The geology and geochemistry of the Maru Banded Iron-Formation, northwestern Nigeria

Journ61 of African Earth Sciences, Vol. 27, No. 2, pp. 241-257, 1998

Pergamon

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The geology and geochemistry of the Maru Banded Iron-Formation, northwestern Nigeria J. A. ADEKOYA Department of Applied Geology, Federal University of Technology, PMB 704, Akure, Nigeria

Abstract--Narrow units (< 1-30 m thick) of banded iron-formation (BIF) occur in the lowgrade schist belt of the Maru district, northwest Nigeria. The schist belt consists of a deformed sequence of pelitic to semi-pelitic phyllites with subordinate iron-formation and quartiztes, and also amphibolites of inferred tholeiitic volcanic origin. The Maru BIF is commonly interlayered with phyllites within an antiformal fold. It consists essentially of a magnetite oxide facies with sporadic silicate facies composed of grunerite, garnet and rare stilpnomelane. In the BIF, Fe203 (as total Fe) ranges from 44.07 to 58.41% and SiO 2 from 24.74 to 42.67%. Relatively high values of AI203 (2.00-8.97%) and MnO (2.29-9.37%) are characteristic of the iron-formation. On the basis of bulk geochemical affinity and rock association, the Maru BIF is similar to the Lake Superior-type BIFs. However, it differs from them in containing comparatively high amounts of AI203, MnO, TiO 2, Ba, Sr, Co and Zr. The close association of the BIF with metasediments, supported by geochemical evidence, suggests its derivation from a deeply weathered peneplaned hinterland. The BIF precursor materials were probably transported by sluggish flowing rivers and deposited under mixed river fresh water-sea water conditions in a restricted or sheltered basin. © 1998 Elsevier Science Limited. R6sum6--Les unitds etroits (< 1-30 m dpais) de la formation du fer 8 bande se produit en grade faible ~ zone du schist ~ la rdgion de Maru, au nord-ouest du Nigdria. La zone de schist consiste de s6quence de phyllites pdlitique et demi-pdlitique avec la formation de fer et quartzites subordonnde, aussi bien qu'amphibolites inf~re de I'origin tholdiitique volcanique. La formation du fer de Maru est toujours entreposeur avec phyllites en dedans de I'antiforme. Ills consistent essentiellement de facies magnetite oxide avec dparpille silicate faci6s composd de grundrite, garnet et rare stilpnomdlane. Dans la formation du fer ti bande, Fe203 (come le Fe totale) rangde de 44,07 & 58,41% et SiO 2 de 24,74 & 42,67%. Relativement la haute valeur de AI203 (2,00 ~ 8,97%) et MnO (2,29 ~ 9,37%) sont la characteristiques de la formation du fer. A cause de I'affinitd g6ochimique et I'association du roches, la formation du fer de Maru est ressemblant ~ la type de lac Supdrieure. Mais il se diff~re parce qu'il contient comparativement un haute quantit6 de AI203, MnO 2, TiO 2, Ba, Sr, Co et Zr. La toute proche association de la formation du fer ~ bande avec mdtasediments, confirme de preuves gdochimique, sugg~,re I'arri~re-pays profondement pdndplaine. Les matdrieux procuseur de la formation du fer 8 bande sont problablement transportd par un rivi~,re que coule lentement et depos6 sous un rivi~re m~ld I'eau frais-I'eau de mer conditions dans un bassin limitd au abtrit6. © 1998 Elsevier Science Limited. (Received 26 October 1995: revised version received 11 November 1997)

INTRODUCTION Banded iron-formations (BIF) occur at several localities in the n o r t h w e s t and central parts of Nigeria. One of the notable occurrences is the Maru BIF, located about 4 km w e s t of Maru (Fig.

1). Owing to an aeromagnetic survey of northern Nigeria in the early 1 9 6 0 ' s , the BIF, w h i c h was then described as ferruginous quartzite (Truswell, 1962), gave a strong magnetic anomaly. For this

Journal o f African Earth Sciences 241

J. A. ADEKOYA

reason, the Fe deposit was selected for an economic geological investigation in 1969 by the Geological Survey of Nigeria. The field investigation, which involved mapping, pitting and trenching, was supervised by Adekoya ( 1 9 6 9 ) . H o w e v e r , the p r o j e c t was later abandoned for an apparently better prospect at Birnin Gwari. On account of its characteristic banding and mineralogy, the Maru Fe deposit was recognised as a banded iron-formation and its general features were described in a regional survey of BIFs in northwestern Nigeria (Adekoya, 1979, 1988). Egbuniwe (1982) studied the geotectonic evolution of the Maru district and concluded that the BIF was a deformed antiform. Renewed interest in the Fe bearing rock between 1983 and 1988 resulted in more detailed field and laboratory studies. A preliminary report on the investigation highlighted the broad mineralogical and geochemical features (Adekoya, 1993). This paper presents a full account of the geology, petrography and geochemistry of the Maru Iron-

5°

Formation, as well as examines its possible genesis.

REGIONAL GEOLOGICAL SETTING

The Maru BIF occurs within the Maru Schist Belt (Maru Formation of Truswell, 1962) in n o r t h - w e s t e r n Nigeria. This belt, w h i c h is about 200 km long and 12-19 km wide, is one of the Proterozoic low-grade (greenschistto lower amphibolite-facies) linear supracrustal remnants in the polycyclic basement complex of Nigeria. It is bounded to the east by a gneiss-migmatite complex and to the west by the Maiinchi granodiorite batholith and the Anka Belt (Fig. 2). The g n e i s s - m i g m a t i t e complex c o n s t i t u t e s the predominant rock group in the basement of Eburnean (ca 2000 Ma) to Liberian (ca 2 8 0 0 Ma) age (Ogezi, 1988). The Anka Belt is also a metasedimentary schist belt which was affected by a Late PanAfrican calc-alkaline volcanism (McCurry and Wright, 1977). Contacts between the schist

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Figure 1. Generalised geological map o f Nigeria, showing the major fithologies and the Iocafities o f Maru and other banded iron-formations.

242 Journal of African Earth Sciences

The geology and geochemistry o f the Maru Banded Iron-Formation

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Figure 2. Geological map of part o f northwest Nigeria showing the Maru Schist Belt (after Holt et al., 1978).

belt and the gneiss-migmatite complex are conformable, but are locally migmatised around intrusive granitic plutons. The basement complex also forms part of a younger rejuvenated Pan-African domain of c a 600 Ma in age, located east of the West African Craton (Kennedy, 1964; Grant, 1971). Older

(Pan-African) Granites intruded the pre-existing basement rocks, including the schist belts, during the Pan-African thermotectonic event. Detailed descriptions of the geology of the northwest region of Nigeria have been provided by Truswell (1962), Ogezi (1977, 1988) and Egbuniwe (1982).

Journal of African Earth Sciences 243

J. A. ADEKOYA

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SYNOPTIC

GEOLOGY

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Geological setting The Maru Schist Belt (Fig. 2) consists predominantly of pelitic to semi-pelitic metasediments with subordinate interlayered psammites, banded ironformation (BIF) and amphibolites. All these rocks strike approximately north-south, parallel to the structural grain of the surrounding basement

244 Journal of African Earth Sciences

complex. Based on the pelite/psammite ratio, Holt e t al. (1978) have differentiated the entire Maru Belt into eastern and western units. While the eastern unit consists of pelites with locally dominant quartzites and iron-formations, the western unit is almost entirely made up of pelites. In both units, the pelites locally grade into semipelites. The lithostratigraphy of the belt has not

The geology and geochemistry o f the Maru Banded Iron-Formation

been worked out an account of poor exposure, repeated deformation and lack of younging criteria. The pelites and semi-pelites are mainly represented by phyllites with minor weaklydeveloped slates and schists, These rocks are locally banded (the bands being 2-3 cm thick) or laminated on a fine scale (1-3 mm thick). The bands or laminations are alternately rich in quartz and phyllosilicates, thus resembling interlaminated siltstones and shales. However, Fitches et al. (1985) have attributed much of the lamination to tectonic pressure solution striping and suggested that the initial sediments were more homogenous, poorly-bedded or unbedded mudstones or shales. Mineralogically, the phyllites predominantly consist of muscovite with subordinate chlorite and quartz, as well as acccessory biotite, epidote and tourmaline. Locally, they contain significant amounts (up to 10% by volume) of magnetite, pyrite/pyrrhotite, finely divided carbonaceous matter and graphite grains. Although the carbonaceous and pyritic phyllites are widespread, they are only mappable southeast of the BIF (Fig. 3). Both the psammites and the BIF locally dominate the eastern unit. The psammites commonly occur as thin bands or lenticles (< 1-10 m thick) of fine-grained orthoquartzite, flaggy micaceous quartzite and ferruginous quartzite, w h i c h probably represent pure sandstone, sandstone with shale partings and ferruginous sandstone or chert, respectively. These rocks show hardly any sedimentary structures, except local lamination and rare graded bedding and ripple marks (Egbuniwe, 1982). H o w e v e r , the q u a r t z i t e lenticles commonly form small (<1 m in length) "local eye structures" within the phyllites. In general, the quartzites contain minor to accessory magnetite, epidote and tourmaline. Amphibolites occur as altered tholeiitic sheets and pillow lavas interlayered with the metasediments in the Maru and Kanoma districts (Figs 2 and 3). They are essentially made up of actinolite-tremolite, epidote, clinozoisite, plagioclase and yellow/green biotite with accessory piedmontite and iron oxide. The Maru Belt is intruded in places by PanAfrican granite plutons (Fig. 2) resulting in contact metamorphism marked by the sporadic development of chiastolite, andalusite and sillimanite in the pelitic metasedimentary aureole. In the Maru district (Fig. 3), the Damaga tonalitic pluton is emplaced east of the BIF. Both the metasediments and associated tholeiitic basaltic rocks (amphibolites) have

undergone multiple deformation and metamorphic episodes. On the basis of the mineral assemblages of the metapelites and metabasites, the grade of metamorphism of the Maru Belt is considered to be greenschist-facies in the range of low to medium pressure baric-type regional metamorphism (Miyashiro, 1973; Winkler, 1974). Structure The structure of the Maru district (Fig. 3), like that of the other Nigerian schist belts, is complex due to polyphase deformation. At least three deformation episodes (D 1, D2 and D3) have been recognised, the second being the major deformation phase (Egbuniwe, 1982). The Maru metasediments, including the iron-formation, are regionally folded, producing D2 antiforms and synforms with north-south trending parallel axes (Fig. 3). The Baraba Antiform is an F2 fold whose axis runs through the BIF. A complimentary synform occurs to the east of the iron-formation (Fig. 3). Numerous small-scale folds are particularly common in the BIF, especially in the Baraba Hills. They indicate that the F2folds are upright, commonly t i g h t to isoclinal, but sometimes also include more open structures with steep to vertical plunges. The open folds usually have smoothly rounded hinges. D1 and D 3 structures are generally less common. While they are rare in the ironformation, their relics are often preserved in the pelitic and semi-pelitic metasediments. The earliest D 1 structure is represented by the first generation folds (F~), which are now reduced to fold relics or contortions observed in the thin sections of the pelitic and semi-pelitic rocks. These fold relics are defined by tiny m u s c o v i t e flakes o u t l i n i n g S 1 f o l i a t i o n s , sandwiched between stronger axial planar S 2 foliations. Chevron folds, formed locally by deforming the S~/S 2 cleavage in the phyllites, and other open folds, resulting from the refolding of F2 folds in the Maru BIF, are examples of the D 3 s t r u c t u r e s . The multiple S~, S2 and S3 folitations in the metasediments suggest that these rocks have also been subjected to polymetamorphism.

THE IRON-FORMATION Occurrence The BIF is best exposed in a range of northeastsouthwest trending discontinuous metasedimentary ridges (Baraba Hills) covering an area of over 14 km 2. Individual ridges are separated by narrow lowlands underlain by

Journal of African Earth Sciences 245

J. A. ADEKOYA

\

Figure 4. A sample of the Maru Banded Iron-Formation showing alternating light and dark bands.

phyllosilicate rich phyllites. Most outcrops of the BIF occur at the top of the ridges as bands, ranging in width from less than 1 m to more than 30 m and traceable, though discontinuously, for several hundreds of metres. The larger b a n d s are a l m o s t i n v a r i a b l y interlayered with phyllites and minor quartzite and ferruginous metachert. Thus on a regional scale the BIF appears as intermittent bands w i t h i n p r e d o m i n a n t pelitic to s e m i - p e l i t i c phyllites. These bands are intensely fragmented, forming boulders and rubble that often litter the pediment surrounding the ridges. The iron-formation consists of the oxide facies, containing sporadic thin laminae of grunerite and garnet. Banding is the most striking internal f e a t u r e of t h e i r o n - f o r m a t i o n , being characterised by alternating light and dark bands (Fig. 4). Three t y p e s of b a n d i n g , w h i c h correspond to mesobanding, micro-banding and striping as defined by Trendall (1973), have been described in detail by Adekoya (1993). Relict sedimentary features w h i c h have survived deformation are present in the iron-formation. They include ripple-like marks (Fig. 5), linear markings and tiny ferruginous concretions (Adekoya, in press, a).

Petrography Extensive sampling and petrographic studies have revealed the following mineral assemblages in the BIF: i) quartz + magnetite; ii) quartz + magnetite + ilmenite-pyrophanite; iii) quartz + magnetite + muscovite + epidote; iv) quartz + magnetite + chlorite + epidote; v)quartz + magnetite + muscovite + chlorite _+ epidote;

246 Journal of African Earth Sciences

Figure 5. A boulder of the Maru Banded Iron-Formation sho wing ripple-like features.

vi) quartz + magnetite + grunerite; vii) quartz + magnetite + grunerite + garnet; viii) quartz + magnetite + goethite + hematite; ix) quartz + magnetite + goethite + hematite + cryptomelane. The fresh unaltered BIF is represented by the assemblages (i) to (vii), while the BIF affected by supergene alteration contains the last two assemblages [(viii) and (ix)]. On the whole, quartz and magnetite predominate in the assemblages, except in those of the altered rock where they may occur as remnants within preponderant secondary goethite and hematite. Muscovite, chlorite, grunerite, garnet, epidote and ilmenitepyrophanite are common minor to accessory minerals. Locally, the silicate bearing facies [(vi) and (vii)l is abundant, particularly south of the Baraba Hills. Stilpnomelane, in the form of either 0.2 mm long flakes (Egbuniwe, 1982) or as relict inclusions in magnetite (M0cke and Annor, 1993) is also reported in the iron-formation.

Mineralogical features The BIF minerals are arranged in alternating quartz rich light and magnetite rich dark layers or bands (Fig. 4) in hand specimen. These mesobands are sometimes microbanded, with quartz, magnetite and, occasionally, grunerite microlaminae. Three mesoband t y p e s are recognised on the basis of mineralogy: quartz, magnetite and quartz-magnetite mesobands. The salient features of the different mesobands have been presented elsewhere (Adekoya, 1993). Quartz Quartz is the most abundant mineral in the fresh unaltered BIF. The grains fall in the size range of 0 . 0 1 - 0 . 0 6 mm, t h u s being considered as metachert following the definition of Klein and

The geology and geochemistry of the Maru Banded Iron-Formation

Figure 6. Subhedral to euhedral magnetite grains (rot) disseminated in an iron oxide rich band. Reflected light, oil immersion.

Figure 7. Densely-packed martitised magnetite aggregates (light to dark grey) enclosing granoblastic quartz grains (black) in regular patches. Reflected light, oil immersion.

Fink (1976). Apart from being the only or substantial constituent of the light bands (quartz and quartz-magnetite mesobands), it constitutes a subordinate proportion of the dark bands. Metachert aggregates are often equidimensional and granoblastic-polygonal with equiangular triple points. Most grains are strained, showing undulose extinction, but a few large ones exhibit patchy extinction resulting from the coalescence of smaller grains.

Magnetite Magnetite occurs as anhedral to euhedral grains, ranging in size from 0.02-0.4 mm, but the majority of them fall within a range of 0.05-0.1 mm. The grains are found in different habits; as discrete commonly subhedral grains (Fig. 6) and as a n h e d r a l to s u b h e d r a l granoblastic aggregates (Fig. 7). These grains

may be concentrated in relatively thick bands (0.5 mm to > 5 mm in thickness) or in thinner microlaminae, or they may be disseminated in an essentially quartz framework (i.e. quartzmagnetite mesobands)o W i t h i n the m a g n e t i t e rich mesobands, magnetite grains are arranged in different patterns. The grains may be distributed rather uniformly (Fig. 6) or may form aggregates of various configurations. The aggregates may be densely or loosely packed, or may be arranged in patchy or reticulate patterns having enclosures of granoblastic polygonal quartz (Fig. 7). In spite of the arrangement of the grains in bands, they are in fact randomly orientated, with some crosscutting the rock banding to suggest post-tectonic recrystallisation. Magnetite grains are martitised to varying degrees (Fig. 7). Complete alteration produces martite characterised by the "Widmanstetten" texture under crossed polars.

Secondary minerals Goethite, hematite and cryptomelane are secondary minerals formed by supergene alteration of the BIF. They vary in proportion to the intensity of alteration. Goethite is the most abundant of the secondary minerals. It occurs as amorphous or colloform masses which may replace gangue minerals or may be replaced by secondary hematite and cryptomelane. Four types of secondary hematite are recognised in the BIF. They include fine-grained platy, coarsegrained platy and colloform hematite, as well as martite. Various i n t e r g r o w t h and textural patterns, depicting different replacement relationships both among the secondary minerals and between them and the primary minerals (including the gangue minerals), have been described and discussed in detail elsewhere (Adekoya, 1991a). These relationships suggest that both secondary hematite and cryptomelane formed from goethite by replacement in the altered rock. GEOCHEMISTRY Seventeen representative samples of the Maru Iron-Formation and associated rocks were analysed for major and trace elements by XRF spectrometry at the Geochemistry Laboratory, Department of Geological Sciences, McGill University, Montreal, Canada. Some of the samples were analysed by titrimetry for FeO contents. The analytical results are presented in Table 1. Where the FeO is not indicated in

Journal of African Earth Sciences 247

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0.77

17 68.16

1-15: Oxide facies iron-formation; 16: banded quartz-magnetite-chlorite-muscovite slate; 17: carbonaceous phyllite; 18: green amphibolite. ND: Not determined; -: not detectable. Where FeO is not determined, FeO3t values represent the total iron oxide present.

202 75 19 < 10 < 10 5 10 <5 48 5 <5 30 27 < 10 55

46.32 4.68 2.53 0.19 0.09 <0.01

2.30

Ba Co Cr Cu Ni Nb Pb Rb Sr Th U V Y Zn Zr

O. 16

O. 12

TiO2

AI2Oz

2.42

12 40.64

11 39.35

Wt% SiO2

Table 1. continued

631 ND 172 107 33 18 45 167 113 12 <5 66 28 147 191

0.069 32.39 100.82 7.89 2.52

4.09

7.89 ND O, 19 1.81 0.82 1.27

17.14

0.60

19 64.55

1O0 ND 405 10 165 9 150 12 132 <5 <5 276 21 1213 100

O. 10 2:39 99.94 15.26 10.67

0.51

15.26 ND 0.22 6.68 10.42 1.17

12,35

0.97

20 49.87

phyllite; 19: pillow lava (metamorphosed); 20:

36 91 36 210 22 11 8 <5 68 6 8 57 15 67 83

0.14 2.85 100.02 13,00 0.09

0.04

13.00 ND 0.26 1.30 0.17 0.02

6.47

0.26

18 76.40

O

J. A. ADEKOYA

Table 2. Averages and ranges of chemical analyses of iron-formations from Maru, Birnin Gwari and Muro in the Nigeria and Yilgarn Block and Hamersley Basin in Australia, the Lake Superior region in the USA and the Algoma Region in Canada Wt% SiO 2 TiO 2 AI203 Fe203

FeO MnO MgO CaO Na20 K20 P205 S -O S Total Fe203t

1 34.35(26.23-43.95) O. 16(0.01-0.29) 4.25(2.25-9.19) 50.70(41.98-60.55 4.72(0.0-13.56 4.83(2.31-9.94 O. 13( < 0 . 0 1 - 0 . 4 4 O. 11 (0.03-0.20 0.01 (0.01-0.06 0.65(0.01-3.01 0.09(0.04-0.20 ND ND 100 .00 54.30(45.16-60.55)

2 35.96(27.85-40.13) 0.21(0.12-.23 3.77(3.20-4.29 46.14(40.62-58.42 8.82(6.89-11.20 3.58(1.17-5.72 0.56(<0.01-1.00 0.55(<0.01-0.92 <0.01 ( <0.01-0.01 0.30(0.13.70) 0.12(0.08-0.20) ND ND 100.00 55.93(47.22-66.35)

3 57.66(51.00-64.96 0.02(0.01-0.05 0.28(0.09-0.55 38.29(34.07-48.62 3.58(0.20-8.60 0.06(0.03-0.12 0.01 ( < 0 . 0 1 - 0 . 0 6 0.02(<0.01-0.10 <0.01 (<0.01-0.01 ) 0.02(<0.01-0.07) 0.05(<0.01-0.15) ND ND 100.00 29.39(23.58-34.00)

4 49.13

5 51.58

0.02 1.45 36.84 8.54 0.76 1.29 1.64 0.12 0.15 O.O6 0.02 ND 100.00 46.33

0.09 3.06 27.43 13.26 0.14 1.56 1.54 0.32 0.59 0.21 0.3 ND 100.00 42.62

Analyses have been recalculated to 100% on a H20 and COz free basis for comparison. 1 : Maru iron-formation (oxide facies), N = 15 (this study). 2: Lake Superior silicate facies iron-formation from North America (Gross and Macleod, 1980, table 3). 3: Algoma oxide facies iron-formation, from North America (Gross and Macleod, 1980, table 3). 4: Orissa iron-formation (oxide facies), India (Majumder e t al., 1982, table 3). 5: Itabirite from Minas Gerais, Brazil (Majumder e t al., 1982, table 3). ND: Not determined or reported; ranges are given in brackets.

the table, the Fe203 values represent the total iron oxide (Fe203t). Major elements Table 1 reveals that the total Fe203 in the BIF ranges from 44.07-58.41%, which corresponds to a calculated total Fe of 30.82-40.85%. The higher values are obtained from samples which show evidence of minor supergene enrichment, as observed in polished section. The SiO 2 content varies from 24.74-42.67%. The total Fe203 and SiO 2 make up over 80% of the analyses and they have an inverse relationship to each other. AI203 accounts for 2.00-8.97%, and MnO for 2.29-9.37% of the rock. The values of these two components are unusually high compared with those of other Precambrian iron-formations (Gross, 1990) (see also, Table 2). In general, other components, viz. TiO 2, MgO, CaO, Na20 and P20~, are each less than O.5%. The K20 content generally ranges from 0.01-0.75%, but an exceptionally high value of 2 . 9 4 % was recorded in one sample (Table 1, No. 4). The sample also yielded the highest AI203 content of all the analyses. These results can be a t t r i b u t e d to a r e l a t i v e l y high a m o u n t of

250 Journal of African Earth Sciences

muscovite, which was observed petrographically in the sample. Characteristically, the K20 values are higher than the Na20 values in the BIF. The LOI ranges from 1 . 7 6 - 5 . 9 8 % , suggesting a paucity of carbonate and sulphide minerals in the BIF. Trace elements Fifteen trace elements (TE) were determined in the BIF (Table 1). The results show that the TE concentrations are generally low and highly variable. Similar observations have been made on Precambrian iron-formations in different parts of the world (James, 1966; Barbosa and Gross Sad, 1973; Gole, 1981 ). The concentrations of Cr ( < 1 0 - 3 6 ppm), Cu ( < 1 0 ppm), Ni ( < 1 0 ppm), Nb ( < 5 - 9 ppm) and U ( < 5 - 9 ppm) are particularly low. A few other elements (Ba, Co, Sr, V and Zr) tend to occur at relatively higher concentration levels than those of the oxide facies of the Lake Superior BIFs (Table 3) reported by Gross and M a c l e o d ( 1 9 8 0 ) . However, the concentrations of these specific elements vary within relatively wide limits: Ba: 164-802 ppm; Co: 75-173 ppm; Sr: 39-179 ppm; V: 25-60 ppm; and Zr: 51-85 ppm.

The geology and geochemistry o f the Maru Banded Iron-Formation

Table 3. Averages and ranges of trace elements in ppm in banded iron-formations from Maru in Nigeria and from other parts of the world. ppm

Maru BIF Lake Superior Algoma-type Orissa BIF Minas Gerais BIF BIF Itabirite Ba 293(164-802) 180 170 70 179 Co 100(80-173) 27 38 35 69 Cr 23( < 10-36) 122 78 30 28.5 Cu <10(<10) 10 96 10 22 *Mn 3 7 4 0 8 ( 1 7 8 9 1 - 7 6 9 8 5 4600 1400 120 1785 Nb 5( < 5-9 ND ND ND ND Ni <10(<10 32 83 15 20.3 Pb 10(<2-55 ND ND ND ND Rb 20(<5-141 ND ND ND ND Sr 51(32-179 42 83 15 20.5 Th 6( < 5-9 ND ND ND ND *Ti 959(60-1739) 160 860 40 216.6 U 5( < 5-9) ND ND ND ND V 44(25-61 ) 30 97 30 35 Y 22(9-45) 41 54 ND ND Zn 26( < 10-90) 2 33 ND ND Zr 60(50-85) 56 84 10 17.3 *: These elements are included here for comparison with other iron-formations from different parts of the world. ND: Not determined or reported; ranges are given in brackets.

DISCUSSION

Comparison with other iron-formations The present study indicates that the Maru BIF is essentially similar to other known iron-formations at Birnin Gwari, Koriga, Muro and Obajana (Fig. 1) in the low-grade schist belts of Nigeria (Adekoya, 1991 b; Okonkwo, 1991 ; MQcke and Annor, 1993). Both the Maru and these other iron-formations are interlayered w i t h metasediments mostly derived from argillites and sandstones. They all characteristically occur in relatively narrow bands and lenses ranging in thickness from less than 1 m to tens of metres and are often discontinuous along strike. In general, they consist mainly of a magnetite oxide facies, interlayered with varying proportions of silicate facies composed of grunerite, garnet and rare stilpnomelane. However, the silicate facies occurs only sporadically in the Maru BIF, whereas it constitutes a substantial part of the Birnin Gwari BIF and predominates in the Koriga IronFormation (Adekoya, 1991 b; Okonkwo, 1991 ). Minor differences exist between the Maru BIF and the Muro and Obajana BIFs of central Nigeria. While magnetite is the only iron oxide in the Maru BIF, both magnetite and specular hematite are present in the Muro and Obajana BIFs. Unlike the Maru BIF, the Muro iron-formation includes

a sporadic minor carbonate facies (sideritemetachert rock) found at the contact of the ironformation with a carbonate (marble) unit present in the sequence (Adekoya, in press, b). The Maru iron-formation also shares some characteristics with the Precambrian BIFs in other parts of the world. The samples of the Maru iron-formation plot in the Precambrian field of Govett (1966) on his AI203-SiO2-Fe203 diagram (Fig. 8), suggesting a general chemical similarity to other Precambrian BIFs. The relationship, observed by Gross and Macleod (1980) and Gross (1990), between the distribution patterns of some elements in the Lake Superior-type ironformations and those of the Algoma-type ironformtions in North America, has been found to hold true for the Maru and Algoma-type BIFs. For example, the average concentration of Mn (4.83%) in the Maru BIF is more than twice the mean content (0.14%) in the Algoma-type oxide facies, as published by Gross and Macleod (1980) (Table 2). Similarly, the mean contents of Cu, Ni, V, Na20 and P2Os of the Algoma-type oxide facies are at least double the corresponding average values for these elements in the Maru BIF. These relationships tend to suggest that the Maru Iron-Formation is similar to the Lake Superior-type BIF. The banding characteristics,

Journal of African Earth Sciences 251

J. A. ADEKOYA

AIz~)3 t

/

/

L

/

\

/

\

/

.~ k eee

Si02/"

V

V

v

v

V

V

.%203

Figure 8. Plot o f the Maru Iron-Formation in the Precambrian field o f an SiO2-AI203-Fe203 ternary diagram (after Govett, 1966).

both meso- and microbanding of the Maru BIF, are also similar to those of the iron-formations of the Lake Superior region of the USA and Canada, the Hamersley Basin and Yilgarn Block of Australia, the Minas Gerais area of Brazil, the Bihar and Orissa Regions of India, and those in South Africa (James, 1954; Gross, 1973; Trendall, 1973; Gole, 1 9 8 1 ; Dorr, 1973; Majumder, 1990; Beukes, 1973). However, the Maru BIF differs from the above mentioned examples of the Lake Superior-type BIFs in that it contains comparatively higher concentrations of AI203, MnO, Ba, Sr, Co and Ti (Tables 2 and 3). The high AI203 is attributable to the admixture of fine-grained clastic materials with the chemogenic constituents of the original iron-formation (Stanton, 1972; Govett, 1966; Dymek and Klein, 1988). Although the high Mn content, indicated by microprobe analyses (MQcke and Annor, 1993), can now be traced to spessartine garnet and Mn rich grunerite in the metamorphosed Maru BIF, its primary source must have i n c l u d e d Mn rich siderite and rhodochrosite, the relicts of which have been found in magnetite porphyroblasts by MQcke and Annor (1993). Barium behaves sympathetically with Mn, and Sr with Ca and Ba, in nature (Rai and Paul, 1990). Therefore, the relatively high values of Ba and Sr in the Maru BIF are also easily understandable.

Major BIF genetic models The origin of the Maru BIF is discussed here in the light of existing genetic models of BIFs in general. Four models are currently en v o g u e and are essentially based on the rock association, the envisaged source of the Fe and silica and the nature of the depositional basin of the BIFs

252 Journal of African Earth Sciences

(James, 1992). These include the continental weathering and erosion, deep ocean source, volcanic exhalative and rift-related hydrothermal models. In the continental weathering and erosion model, the Fe and silica are assumed to be leached from the weathered continent and transported under an anoxic atmosphere of the Archean and Palaeoproterozoic age to the sea (Gruner, 1922; Woolnough, 1941; Sakamoto, 1950; Lepp and Goldich, 1964; Govett, 1966; Lepp, 1987). To avoid dilution of the BIF minerals with other detritals, the continent is envisaged as a mature landscape, characterised by a sluggish river flow transporting only dissolved and very fine-grained particles, which were ultimately deposited in shallow basins of different types, namely marine continental shelves (Sakamoto, 1950), marginal restricted or sheltered basins (Woolnough, 1941 ; H/~lbich and Alterman, 1991; H~lbich e t al., 1992) and playa-like lacustrine basins. The deep ocean source model is also predicated on the existence of a reducing atmosphere in the Archean and Palaeoproterozoic times when the ocean was presumably saturated with Fe and silica obtained from diverse sources: marine, terrestrial and volcanic. As a result of upwelling of the charged oceanic waters from the deep ocean to the locally oxygenated shallow continental shelves or restricted marginal basins, the Fe and silica were precipitated through one mechanism or the other (James, 1992). The key element of the volcanic exhalative model is the supply of Fe and silica through volcanic exhalations from submarine volcanoes. In this case, the BIFs are temporally and spatially related to volcanic sources. The precipitation of the BIFs took place in relatively small basins,

The geology and geochemistry o f the Maru Banded Iron-Formation

3-50'

3-oo.

o~ ,=~ z.so.

2.00'

1.50

000

0"04

(~'08

'

().12

(~.16

o'zo

Ti 02 wt°/o Figure 9. Plot o f AI203 against TiO2 showing a positive correlation.

apparently during lulls in volcanic activity. The volcanogenic BIFs (the Algoma-type) are interbanded with acid, intermediate and basic volcanic rocks, as in the Michipicoten district, Canada (Gross, 1973). For the rift-related hydrothermal model, the underlying concept is that the hydrothermal fluids that supplied Fe and silica for the formation of the BIFs came through rifted continental margins and mixed with sea water (James, 1992). The BIFs were presumably precipitated in the rift trench at the continental margins. This model is proposed to explain the enigmatic Neoproterotozic Rapitan-type (Snake River) BIF in the Northwest Territories, Canada and the Urucum and Porteirinha itabirites of Brazil. So far no satisfactory explanation has been offered for the peculiar association of the BIFs with glacial deposits. Genesis of the Maru BIF

When the Maru BIF is considered in the light of the major genetic models highlighted above, it is difficult to assign the BIF strictly to a particular model as the rock association and geochemical evidence suggest t w o possibilities: the continental weathering and erosion model and the volcanic exhalative model. The possibility of

continental derivation of the BIF precursor materials (iron and silica) is based largely on the interlayering of the BIF with rocks of apparently metasedimentary origin. The close association of the BIF with phyllosilicate rich phyllites, considered to be derived from argillaceous source rocks, suggests a terrigenous hinterland. Such detritus, including Fe and silica, could have been eroded from a nearby deeply-weathered but mature landscape, which produced mostly dissolved and fine-grained particles for transport by sluggish flowing rivers, presumably under an anoxic atmosphere. The fine-grained detritus presumably formed the pelitic and semi-pelitic phyllites with which the Maru BIF is associated. A plot of the Al=O3 against TiO 2 in the Maru BIF (Fig. 9) reveals a positive correlation probably attributable to the residual concentration of Ti in aluminous (clay) deposits (Sherman, 1952), which might have constituted the bulk of the terrigenous detritus. Relatively very high AI=O3 and TiO= contents of the BIF (Table 2), particularly at the gradational contact (Table 1, No. 16) with the enclosing phyllites, probably represent detrital dilution during the chemical precipitation of the BIF. The clay minerals with which the TiO 2 was associated would presumably have been the precursor of

Journal of African Earth Sciences 253

J. A. ADEKOYA

the sedimentary Fe bearing silicates that were converted to garnet, stilpnomelane, chlorite and muscovite in the metamorphosed iron-formation. This suggestion is supported by the fact that a clay mineral (montmorillonite) has been transformed into stilpnomelane experimentally (Grubb, 1971). The BIF was probably precipitated periodically, apparently during lulls in clastic deposition under locally oxygenated conditions (James, 1992). Such lulls were most likely controlled by epeirogenic movements. The relatively high Mn content of the BIF could have been derived from the same source as the Fe, which probably included weathered Mn bearing volcanic rocks. Under anoxic conditions, the Mn would be transported as Mn 2+, just like Fe2+, and might have been deposited as an oxidate, perhaps synchronously with the iron oxide in the locally oxidising environment (Mason, 1966; Dorr, 1973; Norton, 1973) or as carbonates, namely Mn rich siderite and rhodochrosite, under reducing conditions. On the other hand, a volcanic exhalative/ hydrothermal origin should also be considered possible in view of the occurrence in the Maru sequence of amphibolites derived from tholeiitic sheets and p i l l o w lavas, w h i c h i n d i c a t e s submarine volcanism (Adekoya, 1979; Baer, 1982; Egbuniwe, 1982; MQcke et al., 1989) that could have supplied the Fe, silica and other BIF constituents. Recent studies have revealed that submarine v o l c a n i s m is a source of metalliferous sediments which are formed partly by v o l c a n i c e x h a l a t i o n s and p a r t l y by hydrothermal leaching caused by seawater-hot lava interactions (Dzotsenidze, 1972; Bostrom, 1980; Bostrom and Widenfalk, 1984). Similar exhalative and hydrothermal processes could have taken place at Maru, leading to the concentration and p r e c i p i t a t i o n of Fe and silica, presumably during lulls in volcanic activity. The comparatively high contents of AI203, TiO 2 and MnO (Table 2), as well as the positive AI203TiO 2 correlation of the BIF, could also be attributed to volcanic derivation. This is because modern exhalative/sedimentary deposits are characterised by similar features (Bostrom and Widenfalk, 1984). However, a major weakness of this volcanogenic model for the Maru BIF is the apparent absence of acidic to intermediate volcanic rocks in the Maru area with which volanogenic BIFs (Algoma-type) are normally associated (Gross, 1973; Goodwin, 1973). Furthermore, very low contents of Cr, Ni and Cu in the Maru BIF, compared w i t h the higher c o n c e n t r a t i o n s

254 Journal of African Earth Sciences

recorded in the pillow lavas and amphibolites of undoubted igneous origin (Table 1 ), interbedded with the Maru metasediments, tend to indicate that the BIF was precipitated under low temperature conditions in a sedimentary environment (Rai and Paul, 1990).

Maru depositional palaeoenvironment model The rock association and the intrinsic characteristics of the Maru Iron-Formation provide some clues to the possible Maru depositional palaeoenvironment model (Adekoya, 1996). Regular banding of the BIF suggests deposition under quiescent conditions, but the occurrence of relict features of ripple marks, linear markings and cutand-fill structures in the iron-formation and associated phyllites and quartzites indicates a shallow water depositional basin. The high phyllosilicate content of the phyllites with which the BIF is associated and the presence of pyrite/ p y r r h o t i t e / m a g n e t i t e bearing carbonaceous phyllites in the Maru sequence support the existence of a low energy, euxinic environment at the time of BIF deposition. The occurrence of BIF as intermittent bands within the Maru phyllites, which were presumably derived from argillites, suggests t h a t the s h a l l o w w a t e r e n v i r o n m e n t was unstable and fluctuated between conditions that favoured chemical precipitation represented by the BIF and t h o s e w h i c h supported deposition of fine clastics (argillites) (H&lbich e t al., 1992). Since the fine clastics, as explained earlier, were probably obtained from topographically subdued hinterland, they were presumably delivered to the depositional basin by sluggish-flowing rivers. On the basis of the foregoing, it is postulated that the BIF was most likely deposited in a shallow, sheltered or restricted marine basin. The inflow of sluggish rivers into the basin could create mixed fresh water-sea water conditions under which the BIF was probably precipitated (H~ilbich and Alterman, 1991 ). The relatively high proportion of quartzite and BIF in the eastern unit of the Maru Belt (Fig. 2) tends to suggest that this area of the Maru Belt experienced more fluctuating conditions during sedimentation than the western unit.

Geological history of the Maru BIF The Maru Belt, like other Nigerian schist belts, is probably a remnant of a small tectonic basin which went through stages of sedimentation, deformation and metamorphism. Sedimentation in the Maru Basin involved the deposition of

The geology and geochemistry o f the Maru Banded Iron-Formation

predominantly fine clastics (shales and mudstones), presumably derived from a nearby mature landscape, in a quiet water environment. On t h e basis of t h e p a l a e o e n v i r o n m e n t deductions made above, the basin is considered to be shallow but restricted or sheltered and is thus able to provide a low energy (quiet), reducing environment. Such an environment was conducive for the f o r m a t i o n of pyrite and carbonaceous matter in the argillites (i.e. black shale) and of magnetite, Fe bearing silicates, Mn bearing siderite and/or rhodochrosite in the iron-formation. The shallowness of the basin also permitted the f o r m a t i o n of s h a l l o w w a t e r sedimentary structures such as ripple marks, cuta n d - f i l l s t r u c t u r e s , s h r i n k a g e cracks and ferruginous nodules, the relicts of which were p r e s e r v e d in t h e BIF and a s s o c i a t e d metasediments (Adekoya, 1996). The deposition of the fine clastics was interrupted intermittently by chemical precipitation represented by narrow bands of i r o n - f o r m a t i o n w i t h i n the Maru sequence. Narrow lenses of sandstone were also interbedded with the argillaceous beds. Because they represent a higher energy regime than other sediments, they probably indicate shallowing water or changes in provenance (Holt et al., 1978). The sedimentation in the basin was accompanied by contemporaneous volcanism during which tholeiitic basalt sheets and pillow lavas were interlayered with the sediments. The iron-formation and associated rocks were subsequently subjected to multiple episodes of deformation and metamorphism resulting in the formation of three generations of folds and f o l i a t i o n s . As i n d i c a t e d by t h e m i n e r a l parageneses of the pelitic rocks and metabasites (amphibolites and pillow lavas), the grade of metamorphism was essentially greenschistf a c i e s , a l t h o u g h it c o u l d reach m i d d l e amphibolite-facies, as shown by the appearance of g r u n e r i t e and g a r n e t in t h e BIF. The m e t a m o r p h i s m c o n v e r t e d the argillaceous sediments into phyllosilicate rich phyllites, with their carbona-ceous and pyritic portions changed into grey carbonaceous or graphitic phyllites, in w h i c h some of the pyrite was reduced to pyrrhotite. The effect of metamorphism on the iron-formation depends on its mineralogy. While the oxide facies merely underwent recrystallisation, with its diluting clay content turned into muscovite and chlorite, the silicate and carbonate bearing facies were converted into assemblages rich in grunerite, garnet and rare stilpnomelane. The tholeiitic sheets and pillow lavas were deformed and altered, forming

amphibolites which display blasto-ophitic texture and elongated pillows with distorted margins. Although the deformation and metamorphism of the Maru Belt was previously dated as Kibaran ( 1 0 6 8 _ 6 5 Ma; Ogezi, 1977), this age is now considered to be geologically meaningless due to the low initial 87Sr/88Sr ratio used in the geochronological determination (Affaton et al., 1991). However, recent studies have revealed that the Nigerian schist belts, particularly the BIF bearing ones (such as the Maru Belt), are at least Eburnean (ca 2000 Ma) in age (Annor, 1995; Adekoya, 1996).

SUMMARY AND CONCLUSIONS The Maru BIF, which consists of a magnetite oxide facies with minor silicate bearing facies, occurs as narrow bands and lenses ( < 1-30 m thick) within a folded package of predominantly pelitic to semi-pelitic matesediments with subordinate quartzites and amphibolites of inferred tholeiitic basalt origin. Although the BIF is similar to the Lake Superior-type BIFs on the basis of e l e m e n t a l d i s t r i b u t i o n p a t t e r n s and rock association, it differs from them in containing comparatively higher concentrations of AI20 z, MnO, TiO 2, Sr, Co, V and Zr, as well as much lower contents of Ni, Cr and Cu. The intermittent occurrence of narrow BIF units within preponderant phyllosilicate rich phyllites, presumably derived from fine clastics, suggests a terrigenous derivation from a hinterland characterised by a very low-lying mature topography which permitted only fine clastics and dissolved particles for transport by sluggish-flowing rivers. A volcanic-exhalative origin of the BIF is ruled o u t based on t h e a b s e n c e of a c i d i c to intermediate volcanics in the Maru area and on the presence of very low Ni, Cr and Cu ( < 10 ppm) concentrations in the BIF. It is postulated that the BIF and associated sediments were deposited in a shallow, restricted or sheltered basin, probably under brackish water conditions (mixing river fresh water with sea water), which is caused by sluggish rivers flowing into a restricted basin. All the rocks including the BIF were deformed and metamorphosed in multiple episodes under conditions of greenschist- to middle amphibolite°facies.

ACKNOWLEDGEMENTS This paper is a direct off-shoot of a Ph. D. project supervised by Prof. A. Badejoko, Department of Geology, University of Ibadan, Ibadan, Nigeria.

Journal of African Earth Sciences 255

J. A. ADEKOYA

The author wishes to thank Prof. Bob Martin, Department of Geological Sciences, McGill University, Montreal, Canada, for his assistance in the geochemical analysis of the rock samples. The author has benefitted from the very useful comments, suggestions and assistance of the reviewers, especially Prof. Pat Eriksson, Department of Geology, University of Pretoria, South Africa. The author is also grateful to Dr W. A l t e r m a n n , D e p a r t m e n t of Geology, University of Munich, Munich, Germany, for the provision of useful information on the South African BIFs. Editorial handling - R Eriksson.

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