Hydrogeochemical mineral exploration in deeply weathered terrains: An example from Mumbwa, Zambia

Locating mineral deposits in areas of thick or transported overburden is notoriously difficult. Post-mineral cover is prevalent in many parts of the globe and has led to prospective geological sequences being missed by traditional methods of exploration. Hydrogeochemistry is particularly applicable for the exploration of Iron Oxide Copper Gold (IOCG) deposits because, when compared to larger porphyry or sediment-hosted systems, IOCG deposits tend to be smaller and high-grade with a limited lateral footprint to intersect with grid-drilling; groundwater interactions and ion dispersion tend to produce a much larger anomaly target than regolith geochemistry alone and require fewer samples. As a case study, we examine the hydrogeochemistry of the Kitumba IOCG deposit, located in the Mumbwa district of west-central Zambia. We present physicochemical data (Eh, pH, TDS, conductivity), major and trace element concentrations, and isotopic compositions (δ⁹⁸Mo, ⁸⁷Sr/⁸⁶Sr, and δ⁶⁵Cu) from groundwaters interacting with the Kitumba deposit and surrounding prospects. A hydrogeochemical footprint of As, Mo, Fe, Mn, and Zn is dispersed from the deposit. Groundwater ⁸⁷Sr/⁸⁶Sr values (0.708832 to 0.731807) reflect the mixing in varying proportions of waters that have interacted with distinct lithological endmembers in the Mumbwa area, corresponding to a complicated tectonic and metamorphic history. We report fractionation of 1.34 to 1.60‰ (∆⁶⁵Cu_{groundwater - chalcopyrite}) between proximal groundwater and primary chalcopyrite, which we postulate may be related to the oxidative dissolution of primary sulfide minerals. The δ⁹⁸Mo₃₁₃₄ values of groundwaters proximal to known ore bodies are isotopically distinct (-1.08 ± 0.18‰ 2SE to 0.64 ± 0.08‰ 2 SE) from background aquifers (2.08 ± 0.12‰ 2SE). The trace element and isotopic hydrogeochemical patterns described in this study document water-rock and water-deposit interactions and demonstrate the potential of non-traditional stable isotopes to be employed in district-scale reduction of exploration ground and vectoring towards undisturbed ore deposits similar to Kitumba.