Mineralogical characterization of podiform chromitite deposit in the Eastern Mirdita Ophiolite, Albania

The Mirdita ophiolite in Albania is part of an ophiolite belt occurring between the Apulian and Pelagonian subcontinents in the Balkan Peninsula. Within the Eastern Mirdita Ophiolite major chromite deposits are currently under production. Chromite is an important mineral for the supply of chromium that is a crucial component for steelmaking (ferrochromite market). The German market is almost entirely dependent on external suppliers. The main chromite production worldwide occurs as stratiform deposits associated with layered ultramafic-mafic intrusions as well as podiform chromite deposits within ophiolites. Chromite deposits in Bulqiza, Batra, Thekna, Katjil and Xisellas are studied here for a mineralogical characterization. Various different chromite ore-textures range from massive, nodular and disseminated mineralization. The Cr# (Cr/Cr+Al) and Mg# (Mg/Mg+Fe) of chromite are useful ratios to study the crystallization history as well as post-magmatic processes affecting the chromite chemistry. The mineral chemistry of these studied chromite deposits analyzed by electron microprobe indicates that pristine chromite cores exhibit a narrow range in Cr# (80 – 85) and are more widespread in Mg# (60 – 80). Ferrian chromite rims show the same variation in Mg#, but reaches higher numbers of Cr# (> 85). Serpentinization is typically observed within all investigated chromite deposits. The degree of serpentinization increases from harzburgitic host – dunite envelope – nodular chromitite – disseminated chromitite to massive chromitite. Serpentine mesh cell pseudomorphs after olivine and pyroxene are frequently observed. The mesh cell rims are composed of lizardite, whereas mesh cores are either relict olivine or distinct serpentine. Different generations of serpentinization and postmagmatic processes are generally present. A two-stage process of serpentinization in sub greenschist facies conditions is proposed. The first stage of serpentinization is the hydration of magmatic silicates to form pseudomorphs. Fe3+ and Mn enter the fluid phase and lead to the formation of a ferrian chromite rim around pristine chromite cores (2. stage of serpentinization). Further cooling and introduction of more fluid cause serpentine recrystallization and replacement with fibrous chrysotile veins. At a last stage, calcite introduced and developed veinlets and replacement of serpentine. BGR – Bundesanstalt für Geowissenschaften und Rohstoffe (2020): Bericht zur Rohstoffsituation in Deutschland 2018 ,148 S.; Hannover. – URL: https://www.bgr.bund.de /ISBN: 978-3-943566-55-0 (print), 978-3-943566-56-7 (PDF)