Assessment of soil boron, ammonia and elemental mercury for geothermal exploration in the Canary Islands

While the sampling of waters and gases from natural thermal discharges remains the standard approach for geochemical characterization, its application is limited in areas where surface geothermal manifestations are scarce or the reservoir's boundaries are poorly defined. In such cases, soil geochemistry and soil-gas surveys emerge as useful tools for delineating the hidden extent of hydrothermal systems. This type of surveys relies heavily on the identification of surface geochemical anomalies that reflect the presence and characteristics of underlying hydrothermal systems. Among the various chemical species utilized in soil geochemistry, Boron (B), Ammonium (NH₄⁺) and Mercury (Hg) are regarded as "pathfinder" elements of primary importance due to their high volatility and mobility in geothermal fluids. Integrating the analysis of these three species allows exploration teams to differentiate between deep-seated geothermal signals and shallow environmental noise.

This study presents the results of soil B, NH₄⁺ and Hg surveys conducted across two specific areas at Tenerife: (1) Madre del Agua (0.7 km²) located within the Tenerife South Rift Zone (TFSRZ) and (2) Abeque (0.8 km²) situated within the Tenerife Northwest Rift Zone (TFNWRZ). A high-density sampling strategy was implemented, with a spatial resolution of 450–550 sites/km², to ensure the detection of small-scale anomalies. The investigation aims to characterize the spatial distribution of these key pathfinder elements to delineate potential upflow zones and identify the structural controls governing the underlying hydrothermal activity. Statistical-graphical analysis was performed to identify distinct geochemical populations within the dataset. To assess the spatial distribution of soil B, NH₄⁺ and Hg concentrations, sequential Gaussian simulations (sGs) were implemented.

Statistical analysis identified two distinct geochemical populations (background and peak) within both study areas. The geometric means for the background populations were estimated as: (1) Madre del Agua (176.4 μg/kg for B, 61.9 mg/kg for NH₄⁺, and 25.4 μg/kg for Hg) and (2) Abeque (109.0 μg/kg for B, 36.5 mg/kg for NH₄⁺ and 9.7 μg/kg for Hg). In both areas, the background levels of B and NH₄⁺ for Madre del Agua are relatively higher than those observed in Abeque, suggesting a different baseline for hydrothermal influence or lithological variations between the TFSRZ and the TFNWRZ. Furthermore, the geometric means values for the peak populations were estimated as: (1) Madre del Agua (723.2 μg/kg for B, 229.9 mg/kg for NH₄⁺, and 122.5 μg/kg for Hg) and (2) Abeque (1,358.1 μg/kg for B, 182.4 mg/kg for NH₄⁺, and 105.5 μg/kg for Hg). These relatively high geometric mean peak values, particularly for B at Abeque, indicate localized zones of enhanced permeability and volatile transport, reinforcing the potential for active hydrothermal upflow in this study area, where the highest absolute soil concentrations of B (1,798.0 μg/kg) and Hg (1,474.9 μg/kg) were also detected. Conversely, soil NH₄⁺ surveys showed relatively higher geometric mean peak values at Madre del Agua. This enrichment may suggest distinct boiling conditions within the hydrothermal reservoir or, alternatively, a more pronounced interaction with organic-rich soil horizons during geothermal steam ascent.