New Mineralogical Data and Geochemical Constraints on the Stypsi-Megala Therma Porphyry-Epithermal Mineralization, Lesvos Island, Greece

Lesvos Island comprises Lower Miocene, post-collisional volcanic to subvolcanic rocks with shoshonitic affinities overlying a metamorphic basement. The Stypsi porphyry Cu-Mo-Au prospect is genetically related to microgranite porphyry, which has intruded into trachyandesitic lavas of the Stypsi caldera. It comprises a stockwork of porphyry-style, black banded quartz- and quartz-actinolite veinlets associated with Na-Ca alteration, and a sericite-carbonate overprint related to IS epithermal quartz-carbonate veins. At higher topographic levels, a lithocap also hosts porphyry-style veinlets. Molybdenite, magnetite, pyrite, chalcopyrite, sphalerite, galena, bismuthinite and fahlores, are the metallic minerals of the porphyry mineralization. EPMA data indicate extremely high Re content (up to 1.92 wt.%) in molybdenite, and low to moderate Fe contents (from 0.26 to 4.74 wt.%) in sphalerite. Fluid inclusion studies demonstrate coexistence of liquidand vapor-rich inclusions in association with NaCl-bearing inclusions in the veinlets, suggesting that boiling occurred during the mineralization. Homogenization temperatures range between 344° and 510° C and salinities reach up to 44% eq. NaCl. Bulk ore analyses revealed up to 843 ppm Cu, up to 76 ppm Mo, up to 0.5 ppm Au, up to 339 ppm Pb, up to 815 ppm Zn, up to 10 ppm Se, up to 4 ppm Te and Bi, and up to 23 ppm Sn. Quartz recrystallization from a silica gel is evidenced by botryoidal textures within banded quartz veinlets, a feature already described from porphyry-Au deposits. The late IS epithermal overprint, in the form of milky quartz-carbonate veins is controlled by NNE-trending faults, and extends further northwards to Megala Therma epithermal deposit, where fluorite in the veins and Au-Ag-Te enrichment occur.