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Oxides of the Pd-Sb-Bi system from the Chiney Massif (Aldan Shield, Russia)

Institute of Geology, Siberian Branch of the Russian Academy of Sciences pr. Ak. Koptyuga 3, Novosibirsk, 630090, Russia

^* e-mail: tolst{at}uiggm.nsc.ru

Pd-Sb-Bi oxides occur in the exocontact sulphide ores of the Chiney anorthosite-gabbronorite massif. Three types of oxides have been distinguished: Pb-Sb, Pd-Sb-Bi, and Pd-Bi oxides. Pd-Sb oxides are complete pseudomorphs after stibiopalladinite. Pd-Sb oxide has a grey-brown colour and is weakly to moderately anisotropic. In some cases there is an optical zonality, which corresponds to the heterogeneity of the the composition. Reflectance in different grains varies from 14.8 to 21.6 % at 380 nm and from 26.0 to 33.7 % at 589 nm. The average VHN₂₀ is 124 kg/mm². The formula of the Pd-Sb oxide exhibiting the highest degree of oxidation is close to Pd₅Sb₂O₄. Pd-Sb-Bi oxide occurs as grey rims, formed on Bi-bearing sudburyite. It is weakly anisotropic. In comparison with Pd-Sb oxide it has a lower reflectance. Its average VHN₅ is 81 kg/mm². The oxide composition varies from Pd_0.49(Sb,Bi)_0.28O_0.24 to Pd_0.33(Sb,Bi)_0.24O_0.43. Pd-Bi oxide, formed after froodite, is grey and weakly anisotropic, heterogeneous in most cases, and has emulsion-like inclusions of a high-reflecting phase. Reflectance spectra are lower than for the other oxides. VHN₅ is on average 156 kg/mm². Compositional points in the Pd-Bi-O diagram lie along the Bi₂O₃-Pd line, which suggests that the studied oxide is a solid solution of Pd in Bi₂O₃ (or a decomposition structure of Bi₂O₃ and Pd). The composition with the lowest concentration of Pd (5.62 wt. %) is similar to that of bismite Bi₂O₃ and the oxide with the lowest concentration of Pd is close to the provisional formula Pd₃Bi₃O₄. The formation of Pd-Sb-Bi-O minerals may have been due to a high-temperature fluid influencing the host sandstones of the exocontact ores.

Key-words: platinum-group minerals, alteration, PGE oxide, electron-probe microanalysis, sulphide ore, Chiney massif, Aldan, Russia.

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