Genesis of the Eastern Iranian bentonite deposits

Numerous bentonite deposits are located in Southern Khorasan (Eastern Iran), especially in Ferdows and Sarayan counties. We studied the genesis of these bentonites at seven deposits (Chah-Taleb, Chah-Keshmir, Chah- Golestan, Chah-Pirouz, Gholeh-Gelia, Kharman-Sar and Khal-Kooh) using petrological, mineralogical and geochemical data (including X-ray diffraction and fluorescence, and inductively coupled plasma-mass spectrometry) from both bentonites and parent rock samples. The deposits mainly consist of Na-montmorillonite and opaline silica, with lesser amounts of quartz, feldspars, carbonates, halite, zeolites, and illite. The studied deposits formed in a volcanic arc setting and their magma series are calc-alkaline, except for the Kharman-Sar deposit, which is tholeiitic in composition. The presence of significant amounts of opaline silica indicates a low fluid flow rate in these systems, in spite of the high leaching of alkalis. The strata-bound nature of the studied deposits, as well as the high amount of montmorillonite, the presence of gypsum layers alternating with those of bentonite and the lack of hydrothermal alteration suggest that these Iranian bentonite deposits were formed through diagenetic processes in lagoon environments, by means of the alteration of intermediate to acidic precursors, trachy-andesite to rhyolite, belonging to two volcanic provinces: one pyroclastic realm in Ferdows, and another volcaniclastic in Sarayan.