Manganese ores in Tunisia: Genetic constraints from trace element geochemistry and mineralogy

Abstract

Northern and central Tunisia hosts few small-scale stratiform and stratabound Mn deposits, low to medium in grade (Mn < 46%), which have a spatial distribution related to major reactivated NS- and NW-oriented faults. The Mn deposits can be broadly classified into sedimentary rock-hosted and karst-hosted. The ores occur in two different structural settings and are hosted by deposits of various age, lithology, size and grade. The geology shows that the Mn deposits are diachronic and were formed in continental and/or shallow marine environments. They commonly occur in close spatial association with Fe ores and are genetically not related to Pb-Zn-(Ba-Sr-F) deposits. Bulk Mn ores are characterized by high and variable Mn/Fe ratios and an enrichment in Pb, Zn, Ba and Sr. Shale-normalized trace element (TESN) patterns of all studied Mn ores show enrichment in the assemblage of Sr, Ba, Co, Cu, Mo, Pb, and Zn and depletion in high-field strength elements Zr, Nb and Th, which is characteristic for hydrothermal ore formation. Total rare earth element (REE) contents show considerable variations and REYSN patterns indicate a complex interplay of hydrothermal, marine and meteoric/supergene processes. LA-ICP-MS analysis of Mn oxides show considerable variations in TE contents and REYSN patterns due to differences in oxide mineralogy, fluid source(s), and their rate of precipitation and related scavenging processes. The TE and REY geochemistry of the Mn ores indicate a complex interplay of primary and secondary processes that were responsible for the formation of the Mn mineralization. In the Tamra Fe-Mn deposit (Nappes zone, polymetallic district and volcanosedimentary context), the Mn ores (stage 1 romanechite-hollandite) occur as porosity and/or microfracture fillings in the ferruginous matrix, as impregnation and massive concretions and also as late cavity fillings (stage 2 coronadite-chalcophanite). Mixing of hydrothermal and predominantly meteoric fluids induced the formation of typical supergene Mn ores. The stratiform Mn ores (pyrolusite) of Jebel Es Stah (southern Tunisian Atlas, Miocene alluvial system) occur as a syngenetic to early diagenetic oxide coating and cementation of detrital sand grains. REYSN patterns reflect the predominant influence of siliciclastic material as detritus in the Mn ore. Minor Mn ores (cryptomelane, hollandite) occurring as duricrust and fracture filling recorded in the Cenomanian-Turonian dolostone in central and southern Tunisian Atlas show flat REYSN patterns and negative CeSN anomalies and suggest remobilization of Mn under supergene conditions. In the Jebel Ank Fe-Mn deposit (southern Tunisian Atlas, hosted in a relatively restricted shallow basin), cryptomelane occurs as centimeter-sized irregular nodules in the late Eocene marls. Based on REY data, a primary hydrogenetic origin is suggested for Mn nodules. The formation of cryptomelane may have occurred at relatively late stages of weathering when local increases in pH and oxidizing conditions prevailed with the input of K+ from phyllosilicate-bearing layers. The stratabound Mn ores (stage 1 hollandite and stage 2 romanechite-coronadite) in the Jebel Aziza Mn deposit (southern Tunisian Atlas, shallow marine platform facies) correspond to karstic filling and stockwork type in the late Cenomanian-early Turonian dolostone. The Mn oxides in the deposit are epigenetic and their formation is related to changes in Eh and pH. The REYSN patterns for Mn ores and Mn oxides are between those of a mixed seawater-hydrothermal- and supergene-type Mn ore. The petrography, mineralogy, TE contents and REYSN patterns of the different Mn ores indicate interplay of hydrothermal, hydrogenetic and supergene weathering processes occurring during formation of the deposits. The data indicates different mineralization stages and different processes that lead to the low-temperature formation of Mn oxides typical of supergene deposits commonly found in North Africa.