The Miocene fluorite–schwazite–cinnabar veins at Jebel Oust dome, Zaghouan Fluorite District, Tunisia: fluid inclusions and evidence of mixing

The Jebel Oust dome F–(Hg–Cu) vein–type deposit in the Zaghouan Fluorite District is located at the intersection zone of the north–south axis (NOSA) and the NW-trending Zaghouan Messinian Rift. Mineralization occurs in E–W and NW–NE-trending fault zones cutting Liassic to Tithonian Age limestones, intercalating between the Triassic salt (below) and the Bir M’Chergua Shale (above). The ores form banded veins (calcite and fluorite) and matrix breccia veins (calcite–fluorite–schwazite–cinnabar). The paragenetic sequence is formed by macrocrystalline sparry calcite (Cal1), followed by massive macrocrystalline fluorite, then by matrix breccia fluorite with Hg minerals (schwazite and cinnabar). Late veins are composed of onyx-like calcite (Cal2), with no fluorite or sulfide minerals. Both fluorites show halite-bearing inclusions (LVH), carbonic inclusion CO 2 gas, and aqueous inclusions (LV). They are also present in sparry calcite and onyx-like calcite. The fluid inclusion microphotometry, coupled with the paragenetic sequence, shows that the mineralization results from the mixing of at least two fluids: a high-salinity and high-temperature fluid (32 to 34 wt% NaCl eq. and Th from 225 to 282 °C) and low-salinity and low-temperature fluid (5 to 12.8 wt% NaCl eq. and Th between 85 and 144 °C). The fluorite ore is formed when the hot waters enriched with fluorine, Hg, Cu, and salt rise towards the calcium-rich Jurassic limestone and mix with low-temperature and low-salinity groundwaters. Fluorite derives its calcium from pre-deposited calcite and from the limestone wall rock. The high-salinity fluid acquires its salinity from underlying Triassic salt. The high temperatures are related to an abnormal high geothermal gradient with thermal heat due to crustal thinning and rifting during the Messinian extensional episode of the Alpine orogen.