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

Manuëlla Frédérique, Dominique Delalande (1) , Laurent Bergonzini (1) , Fabrizio Gherardi (2) , Massimo Guidi (3) , Luc André (4) , Issah Abdallah (5) , David Williamson (6)

1 IDES - Interactions et dynamique des environnements de surface
2 UNIVERSITY OF FIRENZE - UNIVERSITY OF FIRENZE Dipt di Biologia Animale e Genetica "Leo Pardi"
3 IGG - CNR Institute of Geosciences and Earth Resources
4 Section de Pétrographie-Minéralogie-Géochimie
5 IRA Institute of Resource Assessment
6 LOCEAN - Laboratoire d'Océanographie et du Climat : Expérimentations et Approches Numériques

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 degrees C up to 185 degrees 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. (C) 2010 Elsevier B.V. All rights reserved.

Domaines: Sciences de la Terre

Manuëlla Frédérique, Dominique Delalande, Laurent Bergonzini, Fabrizio Gherardi, Massimo Guidi, Luc André, et al.. Fluid geochemistry of natural manifestations from the Southern Poroto-Rungwe hydrothermal system (Tanzania) : preliminary conceptual model. Journal of Volcanology and Geothermal Research, 2011, 199 (1-2), pp.127-141. ⟨10.1016/j.jvolgeores.2010.11.002⟩. ⟨hal-00548780⟩