Ferruginous accumulations in hypogene karst conduits of Crimean Piedmont: Evidence for a deep iron source for the Kerch-Taman iron-ore province, north Black Sea region

Abstract

This study documents ferruginous accumulations in hypogene karst cavities in the Eocene limestones of central-eastern Crimean Piedmont, including unusual quartz-goethite tubes in Tavrida Cave, and aims to reveal their origin and significance to the problem of iron source for the adjacent Kerch-Taman iron ore province. We employed speleological, petrographic, mineralogical, and geochemical (including REE analysis) methods. Ferruginous materials dominated by goethite precipitated in open cavities, although metasomatic replacement of the host rock also occurred. Although the formation of conduits/cavities enclosing ferruginous materials clearly predated the main phase of hypogene speleogenesis in Tavrida Cave, both speleogenetic phases were controlled by the same cross-formational flow paths. Geochemical characteristics of the studied materials in all sites are largely similar, pointing to their genetic affinity. REEY patterns in most of the samples are remarkably similar to those found in iron formations worldwide, for which significant hydrothermal contribution is established. We show that these accumulations had been formed from acidic, reduced, hydrothermal fluids rising from a deep source and intruding oxidizing environment of the karstified Eocene aquifer, where goethite had been precipitated at the boundary of the iron reduction/oxidation zone. This mechanism is consistent with the hypogene origin of host karst conduits. These findings have implications to the origin of iron ores in the adjacent Kerch-Taman province, where deposits are closely associated with mud volcanoes operating there since at least the Middle Miocene. Although the marine sedimentary (hydrogenetic) origin of these deposits is universally accepted, the source of iron is a matter of debate. Revealing of the deep fluid source of iron for the ferruginous accumulations in hypogene caves in Piedmont suggests that the deep fluid system associated with mud volcanism in the Kerch-Taman region could have been a significant, if not the major, supplier of iron for ore deposits in the Cimmerian basin.