THE CHEMISTRY OF TRACE ELEMENTS IN GEOTHERMAL WATERS, ICELAND

Trace element composition of geothermal fluids may be useful in tracing processes occurring in geothermal systems. We studied trace element chemistry in geothermal waters in Iceland. Samples were collected from hot springs, mud pots, soil water and wells with wide range of temperatures (4- >200°C), pH (2.0-9.3) and chemical composition. The waters were categorized into NaCl, steam-heated acid-sulfate and mixed waters. NaCl waters have neutral to alkaline pH, and Na, Cl, Si, Ca and SO₄ as the major ions but low Fe and Mg. They are discharged by wells and hot springs and largely represent aquifer fluids that have boiled and often mixed with cold ground waters. Concentrations of most trace metals (Co, Cr, Cu, Ni, Pb, Zn) was low, whereas Ba, Sr, Cs, Rb, Li, As, Sb, Mo, W were higher. Steam-heated acid-sulfate waters form when H₂S-rich steam condenses and mixes with shallow non-thermal waters. At the surface, oxidation of H₂S results in pH<4 and acid leaching. Thus, they have high SO₄, SiO₂, Mg, Al and Fe but low Cl concentrations. Concentrations of Al, Fe, Mn, V, Cr, Zn, Ni, Cu, Co, Ba, Sr were elevated compared to NaCl waters, but Li, Cs, Rb, As, Mo, Sb and W were lower. Mixed waters are mixtures of NaCl, acid-sulfate and non-thermal water, and showed characteristics of the endmembers.

Rock leaching dominated the chemistry of Li, Cs, Rb, As, Mo, Sb and W, together with mixing of steam and nonthermal surface water in steam-heated environment and with incorporation into secondary minerals in NaCl waters. In contrast, Ba and Sr are considered to be largely influenced by secondary mineral formation in all water types. In steam-heated environment, Al, Fe, Mn, Co, Ni, Zn, Cd, Cu, Pb, Cr and V were generally dominated by rock leaching, though occasionally by mineral precipitation (Fe, Al, Cu, Co). With increasing pH, they became immobilized by secondary mineral formation and mineral precipitation induced by boiling and cooling of the aquifer fluid. Acid supply played an important role in steam-heated environments, whereas mass removal of chalcophile metals from the aquifer fluids seem important in NaCl waters. However, speciation of redox sensitive elements such as Fe may not follow equilibrium distribution, setting challenges for interpretations. It may be crucial to separately analyze the oxidation states, this being the aim of our current project.