Geochemical Characteristics of Thermal Springs in Volcanic Areas of Antsirabe-Itasy, Madagascar: Preliminary Results

Abstract

There are several thermal springs in Madagascar for which a few locations are known, but for which there are not enough available temperature and chemical measurements.
Two types of geothermal systems are recognized in this country: those driven by magmatic heat (magmatic systems) and those associated with high regional heat flow and active faulting (extensional systems). In volcanic areas of Antsirabe-Itasy, it is clear that many of the geothermal fields are related to volcanism and also controlled by fault zones.
Thermal springs in these volcanic areas offer the possibility of geothermal energy. Many of these sites are not so located and require evaluation before the geothermal potential of the areas can be assessed. In order to begin filling in data gaps, inter-comparison of data sets from previous analyses has been undertaken. Over 20 available chemical analyses were obtained from these springs with previously limited data.
There is not a considerable variation in the chemical composition of the springs. Fluids from volcanic area of Antsirabe (magmatic-associated system) have higher Na, K and Cl concentrations and have lower Mg and Ca concentrations compared to their extensional counterparts (magmatic area of Itasy). These differences are observed in spite of the fact pH and salinity ranges are broadly similar. CO2 or HCO3 always shows high concentration whereas SO4 tends to increase.
A preliminary evaluation of temperatures indicates most of the sites have relatively low to medium geothermal potential. Temperatures of the reservoir at depth are estimated to be between 75-152°C and were calculated using chalcedony geothermometer. Some of the reservoir temperatures of these magmatic-associated systems appear promising and may support geothermal power plants.
Cold springs can also provide anomalous geothermometer temperatures indicating a geothermal resource, and that sampling of cold springs can be used as an exploration tool in areas with no surface expression of geothermal systems.
Further investigation based mainly on thermal gradient drilling, geophysical and geological explorations, remote sensing and geothermal GIS, is necessary to determine the geothermal resource potential of selected sites (resource assessment) and to help identify if they have commercial reservoirs (production wells, capacity, nature of the waters, depth and temperature of the reservoir).