Laterite geochemistry for regional exploration surveys – a review, and sampling strategies

Numerous orientation studies and regional sampling campaigns by CSIRO and, more recently, the Cooperative Research Centre for Landscape Evolution and Mineral Exploration (CRC LEME), over the past 20 years, have demonstrated that residual lateritic gravels and duricrust are useful sampling media for regional, district and detailed exploration for gold, base metals and rare metals in the Yilgarn Craton. This paper focuses on regional exploration. A regional approach to geochemical sampling, using lateritic duricrust and lateritic gravels, based on regolith–landform models, is advocated. Sampling of areas of 10 000 km 2 or more, or integrating several data sets, is intended to identify geochemical trends and detect large geochemical haloes several hundred square kilometres in size, arising from concealed ore deposits or mineralized districts. In unexplored or underexplored regions, sampling at 3 km spacing on a triangular grid is appropriate at the reconnaissance stage. Results of this study show that, even if the sampling pattern is opened up to 9 km spacing, major trends can still be identified. In relict landform regimes, follow-up sampling should progressively close the spacing through 3 km, 1 km, 300 m, and less, in areas of interest. The effectiveness of a broad scale approach to laterite sampling is demonstrated by using As and Ni data for the southwestern part of Western Australia, where regional geochemical trends are apparent even at a 9 km sample spacing. For regional sampling in semi-arid terrains, use of a helicopter is effective because it provides the geologist with the optimum view of the landscape for regolith–landform control, as well as flexibility to choose an appropriate site for sampling. Whereas lateritic residuum (ferruginous gravels and duricrust) represents the preferred sample medium for regional surveys, other ferruginous materials are also suitable, particularly at the follow-up stage. Assigning regolith codes to all sample media and categorizing each sample are essential. The codes become part of the geochemical database and allow valid statistical evaluation of data for specific regolith materials. In all cases, analysis of a multi-element suite, including target and pathfinder elements, is recommended. Multi-element analysis allows interpretation that can take into account variable characteristics due to sample matrix, landform position, weathering environment and gross bedrock lithology. Multi-element geochemistry also provides the opportunity for multi-commodity exploration.