Origin of Fe-rich clay minerals in Early Devonian volcanic rocks of the Northern Minusa basin, Eastern Siberia

Abstract

The formation of clay minerals in volcanic successions follows various evolutionary trends, the understanding of which is crucial for estimating the age of volcanism and the prospecting clay deposits. Early Devonian volcanics within the Byskar Series of the Minusa Depression (Khakassia) are variably altered to a range of phyllosilicate mineral assemblages. This study highlights the origin and formational conditions of the clay minerals, including illite, glauconite, smectite (saponite) and chlorite. Detailed mineralogical and chemical investigation identifies the predominant phyllosilicates, including 1 M illite, illite-smectite (R = 3), 1 M glauconite, smectite, chlorite and kaolinite within trachyandesite, trachyte, and rhyodacite. The phyllosilicates formed by the diagenetic transformation of volcanics after eruptive phases. Evolution trends of phyllosilicates depend largely on the composition of the volcanic substrate. The smectite to illite trend developed due to the dissolution of felsic volcanics and the alteration of smectites into illites with the K+, Al3+ and Mg2+ dominating the mineral formation system. The illites evolved by the substitution of Fe2+ and Mg2+ for Al3+ in the octahedral sheets of smectites. The K+, which was released due to devitrification of felsic substances, was introduced into the interlayer sites for compensating the layer charge originated by the isomorphic substitution in tetrahedral and octahedral sheets. The smectite to glauconite transition trend corresponds to the devitrification and dissolution of alkaline intermediate-felsic volcanics. Early formed dioctahedral smectites evolved into glauconite probably with the predominance of K+ and Fe2+/3+ ions in the formation environment. The substitution of Fe2+/3+ to Al3+ and Mg2+ in the octahedral sheets of smectite contributed to the formation of a layer charge and the entry of exchangeable K+ into the interlayers of glauconite. The third trend involved the formation of smectites (saponites) or chlorites within the intermediate volcanics possibly with Mg-rich but K-poor mineral system.