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Journal of African Earth Sciences 50 (2008) 215–233

Gold metallogeny in the Birimian craton of Burkina Faso (West Africa)

Didier Beziat a,*, Michel Dubois b, Pierre Debat a, Serge Nikiema c,Stefano Salvi a, Francis Tollon a

a LMTG, Universite de Toulouse, CNRS, IRD, OMP, 14 Avenue Edouard Belin, 31400 Toulouse, Franceb UMR 8110, Universite de Lille1, Bat SN5, 59655 Villeneuve d’Ascq, France

c Goldfield Exploration Johannesburg, South Africa

Received 6 April 2006; received in revised form 20 November 2006; accepted 13 September 2007Available online 1 November 2007

Abstract

Primary gold deposits in Burkina Faso occur in Paleoproterozoic Birimian belt formations (2.0 Ga). Mineralization was synchronouswith regional metamorphism and deformation, and is either hosted within, or is adjacent to, quartz-bearing veins. These are classicalcharacteristics of epigenetic gold deposits in Precambrian metamorphic terranes and permit to classify the mineralized sites from BurkinaFaso as orogenic-type gold deposits. A review of data collected over the past decade by our team permits to recognize two main styles ofgold mineralization: (1) Quartz-vein hosted; this style occurs in all lithologies, the veins are deformed and gold is principally concentratedwithin the veins, associated with either sulfides or tourmaline. (2) Disseminated; this style occurs exclusively in albitites (and to a lesserextent listvenites) with gold occurring mainly within alteration halos of generally undeformed quartz-albite-carbonate vein. Quartz-veinand disseminated styles of mineralization can be associated within the same deposit. Albitites and listvenites are alteration products ofmainly calc-alkaline igneous rocks of felsic to ultramafic composition, respectively. The predominant alteration assemblage consists ofchlorite, albite, carbonate, and pyrite. Sulfides occur as fine masses commonly in the alteration halos close to vein margins and consistmainly of pyrite and arsenopyrite, depending on host-rock composition. Gold occurs as free native metal and, locally, in form oftellurides, in fissures or as inclusions within pyrite and arsenopyrite. Two main populations of fluid inclusions are associated with thegold deposits, independently of the mineralization style: (1) carbonic inclusions consisting of up to 90 mol% CO2 (plus N2 and CH4)and (2) aqueous-carbonic fluid inclusions with moderate salinities. Interestingly, the disseminated gold style deposits of Burkina Faso,which have the highest economic potential, show strong similarities with the world-class Ashanti deposit, in neighboring Ghana.� 2007 Elsevier Ltd. All rights reserved.

Keywords: Orogenic gold deposit; Tourmaline; Albitite; Paleoproterozoic; West African craton; Burkina Faso

1. Introduction

West Africa (Fig. 1), in particular Ghana (the formerGold Coast), has produced gold since antiquity. Duringthe sixteenth century, the Gold Coast accounted for asmuch as 35% of the entire world’s gold production (Ame-dofu, 1995). However, since then, gold production hadbeen steadily decreasing until the twentieth century. Eco-nomic interests in the region only restarted around 1980,

1464-343X/$ - see front matter � 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.jafrearsci.2007.09.017

* Corresponding author.E-mail address: dbeziat@lmtg.obs-mip.fr (D. Beziat).

due to more favorable global gold prices. Consequently,and also thanks to the development of new mining codes,investment by foreign companies became much moreattractive and a new gold rush ensued, resulting in anincreased effort towards the understanding of gold depositsin this part of the world.

After the classic contributions of Junner (1932, 1935,1940) on the deposits of Ghana, numerous studies havebeen published since 1980, characterizing gold deposits inPaleoproterozoic formations of Ghana (Kesse, 1985; Milesiet al., 1989, 1992; Leube et al., 1990; Eisenlohr and Hirdes,1992; Blenkinsop et al., 1994; Davis et al., 1994; Hirdes and

Fig. 1. Simplified geological map of the West African Craton. The shaded area delimits the studied area.

216 D. Beziat et al. / Journal of African Earth Sciences 50 (2008) 215–233

Nunoo, 1994; Hohndorf et al., 1994; Hunken et al., 1994;Mucke and Dzigbodi-Adjimah, 1994; Oberthur et al.,1994, 1996, 1997, 1998; Hammond and Tabata, 1997;Klemd and Hirdes, 1997; Osae et al., 1999; Schmidt-Mumm et al., 1997; Klemd et al., 1998; Barritt and Kuma,1998; Yao and Robb, 2000; Allibone et al., 2002, 2004;Feybesse et al., 2006), Mali (Dommanget et al., 1985,1993; Milesi et al., 1989, 1992), Senegal (Sylla and Ngom,1997), and Burkina Faso (Huot et al., 1987; Sanogo andProst, 1993; Bourges et al., 1994, 1998; Bamba et al.,1997; Klemd and Ott, 1997; Klemd et al., 1997; Beziatet al., 1998, 1999; Wille and Klemd, 2004). Amongst theseare the review articles of Milesi et al. (1989, 1992) on theentire West African craton, and of Leube et al. (1990)and Oberthur et al. (1994) on the Ashanti deposit inGhana, a world class deposit and the largest gold producerin West Africa. An early classification of Milesi et al.(1989), based on host-rock type, structure, ore-body geom-etry and paragenesis, distinguished five types of primarygold deposits: type 1 – mineralization in tourmalinizedturbidites; type 2 – mineralization within disseminated sul-fides in volcanic or plutonic rocks; type 3 – gold-bearingconglomerates; type 4 – lode mineralization with gold-bearing arsenopyrite; type 5 – quartz-lode mineralizationwith native gold and polymetallic sulfides. Subsequently,Milesi et al. (1992) revised their classification and proposeda threefold typology of gold deposits in West Africa, relat-ing mineralization to regional tectonic events: type 1 –‘‘pre-orogenic’’ stratiform deposits, associated with early

extensional zones. This type corresponds to type 1 of Milesiet al. (1989); type 2 – ‘‘syn-orogenic’’ deposits in exten-sional zones with disseminated Au-sulfide mineralizationin metavolcanites or metadiorite, and auriferous paleoplac-ers in Tarkwaian conglomerate. This type regroups types 2and 3 of Milesi et al. (1989); and type 3 – ‘‘late-orogenic’’,discordant mesothermal gold mineralization, which repre-sents the most economically important deposit type andregroups types 4 and 5 of Milesi et al. (1989). The classifi-cation of Leube et al. (1990) and Oberthur et al. (1994) dis-tinguishes two major types of gold occurrences in theBirimian of Ghana: (1) disseminated-sulfide type (or sulfideores) and (2) free-gold bearing quartz-vein type. Thequartz-vein type generally carries better grades than thedisseminated-sulfide type. Oberthur et al. (1994) show thatthe hydrothermal mineralization is structurally controlled,and largely synmetamorphic and synkinematic, contempo-raneous with the Eburnean tectonothermal event around2.0 Ga.

Although in Burkina Faso, like in the rest of WestAfrica, gold has been exploited for thousands of years(e.g. Kiethega, 1983), production is still mostly artisanal.The only significant mining operations have been at themine of Poura (Fig. 2), where 11 t of gold and 1.5 t of silverwere produced between 1981 and 1991 (Sanogo and Prost,1993). Poura was shut down at the end of the 1990s. Since1990, the only available information on gold deposits inBurkina Faso is limited to unpublished internal companyreports, in addition to a handful of papers or conference

Cambro-ordovician

Greenstone belts

Granitoids

Orebodies

13°W

MALI

IVORY COAST

GHANA TOGO

BENIN

NIGER

5°W 0°

200 km

OUAGADOUGOU

A.E.G.B.

B.Y.G.B.H.

G.B.

B.G.B

. F.N'G.G.B.

: Aribinda-Essakhane Greenstone Belt

: Bouroum-Yalogo Greenstone Belt: F.N'Gourma Greenstone Belt: Houndé Greenstone Belt

: Boromo Greenstone BeltA.E.G.B.B.G.B.B.Y.G.B.F.N'G.G.B.H.G.B.

Diabatou-PielaBoulougouBahonga

Bangou

Sebba

Youga

KamigoueraDoudou

GongondyDienemera

Laro

EssakhaneBaillata

Oursi-Tin Edia

AribindaYalanga

Fété Kolé

BelahouroGangaolBaylidiagaGuiroTaparko

Bouroum

SianGoren

Bonda

InataSouma

SeguenegaKaumbri

Margo

KoupelaGuibaré

Boboulou

KoursieraSoukoura

SikangoDiakadougouDiosso

Tankiedougou

Dohoum

Kari

KiereDossi

Bagassi

Tienzan

Laba

Poura

SombouLoraboué

Larafella

Zwademen

Zoula

Semapoum

Koudma

Nongotayre

Gambo

Patiri

Legend

Fig. 2. Location map of the Burkina Faso gold bodies. The studied deposits are labelled in bold and are marked by a large point.

D. Beziat et al. / Journal of African Earth Sciences 50 (2008) 215–233 217

abstracts. This information concerns of course the PouraMine (Sanogo and Prost, 1993), and a number of prospects,i.e. Gangaol and Taparko (Bourges et al., 1994, 1998), Lar-afella (Bamba et al., 1997; Klemd and Ott, 1997), Loraboue(Beziat et al., 1998), Fete Kole and Guibare (Beziat et al.,1999), Diabatou (Klemd et al., 1997) and Essakhane (Lero-uge et al., 2004). The only reviews on gold in Burkina Fasoremain those of Huot et al. (1987) and Milesi et al. (1989,1992). Huot et al. (1987) distinguish two types of primarymineralization: (1) a syngenetic type, associated with man-ganese in carbonaceous schist and strata-bound sulfidesand (2) an epigenetic type, including vein gold depositsand gold-bearing shear zones. Milesi et al. (1989, 1992)describe two gold deposits in Burkina Faso: the Dienemeradisseminated gold prospect, hosted in tholeiitic volcanicrocks, and the Poura deposit, which they describe as a mes-othermal gold-bearing quartz vein deposit controlled byshear structures. Milesi et al. (1992) classified the Dienemer-a prospect and the Poura deposit as ‘‘syn-orogenic’’ and‘‘late-orogenic’’ deposit types, respectively.

The aim of this paper, is to provide an overall under-standing of the primary gold mineralization in the Paleo-proterozoic metamorphic terranes of Burkina Faso andcompare it to similar mineralization elsewhere in WestAfrica, notably the world-class deposits in neighbouringGhana (i.e. Ashanti). We use a compilation of publishedas well as unpublished data from our team on a numberof gold deposits from several greenstone belts in Burkina

Faso. From North to South, the studied deposits are: FeteKole, Souma, and Inata in the Aribinda–Essakhane belt;Gangaol, Bayildiaga, Guiro, Taparko, and Guibare inthe Bouroum–Yalogo belt; Diabatou in the Fada–N’gour-ma belt and the Loraboue and Larafella deposits whichneighbor the Poura Mine, in the Boromo belt (Fig. 2).

2. Geological setting

The Paleoproterozoic (Birimian) formations of the Manshield form a major part of the West African craton (Bes-soles, 1977) at the eastern and northern boundaries of theArchean Liberian cratonic nucleus (Fig. 1). The Birimianterranes, which make up the Baoule–Mossi domain andthe Kedougou inlier, consist of narrow sedimentary basinsand linear to arcuate volcanic belts intruded by several gen-erations of granitoids (Leube et al., 1990; Doumbia et al.,1998; Gasquet et al., 2003; Dioh et al., 2006). They corre-spond to a period of accretion during the Eburnean orog-eny around 2.0 Ga (Abouchami et al., 1990; Boher et al.,1992; Taylor et al., 1992).

The lithostratigraphic succession of the Paleoproterozo-ic formations of the Man shield, although previously muchdebated (see discussion in Vidal et al., 1996) is now consid-ered to be well established. The Birimian crust s.l. com-prises the following lithologies from bottom to top(Hirdes et al., 1996; Pouclet et al., 1996; Vidal et al.,1996; Doumbia et al., 1998; Beziat et al., 2000; Debat

218 D. Beziat et al. / Journal of African Earth Sciences 50 (2008) 215–233

et al., 2003; Feybesse et al., 2006): (1) a thick sequence ofmafic rocks, including basalt, locally pillowed, as well asdolerite and gabbro, all of tholeiitic composition, locallyinterlayered with immature detrital sediments and lime-stone; (2) a detrital sedimentary pile (volcanics, turbidite,mudstone and carbonate) including interbedded calc-alka-line volcanics; and (3) a coarse clastic sedimentary sequencebelonging to the Tarkwaian group (Oberthur et al., 1998).During the Eburnean orogeny, the volcanic and metasedi-mentary rocks were subjected to crystal shortening associ-ated with greenschist-facies regional metamorphism(Bessoles, 1977; Oberthur et al., 1998). Locally, amphibo-lite metamorphic facies are reached, but these occurrencesare interpreted as resulting from contact metamorphism(Debat et al., 2003).

All deposits studied in Burkina Faso are located in Biri-mian belts and all the above mentioned rock types are rep-resented. Similarly to the rest of the West African craton,the metavolcanic and metasedimentary series are intrudedby granitoids (Hottin and Ouedraogo, 1976; Beziat et al.,2000; Naba et al., 2004). In Burkina Faso, however, grani-toids are particularly abundant and represent 70–80% ofthe Birimian formations (Fig. 2). All rock units show meta-morphic mineral assemblages of greenschist facies grade(Table 1).

3. Styles of gold mineralization

All primary gold deposits, prospects and showings thatwe studied in Burkina Faso occur within or adjacent to

Table 1Primary mineral assemblages and metamorphic assemblages in thedifferent lithologies (Beziat et al., 2000)

Rock types Primary mineralassemblages

Greenschistfaciesmetamorphism

Ultramafic rocks Olivine SerpentineClinopyroxene TremoliteAmphibole ChloriteChromite Magnetite

Gabbro–Diorite ClinopyroxeneAmphibole TremolitePlagioclase AlbiteIlmenite ChloriteBiotite ZoısiteApatite, Zircon Titanite

Basalt andintermediate tofelsic volcanites

No relicts present ChloriteActinoliteAlbiteEpidoteSericiteQuartz

Sedimentary rocks Detrital lithic, plagioclase, quartzand muscovite grains and blackshale

ChloriteEpidoteAlbiteQuartzGraphite

quartz-veins. Therefore, we consider that all deposits areepigenetic in origin. Two main styles of mineralizationencompass all deposits mentioned above, and are largelycontrolled by the nature of the host rock: (1) a quartz-veinmineralization style, in which gold is found withindeformed quartz veins, principally associated with pyriteor tourmaline and (2) a disseminated mineralization style,where gold occurs as disseminated particles dominantlyassociated with sulfides within the alteration halo ofquartz ± albite ± carbonates unfolded veins. The veinsthemselves contain little or no gold. The latter mineraliza-tions have lower grades but much higher tonnage, andoccur in highly carbonatized and albitized rocks or purealbitites.

4. Characteristics of the quartz-vein deposits

Quartz-vein hosted deposits in Burkina Faso, as in therest of West Africa, occur in all lithologies, i.e. in maficformations dominated by tholeiitic series (e.g. Taparko,Gangaol, Bayialdiaga), and in felsic units dominated bycalc-alkaline series, and in metasediments (e.g. FeteKole).

4.1. Structure

In outcrop, three structural types of veins can be distin-guished (Fig. 3): (1) steeply dipping, boudinaged veins(type-1 veins); (2) shallowly dipping, folded veins (type-2veins); and (3) shallowly dipping, undeformed veins (type-3veins). Types-1 and -2 veins are mineralized whereastype-3 veins are barren.

Type-1 veins (e.g. Taparko, Gangaol) trend parallel tothe regional foliation (N50–N80, dipping 60�–80� SE).These veins show an asymmetrical double boudinage withboudins in the dip direction but only pinch-and-swell struc-tures along strike (Fig. 4). This double boudinage charac-terizes a major vertical extension which is also underlinedby a prevalent vertical lineation and a minor horizontalextension, both resulting from horizontal shortening. Inthese veins, quartz is highly deformed (ribbon structureand recrystallization features) (Bourges et al., 1998).

Type-2 veins (e.g. Fete Kole, Guibare) develop perpen-dicularly to the main foliation (Fig. 3). They are stronglyfolded with ptygmatic folds evolving into flattened foldswith thinned limbs and thickened hinges (Fig. 5B and C).In tourmaline-bearing veins (see below), tourmaline crys-tals are folded with the vein whereas quartz crystals areelongated parallel to the axial-plane cleavage of the fold(Fig. 5A).

Type-3 veins are filled with white quartz which has a typ-ical buckshot texture (Boyle, 1979; Dowling and Morrison,1989) (Fig. 3). As mentioned above they are not deformedand can be observed to cross-cut types-1 and -2 veins. Thissuggests that they occurred at the end of the regional defor-mation events.

Diouga

1,5 km

1,8 kmGangaol

70

80

80

6060 20

20

8075

-75

70 80

100m

A

B

35

40

40

30

70

7032

75

65

20

70

70

25

METABASITES

S1

to Dori

N

Barren white quartz

Auriferous dark quartz

Regional cleavage

Later crenulation cleavage

Legend

Barrenquartz

veinAuriferousquartz vein

Metabasite

Selvage4m515 ppb 310 ppb

5,48 to 10.77 g/t(Vairon,1984)

0 to 47ppb

A B

Fig. 4. Level plan and cross-section showing the orientation and dipping of quartz-veins and gold grade distributions at the Gangaol deposit.

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Quartz-vein style Disseminated style

Type-1 vein Type-2 vein Type-3 veins

Type-2 veinsType-2 veinType-3 vein

Type-1 vein

Regionalcleavage

Regionalshortening

Quartz-vein gold mineralization

Disseminated gold mineralization^ ^ ^ ^^

^

LegendMetasediments and metavolcanics

Albitite

not to scale

Fig. 3. Schematic diagram showing simplified structural pattern for the two gold mineralization styles. Darker greys correspond to higher gold grades.

D. Beziat et al. / Journal of African Earth Sciences 50 (2008) 215–233 219

4.2. Petrography

Types-1 and -2 veins can be further subdivided based ontheir mineralogy into quartz-sulfide and quartz-tourmalineveins (sulfides and tourmaline rarely occur in the sameveins) (Table 2).

Quartz-sulfide veins are exemplified by the Taparko andGangaol deposits. At Taparko, veins are hosted by meta-gabbro–diorite and metagranite whereas at Gangaol theyare hosted by a metabasalt with tholeiitic affinities. Host

rocks are highly altered in the selvage of the veins andshow, particularly in the Gangaol deposit, high SiO2 andK2O contents.

Quartz-tourmaline veins are exemplified by two deposits:the Guibare deposit, where most veins are of type-2, andthe Fete Kole deposit, where both boudinaged veins(type-1) and folded veins (type-2) occur. Veins in the Gui-bare deposit are found in highly altered ultramafic to maficrocks (occurrence of chromite). In the Fete Kole depositveins are hosted by metasedimentary rocks and mafic to

220 D. Beziat et al. / Journal of African Earth Sciences 50 (2008) 215–233

intermediate metavolcanic rocks. Carbonate and chloriteoccur in the vein alteration halos in both locations; how-ever, the chemical composition of these minerals as wellas that of tourmaline varies with respect to host-rock lithol-ogy. At Guibare, chlorite displays a Fe/(Fe + Mg) ratioaround 0.25 with a Cr2O3 content higher than 2 wt%, thecarbonate is ferroan dolomite with Mg/Fe > 4 and tourma-line is Mg-rich dravite. The mafic to intermediate volcanicrocks at Fete Kole contain chlorite with Fe/(Fe + Mg)around 0.60, carbonates are predominantly ankerite (cal-cite in metasediments) and tourmaline compositions tendtoward schorl.

4.3. Ore mineralogy

Gold grades in types-1 and -2 veins range from 8 to12 g/t, with generally <1 g/t in the alteration halos (e.g.in the Gangaol deposit, gold concentrations within the

Fig. 5. Petrographic observations of samples and thin sections (transmitted li(crossed polar); Ptygmatic fold in quartz-vein and tourmaline crystals from GInata. (C) Sample LAS91; relict hinge of quartzite layer in metasediment (Lalight); albitite sample from the supergene zone showing high density of shallowSample Las78; Quartz (1)–albite (2) veins cross-cutting the albitite (Larafella(Larafella). (G) Sample Lor13b64; pyrite alteration halo in the selvage of a qu

veins are 5–11 g/t but only 500 ppb in the alteration sel-vages; Fig. 3).

In the quartz-sulfide veins, pyrite is by far the most com-mon sulfide. Gold occurs as native metal: (i) as cloudedgrains associated with microsaccharoidal quartz, fillingthe dense set of tension gashes; (ii) at the contact betweenpyrite aggregates and quartz-fibers in pressure shadowzones, and along pyrite faces (Fig. 6A); and (iii) in smalltension fissures cross-cutting early pyrite crystals(Fig. 6B). Gold is locally accompanied by subordinateamounts of ore minerals including chalcopyrite, sphalerite,pyrrhotite and galena.

In quartz-tourmaline veins, gold is concentrated withinlayers of tourmaline as globular grains or in films(<2 mm), typically at the terminations of tourmaline crys-tals (Fig. 6C1), and as small grains (<100 lm) filling frac-tures or cleavages (Figs. 6C2 and 7A). Uncommonly,gold is accompanied by chalcopyrite, which is pseudomor-phed by chalcocite and covellite during weathering.

ght) from various lithologies. (A) Sample Gui5: A1 (hand specimen), A2uibare. (B) Sample H4-212; quartz-feldspar pebble conglomerates from

rafella). (D) Sample Lor96-21; D1 (hand specimen), D2 (plane polarizedly-dipping quartz-veins and the corresponding thin section (Loraboue). (E)). (F) Sample Las78; Calcite (1)–pyrite (2) veins cross-cutting the albititeartz (1)–ankerite (2) vein (Loraboue).

Fig. 5 (continued )

D. Beziat et al. / Journal of African Earth Sciences 50 (2008) 215–233 221

5. Characteristics of the disseminated style deposits

Disseminated gold deposits occur in felsic rocks of thecalc-alkaline and metasedimentary series. Loraboue is anexample of a deposit dominated by calc-alkaline series,whereas at Larafella and Inata metasediments constitutethe predominant lithology. This style of mineralization ischaracterized by the association of a peculiar rock, com-posed almost entirely of albite (termed albitites). In thedeposits studied, albitites can form bodies varying from afew centimeters to several meters in thickness (<25 m)and can extend over several hundreds of meters in lengthfor the most important units. Although the size of thesedeposits varies, they are generally rather large (Loraboueis 1.5–2 km wide and 8 km long; Larafella is �300 m thickand can be followed for some 3 km along strike, and Inatais somewhat larger, Fig. 8). Although in these deposits goldoccurs principally as disseminated mineralization, mineral-ized folded quartz-veins can also be found within interca-lated metasedimentary and metavolcanic units (Fig. 3).

5.1. Structure

The disseminated deposits occur in strongly deformedzones developed during regional deformation (the majorfoliation in the deposits is parallel to the regional foliation).

Within the deposits, deformation is heterogeneous with: (i)zones of highly ductile deformation affecting the metavolca-nics and the metasediments and (ii) zones of brittle deforma-tion affecting the albitites. Large scale structures include: (1)a major steeply-dipping to vertical cleavage and (2) presenceof vein sets with composition varying according to vein ori-entation and to the nature of the host rocks.

Cleavage: In the deposits, the major cleavage is concor-dant to the regional foliation (10–20�N, dipping verticallyin the Inata deposit; N–S and vertical in the Lorabouedeposit; N–S, dipping 60� east in the Larafella deposit).Within mineralized zones, cleavage planes are more ser-rated, reflecting stronger deformation. Cleavages are well-developed within the metavolcanics and particularly sowithin the metasediments, where they transpose bedding.Commonly, the cleavage surfaces show a steep dip linea-tion. In places, the major cleavage is affected by a secondcrenulation cleavage which, however, does not disturb thestructural pattern (Fig. 8). On the other hand, in the albi-tites, cleavages are only moderate to absent.

Veins: Veins of similar generation to the vein-type styleof mineralization can be found throughout the albitites(Fig. 3). However, albitites are very competent bodies,i.e. they show essentially a brittle behavior. Therefore,although in these rocks shallowly-dipping veins formedsynchronously with the type-2 veins described above, they

Table 2Alteration and mineralization paragenetic sequence. Major ( ), minor ( ) and rare (- - -) phases

(Constructed using textural data in Bamba et al., 1997; Beziat et al., 1998; Beziat et al., 1999).

222D

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eziat

eta

l./

Jo

urn

al

of

Africa

nE

arth

Scien

ces5

0(

20

08

)2

15

–2

33

Fig. 6. Petrographic observations of ore minerals in hand sample and polished section (reflected light). (A) Sample Tap7; gold along pyrite faces andbetween quartz-fibres (Taparko). (B) Sample GanFB1; gold in fractures cross-cutting pyrite crystals (Gangaol). (C) Sample GuiBKP; C1 (hand specimen),C2 (polished section); gold at the tips of tourmaline crystals (Guibare). (D) Sample GanFB7; supergene gold in pyrite boxworks within quartz fibres(Gangaol).

D. Beziat et al. / Journal of African Earth Sciences 50 (2008) 215–233 223

were not folded by further deformation. As a consequence,it is impossible to distinguish between type-2 (mineralized)and type-3 (barren) veins based solely on structural data.

However, the latter veins do not contain sulfides. In theserocks, type-1 veins are only slightly boudinaged, and arefilled by quartz or by quartz + albite. Types-2 and -3 veins

Fig. 7. Backscattered electron SEM images. (A) Sample GuiBKP; gold (Au) as globular grains and thin films in fractures within tourmaline(To) (Guibare). (B) Sample Lors18/235; B1, gold (Au) and silver tellurides (AgTe) as inclusions in pyrite (Py); Ga, galena; mnz, monazite; B2,detail of B1 showing the fissure with quartz (Qz) and muscovite (Mu) cementing some fragments of pyrite and hosting two gold grains; B3,gold and silver tellurides as inclusions and along grain boundaries of pyrite crystals (Loraboue). (C) Sample Lors118/255; pyrite (Py)containing gold inclusions (Au) partly associated with chalcopyrite (Cp) and as thin films in fractures (Loraboue). (D) Sample Lors118/255;gold (Au) on fissures in pyrite (Py), as free-gold grains or intergrown with chalcopyrite (Cp); Ab, albite; Qz, quartz (Loraboue). (E) SamplePRF-6; Cluster of tiny gold grains (Au) localized within an early pyrite crystal (Py); the latter is corroded by an As-rich pyrite (As–Py) thenovergrown by a new pyrite crystal (Poura). (F) Sample SOx; Supergene gold (Au) in goethite (G) derived from a quartz-sulphide vein(Souma).

224 D. Beziat et al. / Journal of African Earth Sciences 50 (2008) 215–233

INATA PROSPECTFété Koléprospect

Soumaprospect

BELAHOURO

+

+

+

++

+

+

++

+

+

++

++ +

+

+

+

+

+

++

+

++

+

+

+

++ +

+

+

+ ++

+ +^ ^

^^

^ ^^^

^

^ ^ ^^ ^

^^

^ ^

^

^^ ^

++

++

+

+

+

+ +

^ ^

Legend

10km

0 500 m

1200

0N

10000 E 11000 E

1200

0N

1100

0N

1000

0N

1000

0N

9000

N

9000

N

1100

0N

S1

S2

S3

5.5m @ 5.390g/t

5.5m@18.500g/t

8m @ 1.080g/t

8m @ 0.570g/t22m @ 2.156g/t

32m @ 1.521g/t10m @ 2.123g/t

6m @2.627g/t

8m @ 1.454g/t

24m @ 3.41g/t56m @ 1.833g/t

40m

10m @ 6.91g/t

9000 E

Thick lateritic cover

INATA GOLDPROSPECT

Thick lateritic cover

LateriticHardcap

BASE

LIN

E

Limite of gold working

Quartz vein

BUMIGEB diamond drill hole

Mineralized interval >0.5g/t in trench

Orpaillage in saprolite

N

Granitoids

Metasediments

Metadifferenciated rocks

Metavolcano-sedimentary rocks

Basic and ultrabasic rocks

Metavolcanic and metaplutonic basicand intermediate rocks

Prospect

Fig. 8. Schematic geological map of Belahouro district, showing the distribution of different host rocks, quartz-veins, mineralized interval in trench, anddata on gold grades, based on scattered outcrops and drill-core data.

D. Beziat et al. / Journal of African Earth Sciences 50 (2008) 215–233 225

can be extremely abundant, and, in place, account for halfthe total rock volume (Fig. 5D).

5.2. Petrography

The Loraboue deposit is dominated by calc-alkalinemetavolcanics associated with mafic to ultramafic units(gabbro and werhlite) (Table 1) and, more rarely, basalt,dolerite and gabbro of tholeiitic affinity (Beziat et al.,2000). The pile also comprises some finely laminated vol-cano-sedimentary rocks alternated with thin (<10 cm) lay-ers of black shale. The Larafella and Inata (Fig. 8) depositsare hosted in volcano-sedimentary rocks dominated byblack shale, conglomerate, sandstone and epiclastic sedi-ments; they also contain various calc-alkaline metavolca-nics including basalt–andesite to rhyodacitic lava andpyroclastites.

In all the deposits, numerous bodies of albitites occur aslenses of white to pale grey homogeneous rocks. Contacts

between albitites and other rock types are parallel to theprincipal cleavage and are commonly transitional; however,locally they can be sharp enough to resemble dikes. Differ-ent facies of albitite are recognized; equigranular with fine-grained (100–200 lm) or coarse-grained (1–2 mm) texture,and heterogranular with large albite crystals (>5 mm) in afine-grained groundmass of albite (Fig. 2 of Beziat et al.,1998). They consist dominantly of almost pure (An0-5)albite which occurs both as plagioclase replacement andas secondary, pore-filling albite. Around early plagioclasephenocrysts, which can exceptionally present an An10 com-position, we observe overgrowth of pure albite showing‘‘chessboard’’ pattern and also a zone of pore-filling equi-granular albite. In addition to residual feldspars, apatiteand zircon, albitite may contain rutile, chlorite and quartz.As a consequence, their chemical composition is very closeto that of albite (Bamba et al., 1997; Beziat et al., 1998).

In addition to albitites, in the Loraboue deposit largepatches (100–200 m across) of carbonatized ultramafic

226 D. Beziat et al. / Journal of African Earth Sciences 50 (2008) 215–233

rocks termed listvenite (Halls and Zhao, 1995) also occur.The listvenite in Loraboue has been described in detail byBeziat et al. (2000). It appears along tectonic contacts, inhighly silicified zones. It is composed of centimeter-sizecrystals of ferroan magnesite including microcrystals ofdolomite, rutile, cobaltite-gersdorffite [(Co,Ni)AsS] and rel-ics of chromite rimmed by chlorite, partially or totally con-verted to fuchsite. Quartz (±pyrite) occurs as interstitialgrains, along magnesite cleavage planes and in late, cross-cutting thin fractures.

5.3. Wall-rock alteration

In the disseminated gold deposits, albitites contain veinswith well developed alteration halos, a few-cm to a few tensof cm in thickness. Although the alteration mineralogychanges slightly with the composition of the rocks sur-rounding the albitites, we were able to define a general alter-ation trend (Table 2): (1) carbonate alteration is importantand consists of extensive replacement of albite by euhedralcarbonate crystals. Relic albite occurs locally as inclusion incarbonates; (2) a sulfidation stage, consisting mostly ofcrystallization of pyrite. This takes place at a subsequentstage, as evidenced by the presence of isolated remnantsof albite crystals, partially replaced by ankerite, within pyr-ite grains; and (3) lastly, a relatively small proportion ofmuscovite occurs as small veinlet swarms cross-cutting ear-lier fabrics, and as overgrowths on pyrite crystals. Quartzcrystallizes throughout the alteration event.

This sequence of crystallization can also be observed inthe numerous zoned veins that cross-cut the albitites. Theseinclude monomineralic albite or quartz-veins; veins com-posed of carbonate plus pyrite (Fig. 5F); zoned albite orcarbonate veins (Fig. 5E and G) with quartz filling the cen-tral parts; and complexly zoned veins with quartz cores sur-rounded by a carbonate-pyrite zone and, at the margins, analbite zone. Pyritized alteration halos surround the Au-bearing veins and extend a few cm to tens of cm into thewall rock. In these halos, pyrite and carbonate contentsincrease progressively toward the vein (up to about 10–20% pyrite by volume) (Fig. 5G). Pyrite also occurs withinthe auriferous veins, as large disseminated grains, whereasin the larger barren quartz-veins, pyrite is much rarer.When albitites occur within mafic to intermediate series,carbonates are dominantly of ankeritic composition,whereas in felsic metasediments nearly pure calcite is theprevailing carbonate phase in the alteration assemblage.

5.4. Ore mineralogy

In the disseminated deposits the most intensely mineral-ized zone is localized on both sides of some shallowly dip-ping undeformed veins within albitites and, less commonly,within listvenites. Gold grades range within 10–6000 ppb inthe albitites, and 10–200 ppb in the listvenites. Sulfidesoccur as millimeter to sub-millimeter sized aggregates,commonly closer to vein margins, more rarely intergrown

within the veins, either in sub-millimeter-thick fracturesor disseminated in sulfide alteration halos (Fig. 5G). Opa-que minerals in the alteration halos are mainly pyrite inbasic to intermediate series (e.g. Loraboue), pyrite andarsenopyrite in metasediments and metavolcanics (e.g.Inata and Larafella). They are accompanied by chalcopy-rite and to a lesser extent by galena, sphalerite, pyrrhotiteand tellurides. Tellurides occur locally and consist essen-tially of petzite (AuAg3Te2), plus silver (AgTe) and lead tel-lurides (PbTe ± S) (Fig. 7).

Native gold occurs mainly as free grains in fractures inpyrite (Fig. 7C), intergrown with pyrrhotite, chalcopyrite(Fig. 7D) and gangue minerals (quartz and muscovitegrains, Fig. 7B). Gold inclusions less commonly occur asclusters of tiny droplets and as ragged inclusions in earlyarsenopyrite and pyrite grains (Fig. 7E), along grain bound-aries, sometimes partly associated with pyrrhotite and chal-copyrite inclusions within pyrite crystals (Fig. 7C). Goldtellurides generally occur as very small particles (up to20 lm) (Fig. 7B1 and B3). In the supergene zones of alldeposits, gold is found as isolated grains or aggregates ingoethite (Fig. 7F) or recrystallized quartz growing fromthe quartz-fibers in the pyrite boxworks (Fig. 6D).

Gold shows a wide range in Ag contents (10–28 wt%Ag), similarly to that of gold from the Ashanti gold mine(Oberthur et al., 1994). However, Ag contents decreasebelow 3 wt% in recrystallized gold grains from the super-gene zone.

6. Fluid inclusions

Several fluid inclusion studies on gold deposits of Burk-ina Faso have been published, mainly on lode depositsassociated with shear zones (e.g. Klemd et al., 1996,1997; Klemd and Ott, 1997; Bourges et al., 1998; Duboiset al., 2001). A review of all data permits to outline threemain fluid-inclusion types (Table 3): type 1 – carbonic fluidinclusions, with CO2–N2(±CH4) P 90 mol%; type 2 –water-rich aqueous-carbonic fluid inclusions; type 3 –aqueous fluid inclusions. In type 1 inclusions, water seemsto be absent or only present in small quantities, whereas N2

is generally subordinate to CO2 but can dominate locally(e.g. Loraboue and Diabatou, respectively; Table 3,Fig. 9). Type 2 fluid inclusion chemistry can be describedby the system H2O–NaCl–CO2–N2, with CO2 being themajor species of the vapor phase (Z(CO2) > 90 mol%)and these inclusions have generally low to intermediatesalinities. Types 1 and 2 inclusions are rarely found in thesame deposits. However, in deposits where these inclusiontypes do coexist, Type 1 inclusions are by far the mostabundant (e.g. Diabatou, 1997; Klemd et al., 1997). BothTypes 1 and 2 inclusions display extremely variable calcu-lated bulk densities (Table 3). These are the consequenceof variable volume fractions of the vapor phase (overallvapor-filling ratio range from 0.1 to 0.5; Table 3) and resultin highly variable bulk homogenization temperatures,which, for all deposits, are below 300–330 �C (Table 3).

Fig. 9. Compilation of volatile fluid-inclusion data for different deposits inBurkina Faso, from the literature sources (cf. Table 3).

Table 3Compilation of fluid inclusion characteristics in gold deposits of Bukina Faso

Deposit Style of mineralization Ref. Inclusion type Bulk inclusion properties

Diabatou Quartz-sulfide veins [1] CO2–N2(–H2O) Raman : X(CO2) = 0.36–0.78X(N2) = 0.26–0.60

Flv = 1 0.35 < d = d(CO2) < 0.99 g/cm3

Aqueous inclusions s = 14–19 wt% eq. NaClTe < �30 �C160 < Th < 190 �C (L)

Taparko [2] H2O–CO2 CO2 + unquantified CH4 and/or N2 and/or H2Ss = 3.0–3.5 wt% eq. NaCl

Flv = 0.1–0.5 0.74 < d(CO2) < 0.93 g/cm3

270 < Th < 300 �C (V)255 < Th < 310 �C (L) + decrepitation

Guibare Quartz-tourmaline veins [3] H2O–CO2(–CH4) Z*(CO2) � 10.95 < d(CO2) < 0.99 g/cm3

Flv = 0.3–0.5 s = 0–6.6 wt% eq NaCl280 < Th < 330 �C (L)

Aqueous inclusions 135.8 < Th < 184.1 �C

Larafella Disseminated [4] CO2–H2O CO2 major species in vapor phaseRaman: X(N2) = 0.064–0.107

Flv = not given X(CH4) = 0.024–0.05160 < Th < 323 �C + decrepitation

Aqueous inclusions s = 0–10.5 wt% eq. NaCl150 < Th < 310 �C

Loraboue [3] CO2 X(CO2) close to 1Flv = 1 0.7 < d = d(CO2) < 1.01 g/cm3

Aqueous inclusions Salinity = 0–3 wt% eq NaCl150 < Th < 200 �C (L)

Symbols: Flv: vapor filling ratio, Tm(CO2) = CO2 melting temperature, Te = eutectic melting temperature, Tmcl = clathrate melting temperature,Th = bulk homogenization temperature, X = bulk mole fraction, Z = mole fraction in the vapor phase generally determined by Raman microspectros-copy or thermodynamic calculation (Z*), s = salinity of the aqueous solution, d = bulk inclusion density, d(CO2) = density of the CO2 dominated phase,V = vapor, L = liquid + decrepitation indicates this process is frequent.Data sources: [1] Klemd et al. (1997); [2] Bourges et al. (1998); [3] Dubois et al. (2001); [4] Klemd and Ott (1997).

D. Beziat et al. / Journal of African Earth Sciences 50 (2008) 215–233 227

Aqueous inclusions (type 3) are considered either sec-ondary or posterior to the CO2-bearing populations. Theymostly contain low-salinity NaCl-dominated solutions.However, higher salinities and complex ionic solution,probably including CaCl2 and MgCl2 in addition to NaCl,have been determined at Diabatou (Klemd et al., 1997).

Homogenization temperatures for type 3 inclusions rangefrom 60 to 300 �C, with most data clustering between 100and 200 �C.

7. Discussion

All primary gold deposits, prospects and showings thatwe studied in Burkina Faso occur in Paleoproterozoicgreenstone belts that have undergone regional metamor-phism and show some degree of structural control (i.e.formed synchronously with deformation). In addition, allmineralizations occur within or adjacent to quartz-veins.These major characteristics are shared by many other epi-genetic gold deposits in Precambrian metamorphic terranes(cf. Goldfarb et al., 2005; for a recent review). We thereforeconsider that all mineralized sites can be classified as oro-genic-type gold deposits, as defined by Groves et al.(1998) (see also Groves et al., 2003; Goldfarb et al.,2005). Homogenization temperatures of fluid inclusionsas well as their low-salinity and CO2-bearing compositionare also consistent with those of many other Paleoprotero-zoic orogenic gold deposits. Although felsic igneous rocksare abundant in the mineralized regions, these depositsare not intrusion-related (cf., Thompson and Newberry,2000) because deposition of gold clearly occurs well after

228 D. Beziat et al. / Journal of African Earth Sciences 50 (2008) 215–233

the emplacements of these igneous bodies (e.g. Beziat et al.,2000).

7.1. Structural control on gold mineralization

A feature that is common to the studied deposits is thatgold can occur in two main mineralization styles, i.e. vein-hosted and disseminated deposits (Table 4). In quartz-veinstyle mineralization, gold is found in strongly boudinaged(type-1) and folded (type-2) veins, i.e. affected by ductiledeformation. The disseminated gold style of mineralizationis characterized by essentially brittle deformation as it ishosted by more competent albitites (Fig. 10). The attitudesof the cleavage and of the deformed and undeformed veinsindicate a coaxial strain history (e.g. pure shear zone) witha horizontal shortening. The cleavage of the two deposittypes is concordant with the regional foliation; the depositsmust therefore be interpreted as a more intense representa-tion of the regional deformation. Furthermore, the mineralassemblages (chlorite–albite–carbonates–quartz–musco-vite) associated with the development of the structures(cleavage, veins) are consistent with PT conditions of regio-nal, greenschist facies metamorphism.

Table 4Summary of the characteristics of the two principal mineralization styles in B

Mineralisationstyle

Occurrences and lithologies Ore bodies Structure

Quartz-veins All rock series (except

granitoids):

Fete Kole:A.E.G.B.

Ductile deformatio

strong verticalcleavage

Metasediments Gangaol,

Bayildiaga,

Guiro,

Taparko,

Guibare:B.Y.G.B.

Type-1 veins:boudinaged, sub-vertical

Metacalc-alkaline rocks Diabatou-

Piela:F.N’G.B.

Type-2 veins: foldesub-vertical fold-axes

Metatholeiitic rocks Poura

Mine:B.G.B.

Type-3 veins:undeformed, sub-horizontal

Disseminated Albitites (to a lesser extent

listvenites): alterationproduct of felsic tointermediate units inmetacalc-alkaline andmetasedimentary series

Souma, and

Inata:A.E.G.B.

Brittle deformation

no apparentcleavage

Type-1 veins:undeformed, sub-vertical

Loraboue

and

Larafella:B.G.B.

Type-2 veins:undeformed, sub-horizontal

Type-3 veins:undeformed, sub-horizontal

It follows that, based on structural and textural evi-dence, formation of quartz-veins and gold precipitation(in veins and in vein halos) is synkinematic and synmeta-morphic, which explains why undeformed (type-3) veinsare barren, as they were emplaced after the peak of theEburnean orogenic event. This interpretation renders obso-lete the distinction between ‘‘syn-orogenic and late-oro-genic deposits’’ suggested by Milesi et al. (1992).

7.2. Two mineralization styles

Although mineralized quartz-veins typically containhigher gold grades than the disseminated mineralizationstyle, the latter contains far greater tonnages, and thus rep-resents a more attractive exploration target. However, thetwo styles of mineralization can coexist in the same miningdistrict and even in the same deposit. This is the case for theBelahouro district, where the Fete Kole prospect is of thequartz-vein style while the Souma and Inata prospectsare of the disseminated style (the latter location also con-tains quartz-vein mineralization). Similarly, the Poura dis-trict comprises both disseminated (Loraboue andLarafella) and quartz-vein (Poura mine) mineralization

urkina Faso gold deposits

Gold Alteration and mineralizationassemblages

Fluid inclusions

n: Concentrated gold:(8–12 g/t) quartzveins

Quartz-sulfide veins: pyrite,chalcopyrite, sphalerite,pyrrhotite, galena, gold (andtellurides)

CO2–N2(–H2O)

d, Low grade: (<1 g/t)vein selvages

Quartz-tourmaline veins:tourmaline ± pyrite,chalcopyrite gold

H2O–NaCl–CO2(–N2–CH4)

Barren H2O–NaCl

: Disseminated gold:(2–3 g/t on average)vein alterationhalos surroundingbarren to low-gradeveins

Associated with mafic rocks:albite! ankerite! pyritegold (and tellurides)

Pure CO2

Associated with felsic and

metasediments:albite! calcite! arsenopyritegold (and tellurides)

H2O–NaCl–CO2–N2–CH4

Barren H2O–NaCl

Stage 1

Stage 2

Stage 3

Regionalshortening

Type-2 vein

Type-2 vein

Type-2 vein

Type-1 vein Type-2 vein Type-1 vein Type-2 vein

Type-2' vein

Type-2' vein

Type-3 veinType-2 vein

^ ^

^^^ ^^

^^ ^

^ ^^

^^ ^^

^^

^^

^^ ^^

^ ^^

^^ ^^

^^ ^

^ ^^

^^ ^

^

^

^^^^

^ ^

^ ^^

^^ ^^

^^ ^

^ ^^

^ ^^

^^

^

^

Albitite

Type-1 vein Type-2 vein Type-2' vein

Type-1 vein

Type-1 vein

Type-2 vein

Type-3 veinType-3 vein

Type-2' vein

not to scale

Fig. 10. Illustration of the effects of successive phases of deformation.Stage 1: In the metavolcanics and metasediments, emplacement of types-1and -2 veins; cleavage in the deposit zone is parallel to the regionalcleavage but more serrated; in the albitite, emplacement of types-1 and -2veins. Stage 2: In the metavolcanics and metasediments, increase of thedeformation with boudinage of type-1 veins, folding of type-2 veins andopening of type-2’ veins; in the albitite, emplacement of type-2’ veins.Stage 3: In the metavolcanics and metasediments, increase of thedeformation and emplacement of type-3 veins, emplacement of type-3veins in the albitites.

D. Beziat et al. / Journal of African Earth Sciences 50 (2008) 215–233 229

styles. In other instances, such as in the Taparko, Gangaoland Guibare prospects, only the quartz-vein mineralizationstyle is represented.

Although the quartz-vein style mineralization is a verycommon type of gold occurrence in orogenic type deposits,in the great majority of these gold is associated with sul-fides. In Burkina Faso, in at least two deposits, gold is inti-mately associated with tourmaline instead of the sulfides.This particular association is extremely uncommon (e.g.Sigma mine, Canada, cf. Robert and Kelly, 1987).

The quartz-vein mineralization style can be observed inall lithologies, however the disseminated gold style occursexclusively in calc-alkaline and metasedimentary series(the latter is also the case for the Ashanti deposit; Milesiet al., 1989). A reason for this could be found in the differ-ences in bulk composition of the rock types that host thesemineralization styles. The tholeiitic series are poorly differ-entiated and mainly of mafic composition. On the otherhand, calc-alkaline series are highly differentiated, rangingin composition from ultramafic and mafic cumulate rocks,to differentiated felsic volcanic and plutonic rocks (Beziatet al., 2000). During wall-rock alteration (see below), onlythe felsic units have a bulk composition that can be easilytransformed into albitite, which we have shown is invari-ably associated with disseminated-style gold deposits, i.e.the most important mineralization, economically.

The question that remains to be addressed is why albi-tites are a favorable host for this style of mineralizationwhereas mafic or intermediate units in general only hostquartz-vein mineralization. The most likely hypothesis is

that the competent igneous rock precursors to the albitite(see below) developed extensive fracture networks sincethe onset of deformation. This results in enhanced perme-ability to hydrothermal fluids, thus development of largeveins and important alteration halos, which is consistentwith the large number of veins observed in these rocks(cf. Fig. 4). An additional reason can be found in the factthat these units represent a chemical anomaly with respectto mafic and intermediate units in the greenstone belts, andthus are more likely in chemical disequilibrium with thefluid. For instance, we can probably assume that the hydro-thermal fluids were quartz-undersaturated (this has beenshown to be the case for many other similar deposits, e.g.Flambeau Lake, Ontario; Mountain and Williams-Jones,1995). As a consequence, when the fluids pass through ten-sion fractures formed in felsic units, quartz is dissolvedfrom the wall rock. This creates added porosity which, inturn, favors the creation of well-developed alteration halos.Conversely, the surrounding metasediments were moreductile and thus much less permeable, explaining the fewerveins observed in these rocks and the near absence of alter-ation around them.

7.3. Wall-rock alteration

The earliest alteration event observed in these deposits isNa-metasomatism (cf. Table 2). The units that are mostaffected by this fluid are most likely calc-alkaline rocks(e.g. diorite, dacite, and felsic metavolcanics) that undergoearly, intense fracturing (see above). In addition to beinghighly permeable, these rocks are rich in plagioclase andtherefore are easily transformed to albitites. Ultramaficrocks on the other hand, are not greatly affected, due tolower permeability and to insufficient Al for the stabiliza-tion of albite. However, both albitites and ultramafic rocksare affected by later, intense carbonatization, probably bythe CO2-rich hydrothermal fluids, similarly to the case ofAlleghany, California (Bohlke, 1989), which, for the latterrocks, gives rise to the listvenite facies.

Pyrite and arsenopyrite are the dominant sulfide miner-als as in the majority of orogenic deposits (cf. review ofGoldfarb et al., 2005). Locally, tellurides are common.We could observe that, within the same district (e.g. Poura),arsenopyrite predominates in sedimentary rock hosted min-eralization (e.g. Larafella), whereas pyrite takes over indeposits dominated by volcanic and plutonic formations(e.g. Loraboue). These differences most likely reflect differ-ent wall-rock composition that interacted differently withthe hydrothermal fluid, rather than different fluid sources.For instance, the presence of arsenopyrite probably reflectshigher amounts of As in pelitic sedimentary rocks com-pared to those in more felsic rocks (Allibone et al., 2004).

7.4. Fluid source and pressure–temperature conditions

Two contrasting fluid-inclusion compositions have beenrecognized in the gold deposits of Burkina Faso (Fig. 9)

230 D. Beziat et al. / Journal of African Earth Sciences 50 (2008) 215–233

and these occur in both mineralization styles. They consist ofa CO2-rich population (X(CO2) generally exceeding90 mol%) and a H2O–CO2 population (X(CO2) < 20 mol%).The salinity of the water-bearing inclusions is low tomoderate (Table 3), and the carbonic phase is commonlyaccompanied by N2 and CH4. In addition to these fluidinclusions, a water-only population has been identified, butdoes not appear to be related to the mineralizing episodes.

The fluid inclusions in gold deposits in Burkina Fasohave characteristics which differ significantly from thoseof fluid inclusions in other orogenic gold deposits. Forinstance, the great majority of fluid-inclusion studies car-ried out on similar deposits in the Canadian and Austra-lian shields describe low-salinity H2O–NaCl–CO2 fluidswith X(CO2) ranging only between 10 and 20 mol%(e.g. Groves, 1993; Groves et al., 2003; cf., Goldfarbet al., 2005 and Robert et al., 2005 for reviews). How-ever, abundance of CO2 (–N2–CH4) fluid inclusions, i.e.very similar to those in Burkina Faso, is a distinguishingfeature of numerous deposits elsewhere in the WesternAfrican craton (e.g. Bowell et al., 1990; Manu, 1991;Oberthur et al., 1994; Schmidt-Mumm et al., 1997; Willeand Klemd, 2004). Indeed, in most Ghanaian deposits ofthe Ashanti Belt, the large majority of inclusions areCO2-dominated (see synthesis in Klemd et al., 1996;Schmidt-Mumm et al., 1997; Wille and Klemd, 2004).In addition, in the Ashanti gold mine H2O–CO2–NaClfluid inclusions have also been described (Oberthuret al., 1994; Wille and Klemd, 2004; see also Bowellet al., 1990), similarly to Burkina Faso.

Authors working on these mineralizations have definedthe fluid chemistry as ‘‘unusual’’ (Klemd and Hirdes,1997) or as a ‘‘new category’’ (Schmidt-Mumm et al.,1997), and the nature of these fluids has been the objectof debate. Schmidt-Mumm et al. (1997) argue in favor ofa mineralizing fluid essentially composed of CO2 with var-iable proportions of N2. The elevated volatile content andthe isotopic data from the inclusions suggest a deep originof the fluids (Oberthur et al., 1996). Schmidt-Mumm (1998)also attributed the chemical nature of the fluid to devolatil-ization at depth and fluid focusing into shear zones withoutre-equilibration with the country rock. A different model toexplain widespread presence of CO2–N2 inclusions, pro-posed by Schwartz et al. (1992), Klemd and Hirdes(1997) and Klemd (1998), considers that the fluid was ini-tially an aqueous-carbonic brine, and that the compositionevolved to CO2-rich due to post-trapping modifications.Wille and Klemd (2004) recently provided convincing tex-tural evidence from the Abawo deposit (Ghana), whichconfirms this theory. Variable H2O/CO2 ratios are attrib-uted to partial immiscibility processes close to the H2O–CO2 solvus (Schwartz et al., 1992; Klemd and Hirdes,1997). Based on the above discussion, we favor the post-entrapment modification hypothesis proposed by Klemd’sgroup. In this scenario, mineralization was operated by aH2O–NaCl–CO2 brine, which is consistent with generallyaccepted models for gold deposits in similar settings. It also

permits to explain the presence of hydrated alteration min-eral assemblages, and the fluid-inclusion density variationsmentioned above.

Temperature (T) and pressure (P) conditions of mineral-ization were determined in some deposits using alterationmineral assemblages and fluid inclusion data (Klemdet al., 1997; Wille and Klemd, 2004). Because of the largedensity ranges and post-entrapment modifications dis-cussed above, the PT estimates are generally affected bylarge errors, and trapping temperatures range from lessthan 300–470 �C. A compilation of P–T estimates for goldmineralization in the Ashanti Belt by Schmidt-Mumm(1998) shows similarly wide ranges, i.e. 150–440 �C for1.7–5.4 kbars.

7.5. Precipitation mechanisms

In both styles of mineralization gold commonly occursin zones of low pressure, such as fissures around or withinpyrite crystals, pressure shadows around porphyroblasts oralteration minerals, open cleavages, tension gashes, and incavities and fractures in tourmaline. In this scenario, gold,which was likely carried as a bisulfide complex (e.g. Sew-ard, 1973; Tagirov et al., 2005), could have precipitatedin response to these local pressure fluctuations. A furtherprecipitation mechanism could be destabilization of thegold sulfide complexes as a consequence of desulfidationof the fluid resulting from pyrite precipitation in the veins(quartz-vein mineralization style) or pyrite replacement ofFe-bearing carbonates in alteration halos (disseminatedmineralization style) (see for instance Phillips and Groves,1984; Bohlke, 1989).

The differences in gold abundances observed betweenalbitite and listvenite could be the result of compositionaldifferences between these two rocks, notably the presenceof ankerite in albitites and magnesite in listvenite, Fe inthe ankerite triggering pyrite precipitation (Bohlke, 1989;Witt, 1992). Pressure fluctuations and desulfidation duringwater/rock interaction are common mechanisms invokedfor gold precipitation in these settings (e.g. Goldfarbet al., 2005).

7.6. Comparison to Ghana and the rest of the West Africa

craton

We have pointed out in previous chapters that golddeposits in Burkina Faso share many features with miner-alizations elsewhere in West Africa (Eisenlohr and Hirdes,1992; Blenkinsop et al., 1994; Allibone et al., 2002, 2004).For instance, our disseminated mineralization style couldbe equivalent to the sulfide-ore type defined for Ashanti(Leube et al., 1990; Oberthur et al., 1994) and to the dis-seminated Au-sulfide (type 2) mineralization described byMilesi et al. (1989) in the Dienemera (Burkina Faso) andYaoure–Angovia (Ivory Coast) deposits and by Alliboneet al., 2004) in the Chirano (Ghana) deposit. Another sim-ilarity consists of the particular fluid chemistry of these

D. Beziat et al. / Journal of African Earth Sciences 50 (2008) 215–233 231

deposits. For instance, Feybesse et al. (2006) pointed outthat the peculiarity of the Ghanaian gold province con-sisted in percolation of CO2-rich fluids in the mineralizedzones.

A discrepancy with the rest of West African deposits isthe association of gold with tourmaline in Burkina Faso.Gold deposits hosted in tourmalinized rocks have also beendescribed in the Loulo deposit in the western border ofMali (Dommanget et al., 1985, 1993), however, here goldis associated with late stockwork sulfide mineralization,and not directly related to the tourmaline. Moreover, thisdeposit is interpreted to predate the main orogenic events,whereas the tourmaline-bearing veins in Burkina Faso aresyn-tectonic.

8. Conclusions

All primary gold deposits, prospects and showings thatwe studied in Burkina Faso can be classified as orogenic-type gold deposits. The observations gathered from severaldeposits have led us to define two main styles of goldmineralization: (1) quartz-vein hosted, in which gold isconcentrated within deformed (boudinaged or folded)quartz-veins, the surrounding rocks being barren and (2)disseminated, larger deposits, where gold mainly occursas disseminated particles in alteration halos surroundingveins, particularly within albitites.

The quartz-vein and disseminated mineralization stylesrepresent variations due to local heterogeneity of the hostlithologies; the mineralizing fluids and precipitation mech-anisms were likely similar. Such fluids are typically moder-ate salinity, NaCl brines saturated with CO2. Albitizationand carbonatization are common hydrothermal alterationassemblages, as is pyrite and/or arsenopyrite precipitation,which is the dominant control on gold deposition. Thegold was likely carried as a bisulfide complex whichdestabilized during wall-rock sulfidation, explaining theassociation of Au with pyrite and arsenopyrite. Thequartz-tourmaline veins, where gold is intimately associ-ated with tourmaline, represent an uncommon style ofmineralization.

Timing of gold mineralization is compatible with golddeposition during a single regional mineralizing event, atthe end of the Eburnean orogenic process. The dissemi-nated-style mineralization forms larger deposits than thequartz-vein style. Their geological and fluid inclusion char-acteristics are very similar to those of the largest golddeposit of West Africa, i.e. the Ashanti and Chirano dis-tricts in the Ghanaian province.

Acknowledgement

The manuscript benefited from discussion with A.E.Williams-Jones and B.W. Mountain. Reviews by twoanonymous reviewers contributed to greatly improve themanuscript. Financial support for this study was providedby the ‘‘Campus research contract’’. We are grateful to

B.H.P. – BILLITON and SOREMIB and for access toexploration properties.

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