GOLD-BISMUTH-TELLURIDE-SULFIDE ASSEMBLAGES AT THE STANOS SHEAR ZONE-RELATED PROSPECT, CHALKIDIKI, NORTHERN GREECE

Cu-Au mineralization in the Stanos area occurs in regional NE-SW trending shear zones within the crystalline Servomacedonian Massif on the Chalkidiki Peninsula, northern Greece. Orebodies are generally located along the contact between orthogneisses of Silurian age of the Vertiskos terrane and marbles and garnet-graphite schists of the Svoula series. These lithologies were intruded by the Triassic Arnea granitoid, which occurs about 3 km southwest of the sulfide mineralization. In this study we report new mineralogical data from three sites along the major mineralized shear zones that include the ancient Cu-Au mines of Paliomylos, Chalkoma, and Karambogia. The Stanos copper-gold mineralization is structurally-controlled and restricted to high-strain shear zones within gneisses that developed late during regional ductile shearing. This deformation event was related to southwestward overthrusting of the Vertiskos unit onto the Svoula lithologies at upper-greenschist to lower-amphibolite facies conditions. The mineralized shear zones are either planar or lensoid, with abrupt transitions to unaltered gneiss, and have a thickness of tens of centimeters across the few meters of lateral exposure. Their internal structure is heterogeneous with moderate- to high-strain bands where the foliation shows a sigmoidal trajectory appearing as S-C′ structures or discontinuity planes. Within the shear zones, the alteration assemblages typically include biotite and muscovite, quartz, ±chlorite, ±siderite and rarely apatite, monazite, xenotime, and zircon. Detailed textural studies of the ore assemblages revealed two stages of hydrothermal mineralization during shearing. Iron-bearing sulfides (pyrite, arsenopyrite and pyrrhotite) were introduced and followed by a copper-bearing association that included chalcopyrite with minor galena, sphalerite, molybdenite and Bi-Au-Te minerals. Ore minerals form disseminated to massive aggregates along foliation planes, asymmetric crenulation cleavages and S-C′ fabrics. In places, they surround or form the strain shadows of s-shaped quartz porphyroblasts indicating synkinematic deposition. The Au-Bi-Te association consists mainly of Bi sulfosalts (bismuthinite derivatives, lillianite homologues, matildite, ikunolite), native elements (bismuth, gold-silver alloy), and bismuth sulfotellurides (joseite-A, joseite-B and telluronevskite). Most of these minerals exist in two-, three-and more rarely four-component blebs or patches often with curvilinear boundaries, mostly disseminated within chalcopyrite. Phase relations in the system Au-Bi-Te suggest the following paragenetic sequence: bismuthinite derivatives + ikunolite → lillianite homologues + matildite + gold-silver alloy + bismuth sulfotellurides → native bismuth + galena → molybdenite + chalcopyrite. The bismuthinite derivatives are mainly bismuthinite (including cuprian members), gladite-krupkaite, hammarite and aikinite (Fig. 1a). Ikunolite is Se-free corresponding to the formula Bi4S3 and is reported here for the first time in Greece. Gold-silver alloy (up to 38.1 wt. % Ag) appears either as 100-micron grains disseminated in chalcopyrite or as rounded droplets coexisting with blebs of Bi-minerals. Matildite with almost ideal composition, lillianite homologues representing members of the lillianite-gustavite solid solutions series, higher order homologues (N=6, N=7), as well as vikingite are the major Ag carrier in the ores (Fig. 1b). The bismuth sulfotellurides joseite-A, joseite-B and telluronevskite (grains up to 10μm) of the joseite- and tsumoite isoseries, respectively, according to the definition of Cook et al. (2007), are closely associated with lillianite homologues (Fig. 2). Native bismuth in symplectitic intergrowths with galena (probably decomposition products of galenobismuthite), rims bismuthinite derivatives and lillianite homologues and probably represents the last Bi-bearing hydrothermal pulse of the system. Molybdenite is Re-free and typical of metamorphic molybdenites. The observed association suggests an evolution of the system towards more reducing conditions and that precious metals may have been scavenged by composite Bi-Te-Pb-S melts in a manner proposed by Ciobanu et al. (2005). The Stanos shear zone-related system shares many characteristics in common with orogenic gold systems; however its accurate classification is a subject of further investigation.