Trace and Isotopic Geochemistry of Groundwaters in Mineral Exploration

Abstract

Total cover (100 % coverage) presents a challenge for mineral explorers, with the effectiveness of traditional geochemical techniques greatly reduced. Hydromorphic mobilization of solutes can produce geochemical footprints much larger than the ore deposits. However, mainstream adoption of hydrogeochemistry has yet to occur, presumably due to a perception that data interpretation is complicated, and targets generated are of low confidence. Therefore, the development of reliable “direct detection” geochemical tools is needed.

Groundwaters were sampled from three case study sites, including porphyry, epithermal, and iron oxide copper gold (IOCG) deposits. Significant geochemical footprints (~ 5 kilometres) were identified. However, results also demonstrate the potential for the contribution of trace elements from endmember mixing and variation in anomalous element concentrations of up to ten times the order of magnitude, relating to contrasting groundwater endmember compositions. In order to quantify this variation, this project proposes a seven-source regional model for the Atacama region of Chile, with the aim of aiding geochemists to interpret hydrogeochemical data.

This project also tests a number of emerging, yet highly promising isotopic systems (S, Sr, Mo, and Cu), and reports the first use of δ98Mo in hydrogeochemical mineral exploration. Mineral exploration requires relatively low costs and high throughput, which form the main barriers to the mainstream use of metal stable isotopes. To that end, this project yields an automated chromatography procedure for δ65Cu suitable for high Na and low Cu concentration samples, yielding precise measurements and low blank contributions.

This work reports that large fractionation factors for δ65Cu and δ98Mo have been observed in case studies between samples proximal to mineralization and distal samples downflow from the source. Following the example of previous work, the processes controlling fractionation in this project are constrained to be oxidation, sorption, and precipitation. These controls appear robust in the oxygenated and sub-alkaline groundwaters often encountered in nature. Strontium isotopes show a clear tracer of water-rock interactions and mixing of endmembers, potentially facilitating geological mapping in the covered environment. All isotopic analysis undertaken in this project demonstrated use for the identification of metal sources, mixing between multiple water sources, and water-sulfide interaction.