Oxygen and hydrogen isotopic composition of clay minerals of the Bahloul Formation in the region of the Bou Grine zinc–lead ore deposit (Tunisia): evidence for fluid–rock interaction in the vicinity of salt dome cap rock

Abstract

The siliciclastic Bahloul Formation of Tunisia is the host for Zn/Pb deposits. A detailed study of 18 O/ 16 O and D/H ratios of kaolinite, quartz and calcite was done to elucidate the conditions and mechanism of deposition of the diapir-related Bou Grine Zn/Pb ores in the formation. D/H and 18 O/ 16 O ratios of the minerals of the host rock have been affected by the mineralizing process. The magnitudes of the isotopic shift in 18 O/ 16 O of kaolinite as well as the intensity of base metal mineralization seem to be governed by the permeability of the Bahloul strata, due to different water–rock ratios. In contrast, the variability in D/H ratios of kaolinite shows no correlation with the estimated permeability of the respective sedimentary section. Published microthermometric fluid inclusions data and organic geochemical maturation parameters both argue for a maximum temperature between 80 and 120°C during ore formation and host rock alteration. The 18 O/ 16 O ratios of the clay minerals of samples taken over a range of distance from the ore body suggest alteration by hot, saline, 18 O -enriched brines. These could have evolved at the flanks of the diapir and would have been capable of remobilizing metals from deeper sedimentary sequences, providing the metals now found in the Bou Grine deposit. The δ 18 O -value of the mineralizing fluid is estimated to have been between +8 and +18‰. The δD-value was between −30 and −55‰. The high δ 18 O -values of these basinal brines provide evidence for a control of the isotopic composition of the fluids by 18 O -rich carbonate minerals of the post-Triassic limestones. Hydrogen isotopic data argue for a meteoric origin of the brines. The variability of the δD-values might be explained by a contribution of evaporated meteoric waters entering the groundwater system, and mixing of these waters with the hot saline brines in the contact zone of the diapir.