The geochemistry and mineralogy of stream sediments as indicators of mineralisation in South Devon.

Detailed geochemical studies of the dispersion of selected elements (Cu, Pb, Zn, Fe, Mn, As and especially Sn) were undertaken in the drainage sediments of an area on the south-eastern flank of Dartmoor, Devon. The study area is one of mixed geology consisting mainly of the Hercynian granites of the Dartmoor mass, the metamorphosed sedimentary rocks of the adjoining "Killas" and some unmetamor-phosed sedimentary rocks of Carboniferous and Devonian ages. Tin mineralization occurs mainly within the granite area, although minor deposits have been found within the killas. Copper-zinc-arsenic mineralization is limited to the metamorphic and sedimentary sequences. Extensive mining of both types of deposits has been carried out in the past, and contamination is relatively widespread. The sediments of all the streams of the area were systematically sampled at an interval of approximately 1 kilometer. At each site two samples were collected - one panned and one unpanned. Each sample was sieved into a number of size fractions which were then divided into heavy and light components using bromoform. The material was then analysed for the elements listed above by atomic absorption spectro-photometry. Mineralogical investigations were employed in order to study the relationship between the geochemical patterns obtained and the mineralogical composition of the various sets of samples. The data obtained were treated by univariate and multivariate statistical-mathematical techniques, which proved to be of great value in making a reliable interpretation of the results obtained. Frequency distribution analysis, moving averages, correlation coefficients and semi-variograms were all employed. It was found that analysis of heavy concentrates can be used very effectively in exploration for tin and lead deposits. However the analysis of light minerals in the search for tin was found to be equally effective. Analysis of monometallic minerals (tourmaline) might also be used as an exploration tool particularly in the search for new stanniferous regions.