An Integrated Geochemical Spatial and Temporal Survey of Low Temperature Thermal Springs to Characterize the Geothermal Resource of a Volcano (Piton Des Neiges, Réunion Island)

49 Pages Posted: 10 Mar 2022

See all articles by Bhavani Bénard

Bhavani Bénard

affiliation not provided to SSRN

Vincent Famin

affiliation not provided to SSRN

Bernard Sanjuan

affiliation not provided to SSRN

Françoise Vimeux

affiliation not provided to SSRN

Bertrand Aunay

affiliation not provided to SSRN

Pierre Agrinier

affiliation not provided to SSRN

Geneviève Lebeau

affiliation not provided to SSRN

Abstract

Geothermal exploration can be challenging in the absence of obvious surface markers of a high enthalpy geothermal resource such as geysers, fumaroles or thermal springs with temperatures exceeding 100°C. This is the case at the Piton des Neiges volcano (Réunion Island), which displays evidence of hydrothermal activity mostly as thermal springs with temperatures not exceeding 50°C, affected by mixing and reequilibration, and emerging in a complex geological framework, resulting from the polygenic history of this old dormant volcano. Yet, these thermal springs keep a partial record of the reservoir geochemistry. Here, we propose an exploration method to set this thermal signature appart from the contribution of other processes and trace back the equilibrium conditions, enabling us to locate thermal fluid circulations, and evaluate their temperature and availability. We report its application at Piton des Neiges volcano. We led an extensive geochemical survey which nearly doubled the number of thermal springs reported on the volcano, and dramatically extended the existing geochemical database in space, time and number of parameters. In tandem, we also monitored δ 18 O and δD values in rainwaters and thermal springs over two years to compare their temporal variations.Our surveys show that the repartition of thermal springs is controlled by only one of the volcanic rift zones (the N30°E axis), which is thus identified as the heat source of hydrothermal circulations. This N30°E rift zone is also a preferential pathway for the degassing of mantellic CO 2 . In addition, the composition of thermal waters is influenced by syenite intrusions along the rim of the caldera, acting as drains for the uprise of hydrothermal waters. O, H and Sr isotopic signatures of springs show that the geothermal resource of Piton des Neiges is in fact made of two independent and unconnected hydrothermal systems on the northern and southern circular depressions of the volcano (the cirques of Salazie and Cilaos, respectively). Their recharge is meteoric, plurennial, and mostly achieved by cyclonic events. Thermal waters experience reequilibration during their ascent and various degrees of CO 2 input or degassing. Last equilibrium temperatures of water/rock interaction, estimated by multicomponent geothermometry, reach up to 125 ± 6°C in the cirque of Salazie, and up to 141 ± 15°C in the cirque of Cilaos, with possibility for hotter waters before reequilibration. Taken together, our results suggest that the most promising target for future exploration is located in the cirque of Cilaos, along the caldera rim, and in proximity of syenite intrusions.Our study demonstrates that a four steps methodology that combines a thorough prospecting of thermal springs with a comprehensive geochemical study, including temporal variations of the different water reservoirs, is a powerful tool to characterize hidden geothermal resources and plan future exploration targets.

Keywords: geothermal, Exploration, geochemistry, Piton des Neiges, Volcano, Thermal springs

Suggested Citation

Bénard, Bhavani and Famin, Vincent and Sanjuan, Bernard and Vimeux, Françoise and Aunay, Bertrand and Agrinier, Pierre and Lebeau, Geneviève, An Integrated Geochemical Spatial and Temporal Survey of Low Temperature Thermal Springs to Characterize the Geothermal Resource of a Volcano (Piton Des Neiges, Réunion Island). Available at SSRN: https://ssrn.com/abstract=4054803 or http://dx.doi.org/10.2139/ssrn.4054803

Bhavani Bénard (Contact Author)

affiliation not provided to SSRN ( email )

No Address Available

Vincent Famin

affiliation not provided to SSRN ( email )

No Address Available

Bernard Sanjuan

affiliation not provided to SSRN ( email )

No Address Available

Françoise Vimeux

affiliation not provided to SSRN ( email )

No Address Available

Bertrand Aunay

affiliation not provided to SSRN ( email )

No Address Available

Pierre Agrinier

affiliation not provided to SSRN ( email )

No Address Available

Geneviève Lebeau

affiliation not provided to SSRN ( email )

No Address Available

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