EUHEDRAL CRYSTALS OF FERROAN PLATINUM, COOPERITE AND PALLADIAN ANTIMONIDE FROM THE DARYA PLACER, ALDAN SHIELD, RUSSIA

Perfectly idiomorphic crystals (up to about 3 mm large) of ferroan platinum, cooperite and palladian antimonide were found in the Darya alluvial Au-PGE placer located in the Aldan Shield (Khabarovskiy Kray, Russia). The Aldan Shield hosts several PGE placer deposits (Kondyor, Inagli, Chad, Sibakh, Darya), which are mostly related to zoned clinopyroxenite-dunite intrusions of Alaskan/Uralian type. Ferroan platinum occurs in cubes and flattened cubes with an average chemical composition of Pt2.64-2.88Fe1.00. Within the accuracy of the experimental conditions, X-ray diffractometry suggests the F-centered lattice type. Os, Ru, Ir, Rh and Pd are below the analytical detection limit of electron microprobe analysis. Occasionally, the crystals contain small inclusions of bornite and chalcosine and have about 100 µm large cooperite overgrowth rims. One sample is covered by a Au-rich Au-Ag alloy. Sulfates and K-bearing silicates are often attached to the surfaces of the ferroan platinum crystals. Cooperite is abundant and occurs both in large euhedral crystals (up to 3 mm across, partly twinned) and as overgrowth aggregates on Pt-Fe crystals. Palladium antimonide is rare. One crystal of homogeneous chemical composition (2.5 mm across) gives an ideal formula of (Pd,Cu)3(Sb,As,Sn)1 although the X-ray analysis confirmed the mertieite-I structure. The crystal is brittle and shows perfect cleavage on 0001. Another crystal, characterized by the formula (Pd,Cu)20(Sb,As,Sn)7, has micron-sized (2-5 µm) inclusions of sperrylite and deformation textures clearly visible in polished section. Analysis by defocused beam gives a bulk composition which corresponds to mertieite-II/stibiopalladinite(?). Previous studies of the Darya heavy mineral concentrate reveal the presence of low-Ir (Ir <0.5 wt.%) Pt-Fe alloy, crystals of sperrylite and cooperite, isomertieite, and very rare Ir-Os-Ru alloy. Euhedral crystals of ferroan platinum alloy (up to 1.5 cm large) were only known from the Kondyor deposit so far. By chemical composition and crystallographic features, the Darya Pt-Fe crystals are similar to those of the Kondyor placer. They both have the general formula of Pt3Fe and are characterized by absence of other PGE elements. This work was supported by INTAS grant YSF 00-73