Early proterozoic ore deposits and tectonics of the Birimian orogenic belt, West Africa

Precambrian Research, 58 (1992) 305-344 Elsevier Science Publishers B.V., Amsterdam

305

Early Proterozoic ore deposits and tectonics of the Birimian orogenic belt, West Africa Jean-Pierre Mil6si, Patrick Ledru, Jean-Louis Feybesse, Alain Dommanget and Eric Marcoux BRGM, B.P. 6009, 45060 OrlOans COdex 2, France (Received May 30, 1990; accepted after revision March 16, 1991 )

ABSTRACT Mil6si, J.-P., Ledru, P., Feybesse, J.-L., Dommanget, A. and Marcoux, E., 1992. Early Proterozoic ore deposits and tectonics of the Birimian orogenic belt, West Africa. In: G. Ga~il and K. Schulz (Editors), Precambrian Metallogeny Related to Plate Tectonics. Precambrian Res., 58: 305-344. The Early Proterozoic of the West African craton comprises a series of volcanic troughs and sedimentary basins (the Birimian ) with granitic terranes accreted on Archean nuclei in the Man and Reguibat shields. Studies of the Birimian of the Man Shield indicate a model of polycyclic evolution, with a major collision event (D~; 2.1 Ga) thrusting part of the Proterozoic terrane over the Archean before individualization of numerous volcanic troughs and clastic-infill basins. The proposed evolution for the Birimian orogenic belt comprises: ( 1 ) deposition of the sedimentary Lower Birimian (B1) with minor tholeiitic volcano-sedimentary intercalations (containing chert and/or Mn-formations), and with most of the detritus being derived from Early Proterozoic sources, apart from contamination near the Proterozoic/Archean contact; (2) pre-B2 crustal thickening related to Dt thrusting; (3) formation, over about 40 Ma, of the Upper Birimian (B2) with numerous volcanic troughs of different composition (tholeiitic and rare komatiitic, bimodal tholeiitic to calc-alkaline, volcano-plutonic) and Tarkwaian clastic-infill basins; and (4) major transcurrent tectonics with sinistral (DE) and dextral (D3) strike-slip faults. Such a tectonic evolution from a collision (D~) phase to a transcurrent (D2, D3 ) phase is typical of collision belts, and in the present case the evolution of the Birimian orogenic belt can be extended into Guyana. The metallogenic history of the Birimian shows a three-phase evolution coinciding with the orogenic evolution, and extends over almost 150 Ma from the Perkoa massive (Zn-Ag) sulfides (2.12 Ga) with a clear mantle affinity to the latemesothermal Au quartz veins ( ~ 2 Ga) with (according to lead isotopes) a high crustal participation. The economic mineralization of belt thus consists of: ( 1 ) "Pre-orogenic" (pre-D~) deposits related to early extension zones. This was diverse with stratiform Au tourmalinite ( type 1 Au: Loulo in Mali; Dorlin in Guyana ), stratiform Fe (Cu) ( Fal6m6 in S6n6gal ) and Mn (Nsuta in Ghana; Tambao in Burkina Faso ), and a single massive Zn-Ag sulfide deposit (Perkoa in Burkina Faso ) associated with regional volcanosedimentary (variably tholeiitic) stratigraphic marker beds; (2) "Syn-orogenic" (post-D~ to syn-D2/D3) deposits with disseminated Au-sulfides (type 2 Au: Yaour6 in the Ivory Coast ) in extensional zones of the B2 followed by auriferous paleoplacers (type 3 Au ) in B2 extensional zones (Tarkwaian Banker conglomerate) or syn-D2 transtensional zones (debris flow of Orapu in Guyana). (3) "Late-orogenic" (post-peak D2/D3 ) deposits with mesothermal Au mineralization evolving from a "disseminated gold-bearing arsenopyrite and Au-quartz lode" type (type 4 Au: Ashanti in Ghana) to a "quartz-vein" type with free gold and Cu-Pb-Zn-Ag-Bi paragenesis. Most of the gold in West Africa comes from this phase. Finally, the metallogeny of the Birimian appears as rich in Au- and Mn-formations and poor in volcanogenic mineralization and BIF. It differs from the Archean metallogeny of West Africa and other regions, through the presence of certain deposit types (such as Au-stratiform tourmalinite, gold-bearing conglomerate with Au and Fe-Ti-oxides but no uranium or sulfides, and Au-arsenopyrite-rich shear-zone deposits) that are very common in, but not exclusive to, the Proterozoic and Paleozoic. Correspondence to: Dr. J.P. Mil6si, BRGM, B.P. 6009, 45060 Orl6ans C6dex 2, France.

0301-9268/92/$05.00 © 1992 Elsevier Science Publishers B.V. All fights reserved.

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Introduction

West African craton The West African craton contains two large areas of Early Proterozoic terrane overlying Archean nuclei--the Man Shield from Ghana to S6n6gal and the Reguibat Shield in Mauritania (Bessoles, 1977; Black and Fabre, 1983; Fig. 1 ), with the Archean of the Man Shield being restricted to the Liberian craton and constituting highly metamorphosed gneiss older than 2.7 Ga (Camil, 1984). The Early Proterozoic terranemthe Birimian--represents large sedimentary basins and linear or arcuate volcanic belts corresponding to a period of accretion around 2.1 Ga (Abouchami, 1990), during the Eburnean orogeny that took place between 2.1 and 2.0 Ga (Bonhomme, 1962 ) and was accompanied by emplacement of large granitic plutons. The deformation and metamorphic grade within the Early Proterozoic are generally rather weak, except (a) near the tectonic Archean-Proterozoic boundary (Feybesse et al., 1989, 1990b), (b) in some presumed collision zones (Lemoine, 1988), and (c) along subsequent transcurrent fault zones (Lemoine, 1982; Vidal, 1986; Ledru et al., 1989b, 1991b).

West African mineralization The Archean of the West African craton is distinguished from other Archean cratons by the notable absence of (a) major volcanogenic pyritic or polymetallic massive sulfide orebodies, (b) disseminated gold mineralization (stratiform or stratabound) related to volcanites or turbidites, (c) magmatic Ni-Cu sulfide deposits, and (d) late-orogenic mesothermal gold. The differences could be due to several factors, such as (a) the limited development of greenstone terranes, (b) subsequent tec-

J.-P. MILESI ET AL.

tonic disruption of the belts, (c) the less complete stratigraphy of the belts which show, in particular, an absence of calc-alkaline facies, and (d) possible, but hypothetical, major differences in the composition of the continental crust. However, our knowledge of Archean metallogeny could be limited simply due to the fact that little exploration has been carried out on the Shield. The Archean does, however, contain most of the iron-ore reserves of the West African craton which are hosted within large BIF (banded iron formation) deposits ( > 6000 Mt of iron ore) in the Man and Reguibat shields (Berge, 1968; Pouit, 1975; Bessoles, 1977; Morel, 1979; Umeji, 1983; Bronner et al., 1990). The Archean also contains occurrences of Cr (numerous in the Man Shield, but rarer in the Reguibat Shield) and deposits of Ni-Co with traces of PGE (well developed in the Man Shield; Tagini and Gobert, 1981) associated with greenstone belts or layered mafic-ultramafic complexes (Umeji, 1983; Camil, 1984). The BIF within greenstone belts are also the source of alluvial and eluvial gold, which is commonly spatially associated with the BIF (Mi16si et al., 1989b). Finally, several discordant Au, Pb, Mo, Sn and W mineralizations are related to various magmatic intrusions---either Liberian or later. The Early Proterozoic (Birimian) of the West African craton is poor in BIF (Tagini, 1971 ), but is characterized by a more extensive range of deposits containing, in particular, Mn, Fe, Au, Zn-Ag, Cu _+ Mo _+ Au (Figs. 2 and 3 ). In the southern part of the West African craton (Figs. 1B and 2), the volcano-sedimentary formations contain stratiform and stratabound deposits such as: (a) Mn-oxide and carbonate deposits at Nsuta, Ghana (Kesse, 1985 ) and Tambao, Burkina Faso; (b) stratiform iron deposits in a carbonate environment later transformed to skarns, e.g. Fa-

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THE BIRIMIAN OROGENIC BELT, WEST AFRICA Ore deposit

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Fig. 3. Type, tonnage, grade and production of the major Early Proterozoic gold deposits of West Africa. ASHANTI: active mine; Bibiani: closed mine; Loulo: under development; BF: Burkina Faso; CI: Ivory Coast; GH: Ghana; M: Mali; S: S6n6gal. Type 1: tourmalinized-turbidite hosted gold deposits; type 2: disseminated Au-sulfide deposits; type 3: Tarkwaian gold-bearing conglomerate; type 4: discordant mesothermal auriferous arsenopyrite mineralization; type 5: mesothermal mineralization with quartz, native gold and polymetallic sulfides; L: lateritic ore. "Au-metal tons" represents past production and inferred reserves. 16m6, S6n6gal (Wade, 1985); (c) a single massive Z n - A g sulfide orebody in Burkina Faso (Napon, 1988; Ouedraogo, 1989); and (d) gold deposits related to tourmalinized turbidite sandstone ( D o m m a n g e t et al., 1985, 1986 ). Several deposits related to mafic or felsic magmatic rocks have been recorded, namely (a) gabbro with F e - T i - V at Tin Edia, Burkina Faso (Neyberg et al., 1980); ( b ) discordant C u - M o , Cu-Ag, Sb, W, Sn, N b - T a mineralizations (Tagini, 1971; Peron, 1975; Tagini and Gobert, 1981; Kesse, 1985; Ouedraogo, 1987);

and (c) spodumene pegmatite at Bougouni in Mali (Bassot et al., 1981 ). Finally, a few rare, late- to post-orogenic quartz veins containing lead and antimony have been reported (Ouedraogo, 1987, 1989) in the Man Shield. In the northern part o f the West African craton (Reguibat Shield) the mineralization (Pouit, 1975 ) includes: (a) some deposits that are similar to those o f the southern p a r t m i n particular, small stratiform Mn orebodies (Bir Caleh) contained within Proterozoic metasedimentary formations (Rocci, 1975 ) and rest-

Fig. 2. West African ore deposits. 1-18. See Fig. 1B for explanation. 19-24. Early Proterozoic and Archean ore deposits: 19= Early Proterozoic disseminated Au-ore deposits, including: type l, tourmalinized turbidite-hosted deposits (Loulo district ); type 2, disseminated Au-sulfide deposits (Yaourr, Syama (pro-parte), Dirnrmrra, Goren (pro-parte) ); type 3, Tarkwaian gold-bearing conglomerate (Tarkwa district, Ntronang); and indeterminate pre-schistose types (Kouprla and Lrro); 20 = Early Proterozoic discordant mesothermal Au ore deposits including: type 4, disseminated auriferous-arsenopyrite ore (Ashanti, Prestea, Marlu-Bogosu, Konongo (pro-parte), Asupiri, Sanoukou (pro-parte); type 5, "quartzvein" ore; and other gold deposits; 21= Archean (white) and Early Proterozoic (black) iron deposits; 22= Early Proterozoic manganese deposits; 23= Early Proterozoic Zn + Ag _+ Pb deposits, including Perkoa massive sulfide deposit and Garango disseminated type; 24 = Other commodities, including: Birimian (?) diamonds (Akwatia, Bonsa river (B), Tortiya); Li pegmatite (Bougouni); volcanogenic disseminated Cu sulfide ore deposit (Zeitouo); Cu-Mo sulfide stockwork ore (Monogaga); Ni _+ Co lateritic ore (Bonga, Syola); disseminated Ni-Co-PGE ore (Biankouma, Archean ultrabasicbasic complexes); Pt traces (Winneba, Axim, Makalondi, Kadiolo); Ta-Cb (Issia); 25 = Late- to post-Eburnean Pb quartz veins. AK=Akrokerri district (Au); NT=Ntronang district (Au Tarkwaian type); OM=Opon Mansi (Fe lateritic ore); M B = Marlu-Bogosu district (Au).

310

ing, commonly with a tectonic contact, on Archean rocks (Deschamps and Rocci, 1975; Deschamps et al., 1986); (b) various quartz veins with native gold and traces of Fe, Zn, Pb sulfides, or with Cu-Zn-Pb-Fe sulfides along northerly-striking (D2?) fracture zones at the margins of granitic plutons; and (c) Mo-FeAs bearing quartz veins and Cu-Sn greisens, also associated with granodiorite or porphyritic granite.

Geologic setting of the Birimian

Lithology In comparison with the classic features of Precambrian shields, the Early Proterozoic of West Africa is distinguished by (a) a paucity of komatiite (Regnoult, 1980; Tegyey and Johan, 1989; Mil6si et al., 1989b), (b) the presence of abundant leucogranite (Casanova, 1973; Cocherie, 1978), and (c) certain metallogenic differences, which are considered later in this article. Whereas the Archean granite-gneiss nuclei of the West African craton typically contain disrupted greenstone belts, the Early Proterozoic Birimian is characterized by an alternation of "sedimentary basins" and "volcanic belts" extending over several hundreds or even thousands of kilometers and by large intrusive magmatic complexes showing polyphase emplacement (Figs. 1 and 2; Mil6si et al., 1989b). The reconstituted pile of the volcanic and sedimentary formations, which were initially defined in the Birim river of Ghana (Kitson, 1928; Junner, 1940), shows (Fig. 4): (1) Sedimentary basins attributed to the Lower Birimian (B 1 ) and consisting, from the base up, of (a) basic volcanic rocks and plutons of tholeiitic character (locally preserved in northwest and west Ivory Coast), (b) flyschoid deposits with volcano-sedimentary intercalations (chemical sediments and subordinate volcanoclastic tholeiite), and (c)

J.-P. MILESI ET AL.

carbonate formations (well developed in S6n6gal, Mali and Guinea); ( 2 ) Volcanic belts (or troughs ) generally attributed to the Upper Birimian (B2) and showing bimodal (tholeiitic and calc-alkaline ) volcanism, polyphase intrusive magmatic complexes and fluvio-deltaic formations (including the Tarkwaian of Ghana) whose abundance and importance varies from one belt to another. Recent geochemical studies of the bimodal volcanic B2 series give controversial results in that they suggest (a) intracontinental rift volcanism at Mako, Bouroum-Yalogo and Tsalabya el Khabra in Mauritania (Deschamps et al., 1986), (b) back-arc volcanism at Mako (Dia, 1988), and (c) oceanic plateau volcanism at Mako, Bouroum-Yalogo, Yaour6, Haute-Como6, Liptako and Tsalabya el Khabra (Abouchami, 1990; Abouchami et al., 1991 ). The tholeiitic and komatiitic suites of Guinea (Kiniero and Niandan) belong to a rift developed in a thick, composite crust (Mil6si et al., 1989b). The calc-alkaline volcanism at the end of each volcanic cycle can be interpreted within the context of either transcurrent faulting (Dal6ma; Bassot, 1987) or subduction; however, the intercalation of continental sediments with the calc-alkaline sequences of the volcanic belts in S6n6gal-Mali and the Ivory Coast (Mil6si et al., 1989b; Ledru et al., 1989b, 1991b), and the absence of major low-angle tectonics would seem to favour the transcurrent context. The geological history of the Early Proterozoic of West Africa is, depending on the latest field and laboratory data, generally described in relation to one of two conceptual models-monocyclic evolution or polycyclic evolution. A comparison of the two models, however, shows an inversion of the proposed lithologic sequence. In the Ivory Coast, for example, the Como6 basin is positioned at the top of the lithologic pile (Tagini, 1971; Alric and Vidal, 1990) whereas its extension in Ghana forms the Lower Birimian (Junner, 1940; Kesse,

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Fig. 4. Location of the main Early Proterozoic ore deposits of West Africa related to host lithologies (paleogeographic reconstruction predating the Eburnean D2 tectono-metamorphic phase). Geology: (unit numbers as in Figs. 1B and 2). 2=Granite (pre- to syn-Di phases). 4=B2 Tarkwaian u n i t - - K = Kawere sandy-conglomerate formation; B = Banket sandy-conglomerate formation containing Au quartz-pebble conglomerate; TH= Tarkwa and Huni formations (mainly phyllite and fine-grained sandstone). 5 = Upper Birimian (B2) predominantly volcanic-5a=tholeiitic volcanic rock; 5b=calc-alkaline or tholeiitic andesite; 5c=felsic volcanic and epiclastic rock; 5d=komatiitic basalt (Niandan; Guinea); 5e= volcano-plutonic complexes (Ghana). 6. Lower Birimian (B 1) predominantly sedimentary--6a = basic tholeiitic volcanic and plutonic rocks; 6b = flysch type rocks; 6c = volcano-sedimentary complexes, including a tourmalinized sandstone (white), b chert, and c "Mnformations" and some intermediate tholeiitic to felsic volcanic rocks; 6d=carbonate formations, including Fe stratiform deposits (black). 7=Archean (9, 10) and/or Early Proterozoic (2, 6) complexes deformed by the Eburnean orogeny (only phase Dl shown). 9=Archean greenstone-belt and ultrabasic-basic complexes--9a = basalt and/or stratified complexes; 9b= banded iron formations with associated metasedimentary rocks. 10 = Archean--10a = granite-gneiss complexes; lob = Late-Liberian granite. Ore deposits: Au ( 1-17 ): 1 = Tourmalinized turbidite-hosted deposit--Loulo district; 2-3. Greenstone-hosted deposits---2=Yaour6-Angovia, Syama, Kokumbo; 3=Di6n6m6ra, Goren; 4=Tarkwaian gold-bearing conglomerate--Tarkwa district, Ntronang. 5-7. Discordant mesothermal auriferous arsenopyrite ore deposits. 5 = Gold Coast Range Ashanti, Prestea, Marlu-Bogosu, Konongo (pro-parte); 6 = Asupiri; 7 = Sanoukou district, Diabarou; 8-16. Discordant mesothermal quartz veins with native gold: 8 = Poura; 9 = Bouroum, Guiro, Aribinda, Zug; 10 = Bayildiaga, Akrokerri; 11 = Bibiani, Konongo, Obuom; 12 = Banora; 13 = Jean-Gobel6; 14 = M6dinandi; 15 = Kalana; 16 = Sabodala; 17 = Lateritic ore--Ity. Other commodities ( 18-27 ): 18 = Fe: Fal6m6; 19 = Mn: Nsuta; 20 = Mn: Tambao, Mokta; 21 = Mn: Zi6mougoula; 22 = Zn-Ag: Perkoa; 23= Cu: Zeitouo; 24 = Diamonds: Birim river (Akwatia), Bonsa river; 25 = Diamonds: Tortiya; 26 = Archean BIF: Nimba, Klahoyo, Simandou, Wologisi Range, Bie, Bagla, Bong range, Putu range; 27 = N i - C o - P t Biankouma (Archean).

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1985 ). Geochronology does not, at present, resolve the problem: numerous Rb/Sr ages favour polycyclism (Lemoine et al., 1985 ), placing the first cycle between 2.3 and 2.1 Ga (but the error margin is wide and a more precise age for the deposits remains uncertain), whilst recent isotopic data, including 2°7pb/2°6pb on single-zircons (Mil6si et al., 1989b; Calvez et al., 1990), conventional U / P b determination on zircons (Boher, written commun., 1990), and N d / S m results (Abouchami, 1990), indicate that accretion was very rapid---detrital zircons of the sedimentary basins have either a localized Archean source (2.93 Ga in Guinea) or a Proterozoic source of between 2.16 and 2.09 Ga, whereas B2 volcanism took place between 2.09 and 2.06 Ga. Structural evolution A fundamental factor in the interpretation of the Birimian in terms ofa monocyclic or polycyclic evolution is the interpretation of the structural evolution of the Early Proterozoic of West Africa. The basis for the monocyclic interpretation is either the geosynclinal model (Arnould, 1959; Bassot, 1969; Tagini, 1971 ) or the Phanerozoic plate-tectonic model (Bertrand et al., 1989; Abouchami et al., 1991 ) with the Birimian volcanic series at the base of the Birimian succession and in general concordance with the Archean greenstone belts. The arguments supporting this model are essentially isotopic (Abouchami, 1990); namely, a very small age range around 2. l Ga, and little or no sign of reworking or recycling of the Archean basement. Leube et al. (1990) consider the sedimentary basins and volcanic belts of Ghana to be lateral, time-equivalent facies variants. A monocyclic evolution implies that deformation of the series would have occurred after the major phase of accretion and before the deposition of the fluvio-deltaic formations; structures would therefore be late and variably

J.-P. MILESI ET AL.

penetrative, according to the competence of the rocks. The basis for the polycyclic evolution is the characteristic polyphase structural and metamorphic sequence, recognized throughout the Ivory Coast (Bard, 1974; Bard and Lemoine, 1976; Lemoine et al., 1985; Lemoine, 1988; Feybesse et al., 1989, 1990b; Fabre et al., 1990), Mali-S6n6gal (Ledru et al, 1989b, 1991b), and Burkina Faso (Ouedraogo and Prost, 1986; Feybesse et al., 1990a), which indicates a major phase of deformation (D1) between the Upper and Lower Birimian. The major arguments supporting this model are: (a) the presence of foliated B1 clasts and inclusions in the fluvio-deltaic conglomerate and in the B2 intrusions; (b) B2 dykes cutting the foliation of the B 1 unit; and (c) the consistent synclinal position of the B2 units and the absence in them of any early D l-related structure (even in the fine-grained volcano-sedimentary facies with similar competence to the B1 rocks). The polycyclic evolution is most controversial in Ghana. According to Ledru et al. (1988), Cozens (1988), and Mil6si et al. ( 1991 ), two major phases of deformation exist--pre-Tarkwaian (DI) and post-Tarkwaian (D2). Leube et al. (1990), on the other hand, although recognizing the existence of two schistosities (S~-$2), lineations and thrust zones in the Birimian, consider the compressive deformation to have been strictly preTarkwaian and deny the existence of a discordance between the Lower and Upper Birimian; they consider the deformation of the Tarkwaian to be associated with a tension-related gravity folding. The discrepancies between the two interpretations are fundamental as they constrain any discussion about the origin of gold mineralization in the Gold Coast Range. As far as the present authors are concerned, the lithologic, structural and metamorphic features that they have observed in the Birimian of Ghana are consistent with a polycyclic evo-

THE BIRIMIAN OROGENIC BELT, WEST AFRICA

lution, and this is presented here as a basis for limited discussion. Thus: ( 1 ) Concerning the existence of a discordance between the Lower and Upper Birimian, polyphase deformation structures, such as fold interference patterns and the development of two schistosities (Fig. 5), exist in the Lower Birimian sedimentary rocks. The first schistosity ( D I - S l ) was related to initially flat-lying isoclinal folds, with the first tectonic event being very intense in the southeastern part of Ghana where the metamorphism grades from biotite-garnet to kyanite-staurolite and finally migmatite. The second schistosity (D2-$2) was subvertical with a NE-SW trend, and was associated with regional anticlinal and synclinal folding; high strain gradients are observed in the vicinity of SE-verging D2 thrust zones and NNE-SSW striking sinistral D2 strike-slip faults in the Bibiani area. The Upper Birimian, on the other hand, which is composed mainly of coarse-grained plutonic rocks and locally associated volcanic rocks, only shows signs of syn-emplacement

313

and D2 structures. In addition, the contact between Lower and Upper Birimian is commonly highly sheared (representing a major lithologic discontinuity) and some Upper Birimian-related dykes observed in the Ashanti and Prestea mines (Adadey, 1985 ) cut the first foliation of the Lower Birimian (Ledru et al., 1988 ). The metamorphic grade is generally of greenschist facies, grading locally to the biotite-garnet zone. (2) Concerning the deformation that affects the Tarkwaian, it can be seen that: (a) the Tarkwaian conglomerates contain metamorphic pebbles from the Lower Birimian (Hirst, 1938; Junner et al., 1942), some of which are foliated (Ledru et al., 1988), whereas the granitic pebbles from the Upper Birimian plutonic and volcanic suite are unfoliated; (b) the Tarkwaian sediments were foliated and recrystallized under prograde metamorphic conditions, from the sericite-chlorite zone up to the chloritoid and kyanite zone; (c) the structures in the Tarkwaian are geo-

Fig. 5. D2 folds in Lower Birimian micaschist, Konongoarea. Plane light (X25). S~: So-~foliation 1; $2: cleavage 2 in axial plane of DEfold; 3: biotite blasts (syn-foliation 1 and pre-cleavage2).

314

metrically and kinematically compatible with the structures developed during the D2 deformation of the Birimian series; and (d) reverse faults are abundant in the Tarkwaian, with some of them (observed in mines; Cooper, 1934) showing a displacement of more than 450 m, and overturned strata are common. Such observations indicate that the Tarkwaian rests unconformably on the Lower Birimian and that it was involved in the second compressive phase of the Birimian. Moreover, the metamorphic conditions that were attained (kyanite isograd) imply a thickening of the series of as much as 15 km which can be correlated with the SE-verging thrusts of the Lower Birimian over the Tarkwaian (Cooper, 1934; Hirst, 1938). In that no evidence of major extensional tectonics has been shown, the model of tension-related gravity folding is unlikely and the deformation of the Tarkwaian is attributed to compression tectonics (D2). Thus the data imply that the sedimentary fluvio-deltaic rocks of the Tarkwaian are at the same structural level as the Upper Birimian, which is composed mainly ofplutonic and rare volcanic rocks associated with minor sedimentary horizons--i.e, later than the Lower Birimian (B1) cycle and the first (D~) deformation event, The Tarkwaian is thus considered as representing the Upper Birimian (B2) cycle. Finally, all the units were affected by NE-SW trending ductile strike-slip faults (Fig. 1 ) correlated with a dextral third deformation event ( D 3 ) that is well defined in Burkina Faso (Feybesse et al., 1990a). The polycyclic character of the Birimian orogenic belt can therefore be established from the lithologic, structural and metamorphic features in Ghana and can also be shown for other areas of West Africa--e.g. the Loulo area in Mali (Ledru et al., 1989b, 1991b) or the Perkoa area in Burkina Faso (Feybesse et al., 1990a). Its evolution can be summarized as follows: ( 1 ) Deposition of the dominantly sedimen-

J.-P. MILESIETAL.

tary Lower Birimian (B1), with basic tholeiitic rocks preserved locally at the base, and tholeiitic volcano-sedimentary intercalations and carbonate formations well developed at the top. This represents a first Early Proterozoic accretionary stage with most of the detrital infill derived from Early Proterozoic material, apart from rare Archean zircons found in Guinea near the contact with the Archean nuclei. (2) A first (D 1) tectonic phase at ,-, 2.1 Ga, which contributed to the cratonization of the terrane around the Archean nuclei. The Early Proterozoic was placed in thrust contact with the Archean basement (Feybesse et al., 1989, 1990b; Figs. 1 and 6), resulting in a tectonic Archean-Proterozoic boundary for West Africa. This phase is attributed to collision tectonics. (3) Deposition, over a period of ~ 40 Ma, of the Upper Birimian (B2) with (a) numerous separate volcanic troughs developed mainly in ensialic sites of different composition (tholeiitic and rare komatiitic volcanism, bimodal tholeiitic to calc-alkaline volcanism, calc-alkaline volcanism, volcano-plutonic zones ), and (b) Tarkwaian clastic-infill basins (syn- to post-calc-alkaline volcanism and preto syn-D2 tectonics). This period corresponded to a second accretionary stage attributed to extensional tectonics. (4) Major transcurrent tectonics (D2-D3) corresponding to a new compressive phase and affecting the whole Birimian series. The D2 deformation is marked by N-S to NNE-SSW sinistral strike-slip faults that are locally associated with SE-verging thrust zones; the D 3 deformation is related to NE-SW dextral strikeslip faults.

Tectonic setting A tectonic evolution passing from a collision (D1) phase to a transcurrent ( D 2 - D 3 ) phase is, according to Shackleton (1986), very typical of collision belts. However, to what extent

315

THE BIRIMIAN OROGENIC BELT, WEST AFRICA

ZIEMOUGOULA (Ivory Coast)

,1I

a - 2480Ma

8

G

~~_ . ,~, - - -~' - - ~ ~ ~_./_.~_ ~ - _ ~.~ u1 thru st ----~_ ~-~-_-~'~7_- --~_.~j • zone 6 - - ~. . . 2. " ----- -: " ~ ~ l . , ~ r ~

Mn 7

6

~

~

2

t 2935Ma --~2880Ma

5 --Fr.~^ ~ r ~ p " q12100Ma~4 -12115Mat

~ 1 /]Ip~~ ~

1 • - - 1 .3041Ma ...... ~

.........

Fig. 6. Lithostructural reconstruction of Zi6mougoula (Ivory Coast ) showing D~ thrust zone and Mn formation. 1, 2. Archean basement: l = A r c h e a n granite-gneiss; 2=Archean granite with increasing rotational deformation toward the D~ thrust zone. 3-9. Early Proterozoic formations: 3=syn-D1 granite. 4-7. Mesothermal volcano-sedimentary formations with increasing rotational deformation toward the D~ thrust zone: 4 = B1 tholeiitic lava; 5 = Early Proterozoic BIF; 6 = B 1 sedimentary rocks; 7= Zi6mougoula Mn-formation; 8 = Epizonal B 1 argillosiltite; 9= B2 conglomerate including B 1 schists and granite boulders: (a) foliated granite boulder (2480 Ma) (b) unfoliated granite boulder (2096 Ma). Ages: U / P b and P b / P b on zircons by Boher and Calvez in Feybesse et al., (1989), Mil6si et al., (1989b).

can plate-tectonic models be applied to the Early Proterozoic? According to Windley (1984), the Archean-Proterozoic boundary defines the stage at which modern-style platetectonic processes were able to begin; this assumption is based on several features of Early Proterozoic terranes in the North American and Australian shields--features such as (a) the presence of a rigid crust, (b) the reconstruction of paleo-environments similar to Phanerozoic environments (rifts, volcanic arcs, accretionary prisms, etc.), and (c) the existence of Himalayan-type collision belts. In the Early Proterozoic Birimian of West Africa it is possible to determine (a) rift zones or oceanic plateaus corresponding to a period of crustal growth (Patchett and Arndt, 1986; Abouchami et al., 1991), (b) flysch-basins, and (c) collision tectonics, but not volcanic arcs or oceanic crust subduction/obduction

zones. The volcanic rocks contain no trace of major tangential tectonics, nor any high-pressure relicts and, although the tectonic evolution now appears to be clearer, too many data are still missing to be able to apply the modern concepts of plate tectonics.

Metallogeny of the Birimian Main economic mineralization

From an economic point of view, the southern part of the West African craton contains: ( 1 ) Gold, which is actively explored for and mined within Birimian terrane. Mining of gold in West Africa dates back to pre-history, with total production up to the beginning of the industrial era (end of the 19th century) being estimated at about 1000 t (Bache, 1984), since when a further ~ 1000 t of gold have been produced from the Ghanaian deposits alone (Bache, 1984; Kesse, 1985; Cappendell, 1987 )--thus greatly exceeding the production from other West African countries where extensive gold panning is still active. In Ghana, two types of gold deposit (Fig. 3) are distinguished: (a) the auriferous quartz-pebble conglomerates of the Tarkwa district, generally interpreted as paleoplacers (Junner et al, 1942; Sestini, 1973; Kesse, 1985), and (b) the Ashanti-type gold deposits (Junner, 1932; Leube et al., 1990) that are well developed along a major strike-slip fault system in the Gold Coast Range. Auriferous quartz-vein deposits have also been described and mined in other regions of West Africa (Sonnendrucker, 1969; Blagonadezdhin, 1975; Pal6, 1979; Kesse, 1985; Huot et al., 1987; Mil6si et al., 1989b). Several occurrences of Au mineralization associated with disseminated sulfides are reported from the Ivory Coast, Mali and Burkina Faso. To complete the picture, mention should be made of locally exploitable laterite deposits, the best known example being Ity in the Ivory Coast (Lajoinie and Fonteilles, 1968) grading 16 g/t Au with reserves of about 20 t gold metal.

316

(2) Predominantly stratiform Mn, Fe, ZnAg deposits that are either being mined (e.g. the Nsuta Mn mine in Ghana) or under development (e.g. the Perkoa massive Zn-Ag sulfide and Tambao Mn mines in Burkina Faso, and the Fal6m6 Fe mine in S6n6gal). ( 3 ) Reputedly Birimian diamonds (Bardet, 1974; Kesse, 1985 ) which are mined from several alluvial and eluvial deposits mainly in Ghana (Birim and Bonsa rivers) and the Ivory Coast (Tortiya) and, with an estimated weight of slightly more than 1O0 t carats, have a not inconsiderable economic significance.

Control of the economic mineralization Analysis of the Early Proterozoic gold deposits in relation to the polycyclic interpretation of the Birimian, which appears to better satisfy the constraints of the field data, shows the distribution of gold between different Birimian lithologic units to be very unequal (Mi16si et al., 1989b; Figs. 2 and 3). Unit B1 contains by far the largest gold metal reserves (close to 1200 t), followed by the B2 conglomerates of the Tarkwaian Banket (about 250 t; Bache, 1984), and then the volcano-plutonic formations of unit B2 (about 110 t); the granitic rocks contain only a minor proportion of the total (less than 50 t). These figures must, however, be considered in relation to the fact that the Ashanti mine, with 710 t of gold, represents almost 60% of the total attributed to unit B 1. This skewed gold distribution is equally apparent when the main deposits and occurrences with more than 1 t of gold metal are considered. Unit B1 contains about 50% and the B2 volcano-plutonic rocks between 25 and 30% of the reserves. Finally, it should be pointed out that 25-30% of the gold deposits occur at structural contacts between the B 1 and B2 units. The lithologic, structural and metamorphic controls of the economic gold and base-metal mineralization can be ascribed to three main

J.-P. MILESI ET AL.

tectonic contexts, described here as: ( 1 ) Pre-orogenic: Pre-D~ mineralization related to early extension zones. The mineralization is diversified--(a) stratiform Au tourmalinite deposits (type 1 Au) in sedimentary settings (Loulo in Mali), (b) stratiform Fe + Cu (Fal6m6 in S6n6gal) and Mn (Nsuta in Ghana, Tambao in Burkina Faso) deposits, and (c) a single massive Zn-Ag sulfide deposit (Perkoa in Burkina Faso)--and occurs in tholeiitic volcano-sedimentary rocks with commonly associated carbonated hydrothermal-sedimentary (Mn formations, chert) regional stratigraphic marker beds. (2) "Syn-orogenic": Post-D~ to syn-D2 mineralization associated with the individualization and subsequent deformation both of the troughs of B2 tholeiitic volcanism (disseminated Au-sulfide deposits or type 2 Au) and of the Tarkwaian clastic infill extension basins and their auriferous paleoplacers (type 3 Au). (3) Late orogenic: Post peak-Dz/D3 mineralization with emplacement of discordant mesothermal Au deposits as auriferous arsenopyrite and gold bearing quartz veins (type 4 Au) and gold bearing quartz veins with traces of Cu-Pb-Zn-Ag-Bi (type 5 Au).

Pre-orogenic mineralization Stratiform Mn, Fe, Cu, Au, and Zn-Ag mineralization took place during the deposition of the Lower Birimian (B 1 ) and is found preferentially in the upper part of the unit in settings that were characterized by sedimentary and paleotectonic instability, as well as by associated hydrothermal and tholeiitic volcanic activity (Fig. 4). Examples of this mineralization are: (a) the Mn layers that are widely distributed throughout West Africa; (b) the Perkoa massive Zn-Ag sulfide deposit in Burkina Faso; (c) the Fal6m6 iron deposits of S6n6gal; and (d) the Loulo tourmalinized sandstones of Mali which host the type 1 Au gold mineralization characterized by an Au-BFe (Ni-Co) association. All these deposits,

THE BIRIMIAN OROGENIC BELT, WEST AFRICA

whose main features are outlined below, were formed prior to the first tectono-metamorphic phase (D 1 )"

Manganese deposits and beds Most of the geological (Bessoles, 1977; Mi16si et al., 1989b) and geophysical maps available for West Africa show the extent of the (B1) manganiferous beds that are widely developed in the central and eastern parts of the Proterozoic domain where they form useful stratigraphic marker horizons appearing at, but not exclusive to, the top of unit B 1. They represent true "manganese formations" in the sense of Kimberley (1989a, b), comprising schist, phyllite, quartzite, gondite and carbonates (Tagini, 1971 ), which may pass laterally into essentially siliceous beds similar to those hosting massive sulfide deposits in numerous provinces (e.g. South Iberia; Routhier et al., 1978). The Nsuta (Ghana), Tambao (Burkina Faso), and Zirmougoula (Ivory Coast; Fig. 6) Mn deposits are located in carbonate beds within unit B 1. The presence of manganese has also been reported in the vicinity of the Perkoa massive Zn-Ag sulfide orebody (Napon, 1988; Ouedraogo, 1989), whereas various sulfides having a Ni _+ Co signature (millerite, linnaeite, pentlandite) have been reported in the primary ores and host rocks at Nsuta (Milrsi et al., 1989b ) where reserves are estimated at around 5 Mt of secondary oxide ore with 48.9% Mn and close to 28 Mt of primary carbonate ore with 15.30% Mn (Kesse, 1985). It is still unclear whether the manganese deposits have a predominantly sedimentary origin or a predominantly exhalative-sedimentary origin due to chemical precipitation from hydrothermal solutions. They show associations with turbidite (mass flow) deposits containing intercalations of more volcanogenic layers (tuff, epiclastite), and also with variably graphitic black shale. The first association would reflect the existence of a talus slope

317

(Ledru et al., 1988; Milrsi et al., 1991), whereas the association with black shale could be interpreted according to the Force and Cannon ( 1988 ) model of"shallow marine chemical sediments on the margin of black-shale (and related) facies of stratified seas" such as has been invoked for the Moanda mineralization of Gabon. However, their common spatial association with tholeiitic volcanoclastite and/or siliceous beds and their presence in the vicinity of Perkoa favour the sedimentary hydrothermal model in combination with Kimberley's ( 1989b ) hypothesis, based on the "Feformations", expanded to include the "Mnformations". A spatial relationship between the Mn beds and certain gold deposits has also been proposed, especially in Ghana (Leube et al., 1990), and will be discussed later.

The Perkoa (Zn-Ag) massive sulfide deposit The Perkoa massive Zn-Ag sulfide deposit in Burkina Faso (Ouedraogo and Franceschi, 1982 ) has been dated by the Pb/Pb method on galena at 2120 + 41 Ma (Marcoux et al., 1988 ). The reserves of this deposit, the only one of its type in West Africa, have been estimated at 4.5 Mt of ore at 17% Zn and close to 60 g/t Ag (Ouedraogo, 1989 ). The host rocks, metamorphosed to amphibolite facies and showing St and $2 deformation in the Perkoa area (Fig. 7 ), have been assigned by Ratomaharo et al. (1988) and Feybesse et al. (1990a) to the Lower volcano-sedimentary Birimian (B1) that underlies the tholeiitic volcanic rocks of the Upper Birimian (B2). According to Ratomaharo et al. (1988), the B1 and B2 volcanic and volcanoclastic rocks belong to a TiO2-poor tholeiitic series similar to modern continental basalts or back-arc basin volcanic suites. The B1 and B2 formations are cut by various intrusions, including a granodiorite showing a dextral D3 mylonitic zone (Feybesse et al., 1990a) that was emplaced in the footwall of the deposit (Fig. 8 ). The host formation itself ( 100-150 m thick) consists mainly of me-

318

J.-P. MILESI ET AL.

Fig. 7. Dt and D2 deformation at Guido, close to Perkoa (Burkina Faso). So-So: Foliation So-S> Py: Pre-S~ sulfides (pyrite). $2: Cleavage2 in axial plane of D2 fold. tasedimentary rock, having the composition of subgraywacke (Ratomaharo et al., 1988) and with volcano-sedimentary and pyroclastic intercalations, that had been affected by early hydrothermal alteration (Ratomaharo et al., 1988; Ouedraogo, 1989 ) as shown by the presence of (a) manganiferous garnet and gahnite in the footwall of the deposit, (b) chloritic stockworks with disseminated sulfides and Ba-

rich magnesian volcanoclastic rocks in the interval between the two orebodies, and (c) disseminated sulfides in a quartz + chlorite + white mica gangue in the hanging wall of the deposit. The hanging wall rocks also contain reworked fragments of pyrrhotite (Ouedraogo, 1989). The mineralization itself (Napon, 1988; Ouedraogo, 1989) occurs as two lenses (20.0

319

THE BIRIMIAN OROGENIC BELT, WEST AFRICA

PERKOA CROSS-SECTION 10075 E

// Om

-fO0

-200

-300

":÷ + \ iF,i, " SULFIDE \ /J I / ++

MASSIVE LENSES

-o- ....

~

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:.

7"-. 2

MYLONITE / / GRANODIORITE

-5OO

-600

Fig. 8. Perkoa massive sulfide deposit: simplified crosssection 10075E after Napon (1988) and Ouedraogo (1989). D3: Dextral N70°E strike-slip fault. B2: Tholeiitic and mafic to pyroclastic rocks and lavas. B l: Hydrothermalized metasedimentary rocks; subgraywacke with volcano-sedimentary rocks and pyroclastic intercalations. In black: the main massive sulfide lenses.

and 5.8 m thick, respectively) separated by barren hydrothermally altered volcano-sedimentary layers (Ratomaharo et al., 1988 ). The paragenesis is sphalerite, pyrite and pyrrhotite with accessory galena, chalcopyrite, tetrahedrite, magnetite, ilmenite, molybdenite and barite. The orebody also appears to have been partially offset by D 3 synmetamorphic shearing in such a way that it is difficult to reconstruct its original shape with any precision. Napon (1988) and Ouedraogo (1989) show, however, that the mineralized bodies consist of locally banded massive ore overlying stratiform breccia facies (locally upward fining in the drill holes) composed of fragments of pyrite or polymetallic ore in a chloritic or sulfide matrix (Ouedraogo, 1989). The paragenesis and nature of the ore per-

mit a comparison of the Perkoa deposit with syngenetic sedimentary-hydrothermal or volcanogenic deposits (Franklin et al., 1981; Pouit, 1984) in spite of numerous post-depositional mineralogical transformations of the syngenetic ores, such as (a) in situ recrystallization; (b) expulsion of the trace elements Pb, Cu, Ag and Sb during remobilization, indicated by the crystallization of galena, chalcopyrite, tetrahedrite and dyscrasite in sphalerite; (c) the crystallization of magnetite at the expense of inclusions of pyrrhotite in the sphalerite, molybdenite and gudmundite; and (d) late emplacement of slightly ferriferous sphalerite, galena, chalcopyrite, tetrahedrite, pyrrhotite, ullmannite and pyrite in fissures. The/.t (238U/2°4pb) (Fig. 9) and W (232Th/ 2°4pb) ratios, characterizing the deposit's lead evolution from 3.7 Ga to 2.12 Ga, are respectively 9.11 and 34.8 in the Stacey and Kramers (1975) model; according to the standard curves of the Zartman and Doe ( 1981 ) model, these values indicate that the lead records a significant mantle contribution (Marcoux et al., 1988). Finally, it appears that the Early Proterozoic of the southern part of the West African craton is abnormally poor in polymetallic volcanogenic mineralization, and especially in Aumassive sulfide deposits. Apart from the Perkoa massive Zn-Ag sulfide deposit hosted within Early Proterozoic manganiferous volcano-sedimentary units, the only notable example is the Tessalit zinciferous deposit (Adrar des Iforas in Mali) that is of Late Proterozoic age (Leblanc and Sauvage, 1986). This latter deposit shows a sulfide association with carbonates and magnetite that is reminiscent of, and of the same age as, the zinciferous and manganiferous deposits of the Khnaiguiyah district in Saudi Arabia (Sabir and Pouit, 1984). Tessalit is considered to have been emplaced in an island-arc setting (Leblanc and Sauvage, 1986) and Perkoa to have been deposited in an intercontinental rift or back-arc basin setting (Ratomaharo et al., 1988 ).

320

J.-P. MILESI ET AL.

15.3 '

I

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'

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/

~-/

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7

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Fig. 9. Initial isotopic composition of lead from Birimian mineralizations (from Marcoux, written commun., 1990). Symbols with arrows represent calculated initial isotopic compositions--i.e, intersection of the line of radiogenic enrichment (arrow indicating the slope R) with the primary isochron (age of the deposit established by U/Pb and Pb/Pb on zircon). No isotopic data on galena are available from these deposits, so it was necessary to use radiogenic-rich sulfides whose isotopic composition defined good radiogenic enrichment lines. Circles represent mesothermal mineralization. Asterisks represent stratiform mineralization. The "Loulo" asterisk represents the initial isotopic composition of the disseminated sulfides hosted by the tourmalinized sandstone at Loulo. The triangle marks the isotopic composition of the Loulouie mesothermal mineralization (Guyana) The dashed line relates to the late-D2 mineralized quartz-carbonate stockworks at Loulo.

The Fal@md iron deposits The Falrm6 district in Srnrgal, with a potential o f more than 800 Mt o f iron ore (Combes, 1980; Milrsi et al., 1989b) is, together with the F e - T i - V mineralization of Tin Edia in Burkina Faso (Neyberg et al., 1980), one of the largest Early Proterozoic iron districts of the West African craton. The Falrm6 iron district, 55 km long by 15 km wide, contains 12 orebodies in eastern Srnrgal and one in Mali. Where outcropping, they consist o f altered ore enriched in martite and iron hydroxides with grades o f between 55 and 60% Fe; the reserves of the three main orebodies ( K o u d r k o u r o u , Karaka~ne and K o u r o u d i a k o ) have been esti-

mated at 340 Mt with an average grade of 58% Fe. The primary magnetite ores show grades in the order of 40-45% Fe, with the two largest orebodies (Farangalia and G o t o ) containing 300 Mt at an average grade o f 43% Fe (Combes, 1980; Milrsi et al., 1989b). The Falrm6 orebodies occur in the top part of unit B 1 (Fig. 10), systematically showing carbonate-bearing rocks in their footwall (Mi16si et al., 1989a). Both the host formation and the mineralized bodies were affected by D i deformation and then intruded by microdiorite dykes of the (B2) calc-alkaline D a l r m a series; andesite dykes from this suite have been dated at around 2.07 Ga ( P b / P b on zircon) by Calvez (in Milrsi et al., 1989b). Finally, the

321

THE BIRIMIAN OROGENIC BELT, WEST AFRICA

Fe FALEMI~-GOTO ORE DEPOSIT

Cross-sections W GOT.24

GOT.

23-21-3

E GOT.14

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Fig. 10. Fal6m6-Goto Fe-ore deposits: schematic map and cross sections (A, B, C), modified from Combes (written commun., 1984).

322

succession was intruded by a granite complex (Boboti) synchronous with D2 deformation (Ledru et al., 1989b, 1991b). The orebodies in the contact aureole of the Boboti granite intrusion comprise a series of magnetite-bearing magnesian skarn lenses with thicknesses from a few meters to as much as 200 m and lengths of several hundred meters. The primary ores are generally banded and contain magnetite associated with sulfides (around 15%) enclosed in a complex gangue of phlogopite, diopside, serpentine, quartz, carbonate, tremolite, chlorite, ilmenite, plagioclase, magnesian hornblende, garnet (grossular andradite ) and spinel. The main sulfides are pyrite and pyrrhotite, with accessory Cu, Ni, Ni-Co, Zn, As, Mo, and Bi sulfides also present (Wade, 1985; Mil6si et al., 1989b). In spite of the absence of any direct association of mafic lithologies with the deposits, this sedimentary setting shows a persistent "mafic signature" which is reflected by the crystallization of pentlandite, smythite, millerite, bravoite and nickeline. The origin of the Fal6m6 deposits is controversial. They were initially interpreted as contact skarn deposits, but it appeared that the contact metamorphism was in fact only responsible for an in situ transformation of the pre-existing stratiform mineralization (Maruejol, 1977; Combes, 1980). Wade (1985) has interpreted the early mineralization as BIF type deposits, whereas Mil6si et al. (1989b), due to the presence of intermediate to silicic tholeiitic volcanic rocks in the carbonate-bearing formations and the existence of lenses of massive banded pyrrhotite-chalcopyrite ore, assign the mineralization to an Fe _+ Cu volcanogenic type.

Tourmalinized turbidite-hosted gold deposits (type 1 Au) Throughout the whole of West Africa, only the Loulo district of Mall and S6n6gal contains gold deposits (Loulo 0, Loulo 3, P64) hosted

J.-P. MILESI ET AL.

by tourmalinized turbidite formations (sandstone and rare conglomerate), and thus identifiable with gold deposits of the "stratiform tourmalinite" and/or "turbidite-hosted deposit" types, which are common in Precambrian shields, particularly in the Proterozoic (Slack, 1982; Taylor and Slack, 1984; Keppie et al., 1986; Plimer, 1986; Hutchinson, 1987). The gold potential of the Loulo district is several tens of tonnes of gold metal; geological reserves in the Loulo 0 deposit are 28.2 t of gold metal in ore with an average grade of 4.38 g/t between the surface and 150 m depth, and with the deposit still open downdip (Dommanget et al., 1985). The tourmalinized turbidite deposits reflect an important change which occurred toward the end of the sedimentation in the extensive Lower Birimian (B1) basins (Fig. 11 ). They mark the top of the B1 flyschoid formations (thus forming a lithostratigraphic marker) and precede variably carbonate-bearing finegrained sedimentary rocks (argillo-siltite) that

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Fig. 11. Loulo 0 (Mali) tourmalinized turbidite-hosted gold deposit (type 1 Au): schematic map (a) and cross section (b) showing D: fold, completed from Dommanget ( 1985 ). 1 = Late dolerite (post Eburnean). 2 = B 1 dolomitic carbonate and carbonate-bearing sedimentary rocks including tholeiitic volcanoclastic rocks. 3=B1 hanging wall: hydrothermalized (chlorite, albite, carbonate) argillo-siltite. 4 = B 1 gold deposit within tourmalinized turbiditic sandstone. 5 = B 1 footwall: green flyschoid sandstone. 6 = Location of section.

THE BIRIMIAN OROGENIC BELT, WEST AFRICA

were followed by the dolomitic carbonates (Dommanget et al., 1986) which host the Fa16m6 stratiform iron deposits (Figs. 4 and 1 l; Mil6si et al., 1989a, b). A roughly N-S trending talus slope controlled the general distribution of the turbidite deposits, which are found along a zone over 200 km long (Mil6si et al., 1989b). This area of instability is also marked by numerous highly tourmalinized hydrothermal zones in which hydrothermal activity commonly preceded the intermediate to felsic tholeiitic volcanic activity (dykes, pyroclastic rocks and volcanoclastic rocks) that persisted during subsequent carbonate sedimentation. The tourmalinized detrital layers were later affected by two phases of synmetamorphic deformation (D~ and D2 ) and, like the Fal6m6 deposit, are cut by Dal6ma-type calc-alkaline dykes that were themselves foliated by D2 deformation. The tourmalinized turbidite-hosted gold deposits reflect two superimposed types of mineralization (Mil6si et al., 1989b): (a) a pretectonic disseminated sulfide mineralization associated with a gangue of pre-D~ cryptocrystalline Mg-tourmaline (dravite) + silica _+ carbonate _+ magnesian chlorite _+ white mica; and (b) a late-D2 Au stockwork sulfide mineralization that overprints the D2 foliation and is cut by polyphase quartz + carbonate _+albite stockworks. The age of the disseminated mineralization has been established at ~2085 Ma, and between 2098_+ I l Ma and 2072 + 9 Ma by the U / P b and Pb/Pb ages of the youngest detrital zircons of the B 1 host rocks and by the Pb/Pb age of zircons from the B2 Dal6ma series dykes cutting the tourmalinized sandstone (ages respectively from Boher and Calvez in Mil6si et al., 1989b). The highly tourmalinized zones correspond to pipes ( 1 to 100 m in diameter) containing pre-tectonic veinlets of tourmalinite in networks (commonly controlled by the strata themselves) and hydraulic breccias that predate foliation. The pipes are associated with massive tourmalinization of the sandstone and

323

conglomerate beds, and extend for as much as several hundred meters from their edges. This pattern, along with the general distribution of the deposits, reflects a control by syn-sedimentary paleofractures. It should be mentioned that less-developed tourmalinite stockworks also occur locally in the hanging wall formations of the tourmalinized sandstones. The early development of host-rock tourmalinization in the feeder zones is confirmed by the presence of tourmalinite pebbles (Fig. 12 ) in the non-tourmalinized volcanoclastic hanging wall, and locally (Loulo 3) of reworked tourmalinized pebbles within conglomerate of the host formation. The gold deposits consist of mineralized bodies in which the highest gold grades correspond to (a) highly tourmalinized sulfide zones fed by pipes with early tourmalinite (Loulo 0, Loulo 3, P64), and (b) zones with a

Fig. 12. Conglomerate, showing black tourmalinite pebbles (To) intercalated in tourmalinized sandstone--Loulo (Mali) (×25).

324

variable density of late-D2 quartz-carbonate and sulfide stockworks. The gold may be discrete (P64) or well developed (Loulo 0) whereupon it is preferentially located in the intensely tourmalinized areas (Loulo 0 ). In both ore types, the sulfide paragenesis contains few mineral species, but is characterized by traces of Ni-Co sulfides; the sulfides are essentially (Dommanget et al., 1985) pyrite, pyrrhotite, pentlandite, gersdorffite and arsenopyrite. The gold is very pure (Ag< 1%) and forms fine particles ( 10-40/.tm) in fissures or at the surfaces of pyrite crystals with which it is generally associated; it is, however, coarser in the quartz and carbonates of the stockworks (0.51.0 mm) and in the oxide ore. The ratio/~ (238U/2°4pb) is 9.35 (Fig. 9) in the Stacey and Kramers (1975) model, using an age of 2085 Ma for the disseminated mineralization; this value, according to the Zartman and Doe ( 1981 ) model, indicates a greater degree of crustal involvement than for the Perkoa massive Zn-Ag sulfide orebody, and the same procedure applied to the sulfides of the Fal6m6 iron deposits, which are of roughly the same age as Loulo, confirms this trend (/t=9.41). It can be suggested that the circulation of mineralizing fluids in a setting consisting of unstable flyschoid basins with little tholeiitic influence would bring about a mixture between the lead of the fluids and that of the host rocks through which the fluid circulated. This tendency is even more apparent with the sulfides of the late-D2 stockworks, as indicated in Fig. 9 by the position of the line of radiogenic enrichment. Thus it can be concluded that the fluids giving rise to the mineralization were subjected to crustal influence and were controlled by (a) active syn-sedimentary paleofracturing within an unstable flyschoid sedimentary basin, leading to early disseminated mineralization, and (b) late-tectonic D2 fracturing controlling the late sulfide and subsequently quartz-carbonate stockworks. The two types of mineralization would appear to belong respectively to the "stratiform

J.-P. MILESI ET AL.

gold-bearing tourmalinites" (Slack, 1982; Plimer, 1986) and the "turbidite-hosted gold deposits" (Boyle, 1986; Keppie et al., 1986 ). The former, common in the Proterozoic (Slack, 1982 ), are considered to be indicators of submarine hydrothermal systems generally associated with exhalative deposits of Pb-Zn, CuZn, Cu-Co, Sn, W and Au (Slack, 1982; Taylor and Slack, 1984; Plimer, 1986; Bone, 1988 ); modern analogs would be hot-spring centers or geothermal systems. This submarine hydrothermal model presumes the early introduction of boron-rich fluids into the deposits. The "turbidite-hosted gold deposits", also common in the Proterozoic (Hutchinson, 1987), contain quartz-carbonate veinlets characterized by variable amounts of tourmaline, chlorite and mica. Certain authors (Haynes, 1986 ) consider such mineralization to derive from syngenetic deposition, although it is more generally interpreted as epigenetic mineralization post-dating the deposition of the host rocks (Keppie et al., 1986; Boyle, 1986 ); the Passagem de Mariana gold mineralization in Brazil has, for example, been considered as both epigenetic (Vial et al., 1988) and syngenetic (Fleischer and Routhier, 1973). This debate could be enlightened by considering the additional factors controlling stratiform and late-tectonic mineralization (multi-stage model) as shown by the polymorphic character of the Loulo district deposits. Finally, it should be noted that the Au-BFe-Ni-Co-As mineralogical association at Loulo is similar to that of the Dorlin gold deposit in Guyana, which is hosted by Paramaca volcanic formations--the probable Early Proterozoic equivalent of unit B I in West Africa (Ledru et al., 1991a).

"Syn-orogenic" mineralization No gold deposits appear to be directly related to the first orogenic phase (D1), but various types of auriferous mineralization took place in localized extensional or transtensional

325

THE BIRIMIAN O R O G E N I C BELT, WEST AFRICA

B2 environments following the D1 phase (Fig. 4). The first of these was the late-B2 Au + Cu mineralization in B2 tholeiitic volcanic to volcano-plutonic areas situated in the hinterland of the major DI thrust zone, and can be assigned to the "greenstone-hosted gold deposits" as described by Hutchinson (1987). It should be noted that (as far as is known) no significant gold or base-metal deposit was emplaced during the deposition of the B2 calc-alkaline series. However, between 2081 and 1968 Ma (Leube et al., 1990 ) the gold was reworked and deposited in Tarkwaian conglomerate again in close spatial association with volcano-plutonic systems that, according to Leube et al. (1990), are mainly tholeiitic with, according to Milrsi et al. (1989b), local preTarkwaian calc-alkaline dacite.

Disseminated Au-sulfide deposits (type 2 Au) Several occurrences of Au _+ Cu mineralization associated with disseminated sulfides are hosted by B2 tholeiite or intrusions (subvolcanic diorite, rhyodacite). They have been reported from Burkina Faso, Mali, and the Ivory Coast and constitute an insufficiently studied but promising type, with several deposits containing geological reserves of between 5 and 10 t of gold metal. The Yaourr-Angovia gold mineralization, in the center of the Ivory Coast is hosted by mafic tholeiitic volcanites (Fabre, 1987; Fabre et al., 1987) with siliceous and manganiferous intercalations. The B2 volcanic rocks are overlain by conglomerate containing clasts of calc-alkaline volcanic rocks and were deformed by the D2 phase that produced an anticlinorium and NNE-SSW striking sinistral strike-slip faults (Fabre et al., 1990). The D i r n r m r r a sulfide mineralization in Burkina Faso (Ouedraogo, 1989) is more cupriferous, but can also be assigned to this type. It is distinguished mainly by its dioritic host rocks that intrude the B2 tholeiitic succession, but which have also been deformed by D2.

However, as at Yaourr, the mineralization was relatively well preserved by the D: deformation which here was of moderate intensity. The mineralization at Syama (Mali), like that at Yaourr, occurs within a mafic tholeiitic volcanic setting rather similar to that at Angovia. However, it seems also to be partly controlled by D2-D3 faults (Pohl, oral comm., 1990). The disseminated gold and sulfide mineralization at Yaourr-Angovia is associated with locally brecciated hydrothermal zones cutting both the volcanic rocks and the suites of dacite to rhyodacite dykes and sills (Fig. 13). The mineralization gangue is characterized by the association quartz _+ white mica __+carbonates _ tourmaline + chlorite, all deposited before the D2 deformation since this affected both the sulfides and the gangue. The sulfide mineralization is characterized by an absence ofPb, Sb, and As minerals (Lafor~t, written comm., 1989); the main sulfides are pyrite (abundant), chalcopyrite (common) and rare pyrrhotite, pentlandite and magnetite, with traces of sphalerite, cubanite, bravoite and linnaeite, and the Bi, Ni, Hg tellurides (tetradymite, melonite, coloradoite). The presence of exsolution blades of cubanite in the chalcopyrite in-

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Fig. 13. Yaour6 (Ivory Coast) Au disseminated sulfides (type 2 Au): schematic cross section along profile 380 m, modified from Trrhin (written commun., 1991). B2: Tholeiitic pillow lava and rare pyroclastic rocks. So-$2: Dip of bedding and cleavage 2 (D2-$2).

326

dicates formation temperatures between 250 and 300 ° C. The gold occurs (a) in particles varying in size from 1 to 150/zm which may be present as inclusions in the gangue minerals (white mica, quartz or carbonates), and (b) as fine inclusions ( 1-50 pm) in pyrite (and exceptionally chalcopyrite) or at the edges of pyrite crystals. With the exception of As, Cu, and Bi, the geochemical signature of the Yaour6-Angovia deposit more closely resembles Loulo with its Au, B, Fe, + Ni-Co, than the late-orogenic mesothermal gold deposits described below; there is also a difference in the abundance of carbonate in the gangue. It should here be pointed out that the Pb isotopic compositions (2°6pb/2°4pb and 2°Tpb/ 2°4pb) of the pyrite and chalcopyrite at Di6n6m6ra (Burkina Faso) define a line of radiogenic enrichment which is very close to that of the disseminated sulfides at Loulo, and which would hence appear to support this comparison between the geochemical signatures of the two deposits (Marcoux, written commun., 1990). The disseminated mineralization and associated alteration at Yaour6 and Di6n6m6ra occurred before the syn-D2 metamorphism and so cannot be regarded as comparable with a "stratabound" epigenetic mineralization (Phillips et al., 1984). However, a hydrotherreal origin can be envisaged for the pre-schistose disseminated mineralization, with precipitation of the metals in early fractures before metamorphic recrystallization; in this respect, they can be assigned to the "greenstone-hosted lode deposits" (Hutchinson, 1976; Valliant and Bradbook, 1986). For other deposits such as Syama, however, it cannot be excluded that enrichment was either "multistage" (Hutchinson, 1987), with early mineralization followed by subsequent preferential concentration during De-D3 tectono-metamorphic events, or else purely epigenetic and related to Da-D3 faulting.

J.-P. MILESI ET AL.

Tarkwaian gold-bearing conglomerate (type 3

au)

The deposits of the Tarkwa district in Ghana (Junner et al., 1942) are located close to the southern end of a northeast-striking D2 synclinorium where they are hosted by quartz-pebble conglomerate of the Banket formation (Fig. 14); total reserves of gold metal are around 250 t (Bache, 1984). From 1878 to 1955 the cumulative production of the Tarkwa and Abosso mines was 87 t at a grade of 9.8 g/t, while between 1969 and 1983 the Tarkwa mine produced 23.1 t. Present annual production is around 600 kg with an average ore grade of about 4.7 g/t (Suttill, 1989; Mil6si et al., 1989b). At Tarkwa, the mineable reefs consist of lenses of quartz-pebble conglomerate averaging 400 m length by 100 m width with orientation being controlled by paleochannel directions (Sestini, 1973 ). The lenses are separated from each other by intervals of virtually barren sandstone. Sestini (1973) considered the mineralization to be of paleoplacer type, based on observations of the lithologic control of the gold enrichment and a positive relationship between the abundance of gold and iron oxides. The matrix of the conglomerate consists of sandstone with iron oxides and, to a lesser extent, ilmenite, rutile and zinciferous chromite (Hirdes et al., 1988) with additional quartz, carbonates, sericite, chlorite, chloritoid, epidote, tourmaline, zircon and garnet (Mil6si et al., 1989b). However, some sulfides (pyrite, chalcopyrite) associated with quartz veins are seen to cut the matrix. It must be kept in mind that the matrix in the Main Tarkwa Reef is composed largely of syn-D2 to post-tectonic minerals (Ledru et al., 1988; Mil6si et al., 1991b). Pre-D2 zircons (dated at 2128___ 14 Ma by the Pb/Pb method; Calvez, written commun., 1990), chromite grains and ilmenite are the only residual detrital components preserved.

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The ilmenite represents only about 20% of the iron and titanium oxides, the majority (80%) being in fact represented by late-D2 euhedral crystals of magnetite and mushketovite. It should also be pointed out that the mafic to intermediate dykes cutting the conglomerate are also locally auriferous and have post-D2 mineral assemblages comparable with those in the gold reef--namely Fe and Ti oxides (in places almost 1 cm in length), chlorite, white mica, quartz, albite and carbonates. Consequently, although some kind of lithologic control seems to have existed since the auriferous concentrations are restricted to certain paleochannels within the conglomeratic lenses of the Banket, the origin and timing of gold mineralization remains open to question. It is possible that the present distribution of gold in the ore is in part dependent on syn- to late-D2 Eburnean tectono-metamorphic phenomena that also affected the matrix of the conglomerate and the volcanic dykes. This idea does not, however, exclude the hypothesis of an original paleoplacer which would then have been modified by late phenomena. It is essential to examine the relative feasibility of several alternative mechanisms, and in particular that which caused, as at Witwatersrand, the

circulation of fluids after deposition of the sediments (Phillips and Myers, 1987; Phillips et al., 1988). With respect to the proposed transtensional geotectonic setting of the Early Proterozoic auriferous conglomerate, the most compelling evidence has been found in Guyana (Ledru et al., 1991 a): "en echelon" basins were formed along the North Guyana Fault Zone where the dynamics of clastic infill appear to be related to the second phase of Guyanaian transcurrent tectonics. In Ghana and the Ivory Coast (Bondoukou), the only well documented feature consistent with an extensional setting is the early intrusion of certain sills and dykes within the B2 conglomeratic sandstone. Moreover, considering the style and intensity of deformation observed in the Tarkwaian deposits, it is natural to conclude that the formation of the basins preceded the peak activity of D2 transcurrent tectonics which obscured the original characteristics of what have generally been interpreted as paleoplacers (Junner et al., 1942; Sestini, 1973; Kesse, 1985). Most authors, such as Kesse (1985), consider the source of gold in the "quartz-pebble conglomerates" to have been derived from Ashanti-type mineralization localized in the au-

328

riferous shear zones bordering the Tarkwaian deposits. This hypothesis appears to be unrealistic because the Ashanti-type mesothermal deposits were formed after the D2 deformation affecting the Tarkwaian sediments and attendant mineralization (Mil6si et al., 1991). Therefore, a gold provenance from B1 sedimentary rocks and/or B2 volcanic rocks and intrusions must be considered, particularly since these sequences are known to contain type 1 and type 2 gold mineralization, albeit in regions far from Ghana.

Late-orogenic mineralization A large number of discordant, mesothermal gold deposits were emplaced following the peak metamorphism of the orogeny and after the most intense stage of compressive deformation in the craton which gave way to brittle deformation towards the end of the D2 and D 3 tectono-metamorphic phases. The emplacement occurred during two main episodes of mineralization, probably partly in continuum, that show a paragenetic evolution from disseminated auriferous arsenopyrite mineralization in columns (type 4 Au), which is particularly well developed in Ghana (Ashantitype), to a gold-bearing "mesothermal quartz lode" mineralization with C u - P b - Z n - A g - B i (type 5 Au) which formed ~2.0 Ga (Pb/Pb on galena) at Poura and Kalana. Both type 4 and type 5 mineralization commonly appear to be restricted to unit B 1 and along structural contacts between units B 1 and B2.

Mesothermal auriferous arsenopyrite and Aubearing quartz vein mineralization (type 4 Au) Discordant mesothermal mineralization (auriferous arsenopyrite + Au-bearing quartz vein lodes) in West Africa includes the Ghanaian deposits such as Ashanti, Prestea, Marlu, Bogosu and Konongo and, with 1100 t of gold metal, represents the economically most important mineralization type. Similar deposits

J.-P. MILESI ET AL.

are also known from the Ivory Coast (Asupiri and Aniuri), within extensions of the Ghana faults and in Mali (Sanoukou district ). The type example of this type of mineralization is the Ashanti deposit, famous in West Africa for having provided (between 1898 and 1986) 584 t of gold metal; production in 1988 was 9.69 t of gold metal (Suttill, 1989) with remaining reserves evaluated at 126 t of gold metal (Cappendell, 1987). The type 4 Au mineralization occurs within tectonic corridors (such as the Gold Coast Range) several tens of kilometers long with a general NE-SW strike, and commonly several kilometers wide. These zones show a complex structural evolution marked by the superposition of several phases of deformation (Ledru et al., 1988; Mil6si et al., 1991 )---e.g. D2 thrust faults, D3 dextral strike-slip faults, and late brittle faults. The host rocks of the mineralized bodies are most commonly metasediments of unit B 1, but may also be mafic to intermediate or (rarely) silicic B2 intrusions cross-cutting the first foliation of the B1 sedimentary rocks (Ledru et al., 1988). The BI metasediments are composed mainly of argillite, graywacke or epiclastic deposits, commonly with intercalations of siliceous or manganiferous "chemical sediments" and black sediments with graphite and pre-D2 disseminated sulfides (pyrite and nonauriferous arsenopyrite at Konongo ). They are locally cut by pre-D2 veins consisting of quartz + (Fe, Mn) carbonates + pyrite + pyrrhotite and rare chalcopyrite, and commonly containing post-Dl/pre-D2 carbonate porphyroblasts. The B2 dykes commonly show chlorite + white mica + carbonate alteration. The Ashanti gold field is almost 7000 m long, which gives some idea of the size of the host structures of the mineralization (Hussey, 1988 ). The mineralized bodies themselves are as much as 500 m long with thicknesses up to 25 m (Kesse, 1985 ), lying along the host structures in columns that can be several hundred meters deep (Fig. 15). Most authors (Kesse,

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1985; Leube and Hirdes, 1988; Mil6si et al., 1991 ) agree in distinguishing two common associations of ore in the mineralized columns: (a) disseminated gold-bearing arsenopyrite, which provides about 60% of the Ashanti production, and (b) quartz veins and stockworks with free gold and polymetallic sulfides, which truncate the disseminated mineralization. Mineralogical and structural observations indicate that the two types of ore were emplaced successively during the later stages of D2 deformation (Mil6si et al., 1989b, 1991). Thus, at Ashanti and Prestea, two principal parageneses belonging to the two ore types can be clearly distinguished: ( 1 ) The first contains disseminated arsenopyrite where gold is incorporated into the crystal lattice of the mineral and where the arsenopyrite itself crystallized as a static fabric overprinting the $2 foliation (Fig. 16); this clearly took place before the formation of the quartz veins since the veins enclose fragments of country rock containing disseminated arsenopyrite. The arsenopyrite occurs as clusters of crystals or as isolated acicular euhedral crystals up to 5 mm long and may include small patches of pyrrhotite. ICP analysis of arsenopyrite concentrates from Ashanti, Prestea and Sanoukou, and microprobe studies of the crystals show that they contain in the order of 150500 ppm of gold, with traces of Ni and Co. The

microprobe analysis also reveals a zoning of As in certain crystals, analogous to that of the goldbearing arsenopyrites in French Hercynian deposits (Bonnemaison and Marcoux, 1987, 1990; Marcoux et al., 1989; Johan et al., 1989) and in synthetic gold-bearing arsenopyrites (Wu and Delbove, 1989 ). The composition of the Ashanti arsenopyrite shows the temperature of the fluid to have been below 350°C, providing the arsenopyrite-pyrrhotite equilibrium was attained (Mil6si et al., 1991 ). The arsenopyrite may be accompanied by chlorite, white mica, (Fe, Mg) carbonate and quartz. (2) The later paragenesis is seen in quartz veins or veinlets as an association of native gold + pyrite with traces of Ni + chalcopyrite + tetrahedrite + pyrrhotite + sphalerite (rare) _+ dolomite _+ graphite It caused, over a distance at least 1 cm from the quartz veins, corrosion of acicular gold-bearing arsenopyrites and the appearance of micron-size patches of native gold within or in contact with the arsenopyrite crystals. Gold which is variably argentiferous also appears commonly as millimeter-size patches in late fractures within the quartz veins. Leube et al. (1990) mention a "vein-related alteration" with paragonite, ferro-dolomite, kaolinite and gibbsite. According to Leube and Hirdes (1988) and Leube et al. (1990) the temperature of the fluids giving rise to the deposition of this second paragene-

330

Fig. 16. Gold-bearing ( 150 ppm Au) arsenopyrite

J.-P. MILES1 ET AL.

(As) overprinting D2 foliation--Prestea

sis would have been between 300 and 400 oC. This succession of a paragenesis with auriferous arsenopyrite followed by a paragenesis with quartz and native gold, has also been described in several deposits of the Hercynian of Europe (Bonnemaison and Marcoux, 1990), and will be discussed later. It must be mentioned here, however, that the second paragenesis is comparable to that which characterizes type 5 described below.

Mesothermal Au-quartz vein deposits with rare polymetallic sulfides (type 5 A u) The type 5 gold mineralization occurs in various host sequences--B1 metasedimentary rocks (Kalana in Mali; Banora in Guinea), B2 volcanic and sandstone-conglomeratic sequences (Poura in Burkina Faso [ Fig. 17A, B ]; Sabodala in S6nrgal), and granite (Hir6 in the Ivory Coast). However, they all show common characteristics, such as (a) having formed during the last brittle to brittle-ductile stages of the Eburnean orogeny, as is evident from the N150°E-striking vein system at Poura which

mine (Ghana) ( × 80).

cuts across an ENE-striking dextral D 3 shear zone, and (b) a paragenesis with polymetallic (Pb, Cu, Zn ) sulfides and cupro-argentiferous sulfosalts. The galenas show the highest Pb isotopic compositions of the Eburnean cycle and give a Pb/Pb model age of 2001 _+ 17 Ma (Mi16si et al, 1989b) confirming their late emplacement, whilst the 238U/2°4pb ratio lz (characteristic of the evolution of Pb) varies between 9.00 (Diabarou in Mali) and 9.44 (Poura in Guinea) in the Stacey and Kramers ( 1975 ) model and, according to the standard curves of Zartman and Doe ( 1981 ), indicates substantial crustal involvement. The type 5 gold mineralization does not attain the economic significance of type 4 (auriferous arsenopyrite + Au quartz vein) deposits. The total reserves of the main deposits (Poura, Kalana, Sabodala) are about 70 t of gold metal. The mineralized bodies consist of quartz _+ carbonate veins, lenses or stockworks with disseminations of polymetallic sulfides and native gold. The gangues show a cataclastic to mylonitic texture and may be

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pyrite, and rare tetrahedrite, as well as a second generation of more argentiferous gold. At Kalana, the paragenesis is completed by magnetite and, in the latest stages, by accessory minerals such as native bismuth and matildite (Blagonadezhdin, 1975 ).

Discussion on late-orogenic gold mineralization In mesothermal gold deposits characterized by brittle-ductile rheology, by the element association of Au, B, W, As, Sb, Se, Te, Bi, Mo,

332

with traces of Cu, Pb, Zn, and by a gangue of quartz, albite, carbonate, muscovite, pyrite and tourmaline (Kerrich, 1989a, b; Bursnall et al., 1989 ), the Au occurs: (a) in the form of native gold; (b) finely disseminated as inclusions within the base-metal sulfides or the gangue; and (c) exceptionally in composite forms with Ag, Cu, Pb, Sb, Te and Se. In certain districts of West Africa (Ashanti-type deposits) and France (Le Chhtelet, Villeranges), however, the gold is contained within auriferous arsenopyrite (Bonnemaison and Marcoux, 1987, 1990; Johan et al., 1989), probably in a nonmetallic chemical state (Marion et al., 1986). Moreover, in West Africa it is evident that two types of post peak-metamorphic discordant mesothermal mineralization succeeded (in continuum ) and locally superposed each other, with the auriferous arsenopyrite mineralization (type 4 Au) being followed by the goldbearing quartz lodes (type 5 Au) characterized by native gold-Cu-Pb-Zn-Ag and Bi (Milrsi et al., 1989b, 1991 ). Numerous authors consider that the Ashanti-type mineralization (type 4 Au), which is recognized as having the highest gold potential in West Africa, is located within shear zones (Junner, 1932, 1935, 1940; Kesse, 1985; Hussey, 1988; Milrsi et al., 1989b) that are broadly similar to those which controlled Archean mesothermal gold deposits (Colvine et al., 1984, 1988; Roberts, 1987; Barley and Groves, 1987, 1990; Bursnall et al., 1989; Eisenlohr et al., 1989). According to Ntiamoah-Agyakwa (1979) and Leube et al. (1990), however, the distribution of disseminated sulfide ores would have been controlled by manganese-rich lithofacies and other chemical sediments such as chert and sulfide layers; Ntiamoah-Agyakwa ( 1979 ) assigns the mineralization directly to a volcanogenic origin, whereas Leube et al. ( 1990 ) invoke the influence of a fluid, formed by metamorphic dehydration, expelled on the submarine floor close to a boundary between a sedimentary basin and a volcanic edifice. The presence of sulfide sediments and of variably

J.-P. MILESI ET AL.

manganiferous chemical sediments is well documented in the B 1 units of Ghana and the Ivory Coast, as in other Precambrian gold districts. According to Oberthiir et al. (1988, 1990), the Archean BIF that are richest in pyrite, pyrrhotite and arsenopyrite are those with associated carbonates and oxides. Applied to Ghana, this hypothesis appears to take into account a definite spatial relationship between the manganese deposits and the auriferous arsenopyrite gold deposits. It does not, however, explain the post-D2 timing of the auriferous arsenopyrite deposition, nor does it account for the fact that manganese layers are widely distributed throughout West Africa whereas Ashanti-type mineralization is rare (Milrsi et al., 1989b). In considering the role of the host rocks in controlling mineralization, it must first be pointed out that there is no direct evidence of the presence of gold (visible or microscopic) in the pre-tectonic mineral assemblages of the Ashanti and Konongo regions. It is true that at Konongo, arsenopyrite associated with pyrrhotite pre-dates foliation development and metamorphism, but unlike the arsenopyrite of the ore mined at Ashanti, it is almost totally devoid of gold. It is possible, though, that the B I volcano-sedimentary and chemical sedimentary rocks, like the B2 dykes, acted as mechanical barriers to fluid flow due to competence contrasts; certain graphitic and manganiferous layers could also have acted as chemical barriers and favoured the precipitation of gold, as was envisaged by Leube et al. ( 1990 ) in Ghana and by Colvine et al. (1984) in Canada. Conclusion

Metallogenic evolution of the southern part of the West African craton The metallogenic (gold and base metal) evolution (Fig. 18) of the Early Proterozoic of

THE BIRIMIAN OROGENIC

333

BELT, W E S T A F R I C A

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Fig. 18. Interpretative timing and setting of Au and base-metal deposits of the West African craton. Black circles= ore deposits; white circles and areas= events; size of circle (large, medium, small) relative to size of deposit or event. West Africa represents a period of less than 150 Ma, ranging from the Perkoa massive Zn-Ag sulfide deposit dated at 2120_+41 Ma ( P b / P b on galena; Marcoux et al., 1988) to the late gold-bearing veins dated at ~ 2000 Ma (Miltsi et al., 1989b). Three broad successive stages are recognized, responding to the evolution of Eburnean tectono-thermal events and corresponding approximately to three major groups of mineralization: (a) pre-orogenic (pre-D]) stratiform (or stratabound) Au, Mn, Fe and Zn-Ag mineralization; (b) "syn-orogenic" gold mineralization within B2 volcanic areas and the Tarkwaian gold-bearing conglomerates that formed within extensional B2 clastic infill basins; and (c) late-orogenic discordant mesothermal gold mineralization emplaced after D2D3 deformation and associated tectono-therreal events (Fig. 19 ). The stratiform a n d / o r stratabound mineralization of the first group (Perkoa massive Zn-Ag polymetallic sulfides; Mn-formations; stratiform Au-tourmalinites (type 1 Au); Fa-

16m6 stratiform Fe + Cu mineralization of probable volcanic origin ) took place in pre-orogenic extensional zones and quite clearly occurs within the predominantly sedimentary Lower Birimian (B 1 ) formations (Fe of Fa16m6; Mn of Nsuta-Tambao) and associated rare (Au of Loulo) to more c o m m o n (Zn-Ag of Perkoa) amounts of tholeiitic volcanoclastic material. One of the significant features c o m m o n to most West African mineralizations within this first group is a close association with siliceous a n d / o r carbonate "chemical" sediments (or facies). These are generally manganiferous and constitute the "Mn-formations" that are widely distributed throughout West Africa (with the exception of the Krdougou inlier) and contain the tourmalinized Loulo deposits. The deposits may also be associated with variably graphitic and pyritic black shales to form the "volcano-sedimentary complexes" (Tagini, 1971 ) which occur principally in the upper part of the Lower Birimian (B1), although also

334

J.-P. MILESIET AL. A

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Fig. 19. Pre- and late-orogenic metallogenic periods. (A) Pre-orogenic period (pre-Di). (B) Late-orogenic period (D2D3 ). 1 = Man Archean granite-gneiss complexes with BIF, greenstone belts and ultrabasic-basic complexes; 2 = Early Proterozoic granites; 3=composite crust (including I and 2 above); 4. Lower Birimian formations: 4=undifferentiated, mainly sedimentary formations; 4a=tholeiitic volcanic rocks; 4b=flysch-type rocks (Zr: rare Archean zircons);

4c="volcano-sedimentary complexes" with: (a) tourmalinized-sandstone (white), (b) chert, (c) Mn-formations; 4d= carbonate-bearingformations; 5. Upper Birimianformations:5 = undifferentiatedvolcanicformations;5a= tholeiitic volcanic rock; 5b---calc-alkalinerocks--rhyodacite dome; 5c=volcano-plutonic zone; 6=Tarkwaian clastic formations (Tarkwa, Bondoukou); 7= syn-D2granite; 8 = undifferentiatedgranite. known in other lithostratigraphic positions, and generally seem to indicate periodically active unstable zones: e.g. the turbidite distribution around talus-slope deposits, the extrusion of tholeiitic volcanites and the irregular distribution of the submarine hydrothermal systems that gave rise to the highly tourmalinized and sulfide-rich zones, indicate the unstable extensional paleogeographic setting the Loulo gold mineralization (stratiform tourmalinite). It should also be mentioned that the disseminated sulfides of this first mineralization group, even though hosted by sedimentary and in some instances volcano-sedimentary formations, have a distinctive Ni-Co signature which possibly indicates a mafic affiliation and which is equally apparent in the gold deposits (Loulo) as in the Mn (Nsuta) and Fe (Fa-

16m6) deposits: a similar feature is present in the late-orogenic gold mineralization, but is less marked and could possibly represent a mantle influence. The low/z (238U/2°4pb) ratios of the Perkoa massive sulfides confirm a strong mantle affinity, which is consistent with the tholeiitic environment, whilst the higher ratios at Loulo and Fal6m6 suggest a greater crustal participation, possibly reflecting the influence of the flyschoid deposits (with only a small tholeiitic component) accessed by the mineralizing fluids. The mineralization of the second group ("syn-orogenic"), formed during the Eburnean ( D r D 3 ) tectono-thermal evolution, took place in extensional environments that gave rise to extensive tholeiitic volcanism and then to the Tarkwaian clastic infill basins that ac-

THE BIRIMIAN OROGENIC BELT, WEST AFRICA

companied or followed a less voluminous phase of calc-alkaline volcanism. It should be pointed out that no mineralization has been found in the calc-alkaline environment which, although still poorly defined, doubtlessly reflects either convergent or transcurrent tectonics. The type 2 gold mineralization and associated hydrothermal alteration of this second group, which is commonly more cupriferous than the mineralization of the first group, was emplaced late in the B2 tholeiitic sequences (with rare Mnformations such as at Yaourr) and occurs in early (pre-D2) fractures cutting the late, commonly sill-like intrusions. The mineralogical and chemical paragenesis (Au, B, Fe, Ni-Co) is more reminiscent of the pre-orogenic group 1 gold mineralization within disseminated sulfides than of the late-orogenic group mineralization. The second group also includes the goldbearing conglomerate (type 3) of the Tarkwa district (Ghana); deposits, generally interpreted as "paleoplacers" (Sestini, 1973; Hutchinson, 1987) and marking a turning point in the Eburnean evolution. The clastic in fill basins containing the gold-bearing conglomerate occur in pre-D2 extension (Tarkwa) zones, some of which experienced bimodal volcanism before or contemporaneous with the clastic deposition (Tarkwa; Bondoukou). The clastic deposits and their paleoplacers were also affected by the paroxysmal Eburnean (D2 and D3) tectono-thermal phases, which considerably modified their primary characteristics. Thus, only certain gross lithologic controls are recognizable today, such as certain quartzpebble conglomerates of the Banker formation controlling the gold mineralization at Tarkwa (Sestini, 1973; Kesse, 1985 ). Consequently, if the paleoplacer hypothesis is accepted, the origin of the gold must be sought in older formations (B 1, B2, pre-orogenic D1 granite) or, more probably, in the mineral deposits of the first group. Moreover, and contrary to the most commonly accepted ideas, the Tarkwa gold could not have been derived from the Ghan-

335

aian shear zones since gold was introduced into these D2-D3 tectonic structures at a late stage and forms the source of the mineralization of the third group. The mineralization of the third group consists of two types of late-orogenic discordant mesothermal gold that are locally superposed: (a) mineralized columns with disseminated auriferous arsenopyrite and Au-quartz veins (Ashanti type or type 4 Au), and (b) quartz veins containing free gold and traces of Cu-PbZn-Ag-Bi (type 5 Au) whose emplacement would have occurred at ~ 2 Ga. In general such mineralization formed after the metamorphic peaks of the D2-D3 tectono-thermal phases, and not at the end of phase 1, as evidenced by the auriferous arsenopyrite blasts (Fig. 16), the quartz veins, and the overprinting and crosscutting fabrics formed during the transcurrent deformation. The type 4 Au mineralization is restricted to tectonic corridors exhibiting multiple D2-D 3 activity (thrust faulting, strike-slip faulting and late brittle faulting) where Au-arsenopyrite crystallized in a static fashion, overprinting the D2 foliation. The mineralizing fluids appear also to have been subjected to lithologic influence where they crossed (a) B1 deposits rich in Mn-formations and graphitic sediments, and (b) zones rich in B2 intrusions. Although such mechanical-chemical lithologic factors would have apparently favoured precipitation from the fluids, this does not imply that these units were the source of the gold; on the other hand, it would explain the spatial Au-Mn relationships observed in Ghana (Ntiamoah-Agyakwa, 1979; Leube et al, 1990). The type 5 Au mineralization occurs within late-orogenic brittle faults cutting various lithologic units (sedimentary and volcanic rocks, granite ) and shows parageneses comparable to certain epigenetic types (e.g. Loulo stockwork). It should, however, be noted that, like the type 4 Au mineralization, the type 5 is common within tectonic corridors cutting the B 1/B2 contact which itself already contains al-

336

most 30% of the West African gold deposits. The observed evolution of the mesothermal mineralization from auriferous arsenopyrite and Au-quartz lode (type 4) to quartz lode (type 5 ) with free gold and traces of polymetallic sulfides (especially galena and late cupro-argentiferous solutions) is similar that described from the auriferous shear zones of the Hercynian of Europe (Bonnemaison and Marcoux, 1987, 1990).

Comparison with other Precambrian shields The review of Birimian mineralization of West Africa can be concluded by a comparison with the mineralization of Proterozoic shields that were formerly contiguous with the West African craton--the Guyanan, Sa6 Francisco and Congolese cratons. The Guyanan craton is the extension of the Man Shield on the South American continent (Bullard et al., 1965; Onstott and Hargraves, 1981; Matheis, 1987; Caen-Vachette, 1988 ). It has a comparable lithology (Ledru et al., 1991 a) of flyschoid sedimentation with volcanic intercalations (Paramaca) overlain by a fluvio-deltaic formation deposited in transtension or transpressure basins (North Guyana Trough). The Guyanan Shield shows the same crustal growth between 2.2 and 2.1 Ga (Bosma et al., 1984; Gruau et al., 1985 ) associated with the Trans-Amazonian orogeny (Gibbs and O1szewski, 1982; Jegouzo et al., 1990) which, like the Eburnean orogeny in Africa (Bonhomme, 1962), was a period of collision tectonics between 2.1 and 2.0 Ga. The mineralization in Guyana is also similar in that: (a) the Paramaca volcanic formations contain a stratabound gold deposit (Dorlin ) associated with disseminated sulfides in an M g - F e tourmaline, quartz and Mg-chlorite gangue (Mil6si, written commun., 1987); (b) the North Guyanan Trough (Ledru et al., 1991 a) contains auriferous conglomerate intercalated in fluvio-deltaic formations (Manier et al., 1992) that show certain similarities

J.-P. M1LESI ET AL.

with those at Tarkwa; (c) the edge of the North Guyanan Trough has several late-orogenic mesothermal gold deposits being either mined or developed (Lasserre et al., 1989); and (d) meta-kimberlite has been reported (Marot, written commun., 1989). However, certain differences are also to be noted between the West African and Guyanan mineralizations, especially with respect to the auriferous conglomerate which in Guyana is contained in transtensional syn-D2 basins where the distribution of the gold was controlled by debris flows (Manier et al., 1992). In Brazil, the same craton contains the Serra Pelada gold deposit, which is hosted by siltite while having a component of structural control in the gold enrichment (Ga~ll and Teixeira, 1988). Correlations with the main mineralization of the Congolese and Brazilian (Sa6 Francisco) cratons, where a collision-type orogenesis is dated at 2 Ga (Feybesse et al., 1986; Ledru et al., 1989a; Figueiredo, 1989), are more tentative because the cratons are separated by the Pan-African mobile belt. Nevertheless, gold and Mn deposits are again present, as for example: (a) the late-orogenic mesothermal gold of the Dondo-Mobi-Et6k6 district in Gabon (Prian et al., 1988) hosted by a Proterozoic greenstone belt (Ledru et al., 1989a); (b) the auriferous conglomerate and Mn-shale at Jacobina in Brazil (Ga~il and Teixeira, 1988); and (c) the Mn district at Moanda in Gabon (Leclerc and Weber, 1980) hosted by Early Proterozoic black metapelite of the Francevillian B (Azziley-Azzibrouck, 1986). Also formed during this epoch was the Mounana uranium district of Gabon situated in Early Proterozoic sandstone of the Francevillian A (Gauthier-Lafaye, 1986 ) and containing the Oklo Uranium deposit's natural reactor, in which spontaneous fission occurred at 1.97 Ma (Bonhomme et al., 1982), contemporaneous with the last stages of the Eburnean orogeny.

THE BIRIMIAN OROGENIC BELT, WEST AFRICA

Specific metallogenic characters of the Early Proterozoic From the economic point of view, the Early Proterozoic Birimian can be characterized as being rich in gold and manganese mineralization and poor in volcanogenic and BIF mineralization. An attempt has been made to show that various patterns within the Birimian mineralization form excellent time markers for the main stages of Eburnean orogenic evolution between 2.1 and 2.0 Ga. From a metallogenic point of view, the Early Proterozoic of the West African craton (Man Shield, Reguibat Shield), as well as its Guyanan-Brazilian counterpart, is comparable to other Precambrian cratons with its high gold potential and diversity of gold deposit types (stratiform tourmalinite, disseminated Ausulfide deposits, paleoplacers, mesothermal lodes) but contrasts with the abnormally Aupoor Archean of West Africa. Like the Proterozoic of South Africa and Central Africa (Gabon), the Birimian of the Man Shield contains numerous manganese deposits associated with siliceous and/or carbonate "chemical" sediments and resulting in extensive Mn-formations which are widely used as regional marker horizons: the presence of the Perkoa massive Zn-Ag sulfides and volcanoclastic rocks in such an environment shows resemblances to other provinces such as the "South Iberian pyrite belt" (Routhier et al., 1978) and favours a hydrothermal-sedimentary origin incorporating Kimberley's (1989b) hypothesis for the Fe- and Mn-formations. On the other hand, the Birimian seems to be abnormally poor in polymetallic volcanogenic mineralization (massive sulfide or SEDEX deposits) in comparison with other Precambrian cratons (Rickard, 1987; Hutchinson, 1987 ), in particular given the absence of auriferous or volcanogenic mineralization in the calc-alkaline volcanic formations. Finally, in comparison with the Archean segments of the Man and Reguibat shields, the

337

Birimian of the West African and Guyanan cratons is unusually poor in iron formations. This, moreover, appears to be a distinctive characteristic of the Birimian, in comparison with a number of other Precambrian shields (see the review of Kimberley, 1989a, b; Thurston and Chivers, 1990). The litho-tectonic history of the West African Early Proterozoic shows characteristics that relate it to that of certain Archean provinces. For example, like the Archean Superior Province of Canada (Thurston and Chivers, 1990), the Birimian contains sedimentary deposits with carbonate-bearing sequences, especially at the base, and sandy conglomeratic deposits at the top associated with calc-alkaline volcanism. Another similarity with the Superior Province is that the Birimian is rich in basic tholeiitic volcanic rocks reflecting extension environments. However, the interpretation of these volcanic rocks still vacillates between oceanic plateau basalt (Abouchami et al., 1991) and/or intracontinental rift basalt (Deschamps et al., 1986) with a composite crust consisting of Archean and pre-orogenic Proterozoic granite and B 1 sedimentary rocks and tholeiites (Milrsi et al., 1989b). The Birimian, however, is poor in BIF and komatiite when compared to the Superior Province, and shows a polyphase granite emplacement. Another characteristic that even more clearly distinguishes the Birimian from the Superior Province is that, in West Africa, the tectono-thermal events began very early with a phase of D I overthrusting, possibly reflecting collision prior to the initiation of both individual bimodal volcanic troughs and the Tarkwaian clastic infill basins. Finally, as in numerous Archean to Phanerozoic provinces, transcurrent tectonics accompanying a granite formation were followed by a major metallogenic event responsible for the mesothermal gold lodes that comprise the most economically significant mineral deposits in West Africa. It should also be noted that the West African

338

and Guyanan Birimian shows types of deposit that are very typical of the Proterozoic and Paleozoic (although certain were already present in the Archean): such are the stratiform tourmalinites (Slack, 1982; Taylor and Slack, 1984), the gold-bearing conglomerates containing oxides but lacking sulfides and uranium (Hutchinson, 1987), and the Au-arsenopyrite-bearing shear zones. These characteristics indicate that the major global pulse of crustal growth ,,- 2.1 Ga (Patchett and Arndt, 1986) was, in West Africa, accompanied by metallogeny that is notably different from that of the Archean.

Acknowledgements The authors would like to thank the technical directors of the BRGM who supported the work as part of BRGM scientific project RM 25 "Lithostratigraphy and evolution of the Birimian cycle of West Africa; assessment of its mineral potential". They would also like to thank all those who helped further this work, in particular the West African authorities from the various Geological and Mining Surveys; the geologists of the mines visited; the West African and French geologists from various universities with whom the project was discussed; and all the BRGM colleagues who gave invaluable support, as well as C. Heinry, Ph. Lagny, S. Matin, and J. Payen. Finally, the authors would like to thank Sir Patrick Skipwith for his translation of this contribution and the journal's reviewers, in particular F. Robert and P. Ward, for their critical and helpful contributions.

References Abouchami, W., 1990. Un 6v6nement volcanique majeur vers 2,1 Ga en Afrique de l'Ouest: un stade pr6coce d'accr6tion crustale. Th6se Doct., Univ. Nancy I, 155 PP. Abouchami, W., Boher, M., Michard, A. and Albar6de, F., 1991. A major 2.1 Ga old event of mafic magma-

J.-P. MILESI ET AL.

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