L. Sankovitch Geology MS Thesis Appendix Files: August 2024, University of Nevada-Reno

Continental hydrothermal systems are critical avenues for the crustal transport of heat and mass captured for geothermal energy and mineral exploration. Thus, understanding their temporal evolution and longevity is important for resource characterization. In particular, high temperature reservoirs (>170°C), which are commonly marked at the surface by deposits of microlaminated siliceous sinter, have the potential to trace hydrothermal histories. Previous attempts to use radiocarbon (¹⁴C) dating in active modern systems have encountered problems matching ages with stratigraphy, possibly due to bacterial mats introducing old carbon leading to older apparent ages. Geothermal reservoirs are often positioned within uranium (U)-rich silicic volcanic rock where subsurface fluid-rock interactions extract U into hydrothermal fluids. As U is precipitated in the surface sinter deposit, assuming closed-system behavior, it begins to produce Th as an intermediate daughter product within the U-Th-Pb decay chain. If this assumption is correct, the U-Th disequilibrium geochronologic method becomes an additional dating option. While U-Th dating has been broadly utilized in carbonates, a systematic methodology has not been established for siliceous sinter deposits. As U-Th ages require only milligrams of material, this would potentially allow detailed chronologies of microlaminated material. For this work, we collected samples from El Tatio geyser field in the altiplano of northern Chile, the largest geothermal system in the Andes. Radiocarbon dating has been conducted at El Tatio and its arid climate offers exceptional preservation of deposits. Our resulting U-Th ages, along with the water and deposit chemistry, suggest trends of U and Th accumulation along the sinter apron. While distal apron facies containing the highest U concentrations (>50 ug/g) show the least effect from detrital Th, they can display suspected open-system behavior. In contrast, more medial apron facies, where bacterial mats and other porous textures are commonly concentrated, have only trace amounts of U (<0.1 ug/g), which leads to unreliable or geologically improbable ages. More proximal facies tend to date most consistently, with repeat age results on one sample even producing a statistically significant isochron. By comparing existing ¹⁴C ages with U-Th results, El Tatio serves as an independent constraint, with U-Th ages trending younger than the ¹⁴C ages, possibly supporting the presence of an old carbon influence.