SPERRYLITE FROM STRUMA RIVER ALLUVIAL PLACERS, BLAGOEVGRAD GRABEN, SWBULGARIA

Sperrylite, PtAs2, associated with a number of platinum-group minerals (PGM) is found in the alluvial placers of Blagoevgrad graben, SW Bulg aria. The morphological, chemical and physical characteristics of the mineral are studied by various methods. It is characterized by a numbe r of crystal forms, homogeneity and stoichiometric composition. Well-f ormed crystals dominated by a combination of (111) and (100) faces, an d complicated by poorly developed faces with indices (110), (210), (22 1), (5.10.2), (310), (421) represent the greatest part of the sperryli te grains. A great diversity of phase inclusions (quartz, pyrite, pyrr hotite, augite, chalcocite, kyanite, etc.) are established in the mine ral studied. The lattice parameter is a = 5.966 angstrom. Pure platinu m is obtained as a result of thermal treatment at 900-degrees-C. A hyd rothermal origin is proposed for the sperrylite from Blagoevgrad grabe n on the basis of the following arguments: wellshaped crystal habit, h omogeneity of Pt and As distribution, and stoichiometric composition c haracteristic of a calm environment of crystallization and sufficient space for crystal growth; presence of low-temperature mineral inclusio ns (chalcocite, quartz, pyrite, pyrrhotite) caught at the time of the sperrylite crystallization as well as lack of relics of high-temperatu re magmatic ones; presence of kyanite inclusions and lack of sperrylit e dearsenization as evidence of its formation after the metamorphism o f the regional rock formations; low Os, Ir and Ru content, and high Pd /Ir ratio in the ultrabasic rocks of the region described being a resu lt of the hydrothermal deposition of platinum-group elements (PGE)