Characterization and genesis of supergene karstic vanadium ores in the Djebba Pb–Zn district (Triassic Diapirs zone, North Eastern Tunisia)

Abstract

The Djebba district hosts hypogene Pb–Zn sulfides and two successive stages of supergene Pb–Zn–Fe–V mineralization. This study mainly focuses on the supergene karstic vanadium mineralization of Jebel Goraa deposit. The hypogene Pb–Zn mineralization is considered to have been formed during Late Miocene (Alpine Orogeny) by Mississippi-Valley-Type (MVT) brines, similar to that proposed for many other Pb–Zn ore deposits of the Diapirs zone in Northern Africa. During the Late Neogene-Pleistocene, several episodes of uplift and erosion enabled the exhumation and karstification of the Early Eocene host rocks, and spatially related hypogene Pb–Zn-(Fe) sulfides, their subsequent weathering, and the formation of supergene ores. Vanadium mineralization is developed in karst cavities filled with oxidized ores (goethite, smithsonite and hemimorphite) and argillaceous materials (smectite). The paragenesis of supergene ores reveals two successive stages of deposition: 1) early supergene weathering with formation of Fe-bearing carbonates in turn oxidized to goethite, smithsonite, hemimorphite, and cerussite; 2) late supergene weathering with deposition of Pb–Zn vanadate minerals (vanadinite, descloizite, and vanadinite-pyromorphite intermediate compositions), minor mimetite and late calcite.Vanadium, phosphorous and probably chlorine of the vanadate ores are inferred to have originated from the immature organic matter of Early Eocene country rocks (Tmax <435 °C and TOC average 3.4 wt%) by descending low-temperature aqueous solutions. Pb, Zn and minor As were derived from primary Pb–Zn sulfides undergoing oxidation. Genesis of supergene vanadium ores is genetically related to post-Miocene uplifting-related weathering episodes, controlled by tectono-morphological and climatic evolution following the Alpine orogeny. The formation of the vanadium mineralization spans through the Late Miocene to Holocene. However, with paleoclimatic considerations it could be generally confined to the Middle Pliocene. Genetically, the supergene vanadium ores of Djebba Pb–Zn district belong to the vanadate ores type, which have been reported in other areas of southern and northern Africa (e.g., Namibia and Morocco). The association of vanadium, particularly in highly fractured and faulted zones with high levels of immature organic matter and spatially close to surface or near-surface MVT Pb–Zn ore deposits, can be used as an exploration tool for new vanadium-bearing deposits.