geothermometry on spring waters from poços de caldas ... · of geothermometer depends on the...
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Geothermometry on spring waters from Poços de Caldas, Minas Gerais , Brazil
Ritter S.M.1, Isenbeck-Schröter M.1, Bonotto D.M.2
1Institute of Earth Sciences - Universtity of Heidelberg Germany; 2 Instituto de Geociências e Ciências Exatas - Universidade Estadual Paulista Campus de Rio Claro, Brasil
Investigated area The City of Poços de Caldas
(PdC) is located in the north of the
PdC plateau, which forms a
distinctive ring structure (Fig.1). It
is underlain by alkaline rocks of
cretaceous age mainly phonolites
and nepheline-syenites, showing a
significantly higher heat-flow
(~32°C/km) than the surrounding
basement (1). Several diffuse
artesian springs occur in the city
of PdC area, discharging thermal
and non-thermal waters.
Geothermometry Reservoir temperatures can be derived
from the water chemistry and were
calculated with SolGeo (5). The selection
of Geothermometer depends on the system’s geology, its dominant processes in geothermal-fluid formation and ascent behavior. Assuming the thermal waters to
be equilibrated with Na-K-Mg-phases of
the host rock (Fig.3), only four Na/K-
Geothermometers give reasonable
estimates for maximum equilibrium
temperatures of 100 to 130 °C (Tab.1).
Hydrogeochemistry Three types of thermal and non-thermal
spring waters reflect different aquifer
zones at PdC:
poorly mineralized, weakly acidic, cold
discharging waters representing a
shallow aquifer
higher mineralized, Na-K-(H)CO3-SO4-
F-type alkaline hot discharging waters
representing a deep aquifer
medium mineralized, alkaline cold
discharging waters representing a
mixture of both, shallow and deep
aquifer
Perspectives Water samples from other thermal springs
at the Poços Caldas plateau:
Genesis and comparability of
geothermal evolution
Determination of REY´s (rare earth and
yttrium) and helium isotope ratios:
Detailed insight on water-rock
interactions, better characterization of
the host rock
heat and mass flow through deep
open fault structures?
Thermal-water formation Suggested processes to form the thermal
waters (Fig.2), derived by inverse modeling
using PhreeqC (4), are:
pyrite oxidation accompanied by iron
precipitation
fluorite (CaF2) or villiaumite (NaF)
dissolution
calcite precipitation at depth
Na-K-alumosilicate weathering
equilibrium at depth of Na and K with
Na-K-alumosilicates (e.g. orthoclase
and nepheline)
References: (1) Araújo, R. (1980), Geothermal research in Pocos de Caldas alkaline caldera. Proceedings of 31 st Brazilian geological congress, p. 1059-1074. Camboriú.; (2) Wikipedia and Google Maps landscape;
(3) Cruz, W. (1987), Hydrogeological and hydrochemical evaluation at Pocos de Caldas area. Belo Horizonte: Fundacao Centro Tecnológico de Minas Gerais; (4) Parkhurst, D. L., & Appelo, C. A. (1999), User´s Guide
to PHREEQC(version2) - A computer program for speciation, batch reaction, one dimensional transport and inverse geochemical calculations. Water investigations Report 99-4259, 312 pp.;
(5) Verma, S. P., Pandarinath, K., & Santoyo, E. (2008), SolGeo: A new computer program for solute geothermomters and its application to Mexican geothermal fields. Geothermics(37), p. 597-621;
(6) Giggenbach, W. F. (1988), Geothermal solute equilibria.Derivation of Na-K-Mg-Ca geoindicators. Geochim.Comochim.(Acta 52), p. 2749-2765; (7) Arnórsson, S. (2000), Isotopic and Chemical Techniques in
Geothermal Exploration, Development and Use: Sampling Methods, Data Handling, Interpretation. Vienna, Austria: International Atomic Energy Agency. 351 pp.; (8) Diaz-González, L., & Santoyo, E. (2008), A new precise
calibration of Na/Ka Geothermometer using a world database of geothermal fluids and improved geochemometric techniques. (Acta 72), p. A215 Abstract.; (9) Arnórsson, S., Gunnlaugsson, E., Svavarsson, H., (1983), The chemistry of geothermal waters in Iceland. III. Chemicalgeothermometry in geothermal investigations. Geochim. Cosmochim. Acta 47, 567–577.
Fig.1: Location of the investigated area (2) and detailed geological map of the Poços de
Caldas area (3).
Fig.2: Schematic cross-section of the study area: Conceptual hydrogeological and hydrogeochemical model.
Fig.3: Ternary plot used to determine reservoir
temperature and equilibrated waters suitable for
Geothermometry (6).
Tab.1: Reservoir temperature estimates with Na/K-
Geothermometers
Na/K
Geothermometers
°C est.
A00 (7)
°C est.
DSR08 (8)
°C est.
A83 (9)
°C est.
2DS08 (8)
Fonte de Macacos 129 105 ± 8 110 102
Pedro Botelho 130 106 ± 9 111 103
Quisisana Sulfurosa 125 100 ± 8 105 97