40Ar/39Ar Geochronology of Supergene Processes in Ore Deposits
From the viewpoint of economic geology, weathering promotes the supergene enrichment of ore deposits and the dispersion of ore and tracer elements in the regolith surrounding an orebody. The reactive nature of the mineralogy associated with ore deposits and the structural complexity of mineralized areas promote the development of extremely deep (in excess of 400 m) and complex weathering profiles. To understand the history and to evaluate possible mechanisms and pathways for the migration of ore or pathfinder elements in supergene enrichment zones, it is often useful to unravel the complex superposition of processes that can occur during weathering. Dating authigenic minerals present in the weathering profile contributes to this goal. In this review, I will illustrate the application of 40Ar/39Ar geochronology of supergene minerals in dating weathering processes.
The supergene minerals most commonly dated by the 40Ar/39Ar method are alunite-group sulfates (alunite and jarosite) and hollandite-group Mn oxides (cryptomelane and hollandite) (Table 1). In addition to their abundance in weathering profiles and their relatively high K contents, alunite-group sulfates and hollandite-group Mn oxides often host ore elements. In some manganese deposits, hollandite-group oxides are the most abundant ore minerals (e.g., in lateritic Mn deposits). In weathering profiles overlying precious and base metal deposits, alunite-group sulfates and hollandite-group Mn oxides often contain lead, silver, copper, and zinc whereas cobalt and nickel are concentrated in supergene Mn oxides associated with lateritic Ni-Co deposits. Dating these supergene minerals provides direct information on the age and duration of the geochemical conditions conducive to dissolution, transport, and redeposition of ore elements in the weathering environment.