Depositional palaeoenvironment of the Middle Jurassic (Aalenian) ooidal ironstones in Labino-Malkin zone (north-western Caucasus)

Abstract

This paper investigates the conditions of formation of the Middle Jurassic iron-bearing strata, in the Labino-Malkin zone in the North-West Caucasus. The mineral composition and geochemical features of the rocks indicate variable conditions of rock formation. The predominance of clay and carbonate minerals in the sedimentary sequence, as well as Ce-anomaly values ranging from 0.9 to 1.4 indicate mainly suboxygenic conditions of sedimentation. Three intervals of anoxia characterised by U, Mo and V enrichment, the presence of siderite and a large amount of pyrite macroframboids, and the formation of phosphorites were identified in the studied section. Oxygen depletion of seawater is the result of upwelling. Formation of oxide minerals was promoted by oxygen saturation of the environment due to sea transgression or meteoric water inflow, which is confirmed by isotope analysis indicators (δ18Ocarb from −11.6‰ to −1.2‰). Eight chemical and two clastic lithofacies were identified based on the mineralogical, geochemical and textural-structural features of the studied strata. Depending on the material input into the basin, different rock types were formed. The constant value of detrital elements Ti and Al indicates a constant input of terrigenous material from the continent. The abundant input of terrigenous material favoured the formation of sandstones and clay minerals. Decomposition of organic matter favoured the formation of limestones and phosphorites due to the input of phosphate- and carbonate-ions into the bottom layers. The presence of iron in seawater favoured the deposition of ironstones. Depending on the level of oxygen and carbon in the bottom environment of the basin, ironstones with siderite, clay or clay-carbonate cement were formed. Calcite and barite contain fluid inclusions of hydrocarbons and hydrogen sulphide gas, indicating the influence of submarine fluids on rock formation. Although the first explorers of the area assumed that the sources of metals come from the continent, the detection of fluid inclusions in the calcite cement, increased content of hydrothermal elements, and the presence of rare minerals such as galena, sphalerite, arsenopyrite, argentotetrahedrite, nugget silver, and coronadite, allows to consider hidden fluid venting as one of the sources of metals for the ooidal ironstones.