Indicator plants as guide to mineralisation in parts of Sikkim-Darjeeling Himalaya, India

Abstract

Systematic geobotanical and biogeochemical studies have not been attempted in any of the basemental prospects in the eastern Himalaya. The conventional geochemical soil and bedrock sampling in this rugged terrain has been proved to be abortive due to steep surface gradient, soil creep, landslide and excessive precipitation. As a supplement to conventional geochemical prospecting, an orientation survey on the population and growth habits of plants may help in the selection of plant species for prospecting. The chemical analyses of systematically sampled plants may possibly lead to the identification of indicator plants which are likely to act as direct guide to mineralisation in the substrate. With these ideas in view, the Pb-Zn-Cu deposit of the Bhotang mine near Rangpo, Sikkim and Pb-Zn-Cu mineralised zone of Malkhola Block, Gorubathan in Darjeeling district, West Bengal have been selected for such studies so as to find out the possible correlation of the elemental concentrations in the substrate and plants growing over them and subsequently delineating the indicator plants for certain elements. The presence and distribution pattern of such indicator plants in adjoining areas or elsewhere may help to detect zones of mineralisation which may not have any surface expression.

The polymetallic Pb-Zn-Cu deposit of the Bhotang mine, Rangpo, Sikkim the mineralisation occurs in the form of veins, disseminations, patches and lenses in chlorite schist and phyllite. Five zones of mineralization have been delineated from southwest to northeast and it has been observed that the chalcopyrite-rich zone increases towards the southwestern direction and galena predominates over the other sulphides in the northeastern part.

The leaves, twigs and roots of different plant species occurring in the hill above the Bhotang mine have been systematically sampled and analysed for basementals and associated trace elements. The ashes of several plant species, viz. Dioscorea batatas, Eupatorium odoratum, Lantana camara, Pteris longifolia, Adiantum sp., grass with hard intemode, grass without hard internode and locally known Koirala (Family: Verbenaceae) show anomalous concentrations of Pb, Zn and Cu and other trace elements. A rare variety of plant, locally known as Koirala (Family: Verbenaceae) grown on the hanging wall side of the lode and Pityrogramma sp., an extremely rare variety of fern, have yielded very high values of Cu Pb and Zn which designates them as possible indicator plants for basemetal mineralisation in this terrain.

The study has thus indicated that Koirala and pytyrogramma sp. In Rangpo area, and Plumbago zeylanica and Micana scandens in Gorubathan area, Darjeeling hold good promise as local indicator plants for identifying zones of basemental mineralisation.