Tuesday, October 29, 2013

Thorium concentration in Ranchi plateau and other parts of Jharkhand State,



Rivers flowing through Ranchi plateau may also contain thorium.

By
Dr. Nitish Priyadarshi



Thorium deposits in India.


River flowing near Ranchi city.




Pegmatite intrusions in host rocks in Ranchi plateau.

Thorium is a naturally occurring radioactive chemical element with the symbol Th and atomic number 90. It was discovered in 1828 by the Norwegian mineralogist Morten Thrane Esmark and identified by the Swedish chemist Jöns Jakob Berzelius and named after Thor, the Norse god of thunder. Thorium produces a radioactive gas, radon-220, as one of its decay products. Secondary decay products of thorium include radium and actinium. In nature, virtually all thorium is found as thorium-232, which undergoes alpha decay with a half-life of about 14.05 billion years.

Berzelius was quite unaware of the tremendous amount of power that this element stores within it. Subsequent development in the field of nuclear science and technology, however, revealed that thorium might well prove to be equal to the god after whom it was named.

Thorium, which is transmutated U-233 in a breeder reactor, can be used as a nuclear fuel. It is presumed that with the development of breeder technology, thorium will come to play a vital role in providing electric power to millions.

Thorium is widely distributed in the earth’s crust with an average abundance of 8 ppm (parts per million) and is usually associated with uranium or the rare earth-earth elements. The principal mode of occurrence is in the form of veins in granites, synites, pegmatites and other acidic intrusions containing thorium- bearing minerals, such as thorite, thorianite, uranothorite and monazite. Detrital monazite occurs in quartz-pebble conglomerates, beach placers, inland placer deposits and dunes.

The largest known reserves of the thorium are contained in the beach and inland placer deposits of monazite, which are exploited for their rare-earth and ThO2 contents. Placer deposits of monazite are found in Australia, Egypt, India, Liberia, Brazil, Malaysia and the USA (Florida).

Among the inland placer deposits containing heavy mineral there are two appreciable concentrations of monazite, which are located in the Ranchi plateau of Jharkhand and the Purulia planes of West Bengal.  These occurrences cover an area of about 608 sq. km. forming a thin cover of an average depth of about 50 cm (which may be locally up to 2m.). These deposits have been formed due to the weathering and erosion of Precambrian gneisses and schists, intruded by pegmatites and porphyritic granites, which are enriched in monazite and other associated heavy minerals. 

The placer minerals are released from their matrix by weathering. The comminuted materials are washed slowly down slope to the nearest stream or to the seashore. Moving stream water sweeps away the lighter matrix, and the heavier placer minerals sink to the bottom or are moved downstream relatively shorter distances. The sands of the rivers like Swarnrekha, Jumar, Potpoto, kanchi, etc. flowing through Ranchi plateau may contain thorium in considerable amount.

Thorium present in the river streambed sediments are mostly of terrestrial origin and their concentrations are related to the type of parent rocks and to the genesis of the sediments. The river sediments generally exhibit large variation in composition. This variation can be related to the chemical and mineralogical evolution of these sediments along the river, influence of tributaries, or different properties of drained soil. The mobility of radionuclides in the aqueous system is an important factor influencing the content of radionuclides in river sediments. Surface run-off waters in the tributaries wash down a part of deposited radionuclides and finally store them in the river sediments.

Most of the radioactive anomalies in the Damodar Valley basins are confined to the Panchet  sandstones with the preponderance of thorium over uranium. A similar pattern has also been observed in the Barakar sediments of the Hutar basin.

The granites of the provenance areas fir the Hutar-Daltonganj basins contain anomalous uranium values. Uranium mineralization has also been observed in the granitic rocks comprising the southern periphery of the Hutar basin. The Proterozoic granitoids, forming the provenance for the Hutar and Auranga sub basin (Jharkhand), have been analysed which revealed uranium content up to 520 ppm, while the clays and sandstones of Barakar Formations have revealed anomalous uranium-thorium values of the order of 120-150 ppm uranium and less than 100-800 ppm thorium.

Reference:

Bateman, A.M. 1955. Economic mineral deposits. John Wiley & Sons, Inc. New York.

Virnave, S.N. 1999. Nuclear geology and atomic mineral resources. Bharti Bhawan, Patna.

Viswanathan, G., Badri, N.S.R., and Virnave, S.N. 1989. Radioelement distribution in the Lower Gondwana sediments of Hutar basin, Palamau district, Jharkhand; its bearing on uranium exploration. Exploration Research Atomic minerals Vol. 2 , pp 121-131.

http://large.stanford.edu/courses/2012/ph241/bordia1/

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