Selenium as an indicator element in geochemical exploration

KOLJONEN, TAPIO 1977: Selenium as an indicator element in geochemical exploration. Bull. Geol. Soc. Finland, 49 (2): 85—88. Selenium is always present in sulphide ores and can be used as an indicator element in the geochemical exploration of those ores. Being an anion selenium also gives information, e.g., about ore type, that cannot be obtained through the analysis of cations alone. When sulphides weather in acid soils selenium is oxidized to elemental form or to selenite, it is adsorbed onto colloids, and migrates with difficulty. Therefore, high selenium contents occur in colloid-rich sediments in restricted areas near sulphide mineralizations. The mineralizations can be located by analysing such sediments as gossan, till, lake and river sediment, and organicand iron-rich layers in podzolic soil. Selenium content is increased also in vegetation growing on selenium-rich soils but the increase observed has been small. When the pH of sediments is neutral or nearly neutral, e.g., in calcareous areas, selenium is oxidized to selenate, which form easily dissolves, and the anomaly near sulphide mineralizations is not as distinct as in acid soils. Tapio Koljonen, Department of Geology and Mineralogy, University of Helsinki, P.O. Box 115, SF-00171 Helsinki 17, Finland. (Address until 2. 11. 1979: University of Dar es Salaam, Department of Geology, P.O. Box 35602, Dar es Salaam, Tanzania.)