Evaluation of critical elements in geothermal systems of the Taupo Volcanic Zone, New Zealand

Abstract

To assess the potential for TVZ (Taupo Volcanic Zone) hydrothermal systems to produce economically critical elements, chemically-analysed well discharges were studied and scales (from geothermal installations and hot springs) petrographically and chemically-evaluated, with the latter back-calculated to reservoir conditions. More than 50 elements/compounds have been analysed in the discharge fluids of geothermal systems in the TVZ. Elements with the highest concentrations (>100kg/a) in the water, from lowest to highest, include Ge, Sb, W, Sr, Rb, Cs, Mg, V, Li, B, Si and As. Although <12kg/a of Tl, Ga, Au and Ag are contained in the discharge fluids their high market price warrants further study. Amongst gases of economic interest in the TVZ are C2H6 and CO2. Helium is present but the production rate at <1000m3/a is too low to be of interest. Without considering mining, production and research costs, fluids from just one producing TVZ geothermal system contains about NZ$1B/a (US$0.67M/a) worth of critical elements. However, specific conditions are required to induce precipitation of specific elements including, among others, changes in fluid flow rates, redox conditions, temperature and pressure; phase separation and fluid mixing. Despite high concentrations of a wide range of critical elements in the TVZ fluids, only a few can be economically extracted at present.