Mohammed Bouabdellah

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 Imourkhssen porphyry Cu±Mo±Au±Ag deposit is located at the Ouzellagh-Siroua Salient (OSS) straddling the boundary between the central Anti-Atlas and the central High Atlas. It is characterized by a typical porphyry-style mineralization. The volcanic rocks are intruded by numerous magmatic rocks of the Ouarzazate Group (580–539 Ma), referred to as the Late Ediacaran magmatic suites (LEMS). Of these, the Askaoun, Imourkhssen, and Imourgane granites are the most significant as they are related to the porphyry mineralization. The entire set is intruded by the Zaghar mafic dyke swarms. Zircon U-Pb dating of the Imourkhssen granite and the ore-bearing granite porphyry shows that these intrusive rocks were emplaced at 558 ± 1 and 550 ± 2 Ma, respectively. Moreover, the whole-rock major and trace element geochemistry reveal a high-K calc-alkaline I-type composition, consistent with an emplacement in a post-collisional setting under a trans-tensional tectonic regime. Ore bodies are hosted by the Askaoun granodiorite as well as the Imourgane granite. The mineralization occurs as fine-grained dissemination and infills of hydrothermally altered NNE–SSW to N–S trending veins and veinlets. Ore-related hydrothermal alteration consists of potassic, chlorite-sericite, serecitic, and propylitic mineral assemblages along with pervasive silicification and pyritization, providing a porphyry-style alteration pattern. The ore periods comprise supergene and magmatic-hydrothermal periods. The latter includes primary dissemination and secondary NNE–SSW to N–S ore-bearing system stages. The occurrence of molybdenite is either restricted to the potassic and chlorite-sericite alteration zones of the ore-bearing granite as fine disseminations or alternatively as veinlet infills within the propylitic halos. The molybdenite occurrences along with pyrite, chalcopyrite, galena, and tennantite dissemination are assigned to the primary ore stage, while the NNE–SSW to N–S ore-bearing system is related to the secondary ore stage. It consists of pyrite, chalcopyrite, bornite, covellite, diagenite, sphalerite, hematite, galena, gold, and chenguodaite. The predominance of cockade and crack-and-seal textures suggest multiple episodes of ore-forming fluid circulations under epithermal conditions. The supergene stage is achieved by subordinate malachite, azurite, barite, hematite, epsomite, and chrysocolla. From the descriptions above, we argue that the Imourkhssen Cu±Mo±Au±Ag mineralization shares many mineralogical and paragenetic attributes of porphyry-copper deposits.

The Roc Blanc Pb-Zn-Ag-Au vein deposit is located in the NW of Marrakech, in the Central Jebilet massif. It is spatially related to Bramram-Tabouchennt-Bamega (BTB) granodioritic pluton (ca. 330 Ma) metamorphism aureole. The main veins hosted in black shales are oriented N-S to NNW-SSE. Pb-Zn-Ag-Au ore is associated with quartz, chlorite, sericite, and carbonate gangue minerals. Two major stages of ore deposition were distinguished. The preore stage (stage I) comprises two quartz-mineralised vein generations with Fe, As, Zn, and Cu ores (vg₁ and vg₂). The main ore stage (stage II) consists mainly on Ag, Au, Pb, Zn, Cu, and Sb ores, which is hosted by carbonaceous vein (vg₃) and by two late quartz generations veins (vg₄ and vg₅ with a geodic quartz). Three types of fluid inclusions have been recognized in silver mineralisation bearing quartz veins according to petrographic investigations, microthermometry, and Raman spectroscopy studies: (i) liquid-rich H₂O-N₂-CH₄±CO₂-(salt) fluid inclusions (type 1), (ii) vapour-rich H₂O-CO₂-CH₄-N₂-(salt) fluid inclusions (type 2), and (iii) aqueous H₂O-(salt) fluid inclusions (type 3). The interpretation of fluid inclusion data shows a mixing of two fluids that are metamorphic and surface to subsurface origin, trapped at boiling state. The first mineralised stage was deposited at (this temperature of ore deposition was supported also by chlorite geothermometry) with salinity of 13.7 wt% NaCl equiv., while the deposition of the argentiferous stage, which consists of the main economic mineralisation of the Roc Blanc deposit, occurs during decreasing temperature at 150°C with a salinity of 12.1 wt% NaCl equiv. The all-mineralised ore was deposited at relatively low pressure, below ~1-1.1 kbar. So, fluid dilution and cooling are probably the main factor for silver deposition in the Roc Blanc polymetallic vein deposit. In addition, fluid inclusion studies reveal that the mineralising fluid corresponds to a mixture of metamorphic fluid (H₂O-CH₄-N₂-CO₂) with surface to subsurface aqueous gas-free fluids (H₂O-salt, meteoric, or brine).

The Igoudrane mine with a total production of 700,000 t of ore grading 485 g/t Ag is currently one of the most productive mines in the Imiter district of the eastern Anti-Atlas in Morocco. The silver-rich ± base metal deposit occurs dominantly as vein- and hydrothermal breccia-hosted orebodies at the interface between the lower Ediacaran turbidites of the Saghro Group and the unconformably overlying, dominantly felsic volcanic, and volcaniclastic rocks of the late Ediacaran Ouarzazate Group. Higher-grade ores are lithologically hosted by the uppermost organic-rich black shale unit and structurally controlled by the intersection of subvertical NW- and NE-trending fault systems. Ore-related hydrothermal alteration includes, in order of decreasing abundance, carbonatization, silicification, sericitization, and chloritization. Three primary paragenetic stages of veining and associated silver ± base metal mineralization have been recognized: (1) early pyrite + quartz + Ag-bearing sulfides and sulfosalts; (2) main Ag-bearing sulfides and sulfosalts + calcite ± fluorite ± dolomite; and (3) late quartz + calcite + base-metal sulfides (galena, sphalerite, pyrite, chalcopyrite). Irrespective of the ore stage, the dominant Ag-bearing ore minerals are Ag-Hg amalgam, argentite, freibergite, acanthite, polybasite, pyrargyrite, and proustite. Fluid inclusion data show a trend of decreasing temperatures with time, from the main silver stage (T_h = 180 ± 12 °C) to late base-metal stage (T_h = 146 ± 7 °C), consistent with fluid mixing, cooling, and/or dilution. The coexistence of aqueous-rich and vapor-rich fluid inclusions together with variations in bulk salinity (NaCl + CaCl₂) of the mineralizing fluids during the main silver stage, at similar temperatures, indicate that boiling and subsequent degassing occurred during the main ore-forming event due to a pressure decrease. Calculated δ¹⁸O_fluid values along with REE+Y and Sr isotope constraints suggest that the ore-forming fluids originated from a predominantly magmatic source, although incursion of meteoric waters during collapse of the hydrothermal system could have contributed to deposition. The post-ore, base-metal quartz-carbonate-dominated mineralization was deposited from dilute Ca-Na-Cl-bearing fluids at temperature below 150 °C. Overall, fluid–rock interaction with the black shales along major faults and thin permeable horizons, boiling-degassing—with subsequent fluid mixing, cooling, and/or dilution—were the main mechanisms of silver deposition.

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.