Precipitation of secondary minerals in acid sulphate-chloride waters traced by major, minor and rare earth elements in waters: The case of Puracé volcano (Colombia)

Major, minor and rare earth elements were analyzed in the acid sulphate - chloride thermal springs associated to Puracé volcano – hydrothermal system. The waters of Puracé were classified in 2 different groups as a function of the physico-chemical parameters and element distributions. Group 1 is characterized by the highest pH (⁓ 3.5), an outlet temperature of ⁓ 81 °C and a strong depletion of Fe, Al, Si and Ba with respect to the isochemical dissolution of the average volcanic local rock. Group 2 waters have lower pH values ⁓ 1.9 and temperature (⁓ 48 °C) compared with Group 1. Moreover, Group 2 is not characterized by a typical pathway representing the congruent dissolution of the rock and shows a distribution of major and minor elements that is more close to the near-congruent dissolution of the average volcanic local rock with respect to Group 1. These geochemical features of major and minor elements allow to propose that the chemical composition of the waters of Group 1 is strongly affected by the precipitation of secondary minerals such as alunite, jarosite, kaolinite, barite and polymorphs of SiO2.

The grouping of waters is also supported by the distribution of dissolved REE normalized to the average volcanic local rock. Group 1 shows REE patterns strongly depleted in light rare earth elements (LREE), typical of water that formed alunitic and/or kaolinitic rocks. On the contrary, Group 2 is characterized by flat patterns, in according to the near-congruent dissolution of the rocks. REE dissolved in waters of Puracé were compared with REE in the acidic waters of Nevado del Ruiz and Azufral Colombian volcanoes and with REE in minerals recognized in advanced argillic alteration (alunite, gypsum and kaolinite). Precipitation of secondary minerals is proposed as a common process depleting LREE in acidic sulphate – chlorine waters in volcano – hydrothermal systems.

Furthermore, the chemical fractionation of the major and minor elements was interpreted together with the corresponding distributions of REE in order to trace the water – rock interaction processes. Saturation indexes of most common secondary minerals identified in advanced argillic alterations were calculated using PHREEQC software in a range of temperature from 25 to 250 °C. This geochemical approach allows to identify the possible mineral precipitation or dissolution of secondary minerals as well as the temperature at which the water reached equilibrium with a given set of minerals. In Group 1, the precipitation of secondary minerals LREE enriched (alunite minerals and kaolinite) was traced at temperature of precipitation higher than ⁓ 101 °C.