Mineralogy, geochemistry and genesis of K-bentonites in Şenköy formation from the Gümüşhane area

In this study, mineralogies, geochemical characteristic, formation environments and conditions of marl/claystones and K-bentonites found in the limestone-claystone-sandstone alternation within the Early-Middle Jurassic aged Şenköy Formation outcropping in Gümüşhane area; were investigated by means of optical microscopy, XRD, DTA-TG, SEM-EDX, whole-rock and stable isotope geochemistry. Clay minerals in marl/claystones are illite, chlorite, illite/smectite, smectite and chlorite/smectite. K-bentonite consist of clay minerals illite/smectite, chlorite/smectite and kaolinite and imogolite indicating volcanic contribution. Non-clay minerals were detemined quartz, calcite, plagioclase and a small amount of pyrite. Illite and I/S in marl/claystones are under high diagenetic conditions (polytype 2M1) and I/S in K-bentonites reflect high and low diagenetic conditions (1M (tv) ve 2M1 polytype). The temperature of the fluids effective in the formation of I/S is similar to the low-grade metamorphism conditions. Marl-claystones are in andesite/basaltic andesite, basalt composition and K-bentonites are in rhyolite/dacite composition. It is suggested that the magma forms the primary rock of K-bentonites has a felsic composition and originates from a subduction-related environment. K-Ar dating of I/S gave smectite transformation age as 103-125 Ma.