ИСТИНА

The Chad concentric zoned alkaline-ultramafic massif located in the northern part of the Khabarovsk Krai (180 km west of Ayan). The Chad massif is composed of dunite core rimmed by olivine and magnetite clinopyroxenite and melanocratic gabbro; diorite and monzodiorite are present at the southeastern flank of the massif. Dunite is the major source for the platinum group minerals (PGM) in alluvial-deluvial sediments; isoferroplatinum is the most abundant among PGM. Silicates are close intergrown with anhedral isoferroplatinum or occur as inclusions in the latter. Clinopyroxene (0.94-0.98 Mg#; 0.3-0.9 wt % Cr2O3), magnesiohastingsite, and partly serpentinized olivine (0.93-0.94 Mg#; 0.2-0.4 wt % NiO) are intergrown with isoferroplatinum. The chemical compositions of clinopyoxene and olivine are consistent with those of the same minerals in dunite core. Silicate inclusions of 10-100 μm in size are polymineralic and are negative crystals in shape. They are composed as rule by clinopyroxene (0.80-0.95 Mg#), amphibole (edenite and less frequent magnesiohastingsite), and phlogopite (0.66-0.93 Mg#); in some cases, albite, K-Na feldspar, apatite, and felsic glass are identified. Rarer inclusions consist of pyroxene compositionally close to omphacite or aegirine-augite, phlogopite (0.58-0.69 Mg#), oligoclase, nepheline, titanite, and apatite. Silicate inclusions are considered to be syngenetic to isoferroplatinum. Mineralogy of silicate inclusions reflects the formation conditions of the placer-forming PGM, isoferroplatinum in the first place. Slightly different silicate associations in each isoferroplatinum grain is are noteworthy. Minerals containing alkalis, P, and Ti in inclusions indicate metasomatic origin of PGE mineralization in dunite. The fluids affecting dunite core of the Chad massif could be derived from the late intrusion phases including those with high alkalinity.