Geology, mineralogy and Fluid inclusions investigation of the fluorite deposit at Jebel Kohol, northeastern Tunisia

摘要

The Jebel Kohol F-(Ba-Pb-Zn) deposit is located in northeastern Tunisia, 90 km South of Tunis. The potential reserves are estimated in 0.4 Mt with 10% CaF2, 2% PbS, 1% ZnS and 2% BaSO4. The geology of the area is dominated by folded and thrusted structures of Cretaceous and Jurassic age which occur as massive limestone units. Fluorite and sulphides mineralizations are mainly associated with dolomitized and silicified Jurassic dolostones. The ore appears as carbonate replacement type and open space fillings type. Aqueous and aqueouscarbonic (CO_2) fluid inclusions from the Kohol fluorite mineralized area were analysed. Type F1 fluid inclusions show homogenization temperature between 105 °C and 137 °C with salinities between of 15 to 19 wt% NaCl equiv. Type F2 primary fluid inclusions of the massive purple fluorite show final ice-melting temperatures (Tm-ice) ranging from -16 °C to -12.5 °C corresponding to a salinity of 18.75 to 16 wt% NaCl equiv. The homogenization temperatures values range from 125 °C to 137 °C. The fluid inclusions in massive white fluorite (Type F3) have homogenization temperatures ranging from 115 °C to 140 °C and salinities ranging between 15 to 18 wt% NaCl equiv. The homogenization temperature and the salinity measured in fluid inclusions of white and purple fluorite in veins and massive types show close similarities. Microthermometric data indicates that the homogenization temperature and salinities of fluid inclusions in purple fluorite are generally higher than those of fluid inclusions in white fluorite. For the fluorite deposit in the Jebel Kohol, two end member fluids can be defined: (A) a moderate-temperature (> 100 °C), moderate salinity (< 20 wt% NaCl equiv.) brine for fluorite F1 and F2, and (B) a high salinity (> 20 wt% NaCl equiv.) fluid with a higher temperature close to 115 °C for the fluorite F3. Raman microspectroscopy shows that aqueo-carbonic fluid inclusions contain CO_2. The Pb-isotopic values from galena are close to upper Crust values. Our data suggest that galena mineralization occurred from remobilization of lead from crustal rocks. The Jebel Kohol fluorite deposit is characterized by minor sulphides and barite and shows epigenetic features. The present fluid inclusion study demonstrates the involvement of basinal brines to form the fluorite deposit of Jebel Kohol. Combined field, mineralogical, isotopic and geochemical data allow to include the Jebel Kohol ore deposit within the large class of Mississippi Valley-type F-(Pb) deposits.