Silica sinter textures in El Tatio geothermal field, Chile: preliminary results on flow reconstruction. Bridget Y. Lynne 1, Diego Morata 2*, Martin Reich 2and Constanza Nicolau 2 1Institute of Earth Science and Engineering, University of Auckland, 58 Symonds St, Auckland 1142, New Zealand. 2Departamento de Geología y Centro de Excelencia en Geotermia de los Andes (CEGA-FONDAP). Facultad de Ciencias Físicas y Matemáticas, Universidad de Chile, Plaza Ercilla 803, Santiago, Chile *E-mail: [email protected] Abstract. Distinctive sinter textures form from silica-rich bounded to the west by the Serranía de Tucle-Loma Lucero alkali chloride hot springs depending on the environmental horst and to the east by volcanoes of the El Tatio volcanic conditions such as flow rate, pH, microbial communities or group (Lahsen and Trujillo, 1976; Lahsen, 1988). Most of water temperature. These textures are normally preserved the hot-springs discharge near local boiling temperature ( ≈ over time and throughout diagenesis. Amorphous silica was 86°C), being siliceous sinter deposits founded sometimes deposited in El Tatio geothermal field around hot springs from silica-saturated, alkali chloride water. Some of the as geyser events or as terraces at the geysers and springs silica sinter textures observed at the high-altitude (i.e. (Phoenix et al., 2006). This high altitude geothermal field ~4000 m a.s.l.) El Tatio geothermal field evidence strong presents some peculiarities concerning silica textures. Few similarities with those found in modern, low-altitude hot studies have been published on high altitude hot spring springs of New Zealand on which detailed studies have settings (Jones and Renaut, 1997; Fernandez-Turiel et al., been already carried out. Field work reconnaissance allows 2005; Phoenix et al., 2006). These systems are us to identify textures associated with different water characterized by some major physical differences with temperatures and also others related to different flow respect to low-altitude silica sinter deposits (lower boiling conditions. Consequently, our preliminary results suggest point, greater evaporation rate and wider thermal daily that El Tatio silica sinter textures and fabric can be used for mapping paleo-flow conditions and establish the locations variations at higher altitude in comparison with low- of paleo hot up-flow zones in these high-altitude hot spring altitude areas) that control silica precipitation. The aim of environments. this preliminary study is to present some of the most important silica sinter textures and microbial mats Keywords: Silica sinters, hot-springs, biomineralization observed in El Tatio geothermal field as a proxy to understand hydrological pattern of this high-altitude silica sinter deposits. 1 Introduction Silica sinter deposits are the consequence of silica 2 Sampling and Results precipitation and accumulation when hot, nearly neutral pH, alkali-chloride springs (derived from hotter deep Paired visible light photographs and infrared images were reservoirs) cool to temperatures less than 100°C (see taken in a field work showing the vent, mid-slope and Lynne et al., 2008 and references therein). Hot spring distal-apron areas (Fig. 1). This clearly shows settings with settings and associated sinter textures are widely high- (>60 °C), mid- (35-59 °C) and low-temperature (<35 documented for low-altitude settings, showing °C) thermal gradients. Microbial mats present different environmentally significant textures that are preserved colours related to temperature and water flow-rate and long after hot spring discharge ceases and even during depth. In this sense, mid-temperature microbial mats are diagenetic processes (e.g. Jones and Renaut, 2003; Lynne characterised by greenish-yellow “bubblemats texture” et al., 2005, 2007, 2008). Therefore, sinters provide a around edges of discharge channel (Fig. 2a). The microbial record of alkali chloride hot spring paleo-flows, while their mat surrounding the bubbles silicifies to produce macro- textural characteristics enable mapping and tracing scale sinter textures of multiple curved laminations with hydrological conditions and broad temperature gradients oval or lenticular voids. Low-temperature alkali chloride from high-temperature vents to low-temperature distal- hot-springs commonly support the formation of “palisade apron areas. texture”, characterized by closely-packed, vertically- stacked, micro-pillar sinter structures. Under these In this work we present some textures and patterns environmental low-temperature conditions, shallow fluid observed in silica sinters from the high-altitude (4200- flowing over micro-terracettes of previously formed sinter 4300 m above see level) El Tatio geothermal field in (Fig. 2b). Distal-apron features are mostly characterized by northern Chile. The El Tatio geothermal field is located in plant-rich sinters (Fig. 2c). the north-south trending Tatio Graben, filled by Mesozoic to Quaternary breccias, dacites and ignimbites, and Other textures and features identified in the El Tatio 448 geothermal field are indicative of water flow rate. Typical textures and features and the mapping of these preserved features of fast flowing water are streamer textures (Fig. environmentally-significant textures in ancient sinters 2d), formed when silicification of microbial communities would provide a useful tool that assists existing exploration that form long strands are aligned along where the flow techniques used in the search for hidden geothermal direction takes place (Smith et al., 2003). resources. Acknowledgements This work is a contribution to the FONDAP-CONICYT Project #15090013 (“Andean Geothermal Center of Excellence-CEGA)” and to the Institute of Earth Science and Engineering (University of Auckland). Special thanks to Pablo Sánchez during field work. References Fernández-Turiel J.L.; García-Valles M.; Gimeno-Torrente D.; Saavedra-Alonso J.; Martinez-Manent S. 2005. The hot spring and geyser sinters of El Tatio, northern Chile. Sedimentary Geology 180: 125-147. Jones, B. and Renaut, R.W. 1977. Formation of silica oncoids around geysers and hot springs at El Tatio, northern Chile. Sedimentology 44: 287-304. Figure 1. Visible light and infrared images of mid to distal-apron areas in the high altitude of El Tatio geothermal field. T (°C) Jones, B. and Renaut, R.W. 2003. Hot spring and geyser sinters: the values shown are measured temperatures integrated product of precipitation, replacement, and deposition. Canadian Journal of Earth Sciences 40: 1549-1569. Moreover, silica sinter textures related to different environments in a geothermal field can also be observed in Lahsen, A. 1988. Chilean geothermal resources and their possible El Tatio. Within this group of textures, the more significant utilization. Geothermics 17: 401-410. are those related with non-overflowing, non-boiling pools Lahsen, A. and Trujillo, P. 1976. The geothermal field of El Tatio, (“Lily pad texture”, developed where small oscillations Chile. In Proceedings, Second United Nations Symposium on the occur due to wind-driven wave surge or small pulses of Development and Use of Geothermal Resources, San Francisco, fluid, but where the water does not overflow the pool or May 1975, Vol 1: 170-177, Washington, D.C., U.S. Government channel rim, Fig. 2e), textures related with intermittently Printing Office (Lawrence Berkeley Laboratory, University of California). overflowing pools (digitate sinter rims, Fig. 2f) and textures related with intermittent flow (oncoidal and Lynne, B.Y.; Campbell, K.A.; Moore, J.; Browne, P.R.L. 2005. pisoidal textures, circular nodules that rotate in alkali Diagenesis of 1900-year-old siliceous sinter (opal-A to quartz) at chloride fluid and grow by accreting silica to their exterior Opal Mound, Roosevelt Hot Springs, Utah, U.S.A. Sedimentary surface, Fig. 2g). All these textures are indicative of Geology 119: 249-278. different environmental deposition conditions. Lynne, B.Y.; Campbell, K.A.; James, B.; Browne, P.R.L.; Moore, J.N. 2007. Tracking crystallinity in siliceous hot-spring deposits. American Journal of Science 307: 612-641. 3 Conclusions Lynne, B.Y.; Campbell, K.A.; Moore, J.N.; Browne, P.R.L. 2008. Silica sinters in the El Tatio geothermal field form from Origin and evolution of the Steamboat Springs siliceous sinter deposit, Nevada, U.S.A. Sedimentary Geology 210: 111-131. discharging alkali chloride hot springs and provide evidence at the surface of a deeper geothermal reservoir. Phoenix, V.R.; Bennett, P.C.; Engel, A.S.; Tyler, S.W.; Ferris, F.G.. Moreover, different silica sinter textures and features 2006. Chilean high-altitude hot spring sinters: a model system for observed in the El Tatio geothermal field are rather similar UV screening mechanisms by early Precambrian cyanobacteria. to those described in some low-altitude geothermal fields, Geobiology 4: 15-28. allowing its interpretation in terms of hot spring paleo-flow Smith, B.Y.; Turner, S.J. and Rodgers, K.A. 2003. Opal-A and conditions and temperature gradient profiles from high- associated microbes from Wairakei, New Zealand: the first 300 temperature vents to low-temperature, distal-apron slopes. days. Mineralogical Magazine 67: 563-579. Moreover, because most of these silica sinter textures and features are preserved long after hot spring discharge cease and even during diagenesis, the recognition and sinter 449 (a) (c) (b) (d) (e) (f) Figure 2. Different silica sinter textures and features observed in the high altitude El Tatio geothermal