Geochemical mapping of magmatic gas-water-rock interactions in the aquiferof Mount Etna volcano

Systematic analysis of major and minor elements in groundwaters from spring s and wells on the slopes of Mt. Etna in 1995-1998 provides a detailed geoc hemical mapping of the aquifer of the volcano and of the interactions betwe en magmatic gas, water bodies and their host rocks. Strong spatial correlat ions between the largest anomalies in pCO(2) (pH and alkalinity) K, Rb, Mg, Ca and Sr suggest a dominating control by magmatic gas (CO2) and consequen t basalt leaching by acidified waters of the shallow (meteoric) Etnean aqui fer. 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 spati al distribution of T degreesC, 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 (CO2, H2S, CH4) and low K/Na ratio, which is likely hosted in the sedimentary basement. This hot brine reaches the surface only at th e periphery of the volcano near the Village of Paterno,where it gives rise to mud volcanoes called "Salinelle di Paterno". However, the contribution o f similar brines to shallower groundwaters is also detected in other sector s to the W (Bronte, Maletto), SW (Adrano) and SE (Acireale), suggesting its possible widespread occurrence beneath Etna. This thermal brine is also cl osely associated with hydrocarbon fields all around the volcano and its ris e, generally masked by the high outflow of the shallow aquifer, may be driv en by the ascent of mixed sedimentary -magmatic gases through the main faul ts cutting the sedimentary basement. (C) 2001 Elsevier Science B.V. All rig hts reserved.