Interplay of magmatic and diapiric environments in the Djebel El Hamra Pb-Zn-Hg ore district, northern Tunisia

The Djebel El Hamra Pb-Zn-Ba-Sr (Hg) deposits in northern Tunisia are hosted in a post-nappe anticline with a core of a Triassic evaporite diapir affected by the NE–SW-trending Ghardimaou-Cap Serrat lineament. Three stages of mineralization occurred in the Triassic dolostone: stages I and II caused alternating deposition of sulfate (Ba, Sr) and sulfide (sphalerite, galena) minerals; stage III formed late-stage calcite-marcasite-cinnabar. Zebra textures record the syntectonic transition from compression to extension in the Late Tortonian-Messinian interval. Two fluid end-members were involved in sulfate deposition: one low-salinity (L1, ~3 wt% eq. NaCl) fluid, probably from a meteoric origin, and a Na-Ca-Cl brine (L2, ~22 wt% eq. NaCl) solution which originated from the Triassic diapiric source. A third end-member fluid (L3) with long residence time in the basement was also involved in the Pb-Zn deposition. The δ 34 S values from a cluster of sulfates around +16‰, show a Triassic evaporate source. The sulfur in sphalerite resulted from bacterial sulfate reduction (BSR); however, crystallization in a closed system resulted in a range of δ 34 S between +1.6 and +26.5‰. The δ 34 S values in galena (−28.4 to +8.2‰) are consistent with a BSR and thermochemical sulfate reduction (TSR) origin of the sulfur. Secondary ion mass spectrometry (SIMS) lead isotope data in galena ( 207 Pb/ 204 Pb: 15.595 to 16.193, 206 Pb/ 204 Pb: 18.673 to 18.939, 208 Pb/ 204 Pb: 38.330 to 40.572) point to local contributions (sedimentary and Cenozoic magmatic rocks) to the main source from the Precambrian basement. Ore deposition occurred at a depth of about 2 km at temperatures between 80 and 250 °C. A shallow magmatic heat source was the cause of these thermal fluctuations.