How to create the highest manganese ore grade in the world? A geodynamic story in the Atlas of Morocco.

<p>The Imini mining district (Morocco) hosts the largest manganese (Mn) ore deposits of North Africa (~120.000 T in 2016). The area is world recognized for hosting several epigenetic karst-type high-grade manganese deposits in a 10-15 meters thick Cenomanian-Turonian dolostone unit. These unconventional Mn oxide deposits occur along a belt of ~25-30&#160;km in the southern foreland of the intraplate Atlasic belt of Morocco. This is due to two laterally extensive ore bodies of nearly pure pyrolusite-rich manganese ores (72-88&#160;wt.% MnO₂) and a third discontinuous medium-grade coronadite-rich Mn ore (40&#8211;48&#160;wt.% MnO₂)(Dekoninck <em>et al.</em>, 2016a, b; Gutzmer <em>et al.</em>, 2006). Our recent works allowed precise dating and geodynamic reconstructions of the ore deposit genesis. The ore depositions occurred during (i) late Cretaceous to late Paleocene (~ 92&#160;Ma, ~ 78&#8211;82&#160;Ma, ~ 65&#8211;67&#160;Ma and ~58&#160;Ma), (ii) late Eocene (c. 36 Ma), and (iii) early Burdigalian to early Serravalian probably in two pulses at c. 19&#8211;20&#160;Ma and c. 13&#160;Ma (Dekoninck <em>et al.</em>, 2021, 2023). This multistage deposition coincides with three geodynamic events linked to the uplift of the Atlas. The late Cretaceous uplift of the Atlas created the required hydraulic head to sustain (1) fluid-rock interactions between O₂-poor acidic ground waters and the Triassic series source, (2) migration of the metal-rich low-temperature hydrothermal fluid from the rock source and (3) overpressure fluid in the Imini depositional site. The vanishing of Triassic series above the Imini anticline forced these hydrothermal fluids to mix with oxygenated ground and alkaline waters resident in the karst system and precipitated the Mn oxides. The N70&#176;-oriented Atlasic tectonic structure controls the orientation of these epigenetic karst-hosted Mn deposits. The late Eocene &#8211; Early/Middle Miocene uplifts generated additional supplies and/or in-situ remobilizations of the primary late Cretaceous medium-grade ore to form the high-grade pyrolusite-rich ore.</p><p><strong>References </strong></p><p>Dekoninck <em>et al.</em> 2023 (accepted). Mineralium Deposita. Dekoninck <em>et al.</em> 2016. Mineralium Deposita 51, 13-23. Dekoninck <em>et al.</em> 2016. in: Mineral Deposits of North Africa. Springer International Publishing, Cham, pp. 575&#8211;594. Dekoninck <em>et al.</em> 2021. Mineralium Deposita 59, 935&#8211;956. Gutzmer <em>et al.</em> 2006. Economic Geology 101, 385&#8211;405.</p>