First Finding of Pt-Pd rich chromitite and platinum group element mineralization in SW Yukon mantle peridotite complexes

ESCAYOLA M.; MURPHY D.; GARUTI G.; ZACARINI F.; PROENZA J. A.; AIGLSPERGER T.; VAN STAAL C. R.

Yukon Geological Survey Current Research

YGS

Lugar: Whitehorse; Año: 2012 vol. 18 p. 101 - 119

1480-3313

abst ract This study reports the first finding of PGE-enriched chromitite veins within mantle dunite in the Slide Mountain terrane of southwest Yukon at Eikland Mountain. Chromitite occurs as irregular centimetrescale veins in dunite bodies within mantle harzburgite. Samples of chromitite veins contain up to 2600 ppb total PGE, with enrichment in PPGE relating to IPGE, and are characterized by a positive slope of the chondrite-normalized patterns. The PGM assemblage in the chromitite veins consists of PPGE-base metal alloys and PPGE and Os-native metal. The PPGE minerals invariably occur associated with the chromitite and Fe-Ni sulphides (pentlandite) in the olivine-rich silicate matrix. The composition of chromian spinel in the chromitite veins is homogeneous although it is highly variable throughout the complex. Vein chromian spinel has low Cr# (~0.45), moderate Mg# (0.58-0.64), low Fe3+# (0.03-0.05) and relatively high TiO2 contents (0.410.62wt%). The chromian spinels contain inclusions of Na-rich amphibole, apatite, rutile and phlogopite. In addition, they host PtFeCu, PtFe alloys, native Pt, abundant Hg-Pd (potarite), isoferroplatinum, tulameenite, sperrylite and Cu sulphides. This mineralization is interpreted as small volumes of alkaline melts extracted at moderate to high oxygen fugacity from deep levels of an upwelling mantle that were trapped in cracks and reacted with the host peridotite crystallizing in the form of discordant chromitite veins. This process took place during, or a short time after, the formation of mantle shear zones at early stages of mantle lithosphere extension, which we infer to have occurred during the initiation of intraoceanic subduction at Slide Mountain Ocean on the passive margin of the Yukon-Tanana terrane.