Geology and geochemistry of carbonate-hosted Pb–Zn–F mineralizations from Jebel Lassel and Bou-Izem in the Kherrata area (Nappe zone of the Tellian Atlas Mountains, northern Algeria)

Abstract

The Jebel Lassel (JL) and Bou Izem (BIZ) Pb–Zn–F mineralizations are widely found in the Djamila nappe which is the dominant structural unit in the Kherrata area (northern Algeria). They are hosted respectively in Upper Cretaceous limestone and marly limestone and in dolomite from a Triassic complex at the contact of the thrust sheets. They occur in tension veins of different fillings and directions which can be divided into several types: (i) galena-calcite veins, (ii) sphalerite-calcite veins, and (iii) fluorite-calcite veins. The mineralizations consist mainly of galena or sphalerite, fluorite, and pyrite. Supergene oxidation minerals include cerusite, smithsonite, and iron oxides. Calcite, dolomite, and to a lesser extent quartz are the main gangue minerals. Fluid inclusions (FI) from Bou Izem fluorite samples occur in three phases at room temperature (L + V + halite) and homogenize by the dissolution of the halite crystal after the vapor bubble has disappeared. They are characterized by high salinities (28–39% wt% NaCl equiv.) and high homogenization temperatures (Th) (222–299 °C). Single-phase fluid inclusions are very abundant in quartz crystals from the Bou Izem borehole samples. These inclusions contain a CO₂ and CH₄ mixture and are nearly pure methane with less than 10% CO₂. At the Jebel Lassel anticline, FIs in fluorite and sphalerite exhibit low ice-melting temperatures (– 11 and – 37 °C) and homogenization temperatures in the liquid phase of 103–174 °C. Primary FIs in calcite studied in late crystals associated with sulfides and fluorite have Th of 141–196 °C, with the final ice-melting temperatures (Tm_(ice)), ranging from − 37 to − 8 °C, corresponding to salinity values ranging between 14 and more than 23.2 wt% NaCl equiv. δ¹³C_V-PDB values of the calcite gangue, which vary between − 1 and + 2‰, indicate an inorganic carbon origin and are consistent with a marine origin of the host rocks and heritage. Most δ¹⁸O_V-SMOW values of calcites are between + 16 and + 25‰ and indicate that the oxygen isotopic compositions of the fluids varied between + 5 and + 16 ‰. These values are typical of deep brines in sedimentary basins. δ³⁴S_V-CDT values vary from 9.5 to 9.8 (galena) and from 12.7 to 14.5‰ (sphalerite), and the high temperature of the fluid exclude all possibilities of bacterial sulfate reduction and indicate that the source of the sulfur is the thermochemical reduction of sulfate from Triassic evaporites (δ³⁴S =  + 15‰). The fluorite samples from JL and BIZ reveal that all patterns fit into two different types without (Eu) and (Ce) anomalies. Combined, geological, mineralogical, isotopic, and geochemical data of vein mineralization at Jebel Lassel and Bou Izem confirm the epigenetic character of the mineralization and enable us to classify them as Mississippi Valley-type deposits.