Download - Oberthur 1996 Gold Mineralization in the Ashanti Belt of Ghana

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

Gold Mineralization in the Ashanti Belt of Ghana:

Genetic Constraints of the Stable Isotope Geochemistry THOMAS OBERTHOR,

Bundesanstah fiir Geowissenschaften und Itohstoffe, Stilleweg 2, D-30655 Hannover, Germany

ANDREAS SCHMIDT MUMM,

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

ULRICH VETTER,

Bundesanstah fo'r Geowissenschaften und Itohstoffe, Stilleweg 2, D-30655 Hannover, Germany

KLAUS SIMON,

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

AND JOE A. AMANOR Ashanti Goldfields Company Ltd., Obuasi, Ghana

Abstract

The Ashanti belt of Ghana is the key district of gold mineralization in the Paleoproterozoic terrane of West Africa. The area considered in southwest Ghana is covered by lithologies of the volcanic-sedimentary Birimian Supergroup and the overlying elastic sedimentary Tarkwaian Group which were jointly folded and metamorphosed under greenschist facies conditions during the Eburnean teetonothermal event at about 9..1 Ga. Regional fotiation m•d subparallel shear zones hosting mesothermal gold mineralization developed during deformation coeval with metamorphism.

Four major ts•oes of primary gold mineralization are present in the Ashanti belt: (1) mesothermal, generally steeply dipping quartz veins in shear zones mainly in Birimian sedimentary rocks, (9,) sulfide ores with auriferous arsenopyrite and pyrite, spatialty dosely associated with the quartz veins, (3) sulfide disseminations and stockworks in granitoids, and (4) paleoplaeers of the Tarkwaian Group.

This study concentrates on types (1) and (9,) of the hydrotherlnal gold mineralization. Stable isotope analyses of host-rock and ore components were performed with the ailn of obtaining parameters relevant to the origin and evolution of the fluids that produced gold mineralization.

Carbonaeeous matter in the Birimian metasediments displays •93C values ranging from -11.4 to -9,8.3 per mil relative to PDB, indicating an organogenie origin. Carbonates display a unimodal distribution of •9aC values ranging from -9.9 to -17.0 per rail relative to PDB. COz extracted from fluid inclusions in the auriferous quartz veins has •93C values ranging from -9.5 to -15.7 per mil relative to PDB. It is proposed that these carbon isotope compositions of carbonates and COz reflect extensive interaction of the CO,2-rieh hydrothermal fluids with reduced carbon in Birimian sediments in the deeper parts of the hydrothermal systems.

Carbonates and auriferous vein quartz have 6•So values ranging from 19,.9 to 9,9,.9, and 19,.8 to 15.6 per mil relative to SMOW, respectively. Carbonates and quartz were deposited in near isotopic equilibrium with respect to 6•So, indicating fluid-dominated conditions during ore formation, from fluids of metamorphic or magmatie origin. Such an origin is corroborated by •SD values of water extracted from fluid inclusions in vein quartz (-37 to -53%• relative to SMOXV).

Pyrite of synsedimentary-diagenetie origin in Birimian schists displays sulfur isotope compositions ranging from +7.3 to -9,0.9 per rail (median ca. -10%• relative to CDT). Similar compositions and wide ranges are usually attributed to sulfide generation by bacterial sulfate reduction from seawater.

Arsenopyrite and eogenetie pyrite from the sulfide ores generally have •534S values in the range -5.3 to -10.9, per rail relative to CDT. The tight unimodal distribution of •534S values indicates a large, homogeneous fluid reservoir. The low (534S values are interpreted as source-inherited, not related to unusual pH, Eh, temperature, or depositional conditions. Sulfides in Birimian sediments represent the most likely sulfur reservoir tapped by the fluid systems.

The C, O, H, and S isotope compositions of ore-related hydrothermal minerals and fluid inclusion components indicate that the mineralizing fluids interacted extensively with the Paleoproterozoie rocks, especially Birimian sediments, at deeper crustal levels and at high temperatures. The isotopic compositions are most compatible with the formation of fluids from devolatilization reactions invoMng Birimian strata during pro- grade metamorphism at depth (metamorphic fluids).

0361-0128/96/1815/289-1355.00 289

290 OBERTHOR ET AL.

Introduction

GOLD mineralization constitutes an important economic fac- tor in the Paleoproterozoic Birimian terratie of West Africa. The largest and most prominent mines, with a cumulative past production in excess of 1,500 metric tons (t) of the noble metal, are located in Ghana. Among these, the Ashanti Gold- fields mine at Obuasi is a world-class gold deposit, having produced more than 800 t of gold historically, with an output of 26,551 kg in 1994.

Light stable isotope studies have gained increasing impor- tance in the description and interpretation of geologic pro- eesses. These studies nowadays form integral parts in the formulation of metallogenetie models for gold deposits and were intensely employed for mesothermal Archcan (e.g., Ker- rich, 1987; Colvine et al., 1988; Colvine, 1989; Golding et al., 1989; de Ronde et al., 1992) and epithermal Phanerozoic (e.g., Rye, 1993) gold deposits (see also contributions in Barnes, 1979, and Ohmoto, 1986). In contrast, little work has been done in the Proterozoic terrane of West Africa.

Exceptions are studies on the stable isotope compositions of carbon in Birimian argillites and cherts from Ghana (Leube et al., 1990), and a stable isotope study on the tourmalinized sandstones and gold ores from Loulo in Mall (Fouillac et al., 1993).

The present work comprises the first comprehensive stable isotope study on host rocks and ore components from the Ashanti gold belt in the Birimian terrane of Ghana. Special emphasis is placed on a distinction between original, synsedimentary-diagenetic isotopic signatures and those due to hydrothermal input and overprint relevant to gold mineralization. The study is based on samples collected underground at the Ashanti and Prestea mines, and in the open pits of Bogosu and Konongo. Drill core was made available from the Ayanfuri concession and from Obenemase north of Ko- nongo. The samples from the old Bokitsi mine near Ayanfuri were collected from mine dumps.

Regional Geology

The relative timing of volcanism, sedimentation, emplace- ment of various granitoid suites, and tectonism in the vast Birimian terrane of West Africa is still controversial and be-

yond the scope of this paper. The following description fol- lows concepts recently developed for the Ghanaian sector of the terrane (Leube and Hirdes, 1986; Eisenlohr, 1989; Leube et al., 1990; Eisenlohr and Hirdes, 1992; Hirdes et al., 1992; Taylor et al., 1992; Blenkinsop et al., 1994; Davis et al., 1994).

Large areas of southern Ghana are covered by Paleopro- terozoie lithologies. The supracrustal rocks are subdivided into the volcanic-sedimentary Birimian Supergroup and the overlying elastic sedimentary Tarkwaian Group. The classical subdivision of the Birimian into Lower Birimian (voleanielas- ties, waekes, argillites, chemical sediments) and Upper Biri- mian (basalts with some interflow sediments), as proposed by Junner (1935, 1940), was reinterpreted by Leube and Hirdes (1986) and Leube et al. (1990). The latter authors regard Lower and Upper Birimian as a coeval sequence with the sedimentary-voleanielastie assemblage (sedimentary basins) representing a distal facies of volcanic belts. Detrital zircons from Birimian and Tarkwaian strata gave maximum sedimen-

tation ages of 2132 _+ 3 and 2135 __+ 5 Ma, respectively, whereas Birimian volcanics yielded an Sm/Nd age of 2166 +_ 66 Ma (Taylor et al., 1992; Da•Ss et al., 1994).

The supracrustal sequence was folded and metamorphosed under greenschist facies conditions during the ca. 2.1 Ga Eburnean tectonothermal event (Leube et al., 1990; Hirdes et al., 1992; Taylor et al., 1992). Structural investigations revealed that Birimian and Tarkwaian rocks were deformed

jointly during a single progressive event. Northwest-southeast-directed crnstal shortening produced major thrusts and shears, commonly close to the basin-belt contacts, which acted as channelways for mineralizing hydrothermal fluids (Eisenlohr, 1989; Blenkinsop et al., 1994). Gold mineralization in the Ashanti belt is largely synkinematic and synmetamorphic, i.e., coeval with the Eburnean event (Oberthiir et al., 1994). The intrusion of two major distinct suites of granitoids, the Dixcove- or belt-type granitoids which occur within the volcanic belts, and the late kinematic Cape Coast- or basin-type granitoids in the sedimentary basins (Fig. 1), took place at different time intervals between ca. 2180 to 2170 Ma and 2116 to 2088 Ma, respectively (Hirdes et al., 1992).

Gold Mineralization

The deposits investigated in this study are situated in a northeast-southwest-trending gold belt which stretches for about 250 km from south of Prestea to Konongo in the north (Fig. 1). The foilroving four major types of primary gold mineralization are present: (1) generally steeply dipping quartz veins in shear zones mainly in Birimian sedimentary rocks, with free-milling gold and fluid inclusions in veins dominated by CO.• (Fig. 2C and D); (2) sulfide ores, with auriferous arsenopyrite + pyrite, closely associated with the quartz veins (Fig. 2B); (3) sulfide disseminations and/or stockworks with free gold, arsenopyrite, and pyrite in granites (e.g., Ayanfuri); and (4) palcoplacers of the Tarkwaian Group.

This study mainly deals with the first two types of mineralization, which were described by Junner (1932, 1935, 1940), Cooper (1934), and Hirst (1941). These authors stressed the structural control of the epigenetic mineralization and their location in Lower Birimian strata close to the contacts with

Upper Birimian or Tarkwaian rocks. Epigenetic quartz vein and sulfide mineralization was also favored by later studies of Eisenlohr (1989), Hirdes and Leube (1989), Milesi et al. (1989, 1991, 1992), Leube et al. (1990), and Oberthiir et al. (1991, 1994). In contrast, Ntiamoah-Agyakwa (1979) put forward a syngenetic-metamorphic concept by proposing that gold and sulfides of volcanic-exhalative origin were initially deposited in the Birimian sediments and subsequently redis- tributed and concentrated to ores by metamorphism. Simi- larly, Leube et al. (1990) postulated that at least some of the disseminated sulfide lode ores are syngenetic.

The Ashanti mine (Junner, 1932; Hirdes, 1989; Leube et al., 1990; Oberthiir et al., 1991, 1994) can serve as a type example for most of the deposits studied because it illustrates the interrelationships between host rocks and ores.

At the Ashanti mine, gold mineralization is hosted in tightly folded, steeply dipping and northeast-southwest-striking Biri- mian metasediments. The greenschist facies rocks comprise alternating argillites (muscovite schists, muscovite-chlorite schists and carbonate-spotted schists, all variably graphitic)

ASHANTI BELT, GHANA, Au MINEBALIZATION 291

Sunyani O i

VOLTAlAN TOGO SERIES

TARKWAIAN

[• sedimentary basins BIRIMIAN volcanic belts

__ DAHOMEYAN

+ + + + •(,O'•'• © + +

basin type granitoids belt type granitoids

+ +

+ +

+ +

+ +

+ +

+

+ + + ++ + + + + + + + + +

+ + + + +

+ + + + + +

+ + + +

+ + + +

+ + +

+ + +

+ + + +

Cape Coast • • S,ko.d, G •3 \ t•

G U L • 0 2•3 40 60Kin I I I

W•nneba

F•c,. 1. Geolo•' of southern Ghana after Leube and Hirdes (1986), modified after Hirdes and Loh (pers. commun., 1995). Also shox•m are major gold mines: A = Ayanfuri and Bokitsi, B = Bogosu, K = Konongo, O = Ashanti mine at Obuasi, P = Prestea.

and feldspathie metasandstones. Irregular boudins and inter- layers of "metavoleanie" or "dyke" rocks (in mine terminol- ogy) are locally present. These rocks, hoxvever, generally are intensely metasomatized or earbonatized and have thus been interpreted to represent earbonatized metasediments (Ober- thiir et al., 1994). This interpretation may be extended to similar rock units at Prestea (Adjimah, 1988). Possibly concor- dant Birimian metavoleanies and unmetamorphosed dikes (dolerites), however, are also present in subordinate amounts at the Ashanti mine.

Gold mineralization is exposed in mine workings for about 8 km along strike and down to ca. 1,650 m below surface. The ore zone consists of several steep orebodies subparallel to the regional strike, which measure up to some hundred meters horizontally and vertically and attain thicknesses of up to 50 m. The ore zone is further characterized by intensive shearing, pronounced sulfide mineralization of country rocks, multiple to massive quartz veining, and the common presence of earbonaeeous schists. Two distinct ore types are recognized

whose relative proportions are variable in the different mine sections:

1. Quartz veins, locally with spectacular showings of visible gold and minor Pb-Sb-Cu sulfides (galena, bournonite, tetra- healrite; Fig. 2C). Single, massive or laminated veins, 0.2 to 5 m wide but locally reaching 25 m in width, or multiple quartz veins with intercalated, sheared, and commonly sulfid- ized wall rocks are present. Selvages in the form of intensely sheared carbonaceous schists are common. Wall-rock alteration takes the form of sulfidization and carbonatization.

2. Sulfide ores, with refractory gold hosted in arsenopyrite as the main ore mineral (Fig. 2B). This mineralization forms either envelopes, up to some meters wide, around quartz veins, or disseminations in metasediments or "dyke" rocks. Ore minerals besides arsenopyrite include pyrite, pyrrhotite, and marcasite, and rare chalcopyrite and sphalerite.

Petrographic and structural investigations have revealed that both ore types are largely synmetamorphic and synki-

292 OBERTHORETAL

bou tet •' '

.

sph

cpy C

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

nematic and were deposited from hydrothermal solutions during one single progressive deformational event, in the PT range of 2 to 5 kbars and 400 ø _ 50øC (Blenkinsop et al., 1994; Oberthiir et al., 1994).

The general geologic setting, types of ore, mineralogy, and the physicochemical conditions of ore deposition at the Ko- nongo, Bokitsi, Bogosu, and Prestea mines are remarkably similar to those at the Ashanti mine: (1) the mineralizations are mainly hosted by Birimian metasediments, close to contacts with Birimian volcanics or Tarkwaian sediments to the

east, (2) the orebodies dip steeply and trend north-northeast- south-southwest to northeast-southwest, i.e., subparallel to the regional strike of the sedimentary lithologies, (3) gold has a bimodal distribution in quartz veins and sulfide ores, and (4) fluid inclusions of the quartz veins are unusual with CO.2 as the dominant phase (Schmidt Mumm et al., 1996, in press).

Differences are a higher metamorphic grade at Konongo (biotite, garnet; Hirst, 1941), whereas the other mines are located in greenschist facies rocks; larger proportions ofpyrite

relative to arsenopyrite occur at Prestea, Bogosu, and Bokitsi (mean about 50/50) compared to Ashanti and Konongo (mean about 15/85).

Mineralization at the Ayanfuri concession comprises im- pregnations of arsenopyrite and free gold in granitic stocks which were intruded into Birimian metasediments.

Analytical Methods Carbon isotope compositions of carbonaceous matter, and

carbon and oxygen isotope compositions of carbonates were determined on whole-rock samples. For carbonates in vein- lets, handpicked concentrates were analyzed. Oxygen isotope compositions of quartz from the auriferous veins were obtained from coarse, clean vein quartz. Sulfide concentrates were produced from sulfide ore samples and pyrite in schists by crushing of the samples and subsequent mechanical concentration (panning).

Isotopic an•yses were performed by GCA Lehrte, Ger- 13 18

many (6 C, 6 O; carbonaceous matter and carbonates) and

ASHANTI BELT, GHANA, Au MINERALIZATION 293

Geochron Labs, Cambridge, United States (834S), using stan- dard methods. Analytical reproducibility was +0.2 per rail for oxygen and carbon and + 0.4 per rail for sulfur. The values of quartz were determined at G6ttingen University by reaction with C1F3, the liberated oxygen was converted to CO2 by reduction on graphite at 600øC;/•D values of water extracted from quartz (grain size 6-12 ram) were determined after degassing adsorbed water at 140øC for 24 h in vacumn. The quartz was subsequently deerepirated at 800øC, the liberated water was reduced to H2 by reaction xvith hot (800øC) uranium, and immediately analyzed for D and H isotope compositions. Typical analytical errors of the method are +_2 per rail. COz froin fluid inclusions •vas extracted by mechanical crushing of quartz at room temperature followed by its separation from H20 by cryogenic methods. The/93C of the obtained CO.2 was measured directly. Gold contents were analyzed by instrumental neutron activation analysis (INAA, ACTLABS, Canada).

Isotopic compositions are denoted as follow: 1513C ...... carbonaceous matter, in per mil relative to PDB; /5t3C = carbonate carbon, in per rail relative to PDB; /9SO = cal'[)

carbonate oxygen, in per rail relative to SMOW, •a4s: sulfide sulfur, in per rail relative to CDT, and/•D = water hydrogen, in per rail relative to SMOW. Results are presented in Tables 1, 2, and3.

TABLE 1. Carbon Isotope Compositions of Carbonaceous Matter (c.m.) and Carbonates (earb), Oxygen Isotope Compositions of Carbonates, Total Organic Carbon (C .... ), and Carbonate Carbon (Cc.,rb) Contents of Rock

Samples from the Ashanti Belt in Ghana

Sample Rock C .... C•.•b no. Locality type (93C•

GH 042

GH 101

GH 107 GH 111

GH 142 GH 199

Glt 209

GH 338

GH 346

GH 368

GH 380

GH 382

GH 384

GH 405 GH 510

GH 604

GH 634

GH 664

GH 684

GH 701

GH 709

GH 710

GH 711

Results GH 017 GH 144

Carbon isotope compositions GH 152 GH 217

Elemental carbon is present as carbonaceous matter in the GH 237 GH 251

various schists (locally called "graphitic" or "graphite schists") GH 25S and concentrated in the shear zones as graphitic gouge or as GH 324 selvages to the veins. The carbonaceous matter is extremely GH 187 fine grained (1-3 by 10-20/•m), flaky, and is often concen- GH 330 trated in irregular schlieren parallel to bedding and/or schis- GH 667 GH 703

tosity. Coal petrographic investigations by Koch (1991, 1992) GH 765 characterized the carbonaceous matter as semigraphite of GH 201 originally sedimentary origin, which, in tectonized rocks, was GH 232 concentrated along shear planes Less conspicious are crys- GH 375 ß GH 387 tallites and small aggregates of carbonaceous matter on grain GH 407 boundaries of coarser hydrothermal carbonates. GH 881

Carbonaceous matter from the Ashanti mine has/93C val- GH 927

ues covering a considerable range from -11.4 to -28.3 per GH 009 rail. The data overlap with values of Birimian argillite and Nsutal Nsuta2

chert samples from Ghana (range -18.3 to -30.2%•) given OH 019 by Leube et al. (1990). Isotopic compositions from the other OH 045 occurrences in the Ashanti belt (Table 1 and Fig. 3) are OH 04S similar to those from the Ashanti mine but display a smaller OH 046 GH 039 variation and tend toward lower •:xC contents (mean GH420 -23.1%o). Collectively, the isotopic coinpositions of carbona- On 436 ceous matter fall into the range of organic carbon (Schidlow- 6009 ski et al., 1983; Hoefs, 1987). 1-40

Carbonates are ubiquitous in all rocks studied. Mg siderite of diagenetic origin constitutes the carbonate augen of the carbonate-spotted schists (Fig. 2A), whereas ankerite with varying Mg/Fe ratios (MgCO3:20.05-36.98 mole %; FeCO3: 8.32-28.51 mole %) is typical for the various schists and metasomatized rocks. Ankerite in late carbonate veins is do-

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

Ashanti sps -24.5 -11.3 16.0 Ashanti carb -26.6 -12.4 14.9 Ashanti carb -9.9 15.3

Ashanti sps -25.8 -12.9 16.3 0.41 1.98 Ashanti sps -24.9 -12.6 18.8 0.58 1.50 Ashanti cs ø -25.3 -14.6 15.9 2.20 1.29

Ashanti sps -22.7 -15.6 15.6 0.63 2.16 Ashanti cs -25.7 -14.2 15.3 1.43 1.29 Ashanti cs -25.2 -15.0 15.1 2.01 1.23 Ashanti s ø -25.6 -15.4 21.2 0.45 0.19 Ashanti mv -12.7 -12.3 12.9 Ashanti mv -11.4 -10.9 14.6 Ashanti cs -24.6 - 13.0 15.3 5.03 0.62 Ashanti s ø - 15.5 - 13.1 14.5 0.69 3.58 Ashanti cs -23.8 - 13.0 22.2 Ashanti s -15.7 -14.8 14.5 Ashanti sst -16.3 -14.4 16.2 0.23 1.54 Ashanti sst -17.9 -13.7 16.4 0.37 0.49 Ashanti sst -15.7 -14.3 16.2 0.33 0.91 Ashanti v - 13.5 14.1 0.21 Ashanti s -20.1 -17.0 13.5 0.62 1.25 Ashanti s -16.4 -15.5 14.3 0.62 1.75 Ashanti cs -21.7 4.16 0.69 Ashanti cs -28.3 Ashanti cs* -25.1 3.64 0.62 Ashanti cs - 25.1 Ashanti cs* -24.5 2.66 0.85 Ashanti cs -23.8 4.67 Ashanti s -23.6 Ashanti es -24.3 Ashanti cs -28.2 4.73 Ashanti cs* -25.2 3.41 1.82 Ashanti cs -20.7 2.92 1.10 Ashanti cs - 18.3 0.50 0.38 Ashanti cs -23.2 0.58 0.34 Ashanti cs -20.5 Ashanti my* -24.8 -13.8 13.5 Ashanti my ø - 13.0 12.4 Ashanti my ø - 12.4 12.8 Ashanti my* - 13.9 13.8 Ashanti my ø - 13.1 13.6 Ashanti cs -27.1 Ashanti cs -27.5

Bogosu s* -28.3 Nsuta s -20.9 0.43 Nsuta s -20.1 0.93 Nsuta Mn-cb -5.5 14.6 Prestea cs -28.1

Prestea cs -28.8 - 15.3 15.3 Prestea cs -19.9 -12.5 16.0 3.79 5.84 Obenemase cs -22.8 5.18 0.29

Ayan furi sps -23.3 - 18.6 11.9 1.32 2.59 Ayanfi•ri gt - 16.6 14.4 Obuom cs -18.6 -12.8 14.4 Huntado cs -20.3 -21.0 19.1

•5•3C in per mil relative to PDB, •5•SO in per mi] relative to SMOW Abbreviations: earb = late carbonate vein, es = earbonaeeous schist, s =

schist, sps = carbonate spotted schist, sst = sandstone, nw = metavoleanie, v = volcanic (dolerite), Mn-eb = manganese carbonate, gt = granitoid, ø = mineralized sample (sulfide ore)

294 OBERTHOR ET AL.

TABLE 2. Oxygen Isotope Compositions of Quartz and Muscovite Samples, Hydrogen Isotope Compositions of Water Extracted

from Quartz, and of Muscovite, as well as Carbon Isotope Compositions of CO2 Extracted fi'om Quartz

Sample Locality •SJSO •SD (H20) •5•3C (COs)

Quartz

GH 145 Ashanti 14.6 GH 159 Ashanti 15.6 GH 160 Ashanti 12.8 GH 172 Ashanti 15.0 GH 191 Ashanti 14.2 GH 239 Ashanti 14.1 GH 255 Ashanti 14.9 GH 269 Ashanti 15.4 GH 931 Ashanti 14.1 GH 952 Ashanti 14.7 GH 970 Ashanti 15.5 GH 914 Prestea 15.4

GH 915 Prestea 15.1 GH 916 Prestea 14.6

GH 367 Ashanti GH 368 Ashanti GH 802 Ashanti

•J s o

11.4

Fluid inclusions

-53 -9.8 -42 - 10.7

-37 - 10.8

-39

Muscovite

•D

-43

-60

-58

-15.7 -9.5

•9So and •SD in per mil relative to SMOW, (•13C in per mil relative to PDB

Sample GH 019 is a chemical sedimentary Mn carbonate, the primary ore of Nsuta manganese mine.

Carbonates from the Ashanti belt have/•aC values from

-9.9 to -17.0 (mean about -14.0%o) and display a unimodal distribution (Table I and Fig. 3). Exceptions are the near- surface samples GH 420 (-18.6%o) and 1-40 (-21.0%o), and the Mn carbonate from Nsuta (-5.5%0).

Carbon isotope interrelationships between carbonaceous matter and carbonates in individual samples are depicted in Figure 4. The plot of 26 pairs of/•3C values of carbonaceous matter and carbonates versus A(•3C) indicates that a subgroup of data closely corresponds to isotopic equilibrium between the carbonaceous matter-carbonate pairs in the temperature range 200 ø to 350øC. Another subgroup is clearly out of isotopic equilibrium; most of these samples are characterized by low ratios of carbonaceous matter to carbonate carbon (cf. Table 1) indicating a certain degree of isotopic overprint and exchange; however, the intercept of the regression lines through all samples (thick lines) is interpreted as the carbon isotope composition of the whole system (/•3C = - 15%o). With respect to those samples closely approximating isotopic equilibrium (thin regression lines), a mean carbon isotope composition of about -20 per rail is indicated in a system dominated by a large amount of carbonaceous matter.

Oxygen isotope compositions

Carbonate •5•sO values range from 12.9 to 22.2 per rail (median 15.5%o) at the Ashanti mine and data from the other occurrences closely match this range (Table 1). The total data set (Fig. 5) approximates a normal distribution.

Quartz from the auriferous veins displays •sO values be-

tween 12.8 and 15.6 per mil (mean = 14.6%o; n = 11) for the Ashanti mine and 14.6 to 15.4 per rail (mean = 15.0%o; n = 3) for the Prestea mine (Fig. 5, Table 2). Studies at the Ashanti mine (Oberth/ir et al., 1994) showed that ore deposition took place in the PT range of 400 ø _ 50øC and 2 to 5 kbars. Accordingly,/•SO values of the water, calculated from/•lSO of quartz (Clayton, 1972), range from 7.4 to 10.2 per rail at 350øC and 9.7 to 12.5 per rail at 450øC, indicating a fluid of metamorphic or magmatic origin.

Comparison of the/•SO values of carbonates and quartz (Fig. 5) reveals closely matching median values at approx 15.0 per rail and a wider range of/•SO values of the carbonates compared to the fight distribution of/•SO values obtained for quartz.

•SD of water extracted from vein quartz Quartz samples from auriferous veins of the Ashanti and

Prestea mines were invesfigated (Table 2). Only four out of ten samples yielded sufficient H•20 for analysis as most inclusions contain mainly mixtures of COs -+ N,2, and aqueous inclusions are rare; however, up to about 15 mole percent of water in gaseous inclusions may remain microscopically undetected. The obtained/•D values range from -37 to -53 per mil, indicating magmatic-metamorphic fluids when plot- ted against the respective values of/•sO (Fig. 6).

Hydrogen isotope coinpositions of muscovite from mineralized muscovite schists (sulfide ores) at the Ashanti mine range from -43 to -60 per rail (Table 2). Recent work by Ojala et al. (1995) indicates that A(D) values of fluid inclusion water and coexistent alteration muscovite should be on the

order of 10 to 20 per rail at temperatures between 300 ø and 350øC, which is in general agreement with the above data.

•5•C of COs extracted from vein quartz Three samples from the Ashanti mine gave/•3C values of

-9.8, -10.7, and -10.8 per rail, and avo samples from Pre- stea yielded -9.5 and -15.7 per rail (Table 2 and Fig. 3). The •'•C isotope compositions of COs from fluid inclusions (mean = -10.9%o) thus overlap xvith the carbon isotope compositions of carbonates, especially from the Ashanti mine (range -9.9 to - 17.0, mean - 13.6%o).

Sulfur isotope compositions The sulfide concentrates obtained from the sulfide ores

and marked "aspy" in Table 3 generally contained between 50 and 80 percent arsenopyrite, the other components being mainly pyrite, pyrrhofite, and marcasite. Sulfide concentrates with larger pyrite contents and handpicked concentrates of other sulfides are denoted accordingly in Table 3, which also contains data of sphalerite and pyrite from the Perkoa volca- hOgchic massive sulfide deposit in Burkina Faso and of pyrite veining Birimian tuffs at the Nsuta manganese mine in Ghana. Table 3 also lists gold contents of the concentrates obtained by INAA.

Pyrite of synsedimentary-diagenefic origin in variably carbonaceous Birimian schists displays a wide scatter of sulfur isotope composifions, from +7.3 to -20.9 per rail (median ca. -10%o, Fig. 7). Pyrite and sphalerite from the Perkoa volcanogenic massive sulfide deposit and pyrite veining Biri- mian tuffs at Nsuta have magmatic values close to 0 per mil.

ASHANTI BELT, GHANA, Au MINEP, ALIZATION 295

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

Ore type/ 6•4S Au Ore type/ •534S Au Sample no. Locality host rock Mineral (%0) (ppm) Sample no. Locality host rock Mineral (%o) (ppm)

GH 002 Ashanti GH 120 Ashanti GH 124 Ashanti GH 131 Ashanti GH 165 Ashanti GH 202 Ashanti GH 202 m Ashanti GH 219 Ashanti GH 231 Ashanti GH 231 m Ashanti GH 301 Ashanti GH 302 Ashanti GH 307 Ashanti GH 308 Ashanti GH 343 Ashanti GH 355 Ashanti GH 368 Ashanti GH 387 Ashanti GH 391 Ashanti GH 407 Ashanti GH 460 Ashanti GH 518 Ashanti GH 520 Ashanti GH 721 Ashanti GH 733 Ashanti GH 891 Ashanti GH 947 Ashanti GH 950 Ashanti GH 956 A Ashanti GH 958 Ashanti GH 959 Ashanti GH 961 B Ashanti GH 962 Ashanti GH 967 Ashanti GH 969 A Ashanti GH 43-1 Ashanti

Sulfide ore mine

mine

mine

mine

mine

mine

mine

mine

mine

mine

mine

mine

mine

mine

mine

mine

mine

mine

mine

mine

mine

mine

mine

mine

mine

mine

mine

mine

mine

mine

mine

mine

mine

mine

mine

mine

mine

mine

aspy -6.4 396

-6.3

-6.3 323 -7.0 167 -6.0 134 -6.4 150

-7.3 86 -6.3 222

-6.5 134

-10.4

-6.2

-6.6 82 -8.9 253

-5.3

-7.5 245 -8.5 100 -6.6 175

-6.0 295 -6.3 268

-7.0 182

-6.4 79

-6.8 198

-7.6 214 -8.9 197

-8.9 55 -10.1 285 -6.9 294

-8.0 93 -6.3 72 -7.1 201 -6.9 306 -7.7 154 -8.0 248

-8.9 164

- 10.2 32 -5.8

GH 357 Ashanti Carb. schi. py -16.0 GH 405 Ashanti Schist in ore py + cpy -9.0 17

GH 888 Ashanti mine Carb. schi. py GH 927 Ashanti mine Carb. schi. py GH 306 Ashanti mine Qtz vein galena GH 780 Ashanti mine Qtz vein galena

Prestea mine Sulfide ore aspy Prestea mine

Prestea mine

Prestea mine

Prestea mine py Prestea mine aspy Prestea mine

Prestea mine Carb. schi. py Prestea mine py

Sulfide ore py aspy py > aspy

Carb. schi. py

Granite aspy Schist py

PY Schist/ore aspy

PY

GH 005

GH 858

GH 860

GH 918 A

GH 918 B

GH 006

GH 10-1

GH 048

GH 046

GH 912 A Bogosu GH 912 B Bogosu GH 920 Bogosu 6181WH Bogosu

GH 785 Ayanfuri GH 420 Bokitsi GH 422 Bokitsi GH 869 A Bokitsi GH 869 B Bokitsi

GH 873 Konongo Sulfide ore aspy GH 036 Obenemase GH 037 Obenemase

GH 035 Obenemase Carb. schi. py GH 039 Obenemase

-20.9 0.4

-19.5 0.3

-12.0

-12.6

-6.9

-6.8

-7.5

-7.1

-7.8 -4.6

-4.7

-11.7

-10.8

-8.2

-8.5

-7.7 -16.8

86

35

195 32

10.1

31

9.7 31

93

137

169

0.7 92

-6.5

-9.6 24

-11.4 7.7

-11.2 29.8

-3.1 32

-0.5 156 -2.8 122

-10.9 1.0

-10.1 0.4

6009 WH Obuom Schist py -4.4 6040 WH Bilpraw Carb. schi. -12.4 6041 WH Bilpraw -14.1 1-44/45 Huntado 5.5 1-49 WH Huntado 7.3

Perkoa Perkoa VMS py 1.2 Perkoa Perkoa VMS sphalerite 1.4 Nsuta Hill B "TufF' py 1.2

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

Arsenopyrite from the sulfide ores at the Ashanti mine is considerably depleted in 34S (range -5.3 to -10.2%•). The data display a tight normal distribution (Fig. 7) indicative of a large, homogeneous source.

The arsenopyrite samples from Prestea, Bogosu, and Boki- tsi conform with and slightly extend the total range of data with respect to the Ashanti mine. However, arsenopyrite from Konongo markedly ranges from -0.5 to -3.1 per rail, and one concentrate from granite-hosted mineralization at Ayan- furi has 0.7 per mil.

Furthermore, cogenetic arsenopyrite and pyrite separated from the same samples of sulfide ores display nearly identical 634S values (arrows in Fig. 7; Table 3). Galena from the gold quartz veins at the Ashanti mine has 634S values of -12.0 and -12.6 per rail.

Discussion

Carbon isotopes Carbonaceous matter in Birimian metasediments is isotopi-

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

grossly fall into the range of organic carbon (Schidlowski et al., 1983; Hoers, 1957), and in accordance with the petrographic studies (Koch, 1991, 1992), lend support to an originally organogenie derivation of the earbonaeeous matter.

Carbonates from unmineralized and mineralized litholo-

gies are considerably depleted in •3C. The •3C values range from -9.9 to -17.0 per mil and display a unimodal distribution (Fig. 3). All data form a eoherant uniform group, with the exception of the synsedimentary Mn carbonate from Nsuta at -5.5 per mil.

An approach toward isotopic equilibrium between earbonaeeous matter and associated carbonates was identified in one

subgroup of samples (Fig. 4), whereas a second subgroup appears to be out of equilibrium. Samples of the latter subgroup are mainly characterized by ratios of earbonaeeous matter to carbonate carbon below unity, indicating an overprint of carbonate carbon isotope compositions on those of the associated earbonaeeous matter.

The light carbon isotope compositions of carbonates from the Ashanti belt are in distinct contrast to those of carbonates

296 OBERTHORETAL.

n-

co 2 mdds) 1-'] •JJJd•

carbonate carbon

-20 -15 -10

,,FI o

lO-

-30 -25 -20 -15

organic carbon

FIG. 3. Histogram of carbon isotope compositions (6]aC in per ]nil relative to PDB) for carbonaceous matter (organic carbon), carbonates, and COs extracted from fluid inclusions. Data from the Ashanti mine (stippled) and other localities in the Ashanti belt. For data source see Table 1.

from chemical sediments, which display relatively constant values close to 0 per mil through earth's history (Schidlowski et al., 1983). Furthermore, they differ from carbon isotope compositions of carbonates associated with Archcan gold deposits in Australia, Canada, and South Africa, which range from +2 to -9 with a mean of about -4 per rail (Kerrich, 1987; Colvine et al., 1988; Golding et al., 1989; de Ronde et al., 1991, 1992). However, similar and even lighter carbon isotope compositions of carbonates like those in the Ashanti belt are reported from the Homestake gold mine in South Dakota (-5.6 to -11.2%< Rye and Rye, 1974) and from gold deposits in the lower Paleozoic Meguma terrane, Nova Scotia (Kontak et al., 1988, 1990; mean -21.5, range -13.5 to

-10

equilibrium temperature 620 ø 450 ø 340 ø 200 ø

• 0

.•.-1• o • -•0

-24 ß

(•C ß ß ß

-28 ''' -2 0 2 4 6 8 10 12 14 16

a(13C) carbonate -carbonaceous matter [%]

ceous matter and carbonates. Equilibrium temperatures after Bottinga (1969). The subgroup of samples with equilibrium fractionation close to and exceeding ca. 10 per mil (thin lines) depicts a system with a mean isotopic composition near -20 per mil dominated by carbonaceous matter. The trend of all samples (thick lines) indicates a carbonate-dominated system with a mean isotopic composition of about -15 per mil.

5 n ii•iD• quartz lO

car.at •o •5 20 •80

Fi•. 5. Histogram of oxygen isotope compositions (6]SO in per mil relative to SMOW) of quartz and carbonates from the Ashanti mine (stippled) and other localities in the Ashanti belt. For data sources see Tables 1 and g.

-24.9%0). Kontak et al. (1990) proposed that the very low 613C values reflect a biogenic origin for this element, probably from oxidation of graphite in the source region.

Ios,> dso. ssd oaon sooph vein carbonates and stated that in gene , 6t3C val s of vein carbonates are dependent on temperature, Eh, and pH during precipitation, as well as the carbon isotope composition of the carbon-bearing species in solution (Ohmoto and Rye, 1979). The fluids depositing hydrothermal carbonates in mesothermal gold deposits are generally considered to be characterized by conditions offo.• close to the QFM buffer, above COs-CH4, at T > 270øC (Kerrich, 1987). At these conditions, carbon isotope fractionation attributable to redox

ß /c13t• -i- effects or tern erature differences is minor •o •o•,on•t• = 13 P ß '• ' -- 6 Cn,,a; Ohmoto and Rye, 1979; Kernch, 1987). The above conditions also hold true for the Ashanti belt gold mineralization, as indicated by the mineral assemblages (pyrrhotite- pyrite-arsenopyrite; no magnetite or sulfates) and the fluid inclusion inventory (mainly COs, traces of CH4).

Oxidized carbon species in hydrothermal fluids may origi-

20 I •' i i o

-20 O•Y • metamorphic water -40 (300- 600øC)

v

r• -oo ,,,•,o 7 .... I.•' I 60 -100 ,•' magmatic /

-120 / water -140

-160 I I I I -20 -10 0 10 20

a"o(o)

FIe. 6. Plot of 6D for aqueous fluids extracted from quartz vs. •sO of 18, o

the fluids calculated from 60 of the host quartz at 400 C. 6D and 6]SO in per mil relative to SMOW. Open circles = Ashanti mine; •11ed circle = Prestea mine.

ASHANTI BELT, GHANA, Au MINERALIZATION 297

ß :i• Ashanti

5 i [] Konongo [] Ayanfuri

[] 8ogosu •' [] Prestea

-15 -10 -5 0 +5

:'•::::• :;:• [-] I I ß I ,, [Al:::.::iil] ,, lad ,r-'l [--I •1 [--I -20 -15 -10 -5 0 +5 +10

FIG. 7. Histogram of sulfur isotope coinpositions (634S in per mil relative to CDT) of arsenopyrite concentrates froin gold mineralization (a, top), and pyrite (b, bottom), from both gold mineralizations and unmineralized Birimian schists. Arro•vs indicate isotopic compositions of coexisting arsenopyrite m•d pyrite. Sample localities are indicated; for data sources see Ta- ble 3.

hate from the decarbonation or dissolution of preexisting carbonate minerals, from magmatie sources, and/or from the oxidation or hydrolysis of reduced carbon in sedimentary or metamorphic rocks. Each of these sources may contribute carbon of differing isotopic compositions to hydrothermal solutions (Ohmoto and Rye, 1979).

The major carbon reservoirs considered possess distinct carbon isotope compositions: (1) seawater-derived carbonate is characterized by average •13C values dose to 0 per mil, (2) magmatie CO.2 mainly shows •13C values between -5 and -7 per rail (Pineau et al., 1976; CoMne et al., 1988), and (3) reduced carbon in sedimentary or metamorphic rocks has mean •13C values of about -25 per mil (Sehidlowski et al., 1983; Hoers, 1987).

The source of carbon in carbonates of Archcan lode gold deposits has been a matter of much debate (Burrows et al., 1986; Kerrieh, 1987; Groves et al., 1988; Nisbet and Kyser, 1988; Golding et al., 1989). The discussion was triggered by the finding that carbonates from Arehean gold deposits with carbon isotope values < -3 per mil are unlikely to originate from seawater-derived carbonate, because dissolution and/or deearbonation of this source would lead to isotopically similar, i.e., 4•3C = __ 0 per mil (Ohmoto and Rye, 1979; Golding, 1989), or even slightly 13C-enriched, CO.2 (by about 3-5%o; Burrows et al., 1986). Therefore, Burrows et al. (1986) sug- gested that the isotopic signatures of Archcan gold-related carbonates rather reflect magmatie carbon reservoirs. How- ever, Groves et al. (1988) and Golding et al. (1989) argue that the relationships derived by Burrows et al. (1986) are not so straightforward. Instead, they identify another carbon reservoir, namely fault-controlled regional alteration thought to reflect mantle outgassing of CO2 along crustal-scale fault systems. In the Norseman-Wiluna belt of Australia, this ear- bonation produced carbonated rocks with a median •13C of about -5 per mil (Groves et al., 1988; Golding, 1989). Meta- morphic dissolution of this regionally extensive soume would

produce a COs-bearing fluid capable of precipitating carbonates, with 4•3C around -3 to -5 per mil (Golding et al., 1989). In the same context of Archcan vein carbonates, Ker- rich (1987) warns that the assumption that the correspon- dence of carbonate •3C values to the magmatie range also implies a genetic equivalence is an unwarranted generaliza- tion, especially since •3C values around -5 per mil are diffi- cult to interpret with regard to the carbon soume, because magmatic, sedimentary, and metamorphic rocks are all characterized by average •l'3C values in this range.

The above discussion is of great value to the situation in the Ashanti belt, where pervasive carbonatization resulting from voluminous streaming of CO2-rich fluids has been identified. Also, Eh and PT conditions of gold mineralization are largely in concordance with those characteristic of Archcan mesothermal deposits.

The uniform and light carbon isotope compositions of carbonates in the Ash anti belt (median about - 14%o), therefore, cannot result from unusual local fluid or depositional conditions, which in any case would only have small effects on isotopic fractionation. Instead, the obtained data point to a specific source of the carbonate carbon, which appears less prominent and underestimated in Archean environments, namely reduced carbon in sedimentary rocks. Ohmoto and Rye (1979) suggest that carbonaceous material may become a source of hydrothermal carbon through hydrolysis and oxidation at elevated temperatures of metamorphism, and that the resultant CO2 would probably have 413C values below - 10 per mil.

In the Ashanti belt of Ghana, sufficient reduced carbon with 4•3C values in the range -11 to -28 per mil is present in the form of carbonaceous matter in Birimian metasedi-

ments (cf. Table 1). Complete inorganic oxidation of that carbon (C + 0.2 • CO2) would result in isotopic compositions of the CO.2 similar to those of the starting material (Ohmoto and Rye, 1979). On the other hand, CO2 produced by hydrolysis reactions (e.g., 2C + 2H20 -* CO2 + CH4) at temperatures between 350 ø and 600øC would have •13C values be-

tween 3 and 12 per mil heavier than that of graphite, de- pending on whether the graphite is completely consumed and whether isotopic equilibrium is attained between the remaining graphite and the CO2 produced (Ohmoto and Rye, 1979).

In conclusion, the light carbon isotope compositions of carbonates in the Ashanti belt are interpreted to result from CO2-rieh hydrothermal fluids which attained their •3C-depleted character through intensive fluid-host rock reactions. These took place at low fluid/rock ratios in the deeper parts of the hydrothermal systems. Both oxidation and hydrolysis of carbon may have played a role, and contributions of CO2 from other sources cannot be ruled out.

Indeed, dewatering and deearbonation of (Birimian) rocks at lower crustal levels undergoing progressive metamorphism, and possibly magmatie input (basin-type granitoids), may all have contributed to the hydrothermal systems which formed the gold deposits in the Ashanti belt. The ultimate source of the CO2 in the fluids, however, is masked by the metamorphic carbon isotope compositions, which were attained during the passage of the fluids through the crust.

The •3C values of CO2 extracted from the quartz veins

298 OBERTHOR ETAL.

(range -9.5 to -15.7%o) are in accordance with known isotopic fractionation between carbonates and CO2 (Ohmoto and Rye, 1979), which indicates that calcite and dolomite which formed at temperatures above about 250øC should be isotopically similar or slightly lower by a few per mil than coexisting CO2, and also in equilibrium with graphite (t93C about -25%0) at temperatures between 300 ø and 500øC.

Oxygen and hydrogen isotopes Oxygen isotope compositions of quartz cluster between

12.8 and 15.6 per mil (median 14.7%o), whereas those of carbonates display a wider scatter and range from 12.9 to 22.2 at a median of 15.5 per mil (Tables i and 2, Fig. 5).

The t9SO values of quartz correspond to the narrow range of data (10-16%o) typical for vein quartz from hydrothermal gold deposits of all ages including the Archcan (Kerrich, 1987; Colvine et al., 1988; Golding et al., 1989; de Ronde et al., 1991, 1992). Kerrich (1987) points out that the distinct uniformity of t9SO values of quartz in Au-Ag vein deposits implies a corresponding isotopic homogeneity of the hydrothermal fluids, and similar ambient temperatures of mineralization. Kerrich (1987) further remarks that a gross covariance exists between average t51SO values of quartz and dolomite. Oxygen isotope compositions of Fe dolomites, however, are generally dispersed in contrast to the restricted range of those of quartz, a finding paralleled by the present set of data from the Ashanti belt. Kerrich (1987) interprets the dispersion in Fe dolomites to reflect varying degrees of reequilibration at temperatures below the ambient thermal conditions of quartz-dolomite precipitation, during later mineral-fluid interaction.

O en isoto e com ositions of water in ore-forming xyg P •s the fluid, calculated from t5 O values of quartz after Clayton et al. (1972), range from 5.8 to 8.6 per rail (300øC) and 8.7 to 11.5 per mil (400øC) in the Ashanti belt. This range is similar to fluid t51SO values (6-10%o) obtained for Archcan gold deposits (Kerrich, 1987; Colvine eta]., 1988; Golding et al., 1989; de Ronde, 1991, 1992). In anology with Archcan hydrothermal gold systems, the oxygen isotope data from quartz and carbonates in the Ashanti belt indicate fluid-dominated

conditions during ore formation. In combination with the tSD values of water extracted from

quartz (-37 to -53%0; Fig. 6), the Ashanti belt oxygen and hydrogen isotope compositions indicate fluids of metamorphic or magmatic origin. However, t93C values of CO2 extracted from fluid inclusions (-9.5 to -15.7%o) unequivocally point to a metamorphic origin of this fluid component. Fur- thermore, the large uniformity of isotopic compositions on the scale of the whole Ashanti belt indicates that the same

giant fluid system or fluid systems of similar genesis pervaded the Ashanti belt structural zone.

Sulfur isotopes

Two distinct groups of sulfides, which will be dealt with separately, are present in the Ashanti belt of Ghana: synsedimentary-diagenetic sulfides, mainly pyrite, in Birimian metasediments, and sulfides related to hydrothermal gold mineralization.

In the first group, the exogenic geochemical cycle of sulfur appears to be basically controlled by life processes over most of the recorded earth history (Schidlowski, 1987). Impor-

tantly, primordial sulfur becomes partitioned between a reduced (sedimentary sulfide) and an oxidized reservoir (marine plus evaporite sulfate) in the exogenic environment, whereby substantial isotopic shifts take place due to kinetic and equilibrium isotopic reactions. The same basic principle holds true in hydrothermal systems where sulfate is generally more enriched in 34S than is the associated sulfide (Ohmoto and Rye, 1979; Lambert and Donnelly, 1990). Three major sulfur reservoirs with distinct sulfur isotope signatures can be distin- guished:

1. Mantle-derived sulfur with t534S values close to 0 per mil. Pyrite and sphalerite from the Perkoa volcanogenic massive sulfide deposit and pyrite from Nsuta possess mantle t534S values (Table 3).

2. The oxidation of mantle-derived sulfur to sulfate will

produce a positive isotopic shift (Ohmoto and Rye, 1979), irregardless of whether the process is inorganic or biologically mediated, leading to the resultant sulfate sulfur in the oceans and in evaporites with (534S = 18 _+ 6 and 19 + 6 per mil, respectively.

3. Tapping of the oceanic sulfate reservoir by sulfate-reducing bacteria leads to the generation of HaS and subsequent precipitation of sulfide sulfur in sediments with t534S = -15 _+ 12 per mil (Schidlowski, 1987).

The kinetic isotopic effect inherent in bacterial sulfate reduction thus brings about an average fraetionation of 30 to 35 per mil and is regarded as being responsible for the isotopic disproportionation of terrestrial sulfur into light, reduced sulfide and heavy, oxidized sulfate (Schidlowski, 1987).

Sulfides in Archcan sedimentary (and igneous) rocks are characterized by sulfur isotope compositions markedly con- centrating in the range -4 to +4 per mil (mantle or magmatic), which implies a general paucity of oxidized relative to reduced sulfur species in the hydrosphere during the Ar- chcan, in a mantle-buffered ocean (Schidlowski, 1987; Lam- bert and Donnelly, 1990).

However, the situation changes after about 2.4 to 2.0 Ga, where wide spreads oftSa4S values (-40 to +30%0) are characteristic of sulfides in sedimentary and exhalative strata. This points to the global transition to a largely oxidized hydrosphere, increasing oxygenation of the atmosphere, and prolif- eration of sulfate-reducing bacteria (Schidlowski, 1987; Lam- bert and Donnelly, 1990).

The observed range of t534S values of pyrite in the Birimian metasediments (-20.9 to +7.3%0; Table 3 and Fig. 7) corrob- orates the statements of Schidlowski (1987) and Lambert and Donnelly (1990) and adds another example to the evolution- ary path of the terrestrial sulfur cycle in the time span between about 2180 Ma (oldest belt-type granitoid) and 2116 Ma (oldest basin-type granitoid). Notably, the black sand mineral assemblage of the Tarkwaian palcoplacers (<2135 Ma and > 2116 Ma; Davis eta]., 1994) indicates that the atmosphere was oxygenated during sedimentation of the Tark- waian (Hirdes and Nunoo, 1994; Krupp et al., 1994). There- fore, it appears that the neccessary ingredients (free oxygen, sulfate in seawater, sulfate-reducing organisms) were available in Birimian times to conduct biological• mediated sulfur isotope fractionations. The wide range of t5 a S values of pyrite and the overall trend toward light isotopic compositions (me-

ASHANTI BELT, GHANA, Au MINEPtALIZATION 299

dian about -10%o) manifest an origin of the sulfides via biogenie (earbonaeeous matter in Birimian sediments) reduction of sulfate from contemporaneous seawater and, furthermore, points to variably dosed systems, whereby more positive 634S values indicate environments with limited replen- ishment of sulfate (Lambert and Donnelly, 1990).

In the second group, sulfur isotope compositions of sulfides in hydrothermal ore deposits are controlled by the isotopic composition of the fluids, as well as temperature, Eh and pH at the site of mineralization (Ohmoto and Rye, 1979; Kerrieh, 1987). •Vhereas the first parameter is a characteristic of the soume, the others relate to the environment of deposition.

With regard to sources, sulfur may originate from magmatie fluids directly or from the leaching or desulfidation of minerals in igneous rocks, hydrothermal sediments or elastic sedimentary rocks (Kerrieh, 1987). Equilibrium isotopic fraetionation between H,•S and most sulfides is small (max _+ 2%0) at temperatures above 250øC (Ohmoto and Rye, 1979). Constraints on ambient temperature, Eh, and pH can be obtained from alteration and mineralization parageneses. Mi- kueki and Ridley (1993) concluded from the investigation of ore and alteration assemblages in Arehean gold deposits that the pH of the ore fluids was near-neutral to slightly alkaline, never far from equilibrium with serieite-albite, at temperatures of low to mid-greensehist fades conditions. Further- more, the fluid inclusion inventory and ore parageneses indicate that most deposits formed from relatively reduced ore fluids (Phillips and Groves, 1983; Mikueki and Ridley, 1993), the fluid redox state generally being below the SO.2/H2S boundary (Kerrieh, 1987) and above the COz/CH4 buffer (Mikueki and IRidley, 1993).

Sulfides (mainly pyrite, pyrrhotite, arsen•o•4Yrite ) in most Archcan mesothermal gold deposits display 6 S values in the range +7 to -3 per mil (Lambert et al., 1984; Kerrieh, 1987; Colvine et al., 1988; de iRonde et al., 1991, 1992). The relatively tight clustering of 6a4S values is interpreted to indicate that the fluid redox state was below the SO•/H2S equal concentration boundary and that the sulfur soume was isotopically uniform (Kerrieh, 1987). A direct derivation of the sulfur from magmas or indirectly by dissolution and/or desulfidation of magmatie sulfides is indicated (Lambert et al., 1984; Ker- rich, 1987).

However, exceptions to the above main range of sulfur isotope compositions are also to be accounted for and are exemplified by the Golden Mile in Western Australia (range -4 to -10%o; Phillips et al., 1986) and the Hemlo deposit in Canada (range 0 to -17.5%o; Cameron and Hattori, 1985). The above unusual light sulfur isotope compositions and the presence of various sulfates and/or hematite in these deposits were interpreted to point to the involvement of oxidized fluids or extensive fluid-wall rock reactions leading to an oxidation of the fluids (Cameron and Hattori, 1985; Phillips et al., 1986; Kerrieh, 1987; Lambert and Donnelly, 1990).

In the Ashanti belt of Ghana, sulfides related to hydrothermal gold mineralization are isotopically light with 634S ranging from -4.6 to -11.4 per mil (Ashanti, Bokitsi, Bogosu, and Prestea mines; Table 3 and Fig. 7). Exceptions are sulfides from Konongo (-0.5 to -3.1%o) and one sample from the granite-hosted Ayanfuri deposit (0.7%0).

Considering the most important factors for these unusual

light sulfur isotope compositions, their depositional environment, and soume, the following statements can be made.

Mineralogieal, geochemical, and fluid inclusion studies of the Ashanti belt mineralization (Oberthiir et al., 1994) suggest a general similari• to Archcan hydrothermal gold deposits. In the Ashanti belt, ore deposition was largely synmetamorphic and took place under greenschist facies metamorphic conditions in the temperature range of 400 ø _+ 50øC. Fluid oxygen fugaeity constraints, as delineated by Mikueki and iRidley (1993), also indicate that the oxidation state of the fluid was above the CO•/CH4 buffer, but below the SO,,/ H,•S equal concentration boundary. This is indicated by fluid inclusion analyses (CO,• >> CH4), the paragenesis arsenopyrite-pyrrhotite-pyrite, the presence of a powerful redox buffer in the form of ubiquitous earbonaeeous matter, and at the same time the absence of hematite and sulfates in the host

rocks and ores (Oberthiir et al., 1994). The light sulfur isotope compositions of sulfides in the

majority of the Ashanti belt gold deposits, therefore, appear neither to be the products of unusual local depositional conditions with respect to pH or temperature nor a reflection of oxidized fluids relative to the SO,•/H2S buffer. Indeed, mineralogieal and geoehelnieal data of host rocks and ores point to fluid properties characteristic of reduced greenschist fades lode gold deposits in the sense of Mikueki and iRidley (1993).

Therefore, the light sulfur isotope compositions of sulfides in the Ashanti belt rather reflect a spedfie soume. Tapping of this soume by hydrothermal fluids leading to leaching and/ or desulfidation, uptake into and transport by the fluids as well as later precipitation of sulfides would result in negligible equilibrium isotopic fraetionation if the assumed process takes place at elevated temperatures (>300øC) and if only reduced sulfur spedes are involved (Ohmoto and iRye, 1979). The most likely candidate for a light 6a4S reservoir is the suite of synsedimentary-diagenetie sulfides present in the Birimian metasediments. These sulfides display a wide range of 6a4S values (+7.3 to -20.9%0), but possess a similar median 6a4S value (-10%o) compared to that of sulfides related to hydrothermal gold mineralization (about -7%0). The latter are also characterized by a tight unimodal distribution of 6a4S values, indicating a large, homogenous fluid reservoir. The depleted light isotopic signatures, therefore, are interpreted as inherited from their soume; i.e., they result from remobilization and isotopic homogenization during transport of preexisting sulfides in Birimian metasediments.

The near-magmatie 6a4S values of gold-related sulfides from Konongo may be related to slightly different mean 6a4S compositions of the sulfides leached (ef. spread of data of sulfides in Birimian metasediments; Table 3). Fluids with magmatie signatures may be implied for the granite-hosted Ayanfuri deposit.

Conclusions

The soume of mineralizing fluids of (Archcan) mesothermal gold deposits is under debate, and the following major hypotheses are in vogue at present: (1) strueturally focused metamorphic dewatering and outgassing at the greensehist- amphibolite facies transition of greenstone piles (e.g., Phillips and Groves, 1983), (2) mantle degassing and granulitization

300 OBERTHORET AL.

at deep crustal levels (e.g., Colvine et al., 1988), and (3) magmatie derivation from felsie intrusions (e.g., Burrows and Spooner, 1987). Notably, one must also take into account that the sources of the fluids, solutes and volatiles may be independent (Kerrieh, 1987).

The isotopic fingerprints of the systems studied (6•:3C, 6•sO, 6D, 634S) in the Ashanti belt of Ghana lead to the following interpretations:

1. Carbonate 615C and gold-related sulfide 654S values indicate metamorphic fluids in the sense of Phillips and Groves (1983), originating from or at least equilibrating with Birimian strata at deeper crustal levels.

2. Quartz and carbonate 61sO and 6D values of water in fluid inclusions point to fluid sources of metamorphic or magmatic origin. Massive streaming of CO2 is manifested in pervasive carbonatization and the CO2-dominant fluid inclusions in the auriferous quartz veins. CO2 may have been expelled by granulitization of the lower crust which, however, appears unlikely in the Birimian terrane of Ghana due to the fact that only juvenile crust is present (Taylor et al., 1992). Alterna- tively, CO• may originate from the decarbonatization of Biri- mian lithologies at depth, or may have been contributed from the mantle directly. Certain magmatic contributions to the fluid systems may be indirectly inferred from the at least temporally close link between basin-type granitoids (2116- 2088 Ma) and gold mineralization (2120-2070 Ma).

Collectively, the stable isotope signatures of hydrothermal systems in the Ashanti belt of Ghana point to a homogeneous and uniform erust-equilibrated fluid reservoir which attained its characteristics from extensive fluid-rock reactions with Bir-

imian strata at deeper erust'al levels. In the Ashanti belt of Ghana, deformation, metamorphism,

gold mineralization, and intrusion of the basin-type granitoids all seem to be interrelated to various degrees, brought about by the Eburnean tectonothermal event at ca. 2.1 Ga.

Substantial northwest-southeast-directed crustal shorten-

ing and subvertical extension took place in southwest Ghana (Eisenlohr, 1989; Blenkinsop et al., 1994), connected with the downfolding of Birimian strata. Dewatering and decarbonatization of this rock pile during progressive metamorphism at depth may have generated the fluids which were focused into tectonically distinct zones, that is, rheologically con- trasting contact zones between Birimian metasediments and metavolcanics.

Deduced from and most compatible with the present data set, is the proposition that metamorphic devolatilization of Birimian strata at depth most probably was the predominant process of fluid generation. Similar fluid-rock reactions and isotopic equilibration, as envisaged here, may have taken place in the case of Archcan mesothermal gold deposits. However, the resulting effects are proposed to be less well recognizable due to quantitative and qualitative peculiarities of the Archcan greenstone belt successions, where carbonaceous schists (6 C) probably constitute <1 percent of the rock assemblages, and an oxygen-poor atmosphere coupled with a sulfate-poor hydrosphere (6•4S) did not promote sulfur isotope fractionations which would lead to sedimentary rocks with distinct isotopic compositions.

Acknowledgments

The present work is a contribution of the Bundesanstalt ftir Geowissenschaften und Rohstoffe (BGR) project "Metal- logenesis of Gold in Africa." Sincere thanks go to the Geologi- cal Survey Department, Accra, Ghana, especially its director, G. O. Kesse, for providing logistic assistance in Ghana. The management of the Ashanti Goldfields Company Ltd. actively supported the work through generous hospitality, access to mine data, and guidance on surface and underground, su- perbly organized by our colleagues of the Geology Depart- ment. The State Gold Mining Corporation, Cluff Minerals, and Canadian Bogosu allowed access to their properties and provided useful information. Joe Areanor, Roger Kumi, Wyl- lie Gyapong (Ashanti), and Jeff Nichols (Cluff) deserve special thanks for their continuous cooperation. The manuscript benefited from critical reviews by A. Techmer, Hannover, and two Economic Geology reviewers. G. GOdecke, Hanno- ver, ably typed the manuscript.

October 2,5, 1994; November 6, 1995

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