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Bou Beker, Touissit, Oued Mekta, Beddiane, El Abed |
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Morocco |
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Zn Pb
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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All papers now Open Access.
Available as Full Text for direct download or on request. |
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The Touissit-Bou Beker district comprises a 16 km long by 100 to 1200 m wide, ENE-WSW-trending, semi-continuous string of carbonate hosted ("Mississippi Valley Type") lead-zinc deposits, centred in far eastern Morocco, a few km west of the border with, and extending 3 km into Algeria, and ~70 km south of the Mediterranean coast line in northwest Africa, #Location: 34° 28'N, 1° 45'W.
From west to east, the constituent deposits are Mekta and Beddiane, separated by a gap of ~1 km from each other and from the continuous, irregular mass that is Touissit-Bou Beker, which is in turn connected by a narrow neck with El Abed in Algeria. A number of smaller offshoots and islands of mineralisation complete the string. This beIt represents the core of a larger, ~20 km diameter cluster of more than 100 small, uneconomic occurrences, all hosted by the broader ENE-WSW-trending Jurassic carbonate platform of "la Chaîne-des-Horsts" Atlantic belt of northeast Morocco and northwestern Algeria.
The ore-bearing Mesozoic and overlying Tertiary rocks of the Touissit-Bou Beker district unconformably overlie a Palaeozoic basement complex, exposed as numerous inliers of metasedimentary and volcanic rocks, locally intruded by Late Hercynian calc-alkaline granitoids emplaced between 344±6 and 333±2 Ma (U-Pb on zircon). The oldest rocks are an Ordovician turbiditic flysch sequence of lithic and sublithic arenite, wacke and siltstone and pelite, overlain by Silurian black shale with intercalated quartzite/chert and Devonian reef limestone. These are followed by up to 1000 m of Early Carboniferous (Visean) bimodal calc-alkaline volcanic and volcaniclastic metasediments, uncomfortably overlain by a mid-Carboniferous basal conglomerate, which is capped in turn by Late Carboniferous flysch-like mudstone and sandstone, and late coalbearing carbonate sediments.
Three stages of Variscan (Hercynian) deformation are recognised, producing a variety of NNW-SSE- to WNW-ESE- and NW-SE-trending moderate to tight, upright to recumbent folds with steep axial planar fracture cleavage or flow schistosity. Peak greenschist-facies metamorphic conditions during D1 was between 368±8 and 372±3 Ma, while weak D2 metamorphism has not been dated. D3 was post Early Carboniferous. A transitional period between the end of the Variscan cycle and onset of the Atlasic cycle ensued from the Late Permian to Early Triassic, during which a NNE-trending, fault-controlled extensional/rift basin was developed.
This basin was filled with an up to 3000 m thick sequence of red-bed littoral, fluvial lagoonal conglomerate, sandstone, siltstone and argillite, locally interbedded with gypsum-bearing layers. The sequence thins rapidly to the north, and is almost absent beneath the Touissit-Bou Beker district. During the ensuing Early and Middle Jurassic, shallow- to deep-water marine carbonates were deposited in an environment with local basement highs against which the Triassic and Lower Liassic sediments pinch out. These carbonates host the mineralisation of the district. They are in turn succeeded by Middle Jurassic massive dolostone strata (Aalenian-Bajocian), and a succession of sandstone and marl (Callovian) followed by Upper Jurassic (Oxfordian-Kimmeridgian) sandstone, marl and carbonate.
The Mesozoic and younger cover rocks remain essentially flat-lying and unmetamorphosed. Neogene (Tortonian-Messinian and Pliocene) sediments, comprising >1000 m of deep to shallow gypsiferous marine and continental deposits, locally intercalated with alkaline basaltic lavas flows, which are well developed in the neighbouring grabens of the Oujda area, are only partially exposed in the Touissit-Bou Beker district.
Post-Variscan igneous activity produced (i) tholeiitic Triassic basalts and hypabyssal dolerite sills; (ii) Eocene alkaline magmatic rocks (alkaline pyroxenites, gabbros, nepheline syenites, lamprophyres) and related carbonatites; and (iii) Plio-Quaternary alkali basalts.
The dominant ENE-trending structural fabric inherited from the Variscan was rejuvenated during the Jurassic, Cretaceous and Eocene to Oligo-Miocene of the Atlasic cycle to produce the current horst-graben configuration of the la Chaîne-des-Horsts. The regional structural trends are ENE-WSW and NW-SE to NNW-SSE, with subordinate E-W elements representing a complex pattern of high-angle normal, reverse and strike-slip displacements.
The succession of repeated Atlasic orogenic pulses resulted in the region being segmented and down-dropped to form the current ENE-trending, 3 to 4 km wide Missouine Graben, within which more closely spaced extensional faulting produced a series of mine district-scale horsts and grabens. The major ENE-trending, steeply north-dipping normal faults have strike lengths of >5000 m and vertical displacements up to 200 m. The intervals between these structures are broken into fault-bounded blocks by a series of NNW- and E-W-trending, steeply dipping, minor normal faults with displacements of <5 m to as much as 20 m, many of which appear to terminate against the major structures. The major faults are interpreted to be kinematically linked and form part of a segmented fault system in which displacements are transferred between structures across faulted relay-ramp zones, with the minor faults representing accommodation structures, resulting from the large displacement gradients and dip changes on the major faults.
Within the immediate Touissit-Bou Beker district, basement consists of lower greenschist facies upper Palaeozoic metasedimentary rocks intruded by resistant Early Carboniferous (Visean) rhyodacite plugs that formed basement islands which were surrounded by shallow-water platformal carbonate rocks in the Triassic and Lower Jurassic seas. These carbonate rocks thin and pinch out against the palaeo-islands such that the Triassic and Lower Jurassic formations are almost absent in the Touissit-Bou Beker district. At Beddiane and Mekta, the basement is exclusively of rhyodacite, whereas at Touissit-Bou Beker and El Abed, schistose pelite is locally intruded by rhyodacite.
All of the economic deposits of the district are located within the Missouine Graben, best developed to the west, becoming less well defined and complicated by graben-parallel and cross-faults to the east. Within the graben, the Palaeozoic basement is overlain by a thin (~5 m) sequence of Lower Jurassic conglomerate and limestone, followed by 20 to 35 m of almost flat-lying (~7°, with local steeper dips adjacent to faults) Aalenian-Bajocian carbonates of reefal origin, composed of fine- to coarse-grained packstone and grainstone interbedded with thin marly horizons. In the Touissit-Bou Beker district these carbonate rocks are pervasively dolomitised and host 95% of the ore.
The Aalenian-Bajocian dolostones are unconformably overlain by a succession of sandstone, marl and carbonate of Late Bajocian-Lower Bathonian age, known locally as the "Toit Jaune," which constitutes the hanging wall to mineralisation. These sediments grade upward into a ~2 m in thick ferruginous Bathonian oolitic regional marker unit which is overlain in turn by a Callovian-Oxfordian section consisting of a thick (600 m) sequence of impermeable shales and carbonate rocks, capped in turn by an up to 100 m thick Kimmeridgian succession of dolostone and limestone.
Dolomitisation is the principal form of wall-rock alteration, with two distinct stages being recognised:
(i) regionally extensive diagenetic replacement dolostone, which in turn comprises an early diagenetic replacement dolostone which forms non-porous and impermeable replacement mosaics of aphanocrystalline to fine-crystalline dolomite, and a late diagenetic medium- to coarsely-crystalline dolostone which obliterated the primary limestone textures, and forms moderately- to highly-porous, permeable mosaics interpreted to have been generated during deep burial. The regional development of secondary porosities, tectonic fractures and breccias, and the deposition of ore-stage minerals are restricted to the late diagenetic dolostones;
(ii) ore-related hydrothermal dolomite cements which are well developed within all deposits of the Touissit-Bou Beker district and are most abundant in secondary matrix porosity and in dissolution structures created within diagenetic replacement dolostones. They occur as ore-related hydrothermal sparry dolomite cement, commonly forming halos around the deposits and extending at least 50 m above the host dolostone unit into the overlying Upper Bajocian lithologic units and laterally for >10 km beyond the orebodies. Three texturally distinct subtypes of this dolomite cement are evident, a pre-ore white- to pink-coloured, medium- to coarse-grained crystals, which comprises ~50% of the hydrothermal dolomites and fills the secondary porosity of the late diagenetic dolostones; a syn-sulphide stage pink to reddish ferroan (0.9 to 3% Fe) which closely follows the ENE-trending mineralised corridor; and a young, post-ore variety occurring only as a widespread, but low volume (<10% of the hydrothermal dolomites), open-space fillings which occlude the remaining porosity and cement fractures.
The Touissit-Bou Beker district deposits are epigenetic and strata-bound. The orebodies are found at a range of stratigraphic levels within the Aalenian-Bajocian carbonates, particularly within the upper 10 m of the late diagenetic dolostone, close to the overlying "Toit Jaune". Mineralisation may extend upward into the overlying Callovian-Oxfordian rocks and the "Toit Jaune" itself for 10 m or more, while generally thin, (<1 cm) uneconomic veins of sulphides penetrate the Visean rhyodacitic basement containing coarse galena, sphalerite, pyrite and chalcopyrite, accompanied by ferroan dolomite cement.
Mineralised breccias form the dominant ore structure in the Touissit-Bou Beker district deposits, comprising saddle dolomite-cemented crackle, mosaic and rubble breccias, and rock-matrix ("trash") breccias, with the ore forming the matrix between angular clasts of host dolostone. The breccias grade outward into net-vein dolomite-sulphide fracture systems. The mineralised structures form interconnected networks of caves and cavities ranging from narrow, small openings to wide ramifying and laterally connected tabular to prismatic openings filled with massive galena and locally clastic carbonate.
Within the mineralised breccias, related open space and porous late diagenetic dolostones, three main styles of mineralisation are distinguished:
(i) Open-space filling - where sulphide ore occurs either in veins, interconnected cavities, or solution collapse breccias related to post-lithification dissolution of late diagenetic dolostone.
(ii) Metasomatic replacement - comprising varied-size grain disseminations and/or tabular orebodies with ore textures that mimic the original stratification of the metasomatically replaced ore-bearing dolostone.
(iii) clay hosted - which is restricted to the Hassi Msidira orebody and selected zones of the Beddiane mine, where it is strongly laminated, with contorted bands of reddish-brown to black illite and kaolinite accompanied by quartz and dolomite. Mineralisation occur as sulphide clasts containing fine disseminations to large, massive galena blocks and, to a lesser extent, sphalerite, pyrite and chalcopyrite. This style has been interpreted to represent internal sediments formed by the postore hydrothermal dissolution of carbonate host rocks.
The primary sulphide mineralisation is essentially composed of galena and sphalerite with pyrite and/or marcasite and lesser chalcopyrite, bornite and tetrahedrite, with minor amounts of Pb-Sb sulphosalts, such as bournonite and stephanite, and Ag-bearing species, such as argentite and native silver. Galena is the principal economic mineral in the Mekta and Beddiane deposits in the west, whereas sphalerite becomes more abundant in the Touissit and Bou Beker deposits in the centre, and dominates to the east of the district at El Abed in Algeria. Chalcopyrite is present as mono- or poly-mineral aggregates associated with tetrahedrite and/or pyrite, occurring as coarse (>2 cm) clots forming anhedral masses or, less commonly, rectangular crystals, although it may also occur as fine-grained inclusions within sphalerite. Tetrahedrite contains the highest concentrations of Ag in the deposits and commonly occurs as irregular inclusions in galena and chalcopyrite. In addition to the lateral Pb-Zn zonation detailed above, there is also a vertical metal differentiation on scales of hundreds of metres within individual deposits, e.g., at Beddiane, Zn-rich mineralisation mainly occurs in the lowermost part of the Aalenian-Bajocian late diagenetic dolomite and gives way through the main-stage hydrothermal event to the predominantly Pb-rich ores which are concentrated largely in the middle and especially in the uppermost Aalenian-Bajocian units, although occasionally extending into the overlying "Toit Jaune" and even into the Callovian-Oxfordian sequences. Cu-rich ore, although restricted to the Beddiane and Bou Beker deposits, is located stratigraphically higher than the Pb-rich ore.
Gangue includes ferroan and nonferroan dolomite and minor calcite, with subordinate quartz, while secondary gypsum resulting from oxidation of sulphides may be locally abundant.
Although no consistent paragenetic sequence is indicated, and the primary mineral assemblage appears to be the result of overlapping multiple pulses, four broad stages of ore deposition/modification are distinguished, namely:
(i) Early fine grained or colloform sulphides fills openings in the organic-rich late diagenetic dolostone, composed of pyrite-1/marcasite, sphalerite-1 and galena-1, with minor chalcopyrite-1 and tetrahedrite;
(ii) Main-stage ore (80% of extracted ore), which is coarse grained and Pb rich, filling interconnected open spaces, lining subvertical fractures and cementing breccias; dominantly galena-2 and sphalerite-2, with bitumen occurring as a minor local phase intimately related to sphalerite;
(iii) Late cuboctahedral stage, which is only a minor phase, dominated by crystalline vug fillings of pyrite-2, sphalerite-3, chalcopyrite-2 and galena-3 occurring as cubic crystals with octahedral modifications encrusted on saddle dolomite, accompanied by minor amounts of calcite and quartz.
(iv) Oxidation stage - all of the Touissit-Bou Beker district deposits are at least partially oxidised, some so deeply that the only surviving sulphide is galena, such that 60% of the total Pb-Zn production is derived from non-sulphide minerals. The secondary mineral assemblage comprises cerussite, anglesite, wulfenite, vanadinite, hemimorphite, pyromorphite, smithsonite, malachite, azurite, Fe oxides and hydroxides, and gypsum.
The main ore controls within the Touissit-Bou Beker district include palaeogeography, stratigraphy, lithology, carbonate dissolution, faulting and the pressence of organic matter. At a district scale, the Aalenian-Bajocian carbonate formation appears critical to the localisation of ore deposits, with economic Pb-Zn occurrences exclusively restricted to the organic-rich late diagenetic dolostone of this unit, with enhanced porosity and permeability associated with the regional pre-ore dolomitisation appearing to have been critical in determining the size and the geometry of orebodies. Conversely, the overlying Callovian-Oxfordian shales presented a relatively impermeable aquitard that restricted upward hydrothermal flow out of the ore-bearing dolostone. In addition, organic-rich carbonates provide an ideal environment for producing reduced sulphur through reduction of sulphate, possibly derived from nearby gypsum deposits, and concomitant metal precipitation. Further to the litho-stratigraphic controls, dissolution and brecciation of the ore-bearing dolostone determined the position and form of the orebodies, with resulting solution-derived structures, and associated high initial porosities and permeabilities, providing channels for ore fluid circulation. The intimate link between paleogeography (i.e., graben margins and resistant basement islands causing host carbonate pinch out), faults and mineralisation suggests that mineralising fluids migrated along both pinchouts and regional and local-scale faulted rocks provided zones of enhanced structural permeability compared to the adjacent rocks. The spatial distribution of ore in the district, which correlates positively with areas of maximum and close-spaced faulting and fracturing, infers that both the ENE- and NW-trending fault zones acted as major mineralising fluid conduits.
These deposits and their reserves/production include, from west to east:
Beddiane - 0.90 Mt of Pb (Dupuy and Touray, 1986);
constituent orebodies: Beddiane - 5.5 Mt @ 17% Pb; Hassi Ennyag - 1 Mt @ 14% Pb;
Hassi M'Sidira - 0.3 Mt @ 18% Pb; (Bouabdellah et al., 2012);
Oued Mekta - 0.21 Mt of Pb (Dupuy and Touray, 1986);
Bou Beker - 0.52 Mt of Pb, 0.59 Mt of Zn (Dupuy and Touray, 1986);
Touissit - 0.37 Mt of Pb, 0.17 Mt of Zn (Dupuy and Touray, 1986),
all of which are in Morocco, while in Algeria:
El Abed - 0.35 Mt of Pb, 1.18 Mt of Zn (Dupuy and Touray, 1986).
Total production from the Touissit-Bou Beker-El Abed group of deposits between 1926 and 1997 was:
~67 Mt @ 7% Pb, 3% Zn (Bouabdellah et al., 2005).
The mean silver content of the deposits of the district was 120 g/t Ag, and locally as high as 600 g/t Ag. The mineralised envelope also contains ~1% Cu. The Bou Beker, Touissit and Oued Mekta deposits contributed 45%, 30% and 2% of the district's total historical production (Bouabdellah et al., 2012).
This summary is largely drawn from Bouabdellah et al., 2012 and presents the observations and interpretations of those authors and other sources quoted therein.
The most recent source geological information used to prepare this decription was dated: 1985.
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.
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Bouabdellah M, Fontbote L, Haeberlin Y and Llinares L, 1999 - Zoned sulphur isotope signatures at the Mississippi Valley-type Touissit - Bou Beker - El Abed District (Morocco-Algeria) - Evidence for thermochemical sulphate reduction and mixing of sulphur sources: in Stanley, et. al., (Eds.), 1999 Mineral Deposits: Processes to Processing Balkema, Amsterdam pp. 821-824
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Bouabdellah M, Sangster D F, Leach D L, Brown A C, Johnson C A and Emsbo P, 2012 - Genesis of the Touissit-Bou Beker Mississippi Valley-Type District (Morocco-Algeria) and Its Relationship to the Africa-Europe Collision : in Econ. Geol. v.107 pp. 117-146
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Claveau J, Paulhac J and Pellerin J, 1952 - The lead and zinc deposits of the Bou Beker-Touissit area, eastern French Morocco: in Econ. Geol. v.47 pp. 481-493
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Dupuy J J, Touray J C 1986 - Multistage ore deposition at the Oued Mekta strata-bound Lead deposit, Touissit-Bou Beker district, eastern Morocco: in Econ. Geol. v81 pp 1558-1561
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Makhoukhi S, Marignac Ch, Pironon J , Schmitt J M, Marrakchi C, Bouabdelli M and Bastoul A, 2003 - Aqueous and hydrocarbon inclusions in dolomite fromTouissit-BouBeker district, Eastern Morocco: a Jurassic carbonate hosted Pb-Zn (Cu) deposit: in J. of Geochemical Exploration v.78–79 pp. 545–551
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Makhoukhi S, Schmitt J M, Bouabdelli M, Bastoul A and Marignac Ch, 2000 - Modelling of an MVT deposit: Touissit-BouBeker district (eastern Morocco): in J. of Geochemical Exploration v. 69-70 pp. 109–113
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Rajlich P, 1983 - Geology of Oued Mekta, a Mississippi Valley-type deposit, Touissit-Bou Beker region, eastern Morocco: in Econ. Geol. v.78 pp. 1239-1254
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