I Congreso de Postgrado fcfm: ingeniería, ciencias e innovación

51 Santiago, 10 al 12 de agosto, 2022 GE/SI RATIOS IN SILICEOUS SINTERS: IMPLICATIONS FOR SILICA PRECIPITATION MECHANISM IN GEOTHERMAL FIELDS Valeria Bustamante¹ , ²*, Alida Perez-Fodich¹ , ², Carolina Munoz-Saez³ ¹Departamento de Geologí , Universi ad de Chile, Santiago, Chile. ²Centro de Excelencia en Geotermia de los Andes (CEGA), Universidad de Chile, Santiago, Chile. ³Department of Geological Sciences and Engineering, University of Nevada, Reno *Email: valeria.bustamante@ug.uchile.cl ABSTRACT Silica sinters are opal deposits formed during the discharge and cooling of underlying geothermal systems, by precipitating a silica-rich hydrothermal f luid, preserving information about the f luid through textural, chemical, and biological evidence¹. This study investigates the chemical composition of sinters formed at El Tatio geothermal field located at 4200m.a.s.l. in the Central Andes. El Tatio is the largest geothermal field in the southern hemisphere and the third largest in the world. Environmental conditions (fast water cooling rates and high evaporation rates) and microorganisms appear to control silica precipitation dynamics², which inf luence the incorporation of trace elements such as Ge. Si and Ge share geochemical characteristics³ that allow the Ge/Si ratio to be an important tracer of the global silicon cycle⁴. Ge/Si ratios in geothermal systems show higher values due to the precipitation of Ge-poor quartz or opal⁵ as Ge stays in the f luid. According to batch experiments of silica precipitation, this fractionation effect is enhanced during rapid silica precipitation⁴. In this research we seek to understand the effect of environmental conditions on the behavior of Ge/Si partitioning during opal precipitation. In our study we have set up in-situ evaporation and opal precipitation experiments plus sampled hydrothermal f luids and fossil sinters to study the partitioning of Ge in opal across a range of extreme environmental conditions in El Tatio. Preliminary results show that our silica concentrations in the chloride-rich hydrothermal f luids range between 90–160 mg/L, and most of the samples are undersaturated with respect to opal. Silica concentrations in the evaporation experiments decreases, which suggests silica precipitation, while Ge behaves conservatively, and Ge/Si ratios increase due to the removal of silica. The precipitated mass in the experiments varies between 2.9-4 mg/day, being considered a high rate². Our results underscore the effect of rapid amorphous silica precipitation on the partitioning of Ge as observed in silica precipitation experiments⁴, and emphasize the role of the f luid temperature, cooling rates and eruption style in the changes of silica’s concentrations. Future work will consist of evaluating Ge/Si ratios in both active and fossil silica sinter deposits. ACKNOWLEDGMENTS This research was conducted in the lands inhabited by the Lickanantay native communities of Caspana and Toconce. REFERENCES [1] Cortecci, G. et al. Geochemical Journal, 39 (6), 547-571 (2005). [2] Nicolau, C. et al. Journal of Volcanology and Geothermal Research, 282 , 60-76 (2014). [3] Goldschmidt, M. Naturwissenschaften 14 , 295-297 (1926). [4] Fernandez, N. et al. Geochimica et Cosmochima Acta 297 , 158-178 (2021). [5] Evans, M. & Derry, L. Geology 30 , 1019-1022 (2002). C I E NC I A S D E L A T I E R R A 03