ARTICLE

A.Y. Billay á A.F.M. Kisters á F.M. Meyer á J. Schneider

The geology of the Lega Dembi gold deposit, southern Ethiopia:implications for Pan-African gold exploration

Received: 26 July 1996 /Accepted: 8 January 1997

Abstract The Lega Dembi deposit is the largest goldproducer in Ethiopia. It is situated in late-Precambrianmetamorphosed sediments of the N-S trending, volcano-sedimentary Megado belt, which forms part of the late-Proterozoic Adola granite-greenstone terrane in south-ern Ethiopia. The lode-gold mineralization occurs in aN-S trending, steep westerly dipping quartz-vein systemthat follows the structural contact between underlyingfeldspathic gneisses and the volcanosedimentary se-quence of the Megado belt. This contact also marks thenorthernmost extension of the regional-scale, sinistralstrike-slip Lega Dembi-A¯ata shear zone. Mineraliza-tion and intense quartz-veining is best developed ingraphite-rich sediments within an area not more than80 m away from this tectonic contact. Hydrothermalwall-rock alteration includes actinolite/tremolite-biotite-calcite-sericite and chlorite-calcite-epidote assemblages.Gold occurs preferentially in the sericite alteration zone,where it is closely associated and intergrown with gale-na. The variable deformation of the gold-quartz veinssuggests a syn-kinematic timing for the gold mineral-ization during transcurrent shearing in a dilational seg-ment of the shear zone. In addition to the structuralcontrol, lithological control on gold deposition is indi-cated by the almost exclusive occurrence of the goldmineralization in graphite-rich metasediments. Thisclose relationship suggests that gold precipitation wasthe result of chemical reduction of regional ore-bearing¯uids. Temperature conditions of mineralization areconstrained by the actinolite-biotite alteration assem-blage and by arsenopyrite chemistry, which indicate that

ore deposition occurred at or close to peak metamorphicconditions at upper-greenschist to lower-amphibolitemetamorphic grades. Rb-Sr dating of sericite indicatesan age of about 545 Ma. for hydrothermal alterationand, thus, for gold mineralization. The style of goldmineralization, structural pattern and lithological as-semblages at Lega Dembi are very similar to lode-golddeposits most commonly reported from Archaeangranite-greenstone terranes. These similarities may opennew perspectives for the exploration of lode-golddeposits, which has previously primarily focused onArchaean greenstone belts rather than Proterozoic oreven Phanerozoic meta-volcanosedimentary belts.

Introduction

Shear-zone hosted, mesothermal lode-gold deposits area major source of world gold production (Woodall1988). The vast majority of these gold-bearing veinsystems is spatially closely associated with the tectono-metamorphic evolution of predominantly late-Archaeangreenstone belts, that are an intricate feature of manyArchaean cratons (e.g. Anhaeusser 1976; Colvine et al.1984, 1988; Robert and Brown 1986; Kerrich 1986;Foster 1989; Groves et al. 1989; de Ronde et al. 1992). Inaddition to their commonly late-Archaean age, thesedeposits bear numerous similarities worldwide, whichled to the term `Archaean lode-gold deposits', that wascoined to describe the styles of gold mineralization, as-sociated alteration patterns and structural controls ofgold deposits in late-Archaean granite-greenstone ter-ranes (comprehensive reviews are given by, inter alia,Groves and Foster 1991, Colvine 1989, Groves 1993,and Kerrich and Cassidy 1994)

In recent years, however, an increasing number ofworkers have questioned the restrictive use of the term`greenstone belt' for linear to irregularly shaped meta-volcanosedimentary rocks of predominantly Archaeanage, since similar lithological associations and tectonic

Mineralium Deposita (1997) 32: 491±504 Ó Springer-Verlag 1997

Editorial handling: P.G. Eriksson

A.Y. Billay (&) áA.F.M. Kisters áF.M. MeyerInstitut fuÈ r Mineralogie und LagerstaÈ ttenlehre,RWTH Aachen, WuÈ llnerstr. 2, 52056 Aachen, Germany(e-mail: kisters@rwth-aachen.de)

J. SchneiderInstitut fuÈ r Geowissenschaften und LithosphaÈ renforschung,Justus-Liebig UniversitaÈ t Giessen, Senckenbergstr.3, 35390 Giessen, Germany

styles are also observed in Proterozoic and Phanerozoicsupracrustal belts (De Wit and Ashwal 1995, and ref-erences therein). The recognition of greenstone-belt typesequences and similar tectonic styles in Proterozoic andPhanerozoic orogens implies that younger Archaean-type mesothermal lode-gold deposits could be far morecommon than has previously been documented (Nesbitt1991). This, in turn, opens new perspectives for goldexploration, that has traditionally focused on Archaeangranite-greenstone terranes, and which has possiblyunderestimated the potential for large-scale golddeposits in younger supracrustal belts.

The Lega Dembi gold mine in the Sidamo region ofsouthern Ethiopia is the largest gold producer in thecountry (Sutton-Pratt 1996). The deposit is situatedsome 500 km south of Addis Ababa, within the late-Proterozoic meta-volcanosedimentary Megado belt ofthe Adola granite-greenstone terrane, which forms thesouthernmost extension of the Neoproterozoic Arabian-Nubian Shield (Kazmin et al. 1978; Worku and Schan-delmeier 1996) (Fig. 1).

Placer gold in the Megado belt was ®rst discovered in1936 and, since then, more than 55 tons of placer goldhas been produced. Primary gold occurrences, includingthe Lega Dembi deposit, were identi®ed during an ex-ploration campaign in the late 1970s by the Adola GoldExploration Project (AGEP) of the Ethiopian MineralResources Development Corporation (EMRDC). TheLega Dembi deposit is currently being mined by theEMRDC in an open cast operation. The gold produc-tion averages 3 t/a (Sutton-Pratt 1996) and reserves ofsome 60 tons of gold at an average grade of 6 g/t havebeen identi®ed up to 200 m below surface (Moudrovet al. 1991).

Various models pertaining to the lithostratigraphicand structural evolution of the region have been pro-posed. However, because of the relatively recent dis-covery of the Lega Dembi gold deposit, little work onthe geological setting, on the style, timing, and controlsof the mineralization and on wall-rock alteration iscurrently available in the literature. The petrochemistryof the igneous rocks of the Adola region has been de-scribed by Bisrat (1993), Gichile and Fyson (1993), Be-raki (1995), and Worku and Schandelmeier (1996). Oreparagenetic, wall-rock alteration and trace-elementgeochemical studies are found in Fiori et al. (1987),Tadesse (1990), and Getaneh (1994), and in reportswhich accompanied the exploration campaign by theAGEP (Morin and Oliver 1986; V/O Tecnoexport 1986;Emelyanov et al. 1987; Moudrov et al. 1991). Fluid in-clusion studies related to the gold mineralization at LegaDembi were carried out by Tadesse (1990). Geneticmodels for the gold mineralization at Lega Dembi in-clude syngenetic gold mineralization in the metasedi-ments and metavolcanics of the Megado belt, followedby a redistribution and concentration of the gold min-eralization during subsequent metamorphic events (Fioriet al. 1987), and a model of a structurally controlled,epigenetic mineralization in a strike-slip shear zonesystem (Emelyanov et al. 1987; Ghebreab et al. 1992;Worku 1993).

This study presents a brief description of the regionalgeology of the Adola granite-greenstone terrane and thegeological setting of the Lega Dembi deposit in partic-ular. The main aim is to focus on the multi-stage quartz-veining and associated gold mineralization and wall-rock alteration at Lega Dembi. Finally, the formation ofthe gold-quartz mineralization is assessed in relation toregional deformational and metamorphic events.

Regional geology

The Adola granite-greenstone terrane covers an area of approxi-mately 5000 km2 in southern Ethiopia. It is characterized by twolinear, closely spaced, N-S trending belts of metamorphosed su-pracrustal rocks, namely the Megado volcanosedimentary belt inthe west and the Kenticha ultrama®c belt in the east (Fig. 1). Theformer consists of ultrama®c and tholeiitic basic volcanics andintrusives which are intercalated with sediments made up pre-dominantly of arkoses, feldspathic quartzites, quartzites, and pe-lites, together with subordinate polymictic conglomerates andgraywackes (Gilboy 1970; Chater 1971; Bisrat 1993; Ghenzebuet al. 1994; Worku and Schandelmeier 1996) (Fig. 2). Small pod- orlens-like bodies of mainly tonalitic composition are intrusive intothe basic rocks of the Megado belt. In contrast, the Kenticha belt isdominated by ultrama®c rocks, with subordinate amphibolites andsedimentary rocks, the latter comprising biotite schists and minorgraphitic schists and marbles (Gilboy 1970; Chater 1971).

The two volcanosedimentary belts are surrounded and sepa-rated by a gneissic terrane which comprises para- and or-thogneisses, including monotonous quartzo-feldspathic biotitegneisses with subordinate muscovite-quartz schists, staurolite-gar-net-biotite schists, impure marbles, and amphibolites (Gilboy 1970;Chater 1971; Kozyrev et al. 1985; Ghebreab 1989; Worku and Yifa1989). Large tonalite bodies also intrude the gneissic terrane.Gneissose granites are con®ned to the gneissic terrain and post-

Fig. 1 Location map and regional geological setting of the Adolagranite-greenstone terrane in southern Ethiopia (modi®ed after Beraki1995)

492

tectonic granites occur marginally to the greenstone belts and in thegneissic terrane (Fig. 1).

The eastern contact between the Megado belt and the gneissicterrane is tectonic and is regionally referred to as the Lega Dembi-A¯ata shear zone (Worku and Yifa 1989; Worku and Schandel-meier 1996). The western margin of the Megado belt is, however,marked by the development of gneissose tonalite which showsprimary intrusive contacts with the supracrustal assemblages.

Metamorphism

Two main metamorphic events (M1-M2) have been recognized forthe Adola granite- greenstone terrane (Gilboy 1970; Chater 1971).Evidence of an early M1 event is only represented by relict mineralparageneses containing cordierite, that have been largely over-printed by the subsequent M2 event which has pervasively a�ectedthe rocks of the region (Gilboy 1970; Chater 1971). In the sedi-mentary rocks of the Megado belt, mineral assemblages typicallycomprise quartz, plagioclase, biotite, muscovite, and accessory ru-tile, chlorite and epidote. In the basic rocks, actinolite/hornblende,plagioclase (albite-oligoclase), epidote and chlorite are common(Gilboy 1970, Chater 1971). In places, however, kyanite-bearingrocks that occur within the greenschist-facies sequence point tolocally higher-grade metamorphic conditions. This juxtaposition ofunits of di�erent metamorphic grades has been interpreted byWorku (1993) to be the result of an imbrication of thrust slices.

The rocks of the gneissic terrane and the Kenticha belt havebeen a�ected by amphibolite±facies metamorphism of the stauro-lite-almandine and kyanite-almandine-muscovite subfacies duringthe M2 metamorphism (Gilboy 1970, Chater 1971). The meta-morphic grade increases from lower- and mid-amphibolite facies inthe NW to upper-amphibolite and lower-granulite facies in the SEof the gneissic terrane (Gilboy 1970). However, no determinationsof P-T conditions of the metamorphism exist for the region.

Structural geology

Five main phases of deformation have been distinguished for theMegado belt and its surrounding gneissic terrane. These include: (1)an early gneissosity-forming D1 event in the gneissic terrane, whichis expressed by early folds in the Megado belt (Gilboy 1970); the D1event was related by Beraki et al. (1989) and Worku and Schan-delmeier (1996) to an early subduction-related thrust event. (2) N-Strending regional-scale, upright D2 folds that dominate the struc-tural pattern of the region (Gilboy 1970; Chater 1971; Gebreab1989; Worku and Yifa 1989; Beraki 1995) were associated with anE-W directed collisional event (Worku and Schandelmeier 1996).(3) Strike-slip shearing (D3) along the contacts between thegreenstones and the gneissic terrane, due to a NW-SE directedtranspressional event (e.g. Beraki 1995; Worku and Schandelmeier1996). (4) E-W trending, upright, moderate-to-steep easterly and/orwesterly plunging folds, referred to by Gilboy (1970), Chater (1971)and Ghebreab (1989) as D3 folds; Worku and Schandelmeier(1996) interpret this folding to be the result of D3 transpressionalshearing, but Beraki (1995) refers to these folds as D4 folds. (5)Late, brittle NW-SE and NE-SW to E-W trending faults whichdisrupt the N-S trending granite-greenstone terrane.

The tectonic evolution of the Adola granite-greenstone terraneand the origin of the supracrustal sequences has been a matter ofcontroversy among di�erent workers. Three di�erent models areproposed for the disposition of the N-S trending linear greenstonebelts which overlie the gneissic terrane. The ®rst model proposes anorigin of the greenstone sequences as ophiolites, that were thrustonto the gneissic terrane. The ophiolites were later refolded bymajor N-S trending folds and subsequently modi®ed by strike-slipshearing (Kazmin 1976; De Wit and Chewata 1981; Beraki et al.1989; Beraki 1995; Worku and Schandelmeier 1996). The secondmodel suggests intra-continental rifting, followed by thick-skinnedtectonics, whereby both the greenstones and the gneisses werethrusted in an eastward direction (Worku and Yifa 1989; Ghebreab1989). The third genetic model proposes intra-continental riftingand multiphase strike-slip shearing (Amenti et al. 1992).

Geochronology

Age determinations on the rocks in the Adola region are scarce.The Megado belt supracrustal series has been correlated with

Fig. 2 Simpli®ed geological map of the Lega Dembi area illustratingthe location of the gold deposit along the contact between thegreenstone sequence of the Megado belt (in the west) and the gneissicbasement (in the east) (modi®ed after Ayalew 1990)

493

similar assemblages of the Upper Proterozoic Arabian-NubianShield (Beraki 1995; Teklay et al. 1996; Worku and Schandelmeier1996). A tonalite body west of the Megado belt yielded an U-Pbage of 765 Ma (EIGS, quoted by Gichile and Fyson 1993), pro-viding a minimum age constraint for the volcano-sedimentary se-quence. Upper Proterzoic ages are also inferred for the gneissicbasement which is correlated with gneisses of the Mozambique belt(Kazmin et al. 1978; Gass 1977, and references therein; Vail 1976,1983). Rb-Sr whole rock ages of 630±680 Ma for the gneisses andsyntectonic granites are interpreted to represent the age of the M2metamorphism (Gilboy 1970; Chater 1971). Post-tectonic granitesyielded whole-rock Rb-Sr ages of 500±550 Ma (Gilboy 1970;Chater 1971).

The geology of the Lega Dembi gold deposit

The Lega Dembi deposit has been divided into four in-terconnected open pit operations that are locally re-ferred to (from south to north) as Southern, Central,Northern and Upper Lega Dembi (SLD, CLD, NLDand ULD, respectively) (Fig. 2). Outcrop is largely re-stricted to the mine area and some resistant lithologiesthat form N-S trending ridges which dominate the geo-morphology of the deeply weathered and densely for-ested area.

The steep westerly-dipping lithostratigraphic se-quence at Lega Dembi can be subdivided into (1) a seriesof quartzo-feldspathic and biotite gneisses and am-phibolites belonging to the gneissic terrane in the east,and (2) the volcanosedimentary sequence of the Megadobelt in the west. In detail, the lithological succession atLega Dembi comprises ultrama®c schists and variousmeta-sedimentary lithologies (Fig 2).

The ultrama®c talc schists are commonly developedalong the contact between the gneissic terrain and thesupracrustal sequence. The moderate-to-steep (40±70°)westerly dipping contact is marked by the developmentof mylonitic fabrics (see below) and, as such, is clearlytectonic and generally sharp, although intercalations ofbands of talc schists within the gneisses are observedlocally. The width of the talc schists at NLD is less than5 m, locally pinching out, but it progressively increasestowards the south and attains a maximum thickness ofabout 180 m south of Reji (Fig. 2).

The succession of meta-sedimentary rocks can besubdivided into a lower leucocratic muscovite-quartz-plagioclase schist, which is overlain by laminated, dark-greyish, graphite-rich, locally graded feldspathic arenitesand quartz wackes. The mineral assemblage of the sed-iments consists of quartz, biotite, muscovite and pla-gioclase, together with accessory rutile, epidote,graphite, tourmaline and chlorite. The latter overgrowsthe main foliation. Kyanite is present locally in prox-imity to quartz veins. The sedimentary succession attainsa maximum thickness of about 280 m at CLD, pro-gressively decreasing to less than 20 m at ULD, where itis buttressed between a massive meta-gabbro in the westand quartzo-feldspathic gneisses in the east (Figs. 2, 3).The massive meta-gabbro, together with minor am-phibolites, forms the western margin of the Lega Dembi

deposit. The ma®c units form a N-S trending ridge,parallel to the structural grain of the Megado belt. Thema®c units are locally intruded by stringer- and pod-liketonalite bodies which display penetrative planar andlinear fabrics, and which are folded together with theamphibolites on a metre scale.

Structural geology of the Lega Dembi mine

The polyphase tectonism of the Adola granite-green-stone terrain is, on the scale of the Lega Dembi deposit,expressed by several generations of folds and faults,together with the development of composite planar andlinear fabrics. Tight to isoclinal, cm-to-dm scale intra-folial folds of the bedding (S0) provide evidence of anearly deformation event (D1). The present attitude ofthe folds, which show moderate-to-steep westerlyplunges, is the result of the subsequent D2 and D3 de-formations.

The Lega Dembi mine is situated in a large, N-Strending fold structure which is part of the regionalpattern of large-scale F2 folds in the Megado belt. Thefold closure is located south of the Lega Dembi depositwhere the ultrama®c talc schists attain their maximumthickness (Fig. 2). The lack of unambiguous indicatorsof the younging direction and parasitic folds hampersthe determination of the synformal or antiformal natureof the fold structure. Although F2 folds show, on a re-gional scale, predominantly shallow northerly andsoutherly plunges, a steep westerly plunge of the mainF2 fold at Lega Dembi is suggested by the westerlyplunge of mineral and mineral stretching lineations (seebelow), together with mesoscale fold axes. A regionallydeveloped, upright, N-S trending S2 fabric is axial pla-nar to the F2 folds (Fig. 3a).

The S2 fabric is intensi®ed to pervasively developedmylonitic foliations towards the eastern margin of theMegado belt, where the greenstone succession is juxta-posed against the gneissic terrane. The development ofmylonitic fabrics underscores the tectonic nature of thiscontact. Field and microstructural evidence (e.g. intra-folial folds of S2 in microlithons) suggest, that this fabricis composite and the result of a transposition of thecoplanar S2 foliation into the mylonitic shear foliation(S3), so that this fabric is henceforth referred to asS2/S3. Macroscopically, the S2/S3 fabric is further ac-centuated by foliation-parallel quartz veins (see below).Shear sense indicators in the meta-sediments at LegaDembi yield ambiguous results (i.e. both a normal and areverse dip-slip component), but with a consistent si-nistral strike-slip component. The orientation of anoblique shear foliation de®ned by biotite, together withthe ubiquitous occurrence of asymmetric, S-shaped,steep westerly plunging folds along the contact betweenthe gneissic basement and the volcano-sedimentary se-quence, also suggest a prominent sinistral strike-slipcomponent. The cm- to dm-scale folds refold the S2/S3fabric, together with the foliation-parallel quartz veins,

494

but can locally be seen to be transected by the N-Strending S2/S3 fabric, indicating the progressive natureof the fabric-forming event. The component of sinistralstrike-slip shearing could possibly also explain the un-usually steep westerly plunge of the large-scale F2 fold atLega Dembi, as a result of the re-orientation and drag ofthe fold along the shear zone. The development of theductile shear fabrics along the contact between thegneissic basement and the greenstone succession, show-

Fig. 3 Simpli®ed geological map of the North Lega Dembi (NLD)open pit. Fabric diagrams of a poles to the S2 and S2/S3 foliationfrom the meta-sediments and meta-volcanics of the Megado belt atLega Dembi; the great circle distribution of the poles is a result of theD4 open folding about moderate-to-steep easterly plunging axes; borientation of prominent quartz-sul®de veins at Lega Dembi,subparallel to the S2/S3 foliation; c summary plot of mineralstretching, intersection and mineral lineations (undi�erentiated) atLega Dembi (all plots are lower-hemisphere, equal-area projections)

495

ing evidence of predominantly sinistral strike-slip kine-matics, indicates that the shear zone forms the northernextension of the Lega Dembi-A¯ata shear zone (e.g.Worku and Yifa 1989).

All rock units have been a�ected by open, upright,E-W trending folds which show moderate westerlyplunges (Fig. 3a). Wavelengths range from <10 m to>100 m. These F4 folds refold the composite S2/S3fabric and the large N-S trending folds, as well as theasymmetric S-shaped folds, resulting in an open typethree interference pattern. Steep-to-moderate westerlyplunging, E-W trending folds are also observed on amm- to cm-scale, crenulating the composite S2/S3transposition fabric. An axial-planar foliation (S4) isexpressed by the growth of biotite, which is at a highangle to the N-S trending S2/S3 transposition fabric.

A set of NW-SE and E-W trending brittle and brittle-ductile faults (D5) a�ects the Lega Dembi deposit in itsnorthern and southern parts, disrupting the continuousextent of the mineralization (Fig. 2). Both the footwallgneisses and the volcano-sedimentary assemblages of thegreenstone belt contain a variety of linear fabrics thatinvariably show moderate to steep westerly plunges, sothat their timing with respect to the deformation eventsdescribed already remains somewhat speculative(Fig. 3c). The various types of lineation include (1) amineral stretching lineation, formed by stretched quartz-feldspar aggregates in the footwall gneisses, (2) a minerallineation de®ned by the preferred growth of, for exam-ple, amphiboles and tourmaline in the supracrustals,together with kyanite in proximity to quartz veins, and(3) an intersection lineation between the bedding (S0),and the S2 and S3 foliations in the greenstones, respec-tively. In addition, the mylonitic foliations close to thecontact between the basement gneisses and the green-stones are characterized by a steep westerly plungingmineral stretching lineation and by quartz-rodding.Worku and Yifa (1989) and Ghebreab et al. (1992) havedescribed a set of subhorizontal lineations which theyrelated to the D3 strike-slip shearing along the contactbetween the Megado belt and the gneissic terrain. Thepresence of this lineation could not be con®rmed in thisstudy.

Gold mineralization at Lega Dembi

The gold mineralization at Lega Dembi is situated alongthe sheared contact between the quartzo-feldspathicgneisses of the gneissic terrain and the volcano-sedi-mentary sequence of the Megado belt, in strongly foli-ated meta-sediments (Fig. 3). The N-S strike extent ofthe mineralization is approximately 2 km, parallel to theS2/S3 fabric and lithological layering. The maximumwidth of the mineralized zone is approximately 140 m inthe central parts of the deposit (i.e. at NLD), butgradually decreases to <20 m in the north, which cor-responds to the northward pinching of the meta-sedi-ments. Except for NLD and ULD, which are connected

by a continuous development of mineralized quartzveins, the various parts of the deposit are separated fromeach other by barren ground as a result of o�sets alongE-W trending D5 faults.

The main mineralization is con®ned to within 80 m ofthe contact between the basement gneisses and the vol-cano-sedimentary succession. The mineralized zone ischaracterized by three main, up to 10 m wide, compositequartz vein systems, which are referred to as the eastern,central and western veins. Thin foliation-parallel quartzveinlets, however, can be observed throughout the meta-sedimentary succession. The main vein system at NLDshows a strike extent of about 250 m and explorationdrilling has delineated a semicontinuous down-dip ex-tent of >350 m (Zemene 1995, personal communica-tion). The eastern vein is hosted in a muscovite-quartz-plagioclase schist and the central and the western veinsare situated in laminated, graphite-bearing arkoses andquartz wackes (Fig. 3). The graphite content of themeta-sediments increases from £ 0.1±0.4 wt.% in prox-imity to the main vein system, up to 1.5 wt.% in some ofthe less altered wall rocks outside the mineralized zone.

Based on the orientation and morphology of thequartz veins, together with their relation to thedominant host-rock fabric, four sets of quartz veinsare identi®ed, including S2/S3-parallel, intensely de-formed, massive to laminated quartz veins which arethe main hosts of the gold mineralization (type 1),veins that are discordant to S2/S3 and which arevariably folded (type 2), breccia quartz veins (type 3),and largely undeformed veins which cut the S2/S3fabric (type 4).

Type 1 veins are the most abundant. They occurparallel to the S2/S3 foliation, or form tight-to-isoclinalfolds with the S2/S3 foliation being axial planar(Figs. 3b and 4). Boudinage and pinch-and-swell of the

Fig. 4 Polished hand specimen of a typical laminated (type 1),foliation- (S2/S3) parallel quartz-veining (light grey), surrounded bysericite and biotite-actinolite � kyanite alteration (dark) (NorthernLega Dembi, central vein system)

496

quartz veins, both down-dip and along strike is com-mon. The quartz veins may occur as isolated veins, butthey are typically closely spaced, showing gradual tran-sitions from laminated or ribbon-textured veins tomassive quartz veins of up to 5 m width. The three mainvein systems show gradual transitions to a more string-er-type mineralization along their lateral terminations,but massive quartzveining may re-occur along strike.

Type 2 veins are subordinate. The thin, commonly<1 cm thick veinlets cross-cut the S2/S3 foliation, andare folded at variable degrees and/or sheared parallel tothe S2/S3 foliation.

Type 3 quartz veins show an extensive brecciation ofwall rocks and are only locally developed. Wall-rockfragments are angular and consist of intensely foliatedwall rocks that contain sul®de mineralization (Fig. 5).The size of the fragments may range from minute in-clusions to rafts of 0.5 m diameter. The internal foliationof the wall-rock fragments has been intensely foldedprior to the brecciation and the subsequent gold-sul®de-quartz mineralization. Type 3 veins are best described asdisaggregation breccias, showing a progressive texturaldevelopment from their margins, where the wall rocksare largely intact, to their centers, where wall-rockfragments occur as isolated and rotated rafts cementedby a massive, milky quartz matrix (Fig. 5).

Type 4 veins are represented by distinct cross-cuttingveinlets. Although the veins appear to be undeformed,on a microscopic scale, deformation is indicated by theundulose extinction of quartz and the formation ofquartz subgrains. Type 4 veins are subordinate in im-portance and their width is commonly <5 cm (majority<0.5 cm). Some of these veins strike parallel to theS2/S3 foliation, but dip at shallower angles (35°±40°) tothe west than the main fabric and type 1 veins, whereasothers dip at steep-to-moderate angles in an easterlydirection.

Wall-rock alteration

Wall-rock alteration at Lega Dembi displays a broadzonation pattern on the deposit scale (Fig. 6). The extentof the various alteration zones is not clearly de®ned dueto the intense and multiple quartz- veining and associatedalteration overprint, as well as due to variations in host-rock lithologies. The following mineral assemblages arerecognized, with increasing proximity to the main quartzveining: (1) chlorite-carbonate-epidote, (2) actinolite/tremolite-biotite�calcite, and (3) sericite (Fig. 6a, b).

Sericite represents the innermost alteration mineral inclose proximity to the massive quartz veins. Within thequartz veins, the locally fuchsitic sericite occurs as thin,commonly less than 3 mm wide, foliation-parallel bandsand laminae. Complete or partial replacement of feld-spars by sericite is common. Minor calcite may bepresent locally in the quartz veins. In places, quartz veinsare bordered by a sericite-biotite assemblage.

The most characteristic alteration at Lega Dembi is apervasive actinolite/tremolite-biotite�calcite paragene-sis that is developed within 15±20 m of the main quartz-veining (Fig. 6a, b). Based on textural evidence and oncross-cutting relationships, at least two main stages ofthis alteration can be distinguished. Stage 1 is commonlycharacterized by a S2/S3 foliation-parallel actinolite-tremolite-biotite�calcite assemblage. It may also occuras strongly folded discrete veins and veinlets that aremarked by their light greenish color. This type of al-teration is locally developed as massive, up to 5 m widezones around the main quartz vein systems. Locally,tourmaline de®nes a steep westerly-plunging minerallineation. A stage 2 actinolite-tremolite overgrows thestage 1 alteration assemblage, as well as the S2/S3 foli-ation, in the form of rosettes or discordant pockets andveinlets. This type of alteration is also developed alongthe margins of type 4 veins.

Mesoscopic propylitic alteration, characterized by thepresence of chlorite, carbonate, and epidote, occurs be-tween the western and the central veins (Fig. 6a,b), anddistal to the veins in the wall rocks, giving the rocks theirdistinct greenish coloration. It may also occur in theform of foliation-parallel anastomosing bands. How-ever, minor chlorite is also developed after muscovite inthe quartz veins.

Sul®de mineralization

Sul®de mineralization occurs mainly as ®ne dissemina-tions in the quartz veins and wall rocks, or parallel to theS2/S3 foliation, in boudin necks or cross-cutting frac-tures. The main gold- and sul®de mineralization is as-sociated with type 1 veins. The sul®de assemblage in thequartz veins consists of (in decreasing order of abun-dance): chalcopyrite, galena, pyrrhotite, and pyrite.Sphalerite, gersdor�te, arsenopyrite, bournonite, mo-lybdenite, tellurides, silver-tetrahedrite, and gold areminor-to-rare constituents. Microscopic gold is sited

Fig. 5 Type 3 (breccia) vein containing angular, variably-sized, wall-rock fragments cemented by massive, milky quartz (Northern LegaDembi, western vein system)

497

along the sericite alteration lamellae and wall-rock sce-ptas within the quartz veins (Fig. 7). Gold grains arecommonly <0.25 mm (mainly <0.05 mm) and areelongated parallel to the S2/S3 foliation (Fig. 7). Gold istypically intergrown or spatially associated with galena,commonly associated with chalcopyrite, pyrrhotite andthe tellurides hessite and stuetzite (Fig. 8). Inclusions ofgold in pyrrhotite, galena, tellurides, and chalcopyrite

are rare. Gold ®neness ranges from 783 to 904, averag-ing 820.

The biotite-actinolite alteration bordering type 1quartz veins is dominated by a pyrrhotite-gersdor�teassemblage, with minor pyrite, chalcopyrite, arsenopy-rite, pentlandite, and niccolite. No microscopically visi-ble gold was observed in the alteration assemblageadjacent to the veins. Less-altered wall rocks betweenthe quartz-vein systems and outside the main mineral-ization contain up to 5 vol.% sul®des, mainly pyrrhotiteand minor chalcopyrite and arsenopyrite. Rare sphale-rite, gersdor�te and molybdenite occur adjacent to fo-liation-parallel quartz veinlets.

Type 2 and 3 veins are characterized by a pyrite-pyrrhotite-chalcopyrite assemblage, with minor galenaand rare microscopic gold. Occasional globular texturesof pyrite possibly indicate open-space ®lling. Featherytextures of pyrite result from the ®lling of foliationplanes in the intensely foliated wall rocks adjacent to thequartz veins. Type 4 veins are commonly devoid of anygold- and sul®de mineralization. However, pyrite,chalcopyrite and pyrrhotite occur in the biotite-actino-lite alteration haloes enveloping type 4 veins.

Fig. 7 Free gold (Au) closely associated with the innermost sericitealteration (Ser) in a quartz vein (Qtz). Note the elongation of the goldgrains parallel to the foliation (subhorizontal) de®ned by the sericite(North Lega Dembi, central vein)

Fig. 6 Schematic map and cross section (line A-B) of the distributionof alteration zones at Lega Dembi North, consisting of (a) aninnermost silici®cation and sericite alteration, (b) predominantactinolite/tremolite-biotite alteration, and (c) locally intercalated anddistal chlorite alteration

498

Fluid inclusions

Fluid inclusion studies were conducted on the variousquartz-vein generations, in order to constrain the com-position of the mineralizing ¯uids and the temperatureof the quartz-vein emplacement and, hence, of the gold-sul®de mineralization. Quartz studied in all samplesexhibits predominantly straight grain boundaries and120° triple junctions, resulting in polygonal, foam-liketextures which, together with the commonly strain-freenature of the quartz, indicates that the textures under-went extensive static recovery after quartz-vein em-placement. As a consequence, no primary ¯uidinclusions were identi®ed that could unambiguously berelated to the mineralizing event. Fluid inclusions aremainly of a secondary nature, and occur predominantlyalong trails that may both cross-cut grain boundaries orterminate at grain boundaries. A total of 177 ¯uid in-clusions was measured. The following types of ¯uid in-clusions can be distinguished and are presented indecreasing order of abundance (all descriptions are givenfor a temperature of 20 °C).

Type 1, mixed CO2-H2O inclusions are 2- and/or 3-phase inclusions, containing either liquid and gaseousCO2 and liquid H2O, or gaseous CO2 nucleates oncooling. CO2: H2O ratios are variable, but CO2 is by farthe dominant component and, in many inclusions, H2Omay only be visible as a thin ®lm along the inclusionwalls. The size of most of the type 1 inclusions that werestudied ranged from 5±10 lm, but they may be as largeas 30 lm. They show predominantly negative-crystal-to-rounded shapes and occur along trails and irregularly-shaped clusters. Melting temperatures of solid CO2 of)57 and )59.5 °C indicate the presence of other com-ponents in the ¯uid such as N2 or CH4. Homogenizationtemperatures (ThCO2,V!L) vary between +18 and+25 °C. Type 1 inclusions occur in all quartz-veingenerations.

Type 2, CO2-rich inclusions, containing no visibleH2O, occur as dark and isolated inclusions in type 1veins only. These single-phase inclusions are irregularlyshaped and range in size from 10 lm up to 70 lm. Finalmelting temperatures range between )57.1 and)59.2 °C, suggesting the presence of other components.Total homogenization into the liquid phase occurs be-tween +13 and +15 °C.

Type 3, two-phase H2O-rich inclusions were identi-®ed in all quartz-vein generations. They occur alongtrails which may cross grain boundaries, but in manycases trails terminate at grain boundaries suggesting thatthey are of pseudo-secondary origin. The average size ofthe inclusions is 5±10 lm, but they may be as large as30 lm. Their shape varies from subrounded to irregular.

First melting temperatures (Teice) of approximately)21 °C indicate that the ¯uids in type 3 inclusions arerelatively pure H2O-NaCl solutions. Final melting tem-peratures (Tmice) show two distinct maxima between)12 and )9 °C (12.8±16 wt.% NaCl eq.) and between )6and )2 °C (3.3±9.2 wt.% NaCl eq.). Homogenizationtemperatures show a very broad range from 190±370 °C.However, individual trails are characterized by relativelyconstant phase ratios and give distinct maxima of 190±225 °C, 245±280 °C, and 370±380 °C. Figure 9 repre-sents a Th-salinity diagram for type 3 inclusions, illus-trating that the more saline inclusions occur only intype 1 veins, while the low-to-moderate salinity inclu-sions are found both in the syntectonic type 1 veins, aswell as in the post-tectonic type 4 veins. The broad rangeof homogenization temperatures of the more saline type3 inclusions may be due to post-entrapment stretching ofthe inclusions (Fig. 9).

Fig. 8 Gold (Au) closely associated with galena (Gn), chalcopyrite(Cpy), stuetzite (Stz) and hessite (Hes) from a quartz vein from thecentral vein system (Lega Dembi North)

Fig. 9 Th-salinity diagram for type 3 (H2O-rich) inclusions fromquartz veins at Lega Dembi; squares: ¯uid inclusions from type 1 veinsonly; circles: ¯uid inclusions from both type 1 and 4 veins; note thebroad range of homogenization temperatures of ¯uid inclusions fromtype 1 veins showing only very small variation in the salinities,possibly indicating post-entrapment stretching of the inclusions

499

Type 4, CH4-rich inclusions are characterized by thenucleation of a gas bubble at temperatures of about)82 °C to )84 °C. They are commonly large (10±70 lm), dark in appearance, irregularly shaped, andoccur along secondary trails and along grain boundaries.Type 4 inclusion are rare and were identi®ed in type 1veins only.

Despite the extensive post-kinematic recovery of thequartz, some indication of the nature of the mineralizing¯uids can be obtained from the ¯uid inclusion data.Although gold mineralization cannot directly be asso-ciated with a particular type of ¯uid inclusion, the ob-servation that type 2 and type 4 inclusions are restrictedto the auriferous type 1 veins may provide an indicationof the composition of the mineralizing ¯uids. The pres-ence of CH4-rich and CO2-rich inclusions only in type 1veins possibly indicates the interaction between a re-gional ¯uid phase and carbonaceous matter in the sur-rounding wall rocks, following the reactions (1) C +2H2O , CH4 + 2(O), where (O) represents oxygenbound in carbon dioxide or carbonate, and (2) C +2H2O , CO2 + 4 (H), where (H) represents hydrogenbound in methane and/or phyllosilicates (Naden andShepherd 1989).

Arsenopyrite thermometry

The composition of 17 arsenopyrite grains (from eightpolished sections) coexisting with pyrrhotite (� pyrite)in quartz veins and wall-rocks, was obtained with anelectron microprobe. Most of the grains contain>1 wt.% Co + Ni and have been rejected for reasonsgiven by Kretschmar and Scott (1976). The composi-tions of arsenopyrite with less than 1 wt.% Co + Ni aredisplayed in a triangular Fe-S-As diagram (Fig. 10),together with the compositions of arsenopyrite fromPanasqueira (Portugal), Oriental mine (California) andHomestake mine (South Dakota), taken from the com-pilation of Sharp et al. (1985). At Oriental and Home-stake, arsenopyrite is associated with pyrite and

pyrrhotite, while at Panasqueira arsenopyrite occurs withpyrrhotite only. The arsenopyrite compositions fromLega Dembi fall within the overlapping ®eld of the ar-senopyrite-pyrite-pyrrhotite and arsenopyrite-pyrrhotiteassemblages. The As content of the arsenopyrites with<1% Co + Ni varies from 31.9 to 33.7 at.%. Thiscorresponds to a temperature range of 320±520 °C,when the modi®ed arsenopyrite geothermometer ofSharp et al. (1985) for arsenopyrite intergrown withpyrrhotite only is used, and to a range from 430±520 °Cassuming a ¯uid system bu�ered by pyrite and pyrrho-tite (Fig. 11).

Rb-Sr isotopes

Rb/Sr analyses were performed on six sericite concen-trates obtained from gold-quartz veins. Analytical re-sults are presented in Table 1. 87Sr/86Sr ratios rangefrom 0.7410 to 1.0915 and 87Rb/86Sr ratios vary between4.7 and 65.2.

The data are portrayed in a Rb-Sr diagram in Fig. 12.The slope of the regression line (York 1969) correspondsto an age of 484 � 67Ma, with a value for the meansquare of weighted deviates (MSWD) of 32. The poorregression cannot be the result of analytical errors, butrather most likely re¯ects non-ideal linear behavior. Thebest explanation is that the samples did not experienceisotopic homogenization even though they are geneti-cally related, or else, that the system was disturbed afterformation of the sericite. Therefore, the regression line isinterpreted to re¯ect mixing between two end-memberisotope compositions and, hence, does not satisfy thecriteria for an isochron.

However, additional information about the timing ofsericite formation can be obtained by model-age calcu-

Fig. 10 Ternary Fe-As-S diagram illustrating the composition ofarsenopyrite from Lega Dembi. Arsenopyrite data from Oriental,Homestake and Panasqueira are plotted for comparison. Furtherinformation is provided in the text

Fig. 11 Activity of S2±T projection of the stability ®eld of arsenopy-rite (from Sharp et al. 1985). The composition of arsenopyrite fromLega Dembi is also shown

500

lations based on assumed initial 87Sr/86Sr ratios (R0). Inthe present case, a value of 0.711 is obtained for R0 (cal-culated from the regression of the data), which appears tobe reasonable geologically, as it represents a typical valuefor crustal Sr. It should be borne in mind, however, thatthe regression line is not considered an isochron and,thus, the calculated initial 87Sr/86Sr ratio can at best beviewed as a rough indication of the actual R0.

Model ages can be estimated, based on the assump-tion that variations in the third decimal point of R0 havenegligible in¯uence on the model age of samples withhigh 87Sr/86Sr and 87Rb/86Sr ratios. This is the case forsamples 232 and 236 at Lega Dembi, which display themaximum and minimum deviations from the regressionline. Using the two data points for two-point calcula-tions and varying R0 within given error limits, results inage values between 410 � 20 and 545 � 40 Ma. Thistime span is considered to represent the interval duringwhich wall-rock alteration as well as gold mineralizationmight have taken place at Lega Dembi. Rb-Sr wholerock ages of 550±500 Ma for post tectonic granites in theAdola granite greenstone terrain mark the end of thePan-African tectonism (Chater 1971; Gilboy 1970), sothat an age of 545 � 40 Ma appears to be the morelikely age for the D3 syn-kinematic mineralization. Thismineralization age is also in accordance with single zir-con Pb-Pb evaporation ages of 650±550 Ma for the last

tectono-magmatic activities during the Pan-Africanevent in southern Ethiopia (Teklay et al. 1996).

Discussion and conclusions

From the structural, lithological and paragenetic datapresented earlier it can be surmised that the localizationof the Lega Dembi gold deposit appears to be the resultof a combination of structural and lithological controls.

The structural control is evidenced by the localizationof the mineralization in the intensely sheared meta-sed-iments, con®ned to within about 150 m of the tectoniccontact between the greenstone terrane of the Megadobelt and the underlying gneissic terrane. This shearingforms the northernmost extension of the regional-scaleLega-Dembi-A¯ata shear zone (Worku and Yifa 1989).The change of strike of the contact between the green-stone succession and the basement gneisses to morenorthwesterly trends to the immediate north of LegaDembi (Fig. 2), indicates that Lega Dembi is located in adilational segment of the shear zone (see also Worku1993). The presence of the quartz-veining, and thus thelocus of ¯uid ¯ow, is largely restricted to the meta-sed-iments, which most likely re¯ects strain partitioning intothe incompetent meta-sediments. The latter are sand-wiched between competent, largely impermeable base-ment gneisses in the footwall and massive meta-gabbrosin the hanging wall to the immediate west (Fig. 6). Pe-riodic supralithostatic pressures during quartz-vein for-mation are indicated by multiple fracture eventsrecorded by the quartz veins, and by the presence ofhydraulic quartz breccias that are associated with themain mineralization (i.e. type 3 veins, Fig. 5). Fluid fo-cusing in the meta-sediments is also indicated by thesystematic increase of gold values from, on average, 3±5 ppm Au at Southern and Central Lega Dembi, wherethe meta-sediments attain their maximum thickness, togold values ³10 ppm at Upper Lega Dembi, where thethickness of the metasediments is reduced to about 20m.This suggests a more pervasive ¯uid ¯ow in the centralparts of the deposit and a more channelized ¯uid ¯ow inthe north.

The presence of both deformed and undeformedgold-quartz veins that occur parallel to the S2/S3transposition fabric, is consistent with a predominantlysyn- and, to a lesser extent, late-D3 timing of the min-eralization. Isoclinally folded and intensely sheared

Table 1 Rb-Sr data from six sericite concentrates from hydrothermally altered wall rocks adjacent to gold-quartz veins at Lega Dembi.

Sample Rb (ppm) Sr (ppm) 87Rb/86Sr 87Sr/86Sr

AT 46 174.58 � 1.03 62.111 � 0.36 8.182 � 0.025 0.76984 � 0.0000550 132.24 � 1.19 81.915 � 0.74 4.688 � 0.014 0.74458 � 0.00005AY 56 106.16 � 0.92 13.278 � 0.12 23.488 � 0.0072 0.86462 � 0.00005231 117.63 � 3.60 66.417 � 2.03 5.141 � 0.022 0.74105 � 0.00005232 127.45 � 1.08 12.616 � 0.11 29.902 � 0.126 0.94307 � 0.00005236 182.37 � 1.48 8.399 � 0.07 65.181 � 0.299 1.09150 � 0.00005

Fig. 12 Rb-Sr isotope plot of sericite concentrates from hydrother-mally altered wall-rock at Lega Dembi. Data are also presented inTable 1. Further information is given in the text

501

quartz veins (type 1) and S2/S3 foliation-parallel, de-formed sul®des indicate a relatively early timing of thequartz-sul®de mineralization during the D3 transpress-ional deformation. A syn-D3 timing of the gold miner-alization is also supported by the alteration mineralassemblage (i.e. biotite-muscovite-amphibole-actinolite,locally tourmaline) which accompanies the gold-sul®demineralization, also de®ning the S2/S3 foliation andmineral lineation. The undeformed geometries of theoblique extensional (type 4) veins imply that this type ofquartz-veining occurred during the waning stages of theD3 deformation or even thereafter. Some tremolite-ac-tinolite (+biotite) alteration that overgrows the S2/S3foliation supports a late-to-post D3 timing for the type 4quartz vein mineralization. The absence of gold in type 4veins and the folding of sul®des in crenulations of S2/S3provide an upper time constraint for the gold mineral-ization at Lega Dembi. Rb-Sr data of 545 � 40 Ma,although poorly constrained, appear to con®rm the syn-to-late Pan-African timing of the mineralization andhence the timing for the waning stages of the regionaltectonism in the Adola granite-greenstone terrain.

The lithologic control of gold mineralization at LegaDembi is manifested by the almost exclusive siting of themineralization in the graphitic meta-sediments. Al-though the ultrama®c talc schists that envelope themeta-sediments contain quartz veins, gold mineraliza-tion in these veins is rare or absent. The signi®cance ofthe graphite-bearing meta-sediments for the localizationof the gold mineralization is also expressed on a regionalscale, by the occurrence of other primary gold depositsin the area, such as the Sakaro prospect, some 4 km tothe southwest of Lega Dembi (Fig. 2). Here, gold min-eralization is hosted in an en-echelon quartz-vein systemthat is developed in graphitic metapelites parallel to thecontact between massive amphibolites and the meta-sediments, the latter being correlated stratigraphicallywith the meta-sediments of the Lega Dembi area.

Both experimental and ®eld studies have indicated thatthe reduction of a ¯uid from an initial redox state down tothe stability of graphite, decreases gold stabilities in termsof both gold-sul®de and gold-chloride complexes over awide range of temperatures (e.g. Naden and Shepherd1989; Ohmoto and Kerrick; 1977, Poulsen and Ohmoto1989), which explains the close relationship between gold-rich quartz veins and the graphitic rocks.

The volcanosedimentary rocks of the Megado beltare characterized by upper-greenschist to lower-am-phibolite facies metamorphism (Gilboy 1970; Chater1971; Beraki et al. 1989; Ghebreab 1989). The temper-atures determined from arsenopyrite analyses indicatesimilar temperatures for the mineralization. The pre-dominance of biotite-muscovite-actinolite alteration andthe paucity of chlorite also suggest that alteration tookplace at temperatures indicative of upper-greenschist tolower-amphibolite facies conditions (Mikucki et al.1990). This is also supported by the presence of kyanitealong the margins of laminated quartz veins parallel tothe mineral-stretching lineation. These observations

imply that gold mineralization occurred synchronouslywith M2 metamorphism (i.e. at upper-greenschist tolower-amphibolite facies), during D3 sinistral strike-slipshearing (see also Worku 1993).

The geologic setting, structural controls and pro-gressive development of the gold mineralization at LegaDembi share numerous regional and deposit-scale char-acteristics of Archaean lode-gold deposits (e.g. Colvine1989; Groves et al. 1989; Groves 1993; McCuaig et al.1993; Kerrich and Cassidy 1994), most notably thestructural control of the gold mineralization at LegaDembi in a second-order structure of a regional-scaletranscurrent shear zone (i.e. in a dilational jog of theLega Dembi-A¯ata shear zone), in an overall trans-pressional geodynamic environment. Aditionally, thelocation of the gold mineralization along a terraneboundary, i.e. along the contact between the supracrustalterrane of the Megado belt and the basement gneisses,and the siting of the gold mineralization in both rheo-logically and chemically favorable lithologies (i.e. ®nelylaminated, well-foliated, graphite-rich meta-sediments)are analogous to Archaean gold-lode deposits. Further-more, the siting of the gold mineralization in syn- to latekinematic quartz veins in a brittle-ductile environmentduring transiently supralithostatic ¯uid pressure cycling,and the syn- to late peak metamorphic timing of the goldmineralization, as well as the predominant quartz-, car-bonate-, mica-, actinolite/tremolite-, chlorite-, and tour-maline alteration mineral assemblages (which indicateupper-greenschist to lower-amphibolite facies metamor-phic conditions of the mineralization), are all comparableto Archaean gold-lode deposits.

Hence, the Lega Dembi deposit illustrates that thestyles of mineralization and regional-scale tectonics thatare commonly discussed for Archaean-type lode goldmineralizations may well be found in younger orogenicbelts. This suggestion is of major importance for goldexploration within the widespread Pan-African terranes.

Acknowledgements This work derives in part from a Ph.D projectby A.Y. Billay funded by the German Academic Exchange Service(Deutscher Akademischer Austauschdienst). The authors thank theEthiopian Mineral Resources Development Corporation (EM-RDC) for providing access to the mine and documents. Appreci-ation is extended to the geologists in the Adola Mineral Evaluationand Development Project and Lega Dembi and Sakaro PrimaryGold Study and Development, Lega Dembi Gold Mine and Sha-kisso Central Laboratory of the EMRDC, including Dr. Z. Desta,T. Tadesse, Z. Asres, A. Getahun, M. Yimenu and A. Zewdu, forinteresting discussions and for access to their properties and vehiclesupport. A. Legesse is thanked for his cooperation and materialsupport in the ®eld. Constructive reviews by L. Robb, W.U. Rei-mold and an anonymous reviewer are greatly appreciated.

References

Amenti A, Hassen N, Yemane T, Genzebu W, Seyid G, Mehari K,Alemu T (1992) Geological evolution of the Proterozoic ofsouthern Ethiopia. 29th Int Geol Congress Kyoto, Japan,Abstracts vol 2

502

Anhaeusser CR (1976) The nature and distribution of Archaeangold mineralization in Southern Africa. Mineral Sci Eng 8: 46±84

Ayalew L (1990) Gold mineralization in East Sakaro: an in-vestigation of a drill hole, Adola gold district, southern Ethio-pia. Ethiopian Institute of Geological Surveys, Addis Ababa

Beraki W (1995) Structural geology and geochemistry of theNeoproterozoic Adobha and Adola Belts (Eritrea and Ethio-pia). PhD thesis, University of Giessen, Germany 218 pp

Beraki WH, Bonavia FF, Getachew T, Schmerold R, Tarekegn T(1989) The Adola fold and thrust belt, Southern Ethiopia: a re-examination with implications for Pan-African evolution. GeolMag 126: 647±657

Bisrat Y (1993) The geochemistry of the meta-basic rocks of theAdola volcano-sedimentary-ultrama®c assemblages: evidencefor supra-subduction (SSZ) ophiolitic sequence of Adola,Southern Ethiopia. Rep EIGS, Addis Ababa, 18 pp

Chater A (1971) The geology of the Megado region of SouthernEthiopia. Unpubl PhD Thesis Leeds University, UK 193 pp

Colvine AC (1989) An empirical model for the formation ofArchean gold deposits: products of ®nal cratonization of theSuperior Province, Canada. Econ Geol Monogr: 37±53

Colvine AC, Andrews AJ, Cherry ME, Durocher ME, Fyon AJ,Lavigne MJ, Macdonald AJ, Marmot S, Poulsen KH, SpringerJS, Troop DG (1984) An integrated model for the origin ofArchean lode gold deposits. Ontario Geol Surv, Open-File Rep5524: 98 pp

Colvine AC, Fyon JA, Heather KB, Marmont S, Smith PM, TroopDG (1988) Archean lode gold deposits in Ontario. OntarioGeol Surv Misc Pap 139: 136 pp

De Ronde CEJ, Spooner ETC, de Wit MJ (1992) Shear-zone re-lated, Au quartz vein deposits in the Barberton GreenstoneBelt, South Africa: ®eld and petrographic characteristics, ¯uidproperties, and light stable isotope geochemistry. Econ Geol 87:366±402

De Wit MJ, Chewaka S (1981) Plate tectonic evolution of Ethiopiaand its mineral deposits: an overview. In: Chewaka S, de WitMJ, (eds.) Plate tectonics and metallogenesis: some guide linesto Ethiopian mineral deposits. Ethiopian Inst Geol Surv Bull 2:115±129

De Wit MJ, Ashwal LD (1995) Greenstone belts: what are they?S Afr J Geol 98: 505±520

Emelyanov EL, Pashkov IA, Abebaw T, Teweldemedhin T (1987)Preliminary exploration of Northern Lega Dembi (1985±1986),vol I Report, Ethiopian Mineral Resources Development Cor-poration (EMRDC), Addis Ababa, Ethiopia

Fiori M, Garbarino C, Grill S, Solomon T, Valera R (1987) Theprimary gold deposit of Lega Dembi (Sidamo): ore mineralassociation and genetic signi®cance (Preliminary report). EM-RDC, Addis Ababa

Foster RP (1989) Archean gold mineralization in Zimbabwe: im-plications for metallogenesis and exploration. Econ GeolMonogr 6: 54±70

Gass IG (1977) The Evolution of the Pan-African crystalinebasement in northeast Africa and Arabia. J Geol Soc London134: 129±138

Getaneh W (1994) Sul®de mineralization in Lega Dembi primarygold deposit, Sidamo. MSc Thesis, Addis Ababa University,Addis Ababa

Ghebreab W (1989) Geological evolution of the Adola Precam-brian greenstone belt, Southern Ethiopia. Report, EIGS, AddisAbaba

Ghebreab W, Yohannes E, Giorgis LW (1992) The Lega Dembigold mine: an example of shear zone-hosted mineralization inthe Adola greenstone belt, Southern Ethiopia. J Afric Earth Sci15: 489±500

Ghenzebu WG, Hassen N, Yemane T (1994) Geology of theAgremaryam area. Ethiopian Institute of Geological Surveys,Addis Ababa

Gilboy CF (1970) The geology of the Garibero region of SouthernEthiopia. Unpubl PhD Thesis, Leeds University, UK, 176 pp

Gichile S, Fyson WK (1993) An inference of the tectonic setting ofthe Adola belt of Southern Ethiopia from the geochemistry ofmagmatic rocks. J Afric Earth Sci 16: 235±246

Groves DI (1993) The crustal continuum model for late Archeanlode-gold deposits of the Yilgarn Block, Western Australia.Mineral Deposita 28: 366±374

Groves DI, Foster RP (1991) Archaean lode gold deposits. In:Foster RP (ed) Gold metallogeny and exploration, Blackie,Glasgow, pp 63±96

Groves DI, Barley ME, HoS (1989) Nature, genesis and tectonicsetting of mesothermal gold mineralization in the YilgarnBlock, Western Australia. Econ Geol Monogr 6: 71±85

Kazmin V (1976) Ophiolites of the basement rocks of Ethiopia,Note 35. Ministry of Mines and Energy, Addis Ababa

Kazmin V Alemu S, Tilahun B (1978) The Ethiopian basement:Stratigraphy and possible manner of evolution. Geol Rundsch,67: 531±546

Kerrich R (1986). Fluid in®ltration into fault zones: chemical iso-topic and mechanical e�ects. J Pure Appl Geophys 124: 225±268

Kerrich R Cassidy KF (1994) Temporal relationships of lode goldmineralization to accretion, magmatism, metamorphism anddeformation-Archean to present: a review. Ore Geol Rev 9:263±310

Kozyrev V, Girma K, Safonov J, Bekele WM, Tuliankin V, Best-ujev A, Baijapov A, Teweldemedhin T, Gurbanivich G, Kai-tukov M, Arijapov A (1985) Regional geological andexploration work for gold and other minerals in the Adolagold®elds. Report, EMRDC, Addis Ababa

Kretschmar U, Scott, SD (1976) Phase relations involving ar-senopyrite in the system Fe-As-S and their application. CanMineral 14: 364±386

McCuaig TC, Kerrich R, Groves DI, Archer N (1993) The natureand dimensions of regional and local gold-related hydrothermalalteration in tholeiitic metabasalts in the Norseman gold®elds:the missing link in a crustal continuum of gold deposits? Min-eralium Deposita 28: 420±435

Mikucki EJ, Groves DI, Cassidy KF (1990) Wallrock alteration insubamphibolite facies gold deposits. In Ho, SE, Groves, DI andBennett JM (eds) Gold deposits of the Archean Yilgarn Block,Western Australia: nature, genesis and exploration. GeologyDepartment University extention, the University of WesternAustralia, Publi 20: 60±78

Morin D, Oliver P (1986) Lega Dembi primary deposit ore pro-cessing study. EMRDC, Addis Ababa

Moudrov AT, Dougalevski VV, Zaitsev NM, Nikulin AI (1991)Brief geological report on the results of exploration of the LegaDembi gold deposit and adjacent localities. Report, EMRDC,Addis Ababa

Naden J Shepherd TJ (1989) Role of methane and carbon dioxidein gold deposition. Nature 342: 793±795

Ohmoto H, Kerrick D (1977) Devolatilization equilibria in gra-phitic systems. Am J of Sci 277: 1013±1044

Poulson SR, Ohmoto H (1989) Devolatilization equilibria ingraphite-pyrite-pyrrhotite-bearing pelites with application tomagma-pelite interaction. Contrib Mineral Petrol 101: 418±425

Robert F, Brown AC (1986) Archean gold-bearing quartz veins atthe Sigma Mine, Abitibi greenstone belt, Quebec: Part I. Geo-logic relations and formation of the vein system. Econ Geol 81:578±592

Sharp ZD, Essene EJ, Kelley WR (1985) A re-examination of theArsenopyrite geothermometer: pressure considerations andapplications to natural assemblages. Can Mineral 23: 517±534

Sutton-Pratt, A (1996) Gold mining in Africa-the ®nal frontier?Trans Inst Mining Metall Sect B: 3±12

Tadesse T (1990) Geochemistry of vein gold mineralization at LegaDembi, Ethiopia. MSc Thesis, Leeds University, UK, 136 pp

Teklay M, KroÈ ner A, Mezger K, OberhaÈ nsli R (1996) Geochem-istry, Pb-Pb single zircon ages and Nd-Sr isotope ratios ofPrecambrian rocks from southern and eastern Ethiopia: Im-plications for crustal evolution in East Africa. Confe Abstra vol1, (1), 1996 V.M. Goldschmidt Conference, Heidelberg, 617 pp

503

Vail JR (1976) Outline of the geochronology and tectonic units ofthe basement complex of northeast Africa. Proc R Soc London,A350: 127±141

Vail JR (1983) Pan-African crustal accretion in north-east Africa.J Afric Earth Sci, 1: 285±294

V/O Technoexport (1986) Results of the laboratory technologicalstudies of gold bearing ores from Lega Dembi deposit in theSocialist Ethiopia. Report, USSR, Ministry of Geology, Zar-ubezhgeologia, Moscow

Woodall R (1988) Gold in 1988. In: Bicentennial Gold 88, Ext AbsOral Programme, Geol Soc Aust Inc Abstr 22, pp 1±12

Worku H (1993) Implication of the structural evolution of theAdola belt on shear zone-hosted gold mineralization. In:

Thorweihe U, Schandelmeier H (eds) Proc int conf on geosci-enti®c research in Northeast Africa, Berlin/Germany 17±19June, pp 139±144

Worku H, Yifa K (1989) The tectonic evolution of the Precambrianmetamorphic rocks of the Adola belt (Southern Ethiopia) andits implication on gold mineralization.Report, EIGS, AddisAbaba, 56 pp

Worku H, Schandelmeier H (1996) Tectonic evolution of theNeoproterozoic Adola Belt of southern Ethiopia: evidence for aWilson Cycle process and implications for oblique plate colli-sion. Precamb. Res 77: 179±210

York D (1969) Least-square ®tting of a straight line with correlatederrors. Earth Planet Sci Lett 5: 320±324

504