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Imini District - Bou Aguioun, Timkit, Tighermit, Boulgir, Bou Tazoult, Lantenois

Morocco

Main commodities: Mn Pb Ba
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Deposit Description

The Imini manganese-lead-barium deposits are located south of the Central High Atlas Mountains of Morocco, about 50 km to the west-northwest of Quarzazate.

Mining started at Imini in 1928 and reached peaks of production of about 90 000 tonnes in the 1970s. Currently (2005), around 48 000 t of high grade concentrate (>92 wt % MnO2) is produced annually from in situ orebodies containing 72 to 88% MnO2.

Manganese orebodies are found along the southernmost exposures of a laterally extensive succession of unmetamorphosed and virtually undeformed shallow marine carbonate rocks that were deposited during theCretaceous, specifically during the Cenomanian (90.4­97.0 Ma) and Turonian (88.5­90.4 Ma) stages. Deposition of this carbonate succession reflects a marine incursion into the ancient Atlas Gulf, a narrow E-W trending sedimentary basin, extending from the Atlantic Ocean in the west to the Tethys Ocean in the east during that time interval. The thin carbonate succession at Imini thickens rapidly to the north and east and to the south, grades into a thin succession of manganese-impregnated coarse arkoses.

The ore-bearing carbonate succession unconformably overlies Lower Cretaceous and Triassic red beds and folded Cambro-Ordovician schists in the foothills of the High Atlas to the north and east of Imini, and Neoproterozoic rhyolites, andesites, granites, and diamictites of the Anti Atlas region to the south. These basement rocks have preserved evidence of deep lateritic weathering under humid and warm climatic conditions immediately from the Atlantic Gulf.

The manganese deposits of the Imini district are confined to a narrow discontinuous east-northeast­striking belt, approximately 25 km long and 400 to 1000 m wide, parallel and close to the southern paleoshoreline of the ancient Atlas Gulf. The carbonate succession in the area is very thin (generally <12 m in thickness) and has a marked terrigeneous environment of deposition, with siliciclastic detritus derived from the Anti Atlas Mountain belt immediately to the south. High-energy carbonate lithofacies indicate deposition in shallow subtidal to intertidal environments.

Toward the close of the Turonian stage regression led to the Cretaceous carbonates being exposed and a marked upper karstic surface developed. It is during this period of karstification that the Imini manganese ores were developed. The karstified carbonate succession was then covered by up to 100 m of red to pink argillaceous sandstones, clays, and evaporites, deposited in arid climatic conditions.

The ore bearing Cenomanian-Turonian succession is made up of a basal transgressive fluvial to marine siliciclastic sandstone, overlain by regressive shallow shelf dolostone which comprises a basal pink sandy dolostone of Upper Cenomanian age, followed by white bioclastic (fossiliferous) dolostone of Upper Cenomanian age, and cherty Lower Turonian dolostone at the top. The dolostones are overlain above a sharp karstic erosional contact by sandstone and conglomerate forming the base of the Senonian red beds.

The pink sandy dolostone at the base of the ore-bearing carbonate succession contains abundant sand-sized siliciclastic detritus (mostly quartz) and is thought to have originated as carbonate sand. The overlying white bioclastic micritic dolostone is massive in texture but contains numerous bivalve and gastropod shells or shell fragments that were deposited together with micritic carbonate mud. The uppermost cherty dolostone unit is finely laminated and beige in color.

While Manganese oxyhydroxides occur throughout the Imini district in veinlets, stockworks, and pseudobreccia bodies that cut the host rock dolostones, economic manganese concentrations are only found in three distinct laterally extensive but discontinuous zones in the dolostone succession representing the three orebodies informally known, from bottom to top, as the C1, C2, and C3 manganese ore zones.

The lower two zones, C1 and C2, are closely spaced and occur in the upper part of the sandy dolostone and the lower part of the fossiliferous dolostone. They are between 1 and 2.5 m thick and account for the majority of the manganese ore resource in the district. The C3 orebody occurs close to the top of the fossiliferous dolostone unit and is laterally less continuous and thinner than the other two, but is otherwise very similar in appearance.

Most of the manganese ores occur in a variable succession of poorly consolidated clastic sediments. The latter include laminated or massive clays, crossbedded sand and silt, and matrix-supported dolostone breccias with local thin flowstone units, all of which accumulated as internal sediment in an ancient cave system in the dolostone succession. Two distinct types of manganese ore are present in the internal sediment, namely:  i). Stratabound manganese ore in the lower part of the internal sediment succession - the top of these orebodies is commonly marked by a discontinuous zone of carbonate nodules near the base of a red- to cream-colored and laminated clay unit in the assemblage of internal sediments; and  ii). Reworked manganese ore granules in breccias and sand in the upper part of the succession.  iii). A third variety of manganese ore occurs as a stockwork of veins or as cement to breccia bodies that cut across dolostone adjacent to the karst caves.

Stockworks of pyrolusite- and manganomelane-filled veins and veinlets are locally very extensively developed in the footwall and hangingwall dolostones of cave zones or laterally adjacent to caves.

The manganese ores are interpreted to have formed when warm, acidic Mn2- bearing meteoric water migrated from the elevated regions of the Anti Atlas into the exposed carbonate succession. Alkali feldspar-rich igneous basement rocks were the source for Mn, Pb, and Ba. Metals were deposited in a zone of mixing between metal-bearing, reducing meteoric water and oxygenated ground water resident in the cave system.

The most recent source geological information used to prepare this summary was dated: 2006.    
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd.   Unauthorised copying, reproduction, storage or dissemination prohibited.


  References & Additional Information
 References to this deposit in the PGC Literature Service Collection: Want any of these papers ? Click Here
Gutzmer J, Beukes N J, Rhalmi M and Mukhopadhyay J,  2006 - Cretaceous Karstic Cave-Fill Manganese-Lead-Barium Deposits of Imini, Morocco: in    Econ. Geol.   v101 pp 385-405


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