Groundwater chemistry is influenced by its contact with host rocks and reveals information on lithology, alteration, and potential mineralisation. It is a prospective exploration tool that samples a derivative of the subterranean geochemistry, with the effectiveness for mineral exploration depending on: (a) the availability of boreholes and borehole density; (b) boreholes with appropriate depth of penetration; (c) hydrologic connectivity of the country rocks; and (d) the availability of a regional hydrogeochemical baseline, to define chemical anomalism. Minerals react with groundwater and the metal solutes can be transported considerable distances from their sourcing ore bodies. Solubility depends on the physiochemical conditions of aquifer, with solutes typically in very low concentrations at levels of parts-per-billion or even parts-per-trillion for precious metals. The highest concentrations of ore-related trace elements tend to be restricted to within 2-5 km of a metal source and tail off with distance from the source, although their distribution is modified by groundwater flow. Isotopic analyses for 18O, 2H and Pb isotopes are useful for determining source region, with the latter applied to fingerprinting a source region, e.g., Proterozoic mineralisation. Hydrogeochemical interpretation applies a variety of techniques: (a) the direct statistical analysis of a single element concentration is employed for metal prospectivity for Cu, Pb, Zn, Ni, Co, Pt, Au, Ag, etc., by comparing the element concentration to its regional-background value; (b) assessment based on a combination of elements (also known as Specific Mineralisation Index), a robust technique that uses prior knowledge of known mineralisation models to predict mineralisation types, e.g., Ni-Co-Pt for NiS systems; Zn-Cu-Pt-Ag for VMS deposits; and Au, Ag, As, Mo, Sb for lode Au deposits; and (c) interpretation of undercover geology using multi-element indices: Higher Cr and V concentrations are indicative of mafic rocks; higher U, of granitic rocks; and B/Na ratios used to differentiate between metamorphic and sedimentary terranes.

Open-Access Online Publication: May 29, 2023