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Geobotany and biogeochemistry in mineral exploration in the tropics
Journal of Geochemical Exploration, 29 (1987) 427-428
427
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
Geobotany and Biogeochemistry in Mineral Exploration in the Tropics E.A.V. PRASAD
Department o[ Geology, Sri Venkateswara University, Tirupati 517.502 ( A.P. ), India (Received June 16, 1986)
The ancient Sanskrit text "Brihat Samhita' (i.e., Master Collection) by Varahamihira (A.D. 505-587) described the fundamental concepts of bioindicators, chiefly plants and termite mounds, to locate groundwater and mineral resources in arid and semiarid regions. Only recently, similar studies have been revived. The occurrence of an aquifer or a mineral deposit gives rise to environmental heterogeneity, with a microenvironment characterized by, respectively, a high relative humidity or heavy-metal toxicity. The ecotypes and the ecophenes with environmentally induced, morphologic or mutational changes constitute the bioindicators for nonbiotic natural resources. Recent studies in southern India have attempted to identify plant indicators in certain selected mineralized areas which include: Kondapalli chromite in Krishna District in Andhra Pradesh (A.P.); Byrapur chromite in Hassan District in Karnataka; barytes deposits in Cuddapah District (A.P.) ; Agnigundala base metal deposits in Guntur District (A.P.), and mica pegmatite in the Nellore District (A.P.). In all these districts there are plant species tolerant to heavy-metal toxicity in the mineralized areas. Some, with full growth, occur even on the concentrated ore dumps. Amongst these plant indicators Tulsi of the Labiaceae (mint) (hyptis and ocimum species) has extraordinary tolerance for soils rich in Cr, Pb, Zn, Cu and Ba, and for a serpentine bedrock. The ubiquitous thorny shrub Prosopisjuliflora has an extraordinary ecologic amplitude and tolerance for a wide variety of heavy metals. Similarly, Calotropis gigantea and Cassia auriculata exhibit tolerance for many heavy metals in the soils and bedrock. Among the tree species, neem (Azadarichta indica) and certain Ficas sp. are tolerant to heavy-metal toxicity. Givotia rottleriformis, which is most common and widespread in the Kondapalli hill range on different rock types, exhibits profuse flowering, whereas in the presence of chromite it is devoid of flowers. In granitic terrain, bush shrubs of thorny Gymnosporia [alconeri indicate a permatite bedrock. When the pegmatite and associated mica, beryl, and tour-
0375-6742/87/$03.50
© 1987 Elsevier Science Publishers B.V.
428
maline is mined, G. [alconeri is replaced by Prosopis juliflora demonstrating the antagonistic relationship between these two thorny species. Gymnosporia [alconeri is common in barytes mineralized zone associated with dolomite and basalt. Where Au concentrations are high and Ba and Sr low, G. falconeri has pink and red foliage; however, where Au is low and there are high concentrations of Ba and Sr, normal green foliage is present. Phyllite and dolomite comprise the country rocks of the Agnigundala base metal deposits. Albizzia amara is an indicator of phyllite, while the presence of Barleria longifolia, indicates domomite. Gymnosporia falconeri, Hardwickia binata, Zizyphus xylopyrus, Chomelia asiatica, and Cassia angusti[olia growing on dolomite have higher metal contents than those growing on phyllite. Termite mounds constitute important tools in mineral exploration. Maintenance of constantly high relative humidity in the mounds is an essential prerequisite for the very survival of the species in arid and semiarid regions. The termites penetrate deep down into the subsoil to reach the water table. Along with the soil particles for the construction of their mounds and groundwater carried out by the termites, any ore elements present are also removed to the surface. Hence, it has been found that the concentration of ore elements in the mounds is locally much higher than in adjacent soils. Furthermore, indicator plants growing on a termite mound have higher concentrations of some ore elements than the same plant species developed on adjacent soils. Thus the excellence and efficacy of the termite mound as an important tool in geochemical prospecting in tropical regions has been firmly established, even in areas devoid of outcrops and covered by a thick soil mantle.
427
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
Geobotany and Biogeochemistry in Mineral Exploration in the Tropics E.A.V. PRASAD
Department o[ Geology, Sri Venkateswara University, Tirupati 517.502 ( A.P. ), India (Received June 16, 1986)
The ancient Sanskrit text "Brihat Samhita' (i.e., Master Collection) by Varahamihira (A.D. 505-587) described the fundamental concepts of bioindicators, chiefly plants and termite mounds, to locate groundwater and mineral resources in arid and semiarid regions. Only recently, similar studies have been revived. The occurrence of an aquifer or a mineral deposit gives rise to environmental heterogeneity, with a microenvironment characterized by, respectively, a high relative humidity or heavy-metal toxicity. The ecotypes and the ecophenes with environmentally induced, morphologic or mutational changes constitute the bioindicators for nonbiotic natural resources. Recent studies in southern India have attempted to identify plant indicators in certain selected mineralized areas which include: Kondapalli chromite in Krishna District in Andhra Pradesh (A.P.); Byrapur chromite in Hassan District in Karnataka; barytes deposits in Cuddapah District (A.P.) ; Agnigundala base metal deposits in Guntur District (A.P.), and mica pegmatite in the Nellore District (A.P.). In all these districts there are plant species tolerant to heavy-metal toxicity in the mineralized areas. Some, with full growth, occur even on the concentrated ore dumps. Amongst these plant indicators Tulsi of the Labiaceae (mint) (hyptis and ocimum species) has extraordinary tolerance for soils rich in Cr, Pb, Zn, Cu and Ba, and for a serpentine bedrock. The ubiquitous thorny shrub Prosopisjuliflora has an extraordinary ecologic amplitude and tolerance for a wide variety of heavy metals. Similarly, Calotropis gigantea and Cassia auriculata exhibit tolerance for many heavy metals in the soils and bedrock. Among the tree species, neem (Azadarichta indica) and certain Ficas sp. are tolerant to heavy-metal toxicity. Givotia rottleriformis, which is most common and widespread in the Kondapalli hill range on different rock types, exhibits profuse flowering, whereas in the presence of chromite it is devoid of flowers. In granitic terrain, bush shrubs of thorny Gymnosporia [alconeri indicate a permatite bedrock. When the pegmatite and associated mica, beryl, and tour-
0375-6742/87/$03.50
© 1987 Elsevier Science Publishers B.V.
428
maline is mined, G. [alconeri is replaced by Prosopis juliflora demonstrating the antagonistic relationship between these two thorny species. Gymnosporia [alconeri is common in barytes mineralized zone associated with dolomite and basalt. Where Au concentrations are high and Ba and Sr low, G. falconeri has pink and red foliage; however, where Au is low and there are high concentrations of Ba and Sr, normal green foliage is present. Phyllite and dolomite comprise the country rocks of the Agnigundala base metal deposits. Albizzia amara is an indicator of phyllite, while the presence of Barleria longifolia, indicates domomite. Gymnosporia falconeri, Hardwickia binata, Zizyphus xylopyrus, Chomelia asiatica, and Cassia angusti[olia growing on dolomite have higher metal contents than those growing on phyllite. Termite mounds constitute important tools in mineral exploration. Maintenance of constantly high relative humidity in the mounds is an essential prerequisite for the very survival of the species in arid and semiarid regions. The termites penetrate deep down into the subsoil to reach the water table. Along with the soil particles for the construction of their mounds and groundwater carried out by the termites, any ore elements present are also removed to the surface. Hence, it has been found that the concentration of ore elements in the mounds is locally much higher than in adjacent soils. Furthermore, indicator plants growing on a termite mound have higher concentrations of some ore elements than the same plant species developed on adjacent soils. Thus the excellence and efficacy of the termite mound as an important tool in geochemical prospecting in tropical regions has been firmly established, even in areas devoid of outcrops and covered by a thick soil mantle.