Hydrothermal Alteration in Diénéméra-Gongondy Paleoproterozoic Porphyry Copper Deposit, Burkina Faso, West African Craton

Abstract

Diénéméra-Gongondy copper-gold deposit is one of the rare Paleoproterozoic porphyry copper deposit of West African Craton. Like most of the Precambrian porphyry copper deposits, the mineralization of Diénéméra-Gongondy copper-gold deposit still remains controversial. In this study, we used thirteen drill cores of “Projet Minier Gaoua” to characterize the petrographic, geochemical signature and ore mineralogy. However, δ34S isotopic analyses has been also conducted on these samples in order to determine the source of fluids associated to the Diénéméra-Gongondy copper-gold deposit. Macroscopic and microscopic observations revealed a complex lithology encompassing a wide range of subvolcanic to volcano-plutonic rocks variable in mineralogy and texture, dominated by several facies of microdiorites which host the mineralization. This mineralization is affected by several phases of hydrothermal alteration dominated at Diénéméra by propylitization. However, at Gongondy, phyllitic-carbonate alteration type is dominant. The geochemical study of alteration minerals confirmed the presence of these alterations, but the potassic alteration is absent, probably destroy by late alteration events or retrograde metamorphism. The δ34S isotopic analyses on pyrite form four samples of pyrite and chalcopyrite of Diénéméra and Gongondy deposits showed that the δ34S values range from −4.1 to 4.7. These values are in accordance with a magmatic origin of the sulfide, with a probable little contribution of late hydrothermal or meteoritic fluids. The (Cs + Rb)/Th normalized diagram confirms the potential mineralization of Diénéméra and Gongondy deposits, whereas the excess Aluminum diagram is confident for a porphyry type of mineralization. In definitive, all the results show that the Diénéméra-Gongondy copper-gold deposit is a porphyry type of birimian age affected by late stage magmatic, structural and hydrothermal events.

Share and Cite:

Belemsobgo, A. , Zongo, W. , Hien, B. , Wenmenga, U. and Naba, S. (2025) Hydrothermal Alteration in Diénéméra-Gongondy Paleoproterozoic Porphyry Copper Deposit, Burkina Faso, West African Craton. Journal of Minerals and Materials Characterization and Engineering, 13, 123-155. doi: 10.4236/jmmce.2025.134009.

1. Introduction

Porphyry copper is a copper, gold and molybdenum mineralization with low grade to high tonnage. The mineralization represents 50 - 60wt.% of copper, 95wt.% of Molybdenum production in the world and other by-product minerals including gold, silver, tungsten, tin. The mineralization is associated with some complex porphyritic intrusions with intermediate to mafic compositions, emplaced at depth of 2 km - 4 km [1] [2]. The deposits are generally associated with subduction zones, preferentially in continental arcs where the crust thickness favors the formation of the mineralization [3]-[5].

Most of the giant porphyry copper deposits are located in Circum Pacific magmatic arc, a relatively young continental arc with a great thickness. The most important criteria for the genesis of porphyry copper, are the fugacity of O2 in the magma, the degree of fractionation of intrusive rocks (high Sr/Y) and the evolution of magmatic-hydrothermal fluid [6]-[12].

In West African Craton (WAC), some porphyry copper deposits are presents, associated with greenstones belts, the most prolific Paleoproterozoic gold province in the world with an average 10,000-metric ton of gold endowment [13]. This omnipresence of orogenic gold deposits in the WAC can be subject to confusion or superposition with other types of mineralization present in the WAC including Iron Oxide Copper Gold [14], Skarns [15]-[17], intrusion-related [18]-[20] and porphyry copper [21]-[33].

In Diénéméra-Gongondy porphyry copper deposit, the characteristic of the copper mineralization is in accordance with porphyry style [22]-[24] which is overprinted by a late orogenic gold mineralization [28] [29] [31]. In this deposit, the great part of the economic copper mineralization (>0.5wt% Cu) is linked with hydrothermal breccia which affects the intrusion complex [24], notably in the contact between andesite and diorite [29]-[31]. The breccia is 200 - 500 metres wide at surface and extend downwards to a maximum of 330 metres. They dip at 50˚ - 70˚ west in the Eastern part in Diénéméra, but in the western part, the dip of the breccia is subvertical and complexly scalloped at the southern end of the Gongondy deposit. They are generally monolithic and show abundant clasts (clasts-hosted type). The reduced matrix is composed of hydrothermal minerals dominated by quartz, K-feldspar, epidote, chlorite, amphibolite, magnetite, calcite and anhydrite. Chalcopyrite and pyrite are the main sulfide minerals and are found in the breccia intra-clasts, generally associated with gold, minor bornite and molybdenite. The copper and gold content of the hydrothermal breccia varies greatly and is between 0.2 to 2wt% for copper. The most important copper mineralization is observed when the matrix become more abundant [23] [24]. Outside of hydrothermal breccia, the copper and gold mineralization decreases considerably with a copper grade between 0.1 to 0.2wt%. In these areas, the mineralization is typically disseminated or in form of stockwork veins (calcite-chalcopyrite, epidote-chalcopyrite, anhydrite-chalcopyrite veinlets) with minor gold [24] [31].

At Gongondy, economic mineralization infers an orebody of 300 to 700 metres wide that is continuous for about 900 to 2000 metres long and extending from 150 to 500 metres downdip. At Diénéméra, the mineralization varies between 100 and 600 metres in width extending continuously between 400 and 1000 metres long, with a depth ranging from 200 to 400 metres [29] [31] [34]. Diénéméra-Gongondy Cu-Au mineralization lacks significant supergene enrichment, even if between 1929 and 1940, 5000 tons of ore grading 4 - 40wt% of copper was mined in Gongondy deposit in the form of chalcocite in the intersection between two inferred fractures zones [22] [35]. Minor oxided copper occur in the two deposits and consists of chalcocite, bornite, malachite, chrysocolla and rare azurite and cuprite [24]-[36]. The total indicated resource was established in Diénéméra and Gongondy deposits which have a total indicated resources of Diénéméra and Gongondy deposits estimated at 22 million metric tons at 0.29wt% Cu and 0.33 ppm of Au, whereas the total inferred resources of the two deposits is estimated at 282 Mt at 0.32wt% Cu for 0.35 ppm Au [34].

Although many studies have been carried out in the Gaoua porphyry copper deposit, the role of the hydrothermal alteration in the formation and evolution of the mineralization has not been clearly established. Most of the studies associated the mineralization with propylitic alteration minerals including chlorite, carbonates, epidote, quartz, white micas [23] [25] [27] [29]. In addition to the propylitic alteration, local potassic and sericitic alterations are distinguished in the study area [30]. Regarding the importance of hydrothermal alteration in the formation of porphyry copper deposit, this study aims to contribute to a better understanding of the hydrothermal alteration in the formation and evolution of the Gaoua (Diénéméra-Gongondy) Cu-Au deposit.

2. Geological Settings

The Diénéméra-Gongondy Cu-Au mineralization is located in the Man-Leo shield, in the southern part of the West African Craton (Figure 1). On the basis of the geology, the shield is divided into two domains. The western part is dominated by Archean rocks [37], marked by an important deformation and granitization defined by plutonic and metamorphic terranes dated between 3500 and 2900 Ma and separated from Paleoproterozoic terranes by the Sassandra fault. These Paleoproterozoic terranes are formed by the Eburnean orogeny between 2200 and 2000 Ma. In Burkina Faso, these terranes represent about 86% of the country and dominated by volcano-sedimentary rocks delimitated by Eburnean batholiths and associated with several generations of granitoïd and mafic intrusions. Most of the Volcano-sedimentary rocks form belts trending NNE-SSW in the western part of the country, and NE-SW in the eastern part. The Diénéméra-Gongondy Cu-Au mineralization occurs in one of these belts, the Boromo greenstone belt (Figure 2(A)). The belt consists of thick basal mafic tholeiitic rocks (Basalt, gabbro) intercalated by detrital or chemical sediments, volcanoclastics and calc-alkaline volcanic rocks (andesite, dacite, rhyolite) [39] [40].

Figure 1. Localization of study zone in Man-Leo Shield [38].

In line with the geology of the WAC, the geology of Diénéméra-Gongondy Cu-Au mineralization consists of an association of volcanic and volcaniclastic rocks crosscut by various intrusions [23] [25] [41] [42].

Volcanic rocks are predominated by basalts and andesites forming pyroclastic flows, tuffs, dacitic or rhyolitic dykes. Basalt with a microlitic, porphyritic or pillow lavas texture, outcrops in the East and Northeast of Gaoua town [30] [39] [42]. Whereas, andesitic rocks are intercalated with volcano-sedimentary and volcanoclastic rocks and are locally brecciated, particularly in Diénéméra-Gongondy deposit [30]. Their frequent association with diorite in the zone, makes them difficultly distinguishable at macroscopic scale, particularly when they are altered.

The Diénéméra-Gongondy Cu-Au mineralization is hosted by subvolcanic intrusions which form an elongated volcano-plutonic complex oriented N-S (Figure 2(B)). The main rocks of this complex consist of quartz diorite and porphyry microdiorites with a large variety of textures, including intersertal, spherulitic, ophitic textures [23] [25] [43]. Quartz diorites form some small lenticular intrusions in Diénéméra and circular bodies in Gongondy covering areas of few tens to one hundred square meters. Microdiorite porphyry also occurs as small intrusions in mafic rocks and is frequently crosscut by small albitic microgranite dykes [25].

Figure 2. Localization of study zone in Boromo greenstone belt modified from [39] (A). Detailed geology of Diénéméra-Gongondy copper-gold deposits showing the dioritic complex according to [30] (B).

Some intrusions also occur in the basaltic rocks dated at 2171.6 ± 9.3 Ma using U-Pb method in the Gongondy prospect [44]. The volcano-plutonic complex locally occurs in the vicinity of the Gaoua batholith, a N-S elongated non-deformed calc-alkaline granite. Four deformation events are locally observed in the study area [30]. The late regional D1 [39] was not observed in the deposit scale, but some local deformations affect the main hosted rocks. The D1Ga deformation event is marked by N-S shortening observed in E-W trending mafic rocks near the Gaoua batholith.

A second deformation phase D2Ga occurred under greenschist facies, responsible of penetrative metamorphic foliation and folding under overall E-W compression. The late stage of this deformation event consists of a transcurrent regime characterized by intense N to NW-trending steeply dipping shear zones. During subsequent D3Ga, a network of brittle to brittle-ductile NW and NE faults were developed. The last deformation event D4Ga resulted in E-W trending thrust faults and crenulation cleavage planes, under overall N-S compression.

3. Materials and Methods

3.1. Data Acquisition and Sampling

The data of this study were acquired from Burkina Faso national geological survey. It consists of twelve drill cores from “Projet Minier Gaoua” (PMG) studies between 1982 and 1985. The drill cores were logged and one hundred and ten representative samples were taken in regards with the previous PMG geochemical copper anomaly contour, lithology, hydrothermal variation and mineralization type. The samples were described, labelled and put in some bags for further several laboratories studies.

3.2. Microscopic Studies

Polished thin sections were done in the laboratory of Geosciences and Environment, in the department of Earth Sciences of University Joseph KI-ZERBO and Burkina Faso national geological survey (BUMIGEB). We used a polarizing and metallographic microscope for sample description and analyses.

3.3. Geochemical Analysis of Whole Rock

Twenty-nine rock samples were analyzed for whole rock geochemistry. Sixteen samples were crushed in Bigs Global laboratory in Burkina Faso, before being sent eleven to Bureau Veritas laboratory in Canada and five in “Laboratoire Magmas et Volcans” (France) for whole rock analysis (Table 1 and Table 2).

Table 1. Major and traces elements of geochemical results of whole rock analyses.

Sample

E2G

E45G

E48G

E33G

EAL

B1

21-08

E2D

E43D

E42D

E8G

E17G

E18G

G4 G24/7

Facies

Porphyry microdiorite

Quartz porphyry microdiorite

Ophitic microdiorite

Microgabbro- diorite

Andesite

Major elements (%)

SiO2

59.37

60.19

61.28

55.85

58.12

58

57.7

60.45

62.43

59.17

54.55

56.46

49.64

54.3

57.7

Al2O3

15.08

13.38

16.05

13.08

11.23

15.3

15.6

15.26

15.7

14.26

13.61

15

11.84

14.95

14.3

Fe2O3

8.12

9.69

7.64

8.86

20.23

7.6

9.38

7.15

7.12

13.61

6.55

5.68

6.98

6.36

9.73

CaO

2.14

0.3

0.58

5.02

0.22

7.31

4.89

6.15

3.35

2.42

5.76

4.77

7.54

5.12

3.11

MgO

4.59

2.06

4.03

2.32

2.02

4.35

4.03

3.16

2.3

1.1

6.21

3.62

6.51

3.53

5.18

Na2O

3.6

2.99

3.64

0.84

0.19

3.85

3.6

3.85

4.57

4.87

3.76

5.63

3.33

4.54

2.29

K2O

1.07

1.4

1.8

3.39

1.04

0.35

1.12

0.34

1.83

1.17

0.22

0.24

0.39

1.11

1.64

Cr2O3

0.019

-

-

-

-

0.03

0.02

0.01

0.009

-

0.044

0.005

0.036

0.02

0.04

TiO2

0.49

0.46

0.53

0.35

0.37

0.5

0.54

0.44

0.39

0.41

0.76

0.64

0.6

0.77

0.47

MnO

0.04

0.06

0.03

0.06

0.06

0.09

0.18

0.06

0.02

0.03

0.1

0.08

0.14

0.09

0.06

P2O5

0.09

0.11

0.12

0.06

0.1

0.07

0.09

0.09

0.08

0.09

0.35

0.36

0.32

0.5

0.08

LOI

5

7.96

4.11

9.36

5.44

2.2

2.65

2.9

1.9

2.68

7.9

7.3

12.4

6.18

5.14

Traces elements (ppm)

Ni

42

-

-

-

-

-

-

35

24

-

150

31

131

Sc

20

12.4

16.9

17.5

16.6

-

-

15

12

13.8

16

13

21

Ba

130

135.87

163.46

457.48

185.31

166.5

323

94

139

260.18

60

163

222

739

644

Be

2

0.58

0.68

0.67

0.45

-

-

<1

<1

0.61

<1

<1

1

Co

28.2

94

103.49

75.8

56.9

62

27.2

12.3

9.5

444.32

25.1

16.7

29.4

24

29.7

Cs

1.2

1.55

1.8

1.96

1.84

1.43

1.25

0.5

2.3

1.66

0.6

0.4

1.2

1.14

2.05

Ga

14.4

14.9

20.1

13

16.4

17.2

16.4

15.1

15.2

17.1

14.3

16

13.7

19.9

18.1

Hf

1.8

1.78

2.58

1.71

1.56

2.1

2.3

1.6

1.8

1.82

3.3

3.6

2.7

4.5

2

Nb

2.2

2.96

3.57

2.09

2.04

1.8

3.5

1.6

1.1

2.47

4.5

3.8

2.7

8.4

3.4

Rb

27.2

38.4

51

70

20.3

10.6

29.4

9

67.2

42.6

6

6

10.6

36.9

53

Sn

<1

1.83

1.13

3.32

1.48

1

-

<1

1

1.09

<1

<1

<1

1

1

Sr

81.9

92.2

81.8

92.8

40.9

371

207

482.7

160.2

324.76

247.7

576

496.2

349

102.5

Ta

0.3

0.19

0.26

0.15

0.14

0.2

0.2

0.1

<0.1

0.15

0.4

0.2

0.2

0.7

0.2

Th

0.8

0.83

1.06

0.8

0.59

0.61

0.94

1

0.6

0.39

3.9

3.3

2.4

4

0.68

U

1.1

0.86

1.4

0.57

0.44

0.22

0.42

0.4

0.5

1.12

1.2

1.4

0.7

1.76

0.49

V

158

101.43

122.71

111.64

120.06

145

162

111

112

124.72

96

105

118

134

170

W

1.3

4.1

1.77

9.45

2.93

218

1

0.8

<0.5

2.06

0.7

0.6

<0.5

30

1

Zr

69.5

68.7

98.9

66.4

60.9

80

75

65.5

72.6

69.7

135.8

141.8

105.3

159

62

Y

10.7

16.2

12.4

7.83

11.4

11.6

12.5

7.9

7.3

17.2

10.4

13.6

13.2

17.6

10.2

La

8.8

5.65

4.24

7.95

3.49

6.3

8

5.1

5

10.9

29.2

28.3

21.8

34.8

2.9

Ce

16.1

12.6

8.84

20.1

8.03

14.5

17.3

12.1

10.1

23.3

56.1

59.4

48.1

75

6.7

Pr

2.13

1.67

1.11

2.9

1.11

1.98

2.31

1.72

1.35

2.79

7.11

7.55

6.52

9.7

1.03

Nd

7.7

7.32

4.69

12.8

4.84

8.7

10.2

6.9

6

10.4

30.2

28.3

27.4

39.4

5.2

Sm

1.96

1.81

1.13

2.88

1.33

2.16

2.07

1.72

1.33

1.94

4.82

5.99

5.68

7.56

1.42

Eu

0.6

0.68

0.47

1.03

0.5

0.71

0.8

0.54

0.46

0.63

1.21

1.6

1.6

2.18

0.46

Gd

2.03

2.34

1.47

2.09

1.83

2

2.46

1.57

1.18

2.22

3.62

4.51

4.31

6.75

1.51

Tb

0.34

0.39

0.24

0.26

0.3

0.33

0.36

0.26

0.22

0.39

0.46

0.54

0.56

0.84

0.27

Dy

2.24

2.68

1.8

1.43

1.94

1.93

2.23

1.35

1.23

2.72

2.39

2.8

2.86

3.84

1.75

Ho

0.44

0.58

0.42

0.29

0.4

0.36

0.5

0.31

0.25

0.62

0.42

0.49

0.44

0.68

0.38

Er

1.3

1.67

1.41

0.83

1.13

1.19

1.38

0.89

0.81

1.99

1.03

1.21

1.4

1.82

1.12

Tm

0.19

0.24

0.23

0.12

0.16

0.18

0.21

0.12

0.11

0.3

0.14

0.17

0.16

0.23

0.19

Yb

1.16

1.5

1.66

0.8

1.04

1.17

1.38

0.76

0.72

2.03

0.92

1.21

1.02

1.59

1.21

Lu

0.2

0.22

0.27

0.13

0.15

0.17

0.2

0.11

0.12

0.31

0.13

0.18

0.15

0.24

0.21

Mo

7

0.83

12.3

4.92

1.21

1

-

0.2

13

17.4

0.2

<0.1

0.1

1

3

Cu

2128.2

200.65

1100.27

46.2

411.69

9

38

101.3

1156.6

10066.16

37.9

10.7

7.1

40

1030

Pb

0.7

2.05

2.5

4.08

1.56

-

23

0.4

0.4

2.29

1

1.6

2.5

-

Zn

43

56.5

50.3

7.16

47.8

34

300

17

16

23.7

70

65

67

79

46

Ag

0.6

0.26

0.48

0.22

2.23

-

-

<0.1

0.2

2.62

<0.1

<0.1

<0.1

-

Ni

42

70.9

55.1

26.6

37.8

63

51

32.4

25.1

25.8

144.4

29.4

131.7

19

84

As

0.9

10.1

22.2

29

144.56

0.3

3.4

<0.5

1.2

11.1

3

1.9

1.7

1.1

3.4

Au

0.0812

0.8

0.16

0.21

0.498

-

-

0.0078

0.0273

0.38

0.002

0.0019

0.0029

Cd

<0.1

-

-

-

-

-

-

<0.1

<0.1

-

<0.1

<0.1

<0.1

Sb

1.6

10.2

5.11

49.9

5.65

0.05

0.06

0.8

1.2

13.8

0.7

0.8

1.3

0.18

0.39

Bi

<0.1

0.14

0.29

0.59

7.43

0.01

0.03

<0.1

<0.1

0.4

<0.1

<0.1

<0.1

0.51

0.12

Hg

0.04

-

-

-

0.173

0.045

<0.01

0.01

-

<0.01

<0.01

<0.01

0.022

0.037

Tl

<0.1

0.14

0.17

0.24

0.08

-

-

<0.1

<0.1

0.18

<0.1

<0.1

<0.1

-

Se

2.3

3.86

4.89

2.18

1.06

0.2

0.2

<0.5

<0.5

1.88

<0.5

<0.5

<0.5

0.2

1.3

Te

-

-

-

-

-

0.01

-

-

-

-

-

-

-

0.02

0.06

Cr

-

-

-

-

-

260

110

-

-

-

-

-

-

Table 2. Suite of Table 1.

Sample

E27D

E23G

G5

G6

B2

G1

G3

B3

M2

D1

D2

E26D

E16G

E57G

Facies

Gabbro. microgabbro

Porphyry Diorite

Quartz Diorite quartzique

Dacite sphérolitique

Major elements (%)

SiO2

49.42

44.28

55.3

51.1

49.7

49.5

57.2

61.6

62.4

60.1

57.6

56.75

78.47

76.95

Al2O3

11.27

11.02

13.15

11.35

13.25

11.5

14.45

15.95

14

14.45

14.3

14.42

13

13.1

Fe2O3

18.93

16.51

9.4

10.8

15.3

8.33

8.33

7.72

6.34

9.77

6.82

7.13

1.17

1.29

CaO

5.89

7.61

6.84

7.14

9.57

6.7

4.16

3.01

5

2.11

5.09

7.09

0.56

0.59

MgO

3.84

5.09

5.64

9.66

6.38

5.83

3.9

3.19

2.97

2.25

3.47

2.31

0.36

0.35

Na2O

3.14

1.66

2.41

2.24

1.72

0.76

2.75

4.13

4.34

3.47

4.02

1.12

1.55

3.26

K2O

0.43

0

2.31

1.52

0.03

2.23

1.11

0.74

0.17

1.7

1.03

3.69

2.4

2.27

Cr2O3

0

0.009

0.03

0.06

0.01

0.06

0.02

0.01

0.02

0.02

0.03

0.015

0

0

TiO2

2.05

2.16

0.63

0.56

0.81

0.42

0.48

0.43

0.33

0.39

0.45

0.38

0.04

0.04

MnO

0.07

0.32

0.16

0.14

0.22

0.08

0.06

0.04

0.05

0.01

0.05

0.04

0.02

0.04

P2O5

0.2

0.17

0.23

0.21

0.07

0.1

0.11

0.12

0.06

0.07

0.08

0.07

0.03

0.02

LOI

4.4

11

2

2.78

2.67

12.7

6.77

3.04

2

2.8

2.28

6.8

2.3

2

Traces elements (ppm)

Ni

32

70

-

-

-

-

-

-

-

-

-

36

<20

<20

Sc

39

28

-

-

-

-

-

-

-

-

-

15

2

3

Ba

49

25

621

1235

11.1

272

189.5

194

97.1

208

150.5

204

250

192

Be

<1

2

-

-

-

-

-

-

-

-

-

<1

1

<1

Co

30.1

27.7

65

52

54.9

28

30

16.3

90

108

64

16.3

0.5

1

Cs

1.1

0.3

1.01

1.64

0.25

1.81

1.07

0.42

0.16

1.35

1.41

7.2

4

2.7

Ga

15.7

18

14.9

12.1

14.6

14.4

17.1

17.3

14.9

20.2

16.6

14.1

12.8

13.2

Hf

3.5

3.1

1.4

1.6

1.4

1.5

2

2.1

1.9

2.2

2.3

1.8

2

2

Nb

5.1

7.9

4.9

3.4

0.9

3.6

4

3.1

2.1

3.2

2.9

1.5

5.5

6.5

Rb

17.5

0.4

72.4

39.1

0.4

66.7

31.7

20.9

4.3

65.1

39.7

142.9

59.7

61.5

Sn

2

3

1

1

-

1

1

-

1

1

1

<1

1

1

Sr

127.8

109.9

324

139.5

131.5

78.1

120.5

379

441

253

388

69.8

96.5

35.4

Ta

0.3

0.5

0.2

0.3

-

0.2

0.3

0.2

0.2

0.3

0.2

<0.1

1

0.9

Th

0.6

0.8

0.76

0.95

0.08

0.81

0.92

0.97

0.77

1

0.8

0.8

2.7

2.5

U

0.9

0.3

0.34

0.71

<0.05

0.4

0.65

0.23

0.31

0.4

0.47

0.4

1.7

2

V

437

350

213

217

309

130

146

124

92

123

120

108

<8

<8

W

2.5

2.6

224

69

-

19

26

1

367

247

253

1.5

1.1

<0.5

Zr

129.3

116.7

46

61

40

52

71

69

70

73

78

62.1

39.7

45.9

Y

43.9

21.5

16

15.7

23.8

10.2

12.2

10.1

7.9

3.8

11.2

6.5

11.8

10.8

La

8

9.8

9.4

8.2

1.2

11.1

5.9

6.1

5.4

4.1

6.3

4.2

7.8

7.7

Ce

21.8

24.5

19.3

18.1

3.8

21.9

12.5

12.6

12.7

8.5

14.7

9.8

14.6

17

Pr

3.24

3.63

2.62

2.38

0.69

2.71

1.63

1.8

1.71

1.03

2.06

1.38

1.69

1.84

Nd

14.4

16.8

11.1

10.4

4.1

10.5

7.1

8.1

7.5

4.4

9.1

6.3

6.2

6.4

Sm

4.79

4.37

2.67

2.72

1.55

2.16

1.79

1.87

1.73

0.92

2.23

1.48

1.58

1.46

Eu

1.42

1.57

0.92

0.9

0.76

0.64

0.53

0.66

0.54

0.43

0.68

0.41

0.2

0.16

Gd

5.8

4.47

2.92

2.62

2.62

2.11

2.04

1.94

1.62

0.87

2.23

1.32

1.63

1.57

Tb

1.11

0.77

0.49

0.44

0.53

0.35

0.36

0.29

0.24

0.12

0.35

0.21

0.34

0.3

Dy

7.29

4.49

2.88

2.69

4

1.93

2.22

1.79

1.46

0.66

2.05

1.22

1.97

2.02

Ho

1.63

0.83

0.58

0.5

0.89

0.39

0.45

0.37

0.3

0.14

0.43

0.25

0.39

0.37

Er

4.84

2.2

1.64

1.64

2.72

1.11

1.32

1.06

0.9

0.4

1.28

0.74

1.06

0.99

Tm

0.71

0.3

0.23

0.25

0.43

0.16

0.19

0.17

0.12

0.06

0.18

0.12

0.14

0.16

Yb

4.85

2.06

1.58

1.68

2.59

1.19

1.41

1.07

0.85

0.38

1.23

0.75

0.98

1.04

Lu

0.77

0.29

0.25

0.25

0.42

0.18

0.22

0.17

0.14

0.06

0.2

0.14

0.13

0.15

Mo

1.2

0.3

1

2

-

1

1

-

1

1

1

0.5

<0.1

0.2

Cu

1012.9

71.8

104

1960

161

753

1305

1620

204

868

160

388.8

4.3

3.6

Pb

1

0.5

-

-

-

-

-

-

-

-

-

0.4

1.5

1.4

Zn

33

59

116

68

98

25

31

24

20

18

30

20

11

8

Ag

0.3

<0.1

-

-

-

-

-

-

-

-

-

<0.1

<0.1

<0.1

Ni

31.5

66.8

31

61

53

76

54

38

32

28

44

31

1.4

1.1

As

1.1

1.7

0.7

0.5

0.8

0.6

0.6

0.7

1

0.6

0.5

0.7

4.1

3.3

Au*

0.033.9

0.004.9

-

-

-

-

-

-

-

-

-

0.0279

0.0072

0.0007

Cd

<0.1

<0.1

-

-

-

-

-

-

-

-

-

<0.1

<0.1

<0.1

Sb

1.1

0.9

0.05

0.05

0.17

0.17

0.11

0.12

0.05

0.09

0.12

0.6

0.7

0.5

Bi

0.3

<0.1

0.02

0.54

0.01

0.03

0.08

-

0.01

0.86

0.03

0.1

<0.1

<0.1

Hg

0.01

<0.01

0.14

0.074

0.011

0.02

0.048

0.012

0.3

0.212

0.215

<0.01

0.1

0.02

Tl

<0.1

<0.1

-

-

-

-

-

-

-

-

-

<0.1

<0.1

<0.1

Se

0.6

<0.5

0.2

0.3

0.5

0.6

2

-

0.2

0.6

0.2

<0.5

<0.5

<0.5

Te

-

-

0.01

0.04

0.04

0.02

0.06

0.02

0.01

0.07

0.01

-

-

-

Cr

-

-

250

480

40

440

160

90

200

150

170

-

-

-

In the two laboratories, major elements were analyzed by inductively coupled plasma-atomic emission spectrometry (ICP-AES), whereas traces elements data were obtained by inductively coupled plasma mass spectrometry (ICP-MS).

These data were supplemented by thirteen whole rock analyses results from [27] and [42] studies in the study area. All of these samples were analyzed in ALS Chemex laboratories in Spain and Canada by ICP-AES for major elements and ICP-MS for trace elements.

3.4. Sulfur Isotopic Analysis

Four rock samples containing pyrite and chalcopyrite were sent to Actlabs in Vancouver (Canada) for extraction and analyses. Dried samples are weighted into tin cups for 34S analysis. Approximately 0.3 mg of sample is used for 34S analysis, with 3 mg of niobium pentoxide added to each sample to ensure complete sample combustion. Samples are loaded into a Fisons Instruments elemental to be flash combusted at 980° C. Released gases are carried by ultrapure helium through the analyzer, then separated by gas chromatography. Clean SO2 gas is carried into the Mat 253, Thermo Scientific, IRMS for analysis. Data is corrected and normalized using three international standards, IAEA SO6, IAEA SO5, NBS 127, and two calibrated internal standards that bracket the samples. Standards are at the beginning and end of every run. The δ34S values are reported relative to the Vienna Cañon Diablo Troilite (VCDT) standard.

3.5. Microprobes Analysis

Microprobe analyses by CAMECA SX100 were carried out at the “Magmas et Volcan” Laboratory of the Clermont-Ferrand University (France). They involved a total of four core samples from the study area. These analyses aim to determine the different mineral phases of these rocks in order to determine their petrogenetic characteristics and their paragenetic sequence. Epidotes, chlorites, feldspars, carbonates and amphiboles are the hydrothermal alteration minerals which have been analyzed.

3.6. Correlation Factor Analyses

Preliminary factor analyses were conducted to define factors related to hydrothermal alteration and mineralization using SPSS 16.0 software to analyze eleven rock samples. The trace elements used for this analysis are copper (Cu), gold (Au), silver (Ag), molybdenum (Mo), selenium (Se), antimony (Sb), nickel (Ni), lead (Pb), zinc (Zn), mercury (Hg), barium (Ba), arsenic (as), strontium (Sr), rubidium (Rb), cesium (Cs), bismuth (Bi). The major elements are represented by Fe2O3, TiO2, loss on ignition (LOI), total carbon (TOT/C) and total sulfur (TOT/S). Alteration indexes were also used. These are those of [45] (AI, SI), [46] (CCPI) and [47] (IP or potential index). The AI generally indicates chlorite-sericite alteration, the CCPI indicates chlorite-carbonate-pyrite alteration and SI indicates the silicification index.

4. Results

4.1. Hydrothermal Alteration Minerals

Hydrothermal alteration is an important factor in the genesis of hydrothermal deposits in magmatic and metamorphic contexts. In the Gongondy and Diénéméra deposits, several types of alteration have been identified on the basis of petrographic observations.

4.1.1. Sericitic Alteration

Sericitic alteration affected spherulitic dacite (Gongondy) and, to a lesser extent, Quartz porphyry microdiorite (Diénéméra) (Figure 3(A)). This alteration is marked by abundant sericite associated with secondary quartz, carbonates and chlorite in minute quantities, accompanied by deposition of abundant metallic opaque crystals, either in fine disseminations or fractured coarse-grains, cemented by quartz and carbonates. Sericite occurs in fine lamellae (40 wt% - 50 wt%), resulting from partial or total destruction of the feldspars. In facies affected by these types of alteration, sericite is also found in the form of veinlets deposited around other veinlets, particularly early quartz-carbonate veinlets. Greenish chlorite is present in small quantities (5 wt% - 15wt%). It is practically associated with disseminated sulfides, which represent around 15-20wt% of the rock minerals (Figure 4(A)).

In dacite and Quartz porphyry microdiorite, a second type of sericite alteration occurs late or coexists with the first. This latter type of alteration is dominated by white micas (50 wt% - 55 wt%) associated with accessory carbonates (5 wt% - 10 wt%). Rocks affected by these types of alteration take on a light green or yellowish color (Figure 3(B)). This type of hydrothermal alteration can also be found in fractures or veinlets (Figure 3(C)). Micas occur in more or less oriented lamellae. Sulfides (5wt%) are rare, fine (80 µm) and automorphs, disseminated in the rock, moulded by phyllites consisting mainly of white mica (Figure 4(B)).

4.1.2. Propylitic Alteration

Propylitic alteration affects quartz porphyry microdiorite, spherulitic microdiorite, intersertal microdiorite and quartz diorite. At a macroscopic scale, altered rocks take on a light green color (Figure 3(D)). At a microscopic scale, it is characterized by an epidote-chlorite-sericite assemblage, with varying content of carbonates, quartz and albite (Figure 4(C)). Epidote (15 wt% - 45 wt%) is the result of the destabilization of plagioclases and amphiboles. Chlorite (10 wt% - 35 wt%) is largely derived from ferromagnesian minerals (biotite, amphibole) and plagioclases, which are also altered into sericite. Carbonates come from the decomposition of feldspars and amphibole. Slight albitization (5 wt% - 10 wt%) related to saussuritisation of the plagioclases is associated with propylitic alteration in these intermediate rocks (Figure 4(H)).

4.1.3. Phyllitic-Carbonate Alteration

The Phyllitic-carbonate alteration is marked by the predominance of phyllites (chlorite and sericite) (50 wt% - 60 wt%) over carbonates (15 wt% - 20 wt%) and, conversely, by the predominance of carbonates (55 wt% - 60 wt%) over phyllites (30 wt% - 35wt%) for the carbonate-phyllitic type. At Gongondy, where these types of alteration are most pronounced, a certain zonation is observed according to the dominance of one of the three phases. Chlorite dominates at the surface, sericite in the intermediate part and carbonates at deeper zones. Depending on the dominance of a particular mineral, the rock can take on a white, grey or light green color (Figure 3(E)). Sericite (15 wt% - 35 wt%) comes mainly from the partial destabilization of feldspars and is present in fine flakes. Chlorite (10 wt% - 25 wt%) results mainly from the alteration of ferromagnesian minerals (amphiboles, biotite) or, to a lesser extent, feldspars. Carbonates (10 wt% - 60 wt%) come from the destabilization of feldspar or ferromagnesian minerals (amphibole, biotite) (Figure 4(D)).

4.1.4. Chloritic Alteration

Chloritic alteration affects microdiorite porphyry and microgabbro (Figure 3(F)). Microscopically, massive chlorite alteration (40 wt% - 60 wt%) almost completely invades the rocks in question and is associated with sericite (10 wt% - 15 wt%), carbonates (10 wt% - 15 wt%) and quartz (10 wt%). Accessory elements (10 wt%) are represented by opaques (pyrite, rutile, etc.). This chlorite can be distinguished from that observed in the mineralized zones by its bluish hue, suggesting a different composition. Chlorite is the product of the destabilization of feldspar or ferromagnesian minerals (amphibole, biotite) (Figure 4(E)).

4.1.5. Potassic Alteration

Traces of potassic alteration were observed in borehole S16G at Gongondy. It affects a pinkish microgabbro-diorite dyke injected into an ophitic microdiorite. Microscopic evidence of this alteration shows the transformation of amphibole to biotite and the perthitisation of plagioclase (Figure 4(F)). This type of alteration is also suspected in a late intrusion of microdiorite porphyry, which shows a similar reddish color.

4.1.6. Silicification

Silicification of rocks is common in the study area. The alteration is frequently visible in the Diénéméra deposit. It is characterized by quartz veins and veinlets, or silica partially or completely invading quartz microdiorite porphyries, spherulitic microgabbros, microgranodiorites and microgranites. Microscopically, silicification leads to the formation of ribbons of quartz, often polygonal, associated with coarse carbonate and opaque crystals (Figure 4(G)). It also appears in the form of coarse quartz veinlets in quartz microdiorites. It forms the cement for the coarse cracked pyrite crystals.

Figure 3. Macroscopic view of altered rocks from Diénéméra-Gongondy copper deposits. Sericitic alteration 1 affecting microdiorite porphyry at Gongondy (A). Sericite alteration 2 affecting dacitic rocks: Pervasive alteration (B) and alteration saw in veinlet (C). Propylitic alteration affecting Quartz porphyry microdiorite in Diénéméra (D). Phyllito-carbonate alteration affecting microdiorite in Gongondy deposit (E). Chloritic alteration affecting porphyry microdiorite in Gongondy (F).

Figure 4. Microscopic view of altered rocks from Diénéméra-Gongondy copper deposits. Sericitic alteration 1 (A), Sericitic alteration 2 (B), Propylitic alteration (C), Phyllitic-carbonate alteration (D), Chloritic alteration (E), Residual potassic alteration (F), Silicification (G) and Albitization (H). Minerals abbreviations: Amp: Amphibole, Bt: biotite, Cb: carbonate, Chl: chlorite, Ep: epidote, Ms: muscovite, Op: Opaques; Pl: plagioclase; Py: pyrite, Qtz: quartz, Ser: sericite, Sulf: sulfides.

Finally, silicification is associated with shearing in the form of centimetric to decimetric quartz veins.

4.2. Geochemistry of Hydrothermal Alteration

Macroscopic and microscopic studies of hydrothermal alteration in the study area show that all samples are subject of hydrothermal alteration. Based on the alteration mineral assemblages, several patterns of hydrothermal alterations have been defined. The geochemical analysis of major and trace elements can be used to define the mobility of some elements such as K2O, Na2O. The binary diagram of [48] modified by [49] discriminated less altered rocks from the very altered ones (Figure 5).

Figure 5. Binary diagram of [48] modified by [49] discriminating the fresh rocks to altered rocks.

Figure 6. CCPI and AI for different types of alteration observed in the study zone. Phyllitic-carbonate alteration (A) (B), Propylitic alteration (C) (D), chloritic alteration (E) (F), Sericite alteration 1 (I), Sericite alteration 2 (G) (H).

Most of the samples analyzed are altered, but the degree of alteration varies. The ophitic rocks are characterized by an exclusive sodic alteration. The alteration indexes of [45] and [46] have been important to distinguish the different types of hydrothermal alteration. Propylitized rocks according to petrographic observations show a low CCPI (60 to 75) and medium AI (25 to 35) (Figure 6(C) and Figure 6(D)) whereas phyllitic-carbonate altered rocks show a relatively high CCPI (75 to 85) and AI (50 to 60) (Figure 6(A) and Figure 6(B)). These two types of alteration are clearly distinguishable from sericitic and chloritic alteration, which have higher AI and CCPI values respectively. Chloritic alteration in relation with mafic rocks has a low AI like propylitic alteration, with high CCPI between 85 and 100 (Figure 6(E) and Figure 6(F)). Sericite alteration 1, on the other hand, shows an AI (50 to 70) that is similar to that of phyllitic-carbonate alteration, for a low CCPI between 20 to 30 (Figure 6(G)). Sericitic alteration 2 differs from the sericitic alteration 1 by a lower value of CCPI which is less than 30 (Figure 6(H) and Figure 6(I)).

In the ternary diagram of [50], an evidence of sericitization is observed in all rocks with a tendency for chloritization, notably for mafic rocks (Figure 7(A)). However, the diagram of [51] shows a clear segregation of rocks in different alteration fields. Dacitic rocks fall in the sericitic and propylitic fields. Gabbroic rocks are generally observed in the propylitic field only. Intermediate rocks are observed in the propylitic fields dominantly quartz microdiorite porphyry, quartz diorite porphyry, and ophitic microdiorites. Porphyric microdiorites, porphyric diorites and andesites are observed in the K-silicate fields in relation with K-Feldspar alteration (Figure 7(B)).

Figure 7. Hydrothermal alteration diagram using major elements. Ternary diagrams of [50] (A) and [51] (B) which define some fields of hydrothermal alterations.

4.3. Alteration Mineral Chemistry

The chemical analyses of some minerals which are frequent in the alteration zone of Diénéméra-Gongondy porphyry copper (plagioclase, chlorite, epidote, amphibole, carbonate) enabled us to define the chemical composition of these minerals and the consequently determine the type of fluid responsible for the alteration and the deposition of the mineralization. The details of chemical analyses of minerals are found in Table 3 and Table 4.

4.3.1. Plagioclase

Four samples were used to study the chemical composition of the plagioclase found in the study area. All analyzed points show a high proportion of SiO2 (66.43 wt% - 71.69 wt%), Al2O3 (18.08 wt% - 20.27 wt%), Na2O (8.81 wt% - 10.91 wt%) and a low proportion of K2O (0.006 wt% - 0.31 wt%) and CaO (0.07 wt% - 0.60 wt%). The percentage of K and Ca are less than 1 wt%, whereas the proportion of Na is higher, varying between 3.08 and 3.66 wt%. According to the ternary Ab-An-Or diagram of [52] for the classification of plagioclase, all the samples analyzed fall in the albite field (Figure 8(A)).

4.3.2. Epidote

Five points of epidotes were analyzed. These epidotes have a homogeneous chemical composition, with higher content of Al2O3 than FeOt and Cr2O3. Moreover, the content of CaO is high, ranging from 22.87 to 23.77 wt%. The Al2O3 content shows very little variation between 21.77 and 22.90 wt%. Total iron content range from 12.82 and 13.82 wt%, whereas the content of chromium is below than 0.1%. The Al-Cr-Fe ternary diagram of [53] shows that the epidotes have a composition of clinozoisite (Figure 8(B)).

4.3.3. Carbonate

Carbonate are fairly ubiquitous in the Diénéméra and Gongondy deposits. Seven carbonates points from three rock samples were analyzed chemically. A wide variation in CaO (28.99 wt% - 58.29 wt%), MgO (0.02 wt% - 9.45 wt%) and FeOt (0.10 wt% - 13.98 wt%) was observed depending on the type of carbonate present in the rocks. The carbonate of sample E11G is distinguished from the other carbonates by high MgO and FeOt contents (9.45 wt.% and 13.98 wt.%, respectively and a correspondingly lower CaO content (28.99 wt%). The Mg-Ca-Fe ternary diagram of [54] shows that all carbonates have a composition close to calcite, except the carbonates of sample E11G, which lies between dolomite and ankerite composition (Figure 8(C)). In the Mg-Fe-Mn diagram of [54], the carbonate of sample E11G falls in the field of Fe-dolomite (Figure 8(D)).

4.3.4. Chlorite

Chlorite is an abundant mineral in the rocks in the study area. The chemical composition of the mineral shows a high proportion of SiO2 (23.83 wt% - 29.78 wt%), Al2O3 (13.18 wt% - 23.30 wt%) and low proportions of CaO (0 wt% - 0.38 wt%), K2O (0 wt% - 0.82 wt%) and Na2O (< 0.20 wt%). The proportions of MgO (9.12 wt% - 20.01 wt%) and FeOt (18.03 wt% - 33.03 wt%) vary depending to the samples. The lowest proportions of MgO are found in the chlorite of samples E22G where this proportion of of FeOt (31.04 wt% - 33.03 wt%) than other chlorites. Other samples have a relatively same proportion of FeOt and MgO. The content of TiO2 of most samples is less than 0.1% except for samples E11G where the content is more than 0.4%. In binary diagram of [55], the chlorites from sample E22G fall into the ripidolite field. whereas the chlorites from the other samples are of types of ripidolite and pycnochlorite fields (Figure 8(E)).

4.3.5. Amphibole

The chemical composition of 3 amphiboles points was analyzed. The contents of SiO2 (52.93 - 54.89 wt%), FeOt (9.07 wt% - 11.32 wt%), MgO (16.63 wt% - 18.01 wt%) and CaO (11.58 wt% - 12.28 wt%) are high. Low contents are observed for TiO2 (0.25 wt% - 0.27 wt%), Na2O (0.46 wt% -0.52 wt%) and K2O (0.03 wt% - 0.11 wt%). The diagram of [56] also shows two different sources of amphiboles: primary and secondary (Figure 8(F)). The diagram of [57] shows that amphiboles are from two types: actinolite and magnesio-hornblende (Figure 8(G)).

Figure 8. Diagrams of alteration minerals chemistry. Ab-An-Or ternary diagram for classifying plagioclase [52] (A). Al-Cr-Fe ternary diagram for epidote classification [53] (B). The Mg-Ca-Fe ternary diagram of carbonates classification [54] (C). the Mg-Fe-Mn diagram of [54] is used to discriminate Fe-carbonates (D). Binary diagram of chlorite classification [55] showing two groups of chlorites (E). Classification binary diagram of amphibole according to the origin [56] (F) and the composition [57] (G).

Table 3. Chemical composition of minerals: major elements.

Plagioclase

Comment

DataSet/Point

SiO2

TiO2

Al2O3

Cr2O3

FeOt

MnO

MgO

CaO

Na2O

K2O

Total

Lame 21D

12

68.6392

0

19.843

0.002

0.0224

0

0

0.2717

10.6663

0.0878

99.5325

Lame E7D

19

68.7006

0.0269

20.0332

0

0.1106

0

0

0.3115

10.7833

0.0632

100.0293

Lame E7D

20

66.4361

0.0077

19.519

0.026

0.5513

0

0.0789

0.5149

8.8177

0.1926

96.1442

Lame E7D

21

71.6964

0.0115

18.0874

0.024

0.0914

0

0

0.0732

9.9697

0.0062

99.9597

Lame E7D

22

70.2831

0

19.5659

0

0.0834

0.0131

0

0.105

10.5183

0.0586

100.6274

Lame E7D

27

68.9615

0

19.8569

0.012

0

0.0185

0.0228

0.3672

10.8533

0.0663

100.1585

Lame E7D

29

68.3212

0

20.1293

0.03

0.0834

0.0361

0

0.6009

10.8576

0.0416

100.1002

Lame E7D

31

69.1764

0.0268

19.8246

0.0334

0.0878

0.0019

0

0.2242

11.0196

0.0487

100.4432

Lame E7D

34

69.4997

0

20.0036

0.013

0.1621

0.0188

0

0.1135

10.9474

0.03

100.7882

Lame E11G

54

68.9643

0.0214

20.0251

0.0112

0.243

0.0103

0.0034

0.4169

10.8048

0.0358

100.5363

Lame E11G

61

68.8879

0.025

19.884

0.006

0.5037

0

0.0083

0.1527

10.82

0.1389

100.4264

Lame E11G

62

69.5917

0.0374

20.1589

0

0.4479

0

0

0.1446

10.9138

0.073

101.3673

Lame E11G

68

68.8152

0.02

20.1057

0.0057

0.1548

0

0

0.2047

10.313

0.0701

99.6893

Lame E11G

69

68.1315

0

20.2778

0.0111

0.1459

0

0.0356

0.3958

10.3469

0.3178

99.6625

Chlorite

Comment

DataSet/Point

SiO2

TiO2

Al2O3

Cr2O3

FeOt

MnO

MgO

CaO

Na2O

K2O

Total

Lame 21D

3

26.7559

0.018

17.8873

0

22.7905

0.1526

16.8718

0.0605

0.0644

0.1278

84.7288

Lame 21D

4

27.3031

0.0879

16.918

0.1701

23.4087

0.2668

16.0471

0.3859

0.0721

0.1245

84.7842

Lame 21D

10

27.5321

0.0154

18.4637

0.0546

22.648

0.1584

17.6807

0.0717

0.0115

0

86.6361

Lame 21D

11

26.9206

0.0325

19.3256

0.0415

21.9077

0.2012

17.8353

0

0.0137

0

86.2782

Lame E7D

17

27.2573

0.0235

20.3247

0.0289

20.9802

0.0545

18.1994

0.0151

0

0.0368

86.9204

Lame E7D

18

29.7875

0.1959

19.563

0.0387

18.0352

0.0681

17.38

0.0613

0.0083

0.8187

85.9566

Lame E7D

23

27.2877

0.0381

20.2588

0.0528

20.7564

0.0632

18.2829

0.0183

0.0085

0

86.7668

Lame E7D

24

25.9168

0.0235

20.8782

0.0362

22.1446

0.0952

17.1202

0.0015

0.0276

0.0517

86.2955

Lame E7D

32

25.0515

0.0352

22.4787

0

22.3364

0.0584

16.4831

0.0352

0.0048

0.0151

86.4984

Lame E7D

33

26.3539

0

21.1799

0.0054

21.661

0.0979

17.4595

0

0.0086

0.0148

86.781

Lame E7D

35

27.3277

0.0405

20.2749

0.0017

20.9877

0.0869

18.4431

0.0411

0.0111

0.0151

87.2297

Lame E22G

44

23.8325

0.0326

22.3008

0

32.9933

0.1935

8.8575

0.0207

0.0053

0.0188

88.255

Lame E22G

45

24.4881

0

21.8799

0.0277

33.0363

0.2646

9.1207

0

0.0495

0

88.8669

Lame E22G

50

25.7404

0.0396

21.4751

0.0504

31.0484

0.1338

9.8946

0.0306

0.0035

0.042

88.4582

Lame E22G

51

26.1132

0.0593

23.3057

0.0439

31.4272

0.152

10.1769

0.0765

0.1092

0.0758

91.5398

Lame E22G

52

25.9103

0.023

22.2617

0.0437

31.621

0.2297

9.6233

0.0846

0.0418

0.0864

89.9255

Lame E11G

66

29.0185

0.5128

16.1821

0.0238

20.0301

0.1808

18.94

0.2012

0.0047

0.1776

85.2717

Lame E11G

67

29.1443

0.4152

16.3992

0.0085

19.9778

0.1898

20.0158

0.1689

0.0142

0.0924

86.4259

Carbonate

Comment

DataSet/Point

SiO2

TiO2

Al2O3

Cr2O3

FeOt

MnO

MgO

CaO

Na2O

K2O

Total

Lame 21D

1

0.0267

0.0258

0.0016

0

0.1023

0.0938

0.0259

57.883

0.0162

0

58.1753

Lame E22G

41

0.2343

0.0455

0.0715

0

1.3306

1.7933

0.3345

58.2378

0.0148

0.022

62.0842

Lame E22G

42

0.0098

0

0.0112

0

1.5568

1.9313

0.4058

58.2958

0.0204

0.0313

62.2623

Lame E22G

43

0.0007

0

0.0037

0

1.209

1.5275

0.294

56.9121

0.0068

0.0204

59.9743

Lame E22G

48

0.0007

0.0349

0.0333

0

1.3335

1.8355

0.2802

57.0587

0

0.014

60.5908

Lame E22G

49

0.0203

0

0

0

1.5601

2.0087

0.4221

58.0076

0.0017

0

62.0207

Lame E11G

55

0.0769

0

0

0

13.9849

4.4293

9.4537

28.9905

0.0262

0.0131

56.9747

Epidote

Comment

DataSet/Point

SiO2

TiO2

Al2O3

Cr2O3

FeOt

MnO

MgO

CaO

Na2O

K2O

Total

Lame E11G

56

37.9187

0.1253

22.902

0

12.8212

0.1168

0.0294

23.2458

0.0126

0.012

97.1837

Lame E11G

57

37.8813

0.0617

22.5317

0

13.1418

0.1075

0.0116

23.7781

0

0

97.5138

Lame E11G

58

37.9829

0.1269

22.102

0.0305

13.2017

0.0707

0

22.8727

0.0412

0

96.4286

Lame E11G

59

37.7682

0.0704

21.7742

0.0322

13.7735

0.0645

0

23.3699

0

0.0173

96.8702

Lame E11G

70

37.7488

0.1596

21.7731

0.0107

13.8206

0.078

0

23.1251

0.0254

0

96.7413

Amphibole

Comment

DataSet/Point

SiO2

TiO2

Al2O3

Cr2O3

FeOt

MnO

MgO

CaO

Na2O

K2O

Total

Lame E11G

60

54.8996

0.2691

2.0425

0

9.0754

0.1828

18.0143

12.2873

0.4672

0.0363

97.2747

Lame E11G

63

52.9398

0.2506

3.4974

0.0199

9.7912

0.2624

17.9715

11.4933

0.4922

0.1157

96.834

Lame E11G

64

53.0791

0.3604

3.325

0.0411

11.3188

0.2027

16.636

11.5877

0.5195

0.0861

97.1565

Table 4. Chemical composition of minerals: trace elements.

Plagioclase

Si

Ti

Al

Cr

Fe

Mn

Mg

Ca

Na

K

Total

11.998

-

4.088

-

0.003

-

-

0.051

3.615

0.020

19.775

11.962

0.004

4.111

-

0.016

-

-

0.058

3.640

0.014

19.806

11.996

0.001

4.154

0.004

0.083

-

0.021

0.100

3.087

0.044

19.490

12.386

0.001

3.683

0.003

0.013

-

-

0.014

3.339

0.001

19.440

12.119

-

3.976

-

0.012

0.002

-

0.019

3.516

0.013

19.658

11.990

-

4.069

0.002

-

0.003

0.006

0.068

3.659

0.015

19.811

11.910

-

4.136

0.004

0.012

0.005

-

0.112

3.670

0.009

19.859

11.997

0.003

4.052

0.005

0.013

-

-

0.042

3.705

0.011

19.829

12.003

-

4.072

0.002

0.023

0.003

-

0.021

3.666

0.007

19.796

11.959

0.003

4.093

0.002

0.035

0.002

0.001

0.077

3.633

0.008

19.812

11.971

0.003

4.072

0.001

0.073

-

0.002

0.028

3.646

0.031

19.827

11.971

0.005

4.087

-

0.064

-

-

0.027

3.640

0.016

19.809

11.994

0.003

4.130

0.001

0.023

-

-

0.038

3.485

0.016

19.689

11.917

-

4.180

0.002

0.021

-

0.009

0.074

3.509

0.071

19.783

Chlorite

Si

Ti

Al

Cr

Fe

Mn

Mg

Ca

Na

K

Si + Al

28 oxygènes

5.776

0.003

4.551

-

4.114

0.028

5.429

0.014

0.027

0.035

8.000

5.920

0.014

4.323

0.029

4.245

0.049

5.187

0.090

0.030

0.034

8.000

5.783

0.002

4.570

0.009

3.978

0.028

5.536

0.016

0.005

-

8.000

5.659

0.005

4.788

0.007

3.851

0.036

5.589

-

0.006

-

8.000

5.642

0.004

4.958

0.005

3.632

0.010

5.616

0.003

-

0.010

8.000

6.126

0.030

4.742

0.006

3.102

0.012

5.329

0.014

0.003

0.215

8.000

5.651

0.006

4.945

0.009

3.595

0.011

5.644

0.004

0.003

-

8.000

5.456

0.004

5.180

0.006

3.899

0.017

5.373

-

0.011

0.014

8.000

5.264

0.006

5.567

-

3.925

0.010

5.163

0.008

0.002

0.004

8.000

5.490

-

5.200

0.001

3.774

0.017

5.422

-

0.003

0.004

8.000

5.637

0.006

4.929

-

3.620

0.015

5.671

0.009

0.004

0.004

8.000

5.208

0.005

5.743

-

6.029

0.036

2.885

0.005

0.002

0.005

8.000

5.308

-

5.589

0.005

5.988

0.049

2.947

-

0.021

-

8.000

5.525

0.006

5.432

0.009

5.573

0.024

3.166

0.007

0.001

0.011

8.000

5.401

0.009

5.681

0.007

5.436

0.027

3.138

0.017

0.044

0.020

8.000

5.478

0.004

5.547

0.007

5.591

0.041

3.033

0.019

0.017

0.023

8.000

6.113

0.081

4.018

0.004

3.529

0.032

5.948

0.045

0.002

0.048

8.000

6.050

0.065

4.012

0.001

3.468

0.033

6.194

0.038

0.006

0.024

8.000

Carbonate

Si

Ti

Al

Cr

Fe

Mn

Mg

Ca

Na

K

Total

6 oxygènes

0.003

0.002

-

-

0.008

0.008

0.004

5.970

0.003

-

-

0.021

0.003

0.008

-

0.101

0.138

0.045

5.654

0.003

0.003

-

0.001

-

0.001

-

0.118

0.149

0.055

5.671

0.004

0.004

-

-

-

-

-

0.095

0.122

0.041

5.739

0.001

0.002

-

-

0.002

0.004

-

0.104

0.145

0.039

5.701

-

0.002

-

0.002

-

-

-

0.119

0.155

0.057

5.665

-

-

-

0.008

-

-

-

1.154

0.370

1.391

3.066

0.005

0.002

-

Epidote

Si

Ti

Al

Cr

Fe

Mn

Mg

Ca

Na

K

Total

13 (O. OH)

3.237

0.008

2.304

-

0.915

0.008

0.004

2.126

0.002

0.001

-

3.234

0.004

2.267

-

0.938

0.008

0.001

2.175

-

-

-

3.272

0.008

2.244

0.002

0.951

0.005

-

2.111

0.007

-

-

3.256

0.005

2.212

0.002

0.993

0.005

-

2.159

-

0.002

-

3.257

0.010

2.214

0.001

0.997

0.006

-

2.138

0.004

-

-

Amphibole

Si

Ti

Al

Cr

Fe

Mn

Mg

Ca

Na

K

Total

7.748

0.029

0.340

-

(1.071)

0.022

3.790

1.858

0.128

0.007

14.992

7.443

0.027

0.579

0.002

(1.151)

0.031

3.767

1.731

0.134

0.021

14.886

7.522

0.038

0.555

0.005

(1.341)

0.024

3.514

1.759

0.143

0.016

14.918

4.4. Hydrothermal Alteration and Mineralization

Using some common metals in mineralization such as copper, gold, silver, molybdenum, lead, zinc in association with some indices such as CCPI, AI and IP, it is possible to correlate the mineralization and hydrothermal alteration types.

Factor 1 shows an association of copper, silver, gold, molybdenum, selenium and antimony mineralization with CCPI and AI (Figure 9(A)). In factor 5 the same mineralization shows a positive correlation with CCPI, TOT/S, FeO3, and TiO2 and a negative correlation with AI (Figure 9(E)). In factor 2, the initial mineralization shows a negative correlation with the phenomena. On the other hand, zinc, nickel, barium mineralization is identified and shows a positive correlation with CCPI (Figure 9(B)). Factor 3 translates a positive correlation of copper and gold with the phenomena to which a strong positive correlation with alkaline minerals (Rb, Cs, IP) can be associated. The positive correlation with AI and negative with CCPI is also remarkable (Figure 9(C)). Factor 4 shows an association of gold and silver positively correlated with the phenomenon, as well as AI where the correlation is positive, but negative with CCPI (Figure 9(D)). Factor 6 is quite similar to factor 4 but differs by a negative correlation of the mineralization with gold, silver, selenium with the phenomena (Figure 9(F)). In the four factors where the mineralization with Cu, Au, Ag shows a positive correlation with the different phenomena, Sr on the other hand shows a negative correlation.

Figure 9. Diagram showing the different factors associated with metals, hydrothermal minerals and alteration index. Evident association of Cu-Au mineralization with alteration indexes is observed in Factors 1 (A), 3 (C) and 5 (E). Factors 2 and 6 show lack of correlation of Cu-Au mineralization with the phenomena (B) (F), whereas, factor 4 shows an association of AI with Se, Au and Ag (D).

The sulfide isotopes for four samples of pyrite are listed in Table 5. The samples consist of one semi-massive pyrite in fractures of quartz veins with few chalcopyrite inclusions which exhibits negative δ34S sulfide (−4,1) for the pyrite. The three other samples consist of disseminated pyrite associated with hydrothermal minerals (carbonates ± chlorite ± epidote). In these sulfides, the δ34S values range from 2.5 to 4.7.

Table 5. Result of δ34S analyses of four samples of Diénéméra-Gongondy deposit.

SAMPLES

Mineral

type

34S

E15D

Pyrite-chalcopyrite

Semi massive

−4.1

E33G

Pyrite

Dissemination

2.5

E39D

Pyrite

Dissemination

4.1

E40D

Pyrite

Dissemination

4.7

5. Discussion

5.1. Hydrothermal Alteration and Mineralization

The use of factor analysis of hydrothermal alteration and mineralization highlighted the association of mineralization and hydrothermal alteration. Indeed, six phenomena are highlighted. The first phenomenon, related to factor 1 correspond to propylitic alteration associated with the Cu-Au-Ag-Mo-Sb mineralization in dioritic intrusions. The same mineralization association is observed in the factor 5 which could be related with phyllitic-carbonate alteration and basic mineralized intrusions represented by microgabbro. The sericitic alteration consists of two types. The first one is represented by factor 3 which is associated with Cu-Au-Mo mineralization and is remarkable by a positive correlation with IP, Rb, Cs and AI. The second is associated with Au-Ag mineralization and Hg, As and SI. This phenomenon can be associated with late orogenic mineralization highlighted by previous studies in the zone [29]-[31]. The last two factors are not associated with the Cu-Au mineralization. The first can be associated with the chloritic alteration regarding the positive correlation with CCPI and negative correlation with AI. The factor 6 corresponds to late non altered diorite which is barren in Cu-Au mineralization. This analysis is in accordance with the ore microscopy studies of the copper mineralization and the study of the hydrothermal alteration at microscopic scale.

The ore microscopy indicated that copper mineralization is associated with hydrothermal alteration. The chalcopyrite is observed in dissemination and associated with alteration minerals, dominated by chlorite and carbonates at Gongondy. However, at Diénéméra, chlorite, carbonates, epidote and muscovite are the most frequent alteration minerals which are associated with dioritic rocks (Figure 10(A) and Figure 10(B)).

In felsic grinding zones, it can be found in subautomorphic crystals and cemented by felsic recrystallization products including carbonates and silica. Moreover, chalcopyrite is also frequently associated with pyrite in crushing zones (Figure 10(C)) or in the form of a trail associated with chlorite and carbonates (Figure 10(D)). The fine crystals are most often crumbled or chamoised while the coarser ones are cracked and filled with chlorite which often acts as cement (Figure 10(E)). In intergranular interstices in dioritic rocks at Diénéméra, chalcopyrite and pyrite are frequently associated with chlorite, carbonates, epidote and muscovite (Figure 10(F)).

Figure 10. Hand specimens and photomicrographs showing the association of sulfides with hydrothermal alteration. The ore microscopy showed a relation between chalcopyrite and pyrite with some quartz in inclusions in pyrite fractures (A). Sulfides in relation with chlorite and cristal of K-feldspar (B). Defromed and fragmented chalcopyrite associated with chloritic alteration (C). Fragmented pyrite in deformed zone associated with quartz in shadow pressure and carbonate (D). Massive chalcopyrite in quartz fractures (E). Chalcopyrite and pyrite observed in intergranular joint observed in quartz microdiorite (F). Silicified microdiorite associated with semi-massive sulfides (G). Fractured quartz with sulfides in the fractures (H).

In silicified zones, chalcopyrite and pyrite are massive to semi-massive associated with silica and chlorite.

5.2. Lack of Potassic Alteration

The study of Gaoua Porphyry copper deposit showed that potassic alteration is absent. The greenschist facies metamorphism or late hydrothermal alteration can explain this lack, regarding the intense albitization of potassic feldspar and possible chloritization of hydrothermal biotite. The albitization is highlighted by microprobes analyses which indicated that the feldspars in the study area have a composition of albite. Even if [21] thought that this albitization can be related to the chemical factor of Precambrian rock composition, we think that this albitization can be associated with late hydrothermal alteration, notably the propylitic alteration generally associated with albitic phase or with retrograde greenschist facies metamorphism. This latter hypothesis can be justified by the posteriority of potassic alteration in relation with the deposition of the porphyry copper deposit around 2161 ± 23 Ma, whereas the greenschist facies metamorphism occurred during the D2Ga deformational phase which took place after −2150 Ma [28] [30] [32] [58].

5.3. Indicator Minerals

The most important alteration minerals frequently associated with the copper mineralization of Gaoua porphyry copper deposit consists of chlorite, carbonates and locally silica, sericite and epidote. The composition of some of these hydrothermal minerals such as chlorite and epidote is an important tool for porphyry copper mineralization exploration [59]-[63]. The analyses of chlorite in the study area enabled us to define two types of chlorite which are low-magnesium chlorite and the relatively high iron-magnesium chlorite, respectively with Ripidiolite and pycnochlorite composition. The composition of chlorite can permit to discriminate the two chlorites and to see the link of these chlorites with the mineralization. Based on its association and other minerals such as epidote, carbonates and sericite, we classify chlorite depending on the propylitic, phyllitic-carbonates and chlorite alteration zones. In this study, Fe and Mg rich chlorite is found in propylitic alteration zone, possibly in phyllito-carbonates alteration zone, whereas, the low Mg chlorite is observed in the chloritic alteration zone where no relation was observed with the mineralization. In some parts of Diénéméra area, the relatively high Mg content of secondary amphibole (actinolite), associated with chlorite and epidote is a proof of origin of Fe-Mg-chlorite in link with the propylitic alteration. This is in accordance with the chlorite study in Naruo porphyry copper deposit [64] where the binary MgO versus FeO diagram determinated a porphyry copper origin of Mg-chlorite. However, Fe-rich chlorite can be associated with a late fluid, notably in relation with greenschist facies metamorphism or orogenic gold mineralization event which overprinted on the porphyry copper mineralization [28] [29]. The presence of Fe-rich dolomite associated with chlorite can permit to confirm this hypothesis regarding the absence of carbonates in greenschist metamorphic facies [65] [66].

5.4. Potentiality of Cu Mineralization

Alkalis such as Sr, Cs, Rb are the most important trace elements to evaluate the impact of hydrothermal fluids on rocks. The mobility of alkalis due to hydrothermal alteration, the primitive mantle normalized formula of Leach and Campbell [47] (Cs + Rb)/Th can be used as alteration index to determine the potential mineralization of a lithology. According to this formula, most of the samples analyzed, show a value of (Cs + Rb)/Th of more than 5 units confirming the potentiality of Diénéméra-Gongondy mineralization (Figure 11(A)).

In the binary diagram of [67] based on the excess of Aluminium content of plagioclase used to determine the fertility of the intrusions, all of the samples fell in the field of fertile intrusion porphyry (Figure 11(B)). The clinozoisite composition of epidote is also a proof of the Al-rich origin of plagioclase in the area zone.

Figure 11. Graph of potential mineralisation index using the primitive mantle normalized formula of [47]. The red line corresponds to limit of potential mineralization which must be more than 5 units (A). Binary diagram of [66] for the determination of the fertility of intrusions. Most of the samples fell in the fertile intrusion field (B).

5.5. Origin of the Fluids

The preliminary δ34S sulfide value of Diénéméra-Gongondy Cu-Au deposit is close to the δ34S mantle value and confirms the magmatic origin of the copper and gold mineralization in comparison with other porphyry copper in the world (Figure 12). Indeed, this conclusion is in accordance with studies of fluid inclusions in quartz associated with the mineralization in the study area [28] [30]. The early mineralization is disseminated associated with dioritic intrusions. During the emplacement of the preserved-structure Fe-rich (magnetite) microgabbro and the possible contact with meteoric groundwater induced a probably sericitic alteration and precipitation of copper mineralization, which corroborates with the study of [68]. The emplacement of several generations of late diorites in the form of dykes and especially of a late gabbro adjacent to the mineralization at Diénéméra and Gongondy, could be the origin of the mobilization of hydrothermal fluids rich in iron and magnesium which caused the chloritic alteration observed in certain zones. It is not excluded that this chloritic alteration often associated with carbonates in relation with the emplacement of the orogenic gold mineralization.

Figure 12. Comparison of the δ34S compositions in the Diénéméra-Gongondy Cu-Au deposit with the other well-known porphyry Cu deposits. Data from [70].

The late fluid which caused the chloritic alteration and released acidic fluids, origin of the dissolution of the rock and carbonates and sulfates filled cavities or breccia. The acidic fluids are the result of interaction of late fluids with the host rocks, responsible of the alteration of albite into chlorite with liberation of acidic fluids according to [69].

The mineralization of Gaoua is however fortunate to have been preserved despite the different phases of late intrusions and the level of erosion that could have destroyed a part of the mineralization. Although the association of copper mineralization with propylitic alteration in Diénéméra is obvious, it does not contradict the conclusions of [1] who defined the propylitic zone as barren. Indeed, the propylitic zone is characterized by disseminated mineralization, and does not constitute an important zone of economically valuable mineralization because the grades are generally low, less than 0.2% of copper. However, the presence of significant mineralization associated with breccia, cavities, intergranular interstices and silicification (Figure 10), reflects a last-stage phenomenon that would be the cause of the remobilization of the copper mineralization or the reconcentration of this mineralization as [25] had stated. The first-order structures associated with the D3Ga deformation phase would have effectively contributed to this remobilization by channeling the fluids rich in gold and copper released by mafic intrusions (Microgabbro) into more permeable and lower pressure zones or zones likely to contain the mineralization such as intergranular interstices, breccia zones or cavities (Figure 13) This conclusion is in accordance with studies of Cu-Au-Mo in Oyu Tolgoï [71] [72] and Cu ± Mo ± Au ± Ag in Imourkhssen [73].

Figure 13. Lithological log of drill cores from Diénéméra and Gongondy deposits showing the relationships between lithologies, copper and gold mineralization, hydrothermal alteration and shearing. Important mineralization (more than 0.2% of copper) can be observed in breccia, cavities, veins and veinlets, intergranular interstices or as massive patches. Abbreviations: Br.: Breccia, Cav.: Cavities, Diss.: Dissemination, Frac.: Fracture, Inters. Intergranular joints, Mas.: Massive, Stock.: Stockwerk.

6. Conclusions

The Diénéméra-Gongondy Cu-Au deposit is a Paleoproterozoic porphyry copper deposit in the West African Craton (WAC). The mineralization is associated with an intrusion complex dominated by dioritic composition with a variable texture.

The petrographic observations and geochemical analyses revealed that the mineralization is associated with the hydrothermal minerals as well as the alteration indexes. Indeed, the copper mineralization is generally associated with some hydrothermal minerals including Mg-Fe-rich chlorite, sericite, carbonates, silica in dissemination, associated with intergranular interstices, breccia or as massive patches. The hydrothermal alterations associated with mineralization are dominated by propylitic alteration in Diénéméra and phyllitic-carbonate alteration in Gongondy. The presence of potassic alteration is not clearly observed. This is probably due to the late-stage albitization associated with retrograde metamorphism or late-stage hydrothermal alteration related to an orogenic gold deposition which overprinted the porphyry copper-gold deposit. Nevertheless, the potential of porphyry copper mineralization is clearly demonstrated even if the late phenomena have attempted to modify or remobilize the copper-gold mineralization as it is the case in many Paleoproterozoic porphyry copper deposits in the world.

Conflicts of Interest

The authors declare no conflicts of interest regarding the publication of this paper.

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