Genesis of the Jurassic carbonate-hosted Pb–Zn deposits of Jebel Ressas (North-Eastern Tunisia): evidence from mineralogy, petrography and trace metal contents and isotope (O, C, S, Pb) geochemistry

摘要

The Jebel Ressas Pb–Zn deposits in North-Eastern Tunisia occur mainly as open-space fillings (lodes, tectonic breccia cements) in bioclastic limestones of the Upper Jurassic Ressas Formation and along the contact of this formation with Triassic rocks. The galena–sphalerite association and their alteration products (cerussite, hemimorphite, hydrozincite) are set within a calcite gangue. The Triassic rocks exhibit enrichments in trace metals, namely Pb, Co and Cd enrichment in clays and Pb, Zn, Cd, Co and Cr enrichment in carbonates, suggesting that the Triassic rocks have interacted with the ore-bearing fluids associated with the Jebel Ressas Pb–Zn deposits. The δ¹⁸O content of calcite associated with the Pb–Zn mineralization suggests that it is likely to have precipitated from a fluid that was in equilibrium with the Triassic dolostones. The δ³⁴S values in galenas from the Pb–Zn deposits range from -1.5 to +11.4‰, with an average of 5.9‰ and standard deviation of 3.9‰. These data imply mixing of thermochemically-reduced heavy sulfur carried in geothermal- and fault-stress driven deep-seated source fluid with bacterially-reduced light sulfur carried in topography-driven meteoric fluid. Lead isotope ratios in galenas from the Pb–Zn deposits are homogenous and indicate a single upper crustal source of base-metals for these deposits. Synthesis of the geochemical data with geological data suggests that the base-metal mineralization at Jebel Ressas was formed during the Serravallian–Tortonian (or Middle–Late Miocene) Alpine compressional tectonics.