δD and δ18O Values of Halloysite-Kaolinite and Goethite-Hematite in the Fe deposit of Tamra (NW Tunisia): New insights for mixing fluids during ore formation

The iron mine of Tamra (Nefza District, NW Tunisia) is located within the approximately 50 m thick Zanclean sedimentary series. This sequence contains Fe-Mn oxide impregnation associated with halloysitekaolinite occurrences. Fluid evaporation during weathering and pedogenesis, and/or mixing of surface water and regional hydrothermal fluids are considered to be related to the concentration of iron in those sediments. The stable isotope geochemistry of oxygen and hydrogen of goethite-hematite, and halloysite-kaolinite, was examined in order to provide new insights to the ore formation. The main outcomes of this study indicate that halloysite-kaolinite and goethite did not equilibrate with the same fluid during supergene conditions. In a δD and δ18O diagram, kaolinite from the Tamra deposit plots to the right side of the kaolinite line, which would correspond to unlikely formation temperatures of below 0°C. δD and δ18O values of goethite allow for the same conclusion. The origin of these unusual compositions could be due to two main factors: 1) the arid Zanclean climate with mineral formation from evaporative brines and/or 2) fluid-mineral fractionations with relatively high δ18O hydrothermal fluids.