A new terrestrial active mineralizing hydrothermal system associated with ore-bearing travertines in Greece (northern Euboea Island and Sperchios area)

The northwestern Euboea Island and the neighboring part of the mainland in eastern Central Greece, i.e. Sperchios area, contain several hot springs and active thermogenic travertine deposits, which are the surface manifestations of an active hydrothermal system, controlled by active tectonics, and supplied with heat by a 7–8 km deep magma chamber, with surface manifestation, the Plio-Pleistocene trachyandesitic volcanic center of Lichades. This hydrothermal system is fueled by a mixture of seawater and deep magmatic fluid with only limited meteoric water contribution. Thermal water samples show extreme pH, temperature and Electrical Conductivity values, with maximum values always recorded in two locations (Aedipsos and Ilia). The observed similarities in fluid compositions for most of the analyzed anions, major and trace elements, suggesting a stronger hydrothermal signature for the Northern Euboea area, perhaps reflecting greater proximity to the heat source of the hydrothermal system. The hydrothermal fluids collected and analysed were found to be highly enriched in a number of metallic and non-metallic elements e.g. up to 100 μg/L As, up to 1.1 wt.% Fe, up to 340 μg/L Ba, up to 65 μg/L Cu, up to 2.1 wt.% Cl, up to 3700 mg/L SO42 −, up to 390 μg/L Se. Some of the enrichments are reflected directly in the travertines lithogeochemistry and the metallic mineral phases found inside the travertines. A number of mineral phases including sulfides (such as pyrite, arsenopyrite, galena, chalcopyrite, sphalerite and stibnite), native elements (such as Pb and Ni), alloys (such as Au ± Cu-Ag) fluorite and REE-bearing phases (such as Ce-, Nd- and La-bearing members of hydroxylbastnäsite, cackinsite, lanthanite and sahamalite) were identified syngenetically enclosed as clastic grains within the pores of all studied travertines; the ore grade concentrations of some iron-rich travertines (up to 28.9 wt.% Fe and up to 1.83 wt.% As), as well as the high concentration of precious and base metals at the hydrothermal fluid, strongly suggest active mineralizing processes throughout the studied system. Travertines containing elevated Fe ± As and consisting of ferrihydrite in addition to aragonite/calcite, were deposited on the surface, most likely after mixing of ascending reducing hydrothermal fluids with cool seawater. The high REE content in the Fe ± As-rich travertines (up to ~ 465 mg/kg ΣREE) is caused by adsorption of REE-bearing phases by iron oxyhydroxides. Mineralogical and geochemical evidence (such as the presence of elements in their native form, of alloys such as Au ± Cu-Ag, the enrichment of metalloids and the abundance of REE) may indicate magmatic contribution to the hydrothermal system and accordingly to the studied travertines. We support the hypothesis that metals and metalloids were mainly derived from magmatic fluids, which successively mixed with heated-, and with oxygenated, cool seawaters at depth and on the surface respectively, resulting in the deposition of carbonate-hosted sulfide mineralization at depth, and of Fe ± As-rich travertines at the surface. The northwestern Euboea Island and Sperchios area hydrothermal system represents the first documented active terrestrial mineralizing hydrothermal system associated with ore-bearing travertines in Greece.