Petrographic and geochemical features of the B3 bauxite horizon (Cenomanian-Turonian) in the Parnassos-Ghiona area: a contribution towards the genesis of the Greek karst bauxites

• Parnassos-Ghiona bauxites are rich in diaspore, deriving from the hypogene alteration of boehmite. • The Variani and Shila 1 deposits contain pyrite veins of epigenetic origin. • Both bauxitization and late epigenetic processes affected the geochemical features of the deposits. • Residual chromite and serpentine, and the Eu/Eu* ratio indicate ophiolites as possible parent rocks. The karst bauxites of the Parnassos-Ghiona Unit (Greece) are hosted within Upper Jurassic to Late Cretaceous carbonate rocks. Three bauxite horizons are intercalated within shallow water limestones. They are named B1, B2 and B3 from bottom to top. In this study we present the characteristics of the B3 horizon (Cenomanian-Turonian) sampled in Agia Anna, Variani and Shila 1 underground mines. The bauxite ore has a prevailing oolitic texture. Diaspore, boehmite, hematite and goethite are the major mineral components of the analyzed samples. Kaolinite, rutile and anatase are common minor phases. Pyrite occurs in a small number of samples. Ilmenite, rutile, chromite, zircon, baddeleyite and serpentine are common detrital minerals. Authigenic minor minerals are mostly represented by REE-phosphates, barite, and gypsum. The presence of pyrite and diaspore suggests that after a first stage of additional in situ weathering and bauxitization, the deposits were affected by early diagenetic reducing conditions (allowing the formation of pyrite) and significant diagenesis, resulting in diaspore formation. This three-stages genesis is reflected in complex correlations between major, minor and trace elements. The occurrence of detrital chromite and serpentine in the bauxite reflects mafic parental rocks. The mafic component likely originated from the ophiolite suites exposed in the vicinity of the Parnassos-Ghiona bauxites.