Table 3 Geological and geothermal characteristics for prospective areas in Madagascar

Dominant geological formations

Sandstone and shale of the Isalo and Sakamena groups

Paragneiss and migmatite of the Sambirano Group

Migmatitic gneiss, paragneiss and schist of the Ambatolampy Group

Orthogneiss and tonalitic-granite of the Dabolava Suite

Sandstone, limestone, clay and marl of the Isalo, Sakamena groups

Interpreted from geological maps

Temperature of hot springs (°C)

47–78

48–72

24–57

38–57

30–55

39–42

Hambinintsoa et al. (2017)

Gunnlaugsson et al. (1981)

Besairie (1959)

Temperature of geothermometer (°C)

140–180

140–200

170–240

150–240

140–170

120–140

Gunnlaugsson et al. (1981)

Dominant chemical composition of fluids

Moderate to high CO₂ (200–1000 ppm) and low Cl (< 300 ppm)

High CO₂ (1000–4000 ppm) and Cl (400–500 ppm)

Low CO₂ < 200 ppm and moderate Cl (500–1200 ppm)

Saline brine, NaCl-dominated

Basinal or infiltration fluids?

Hambinintsoa et al. (2017)

Gunnlaugsson et al. (1981)

Mobile fluid phase in the geothermal reservoir

Liquid-dominated

Interpreted according to temperature of geothermal reservoir