Periferakis, in a Master's Thesis for the National and Kapodistrian University of Athens, Greece, reported in 2019 that Lesvos Island comprises Lower Miocene, post collisional volcanic to subvolcanic rocks withshoshonitic affinities overlying a metamorphic basement. The Stypsi porphyry-Cu-Mo-Au prospect is genetically related to a 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 asericite-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 ofthe porphyry mineralization. EPMA data indicate extremely high Re contents (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 liquid- and 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 the Megala Therma epithermal deposit, where fluorite in the veins and Au-Ag-Te enrichment occur.
The author concluded that the Stypsi porphyry-epithermal deposit is a telescoped porphyry-epithermal IS system which shares similarities with other deposits of the Aegen Area and of Anatolia . There is significant enrichment in Au, Ag, Te, Sn, Zn and Pb. Zn and Pb are unrelated to the epithermal overprint. It is believed that the Stypsi porphyry and epithermal system formed inside the collapsed Stypsi caldera, in accordance with the proposed method of formation for such deposits. The Cu:Mo ratios of the system vary, between 11 and 60, hindering the system's classification.
The area of Stypsi is characterized by intense hydrothermal alteration, ranging from phyllic, to propylitic to silicic and advanced argillic. In the center of the system, Na-Ca alteration has overprinted the host rocks and is associated with black banded quartz veinlets. The molybdenite + pyrite veins, termed D veins, are associated with phyllic alteration and postdate the black banded quartz veins. The porphyry system at Stypsi is associated with a microgranite porphyry, which has intruded the trachy-andesitic Lower Lavas of the Skoutaros Unit. The area of Stypsi is, in reality, the caldera of a Miocene stratovolcano. The NNE trending fault zone, with a total length of about 5 Km, from Stypsi to Megala Therma, is associated with magma emplacement and the subsequent circulation of hydrothermal fluids. The repetitive reactivation of this fault resulted in the strong telescoping of the Stypsi porphyry-epithermal system.
The whole system is not of economic importance, insofar as the bulk ore contents of D-type and E-type veins are concerned. However, late stage molybdenite in the porphyry D veins is Re-rich in comparison to early stage molybdenite in the banded quartz veins. This feature is in accordance with the late stage molybdenites of Pagoni Rachi, Greece. According to relevant data the early stage magmatic molybdenite is Re poor and the late stage epithermal molybdenite is Re rich, probably because of sediment contamination of hydrothermal fluids. It must be born in mind that the spider plot patterns of altered samples exhibit indications of upper crustal rocks' contamination. One possible interpretation of Re enrichment is the interaction of hydrothermal fluids with the ophiolites of Lesvos. This assertion however raises some spatial and temporal constraints, regarding the path followed by the ascending hydrothermal fluids, given that the bulk of ophiolites is found some 15 Km to the SE of the porphyry-epithermal system at Stypsi. Xiong & Wood (2002) concluded that Re-enrichment in associated with oxidized mineralizing fluids, so this must be the case at Stypsi.
Although there is a slight Te enrichment at Stypsi (up to a maximum of 3.46 ppm) there are no identifiable Te minerals. The fluid inclusion analyses point to fluids boiling at a shallow level, a common fact of subvolcanic magmatic reservoirs according to the data presented by Voudouris et al. (2019). According to Henley & McNabb (1978) and Hedenquist et al. (1998) such a procedure is responsible for host rock fracturing, thereby increasing the volume of hydrothermal fluids able to permeate the rock. Thus are new veins created and older veins mineralised.
Additional fluid inclusion data indicate the rapid decompression of the whole system at moderately high temperatures (between 440 ºC and 520 ºC). Based on the assertions of Monecke et al. (2018) this is probably the impetus for the creation of the banded quartz veins. Cu and Mo can be precipitated by the condensation of brines, a fact already aknowledged by Seo et al. (2012). At Stypsi, the hypersaline fluid inclusions contain chalcopyrite which confirms the assumption of transportation of Cu through the brines (Voudouris et al., 2019). It can therefore be presumed that the Cu-Mo mineralizing event was associated with boiling events.
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