Petrography and origin of the Mn(-Pb) Tasdremt deposit (High Atlas, Morocco)
Résumé
447Petrography and origin of the Mn(-Pb) Tasdremt deposit (High Atlas, Morocco)Augustin Dekoninck∗1, Gilles Ruffet 2, Mohamed Magoua 3, Yves Missenard 4, Oriane Parizot 4,AbdellahMouttaqi 3, Omar Saddiqi 5, Johan Yans 11 Université de Namur, Département de Géologie, ILEE – Namur – Belgique2 Université de Rennes 1, Géosciences Rennes – CNRS UMR 6118 – France3 Office National des Mines et des Hydrocarbures, Rabat (ONHYM) – Maroc4 Université Paris Sud XI, Orsay (GEOPS) – 8148 – France5 Université Hassan II, Géosciences, Casablanca – MarocThe Tasdremt manganese deposit is located in the southern foreland of the High Atlas at 70 km to the E of Taroudant, and 90 km to the SW of the Imini manganese district. The manganese ores are located in the Cretaceous strata of Tasdremt [W] and Aoufour [E]. The Cretaceous sedimentation is a succession of i) a 0–30 m thick detritic serie (red clays, sandstones and conglomerates, infra-Cenomanian), ii) a 0–20 m thick dolostone Cenomanian-Turonian in age, and iii) a Senonian detritic succession (20–50 m thick) composed of dolomitized sandstones and conglomerates interbedded with sandy dolostones. The Cenomanian-Turonian dolostone is subdivided into a yellow/gray dolostone at the base, which is covered by ii) a pinkish/white fossiliferous dolostone, the top being brecciated in contact with the Senonian conglomerates. The variable thickness of the whole Cretaceous serie is attributed to paleotopographic hights of the Cambrian dolostones. These rocks were deformed during a post-Cretaceous tectonic phase, probably the Late Eocene Atlasic one, inducing thick-skin thrusting of Cambrian dolostones over meso-cenozoic series. The present study shows that the stratigraphic position and the lateral extension of the manganese levels are quite variable along the district. The main orebeds are exposed at the top of the yellow and fossiliferous dolostone, while other manganese occurrences are located at the top of the infra-Cenomanian sandstones, at the base of the Senonian conglomerates and dolostones. X-ray diffraction points out that the ore is composed of hollandite group minerals [(K,Ba,Pb)Mn8O16], with only small amounts of lithiophorite. Pyrolusite is nearly absent. Petrographic observations indicate that hollandite group minerals replace dolomite crystals, and fill open cavities in the dissolved dolostone, while brecciated fragments of the Mn-ore, dolomite and quartz at the top of the fossiliferous dolostone are cemented by a late calcite. This breccia is then covered by the Senonian conglomerates. This suggests that the mineralization postdates the dolomitization of the host dolostone, but predates the deposition of the Senonian sediments. This could be linked to the post-Turonian karstification period. However, the occurrence of mineralized Senonian sandstones and dolostones, as well as Mn-ore cemented by colloform hol- landite group minerals or Mn-ore in tectonic joints may indicate a second mineralization stage, postdating the Cretaceous sedimentation.