PbZn mineralization associated with diapirism: Fluid inclusion and stable isotope (H, C, O) evidence for the origin and evolution of the fluids at Fedj-el-Adoum, Tunisia

Abstract

Pb-Zn mineralizations occur in the contact zone to a saliferous diapir and its surrounding Cretaceous limestonerich sediments. The diapir of Triassic formations was initially intruded hot from a depth of 3–7 km during the Cretaceous. Three principal types of mineralization occur: bedded, vein and massive. Combined microthermometric and stable isotope (H, C, O) studies have characterized three principal thermal events before cool meteoric waters entered the system: 1. (1) Hot (170–250°C) saline (22–33 eq. wt.% NaCl) fluids of probable deep formation water origin with δ D∼ −43‰ and δ 18 O∼ +20‰ precipitated calcite (with δ 13 C∼ −8‰ ) and celestite in the inner part of the contact zone with the diapir. 2. (2) Hot (100–130°C) saline (18–23 eq. wt.% NaCl) fluids with δ D∼ −60‰ and δ 18 O∼ +13‰ (containing CO 2, N 2 and hydrocarbons) precipitated sphalerite, calcite ( δ 13 C∼ −9‰ ), quartz and minor galena as banded mineralization in the outer part of the contact zone. More galena-rich vein mineralizations in the contact zone were deposited at similar temperatures from comparable fluids. Mineralizing solutions are interpreted to be formation waters with the carbonate carbon being derived from organic carbon. The colloform textures in the banded mineralization suggest that the nearby sedimentary basin underwent episodic dewatering. 3. (3) Warm (70–90°C) saline (16–22 eq. wt.% NaCl) fluids with δ D> −110‰ and δ 18 O∼ +10‰ precipitated calcite (with δ 13 C∼ −16‰ ), galena-rich ore in the massive mineralizations and vein mineralizations in the Cenomanian-Turonian limestones, and essentially sulphide-free calcite in the banded and contact vein mineralizations. A locally derived organic water origin is proposed where both hydrogen and carbon were derived from organic constituents such as petroleum during bacterial sulphate reduction reactions. Variable δD and salinity values suggest mixing processes between the D-depleted organic water and a more D-rich formation or meteoric water. 4. (4) Low-temperature (<50°C) meteoric waters with δD∼ −15 to −25‰ introduced a second generation of calcite, with the carbonate largely coming from the sedimentary carbonates, in the massive and Turonian vein mineralizations. Anhydrite was probably converted to gypsum at about this time and before Fedj-el-Adoum arrived at its present altitude.