20     • BULGARIAN ACADEMY OF SCIENCES GEOCHMISTRY, MINERALOGY AND PETROLOGY • 43 • SOFIA • 2005 Au-Ag-Te-Se deposits IGCP Project 486, 2005 Field Workshop, Kiten, Bulgaria, 14-19 September 2005 Au-Ag-Te-Se mineralization in the Potashnya gold deposit, Kocherov tectonic zone, Ukrainian Shield Sergiy Bondarenko 1 , Oleksandr Grinchenko 2 , V. Semka 1 1 Institute of Geochemistry, Mineralogy and Ore Formation, NAS of Ukraine; 2 Geological Department, Kiev National Taras Shevchenko University, Kiev Ukraine; E-mail: alexgrin@univ.kiev.ua Abstract. Bismuth tellurides are widely abundant in most Precambrian orogenic gold deposits of the Ukrainian Shield. The recently discovered Potashnya gold deposit of the Volyn’ Megablock, however, is conspicuous because it is typified by a Au-Ag-Te-Se type of mineralization and the practical absence of Bi tellurides. This unusual mineralogical signature gives the deposit additional interest. Telluride formation occurs as the result of an unusual ‘telluric metasomatism’, not observed anywhere else in the Ukrainian Shield deposits. Phases including weissite (Cu 2-x Te), hessite (Ag 2 Te), altaite (PbTe), frohbergite (FeTe 2 ), melonite (NiTe 2 ), tellurobismuthite (Bi 2 Te 3 ) and bohdanowiczite (AgBiSe 2 ) and unnamed Au 3 TlTe 2 have been found. Key words: Au-Ag-Te-Se mineralization, Bi-tellurides, Ukrainian shield, Potashnya gold deposit Introduction Most Precambrian orogenic gold deposits of the Ukrainian Shield show the persistent presence of bismuth tellurides (Bondarenko et. al., 2004). Hedleyite is predominant, with lesser amounts of pilsenite. As a result, the first identification, in the Precambrian Potashnya deposit, of a Au-Ag-Te-Se type of minera- lization, in which Bi-tellurides are virtually absent has attracted not only mineralogical interest to this deposit, but also raised practical interest in connection with the estimation of possible economic prospects in the area, and in particular, the possible presence of richer ore bodies at depth. Geological setting The Potashnya deposit is situated in the western part of the sub-meridional Kocherov regional tectonic zone, which strikes almost parallel to the geographical meridian and which is spatially confined to the southeastern flank of Korosten rapakivi granite pluton (Fig. 1). Metamorphic rocks of Teterev Group (PR 1 tt) and anatectic formations of the Zhitomir Complex (PR 1 zt) comprise the main geological structure of the zone. Metamorphic rocks are commonly found as small relict bodies among granitoids and migmatites, and are mostly represented by rocks of the Kocherov suite (PR 1 kc). The latter includes dominant crystalline schists, biotite and biotite- amphibolite gneisses with thin layers of calciphyres and marbles. Rocks of the Kocherov suite are regio- nally metamorphosed at the middle to upper amphibolite facies. Gold mineralization is commonly found at the exocontact zones of the apical part of granite massifs, which are characterised by the widespread presence of