Occurrence of sepiolite in the Hirsizdere sedimentary magnesite deposit, Bozkurt-Denizli, SW Turkey

The Hirsizdere magnesite deposit is found in the upper Miocene-Pliocene lac ustrine sediments of the Cameli formation in the western Aegean depression zone, which was uplifted to form a broad anticline and in which consecutive faults and fractures developed due to the neotectonic activities. Thick al ternation of carbonate cemented and weakly cemented conglomerate, sandstone , mudstone intercalated with a few thin dolomite and magnesite layers is ex posed at the lower part of the sequence. These units are overlain by magnes ite and dolomite in association with thin claystone and sandstone layers an d lenses. Green and claret red-brown smectite is observed in detrital mater ials as well as in sandy carbonate units, and is accompanied by serpentine, amphibole, illite, feldspar and quartz, whereas sepiolite is developed in the form of 3-5 cm thick, yellow and greenish brown colored veins, or as th in films lining parallel to the vein axis within the magnesite unit. SEM st udies show that sepiolite fibers are grown in the form of meshworks and bri dges between relict carbonate mineral grains. The presence of plant stem im prints, and the abundance of gastropod and ostracode shells in carbonate un its indicate shallow and alkaline lake conditions. Field observations and d etailed mineralogical, chemical and micromorphological determinations indic ate that sepiolite is formed by diagenetical transformation of magnesite du ring the movement of silica-rich meteoric water through the fractures of ca rbonate units under near-surface alkaline conditions.