Geochemistry of the Oligocene-hosted manganese ores and the host sediments in the Thrace Basin, Türkiye: Implications for genesis and exploration of the Paratethyan Mn deposits

Abstract

The Early Oligocene Binkılıç Mn deposit occurs within the laminated clays and clayey carbonates of the Thrace Basin, NW Türkiye. Carbonatic and oxidic ores generally alternate with greenish to brownish grey laminated clays. The Mn-bearing laminated clays are unconformably overlain by deltaic Miocene sediments consisting of clay, sand and lignite.

Trace element and REE contents of the Mn oxides and the laminated host Oligocene clays of the Binkılıç Mn deposits are quite similar. Mn-bearing Oligocene clays and Mn oxides typically show enrichment in Ni, U, Tl, Mo, Co, W, As, Hg, Cu and V relative to PAAS. While Sr and Ba extremely enriched in Mn oxide ore relative to PASS, such enrichment is not observed in the Oligocene clay. A similar trace element enrichment of the Mn oxides and the host Oligocene clays indicates that both were formed under similar geochemical conditions. The fact that the oxidic manganese ore is rich in these trace elements, but the carbonate ore is poor, may indicate that a local reducing environment developed as a result of organic matter decomposition, and that Mn carbonates were formed secondarily in the post-burial stage.

Trace elements are highly enriched in Mn oxides and clays, are clearly organophylic elements and indicate that the bioproductivity of Paratethyan sea was high in the early Oligocene and that the Mn deposit was formed by anoxic processes as a result of the development, precipitation and decomposition of organic substances. Organic matter-rich sediments at the bottom of deep basins must have ensured the continuity of reducing conditions while transferring the high amount of organophylic trace elements to the seawater. Mn and other anoxia proxy elements were concentrated in the anoxic water column and then carried to shallow marine environments by upwelling deep currents, where they may have been precipitated as Mn oxides in the early Oligocene. In addition to Mn, we propose that a trace element association consisting of Ni, U, Tl, Mo, Co, W, As, Hg, Cu, V anomaly of the Oligocene clays can be used as a prospecting tool for Oligocene-hosted manganese deposits around the Paratethyan Basin.