The Tertiary volcano-sedimentary sequence of the Feres basin (Thrace, NE Greece), includes lavas of andesitic–rhyolitic composition as well as volcaniclastic rocks, pyroclastic flows and pyroclastic fall deposits principally of dacitic–rhyodacitic composition. The pyroclastic flow deposits frequently show intense devitrification, vapour-phase crystallization and evidence of fumarolic activity, which involves deposition of scapolite in pore spaces. The Feres basin can be subdivided on the basis of mineral alteration assemblages: (1) the Pefka region; characterized by intense hydrothermal alteration of the volcanic rocks and mineral zoning (silicic, argillic, sericitic and propylitic zones) with polymetallic mineralization, and (2) the remainder of the basin; where the volcaniclastic rocks are characterized by the alteration of volcanic glass to zeolites (clinoptilolite, heulandite, mordenite, analcime), clay minerals (smectite, illite, celadonite, chlorite), SiO2 polymorphs (cristobalite, opal-CT, quartz), K-feldspar and calcite. Laumontite is also present as an alteration product of plagioclase, with stilbite sporadically occurring in veinlets. Locally, rhyolites are also altered to zeolites (clinoptilolite and/or mordenite). The zeolitization process has occurred rapidly with the depositional environment, temperature, rate of cooling (of the volcanic rocks), nature and temperature of the mineral-forming fluids and composition of the parent material controlling the formation of zeolites.

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