Mineralogical and isotopic study of magnesite veins in the Evia and Gerakini ultramafites (Greece)

Magnesite ore deposits are the main source of Mg for the industry and are used in the production of various kinds of magnesia. They are closely associated to ophiolite geological contexts, and are usually hosted within variously altered harzburgites and dunites. Greece hosts exploitable ophiolite-related magnesite deposits in two areas, located in the Evia island and in the Chalkidiki peninsula. In the present contribution we take into consideration magnesite veins at Kymasi (Evia) and Gerakini (Chalkidiki). The Kymasi magnesites are hosted in peridotites in the tectonic unit of the Pelagonian Zone of Northern Evia. Gerakini is located in the Vardar Zone ophiolite belt, and forms the Gerakini-Ormylia ophiolite complex. Both areas host major magnesite deposits, mainly comprising shallow stockwork-type ores hosted within variably altered dunites and harzburgites locally intruded by pyroxenitic and gabbroic veins. At Kymasi, field work was focused on a well exposed network of magnesite veins that show the following zoning from core to rim: a thick massive magnesite central portion, a serpentinized hydraulic breccia with magnesite cement and a partially dolomitized peridotite. The host rock is unaltered peridotite. All contacts are irregular and sharp. At Gerakini sampling was focused on two active open pits, Ugo and Antonium. Magnesite samples show a variety of textures, comprising massive magnesite, cauliflower magnesite, fibrous magnesite and brecciated magnesite. Host rocks are serpentinized dunites. XRD data show that, differently from Kymasi, at Gerakini magnesite veins contain a variable amount of dolomite up to 16%. Mineral chemistry data show that, at Kymasi, high-Cr spinels occur in the breccia, while partially dolomitized peridotite and unaltered peridotite spinels are lower in Cr. The isotopic analyses show very similar δ18O and δ13C values in Kymasi and Gerakini, also consistent with other ophiolite-related magnesite mineralizations in the Balkans. The isotopic values suggest that the CO2 was originated from the decarboxylation of organic matter present in the sediments of the area. Considering the origin of the CO2 and the shallow nature of the stockwork mineralization, it can be concluded that both Kymasi and Gerakini magnesites were formed by infiltration of CO2-rich meteoric waters which interacted with peridotites. The interaction caused the leaching of Mg, which was transported by the fluids through preferential pathways, possibly reactivating older shear zones, and finally deposited in the form of magnesite once it reached saturation at shallow depths.