargipalli, Orissa, India. Econ. Geol. v.69, p.206-217. Sarkar, S.C., Bhattacharya, P.K., and Mukherjee, A.D, 1980. Evolution of the sulfide ores of Saladipura, Rajasthan, India. Econ. Geol., v. 75, p.1152-1167. Sarkar, S.C., 1984. Geology and ore mineralization of the Singhbhum copper-uranium belt, Eastern India. Jadavpur University Press, 263p. Sarkar, S.C., 1985.Concepts of tectonic control of metallization: yesterday and today. Proc. of symp. On ‘Megastructures and plate tectonics and their role as a guide to ore mineralization’. Bull. Geol. Min. Met. Soc. India, No. 53, p.181-204. Sarkar, S.C., Kabiraj, S., Bhattacharya, S., Pal, A.B., 1996. Nature, origin and evolution of the granitoid-hosted early Proterozoic copper-molybdenum mineralization at Malanjkhand, Central India. Mineral. Deposita, v.31, p.419-431.

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Sarkar, S.C., 2000. Geological setting, characteristics, origin and evolution of the sediment-hosted sulfide ore deposits of Rajasthan: a critique, with comments on their implications for future exploration. In Deb, M. (ed.) Crustal evolution and metallogeny in the northwestern Indian shield. p.240-292. Narosa Publ. House, Delhi. Sarkar, S.C., 2002. Mineral deposits, their exploration and environmental management. CSME Bulletin, v.13, p.6-28. Sawkins, F.J., 1984. Metal deposits in relation to plate tectonics. Springer-Verlag, Berlin, 325p. Sikka, D.B., 1989. Malanjkhand: Proterozoic porphyry copper deposit, M.P., India. Jour. Geril. Soc. India, v.34, p.487-504 Valdiya, K. S., 1968. Origin of the magnesite deposits of southern Pithoragarh, Kumauon Himalayas. Econ. Geol. v. 63, p. 924-934. Wright, J.B., 1992. Continental drift and the origin of mineral deposits. In: Nierenberg, W. (ed.). Encyclodedia of Earth System. Science 1, p.603-608.