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Diagenesis of phyllosilicates in the sedimentary Fe-Ni deposits of Central Euboea, Greece and mobilization of critical metals

Cheliotis-Chatzidimitriou Spyridon

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Summary

The sedimentary Fe-Ni deposits of Central Euboea were formed from lateritic weathering of ultrabasic-basic rocks of Jurassic ophiolites. After the formation of lateritic mantles, followed erosion, transportation and deposition in swallow marine/lagoonal environment in the Upper Cretaceous, over Triassic limestones or ophiolites or rarely, chert-schist stones. 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. Therefore, 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. Moreover, about smectite’s presence and it’s role in the deposits, there are contadictory reports. In this study we report on the phyllosilicate mineralogy of the deposits of Katsikiza, Pagondas, Koutos, Platani, Vrysakia, Rekavetsi and Zygos, Central Euboea, and examine it’s significance on the evolution of the deposits during diagenesis. The deposits are characterized by both 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, as well as, pyrite in certain sectors. The shape of grains and the texture of the main phases designate limited transportation from the source rocks. The bulk mineralogy was determined by PRXD, while for clay mineralogy was carried out clay fraction separation and ethylene-glycol solvation, before PXRD analysis. Mineral texture characterization and semi-quantitative analysis were conducted with SEM-EDS. Complex peaks from diffraction patterns were analyzed into individual mineral phases with Fityk software. The main phases of the phyllosilicate mineralogy are randomly ordered(R0) mixed layered chlorite-smectite (Chl/Sme) 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 most of the samples, but in small amounts. Corrensite (Corr), the perfectly ordered(R1) mixed layered chlorite-smectite (50% chlorite-50% smectite), is present in Vrysakia and Platani deposits. Discrete smectite(Sme) was identified as major phase in sectors of the Vrysakia deposit, and discrete illite in Katzikiza and Rekavetsi deposits. 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 R0 mixed layer chlorite-sme, corrensite, discrete smectite and discrete chlorite, as well as, 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. Also, the occurrence of abundant talc layers might be attributed to the transformation of stevensite(Mg-Ni-rich trioctahedral smectite), which probably existed in the primary lateritic mantle, to talc during diagenesis. Therefore, chlorite, R0 mixed layered Chl/Sme and Corr phases are probably products of diagenetic processes and were not formed during lateritization. This is the first report for chloritization of Fe-rich dioctahedral smectite during diagenesis. It is suggested that the smectite to chlorite transformation, the dissolution of serpentine and the transformation of stevensite to talc are related to the mobilization of Ni and other critical elements, such as Co. The reaction paths for corrensite, R0 mixed layered Chl/Sme, discrete chlorite and data from literature may be used for us to suggest a burial temperature of sediments between 70-100°C. Additional work would be necessary to in order to confirm the exact reaction paths of phyllosilicates and the mobilization of Ni and critical metals during diagenesis.

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