Geochemical, mineralogical and isotopic signatures of the Semikan, West KasrA±k Turkish phosphorites from the Derik-MazA±dagA±-Mardin area, SE Anatolia

The geochemistry and mineralogy of a condensed section, similar to 6 m thick of the West Kasrik member (Coniacian-Santonian) near the uplifted northern flank of the Mardin-Derik anticline (south-eastern Turkey) was studied. The only deposit exploited in Turkey is found in this area. The sediment textures as well as the mineralogical and the geochemical results collectively suggest that these recycled phosphorites accumulated in areas of intensive very early diagenesis of the sediments in highly oxic bottom waters, and almost no detrital apport. The total concentration of redox-sensitive trace metals is very low (< 600 mu g/g); in addition the structural CO2 and F/P2O5 are unusually high (similar to 5 wt % and 0.14, respectively). REE distribution shows a clear seawater pattern with a strongly negative Ce-anomaly (0.20 +/- A 0.02) and heavy REE enrichment (Lu-N/La-N = 1.50 +/- A 0.12), however their total concentration is very low. The low REE contents, quite unexpected in recycled phosphorites, are explained by the scarcity of terrigenous components which leads to minimal incorporation of REEs from detrital clastic phases in the CFA fraction. Weathering previously suggested as responsible for the high P enrichment of the Mazidagi phosphorites is rather improbable in the samples we studied, in view of their high structural CO2, high F/P2O5, high (La/Nd)(N) and (La/Sm)(N) ratios, and high Sr/P and Ca/P of separated CFA fractions that all negate post-depositional weathering. More likely, the high P enrichment of some rocks (P2O5 content reaches 34%) was produced by sedimentary and early diagenetic processes acting in oxygenated areas of starved sedimentation on and around tectonic highs.