Exploration methods
Soil and stream
Systematic soil and stream sediment sampling for geochemical analysis has proven an effective method to detect and delineate mineral deposits in both greenfield as brownfield context. Regional stream sediment sampling can be applied to hone in on regions with high deposit potential, that can benefit from more detailed investigation. Soil sample grids can be interesting as a first pass widely spaced grid over an underexplored area, as well as defining known anomalies in more detail.
Our sample collection procedure is 100% standardised, ensuring consistent data, with quality control being guaranteed by the use of field duplicates and reference samples.
Selective Leach Methods
Advancement and understanding of element mobility in soil profiles now sees near surface sampling playing a greater role around known mineralisation (brownfields surveys) by providing vectors towards deeply buried targets. One method that excels in this is Selective Leach geochemistry. Extractants containing strong ligands are used to detach unbound or weakly attached metal ions from soils, resulting in mobile metal ions. The idea is to measure metal ions that are released from mineralised material and travel upward to unconsolidated surface materials such soil, till, sand and so on.
Mobile forms of metals Cu, Pb, Zn, Ni, Cd, Au, Ag, Co and Pd show strong contrast over mineralisation compared to background, producing anomalies that are generally sharper and of higher contrast than conventional methods. This makes it favourable as a targeting tool to facilitate discovery of mineralisation in early stages of exploration programmes.
BLEG
Another useful geochemical sampling tool, mainly used for gold exploration, is BLEG (Bulk Leach Extractable Gold). A sufficient large sample of at least 2 to 5 kg is digested or leached with cold sodium cyanide solution, for one to several days. The gold is dissolved through its formation of a cyanide complex, which is concentrated through the solvent exchange process into an organic solvent and subsequently analysed. The use of large sample weights and solvent extraction enables low detection limits, as low as 0.1 ppb.
Although the precision of BLEG test results is relatively high, this method is not a total assay and does not compare to fire assay methodology. Gold values in BLEG are usually lower than those obtained through fire assay, so it’s best used as an exploration tool to spot anomalies of the more mobile fine gold rather than total gold content. In addition, it also addresses problems associated with sample heterogeneity (“nugget effect”) encountered in traditional sampling methods.
Soil gas
The study of the spatial distribution of soil gas anomalies at the surface, can give important and interesting information on the origin and processes involving deep and superficial gas species.
Field methods of soil-gas analysis for CO2, O2, Rn and Th can be applied to the exploration for copper sulphide and gold/sulphide mineralisation in terrains ranging from tropical rain forest to sand desert. For instance, CO2 concentrations are high in soil gases overlying sulphide-bearing mineralisation and the patterns can be followed under increasing cover of sand to a depth of several tens of metres. The radioactive gases also show promise in the mapping of basic rocks which are potential hosts to gold mineralisation.
Furthermore, anomalous gas species can also indicate the presence of fractured zones due to preferential migration along preferential pathways.
Geobotanics and biogeochemistry
Certain plant species are known to provide vectors to mineralised zones, mainly through two methods. Firstly, geobotanical prospecting refers to prospecting based on indicator plants like metallophytes, which thrive or at least survive on soil with high metal contents. Well known examples are the galmei flora in Eastern Belgium and Western Germany (Viola calaminaria and Thlaspi calaminare) and the copper flower (Ocimum centraliafricanum) in Central Africa.
Secondly, in biogeochemical prospecting (biogeochemistry or phytogeochemistry), parts of a specific plant species are sampled and analysed for their metal content. Through the uptake of groundwater, plants sample the soil and underlying rocks. Organic acids around the root system dissolve minerals for the uptake of nutrients. Absorbed waste elements are subsequently segregated and often concentrated in specific parts of the plant. Easy sampling of the vegetation allows for fast and non-invasive geochemistry, even with transported cover.
Pitting and trenching
Next to traditional pitting, we use hand-portable percussion drills to sample overburden and bedrock through cover, cutting down on time and cost required to excavate pits and trenches. A series of closely spaced drill holes can replace trench sampling, avoiding the impact of a long trench.