Key geochemical processes in typical magmatic hydrothermal systems explaining REE patterns in the neutral boiling springs

Abstract

The mobility and fractionation of REE in hydrothermal system have been studied by a number of researchers. A major effort has been made in this study to investigate the genesis of REE in the parent geothermal liquid-fed (PGL-fed) neutral boiling springs. The Daggyai and Rehai are two typical magmatic hydrothermal systems in which several representative PGL-fed boiling springs have been collected. PHREEQC V3.0 with self-constructed database was used to calculate the aqueous REE species in the PGL and construct models to study the impacts of pH, temperature, and Fe/Mn (hydr)oxides on the distribution of REE. Simulation results showed that NdCl²⁺, NdF²⁺, NdSO₄⁺, Nd³⁺, NdCl₂⁺, and NdOH²⁺ are dominant species under strong acid condition. Solid REE oxides start to precipitate at pH of higher than 2.9, meanwhile, the content of all the REE species decrease rapidly to lower than 1 ppb. The impact of temperature on REE fractionation in PGL-fed boiling springs was analyzed under a nearly neutral pH condition (fixed at pH 6.3). The simulation results showed that high temperature (over 200℃) favors the REE hydroxyl over the other species; however, REE fluorides are dominant followed by REE free ions, carbonates, and sulfates at temperature lower than 150℃. The presence of Fe/Mn (hydr)oxides can change the distribution of REE in geothermal fluids due to its strong affinity to dissolved REE species. The result given by a simplified 1-D model showed that after interaction of 150 aliquots of PGL, the REE_N patterns have a strong trend of depletion of LREE; as this simulation continues until 6000 aliquots have passed, the REE_N patterns tend to be flat especially at temperature of 100℃. This can help to explain the differences of REE_N patterns in Daggyai and Rehai geothermal fields that the relatively flat REE_N patterns in Rehai is probably because the presence of hydrothermal alteration minerals such as pyrite, rhodochrosite, kaolinite and illite-montmorillonite interstratified clays.