Abstract

A 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 Misséni gold anomaly area, Mali. The bedrock in this area includes Birrimian volcano-sedimentary rocks (lower Proterozoic). At Misséni, primary gold is associated with disseminated copper and iron sulfides 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 less than 1500 mm. Misséni area is characterized by low plateaus 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 (i) saprolith ("pistache" laterite), variegated and mottled clays, (ii) iron hardpan, (iii) iron cuirasse, and (iv) eluvium. Mineralogical analyses reveal that the laterite is dominated by kaolinite, goethite, quartz, mica, and hematite. The mean chemical composition is 46% SiO2, 22% Al2O3, 20% Fe2O3, 1% TiO2, 1% K2O, 0.05% CaO2, and 0.03% Na2O. The laterite profile showed an iron enrichment and a silica depletion (upper part: 0–2 m) and a very weak variation in alumina. This 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 provides 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 gold reveals (i) anomalous gold in latérite (0.2–1.0 ppm) and (ii) 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. These preliminary results would call for further studies to explain the concentration of oxide minerals and gold in some soprolith levels inasmuch as they may lead to potential gold deposits in laterite soils. [Journal Translation]

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