Abstract

The compositions of PGE and precious metal minerals located in sulfide blebs from the base of the melagabbro unit of the Rudniy ultramafic-mafic intrusion in northwest Mongolia were studied. Sulfide blebs occur in the elongated narrow zone (∼30 × 0.2 m), which extends along the contact with the leucogabbro. There are two layers in this zone, the upper (present throughout the zone) and the lower (present fragmentarily). The layers differ in bleb shape, size, and distribution. The layers also have different types of olivine crystals and PGE mineral abundances, but exhibit nearly identical whole-rock metal contents. Approximately 240 grains of PGE and precious metal minerals were studied, and at least eight different minerals were identified. Melonite group minerals (PtTe2–PdTe2–NiTe2) dominate the majority of the single and composite grains in blebs from both layers. A correlation exists between the melonite group mineral compositions based on the host mineral and bleb layer. No other PGE minerals were identified in the lower layer. Three Ag phases, hessite (Ag2Te), sopcheite (Ag4Pd3Te4), and telargpalite [(Pd,Ag)3Te], and three Sn phases, paolovite (Pd2Sn), taimyrite [(Pd,Cu,Pt)3Sn], and an unnamed Pt stannotelluride mineral (probably Pt3SnTe4), were identified with hessite in the composite grains of the upper layer. A comparison of compositional data obtained from the melonite group and data from other occurrences indicates an uncommonly wide range of compositions, most likely indicating supercooling conditions during crystallization.

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