PGE Distribution in Chromite Placers from Andaman Opliiolite and its Boninitic Parentage

摘要

Chromite placers occurring in Rutland coast of Andaman and Nicobar Islands, India, show PGE incidence. Serpentinised dunite containing thin stringers/lenses of chromite occurring adjacent to placer was possibly the source rock of the chromite placers. Chemical analysis of these chromite placers show PGE elements incidence in the range: Ru <10-56 ppb, Ir <10-12 ppb, Rh <10-14 ppb, Pd <10-136 ppb, Pt <10-28 ppb. Normalization of these PGE values against Cl-chondrite indicates enrichment of Ir and Ru and depletion of Rh and Pt, a typical character of ophiolitic chromite. Except few Pd values, the PGE values depict highly fractionated PGE distribution pattern. High Mg-olivinc (Fo_(92-90)) and highly fractionated PGE pattern may suggest derivation of the source rock from boninitic melt. Ru-Ir-Os bearing sulphides or alloys were crystallised at early magmalic stage, Pt, Pd were remobilized during hydrothermal activity and concentrated in serpentine.