EXPERIMENTAL STUDY ON RARE EARTH ELEMENT MOBILITY DURING GREYWACKE - WATER INTERACTIONS

Abstract

Due to the increase demand in Rare Earth Elements necessary in solar panels or wind turbine magnets, the source and mobility of REE in geothermal fluids is becoming increasingly interesting. Previous studies of geothermal fluids from Taupo Volcanic Zone (TVZ) indicate that some REE are released during the breakdown of primary phases due to hydrothermal alteration. However, there is limited experimental data on the processes controlling the behaviour of REE in geothermal systems of Aotearoa New Zealand. The
results presented here comprise part of a larger project that aims to broaden the understanding of the chemical exchange between geothermal fluids and reservoir rocks occurring during recharge of the geothermal plume.
In this preliminary study, we have investigated experimentally the release of REE during fluid-rock interaction using a continuous flow apparatus under hydrothermal conditions. The experiment used a “unaltered” greywacke as a proxy for the dominant basement
rock in the Taupo Volcanic Zone. Distilled water was used as the fluid. The greywacke was progressively heated from ambient temperature to 400°C in 50°C increments over a period of 126 days. The fluid flow rate was 1 ml hr-1 for the entire experiment.
Results indicate that measurable concentrations of REE are released to the fluid from the greywacke. The similarity of REE patterns to other species present in the solution (i.e. Fe, H2S, SO42-) suggests that their trends are controlled mainly by mineralogy. However,
the concentration of REE is strictly dependent on temperature and pH. These results show that the basement greywacke in the TVZ does influenced the REE composition of the geothermal fluid. The research findings are discussed and compared with available historical data.