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 delta(18)O 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 delta(34)S 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.