P-T-X reconstruction for ore deposits using petroleum-rich fluid inclusions in fluorite: A case study in the Bou Jaber diapir-related Ba-Pb-Zn-F deposit, Northern Tunisia

摘要

The Bou Jaber ore deposit is one of the numerous diapir-related Pb-Zn-F-Ba deposits of the Dome Zone in Northern Tunisia. Its location is controlled by the regional NE-SW Tajerouine Fault.Ore minerals are hosted in the Late Aptian limestones (Serdj Formation) as open space filling and stratabound replacement bodies. According to Bouhlel et al. (2016), the poly-phase mineralization resulted from the successive activity of three mineral systems, a Pb-Zn, then a barite, and eventually a fluorite deposition system. The latter, from the Late Miocene, is demonstrated in the present study.The fluorite system is characterized by the involvement of oil in the hydrothermal fluids. This oil was produced in the local environment of the deposit from the thermal maturation of the Albian Fandene black shale source-rock at temperature range of similar to 140 degrees C-similar to 100 degrees C. Two brines were involved in the fluorite hydrothermal system. The first one (L1) is a Ca-rich brine (>= 20 wt % bulk salinity), with Na/Ca <= 0.18, which is thought to have long resided in the basement, before its transfer into the Jurassic reservoir (Upper Nara Formation) and its eventual mobilization at the time of ore deposition. The second brine (L2), less saline (<= 14 wt % bulk salinity) is more sodic, with Na/Ca up to 0.53, and represents unmodified brine originated from the nearby Triassic salt. The L1 brine was F-bearing, whereas the L2 brine was associated with oil. Fluorite deposition occurred at the estimated shallow depth of 1.7 km from cooling of the L1 brine due to the first isobaric cooling from similar to 160 degrees C to 135 degrees C under sub-lithostatic conditions (36 MPa). This stage is followed by mixing with the newly incoming L2 brines (transporting oil) along a sub-isochoric decompression path (down to the hydrostatic pressure at 17 MPa) and continuously cooling from 135 degrees C to 125 degrees C. The cooling and mixing caused fluorite deposition after which a transient heat advection episode (up to 145 degrees C) caused late calcite deposition prior to the end of hydrothermal circulation.