THE GEOLOGY, MINERALOGY, FLUID INCLUSIONS, AND C-O-S-PB ISOTOPES STUDIES AT THE BOU-DAHAR PB-ZN-(BA) ORE DISTRICT (CENTRAL HIGH ATLAS, MOROCCO): IMPLICATIONS IN THE GENESIS OF MINERALIZATION AND ITS RELATION TO BASIN INVERSION

The Bou-Dahar Mississippi Valley-type Pb-Zn-(Ba) is hosted in Lower and Middle Liassic carbonate platform initiated probably by a local early Liassic halokenisis activity. The paragenetic sequence is simple: quartz-calcite-pyrite-melnicovite-sphalerite-galena-barite-fluorite-celestite.

The fluid inclusions analysis carried out on sphalerite, barite, and celestite reveals two end-members fluids: one ascending hot, saline fluid (brine) and one descending cooler oxidizing meteoric fluid. The mixing ratio brine/meteoric fluid decreases with time along the paragentic sequence. At the early stage, the mixing produced a hot (165 ^oC) and saline fluid (23 % Wt. eq NaCl) responsible for the precipitation of sulfides (galena, sphalerite). As this cooling-dilution process continues, phase separation or immiscibility occurs giving rise to cooler and less saline fluid responsible for barite and celestite.

Sulfur isotopes recorded in galena, sphalerite, barite, and celestite, revealed δ³⁴S values ranging from -4.0 to 6.8‰, 0.7 to 5.0‰, 17.2 to 20.4‰, and 15.4 to 15.8‰ respectively. Sulfur derived mainly from Triassic and Jurassic sulfates and has been reduced through thermochemical sulfate reduction.

Carbon and oxygen isotopes studies performed on Liassic host limestone and hydrothermal calcite reveals that carbon of calcite derived from the local host limestone rock and that δ¹⁸Ofluid _SMOWis around 6.5 ‰ average typical of the brine-type fluid.

Lead isotopes on samples of galena reveal lead isotopes ratios ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb, and ²⁰⁸Pb/²⁰⁴Pb ratios ranging from 18.49 to 19.24, 15.51 to 16.16, and 38.39 to 39.62. They reveal a linear positive array in the ²⁰⁸Pb/²⁰⁴Pb vs. ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁷Pb/²⁰⁴Pb vs. ²⁰⁶Pb/²⁰⁴Pb diagrams and plot in the upper crustal and orogene reservoirs. This linear array is explained by mixing of two sources reservoirs of lead with distinct isotopic compositions.

The age of the mineralization is Miocene (~23 Ma) and is related to the collision between African-European plates during which the orogenic metal-rich brines were expulsed and migrated, along thrusting regional E-W and NE-SW deep-seated faults, to the carbonate-Liassic reservoir where they progressively reacted and mixed with a cooler meteoric fluid in the Mesozoic cover.