Organic matter and metal contents within the Cretaceous rocks of the Slata-Guern Halfaya area, North-Central Tunisia: Implication for ore genesis

Abstract

The proximity of ore deposits, in the Slata-Guern Halfaya area (north-central Tunisia), to the Cretaceous Fahdene and Bahloul Formations raises the question of the role that these formations played in the emplacement of the ore. These formations are rich in organic matter (OM) with total organic carbon (TOC) values in the range of 0.2–1.4% and 0.62–5.46%, respectively, for the Albian–Vraconian facies of the Fahdene Formation at Slata and the Cenomano–Turonian facies of the Bahloul Formation at Guern Halfaya. Rock-Eval pyrolysis data indicates that the OM in the Fahdene Formation at Slata is relatively mature (oil window, 431 °C < Tmax < 447 °C) and with variable Production Index (PI, %)). Conversely, the OM preserved in the Bahloul Formation at Guern-Halfaya is immature (423 °C < Tmax < 433 °C). The analysis of whole rocks and the LA-ICP-MS analysis of inorganic fraction reveal high concentrations of trace elements (e.g., Pb, Zn) in both formations. We infer that seawaters were enriched in trace elements during the deposition of both formations. These trace elements were then sequestered by both organic and inorganic matter. Given the similar OM-rich lithologies (carbonates and marls) of these formations, the difference in trace element concentrations can be due to their contrasting thermal maturities. Concentrations of trace elements (Pb, Zn, Cd, and Ni) moderately correlate with OM content (TOC, pyrolyzable hydrocarbons and residual OM) positively in the organically-immature Guern-Halfaya ore district and negatively in the organically-mature Slata mining district. Subsidence of the OM-rich Cretaceous Formations (Fahdene and Bahloul), located in the deep part of the basin, resulted in thermal cracking of kerogen in these formations. This catagenesis allowed the dissociation of organo-metal ligands and subsequent expulsion of metals in low viscosity hydrocarbons during the Alpine orogeny. Metals and hydrocarbons migrated together from the subsiding areas of the basin and ultimately encountered salt diapirs. The mixing between metal-rich hydrocarbons and ore-bearing associated fluids, on the one hand, and the Triassic-derived sulfate-rich fluids, on the other hand, led to the precipitation of sulfides and sulfates at different times. A potentially effective trilogy to be targeted for base metal exploration is the juxtaposition of (i) mature OM-rich formations – potential sources of metals and hydrocarbons – with (ii) diapiric evaporites that served as a source of sulfur and sulfate, and (iii) a permeable reservoir that acted as a suitable site for ore precipitation.