Fluid geochemistry of natural manifestations from the Southern Poroto–Rungwe hydrothermal system (Tanzania): Preliminary conceptual model |
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| Authors: | Manuëlla Delalande Laurent Bergonzini Fabrizio Gherardi Massimo Guidi Luc Andre Issah Abdallah David Williamson |
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| Institution: | 1. Department of Life Sciences and Biotechnology, University of Ferrara, Via Luigi Borsari 46, 44121 Ferrara, Italy;2. Department of Environmental, Biological and Pharmaceutical Sciences and Technologies, University of Campania “Luigi Vanvitelli”, Via Vivaldi 43, 81100 Caserta, Italy;3. School of Earth Sciences, Addis Ababa University, P.O. Box 1176, Addis Ababa, Ethiopia;1. Istituto Nazionale di Geofisica e Vulcanologia, sezione di Palermo, Italy;2. The Queensland Geothermal Energy Centre of Excellence, The University of Queensland, QLD, Australia;3. Eskisehir Osmangazi University, Department of Geological Engineering, Meselik, Turkey;4. Hacettepe University, Department of Geological Engineering, Beytepe, Ankara, Turkey;5. School of Earth Sciences, The University of Queensland, QLD, Australia;6. Australian National Centre for Groundwater Research and Training, Australia;1. Key Laboratory of Earthquake Prediction, Institute of Earthquake Science, China Earthquake Administration, Beijing 100036, PR China;2. State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, PR China;3. University of Chinese Academy of Sciences, Beijing 100049, PR China |
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| Abstract: | 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. |
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