Alluvial Gold: Prospecting and Evaluation

The limitations of the conventional practices, and their effects, in prospecting for and in the evaluation of alluvial gold deposits are briefly reviewed. Geophysical prospecting methods have demonstrated their complementary role in the delineation of deposits by reducing the number of boreholes which are primarily of a 'probing' type. Larger diameter boreholes, although more expensive on a linear basis, are much cheaper on a volume recovered basis. The need to have sufficient volume as well as number of samples is stressed. Classical statistical methods are not appropriate for such deposits as there is sufficient structure, or spatial correlation, to require the application of geostatistics. Quite reliable semi-variograms, but not in all cases, of the spherical scheme have been obtained. Subsequent kriging has, to date, been confined to unworked deposits: retrospective estimates to compare to production have not therefore been produced. However, the technique shows promise but indicates a considerably greater borehole density (of the conventional diameter) may be necessary to produce confident estimates. Used in conjunction with larger diameter boreholes, a lesser borehole density may be sufficient, through radical reductions of the sampling error.