Extreme alteration in an acid-sulphate geothermal field: Sulphur Springs, Saint Lucia

Abstract

Sulphur Springs is a vigorous geothermal field associated with the Soufrière Volcanic Centre in southern Saint Lucia. Intensely altered rocks occur over an area of at least 200 × 400 m, together with bubbling hot pools and fumaroles. The pools are sodium‑calcium-sulphate type, with pHs of 3–7 and temperatures of 41–97 °C. Fumaroles have temperatures up to, and at times above 100 °C. Gases collected from both fumaroles and bubbling pools have high contents of CO2 (601–993 mmol/mol) and commonly high H2S (3–190 mmol/mol).Least-altered host rocks surrounding Sulphur Springs comprise calc-alkaline, feldspar-quartz-porphyritic dacites of near-uniform chemical composition. These rocks, which form massive domes and volcaniclastic units, were emplaced 13–30 ka ago (Lindsay et al., 2013). Within the geothermal field, the dacites have been moderately to highly altered to acid-sulphate assemblages of kaolinite, illite, smectite, alunite, natroalunite, cristobalite, opaline silica, native sulphur, jarosite and amorphous compounds. Muddy sediments from grey to blackish thermal pools additionally contain iron sulphides.Despite intense alteration of the volcanic rocks (and derived volcaniclastic sediments), Zr and Ti have remained essentially immobile, which allows identification of the precursors (dacites) and calculation of mass changes. Many of the altered rocks and sediments show major depletions of Fe, Mg, Ca, Na. Extremely altered samples additionally show notable losses of Al, P, K, REE and Y, implying leaching by highly acidic waters. A few of the highly altered rocks, however, have gained Al, together with P, presumably due to neutralization or evaporation of acidic thermal waters. Also present within the geothermal field are metre-scale zones with very high contents of silica and native sulphur, and <1% Al2O3. Based on mass change results, the silica in the silica-sulphur zones, although recrystallized, was mainly inherited from the precursor dacites, rather than added as new hydrothermal precipitates.Surface thermal waters define a linear trend in a plot of δ18O vs. δD, and reach high levels of isotopic enrichment. The enrichment trend is ascribed to progressive evaporation in the thermal pools. The low slope of the isotopic trend (1.4) is interpreted as being due to surficial mixing between meteoric waters and steam condensates derived at about 230 °C from the deep brine that underlies the geothermal field. The whole-rock O-H-isotopic composition of four highly altered samples (kaolinite-smectite-opal-sulphates) can be accounted for if the alteration fluid was a 50:50 mixture of steam and meteoric water, and alteration occurred at 50–125 °C. Whole-rock δ34S values of samples containing variable amounts of sulphates, sulphides and native S are all notably negative (−5 to −16‰), consistent with derivation of sulphur from volcanic gases such as SO2 and H2S. Altered rocks and thermal pool sediments in the geothermal field are commonly enriched in Hg and Se (up to 50 mg/kg).At Sulphur Springs, primary dacitic rocks have been chemically and mineralogically transformed by reaction with acidic waters that formed when uprising volcanic gases condensed into neutral meteoric waters. In many cases, this has led to substantial mass losses or gains in Al, P and REEs. Regardless of the degree of alteration, however, the precursor rocks can still be identified using Ti-Zr relations.