Geochemical signatures of stream sediments within the main geological domains and terranes of North and Central Madagascar

摘要

Geochemical mapping of North and Central Madagascar was carried out using stream sediments at an average density of one sample per 11 km~2. Over 50 elements were determined from some 13 300 stream sediments using a sample size fraction of <150 ìm following hot aqua regia digestion. Partially extractable concentrations of six major elements and seven base metals reveal that the Andriamena 'greenstone' Belt of the Tsaratanana Complex is geochemically distinct from the other geological divisions within the Precambrian basement of central and northern Madagascar. In particular, this study has shown the detailed spatial distribution of anomalous concentrations of base metals which confirm known areas of chromite mineralisation within the Andriamena Belt. Base metal anomalies also occur in relation to Cretaceous and Neogene volcanic rocks, for example at Nosy Be, and mafic-ultramafic intrusions such as along the Maroala deformation zone. The distribution of anomalous Au in stream sediments showed good correspondence with known gold districts of central and northern Madagascar. Highest concentrations were measured in stream sediments from within the Ampasary-Mananjary (southern Androna-Mandritsara) and Andavakoera (northern margin of North Bemarivo) gold districts. The results of the stream sediment geochemistry show that these new data provide valuable information for current and future mineral exploration and environmental studies in Madagascar, at both regional and local scale.