Fumarolic alteration products at Sousaki (Greece): occurrence and environmental impact

Abstract

Sousaki (Corinthia, Greece), is a presently inactive volcanic area hosting a geothermal reservoir. Geothermal activity, still recognizable by a series of low temperature gas manifestation, is responsible of the widespread alteration of the outcropping rocks in the area. The main manifestations are hosted within caves whose walls are covered by alteration products in the form of crusts and efflorescences. This study presents the results of mineralogical and chemical analyses of the alteration products collected in the area. Leaching experiments with distilled water were also performed to get insights on the mobility of the elements incorporated in the alteration products. X-ray diffractometry allowed us to recognize a series of secondary minerals deriving from the alteration of the ophiolithic host rocks (altered peridotites to serpentinites) and whose composition depends mainly on the microenvironmental conditions in which they were formed. Elemental sulphur is the main mineral phase at the bottom of the caves where the atmosphere is anoxic. In the upper part of the caves, where oxygen is readily available, efflorescences are composed of many highly soluble acid sulphate minerals. In the oxidised part of the cave the stability of the mineral phases is mainly controlled by the relative humidity. The most hydrous mineral phases occur in the lower part of the cave, which is characterised by higher humidity values. Outside the caves highly soluble sulphates have been found in two samples collected in relatively sheltered position while a further sample collected outside the caves, but exposed to atmospheric agents, is composed almost exclusively by gypsum. Chemical analyses (ICP-MS after digestion with HNO3) revealed high contents of Al (up to 55,000 µg/g), Co (up to 655 µg/g), Cr (up to 7400 µg/g), Fe (up to 105,000 µg/g), Mg (up to 147,000 µg/g), Mn (up to 3700 µg/g) and Ni (up to 8800 µg/g) in the sample collected in the oxidised part of the caves. These strong enrichments confirm that the alteration products derive from the ophiolithic rocks. Leaching experiments evidenced the high mobility of these elements. Due to the extreme solubility of the mineral phases, on average between 70 and 94% of Al, Ca, Co, Cr, Cs, Cu, Fe, Li, Mg, Mn, Ni, Rb, Sr, U and Zn is in water soluble form. On the contrary, As, B, Ba, K, Mo, Na, Pb and V display lower solubilities (4 – 56%). Toxic elements’ mobility, favoured by the strongly acidic environment of the fumarolic area, may have severe environmental consequences.