The chemistry and hydrology of thermal springs on efate, vanuatu, sw pacific

Abstract

Two zones of geothermal spring activity on the island of Efate are described. The hydrothermal systems are associated with a deep-seated graben through Pleistocene volcanics and fringing reef limestones. The Teuma group of springs have temperatures up to 59°C and discharge around 1 MW of heat, although total discharge into rivers is estimated from conductivity changes to be 10 MW. The hydrochemistry and stable isotope hydrology of these springs indicate that there are two thermal components: residual sea-water with estimated base temperature no more than 114°C and a less saline thermal water which has a much smaller surface discharge but is indicated by cation and SiO 2, geothermometry to have a base temperature up to 218°C. The Takara group of springs discharge in the coastal reef limestone where the meteoric water table intersects the ground surface; the proximity of this thermal water table also gives rise to an expanse of “hot ground” inland of the spring line. The excess heat flow from this system is around 20 MW, and varies according to the seasonality of fresh-water recharge and flux. A heat source at depth to the landward side of the thermal area is indicated. The hydrochemistry of the Takara system shows that it comprises sea-water, possibly mixed with meteoric water prior to circulation in the thermal reservoir. Mg 2+ and SO 2− 4 are strongly depleted in the thermal water relative to unreacted sea-water; however there is no detectable δ 18O shift which suggests a high water:rock ratio in the thermal reservoir. Quartz geothermometry indicates a base temperature up to 174°C which is similar to that indicated by cation geothermometry although the unusual absence of K + enrichment casts some doubt on use of the Na—K—Ca method. The interpretation emphasizes the need to examine and understand alteration mineralogy in order to place more confidence in geothermometry and to obtain more information on evolution of the geothermal systems. No drilling has yet taken place and much of the reservoir geology and description is speculation. The Teuma system probably comprises circulation in fractured basalt dykes associated with graben activity, and flow may therefore be very limited at depth. The Takara system has some similarities with other marine systems, mostly at oceanridge locations, where sea-water circulates convectively in basalts with a regionally-high geothermal gradient.