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Rendiconti online Soc. Geol. It., Vol. n (2012), n-n (Stile: intestazione prima pagina)
The geothermal System of Rosario de la Frontera (Salta, Argentina):
preliminary geochemical and hydrogeological results
P. P. PIERANT ONI (*), C. INVERNIZZI (*), G. GIORDANO (**), S. CORRADO (**), R. M AFFUCCI (**),
A. CHIODI (***), F. TASSI (****) & J. VIRAMONT E (***)
RIASSUNTO
Il siste ma ge ote rmico di Rosario de la Frontera (Salta,
Arge ntina): risultati geochimici e idrogeologici preliminari
Questo studio è parte di un'indagine multidisciplinare delle sorgenti
termali emergenti nel territorio di Rosario de La Frontera (La Candelaria
Ridge, Salta, Argentina; Progetto CUIA bando 2010-11); esso è stato
condotto al fine di produrre una valutazione preliminare del potenziale
geotermico di questo sistema. Queste manifestazioni termali di superficie,
la cui temperatura varia tra 22,7 e 93,3°C, si trovano nel settore
settentrionale dell’anticlinale a direzione N-S che caratterizza l’intera
Sierra Candelaria nella provincia di Salta (Argentina NW, SEGEMAR,
Catálogo de Manifestaciones T ermales de la República Argentina,
SEGGIARO et alii, 1997). Lo studio si basa su dati analitici relativi alla
composizione chimica e isotopica ((18O2, D e 13C-CO2) di 13 campioni
di acqua e 5 campioni di gas prelevati da sorgenti termali che emergono in
questa area riservata. Inoltre, più di 20 stazioni strutturali sono state
realizzate per caratterizzare il principale sistema di fratture. Questi dati
sono importanti per valutare la permeabilità del serbatoio e per
determinare le principali direzioni di flusso del fluido in profondità.
Key words: Hydrothermal fluid, Rosario de la Frontera,
Argentina, Geochemistry, Hydrogeologic modeling.
INTRODUCTION
This study is part of the multidisciplinary investigation of
the thermal springs emerging in the area of Rosario de La
Frontera (La Candelaria Ridge, Salta, Argentina; CUIA
PROJECT call 2010-11); it is carried out in order to produce a
preliminary evaluation of geothermal potential of this system.
These surface thermal manifestations, whose temperature
ranges from 22.7 to 93.3 °C, are located in the northern sector of
the La Candelaria Ridge anticline in the province of Salta (NW
Argentina; SEGEMAR, Catálogo de Manifestaciones Termales
de la República Argentina; SEGGIARO et alii, 1997). The study
is based on analytical evidence of the chemical and isotopic
composition ((18 O2 , D e 13 C-CO2 ) of 13 water samples
_________________________
(*) Scuola di Scienze e T ecnologie, Sezione di Geologia, Università degli
Studi di Camerino, 62032 Camerino (MC).
(**) Dipartimento di Scienze Geologiche, Università degli Studi “Roma
T re”, 00146 Roma.
(***) Instituto Geonorte, Facultad de Ciencias Naturales, Universidad
Nacional de Salta, Salta, Argentina.
(****) Dipartimento di Scienze delle T erra, Università di Firenze, 50121
Firenze.
and 5 gas samples taken from hot springs emerging in this
restricted area.
Furthermore, more than 80 stations for structural data
acquisition have been performer in order to recognize main
fracture system network. These data are important to evaluate
the reservoir permeability and to determine main fluid flow
directions at depth.
GEOLOGICAL SETTING
The thermal area of Rosario de la Frontera (25 º 50,047 'S - 64
º 55,715' W) is located north of La Candelaria Ridge, which
pertains to Santa Bárbara System (Salta, Argentina; Fig. 1).
Santa Barbara System is a thick-skin fold and thrust belt formed
by relatively short and broad anticlines with axes striking from
NS and NE-SW.
The main structure of the Sierra de La Candelaria is a N-S
trending periclinalic and asymmetric macro-anticline (Fig. 1),
strongly plunging either to the North (below the Metan alluvial
plain) and to the South, cored by neo-Proterozoic phyllite
basement and draped by a Cretaceous -Quaternary sedimentary
succession (ESPELT A et alii, 1975; SEGGIARO et alii, 1997). The
sedimentary succession is made up of continental syn - and
post-rift sequences (Cretaceous to Eoecene in age) and syn orogenic siliciclastics (Oligocene-to present in age).
The anticline shows an high angle tectonic contact along
the eastern limb, which can be interpreted as a reverse fault
probably due to the inversion of a previous extensional
structure related to the Cretaceous rifting (CRIST ALLINI et alii,
1997 and references therein; Fig. 2). An important strike-slip
component along this features is probable. E-W trending
extensional faults witch realize a general N-S extension offset
the stratigraphic succession and the other discontinuities. They
are probably related to a younger deformation phase.
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P. AUT ORE ET ALII
(ST ILE: INT EST . PAGINE PARI)
Fig. 1 – La Candelaria Ridge, Argentina. Blue dots indicates the thermal springs.
STRATIGRAPHY
The stratigraphic succession of La Candelaria Ridge
comprises three major sequences extensively outcropping
along a ca. 60 km long N-S anticline.
The older unit crops out in the core of the anticline. It is the
Precambrian basement made up of low grade metasedimentary
rocks (Medina Fm).
M ARQUILLAS, 1994). The Salta Group is subdivided from
bottom to top in the Pirgua, Balbuena and Santa Barbara
subgroups (SALFIT Y AND M ARQUILLAS, 1994). The Early to
Late Cretaceous Pirgua subgroup consists mainly of red
continental conglomerates and sandstones of 1000 m average
thickness (ESPELT A et alii, 1975) and represents the syn-rift
stage. The Latest Cretaceous to Early Paleocene Balbuena
Subgroup is 180 m thick and comprises sandstones and
limestones, whereas the Paleocene to Early Eocene Santa
Barbara subgroup 330 m thick, is dominantly shaly with rare
carbonate intercalations. The Balbuena and Santa Barbara
subgroups represent the post-rift thermal subsidence stage
(BIANUCCI et alii, 1981, COMINGUEZ AND RAMOS, 1995).
Post-rift deposits are in turn overlain by a thick continental
foreland basin fill, related to the Andean mountain building,
that was shed from Middle Miocene to Plio-Pleistocene times
(GEBHARD et alii, 1974). The retrowedge basin fill includes two
subgroups: Metán and Jujuy. They belong to the Oran Group
(GEBHARD et alii, 1974). Main lithotypes include sandstones,
siltstones and mudstones of the Anta Fm (Metán subgroup)
with maximum thicknesses of about 700 m.
PRELIMINARY RESULTS
Fig. 2 – Geological section across the northern sector of La
Candelaria Ridge.
These strata are disconformably overlain by a
predominantly continental succession of red beds with minor
calcareous intercalations (Salta Group) of Cretaceous to
Paleogene age. This marks a rift stage (S ALFIT Y AND
Our results are relative to 13 hot springs water samples
which show a relatively low salinity (TDS < 1000 mg/L), with the
exception of three hot springs (Agua Laxante, Vicky e Salada).
These latter are characterized by TDS values in the range of
5500 and 30000 mg/L. All the sampled waters have a prevalent
sodium chloride composition (Fig. 3a), although the presence of
sulfate ion is significant (up to 3000 mg/L). Concentration of
ammonium (<0.28 mg/L) and boron (<0.51 mg/L) ions, generally
used as tracer for hydrothermal origin of water, are negligible.
The isotopic ratios of water (-6.7 and -37 ‰ V-SMOW for δ 18O
and δD respectively), suggest a prevalent meteoric origin of
fluids (Fig. 3b). Gaseous phases, associated with hot springs as
bubbling gases, are mainly CO2 (between 550 and 960
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T IT OLO DEL LAVORO (ST ILE: INT EST . DISPARI)
plunging nose of the anticline where several hot springs occur.
E-W system probably exerts a structural control on the
distribution of the chemical and physical features of waters. On
the other hand N-S, WNW-ESE, and NNE-SSW, E-W trending
fractures are predominant in the reservoir rock cropping out
along the eastern and western limbs of the anticline,
respectively.
This unit represents the main reservoir due to fractures
continuity showing typical spacing of approximately 30 cm and
aperture values that range from 3 mm up to 1 cm.
CONCLUSIONS
Our study confirms that the Pirgua subgroup is the reservoir
due to its lithology, secondary permeability (continous
fractures) and thickness, while the Metán subgroup, although
fractured, can still be a cap rock able to preserve temperature
and pressure conditions of geothermal fluids where it is kept
and mainly below the Quaternary deposits surrounding La
Candelaria Ridge.
As a whole, the detected fracture systems affecting the
principal reservoir enhanced its permeability, and allows deep
circulation of meteoric water.
Hydro-geochemical and isotopic preliminary results indicate
a prevalent meteoric signal of the spring waters.
Comparison between preliminary data from values of spring
discharge in the area of Rosario de La Frontera and values of
average annual rainfalls, which in La Candelaria Ridge is
about 900 mm/a, allow to estimate recharge time of the reservoir
in the order of some years.
Further data may allow to obtain an hydraulic conductivity
value for the characterization of the rocks heaps .
ACKOWLEDGEMENTS
Fig. 3 – a) Ion composition of watersamples (Piper diagram). b)
Isotopic composition of water samples.
mmol/mol), with relevant concentration of N2 (>28 mmol/mol).
Sulfurosa de la Vieja Quemada, Sulfurosa and Aguas Chicas
springs are characterized by relatively high concentrations of
CO2, H2S (up to 3.6 mmol/mol), H2 (up to 0.022 mmol/mol) and
CH4 (up to 0.087 mmol/mol), and low abundances of
atmospheric gases (O2, Ar and Ne). 13C/ 12C ratio in CO2 (δ 13CCO2 from -5.16 to -3.66 ‰ V-PDB) are consistent with a
prevalent mantle origin of this gas.
Preliminary results from structural analyses carried out by
scan lines within different lithologies allowed us to define the
main characteristics of the potential geothermal reservoir and
cap-rock and to identify the main fracture systems and their
relationships with the macro-scale structure.
Fractures orientations show six main different trends: N-S, NNESSW, WNW-ESE, NW-SE, NE-SW and E-W. Furthermore NWSE and E-W trending fractures are concentrated in the northern
We kindly acknowledge Walter Baez, Lea Di Paolo and
Pablo Caffe for useful discussions and their precious help in the
field.
Fundings: Project C.U.I.A. 2011-12 “Esplorazione e utilizzo
di risorse geotermiche di media e bassa entalpia in area sub andina per lo sviluppo energetico sostenibile delle città delle
province di Jujuy e Salta”, responsable C. Invernizzi.
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