Geochemical mapping of magmatic gas–water–rock interactions in the aquifer of Mount Etna volcano

Abstract Systematic analysis of major and minor elements in groundwaters from springs and wells on the slopes of Mt. Etna in 1995–1998 provides a detailed geochemical mapping of the aquifer of the volcano and of the interactions between magmatic gas, water bodies and their host rocks. Strong spatial correlations between the largest anomalies in pCO 2 (pH and alkalinity) K, Rb, Mg, Ca and Sr suggest a dominating control by magmatic gas (CO 2 ) and consequent basalt leaching by acidified waters of the shallow (meteoric) Etnean aquifer. Most groundwaters displaying this magmatic-type interaction discharge within active faulted zones on the S–SW and E lower flanks of the volcanic pile, but also in a newly recognised area on the northern flank, possibly tracking a main N–S volcano-tectonic structure. In the same time, the spatial distribution of T °C, TDS, Na, Li, Cl and B allows us to identify the existence of a deeper thermal brine with high salinity, high content of B, Cl and gases (CO 2 , H 2 S, CH 4 ) and low K/Na ratio, which is likely hosted in the sedimentary basement. This hot brine reaches the surface only at the periphery of the volcano near the Village of Paterno, where it gives rise to mud volcanoes called “Salinelle di Paterno”. However, the contribution of similar brines to shallower groundwaters is also detected in other sectors to the W (Bronte, Maletto), SW (Adrano) and SE (Acireale), suggesting its possible widespread occurrence beneath Etna. This thermal brine is also closely associated with hydrocarbon fields all around the volcano and its rise, generally masked by the high outflow of the shallow aquifer, may be driven by the ascent of mixed sedimentary–magmatic gases through the main faults cutting the sedimentary basement.