Abstract
A procedure for sequential leaching of various mobile forms of metals in overburden (MOMEO) has been developed. Previous work showed that selective extraction techniques developed for base metals are ineffective for Au extraction. The mobile forms of Au in overburden occur not only as ionic or complex forms but also as ultrafine particles (submicron to nanometer) dispersed in, or bound onto, water-soluble salts, colloidal particles, organic matter, iron and manganese oxides or clay minerals. The conventional approach of partial leach methods, developed for base metals, involves using appropriate extractants to attack the host materials and release the trapped mobile forms of metals into solution. But the ultrafine Au particles thus released from the host are either insoluble in the mild extractant or reabsorbed by colloids. Therefore a two-stage selective leaching scheme has been developed for Au, a sequential leaching procedure to destroy the host materials, followed by a second treatment to place the Au released from the first stage into solution for analysis. This method has been used widely in detailed, regional and strategic exploration in overburden terrains. Case studies from different climatic and geographic settings show that selective leaching of optimum mobile forms of metals in different settings can be effective in finding indications of mineralization. In temperate alluvial terrains, water-extractable and adsorbed or exchangeable forms of metals give the most prominent expression. In arid sandy desert terrains, water-extractable metals and metals in oxides are the best indicators. In cold grasslands, organically bound and water-extractable metals are the most effective indicators of mineralization at depth.
Published Version
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