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

Abstract

Geochemical mapping of North and Central Madagascar was carried out using stream sediments at an average density of one sample per 11 km². Over 50 elements were determined from some 13 300 stream sediments using a sample size fraction of <150 μm 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.

Keywords:

• Stream sediment geochemistry
• Base metals
• Gold deposits
• Precambrian basement
• Geochemical signature

The authors are grateful to the Ministère de L’Energie et des Mines of the Republic of Madagascar for granting permission to publish the stream sediment data. The authors also thank all Madagascan student samplers and counterparts from Projet de Gouvernance des Ressources Minérales (PGRM) involved in mapping campaigns. The authors thank staff of GLW Conseil for their support during the duration of the project, in particular, the valuable contribution of Navoko Rakotomahefa in coordinating sampling campaigns, processing, preparation and shipment of samples. Many thanks to Roger Key, Paul Lusty and two external reviewers for their valuable comments on earlier versions of this manuscript. This paper is published with the permission of the Director of the British Geological Survey (NERC).