oberthur 1996 gold mineralization in the ashanti belt of ghana

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Economic Geology Vol.91, 1996, 289-301 pp

Gold Mineralization

in the Ashanti Belt of Ghana:

Genetic Constraints the Stable of Isotope GeochemistryTHOMAS OBERTHOR,

Bundesanstah Geowissenschaften fiir undItohstoffe, Stilleweg D-30655 2, Hannover, GermanyANDREAS SCHMIDT MUMM,

Institut Geologische fiir ]Vissenschaften Geiseltalmuseum, und Domstrafle D-06108 5, Halle(Saale), GermanyULRICH VETTER,

Bundesanstah Geowissenschaften fo'r undItohstoffe, Stilleweg D-30655 2, Hannover, GermanyKLAUS SIMON,

Geochemisches der Universitiit Institut GSttingen, Goldschmidtstrasse 1, D-37077 GSttingen, Germany

ANDJOEA. AMANORAshanti Goldfields Company Ltd., Obuasi, GhanaAbstract

The Ashanti of Ghana thekeydistrict goldmineralization thePaleoproterozoic belt is of in terrane West of Africa. area The consideredsouthwest in Ghana covered lithologies thevolcanic-sedimentary is by of Birimian Supergroup theoverlying and elastic sedimentary Tarkwaian Group which werejointly folded metamorand phosed undergreenschist facies conditions during Eburnean the teetonothermal at about9..1Ga. event Regional fotiation subparallel zones md shear hosting mesothermal mineralization gold developed during deformation coeval withmetamorphism. Fourmajor tsoes primary mineralization present theAshanti (1)mesothermal, of gold are in belt: generally steeply dipping quartzveinsin shearzones mainlyin Birimiansedimentary rocks,(9,)sulfide oreswith auriferous arsenopyrite pyrite, and spatialty dosely associated thequartz with veins, sulfide (3) disseminations andstockworksgranitoids, (4) paleoplaeerstheTarkwaian in and of Group. This study concentratestypes and(9,) thehydrotherlnal mineralization. isotope on (1) of gold Stable analyses of host-rock orecomponents performed theailnof obtaining and were with parameters relevant theorigin to andevolution the fluids of thatproduced mineralization. gold

Carbonaeeous in theBirimian matter metasediments displays values 93C ranging from-11.4 to -9,8.3 permilrelative PDB,indicating organogenie Carbonates a unimodal to an origin. display distribution9aC ofvalues ranging from -9.9 to -17.0 per rail relative PDB. COz extracted to fromfluidinclusions the in

auriferous veins 93C quartz has values ranging -9.5 to -15.7 permilrelative PDB.It isproposed from tothatthese carbon isotope compositions of carbonates COzreflect and extensive interaction theCO,2-rieh of hydrothermal with reduced fluids carbon Birimian in sediments the deeper in partsof the hydrothermalsystems.

Carbonates auriferous quartz 6So and vein have values ranging 19,.9 9,9,.9, 19,.8 15.6 mil from to and to per relative SMOW,respectively. to Carbonates quartz and weredeposited nearisotopic in equilibrium with respect 6So, to indicating fluid-dominated conditions during formation, fluids metamorphic ore from of or magmatie origin. Such origin corroborated SD an is by values waterextracted of fromfluidinclusions vein in quartz (-37 to -53% relative SMOXV). to Pyrite synsedimentary-diagenetie Birimian of origin in schists displays sulfur isotope compositions ranging from+7.3 to -9,0.9perrail(median -10% relative CDT). Similar ca. to compositions wideranges and are usually attributed sulfide to generation bacterial by sulfate reduction seawater. from Arsenopyrite eogenetie and pyrite fromthesulfide generally 534S ores have values therange in -5.3 to -10.9,perrailrelative CDT.Thetight to unimodal distribution534S indicateslarge, of values a homogeneous

fluidreservoir. low(534S The values interpreted source-inherited, are as notrelated unusual Eh, to pH,temperature, depositional or conditions. Sulfides Birimian in sediments represent mostlikelysulfur the reservoir tapped the fluidsystems. by TheC, O, H, andSisotope compositions of ore-related hydrothermal minerals fluid and inclusion components indicate the mineralizing interacted that fluids extensively the Paleoproterozoie especially with rocks, Birimian sediments, deeper at crustal levels at hightemperatures. isotopic and The compositions most are compatible the formation fluids with of fromdevolatilization reactions invoMng Birimian strata during prograde metamorphism at depth (metamorphic fluids).

0361-0128/96/1815/289-1355.00

289

290

OBERTHOR ET AL.

tationagesof 2132 _+3 and 2135 __+ Ma, respectively, 5 whereas Birimian volcanics yielded Sm/Nd of 2166+_ an age GOLDmineralization constitutes important an economic fac- 66 Ma (Taylor al., 1992;DaSs al., 1994). et et tor in the Paleoproterozoic Birimian terratie WestAfrica. Thesupracrustal of sequence folded metamorphosed was and The largest mostprominent and mines, with a cumulativeundergreenschist facies conditions duringthe ca. 2.1 Ga past production excess 1,500 in of metric tons of thenoble Eburnean tectonothermalevent (Leube et al., 1990; Hirdes (t) metal, located Ghana. are in Among these, Ashanti the Gold- et al., 1992; Taylor al., 1992). et Structural investigations refields mineat Obuasi a world-class deposit, is gold having vealed that Birimian and Tarkwaian rocks were deformed produced more than800t of goldhistorically, anoutput jointlyduring single with a progressive event.Northwest-southof 26,551kg in 1994. east-directed crnstal shortening produced major thrusts and Lightstable isotope studies gained have increasing impor- shears, commonly closeto the basin-belt contacts, which tancein the description interpretation geologic and of pro- actedas channelways mineralizing for hydrothermal fluids eesses. Thesestudies nowadays form integral partsin the (Eisenlohr, 1989; Blenkinsop al., 1994).Goldmineralizaet formulation metallogenetie of models golddeposits for and tionin theAshanti is largely belt synkinematic synmetaand wereintensely employed mesothermal for Archcan (e.g., Ker- morphic, coeval i.e., withthe Eburnean event(Oberthiir et rich,1987; Colvine al., 1988; et Colvine, 1989; Golding al., al.,1994). intrusion twomajor et The of distinct suites granitof 1989;de Rondeet al., 1992) and epithermal Phanerozoicoids, Dixcove- belt-type the or granitoids which occur within (e.g., Rye, 1993)gold deposits alsocontributions the volcanic (see in belts,and the late kinematic CapeCoast-or Barnes, 1979, and Ohmoto, 1986). In contrast,little work basin-type granitoids thesedimentary in basins 1),took (Fig. has been done in the Proterozoic terrane of West Africa. placeat different time intervals between 2180 to 2170 ca. Exceptions studies the stable are on isotope compositions Ma and2116to 2088Ma, respectively of (Hirdes al., 1992). et carbon Birimian in argillites cherts and fromGhana (Leube Gold Mineralization et al.,1990), a stable and isotope study thetourmalinized on sandstones goldores and fromLoulo Mall (Fouillac al., in et The deposits investigated this studyare situated a in in 1993). northeast-southwest-trending whichstretches goldbelt for Thepresent work comprises firstcomprehensive the stable about km fromsouth Prestea Konongo thenorth 250 of to in isotope study hostrocks on andore components the (Fig.1).Thefoilroving major from four types primary minof gold Ashanti goldbelt in the Birimian terrane Ghana. of Special eralization present: generally are (1) steeply dipping quartz emphasisplaced a distinction is on between original, synsedi- veins shear in zones mainly Birimian in sedimentary rocks, mentary-diagenetic isotopic signatures those to hy- withfree-milling andfluidinclusionsveins and due gold in dominated drothermal inputandoverprint relevant goldmineraliza- by CO.(Fig. 2C and D); (2) sulfide to ores,with auriferous tion.The study based samples is on collected undergroundarsenopyritepyrite, + closely associated thequartz with veins at the Ashanti Prestea and mines, in the openpitsof (Fig.2B); (3) sulfide and disseminations stockworks and/or with Bogosu Konongo. and Drill corewasmadeavailable from freegold, arsenopyrite, pyrite granites Ayanfuri); and in (e.g., the Ayanfuri concession fromObenemase and northof Ko- and(4) palcoplacers the Tarkwaian of Group. nongo. samples theoldBokitsi The from minenearAyanfuri Thisstudy mainly deals withthefirsttwotypes mineralof werecollected fromminedumps. ization, which weredescribed Junner by (1932,1935,1940), Cooper (1934), Hirst(1941). and These authors stressed the Regional Geology structural control the epigenetic of mineralization their and The relative timing volcanism, of sedimentation, emplace- location in Lower Birimian strata close to the contacts with or rocks. Epigenetic quartz vein mentof various granitoid suites, tectonism the vast UpperBirimian Tarkwaian and in Birimian terrane of West Africa is still controversial and beandsulfide mineralization alsofavored laterstudies was by yondthe scope thispaper. of The following description fol- of Eisenlohr (1989), Hirdes and Leube (1989), Milesi et al.

Introduction

lows concepts recently developedtheGhanaian of for sectorthe terrane (Leube and Hirdes, 1986; Eisenlohr,1989; Leube et al., 1990; Eisenlohrand Hirdes, 1992; Hirdes et al., 1992;

(1989, 1991, 1992), Leube et al. (1990), and Oberthiir et

al. (1991,1994).In contrast, Ntiamoah-Agyakwa (1979)put

forward syngenetic-metamorphic byproposing a concept that

Taylor al.,1992; et Blenkinsop al.,1994; et Davis al.,1994). goldandsulfides volcanic-exhalative wereinitially et of origin in sediments subsequently and redisLarge areas southern of Ghana covered Paleopro- deposited theBirimian are by and to by Simiterozoie lithologies. supracrustal are subdividedtributed concentrated ores metamorphism. The rocks et postulated at least that some the of into the volcanic-sedimentary Birimian Supergroup the larly,Leube al. (1990) and sulfide lodeoresaresyngenetic. overlying elastic sedimentary Tarkwaian Group. classicaldisseminated The subdivision the Birimian into Lower Birimian (voleanielasof The Ashanti mine(Junner, 1932;Hirdes,1989;Leubeet ties, waekes, argillites, chemical sediments) UpperBiri- al., 1990;Oberthiiret al., 1991, 1994)can serveas a type and for of studied becauseillustrates it mian(basalts some with interflow sediments),proposed example most thedeposits as by between hostrocks ores. and Junner (1935,1940), reinterpreted Leube Hirdes the interrelationships was by and mine, gold mineralization ishosted tightly in (1986)and Leubeet al. (1990).The latter authors regard AttheAshanti Lowerand UpperBirimian a coeval as sequence the folded, with steeply dipping northeast-southwest-striking and BiriThe facies comprise sedimentary-voleanielastie assemblage (sedimentary basins) mianmetasediments. greenschist rocks representingdistal a facies volcanic of belts. Detritalzircons alternating argillites (muscovite schists, muscovite-chlorite fromBirimian Tarkwaian and strata gave maximum sedimen- schists carbonate-spotted and schists, variably all graphitic)

ASHANTI BELT, GHANA,Au MINEBALIZATION

291

VOLTAlAN TOGO SERIES TARKWAIANBIRIMIAN

basintype

granitoidsbelttype

belts [ volcanic basins sedimentary granitoids

i

SunyaniO

__

DAHOMEYAN

+

++ + ++

(,O''

+

+

++

++ + + + +

+++

+++

+ ++++

++ ++

++

+

++

++ +

++

++

+

+

+

++ +

++ +

++ +

+Wnneba

++

+

+

GUL

S,ko.d,G 3\ tI

Cape Coast

I I

0 2340 60Kin

Fc,.1. Geolo' of southern Ghana afterLeubeandHirdes(1986),modified afterHirdesandLoh (pers. commun., 1995).Alsoshoxm majorgoldmines: = Ayanfuri Bokitsi, = Bogosu, = Konongo, = Ashanti are A and B K O mineatObuasi, P = Prestea.

relative proportions variable the different are in mine andfeldspathie metasandstones. Irregular boudins inter- whose and layers "metavoleanie""dyke" of or rocks mineterminol- sections: (in ogy) locally are present. These rocks, hoxvever, generally are 1. Quartz veins, locally spectacular with showingsvisible of intensely metasomatizedearbonatized have or and thus been gold minor and Pb-Sb-Cu sulfides (galena, bournonite, tetrainterpreted represent to earbonatized metasediments (Oberhealrite; 2C). Single, Fig. massive laminated or veins, to 0.2 thiir et al., 1994).Thisinterpretation be extended may to reaching m in width,or multiple 25 similar units Prestea rock at (Adjimah, 1988). Possibly concor- 5 m widebut locally quartz veins withintercalated, sheared, commonly and sulfiddant Birimianmetavoleanies unmetamorphosed and dikes izedwallrocks present. are Selvages theformof intensely in (dolerites), however, also are present subordinate in amounts sheared carbonaceous schists are common. Wall-rock alterat the Ashanti mine.

Goldmineralization is exposed mine in workings about for ores, withrefractory hosted arsenopyrite gold in 8 km along strike down ca. 1,650m below and to surface. 2. Sulfide asthemainoremineral (Fig.2B).Thismineralization forms The orezoneconsists several of steep orebodies subparallel eitherenvelopes, to somemeters up wide, around quartz to the regional strike, whichmeasure to some up hundred veins, disseminationsmetasediments "dyke" or in or rocks. meters horizontally vertically attainthicknesses and and of Oreminerals besides arsenopyrite include pyrite, pyrrhotite, upto 50 m.Theorezone further is characterized intensive by andmarcasite, rarechalcopyrite sphalerite. and and shearing, pronounced sulfide mineralizationcountry of rocks, multiple massive to quartz veining, thecommon and presence Petrographic structural and investigations revealed have are synmetamorphic synkiand ofearbonaeeous Twodistinct types recognizedthatbothore types largely schists. ore are

ation takes the form of sulfidization and carbonatization.

292

OBERTHORETAL

bou

tet

' '

.

sph

cpy

C

FG.2. A. Carbonate spotted schist consisting augen monocrystalline siderite a fine-grained of of Mg in matrix of quartz, sericite, carbonaceous and matter. Transmitted onepolarizer, light, horizontal width= 1.2cm.Ashanti mine.B. Well-developed crystals arsenopyrite of (white) minor and hypidiomorphic (white) sulfide fromPrestea. pyrite in ore Note abundant carbonates (lightest gray). Reflected in air,onepolarizer, light, horizontal width= 1.4min.C. Sulfide paragenesis in the goldquartz veins the Ashanti of mine.Bournonite (bou), chalcopyrite galena (cpy), (ga),gold(white), sphalerite (sph), tetrahedrite Gangue quartz. and (tet). is Reflected oilimmersion, polarizer, light, one horizontal width= 700tim. D. Primary gaseous +__ +__ inclusions quartz, CO N CH4 in Ashanti mine. Transmitted horizontal light, width= 250tim.

nematic and were deposited from hydrothermal solutions relative arsenopyrite at Prestea, to occur Bogosu, Bokitsi and during single one progressive deformational event, thePT (mean in about 50/50)compared Ashanti Konongo to and (mean range 2 to 5 kbars 400 _ 50C(Blenkinsop al., about 15/85). of and et 1994; Oberthiir et al., 1994). Mineralization the Ayanfuri at concession comprises imThegeneral geologic setting, types ore,mineralogy, pregnations arsenopyrite freegoldin granitic of and of and stocks

Analytical Methods aremainly hosted Birimian by metasediments, to conclose Carbon isotope compositions of carbonaceous matter, and tacts with Birimian volcanics or Tarkwaian sediments to the carbon oxygen and isotope compositionscarbonates of were east, theorebodies steeply trend (2) dip and north-northeast-determined whole-rock on samples. carbonates veinFor in south-southwest to northeast-southwest, i.e., subparallel lets,handpicked to concentrates analyzed. were Oxygen isotope theregional strike thesedimentary of lithologies, gold (3) has compositions quartzfromthe auriferous of veins wereoba bimodal distribution quartz in veins sulfide and ores, and tainedfromcoarse, cleanveinquartz.Sulfide concentrates (4) fluidinclusions the quartz of veins unusual CO.2 wereproduced sulfide samples pyrite schists are with from ore and in asthedominant phase (Schmidt Mumm al.,1996, press). by crushing the samples subsequent et in of and mechanical conDifferences a higher are metamorphic at Konongo centration grade (panning). (biotite, garnet; Hirst, 1941),whereas othermines the are Isotopic 18 anyses wereperformed GCA Lehrte,Gerby 13 located greenschist rocks; in facies larger proportions ofpyrite many C, 6 O; carbonaceous (6 matterandcarbonates) andsimilarto thoseat the Ashantimine: (1) the mineralizations

thephysicochemical conditions oredeposition theKo- which of at nongo, Bokitsi, Bogosu, Prestea and mines remarkably are

were intruded into Birimian metasediments.

ASHANTI BELT,GHANA, MINERALIZATION Au

293

values quartz of weredetermined G6ttingen at University by reaction with C1F3, liberated the oxygen converted was to Sample Rock Locality type (93C CO2by reduction graphite 600C;/D on at values water no. of extracted quartz from (grain 6-12 ram) size were determinedGH 042 afterdegassing adsorbed water 140C 24 h in vacumn. GH 101 Ashanti sps -24.5 at for Ashanti carb -26.6 carb Thequartz subsequently was deerepirated at800C, liber- GH 107 Ashanti the atedwaterwasreduced H2 by reaction to xvith (800C) GH 111 Ashanti sps -25.8 hot sps -24.9 uranium, and immediately analyzed D and H isotope GH 142 Ashanti cs for GH 199 Ashanti -25.3 compositions. Typical analytical errors themethod +_2 Glt 209 Ashanti sps -22.7 of are perrail.COzfroinfluidinclusions extracted mechani- GH 338 Ashanti vas by cs -25.7 cs -25.2 calcrushing quartz roomtemperature of at followed its GH 346 Ashanti by

1. Isotope Compositions of Carbonaceous (c.m.) Matter Geochron Cambridge, Labs, United States (834S), stan- TABLE Carbon using (earb), Oxygen Isotope Compositions of Carbonates, Total dardmethods. Analytical reproducibility +0.2 per rail andCarbonates ....), andCarbonate was Organic Carbon (C Carbon (Cc.,rb) Contents Rock of foroxygen carbon + 0.4perrailforsulfur. and and The Samples theAshanti in Ghana from BeltC....-11.3-12.4 -9.9

C.b

16.014.9 15.3

-12.9 -12.6-14.6

16.3 18.815.9

0.41 0.582.20

1.98 1.501.29

-15.6-14.2 -15.0 -15.4-12.3 -10.9 - 13.0

15.615.3 15.1 21.212.9 14.6 15.3

0.631.43 2.01 0.45

2.161.29 1.23 0.19

separation H20bycryogenic from methods. The/93C the ofobtained CO.2 wasmeasured directly. Goldcontents were analyzed instrumental by neutron activation analysis (INAA,ACTLABS, Canada).

GH GH GH GH

368 380 382 384

AshantiAshanti Ashanti Ashanti

smv mv cs

-25.6-12.7 -11.4 -24.6

Isotopic compositionsdenoted follow: ...... are as 1513C carbonaceous in permilrelative PDB; matter, to /5t3C = GH GH carbonate carbon, per rail relative PDB;/9SO = GH in to carbonate oxygen,perrailrelative SMOW, in to a4s: sulfide GHcal'[)

GH 405 GH 510604 634 664 684

sulfur, perrailrelative CDT, and/D= water in to hydrogen,GH in perrailrelative SMOW. to Results presentedTables GH are in1, 2, and3.GH

701 709 710

GH 711

Ashanti Ashanti Ashanti Ashanti Ashanti Ashanti Ashanti Ashanti Ashanti Ashanti

s cs s sst sst sst v s s cscs cs* cs

- 15.5 -23.8 -15.7 -16.3 -17.9 -15.7 -20.1 -16.4 -21.7-28.3 -25.1 - 25.1

- 13.1 - 13.0 -14.8 -14.4 -13.7 -14.3 - 13.5 -17.0 -15.5

14.5 22.2 14.5 16.2 16.4 16.2 14.1 13.5 14.3

5.03 0.69

0.62 3.58

0.23 0.37 0.33 0.62 0.62 4.163.64

1.54 0.49 0.91 0.21 1.25 1.75 0.690.62

Results

GH

GH017 Ashanti 144 AshantiAshanti

various schists (locally called "graphitic""graphite or schists") 25SAshanti GH

Carbon isotope compositions GH Ashanti 152 GH 217 Ashanti Elemental ispresentcarbonaceous inthe GH Ashanti carbon as matter 237 GH 251andconcentrated the shear in zones graphitic as gouge as GH324 Ashanti or selvages the veins. carbonaceous to The matter extremely GH187 Ashanti is finegrained (1-3 by 10-20/m), flaky, is often and concen- GH330 Ashanti Ashanti

cs* cs ses cs cs* cs cs cs cs my* my my my* my cs cs

-24.5 -23.8 -23.6-24.3 -28.2 -25.2 -20.7 - 18.3 -23.2 -20.5 -24.8

2.66 4.67

0.85

trated irregular in schlieren tobedding schis- 667 parallel and/or GH Ashanti GH 703tosity. petrographic Coal investigations byKoch (1991, 1992) GH765 Ashanti characterized carbonaceous the matteras semigraphite GH201 Ashanti of originally sedimentary origin, which, tectonized in rocks, was GH232 Ashanti Ashantiboundaries coarser of hydrothermal carbonates. GH881 AshantiGH 927Ashanti

4.73 3.41 2.92 0.50 0.58 -13.8 - 13.0 - 12.4 - 13.9 - 13.1 13.5 12.4 12.8 13.8 13.6

1.82 1.10 0.38 0.34

concentrated shear along planes conspicious arecrys- GH Ashanti Less GH 375 387tallites small and aggregates ofcarbonaceous ongrain GH Ashanti matter 407Carbonaceousmatter from the Ashanti mine has/93C val-

-27.1 -27.5

uescoveringconsiderable from-11.4 to -28.3 per GH009 Bogosu a range Nsuta

s*s s Mn-cbcs

-28.3-20.9 -20.1 0.43 0.93

rail. data The overlap values Birimian with of argillite Nsutal and Nsuta2 Nsuta

chertsamples Ghana from (range -18.3 to -30.2%) given OH019 Nsuta by Leube al. (1990). et Isotopic compositions theother OH045 Prestea from occurrences the Ashanti in belt (Table 1 and Fig. 3) are OH04S Prestea Prestea

-5.5-28.1

14.615.3 16.0

similar those theAshanti butdisplaysmaller OH Obenemase to from mine a 046 GH 039 variation tend toward and lower:xC contents (mean GH420 furi Ayan-23.1%o). Collectively, isotopic the coinpositions ofcarbona- On436 Ayanfiri ceous matter intotherange organic fall of carbon (Schidlow-6009 Obuomski et al., 1983;Hoefs, 1987). 1-40Huntado

cs cs

-28.8 -19.9

- 15.3 -12.5

cs

-22.8

3.79 5.18

5.84 0.29

sps gtcs cs

-23.3-18.6 -20.3

- 18.6 - 16.6-12.8 -21.0

11.9 14.414.4 19.1

1.32

2.59

Carbonates ubiquitous all rocks are in studied. siderite Mg 53C permilrelative PDB,5SO permi]relative SMOW in to in to of diagenetic origin constitutes carbonate the augen the of Abbreviations: earb = late carbonatevein, es = earbonaeeous schist,s = carbonate-spotted (Fig. 2A), whereas schists ankerite with schist, = carbonate sps spotted schist, = sandstone, = metavoleanie, sst nw (dolerite), Mn-eb= manganese carbonate, = granitoid,= gt varying Mg/Feratios (MgCO3:20.05-36.98 mole%; FeCO3: v = volcanic sample (sulfide ore) 8.32-28.51mole%) is typical the various for schists and mineralizedmetasomatized rocks. Ankerite in late carbonate veins is do-

lomitic (MgCO3:31.53-42.91 %) andhas mole elevated contents of SrCOa (mean 0.60 mole %; Oberthfir et al., 1994).

294

OBERTHOR ET AL.

TABLE Oxygen 2. Isotope CompositionsQuartz Muscovite of and Samples, Hydrogen Isotope CompositionsWaterExtracted of fromQuartz, andof Muscovite, well asCarbon as Isotope Compositions CO2Extracted of fi'omQuartz

tween12.8and 15.6per mil (mean= 14.6%o; = 11) for n the Ashanti mineand 14.6to 15.4per rail (mean= 15.0%o; n = 3) for the Prestea mine (Fig. 5, Table2). Studies atthe Ashanti mine (Oberth/ir et al., 1994) showedthat ore

Sample

Locality

SJSOQuartz

SD (H20)

took in of and2 53C (COs) deposition place the PT range 400 _ 50C-9.8 - 10.7

Fluid inclusions-53 -42

to5 kbars. Accordingly,/SO ofthewater, values calculated from/lSO quartz of (Clayton, 1972), range from7.4to 10.2Comparisonthe/SOvalues carbonates quartz of of and

GH GH GH GH GH GH GH GH GH GH GHGH

145 159 160 172 191 239 255 269 931 952 970

Ashanti Ashanti Ashanti Ashanti Ashanti Ashanti Ashanti Ashanti Ashanti Ashanti AshantiPrestea

14.6 15.6 12.8 15.0 14.2 14.1 14.9 15.4 14.1 14.7 15.515.4

per railat 350C 9.7 to 12.5per railat 450C, and indicating a fluidof metamorphic magmatic or origin.

-37

- 10.8

(Fig.5) reveals closely matching median values approx at 15.0 per railanda widerrange of/SOvalues thecarbonates of

compared thefightdistribution to of/SOvalues obtainedfor quartz.SD waterextracted of from veinquartz Quartzsamples fromauriferous veins the Ashanti of and-39 Muscovite

914 GH 915 GH 916

Prestea Prestea

15.1 14.6

-15.7 -9.5

JsoGH 367 GH 368 GH 802 Ashanti Ashanti Ashanti

D-43 -60

11.4

-58

Prestea mines wereinvesfigated (Table2). Onlyfour out of ten samples yieldedsufficient H20for analysis most as inclusions contain mainly mixtures COs-+N,2, aqueous of and inclusions rare;however, to about15 molepercent are up of waterin gaseous inclusions remainmicroscopically may undetected. obtained/D The values range from-37 to -53 per mil,indicating magmatic-metamorphic whenplotfluids

9SoandSD per mil relative SMOW,(13C per mil relative in to in toPDB

tedagainst respective of/sO (Fig.6). the values Hydrogen isotope coinpositions of muscovite minerfromalized muscoviteschists(sulfide ores) at the Ashanti mine

range from-43 to -60 per rail (Table2). Recent workby Ojala al.(1995) et indicates A(D) values fluidinclusion that ofSample GH 019 is a chemical sedimentary carbonate, water and coexistent alteration muscovite should be on the Mn order 10to 20 per railat temperatures of between and 300 theprimary of Nsuta ore manganese mine. Carbonates from the Ashanti belt have/aC values from 350C, which in general is agreement theabove with data. -9.9 to -17.0 (mean about -14.0%o)anddisplay unimodal a 5Cof COs extracted veinquartz from distribution (TableI and Fig. 3). Exceptions the nearare Three samples theAshanti gave/3C from mine values of surface samples 420 (-18.6%o)and 1-40 (-21.0%o), GH and the Mn carbonatefrom Nsuta (-5.5%0). -9.8, -10.7, and-10.8 per rail,andavosamples Prefrom stea Carbonisotope interrelationships between carbonaceous yielded-9.5 and -15.7 per rail (Table2 andFig. 3). 'C isotope compositions from inclusions ofCOs fluid matter andcarbonates individual in samples depicted The are in (mean -10.9%o) = thus overlap thecarbon xvith isotope comFigure Theplotof26pairs 4. of/3Cvalues carbonaceous of matterand carbonates versus A(3C) indicates that a sub- positions carbonates, of especially from the Ashanti mine -9.9 to - 17.0,mean- 13.6%o). group data of closely correspondsisotopic to equilibrium be- (range tweenthe carbonaceous matter-carbonate in the tempairs Sulfurisotope compositions perature range to 350C. 200 Another subgroup clearly is The sulfide concentrates obtained from the sulfide ores outofisotopic equilibrium; ofthese most samples characare "aspy" Table3 generally in contained between terizedby low ratiosof carbonaceous matterto carbonate andmarked arsenopyrite, othercomponents the being carbon Table 1) indicating certain (cf. a degree isotopic 50 and80 percent of pyrite, pyrrhofite, marcasite. and Sulfide concentrates overprint exchange; and however, intercept theregres- mainly the of pyrite contents handpicked and concentrates of sion linesthrough samples all (thicklines) interpreted withlarger is as are accordinglyTable3, which in also thecarbon isotope composition whole ofthe system = othersulfides denoted (/3C contains of sphalerite pyrite data and fromthe Perkoa volca- 15%o). Withrespect those to samples closely approximating massive sulfide deposit Burkina in Faso ofpyrite and isotopic equilibrium (thin regression lines),a meancarbon hOgchic mine in isotope composition about-20 per rail is indicated a veiningBirimiantuffs at the Nsuta manganese of in Table3 also listsgoldcontents the concentrates of system dominated a large by amount carbonaceous of matter. Ghana. obtained INAA. by Oxygen isotope compositions Pyriteof synsedimentary-diagenefic in variably origin carBirimian schists displayswidescatter sulfur a of Carbonate values 5sO range from12.9to 22.2per rail bonaceous (median 15.5%o)at the Ashantimine and data from the other isotope composifions, +7.3 to -20.9 per rail (median from occurrences closely match range this (Table Thetotaldata ca. -10%o,Fig. 7). Pyriteandsphalerite 1). fromthe Perkoa volcanogenic massive sulfide deposit pyrite and veining Biriset(Fig.5) approximates a normal distribution.

Quartz fromtheauriferous displays values veins sO be- mian tuffs Nsuta at have magmatic values close 0 permil. to

ASHANTI BELT,GHANA, MINEP, Au ALIZATION

295

TABLE Sulfur 3. Isotope Compositions Gold and Contents Various of Sulfides theAshanti in Ghana from Belt

Oretype/Sample no.GH GH GH GH GH GH GH GH GH GH GH GH GH GH GH GH GH GH GH GH GH GH GH GH GH GH GH GH GH GH GH GH GH GH GH GH 002 120 124 131 165 202 202 m 219 231 231 m 301 302 307 308 343 355 368 387 391 407 460 518 520 721 733 891 947 950 956 A 958 959 961 B 962 967 969 A 43-1

64SMineralaspy-6.4 -6.3

AuLocalityAshanti mine Ashanti mine Ashanti mine Ashanti minePrestea mine

Oretype/host rockCarb.schi. Carb.schi. Qtz vein Qtz vein

534S AuMineralpy py galena galena aspy

LocalityAshanti mine Ashanti mine Ashanti mine Ashanti mine Ashanti mine Ashanti mine Ashanti mine Ashanti mine Ashanti mine Ashanti mine Ashanti mine Ashanti mine Ashanti mine Ashanti mine Ashanti mine Ashanti mine Ashanti mine Ashanti mine Ashanti mine Ashanti mine Ashanti mine Ashanti mine Ashanti mine Ashanti mine Ashanti mine Ashanti mine Ashanti mine Ashanti mine Ashanti mine Ashanti mine Ashanti mine Ashanti mine Ashanti mine Ashanti mine Ashanti mine Ashanti minemine Ashanti mine Ashanti

host rockSulfide ore

(%0) (ppm) Sample no.396

(%o) (ppm)-20.9 0.4

-6.3 -7.0 -6.0 -6.4 -7.3-6.3

323 167 134 150 86222

GH 888 GH 927 GH 306 GH 780GH GH GH GH GH GH GH GH GH 005 858 860 918 A 918 B 006 10-1 048 046

-19.5-12.0 -12.6-6.9

0.3

Prestea mine Sulfide orePrestea mine Prestea mine

86

-6.8

35 19532

-7.5 -7.1

-6.5-10.4 -6.2

134

Presteamine PresteaminePrestea mine

py aspy

-7.8-4.6

10.131

-6.6 -8.9 -5.3 -7.5 -8.5 -6.6 -6.0 -6.3 -7.0 -6.4 -6.8 -7.6 -8.9 -8.9 -10.1-6.9

82 253 245 100 175 295 268 182 79 198 214 197 55 285294

-4.7

Prestea mine Carb.schi.Presteamine Bogosu Bogosu Bogosu Bogosu

pypy

-11.7 -10.8 -8.2

9.731

GH 912 A GH 912 B GH 920 6181WH

Sulfide ore

pyaspy

93 137169

-8.5 -7.7 -16.80.7

py > aspy

Carb.schi.Granite

pyaspy

GH 785GH GH GH GH 420 422 869 A 869 B

AyanfuriBokitsi Bokitsi Bokitsi Bokitsi

92 24

SchistSchist/ore

pyPY

-6.5-9.6

aspyPY

-11.4 -11.2 -3.1

7.7 29.8 32 156122 1.0 0.4

GH 873GH 036 GH 037

KonongoObenemase Obenemase

Sulfide ore

aspy py py

-0.5-2.8 -10.9 -10.1

-8.0 -6.3 -7.1 -6.9 -7.7 -8.0 -8.9 - 10.2 -5.8

93 72 201 306 154 248 164 32

GH 035GH 039

Obenemase Carb.schi.Obenemase

6009WH 6040WH 6041WH1-44/45 1-49 WH

Obuom Bilpraw BilprawHuntado Huntado

Schist Carb.schi.

-4.4 -12.4 -14.15.5 7.3

GH 357 GH 405

Carb.schi. py Schist ore py + cpy in

-16.0 -9.0

17

Perkoa Perkoa Nsuta

Perkoa Perkoa Hill B

VMS VMS "TufF'

py sphalerite py

1.2 1.4 1.2

34S permil relative CDT in to Abbreviations: schi. carbonaceous qtzvein= quartz carb. = schist, vein; aspy arsenopyrite, = chalcopyrite, = pyrite = cpy py

fall of carbon (Schidlowski et Arsenopyrite the sulfide from oresat the Ashanti mineis grossly intotherange organicwith considerably depleted 34S in (range -5.3 to -10.2%). The al., 1983;Hoers,1957),and in accordance the petro-

studies (Koch, 1991, 1992), lendsupport anorigito datadisplay tightnormal a distribution 7) indicative graphic (Fig. of nallyorganogenie derivation the earbonaeeous of matter. a large, homogeneous source. Carbonates from unmineralized and mineralized litholoThearsenopyrite samples Prestea, from Bogosu, Bokiand are depleted 3C. 3C values in The range tsi conform with andslightly extend totalrange data gies considerably the of a distribuwithrespect theAshanti to mine. However, arsenopyrite from from-9.9 to -17.0 permilanddisplay unimodal form eoherant a uniform group, the with Konongo markedly ranges from -0.5 to -3.1 per rail, and tion(Fig.3).Alldata of Mn from at oneconcentrate granite-hosted from mineralization Ayan- exception thesynsedimentarycarbonate Nsuta at -5.5 per mil. furi has0.7 permil. Furthermore, cogenetic arsenopyrite pyrite and separated Anapproach toward isotopic equilibrium between earbonafromthesame samples sulfide display of ores nearly identical eeous matter and associated carbonates was identified in one

634S values (arrows Fig.7;Table Galena thegold subgroup samples 4), whereas second in 3). from of (Fig. a subgroup quartz veins theAshanti at minehas634S values -12.0 appears be outof equilibrium. of to Samples thelattersubofand-12.6 per rail.Discussion

Carbon isotopes Carbonaceous in Birimian matter metasediments isisotopi-

group mainly are characterizedratios earbonaeeous by of matter to carbonate carbon belowunity,indicating overprint an of carbonate carbon isotope compositions those the on ofassociated earbonaeeous matter.

Thelightcarbon isotope compositions of carbonates fromthe Ashanti belt are in distinct contrast to those of carbonates

cally light(6aC -11.4 to -28.8%).The data(Fig.3) =

296

OBERTHORETAL.

co 2 mdds)1-'] JJJdn-

carbonate

carbon

lO

5 iiiD n quartzo 5 20 80

o lO-20 -15 -10

,,FI

organic carbon

car.at

Fi. 5. Histogram oxygen of isotope compositions in permilrela(6]SO tiveto SMOW)of quartz carbonates theAshanti and from mine(stippled)-30 -25 -20 -15

and other localities in the Ashanti belt. For data sources see Tables 1and g.

FIG.3. Histogram carbon of isotope compositions in per]nilrela(6]aC tiveto PDB)forcarbonaceous (organic matter carbon), carbonates, COs and extracted fluidinclusions. fromtheAshanti from Data mine(stippled) andother localities in the Ashanti belt. For data source see Table 1.

-24.9%0).Kontak al. (1990)proposed the verylow et that

613C values reflectbiogenic forthis a origin element, probablyfromoxidation graphite the source of in region.

fromchemical sediments, whichdisplay relatively constant veincarbonates stated in gene , 6t3Cval s of and that values close 0 permilthrough to earth's history (Schidlowski veincarbonates dependent temperature, andpH are on Eh, et al., 1983). Furthermore, differfromcarbon they isotope precipitation, wellasthecarbon as isotope composicompositions of carbonates associated Archcan de- during with gold tionof the carbon-bearing species solution in (Ohmoto and posits Australia, in Canada, South and Africa, whichrange Rye,1979).The fluids depositing hydrothermal carbonates from+2 to -9 witha mean about per rail (Kerrich, of -4 gold are consideredbe to 1987; Colvine al.,1988; et Golding al.,1989; Ronde in mesothermal deposits generally et de et by offo.close the QFM buffer, to al., 1991,1992).However, similar evenlighter and carbon characterized conditionsisotope compositions carbonates thosein the Ashanti above COs-CH4, at T > 270C (Kerrich, 1987). At these of like -ieffectsor tern eraturedifferences minor o o,ont= -is /c13t ' ' 13 P

Ios,> ssd sooph dso. oaon

beltarereported fromthe Homestake minein South gold Dakota (-5.6 to -11.2%