A review of metal transfer mechanisms through transported cover with emphasis on the vadose zone within the Australian regolith

There is a growing trend to try to make surface geochemistry effective for exploring areas of transported cover using sensitive techniques such as partial extractions of soil and gas analysis. However, these techniques have had mixed success in delineating buried ore bodies, because the particular mechanisms and their effectiveness in transferring ore-related metals upwards through transported cover are poorly understood. This precludes discriminating null results (a potentially ineffective technique) from negative results (no mineralization). A review of mechanisms capable of transferring metals through barren transported cover to the surface identified those well documented and others not well studied but nonetheless promising. The mechanisms are classified according to two main processes: phreatic process involving groundwater flow, convection, dilatancy, bubbles, diffusion and electromigration; and vadose processes involving capillary migration, gaseous transport and biological transfer. Microbial metabolism affects the kinetics of many hydrochemical processes, especially sulfide oxidation and other redox transfers, and also impacts, negatively and positively, on the generation of gases throughout the entire cover sequence. Phreatic mechanisms require groundwater to transfer solutes and are most effective. These have the most predictive capability where there are shallow water tables. In the Australian landscape, groundwater occurs commonly more than 5 m below surface except in lower, discharge landform sites, and therefore other mechanisms (biological, capillary, gaseous), are necessary to transfer metals up from the water table. Thus, much of the emphasis in this paper is on vadose processes. An integrated approach is necessary, combining different mechanisms with the nature and evolution of the transported cover and climatic settings. Regions and landforms of highly weathered transported cover with current or past water tables residing within the cover and long-standing vegetation will favor combined mechanisms such as electrochemical, plant uptake, capillarity, and bioturbation. Fresh, relatively unweathered and thick (> 30 m) transported cover may prove the most unlikely to develop surface geochemical anomalies. Gas (e.g. CO2, H2S) mechanisms may work, provided that sufficient gases are generated from the oxidation of ore to produce a surface signature.