In this study the main focus was in Katsimouti in Kondaros and

In this study, the main focus was in Katsimouti, in Kondaros and mainly in Profitis Ilias areas (Figure 10). In Katsimouti area there are two crosscutting mineralized striking directions (NE-SW and NW-SE, i.e. a stockwork texture), which is the main feature of the area. From NW to SE, along the shore, there are several small faults and fissures crosscutting the underlying dacites (Dado) subvolcanics and the overlying tuffs, which are filled mainly with quartz-baryte-Mn veins and veinlets. The NE-SW trending mineralized veins consist of base metal sulfides with chalcedony-baryte-Mn and/or Mn-baryte veins and are developed along, and in between, Katsimoutis and Vani zone.

The dominant alteration style is that of adularia-sericite (Melidonis, 1963; Liakopoulos, 1987).

The latter NW-SE mineralized fault/lineament system (the western bounding fault of the Milos gulf (graben), consists of sheeted and banded veins and stockworks. The characteristic alteration style of the subvolcanic domes is propylitic to argillic and hosts the above-mentioned NW-SE trending mineralization, which is an epithermal Ag-bearing intermediate sulphidation (IS) type.

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It shows features like cockade structures, colloform banding and consists of carbonate, adularia and chalcedony gangue. At the SE end of Katsimoutis bay, quartz-baryte-Mn vein lets in a stockwork configuration have been developed (Melidonis, 1963; Liakopoulos, 1987).?

In Kondaros area (Figure 17a), the main geological units are andesite/dacite flow dome complexes (Anda), which are being overlained by submarine lower pumiceous lapilli tuffs (Lmp), and locally dacitic flow domes (Dado) (Figure 17b). Structures of probable monogenic volcanic activities (necks and coarse pyroclastic breccias) and vent like breccia structures, which are post andesite/dacite dome emplacement (Figure 17c).

The base-precious metal mineralization is hosted within a propylitic and intermediate argillic altered dacitic flow dome (Anda), which constitute the lowest outcropping trending formation (Alfieris, 2006)?.

At the upper parts of Profitis Ilias mountain, various flow mineralization (quite similar to Katsimoutis), containing base metal sulfides and Ag-sulphosalts along the western lineament (NW-SE) of Milos gulf, and most probably another precious metal rich vein style mineralization at depths below the sinters, are the main features of the area. In the Profitis Ilias area, ignimbrite (PIi) dominates the surface geologic picture and overlies fine tuffs (Fat), pumice flow tuffs (Pfl) and the older lithic/ pumice tuff (Lpt), while at the dykes (Rhyda) are deformed and folded during flow, which implies that the cooling rates were low (i.e. subaerial environment) and the material had a sufficiently ductile behaviour (Figure 10) (Alfieris, 2006)?.

Mineralization at Profitis Ilias comprises mainly pyrite, chalcopyrite, galena, Fe-poor sphalerite, native gold and electrum. Surface hand specimens indicated deposition of base metal sulfides (galena and sphalerite) (Figure 17d,e). Galena includes chalcopyrite, tetrahedrite and sphalerite (Figure 18a) (Constandinidou et al., 1998; Dimou, 2001; Kilias et al., 2001; Alfieris, 2006)?.

The mineralization is characterized by an early deposition of pyrite followed by an assemblage composed of the tellurides hessite, altaite and petzite and by native gold, chalcocite, galena, chalcopyrite and finally by sphalerite. A second mineralizing event, which introduced copper sulfides and second-generation galena overprints both the tellurides and the earlier sulfides. The tellurides are included either in galena, chalcopyrite and sphalerite, or they occur at the grain boundaries between sphalerite and galena (Figure 18d). In a broader sense, the telluride assemblage is separated from galena by the synchronous growth of chalcocite.

The following contact assemblages of gold-, silver- and tellurium-bearing phases have been observed: hessite-petzite, altaite-petzite, altaite-hessite, native gold-petzite and native gold-hessite. Hessite mainly occurs as composite blebs within galena, within sphalerite and chalcopyrite (Figure 18a-c). Hessite also occurs along grain boundaries between early pyrite and galena. Most hessites contain exsolved petzite (Figure 18a-d), and/or Au, altaite. Native gold occurs as filaments in blebs of hessite, as isolated inclusions in chalcopyrite, or at grain boundaries between hessite and chalcopyrite (Figure 18c). Petzite is present as minute grains within blebs of hessite, or as coarser inclusions surrounded by hessite (Figure 18b). The round shape of some petzite inclusions, suggests co-crystallization of hessite-petzite. Petzite occurs also as skeletal flakes in hessite or as inclusions with a square cross-section. Altaite occurs in the form of exsolved grains in hessite, where it is closely associated with petzite (Figure 18d) (Dimou, 2001; Alfieris, 2006).

Galena is a major constituent of Profitis Ilias mineralization. It is present in two generations. Early galena form subhedral to anhedral grains, which accompany chalcopyrite, sphalerite and the precious metals in the ore. Late galena occurs in veinlets and vugs, crosscutting earlier sulfides and is closely associated with the late copper sulfides. Chalcopyrite similarly to galena was introduced from both early and late stage fluids. Early coarse-grained chalcopyrite accompanies sphalerite and is precious metal carrier in the ore. Late chalcopyrite occurs either in veinlets, closely related with the second-generation galena, or replaces late bornite (Alfieris, 2006).?

Sphalerite of both first and second generation is also present in Profitis Ilias ore. Early sphalerite is a major carrier of precious metal tellurides, whereas late-stage sphalerite forms small subhedral grains associated with copper sulfides. Pyrite was also introduced during early and late hydrothermal events in the mineralization. It occurs either as coarse-grained euhedral crystals or as small subrounded grains accompanying galena and chalcopyrite. The copper sulfides bornite, covellite, digenite and chalcocite replace chalcopyrite and sphalerite (Alfieris, 2006).?

The formation of Profitis Ilias (at least the lower telluride-bearing parts), most likely, is a typical sub-seafloor precipitation of sulfides forming stockwork zones. Later low-temperature circulation of seawater and a renewed hydrothermal pulse led to the formation of second-generation phases, including copper sulfides, galena and sphalerite. Atacamite and native copper in the upper parts of Profitis Ilias may be the result of direct oxidation of the sulfides by seawater (Alfieris, 2006).