摘要
Extremely abundant PGE-minerals(PGM)hosted in chromitites from the Veria ophiolite complex in Macedonia(N.Greece)may be unique among ophiolite complexes.This study focuses on differences between the low-and high-PGE chromitites.New textural,mineralogical and geochemical constraints from those ores are presented,aiming to define factors controlling the PGE enrichment in a supra subduction environment,in the light of postmagmatic processes.The whole ore analyses for mmajor and trace elements indicated an unusually high-IPGE content(up to 25 ppm)and higher Fe,Ca,Mn,Zn and V contents in high-PGE compared to low-PGE in massive chromitites.The wide compositional variation of chromite,even in the same polished section,the occurrence of very fine PGM(less than 20μm)as inclusions within chromite and extremely large(>1000μm),angular or fine-grained PGM aggregates ones within a matrix of highly fragmented chromite-Cr-garnet matrix,may indicate crystallization/recrystallization of chromite from more than one precursor phases.Laurite(RuS2)is very limited,occurring as remnants surrounding by Ru–Os–Ir oxides/hydroxides,of a wide compositional variation.Irarsite occurs as euhedral crystals up to 200μm,surrounding by chromite,as anhedral exsolutions 1–200μm within laurite,or creating segregates with platarsite and relics of(Ru,Pt,Rh,Os)sulfarsenides.Platinum–Ru–Rh–Pd-minerals occur commonly as relatively fine-grained assemblages,up to 50μm,along with irarsite and other relics of(Ru,Pt,Rh,Os)sulfarsenides.Pt-alloys show a variation ranging from tetraferroplatinum to Pt–Ir–Fe–Ni alloys.The presence of laurite relics in large IPGM,awaruite,heazlewoodite,and carbon-bearing material reflecting a super-reducing environment,and the transformation of primary PGM into Os–Ir–Ru-alloys and oxides/hydroxides in association with Fe-chromite and Fe3t-bearing garnet(andradite-uvarovite solidsolution series)may reflect changes of the redox conditions from reducing to oxidizing.The relatively high Na content in hydrous mineral inclusions within high-PGE chromitites suggest a hydrous mantle source and provide the possibility for estimation of the P(average 3.0 kbar)and T(average 874C),indicating formation at a shallow mantle environment.
基金
The National and Kapodistrian University of Athens(NKUA)is greatly acknowledged for the financial support(Grant No.KE_11078)of this work.
