The genesis of the Jurassic-hosted Mississippi Valley-type Pb-Zn ore deposit, Tigrinine-Taabast district (Central High Atlas, Morocco): Insights from fluid inclusion and C-O-S-Pb isotope studies

摘要

The Mississippi Valley-type Pb-Zn ore deposit of the Tigrinine-Taabast ore district, located in the southern border of the Moroccan Central High Atlas, is hosted in hydrothermally dolomitized Hettangian-Sinumerian carbonates. The mineralogical paragenetic sequence is simple: Quartz-dolomite-pyrite-galena-sphalerite. Fluid inclusions carried out on sphalerite reveal that sulfide ore precipitated from a NaCl-CaCl2-MgCl2 basin-derived warm (78 degrees-168 degrees C) and saline (21-36 wt% NaCl equiv.) brine. The carbon and oxygen isotopes of the ore-related calcite (Cal-1) suggests an equilibrium between the calcite-forming fluid and carbonates. The late post-ore calcite (Cal-2) reveals the influx of the isotopically light meteoric fluid during the precipitation of this calcite. The delta S-34 values range from 6.6 to 9.4 parts per thousand and from 12.7 to 13.0 parts per thousand for galena and sphalerite, respectively. Sulfur derived from dissolved Triassic-Jurassic sulfates and/or coeval seawater pore sulfates primarily through thermochemical sulfate reduction and possibly bacterial sulfate reduction. Galena samples have a homogeneous Pb isotopic composition with (206)pb/(204)pb, (207)pb/Pb-204, and Pb-208/Pb-204 ratios ranging from 18.229 to 18.236, 15.643 to 15.655, and 38.448 to 38.527, respectively. This uniform Pb isotope composition suggests that Pb and by inference other metals derived primarily from the Paleozoic rocks. During the African-European Cenozoic collision, the metal-bearing fluid was remobilized and migrated through the major faults and mixed with a shallow, diluted, cooler fluid, leading to the precipitation of ore sulfides.