618 MINING GEOLOGY/Exploration Table Main geochemical tools in mineral exploration Tool Basis Coverage Applicability Target Soil geochemistry Systematic sampling of soil, usually of a selected size fraction Large to small areas, depending on sampling density Areas with residual soils; a very commonly used method Most metallic deposit styles Rock chip geochemistry Systematic sampling of exposed bedrock as composites of small chips Small areas only Requires extensive exposure of bedrock Most metallic deposit styles Bedrock geochemistry Systematic sampling of the upper parts of bedrock, usually by drilling through shallow cover Large to small areas, depending on sampling density and depth of cover Areas of strong near surface leaching, or covered by transported overburden Most deposit styles Stream sediment geochemistry Systematic sampling of stream sediments, usually of a selected size fraction Mostly large areas; application to small areas may be limited by the stream pattern density Widely applicable in regional exploration; requires bedrock outcrop or shallow subcrop Most metallic deposit styles Water geochemistry Sampling of water in streams or wells and bore holes May be applied over large areas; sample density typically low Depends on access to water samples in streams or bores Limited application for base metal and uranium deposits Soil gas geochemistry Systematic sampling and analysis of gases trapped in soil Mostly small areas; sampling density is high Suitable for areas with deep soil and transported cover Locates structures and may help find some sulphide rich deposit styles Plant geochemistry Sampling of one plant species or part of one plant species, followed by chemical analyses (mostly after ashing) Mostly over restricted areas Best in areas with high plant density but low plant diversity (usually temperate to cold areas) Most base metal deposit styles Figure Geochemical sampling in eastern Taiwan Sediments are dug from the creek bed, sieved to remove the largest pieces, and the finer fraction is bagged and sent to a laboratory for chemical analysis