Mineralogy and geochemistry of the pliocene iron-rich laterite in the Vatera area, Lesvos Island, Greece and its genesis
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A nickel laterite deposit occurs in the Vatera area of Lesvos Island, Greece, and is transgressively developed on serpentinized basic rock (norite). The overlying sedimentary rocks include marls and marly limestones with sandstone intercalations and belong to the Pliocene sediments. The following lateritic zones are defined from the bottom to top layers: a) bedrock (norite); b) serpentinized zone; c) goethitic zone. The bedrock consists of the following primary minerals: basic plagioclase, orthopyroxenes and clinopyroxenes. The serpentinized zone includes clinochrysotile, lizardite, antigorite clinoenstatite, calcite and dolomite while in the goethic zone there are goethite, quartz, pyrite, chromite, dolomite. A12O3, Fe2O3, CaO, Na2O, K2O, Ba, Sr, Ni, C and S are enriched in the goethitic zone. Nickel enrichment is related to the formation of nickeliferons minerals substituting for Mg or/and Fe in the goethite and pyrite. Enrichment of Ni in the matrix may be due to the presence of amorphous Ni-silicates (pimelite). There is a significant change (decrease) in the concentration of Ni from the top to bottom parts of the laterite formation, indicating that there was no tendency to migrate downwards (immature laterite). A second support of the immaturity of the Vatera laterite is the incomplete oxidation of ferrous iron to form ferric iron hydroxides.
Under tropical/subtropical conditions, which dominated from the end of Miocene to the Pliocene the norite rocks of the Vatera area altered in response to reaction with acid solutions enriched in CO2. Due to hydrolysis and oxidation of pyroxenes, Mg2+, P4SiO4 and Ni2+ were removed in the continental acid solutions.
Key words:mineralogy geochemistry laterite genesis Lesvos Island Greece
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