Transformation of PGM in supra subduction zones:Geochemical and mineralogical constraints from the Veria (Greece) podiform chromitites

• 来源期刊: 地学前缘（英文版）
• 年，卷（期）: 2021,（2）
• 所属期刊栏目: Research Paper
• 页码范围: 827-842
• 页数: 16页
• 语种: 英文

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 post-magmatic 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 vari-ation. 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. Plati-num–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 Fe3+-bearing garnet (andradite-uvarovite solid-solution 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 874 °C), indicating formation at a shallow mantle environment.