Geothermal Exploration of La Soufriere Volcano, St. Vincent, West Indies

摘要

Geothermal exploration in the northern half of the island on St. Vincent, West Indies revealed a possible geothermal system centered in the vicinity of the La Soufriere volcano. Mapping has shown abundant pyroclastic and other volcanic deposits throughout the island, indicating a prolific volcanic history, one which has included two eruptions of La Soufriere within the past 40 years. Almost all eruptions are of the central vent type, providing a spatial constraint on the extent of a related geothermal system. Geothermal exploration included geologic mapping, remote sensing and geochemical sampling and analysis of surface manifestations of the geothermal and hydrological system. Three areas were selected for a detailed, controlled source audiomagnetotelluric survey of which two were surveyed. A regional study was completed to evaluate the electrical structure of the volcano. Limited surface manifestations of the potential geothermal system in the vicinity of La Soufriere included: fumaroles discharging from the center of the La Soufriere carter; warm waters discharging along the Wallibou River; and possible evidence of geothermal influx in other springs. Results of chemical and isotopic analysis of fumarolic steam and gas samples suggest that the fumarolic steam is locally recharged by rainwater boiling at approximately 170°C and is discharged along with an influx of geothermal gases from high-temperature sources above 250°C. Chemical analysis of warm spring waters that discharge as a series of high volume, warm springs occurring along the northern bank of Wallibou River suggest that these springs are a mixture between meteoric water and geothermal brine. The geothermal brine appears to be characterized as a neutral, hot (>150°C), and low TDS (<1000 mg/kg) with a magmatic influx. Results of the initial exploration effort suggest that a potential geothermal system underlies the La Sourfriere volcano consisting of a deeper geothermal brine, which flows up along fractures or other permeable zones on the southern flanks of the volcano, mixes with meteoric water and discharges along the Wallibou River. A small amount of shallow local boiling in the immediate vicinity of the volcanic vent discharges as fumaroles with higher temperature geothermal gases along with steam. The hot-water system appears to be laterally confined, possibly between the crater and the Wallibou River.