The Bougrine Zn-Pb deposit, the largest salt diapir-related Mississippi Valley-type deposit in the Eastern Maghreb salt diapir province, Tunisia

Abstract

The Bougrine Zn-Pb sulfide deposit (5.5 Mt @ 12 % Zn + 2.5 % Pb) in the Eastern Maghreb salt diapir province of Tunisia is located at the northeastern end of the Lorbeus-Bougrine diapir. The several-km large diapir is composed of Triassic evaporite that intruded about 10 km-thick Jurassic and Cretaceous marine sedimentary rocks during the Cretaceous and Tertiary. Mineralization occurs in breccia zones at the contact with the Triassic diapir and in the peridiapiric Cretaceous strata. Most of the ore is stratabound (F2 orebody) with microsphalerite disseminations in organic-rich laminated black limestone of the Cenomanian-Turonian Bahloul Formation, where sphalerite occurs as infill of Globigerina shells, as massive replacement of Globigerina-rich layers, as banded colloform sphalerite and in voids as euhedral, coarse-grained crystals together with skeletal galena and minor marcasite. Abundant degraded oil seepages in the orebodies and their host rocks testify to the existence of a (former) hydrocarbon reservoir.The various sphalerite generations have highly variable Mn and Fe contents; Cd has an average content of 445 ppm; As and Hg contents are in the 200 ppm range; Tl content is about 40 ppm; Ga content is about 10 ppm; and In content is consistently <1 ppm. Fluid inclusions in celestite, coarse-grained late-stage sphalerite and calcite indicate moderate fluid salinities (16 ± 2 wt% NaCl eq.) and moderate temperatures (85 to 146 °C). The empirical trace-element sphalerite geothermometer gives 184 ± 38 °C for the main-stage microsphalerite and the banded colloform sphalerite which have no measurable fluid inclusions.A wide range of δ13C (-13.6 to 5.6 ‰) and δ18O (-19.8 to 33 ‰) values is observed in ore-stage and post-ore calcite. Ore-stage calcite commonly has negative δ13C values reflecting involvement of carbon from organic matter; its δ18O composition with an average of 22.6 ± 2 ‰ (19.7 to 26.0 ‰) may be explained by a moderate-temperature hydrothermal fluid. There is no systematic δ34S variation between and across the different styles of mineralization. The range of δ34S of 6–19 ‰ with a mean of 13 ‰ is close to the δ34S value of local and regional Triassic marine sulfate (δ34S = 16 ‰). Thermochemical reduction of Triassic sulfate, likely mediated by hydrocarbons, is the most probable source for sulfur in the ores. Lead isotope data on galena are within a narrow field of upper crustal signature (206Pb/204Pb = 18.71–18.77; 207Pb/204Pb = 15.66–15.70; 208Pb/206Pb = 38.81–38.97) indicating a homogeneous ore fluid.The major characteristics of the Bougrine ore deposit are typical of Mississippi Valley type Pb-Zn deposits and indicate large-scale fluid circulation likely related to stacking and uplift in the northern Tunisian Nappes Zone during the Miocene. Both the hydraulic load of meteoric water and over-pressured basinal and basement fluids during compressional movements favored southward fluid migration along basement discontinuities and most permeable strata in the Mesozoic-Cenozoic sedimentary pile. Low-pressure domains around salt diapirs in a rifting environment allowed focused upward fluid flow, and the hydrocarbon reservoir of the Bahloul Formation then provided the geochemical trap for efficient base metal deposition.