LATERITIC WEATHERING AND ORE-FORMING PROCESSES

Supergene lateritic deposits have played an important role in the global mineral resource economy for over 50 years, with lateritic Al, Fe, Ni, and Au deposits having a significant input to global metal production and reserves. This paper reviews the contribution of the knowledge acquired on lateritic weathering processes on the understanding of supergene deposits. The formation of lateritic deposits is controlled by four major factors: parent-rock characteristics (chemical and mineralogical composition); geomorphological conditions (location in the landscape, drainage conditions, local erosion rate); paleoclimatic history, and the age of lateritic weathering.

Increased knowledge of paleoclimates and global climatic changes has allowed weathering scenarios to be applied to the main lateritic provinces and to identify the principal stages in the development of secondary mineralization. This has been particularly helpful in understanding the formation of supergene Au and Ni laterite deposits in Western Australia, where major climatic changes from sub-tropical humid to arid conditions during the late Mesozoic and Tertiary caused significant changes in the soil-water systems. In Amazonia, by comparison, there was a change from a seasonally humid, savanna climate to a humid rainforest climate, which not only caused surficial enrichment of Au, but also the upgrading of near-surface horizons of bauxitic profiles.

The relationship between the stability fields of principal secondary minerals, based mostly on the humidity of the paleoclimates, provided the key information on the formation processes of lateritic profiles and the related deposits. This led to a comprehensive understanding of the vertical evolution in the profiles and of the changes of the top horizons issued of the paleoclimatic history. These development models greatly influenced the interpretation of the genesis of lateritic deposits, their classification and the procedures in exploration geochemistry.