Through integration of Pb-Zn ± Cu non-sulfide mineralogy, texture, and stable isotope (C, O, S) geochemistry, the world-class Touissit- Bou Beker and Jbel Bou Dahar Mississippi Valley-type districts of the Moroccan Atlasic system have been investigated in order to gain insights into the origin and processes that contributed to the formation of the base metal non-sulfide mineralization. In both districts, direct replacement (“red calamine”) and wallrock replacement (“white calamine”) ores are observed. Based on the mineral assemblages, ore textures, and crosscutting relations, three distinct mineralizing stages are recognized. The earliest, pre-non-sulfide gossanous stage was a prerequisite for the following supergene stages and constituted the driving force that ultimately promoted the leaching of most base metals such as Zn and Cu and alkalis from their rock sources. The following two stages, referred to as the main supergene “red calamine” and late “white calamine” ore stages, generated the bulk of mineable “calamine” ores in the Touissit-Bou Beker and Jbel Bou Dahar districts. Stable isotope compositions (δ¹³C_V-PDB, δ¹⁸O_V-SMOW, δ³⁴S_CDT) support a three-stage model whereby metals were released by supergene acidic fluids and then precipitated by bacteria and archaea-mediated metal-rich meteoric fluids due to a decrease in temperature and/or increase of f_O2. Oxygen isotope thermometry indicates decreasing precipitation temperatures with advancing paragenetic sequence from 33° to 18 °C, with wet to semi-arid to arid climatic conditions. The close spatial relationships between coexisting sulfide and non-sulfide mineralization along with stable isotope constraints suggest that the oxidation of sulfides occurred concurrently after the main stage of the Alpine orogeny between 15 Ma and the present. More importantly, the current data show for the first time the involvement of biologically controlled activity as the major driving process that triggered both oxidation and deposition of supergene mineralization at Jbel Bou Dahar and Touissit-Bou Beker districts. Conclusions drawn from this study therefore have implications for supergene Mississippi Valley-type (MVT) -derived non-sulfide deposits worldwide and account for the prominent role of biological processes in the genesis of this category of ore deposits.

Unusual Ag-(Ni-Co-Sb-As-Hg ± Bi)-bearing fault-fill vein ore shoot mineralization set in a gangue of quartz, fluorite, and barite has been identified in Morocco’s Aouli deposit. The Paleozoic host rocks consist of a succession of Cambrian to Ordovician-aged folded and low- to medium-grade metasediments and metavolcaniclastic rocks with tuff interbeds and amphibolite sills, locally intruded by late Visean calc-alkaline to alkaline granitoid intrusions. Paragenetic relationships indicate that the sequence of ore precipitation comprises a succession of Ni-Co-Fe arsenides, followed by Pb-Sb-As-Ag-Hg sulfarsenides/sulfosalts and then Zn-Pb-Fe sulfides. Results indicate that the ore shoot mineralization formed from episodic stages of fracturing and subsequent fluid migration. Precipitation of ore phases is thought to have occurred as a result of isothermal mixing and subsequent fluid–rock interactions. The timing of mineralization is thought to have occurred between Late Triassic and Late Miocene, coinciding with major crustal extension and Middle Jurassic–Upper Cretaceous alkaline magmatism. Thermal convection and seismic pumping are proposed as the main driving force for the large-scale migration of the ore-forming brines. This research bears directly upon the potential for new exploration targets in Pb-Zn ± fluorite ± barite deposits hosted in Variscan inliers throughout North Africa.

The Beni Snassen belt (northeastern Morocco) hosts several Mississippi Valley-type Pb-Zn ± Cu occurrences localized along the Variscan basement/Lower Liassic carbonate interface within the Atlasic foreland system. This study integrates geological observations with organic petrography and C-O-S-Pb isotopic systematics to constrain the origin of mineralizing fluids, metal source, and ore-forming processes within a basin-scale metallogenic system. The host sequence consists of unmetamorphosed, dolomitized Pliensbachian carbonates with marl interbeds and organic-rich black shales. Mineralization is structurally focused along ENE-WSW and E-W faults and occurs as massive calcite-galena veins, “en échelon” tension gashes, vug fillings, and solution-collapse breccias. Ore-stage calcite exhibits restricted isotopic variability (δ¹³C = −4.7 to +1.2‰; δ¹⁸O = 14.9 to 19.7‰), consistent with rock-buffered basinal fluids and extensive fluid–carbonate interaction. Calculated δ¹⁸OH₂O values indicate precipitation from evolved saline brines variably mixed with meteoric waters. Galena δ³⁴S values (−20.9‰ to +10.3‰) reflect thermochemical sulfate reduction (TSR) under fluctuating redox conditions. Pb isotope compositions define a tight linear cluster between upper crust and orogene growth curves, indicating a predominantly upper crustal metal source, notably Triassic dolerite–diabase lithologies, with a possible contribution from organic-rich black shales. High-reflectance pyrobitumen (VR₀ up to 4%) indicates thermal conditions exceeding those predicted by local burial history, supporting long-distance migration of hydrocarbon-bearing metalliferous fluids from overpressured basin compartments, most plausibly the adjacent Neogene Guercif Basin. Fault reactivation during Late Miocene transtension fostered basin-scale fluid focusing and ore deposition. Hence, the Beni Snassen district represents a basin-integrated MVT system involving crustal metal leaching, organic-assisted metal transport, TSR-mediated sulfur reduction, and structurally focused fluid flow. These results refine metallogenic models for the Atlasic belts and highlight the exploration potential of structurally reactivated foreland basins hosting coupled hydrocarbon-hydrothermal systems.

The trace-element geochemistry of sphalerite is commonly used as a proxy for the sulfur fugacity and temperature evolution of ore-forming fluids. In this study, in-situ laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) analyses of paragenetically well-constrained sphalerite crystals spanning the sequence of mineral deposition in the Aouli polymetallic Pb-Zn-Cu (Ag, Ni, Co, As) deposit, with indicated resources of 10 Mt of ore at an average grade of 5% Pb, 150–600 g/t Ag and 200–700 g/t Bi, were performed to track back the physico-chemical characteristics of the ore-forming fluids responsible for the deposition of Cu-Ag-Ni-Co-As-rich and Pb-Zn-rich ores. The sulfide/sulfosalt Pb-dominant mineralization consists of an array of open-space fillings of transtensional veins, veinlets, stockwork, and breccia veins. The host rocks of presumed Cambrian–Ordovician age comprise a variably deformed and weakly metamorphosed succession of interlayered low- to very-low-grade siliciclastic and volcaniclastic rocks with interbedded tuffs, and amphibolites. Several syntectonic to late tectonic granitoid batholiths and stocks (ca. 340–330 Ma), ranging from calc-alkaline to alkaline and metaluminous to peraluminous, as well as mafic rocks (gabbro-diorite) and cordierite-garnet anatectic granites, intrude the host rocks. The idealized paragenetic sequence includes an early pre-ore iron-sulfide-rich stage, followed by the main stage of Zn-Pb ore and the later Cu-Ag-Ni-Co-As-rich stage. Gangue minerals comprising quartz, fluorite, and barite were formed during the main ore mineralization stages. The trace element composition of sphalerite is used to compare the mineralization conditions between Pb-Zn-rich and Cu-Ag-Ni-Co-As-rich ores. Sphalerite typically occurs as greenish to brownish-colored, closely packed patches of anhedral to euhedral sphalerite crystals ranging from 100 μm to up to 5 cm in size, embedded in quartz ± fluorite ± barite. Based on paragenetic position, mineral color, textural features, and luminescence in CL alongside chemical compositions, three distinct generations of sphalerite referred to as Sp-1, Sp-2, and Sp-3 are recognized. Statistical analyses of the compositional data show systematic differences between the three generations of sphalerite. Compared to Sp-2 and Sp-3, Sp-1 shows high Co, Ga, and In concentrations, while Sb, As, Cu, Hg, Pb, Mn, and Ag concentrations are lower. Cu, Ga, Sb, and Mn concentrations of Sp-2 and Sp-3 display similar distribution trends. Conversely, Sp-3 relative to Sp-2 is enriched in As, Ag, and Hg but more depleted in Fe. Formation temperatures and sulfur fugacity estimated from the GGIMFis (Ga, Ge, In, Mn, and Fe in sphalerite) geothermometer and Fe contents in sphalerite indicate similar T and fS2 conditions for the Cu-Ag-rich and Pb-Zn-rich ores, with temperatures < 250 °C. More importantly, the evolutionary trend shown by the three generations of sphalerite which plot within the intermediate-sulfidation field along the “rock buffer” line, argues for a common origin for the Cu-rich and Zn-rich ores, and that the hydrothermal ore fluids interacted extensively with the enclosing host rocks.