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'Heavenly Bounty' Some Thoughts on Impact Metallogeny

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'Heavenly Bounty' Some Thoughts on Impact Metallogeny HEAVENLY BOUNTY 705 ‘Heavenly Bounty’ Some thoughts on Impact Metallogeny Impact cratering must be one of the most spectacular displays ever witnessed in the solar system. Although the ancient Indian astronomers, the ancient Vedic texts and the poets who have chronicled the history of India in the great Ra¯ma¯yana and Maha¯bha¯rata, described heavenly events in the past in lines of great poetic beauty, the subject has not evoked much interest in the modern generation of geoscientists. India has a hoary tradition of study of astronomical bodies, but these studies have come to be neglected, considered more as myths deserving no serious attention. Impact Metallogeny “Impact Metallogeny” may be defined as the study of major metallic and diamond deposits from the perspective of extra-terrestrial impacts that have been directly or indirectly responsible for metallogeny. An increasingly large body of evidence – geochemical (trace and rare element data, stable isotope systematics, field and structural data, geographical distribution patterns) is pointing to a causal link between impact processes and metallogeny either directly or indirectly. My plea is to re-examine our known large metallic and diamond areas from this fresh perspective so that we identify and characterise major impact structures and related metallogeny in the Indian sub-continent. The Mystery of Abnormal Concentration of Metals at Selected ‘Hot Spots’ in the Earth Spectacular concentrations of gold and uranium are to be found in the Witwatersrand Basin in South Africa, from where over 50,000 tonnes of gold have already been extracted and another 50,000 tonnes indicated in reserve; at Homestake Mines, (South Dakota); Colorado Mineral belt with its several large deposits; at Porcupine district of Ontario, (Canada); at Cadea (New South Wales), Kalgoorlie, Bendigo and Ballarat of Victoria, (Australia); nickel and platinum group elements at Sudbury (Canada); chromite, nickel and platinum at Bushveld (South Africa); nickel in Zimbabwe (Africa). Lead and zinc deposits (Australia). Quite a few remarkably large concentrations of metals in Russia, China and Indonesia and uranium-bearing conglomerate at Blind River, (Canada); tin deposits in Malaysia; gold deposits at Kolar and Hutti (India). The distribution of rich concentrations appears to be at random and no particular reason could currently be given for such extraordinary enrichment within a very restricted area. Why is this randomness in distribution and exceptional enrichment confined to selected hot spots in the outer skin of planet Earth? Let us take a look at how Earth originated. JOUR.GEOL.SOC.INDIA, VOL.72,DEC.2008 706 B. P. RADHAKRISHNA ‘Big Bang’ Theory of the Origin of the Solar System According to the currently accepted theory, the starting point of the universe was the Big Bang which occurred some 12-15 billion years ago. By 4,600 m.y., the released energy had evolved into several galaxies, one of which was the solar system with the Sun at its centre and a group of planets revolving round it. Planet Earth was one such accumulation of planetary solar dust. Such of the material which did not segregate as planets is believed to have floated in outer space and continued to orbit as planetary debris and is recognized as asteroids and comets. Individual asteroids could be as large as 1000 km in diameter. When forced out of their orbits, they collide with nearby planets, the impact carving huge circular hollows. The pock-marked face of the Moon, disfigured by circular basins of various sizes, is evidence of this early history of bombardment which the planets went through. Greenstone belts on Earth were probably the result of bombardment similar to lunar maria. It is reasonable to assume that Earth, too, went through a similar process of brutal battering which destroyed the originally formed crust. What we now see appears to be a secondary crust largely made up of material derived from extra-terrestrial sources subjected to further erosion and tectonic change. The oldest components which we recognize on this secondary (?) crust today are largely igneous, made up of an unusual rock type called komatiite and its variants, cherts, and graphite schists which, in all likelihood, represent material derived from outer space as remnants of the early phases of bombardment. The first continents which floated over the oceanic crust were necessarily limited in extent. With the advent of plate tectonic processes, rifts developed in the crust and subducting slabs, caused basins to form, in which sediments accumulated, creating more new crust. These are recognized as the Trondjhemite-Tonalite-Granodiorites (TTGs) and greenstone belts which floated as micro-continental rafts on an oceanic base. The thickened crust melted, produced granites and in this way the first shields were formed and stabilized. This brings us to the end of the Archaean period (2500 to 3000 m.y. ago) by which time the process of stabilization was complete and bombardment appears to have been greatly reduced in frequency and intensity. The Archaean- Proterozoic boundary, very probably represents an extended period of catastrophic events marked by major impacts from outer space, creating a series of large sedimentary basins. Uranium mineralization, so widely witnessed in the Proterozoic basins, appears to be the result of such major impacts. Impact Structures on Earth and their Economic Significance In order to follow the above course of events it would be necessary to locate impact structures, examine them in detail and determine their characteristics. The apparent paucity of impact structures in India is surprising since the Earth is believed to be located in the belt of asteroids and millions of pieces of planetary debris travelling at enormous speeds must have struck the Earth during the earliest phase of its recorded history. With the availability of excellent high resolution statellite and remotely sensed data, a diligent search for such impact structures in India is warranted. JOUR.GEOL.SOC.INDIA, VOL.72,DEC.2008 HEAVENLY BOUNTY 707 Views are expressed about periodicity in the frequency of such impact structures, causing changes in climate, reversal in magnetization, mass extinction of plants and animals. The subject is still in its infancy and a clear picture may emerge only after further study. The purpose of this essay is not about these matters but is restricted specifically to the possible concentration of metals and minerals in such structures. A few instances are quoted by way of illustration. Gold: Witwatersrand Basin, S. Africa: The world’s largest and oldest structure is the Vredefort Dome in South Africa at the centre of the Witwatersrand Basin. The structure is 2 million years old and is believed to have been caused by a huge asteroid which struck the Kapvaal craton, forming a crater 300 km in diameter, the largest impact structure so far known. Mineralisation is confined to conglomerates and the world’s largest concentration of gold, over 50,000 tonnes extracted to date. Uranium resources are also considerable. Most of the gold and uranium are currently considered to be of placer origin but the parent rocks which could have contributed to this amount of gold have not been identified. Australia: Rich pockets of gold close to the surface have been worked at Bendigo, Ballarat and the Klondike. The Super pit at Kalgoorlie in Western Australia is the world’s largest operation for gold. The pit is 3 km long and 1.5 km wide, has reached a depth of 290 m below surface and is proposed to be taken down to a depth of 600 m. The pit has produced 1600 tonnes of gold so far. Olympic Dam in South Australia is another large accumulation of metals containing resources of approximately 2500 tonnes of gold. Presently an underground mine, it is proposed to be converted into a large open pit, one of the biggest of its type in the world. India: Kolar Gold Field: Heavy concentration of gold is confined to narrow zones at Kolar, India. The early prospectors spotted a peculiar mylonitic gneiss charged profusely with opalescent blebs of quartz. Nickel, Copper and PGE – Sudbury Basin, Canada: This is another large basin believed to have been caused by impact of an extraterrestrial object more than 12 km in diameter, and is known for its nickel, copper and significant concentration of Platinum Group Elements. The basin is elliptical, 11 km long and 5 km wide. The structure dips inward. Mineralisation is in komatiites, rifted continental basalts and other ultramafic rocks. Major and rare earth element geochemistry is stated to be indicative of crustal rather than mantle source (Grieve, R.A.F, Geol. Soc. London Spec. Publ., 2005, no.248, pp.16-18). Lead-Zinc: Mississippi Valley: These are generally carbonate-hosted and strata-bound. Major deposits are found in Canada (Pine Point) and in the Upper Mississippi valley of the United States. Ore occurs at shallow depths, not greater than 600 m. The well-known Rampur-Agucha deposit of Rajasthan, India, probably belongs to this category. Mineralization is in breccia and fractures and the origin of this type of deposit is not very clear. There have been no attempts to trace signs of extra-terrestrial impact. Century (Lawn Hill) Zn-Pb Deposit, Queensland, Australia: A 19.5-km-wide impact structure has been described in close association with a major Zn-Pb deposit of world class rank (Australian Jour. Earth Sciences, v.55, 2008, pp.587-603). The exact relationship of the impact structure to the segregation of metals has yet to be determined Diamond: Diamond is specially characteristic of impact structures, the notable example being that at Popagai (100 km in dia), in Siberia. This large structure is profusely charged with micro JOUR.GEOL.SOC.INDIA, VOL.72,DEC.2008 708 B. P. RADHAKRISHNA diamonds.
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