The Mississippi Valley-type fluorite ore at Jebel Stah (Zaghouan district, north-eastern Tunisia): Contribution of REE and Sr isotope geochemistries to the genetic model

摘要

The fluorite district of Zaghouan, north-eastern Tunisia, is characterized by F-(Ba-Pb-Zn) ore deposits. Mineralization occurs within stratabound to stratiform bodies, either within Jurassic back-reef limestone uplifted blocks, or along unconformity surfaces that separate them from the overlying sequence. Fluorite mineralization also occurs within fractures crosscutting the uplifted limestone blocks and their overburden. Despite the similarity of the Liassic series throughout the major part of the district, stratabound fluorite deposits formed subsequent to a strong epigenetic dolomitization (Souissi et al., 1998), and occurring along the Lower-Middle Liassic unconformity, are known only at Jebel Stah and at the adjacent Jebel Kohol. Four petrographic types of fluorite are recognized: the finely-laminated karst deposits (Fl1a), the macrogranular fluorite (Fl1b), resulting from the recrystallization of Fl1a, and the megacrystalline fluorite (Fl2) associated to calcite in lodes, and fluorite in geodes (Fl3). This study gives new arguments in terms of the genetic model of the fluorite ore of Jebel Stah and by using trace metal elements (MTE), rare earth elements (REE) and Sr isotope geochemistry. The MTE geochemistry shows, that the epigenetic dolomites and the fluorite Flla bear higher concentrations in Mn and Zn, with respect to the fluorites Fl1b, Fl2 and Fl3, while the latter are depleted in all MTE, but enriched in Co. The similarity of their REE patterns indicates that the latter are coeval, and that Fl1a fluorite may be considered as the early ore generation. The same argument confirms that this geochemical imprint has been inherited from a hydrothermal fluid that had remained in equilibrium with shales at depth. Fluorite deposition appears related to an oxidized fluid derived from a sedimentary basin. Fluid circulation may have been triggered in response to hydraulic fracturing within a high geothermal gradient setting, especially along the major structural trends. Fl1b, F12 and Fl3 fluorites have been generated from the residual fraction of the ore fluid at later stages of mineralization and in a closed system. Both mineralization and the epigenetic dolomites are characterized by Sr isotope ratios that are higher than the barren Lower-Middle Liassic rocks, indicating that the Sr cannot be derived from the wallrocks. However, these ratios are higher than the Triassic seawater; yet consistent with those of the Paleozoic period. Thus, the fluorite ore of Jebel Stah, is believed to be remobilized from primary ore deposits which are suspected to be hosted in the late series within this period.