GOLD MASS-BALANCE IN LATERITIC PROFILES FROM SAVANNA AND RAIN-FOREST ZONES

The gold mass balance was studied in two areas gold mineralization; one in a savanna zone in South Mali, the other in a rain forest zone in French Guiana. In the ferricrete profile of the savanna zone, the gold mass balance is strongly negative within the ferricrete itself, where almost all the primary gold stock is leached. Beneath the ferricrete, an absolute accumulation of fine secondary gold is found in the mottled zone. The gold cycle is controlled by the migration of Fe and associated with the neogenesis of iron oxides, particularly hematite. The lateritic profile in the rain forest zone is truncated, and a soil is developing from the underlying saprolite in equilibrium with the equatorial conditions. An absolute accumulation of gold is found in the soil, with gold migration and reprecipitation being controlled by the evolution of the macroporosity. The global mass balance of gold in the Guiana deposit shows a leaching rate of 40 % in the weathered part of the mineralization. This process occurs mostly at the weathering front. In the two examples, gold shows opposite behaviors due to different pedogenic and paleoclimatic conditions. This might influence the interpretation of geochemical soil surveys in various intertropical environments.