Using stable isotopes of Cu, Mo, S, and 87Sr/86Sr in hydrogeochemical mineral exploration as tracers of porphyry and exotic copper deposits

摘要

Groundwaters collected from the undisturbed Picaron Cu?Mo porphyry and exotic prospect (Atacama region, Chile) were analyzed for major and trace elements, along with isotopes of Cu, Mo, S, and 87Sr/86Sr. Sulfide minerals at Picaron have undergone oxidation by meteoric waters, resulting in the generation of acid and the lateral and downward migration of dissolved trace cations (Cd, Co, Cu, Ni, Pb, and Zn) and (oxy)anions (As, Mo, Sb, and Se). This paper presents a case study utilizing stable metal isotopes in hydrogeochemical mineral exploration to ascertain the possible hydrogeochemical footprints that can be detected around known mineralization. The groundwaters are weakly brackish (TDS < 3500 mg/L), reflected in relatively low trace metal concentrations. Despite this, the groundwaters reflect the dissolution of ore, as well as alteration and wall rock minerals proximal to porphyry mineralization with elevated dissolved concentrations of Ca, Mg, HCO3? , and SO42- . 865Cu values range from 0.41 to 2.80%o, with values above 1%o proximal to porphyry mineralization reflecting Cu fractionation during sulfide oxidation and the highest values (up to 2.80%o) proximal to exotic mineralization likely from Cu desorption. Downgradient, 865Cu values decrease; possibility resulting from adsorption of the heavier 65Cu onto reactive surfaces or mixing with other sources. 898Mo values proximal to porphyry mineralization (2.41 and 2.42%o) are much higher than the regional average for molybdenite reported in previous studies (0.11%o), suggesting acid neutralization and isotopic fractionation related to adsorption may be occurring close to the source. Much lower 898Mo values downgradient (<1.0%o) are likely indicative of Mo input from surrounding country rocks. The hydrogeochemical trends observed in the Picaron groundwaters highlight the chemical processes of mineral dissolution, precipitation, and recharge pathways and indicate that the leaching of the porphyry mineralization could be the source of exotic mineralization. Ultimately, the results indicate the need for a multiple isotope approach to hydrogeochemical mineral exploration.