Low sampling density exploration geochemistry for gold in arid and tropical climates: comparison between conventional geochemistry and BLEG

Exploration of large areas for Au has led the Bureau de Recherches Géologiques et Minières to compare the efficiency of conventional fine-fraction geochemistry with BLEG (bulk cyanide leach extractable gold) sampling for low-density drainage surveys. Comparative test work was carried out on different types of mineralization in arid (Sudan) and tropical (Guinea, Indonesia) regions. Conventional geochemistry and BLEG give broadly similar results in all the test areas where Au anomalies of comparable size delineate the mineralized zones. In arid conditions, the mineralized zones can be detected by simulated surveys with a density of 1 sample per 10 km 2 , and such a sampling density is recommended for evaluating the Au potential in large remote areas. Under the tropical conditions of Indonesia (Irian Jaya), a lower density of one sample per 15 km 2 or more might be used in ‘grass-root’ surveys because of the large size of the potential mineralized zones and the mountainous morphology of the region. In most of the test areas, the total Au contents of the stream sediment fine fractions (<63 μm, <125 μm or <180 μm) are higher than those extracted by BLEG from coarser size fractions (<500 μm or <2 mm) which means that cyanidation extracts only part of the free fine Au and/or that BLEG results are influenced by dilution of Au by barren material. On the other hand, the BLEG method extracted more Au than was present in the fine fraction from several sampling sites in Sudan and Iran Jaya, which indicated that the cyanidation extracted some coarse Au. The nugget effect so generated, however, does not hamper the recognition of mineralized zones. From an operational point of view, conventional geochemistry on fine fractions is less costly than BLEG for remote areas, and has several other advantages including the ability to carry out multi-element analyses in order to rank anomalies, and the relatively easy transport and storage of samples.