First finding of merenskyite (Pd,Pt)Te2 in porphyry Cu-Mo ores in Russia

Contents of Pt and Pd were determined in weakly mineralized rocks, ores, and flotation concentrates of the Aksug porphyry Cu-Mo deposit, northeastern Tuva. In all studied samples they are above the detection limits: Pt = 17–96 ppb and Pd = 9–924 ppb. These elements are unevenly distributed throughout the rocks and ores, with Pd/Pt varying from 0.5 to 37. Study of Pd-rich ores (up to 924 ppb, Pd/Pt = 37) on a JEOL JSM 5600 scanning electron microscope revealed finest (2–5 μm) merenskyite inclusions (25.20% Pd, 1.21% Pt, 72.31% Te) in chalcopyrite. The calculated crystallochemical formula of merenskyite from ores of the Aksug deposit is (Pd_0.862Pt_0.023Cu_0.026Fe_0.025)Te_2.064. The merenskyite is associated with electrum (79.92% Au, 18.96% Ag), monazite, cobaltite, tennantite, and Sr-containing barite (4.6–18.0% Sr). Palladium mineralization occurs in massive chalcopyrite veinlets in zones of intensely propylitized rocks. The Devonian Aksug ore-bearing porphyry complex developed in the field of Early-Middle Cambrian intrusions of gabbro-diorite-plagiogranites associated with basalt-andesite effusions of island-arc complex. This might have led to high PGE contents in the Aksug rocks. The deposit formation proceeded with the participation of ore-bearing Cl-enriched fluids favoring the concentration and transport of PGE in porphyry copper systems.