Hydrogeochemical characterization of thermal springs, Los Lagos, Chile

Abstract

Liquiñe-Ofqui Fault System (LOFS) and the Arc-Oblique Long-lived Faults System (ALFS) with a WNW-ESE trending and a mostly siniestral dynamic make the Los Lagos District an area of interest for geothermal development. There are two identified groups of hot springs which are formed according to their geological localizations. The first one corresponds to those situated over the ALFS (Porcelana Chico, El Comau and Porcelana Grande), characterized for having a direct volcanic influence relative to fluid horizontal transport (expressed by bigger B/Cl ratios), a geothermal source of chlorides and a minor cation exchange. These thermal manifestations record the highest temperatures. The second group is comprised by thermal waters over the LOFS and is subdivided in to subsets: (i) hot springs localized in the intertidal zone (Rollizos, Cochamó, Sotomó y El Yate) which have a slight interaction with saline water and a chloride mix origin. and (ii) thermal waters located in continental zone, especially those that are far from estuaries and fjords (Puyehue, Aguas Calientes, Rupanco, El Callao, Cayetué, Ralún, Puelo, Pichicolo, Llancahué, Cahuelmó y El Amarillo), where waters show an important cation exchange, and they present a basically vertical fluids transport with a great interaction with North Patagonian Batholith (NPB) rocks.

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la serena octubre 2015
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Hydrogeochemical characterization of thermal springs,
Los Lagos, Chile.
Barbara Ruiz a*, Diego Morata a and Juan Rojas b
a Departamento de Geología y Centro de Excelencia en Geotermia de los Andes (CEGA), Facultad de Ciencias Físicas y
Matemáticas, Universidad de Chile, Plaza Ercilla 803, Santiago
b Servicio Nacional de Geología y Minería (SERNAGEOMIN), Santa María 0104, Santiago, Chile. At present: Juan Rojas,
Consultant
* Contact email: bruiz@ing.uchile.cl
Abstract. Liquiñe-Ofqui Fault System (LOFS) and the
Arc-Oblique Long-lived Faults System (ALFS) with a
WNW-ESE trending and a mostly siniestral dynamic make
the Los Lagos District an area of interest for geothermal
development. There are two identified groups of hot
springs which are formed according to their geological
localizations. The first one corresponds to those situated
over the ALFS (Porcelana Chico, El Comau and
Porcelana Grande), characterized for having a direct
volcanic influence relative to fluid horizontal transport
(expressed by bigger B/Cl ratios), a geothermal source of
chlorides and a minor cation exchange. These thermal
manifestations record the highest temperatures. The
second group is comprised by thermal waters over the
LOFS and is subdivided in to subsets: (i) hot springs
localized in the intertidal zone (Rollizos, Cochamó,
Sotomó y El Yate) which have a slight interaction with
saline water and a chloride mix origin. and (ii) thermal
waters located in continental zone, especially those that
are far from estuaries and fjords (Puyehue, Aguas
Calientes, Rupanco, El Callao, Cayetué, Ralún, Puelo,
Pichicolo, Llancahué, Cahuelmó y El Amarillo), where
waters show an important cation exchange, and they
present a basically vertical fluids transport with a great
interaction with North Patagonian Batholith (NPB) rocks.
Keywords: Geothermal, hot springs, water chemistry,
Liquiñe-Ofqui Fault System (LOFS), Arc-oblique Long-
lived Fault Systems (ALFS), Los Lagos
Introduction
Thermal springs in Southern Chile are mostly related
with both active volcanism and the Liquiñe-Ofqui Fault
System (LOFS). In this sense, thermal springs in the Los
Lagos district (Figure 1) are a good opportunity to check
the relationships between active volcanism and fault
constraints. Moreover, some of the thermal springs are
close to sea level and a probable influence of marine
water could be expected when analyze its geochemistry.
The present work is mostly focused on the geochemistry
of thermal waters existing in the Los Lagos district. The
main objectives of this work are to generate a
geochemical characterization of some hot springs from
this area, finding a possible relation with their
localization and the present geological structures. Data
obtained with this work would allow us a correction
index for the applied geothermometers due to the high
dilution expected in thermal waters as consequence of
normal intense raining regime happened in the area.
Finally, a geothermal favorability map would be
proposed based on obtained data.
Geological background
Los Lagos district, Chile, is located in the South Volcanic
Zone (SVZ) (40.5 – 44°S), showing a large and active
volcanism.
The oblique convergence between the plates produces
important stresses which generate numerous folds and
faults with different orientation depending on the tectonic
conditions during their formation. In the studied area
there are two big structural first order domains one with a
NNE-SSW trending and mostly dextral faults, clustered
in the Liquiñe-Ofqui Fault System (LOFS) (Cembrano et
al., 1996; Cembrano et al., 2000; Cembrano and Lara,
2009). As the dynamic of this fault system is produced by
oblique stress tensors, there exist zones with trastensive
basins, helping fluid ascent (Cembrano and Lara, 2009).
The second group consists of faults with a WNW-ESE
trending and a mostly siniestral dynamic, which are
grouped in the Arc-Oblique Long-lived Faults System
(ALFS). They were formed by the reactivation of pre-
andean faults as siniestral strike-slip faults (Cembrano
and Moreno 1994; López-Escobar et al. 1995; Lara et al.
2006; Rosenau et al. 2006; Sánchez et al., 2013).
Different volcanic complexes (from basaltic to rhyolitic
compositions) are present over the fault systems
mentioned above, because of the permeability granted by
the present structures. The volcanic systems aligned
along NNE-SSW correspond to Pleistocene-Holocene
stratovolcanoes with a basaltic to andesitic composition,
related to a less magmatic differentiation during the
ascent through structures of LOFS (e.g. Cembrano and
Moreno, 1994; López-Escobar et al., 1995; Cembrano
and Lara, 2009). Some examples of volcanoes which
would affect the geothermal studied systems are
Casablanca, Yate and Hornopirén. On the other hand,
there are volcanic complexes localized over ALFS
structures, which have a NW-SE orientation and a
variable chemical composition, from basaltic to rhyolitic.

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AT 2 geología económica y recursos naturales
Calbuco, Huequi and Porcelana volcanoes are some
examples of this group.
Analyzed hot springs
Eighteen hot springs and their closest superficial waters
were chosen to obtain their geochemical analysis, which
could show possible interaction between them. The
method used was the sampling of water, acidifying and
filtering those ones used to obtain concentrations of trace
elements. These samples were sent to the
SERNAGEOMIN laboratory using atomic absorption
spectrophotometry (AAS), ion chromatography,
inductively coupled plasma mass spectrometry (ICP-MS)
and isotope-ratio mass spectrometry (IRMS) for cations
and silicon, anions, trace elements and H and O isotopes,
respectively; with an analytical error ≤5%. These results
were normalized according to the VSMOW (Vienna
Standard Mean Ocean Water).
At first, three main groups can be formed using their
geographical localizations: which are over or close to
important volcanic complexes (red symbols); the hot
springs which reach the surface in distant areas of saline
waters as estuaries and fjords (green symbols); and which
reach the surface in intertidal zones (blue symbols). The
water samples of the last group were obtained while low
tide periods, when it is possible to see the geothermal
water separated from superficial one.
Geothermal waters classifications
In the anion ternary diagram (Figure 2), it is possible to
distinguish the blue group as chlorine waters.
Nevertheless, it is necessary to remark that these hot
springs are really close to the sea level; consequently we
need to confirm if these hot springs are truly mature
waters or the result of a mixture of waters. For this
proposal, it is useful to compare the isotopic chemical
compositions with the Global meteoric water line
(GMWL) (Figure 3), where the hot springs appear as
meteoric and not mixed waters.
On a Li-B-Cl diagram, it is possible to relate Porcelana

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SIM 4 SISTEMAS GEOTERMALES ANDINOS
Chico, El Comau and Porcelana Grande (Figure 4) to
more proximal geothermal zones, because of its higher
composition of boron (Giggenbach, 1991). While on the
other hand Rollizos and Sotomó geothermal waters show
a probable interaction with saline/marine waters, because
their B/Cl ratios (0.00004 and 0.0001, respectively) are
very similar to the fjord and estuary waters (~0.0001) and
they are the lowest values (Figure 5).
Geothermometry
Another feature which could show some kind of mixing
is the different temperatures obtained by
geothermometry. Two geothermometers were chosen
because of their quasi-linear relationship with surface
temperatures: silica (Verma and Santoyo, 1997) and
sodium-potassium (Díaz-González et al., 2008); where
the first method show the best correlation. The highest
estimated temperatures were obtained from Porcelana
Chico (between 180 and 192°C, respectively) and
Porcelana Grande thermal waters (over 130°C); on the
other hand, Sotomó and Pichicolo have values <14°C
with the Na-K geothermometer. From the difference
between the two geothermometers temperatures a
correction index of external contribution (CIEC) was
created for each value generated by the Na/K
geothermometer. The non-linear correlation of this
geothermometer could be generated by cation exchange
produced by rock interaction or water mixing.
Discussion and conclusions
Sánchez et al. (2013) proposed a structural model to
explain the different geochemical signature founded in
some thermal waters in the Villarrica area, also
controlled by the LOFS structure. A similar structural
control for the fluids circulation is proposed in our
studied zone to explain the different geochemical
signatures found in our thermal springs (Figure 7).
Consequently, We define two groups of hot springs
according their chemical composition and their kind of
structure above they are: ALFS domain (PoC, Com and
PoG) with a lateral fluid transport, and LOFS domain
(Puy, Agu, Rup, Cal, Rol, Coch, Sot, Yat, Pich, Llan,
Cah, Cay, Pue and Ama) with a mainly vertical-
dominated fluid transport. The first group is characterized
by high B/Cl and intermediate Br/Cl ratios (Figure 5).
Considering the volatile behavior of boron and the
conservative behavior of bromine (Giggenbach, 1991;
Vengosh, 2003; Sánchez et al. 2013), these ratios
indicate a hydrothermal source of chlorides; also, as the
cation compositions show a linear relation with chlorides,
this imply no important cation input, therefore these
thermal waters would not have a relevant cation
exchange in depth. On the other hand, the geothermal
waters of the second group are characterized by a lower
B/Cl ratios, higher Na/K ratios (Figure 6) and extreme
chloride concentrations (very low or very high), showing
a probable rock interaction and, for the blue symbols
waters, a possible saline water mixing, specifically for
Rollizos and Sotomó spring waters.
Finally, with the obtained data in this work and those
available data from literature, we propose a favorability
map which indicates the zones with more favorable

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factors for the existence of a geothermal system. The
method was initially proposed by Aravena (2012) and
modified by Ruiz (2015). According this methodology,
the areas with best favorability for the location of a
geothermal resource in depth are located north of the
Reloncaví fjord and over the northern coast of Huequi
peninsula. This result can be related to less mixed
sources of thermal water formed mainly by infiltrated
rain water heated by volcanic gases with a horizontal
transport of fluids; meanwhile Rollizos and Sotomó
represent mixed systems with an important externa l
influence, throw the LOFS vertical structures.
Acknowledgments
This paper has been supported by the Geothermal
Program of SERNAGEOMIN, during the development of
“Informe Registrado IR-14-57”; and is a contribution to
the Fondap-Conicyt Project 15090013 “Centro de
Excelencia en Geotermia de los Andes (CEGA)”.
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