PLATINUM-GROUP-ELEMENT GEOCHEMISTRY AND MINERALOGY OF OPHIOLITIC CHROMITITES FROM THE KOP MOUNTAINS, NORTHEASTERN TURKEY

Abstract

Different types of chromitite, including texturally massive, disseminated and banded varieties, are present in the Kop ophiolitic ultramafic rocks, which extend from Erzincan to Erzurum in northeastern Turkey. The chemical composition of the chromian spinel in these deposits is relatively homogeneous, with 0.64 2 O 3 and FeO/MgO values in the range 0.71–1.12) from which the Kop chromitites precipitated is very similar to picritic basalt and boninite, in distinct contrast to MORB-type magma. Total contents of platinum-group elements (PGE) in different deposits range from 87 to 520 ppb, with an average of 199 ppb. Values normalized to chondrite (CI) show negatively sloping PGE patterns with slight positive Ru and Pd anomalies. Primary solid inclusions of silicates consist of olivine, amphibole, phlogopite and rare clinopyroxene, randomly distributed within the chromitites. Chlorite and a few grains of garnet were also identified as secondary phases. Mostly IPGE-dominated single and composite inclusions of sulfides and alloys are the main PGM phases in chromian spinel. Some PPGE phases, which are uncommon for ophiolitic chromitites, also were found as inclusions in unaltered chromian spinel. Stabilization of PGE alloys and laurite during the early stage of PGM crystallization in the Kop chromitites is explained by the relatively high temperature (T) and low fugacity of sulfur f (S 2 ) of the melt during the precipitation of the chromian spinel. Low-temperature in situ alteration and modification of PGM by loss of S and addition of base metals led to the transformation of laurite to alloys or S-depleted laurite in some samples of chromitite. The calculated composition of the parental melts, the high-Cr content (Cr# > 0.60) and low values of Ti in the chromian spinel in the presence of primary hydrous silicate inclusions suggest that the chromian spinel crystallized from a hydrous melt, enriched in Na and K, formed as a result of high degree of partial melting of upper mantle or of peridotite–melt interaction, probably in a suprasubduction setting.