Apatite and Mn, Zn, Co-enriched chromite in Ni-laterites of northern Greece and their genetic significance

Abstract

The SEM-microprobe investigation of pseudo-autochthonous Fe–Ni deposits from the areas east Vermion, Edessa and Olympos, northern Greece, reveals the presence of apatite, and a significant Mn, Zn and Co enrichment in chromite, up to 14.0, 3.3 and 4.1 wt.%, respectively, due to the substitution for Mg 2+ and Fe 2+ by Mn, Zn and Co in the chromite lattice. Despite the significant Mn, Zn and Co contents (whole ore analysis) of some Ni-laterite deposits of the Balkan peninsula, Mn, Zn and Co enrichment in chromite is restricted to certain deposits, and its level seems to be a characteristic feature of each deposit. This may indicate that the Mn±Zn±Co enrichment has taken place after the re-deposition of the weathered material rather than during serpentinization and weathering of parent rocks. Mn±Zn±Co enrichment is commonly higher in the ferrian-chromite zone of chromite grains, and in high-Cr than high-Al chromite. The availability of Mn 2+±Zn 2+±Co 2+ in solution during the diagenesis–metadiagenesis stage, due probably to the decomposition of organic matter, and the Eh–pH conditions may be major controlling factors for the incorporation of Mn, Zn or Co in the lattice of chromite. Apatite occurs as fine grained crystals (<10 to 50 μm) dispersed throughout the matrix of the ore and as inclusions within zoned crystals of chromite, between chromite core and magnetite rim. It is associated with Fe-oxides (goethite, hematite, magnetite), chamosite, and quartz. A salient feature of the apatite is its poikiloblastic texture (with inclusions of quartz and iron oxides). The apatite crystals exhibit both plastic and brittle deformation textures. The composition of apatite is homogeneous, either in different apatite types or within crystals themselves. Apatite is Cl-free and corresponds to the fluor-hydroxyl-apatite type with the following average composition: 51.35 wt.% CaO, 2.80 wt.% FeO, 41.4 wt.% P 2O 5 and 5 wt.% (F+H 2O). The inclusions of apatite between chromite core and magnetite rim and the inclusions of magnetite and quartz dispersed throughout apatite suggest that these minerals crystallized at the same time. The association of apatite with Fe-chlorite and magnetite points to their formation during the diagenesis–metadiagenesis stage, while the gradual dissolution of apatite crystals dispersed throughout the matrix suggests that it was unstable during a subsequent stage of its formation.