CHROMITITE AND PLATINUM-GROUP-ELEMENT MINERALIZATION AT MIDDLE ARM BROOK, CENTRAL ADVOCATE OPHIOLITE COMPLEX, BAIE VERTE PENINSULA; NEWFOUNDLAND, CANADA

摘要

We report the first find of chromitite in the mantle harzburgite of the Advocate ophiolite complex, Baie Verte Peninsula, Newfoundland, Canada. Chromitite occurs as centimetric veins, pods and schlieren enveloped by dunite at Middle Arm Brook. The composition of the chromian spinel is homogeneous, with relatively high Cr# and low TiO_2, and is consistent with formation of the chromitite by interaction of boninite-type basaltic melt with residual mantle in a supra-subduction arc setting. Chromitites contain up to 1028 ppb total PGE, with significant enrichment in PPGE relative to IPGE, and are characterized by a positive slope of the chondrite-normalized patterns. This is uncommon in typical ophiolitic chromitites, and distinguishes the Middle Arm Brook chromitites from those in the mantle harzburgite of other Appalachian ophiolites (Bay of Island and Thetford Mines), which usually display negative PGE patterns with low Pd/Ir values. The PGM assemblage consists of sulfides (laurite and an undefined Ir-S phase), native PGE (osmium, ruthenium), PPGE - base-metal alloys and PPGE antimonides. The textural characteristics of the IPGE sulfides suggests that they are magmatic in origin. In contrast, the PPGE minerals invariably occur associated with low-temperature alteration phases (ferrian chromite, Fe oxides, serpentine, chlorite, brucite, awaruite, heazlewoodite, and Co-rich pentlandite). The PPGE alloys are interpreted to represent the product of desulfurization of a primary sulfide precursor, which had segregated at a high temperature interstitially to the chromite. This fact is unusual for sulfide-undersaturated boninites, and indicates that sulfide saturation was reached during the late stage of chromite precipitation, possibly by assimilation of sulfur from the country-rock mantle peridotite.