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

MONICA ESCAYOLA; GIORGIO GARUTI; FEDERICA ZACCARINI; JOAQUÍN PROENZA ; JEAN BÉDARD; CEES VAN STAAL

CANADIAN MINERALOGIST

MINERALOGICAL ASSOC CANADA

Lugar: Montreal; Año: 2010

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 centimetric veins, pods and schlieren enveloped by dunite. The composition of chrome spinel is homogeneous, with relatively high #Cr and low TiO2, is consistent with formation of the chromitite by interaction of boninite-type basalt with residual mantle, in a supra-subduction arc setting. Chromitite contains up to 1028 ppb total PGE, with significant enrichment in PPGE relative to IPGE, giving raise to positive slope of the chondrite-normalized patterns. This is uncommon in typical ophiolitic chromitites, which usually display negative sloping PGE patterns with low Pd/Ir ratios. The PGM assemblage consists of sulfides (laurite and undefined Ir-S), native PGE (osmium, ruthenium), PPGE-base metal alloys and PPGE-antimonides. The mode of occurrence 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, 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, unusual for S-undersaturated boninites, indicates that S-saturation was reached during the late stage of chromite precipitation, possibly by assimilation of sulfur from the country rock mantle peridotite.