The podiform chromitites in the Dagküplü and Kavak mines, Eskisehir ophiolite (NW-Turkey) : genetic implications of mineralogical and geochemical data

Abstract

Mantle tectonites from Eskisehir (NW-Turkey) include high-Cr chromitites with limited variation of Cr#, ranging from 65 to 82. Mg# ratios are between 54 and 72 and chromite grains contain up to 3.71 wt% Fe2O3 and 0.30 wt% TiO2. PGE contents are variable and range from 109 to 533 pbb. Chondrite-normalized PGE patterns are flat from Os to Rh and negatively sloping from Rh to Pd. Total PGE contents and low Pd/Ir ratios (from 0.07 to 0.41) of chromitites are consistent with typical ophiolitic chromitites. Chromite grains contain a great number of solid inclusions. They comprise mainly of highly magnesian (Mg# 95-98) mafic silicates (olivine, amphibole and clinopyroxene) and base-metal sulfide inclusions of millerite (NiS), godlevskite (Ni7S6), bornite (C5FeS4) with minor Ni arsenides of maucherite (Ni11As8) and orcelite (Ni5-xAs2), and unnamed Cu2FeS3 phases. Heazlewoodite, awaruite, pyrite, and rare putoranite (Cu9Fe,Ni9S16) were also detected in the matrix of chromite as secondary minerals. Laurite [(Ru,Os)S2] was the only platinum-group minerals found as primary inclusions in chromite. They occur as euhedral to subhedral crystals trapped within chromite grains and are believed to have formed in the high temperature magmatic stage during chromite crystallization. Laurite has limited compositional variation, range between Ru0.94Os0.03Ir0.02S1.95 and Ru0.64Os0.21Ir0.10S1.85, and contain up to 1.96 at% Rh and 3.67 at% As. Close association of some laurite grains with amphibole and clinopyroxene indicates crystallization from alkali rich fluid bearing melt in the suprasubduction environment. The lack of any IPGE alloys, as well as the low Os-content of laurite, assumes that the melt from which chromite and laurite were crystallized had relatively high fS2 but never reached the fS2 to crystallize the erlichmanite. The presence of millerite, as primary inclusions in chromite, reflects the increasing fS2 during the chromite crystallization.