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

M. ESCAYOLA; G. GARUTI; F. ZACCARINI; J. A. PROENZA; J. BÉDARD; C. VAN STAAL

CANADIAN MINERALOGIST

MINERALOGICAL ASSOC CANADA

Lugar: Montreal; Año: 2011

0008-4476

We report the first finding of chromitite in the mantle harzburgite of the Advocate ophiolite complex (Baie Verte, Newfoundland, Canada). Chromitite occurs as centimeter veins, pods and schlieren enveloped by dunite at the locality of Middle Arm Brook. The composition of chrome spinel is homogeneous, with relatively high #Cr and low TiO2, and is consistent with formation of the chromitite by interaction of boninite-type basalt 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 ratios. 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-pentlandite). The PPGE alloys are interpreted to represent the product of desulfurization of a primary sulfide precursor which segregated at high temperature interstitial to chromite. This fact is unusual for S-undersaturated boninites, and indicates that S-saturation was reached during the late stage of chromite precipitation, possibly by assimilation of sulfur from the country rock mantle peridotite.