Evolution of regoliths and landscapes in deeply weathered terrain - implications for geochemical exploration

Thick, commonly lateritic, regoliths are widespread in inter-tropical regio ns of the world and present particular challenges in exploration. These are best tackled through a sound understanding of the evolution of the landsca pes in which they occur. The regoliths formed under humid, warm to tropical conditions and, although they may have been modified by later climatic cha nges, i.e., to more humid or more arid conditions, many chemical and minera logical characteristics are retained. These include the geochemical express ions of concealed mineralization. Erosional and depositional processes cont rol the preservation and occurrence of specific regolith units that may be used as sample media and, in turn, target size, element associations and co ntrast, thereby influencing sampling procedures, analysis and data interpre tation. These parameters are best summarized in terms geochemical dispersio n models based on the degree of preservation of the pre-existing lateritic regolith. Regolith-landform mapping permits an assessment of he terrain in terms of such models. In relict regolith-landform regimes, in which the lat eritic regolith is largely preserved, broad multi-element anomalies in the upper ferruginous horizons (lateritic residuum) can be detected using sampl e intervals of 1 km or more. In contrast, in erosional regimes, where this material is absent, anomalies in upper saprolite, and the soil and lag deri ved from it, are more restricted in area and closer sampling intervals, (20 0 X 40 m or less) may be necessary. Lag and soil are, generally, ineffectiv e in depositional areas, except where the sediments are very thin (e.g., < 2 m) or overburden provenance can be established. Stratigraphic drilling is necessary to establish whether the overburden overlies a buried lateritic horizon or an erosion surface cut in saprolite. Lateritic residuum remains an excellent sample medium if present, again with widespread haloes, but wh ere it is absent, leaching and the restricted haloes in upper saprolite pre sent formidable problems. Ferruginous saprolite or composites across the un conformity may be effective, but otherwise carefully targeted drilling and sampling through saprolite and saprock may be necessary. Partial extraction analyses have yet to demonstrate significant results except in very specif ic environments. In arid regions, pedogenic carbonate (calcrete, caliche) m ay be a valuable sample medium for Au exploration, principally in erosional regimes, and in depositional areas where the overburden is shallow. Sample intervals range from 1 km for regional surveys, through to 100 X 20 m in p rospect evaluation. Saprolite is an essential sample medium in all landform environments. bur the restricted halos and possibility of leaching require s that drilling and sampling should be at close intervals. (C) 2000 Elsevie r Science B.V. All rights reserved.