Geochemistry of Precarnbrian basaltic rocks from the Central African Republic (Equatorial Africa)

J . DOSTAL Department of Geology, Saint Mary's University, Halifax, N.S., Canada B3H 3C3

C. DUPUY Centre géologique et géophysique, USTL, Place E . Bataillon, 34060 Montpellier, France

AND

J. L. POIDEVIN Departement de géologie et mineralogie, Université de Clermont, 63038 Clermont-Ferrand, France

Received April 9, 1984 Revision accepted November 6, 1984

The two Archaean greenstone belts (Bandas and Bogoin) in the Central African Republic (Equatorial Africa) are 250 and 150 km long. The metavolcanic rocks in the belts are predominantly komatiitic and tholeiitic basalts. Komatiites include both Al-depleted and Al-undepleted types. The komatiites and light-REE-depleted tholeiites were probably derived from a similar upper mantle source. However, the tholeiitic basalts enriched in light REE from the upper volcanic strata of the Bandas belt were generated from a different source. The dolerites from Proterozoic dyke swarms and sills differ from the basalts mainly in their abundances and ratios of several incompatible elements such as K, Rb, Th, and light REE. They were derived from a distinct, incompatible-element-enriched upper mantle source.

The average background gold levels in th;Ëandas belt and dolente dyke swarms are comparable to those in equivalent rocks from North America. The exception is the Bogoin greenstone belt, which has anomalously high gold abundances.

Les deux ceintures de roches vertes archéennes (Bandas et Bogoin) présentes dans la République africainne centrale (Afrique équatoriale) s'étendent sur une longueur de 250 et 150 km. Les roches métavolcaniques dominantes dans les ceintures sont des basaltes komatiitiques et tholéiitiques. Les komatiites et les tholéiites appauvries en éléments des terres rares légers émanent probablement d'une source commune correspondant au manteau supérieur. Cependant, les basaltes tholéiitiques enrichis en éléments des terres rares légers de la couche volcanique supérieure de la ceinture de Bandas prennent leur origine à une source différente. Les dolérites formant les groupes de dykes et les filons-couches protérozoïques diffèrent des basaltes principalement par leurs teneurs et leurs rapports de plusieurs éléments incompatibles, tels le K, Rb, Th et les éléments des terres rares légers. Elles doivent leur origine à une source distincte qui serait un manteau supérieur enrichi en éléments incompatibles. Les concentrations moyennes en or des roches de la ceinture de Bandas et des groupes de dykes sont comparables à celles de roches équivalentes en Amérique du Nord. Cependant la ceinture de roches vertes de Bogoin se distingue par des teneurs en or plus élevées que la normale.

[Traduit par le journal] Can. 1. Earth Sci. 22, 653-662 (1985)

Introduction Geology Abundant geochemical data are available on lavas of

Archaean greenstone belts, particularly those frorn Australia, Canada, and South Africa. However, there is little comparable information on many of these belts in central and eastem Africa. Likewise, information on Proterozoic basaltic dykes from central Africa is sparse.

The purpose of this paper is to provide some geological and geochemical data on the Archaean greenstone belts and on the Proterozoic doleritic dyke swarms and sills frorn the Central African Republic (Equatorial Africa). Together, these data rnight yield information on the temporal evolution of sub- continental upper mantle sources during the Precambrian. The paper also provides data on the abundances of Au in the basalts and dolerites in order to establish their background levels. Not only is such information rather limited but also the currently available data are ambiguous. Some data (e.g., Boyle 1961; Viljoen et al. 1970; Saager et al. 1982) suggest that the average Au abundances in the mafic volcanics of the belts are higher than those of equivalent Phanerozoic rocks (e.g., Gottfried et al. 1972), whereas other results (e.g., Anhaeusser et al. 1975) indicate that the abundances in al1 these rocks are comparable.

The Precarnbrian rocks that are part of the Equatorial African craton in the Central African Republic (CAR) form a typical granite-greenstone terrain (Fig. 1). They can be subdivided into seven main lithostratigraphic units (Mestraud 1964, 1971). The oldest two units, one composed of granulites and the other composed of granitic (rnainly tonalitic) gneisses and amphib- olites, represent a polyrnetamorphic basement. The basernent extends into the south Cameroon, where it is dated at 2890 + 230 Ma (Delhal and Ledent 1975) and 2920 i 40 Ma (Lasserre and Soba 1976), and into Zaire, where its age is 3086 + 94 Ma (Lavreau and Ledent 1976).

The third unit includes two subparallel greenstone belts (Bandas and Bogoin) that run across the CAR in a roughly west-east direction for distances of 250 and 150 km. They are considered to be the northem extension of the Kibali- Ituri belt of Zaire. The belts lie unconformably upon the basement com- plex (Poidevin 1977) and were intruded by nurnerous granitic bodies. They are composed of metavolcanic - metasedirnentary sequences, which always include volcanic rocks, itabirites, and graywackes. The longer northem belt of Bandas was cross-cut by a granodiorite pluton dated at 2828 2 78 Ma (Lavreau et al. 1980). The ages of several other plutons that intersect the belts

l 654 CAN. J. EARTH SCI. VOL. 22, 1985

FIG. 1. Generalized geological map of the Central Afncan Republic and third (3) generations; (4) greenstone belts (upper: Bandas; lower: (A) La Basse Vallée de la Lobaye; (B) Les Rapides de 1'Eléphant; (C

range from 2.2 to 2.6 Ga (Cahen et al. 1976; Poidevin 1979~) . The fourth unit is composed of quartz schists that un-

conformably overlie the greenstone belts and the basement complex. The schists are locally migmatized and granitized and contain intercalated amphibolites and orthogneisses. The preliminary data indicate an age of more than 2500 Ma. Unit 5 consists of quartzites and schists metamorphosed under greenschist-facies grade. They are probably older than 2.1 Ga (C. Pin, personal communication, 1983). The upper Proter- ozoic sequences include schists and carbonates (unit 6), which extend over most of the CAR, and an overlying series of pelites and carbonates (unit 7), which are restricted to the east-central part of the CAR. The Precambrian terrain has also been in- vaded by three generations of dolerite intrusions. The first generation of dolerite dykes is contemporaneous with unit 6, whereas the second dolerite generation cross-cuts al1 Pre- cambrian units but is affected by the Pan-African orogenic event. The last generation postdates the Pan-African orogeny.

Greenstone belts Both greenstone belts of the CAR are typical Archaean

volcano-sedimentary sequences surrounded by various gran- itoids. The sequences are not continuous but occur as isolated bodies several kilometres in size. The lithostratigraphy of the Bandas belt is most complete in its southernmost part (about 50 km north-northeast of Bambari), where it reaches a total thickness of more than 4 km. The belt consists of two units

I (Poidevin 1979 b; Poidevin et al. 198 1); the lower unit includes three stratigraphic cycles. The lowermost cycle exposed at Goumbrou reaches a thickness of about 500 m and is made up of biotite schists intercalated with amphibolites and itabirites. The second cycle, about 2000 m thick, is represented by alter- nating basalts and itabirites, whereas the third cycle, about 1000 m thick, is formed by basalts, andesites, and itabirites. The upper unit is mainly metasedimentary rocks and some interbedded acid volcanic tuffs.

The southern greenstone belt of Bogoin is located about 100 km north of Bangui. Here, two units have also been identified. The lower unit is formed by tremolite-chlorite schists (with relict primary textures), which originally corresponded to py- roxenic and basaltic komatiites. Towards the top, komatiites

:. (1 -3) Areas with swarms of dolente dykes of the first ( l ) , second (2), Bogoin); (5) Precambrian terrains; (6) Cambnan and younger terrains.

1) Bakouma.

are intercalated with tholeiitic basalts. The thickness of this sequence is 400-700 m. The upper unit consists of graywackes overlain by itabintes, and its total thickness is 100-200 m. The samples of the greenstone belt volcanics were collected from the southern Bogoin belt and from the northem Bandas belt at Boufoyo. Data on rocks from another area (southernmost part) of the Bandas greenstone belt were previously given by Poidevin et al. (1981).

Only the two earliest generations of dolerites were studied. The dolerites of the first generation were emplaced about 950 Ma ago. Samples were collected from two localities: La Basse Vallée de la Lobaye, where dolerites mainly fqrm dykes, usu- ally several metres thick; and Les Rapides de I'Eléphant, where dolentes occur mainly in the form of sills, typically a few metres thick. The second dolerite generation was emplaced around 700 Ma (Poidevin 1979a) after the end of the main Precambrian sedimentation. Most of the intrusions are located in the southeastern part of the CAR and extend into Zaire. The dolerites studied were collected in the vicinity of Bakouma.

Gold mineralization The Precambrian terrain of the CAR has been extensively

prospected for gold from 1912 to 1957, and several minor gold deposits were discovered. These deposits are either alluvial or eluvial; no deposit was found in solid bedrock or in quartz veins. Most gold mineralization is associated with the greenstone belts. In the Bandas area, gold was mined in two small alluvial deposits. One deposit, located near Goubadjia, is associated with strongly carbonatized metabasalts, whereas the other gold showing, near Roandji, is related to silicified itabirites, rich in pyrite. In the Boufoyo area, several minor gold showings in the greenstone belt were discovered during regional reconnaissance although none was developed. In the southern greenstone belt, in the vicinity of Bogoin, a small gold deposit was mined until 1950. The eluvial deposit is hosted by strongly altered komatiites containing quartz veins.

Petrography Most Archaean metavolcanic rocks occur as massive

DOSTAL ET AL.

F e 0 + F e 2 O 3 + T i O 2

FIG. 2. Plot of Mg-Fe + Ti-AI (after Jensen 1976; Viljoen et al. 1982) for the basaltic rocks of CAR greenstone belts. (0) Boufoyo; (X ) Bandas (Poidevin et al. 1981); (@) Bogoin.

flows; less frequent are pillow lavas and vesicular flows. The rocks are mainly composed of secondary minerals. In the rocks with kornatiitic composition, the relicts of prirnary magrnatic assemblages, chiefly pyroxene, are very rare, although relicts of prirnary texture are more frequent. The komatiites are predorninantly composed of trernolite-actinolite and chlonte and variable arnounts of serpentine, plagioclase, carbonate, and rnagnetite. At higher metamorphic grades, the koma- tiites have homblende-trernolite-chlorite-dorninated rnineral assemblages.

The mafic rocks are mainly "low-grade" amphibolites formed chiefly by blue-green homblende and plagioclase (oli- goclase). Biotite, quartz, zoisite, and opaque rninerals are fre- quently present in minor amounts. The intercalated pyroclastic rocks contain rnineral assemblages that include quartz, biotite, blue - green homblende, and gamet. In several areas, however, the rnafic volcanics were rnetamorphosed into greenschist facies. Such rocks are composed mainly of actinolite and plagioclase (oligoclase) and even occasionally retain relicts of prirnary minerals-pyroxene and plagioclase (labradonte- bytownite).

The metavolcanic rocks were subsequently affected by retro- grade metamorphisrn, which led to the appearance of variable arnounts rnainly of actinolite, chlonte, carbonate, and leuco- xene and locally to carbonitization and epidotization. Minerals formed during this process are typical of greenschist facies but they cross-cut foliation, indicating a postrnetarnorphic origin. In carbonitized and epoditized rocks, carbonates and epidote are disseminated or form veinlets. As in the Barberton green- Stone belt (Condie et al. 1977), carbonates, epidote, and chlo- rite replace plagioclase and amphibole. The opaque rninerals in these rocks are rnainly hernatite-limonite and sulphide (chiefly pyrite).

The Proterozoic dolentes are fine-grained rocks that usually have well preserved ophitic and subophitic textures. Their pri-

Y o m a t i i t e c " 3 F i e l d

~holeii:: \

F i e l d ' '. 1 ' \ \ ' .

FIG. 3. A1203-Mg0 diagram (after Viljoen et al. 1982) for the basaltic rocks of CAR greenstone belts. (1) Al-depleted trend of kornatiites; (2) AI-undepleted trend of komatiites. (0) Boufoyo; (x) Bandas (Poidevin et al. 1981); (e) Bogoin. (A) Mg tholeiite; (B) normal tholeiite; (C) Fe tholeiite.

mary rnagmatic assemblages include plagioclase, augite, and opaques. Quartz is rarely found and olivine is absent. The dolentes have been subjected to low-grade regional meta- morphisrn during the Pan-African orogeny. The rnost frequent rnetamorphic assemblages are purnpellyite-prehnite in do- lentes of the second generation frorn Bakouma and albite- tremolite in the dolentes of the first generation frorn La Basse Vallée de la Lobaye and Les Rapides de l'Éléphant. Relict

656 CAN. J. EARTH SCI. VOL. 22, 1985

TABLE 1. Comparative geochemical data on koma- tiitic basalts from the Central African Republic and

Barberton greenstone belt (South Africa)

RG. 4. Chondrite-normalized REE abundances in the metavol- canic rocks of the greenstone belts. ( a ) Strongly altered komatiitic basalts (5490-Boufoyo; 5435-Bogoin); (b) Al-depleted komatiitic basalts (5428, 5433-Bogoin); ( c ) 5436-komatiite, Bogoin; 5492, 5493-tholeiites, Boufoyo; (d) 5434-tholeiite, Bogoin; 5491- tholeiite, Boufoyo.

igneous mineralogy is present in both generations.

Analytical notes The major elements were analyzed by wet methods, and Li,

Rb, Sr, Ba, V, Cr, Co, Ni, Cu, and Zn were determined by atomic absorption spectroscopy. Instrumental neutron activa- tion was used to analyze rare-earth elements (REE), Sc, Hf, and Th. The precision and accuracy of the trace-element determination have been reported by Dostal et al. ( 1 9 8 3 ~ ) . The precision is generally better than 10%. The gold was determined by neutron activation at X-ray Assay Laboratories, Toronto. The technique involves preconcentration by fire assay followed by irradiation. Included in the analyzed suite were three aliquots of a single sample, which gave a reproducibility of about 35%.

Geochemistry Greenstone belts

Major elements The analyzed rocks of the greenstone belts have a basaltic

composition and display variable contents of major elements. They can be divided into two groups. Group 1 has a high Mg0 content and plots in the komatiite fields of the Mg-Fe + Ti- Al (Fig. 2) and A120,-Mg0 (Fig. 3) diagrams. The second group has a higher content of A1203 and falls into the tholeiitic fields of Figs. 2 and 3.

The rocks of group 1 have Mg0 contents ranging between 13

Central African Barberton Republic (Smith and

Erlank 1982) (1) (2)

Sample: 5428 5437 (1) (2)

NOTES: (1) Al-depleted komatiitic basalt; (2) Al-undepleted komatiitic basalts.

and 25% and their composition is typical of komatiitic basalts (Nesbitt et al. 1979; Viljoen et al. 1982). On the A1203-Mg0 diagram (Fig. 3), most samples of this group plot along the komatiitic A120,undepleted trend of Viljoen et al. (1982). Like most komatiites they have CaO/AI2O3 ratios of about 1 and A120,/Ti02 ratios around 20 (i.e., similar to those of chondrites). Two other komatiitic samples fa11 on the depleted AI2O3 trend of Viljoen et al. (1982) in the A1203-Mg0 diagram, have lower A1203/Ti02 ratios (10- 11) and are very similar to the komatiites of the Barberton belt (Nesbitt et al. 1979). Al1 the rocks of this group have CaO/Ti02 (18-20) close to chondritic values. The exceptions are samples 5435 and 5490, which are depleted in Cao, probably because of their extensive alteration (chloritization and serpentinization).

The second group is composed of tholeiitic basalts, which are compositionally distinct from komatiites (Figs. 2, 3), sug- gesting the existence of two discrete volcanic suites. In general al1 the rocks show an increase of Fe and Ti with differentiation (typical of tholeiites), have variable Na20 contents, and are depleted in K20. In addition, the basalts from Bogoin are low in Ti02.

In the southernmost part of the Bandas greenstone belt, Poi- devin et al. (1981) have also identified basalts and andesites distinctly enriched in some incompatible elements including light REE (LREE). These rocks occur in the higher strati- graphie levels of the belt and have not yet been found in the Bogoin and Boufoyo areas. The basalts and calc-alkali an- desites resemble, respectively, the enriched Archaean tho- leiites and low-alkali Archaean andesites of Condie (1976). Except for their higher contents of transition elements, the andesites are also similar to recent orogenic andesites.

Alteration The analyzed volcanic rocks of the greenstone belts were

affected by alteration processes such as hydration, epi- dotization, and carbonatization, which probably modified the primary chemical composition of the rocks, particularly the abundances of the alkali and alkali-earth elements. Among the analyzed komatiites, two (5435 and 5490) that are strongly chloritized and serpentinized have a high loss on ignition (LOI) and are depleted in Cao, resulting in anomalously low CaO/A1203 (-0.8). The chondrite-normalized REE patterns of these two rocks display a small negative Eu anomaly, a distinct depletion in Ce, and an enrichment in La, probably as a result of secondary processes. It has been shown that under certain conditions REE may be mobile during alteration (e.g., Hellman et al. 1977, 1979; Dostal et al. 1980).

TA

BLE

2.

Che

rnic

al c

ompo

sitio

ns o

f m

etav

olca

nic rocks

from

Bog

oin

and

Bou

foyo

Bog

oin

Bou

foyo

,542

8 54

33

5436

54

37

5435

54

31

5432

54

29

5434

54

30

FE16

54

90

5493

54

89

5492

54

94

5491

54

88

Sio

z(%

) 47

.65

51.0

8 47

.29

50.6

4 45

.79

53.8

4 52

.61

50.8

4 48

.04

50.9

7 50

.40

47.0

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.54

49.8

6 49

.16

50.1

5 45

.18

55.4

7 A

1203

5.

84

7.30

8.

10

7.57

7.

07

13.9

6 17

.09

15.8

9 15

.64

15.8

5 16

.37

7.15

16

.00

13.8

9 13

.94

13.6

3 14

.61

13.1

8 Fe

&3

13.3

1 12

.22

10.4

3 10

.26

14.4

4 7.

90

6.80

11

.56

13.0

5 11

.92

14.5

1 4.

19

3.97

1.

75

2.08

2.

96

3.00

1.

39

Fe0

5.

97

4.42

9.

06

10.2

3 9.

60

11.8

2 6.

01

Mn

0 0.

20

0.19

0.

18

0.22

0.

22

0.17

0.

12

0.18

0.

20

0.18

0.

51

0.19

0.

18

0.18

0.

20

0.20

0.

20

0.18

M

g0

18.2

9 13

.27

21.0

5 18

.04

20.1

2 11

.42

10.0

2 7.

23

7.73

6.

26

4.25

24

.93

8.89

9.

56

8.67

7.

53

8.42

3.

66

Ca0

9.

63

13.0

2 7.

69

9.85

5.

84

5.88

6.

86

10.0

2 10

.85

10.3

8 11

.26

5.44

10

.67

10.4

8 10

.86

10.6

8 10

.21

14.6

9 N

azO

0.

14

0.61

0.

50

0.88

0.

20

1.75

2.

58

2.20

2.

23

1.81

0.

85

0.57

1.

37

2.17

1.

93

1.94

2.

76

1.31

K20

0.02

0.

06

0.08

0.

06

0.01

0.

02

0.06

0.

23

0.20

0.

25

0.18

<

0.02

0.

03

0.04

0.

04

0.04

0.

34

<0.

01

Ti0

2 0.

55

0.69

0.

42

0.42

0.

52

0.28

0.

31

0.66

0.

98

0.69

0.

58

0.41

0.

77

0.77

1.

03

1.00

1.

72

1.77

pz

05

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0.

09

0.08

0.

09

0.08

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I 4.

21

1.40

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01

2.07

5.

05

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0.95

0.

92

1.14

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1.

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1.57

1.

61

1.25

1.

75

0.92

2

99.8

4 99

.84

99.7

5 10

0.01

99

.26

99.5

2 99

.83

99.7

6 99

.84

99.4

5 99

.75

100.

91

99.9

1 99

.41

99.8

4 99

.06

100.

14

98.7

4

Li (

ppm

) 10

27

24

17

13

15

11

54

25

48

40

60

64

34

18

14

2 P

Rb

< 1

1 1

1 1

2 1

2 4

2 1

8 14

18

10

6

32

F

Sr

11

102

8 3

<2

58

110

100

97

106

3 13

6 14

3 11

4 93

16

4 77

2

Th

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1.5

0.7

<O. 1

C

O.1

0.

3 0

.3

0.1

0.6

0.9

0.6

0.3

0.3

0.

4 1.

5 r

Hf

1.4

1.2

0.5

0.6

0.

7 0.

4 0.

5 1.

2 1.

7 1

.3

0.70

1.

1 1.

7 2.

9 La

12

.3

4.59

0.

96

2.30

3.

3 1.

70

1.5

4.9

3.8

5.6

6.5

2.0

4.1

8.6

Ce

21.4

9

.4

3.10

4.

7 5.

9 4.

5 4.

1 8.

6 9.

3 9.

7 5.

5 5.

6 8.

4 16

.5

Sm

3.47

1.

94

0.79

1.

11

2.22

0.

92

1.08

2.

00

2.29

1.

86

2.58

1.

55

2.37

4.

01

Eu

0.72

0.

64

0.29

0.

33

0.69

0.

31

0.34

0.

68

0.79

0.

67

0.70

0.

66

0.87

1.

33

Tb

0.57

0.

36

0.22

0.

28

0.49

0.

22

0.28

0.

51

0.53

0.

47

0.73

0.

43

0.71

0.

99

Yb

1.20

1.

20

0.85

1.

20

1.71

1.

38

1.48

2.

11

2.32

2.

17

2.26

1.

70

2.43

3.

43

Lu

0.19

0.

21

0.15

0.

20

0.27

0.

25

0.27

0.

37

0.36

0.

35

0.36

0.

28

0.40

0.

57

Sc

21

39

25

27

34

42

42

49

38

46

36

38

45

36

V

123

219

185

195

178

153

130

300

295

300

181

235

254

302

294

293

417

Cr

1055

15

10

1815

17

85

2070

46

5 69

0 19

8 30

0 24

5 18

50

259

352

376

264

79

106

Co

55

58

78

78

85

44

37

42

62

46

78

79

46

46

46

62

Ni

835

320

835

1065

10

35

176

203

93

228

100

900

433

95

94

103

118

222

Cu

3 19

28

42

23

1 4

4 19

10

6 78

3

82

59

89

87

102

117

Zn

168

86

120

70

145

101

40

91

104

103

123

68

77

90

90

125

82

Au (PP~)

44

20

36

40

38

53

38

42

42

18

3 1

2 4

7 1

658 CAN. 1. EARTH SCI. VOL. 22, 1985

Among the tholeiitic basalts, two sarnples (5431 and 5432) that are strongly chloritized and carbonatized have high LOI and display a strong depeletion of Cao. However, the con- sistency of the corresponding REE patterns indicates that the REE distribution is not signifjcantIy affected by alteration. The one exception is Eu. which often displays a negative anomaly geneerally interpreted to he either the e f k t of alteration (Sun and Nesbitt 1978) or the result of an Eu-depleted source (Jahn et al. 1980). The high and variable content of Li in the annlyzed rocks is probably due ta secondary processes. Such high abundances are unusual in fresh basaltic rocks and are mostly restricted to those having undergone zeolite- and (or) greenschist-facies rnetamorphism (Dostal et al. 1983b).

Trace elemcnts Ni, Cr, and, to a lesser extent. Co show a positive correlation

with MgO. For a given Mg0 content, the Ni, Cr, and Co contents of tholeiitic and komatiitic basalts are sirnilar to those in other greenstone belts (Nesbitt and Sun 1976; Jahn ei al. 1980). Cu and Zn are strongly dispersed, probably because of metamorphic alteration; in general the two elements have contents similar to those reported from altered rocks of the Barberton greenstone belt by Glikson (1979). V increases with degree of differentiation and is strongly correlated with TiOz. The Ti/V ratio ranges between 10 and 20 for most samples. These values are common in the basalts of greensione belts (Nesbitt and Sun 1976), but they are higher than chondritic values and lower than those of mid-ocean ridge basalts (MORB). The two kornatiitic basalts with high CaO/AI2O3 also have higher Ti/V ratios (19-27) than the other komatiitic basalts, which have a Ti/V range of 13- 14. A similar re- lationship among komatiitic basalts was found in the Barberton belt (Smith and Erlank 1982).

In addition to K and Ti, the analyzed rocks are depleted in several incompatible elements, especially Rb, Sr, and Th; these low abundances are typical of greenstone belts (Nesbitt et al. 1979: Jahn et al. 1980). The REE distributions in the basalts are not uniform. With the exception of the two strongly altered samples (Fig. 4a) , three main pattems can be discemed in the komatiitic and tholeii tic basalis:

(a) REE patterns with LREE enrichment and fractionated heavy REE (HREE) (Fig. 4h). These patterns are found in the komatiites depleted in Al and are also encountered in Al-depleted samples from the Barberton greenstone k l t (e.g., sample 331/78 of Sun and Nesbitt 1978).

((7) Concave REE patterns with slightly depleted LREE and HREE (Fig. 4c). The patterns that are present in both koma- tjites and tholeiites are very similar to those of sample 87 J of Nesbiît et al. ( 1 979) f m the Barberton area.

( c ) Rat REE patterns wjthoui any significant fractionation (slightly affected by alteration), which are also encountered among the rocks of the Barberton belt (Fig. 4d).

D f scussion The available geochemicai data can be used to put

constraints on the petrogenesis of various rock types of the greenstone belts, particujarly on the origin of komatiites with variable CaO/A1201 ratios and their relationship with closely associated tholeiites. Hawever, any petrogenetic model should take into account the fact that most of the rocks are altered and their corresponding trace-element distribution has been af- fected to variable d e p s . Furthemore. as these rocks were probably formed under high temperature, the comspnding partition coefficients are not well known.

Whereas most komatiites are undepleted in aluminum, the komatiites from the Barberton bel( and also frorn some other South African belts (Viljoen et al. 1982) are characterized by AI depletion accompanied by high CaO/AI20, ratios. Some of the studied komatiites are also Al depleted. ln fact, the rocks of the greenstone belts fmm the CAR have several features in common with the Barberton rocks (Table l), including the presence of both aluminurn-depleted and aluminum-undepleted komatiitic lavas. Several hypotheses have been invoked to account for the occurrence of both types of komatiites. Nesbitt et al. (1979) have argued that the presence of gamet in the residue after melting can explain the AI20, depletion of some komatiites; altematively, the difference between the two types may reflect significant lateral mantle beterogeneity. Smith and Erlank (1982) have suggestcd a model involving a high degree of melting, with olivine as the only residual phase for the Al-undepleted komatiite, whereas rnodels assuming harz- burgite residue and a magma mixing process (e.g., Cox 1978) have k n invoked for the Al-depleted komatiites.

In a cornparison of AI20,-undepleted and Al103depleted rocks (e.g., sarnple 5437 versus sample 5428) that have the same Mg0 contents (Table 2) it appears that the Al-dcpleted kornatiites are characterized by lower Sc, V , Cr, Co, Ni, and higher incompatible elements such as Ti, Th, Hf, and LREE. In addition, these rocks have higher ratios involving elements of variable degrees of incompatibility (e.g., Ti/V). These characteristics and the difference in LREE contents between samples 5437 and 5428 cannot be explained in ternis of variable degrees of partial melting of the same source rock. The higher contents of LREE and some other incompatible ele- ments in sample 5428 are probably due partly to its more extensive alteration and partly to the heterogeneity of the upper mantle (cf. Sun and Nesbitt 1978; Jahn et al. 1980).

It is probable that there is a genetic link between komatiites and basalts of greenstone belts, but the nature of this re- iationship i ç still in dispute. Naldrett and Turner (1977) and Whitford and Amdt (1978) argued that al1 these rocks can be derived from ultramafic parental magma by low-pressure frac- tional crystallization. On the other hand, because of a com- positional gap in the range of about 12- 18% MgO, Nesbin and Sun (1976) suggested that the tholeiitic basalts are not the product ~Fdifferentiatjon of a high-Mg0 suite but were derived from a primary melt (witti about 12% MgO) produced by a lower degree of partial melting. Such a liquid would sub- sequently generate a specmrn of basaltic lavas with Mg0 in the range of 4 - 8%.

The data in Table 2 show that several samples with variable Mg0 contents (4-22%) are genetically related. For example, this is the case with samples 5437,5430, and 5429, which have the same Ti/V (13- 14) and Sc/Yb (21 -23) ratios. In these samples, the conaancy of the [Srn/YblN ratio (-1) and the increase of Sc towards the most differentiated rocks indicate that the fractionation trend was dominated by olivine, as suggested for mafic rocks of greenstone belts by Whitford and Amdt (1978).

The basalts e ~ c h e d in incompatible elements from the southernmost part of the Bandas greenstone belt are not related to the other tholeiitic basalts by fractional crystallization or partial melting of a single upper mantle source (Poidevin et al. 198 1). These processes cannot explain the differences in LREE contents and incompatible-element ratios and the sirnilarities in the major- and transition-element abundances between the two groups of rocks. Poidevin et al. (1981) concluded that the

WSTAL ET AL.

TABLE 3. Chemical compositions of representative samples of dolerites

La Basse Vallée de la Lobaye Les Rapides de l'Éléphant Bakouma

5475 5477 5479 5480 5483 5487 D85 D96

SiOP (%) Al203 Fez03 Fe0 Mn0 Mg0 Ca0 NatO KzO TiO, Pz05 LOI Z

Li ( P P ~ ) Rb Sr Th Hf La Ce Sm Eu Tb Yb Lu Sc v Cr Co Ni Cu Zn Au ( P P ~ )

normal and enriched tholeiites were denved by partial melting from different upper mantle sources. The source rocks differed mainly by relative abundances of LREE. The origin of the andesites with calc-alkaline characteristics (including a distinct enrichment of incompatible elernents) by upper mantle anatexis would reqaire either a very small degree of melting or a strongly LREE-enriched source. More probably they were formed either by fractionai crystallization of basaltic magma enriched in LREE by equilibrium melting of eclogite or gamet amphiboIite.

Dolerites The major- and trace-element data for eight representative

dolentes are given in Table 3. Al1 the rocks have a basaltic composition and most are olivine tholeiites. The others are more differentiated and contain quartz in their noms. The tholeiitic character of the dolerites is further supported by the increase of Ti and Fe with the degree of differentiation. Al1 the analyzed m k s are relatively highly fractionated; their [Mg] values (= Mg/(Mg -t Fezt)) range between 0.6 and 0.2. Compared with basdts from the greenstone belts, the dolerites have higher Fe. Ti, and other compatible-element contents for quivalent MgO. Chondrite-normalized REE patterns of dolentes (Fig. 5) are subparallel and are characterized by LREE enrichment and fractionated HREE, a shape typical of continental tholeiites (Dupuy and Dostal 1984). Some more

FE. 5. Chondrite-normalized REE abundances in $lerites. (a) La Basse Vallée de la Lobaye; (b) Les Rapides de I'Eléphant; (c) Bakouma.

660 CAN. J. EARTH SCI. VOL. 22, 1985

TABLE 4. Gold contents of Precambrian basaltic rocks €rom CAR

Gold contents (ppb)

Greenstones Bandas 7 8.4 8.6 Boufoyo 6 3.0 2.3 Bogoin 10 37.1 10.6

Dolerites La Basse Vallée de la Lobaye 6 6.8 8.4 Les Rapides de I'Eléphant 9 5.4 4.6 Bakouma 7 3.7 1.6

- NOTES: N = number of samples; X = average; SD = standard

deviation.

differentiated dolerites also have negative Eu anomalies. Although the dolerites are of different ages, their REE patterns and contents are similar: the La/Yb ratio evolves from 4.4 to 7.4, increasing with differentiation. The abundances of other immobile incompatible elements such as Th and Hf are also similar in dolerites of different ages but equivalent degree of differentiation.

The abundances of V and Sc increase with differentiation until [Mg] = 0.4, then decrease in more differentiated samples. Ni decreases with differentiation, whereas Cr and Zn remain fairly constant, indicating the fractionation of olivine. The high Cu content is typical of continental tholeiites. In doleetes from La Basse Vallée de la Lobaye and Les Rapides de l'Eléphant, Cu content varies between 100 and 300 ppm irrespective of differentiation. On the other hand, in the dolerites from Bakouma, Cu is strongly enriched (292-1493 ppm) and in- creases with differentiation. The Ti/V ratio varies between 35 and 50. These values are twice as high as those in greenstone basalts because of higher Ti contents in dolerites.

Discussion The major difference between the basalts of greenstone belts

and dolerites is the distinct enrichment of some incompatible elements including LREE in the latter rocks. The dolerites have several other geochemical features that are also found in Pre- cambrian dolerites from areas such as Antarctica (Sheraton and Black 1981) and Scourie (Weaver and Tarney 1981). In these instances, the incompatible-element enrichment has been at- tributed to metasomatism of an upper mantle source (Sheraton and Black 1981). Altematively, the high contents of in- compatible elements in dolerites could be attributed to crustal contamination, as argued for some continental tholeiites by Dostal et al. (19836) and Dupuy and Dostal (1984).

Distribution of gold l The average abundances of gold in the basalts and dolerites l from the studied areas are given in Table 4, and the Au contents

in some representative samples are reported in Tables 2 and 3. In addition, the tholeiitic basalts from the southernmost part of the Bandas belt were analyzed for Au. The samples were se- lected from a suite of rocks studied by Poidevin et al. (1981). Because of the large variations of the values within the indi- vidual suites and the limited number of samples analyzed, the averages should be taken with caution. Furthermore, some samples have Au abundances below detection limits. Thus, their contents were arbitrarily taken as 0.5 ppb.

The data in Table 4 show that there are significant differ- ences in Au abundances among the individual suites and also within the suites. in the two areas of the Bandas belt, the average gold abundances differ by a factor of more than two, and even within individual suites there is a large scatter of values, as indicated by the large standard deviation. The gold in .the Bandas suite displays a very strong correlation with Cu (correlation coefficient, CC, = f 0.94), suggesting that gold is related to the occurrence of Cu sulphides (cf. Sighinolfi and Gorgoni 1977). On the other hand. in the mcks from Boufoyo, Au does not correlate with the major and trace elements, and its content is close to the worldwide tholeiite average of 2.5 ppb (Gottfried et al. 1972). The volcanic rocks of the Bogoin belt have anomalously high contents of Au, stiowing litîle cor- relation with the major and trace elernents. These values are significantly higher than the values reported for typical green- stone belts (e.g., Anhaeusser et al. 1975; Kwong and Crocket 1978; Saager et al. 1982) but well within the range of meta- volcanic rocks From the greenstone belt of the Red Lake area (Crocket PI al. 1980). It is also possible that they reflect only a localized anomaly.

The dolerites have gold abundances similar to those of the basalts from the Bandas belt. The samples from La Basse Vallée de la Lobaye have a slightly higher average Au content and a morc scaitered distribution than rhose from Les Rapides de l'Éléphant and Bakouma. The gold in La Basse Vallée de la Lobaye dolerites correlates with Li (CC = +0.92) and Na (CC = +0.79), suggesting that its distribution is affccred by sec- ondary processes. On the other hand, in the rocks from Les Rapides de l'Éléphant, Au displays a positive correlation with M g 0 (CC = 4-03 1 ) and Co (CC = t-0.78). indicating that Au is associated with mafic rock-foming minerais. In dolerites [rom Bakouma, Au does not correlate with any other analyzed elements. Thus it appears that the gold abundance levels in the basalts and dolentes are related to different factors in vanous areas.

Conclusion Metavolcanic rocks f o m a significant part of the two Ar-

chaean greenstone belts in the Equatorial African craton. On the basis of geochemistry and stratigraphic position, the rocks can be subdivided into four p u p s . The lowesr two groups consist of komatiites and tholeiites. Komatiites include both Al-depleted and Al-undepleted types, similar to those de- scribed from the Barberton area (Nesbitt er al. 1979; Viljoen et al. 1982). The tholeiites have trace-element abundances in- cluding LREE depletion comparable to those of other Archaean low-K tholeiites or to those of the depleted Archaean tholeiites of Condie (1976). The komatiites and tholeiites were probably derived from a similar upper mantle source. The rocks of the upper two groups, which were found only in the southernmost part of the Bandas bel! (Poidevin et al. 1981), are tholeiitic basalts enriched in LKEE and andesites. Poidevin et al. ( 198 1) have concluded that the enriched tholeiites were generated from a source with higher LREE contents than those of the komatiite-tholeiite source. Minor andesites, the youngest rnetavolcanics, could have resulted from the fractional crystal- lization of basaltic magma enriched in LREE.

The dolentes differ from the basalts of greenstone belts rnainly in their abundances and ratios of several incompatible elements such as K, Rb, Th. and LREE. The differences cannot be explained only by variable degrees of melting or crystal fractionation; they also imply derivation from a distinct upper

DOSTAL ET AL. 66 1

mantle source. Such heterogeneity of the upper mantle may be due to metasomatism by a volatile fluid phase enriched partic- ularly in K, Rb, Th , and LREE (Mysen 1979; Wendlandt and Harrison 1979). The degree of enrichment of the Precarnbrian

l upper mantle sources for the greenstone metavolcanics and dolerites increases with decreasing age and suggests a complex history of the upper mantle.

l The average abundances of Au in basalts of the Bandas belt

are close t o the average Au contents of the Superior Province greenstone beIts (Goodwin 1982). Likewise, the Au abun- daoces of the doIcrite dykes are not v e y difkrenr fmm those of many continental tholeiitic rocks including dolentes (e .g . , United States FeologÎcal Survey standard rock W-1, Kwang and Cracket 1978). The exceptions are basalts from the Bogoin belt, which are strongly enriched in gold relative to the other analyzed rocks and to many rocks found in Archaean green- Stone belts. The data in hand cannot explain whether the anom- alously high Au content in the Bogoin klt is due either to its enrichment in the iipper mantle source or to local secondary dispersion. Howevez, it is suggested that these high values migha repsent the "prirnary" Au contents in the rocks, as advocated recently by Keays (1982) for some Archaean greenstone belts.

Acknowledgments The work was supported by the Natural Sciences and

Engineering Research Council of Canada (operating grant A3782) and the Centre géologique et géophysique, Mont- pellier, France.

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