For winning the diamonds, the ancients mined mostly secondary (sedimentary) rocks i.e., the river gravels and conglomerates, a few primary (igneous) rocks i.e., kimberlites and lamproites and surface soils distributed mainly in the districts of Krishna, Guntur, Mahaboobnagar, Kurnool, Prakasam, Cuddapah and Anantapur districts. The mines at Wairagarh were in the secondary rocks and those at Raichur are not clear.
Along the banks of the Krishna River, aptly termed as Diamond River, for a length of about 300 km between Sangam, the confluence of the Krishna and Tungabhadra Rivers, and Vijayawada in A.P., particularly Kolluru - Paritala belt, was the scene of intense mining activity producing a galaxy of historically world famous diamonds, such as the Koh -i- Noor (186 ct), the Great Mogul (787 ct), the Pitt or Regent (410 ct), the
Orloff (300 ct), the Nizam (440 ct) and the Hope (67 ct). Of all the diamonds, the Koh-i-Noor is more famous and perhaps it is the only diamond, which fell into many hands and crossed borders of several countries without being sold and ruled the destinies of many kingdoms. Historical accounts though differ on the authenticity of the period of their recovery, and their weights, all point to the Krishna Valley as the source for these celebrated diamonds. Famous conglomerate mines were at Ramallakota, Banganapalle, Vajragiri and Munimadugu in Kurnool district and Kolluru in Guntur district. The surface soils of the Wajrakarur and Jonnagiri areas in Anantapur and Kurnool districts and a few of the Wajrakarur kimberlite pipes in Anantapur district and the Chelima lamproite dykes in Kurnool district were also mined for diamonds. Large size diamonds were not only recovered from the Krishna gravels but also from the mines of Wajrakarur area known for many kimberlites bodies, under the Vijayanagar Kingdom. Many, in the earlier times, mentioned about the occurrence of diamonds in Raichur area of Karnataka during Vijayanagar empire (1420-21), It is given to understand that -the Kingdom had a regular income from the diamond mines of Raichur Doab (Area between two Rivers). It was because of these and other mineral deposits that the Raichur Doab was always coveted by the Bahmanis and after them by Bijapur and Vijayanagar rulers. The central -eastern part of India had also witnessed ancient diamond mining activity centered around Wairagarh, known as Beiragarh in the earlier times. They are located about 125 km northeast of Chandrapur in Maharashtra and at the confluence of Satti Nadi and Kobragarhi, tributaries of the Wainaganga River. The gravels of Baghain River, superficial lateritic gravels and Vindhyan conglomerates in the Panna belt were worked for these precious gems. The Mahanadi River gravels in Sambalpur-Hirakud area in eastern India also witnessed diamond mining in the medieval period.
SOME FAMOUS INDIAN DIAMONDS FROM THE KRISHNA VALLEY
DIAMOND HOST ROCKS
Diamonds are found in a variety of rock types including meteorites. The kimberlites and the lamproites, considered as primary source rocks for diamond, are magnesium, potassium and titanium rich ultra basic rocks. These rocks have become fascinating objects for the earth scientists not only for their diamonds in economic quantities but also provide interesting clues about formation of upper mantle. These rocks come to earth’s surface through violent eruption in the form of small volcanoes and ultimately solidify giving rise to vertical pipe like, carrot shaped and champagne glass shaped bodies and at places linear shaped bodies, called "dykes". Kimberlites are mantle derived ultrabasic igneous rocks produced by intraplate alkaline magmatism which form small bodies relative to other plutonic rocks.
Model of a Kimberlite pipe (after Mitchell)
Model of a Lamproite pipe (after Mitchell)
Majhgawan diamond mine in Panna district, Madhya Pradesh.
Diamond bearing source rocks (kimberlites/lamproites) of India
Besides these primary source rocks, there are also secondary source rocks containing diamonds, derived from disintegration or breaking of primary rocks. These include conglomerates (hardened gravel) and gravels (river as well as offshore).
Conglomerates formed by the disintegration of pre existing rocks containing diamonds.
Gravels formed by the transportation and deposition of weathered and disintegrated pre-existing rocks by major rivers. Kollur gravel developed by River Krishna. Site of ancient diamond mining activity and home for many a famed diamonds
LOCATING DIAMONDIFEROUS ROCKS
Although there are three types of major source rocks containing diamonds i.e. kimberlites, lamproites, conglomerates and gravels, the exploratory efforts are mostly focused on locating primary source rock as the expectancy level of mine life in these rocks is generally much higher than in other source rocks. But locating kimberlites is an arduous task as they occur sporadically and that too as small volume bodies. The problem of its detection is further complicated by their fragile nature which renders them easily vulnerable to disintegration and near obliteration of the evidences of its existence. As a result, one has to depend on some specialised techniques to detect such hidden and obscure bodies. Many kimberlites (Ks) and lamproites (Ls) are too small to be observed directly. Traditional mapping techniques are less effective in view of relatively small size and restricted distribution of the (Ks) and (Ls). Therefore, Earth scientists deploy a combination of techniques/methods comprising remote sensing and aerial photography, geochemistry, heavy mineral studies and geophysics, of which the last two techniques are found to be quite successful. Satellite imagery and aerial photographs help in identifying regional deep-seated fractures, if any. The physical properties of kimberlites, like magnetic susceptibility, specific gravity, electrical resisitivity or conductivity, in general have significant contrast with the enveloping rocks and this contrast is picked up by geophysical surveys. Such surveys are usually carried out initially by air, covering very large areas and then followed up by detailed ground surveys in selected areas of interest. Stream sediment sampling is by far the most widely followed technique all over the world for locating kimberlites. The pipe rocks generally contain a diagnostic suite of heavy minerals or tracer minerals, such as pyrope garnet, chromite, ilmenite, and chrome diopside. These minerals, once released from the kimberlites through weathering and disintegration, get transported for long distances along the streams. Geologists follow the trails of these minerals upstream to locate the source rocks. Modern diamond exploration is very expensive and time consuming, which requires investment of large amounts. Diamond exploration, apart from being a high-investment venture, is a high-risk venture too. Going by the available data on the number of kimberlite/lamproite pipes known and the number of diamond producing mines the world over, it emerges broadly that out of every 100 pipes discovered, only 10 turn out to be diamondiferous. Of these 10, only one would eventually be economically viable. The five to six thousand pipes discovered till date the world over, resulted in about 20 major mining operations and an additional 70 to 100 smaller operations. These figures eloquently reflect the magnitude of risk involved in diamond exploration. That being the case, anyone would readily appreciate that hitting upon an economically viable diamondiferous pipe would be very fortuitous.
INTEGRATED DIAMOND EXPLORATION PROGRAMME
Diamond Tracer Minerals
Worldwide distribution of Diamond bearing source rocks
Kimberlite intrusions commonly occur in clusters. Each cluster is composed of 1-20 distinct intrusions in close proximity to each other and separated by distances of > 20 km from other similar clusters. One or more clusters constitute what is known as kimberlite field. A geographically and genetically closely related group of kimberlites of similar age is known as kimberlite field. All the kimberlites within a given field originated most likely from a single source in the mantle. Variations in mineralogy and texture within fields are caused by diverse styles of intrusion, and differentiation of different batches of magma. Most kimberlite fields have a diameter of up to 50 km. One or more kimberlite fields of similar or differing ages constitute a kimberlite province. Kimberlites and Lamproites are known from every continent. These Archons are distributed globally in 12 potentially diamond bearing regions scattered on seven continents. The most prominent among them are those of the African continent. Kimberlites and lamproites are known from 16 African countries.
World map showing diamond occurrences. Colored areas demark "Archons"-cratons older than 2.5 Ga years, "Protons"-cratons 1.6-2.5 Ga years, and "Tectons"-0.8-1.6 Ga years, important in prospecting for mantle-derived diamonds. Black diamond symbols are used for mantle derived sources and red diamonds for ultra high pressure (UHP) metamorphic sources. (adapted from Levinson, 1998)
Russia has a major kimberlite province with 20 fields and more than 1000 intrusions. Canada, though came on to the kimberlite map of the world in 1956, has by far the most significant kimberlite province in the world with over 200 kimberlite pipes ,with some being of high economic potential. Australia contains several kimberlites and lamproites, however, the only diamond producing mine of Australia is the Argyle olivine lamproite mine, which is one of the main global contributors of diamonds. The other areas of kimberlite occurrence in the world include China (with two working mines), USA (with more than 25 kimberlite intrusions), and Ukraine (the Arkhangel province), Belorus, Finland and Sweden with diamondiferous and barren kimberlites.
Indian distribution of Diamond bearing source rocks
Though India is known from antiquity for its most beautiful, famous and large size diamonds, existence of the primary diamond source rocks came to light only in 1930 with the identification of the present outline of the Majhgawan pipe (where diamond production can be traced back to 1829) as Kimberlite (Sinor, 1930), which was later termed as lamproite (Scott Smith, 1989). This was followed by identification of tuffacious rock near Wajrakarur in southern India, where diamond mining was known since centuries, as kimberlite (Rao and Phadtare, 1966). The only working diamond mine in India today is located in the Majhgawan pipe. Kimberlite and lamproite emplacements are known from three Precambrian cratonic blocks of India viz. Dharwar in southern India, and Bastar and Bundelkhand in central India. Consistent efforts by the Geological Survey of India to locate kimberlites have resulted in the discovery of more than 80 kimberlite bodies in the Indian Sub continent, i.e. over 60 kimberlite bodies in Dharwar craton, five kimberlite bodies in the Bastar Craton and over 11 lamproite bodies in the southern part of the Bundelkhand Craton in recent years. Besides, a few lamproite bodies were also reported from the eastern margin of the Dharwar craton.
Present Indian scenario
In the recent years, India has developed as a major centre for diamond industry. It has rapidly become the world’s leader in fashioning the imported rough diamonds. The growth of the Indian diamond industry is one of the most successful stories in the country. Indian global business in the diamond industry progressed leaps and bounds to become the second largest foreign exchange earner, next only to software products and services. The Indian diamond industry, because of high skills of the artisans and cheap cost of labour, has an edge over other countries and placed itself much ahead of others. The industry, expanded vastly in the 1980s, consumes about 85 to 90% by volume of world production. In the last four decades, the volume and value of the diamond business in the country has increased four times, as a result India has now become the Global King of Diamonds.
Cratonic blocks of India (modified after Radhakrishna, 1989) showing locations of kimberlites /lamproites. EGMB:Eastern Ghats Mobile Belt;B:Basna kimberlite field; Bu:Bunder lamproites;D:Damodhar valley lamproites;K:Krishna lamproite field;M:Majhgawan lamproite field;Mp:Mainpur kimberlite field;Na:Nawapara lamproite field;N:Narayanpet kimberlite field:Nl:Nallamalai lamproite field;Ra:Ramadugu lamproite field;R:Raichur kimberlite field;T:Tungabhadra kimberlite field;Tk:Tokapal kimberlite field;W:Wajrakarur kimberlite field.
The Indian diamond cutting and polishing industry, which produces polished diamonds worth Rs.40, 000 crores (~ $8 billion), is located in the west coast of India. Gujarat is the main hub of diamond cutting and polishing with units located at Surat, Bhavnagar, Navsari and Palanpur. Mumbai has developed into a major trading centre for import of rough diamonds and export of polished goods. The gem and jewellery industry employs about 10 to 12 lakh workforce of which about 80% is engaged in diamond cutting and polishing industry.
GEOLOGICAL SURVEY IN DIAMOND EXPLORATION
The Indian Diamond industry is heavily dependent on import of raw diamonds for the cutting and polishing industry. The domestic production does not even meet 0.1% of our annual requirement. In order to meet the diamond rough requirement for the fast-growing Indian diamond industry and lessen our increasing dependence on imports, the Government of India, under the auspices of the Ministry of Mines has prioritised diamond exploration with emphasis on assessing diamond potential of the known primary and secondary deposits and evaluating them and to locate the two important primary source rocks for diamond i.e., kimberlite and lamproite. The Geological Survey of India (GSI), the premier Earth science organisation and the second oldest survey in the country, established in 1851, has contributed a lot to the country’s economic development, both before and after Independence. During the post Independence period, GSI has embarked upon the exploration for diamonds in favourable geological milieu spread over the Dharwar, Bastar, Singhbhum and Aravalli cratons. The investigations carried out since 1960s provide us first hand information of different diamond source rocks as also their potential. Since commencement of investigations for locating the primary source rocks for diamond, more than 100 kimberlite and lamoproite bodies have been located in India. Kimberlite generally occur either as pipes or as dykes, where as Lamproites are found as thin and impersistent dykes and sometimes as bowl shaped bodies as in Bunder and Majhgawan. Kimberlites are found mostly in Wajrakarur, Chigicherla, Timmasamudram, Kalyandurg Siddanapalle, Chagapuram, Maddur, Kotakonda and Narayanpet areas in Andhra Pradesh, Behradih, Payalikhand, Kodomali, Tokapal in Chattisgarh. Lamproites are found in Jaggayyapeta, Ramadugu, Aliabad, Banaganapalle, Chelima in Andhra Pradesh, Nawapara in Orissa, Bunder and Majhgawan in Madhya Pradesh and Damodar Valley in Jharkhand. Keeping in tune with the modern trends of diamond exploration, the GSI oriented its programmes through multidisciplinary surveys and from time to time equipped with state-of-the-art laboratories to back up the diamond exploration programmes. The efforts have led to discovery of several primary source rocks for diamond in virgin areas in different parts of the country. A few other central and state government organisations were also involved in diamond exploration now and then, mostly in collaboration with foreign organisations. The liberalization of our National Mineral Policy in 1993 paved the way for entry of private entrepreneurs, including those from overseas for carrying out diamond exploration. The database developed by GSI has been found very useful for taking investment decisions by the MNCs. A total of 4,810 diamonds were recovered by GSI during its exploration activities over a period of time weighing 923 ct (ranging in weight from 0.01- 16.3ct), out of which a total of 4,085 diamonds weighing 753.47 ct were handed over to Government of Andhra Pradesh. The valuation of which was estimated as Rs.33,58,805/=.Remaining diamonds consisting of 725nos were retained by GSI for a detailed research studies.
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