Using groundwater in the hyper-arid Atacama Desert to discover buried hydrothermal mineral deposits

ABSTRACT

Epithermal deposits occur in many metallogenic belts globally and are economically important sources of precious and base metals. However, post-mineral cover (both transported and younger deposits) also occurs in many prospective districts and hinders exploration efforts. Hydrogeochemistry is a potentially useful exploration tool in these environments, with water–rock interaction shown from other studies to produce large hydrogeochemical footprints. Groundwaters from in and around the Vaquillas (Au-Ag) epithermal deposit in the pre-cordillera of northern Chile are typically Ca-SO₄^2- type with meteoric upflow groundwaters interacting with the ore body and producing a significant downflow (down hydraulic head) mobility (up to 3 km) of (oxy)anions of arsenic and molybdenum, which contrasts with proximal trace cation anomalies. Multiple sulphur endmembers contribute to the dissolved sulphate concentrations, including evaporites, primary hypogene sulphide minerals, and a single δ³⁴S_(SO4) value of 30.1 ‰ which is potentially associated with disproportionated sulphur in sulphate. This study reports anomalism from one of the most dilute endmember groundwater compositions from Chile reported to date. The sub-alkaline and weakly oxygenated groundwaters are common in many aquifers globally, making the results applicable to many other metallogenic belts. It is also the first hydrogeochemical case study of an intermediate-sulfidation epithermal deposit. Despite the highly dilute groundwaters recharging aquifers at Vaquillas, we conclude hydrogeochemistry to be a highly effective exploration tool for epithermal and porphyry deposits in areas of significant post-mineral cover.

KEYWORDS:

• Groundwater
• epithermal
• porphyry
• hydrogeochemistry
• stable isotopes
• mineral exploration

Acknowledgments

This project was funded by First Quantum Minerals LTD (FQM). Assistance was provided by field technicians Raul Araos (Jr), Wido Alvarez Alvarez Trujillo, Ricardo Gonzalez, and Juan Carlo Cocha. Additional thanks are expressed to James Banyard, Matthew Hope, Mclean Trott, and Steve Andersson for assistance during fieldwork, as well as all the FQM Chile exploration team. Special thanks to FQM exploration director Mike Christie. The authors wish to thank April Vuletich, Alexandre Voinot, Donald Chipley, Evelyne Leduc, and Marissa Valentino from Queen’s Facility for Isotope Research (QFIR), for assistance and advice during the project. This paper was vastly improved by the efforts of Dr Ryan Nobel and two anonymous reviewers.

Author Contribution statement

JK collected the samples and wrote the initial draft; ML and DLM helped revise the manuscript and with data interpretation.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Supplementary material

Supplemental data for this article can be accessed here

Additional information

Funding

This work was supported by the Arthur B. McDonald Canadian Astroparticle Physics Research Institute and First Quantum Minerals.