Metamorphic evolution of sulphide-rich chromitites from the Chernichevo ultramafic massif, SE Bulgaria

Abstract

The upper mantle rocks of the metamorphosed ophiolite of Chernichevo, Rhodope Metamorphic Complex in southern Bulgaria, host small chromite ores with unusual mineralization of base-metal sulphides rich in platinum-group elements. Mineralogical and chemical data indicate that after their formation in the mantle the Chernichevo’s chromite ores were modified by the intrusion of an alkaline mafic melt, which resulted in the precipitation of a suite of metasomatic minerals (sulphides, calcite, apatite and ilmenite), accompanied by an increase in FeO, TiO2, Ga, Zn, Vn, Mn, and especially Ti and Fe2O3 contents in the chromite. The degree of chemical modification and abundance of metasomatic minerals are positively correlated and mark the extent of reaction of the chromitite with the intruding melt. Sulphide segregation promoted the concentration of high amounts of PGEs (up to 3661 ppb), particularly Pt and Pd, yielding chromite ores with a typical flat to positive-sloped chondrite-normalized pattern. Subsequently, the chromite ores were deformed and metamorphosed together with their host rocks at ultra-high pressure (UHP) (>2.5 GPa, >1200 °C) to be later retrograded under eclogite and finally hydrous amphibolite-facies conditions, giving rise to three microstructural types. Metamorphism of the most metasomatized (i.e., sulphide-rich) chromitites at temperatures >700 °C within the conditions of UHP and eclogite-facies resulted in the formation of (1) non-porous recrystallized chromite, consisting of a granoblastic microstructure made-up of coarse-grained blasts and finer-grained chromite neoblasts. In contrast, hydrous metamorphism on the less metasomatized (i.e., sulphide-poor) chromitite under the conditions of amphibolite-facies (ca. 482–483 °C) resulted in the formation of (2) partly altered chromite, characterized by unaltered cores surrounded by Fe2+-rich and Al-depleted porous chromite containing abundant clinochlore, and (3) porous chromite corresponding to a chromite that was entirely transformed by the metamorphic alteration to Fe2+-rich and Al-depleted porous chromite. During metamorphism magmatic Ni-Fe-Cu sulphides originally formed during the metasomatic event in the mantle were altered, resulting in a major leaching of Cu-rich sulphides, leading to significant remobilization of Pt and Pd.