Wednesday, September 8, 2010

Geobotanical methods for prospecting uranium deposits.


Plants can also help us in finding uranium.
by
Dr. Nitish Priyadarshi
Fig. Aster Venusta
Name of the plant in the figure below is Astraqualus sp.

Geobotanical methods of prospecting involve the use of vegetation for identification of the nature and properties of the substrate. Paradoxically, these methods are among the easiest to execute and yet the most difficult to interpret of all the methods of exploration available at the present time. In terms of execution, the basic requirement is merely a pair of human eyes; but in the interpretation of the visual (or photographic) image, some knowledge is required of a number of different disciplines such as biochemistry, botany, chemistry, ecology, geology, and plant physiology.

Geobotanical methods of prospecting are based on the visual observation and identification of vegetation or plant cover that may reveal the presence of a specific type of sub-surface mineralization. In this method, it is presumed that a particular variety of plant species is an indicator of a sub-surface uranium molecules. In recent years, geobotanical methods have become useful in the identification of uranium- ore deposits particularly in areas of dense vegetation.

The method dates back to the eight or ninth century, when the Chinese had observed the association of certain plant species with mineral deposits. In the early nineteenth century, the Russian geologist, Karpinsky observed that different plants or plant communities could be indicators of rock formations and that the characteristics of the plants of an area could be used to decipher the geology of the area. The method has evolved over the years and now more than a hundred species have been recognized as indicators of the presence of a number of elements, including ore metals.

Two distinct approaches to geobotanical prospecting for uranium have been developed to cope with specific problems of exploration. The first method is based upon the presence of uranium in all plants in small but measurable amounts. It has been observed that the uranium content of plants rooted in mineralized ground is detectably higher than the uranium content of plants rooted in unmineralized ground. Plant ash is analyzed directly for the determination of uranium content. The uranium content of the ash of plants growing above unmineralized formations is generally less than 1 ppm, whereas that of the plants rooted in ore bodies contain several parts per million (ppm). This technique helps in the broad outlining of mineralized areas.

The second method involves mapping the distribution of certain indicator plants growing in ecologically favourable areas. A plant may be used as an indicator, provided it is established that its growth is controlled by certain factors which are related to the chemistry of the ore deposit. The sandstone type of uranium deposit contains an appreciable amount of selenium and sulphur. The distribution pattern of plants, which require one of these elements for normal growth, may indicate favourable ground for sub-surface uranium mineralization. Plant morphology and physiology are profoundly influenced by the chemical composition of sub-surface ore bodies and groundwater regime.

This method has been used in the USA for locating sandstone type of uranium deposits, particularly in areas where surface expression is lacking. Astragalus pattersoni, which thrives on the direct intake of selenium from ore bodies located up to a depth of 75 feet, was identified as one of the indicator plants for uranium.

Prospecting by both plant analysis and indicator plant mapping in widely separated areas of the Colorado plateau has shown a positive correlation between botanically favourable ground and major ore deposits.

Geobotanical surveys have been carried out in the Satpura-Gondwana basin of Madhya Pradesh and the foot hills of the Himalayas to demarcate mineralized (uranium) sandstone facies. Surveys conducted in the Kangoo basin of Hamirpur district in Himachal Pradesh revealed uranium values in plant ash samples ranging from 4.3 ppm to 96 ppm. Two- fern plants belonging to Adiantum venustum analyzed uranium values of 194 and 634 ppm respectively.

Perhaps the most obvious of all plant mutations is that of changes in the colour of the flowers. Colour changes in flowers are usually the result of either radioactivity or of the presence of certain elements in the soils.

Metal ions as well as radioactivity can affect the colour of flowers. The gardener’s trick of adding iron or aluminium to red hydrangeas to turn them blue is of course well known. The theory behind such colour changes is interesting and may have bearing on mineral exploration.

The majority of flower colours are produced by a surprisingly small number of pigments. Apart from Carotenoids, which are important in yellow and orange flowers, it is mainly the anthocyanins that are responsible for the colour range from orange to deep blue. It was suggested that in the absence of certain metals, the anthocyanins for red oxonium salts which become blue when they are complexed with excessive amounts of iron, aluminium, or other elements.

Besides iron and aluminium, other elements such as chromium and uranium can form stable complexes with anthocyanins. It is therefore possible that excessive amounts of some of these metals could produce a blue tint in flowers that are normally red or pink, and this could be useful field guide in prospecting.

Unusual and unpredictable changes of form are produced by radioactivity. The first result of mild doses of radioactivity is a stimulatory effect on the vegetation. After the nuclear explosion at Hiroshima, exceptional yields of various crops were obtained.

Fortunately, however, natural radiation is never as high as that encountered at Hiroshima in 1945 and levels normally encountered are therefore seldom sufficient to produce an obvious stimulatory effect on vegetation. There is, however, ample evidence that even fairly low levels of radiation can produce morphological changes in plants over a prolonged period. Variations was found in fruit of the bog bilberry ( vaccinium uliginosum) growing in a radioactive area at Great Bear Lake in Canada.

Plants are the only parts of the prospecting prism which extend through several of the layers simultaneously. It is claimed that the main advantage of biogeochemical prospecting compared with other geochemical methods lies in its power of penetration through a non mineralized over burden.

Reference:

Brooks, R.R., 1972. Geobotany and biogeochemistry in mineral exploration. Harper and Row publishers, New York.

Virnave, S.N. 1999. Nuclear geology and atomic mineral resources. Bharati Bhavan, Patna.
Post a Comment