Diagenesis of phyllosilicates in the sedimentary Fe-Ni deposits of Central Euboea, Greece and mobolization of critical metals
Τριμελής Εξεταστική Επιτροπή:
Χρηστίδης Γεώργιος, Καθηγητής ΜΗΧΟΠ
Laurent Caner, , Γεωεπιστήμες, Μέλος ΔΕΠ Université de Poitiers
Μάραντος Ιωάννης, Γεωλόγος ΙΓΜΕ
The sedimentary Fe-Ni deposits of Euboea were formed from lateritic alteration of ultrabasic-basic rocks of Jurassic ophiolites, subsequent erosion of the lateritic mantles and deposition in swallow marine/lagoonal environment in the Upper Cretaceous. The deposits have been studied extensively in the past and their mineralogy has been relatively well established regarding the ore minerals and the hosts of Ni. Hence, it is widely accepted that the main hosts of Ni are phyllosilicates, mainly chlorite, serpentine and talc, whereas Fe-oxides and Fe-oxyhydroxides are of minor importance. In contrast, there are contradictory reports about the presence and significance of smectite in the deposits. In this study we report on the phyllosilicate mineralogy of the deposits of Katsikiza, Pagondas, Koutos, Platani, Vrysaki, Rekavetsi and Zygos, Central Euboea, and examine its significance on the evolution in the deposits during diagenesis.
The deposits are characterized both by pisolititic and pelitic textures. They consist of complex (peloids, scarce pisoids) and individual grains of hematite with variable sphericity (angular to rounded) as basic component. They also contain angular to rounded quartz, calcite and accessory chromite fragments. The texture of the main phases indicates limited transportation from the sources. Phyllosilicate mineralogy is comparable in all deposits, regardless of the ore texture. The main phases are mixed layer chlorite-smectite (C/S) with 10-20% smectite layers and discrete chlorite. Talc (kerolite-pimelite) is minor phase in all deposits, being a main phase locally in the Vrysakia deposit, while serpentine is present in all deposit in small amounts. Corrensite (Cor) is present in Vrysaki and Platani deposits. Smectite was identified as major phase in sectors of the Vrysaki, Koutos and Rekavetsi deposits, and discrete illite in the Zygos deposit. Finally, a (La, Ce, Nd)-rich phosphate, monazite-like mineral phase have been identified in Vrysakia deposit, fact that suggests a rather diverse origin of the lateritic material in the Fe-Ni sedimentary deposits of Central Euboea, not only ultramafic ophiolithic rocks.
The coexistence of mixed layer C/S, Cor, the end-member phases (smectite and chlorite) and the limited abundance or lack of serpentine suggest a) a smectite to chlorite conversion during diagenesis of the ore, b) lack of equilibrium in the smectite-to-chlorite transformation and c) dissolution of serpentine during diagenesis. Therefore, chlorite and mixed layer C/S and Cor phases are rather of diagenetic origin and were not formed during lateritization. It is suggested that the smectite to chlorite transformation and the dissolution of serpentine are related to the mobilization of Ni and other critical elements, such as Co. The presence of Cor and mixed layer C/S may be used for estimation of burial depth and temperature. Further work is in progress on this issue.