Augustin Dekoninck

Detailed mineralogy and petrology of manganese oxyhydroxide deposits of the Imini district (Morocco)

Manganese ore in the Cenomanian-Turonian dolostone of the Imini district (south of the High Atlas, Morocco) displays a high Mn content due to the occurrence of pyrolusite, cryptomelane, hollandite sensu stricto, coronadite, romanechite, and lithiophorite. The orebodies occur mainly as three stratabound layers along the ~25-km-long ore belt following a WSW-ENE direction. Using scanning electron microscopy (SEM-EDS) coupled with X-ray diffraction (XRD), we here refine the mineralogy and petrology of Mn oxides and oxyhydroxides in order to constrain the paragenetic sequence and define the main processes of ore formation. Deposition and concentration of Mn oxyhydroxides follow a two-step sequence after dolomitization: (1) replacement of dolomite fabric (dolomite micrite and dolomite rhombs) by hollandite group minerals leading to a textural conservation and then (2) neoformation of collomorphous aggregates in an opened and brecciated system. These observations are consistent with a multistage evolution including multiple reworking and brecciation. Pyrolusite is the main Mn oxide observed in the Imini district during both early and late stages. The superficial environment provides supergene conditions suitable for the deposition of only Mn oxyhydroxides. This supergene environment and the occurrence of multiple bands of hollandite group minerals indicate an external migration (allochthonous supply) of Mn and associated elements, contemporaneous to an in situ chemical and mechanical dissolution of the host dolostone. The lithological heterogeneities of dolostone and its chemical environment compared to less permeable surrounding rocks may have provided the conditions to concentrate Mn oxides and oxyhydroxides.

Mineralogical and geochemical characterization of supergene Cu-Pb-Zn-V ores in the Oriental High Atlas, Morocco

In the Moroccan High Atlas, two sulfide deposits hosted by Jurassic dolostones underwent significant weathering. In the Cu deposit of Jbel Klakh, several stages of supergene mineralization are distinguished: (1) the replacement of hypogene sulfides in the protolith (chalcopyrite) by secondary sulfides in the cementation zone (bornite, digenite, chalcocite, covellite), (2) the formation of oxidized minerals in the saprolite (malachite, azurite, brochantite) where the environment becomes more oxidizing and neutral, and (3) the precipitation of late carbonates (calcite) and iron (hydr-)oxides in the laterite. The precipitation of carbonates is related to the dissolution of dolomitic host rocks, which buffers the fluid acidity due to the oxidation of sulfides. In the Jbel Haouanit Pb–Zn deposit, the mineral assemblage is dominated by typical calamine minerals, Cu minerals (chalcocite, covellite, malachite), and a Cu–Pb–Zn vanadate (mottramite). Galena is successively weathered in anglesite and cerussite. Sphalerite is weathered in smithsonite, which is rapidly replaced by hydrozincite. Late iron (hydr-)oxides are mainly found at the top of both deposits (laterite). Both deposits are thus characterized by specific mineral zoning, from laterite to protolith, related to variations in the mineralogy and ore grades and probably caused by varying Eh-pH conditions.

The High-Grade Imini Manganese District—Karst-Hosted Deposits of Mn Oxides and Oxyhydroxides

For more than 80 years of mining the economically important Imini district has been the main producer of manganese in Morocco. Three stratabound manganiferous orebodies in the district are hosted in a 10-m-thick Cenomanian-Turonian dolostone that records the effects of laterally extensive dissolution and epigenetic processes. The predominance of pyrolusite, accompanied by minor amounts of hollandite group minerals (cryptomelane, hollandite s.s., coronadite), lithiophorite, and romanechite, provide a high grade ore having more than 70 wt% MnO and low contents of SiO2, Fe2O3, and P2O5. One metallogenic model suggests the main mineralization stage was related to a period of terrestrial exposure and weathering, leading to formation of an extensive karst system, following host rock deposition and dolomitization. This model envisages Mn mineralization prior to the deposition of Coniacian-Maastrichitan (Senonian) red beds, carbonates, and evaporites. A variety of data shows that the ore beds formed by epigenetic processes, in which Mn precipitation occurred with increasing pH and Eh. These physicochemical conditions are linked to dolomite dissolution and the influx of oxygenated meteoric waters, probably in contact with deeper O2-depleted meteoric groundwaters, and to surficial saline solutions and/or microbial activity. The Cenozoic construction of the Atlas Mountains probably remobilized or at least reworked previously formed manganese accumulations.

Erosion surfaces in the Ardenne-Oesling and their associated kaolinic weathering mantle

This chapter deals with the long-term geomorphology of the Paleozoic Ardenne–Oesling massif of S Belgium and the landforms that currently attest the very long persistence and high resistance to erosion of landscape elements created as far back in time as the Lower Cretaceous in a region of predominantly continental regime and constant low to moderate elevation since the Permian. We first present recent results regarding the antiquity of kaolinic weathering mantles preserved in four sites on or close to the plateau surfaces characteristic of the Ardennian landscape, showing that they record at least three main weathering phases, namely in the Early Cretaceous (extending in the early Late Cretaceous), the Late Paleocene, and the Early Miocene. Then, having provided a working definition of the erosion surface and listed criteria helping recognize them in the landscape, we focus on the Hautes Fagnes massif, the highest NE part of the Ardenne–Oesling. We describe the geometric and sedimentary evidence that allow one to recognize a succession of four erosion surfaces formed during the Cretaceous and the Cenozoic and reconstruct the morphogenetic evolution of the area that responded to the variable interplay of the marine transgression-regression, climatic, and tectonic controls. Finally, we present the few estimates of long-term denudation rates available for the Ardenne massif, fission track and cosmogenic nuclide studies both suggesting Cenozoic denudation rates in the order of a few 10 m/Myr, and comment on their compatibility with field observations.