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Chemical Geology, 40 (1983) 25--42

Elsevier Science Publishers B.V., Amsterdam

25,

Printed in The Netherlands

MANGANITE FORMATION IN THE FIRST STAGE OF THE LATERITIC

MANGANESE ORES

IN AFRICA

-t

DANIEL NAHON' , ANICET

BEAUVAIS' , JEAN-LOUP ~OEGLINZ '

JACQUES DUCLOUX' and PACOME NZIENGUI-MAPdNGOU3

86022 Poitiers Cedex (France)

'Office Recherche Scientifique et Technique Outre Mer, Paris (France) "COMILOG, Service Prospections Extérieures, Moanda (Gabon) (Received June 23, 1982; revised and accepted November 25,

1982) 1

I Laboratoire de Pétrologie de la Surface, E.R.A. 220, Université de Poitiers,

ABSTRACT

Nahon, D., Beauvais, A., Boeglin, J.-L., Ducloux,

J. and Nziengui-Mapangou, P., 1983.

Manganite formation in the first stage of the lateritic manganese ores in Africa. Chem.

Geol.,

40: 25-42. Lateritic profiles on manganiferous rocks from two areas in central West Africa permit

the observation of manganite formation on rocks of different primary nature. In an area of gondites and tephroitites (Ziemougoula, Ivory Coast) manganites are found to replace tephroites and manganocalcites, whereas in a weathered carbonate sequence (Moanda, Gabon) manganite replaces rhodochrosite. In both areas manganite occurrence is limited to a very narrow and basal portion of the altered profile, in conformity with the limited stability field established for this mineral. It is found that manganites of different mineral lineages may be distinguished chemically from one another by variation in the ratio

MnO/(CaO + MgO).

INTRODUCTION

The Lateritic weathering of manganesé-bearing rocks was studied in tropi- cal and equatorial Africa. Two areas were especially studied. The first one was near Ziemougoula in the northwest of

Ivory Coast, where a thick

weathered cover develops at the expense of a Birrimian metamorphic parent- rock made up of gondites and tephroitites. The second is the minefield of Moanda in Gabon where the weathering profile overlies Precambrian, un- metamorphosed calcareous shales. Thus, the genesis of manganite in rocks of diverse origins and mineralogy has been studied under similar regimes of lateritic alteration. This permits an examination of the influence of parent- rock composition on the secondary manganite phases and provides an op- portunity to circumscribe and differentiate manganites of different para- geneses. 26
MANGANITE FORMATION IN WEATHERING PROFILES OF ZIEMOUGOULA (IVORY COAST) The Birrimian mebamorphic parent rocks consist essentially of 65% tephroite, 20% spessartite garnet and 10% manganocalcite. The remaining differently to weathering. Thus, at the base of weathering profiles, spessar- tites are unweathered during the transformation of tephroite, manganocal- cite and chlorite. These latter minerals weather into Mn-oxyhydroxides at the earliest stages, but garnet weathers higher in the profile, in a zone where these first Mn-oxyhydroxides are replaced by secondary Mn-oxides. In this paper we deal only with the first transformation of tephroite. manganocal- cite into manganite. Chlorite is replaced by a mixture of Mn-oxyhydroxide + kaolinite.

5% are manganese-bearing chlorites and sulphides. These minerals respond I'

-J 8.

Paren t-ln in erab

Tephroite crystals are anhedral and tightly cemented into a fine-grained mosaic. The chemical compositions obtained by means of the microprobe (Table I) yield the following average formula for the tephroite: Nests of chlorite appear at the grain junctions of tephroite crystals, and from their compositions (Table

II) we can note the following average for-

mula for chlorite: ) O4 3 1 @r3+n.ool Fe2 + o.oloMn1 .WÓM~O.I~Ó@~O.OO~~

C(Si3.017A10.983)

O101 [A11.063 Mn0.680 Mg4.140 Ca0.002) (OH),]

The manganocalcite evolves in patches which can locally replace the bor- der of other parent-minerals. Thus, the manganocalcite appears as the last

TABLE I

Chemical composit,ions of t.ephroites

Sample

No. Average

Total I 2 3

30.23 29.95 30.43

0.27

0.43 0.12

0.08

0.01 0.08

0.48

0.73 0.03

66.18 66.07 66.64

2.43

3.48 2.47

0.15

0.20 0.15

-1 30.01
0.33 0.01 0.60 66.72
2.85 0.20 5

30.69 30.26

0.21 0.27

0.01 0.04

0.03 0.37

66.25 66.37

2.51 2.75

0.27 0.19

99.81 100.87 99.92 100.72 99.97 100.35

9 27

TABLE II

I

Chemical compositions of chlorites

Sample

No. Average

I

9 1 2 3 4 5 6 7 8

I J- I

SiO, 31.09

Alzo, 17.68

Fe,O, n.d.

Cr,O,

Fe0 0.51

MnO 7.78

MgO 29.81

Ca0

Na,O n.d.

K,O - coo - Ni0 Cu0 0.14

ZnO 0.27

H,O* 12.72

Tio, -

32.68
n.d. l7.60 31.25 17.98 n.d. 31.11
17.96 n.d. 30.53
17.60 n.d. 33.40
18.71 n.d. 30.98
18.23 n.d. 30.54
19.39 n.d. 0.31 9.27 28.27
n.d. ' 31.45
18.14 n.d. 0.83 8.37 28.97
0.02 n.d. , 0.93 s 8.10 28.40
, 0.02 ,'i n.d. i 0.02 0.13 I I 1 .o9 8.50 29.05
n.d. 1.18 8.27 28.85
0.04 n.d. 0.43 8.28 27.19
0.13 n.d. 10.98 8.25 30.04
n.d . 0.03 1.21 8.51 30.16
0.02 n.d. 0.02 0.05 0.04 0.14

11.97 0.04

0.03 12.15 , 0.21 0.06

0.09 0.05

0.10 0.04 13.34 0.11 11.75 0,80

10.69 10.66' 12.14 12.12

I Total 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 *Obtained by difference. n.d. = not determined; - = beyond detection limit. TABLE III

Chemical compositions of manganocalcite

Sample No. Average

1 2 3 4 5 6

SiO, 0.05

Alzo, Fe,O, Cr,O,

Fe0 0.04

MnO 15.50

MgO 0.34

Ca0 47.49

Na,O n.d.

0.02

Tio, -

coo -

CO,* 36.56 0.05 0.01

0.03 0.01 0.07 0.04

10.87 13.00

0.21 0.26

51.02 48.24

n.d. n.d. 0.02 0.13

37.75 38.29

100.00 100.00

0.12 0.02 0.05 13.22 0.25 48.24
n.d. . 0.07 0.10 11.84 0.14 49.88
, n.d. 0.01

37.96 0.12

11.48 0.20 49.84
n.d. 0.11 0.19 38.06
16.63 0.36 42.99
n.d. 0.07 39.94
I.? 7 0.02 0.06 38.12

Total 100.00 100.00 100.00 100.00 100.00

*Obtained by difference. n.d. = not determined; - = beyond detection limit. 28

PLATE I

29
crystallized phase of the parent-rock. Here too, chemical compositions were obtained by means of the,electron microprobe and are reported in Table III. The average formula, calculated on the basis of two oxygens, is 1, I ,. Transformation of tephroite into manganite Ji In some pits we have noted an ephemeral weathering of tephroite into

Mn2+-smectites (Nahon et

al., 1982) which precede the manganite stage. But generally the manganite appears as the first product of the replacement of

TABLE IV

Chemical compositions of manganite crystals generated from tephroites

Sample

No. Average

1 .2 3 4 5 6 7

4.02 84.52
0.15 n.d.

11.31 2.74

84.30
0.02 0.32 n.d.

0.02 '

12.60 2.27

0.03

85.22 3.01

0.12 0.28 83.08
0.25 0.03 0.06 n.d.

12.39 0.12

0.36 n.d. 0.01 12.77

1.84 2.81

0.04

0.09 0.33

0.03

84.63 85.43

0.23

0.01 0.02

0.15 0.11

n.d. n.d.

13.02 11.26 2.15

85.70
0.10 n.d.

12.05 2.69

0.03 0.10 84.70
0.07 0.03 0.18 n.d. 12.21 -T Total 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 *Obtained by difference. n.d. = not determined; - = beyond detection limit. PLATE I Manganite formation in weathering profiles of Ziemougoula (Ivory Coast). a. Transformation of tephroite into manganite: T = tephroite; M = mosaic of euhedral manganite crystals (one can note tephroite relict among manganite crystals);

CH = chlo-

rite; F = microfissures from opening-up and buckling of the chlorite lamellae. Thin b. Details of manganite crystals: M = manganite crystals; T = tephroite; G = unweathered spessartite garnet. Thin section under crossed polarizers. c. Transformation of manganocalcite into manganite: MCu = manganocalcite; M = manga- nite crystals developed from cleavages; b) section under crossed polarizers. J = tephroite. Thin section in plain light. b .I 30
tephroite. These modifications begin in microfissures traversing tephroite crystals and develop as a mosaic of euhedral manganite crystals easily recog- nized under reflected light (Plate I, a and b). Table IV gives the chemical compositions obtained on such manganite with the electron microprobe. One can note that silica is not entirely leached when manganite appears. The irregular distribution of silica among manganite crystals suggests a scat- tering of microconcretions of SiO,. In addition, the distribution pattern of Si shows that silica is more abundant near the edges of unweathered tephroite. L

Transforinatioiz of mmzganoealeite into manganite

A well-crystallized manganite mosaic develops from cross-cleavages and microfractures towards unweathered manganocalcite (Plate I,

C). These

manganite crystals are similar, under reflected light, to those observed in the weathering products of tephroite.

With the advance

of weathering, the manganite develops at the expense of manganocalcite, eventually isolating residual fragments of the parent-carbon ate. Each fragment retains the crystallographic orientation of the original carbonate. This proves the in situ weathering of manganocalcite. The compositions of these manganite crystals (Table V) compared to those of manganocalcite (Table III) enlightens us regarding the source of the silica impurities. As the parent-manganocdcite is almost depleted in this ele- ment, we can consider a silica pollution from the surrounding tephroites which weather at the same time as manganocalcite.

Manganite crystallized in fissures

Many fissures cross the rock and cut each parent-mineral and its weather- ing product. These fissures are - 1 mm thick and are covered or filled by manganite crystals which are perpendicular to the walls. Under the micra- scope these crystals present optical features similar to those observed in weathering products of tephroite or manganocalcite, though they are larger in size. Their compositions are given in Table VI. One can note that these manganite crystals are the richest in Mn. X-ray eharaeterization of ie mo ugou la mangani te The manganite phase was separated by hand-picking from crushed weath- ered rocks, analysed by X-ray diffraction (XRD) and infra-red (IR) spectro- metry. We could not, however, differentiate manganite crystals originating from tephroite from tthose of manganocalcite or from fissures. X-ray pat- terns obtained are compared to ASTM data, and crystallographic param- eters are calculated by the

UNIT-CELL program (Table VII). We can note a

slight difference between the parameters of standard manganite and the manganite phase of Ziemougoula. b

TABLE V

Chemical compositions of manganite crystals generated from manganocalcite

Sample No. Average

I 1 2 3 4 5 6 7 8 9 10 11 12 13 14

0.35 0.02 0.04 74.69
0.32 0.65 5.32 0.08

18.53 0.48

0.29 0.05 74.03
0.19 0.56 5.19 19.21 O .42 0.01 73.53
0.23 0.47 5.12 0.07 20.15
0.67 0.01 73.76
0.01 0.61 5.59 0.02 0.05

19.28,

0.37 0.08 75.76
O .40 0.51 4.75

18.13 0.57

0.03 0.01quotesdbs_dbs17.pdfusesText_23

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