Fulbright Research – Chile

In 2016, I had the amazing opportunity to spend 9 months living in Santiago, Chile, doing geochemistry research at the Universidad de Chile supported by a US Student Fulbright Grant. Working in Dr. Martin Reich’s group at U Chile, I met geoscientists from all over South America and was able to contribute to two research projects. Besides doing science, I also learned more about the world and different cultures than I ever had before in my life. It was the most challenging, rewarding, eye-opening, demanding, incredible experience of my life. I kept a blog on my time there, which can be found under Travel – Santiago, Chile.

Fig. 2
Photos from the SEM in Tardani et al (2017).

Research-wise, the first project I contributed to was part of Dr. Daniele Tardani’s doctoral research on the fluid geochemistry of the Tolhuaca geothermal system in south-central Chile. I photographed pyrite from cores using the Scanning Electron Microscope (SEM) and reflected and transmitted light microscopy for publication. Pyrite is commonly found in geothermal systems and Daniele’s study focused on investigating the various trace element concentrations in the pyrite crystals in order to learn about past fluid compositions. He found that there were distinct zones of Cu-rich, As-poor and As-rich, Cu-poor areas in pyrite, suggesting that these two zones record a change in fluid chemistry that was recorded as the pyrite grew (Tardani et al., 2017).

Figure 1
Fig. 1 – Shows the location and nature of the faults and thermal waters sampled in the SVZ.

The second half of my stay in Santiago consisted of developing my own project. Water chemistry data from 30 thermal springs collected by Daniele in the Southern Volcanic Zone of Chile was analyzed to explore the influence of major fault systems on water chemistry. The study focused on a ~450 km region between 36-41 degrees south in the Southern Volcanic Zone (SVZ; see Fig. 1). The SVZ is an outstanding natural laboratory to study the interplay between tectonic activity, volcanism, geothermal systems, and fluids as all of these aspects are closely related in this region. The two major fault systems in the SVZ are the Liquiñe-Ofqui Fault System (LOFS) and the Andean Transverse Faults (ATF). The NNE-striking LOFS and WNW-striking ATF are preferentially oriented and severely misoriented to the prevailing stress regime of today, respectively, and therefore exhibit contrasting characteristics in regard to volcanism, permeability, and fluid flow. While faults of the LOFS have high vertical permeability networks that enhance fluid flow, faults of the ATF tend to inhibit fluid flow and instead store fluids until supralithostatic pressures are reached that release and transmit fluids. Previous studies have confirmed that these inherent structural differences in the two major fault systems of the SVZ affect isotope signatures, volcanism, mineral precipitation, and water chemistry in a localized area. Our goal was to investigate the processes controlling the chemistry of thermal surface waters in the SVZ by analyzing the geochemistry and relationships of major and selected trace elements to explore the influence of the major fault systems on water chemistry.

Three sets of waters were identified based on physicochemical characteristics: neutral NaCl waters, acid-sulfate waters, and bicarbonate waters. Each of these three types had specific pH and temperature ranges and their chemistries were related to their formation mechanisms. The waters were also characterized according to their spatial relationship to the two major fault systems described above. This was done using detailed maps and previous publications. Using elemental relationships and ratios, differences in water chemistry between LOFS-associated waters and ATF-associated waters suggest that the inherent differences between the two fault systems constitutes the primary structural control on surface water chemistry in the SVZ. Specifically, LOFS-associated waters had a higher degree of correlation between trace alkali metals and Cl than did ATF-associated waters. This is due to the enhanced vertical permeability of the LOFS that directs fluid flow efficiently. ATF-associated waters showed higher B/Cl ratios than waters of the LOFS due to the increased storage capacity of faults of the ATF and their proximity to degassing, more evolved magma chambers.

Geothermal systems have been largely understudied in the Andean region, and in particular there is limited scientific information about the role of local kinematic conditions on fluid flow and mineralization during the development and evolution of geothermal reservoirs. Therefore, the results from this study increase our knowledge about structurally controlled geothermal systems in southern Chile, and particularly, provide new insights about the effects of both regional and local tectonics on fluid flow and permeability.

The results of this study were published in Chemical Geology (Wrage et al. 2017).

Citations

Tardani, D., Reich, M., Deditius, A.P., Chryssoulis. S., Sanchez-Alfaro, P., Wrage, J., Roberts, M.P. (2017) Copper-arsenic decoupling in an active geothermal system: a link between pyrite and fluid composition. Geochimica et Cosmochimica Acta, v. 204, 179-204.
Wrage, J., Tardani, D., Reich, M., Daniele, L., Arancibia, G., Cembrano, J., Sanchez-Alfaro, P., Morata, D., Perez-Moreno, R. (2017) Geochemistry of thermal waters in the Southern Volcanic Zone, Chile – Implications for structural controls on geothermal fluid composition. Chemical Geology, v. 466, 545-561.