Influence of wall-rock alteration and fluid mixing mechanisms in the chemistry of thermal fluids and mud-pool sediments at Caldeiras da Ribeira Grande (S. Miguel Island, Azores)

The Caldeiras da Ribeira Grande site is placed at the north-eastern side of the Ribeira Grande geothermal system (S. Miguel Island) and includes a fumarolic field with multiple discharge points, closely associated with a mud pool. The major discharge is artificially confined to a tank, also fed by a cold (≈19 °C) spring water, allowing the accumulation of diluted thermal fluid and mud. In the open fumarole site placed to the NE of the tank another persistent thermal fluid discharge can be observed, whose temperature (70 °C), pH (≈2–3) and electrical conductivity (185.4–3,300 μS/cm) values, as well as major element contents, are within ranges bracketed by measurements performed at the same site over time. Despite of differences imposed by inhomogeneous dilution processes within tank, the compositional fingerprints of both thermal fluids are similar, pointing to a common origin. Variations in Br, Mo, W, Nb, Sn and Sb contents are minimal, but the thermal fluid discharging at the open fumarole site reveals concentrations above 100 μg/l in Zn, Ti, Mn, Rb, Y, Ce and Nd and up to 11 μg/l in As, Hg and Pb. The chondrite-normalised REE patterns are equivalent to those usually reported for acid, SO₄ ²⁻-bearing geothermal fluids and the available data show that the fluid chemistry strongly depends on the alteration experienced by the hosting volcanic rocks. This alteration led to relative [Hg, S, Ag, Cu, ±Ge] enrichments, but a significant part of the [K, Na, Ca, Mn, Mg, Zn, Co, ±W ± REE] original rock contents was carried out by the modified fluid. Kaolinite and alunite(-jarosite), besides fine-grained relics of K-feldspar and quartz, are the fundamental components of the mud forming the boiling pool or the mounts accumulated in tank. This mineralogical similarity is consistent with bulk chemical compositions and differences in concentration rarely exceed 1.5–2 times for the large majority of the elements analysed. Some compositional features displayed by thermal fluids and coexisting muds are sensitive to pH and Eh variations, reflecting also changes in fluid/rock reaction paths due to variations in acid steam production/composition and relative proportion of steam subjected to condensation. These chemical features can be used as proxies to monitor the Ribeira Grande geothermal system, provided that adequate time series data exist for pertinent parameters in representative sampling sites.