Preliminary results of a study on gold dispersion in laterite around the Misseni gold indicator, Mali
Resultats preliminaires d'une etude sur la dispersion de l'or en milieu lateritique autour de l'indice aurifere de Misseni, au Mali (in Recent developments in the study of Archean gold deposits--Developpements recents dans l'etude des gisements d'or archeens, Pierre Trudel (editor))
Canadian Journal of Earth Sciences = Revue Canadienne des Sciences de la Terre (December 1990) 27 (12): 1686-1698
The study of gold dispersion in laterite above a gold zone is based on 58 analyses of soil samples from two shafts and one trench located in the Misseni Au anomaly area, Mali. The bedrock in this area includes Birrimian volcano-sedimentary rocks (Lower Proterozoic). At Misseni, primary gold is associated with disseminated copper and iron sulphides in a silicified shear zone within andesites. The area is dominated by a dry tropical climate, including two contrasting seasons (a short rainy season and a long dry season) and a mean annual rainfall of < 1500 mm. The Misseni area is characterized by low plateaux averaging 350 m in altitude and by a very thick weathered cover (> 35 m) of laterite topped by iron cuirasse. In general, the laterite profiles show, from bottom to top: 1) saprolite ('pistache' laterite), variegated and mottled clays; 2) iron hardpan; 3) iron cuirasse, and 4) eluvium. Mineralogical analyses reveal that the laterite is dominated by kaolinite, goethite, quartz, mica and hematite. The mean chemical composition is SiO (sub 2) 46, Al (sub 2) O (sub 3) 22, Fe (sub 2) O (sub 3) 20, TiO (sub 2) 1, K (sub 2) O 1, CaO 0.05 and Na (sub 2) O 0.03%. The laterite profile showed an iron enrichment and a silica depletion (upper part: 0-2 m) and a very weak variation in alumina. The iron enrichment is correlated with increasing goethite toward the surface, whereas silica decreases with lower mica content. The gold concentration increases in the iron hardpan immediately below the cuirasse. The upper saprolith provided low gold values, but these increase with depth. Conversely, As and Co tend to increase towards the surface (0-2 m) in the hardpan, cuirasse, or eluvium. A deep core (> 35 m) assayed for Au reveals: 1) anomalous gold in laterite (0.2-1.0 ppm), and 2) higher gold values (5.5-8.8 ppm) occurring between 11.45 and 17.75 m depth, associated with oxide minerals in certain saprolith levels, reflecting probably an ancient groundwater table.