Abstract
The Imini mining district (southern foreland of the intraplate Atlasic belt of Morocco) hosts the largest Mn resources of North Africa, consisting of two laterally extensive bodies of high-grade pyrolusite-rich manganese ore and a third discontinuous medium-grade coronadite-rich Mn ore in a ~ 10–15-m-thick Cenomanian–Turonian dolostone unit. Until now, the origin and timing of the Mn ore have been poorly constrained. New Pb isotopic ratios show that Triassic series (basalts and ferruginous sandstone) are likely the source of the metals. 40Ar/39Ar dating of K-Mn oxides shows that the Mn-rich orebodies formed during at least three periods: late Cretaceous to late Paleocene (> 58 Ma), late Eocene (ca. 36.3 Ma), and early Burdigalian to early Serravalian probably in two pulses at ca. 19–20 Ma and ca. 13 Ma. These periods coincide with three known building phases of the Atlasic relief during late Cretaceous, late Eocene, and the Early(?)-Middle Miocene. We therefore propose the Atlasic tectonics as the first-order control of the Mn mineralization. Periods with regionally high elevations modified the climate to wetter conditions that supplied meteoric water to feed temporary aquifers. Relief building created the required hydraulic head to sustain (1) fluid-rock interaction between O2-poor acidic fluids and the Triassic series, (2) migration of the metal-rich fluid, and (3) to overpressure fluid in the Imini depositional site. The decreasing thickness of Triassic series in front of the Imini anticline forced these low-temperature (< 100 °C) fluids to mix with oxygenated and alkaline ground waters in the karst system and precipitate Mn oxides. The N70°-oriented Atlasic tectonic structure controls the orientation of the Mn deposits. The late Eocene–Early/Middle Miocene uplifts generated additional supplies and/or in situ remobilization of the primary late Cretaceous medium-grade ore to form the high-grade pyrolusite-rich ore.
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Acknowledgements
We are grateful to all those involved in the field missions carried out between 2012 and 2018. We acknowledge the “Office National des Mines et des Hydrocarbures” (ONHYM) for having shared their logistical and scientific support. We are very grateful to the Imini mine (SACEM) for providing field access and assistance in the field. We are thankful to Prof. M. Essalhi (University Moulay Ismail Errachidia), Th. Mortier (UMons), Z. Pirotte (ULB), B. Saint-Bézar (PSUD), and G. Delpech (PSUD) for field assistance. We appreciate the help of Prof. J. Wouters and Dr. N. Tumanov of the PC2 platform (UNamur) for collecting XRD patterns. We also thank G. Rochez for gathering field pictures and the preparation of sections and sample powders. Co-authors would like to warmly thank Jeroen De Jong for his assistance during the isotopic analyses and the “Fonds National de la Recherche Scientifique” (FNRS) for funding support in lab instruments. This research used the resources of the Electron Microscopy Service located at the University of Namur (“Plateforme Technologique Morphologie — Imagerie”). This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. We are very grateful to Prof. M. Bouabdellah and Prof. J. Gutzmer for their careful review of the paper, as well as Prof. G. Beaudoin for editing the final version.
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Supplementary file1 Additional file 1: Details of the 40Ar/39Ar and Pb isotope analytical procedures. Additional file 2: Lead, uranium, and thorium abundances and lead isotope compositions of the Tasdremt-Imini Mn ores and rocks from Neoproterozoic to late Cretaceous. AA=Anti-Atlas; S=Siroua; S-MHA= Southern Marrakech High Atlas; M-MHA= Middle Marrakech High Atlas; MA= Middle Atlas. Initial ages of Neoproterozoic volcanic rocks are from Gasquet et al. (2005), Neoproterozoic Mn ores are from Lippolt and Hautmann (1995), Cambrian basalts are from Landing et al. (1998), Ordovician shales are from Cornée and Destombes (1991), Triassic basalts are from Marzoli et al. (2019), Triassic red sandstones are from El Arabi et al. (2006), CT dolostones are from Rhalmi et al. (2000, 1997), Senonian sediments are from Algouti et al. (1999), Tasdremt Mn ores are from Dekoninck et al. (2021). Additional file 3: Analytical data and parameters used for calculations of 40Ar/39Ar results. Pure K, Ca, and Cl salts; mass discrimination; atmospheric argon ratios; J-parameter; and decay constants. Additional file 4: Major and trace element geochemistry. Additional file 5: K-Ar ages of coronadite group minerals in the Imini deposits. No ages are relevant for this study given the complexity of the Ar/Ar spectra. Additional file 6: 208Pb/204Pb vs 207Pb/204Pb and 208Pb/204Pb vs 206Pb/204Pb plots of the Imini and Tasdremt Mn ores and potential source rocks. Additional file 7: Altered basalts in the Atlas range. a Microscopic view of blue chayesite [K(Mg,Fe2+)4Fe3+Si12O30] under polarized light in Triassic basalt at Telouet (Figs. 1b and 2a). b Microscopic view of silicified Triassic basalt with chlorite-quartz druses under polarized light at Toufliht (Fig. 1b) (PDF 3238 KB)
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Dekoninck, A., Barbarand, J., Ruffet, G. et al. Intraplate orogenesis as a driver of multistage karst-hosted mineralization: the Imini manganese case (Atlas, Morocco). Miner Deposita 59, 453–472 (2024). https://doi.org/10.1007/s00126-023-01212-9
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DOI: https://doi.org/10.1007/s00126-023-01212-9