87     • 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 The formation of Pt, Pd and Ni tellurides during cooling of Fe-Ni- Cu sulfide: Results of experiments and implications for natural systems Hassan M. Helmy 1 , Chris Ballhaus 2 , Jasper Berndt-Gerdes 2 1 Geology Department, Faculty of Science, Minia University, Minia, Egypt; 2 Institute of Mineralogy, University of Münster, Münster, Germany Abstract. In this study we evaluate the role of temperature and Te concentration in the crystallization of Pt, Pd and Ni tellurides (melonite-group minerals). We add Pd, Pt and Te at two Pt+Pd/Te ratios of 0.50 ‘mixture A’ and 0.65 ‘mixture B’ to the (Fe,Cu,Ni) 1-x S melt at various temperatures from 1015 to 370 °C. Significant differences in the phase chemistry and the tellurides assemblage are observed in the two experimental sets. 1) The solubility of Pt and Pd in the monosulfide solid solution (Mss) decreases with the increase of Te concentration in the starting material. 2) The solubility of Te in Mss decreases from 0.3 wt.% (mixture A) to 0.2 wt.% (mixture B). 3) The Pt and Pd telluride assemblage is moncheite-merenskyite-melonite solid solution (mixture A) and moncheite-kotulskite-unnamed Pd 3 Te 2 phase (mixture B). The relatively high solubility of Te in the mss at a wide-range of temperatures makes it is likely that no PGE tellurides form before the decomposition of the mss to pentlandite and pyrrhotite (below 250°C). The melonite-group mineralogy and chemistry is mainly controlled by the Te concentration in the ore fluids. Key words: Melonite group, platinum, palladium, nickel, tellurides, synthetic experiment Introduction Pd, Pt and Ni-tellurides have been described from magmatic, hydrothermal and metamor- phosed Cu-Ni-PGE deposits. Among Pt and Pd tellurides, moncheite and merenskyite (mine- rals of the melonite group) are the most abundant. In Cu-Ni-PGE deposits, which preserve magmatic textures, moncheite and merenskyite are commonly hosted in pyrrhotite and pentlandite. In hydrothermal and metamor- phosed deposits, these minerals are commonly hosted in mobilized chalcopyrite, violarite and secondary silicates. A wide-range of temperatures (>600 to 300°C) was suggested for crystallization of the melonite-group members based on ore texture and associated silicate assemblage. In the present study, we evaluate the role of temperature and Te concentration in the crystallization of Pt, Pd and Ni tellurides (the melonite-group minerals; MGM). Because of the common association of MGM with base- metal sulfides, it is reasonable to consider the problems of MGM formation in close connection with the processes of crystallization in the Fe-Ni-Cu sulfide systems. We add Pd, Pt and Te at two Pt+Pd/Te ratios: 0.50 (‘mixture A’) and 0.65 (‘mixture B’) to the sulfide melt at various temperatures; 1000, 900, 700; 600, 500 and 400°C (for mixture A) and 1015, 920, 825, 725, 625, 500 and 370°C (for mixture B).