Geological and ore microscopic evidence on the epigenetic origin of the manganese occurrences in northern Nigeria

0731-7247/84 $3.00 + 0.00 Pergamon Press Ltd.

Journal ofAfrican Earth Sciences, Vol. 2, No.3, pp. 209 to 225,1984 Printed in Great Britain

Geological and ore microscopic evidence on the epigenetic origin of the manganese occurrences in northern Nigeria A.

MOCKE

Department of Mineralogy and Petrography, Georg-August University of G6ttingen, Goldschmidstra13e 1 3400 Gottingen, F.R.G.

and C.OKUJEN1

Department of Geology, Ahmadu Bello University, Zaria, Nigeria (Received 14 March 1984)

Abstract-Northern Nigerian manganese occurrences closely associated with the metasedimentary belts are described and compared. The vein-like mineralizations show sharp contacts to the phylIites and they are exclusively arranged parallel to the foliation of the host-rock phyllite. In the Maru Belt, the localities of Ruwan Doruwa and Maraba Hill are linked up with the banded iron formation. The Tudun Kudu and Ungwan Malam Ajuba localities are situated in the Maska Belt where banded ores do not occur. All occurrences are rich in spessartite/almandine quartz, which is why Tudun Kudu was classified as gondite type [Wright and McCurry 1970 (Econ. Geol. 65, 103-106)]. Apart from gamet, typical minerals for gondites are not found in the Nigerian localities. Instead of these minerals other minerals abundantly appear, which generally either do not occur in manganese ore deposits or are extremely rare. These are manganoan ilmenite and ferrian pyrophanite. The primary manganese minerals (spessartite-almandine, pyrophanite-ilmenite, manganiferrous magnetite) are superficially replaced by secondary manganese minerals (cryptomelane, lithiophorite, pseudorutile, etc.). Besides some rare minerals, additional new ones were discovered. Special attention was directed to the diadochism of elements in cryptomelane (e.g. K, Ba, Ca, Al and Ti). On the basis of field observations, thin sections and ore microscopic investigations (stock of minerals, intergrowth of the minerals and textures), a syngenetic formation hypothesis for the Nigerian manganese mineralizations in the northern metasedimentary belt is probably not acceptable.

INTRODUCTION FROM THE formational point of view, the manganiferrous mineralizations in Pre-Cambrian rocks are not only of scientific but also of economic significance. They either occur in connection with banded iron formations or as so-called gondite or related types (excluded from this discussion are the queluzite and kodurite). Both types are characteristic of metasediments spread all over the world. For the formation of the gondite type, it is suggested that there are either manganese rich layers in the original sediment, which are both transformed during metamorphism into gonditic composed layers and phyllite, or that a high manganese concentration in the original sediment was mobilized and enriched during metamorphism. A common syngenetic formation of phyllite and spessartite is supported by some experimental data and this mode of formation is generally accepted. Occasionally, the assumption is made that host rock and ore mineralization are not of the same age (epigenetic formation). Roy (1965) discussed the gondite type comprehensively stating that some minerals were formed from later pegmatite intrusions into gondite, especially those related to alkali and Fe3+ metasomatism (Roy 1981). On the basis of field observations, thin section and ore microscopic investigations Miicke and Bar (1982) pointed out that a syngenetic origin for the Nigerian manganese mineralizations in the northern metasedimentary belts does not seem to be feasible.

Tudun Kudu was first mentioned by Wright and McCurry (1970) who linked the formation of the occurr~nce to the gondite type. Then Moneme et al. (1982) supported the same view. In both papers there were no detailed investigations on the genesis. All the other localities described in this paper were first mentioned by Miicke and Bar in 1982. Then Moneme and Scott (1983) gave a brief description of the mineral paragenesis (garnet, amphibole, cryptomelane, hollandite, quartz, kaolinite, goethite) in Ungwan Malam Ajuba, but did not discuss the genesis. This paper strongly supports the epigenetic origin for the mineralization and completes the results of Miicke and Bar (1982). The present results do not, and cannot, mean that for every mineralization described as gondite a syngenetic formation is excluded since different natural processes could lead to similar results.

THE MANGANESE OCCURRENCES IN NORTHERN NIGERIA The occurrences are in the northern part of Nigeria and are closely associated with the metasedimentary belts, which consist almost exclusively of phyllites. Tudun Kudu Hill and Ungwan Malam Ajuba are situated in the Maska Belt, Ruwan Doruwa and Maraba Hill in the Maru Belt (Fig. 1). Whereas Ruwan Doruwa and Maraba Hill are linked up with the banded iron formation, such iron mineralizations are unknown in Tudun Kudu and Ungwan Malam Ajuba. Therefore, 209

A. MOcKE and C. OKUJENI

210

1 TUDUN KUDU HILL 2 UNGWAN MALAM AJUBA 3 RUWAN DORUWA 4 MARABA HILL mnn meta-sedimEnts CJ basement complllx ETI oldEr granite

,.,,----

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------.....",

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\

/

,,/

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I

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\

, I

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\

I \

\

,/-'

(

.) \

,J

100 km

Fig. I. Geographical and geological setting of the manganese occurrences in northern Nigeria.

contrary to Tudun Kudu Hill and Ungwan Malam Ajuba, Ruwan Doruwa and Maraba Hill are rich in magnetite (see Figs 6 and 30). Magnetite which is included in the manganese mineralization contains about 0·5-2 wt. % Mn in solid solution as well as Al and traces ofTi (see Fig. 34). Magnetite found in the banded iron formation is free of manganese. That means that there are at least two generations of magnetite. In Ungwan Malam Ajuba magnetite occurs sporadically, Tudun Kudu seems to be free of magnetite.

COMMON FEATURES OF ALL OCCURRENCES The analogies of all the manganese mineralizations mentioned can be briefly summarized as follows. (1) The manganese mineralizations are arranged parallel to the foliation of the phyllites, that means generally north-south and therefore analogous to structural trends of the Pan-African tectonics. (2) The "wall paper like" outcrops of the mineralizations are situated more or less perpendicularly to the earth surface (Fig. 2). They are often banded or zoned parallel to the vein wall, even in those cases where adjacent phyllites show a fine refoliation. (3) The veins are visible over long distances (as far as about 100 m). (4) The thickness of the veins varies between several em andm. (5) The contact between phyllite and manganese veins is sharp (Fig. 2).

(6) Quartz is the only mineral which occurs in the veins as well as in the phyllite. All other minerals are restricted either to the phyllite or the vein. (7) Quartz and spessartite/almandine (mixed crystals) are the most common minerals in the veins. (8) In the localities, one mineral abundantly occurs which in all types of manganese deposits is of no significance (or it does not occur at all): pyrophanite-ilmenite. These mixed crystals occur independently (Fig. 3) and they are also intergrown with magnetite, mostly in orientation (Fig. 4). The manganese concentration varies between 27 and 8 wt. % corresponding to the structural formula between (Mno.74 Feo.26) Ti03 and (Feo.78 Mno.22) Ti0 3 • (9) The mineralization near the earth surface in the veins is altered and the original minerals are more or less replaced by secondary minerals.

ORE MICROSCOPIC INVESTIGATIONS (a) The primary minerals The garnet crystals [consisting of spessartite (-50%)almandine (-50% )-grossulare] are mostly uncorroded, sometimes partially, sometimes completely replaced by cryptomelane and lithiophorite. The other manganese bearing and obviously primary minerals (ilmenite-pyrophanite, pyrophanite-ilmenite and magnetite) are generally nearly completely replaced (Fig. 5). They are only partially preserved as relics

Epigenetic origin of manganese occurrences, N Nigeria

--

211

1/

--:-~

_ _

Fig. 2. The quartz/garnet rich vein shows sharp contacts to the phyllite host rock. Thc middle part of the zoned vein consists only of quartz. Due to its resistance against weathering effects as compared with phyllite the vein is outcropping. Perpendicular to the vein there is a trench . (Tudun Kudu Hill.) Fig. 3. Polycrystall ine aggregate of manganoan ilmenite (with distinct dichroism). (Reflected light, oilimmersion ; Ungwan Malam Ajuba.) Fig. 4. Manganoan ilmenite (lamellae) in orientated intergrowth with magnetite nearly completely martitisized and partially altered into limonite. The lamellae lies parallel to one of the three visible systems of rnartite orientated parallel to (lll)-magnetite. (Reflected light, oilimmersion; Ungwan Malam Ajuba, rna = martitc, mil = manganoan ilmenite, Ii = limonite, mg = relics of magnetite.)

212

cry

100 J.1111

Fig. 5. Cryptomelane with relics of ferrous pyrophanite. (Reflected light, oilimmersion; Ruwan Doruwa , pyr pyrophanite, cry = cryptomelane.)

= ferrous

Fig. 6. Island-like inclusions of replaced magnetite (partially martitisized) in the matrix of the replacing cryptomelane (zoned) . (Reflected light. oilimmersion; Ruwan Doruwa, mg = rnartitisized magnetite. cry = cryptornelanc.) Fig. 7. Pseudorutile with relics of manganoan ilmenite in the centre of some grains . The texture of ilmenite is the foam texture (triple-junction configuration) . Included is the traverse along which the microprobe analysis was carried out (see Fig. 8). (Refleeted light. oilimmersion;Tudun Kudu Hlll.mil = manganoan ilmcnitc.psr = pseudorutile,li = limonite.)

213

goe

cry

Fig. 9. The replacement takes place originating from grain boundaries of garnet. Included is the traverse along which the micro-probe analysis was carried out (see Fig. 10). (Reflected light, oilimmersion; Ungwan Malam Ajuba, ga = garnet (spcssartite-almandine), unk = unknown mineral.) Fig. 11. Ooidic zoned and X-ray amorphous aggregates are replaced by zoned cryptomelane. Included are the traverses along which the microprobe analyses were carried out (see Figs 12 and 21). (Reflected light, oilimmersion; Tudun Kudu Hill, mil = manganoan ilmenite .) Fig. 13. Cryptomelane (chemical composition, see Fig. 14) surrounded by goethite. (Reflected light, oilimmersion ; Tudun Kudu Hill, cry = cryptomelane, goc = goethite.)

214

100 pm

ra

Fig. IS. Zoned cryptomelane. With increasing Sa content the reflectance decreases (see Fig. 16). In the middle part (with the lowest reflectance) the Ba concentration is higher than that of potassium (=psilomelane). Included is the traverse along which the microprobe analysis was carried out. (Reflected light, oilimmersion; Ungwan Malam Ajuba.) Fig. 17. Cryptomelane and psilomelane in alternative arrangement. Included is the traverse along which the microprobe analysis was carried out (see Fig. 18). (Reflected light, oilimmersion; Ungwan Malam Ajuba.) Fig. 19. Ca-rich cryptomelane (rancieite?) is replaced by Co-bearing cryptomelane. The grainboundaries (black) between both are papered by calcite. Included is the traverse along which the -microprobe analysis was carried out (see Fig. 20) . (Reflected light, oilimmersion; Ruwan Doruwa, ra = Ca-rich cryptomelane (r:mcieite?), cry = cryptomelane.)

Epigenetic origin of manganese occurrences, N Nigeria TUDUN KUDU /

20pm

UNGWAN HALAH AJUBA

PRIHARY ORES

Ti 37%

33%

215

Al

35%Ti ZONE of REPLA CEHENT

GARNET Hn

Si AI

Mn

Hn

3,7"10

Fe

Fe

Fe

22,% Si Fig. 8. Distribution of clements in manganoan ilmenite and the replacing pseudorutile (linescan).

which occur island-like in the matrix of the replacing mineral (Fig. 6). It is obvious from the abundance of garnet, especially, and the other primary manganese minerals that: (1) during their formation there was a high supply of manganese and (2) manganese which was necessary for the formation of the secondary minerals originated from the replacement of pyrophanite-ilrnenite, manganese bearing magnetite and spessartitealmandine. Through the decomposition of ilmenite-pyrophanite, the mineral pseudorutile is newly formed (Figs 7 and 8): 3(Fe,Mn)Ti03 ~(Fe,Mn)2Ti309 + (Fe,Mn). Ilmenite-pyrophanite pseudorutile The leached part of manganese and iron is used for the formation of limonite and cryptomelane, respectively. Until now the mineral pseudorutile is only known from placer deposits. Tudun Kudu Hill where this replacement was observed, is thus the first occurrence where pseudorutile is an ill situ replacement product in the original host rock. As mentioned above, the mineral spessartite-almandine is also replaced, mostly by cryptomelane, sometimes by lithiophorite. Only in Ungwan Malam Ajuba does a special and restricted type of replacement occur (Fig. 9). Originating from the grain boundaries, garnet is replaced by an unknown and most probably optically isotropic mineral with a reflectance of about 20%. The newly formed mineral shows relative enrichment of iron and manganese and a decrease of Si and Al (in comparison with garnet) (Fig. 10). A spinel-like composition and structure of this mineral is suggested (Mn, Fe) (Fe, AI, Si, Dh o.. Other primary minerals are chalcopyrite, pyrrhotite and pyrite. These minerals occur as small inclusions in unaltered garnet.

(b) The secondary manganese minerals Ooidic zoned and X-ray amorphous aggregates, the

Fig. 10. Distribution of elements in garnet and the replacing phase (linescan).

first newly formed secondary mineralization, only observed in Tudun Kudu, are replaced by cryptomelane (Fig. 11). Besides Al and the main components Mn and Fe, these aggregates contain up to 4 wt. % titanium. The Ti concentration proves that ilmenite-pyrophanite and garnet were decomposed because Al is also found in the newly formed product (Fig. 12). Commonly, cryptomelane is free of titanium; titanium bearing types are the exception (Figs 13 and 14). Almost every cryptomelane-the main mineral in all occurrences-is potassium-, barium- and calcium-bearing. While potassium and barium show a negative correlation, that means potassium replaces barium and vice versa (Figs 15 and 16), calcium shows a different behaviour (Figs 17 and 18). If there is a replacement recognizable, calcium correlates with barium (see Fig. 16). In some grains, or zones, of secondary manganese minerals the barium concentration is higher than the concentration of potassium, as detected at Malam Ajuba and Ruwan Doruwa. In these cases psilornelane is found. In other cases, the calcium amount is higher than that of potassium linked in zoned aggregates from Ruwan Doruwa. Probably, this is due to the mineral rancieite (Ca, Mn)O . 4Mn02 . 3H2 0 (Figs 19 and 20). Of mineralogical and crystallochemical interest is potassium-aluminium-<:ryptomelane (see Fig. 11) where the main components Fe and Mn show the same negative correlation as potassium and aluminium. In the outer part of this zoned crystal the AI concentration is far higher than that of potassium (Fig. 21). In Tudun Kudu Hill besides cryptomelane, Felithiophorite is the second main mineral. It replaces garnet as well as cryptomelane (Fig. 22). In the other locations, the mineral occurs only sporadically. In Ungwan Malam Ajuba this concerns bariurn-Iithiophorite (Figs. 23 and 24). <,

A.

216

MOCKE and

C.

OKUJENI

TUOYN KUOU HILL I HANGANIFEROUS 0010 Hn

27% 24% Hn

18,5% Fe

15%

Ti

4,0%

AI

2.2%

I1n

Fe

Ti

14% 8.5%

4,5

Fe

3,0%

SHELL

CORE

Fig. 12. Distribution of elements in an ooidic and zoned aggregate (linescan).

TUDUN KUDU HILL !CRYPTOMELANE

UN6WAN HALAt1 AJUBA

46%

I1n

1-----.._

-

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45%Mn

-

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53 5J1m

3.6 K

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0.3%

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\ \ Ba \ .../ ' 1.5

Ti

-~

Al

[a

[a

--K

Ca

At

Sa

At

Fig. 18. Distribution of elements of cryptomelane and psilomelane in alternative arrangements (linescan).

'Fig. 14. Distribution of elements of cryptomelane along a traverse perpendicular to the wall of the vcinlet (see Fig. 13) (Iinescan).

UN6WAN HALAH AlYBA: zoned CRYPTOHELANE Hn

53

I

'1, /,I

Hn

43%

10%

~

Calcite f?J

,III II I I ,I'I 1I 1\

............J

\

CRYPTOMELANE

,

~5% K ~~--------------

\ \

...... /

/---"_

1,0%,./

1.5

AI

Fig. 16. Distribution of clements in a zoned cryptomelane (Iinescan). The middle part (see Fig. 15) consists of psilomelane.

,.,----

/

52% I1n

20pm

\

RANCEI TE

AI

\

6,5% Fe

5,5%

0

1,5Yo

-------

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Fe

RUWAN DORUWA / Mn-t1INERALS Fig. 20. Element distribution along the traverse of Fig. 19.

Epigenetic origin of manganese occurrences, N Nigeria

,

r. 4,5 %

/I

217

TUDUN KUDU HILL I ZONED CRYPTOHfLANE

AI- CRYPTOHfLANE

I I I I

AI"

,'\11 JII,

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CRYPTOHELANE

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I

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Fe Fe

4,0%

('-v'

OUTER PART

INNER PART

Fig. 21. Distribution of elements in the zoned cryptomelane (see traverse 2 in Fig. 11).

Table 1. Trace element contents of phyllites

UNGWAN MALAH AJUBA

-

Mn

Hn

5pm

10.0

8.5

Ruwan Doruwa (ppm) Ca Ti V Cr

7326 3580 77 124

5637 5254

Mn

260

188

Ni

29 46 265 163 33 20 III 550 194

27

Cu Zn Rb Sr

Y Zr Ba Ce Fe

AI

4.0

3.2

Fig. 24. The element distribution in lithiophorite (see Fig. 23) shows zoning and a negative correlatiorfbetween AI and (Ba + Fe + 1\In). The resulling formula is: (AI, Ba. Fe, 1\In) (OH)z MnOz . Ats Z:3-S

Maraba Hill (ppm)

3.1 wt.%

279

322 78 61 22 144 949 1.8 wt.%

Limonite (goethite) occurs at every locality. It is of the same age as cryptomelane, but during its collomorphic condition it separates itself from cryptomelane (Fig. 25). Minerals from Tudun Kudu Hill and Ungwan Malam Ajuba and Maraba Hill are summarized in Table 1. Ruwan Doruwa has the greatest number of different minerals, including some minerals which are unknown (Table 1). The unknown minerals are older than cryptomelane because they are replaced by it (Fig. 26). They will be described in greater detail after their approval by the IMA Commission on New Minerals and Mineral Names.

218

A.

MOCKE

and C. OKUJENI

Table 2. Ore min erals from Tudun Kudu Hill (TK), Ungwan Mal am Ajuba (MA), Ruwan Doruwa (RD) and Maraba Hill (MH) Locality Minerals Man ganoan magnetite Martire Ferrous pyrophanite Man ganoan ilmenite Cryptomelane

Psilornelane Ho llandite Lithiophorite Rancieite (7) Ferrous todorokite Mang anoan pseudorutile Limonite Limonite/groutite Goethite Unknown mineral! Unknown mineral 2

Formula

TK

(Fe , Mn) FezO~ FeZOJ (Mn,Fe)TiO J (Fe , Mn) TiO J (K, Ba, Ca)
Fe-Mn/oxide-hydrate Fc-Mn-Al-Si/oxide-hydrate

OTHER MINERALS In Tudun Kudu Hill and Ungwan Malam Ajuba, needle-like crystals occur which form radially fibrous aggregates. They are included in quartz and sometimes the crystals are colourless, sometimes brown and dull, probably due to oxidation . By the electron microprobe analysis the following elements were found: Fe, K, AI, Si, Mg and Mn, but Ca could not be detected . To date, the X-ray powder diagram of this mineral could not be identified. Moneme and Scott (1983) named this mineral anthophyllite, Bar (1982) called it disthene. In some parts of the Tudun Kudu vein large quantities of kaolinite occur together with secondary manganese oxides (so called black and white ore) (Fig. 27). Kaolinite is also found in Ungwan Malam Ajuba. In all the OCCUrrences the grain boundaries of quartz crystals are sometimes papered with thin biotite layers.

TEXTURAL OBSERVATIONS OF THE PRIMARY MINERALS The garnet crystals are embedded and surrounded by undeformed and unaligned quartz crystals (Fig. 28). In pure quartz and garnet-rich areas of microscopical scale, both minerals are arranged in a triple-junction configuration (Fig. 29). This texture may occur in high temperature epigenetic mineralization or in a strongly metamorphosed environment. This is due to either slow cooling after deposition or slow heating and cooling during and after metamorphism, respectively. Since quartz and garnet are more or less refractory .minerals, epizonal conditions of metamorphism are too low to re-equilibrate texturally both minerals during annealing. Recrystallization, especially for refractory minerals, is unusual for this low grade process.

x x

x x x x x x

MA

RD

MH

x x

x x x

x x x

x x x

X

x

x x

x x

x x

x

x

x

x

x x x

x x

x

x

x

x

x

x x x x x

x x x

In low grade met amorphic rocks like phyllites, refrac tory minerals, e.g. Ti minerals, are considered to be of sedimentary origin. Their chemical composition resists during the process of metamorphism; elongated crystals were aligned into the developing texture. All primary oxidic minerals (ilmenite-pyrophanite and magnetite) of the described occurrences are also refractory minerals and exhibit triple-junction configuration (Figs 7 and 30), the formation of which, during and after low grade metamorphism, is impossible. Therefore, the texture of these minerals has to be of primary and high temperature origin.

GENETICAL ASPECTS OF THE FORMATION AND THE ORIGIN OF THE ORE Tudun Kudu and Ungwan Malam Ajuba belong-in accordance with.the classification of metamorphic manganese ore deposits-to the gondite type in so far as the simple existence of spessartite is overemphasized. Mamba Hill and Ruwan Doruwa being located close to each other belong to the Pre-Cambrian banded-iron formation. The last two localities, also rich in spessartite (comparable with Tudun Kudu and Ungwan Malam Ajuba), likewise belong to the gondite type. Phyllites, in which manganese mineralization occur, were formed under epizonal conditions of regional metamorphism. Matthes (1961) has shown that almandine bearing spessartite starts to form under a pressure of 2001500 atm and a temperature range of 410-480°C. Under these conditions, a syngenetic formation of phyllite and the garnet/quartz mineralization is possible. The Nigerian Mn occurrences show some features which seem to be contradictory to this type of genesis: (1) The average contents of manganese in the original sediments are too low (phyllite from Ruwan Doruwa: 260 ppm, from Maraba Hill: 188 ppm; see also Table 1)

Epigenetic origin of manganese occurrences, N Nigeria

Fig. 22. Lithiophorite (strong reflection dicroism) replaces cryptomelane (in the middle part). (Reflected light, oilimrnersion; Tudun Kudu Hill.) Fig. 23. Ba-lithiophorite and cryptomelane. (Reflected light, oilimmersion; Ungwan Malam Ajuba.) Fig. 25. Separation of limonite (grey) from cryptomelane (white), or vice versa, during their collomorphic conditions. (Reflected light, oilimmersion; Tudun Kudu Hill.)

219

220

Fig. 26. Todorokite (the iron content is higher than this for manganese) replaced by cryptomelane. (Reflected light, oilimmersion: Ruwan Doruwa.) Fig. 27. Black and white ore (mixture of kaolinite and secondary manganese minerals). (Tudun Kudu Hill.) Fig. 28. Idiomorphic garnet crystals are embedded in homogeneous medium, grained, undeformed and unaligned quartz crystals. (Transmitted light, crossed polars; Tudun Kudu Hill.)

221

Fig. 29. Grain boundaries of quartzcrystals are covered with cryptomelane. The crystals are arranged in triple-junction configuration. (Reflected light; oilimmersion; Tudun Kudu Hill.) Fig. 30. Triple-junction configuration of magnetite (Mn-, Ti-, At-bearing) partially martitisized. (Reflected light, oilirnmersion; Ruwan Doruwa.) Fig. 31. Symmetrically zoned vein of manganese mineralization/quartz with a pure quartz band in the middle part. (Tudun Kudu Hill.)

222

A.

MUCKE

and C.

OKUJENI

Fig. 32. Texture of the host rock phyllite consisting of quartz and muscovite. (Transmitted light; Ruwan Doruwa.) Fig. 33. Orientated intcrgrowth of garnet (=host crystal) and lamellae of manganoan ilmenite . (Reflected light, oilimmersion ; Ungwan Malam Ajuba, ga = garnet , mil = manganoan ilmenite, qu = quartz.) Fig. 35. Some parts of the quartz veins consist nearly completely of tourmaline. (Tudun Kudu H ill.)

Epigenetic origin of manganese occurrences, N Nigeria to produce so much spcssartite as a result of metamorphism. No type of metamorphism is known to produce so high a manganese concentration, and under epizonal conditions of metamorphism, the migration of titanium is most uncommon. (2) In spite of this, it should still be taken into consideration, that the above mentioned ore deposition came into existence through mobilization. It should then be assumed that the migration must be perpendicular to the foliation plane of the phyllites . As the enrichment of manganese could not take place in the described manner, it must be considered that there were manganese rich horizons in the original sediment thus assuming a syngenetic origin. (3) Metamorphism of these manganese rich horizons could not only produce quartz, garnet and i1menite/pyrophanite. The paragenesis is by far too selective. (4) If the manganese occurrences were of sedimentary-metamorphic origin they would be analogous to the phyllites which are more strained due to D. deformation . They would not only show small drag folds (only occasionally visible) and would not only be confined to the foliation of the phyllites. (5) The contacts between manganese mineralization and phyllite are sharp (see Fig. 2). There are no transitional zones. The phyllite is free of garnet. (6) The garnet/quartz mineralization is sometimes zoned symmetrically with a pure band of quartz in the middle portion (Fig. 31, see also Fig. 2). (7) The following minerals are characteristic in parageneses with the banded-iron formation as well as with the gondite type: hausmannite, rhodonite, jakobsite , vredenburgite, braunite, bixbiite, pyrolusite and several manganese silicates. None of these minerals occur in the Nigerian localities . (8) The texture of manganese rich zones is not comparable with the texture of the phyllite, in which all minerals are strongly aligned (Fig. 32; compare with Figs 28 and 29). (9) Apart from garnet and quartz, manganoan ilmenite and ferrous pyrophanite also occur. Down to a minimum temperature of 500°C ilmenite formation is possible. Pyrophanite MnTi0 3 and ilmenite FeTi03 are isomorphic and are able to form complete solid solution at temperatures considerably higher than 500°C. The presence of ferrous pyrophanite and manganoan ilmenite is generally an indication of late origin and it is suggested that the element manganese was concentrated during the natural process of magmatic differentiation. The large size of the Mn2+ ion (0.80 A) compared with Fe 2 + (0.74 A) favours the tendency of its concentration in later, more silicic liquids, as compared to the original melt. Manganoan ilmenite (MnO content ranges from 814 wt. %) occurs e.g. in a quartz rich adamellite from Yosemite Valley, California (Snetsinger 1969). In the same paper, Snetsinger mentioned that ilmenites with MnO higher than 5 wt. % are rare and are restricted to three other localities; outside the type locality of

223

MARABA HILL

Fe

72.0 "";--Ttt

(I

Fe

32.0

II

II

II MAGNETITE: \

,i~;'-Mn ., I

,.

18.5

I

I

'I,; lj-Fe ,)

18.0

,. II II • I

5J1m

I

Ii

!I

Ii

'I

Ii \1

Ii

\I (Mn.Fe)TiO~ ~ .;J ...?--' <1

·I :

Ti TRACE At'" 0-: 5 ,,~. . .. ..... '"

.:.: -': ====--'

/Mn 0.7 . ,.-.-::: ..::".:':,::.:"-=--=::'- - -

Fig. 34. Distribution of elem ents in magnetite and a lamella (in orientated intergrowth with magnetite) of ferrous pyrophanite. (Lines can; Marab a Hill.)

pyrophanite at the Harstig Mine, Pajsberg, Sweden. Another locality (Itakpe Hill, iron ore deposit, Kwara State, Nigeria) of manganoan ilmenite and ferrous pyrophanite (as exsolution bodies in manganiferrous magnetite) was mentioned by Miicke (1982). Spessartite is of the same age and formation temperature as observed from orientated intergrowth between both spessartite and manganoan ilmenite (Fig. 33). (10) Orientated intergrowth between magnetite and i1menite/pyrophanite as formed at Ungwan Malam Ajuba and Maraba Hill is usually of high temperature origin (see Fig. 4) . Magnetite contains 0.5-2 wt. % of Mn , Al and traces ofTi (Fig. 34). (11) In phyllites there are also quartz rich zones free of ore inclusion. They are known as barren quartzite and are definitely vein quartz, partially rich in tourmaline (Fig. 35). A metamorphic origin must be excluded and, as well as the assumption of originally, pure sandy horizons with an extremely high content of boron. They are of pegmatitidpneumatolytic to katathermal origin . and mineralized along weak zones of the phyllites which means along the foliation . The manganese mineralizations occur in the same arrangement, as already mentioned, and are partially closely associated with the tourmaline-bearing vein quartz. This is another argument in favour of high-temperature origin for the manganese rich mineral associations. It differs, however, from the conditions related to metamorphism leading to the formation of phyllite. What we observe now in the field, is the superficial enrichment of manganese due to leaching processes of

224

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the prim ar y manganese bearing minerals as well as the formation of secondary ones during geological time (Fig. 36).

FOLLOW-UP PROJECT The programme in the future is to investigate and compare on a larger scale in one or two areas host rock and ore mineralization by means of geochemical, petrographic and ore microscopic methods and electron microprobe analyses. Other localities are to be included at random in order to facilitate comparisons. If the expected epigenetic formation of the ore is reconfirmed then the problem of the origin of ores has to be solved. In each of the studied areas, there are outcrops of acidic rocks which may be related to the origin of the ore. However, the answer to the question of the origin must not be unconditionally related to the twodimensional view. The tourmaline-bearing vein quartz has to be included in th e discussion on the origin of the ore. A dominant part of the discussion of the origin of the ore may include some ore rich zones in the Maru area which are to be distinguished through their shape (rounded) and location (occurring in the bas ement complex) from the others that only occur in the phyllites (Fig. 37).

Fig. 37. Geological and geographical setting of Maraba Hill and Ru wan Doruwa. Note tf:e two orcbodics at the lower boarder of the D = hypersth ene-quartz-diorite, figure. A = amphibolite, QS = quartz-syenite.

Acknowledgements-For their part in the field work and in several discussions on the subj ect of this paper, we are indebted to Mr. M. Woakes and Dr. R. Glaeser. both Crom the Department of Geology, Ahmadu Bello University Zari a. Nigeria , Dr. P. Bar, Department of Geological and Mineral Sciences . University of Ilorin, Nigeria and Mr. P. C. Moneme, Nigerian Steel Council, Kaduna, Nigeria. We should like to express our thanks to the Head of Department of Geology, ABU. Zaria. Prof Dr. C. A. Kogbe and Dr. D. Osijuk (Ag. Head since 1982) Cor the ir encouragement and assistance with field vehicles.

Epigenetic origin of manganese occurrences, N Nigeria We arc also grateful to Mr. A. Koch, Institut fUr Mctalllorschung, Bereich Elmi, Technische Universitat Berlin , who ' carried out the microprobe analyses (CAMECA MS 46) and to Ch. Agthc, Institut fUr Mineralogic und Kristallographic, Technische Universitat Berlin, for the XRF results (trace elements of the phyllites). For reading our original English manuscript we had the assistance of Mrs. G. Seidensticker, Zaria. We are very grateful to her.

REFERENCES Bar, P. 1982. Tektono-stratigraphischc Untersuchungen im prakarnbrisch/alt-palaozoischen Grund gebirgc von NW Nigeria. AfrikaGruppe deutscher Geowissenscahftler, Kolloq. in Gottingcn, 25-26 June. Bartholomew, A. 1982. The geology and geochemical secondary dispersion pattern of manganese occurrence in Ugoge (South Western Sector), via Giwa, Kaduna State, Nigeria. Special Project, Department of Geology, Ahmadu Bello University, Zaria, Nigeria (unpublished). Matthes, S. 1961. Ergebnisse zur Granatsynthese und ihre Beziehungen zur naturlichen Granatbildung innerhalb der Pyralspit-Gruppe. Geochim . cosmochlm. Acta 23, 233--294.

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Monernc, P. c.,Scott, P. W. and Dunham, A. C. 1982. Occurrence of manganese ores in Zaria Area, Kaduna State. Nigerian Mining Geosciences Soc. Abstracts, 18th Annual ConL Kaduna, Moncme, P. C. and Scott, P. W. 1983. The mineralogy of Malam Ajuba manganese mineralization in Snect (101) (Maska), Kaduna State. Nigerian Mining Geosciences Soc. 19th Annual Conf. Warri. Mucke, A. 1982. Some remarks of the iron ore deposit of Itakpe Hill ncar Okene (Kwara State/Nigeria). Nigerian Mining Geosciences Soc. Abstracts, 18th Annual Conf , Kaduna. Mucke, A. and Bar, P. 1982. Mineralisation und Genese einiger nigerianischer Manganvorkornrncn (Tudun Kudu Hill, Ungwan Malam Ajuba bei Gangara, Ruwan Doruwa and Maraba Hill). Fortschr. Mill. 60, Bh. 1, 148-149. Roy, S. 1965. Comparative study of metamorphosed manganese protores 'If the world-the problem of the nomenclature of the gondites and kodurites. Econ . Geol. 60, 1238-1260. Roy, S. 1981. Manganese Deposits, pp . 333--334. Academic Press, London. Snetsinger, K. G. 1969. Manganoan ilmenite from a sierran adamel. lite. Am. Miller. 54,431-436. Widadason, G. A. 1982. The geology and geochemical secondary dispersion pattern of the manganese occurrence at Ugoge (Tudun Kudu) south eastern sector. Special Project, Department of Geology, Ahmed Bello University, Zaria, Nigeria (unpublished). Wright, J. B. and McCurry, P. 1970. First occurrence of manganese orcs in northern Nigeria . Econ. Geo/. 65, 103--106.