HYDROGEOCHEMISTRY OF THE SULPHUR SPRINGS REGION, ST. LUCIA, LESSER ANTILLES

The Caribbean island of St. Lucia is located in the Lesser Antilles, an island arc formed by the subduction of the Atlantic Plate beneath the Caribbean Plate. The Sulphur Springs Park (managed by the Soufriere Foundation) is located in the Southwest region of the island near the city of Soufrierre. This region is characterized by the most recent volcanic activity on the island, dated from 34,000 to 21,000 BP by ¹⁴C. This recent period of volcanism is represented by K-rich and highly silica-oversaturated magmas ranging in composition from andesitic to dacitic. The Sulphur Springs reside within an extinct caldera and, due to high surface temperatures (up to 93°C) and water-rock interactions, represent an extreme environment for investigation of geochemical and microbial processes. This study focuses on elemental and isotopic exchange between the rapidly altering sediments and bedrock and the caldera waters.

Water was sampled from caldera pools, caldera streams, fumorales, and sulfur/fresh water stream mixing zones. Surface temperatures and pH measurements were taken on site. Samples ranged in temperature from 33 to 88 degrees C and from approximately pH 3 to 7. Sediments were also collected from sites that allowed direct access. Petrographic examination of several thin sections revealed extreme hydrothermal alteration of primary minerals. SEM analysis showed that feldspars had been almost completely replaced by silica. This evidence is also supported by high silica concentration in the cauldron waters, ranging from 50 to 60 ppm. These waters were also enriched in Ca and SO₄ relative to the incoming streams and yielded extremely high boron concentrations with one sample containing >2,000 ppm B. While boron enrichment in volcanic springs is a common occurrence, these values are among the highest recorded, due to both source and evaporative concentration. This evaporative concentration is clearly demonstrated by the enrichment in d ¹⁸0 and d D in the waters relative to the meteoric water line. Further analysis will include the d ¹¹B composition of the fluids that will help to attribute the high B concentration to one or more of the following sources: magma, marine sediments, oceanic crust and recycled seawater.