LEACHING OF ARSENIC, LITHIUM, AND OTHER TRACE ELEMENTS FROM IGNIMBRITES IN THE ANDEAN CENTRAL VOLCANIC ZONE

Ignimbrites are volcanic deposits, generally of felsic composition, which are widespread in the Andean Central Volcanic Zone (ACVZ). Massive ignimbrites displaying some of the highest concentrations of arsenic (As) have been found in the ACVZ and in the Altiplano-Puna plateau, a tectono-geomorphological feature linked to the ACVZ. To explore the elevated concentrations of As and other trace elements related to the volcanic activity of the ACVZ, a bench-scale experiment was conducted with ignimbrite samples from the Altiplano of northern Chile (-68.366193° W; -21.070308° S) in a water tank with 4 compartments, 3 containing water and the sample as well as a control division containing just MilliQ water type. MilliQ water type (3 L) + 300 g of a welded ignimbrite were agitated constantly until the water completely evaporated lasting from 36 up to 98 days. The mineralogy of ignimbrite samples was studied using thin sections and traditional microscopy. Physicochemical parameters (temperature, conductivity, pH, total dissolved solids (TDS), and oxidation reduction potential (ORP)) were measured daily, 15 mL of water were sampled monthly, main water cations and anions were measured by liquid chromatography, and As, lithium (Li), copper (Cu), and antimony (Sb) concentrations were measured by ICP-MS.

Main results showed that pH and temperature increased moderately over time, while ORP showed slight increases or decreases over time. On the other hand, conductivity and TDS increased nearly exponentially over time. Regarding As and Li, their concentrations tended to increase, while Cu and Sb showed variable behaviors. The analysis of main cations and anions, as well as microscopic observations, are currently in process. These preliminary results show that ACVZ ignimbrites can represent an important source of As and Li when outcrops are exposed to water, which can occur in superficial or shallow environments. More experiments are needed to simulate the natural groundwater conditions and the potential for ignimbritic aquifers to release these elements.

Acknowledgements: this research was financed by Fondecyt Regular 1240245 granted by ANID Chile.