- Email: [email protected]
Host rock geology and geochemistry of the Zona uranium occurrence, Peta Gulf Syncline (Upper Benue Trough), northeast Nigeria
Journal
Pergamon
PII:s0899-5262(00)00010-0
of African
Earth Sciences.
Vol. 31. No. 314. pp. 619-632. 2COO 2001 Elsevier Science Ltd All rights reserved. Printed in Great Britain 08995362/01 b- sea front matter
o
Host rock geology and geochemistry of the Zona uranium occurrence, Peta Gulf Syncline (Upper Benue Trough), northeast Nigeria C.E. SUH’,*, S.S. DADA2 and G. MATHEIS3 ‘Department of Geology, University of Buea, Buea, Cameroon 2Geology Programme, Abubakar Tafawa Balewa University, Bauchi, Nigeria 3Technische Universitgt, Berlin, Serkr, BH4, D-l 0587, Berlin, Germany
ABSTRACT-The Peta Gulf Syncline (Upper Benue Trough, northeast Nigeria) is a faultbounded pull-apart sub-basin. The boundary faults are mainly northeast-southwest-trending en echelon strike-slip faults, truncated along their lengths by normal and tear faults with stepovers. The eastern marginal faults underwent rotation during sedimentation, whereas the steeply dipping western marginal faults were inactive. The Peta Gulf Sub-basin is filled by the Bima Sandstone Formation (Lower Cretaceous) which has three siliciclastic members: (i) B,: medial fan coarse-grained to microconglomeratic sandstones; (ii) B,: full fluvial medium-grained sandstones with minimal fines; and (iii) B,: lacustrine and flood basin deposits comprising alternating fine-grained sandstones and siltstones/claystones. Sediment supply was from east to west and facies changes show a general fining in this direction. B, offers the most favourable environment/lithology for U concentration. The only significant U occurrence in the Peta Gulf Syncline is the Zona U anomaly, which occurs within transitional B,-B, brecciated sandstones with wall rock alteration zones. The mineralised zone has elevated SiO,, Fe, As, Ba and W levels but is depleted in the alkalis, Zr, Rb and Sr. This chemical zonation supports the epigenetic origin of this anomaly. e 2001 Elsevier Science Limited. All rights reserved. RESUME-Le synclinal du Golfe de P&a (Depression du Benue superieur, NE Nigeria) est un sous-bassin en ‘pull-apart’ delimit6 par failles. Les failles bordieres en echelon presentent des decrochements et sont majoritairement de direction NE-SO. Ces failles sont tronquees par des failles normales formant des decrochements et des gradins. Les failles de la marge Est subirent une rotation synsedimentaire tandis que les failles verticales de la marge Ouest ne furent pas affectees par ces mouvements. Le Golfe de P&a est rempli de formations composees des gres de Birma dates du C&ace inferieur. Ces formations gr4seuses sont decomposees en trois unites silicoclastiques: (i) B,: zone deltdique a grains grossiers et microconglomerats; (ii) B,: facies fluviatiles a grains moyens avec une faible proportion de grains fins; et (iii) B,: zone d’innondation lacustre composee d’une alternance de gres fins, de silts et d’argilites. Le materiel de remplissage de ce sous-bassin provient d’Est en Ouest, avec des proportions croissante en grains de plus en plus fins. L’unite B, presente I’environnement et la lithologie les plus favorables B la concentration d’U. La seule concentration significative d’U rep&be dans le synclinal du Golfe de P&a se trouve dans des zones de transition situee a I’interface des unites B,-B,. II s’agit de zones greseuses
*Corresponding author [email protected]
Journal of A f&an Earth Sciences 6 19
C. E. SUH et al. et brechiques presentant une alteration pervasive. La zone mineralisee contient des teneurs Bl6ves en SiO,, Fe, As, Ba et W mais appauvries en Zr, Rb et Sr. Cette zonation chimique est en accord avec une origine Bpigenetique de cette zone mineralisee. o 2001 Elsevier Science Limited. All rights reserved. (Received 1 B/l 199: revised version received 1/I 2199: accepted 3/4/00)
INTRODUCTION Uraniferous
sandstones
constitute
a significant
source of U. For example, the Karoo Basin, South Africa, has recoverable resources of at least 41,500 tonnes of U (le Roux, 1993). Sandstone-type deposits account for about 75% of the world’s U resources and they are known in many parts of the world (Dahlkamp, 1978). The sources of U in such sandstones are varied. Uranium can be deposited simultaneously with granite debris and volcanics or epigenetically derived from detrital U minerals within the sediments (Turner, 1978; Sherborne et a/. , 1979; Zielinsky, 1981; Visser, 1989). Its precipitation is often related to a redox boundary (Rackley, 1976; Nash et a/. , 1981). Most often, within a given basin, sandstone-hosted U mineralisations occur as scattered bodies. The mineralisation may be stratiform or of the post-fault U ore type. Sandstones are elastic sedimentary rocks, and for most sandstone-hosted mineralisations there is a direct link between the general sedimentological aspects (e.g. texture, architecture, lithofacies) of the sediments and the zone of U concentration (Evans, 1987). Also, an understanding of facies distribution, depositional environments and architecture of sedimentary sequences is a sig-nificant aspect of exploring for sandstone-hosted U deposits. These aspects are examined in this paper as they relate to the Zona U occurrence. The Zona U anomaly occurs within Cretaceous sandstones of the Peta Gulf Syncline, Upper Benue Trough, northeastern Nigeria. The main U mineral here is autunite. Three main zones of wall rock alteration have been identified for the Zona anomaly (Suh, 1997; Suh et al., 1998): a silicified zone; a hematite zone and a goethite zone., However, complete major and trace element geochemical data on these various zones were hitherto unavailable. These data are presented and discussed in this paper.
GEOLOGICAL SETTING The Zona U anomaly occurs within the northeastsouthwest striking intracontinental Benue Trough (Fig. 1 a). The trough is well over 1000 km long and 700 km wide and is geographically divided into three segments: the Lower, Middle and Upper Benue Trough. The Peta Gulf Syncline lies within the Upper
620 Journel of African Eerth Sciences
Benue Trough. Earlier work on the geodynamic evolution and structure of the Benue Trough show that its origin is linked to an Early Cretaceous opening of a rift structure related to a late tectono-magmatic phase (Olade, 1975; Benkhelil, 1989). Benkhelil and Robineau (1983) demonstrated that the principal structures of this trough are grabens, horsts and halfgrabens, whose individual dispositions are controlled by mostly north-south-trending normal faults and northeast-southwest strike-slip faults. These strikeslip faults divide the Benue Trough into sub-basins (Fig. 1b), often separated from one another by either extensive (lengthwise) strike-slip faults or basement horst structures. The Peta Gulf Syncline is one such sub-basin. The distribution of lithofacies, sedimentary structures and sedimentary environments are not necessarily the same within these sub-basins. Because most of these strike-slip faults remained active during the Early Cretaceous infilling of the subbasins, sedimentological patterns and lithofacies distribution were, to a large extent, affected by the faults. A summary of the stratigraphy of the Upper Benue Trough is presented in Fig. 2. Because there exists published work available on the stratigraphy of the Benue Trough (for example, Offodile, 1975; Petters and Ekweozor, 1982; Obaje et al., 1994, 19961, a detailed discussion of this aspect is not included here. The Zona U anomaly occurs within the Bima Sandstone Formation, the oldest sedimentary unit in the Upper Benue Trough (Fig. 2). The Bima Formation has three siliciclastic members (Guiraud, 1990); namely B,, B2and B, (Fig. 3). All three members do not necessarily outcrop in any given sub-basin. In the Peta Gulf Syncline, both the western and eastern margins are defined by faults (Fig. 3). However, because sediment supply was from east to west, most faults on the eastern margin have been obscured by sediments. The sub-basin is extensive along its strike but has a very limited width. This is a common feature of strike-slip fault controlled sedimentary basins as described by Reading (19801, Hempton and Dunne (1984) and Steel (1988). The boundary faults of the Peta Gulf Syncline are normal faults and strike-slip faults. The normal faults on the eastern boundary are locally sub-horizontal faults, whereas on the western boundary they are steeply dipping. Most of these normal faults have an
Host rock geology andgeochemistry of the Zona uranium occurrence, Peta
GulfSyncline, northeast Nigeria
6“
a)
12O
I crrr
E
‘Megatension gash’ mineralisetion Normal fault
0’
N
: ’ : Extensional Basin Cl Sedimentary cover cl .., A% Basement complex tzl
b)
lzlPeta Gulf Syncline
Figure 1. la/ Regional geology of the Benue Trough finset sketch map of Nigeria shows northeast-sduthwest-trending Benue Trough) and the position of the Pets Gulf Synclind: Ibl Benue Trough sub-basins resulting from north-south normal and northeastsouthwest strike-slip faults (after Benkhelil and Robineau, 1983).
Journal of African Earth Science
62 1
C.E. SUHet al.
Age
Tertiary
Formation
Litho.
Palaeo-environment
.. . ,. .. .. .. .. ... .. .. ..a. .. .. . . . . . : L_‘,‘,..
Continental
Formation
Gongola Arm
Yola Arm
Keri-Keri
Keri-Keri
.-.-. .-. ._.-. .-.d.
-. (fluviatilellacustrine)
. . . . ._ . . . . . . . . -a
. . .. .. .. .. .. Maastrichtian
Gombe
G om be
‘.* .‘:* .i* :
Continental/ Transitional (fluviatile,
lacustrine,
d Numanha
Cenomanian
q
_. _.
= =
-.-._
Sandstone Siltstone Claystone Shale
Transitional
Coal Continental
Pre-Albian
Basement
Complex
Ifr
311
Igneous/Metamorphic
-1 d
Limestone Granite, gneiss Unconformity Deformation
Fi@ue 2. Stratigraphical succession of the Upper Benue Trough. The Gongoia and Yola Arms are structural units in the Upper Benue Trough (after Petters and Ekweozor, 19821.
average north-south trend (N05E-N2OE) and are often associated with tear faults. These tear faults are subparallel to the regional northeast-southwest strike of the sub-basin and are relatively small-scale and localised strike-slip faults. The major boundary faults in the Peta Gulf Syncline Sub-basin are strike-slip faults, which trend northeast-southwest (Fig. 3). These faults are planar and vertical with distinct fault traces, which run uninterrupted for over 50 m in many instances, seen as straight lines in the field. The north-south normal faults often truncate these strike-slip faults. In some spectacular instances, the strike-slip faults are truncated by stepovers (areas where one en echelon fault ends and another with the same orientation begins). Evidence of strike-slip displacement parallel to the fault trace is common on the adjoining basement rocks. However, where most of these fault surfaces are exposed, striations indicating an appreciable amount of vertical displacement are also observed. Such oblique strike-slip/dip-slip
motion may be due to a geometric effect (Passchier and Williams, 1996) or may result from extension perpendicular to the strike trend of the sub-basin. Structural analysis of en echelon pegmatitic veinlets in the adjoining granitic basement show net extension in the northwest-southeast direction, perpendicular to the length of the sub-basin. This sub-basin is, therefore, of the pull-apart type and its evolution was accompanied by basement reactivation along strikeslip faults. Lithological units of the Peta Gulf Syncline and favourable environments of U concentration The general geology of the Peta Gulf Syncline Subbasin is depicted in Fig. 3. This sub-basin is filled by a spectrum of interrelated coarse to micro-conglomeratic sandstones, medium- to fine-grained sandstones, siltstones and purplish sandy shales and clayey beds. All three members (B,, B,, and BJ of the Bima Formation are present and a full description of these members is given in Table 1.
Figure 3. Structure and geology of the Peta Gulf Syncline Sub-basin, Upper Benue Trough.
622 Journal of Afrfcan Earth Sciences
Host rock geology andgeochemistry of the Zona uranium occurrence, Peta Gulf Syncline, northeast Nigeda
10°15' N
/
+
+
+
. . . . . *.
.
. B
Basalt
A-B Section shown in inset
Journal ofAfican
Earth SC/~
623
C.E. SUHet al. Table 1. Description and interpretation of B,, B, and B, members of Bima Formation, Upper Benue Trough Silicidastic members (Bima Formation) B3
BZ
Bl
Matrix-supported
Main rock types and characteristics
Well-sorted, well-rounded, distinctly bedded, uniform fine-grained ferruginised reddish-brown to yellowish arkosic sandstone alternating with more clayey to shaly, purplish to light grey, carbonaceous and pyritic beds. The sandstone grades rapidly into siltstones both laterally and vertically. Small-scale trough cross lamination is prevalent. Fining-upward trends towards the top. Lower parts have small-scale upward coarsening. Coarse- to medium-grained; rarely microconglomeratic sandstone with low angle bedding planes. Simple cross lamination is common and there is sporadic trough cross bedding. Beds have good lateral continuity. Red-dish to purplish silty beds are present, as well as claystone lenses. Sandstone beds get silty towards the top. Sequences are poorly developed and generally finingupward. Palaeocurrent direction is east-southeast-westnorthwest. Grain-supported coarse-grained conglomeratic to finer clast-supported open-framework microconglomeratic sandstones with internal scouring surfaces. Lenses of finer sandy material are present. Cross-bedding is rare. Poorly sorted with coarse-grained matrix. Lateral facies change fining from east-southeast to west-northwest. Upward-coarsening sequences are common with subordinate upward-fining sequences. Beds have poorly defined contacts and lack good lateral continuity. Palaeocurrent direction is east-southeast-westnorthwest.
conglomerates
commonly
found
in proximal fan environments and typified by debris flow deposits (Larsen and Steel, 1978; Collinson, 1986) are absent in B,. Sediments similar to B, have been described in other parts of the world (Steal and Thomson, 1983; Steel, 1988) and also interpreted to most likely represent medial fan detritus dominated by stream channel deposits. Steel and Thompson (I 983) further postulated that basin forming tectonism of increasing intensity can cause coarseningupward trends in sedimentary sections, whilst more stable tectonic conditions account for upward-fining trends. This interpretation adequately explains the spatial distribution of upward-coarsening and upwardfining trends within B, sediments of the Peta Gulf Syncline. Hare, in areas where distinct strike-slip fault surfaces are observed on the eastern margins of the sub-basin, the B, sediments have upward-coarsening trends, whereas areas not affected by these faults have upward-fining trends. Towards the top, B, sediments generally become siltier and grade into indistinguishable B,-B, transitional sediments.
624 Journal
of A f&an Earth Sciences
Interpretation (depositional environments1 Flood plains. Lacustrine sediments formed in distal lakes.
Full fluvial deposits. Silty and clayey pockets represent proximal lacustrine environments or small overbank deposits.
Medial fan detritus dominated by stream channel deposits without debris.
Based on the characteristics presented in Table 1, B, sediments are interpreted to represent full fluvial deposits, with the alternating coarse and fine layers representing strong variations of energy levels. Also, the presence of coarse-grained facies and poorly developed general fining-upward sequences in B2are interpreted to be the result of deposition in channels with low sinuosity. The fine member deposits of B, are typical of interchannel areas, which receive sediments during floods (Collinson, 1986). The areas may be floodplains or perennial swamps or shallow lakes. The small-scale upward-coarsening units probably record the infilling of shallow floodplain lakes by small deltas (Flores, 1981; Gersib and McCabe, 1981). The background radiometric readings for the various facies of B,, B, and B, described earlier are, on average, between 90 and 150 cps. The finer, muddy materials generally have higher background values. In sandstone-hosted U deposits, reduced claystones in lacustrine and deltaic environments often show a high background. For example, le Roux (I 991) has
Host rock geology and geochemistry of the Zona uranium occurrence, Peta Gulf S yncline, northeast Nigeria demonstrated that most of the U-bearing sandstones of the Karoo Basin were deposited by distal braided rivers with significant inputs from lacustrine delta deposits. Furthermore, le Roux (I 993) argues that if the permeability of the sandstone is too high, then the ore-bearing fluids would pass through too rapidly without sufficient time for U precipitation. Conversely, predominant fine- to very finegrained (relatively impermeable) sandstones are good host rocks. Following these arguments, B, is definitely not a good host rock for U in the Peta Gulf Syncline, owing to its high permeability and absence of fine sediments. B2offers moderate potential. B, best satisfies the sedimentological controls for U precipitation in sandstones: sandwiching of sandstones and claystones of reduced and oxidised facies, organic (lacustrine-type) matter and moderate permeability. The fine sediments would create permeability barriers, necessary impediments to fluid-flow, so that U can precipitate. The only significant U occurrence so far discovered in the Peta Gulf Syncline is the Zona U anomaly (Figs 3 and 4) with radiometric readings of 600- > 2500 cps (Sub, 1997; Suh et a/., 1998). The Zona anomaly occurs within B,/B, transitional fine-grained sand-
0
stones. The mineralisation is confined to fault trends with distinct alteration zones (Fig. 4). Therefore, even if the U was derived from B, sediments, its concentration was not only controlled by sedimentological factors but also by post-depositional deformation. Frac-ture/fault zones within B, and B,/B, transitional sequences are favourable sites for U concentration in response to fluid circulation initiated by this deformation within the Peta Gulf Syncline Sub-basin. The ore was epigenetically remobilised from within the sediments and concentrated along these open structures.
GEOCHEMICAL INVESTIGATION: ZONA U ANOMALY samplepreparatim8ndanalytiwlprocedum Carefully chosen samples representing the entire spectrum of fresh to altered sandstones from the various alteration zones of the Zona anomaly (Fig. 4) were used. The choice of the samples to be analysed was guided by careful petrographic examination of thin sections, integrated with field data, fabric development and radiometric levels of the whole
200 m
cl
Fresh, unbrecciated, coarse-grained sandstone
cl‘,O
Brecciated, red ferruginised zone
D-
Brecciated, silicified zone
:<
Uranium mineralisation (visible) Fault/fractures Facies boundary (radiometric) Topographic contour line
F&we 4. Structural pattern and deformation/a/t ‘eradon zones of the Zona U anomaly (after Suh et al., 19981.
Joumal of African E&h Sciences625
CL SUH et al. Berlin, Germany, was used following the techniques of Norrish and Chappel(1977). Errors obtained for the pressed powder pellet analyses were high, especially for the major elements ( f 2 wt% at 20) as opposed to data obtained from the fused discs. Thus, the fused glass disc data were regarded as ‘secure data’ and are used in this article. With these data the errors at 2u were generally < f 1.05 wt% for major elements and f 0.5 ppm for trace elements. Also, to check the randomly selected samples (powders and/ or consolidated rock samples), further analyses were done using particle induced y-ray emission (PIGE), energy-dispersive X-ray detector (EDAX) and particle induced X-ray emission (PIXE) (Sub, 1997). Evaluation
sample set. The results of these earlier studies (petrography, field occurrence, fabric and radiometry) are presented elsewhere (Sub et al., 1998). Each sample, weighing l-3 kg, was crushed using standard jaw-crushing techniques and subsequently milled to 543pm powder in a W carbide coated mill at Obafemi Awolowo University, Ile-lfe, Nigeria. 40 f 0.1 g of each powdered sample were carefully weighed into precleaned Teflon@sample bottles and tightly closed. Pressed powder pellets and fused glass discs made from these samples were analysed for major and trace elements. An X-ray flourescent (XRF) unit at the Geochemistry Laobratory, Technical University of
Table 2. Major, trace and REE concentration of representative samples from the various alteration zones of the Zona U anomaly MAJOR ELEMENTS
(wt%)
Fresh sandstrme SZI Sd
SZ3
sz4
SZ5
SZS
SZ7
silEihd SZS
SZS
zone Sr10
fall
66.50
0.24
0.35
0.13
14.00
10.4
IS.80
7.56
72.90
79.40
74.50
74.00
72.60
76.50
60.60
67.03
63.16
0.01
0.41
0.15
0.46
0.70
0.17
0.43
0.26
0.26
0.21
0.33
A124
17.76
19.61
16.40
17.20
9.00
11.30
hQ3
0.76
0.62
1.35
MnO
0.67
0.32
wJ CeO
0.33 0.01
0.20 0.38
Na2C
0.10
2.30
1.30
KzC
3.64
3.35
4.50
14.70
16.40
IS.80
18.30
sz14
77.40
72.80
ma
SZl3
sr12
77.Qo
Sio,
66.B
0.32
1.12
2.00
1.30
0.11
0.62
0.01
1.30
1.6
2.30
3.30
-
0.77
0.51
0.66
0.41
0.44
0.32
0.01
0.34 0.35
0.21 0.25
0.71 0.31
0.04 0.22
0.21 0.51
0.05 0.04
0.06 0.19
-
0.03 0.06 0.11
0.08 0.02 0.01
0.25 0.04 0.15
0.22 0.01 0.02
3.20
2.20
2.10
3.40
0.25
0.32
0.20
0.20
0.35
-
0.16
0.11
2.40
3.21
2.x)
2.12
0.13
-
-
0.04
-
0.35
P&5
0.02
-
0.12
-
0.01
0.30
0.33
0.05
0.03
0.06
0.12
-
0.11
LOI
0.21
0.01
0.01
0.50
0.65
0.50
0.20
0.51
0.41
0.77
0.34
0.3
0.51
0.64
101.60
loo.20
69.50
Qs.62
99.50
99.61
loo.40
94.65
101.50
99.56
loo.17
loo.60
101.20
TOtal
MINOR/TRACE
ELEMENTS
Fresh sandstone SZl Sr2
sz3
sz4
Sr5
SZ6
sr7
SZ6
.siliiiIi6dzom SZQ SZlO 17 54
35
56
30
11
52
7 41 3
41 19 5
15 16 6
6 10
20 19
7 6
12 4
1 13
3 3
4 2
5 -
4 -
320
141
164
108
113
Sr Se V Cr
369 63 33 10
167 78 41 18
166 26 33 14
352 68 54 32
189 215 36 13
166 71 32 16
142 23 55 10
232 66
98
62 315
41
29
CO Ni CU al AS
16 4 2
21 3 -
22 3 3 3
17 4 1 4
23 9 4 7
11 4 1 6
20 6 4 2
13 12 5
16 13 9
-_-_ -
2 2
Bi Cd -
1
-_
sz3
Sr2
Ga
13.0
11.0
Hg zr MO
0.3 549.0 3.0
0.2 3B.O 3.0
Sb Sfl Br
4.0
4.0
10.0
12.0
0.1 550.0 4.0. 5.0 1QO
sz5
Sr6
s27
zone SZIO
SZll
sz12
7.0
5.0
3.0
3.0
6.0
sz13 6.0
0.1 330.0 7.0
0.2 395.0 3.0
0.3 460.0
0.3 451.0
0.2 320.0
0.1 428.0 4.0
0.2 216.0 2.0
0.3 366.0 2.0
0.4 166.0 4.0
0.3 195.0 2.0
0.3 310.0 2.0
fir14 -
5.0
6.0
11.0 4.0
6.0 4.0
5.0 2.0
4.4
3.5
4.0
4.0
2.0
2.0
6.0
6.0
6.0
6.0
10.0
5.8 0.2
6.0 -
4.0 -
4.0 -
4.0 -
7.0 _
51.0 10.0 10.0
-
73.0
60.0
75.0
74.0
50.0
6.0 19.0
32.0 20.0
22.0 11.0
56.0 11.0
10.0
10.0
20.0
65.0 6.0 13.0
13.0
5.0
15.0
10.0
1340 7.0
47.0 6.0
140.0 9.0
234.0 6.0
201.0 10.0
256.0 6.0 13.0
262.0 9.0 15.0
159.0 9.0 6.0
26.0 20.0
24.0
25.0
15.0
16.0
10.0 4.0 66.0 10.0
10.0 4.0 54.0 10.0
13.0
11.0
19.0
30.0
73.0
60.0
27.0 14.0
20.0 16.0
6.0 11.0
W
177.0
140.0
135.0
40.0
234.0
Nb m
6.0 10.0
9.0 6.0
6.0 11.0
10.0 9.0
11.0 6.0
9.0 10.0
LS Nd Pr
23.0 15.0 10.0
21.0 13.0 10.0
23.0 15.0 10.0
20.0 31.0 10.0
16.0 25.0 10.0
20.0 13.0
Sm Ce
4.0 55.0
4.0 54.0
4.0 50.0
4.0. 53.0
4.0 91.0
SC Y
9.0 16.0
10.0 15.0
10.0 16.0
10.0 13.0
10.0 13.0
626 Journal of Afticon Earth Sciences
Siliiikid SZQ
SZ6 6.0
11.0 10.0
iOl:
3
6.0
6.0 11.0
Samples l-29;
3 2 -
6.0
367.0
Szl-SZ29:
1 3 -
6.0
U m
Pb
10 60 340
1
S&4 3.0
16
1
Freshsandstom, SZI
sr14
5 25 141
loo
3 -
sz13
sz12 21 42 78
lb
6
call 40 63 110
Rb
323
Ag
lW.12
(ppm)
loss on ignition.
17EO
9.0
10.0
15.0
9.0
10.0 4.0
15.0 10.0 4.0
20.0 14.0 10.0 4.0
25.0 15.0 10.0
27.0 36.0 11.0
25.0 15.0 10.0
55.0 9.0
B2.0 10.0
61.0 10.0
4.0 54.0
6.0 92.0
4.0 55.0
10.0 4.0 53.0
16:O
15.0
11.0,
9.0 13.0
10.0 13.0
10.0 12.0
10.0 15.0
10.0 21.0
Host rock geology andgeochemistry of the Zona uranium occurrence, Peta Gulf Syncline, northeast Nigeria a predominantly granitic basement (Banerjee and Bhattacharya, 1994). However, the fresh sandstones are poor in P,O, and TiO, and have low loss on ignition (LOU values (Table 2). For all the Zona samples, A&O, generally decreases with increasing SiO, (Fig. 5). Samples from the silicified zones, SZS-SZI 6 (Table 2) show elevated SiO, values ranging from 77.4-89.9 wt%. These samples also show a depletion in K,O and Na,O concentration (Fig. 51, as well as in AI,O, and P205consentrations. Conversely, there is a slight increase in Fe,O, and LOI values in the silicified zone (Fig. 5). Similar trends described above for the silicified zone are discernible in the hematite and goethite zones. In
of the paired analyses (XRF and EDAX/PIGE/PIXE) of the replicate samples showed no significant differences. Resultsand interpretation Major elements Results of major analysis of 29 representative samples from the Zona anomaly are provided in Table 2. The fresh sandstone samples are rich in SiO, and K,O but relatively poorer in Na,O. They are, thus, of the potassic sandstone type (Bhatia, 1983; Banerjee and Bhattacharya, 1994). These fresh sandstones are also poor in CaO and MgO, indicating the absence of carbonates. These chemical features are indicative of debris derivation from Table 2. continued MAJORELEMENTS siBiedzon.3 SZlB Sr15 SiOl
Hemathitiezone sz17 SZ16
na
79.95
78.22
69.25
0.B
0.26
0.46
0.35
0.25
0.13
0.06
0.07
0.71
0.13
OX5
0.62
14.30
12.10
15.00
13.65
14.00
3.76
3.98
2.40
2.05
5.57
3.98
3.15
4.59
2.30
10.10
5.23
9.40
13.40
10.00
2.77
3.51
5.40
5.24
11.10
5.15
4.11
10.01
0.03 0.07 0.22
0.11 0.03 0.01
0.35 0.01 0.16
0.31 0.05 0.13
0.25 0.04
0.30 0.07 0.01
0.02 0.01 0.01
0.02 0.01 0.01
0.02 0.01 0.01
0.02 0.01 0.01
0.02 0.01 0.16
0.01 0.01 0.07
0.W 0.01 0.01
0.02 0.11 0.15
0.34
0.41
0.13
0.12
0.31
0.19
0.16
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.56
0.01
0.12
0.33
0.12
0.11
0.42
0.15
0.42
0.46
0.36
011
0.13
0.33
0.44
0.41
-
0.67
0.28
0.29
0.04
0.31
0.19
0.03
0.04
0.02
0.12
0.03
0.05
0.13
1.00
3.31
4.46
2.10
2.46
1.59
1.04
99.14
99.96
99.94
90.46
1.12 9993
3.34 59.57
2.11 loo.66
1.10 loo.14
3.36 99.39
0.67
0.15
0.17
17.10
2.20 0.04 0.06 0.31
0.91
1.10
0.30
loo.70
101.36
100.17
60.96
lW.60
9907
99.20
MlNOR/lRACEELEMENTS(ppm) Hemathiiezone SZl6
5217
SZ16
1
Godkte zcm - miner&red
SzlQ
Sz20
sz21
5224
SZ23
sz25
szm
sz27
sz?B
55 94 108
12 150 614
10 155 359
12 160 623
10 95 514
14 101 604
15 173 327
26 105 126
27 94 146
25 73 138
gs 150
6 163 133
12 95 131
13 113 168
6 5
25 13
27 15
30 6
31 9
25 10
22 12
31 11
10 10
15 10
15 6.5
22 9
11 10
11 10
18 Q
12
21
16
4
2
3
43 5 1
64 4 5 11
45 6 3 29
52 9 5 12
74 3 3 -
71 5 5 -
61 1 2 -
9
37 6 10 16 27
55 3 7
4
61 3 3 --_ 10
11 3 4
7
27 1 9 3 24
-
25 3 1.2 2
22 2.5
13 -
27 -
22 -
3
3 2 0.9
16 3
1
-
-
sz24 3.0
sz26 1.0
~Hemathiizone sz17 SZ16
-I
0.2 225.0
0.3 330.0
-
3.0
4.0
20.0 11.0 152.0 10.0 10.0 25.0 31.0 10.0
13.0 133.0 6.0 15.0 21.0 25.0 10.0
Sill Ca SC Y
4.0 51.0 10.0 13.0
4.0 91.0 10.0 12.0
Szl-Sz29:
Samples
-
Pb
-__ 1
l-29;
2.5
sz19
sz20
6.0
3.0
4.0
0.3 215.0 3.0 2.7
4.0 160.0 3.0 2.0
0.2 210.0 2.0 0.9
3.5
6.0
4.0
sz27 3.0
SrzB 20
sz29 20
0.2
0.1
0.;
0:;
193.0 2.0 4.4
130.0 2.0 2.7
131.0 2.0 2.2
370.0 295.0 17.0 316.0 3.0 6.0
7.0 159.0 16.0 513.0 2.5 4.0
4.1 1.7 2.0 369.0 14.0 650.0 3.0 1.5
6.4 4.2 15.0 162.0 15.0 5990 3.0 3.0
6.3 0.3 3.0 346.0 3.0 EQ9.0 3.0 5.0
6.4 0.2 9.0 546.0 4.0 599.0 2.0 3.0
6.0 2.0 660.0
loo.0 2.5 2.7 4.4 0.3 3.0 547.0
4.0 513.0 3.0
3.0 608.0 3.0
20.0 9.5 10.0 4.0 30.0
19.0 6.5 10.0 3.5 16.0
16.0 10.0 10.0 4.0
16.0 6.5 10.0 4.0 20.0
2.0 16.0 15.0 10.0
20.0
15.0 7.0 10.0 4.0 16.0
4.0 m.0 9.0 10.0
20.0
12.0 10.0 4.0 16.0
16.0 31.0 10.0 5.0 60.0
10.0 6.0
10.0 6.0
11.0 14.0
10.0 10.0
9.5 10.0
10.0 6.0
10.0 10.0
10.0 9.0
4.0 16.0 10.0
4.0 16.0 10.0
4.0
4.0 16.0 10.0 6.0 on
sz26 3.0
0.2
73.0
loss
4.0
67.0 2.0 3.3
289.0 4.0 607.0 2.0 6.0 15.0 13.0 10.0
LOI:
sz23
0.1
6.0
16.0 9.0 6.0
7.0
97.0 4.0 2.9
192.0 10.0 270.0 2.0 4.0
10.0
3.01
0.1
50.0
20.0
1 sm
137.0 2.0 3.6 4.5
297.0 4.0 513.0 2.5 5.0 20.0
16.0 15.0 10.0 4.0
sz21
0.2
24.0
10.0 10.0
-
-_
216.0 7.9 2.0 10.4
269.0 6.0 316.0 3.0 5.0 16.0 13.0 10.0
20.0
10 33
lGoethitezone- mineded
1
2.0
9
30
sz26
22 41 121
4.0
Pr
sz2s
91.20
91.00
5.0
Nd
sz26
90.01
2.0
U Th w Nb In La
sz27
90.23
0.2 195.0 2.0 3.6
WI sr
5226
90.29
SiMiedron6 Sr16 sz15
Zr MO Sb
St25
71.40
74.03
5x15
I
Goethiizone-minerdiwd sz22 s?23 s.724
sz21
E8.53
60.00
Siizone
Ga
s7.20
70.00
60.20 lJ.M
Rb Sr Ba
sz19
4.0
0.3 150.0 3.0 3.2 6.0 54.0 270.0 3.0 310.0 3.0 5.0
20.0
9.0
6.0,
ignition.
Journal of Afrfwn Earth Sciemzes 627
C. E. SU. et al. 20 ,
0 CvlO ’
1.00
0
l
(d)
0”
-
0
8-
0
6 4-
+
2-r.
I.,
64
.
68
,
72
14.
.
76
SiO,
0
+ ,
.
80
, 84
.
+++ ,t+ 88
0.00
92
4.00 ~3.50 o 3.00 -2.50 3 OJ.00
+
8 -
0” $’ IL
6 -
64
A
A
+
m 1.50 z 1.00 0.50 0.00 -0.50 -1.00
A
68
72
68
72
76
80
SiO 2(Wt
84
88
92
0.00
-
SiO 2(Wt %)
,
a&$
84
88
92
84
88
92
0
‘4
j -
* A
.A
A
&
0
Qo&
. , . , . , . , . , . , . , . ( 64
68
72
3 -
76
g e
2.50
yR
2.00 1.50 1.00
-
0.50 0.00
-
80
84
88
92
(Wt %)
0
(f) 0
00 00
-
A
A
0
A
QA
GOa
A-0
-
-1 .oo
80
80
l
5.00 4.50 4.00 3.50 3.00
-0.50
76
.
,
(e)
%)
1
72
r
76
SiOp
(c)
68
,
-
60
1 .oo
64
t
5.00 4.50 -
(b) A
. A,
SiO 2(Wt %)
A
-
E
64
+
.’ ,
(Wt %)
12 s&O
7
(a)
,
60
64
68
72
-
I-
I.
76
80
I
84
.(
.I
88
92
Sio;! (Wt %)
Figure 5. Harker-type variation diagrams for the Zona samples. 0: Fresh sandstone; 0: altered sandstone lsilcified zone); A: altered sandstone (hematite zone); +: altered sandstone Imineralised goethite zone).
628 Journal of African Earth Sciences
Host rock geology andgeochemistry of the Zona uranium occurrence, Peta Gulf Syncline, northeast Nigeria
5.00 4.50 4.00 3.50 g 3.00 2.50 3 - 2.00 2 1.50 1.00 0.50 0.00 -0.50 -1.00
7
(g) A ++ A + + A + $ ++o :O: , . I . 1 . , . , . , . , . , . , 60
64
68
72
76
80
84
88
92
SiO2 (Wt %) Fipre 5. Continued. Harker-type variation diagrams for the Zona samples. LOI: Loss on ignition; l : fresh sandstone; 0: altered sandstone Isilcified zone); A: altered sandstone (hematite zone); +: altered sandstone Imineralised goethite zonel.
fact, SiO, concentration in the goethite zone attains a maximum of 91.2 wt%, while Fe,O, reaches 13.4 wt% in the hematite zone (Table 2). These features suggest that fluid-rock interaction, which led to the precipitation of U, also induced remobilisation of Fe,O, and SiO, and later concentrated them in the mineralised zone. Because of their overall low alkali content and high Fe,O, contents, the sandstones of the hematite and goethite zones belong to the ferromagnesian potassic sandstone type of Bhatia (1983). LO1also increases in the altered samples (Fig. 5), implying that neo-crystallisation led to the formation of predominantly hydrous mineral phases such as chlorites, micas, clay minerals and hydrated Fe compounds. This is in agreement with petrographic results (Sub, 1997). The hematite and mineralised goethite zones show significant depletion in Na,O, CaO, MgO and K,O (Fig. 5). These elements are relatively very mobile in the supergene environment (Hawkes and Webb, 1962; Orajaka, 1981). Trace and minor elements The results of trace and minor elements are also provided in Table 2; the trace elements worthy of close examination are Rb, Ba, Sr, V, Co, As, Zr, Pb, Th, U and W. All the other elements yield rather nonsystematic results as far as elemental zoning patterns are concerned. Rb, Sr, VandZr. Rubidium and Sr, on the whole, show a pat-tern of decrease from the unaltered fresh
sandstone zone to the central mineralised goethite zone (Table 2). Uranium concentration increases with decreasing Rb and Sr concentration. For example, Rb concen-tration in the fresh sandstone ranges from 108-328 ppm, whereas the range in the goethite zone is 6-30 ppm (Table 2). Riese et a/. (I 978) noted that the Rb released during rock disintegration is often removed quickly by solution. This mechanism most likely operated in the Zona anomaly to effect such systematic Rb and Sr dispersion patterns. Vanadium and Zr also show depletion with increasing U concentration. That Zr is mobile in the Zona anomaly is significant because it provides evidence of leaching and chemical mass transfer processes. The mobility of Zr in the Zona anomaly indicates that the envi-ronment was acidic, since the solubility of Zr is restricted to the pH range of 3-6.5 (Colin et al., 1993). Ba, Co, W, As and Pb. These elements are enriched in the mineralised goethite zone (Table 2). Barium is often associated with sandstone-type U deposits and it is concentrated in the ore zone, e.g. Wyoming deposits USA (Riese et al., 1978). High concentrations of Ba in the ore zone may be attributed to Ba fixation as BaSO,, indicative of cation exchange reactions during the precipitation of U. Fischer (1970) reported high Co concentrations within the mineralised zone of the Colorado and Wyoming sandstone-type U deposits. The Zona samples also show elevated Co levels in the goethite zone (Table 2). Cobalt in the supergene zone may be absorbed by Fe-based hydroxides (Fischer, 1970; Olorunfemi, 1984). This is by far the most common mechanism of Co precipitation in the surficial environment. A similar mechanism of Co enrichment was probably involved for the Zona mineralised zone. Tungsten concentration in the goethite zone is also higher than in all the other zones. Arsenic concentration in the Zona samples is higher in the hematite and goethite zone than in the fresh sandstones. Arsenic is commonly associated with sandstonetype U deposits (Miller et a/. , 1984). Indeed, As, in terms of abundance ratios, is one of the most important intrinsic elements in these deposits (Shoemaker et al., 1959). In sediments, As is known to be precipitated in hydrolysate and oxidate states (Riese et al., 1978) by adsorption on ferric hydroxides. Because the hematite and goethite zones of the Zona anomaly are rich in As, this element, therefore, was most probably adsorbed on these Fe-bearing minerals. This evidence in support of adsorption reactions confirms the epigenetic origin of the Zona anomaly and the intrinsic genesis of U. Lead exhibits similar dispersion patterns to As and the mechanisms
Journal of African Earth Sciences 629
C. E. SlJH et al. governing its mobility are the same as those discussed above for As. REEs andhalogens. The concentrations of REEs in all the alteration zones of the Zona anomaly are quite low. Also, no zone shows any REE enrichment over the other (Table 2). There is, however, one element, Ce, which shows a relatively significant depletion in the mineralised goethite zone (Table 2). While conceding that REE enrichment in sandstone-type U deposits is rare, Riese et al. (1978) noted that Ce and Eu often show distinct depletion in the ore zone. Of all the REE, only Ce and Eu exist in the + 4/ + 3 and + 3/ + 2 oxidation states, respectively. All the others are + 3. This duality in oxidation states has variable influences on their mobility over other REE, based on the redox potential of the system. The conclusion to be drawn, therefore, is that Ce underwent an open system depletion at the Fe redox boundary, contemporaneous with U precipitation. Cerium depletion may, therefore, be a useful index to identifying such Fe redox boundaries, hence enhancing exploration efforts in the Peta Gulf Syncline.
DISCUSSIONAND CONCLUSIONS The Peta Gulf Syncline Sub-basin is of the pull-apart basin class. Pull-apart basins dominated by strikeslip faults often show organisation of facies into marginal fault-bounded fanglomerates/conglomerates and central flood basin/playa/lacustrine deposits (Hempton and Dunne, 1984). In the Peta Gulf Syncline, the facies organisation defines a spectrum of progressive fining from the eastern margin conglomerates to fine sandstones/siltstones on the western margin. This suggests that the strike-slip/normal faults on the eastern margin had low angles and were tectonically active during sedimentation. Several authors (e.g. Ganz, 1987; Wernicke and Axen, 1988; Bosworth, 1994) have demonstrated that such low angle boundary faults in extensional basins can be developed from initially steeply dipping faults by rotation. Such rotational motion on the eastern margin of the Peta Gulf Syncline during sedimentation most likely influenced the facies pattern within the basin. The steep faults of the western margin were dormant throughout this process. Because the individual facies have a limited lateral extent with restricted deposition of locally derived microconglomerates, sedimentation in a fault-bounded rapidly subsiding basin is indicated, as noted also by Reading (1980). Three members of the 8ima Formation are discernible within this sub-basin; from bottom to top: B, (microconglomerates, no fines); B, (coarse-grained
630 Journal of African Earth Sciences
sandstones; minimal fines) and B, (alternating finesandstones and siltstones/claystones). B, sediments are inferred lacustrine deposits and offer the best lithofacies favourable for U precipitation. However, the Zona U anomaly within this sub-basin seems to be controlled more by structural geology, rather than solely by sedimentological aspects. Therefore, even if U was disseminated in B, sediments, its concentration to yield the Zona U occurrence was enhanced by postdepositional deformation. In terms of whole rock geochemistry of samples collected from the Zona anomaly, the fresh sandstones are of the potassic sandstone-type and are poor in carbonates, indicated by the low levels of CaO and MgO in them. The CaO and MgO concentration in all the altered zones is also very low. This supports the absence of carbonates inferred from the petrographic examination (Sub, 1997) and suggests that U was not transported in the hydro-thermal fluid as soluble carbonate complexes. Uranium has a tendency to form complexes with a large number of naturally occurring anions. These complexes greatly enhance the solubility and mobility of U in low temperature and pressure secondary environments (le Roux, 1993). Langmuir (1978) indicated that for U complexes to attain significant concentrations in groundwater, pH must be C 7. Furthermore, carbonate complexes are important U complexing agents in alkaline environments (pH > 7) and not in acidic environments (Langmuir, 1978). The absence of carbonates around the Zona anomaly, coupled with the mobility of Zr in this region, are good indications that the environment was acidic during the U precipitation stage. With progressive deformation, the altered Zona sandstones have variable degrees of SiO, and Fe,O, enrichment. In addition to Fe,O,, Ba, W, As, Co and Pb have anomalously high concentrations in the mineralised goethite zone. This zone is significantly impoverished in the alkalis. These elemental dispersion patterns show clearly that chemical leaching and mass transfer were significant processes in the formation of this U anomaly. The Zona U anomaly is hosted by a brecciated and fractured sandstone and it is therefore of the ‘postfault’ ore type. This class or type of U deposit is characterised by discordant relations with host rock primary features (bedding planes, stratigraphical boundaries) and spatial association with faults (Ludwig and Simmons, 1984). They are also termed joint controlled sandstone-type U deposits (Allen and Thomas, 1984) marked by concentration of U along brittle discontinuities. The Zona anomaly, therefore, is not an unconformity type deposit, sensu stricto. The source of the U in post-fault ore types is principally primary ore bodies that have been destroyed
Host rock geolog y and geochemistry of the Zona uranium occurrence, Peta and redistributed by groundwater. However, for the Zona anomaly, the U comes from detritus in the host sediments, derived from the adjoining granitic basement terrain. Uranium mineralisation occurs within this basement and can be regarded as a provenance for uraniferous debris during the deposition of the Bima Sandstone. The Quirke ore zone, Ontario (Robinson and Spooner, 1984) is a classic example of a U deposit with detrital uraninite in the host sediment. In the Quirke ore zone, the ore concentration process was initiated by complete fluid-mediated leaching of Fe, SiO,, U, Th and Y (Robinson and Spooner, 19841, followed by the precipitation of secondary quartz and sericite in pore spaces, as well as the precipitation of U in favourable structures. Mineral alteration studies of the Zona U anomaly (Sub et al., 1998) have provided evidence that U concentration is associated with hydrous Fe compounds and feldspar dissolution and alteration to kaolinite. This supports the epigenetic origin of this anomaly. The main geochemical pathways envisaged for the deposition of U at Zona can, therefore, be summarised as follows: i) Uranium is leached from detrital material within the sandstone host rock by fluids, possibly heated, and slightly acidic connate and groundwater affinity. The fluid was probably heated by simple diagenetic compaction, though renewed tectonic movement along the fractures and faults probably supplied a lot of frictional heat. i) Groundwater moving along the brittle openings transported U in the form of uranyl complexes in association with Fe, As, Ba and W. Such uranyl complexes often bear carbonates (for example, Miller et a/. , 1984; Smith, 1990). However, because carbonates are not present in the Zona anomaly, these uranyl complexes were not carbonate-based. i) Interaction of these groundwaters with organic matter (especiallyin B3sediments)and possiblysulphii, led to freed uranyl cation (subsequently oxidised by alkalis, Na+, K+, Ca2+, etc) -bearing solutions to form insoluble secondary U minerals. Autunite is the principal U mineral at Zona. Bariumconcentrations are high in the mineralised samples as discussed earlier, and this is attributed to fixation of Ba as BaSO,. This underlines the role of sulphides in the U precipitation, as these sulphides were oxidised to sulphates. Editorial handling - P. Eriksson
REFBtENcEs Allen, RF., Thomas, R.G., 1984. The uranium potential of diageneticaily altered sandstones of the Permain Rush Springs Formation, Cement district, Southwest Oklahoma. Economic Geology 79, 284-296.
GulfSyncline, northeast Nigeria
Banerjee, D.M., Bhattacharya, P., 1994. Petrology and geochemistry of greywackes from the Aravalli Supergroup, Rajasthan, India, and the tectonic evolution of a Proterozoic sedimentary basin. Precambrian Research 67, 1 l-35. Benkhelil, J., 1989. The origin and evolution of the Cretaceous Benue Trough (Nigeria). Journal African Earth Sciences 8, 251-282. Benkhelil, J., Robineau, B., 1983. Le fosse de la Benoue est-il un rift? Bulletin Centres Recherches Exploration Production Elf-Aquitaine 7, 315-321. Bhatia, M.R., 1983. Plate tectonics and composition of sandstones. Journal Geology 91, 61 l-627. Bosworth, W., 1994. A model for the three-dimensional evolution of continental rift basins, north-east Africa. Geologische Rundschau 83, 671-688. Colin, F., Alarcon, C., Vieillard, P., 1993. Zircon: an immobile index in soils? Chemical Geology 107, 273276. In: Reading, Collinson, J.D., 1986. Alluvial sediments. H.G. (Ed.), Sedimentary Environments and Facies. Blackwell Scientific Publications, Oxford, pp. 20-82. Dahlkamp, F.J., 1978. Classification of uranium deposits. Mineralium Deposita 13, 83- 104. 1987. An introduction to Ore Geology. Evans, A.M., Blackwell Scientific Publication, Oxford, 358~. Fischer, R.P., 1970. Similarities, differences and some genetic problems of the Wyoming and Colorado plateau types of uranium deposits in sandstones. Economic Geology 65, 778-784. Flores, R.M., 1981. Coal deposition in fluvial palaoenvironments of the Paleeocene Tongue River Member of the Fort Union Formation, Powder River area, Powder River Basin, Wyoming and Montana. In: Ethridge, F.G., Flores, R.M. (Eds.), Recent and Ancient Non-marine Depositional Environments: Models for Exploration. Society of Economic Palaeontology and Mineralogy, Special Publication, pp. 169- 190. Ganz, P., 1987. An open system, two-layer stretching model for the eastern Great Basin. Tectonics 6, l-l 2. Gersib, G.A., McCabe, P.J., 1981. Continental coal-bearing sediments of the Port Hood Formation (Carboniferous), Cape Linzee, Nova Scotia, Canada. In: Ethridge, F.G., Flores, R.M. (Eds.), Recent and Ancient Non-marine Depositional Environments: Models for Exploration. Society of Economic Palaeontology and Mineralogy, Special Publication, pp. 95-l 08. Guiraud, M., 1990. Tectono-sedimentary framework of the Early Cretaceous continental Bima Formation (Upper Benue Trough, N.E. Nigeria). Journal African Earth Sciences 10, 341-353. Hawkes, H.E., Webb, J.S., 1962. Geochemistry in Mineral Exploration. Harper and Row, London, 416~. Hempton, M.R., Dunne, L.A., 1984. Sedimentation in pullapart basins: active examples in Eastern Turkey. Journal Geology 92, 513-530. Langmuir, D., 1978. Uranium solution-mineral equilibria at low temperatures with applications to sedimentary ore deposits. Geochimica Cosmochimica Acta 42, 547-567. Larsen, V., Steel, R.J., 1978. The sedimentary history of a debris flow-dominated, Devonian alluvial fan-A study of textural inversion. Sedimentology 25, 37-59. Le Roux, J.P., 1991. Flume experiments on permeability and organic matter as related to the genesis of uranium deposits in the Beaufort Group. South Africa Journal Geology 94, 212-219. Le Roux, J.P., 1993. Genesis of stratiform U-MO deposits in the Karoo Basin of South Africa. Ore Geology Reviews 7, 485-509.
JwmalotAttican
Earth Sciences 63 1
C. E. SW et al. Ludwig, K.R., Simmons, K.R., 1984. U-Pb isotope systematics and apparent ages of uranium ores, Ambrosia Lake and Smith districts, Grants Mineral belt, New Mexico. Economic Geology 79, 322-331. Miller, R.W., Wanty, R.8.. Mchugh, J.B., 1984. Application of mineral solution equilibria to geochemical exploration for sandstone-hosted uranium deposits in two basins in West Central Utah. Economic Geology 79, 266-283. Nash, J.T., Granger, H.C., Adams, S.S., 1981. Geology and concepts of genesis of important types of uranium deposits. Economic Geology, 75* Anniversary Volume, 63-l 16. Norrish, K., Chappel, B.W., 1977. X-ray fluorescence spectrometry. In: Zussman, J. (Ed.), Physical Methods in Determinative Mineralogy. 2”6 Edition, New York, Academic Press, pp. 201-273. Obaje, N.G., Funtua, I.I., Ligouis, B., Abaa, S.I., 1996. Maceral association, organic maturation and coalderived hydrocarbon potential in the Cretaceous Awgu Formation, Middle Benue Trough, Nigeria. Journal African Earth Sciences 23, 89-94. Obaje, N.G., Ligouis, B., Abaa, S.I., 1994. Petrographic composition and depositional environments of Cretaceous coals and coal measures in the Middle Benue Trough of Nigeria. International Journal Coal Geology 26, 233-260. Offodile, M.E., 1975. A review of the geology of the Benue Trough, Nigeria. Journal African Earth Sciences 3, 283291. Olade, M.A., 1975. Evolution of Nigeria’s Benue Trough (Aulacogen): a tectonic model. Geological Magazine 112, 575-581. Olorunfemi, B.N., 1984. Factors influencing the geochemistry of lateritic soils in the greenschist amphibolite metatectonic belt, South Western Nigeria. Chemical Geology 42, 261-275. Orajaka, I.P., 1981, Mineralogy and uranium geochemistry of selected volcaniclastic sediments in the Western United States-an exploration model. Ph.D. thesis (unpubl.), Univerity of Texas, El Paso, 365~. Passchier, C.W., Williams, P.R., 1996. Conflicting shear sense indicators in shear zones: the problem of non-ideal sections. Journal Structural Geology 18 (IO), 1281- 1284. Petters, C.W., Ekweozor, C.M., 1982. Petroleum geology of the Benue Trough and southeastern Chad Basin, Nigeria. American Association Petroleum Geologists, Bulletin 66, 1141-I 149. Rackley, R.I., 1976. Origin of western states-type uranium mineralisation. In: Wolf, K.H. (Ed.), Handbook of StrataBound and Stratiform Deposits 7. Elsevier, Amsterdam, pp. 89-156. Reading, H.G., 1980. Characteristics and recognition of strike-slip fault systems. International Association Sedimentologists, Special Publication 4, 7-26. Rebelle, M., 1990. The marine transgression in the Benue Trough (northeast Nigeria): a palaeogeographic interpretation of the Gongila Formation. Journal African Earth Sciences 10, 643-656.
632 Journal of Afrfcan Eanh Sciences
Riese, W.C., Lee, M.J., Rookins, D.G., Della-Valle, R., 1978. Application of trace element geochemistry to prospecting for sandstone-type uranium deposits. In: Watterson, J.R., Theobald, P.K. (Eds.1, Geochemical Exploration 1978. Proceedings of the 7* International Geochemical Exploration Symposium, AEG, pp. 47-64. Robinson, A., Spooner, E.T.C., 1984. Post-depositional modification of uraninite-bearing quartz-pebble conglomerates from the Quirke ore zone, Elliot Lake, Ontario. Economic Geology 79, 297-321. Sherborne, J.E., Buckovic, W.A., de Witt, D.B., Hellinger, T.S., Pavlak, S.J., 1979. Major uranium discovery in volcaniclastic sediments, Basin and Range Province, Yavapai County, Arizona. Bulletin American Association of Petroleum Geologists 63, 821-648. Shoemaker, E.M., Miesch, A.T., Newman, W.L., Riley, L.B., 1959. Elemental composition of the sandstone-type deposits. In: Garrels, R.M., Larsen, E.S. (Eds.), Geochemistry and Mineralogy of the Colorado Plateau Uranium Ores. US Geological Survey, Professional Paper 320, 2554. Smith, R.M.H., 1990. A review of stratigraphy and sedimentary environments of the Karoo Basin of South Africa. Journal African Earth Sciences 10, 117-l 38. Steel, R.J., 1988. Coarsening-upward and skewed fan bodies: symptoms of strike-slip and transfer fault movement in sedimentary basins. In: Nemec, W., Steel, R.J. (Eds.), Fan Deltas: Sedimentology and Tectonic Settings. Blackie and Son, pp. 75-83. Steel, R.J., Thompson, D.B., 1983. Structures and textures in Triassic braided stream conglomerates (‘Bunter’ Pebble Beds) in the Sherwood Sandstone Group, North Staffordshire, England. Sedimentology 30, 341-367. Suh, C.E., 1997. Structural, geochemical and fluid-rock interaction studies of uranium mineralisation in N.E. Nigeria: Implications for uranium metallogenesis and exploration. Ph.D. thesis (unpubl.), Abubakar Tafawa Balewa University, Bauchi, Nigeria, 221 p. Suh, C.E., Dada, S.S., Ajayi, T.R., Matheis, G., 1998. Integrated structural and mineral alteration study of the Zona uranium anomaly, northeast Nigeria. Journal African Earth Sciences 27 (I), 129-140. Turner, B.R., 1978. Sedimentary patterns of uranium mineralisation in the Beaufort Group of the Southern Karoo (Gondwana) basin, South Africa. Canadian Society Petroleum Geologists Memoir 5, 831-848. Visser, J.N.J., 1989. Discussion on U-Pb isotope systematics, ages and genesis of Karoo uranium deposits, South Africa. South African Journal Geology 92, 466469. Wernicke, B., Axen, G.J., 1988. On the role of isostacy in the evolution of normal fault systems. Geology 16, 848851. Zielinsky, R.A., 1981. Experimental leaching of volcanic glass: Implications for evaluation of glassy volcanic rocks as sources of uranium. American Studies in Geology, Association Petroleum Geologists 13, 1- 12.
Pergamon
PII:s0899-5262(00)00010-0
of African
Earth Sciences.
Vol. 31. No. 314. pp. 619-632. 2COO 2001 Elsevier Science Ltd All rights reserved. Printed in Great Britain 08995362/01 b- sea front matter
o
Host rock geology and geochemistry of the Zona uranium occurrence, Peta Gulf Syncline (Upper Benue Trough), northeast Nigeria C.E. SUH’,*, S.S. DADA2 and G. MATHEIS3 ‘Department of Geology, University of Buea, Buea, Cameroon 2Geology Programme, Abubakar Tafawa Balewa University, Bauchi, Nigeria 3Technische Universitgt, Berlin, Serkr, BH4, D-l 0587, Berlin, Germany
ABSTRACT-The Peta Gulf Syncline (Upper Benue Trough, northeast Nigeria) is a faultbounded pull-apart sub-basin. The boundary faults are mainly northeast-southwest-trending en echelon strike-slip faults, truncated along their lengths by normal and tear faults with stepovers. The eastern marginal faults underwent rotation during sedimentation, whereas the steeply dipping western marginal faults were inactive. The Peta Gulf Sub-basin is filled by the Bima Sandstone Formation (Lower Cretaceous) which has three siliciclastic members: (i) B,: medial fan coarse-grained to microconglomeratic sandstones; (ii) B,: full fluvial medium-grained sandstones with minimal fines; and (iii) B,: lacustrine and flood basin deposits comprising alternating fine-grained sandstones and siltstones/claystones. Sediment supply was from east to west and facies changes show a general fining in this direction. B, offers the most favourable environment/lithology for U concentration. The only significant U occurrence in the Peta Gulf Syncline is the Zona U anomaly, which occurs within transitional B,-B, brecciated sandstones with wall rock alteration zones. The mineralised zone has elevated SiO,, Fe, As, Ba and W levels but is depleted in the alkalis, Zr, Rb and Sr. This chemical zonation supports the epigenetic origin of this anomaly. e 2001 Elsevier Science Limited. All rights reserved. RESUME-Le synclinal du Golfe de P&a (Depression du Benue superieur, NE Nigeria) est un sous-bassin en ‘pull-apart’ delimit6 par failles. Les failles bordieres en echelon presentent des decrochements et sont majoritairement de direction NE-SO. Ces failles sont tronquees par des failles normales formant des decrochements et des gradins. Les failles de la marge Est subirent une rotation synsedimentaire tandis que les failles verticales de la marge Ouest ne furent pas affectees par ces mouvements. Le Golfe de P&a est rempli de formations composees des gres de Birma dates du C&ace inferieur. Ces formations gr4seuses sont decomposees en trois unites silicoclastiques: (i) B,: zone deltdique a grains grossiers et microconglomerats; (ii) B,: facies fluviatiles a grains moyens avec une faible proportion de grains fins; et (iii) B,: zone d’innondation lacustre composee d’une alternance de gres fins, de silts et d’argilites. Le materiel de remplissage de ce sous-bassin provient d’Est en Ouest, avec des proportions croissante en grains de plus en plus fins. L’unite B, presente I’environnement et la lithologie les plus favorables B la concentration d’U. La seule concentration significative d’U rep&be dans le synclinal du Golfe de P&a se trouve dans des zones de transition situee a I’interface des unites B,-B,. II s’agit de zones greseuses
*Corresponding author [email protected]
Journal of A f&an Earth Sciences 6 19
C. E. SUH et al. et brechiques presentant une alteration pervasive. La zone mineralisee contient des teneurs Bl6ves en SiO,, Fe, As, Ba et W mais appauvries en Zr, Rb et Sr. Cette zonation chimique est en accord avec une origine Bpigenetique de cette zone mineralisee. o 2001 Elsevier Science Limited. All rights reserved. (Received 1 B/l 199: revised version received 1/I 2199: accepted 3/4/00)
INTRODUCTION Uraniferous
sandstones
constitute
a significant
source of U. For example, the Karoo Basin, South Africa, has recoverable resources of at least 41,500 tonnes of U (le Roux, 1993). Sandstone-type deposits account for about 75% of the world’s U resources and they are known in many parts of the world (Dahlkamp, 1978). The sources of U in such sandstones are varied. Uranium can be deposited simultaneously with granite debris and volcanics or epigenetically derived from detrital U minerals within the sediments (Turner, 1978; Sherborne et a/. , 1979; Zielinsky, 1981; Visser, 1989). Its precipitation is often related to a redox boundary (Rackley, 1976; Nash et a/. , 1981). Most often, within a given basin, sandstone-hosted U mineralisations occur as scattered bodies. The mineralisation may be stratiform or of the post-fault U ore type. Sandstones are elastic sedimentary rocks, and for most sandstone-hosted mineralisations there is a direct link between the general sedimentological aspects (e.g. texture, architecture, lithofacies) of the sediments and the zone of U concentration (Evans, 1987). Also, an understanding of facies distribution, depositional environments and architecture of sedimentary sequences is a sig-nificant aspect of exploring for sandstone-hosted U deposits. These aspects are examined in this paper as they relate to the Zona U occurrence. The Zona U anomaly occurs within Cretaceous sandstones of the Peta Gulf Syncline, Upper Benue Trough, northeastern Nigeria. The main U mineral here is autunite. Three main zones of wall rock alteration have been identified for the Zona anomaly (Suh, 1997; Suh et al., 1998): a silicified zone; a hematite zone and a goethite zone., However, complete major and trace element geochemical data on these various zones were hitherto unavailable. These data are presented and discussed in this paper.
GEOLOGICAL SETTING The Zona U anomaly occurs within the northeastsouthwest striking intracontinental Benue Trough (Fig. 1 a). The trough is well over 1000 km long and 700 km wide and is geographically divided into three segments: the Lower, Middle and Upper Benue Trough. The Peta Gulf Syncline lies within the Upper
620 Journel of African Eerth Sciences
Benue Trough. Earlier work on the geodynamic evolution and structure of the Benue Trough show that its origin is linked to an Early Cretaceous opening of a rift structure related to a late tectono-magmatic phase (Olade, 1975; Benkhelil, 1989). Benkhelil and Robineau (1983) demonstrated that the principal structures of this trough are grabens, horsts and halfgrabens, whose individual dispositions are controlled by mostly north-south-trending normal faults and northeast-southwest strike-slip faults. These strikeslip faults divide the Benue Trough into sub-basins (Fig. 1b), often separated from one another by either extensive (lengthwise) strike-slip faults or basement horst structures. The Peta Gulf Syncline is one such sub-basin. The distribution of lithofacies, sedimentary structures and sedimentary environments are not necessarily the same within these sub-basins. Because most of these strike-slip faults remained active during the Early Cretaceous infilling of the subbasins, sedimentological patterns and lithofacies distribution were, to a large extent, affected by the faults. A summary of the stratigraphy of the Upper Benue Trough is presented in Fig. 2. Because there exists published work available on the stratigraphy of the Benue Trough (for example, Offodile, 1975; Petters and Ekweozor, 1982; Obaje et al., 1994, 19961, a detailed discussion of this aspect is not included here. The Zona U anomaly occurs within the Bima Sandstone Formation, the oldest sedimentary unit in the Upper Benue Trough (Fig. 2). The Bima Formation has three siliciclastic members (Guiraud, 1990); namely B,, B2and B, (Fig. 3). All three members do not necessarily outcrop in any given sub-basin. In the Peta Gulf Syncline, both the western and eastern margins are defined by faults (Fig. 3). However, because sediment supply was from east to west, most faults on the eastern margin have been obscured by sediments. The sub-basin is extensive along its strike but has a very limited width. This is a common feature of strike-slip fault controlled sedimentary basins as described by Reading (19801, Hempton and Dunne (1984) and Steel (1988). The boundary faults of the Peta Gulf Syncline are normal faults and strike-slip faults. The normal faults on the eastern boundary are locally sub-horizontal faults, whereas on the western boundary they are steeply dipping. Most of these normal faults have an
Host rock geology andgeochemistry of the Zona uranium occurrence, Peta
GulfSyncline, northeast Nigeria
6“
a)
12O
I crrr
E
‘Megatension gash’ mineralisetion Normal fault
0’
N
: ’ : Extensional Basin Cl Sedimentary cover cl .., A% Basement complex tzl
b)
lzlPeta Gulf Syncline
Figure 1. la/ Regional geology of the Benue Trough finset sketch map of Nigeria shows northeast-sduthwest-trending Benue Trough) and the position of the Pets Gulf Synclind: Ibl Benue Trough sub-basins resulting from north-south normal and northeastsouthwest strike-slip faults (after Benkhelil and Robineau, 1983).
Journal of African Earth Science
62 1
C.E. SUHet al.
Age
Tertiary
Formation
Litho.
Palaeo-environment
.. . ,. .. .. .. .. ... .. .. ..a. .. .. . . . . . : L_‘,‘,..
Continental
Formation
Gongola Arm
Yola Arm
Keri-Keri
Keri-Keri
.-.-. .-. ._.-. .-.d.
-. (fluviatilellacustrine)
. . . . ._ . . . . . . . . -a
. . .. .. .. .. .. Maastrichtian
Gombe
G om be
‘.* .‘:* .i* :
Continental/ Transitional (fluviatile,
lacustrine,
d Numanha
Cenomanian
q
_. _.
= =
-.-._
Sandstone Siltstone Claystone Shale
Transitional
Coal Continental
Pre-Albian
Basement
Complex
Ifr
311
Igneous/Metamorphic
-1 d
Limestone Granite, gneiss Unconformity Deformation
Fi@ue 2. Stratigraphical succession of the Upper Benue Trough. The Gongoia and Yola Arms are structural units in the Upper Benue Trough (after Petters and Ekweozor, 19821.
average north-south trend (N05E-N2OE) and are often associated with tear faults. These tear faults are subparallel to the regional northeast-southwest strike of the sub-basin and are relatively small-scale and localised strike-slip faults. The major boundary faults in the Peta Gulf Syncline Sub-basin are strike-slip faults, which trend northeast-southwest (Fig. 3). These faults are planar and vertical with distinct fault traces, which run uninterrupted for over 50 m in many instances, seen as straight lines in the field. The north-south normal faults often truncate these strike-slip faults. In some spectacular instances, the strike-slip faults are truncated by stepovers (areas where one en echelon fault ends and another with the same orientation begins). Evidence of strike-slip displacement parallel to the fault trace is common on the adjoining basement rocks. However, where most of these fault surfaces are exposed, striations indicating an appreciable amount of vertical displacement are also observed. Such oblique strike-slip/dip-slip
motion may be due to a geometric effect (Passchier and Williams, 1996) or may result from extension perpendicular to the strike trend of the sub-basin. Structural analysis of en echelon pegmatitic veinlets in the adjoining granitic basement show net extension in the northwest-southeast direction, perpendicular to the length of the sub-basin. This sub-basin is, therefore, of the pull-apart type and its evolution was accompanied by basement reactivation along strikeslip faults. Lithological units of the Peta Gulf Syncline and favourable environments of U concentration The general geology of the Peta Gulf Syncline Subbasin is depicted in Fig. 3. This sub-basin is filled by a spectrum of interrelated coarse to micro-conglomeratic sandstones, medium- to fine-grained sandstones, siltstones and purplish sandy shales and clayey beds. All three members (B,, B,, and BJ of the Bima Formation are present and a full description of these members is given in Table 1.
Figure 3. Structure and geology of the Peta Gulf Syncline Sub-basin, Upper Benue Trough.
622 Journal of Afrfcan Earth Sciences
Host rock geology andgeochemistry of the Zona uranium occurrence, Peta Gulf Syncline, northeast Nigeda
10°15' N
/
+
+
+
. . . . . *.
.
. B
Basalt
A-B Section shown in inset
Journal ofAfican
Earth SC/~
623
C.E. SUHet al. Table 1. Description and interpretation of B,, B, and B, members of Bima Formation, Upper Benue Trough Silicidastic members (Bima Formation) B3
BZ
Bl
Matrix-supported
Main rock types and characteristics
Well-sorted, well-rounded, distinctly bedded, uniform fine-grained ferruginised reddish-brown to yellowish arkosic sandstone alternating with more clayey to shaly, purplish to light grey, carbonaceous and pyritic beds. The sandstone grades rapidly into siltstones both laterally and vertically. Small-scale trough cross lamination is prevalent. Fining-upward trends towards the top. Lower parts have small-scale upward coarsening. Coarse- to medium-grained; rarely microconglomeratic sandstone with low angle bedding planes. Simple cross lamination is common and there is sporadic trough cross bedding. Beds have good lateral continuity. Red-dish to purplish silty beds are present, as well as claystone lenses. Sandstone beds get silty towards the top. Sequences are poorly developed and generally finingupward. Palaeocurrent direction is east-southeast-westnorthwest. Grain-supported coarse-grained conglomeratic to finer clast-supported open-framework microconglomeratic sandstones with internal scouring surfaces. Lenses of finer sandy material are present. Cross-bedding is rare. Poorly sorted with coarse-grained matrix. Lateral facies change fining from east-southeast to west-northwest. Upward-coarsening sequences are common with subordinate upward-fining sequences. Beds have poorly defined contacts and lack good lateral continuity. Palaeocurrent direction is east-southeast-westnorthwest.
conglomerates
commonly
found
in proximal fan environments and typified by debris flow deposits (Larsen and Steel, 1978; Collinson, 1986) are absent in B,. Sediments similar to B, have been described in other parts of the world (Steal and Thomson, 1983; Steel, 1988) and also interpreted to most likely represent medial fan detritus dominated by stream channel deposits. Steel and Thompson (I 983) further postulated that basin forming tectonism of increasing intensity can cause coarseningupward trends in sedimentary sections, whilst more stable tectonic conditions account for upward-fining trends. This interpretation adequately explains the spatial distribution of upward-coarsening and upwardfining trends within B, sediments of the Peta Gulf Syncline. Hare, in areas where distinct strike-slip fault surfaces are observed on the eastern margins of the sub-basin, the B, sediments have upward-coarsening trends, whereas areas not affected by these faults have upward-fining trends. Towards the top, B, sediments generally become siltier and grade into indistinguishable B,-B, transitional sediments.
624 Journal
of A f&an Earth Sciences
Interpretation (depositional environments1 Flood plains. Lacustrine sediments formed in distal lakes.
Full fluvial deposits. Silty and clayey pockets represent proximal lacustrine environments or small overbank deposits.
Medial fan detritus dominated by stream channel deposits without debris.
Based on the characteristics presented in Table 1, B, sediments are interpreted to represent full fluvial deposits, with the alternating coarse and fine layers representing strong variations of energy levels. Also, the presence of coarse-grained facies and poorly developed general fining-upward sequences in B2are interpreted to be the result of deposition in channels with low sinuosity. The fine member deposits of B, are typical of interchannel areas, which receive sediments during floods (Collinson, 1986). The areas may be floodplains or perennial swamps or shallow lakes. The small-scale upward-coarsening units probably record the infilling of shallow floodplain lakes by small deltas (Flores, 1981; Gersib and McCabe, 1981). The background radiometric readings for the various facies of B,, B, and B, described earlier are, on average, between 90 and 150 cps. The finer, muddy materials generally have higher background values. In sandstone-hosted U deposits, reduced claystones in lacustrine and deltaic environments often show a high background. For example, le Roux (I 991) has
Host rock geology and geochemistry of the Zona uranium occurrence, Peta Gulf S yncline, northeast Nigeria demonstrated that most of the U-bearing sandstones of the Karoo Basin were deposited by distal braided rivers with significant inputs from lacustrine delta deposits. Furthermore, le Roux (I 993) argues that if the permeability of the sandstone is too high, then the ore-bearing fluids would pass through too rapidly without sufficient time for U precipitation. Conversely, predominant fine- to very finegrained (relatively impermeable) sandstones are good host rocks. Following these arguments, B, is definitely not a good host rock for U in the Peta Gulf Syncline, owing to its high permeability and absence of fine sediments. B2offers moderate potential. B, best satisfies the sedimentological controls for U precipitation in sandstones: sandwiching of sandstones and claystones of reduced and oxidised facies, organic (lacustrine-type) matter and moderate permeability. The fine sediments would create permeability barriers, necessary impediments to fluid-flow, so that U can precipitate. The only significant U occurrence so far discovered in the Peta Gulf Syncline is the Zona U anomaly (Figs 3 and 4) with radiometric readings of 600- > 2500 cps (Sub, 1997; Suh et a/., 1998). The Zona anomaly occurs within B,/B, transitional fine-grained sand-
0
stones. The mineralisation is confined to fault trends with distinct alteration zones (Fig. 4). Therefore, even if the U was derived from B, sediments, its concentration was not only controlled by sedimentological factors but also by post-depositional deformation. Frac-ture/fault zones within B, and B,/B, transitional sequences are favourable sites for U concentration in response to fluid circulation initiated by this deformation within the Peta Gulf Syncline Sub-basin. The ore was epigenetically remobilised from within the sediments and concentrated along these open structures.
GEOCHEMICAL INVESTIGATION: ZONA U ANOMALY samplepreparatim8ndanalytiwlprocedum Carefully chosen samples representing the entire spectrum of fresh to altered sandstones from the various alteration zones of the Zona anomaly (Fig. 4) were used. The choice of the samples to be analysed was guided by careful petrographic examination of thin sections, integrated with field data, fabric development and radiometric levels of the whole
200 m
cl
Fresh, unbrecciated, coarse-grained sandstone
cl‘,O
Brecciated, red ferruginised zone
D-
Brecciated, silicified zone
:<
Uranium mineralisation (visible) Fault/fractures Facies boundary (radiometric) Topographic contour line
F&we 4. Structural pattern and deformation/a/t ‘eradon zones of the Zona U anomaly (after Suh et al., 19981.
Joumal of African E&h Sciences625
CL SUH et al. Berlin, Germany, was used following the techniques of Norrish and Chappel(1977). Errors obtained for the pressed powder pellet analyses were high, especially for the major elements ( f 2 wt% at 20) as opposed to data obtained from the fused discs. Thus, the fused glass disc data were regarded as ‘secure data’ and are used in this article. With these data the errors at 2u were generally < f 1.05 wt% for major elements and f 0.5 ppm for trace elements. Also, to check the randomly selected samples (powders and/ or consolidated rock samples), further analyses were done using particle induced y-ray emission (PIGE), energy-dispersive X-ray detector (EDAX) and particle induced X-ray emission (PIXE) (Sub, 1997). Evaluation
sample set. The results of these earlier studies (petrography, field occurrence, fabric and radiometry) are presented elsewhere (Sub et al., 1998). Each sample, weighing l-3 kg, was crushed using standard jaw-crushing techniques and subsequently milled to 543pm powder in a W carbide coated mill at Obafemi Awolowo University, Ile-lfe, Nigeria. 40 f 0.1 g of each powdered sample were carefully weighed into precleaned Teflon@sample bottles and tightly closed. Pressed powder pellets and fused glass discs made from these samples were analysed for major and trace elements. An X-ray flourescent (XRF) unit at the Geochemistry Laobratory, Technical University of
Table 2. Major, trace and REE concentration of representative samples from the various alteration zones of the Zona U anomaly MAJOR ELEMENTS
(wt%)
Fresh sandstrme SZI Sd
SZ3
sz4
SZ5
SZS
SZ7
silEihd SZS
SZS
zone Sr10
fall
66.50
0.24
0.35
0.13
14.00
10.4
IS.80
7.56
72.90
79.40
74.50
74.00
72.60
76.50
60.60
67.03
63.16
0.01
0.41
0.15
0.46
0.70
0.17
0.43
0.26
0.26
0.21
0.33
A124
17.76
19.61
16.40
17.20
9.00
11.30
hQ3
0.76
0.62
1.35
MnO
0.67
0.32
wJ CeO
0.33 0.01
0.20 0.38
Na2C
0.10
2.30
1.30
KzC
3.64
3.35
4.50
14.70
16.40
IS.80
18.30
sz14
77.40
72.80
ma
SZl3
sr12
77.Qo
Sio,
66.B
0.32
1.12
2.00
1.30
0.11
0.62
0.01
1.30
1.6
2.30
3.30
-
0.77
0.51
0.66
0.41
0.44
0.32
0.01
0.34 0.35
0.21 0.25
0.71 0.31
0.04 0.22
0.21 0.51
0.05 0.04
0.06 0.19
-
0.03 0.06 0.11
0.08 0.02 0.01
0.25 0.04 0.15
0.22 0.01 0.02
3.20
2.20
2.10
3.40
0.25
0.32
0.20
0.20
0.35
-
0.16
0.11
2.40
3.21
2.x)
2.12
0.13
-
-
0.04
-
0.35
P&5
0.02
-
0.12
-
0.01
0.30
0.33
0.05
0.03
0.06
0.12
-
0.11
LOI
0.21
0.01
0.01
0.50
0.65
0.50
0.20
0.51
0.41
0.77
0.34
0.3
0.51
0.64
101.60
loo.20
69.50
Qs.62
99.50
99.61
loo.40
94.65
101.50
99.56
loo.17
loo.60
101.20
TOtal
MINOR/TRACE
ELEMENTS
Fresh sandstone SZl Sr2
sz3
sz4
Sr5
SZ6
sr7
SZ6
.siliiiIi6dzom SZQ SZlO 17 54
35
56
30
11
52
7 41 3
41 19 5
15 16 6
6 10
20 19
7 6
12 4
1 13
3 3
4 2
5 -
4 -
320
141
164
108
113
Sr Se V Cr
369 63 33 10
167 78 41 18
166 26 33 14
352 68 54 32
189 215 36 13
166 71 32 16
142 23 55 10
232 66
98
62 315
41
29
CO Ni CU al AS
16 4 2
21 3 -
22 3 3 3
17 4 1 4
23 9 4 7
11 4 1 6
20 6 4 2
13 12 5
16 13 9
-_-_ -
2 2
Bi Cd -
1
-_
sz3
Sr2
Ga
13.0
11.0
Hg zr MO
0.3 549.0 3.0
0.2 3B.O 3.0
Sb Sfl Br
4.0
4.0
10.0
12.0
0.1 550.0 4.0. 5.0 1QO
sz5
Sr6
s27
zone SZIO
SZll
sz12
7.0
5.0
3.0
3.0
6.0
sz13 6.0
0.1 330.0 7.0
0.2 395.0 3.0
0.3 460.0
0.3 451.0
0.2 320.0
0.1 428.0 4.0
0.2 216.0 2.0
0.3 366.0 2.0
0.4 166.0 4.0
0.3 195.0 2.0
0.3 310.0 2.0
fir14 -
5.0
6.0
11.0 4.0
6.0 4.0
5.0 2.0
4.4
3.5
4.0
4.0
2.0
2.0
6.0
6.0
6.0
6.0
10.0
5.8 0.2
6.0 -
4.0 -
4.0 -
4.0 -
7.0 _
51.0 10.0 10.0
-
73.0
60.0
75.0
74.0
50.0
6.0 19.0
32.0 20.0
22.0 11.0
56.0 11.0
10.0
10.0
20.0
65.0 6.0 13.0
13.0
5.0
15.0
10.0
1340 7.0
47.0 6.0
140.0 9.0
234.0 6.0
201.0 10.0
256.0 6.0 13.0
262.0 9.0 15.0
159.0 9.0 6.0
26.0 20.0
24.0
25.0
15.0
16.0
10.0 4.0 66.0 10.0
10.0 4.0 54.0 10.0
13.0
11.0
19.0
30.0
73.0
60.0
27.0 14.0
20.0 16.0
6.0 11.0
W
177.0
140.0
135.0
40.0
234.0
Nb m
6.0 10.0
9.0 6.0
6.0 11.0
10.0 9.0
11.0 6.0
9.0 10.0
LS Nd Pr
23.0 15.0 10.0
21.0 13.0 10.0
23.0 15.0 10.0
20.0 31.0 10.0
16.0 25.0 10.0
20.0 13.0
Sm Ce
4.0 55.0
4.0 54.0
4.0 50.0
4.0. 53.0
4.0 91.0
SC Y
9.0 16.0
10.0 15.0
10.0 16.0
10.0 13.0
10.0 13.0
626 Journal of Afticon Earth Sciences
Siliiikid SZQ
SZ6 6.0
11.0 10.0
iOl:
3
6.0
6.0 11.0
Samples l-29;
3 2 -
6.0
367.0
Szl-SZ29:
1 3 -
6.0
U m
Pb
10 60 340
1
S&4 3.0
16
1
Freshsandstom, SZI
sr14
5 25 141
loo
3 -
sz13
sz12 21 42 78
lb
6
call 40 63 110
Rb
323
Ag
lW.12
(ppm)
loss on ignition.
17EO
9.0
10.0
15.0
9.0
10.0 4.0
15.0 10.0 4.0
20.0 14.0 10.0 4.0
25.0 15.0 10.0
27.0 36.0 11.0
25.0 15.0 10.0
55.0 9.0
B2.0 10.0
61.0 10.0
4.0 54.0
6.0 92.0
4.0 55.0
10.0 4.0 53.0
16:O
15.0
11.0,
9.0 13.0
10.0 13.0
10.0 12.0
10.0 15.0
10.0 21.0
Host rock geology andgeochemistry of the Zona uranium occurrence, Peta Gulf Syncline, northeast Nigeria a predominantly granitic basement (Banerjee and Bhattacharya, 1994). However, the fresh sandstones are poor in P,O, and TiO, and have low loss on ignition (LOU values (Table 2). For all the Zona samples, A&O, generally decreases with increasing SiO, (Fig. 5). Samples from the silicified zones, SZS-SZI 6 (Table 2) show elevated SiO, values ranging from 77.4-89.9 wt%. These samples also show a depletion in K,O and Na,O concentration (Fig. 51, as well as in AI,O, and P205consentrations. Conversely, there is a slight increase in Fe,O, and LOI values in the silicified zone (Fig. 5). Similar trends described above for the silicified zone are discernible in the hematite and goethite zones. In
of the paired analyses (XRF and EDAX/PIGE/PIXE) of the replicate samples showed no significant differences. Resultsand interpretation Major elements Results of major analysis of 29 representative samples from the Zona anomaly are provided in Table 2. The fresh sandstone samples are rich in SiO, and K,O but relatively poorer in Na,O. They are, thus, of the potassic sandstone type (Bhatia, 1983; Banerjee and Bhattacharya, 1994). These fresh sandstones are also poor in CaO and MgO, indicating the absence of carbonates. These chemical features are indicative of debris derivation from Table 2. continued MAJORELEMENTS siBiedzon.3 SZlB Sr15 SiOl
Hemathitiezone sz17 SZ16
na
79.95
78.22
69.25
0.B
0.26
0.46
0.35
0.25
0.13
0.06
0.07
0.71
0.13
OX5
0.62
14.30
12.10
15.00
13.65
14.00
3.76
3.98
2.40
2.05
5.57
3.98
3.15
4.59
2.30
10.10
5.23
9.40
13.40
10.00
2.77
3.51
5.40
5.24
11.10
5.15
4.11
10.01
0.03 0.07 0.22
0.11 0.03 0.01
0.35 0.01 0.16
0.31 0.05 0.13
0.25 0.04
0.30 0.07 0.01
0.02 0.01 0.01
0.02 0.01 0.01
0.02 0.01 0.01
0.02 0.01 0.01
0.02 0.01 0.16
0.01 0.01 0.07
0.W 0.01 0.01
0.02 0.11 0.15
0.34
0.41
0.13
0.12
0.31
0.19
0.16
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.56
0.01
0.12
0.33
0.12
0.11
0.42
0.15
0.42
0.46
0.36
011
0.13
0.33
0.44
0.41
-
0.67
0.28
0.29
0.04
0.31
0.19
0.03
0.04
0.02
0.12
0.03
0.05
0.13
1.00
3.31
4.46
2.10
2.46
1.59
1.04
99.14
99.96
99.94
90.46
1.12 9993
3.34 59.57
2.11 loo.66
1.10 loo.14
3.36 99.39
0.67
0.15
0.17
17.10
2.20 0.04 0.06 0.31
0.91
1.10
0.30
loo.70
101.36
100.17
60.96
lW.60
9907
99.20
MlNOR/lRACEELEMENTS(ppm) Hemathiiezone SZl6
5217
SZ16
1
Godkte zcm - miner&red
SzlQ
Sz20
sz21
5224
SZ23
sz25
szm
sz27
sz?B
55 94 108
12 150 614
10 155 359
12 160 623
10 95 514
14 101 604
15 173 327
26 105 126
27 94 146
25 73 138
gs 150
6 163 133
12 95 131
13 113 168
6 5
25 13
27 15
30 6
31 9
25 10
22 12
31 11
10 10
15 10
15 6.5
22 9
11 10
11 10
18 Q
12
21
16
4
2
3
43 5 1
64 4 5 11
45 6 3 29
52 9 5 12
74 3 3 -
71 5 5 -
61 1 2 -
9
37 6 10 16 27
55 3 7
4
61 3 3 --_ 10
11 3 4
7
27 1 9 3 24
-
25 3 1.2 2
22 2.5
13 -
27 -
22 -
3
3 2 0.9
16 3
1
-
-
sz24 3.0
sz26 1.0
~Hemathiizone sz17 SZ16
-I
0.2 225.0
0.3 330.0
-
3.0
4.0
20.0 11.0 152.0 10.0 10.0 25.0 31.0 10.0
13.0 133.0 6.0 15.0 21.0 25.0 10.0
Sill Ca SC Y
4.0 51.0 10.0 13.0
4.0 91.0 10.0 12.0
Szl-Sz29:
Samples
-
Pb
-__ 1
l-29;
2.5
sz19
sz20
6.0
3.0
4.0
0.3 215.0 3.0 2.7
4.0 160.0 3.0 2.0
0.2 210.0 2.0 0.9
3.5
6.0
4.0
sz27 3.0
SrzB 20
sz29 20
0.2
0.1
0.;
0:;
193.0 2.0 4.4
130.0 2.0 2.7
131.0 2.0 2.2
370.0 295.0 17.0 316.0 3.0 6.0
7.0 159.0 16.0 513.0 2.5 4.0
4.1 1.7 2.0 369.0 14.0 650.0 3.0 1.5
6.4 4.2 15.0 162.0 15.0 5990 3.0 3.0
6.3 0.3 3.0 346.0 3.0 EQ9.0 3.0 5.0
6.4 0.2 9.0 546.0 4.0 599.0 2.0 3.0
6.0 2.0 660.0
loo.0 2.5 2.7 4.4 0.3 3.0 547.0
4.0 513.0 3.0
3.0 608.0 3.0
20.0 9.5 10.0 4.0 30.0
19.0 6.5 10.0 3.5 16.0
16.0 10.0 10.0 4.0
16.0 6.5 10.0 4.0 20.0
2.0 16.0 15.0 10.0
20.0
15.0 7.0 10.0 4.0 16.0
4.0 m.0 9.0 10.0
20.0
12.0 10.0 4.0 16.0
16.0 31.0 10.0 5.0 60.0
10.0 6.0
10.0 6.0
11.0 14.0
10.0 10.0
9.5 10.0
10.0 6.0
10.0 10.0
10.0 9.0
4.0 16.0 10.0
4.0 16.0 10.0
4.0
4.0 16.0 10.0 6.0 on
sz26 3.0
0.2
73.0
loss
4.0
67.0 2.0 3.3
289.0 4.0 607.0 2.0 6.0 15.0 13.0 10.0
LOI:
sz23
0.1
6.0
16.0 9.0 6.0
7.0
97.0 4.0 2.9
192.0 10.0 270.0 2.0 4.0
10.0
3.01
0.1
50.0
20.0
1 sm
137.0 2.0 3.6 4.5
297.0 4.0 513.0 2.5 5.0 20.0
16.0 15.0 10.0 4.0
sz21
0.2
24.0
10.0 10.0
-
-_
216.0 7.9 2.0 10.4
269.0 6.0 316.0 3.0 5.0 16.0 13.0 10.0
20.0
10 33
lGoethitezone- mineded
1
2.0
9
30
sz26
22 41 121
4.0
Pr
sz2s
91.20
91.00
5.0
Nd
sz26
90.01
2.0
U Th w Nb In La
sz27
90.23
0.2 195.0 2.0 3.6
WI sr
5226
90.29
SiMiedron6 Sr16 sz15
Zr MO Sb
St25
71.40
74.03
5x15
I
Goethiizone-minerdiwd sz22 s?23 s.724
sz21
E8.53
60.00
Siizone
Ga
s7.20
70.00
60.20 lJ.M
Rb Sr Ba
sz19
4.0
0.3 150.0 3.0 3.2 6.0 54.0 270.0 3.0 310.0 3.0 5.0
20.0
9.0
6.0,
ignition.
Journal of Afrfwn Earth Sciemzes 627
C. E. SU. et al. 20 ,
0 CvlO ’
1.00
0
l
(d)
0”
-
0
8-
0
6 4-
+
2-r.
I.,
64
.
68
,
72
14.
.
76
SiO,
0
+ ,
.
80
, 84
.
+++ ,t+ 88
0.00
92
4.00 ~3.50 o 3.00 -2.50 3 OJ.00
+
8 -
0” $’ IL
6 -
64
A
A
+
m 1.50 z 1.00 0.50 0.00 -0.50 -1.00
A
68
72
68
72
76
80
SiO 2(Wt
84
88
92
0.00
-
SiO 2(Wt %)
,
a&$
84
88
92
84
88
92
0
‘4
j -
* A
.A
A
&
0
Qo&
. , . , . , . , . , . , . , . ( 64
68
72
3 -
76
g e
2.50
yR
2.00 1.50 1.00
-
0.50 0.00
-
80
84
88
92
(Wt %)
0
(f) 0
00 00
-
A
A
0
A
QA
GOa
A-0
-
-1 .oo
80
80
l
5.00 4.50 4.00 3.50 3.00
-0.50
76
.
,
(e)
%)
1
72
r
76
SiOp
(c)
68
,
-
60
1 .oo
64
t
5.00 4.50 -
(b) A
. A,
SiO 2(Wt %)
A
-
E
64
+
.’ ,
(Wt %)
12 s&O
7
(a)
,
60
64
68
72
-
I-
I.
76
80
I
84
.(
.I
88
92
Sio;! (Wt %)
Figure 5. Harker-type variation diagrams for the Zona samples. 0: Fresh sandstone; 0: altered sandstone lsilcified zone); A: altered sandstone (hematite zone); +: altered sandstone Imineralised goethite zone).
628 Journal of African Earth Sciences
Host rock geology andgeochemistry of the Zona uranium occurrence, Peta Gulf Syncline, northeast Nigeria
5.00 4.50 4.00 3.50 g 3.00 2.50 3 - 2.00 2 1.50 1.00 0.50 0.00 -0.50 -1.00
7
(g) A ++ A + + A + $ ++o :O: , . I . 1 . , . , . , . , . , . , 60
64
68
72
76
80
84
88
92
SiO2 (Wt %) Fipre 5. Continued. Harker-type variation diagrams for the Zona samples. LOI: Loss on ignition; l : fresh sandstone; 0: altered sandstone Isilcified zone); A: altered sandstone (hematite zone); +: altered sandstone Imineralised goethite zonel.
fact, SiO, concentration in the goethite zone attains a maximum of 91.2 wt%, while Fe,O, reaches 13.4 wt% in the hematite zone (Table 2). These features suggest that fluid-rock interaction, which led to the precipitation of U, also induced remobilisation of Fe,O, and SiO, and later concentrated them in the mineralised zone. Because of their overall low alkali content and high Fe,O, contents, the sandstones of the hematite and goethite zones belong to the ferromagnesian potassic sandstone type of Bhatia (1983). LO1also increases in the altered samples (Fig. 5), implying that neo-crystallisation led to the formation of predominantly hydrous mineral phases such as chlorites, micas, clay minerals and hydrated Fe compounds. This is in agreement with petrographic results (Sub, 1997). The hematite and mineralised goethite zones show significant depletion in Na,O, CaO, MgO and K,O (Fig. 5). These elements are relatively very mobile in the supergene environment (Hawkes and Webb, 1962; Orajaka, 1981). Trace and minor elements The results of trace and minor elements are also provided in Table 2; the trace elements worthy of close examination are Rb, Ba, Sr, V, Co, As, Zr, Pb, Th, U and W. All the other elements yield rather nonsystematic results as far as elemental zoning patterns are concerned. Rb, Sr, VandZr. Rubidium and Sr, on the whole, show a pat-tern of decrease from the unaltered fresh
sandstone zone to the central mineralised goethite zone (Table 2). Uranium concentration increases with decreasing Rb and Sr concentration. For example, Rb concen-tration in the fresh sandstone ranges from 108-328 ppm, whereas the range in the goethite zone is 6-30 ppm (Table 2). Riese et a/. (I 978) noted that the Rb released during rock disintegration is often removed quickly by solution. This mechanism most likely operated in the Zona anomaly to effect such systematic Rb and Sr dispersion patterns. Vanadium and Zr also show depletion with increasing U concentration. That Zr is mobile in the Zona anomaly is significant because it provides evidence of leaching and chemical mass transfer processes. The mobility of Zr in the Zona anomaly indicates that the envi-ronment was acidic, since the solubility of Zr is restricted to the pH range of 3-6.5 (Colin et al., 1993). Ba, Co, W, As and Pb. These elements are enriched in the mineralised goethite zone (Table 2). Barium is often associated with sandstone-type U deposits and it is concentrated in the ore zone, e.g. Wyoming deposits USA (Riese et al., 1978). High concentrations of Ba in the ore zone may be attributed to Ba fixation as BaSO,, indicative of cation exchange reactions during the precipitation of U. Fischer (1970) reported high Co concentrations within the mineralised zone of the Colorado and Wyoming sandstone-type U deposits. The Zona samples also show elevated Co levels in the goethite zone (Table 2). Cobalt in the supergene zone may be absorbed by Fe-based hydroxides (Fischer, 1970; Olorunfemi, 1984). This is by far the most common mechanism of Co precipitation in the surficial environment. A similar mechanism of Co enrichment was probably involved for the Zona mineralised zone. Tungsten concentration in the goethite zone is also higher than in all the other zones. Arsenic concentration in the Zona samples is higher in the hematite and goethite zone than in the fresh sandstones. Arsenic is commonly associated with sandstonetype U deposits (Miller et a/. , 1984). Indeed, As, in terms of abundance ratios, is one of the most important intrinsic elements in these deposits (Shoemaker et al., 1959). In sediments, As is known to be precipitated in hydrolysate and oxidate states (Riese et al., 1978) by adsorption on ferric hydroxides. Because the hematite and goethite zones of the Zona anomaly are rich in As, this element, therefore, was most probably adsorbed on these Fe-bearing minerals. This evidence in support of adsorption reactions confirms the epigenetic origin of the Zona anomaly and the intrinsic genesis of U. Lead exhibits similar dispersion patterns to As and the mechanisms
Journal of African Earth Sciences 629
C. E. SlJH et al. governing its mobility are the same as those discussed above for As. REEs andhalogens. The concentrations of REEs in all the alteration zones of the Zona anomaly are quite low. Also, no zone shows any REE enrichment over the other (Table 2). There is, however, one element, Ce, which shows a relatively significant depletion in the mineralised goethite zone (Table 2). While conceding that REE enrichment in sandstone-type U deposits is rare, Riese et al. (1978) noted that Ce and Eu often show distinct depletion in the ore zone. Of all the REE, only Ce and Eu exist in the + 4/ + 3 and + 3/ + 2 oxidation states, respectively. All the others are + 3. This duality in oxidation states has variable influences on their mobility over other REE, based on the redox potential of the system. The conclusion to be drawn, therefore, is that Ce underwent an open system depletion at the Fe redox boundary, contemporaneous with U precipitation. Cerium depletion may, therefore, be a useful index to identifying such Fe redox boundaries, hence enhancing exploration efforts in the Peta Gulf Syncline.
DISCUSSIONAND CONCLUSIONS The Peta Gulf Syncline Sub-basin is of the pull-apart basin class. Pull-apart basins dominated by strikeslip faults often show organisation of facies into marginal fault-bounded fanglomerates/conglomerates and central flood basin/playa/lacustrine deposits (Hempton and Dunne, 1984). In the Peta Gulf Syncline, the facies organisation defines a spectrum of progressive fining from the eastern margin conglomerates to fine sandstones/siltstones on the western margin. This suggests that the strike-slip/normal faults on the eastern margin had low angles and were tectonically active during sedimentation. Several authors (e.g. Ganz, 1987; Wernicke and Axen, 1988; Bosworth, 1994) have demonstrated that such low angle boundary faults in extensional basins can be developed from initially steeply dipping faults by rotation. Such rotational motion on the eastern margin of the Peta Gulf Syncline during sedimentation most likely influenced the facies pattern within the basin. The steep faults of the western margin were dormant throughout this process. Because the individual facies have a limited lateral extent with restricted deposition of locally derived microconglomerates, sedimentation in a fault-bounded rapidly subsiding basin is indicated, as noted also by Reading (1980). Three members of the 8ima Formation are discernible within this sub-basin; from bottom to top: B, (microconglomerates, no fines); B, (coarse-grained
630 Journal of African Earth Sciences
sandstones; minimal fines) and B, (alternating finesandstones and siltstones/claystones). B, sediments are inferred lacustrine deposits and offer the best lithofacies favourable for U precipitation. However, the Zona U anomaly within this sub-basin seems to be controlled more by structural geology, rather than solely by sedimentological aspects. Therefore, even if U was disseminated in B, sediments, its concentration to yield the Zona U occurrence was enhanced by postdepositional deformation. In terms of whole rock geochemistry of samples collected from the Zona anomaly, the fresh sandstones are of the potassic sandstone-type and are poor in carbonates, indicated by the low levels of CaO and MgO in them. The CaO and MgO concentration in all the altered zones is also very low. This supports the absence of carbonates inferred from the petrographic examination (Sub, 1997) and suggests that U was not transported in the hydro-thermal fluid as soluble carbonate complexes. Uranium has a tendency to form complexes with a large number of naturally occurring anions. These complexes greatly enhance the solubility and mobility of U in low temperature and pressure secondary environments (le Roux, 1993). Langmuir (1978) indicated that for U complexes to attain significant concentrations in groundwater, pH must be C 7. Furthermore, carbonate complexes are important U complexing agents in alkaline environments (pH > 7) and not in acidic environments (Langmuir, 1978). The absence of carbonates around the Zona anomaly, coupled with the mobility of Zr in this region, are good indications that the environment was acidic during the U precipitation stage. With progressive deformation, the altered Zona sandstones have variable degrees of SiO, and Fe,O, enrichment. In addition to Fe,O,, Ba, W, As, Co and Pb have anomalously high concentrations in the mineralised goethite zone. This zone is significantly impoverished in the alkalis. These elemental dispersion patterns show clearly that chemical leaching and mass transfer were significant processes in the formation of this U anomaly. The Zona U anomaly is hosted by a brecciated and fractured sandstone and it is therefore of the ‘postfault’ ore type. This class or type of U deposit is characterised by discordant relations with host rock primary features (bedding planes, stratigraphical boundaries) and spatial association with faults (Ludwig and Simmons, 1984). They are also termed joint controlled sandstone-type U deposits (Allen and Thomas, 1984) marked by concentration of U along brittle discontinuities. The Zona anomaly, therefore, is not an unconformity type deposit, sensu stricto. The source of the U in post-fault ore types is principally primary ore bodies that have been destroyed
Host rock geolog y and geochemistry of the Zona uranium occurrence, Peta and redistributed by groundwater. However, for the Zona anomaly, the U comes from detritus in the host sediments, derived from the adjoining granitic basement terrain. Uranium mineralisation occurs within this basement and can be regarded as a provenance for uraniferous debris during the deposition of the Bima Sandstone. The Quirke ore zone, Ontario (Robinson and Spooner, 1984) is a classic example of a U deposit with detrital uraninite in the host sediment. In the Quirke ore zone, the ore concentration process was initiated by complete fluid-mediated leaching of Fe, SiO,, U, Th and Y (Robinson and Spooner, 19841, followed by the precipitation of secondary quartz and sericite in pore spaces, as well as the precipitation of U in favourable structures. Mineral alteration studies of the Zona U anomaly (Sub et al., 1998) have provided evidence that U concentration is associated with hydrous Fe compounds and feldspar dissolution and alteration to kaolinite. This supports the epigenetic origin of this anomaly. The main geochemical pathways envisaged for the deposition of U at Zona can, therefore, be summarised as follows: i) Uranium is leached from detrital material within the sandstone host rock by fluids, possibly heated, and slightly acidic connate and groundwater affinity. The fluid was probably heated by simple diagenetic compaction, though renewed tectonic movement along the fractures and faults probably supplied a lot of frictional heat. i) Groundwater moving along the brittle openings transported U in the form of uranyl complexes in association with Fe, As, Ba and W. Such uranyl complexes often bear carbonates (for example, Miller et a/. , 1984; Smith, 1990). However, because carbonates are not present in the Zona anomaly, these uranyl complexes were not carbonate-based. i) Interaction of these groundwaters with organic matter (especiallyin B3sediments)and possiblysulphii, led to freed uranyl cation (subsequently oxidised by alkalis, Na+, K+, Ca2+, etc) -bearing solutions to form insoluble secondary U minerals. Autunite is the principal U mineral at Zona. Bariumconcentrations are high in the mineralised samples as discussed earlier, and this is attributed to fixation of Ba as BaSO,. This underlines the role of sulphides in the U precipitation, as these sulphides were oxidised to sulphates. Editorial handling - P. Eriksson
REFBtENcEs Allen, RF., Thomas, R.G., 1984. The uranium potential of diageneticaily altered sandstones of the Permain Rush Springs Formation, Cement district, Southwest Oklahoma. Economic Geology 79, 284-296.
GulfSyncline, northeast Nigeria
Banerjee, D.M., Bhattacharya, P., 1994. Petrology and geochemistry of greywackes from the Aravalli Supergroup, Rajasthan, India, and the tectonic evolution of a Proterozoic sedimentary basin. Precambrian Research 67, 1 l-35. Benkhelil, J., 1989. The origin and evolution of the Cretaceous Benue Trough (Nigeria). Journal African Earth Sciences 8, 251-282. Benkhelil, J., Robineau, B., 1983. Le fosse de la Benoue est-il un rift? Bulletin Centres Recherches Exploration Production Elf-Aquitaine 7, 315-321. Bhatia, M.R., 1983. Plate tectonics and composition of sandstones. Journal Geology 91, 61 l-627. Bosworth, W., 1994. A model for the three-dimensional evolution of continental rift basins, north-east Africa. Geologische Rundschau 83, 671-688. Colin, F., Alarcon, C., Vieillard, P., 1993. Zircon: an immobile index in soils? Chemical Geology 107, 273276. In: Reading, Collinson, J.D., 1986. Alluvial sediments. H.G. (Ed.), Sedimentary Environments and Facies. Blackwell Scientific Publications, Oxford, pp. 20-82. Dahlkamp, F.J., 1978. Classification of uranium deposits. Mineralium Deposita 13, 83- 104. 1987. An introduction to Ore Geology. Evans, A.M., Blackwell Scientific Publication, Oxford, 358~. Fischer, R.P., 1970. Similarities, differences and some genetic problems of the Wyoming and Colorado plateau types of uranium deposits in sandstones. Economic Geology 65, 778-784. Flores, R.M., 1981. Coal deposition in fluvial palaoenvironments of the Paleeocene Tongue River Member of the Fort Union Formation, Powder River area, Powder River Basin, Wyoming and Montana. In: Ethridge, F.G., Flores, R.M. (Eds.), Recent and Ancient Non-marine Depositional Environments: Models for Exploration. Society of Economic Palaeontology and Mineralogy, Special Publication, pp. 169- 190. Ganz, P., 1987. An open system, two-layer stretching model for the eastern Great Basin. Tectonics 6, l-l 2. Gersib, G.A., McCabe, P.J., 1981. Continental coal-bearing sediments of the Port Hood Formation (Carboniferous), Cape Linzee, Nova Scotia, Canada. In: Ethridge, F.G., Flores, R.M. (Eds.), Recent and Ancient Non-marine Depositional Environments: Models for Exploration. Society of Economic Palaeontology and Mineralogy, Special Publication, pp. 95-l 08. Guiraud, M., 1990. Tectono-sedimentary framework of the Early Cretaceous continental Bima Formation (Upper Benue Trough, N.E. Nigeria). Journal African Earth Sciences 10, 341-353. Hawkes, H.E., Webb, J.S., 1962. Geochemistry in Mineral Exploration. Harper and Row, London, 416~. Hempton, M.R., Dunne, L.A., 1984. Sedimentation in pullapart basins: active examples in Eastern Turkey. Journal Geology 92, 513-530. Langmuir, D., 1978. Uranium solution-mineral equilibria at low temperatures with applications to sedimentary ore deposits. Geochimica Cosmochimica Acta 42, 547-567. Larsen, V., Steel, R.J., 1978. The sedimentary history of a debris flow-dominated, Devonian alluvial fan-A study of textural inversion. Sedimentology 25, 37-59. Le Roux, J.P., 1991. Flume experiments on permeability and organic matter as related to the genesis of uranium deposits in the Beaufort Group. South Africa Journal Geology 94, 212-219. Le Roux, J.P., 1993. Genesis of stratiform U-MO deposits in the Karoo Basin of South Africa. Ore Geology Reviews 7, 485-509.
JwmalotAttican
Earth Sciences 63 1
C. E. SW et al. Ludwig, K.R., Simmons, K.R., 1984. U-Pb isotope systematics and apparent ages of uranium ores, Ambrosia Lake and Smith districts, Grants Mineral belt, New Mexico. Economic Geology 79, 322-331. Miller, R.W., Wanty, R.8.. Mchugh, J.B., 1984. Application of mineral solution equilibria to geochemical exploration for sandstone-hosted uranium deposits in two basins in West Central Utah. Economic Geology 79, 266-283. Nash, J.T., Granger, H.C., Adams, S.S., 1981. Geology and concepts of genesis of important types of uranium deposits. Economic Geology, 75* Anniversary Volume, 63-l 16. Norrish, K., Chappel, B.W., 1977. X-ray fluorescence spectrometry. In: Zussman, J. (Ed.), Physical Methods in Determinative Mineralogy. 2”6 Edition, New York, Academic Press, pp. 201-273. Obaje, N.G., Funtua, I.I., Ligouis, B., Abaa, S.I., 1996. Maceral association, organic maturation and coalderived hydrocarbon potential in the Cretaceous Awgu Formation, Middle Benue Trough, Nigeria. Journal African Earth Sciences 23, 89-94. Obaje, N.G., Ligouis, B., Abaa, S.I., 1994. Petrographic composition and depositional environments of Cretaceous coals and coal measures in the Middle Benue Trough of Nigeria. International Journal Coal Geology 26, 233-260. Offodile, M.E., 1975. A review of the geology of the Benue Trough, Nigeria. Journal African Earth Sciences 3, 283291. Olade, M.A., 1975. Evolution of Nigeria’s Benue Trough (Aulacogen): a tectonic model. Geological Magazine 112, 575-581. Olorunfemi, B.N., 1984. Factors influencing the geochemistry of lateritic soils in the greenschist amphibolite metatectonic belt, South Western Nigeria. Chemical Geology 42, 261-275. Orajaka, I.P., 1981, Mineralogy and uranium geochemistry of selected volcaniclastic sediments in the Western United States-an exploration model. Ph.D. thesis (unpubl.), Univerity of Texas, El Paso, 365~. Passchier, C.W., Williams, P.R., 1996. Conflicting shear sense indicators in shear zones: the problem of non-ideal sections. Journal Structural Geology 18 (IO), 1281- 1284. Petters, C.W., Ekweozor, C.M., 1982. Petroleum geology of the Benue Trough and southeastern Chad Basin, Nigeria. American Association Petroleum Geologists, Bulletin 66, 1141-I 149. Rackley, R.I., 1976. Origin of western states-type uranium mineralisation. In: Wolf, K.H. (Ed.), Handbook of StrataBound and Stratiform Deposits 7. Elsevier, Amsterdam, pp. 89-156. Reading, H.G., 1980. Characteristics and recognition of strike-slip fault systems. International Association Sedimentologists, Special Publication 4, 7-26. Rebelle, M., 1990. The marine transgression in the Benue Trough (northeast Nigeria): a palaeogeographic interpretation of the Gongila Formation. Journal African Earth Sciences 10, 643-656.
632 Journal of Afrfcan Eanh Sciences
Riese, W.C., Lee, M.J., Rookins, D.G., Della-Valle, R., 1978. Application of trace element geochemistry to prospecting for sandstone-type uranium deposits. In: Watterson, J.R., Theobald, P.K. (Eds.1, Geochemical Exploration 1978. Proceedings of the 7* International Geochemical Exploration Symposium, AEG, pp. 47-64. Robinson, A., Spooner, E.T.C., 1984. Post-depositional modification of uraninite-bearing quartz-pebble conglomerates from the Quirke ore zone, Elliot Lake, Ontario. Economic Geology 79, 297-321. Sherborne, J.E., Buckovic, W.A., de Witt, D.B., Hellinger, T.S., Pavlak, S.J., 1979. Major uranium discovery in volcaniclastic sediments, Basin and Range Province, Yavapai County, Arizona. Bulletin American Association of Petroleum Geologists 63, 821-648. Shoemaker, E.M., Miesch, A.T., Newman, W.L., Riley, L.B., 1959. Elemental composition of the sandstone-type deposits. In: Garrels, R.M., Larsen, E.S. (Eds.), Geochemistry and Mineralogy of the Colorado Plateau Uranium Ores. US Geological Survey, Professional Paper 320, 2554. Smith, R.M.H., 1990. A review of stratigraphy and sedimentary environments of the Karoo Basin of South Africa. Journal African Earth Sciences 10, 117-l 38. Steel, R.J., 1988. Coarsening-upward and skewed fan bodies: symptoms of strike-slip and transfer fault movement in sedimentary basins. In: Nemec, W., Steel, R.J. (Eds.), Fan Deltas: Sedimentology and Tectonic Settings. Blackie and Son, pp. 75-83. Steel, R.J., Thompson, D.B., 1983. Structures and textures in Triassic braided stream conglomerates (‘Bunter’ Pebble Beds) in the Sherwood Sandstone Group, North Staffordshire, England. Sedimentology 30, 341-367. Suh, C.E., 1997. Structural, geochemical and fluid-rock interaction studies of uranium mineralisation in N.E. Nigeria: Implications for uranium metallogenesis and exploration. Ph.D. thesis (unpubl.), Abubakar Tafawa Balewa University, Bauchi, Nigeria, 221 p. Suh, C.E., Dada, S.S., Ajayi, T.R., Matheis, G., 1998. Integrated structural and mineral alteration study of the Zona uranium anomaly, northeast Nigeria. Journal African Earth Sciences 27 (I), 129-140. Turner, B.R., 1978. Sedimentary patterns of uranium mineralisation in the Beaufort Group of the Southern Karoo (Gondwana) basin, South Africa. Canadian Society Petroleum Geologists Memoir 5, 831-848. Visser, J.N.J., 1989. Discussion on U-Pb isotope systematics, ages and genesis of Karoo uranium deposits, South Africa. South African Journal Geology 92, 466469. Wernicke, B., Axen, G.J., 1988. On the role of isostacy in the evolution of normal fault systems. Geology 16, 848851. Zielinsky, R.A., 1981. Experimental leaching of volcanic glass: Implications for evaluation of glassy volcanic rocks as sources of uranium. American Studies in Geology, Association Petroleum Geologists 13, 1- 12.