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Oulad Abdoun, Khouribga, Sidi Daoui, Ganntour, Youssoufia, Benguerir, Meskala, Bou Craa |
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Morocco |
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Morocco is estimated to contain 30% of the world's known phosphate, with measured phosphorite resources of 85 000 Mt (USGS Minerals Yearbook, 2009). These deposits occur within a series of plateaux which comprise irregular erosional remnant outliers of Upper Cretaceous to Eocene sediments containing phosphorite beds deposited in two long, narrow gulfs on the palaeo-Atlantic coast.
In Morocco, an ~400 x 100 km, NE-SW trending, embayment is preserved in three main, irregular, eroded plateaux clusters, as follows, from NE to SW:
Oulad Abdoun plateau, which occupies ~60% of an ~90 x 60 km area, ~140 km SE of Casablanca and includes the:
Khouribga, Mera el Arech, Sidi Daoui and Sidi Chennane Mines, #Location: 32° 45'N, 6° 42'W.
Ganntour plateau, comprises two main (and a number of smaller) semi-contiguous segments of the 100 x 10 to 20 km, east-west elongated outliers separated from the Oulad Abdoun plateau to the NE by <50 km, and 70 km NE of Marrakesh, and includes the:
Youssoufia underground and Benguérir open pit mines, #Location: 32° 15'N, 8° 12'W.
Meskala group of small plateaux, ~100 km SW of the Ganntour plateau, and 50 km WSW of Marrakesh, and includes the:
Meskala mine, #Location: 31° 23'N, 9° 25'W.
These plateaux are located within the Western Meseta, between the Atlantic Coastal Ranges to the NW, and the ENE-WSW-trending High Atlas range to the SE. The Western Meseta is obliquely seperated from the Eastern Meseta by the NE-SW-trending Middle Atlas Range, and is truncated to the north by the overthrust, structurally complex Rif Domain emplaced during the Oligocene-Miocene.
In Morocco administered Western Sahara, remnants of a larger >1000 km long ENE-WSW-trending gulf hosts phosphorite deposits as the:
Bou Craâ plateau, 50 km SE of the Atlantic port of Laâyoune, and includes the:
Oued Eddahab or Bou Craa mine, #Location: 26° 22'N, 12° 55'W.
This plateau is one of a string occurring to the south of the Atlas and Anti-Atlas Mountains, bounded on its southern margin by the Reguibat Shield of the West African craton.
In addition, Late Cretaceous, Eocene and Miocene phosphorites and phosphatic rocks occur as submarine outcrops on the continental margin, offshore from Morocco between 31° and 33° 45'N, being most abundant off Cap Blanc and Cap Sim at depths of from 80 to 900 m below sea level. These occurrences appear to represent two separate embayments, one of which is the western extension of the same palaeo-embayment as the Moroccan plateaux listed above, the other 350 km to the north.
The phosphate deposits of Morocco are part of a greater Mediterranean Tethyan phosphogenic province that extends from western North Africa within Morocco and Western Sahara, through Algeria, Tunisia and Egypt, to Israel, Jordon and Syria in the eastern Mediterranean.
The phosphatic sediments within the Morocco plateaux listed above were deposited on the African plate, in a platformal environment passive margin accompanying the break-up of Pangea and formation of the Central Atlantic and Alpine Tethys oceans.
The initial rifting stage is represented by Middle- to Late-Triassic evaporite basins on both sides of the Central Atlantic rift, as well as in the Atlas and Maghrebides areas of the Tethyan basins. In Morocco these sequences are characeterised by as much as 3000 m of red conglomerates and sandstones, and evaporite beds, unconformably overlying a Precambrian and Palaeozoic basement complex, exposed as numerous inliers of metasedimentary and volcanic rocks, locally intruded by Late Hercynian calc-alkaline granitoids. The rift climax is recorded by the short-lived, but voluminous basaltic magmatism of the Cetral Atlantic Magmatic Province (CAMP) at the Triassic-Jurassic boundary. The CAMP magmatism was particularly well developed on the African side of the rift, where it occurs as mostly 200±1 Ma dykes, sills and lava flows, now exposed in the uplifted High and Middle Atlas Mountains and the Coastal Range. Rifting was assymetric, with a SW-dipping main detachment that caused a strong uplift of the Moroccan shoulder of the rift. This led to extreme erosion of the Mesozoic cover in western Morocco and extensive exposure of basement, although evaporite deposition continued in intervening basins until 195 to 190 Ma. Following this initial break-up phase, spreading developed during the Jurassic in both the Central Atlantic to the NW, and the semi-parallel Alpine Tethys to the east, connected by a NW-SE transform fault system between Morocco and the Iberian block. A broad Jurassic carbonate shelf of >1500 m in thickness developed over much of Morocco and into Algeria throughout the Jurassic, grading laterally into detrital facies approaching the continent in the east, and to marly facies towards the basin in the west and a thin pelagic basinal seqeuence further to the west on the continental slope and deep basin.
During the Cretaceous, the onset of spreading in the North Atlantic resulted in Iberia being displaced to the east. In the Late Cretaceous, the opening of the South Atlantic caused Africa to cease translating parallel to Eurasia and to commence to converge with it to form the Pyrenean-Alpine and Maghrebide-Atlasic shortening events with the closure of the Tethyan basins to the NE. The same event also led to the onset of Tethyan ocean subduction and roll-back. Throughout the Late Cretaceous and Cenozoic, Morocco was pinned against Iberia retarding convergence and forming a pivot for the anticlockwise rotation of Africa. This contrasted with the unrestricted convergence further to the east in Libya and Egypt, where wide oceans persisted to the north of the convergent margin. During this same period, while much of northern Africa was a convergent margin, accompanied by Atlasic uplift, the passive Atlantic margin to the west persisted, with extension related to the continued opening of the Central Atlantic, and associated subsidence which allowed sediments to accumulate from the Triassic onwards in the western Moroccan Coastal Basins, which were only partially deformed and emergent during the Neogene.
From the Early Cretaceous an important regression resulted in deposition of a thick (up to 2300 m) of widespread continental transtion (lagoonal and deltaic) sedimentary rocks unconformably overlying the Jurassic carbonate platform. The high sea levels of the Late Cretaceous to Eocene flooded much of Morocco, with the unconformable deposition of a thinner (~750 m) sequence of shallow marine to lagoonal sequence of sandy sediments to marly-limestones of the Saharan Hamadas and eastern Atlas domains, passing westward into organic-rich black shales at the shelf edge during about the Cenomanian-Turonian boundary. This sequence is also deeply truncated at the shelf edge during the Late Cretaceous.
These Late Cretaceous sedimentary rocks were overlain by a series of phosphorites within two shallow, extensional, fault controlled gulfs connected to the Atlantic oceanic basin to the west, comprising: (i) a northern, ~400 km long by 100 km wide, NE-SW trending, embayment over the Western Mesata, between the current Coastal Ranges and the Atlas Mountains in Morocco, now represented by the "Plateau des Phosphates" flat lying outliers of Oulad Abdoun, Ganntour and Meskala; and (ii) a southern broad gulf, which is more extensive and 500 km to the south with a >1000 km long ENE-WSW-trend to the south of the Atlas and Anti-Atlas Mountains, in Western Sahara, bounded on its southern margin by the Reguibat Shield of the West African craton. The latter are now represented by plateau on the Sahara cratonic domain, from the Bou Craa plateau near the present Atlantic coast, through the Hamada Du Draa to the Kem Kem in the east. These same embayents were centres of deposition from the Mid-Cretaceous to the Oligocene.
The late Cretaceous to Eocene sediments are followed by a Miocene sequence which thickens abruptly westward beyond the shelf edge. In contrast, during the Late Cretaceous to the Eocene and Oligocene, the Tethyan margin continued as an oblique subduction-collision domain. The present Mediterranean framework was established during the Neogene.
The phosphatic sequences in both Morocco and Western Sahara were deposited over a broad window extending from the Late Cretaceous Maastrichtian (~80 Ma) to the Mid-Eocene Lutetian (~40 Ma), although deposition was irregular and not contiuous throughous this period. The sequence and sub-stratum to the phosphorite changes from west to east and north to south within the series of plateaus. The thickness and facies of the phosphorite-bearing sequence changes laterally from the wester to eastern parts of the gulf. The western part of the gulf contains the main and thickest phophorites, which are interbedded with black shales which progressively decrease to the east.
The Maastrichtian phosphatic series in the northeasterly part of the Oulad Abdoun plateau near Khouribga is very condensed/thin, being only about 2 to 5 m thick, and phosphate-rich. It consists of a basal grey limestone and yellow, soft phosphates as well as grey, soft phosphates with brown streaks and marls. The series is late (but not latest) Maastrichtian in age. This part of the sequence is interpreted to have been deposited in a high energy, near-shore environment (Bardet et al., 2005). The phosphorite where mined underground at Kourigha, averages 1.7 m in thickness but thins and thickens through a range of ~1 m. The immediatey underlying rocks are characterised by a layer of siliceous nodules in many parts of the mine, while in others an underlying thin layer of white clay separates the ore from this layer of siliceous nodules. Similar siliceous nodules are sprasely distributed through the phosphorite (Lawson, 1931). Oulad Abdoun is the most northeasterly of the Western Meseta plateaux, with a ragged, erosion-dissected outline, covering aound 60% an area of ~90 x 60 km.
The Maastrichtian phosphatic series to the SW Oulad Abdoun and in the Ganntour plateau is not condensed and is better developed than that in the northeasterly part of the Oulad Abdoun plateau. It may reach as much as 45 m in thickness, although it becomes more marly phosphatic. The soft phosphatic beds (<1 to 3 m in thickness) are separated by 1 to 3 m thick barren calcareous and marly bands. These parts of the sequence are interpreted to have been laid down in a deeper open sea setting. On fossil evidence, this series ranges from earliest to latest Maastrichtian in age (Bardet et al., 2005). The Ganntour plateau comprises two main (and a number of smaller) semi-contiguous segments of the 100 x 10 to 20 km, east-west elongated outliers separated from the Oulad Abdoun plateau to the NE by <50 km.
Overall, the Maestrichtian to Eocene phosphatic sequence where fully developed, varies from 30 to 50 m in thickness. The earlier deposits, i.e. the lower 5 to 28 m, are clayey phosphates of Maastrichtian age, the upper 20 to 30 m are less homogeneous, comprising layered phosphate marls and sandstones with some limestones of Eocene age (Bakkali, 2006). Mineable phophate beds (as at the Benguérir mine in the Ganntour plateau) range from 1 to 3m in thickness and grade from 22 to 28% P2O5.
Phosphatic ore in Morocco occurs as either oolitic sand or pelletal phosphate rock. Mineralisation resulted from the formation of phosphatic muds, which in the case of the pelletal varieties are interpreted to have been exposed during low-stands, and subsequently fragmented and reworked as tiny chips by storms during high-stands, and transported to accumulations centres by tidal currents. As many as eight eustatic cycles have been identified based on minor transgressive-regressive sequences (Zouhri et al., 2008).
The Moroccan phosphatic ores contain by-product uranium, e.g., the Eocene phosphorites at Khouribga carry around 150 ppm U, with local enrichment in some layers to 500 to 600 ppm. The phosphate consists of 0.1 to 0.2 mm diameter grains of F-collophanite enclosed in a clayey and calcareous matrix that is locally impregnated with opal. The phosphate grains often have quartz or zircon grains at their centres, which served as the nucleus of phosphate precipitation. Phosphate related resources of this type in Morocco reportedly total 6.53 Mt of contained U (IAEA, 2009).
The ores within the Moroccan phosphorites are interrupted by large solution collapse breccia masses containing largely barren carbonate blocks which disrupt mining. Two main types are found:(i) The first type is found throughout the mineral deposit and appears to be a random mixture of limestones, marls, clays, cherts and low-grade phosphate with large amounts of cherty limestone. (ii) a second type is highly disturbed and lacks any dominant facies, occuring as accumulations of low-grade phosphatic limestone blocks with large nodules of chert, marl, some fragments of chert and phosphate rock. The latter type forms volumes that are 10 to more than 150 m across cutting the phosphate beds and is the most frequently encountered (Summerhayes and McArthur, 1990).
The phosphorites found offshore on the continental margin appear to include units of Miocene age that are not found onshore in the "Plateau des Phosphates". These younger phosphatic rocks which sit above a major Oligocene unconformity. The three sets of phosphatic units occur in gently folded concordant strata that strike parallel to the coast line and dip gently seaward (northwest) beneath a relatively smooth sea floor, although further to the south they become more strongly folded and the sea floor has a greater relief in response to Atlasic deformation and to salt diapirism (Summerhayes and McArthur, 1990).
Four types of phosphorite have been recovered from the Moroccan margin. These include: (i) glauconitic phophatic conglomerates which contain rounded tabular cobbles and boulders weighing up to 50 kg; (ii) pelletal conglomeratic phosphorites; (iii) ferruginous dolomitic phosphatic limestone; and (iv) various types of non-ferrous phosphatic limestone. The last three types most commonly occr as rounded, spheroidal-discoidal cobbles and pebbles and probably represent the product of erosion during sea level low-stands (Summerhayes and McArthur, 1990).
Reserve and resource figures include:
Oulad Abdoun Plateau (Khouribga district) - 4.70 Gt @ 24.7% P2O5 (USGS, 2002),
Ganntour Plateau (Youssoufia, Benguérir) - 4.18 Gt @ 29.7% P2O5 (USGS, 2002),
Meskala Plateau - 5.00 Gt @ 32% P2O5 (USGS, 2002),
Bou Craâ Plateau (Oued Eddahab) - 688 Mt @ 32% P2O5 (USGS, 2002),
The tectonic and regional setting is drawn from Michard et al., 2008.
The most recent source geological information used to prepare this decription was dated: 2011.
This description is a summary from published sources, the chief of which are listed below.
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