Terrestrial impact craters are relatively new objects for multi disciplinary investigations. Their discovery and study represents one of the most interesting episodes in the history of geological science.
Economic deposits associated with terrestrial impact structures range from world-class to relatively localized occurrences. The more significant deposits are introduced under the classification : progenetic, syngenetic or epigenetic, with respect to the impact event.
Natural impact craters are the result of the hypervelocity impact of an asteroid or comet with a planetary surface. Impact is an extraordinary geological process involving vast amounts of energy, and extreme strain rates, causing immediate rises in temperature and pressure that produce fracturing, disruption and structural redistribution of target materials. Some economic deposits of natural resources occur within specific impact structures or arc, in someway, impact related. Masaitis (1992) noted approximately 35 known terrestrial impact structures that have some form of potentially economic natural resource deposits.
In a review of the economic potential of terrestrial impact structures, Grieve and Masaitis (1994) reported that there were 17 known impact structures that have produced some form of economic resources.
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 Ramayan and Mahabharata, described heavenly events in the past in lines of great poetic beauty, the subject has not evoked much interest in 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 (Radhakrishna,2008.)
Earth is a part of the solar system and solar gaseous matter in the form of planetesimals floated and orbited in the region between planets. The largest of these planetesimals could be as large as 1000 km in diameter. These planetesimals, although part of the solar nebula, may not all have had similar chemical composition. It is possible some planetesimals formed in special zones of metal enrichment. One possibility of the special concentration of metals to a specific region on earth could be due to impact of such metal–rich asteroids hitting the earth especially during its early history (Radhakrishna, 2008). They had the same composition as the earth’s mantle but with additional attribute of having segregated metals such as iron, nickel, chromium and gold in the form of metal droplets. This is very likely in view of the abnormal conditions of enormous heat and pressure prevailing which permits segregation in asteroids. Such asteroids when they struck the earth gave rise to anomalous concentration of metals at points where they struck. If this view is considered feasible, the importance of the study of impact structures as a possible source of minerals and metals of economic importance will become evident. It is reported that a German Professor, Gerhard Schmidt from the University of Mainz has been investigating surface samples from impact sites for highly siderophile elements rich in elements like gold and PGE (Platinum Group Elements) and concluded that the metals have a Cosmochemical source (Times of India, 24 Sept. 2008, p.19).
The enrichment of heavy metals in the outer crust appears to be the result of extra-terrestrial impact especially in the earlier part of the history of the earth. Impact Structures are not just some special features; they are to be recognized as the result of normal processes operating on the earth and have to be studied in detail with all care to locate them. In the present age where satellite images and aerial photographs on different scales are available, such studies are specially warranted.
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 near by 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.
We should be thankful for the keen minds of the investigators who first recognized the significance of the circular depressions surrounded by rims that result from meteorite impact. We should remember also that these gifts from space were used by people in the early stage of human evolution. The fragments of iron meteorites, along with impact glasses and tektites, were used for making tools and for ceremonial purposes. In some cases, the impactites ( suevites and tagamites) are used as building materials. The Rathaus and Church of St. George’s in Nördlingen (Ries basin), together with the Rochechouart Castle (Rochechouart astrobleme, France) are magnificent examples of the application of Suevite for wall blocks and architectural detail. In many astroblemes the allogenic breccias, and the blocks of dense rocks in them, are utilized quarrying building stone and road materials. Pure silica which originated from shocked sandstone was mined for glass production in Meteor Crater, Arizona.
Moldavites are tektite glasses that are most probably the produce of ejecta from Ries basin. They are well known as the raw material for the manufacture of fashion decoration. Every women wearing accessories made from moldavites will no doubt give a positive answer to the question, ‘Are impact craters useful?”.
For the present, the assessment of the use of impact craters and their study has two related aspects in terms of their importance to modern society. On one hand, there is the possibility of direct industrial and economic use, while on the other hand, knowledge about the Earth and Space can be developed by using the data obtained during the investigation of impact craters.
Characteristics of natural resource deposits:
The location and origin of economic natural resource deposits in impact structures are controlled by several factors related to the impact processes and the specific nature of the target. The types of deposits are classified according to their time of formation relative to the impact event: progenetic, syngenetic, and epigenetic.
Progenetic economic deposits are those that originated prior to the impact event by purely terrestrial concentration mechanisms. The impact event caused spatial redistribution of these deposits and, in some cases, brought them to surface or near-surface position. From where they can be exploited.
Syngenetic deposits are those that originated during the impact event, or immediately afterwards, as a direst result of impact processes. They owe their to energy deposition from the impact event in the local environment, resulting in phase changes and melting.
Epigenetic deposits result from the formation of an enclosed topographic basin, with restricted sedimentation, or the long term flow of fluids into structural traps formed by the impact structure.
Impact Structures and their economic significance:
Gold in the Witwatersrand Basin, South Africa:
Vredefort crater :The multi-ringed Vredefort crater
The world’s largest and oldest structure in 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. Mineralization is confined to conglomerates and the world’s largest concentration of gold, over 50,000 tonnes extracted to date.
The asteroid that hit Vredefort is one of the largest to ever impact Earth (at least since the Hadean) estimated at over 10 km (6 miles) wide. The crater has a diameter of roughly 250 - 300 km (155 - 186 miles), larger than the 200 km (124 miles) Sudbury Basin, and the 170 km (106 miles) Chicxulub crater. This makes Vredefort the largest known impact structure on Earth (though the Wilkes Land crater in Antarctica, if confirmed to have been the result of an impact event, is even larger at 500 kilometers across). The age is estimated to be over 2 billion years (2,023 ± 4 million years), impacting during the Paleoproterozoic era. It is the second oldest known crater on the Earth, a little less than three hundred million years younger than the Suavjärvi crater in Russia.
It was originally thought that the dome in the center of the crater was formed by a volcanic explosion, but in the mid 1990s evidence revealed that it was the site of a huge bolide impact, with telltale shatter cones often discovered in the bed of the nearby Vaal River.
The Vredefort crater site is one of the few multi-ringed impact craters on Earth, though they are more common elsewhere in the solar system. Perhaps the best example of one is Valhalla crater on Jupiter’s moon Callisto, though Earth's Moon has a number as well. Geological processes, such as erosion and plate tectonics, have destroyed most multi-ring craters on Earth.
Vredefort structure is the eroded remnant of a very large complex impact structure. The Witwatersrand Basin is the world’s largest goldfield, having supplied some 40% of the gold ever mined in the world. Since gold was discovered there in 1886, it has produced 47,000 tonnes of gold. The annual Witwatersrand gold production for 2002 was approximately 350 tonnes, or approximately 13.5 % of the global gold supply, and current reserve estimates are around 20,000 tonnes of gold (Grieve,2005).
Gold in 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.
Gold in Kolar gold field, India.:
Heavy concentration of gold is confined to narrow zones of Kolar, India. The early prospectors spotted a peculiar mylonitic gneiss charged profusely with opalescent blebs of quartz.
The first indication of impact diamonds was the discovery in the 1960s of diamond with lonsdaleite, a high pressure (hexagonal) polymorph of carbon, in placer deposits, e.g. in the Ukraine, although their source was unknown (Cymbal and Polkanov,1975).
In the 1970s, diamond with lonsdaleite was discovered in the impact lithologies at the Popigai (100 km in diameter) impact structure in Siberia. This large structure is profusely charged with micro diamonds. Since then, impact diamonds have been discovered at a number of structures, e.g. Kara, Puchezh-Katunki, Reis, Sudbury, Ternovka, Zapadnaya, and others.
One of the more interesting features of Popigai is that diamonds are found in and around the crater environs. The pressure waves released by whatever phenomenon caused the crater are thought to have compressed the graphite within the gneiss formations and instantaneously transformed it into diamond. The blast that created the precious gems also threw them over 150 kilometers to the east, where they can be found loose in the soils and the rivers.
The diamonds not only inherit the tabular shape of the original graphite grains but they additionally preserve the original crystal's delicate striations
Impact diamonds originate as a result of phase transitions from graphite, or crystallization from coal, and occur when their precursor carbonaceous lithologies were subjected to shock pressures greater than 35 G Pa (Masaitis, 1998).
The diamonds from graphite in crystalline targets usually occur as paramorphs with inherited crystallographic features and as microcrystalline aggregates. At Popigai, these aggregates can reach 10 mm in size but most are 0.2-5 mm in size (Masaitis, 1998). The diamonds, generated from coal or other carbon in sediments, are generally porous and coloured. In the case of Popigai, the original source of carbon is Archaean gneisses with graphite.
The Reis structure in Germany is a diamond –producing mine and is believed to be the result of impact, with a pipe 24 km in a diameter. Canada is rich in diamonds in its northern territories. It is recently reported that a carbon-rich comet crashed over diamond fields of Canada 12,900 years ago and caused scattering of diamonds over a large area. Besides diamond, gold and silver too are reported to be present (Peizer Benny, CCNet,110, Science Daily, 3 July 2008, Tankersbay, Cincinatti Newsletter).
The oldest working for diamonds known in India is at Panna, Madhya Pradesh. The structure is a cup-shaped open pit with rings of breccia in which diamonds are distributed (Radhakrishna,2008)
Of more importance are the diamonds gravels exposed in the banks of the lower reaches of Krishna River in Andhra Pradesh. A cluster of Kimberlite pipes have been identified but these could not have provided all the diamonds. More likely, the source region lies within the Proterozoic Cuddapah Basin, itself of possible impact origin, as has been suggested based on geophysical evidence.
Hydrocarbons occur at a number of impact structures. In North America, approximately 50% of the known impact structures in hydrocarbon-bearing sedimentary basins have commercial oil and/or gas fields.
The Ames structure is located in Oklahoma, USA, and is a complex impact structure about 14 km in diameter, with a central uplift, an annular trough, and slightly uplifted rim. It is buried by up to 3 km of Ordovician to Recent sediments (Carpenter and Carlson, 1992) making an exploration difficult. However, some rocks recovered from the central uplift were reported to shows quartz grains with shock-diagnostic planar formation features, indicating an impact origin for the structure. The feature has morphological characteristics, such as a central uplift surrounded by a circular depression and an outer ring, which are typical of those of complex impact craters. The central uplift is about 5 km in diameter and is collapsed or eroded in center.
Its origin has been variously attributed to meteorite impact, volcanic activity, dissolution collapse and other causes. Geological arguments for an impact origin have been based on geomorphology, rock textures, mineral deformation, and stratigraphic relations( Nick, 1994).
The first oil and gas discoveries were made in 1990 from an approximately 500 m thick section of Lower Ordovician Arbuckle dolomite in the rim. Due to impact –induced fracturing and Karsting, the Arbuckle dolomite in the rim of Ames has considerable economic potential (Grieve, 2005)
Other impact structures also produce hydrocarbons. For example, the 25 km diameter Steen River structure, Canada, produces oil from two wells on the northern rim. The large diameter (180 km) Chicxulub in Mexico is known for its oil and gas accumulation. The Chicxulub crater is an ancient impact crater buried underneath the Yucatán Peninsula in Mexico. Its center is located near the town of Chicxulub, after which the crater is named — as well as the rough translation of the Mayan name, "the tail of the devil."The crater is more than 180 kilometers (110 mi) in diameter, making the feature one of the largest confirmed impact structures in the world; the impacting bolide that formed the crater was at least 10 km (6 mi) in diameter. The crater was discovered by Glen Penfield, a geophysicist who had been working in the Yucatán while looking for oil during the late 1970s. Evidence for the impact origin of the crater includes shocked quartz, a gravity anomaly, and tektites in surrounding areas.
Structures in the Gulf of Mexico are known to be producers of vast amounts of oil and gas in brecciated rocks at the Cretaceous- Tertiary boundary.
A report from NASA (Nature, v.454,2008, p. 589 and 609-610) states that the US-Europe cosmic spacecraft CASSINI has spotted large lakes containing liquid hydrocarbons and ethane on Titan, the largest moon of Planet Saturn. This open up the possibility of identifying extra-terrestrial object being possible source of hydrocarbons.
The other case is represented in the Red Wing Creek astrobleme (North Dakota, USA). Here the authigenic breccia of the central uplift is covered by a clay layer, which forms a structural trap. Oil has migrated from the original Mississippi deposits into this trap and has been exploited. It is possible that the oil-bearing structures Viewfield, Eagle Butte and some others situated in that basin have the same impact nature.
Nickel, Copper and Platinum group elements in Sudbury Basin Canada:
Fig. of Geological map of Sudbury Basin
This is another large basin believed to have been caused by impact of an extraterrestrial object more than 12 km in diameter, and its 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. Mineralization is in Komatiites, rifted continental basalts and other ultramafic rocks.
It was created as the result of a 10 km meteorite impact that occurred 1.85 billion years ago in the Paleoproterozoic era. Debris was scattered over an area of 1.6 million square kilometers and travelled over 800 kilometers away — rock fragments ejected by the impact have been found as far as Minnesota. Its present size is believed to be a smaller portion of a 250 km round crater that the bolide originally created. Subsequent geological processes have deformed the crater into the current smaller oval shape. Sudbury Basin would then be the second largest crater on Earth, after the 300 km Vredefort crater in South Africa, and larger than the 170 km Chicxulub crater in Yucatán, Mexico.
Copper-Nickel were first noted at Sudbury in 1856. It was not until they were ‘rediscovered’ during the building of the trans-Canada railway in 1883 that they received attention, with the first production occurring in 1886 (Naldrett, 2003). By 2000, the Sudbury mining camp had produced 9.7 million tonnes of Cu, 70 thousand tonnes of Co, 116 tonnes of Au, 319 tonnes of Pt, 335 tonnes of Pd, 37.6 tonnes of Rh, 23.3 tonnes of Ru, 11.5 tonnes of Ir, 3.7 tonnes of Ag, 3 thousand tonnes of Se and 256 tonnes of Te (Lesher and Thurston 2002).
At Sudbury the nickel-copper deposits are associated with an elliptical basin that is about 65 kilometres long and 27 kilometres wide, containing layers of norite and diorites up to 3 kilometres thick, called the "Sudbury Igneous Complex".
Lead-zinc in 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 deposits at Rajasthan, India, probably belongs to this category.
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 generation of metals has yet to be determined.
The famous Carswell structure (side figure) in Saskatchewan, Canada, known for its uranium deposit, is ascribed to an impact structure eroded below the floor of the original crater.
The Athabasca Basin is the largest and richest uranium producing basin in the world. The basin is located just to the south of Lake Athabasca. The basin covers about 100,000 square kilometres in Saskatchewan and a small portion of Alberta. The surface of the basin consists of main sandstone sediment varying from 100 to 1000 metres in depth. The uranium is mostly found at the base of this sandstone, at the point where it meets the basement.
Uranium was discovered in the region in the 1940s. The first mine in the area was the Rabbit Lake Mine, which was discovered in 1968 by Gulf Mineral Resources and opened in 1975. The most important current mine is Cameco's McArthur River mine, the world's largest high-grade uranium mine.
Cumulative uranium production from the basin is approximately 1.5 billion pounds of uranium oxide, with a value of close to US $ 1.5 billion.
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Cymbal, S.N. and Polkanov, Yu. A. 1975. Mineralogy of titanium-zirconium placers of Ukraine. Nauk Press, Kiev [in Russian]
Grieve, R.A.F. and Masaitis, V.L., 1994. the economic potential of terrestrial impact craters. International Geology Review, 36, 105-151.
Grieve, R.A.F., 2005. Economic natural resource deposits at terrestrial impact structures. Mineral deposits and Earth evolution. Geological Society, London, Special publications, 248,1-29.
Lesher, C.M. and Thurston, P.C. (eds.) 2002. A special issue devoted to mineral deposits of the Sudbury Basin. Economic Geology, 97. 1373-1606.
Masaitis, V.L. 1992. Impact craters: Are they useful?. Meteoritics, 27, 21-27.
Masaitis, V.L., 1998. Popigai crater. Origin and distribution of diamond-bearing impactites. Meteorities and Planetary Science,33, 349-359.
Naldrett, A.J.,2003. From impact to riches: Evolution of geological understanding as seen at Sudbury Canada. GSA Today, 13, 4-9.
Nick, K. E., 1994. Lithologic and stratigraphic evidence for the impact origin of a buried Ordovician age crater and reservoir near Ames, Major County, Oklahoma: American Association of Petroleum Geologists 1994 Annual Convention Official Program, p. 224.
Radhakrishna, B.P. 2008. Heavenly Bounty, some thoughts on impact metallogeny. Jr. of Geol. Soc. of India. v.72, no.6. 705-712.