Origin of iron-rich minerals, ooids and pisoids in the Jurassic ooidal ironstones of the Labino-Malkin region (Caucasus)

This study investigates the Middle Jurassic rocks of the Labino-Malkin zone (Caucasus), represented by ooidal ironstones, biogenic limestones, and nodular phosphorites. All rocks contain ooids/pisoids. Based on the mineral composition, nine types of ooids and three possible scenarios for their formation have been identified. The mineral composition of ooids indicates the variable conditions of their deposition and repeated changes in physical and chemical conditions of the environment, including fluctuations in theredox regime. The access of oxygen in the bottom environment as a factor of increasing oxic conditions was associated with the regression of the sea and/or the inflow of meteoric waters. The main sources of ooids are the drift of material from the continent and convecting metal-bearing meteoric and/or deep solutions.