Gold scavenged by bismuth melts: An example from Alpine shear-remobilizates in the Highis Massif, Romania

Gold mineralization occurs in the Soimus Ilii vein, the main Cu prospect in the Highis Massif, Western Apuseni Mts., Romania. The Highis Massif is part of the Highis Biharia Shear Zone, a 320 - 300Ma Variscan greenschist belt, with a 114 - 100Ma Alpine overprint. In Highis, phyllonites enclose an igneous core consisting of an Early Permian basic complex intruded by Middle Permian granitoids. The vein is hosted within basalt hornfels at its contact with the 264Ma Jernova granite. Gold is not only present as native gold, but also as jonassonite ( ideally AuBi5S4). The latter occurs as inclusions 1 - 30 mm in size in chalcopyrite; microanalysis gives the empirical formulae Au-1.02(Pb0.47Bi4.51)(4.98)S-4. The two Au minerals are spatially associated with Bi -(Pb) sulfosalts ( oversubstituted bismuthinite, cosalite) and sulfotellurides/selenides ( ingodite, ikunolite and laitakarite) in blebs/patches, mainly hosted in chalcopyrite. This Au - Bi - Te association overprints an earlier, chalcopyrite-quartz assemblage, occurring as trails along discrete zones of brecciation that crosscut former mineral boundaries. Curvilinear and cuspate boundary textures within the blebs/patches suggest deposition in a molten form. Mineral associations in combination with phase relations indicate that the Au - Bi - Te association formed as a result of melting of pre-existing native Bi ( and possibly sulfosalts) at 400 degrees C under sulfidation conditions. These melts incorporated Au, Pb, Te and S as they moved in the vein during shearing and were locked within dilational sites. Native Bi occurs as coarse aggregates along vein margins, but in the Au - Bi - Te association, it is present only as small droplets in shear gashes, never together with other Bi- and Au-minerals. The Bi- derived melts are part of an internal remobilizate which also includes chlorite and adularia. Minerals in the system Au - Bi - Te were deposited from a neutral low reducing fluid during Alpine shearing in the Early Cretaceous. The fluid also assisted solid-state mobilisation of chalcopyrite and cobaltite. This study illustrates the significant potential of Bi, a low melting-point chalcophile element (LMCE), to act as Au scavenger at temperatures as low as 400 degrees C.