Platinum-group element distribution in chromite ores from ophiolite complexes, western Turkey

Abstract

Selected chromite mines in western Turkey (Fig. 1) were the focus for a study of platinum-group element (PGE) and Au concentrations in chromite ores and host rocks. Forty-six underground chromite mines were sampled in ten districts. The chromite is hosted in peridotite, dunite, serpentinized harzburgite and lherzolite rocks of late Jurassic to late Cretaceous age. A total of 113 ore and rock samples were collected and examined.High PGE concentrations are a feature of chromite-rich rocks worldwide in ultramafic complexes of Alpine type. 1,6,8,23,29 Knowledge of the geology and geochemistry of these deposits is, therefore, critical to understanding of the mechanism of PGE mineralization and its genesis. The study area contains numerous chromite deposits in a complex, ENE-trending suture zone between the I . zmir-Ankara ophiolite zone in the north and the Taurides platform in the south. More than 200 chromite occurrences, prospects and mines are scattered throughout the peridotites, dunites and serpentinized lherzholite-harzburgites. 10 General geological settingOphiolites of the I . zmir-Ankara zone differ from others in the study area in that they have been subjected to high-pressure, low-temperature metamorphism with the development of glaucophane, lawsonite and jadeite. 14 The ophiolitic massifs of the I . zmir-Ankara zone consist mainly of ultrabasic rocksmore than 70% harzburgite and dunites-all of which are strongly serpentinized and show a metamorphic schistosity. 24 They also include more or less recrystallized and layered metagabbros and pyroxenite; scattered dolerite dykes, which intersect all coarse-grained rocks; and crushed and dismembered volcanic and sedimentary units, all of which appear as mélanges tectonically associated with the ultrabasic masses. Finally, a discontinuously exposed blue-schist metamorphic belt immediately flanks the northern margin of the large ultrabasic massifs in the eastern part of the I . zmir-Ankara ophiolite zone. Moreover, the ophiolites are intersected by intrusive plutons, stocks and dykes of dioritic, granodioritic and tonalitic composition.The Tauric ophiolite belt, which includes ophiolites of Lycian nappes and Antalya nappes, was affected by Alpine tectonics. The ophiolite sequence at Antalya (Fig. 1) extends from harzburgitic tectonites to the dyke complex and includes a well-developed cumulate sequence. All massifs in the Tauric ophiolite belt are allochtonous and belong to a complex system of nappes, thrust over the Tauric calcareous axis. The oldest tectonic emplacement is dated as late Cretaceous and the first post-tectonic transgressive deposits are Palaeocene. 14 The ophiolites of the Lycian nappes ( Fig. 1) are mainly peridotite. They cover more than 3000 km 2 with a thickness of between 1 and 2 km. The lithology of this giant ultramafic mass is known to be extremely monotonous. In the bay of Marmaris and the Datça peninsula the whole mass is a complex of dunite-harzburgite-orthopyroxenite. Most of the harzburgites are folded, foliated and lineated tecton...