Abstract

Arsenic-copper-gold-bismuth mineralization in the Stanos area is hosted by a km-long, NW-SE trending ductile to brittle shear zone within Silurian orthogneisses of the Vertiskos terrane of the Servomacedonian Massif, Chalkidiki Peninsula, northern Greece. Shearing was accompanied by iron-potassic alteration of the gneisses including biotite, muscovite, chlorite, apatite, zircon, quartz, and minor rare-earth element phosphates. Metallic minerals form disseminated to massive aggregates along foliation planes, asymmetric crenulation cleavages, and S-C' fabrics. Detailed textural investigation of the ore assemblages revealed two stages of hydrothermal mineralization during shearing. An initial introduction of iron sulfdes (pyrite, arsenopyrite, and pyrrhotite) was followed by a copper-bearing stage that is associated with the formation of chalcopyrite, minor sulfdes (galena, sphalerite, and molybdenite), and Bi-Cu-Pb-Au-Ag-Te minerals. The stage II metallic mineral association consists mainly of Bi sulfosalts (bismuthinite derivatives, lillianite homologues, matildite), native elements (bismuth, electrum), and bismuth chalcogenides (joséite-A, joséite-B, ikunolite, and unnamed Bi-Pb-bearing sulfotellurides). The bismuthinite derivatives are mainly bismuthinite (including cuprian varieties), gladite-krupkaite, paarite, salzburgite, and an unnamed CuPbBi7S12 phase. The ikunolite is Se-free, corresponding to the formula Bi4S3, and is the first known occurrence in Greece. Most stage II minerals exist in two-, three-, and, more rarely, four-component blebs or patches generally with curvilinear boundaries, suggesting the possibility that they were precipitated in a molten form. Phase relationships among minerals in the system Au-Bi-Te-Pb-Ag-Cu indicate that they formed by successive hydrothermal pulses from fluids that penetrated the shear zone at different times contemporaneous with ductile deformation. These pulses deposited the following assemblages in paragenetic order: molybdenite + cosalite + native bismuth + galenobismutite → gustavite/lillianite + native gold + native bismuth + bismuth sulfotellurides → bismuthinite-aikinite solid solution series→ matildite + native bismuth + galena → chalcopyrite + bornite. The observed association suggests that the system evolved under fluctuating f(S2) and f(O2) conditions and precious metals may have been scavenged by composite Bi-Te-Pb-S melts at temperatures well above 350 ºC. The enrichment of Bi, Mo, Pb likely indicates a magmatic contribution to the ore-forming fluid.

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