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