Dacitic andesitic calc-alkaline lavas and their pyroclastic rocks of Upper Cretaceous age are widespread in the Ordu area, eastern Black Sea Coast of Turkey. Ca-bentonite and Ca/Na-bentonite deposits with significant economic potential formed in the broader region. The mineralogy and geochemistry of these deposits were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM) and chemical analyses. The parent rocks (PR) and bentonite samples were analysed for the major and trace element contents, including rare-earth elements, to understand the effect of differential alterations on element mobility. The bentonites contain mainly di-octahedral Ca-montmorillonite with minor amounts of illite, quartz, calcite, dolomite and feldspar. Primary K-feldspar and plagioclase phenocrysts were altered in the bentonite horizons, and were completely leached at greater depth. For this reason, the higher quality bentonites are found at deeper zones. Bentonite horizons are characterized by relatively high loss on ignition (LOI: 9.8–20.8%) and MgO content (3–5%) and low K2O (<0.5%) and Na2O (<2%) contents compared with the least altered parent rocks. Silicon, Ca, Mg, Mn and P are markedly enriched whereas Fe, Na, K and Ti are depleted in the bentonite samples. The chondrite-normalized rare earth element (REE) patterns show identical trends characterized by strong light rare earth element (LREE) enrichment in both the PR and bentonite samples. The LREE contents are higher than those of their heavy counterparts (HREE) in the samples. Alteration of the PR to bentonite is associated with intense chemical leaching and subsequent removal of K and Na in open-system conditions.

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