Irregular masses, veins and leaf replacements composed of the phosphate minerals vivianite, mitridatite, anapaite and hydroxylapatite occur in the Sarantaporo-Elassona Upper Miocene lacustrine basin, Thessaly, central Greece. They are hosted in clayey diatomite. Grey to dull green irregular veins of anapaite up to 40 cm thick and spherical concretions up to 2 cm in diameter sporadically occurred at shallow depths. Minor hydroxylapatite rims anapaite masses. Mitridatite occurs in the near surface as greenish-brown veins 5 cm thick, cutting the diatomite or oriented sub-parallel to bedding. The mitridatine veins are rimmed by, and grade into, iron oxides-hydroxides at the surface. In borehole samples, at depths greater than 40 m, bluish vivianite frequently replaces leaves and faecal pellets up to 2 in diameter.

Conditions permitting the precipitation of phosphate minerals in the basin were the deposition of organic matter, a highly reducing environment at depth (possibly in the lake water but certainly in the bottom sediment), and a low availability of calcium during sedimentation, but its subsequent moderate enrichment in groundwater.

The kind of phosphate formed depended on the diagenetic microenvironment. The diagenetic environment was slightly alkaline (8>Ph>7). The Ca2+ / Fe2+ and Ca2+ phosphates (i.e.anapaite and hydroxylapatite). That occur at shallow burial depths, most likely formed at the expense of the originally formed Fe2+ -phosphate vivianite. The progressive substitution of Fe by Ca is attributed to the action of Ca-rich groundwater. The alteration of the ferrous phosphate anapaite to the ferric phosphate mitridatite, plus amorphous iron hydroxides and goethite at the surface, is an indicator of strongly oxidising conditions.

The Fe2+ ions of the phosphates were derived from the leaching of the ultrabasic and schist basement rocks. The absence of calcite in the diatomite reflects the low calcium content of the lake water during sedimentation. The presence of smectite and vermiculite promoted the adsorption of phosphate ions in the sediments of the basin.

The accumulation of Fe indicates the presence of a eutrophic lake environment. The phosphate minerals further suggest stagnant bottom waters and high biological productivity that was promoted by warm-humid conditions.

Keywords : Phosphate, Clayey diatomite, Upper Miocene, Lake sediments, Greece