Chromites from Vourinos complex mines and their alteration

Chromite ore deposits have been largely exploited in Greece and the presence of tens of abandoned mines witnesses the important role of the country in the chromite mining industry past scenario.
   Numerous studies about the geochemical and textural features have been carried out so far of different ores. With this contribution we attempt to summarize the main geochemical and textural features of Vourinos ophiolite complex chromite deposits focusing our attention on new data on the later alteration processes occurring in almost all ores.
   Vourinos complex covers 400 km² and in spite of predominating mantle outcrops it shows a full ophiolite sequence with a well exposed petrographic Moho. The crustal sequence comprises mafic and ultramafic cumulates, gabbro, dykes, some pillow lavas and a carbonatic sedimentary cover. All chromite bodies are set in dunite bodies or in dunite enveloped by harzburgite and are concentrated within the metalliferous zone. Geochemically, Vourinos chromite presents quite homogeneous features, among the different mines, with Mg# and Cr# ranging between 0.45 and 0.64 and between 0.75 and 0.83 respectively. Cr₂O₃ contents range between 57 and 60 wt%. No differences in primary chromite between the different mines were detected except for a generally lower Cr# for Rizo mine. All chromitite bodies independently of their texture (massive, schlieren or disseminated) are made of euhedral to subeuhedral chromite crystals with size varying mainly between 0.1 and 2 mm, with some rare nodular textures showing crystals of up to 5 mm in size.
   Chromite from all studied mines shows some important features highlighting the presence of Fe-chromite and magnetite alteration. Sometimes completely altered chromites where only the shapes of the habits were preserved were detected. Fe-chromites show a wide range of compositions. They are characterized by an increase in Cr# and/or a decrease in Mg# compared to their chromitic cores. In spite of the low range of primary chromite compositions, Fe-chromite can span over the full range of possible Cr# increase and Mg# decrease. Extreme compositions comprise virtually MgO-free Cr-magnetites and virtually Al₂O₃-free chromites s.s. Anomalous compositions were also detected in few samples with high NiO and MnO contents. MnO content of primary chromite is very low and no MnO has been detected in silicate phases. Cr₂O₃-free magnetites are often found as small crystal within the serpentinite matrix and are not a product of alteration of chromite but are related to release of iron during serpentinization. Fe-chromite always grows at the expenses of primary chromite as chromite and Fechromite together, drawing the shape of the original chromite crystal. The close association of Fe-chromite and chromian-chlorite (kammererite) independent of the degree of serpentinization of chromitite silicate matrix and peridotite host rock suggests that alteration of chromites pre-dates serpentinization.