Un gisement polymétallique en milieu volcanosédimentaire du Protérozoïque supérieur: Tessalit (Adrar des Iforas, Mali)

The Tessalit deposit comprises a massive sulphide mineralization (up to 7% Zn, 1–2% Pb and less than 1% Cu) associated with carbonate and magnetite layers. It lies within a submarine acid dominated volcanic and pyroclastic complex belonging to an Upper Proterozoic island arc system (Tilemsi Group). A dome of porphyric rhyolite, brecciated in situ and pyritized occurs 1 km north from the Tessalit deposit; this dome is surrounded by laminated acid tuffs and tuffites. It is proposed that like many massive base metal sulphide bodies, the Tessalit deposit formed by volcanogenic and exhalative processes. The Tessalit mineralization comprises bedded sulphides associated with carbonate and iron rich exhalites and with waterlain acid tuffs. The orebody displays a mineral zoning with an upward change from a magnetite facies to a carbonate-copper sulphide, then a carbonate-sphalerite facies. Pan-African deformations have determined the actual complex geometry of the ore lenses. Other sulphide mineralization occurrences are known in other parts of the Tilemsi Group or equivalent volcano-sedimentary series. Gold occurrences may be also expected in these series.