Fluid geochemistry of natural manifestations from the Southern Poroto–Rungwe hydrothermal system (Tanzania): Preliminary conceptual model

The South Poroto–Rungwe geothermal field, in the northern part of the Malawi rift, Tanzania divides in two main areas. The relatively high altitude northern area around the main Ngozi, Rungwe, Tukuyu and Kyejo volcanoes, is characterised by cold and gas-rich springs. In contrast, hot springs occur in the southern and low-altitude area between the Kyela and Livingstone faults. The isotopic signature of the almost stagnant, cold springs of the Northern district is clearly influenced by H2O–CO2(g) exchange as evidenced from negative oxygen-shifts in the order of few deltas permil. In contrast, the isotopic signature of waters discharged from the hot springs of the Southern district is markedly less affected by the H2O–CO2(g) interaction. This evidence is interpreted as an effect of the large, permanent outflow of these springs, which supports the hypothesis of a regional-scale recharge of the major thermal springs. Measurements of carbon isotope variations of the dissolved inorganic carbon of waters and CO2(g) from the Northern and Southern springs support a model of CO2(g)-driven reactivity all over the investigated area. Our combined chemical and isotopic results show that the composition of hot springs is consistent with a mixing between (i) cold surface fresh (SFW) and (ii) Deep Hot Mineralised (DHMW) Water, indicating that the deep-originated fluids also supply most of the aqueous species dissolved in the surface waters used as local potable water. Based on geothermometric approaches, the temperature of the deep hydrothermal system has been estimated to be higher than 110 °C up to 185 °C, in agreement with the geological and thermal setting of the Malawi rift basin. Geochemical data point to (i) a major upflow zone of geothermal fluids mixed with shallow meteoric waters in the Southern part of the province, and (ii) gas absorption phenomena in the small, perched aquifers of the Northern volcanic highlands.

Research Highlights
► The aim of our manuscript VOLGEO-S-08-00393-1 is to characterise the hydrothermal activity and to provide a general framework for future studies of the South P-RVP hydrothermal system, one of the provinces of the East African Rift System.
► This paper presents a valuable and compelling body of data and interpretation about the origin of gassy cold and hot spring waters in the southern East African Rift Zone.
► The origin of parent liquid is discussed in terms of water and carbon isotopes and a particular section, about the modelling of the degassing/precipitation processes to reconstruct the δ13C-TDIC and δ13C-CO2(aq) at discharge conditions, is addressed in order to support the main conclusion of our manuscript.
► The parent thermal fluids origin is a meteoric water infiltrated above 2000 m in the Poroto–Rungwe highland or under colder conditions with relative high amount of deep and most likely magmatic CO2.