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

Augustin Dekoninck, Jocelyn Barbarand, Gilles Ruffet, Yves Missenard, Nadine Mattielli, Rémi Leprêtre, Abdellah Mouttaqi, Omar Saddiqi, Michèle Verhaert, Johan Yans

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## To cite this version:

Augustin Dekoninck, Jocelyn Barbarand, Gilles Ruffet, Yves Missenard, Nadine Mattielli, et al. How to create the highest manganese ore grade in the world? A geodynamic story in the Atlas of Morocco. EGU General Assembly 2023, Apr 2023, Vienna, Austria. pp.EGU23-5478, 2023, 10.5194/egusphere-egu23-5478 . insu-04083146

HAL Id: insu-04083146

https://insu.hal.science/insu-04083146

Submitted on 27 Apr 2023

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

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Augustin DEKONINCK(1,2), Jocelyn BARBARAND(3), Gilles RUFFET(4,5), Yves MISSENARD(3), Nadine MATTIELLI(1), Rémi LEPRETRE(3,6), Abdellah MOUTTAQI(7), Omar SADDIQI(8), Michèle VERHAERT(2), Johan YANS(2)

## Introduction

The Imini mining district hosts the **largest Mn resources of North Africa** due to two laterally extensive ore bodies (~25-30 km) of **nearly pure pyrolusite** ($MnO_2$). A third medium-grade coronadite-rich (K,Ba,Pb$Mn_8O_{16}$) Mn ore occurs discontinuously over a wider area of ~100 km. The two ore types are hosted in a poorly deformed 10-15m-thick Cenomanian-Turonian dolostone unit located in the southern foreland of the High Atlas of Morocco. Until now, the **source of the metals and the timing** of ore deposition remains unclear for this unconventional karst-hosted deposit.

## Objectives and methods

We examine the **Pb isotope** composition of the Mn ores and the most representative country rocks to track down potential sources of the metals. We also study the age of the K-bearing Mn oxides present in the Mn ore by $^{40}$Ar/$^{39}$Ar. By combining these two methods we are able to produce a consistent model of the ore deposition.

<figure><img src="image_4.png" /><figcaption>Fig. 1 Location map of the Imini Mn district and Mn occurrences in the Cenomanian-Turonian dolostone unit.</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>Fig. 2 Field photograph of the Lantenois mine showing the high-grade pyrolusite-rich ore (C2) and the medium-grade coronadite-rich Mn ore (C3)</figcaption></figure>

## Results

The new Pb isotopic ratios show that **Triassic series (basalts and Fe-rich sandstones) are likely the source** of the metals. The $^{40}$Ar/$^{39}$Ar dating of these K-Mn oxides shows that the Mn-rich ore bodies formed during at least **three periods**:

* late Cretaceous to late Paleocene (>58 Ma)

* late Eocene (c. 36.3 Ma)

* early Burdigalian to early Serravalian (c. 19–20 Ma and c. 13 Ma)
These periods **coincide with three known Atlas geodynamic deformation** events during late Cretaceous, late Eocene and the Early(?)-Middle Miocene.

<figure><img src="image_6.png" /><figcaption>Fig. 3 Pb isotope composition of the Imini Mn oxide ores and different country rocks.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>Fig. 4 Chronology chart of Mn deposition based on $^{40}$Ar/$^{39}$Ar dating of coronadite group minerals and other events of the area. See the best fit between uplift events and K-Mn oxide ages.</figcaption></figure>

## Key interpretations

* **Atlas tectonics** is the first-order control of the Mn mineralization.

* Periods with regionally high elevations modified the climate to wetter conditions that brought rather **continuous supplies of meteoric water** to feed temporary aquifers.

* **Relief building created the required hydraulic head** to sustain (1) fluid-rock interaction between $O_2$-poor acidic fluids and the Triassic series, (2) migration of the metal-rich fluid and (3) overpressure fluid in the Imini depositional site.

* The disappearance of Triassic series in front of the Imini anticline forced these low-temperature (<100°C) fluids to **mix with oxygenated ground and alkaline waters** resident in the karst system and precipitate Mn oxides.

* The N70°-oriented **Atlasic tectonic structure controls the orientation** of the Mn deposits.

* Post-Cretaceous 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 in Imini.

<figure><img src="image_8.png" /><figcaption>Fig. 5 Formation model of the high-grade Imini Mn ore deposits due to multiple orogenic phases of the Atlas.</figcaption></figure>

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(3)
université
PARIS-SACLAY

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