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Integrated structural and mineral alteration study of the Zona uranium anomaly, northeast Nigeria
Journal of African
Vol.
Earth Sciences.
All rights
Pll:SO899-5382(98)00051-7
27, No. 1, pp. 129-l 40, 1998 o 1998 Elsaviar Science Ltd reserved. Printed in Great Britain 0899~5362/98 $19.00 + 0.00
COMMUNICATION
Integrated structural and mineral alteration study of the Zona uranium anomaly, northeast Nigeria C. E. SUH,’ S. S. DADA,’ T. R. AJAY12 and G. MATHEIS3 ‘Geology Programme, Abubakar Tafawa Balewa University, Bauchi, Nigeria 2Department of Geology, Obafemi Awolowo University, Ile-lfe, Nigeria 3Technische Universitat, Berlin, Serkr, BH4, D-l 0587, Berlin, Germany
Abstract-The Zona U anomaly is a sandstone-hosted anomaly which is structurally controlled. Three main zones of mineral alteration are recognised, namely a silicified zone; a red-brown ferruginised (hematite) zone and a brownish/yellowish-brown ferruginised (goethite) zone. These zones are typified by brecciation and kaolinitisation. The U mineralisation is epigenetic in origin and post-dates the main tectonic deformations. Uranium was leached from the Basement Complex granites and syngenetically disseminated in the sandstones. The ore was subsequently concentrated by percolating groundwater accompanied by the formation of kaolinite and goethite at low temperatures. The main U mineral is autunite. o 1998 Elsevier Science Limited. Resume-L’anomalie en U de Zona est incluse dans des gres et contr8lee par la structure. Trois zones principales d’alteration ont bte reconnues: une zone silicifiee, une zone ferrugineuse brun-rouge (hematite) et une zone ferrugineuse brun-jaune (gcethite). Ces zones sont brechifiees et kaolinisees. La mineralisation dpigenetique d’U est posterieure aux phases tectoniques majeures. L’uranium a Bte lessive des granites du Complexe de Base et dissemine syngenetiquement dans les gres. Le minerai s’est ensuite concentre par percolation d’eau profonde accompagnee de la formation de kaolinite et de gosthite a basse temperature. Le mineral majeur porteur d’U est I’autunite. o 1998 Elsevier Science Limited. (Received 19 June 1996:
revised version received 16 September
1997)
INTRODUCTION As the power
principal plants,
of energy
fuel
in many
With the relaxation Europe
and
stockpiles
for the
U hasebecome countries of tension
Eastern
Europe,
of U on the world
environmental
concerns
the price
remained
very low. However,
not all that and
Muller-Kahle
cumulative
aggregate
the
1993-2010
period
whereas planned
basis,
(Tauchid,
1993).
567,000
tonnes
of U is needed.
supply
is expected
between the
Western
presence
market
of
and global the
nuclear scale
the future
has
for U is
existing
stockpiles,
occurrences mapped
this optimistic
that
the
non-producing
U requirement
for
tonnes,
production capacity from existing and centres over this same period is only
short
countries
term
to come
as welil as from
but in the long term,
new
be sought,
and
forecast
explored
It is perhaps
that has led to a renewed
in U exploration, countries
especially
in currently
with high future energy
requirements like Nigeria. The exploration for U in Nigeria, in the period establishment
additional
This additional
out ready for production.
interest
is 1,242,950
must
a cumulative
in the
from Eastern European
Underhill
projected
world
supply and demand
on a worldwide
For example,
(1993)
To balance
source
of U on a global
pessimistic.
tonnes.
nuclear
around
industry,
676,000
world’s a valuable
which
peaked
between 1976-1983 with the of the Nigerian Uranium Mining
Journal
of African
Earth
Sciences
129
??
.
1
Yolde
3asement
vith in&be&~
Formation
Bima
Lower
-----
Formation
Bima
Mid
----_
Formation
Bima
Upper
Formation
y
Figure 1. Geological map and stratigraphical sequence of the Upper Benue Trough, Nigeria (from Rebelle, 19901. insert shows the location of the study area.
??
??
.-
BASIN
.......... . il.
.
.
P‘ir@a
! Fika I Shale 1. 6-n. jeiii
i
Kerri - Kerri Formation Gombe Sandstones
__jktiary Bass%
AGE
-APTIAN
NEOCOMIAN
--_?--_
---
ALBIAN
:ENOMANIAN
TURONIAN
SANTONIAN CONIACIAN
CAMPANIAN
dAASTRlCHTlA
PALEOCENE
MIOCENE
Integrated
strut Ural and mineral alteration study of the Zona uranium
Company (NUMCO), is on a steady decline for several reasons, key amongst which was the failure to find an economic deposit during the 1976-I 983 period. During the post 1983 years, U research has been rather scanty and to a large extent confined to universities. However, with the creation of a government Ministry of Solid Minerals in 1995, with an arm charged with the responsibility of radioactive minerals exploration, the future for U research and exploration in Nigeria, now and in the years ahead is bright. Although some significant U occurrences were noted during the 1976-I 983 search period, e.g. the Mika and Ghumchi anomalies (Maurin and Lancelot, 1990; Funtua and Okujeni, 19961, they were mostly confined to granitic bodies. The hope of finding significant sandstone type deposits, though still elusive, is not lost however. The Precambrian basement/Cretaceous sediment unconformity is a possible secondary U mineralisation zone and it is the focus of a new Uranium Research Programme in Nigerian institutions. The Zona uraniferous anomaly is arguably the most significant sandstone-hosted U anomaly in northeast Nigeria. Zona lies within the well known Benue Trough (Fig. 11, which is a major intracontinental basin in the west African subregion. The host rocks are fluvial arkosic sandstones (Bima Sandstone Formation) interbedded in places with silty lenses and mudstones and occupying a more or less N65E trending structural basin. This basin around Zona is bound by a very extensive fault which separates the Precambrian Complex from the overlying Bima Sandstone sedimentary cover (Fig. 2). The Bima Sandstone Formation is Cretaceous (Upper Aptian-Lower Albian) in age and comprises a lower sequence of massively bedded, poorly-sorted conglomeratic and medium- to coarse-grained sandstones, passing upwards into an upper sequence of moderately sorted sandstones. Pebble density decreases from bottom to top and trough cross-lamination is a prevalent primary structure. It is certain from field studies that the Zona anomaly, which occurs on a gently sloping broad-top ridge, is structurally controlled by fractures and faults trending in three main directions (Figs 2 and 3). The mineralisation is also associated with different mineral alterations. Previous geochemical studies show that the ore zone has enhanced Fe3+ and Mn2+ levels (Ogunleye and Okujeni, 1993). This paper examines the overall mesoscopic and microstructural setting of the Zona
anomaly
mineralisation. It also describes the associated mineral alteration zones and demonstrates that the U mineralisation is epigenetic in origin and that it postdates the deformation episodes. The mineralisation occurred at low surface temperatures accompanied by kaolinitisation and goethite formation, which are bot:h surface hydrolysis reactions.
GEOLOGICAL CONTEXT Regional aspects of Bima Sandstone Formation The Bima Sandstone Formation is the most extensively outcropping formation of the Upper Benue Trough. The Benue Trough forms part of the West African Rift subsystem, which extends from the Gulf of Guinea northwestwards for over 1000 km in a northeast-southwest direction, along strike-slip wrench faults (Binks and Fairhead, 1992). On a very broad scale, the infilling of the upper part of the Benue Trough is characterised by a succession of continental to marine deposits ranging from Aptian to Palaeocene in age (Rebelle, 1990). The Bima Formation rests unconformably on the PanAfrican basement (Fig. I) and forms the basal succession. of the Cretaceous part Sedimentological studies (Guiraud, 1990) show that the continental Bima Sandstone Formation can be divided into three siliciclastic members, namely the Lower, Middle and Upper Bima Members, designated as B,, B, and B, respectively. B, represents alluvial fan deposits, B, represents high energy braided river deposits and B, represents shallow water sand barlacustrine deposits. The Lower Bima (B,) basement-derived comprises mainly fanglomerates with granite boulders supported by a rather scanty muddy to sandy matrix. This sequence is succeeded by B, which consists of gravels to coarse-grained sandstones passing upwards into the homogeneous, well-sorted, medium- to fine-grained sandstones of the Upper Bima (B,) Member. These lithological subunits of the Bima Sandstone are generally conformable and have gradational boundaries. B, is rich in clasts of basalt, rhyolite and rhyolitic to basaltic volcanic lenses, indicating contemporaneous volcanic activity (Guiraud, 1990; Guiraud and Maurin, 1992). The palaeosols of B, are associated with silicified wood, which ranges in size from complete tree trunks to tiny fragments. Palaeocurent direction studies indicate that the detrital deposits were transported along varying directions from uplifted basement areas towards neighbouring ‘rhomb grabens’ or ‘pull-apart’ sub-
Journal of African Earth Sciences 13 1
C. E. SW
et al.
-
L \-
-
-
KubukuGaru
- -
-
n.--
21
,-
;Y--
3
--
+
-
ll >-Fig.
Yimirdllang
50’
_
3 ‘O
+
+
/<
\I+
*
!I+
“/I
+
Jauro
.
+
+
Yaya
/
/
LO0
Peta +&%I
10Vl’N
+
2 Km.
0 EXPLANATION
_-_-
Granite
gneisses
Granites
( undifferentiated
Fractures
Z ++
and
migmatitic
1
C 3
0
0
0:
1 Bima
Iu
Sandstone
-
_T-
Dip
and
strike
of
bedding
Uranitim
mineralisation
I foliation
Figure 2. (Al Map of Nigeria showing the location of study area (shaded portion). iBI Generalised geology and structural setting of the Mona lJ anomaly and its surroundings. (Cl Rose diagram showing the main fracture directions in the Zona area.
132 Journal of African Earrh Sciences
Integrated structural and mineral alteration study of the Zona uranium anomaly
0
200 m
??
Fresh,
unbrccciated,
cootse
-*
Brccciatcd,
red
-
Brecciated,
siliciticd
a a
Brccciatcd,
fcrruginiscd
brown
Fault / Fractures Uranium
m
Facics
boundary
sandstone
zone
zone I yellowish
(
minerallsatlon :
-graincd
m
(
brown
vtsiblc
Topographic
radiometric
ferruginiscd
zone
) contour
line
)
Figure 3. Structural deformation pattern and alteration zones of the Zona U anomaly.
basins (Benkhelil, 1989) characteristic of the Benue Trough. The sandstones around the Zona anomaly represent transitional B,/B, sediments. In the Zona area, a northwest-southeast current direction is constant and the source of the sediments is the adjoining granitic basement (Fig. 1). The sedimentology of the Bima Sandstone around Zona indicates derivation of coarse detrital materials from erosion of the granitic basement and deposition in the medial part of an alluvial fan. This was followed by a fluviatile episode with very strong variations of energy levels resulting in alternating sandstone beds and silty-shaly lenses. Towards the top of the sequence, the sediments become thinner, low energy deposits (NUMCO, pers. comm., 1994) with fossilised remains of vegetation, probably of lacustrine type, in which the streams would have been blocked up by a fluviatile bar and would have made flood plains which have been filled with fine-grained sediments. Such confined environments rich in carbonaceous material are chemically reducing and favourable for U precipitation. Mesoscopic features of the Zona anomaly The fresh Bima Sandstone in this area is generally whitish to buff in colour, poorly-sorted and medium- to coarse-grained with excellent permeability. Although organic debris (petrified wood) is prevalent in the Bima Sandstone, only
a few silicified wood fragments are known to occur within the vicinity of this anomaly. Three main structural trends are present in the Zona area (Figs 2 and 31, namely: i) An inherited north-south trend related to the late reactivation of the Pan-African ‘basement complex. This trend is very prominent in the lineaments of northeast Nigeria, especially on the Kaltungo inlier (Fig. 1). ii) A general N65OE trend which is subparallel to the Benue Trough and defines the basement/ sediment boundary. This is the main mineralised trend and has a predominant sinistral shear displacement. iii) Numerous but less extensive N40°WN60°W fractures and faults. These faults offset the N65OE trends and the zones of intersection of these two trends yield relatively higher radiometric readings. The N40°W-N60°W fractures are low angle sinistral and dextral shearing surfaces which are curviplanar in a few cases. There is evidence of widespread cataclasis and mineral alterations associated with these tectonic trends. The Zona anomaly shows a te’xtural and mineralogical variation from the undeformed peripheral sandstone towards a highly brecciated centre. Three main alteration zones are recognised (Fig. 3). These are: i) Silicified zone: This zone is surrounded by fresh unbrecciated sandstone. The rocks in this zone are whitish, indurated and generally yield
Journal of African Earrh Sciences 7 33
C. E. SUH et al.
Table 1. Summary of radiometric from the Zona U anomaly 1 Radiometry (counts oer second)
60-90
and mean spectrometric
2
3
80- 140 1000-2000
readings
4 > 2500
Spectrometry U
0.83
0.73
1.83
Th
0.44
1.07
0.34
17.59 2.10
K
5.11
0.15
-0.11
-0.14
Spectrometric readings corrected for background. 1: fresh unbrecciated sandstone; 2: silicified zone; 3: red-brown ferruginised (hematite) zone; 4: brownish/yellowish-brown ferruginised (goethite) zone.
only background radiometric readings (80-l 40 cps; SPP2 Scintillometer). The quartz and feldspar grains are held together by microcrystalline and cherty silica. ii) Red-brown ferruginised (hematite) zone: Texturally, the sandstones in this zone are fine-
grained due to intensive brecciation. The rocks are reddish and indicative of oxidation reactions. Hematite, though not detected in hand specimen, is a major alteration product, as shown by X-ray diffractometry. Radiometric readings in this zone range from 1000 cps to 2000 cps.
Figure 4. Ial Quartz crystals (white) with intragranular cracks due to brittle deformation. Scale bar= 1 mm. (bJ Brecciated sandstone with quartz crystals (lower left half of figure) showing 120° grain boundary angles. Scale bar = 1 mm. (cl Kaolinite (white) haloes around disintegrating feldspar crystals (grey rounded grains; centre of figure). Left half of the figure shows kaolinite (white) surrounded b y autunite coating (white portions of veinlet) traversing the groundmass. Scale bar= 1 mm. Id) Ferruginous veinlet (opaque) in a fine-grained brecciated sandstone sample. To the left of the veinlet is a mass of kaolinite (pure white) and autunite (light greyl. Scale bar= 1 mm.
Figure 5. Diffractograms of whole rock samples from the different alteration zones of the Zona U anomaly. (e) From a silicified/ferruginised zone con tact; (bJ silicified zone; (cJ red ferruginised zone; Id) brownish/yellowish-brown zone: mineralised. Cu Ka radiation.
134 Journal of African Earth Sciences
1
I
56
I
60
30
I
,
I
56
,
,
11
18
t
20
50
52
,
22
I1
(tow)
24
54
26
1.64&7 Quartz
28
4.210 Kaolinite
,
,
-
44
11
16
ze-
46
28
16
42
1C
,
”
40
12
,
38
10
,
6.9700 Kaolinite (naCritC)
,
’
36
8
,
“1 34
6
,
*
32
,
2
30
f
0
1
.
51
57
55
I
53
51
I1
47
.
h
49
47
1,8953 Kaolinitc
19
65
I
43
43
2e-
45
2e-
1.6679 Kaolinite
I I ‘.:?e!: _ II .
1.7039 Quartz
53
I”
1.7565
~~,,‘I~,‘,‘,‘l’l”““““““r
Godhitt
I
55
1.681
57
1.700 Ouartz
11
39
41
39
2.2758
2.2758 Quartz
41
II
37
37
-..-‘-
I
.._A.
33
35
33
I.%#“PA, A
35
I
31
J’
31
I
29
n
29
I
27
C. E. SUH et al.
iii) Brownish/yellowish-brown ferruginised (goethite) zone: This zone has the highest radiometric reading (>2500 cps), is rather discontinous and occurs as enclaves within the central axis of the hematite zone (Fig. 3). Boulders from this zone are fine- to mediumgrained and when split open reveal bright yellowish flakes of secondary U minerals associated with brownish goethite. A summary of the radiometric and spectrometric readings ,for the alteration zones of the Zona anomaly is given in Table 1. Although the radiometric readings are generally low, they define a U related trend. Microscopic and X-ray diffractometry studies Laboratory procedures Representative samples from the zones described above were thin sectioned for petrographic analysis, All the samples were cut normal to the apparent foliation and parallel to the weakly developed lineation defined by seams of finegrained clayey, goethite and opaque mineral aggregates. A fraction of each sample was crushed and thoroughly milled into a fine powder in a carbidecoated mill. The powders were then analysed using a Ni filtered Cu Ka radiation X-ray diffractometer at the Department of Geology, Obafemi Awolowo University, Ile-lfe. Both the unorientated and orientated pressed powder mounts were analysed. Further analyses were carried out on heated and glycol-treated replicate samples (Fig. 5).
RESULTS AND INTERPRETATIONS Under the microscope,‘the fresh sandstone is very rich in quartz (>70%), K-feldspar (15%), muscovite (I 0%) and plagioclase feldspars (5%). The quartz content of the brecciated samples is still high (-60%) with some resistant plagioclase clasts and interstitial primary muscovite. Kfeldspars are rare or totally absent. Accessory zircon with a distorted tetrahedral to rounded morphology is a common feature. The effects of brecciation on a microscopic scale are very evident. In these samples, most of the quartz crystals show transgranular fracturing (Fig. 4a), whereas the feldspars are fractured along their cleavage planes. In the silicified samples these microcracks are filled by winding seams of fine-grained quartz (silica). Such intergranular authigenic quartz represents a significant sink for silica in sandstones in response to brittle deformation at shallow depths
136 Journal of African Earth Sciences
(Dickinson and Milliken, 1995). Both the resistant quartz and feldspar clasts have arcuate and interpenetrative contacts. Although concavoconvex interpenetrative contacts commonly result from pressure solution due to compaction, the presence of quartz aggregates with individual crystals making 120° contact with each other (Fig. 4b) represents deformation rather than diagenetic compaction. By contrast, in the ferruginised samples most of the K-feldspars have disintegrated into a clayey mass which cannot be distinguished under the microscope (Fig. 4~). The few resistant quartz clasts are rimmed by veinlets of earthy reddish/brownish ferruginous compounds criss-crossing randomly, although a local alignment is observed in some of the samples (Fig. 4b). The yellowish flakes of the U ore occur as coatings around these ferruginised seams, as well as in areas of feldspar disintegration (Fig. 4c, d). This association suggests a metallogenetic link between the secondary Fe compounds and the disintegration of the feldspars in response to the invading U bearing fluids. Examples of the X-ray diffractograms are shown in Fig. 5a-d. Glycolated peaks of the kaolinite group of clays do not differ significantly from the orientated mounts peaks, indicating the absence of halloysite. No illite was detected, confirming the continental nature of the sandstones. In the brecciated samples Kfeldspars have also not been detected from the diffractograms and this tallies with the hand specimen and petrographic examinations. Quartz, kaolinite and muscovite are the minerals with significant peaks (Fig. 5a-d). A goethite peak is observed on the diffractogram of the sample from the brownish/yellowish-brown zone (Fig. 5d). This particular sample, as well as others from this zone, are mineralised and therefore confirm the link between goethite formation and U precipitation. Hematite is prevalent in the samples from the ferruginised zone (Fig. 5~). Because of its disorganised nature, it could not be distinguished under the microscope. An autunite peak (Fig. 6) was detected in diffractograms obtained from the non-glycolated, unorientated powder of a sample from the goethite zone. In the hand specimen of this sample, the U mineral is yellowish, flaky to micaceous and occurs as crusts and powdery aggregates coating the goethite-stained quartz crystals. These physical properties agree with those of autunite. No other U ore mineral was detected.
Integrated structural and mineral alteration study of the Zona uranium anomaly
3.339 Quartz
1.661
3.58 Au tunite
Goethite
Figure 6. Diffractogram from the brownish/yellowish-brown zone) with a prominent autunite peak. Cu Ku radiation.
DISCUSSION
AND CONCLUSIONS
The nature of the fracturing of the quartz grains as described here, assuming normal lithostatic and hydrostatic pressures, corresponds to deformation at very shallow depths. Tada and Sievers (I 989) and Zhang et a/. (1990) attributed such features to brittle deformation at < 1 km depth of burial (about 1 O-l 00 bars pressure) and 20-60°C, which are minimum conditions for pressure solution in quartzose rocks. Generally, K-feldspars at surface temperatures disintegrate more rapidly than plagioclase feldspars (Tullis and Yund, 1980) and it is also the case here. The petrographic and X-ray diffractogram results of this study indicate that most of the K-feldspars in the brecciated host rock have been altered to kaolinite. Kaolinitisation is a near-surface low pressure/ low temperature reaction. This could indicate that the U ores were precipitated from a rather low temperature fluid; probably groundwater percolating after the main deformation episodes. The absence of neo-crystallised mineral phases such as micas and chlorites also supports this argument. Feldspar alteration at much higher temperatures would normally lead to the formation of secondary micas, epidotes and/or chlorite associations. This secondary mineral association typifies the Central Pyrenees U anomaly, for example, which is hosted by hornfels formed at much higher temperatures (Charlet, 1992). Under the microscope, as in hand specimen, the U ore is concentrated in the goethite alteration zone. Brownish goethite forms as a result of water reacting with Fe bearing minerals,
zone (goethite
commonly at surface temperature conditions (Ramdohr, 1980). Hematite in the Zona sandstones readily provided the reactant for the formation of this goethite. Fe,O, + H,O -+ 2FeO(OH) (Hematite) (Goethite) Paragenetic studies (Fig. 7) clearly show that kaolinite and goethite are secondary minerals typical of the ore stage. The U ore occurs in areas of K-feldspar alteration to kaolinite and as veinlets and/or haloes around rims of goethite and kaolinite. This therefore suggests that the genesis of goethite and kaolinite is synchronous to that of the U ore. Because these secondary minerals (goethite and kaolinite) form from groundwater reactions with primary minerals, it is postulated that the U ore must have been precipitated from this fluid. This supports the epigenetic origin of this anomaly. Initially, the U was leached from the Pan-African basement complex granites, followed by syngenetic dissemination in the sandstones and subsequent epigenetic concentration by groundwater. The numerous fractures and faults in the vicinity acted as good conduits along which the mineralising fluids were channelled. Organic debris probably aided the precipitation of the U by reduction. Epigenetic ore deposits often show evidence of leaching and element remobilisation. Although the geochemical data so far obtained on the Zona anomaly are incomplete, two elements, Fe and Zr, clearly show that chemical leaching and mass transfer were significant processes in the genesis of this anomaly (Table
Journal of African Earth Sciences 137
C. E. SlJH et al.
Post-Ore
Ore - stage
Pre - ore stage
stage
Quartz K -feldspar Ptagioclase
Wdspars
Muscovite
Coethite
--
--
Kaotinite
--
-
-
------
-
I
Autunite Quartz
--
----
Hematite
(Chert
I Silica
,-,__-
)
-_
-ate
Figure 7. Paragenetic sequence of the Zona lJ anomaly, northeastern Nigeria.
Table 2. Mean concentration alteration
of selected
2 (r-r= 6)
1 (n=lO)
Si02 (wt%) Fe203r (wt%) U (ppm)
elements
of the various
zones of the Zona anomaly
3 (n=5)
4 (n=6)
66.53
79.40
72.81
87.03
0.34
2.81
10.69
4.72 548
19
192
269
Th (ppm)
13.9
10.7
4.3
3.4
W (ppm)
140
177
513
608
Zr (ppm)
549
395
130
193
1: fresh unbrecciated sandstone; 2: silicified zone; 3: red-brown ferruginised (hematite) zone; 4: brownish/yellowish-brown ferruginised (goethite) zone.
2). The concentration of Zr decreases progressively from the fresh unbrecciated sandstone to the extensively altered and mineralised central sandstone (Table 2). Zirconium in sedimentary formations is found principally in sandstones that have igneous rocks in their provenance and its behaviour and distribution in these sediments is determined by the high weathering resistance of zircon, the principal source of resistate Zr (Riese et a/., 1978). Because the concentration of Zr in the alteration zones of the Zona anomaly shows a general decrease from the fresh sandstone to the mineralised and brecciated centre, Zr must have been leached from this central goethite zone. Zircon dissolves under acid pH with significant chloride concentrations (Colin ef al., 1993) and
138 Journal of African Earth Sciences
in the presence of organic matter (Tejan Kelia, 1991) leading to the release of Zr. The mobility of Zr in the Zona anomaly may therefore indicate that such surficial acid conditions prevailed during the ore concentration process. It is therefore proposed that an epigenetic model for the Zona anomaly is used in which U is derived from granitic fragments in the sandstone, remobilised by groundwater and precipitated in the brecciated zone. The finegrained nature of the sandstone in this zone, coupled with probable organic debris, provided a suitable geochemical substrate for the precipitation of U. The fine-grained sandstones presumably offered a greater surface area for adsorption of ions, notably U. In sandstone type
Integrated
structural
and mineral
alteration
U deposits, evidence for formation and/or meteoric waters having transported U comes from the presence of U in clay seams (Dunn and Ramaekers, 1978) or fine-grained materials, as in the Zona anomaly. The association of U with Fe (Table 2) indicates that the ferromanganese component does play a significant role in scavenging U in this environment (Dunn and Ramaekers, 1978). Although the volcanics associated with the Lower 8ima Member are widely assumed to be the source of the U which was leached and subsequently concentrated around the Zona fracture zone (see Ogunleye and Okujeni, 19921, no evidence of U enrichment in these volcanics has been documented. However, vein type U mineralisation does exist within the Pan-African basement, e.g. the Kanawa Violaine uraniferous vein (Sub and Dada, in press) which lies southwest of Zona. Mindful of the fact that this area is the major provenance of the Bima Sandstone around Zona, debris from such mineralisation are the most plausible source of primary U ores, syngenetically disseminated in the Bima Sandstone and subsequently remobilised by groundwater. In the Karoo Basin, for example, sandstones of the Abrahams Kraal Formation (South Africa) and K7/K8 formations of the Beaufort Group (Tanzania; Kreuser et a/., 19901 are enriched in uraninite and coffinite, concentrated mainly within high sinuosity multistoried channel sandstones. However, le Roux (1993) showed that most of the U bearing sandstones throughout the Karoo Basin were deposited by distal braided rivers, although anastomosing river and lacustrine delta deposits also host U orebodies in these formations. These formations are exceptionally rich in organic matter, which acted as oxidising agents precipitating U from mildly reducing groundwater carrying uranylcarbonate complexes in solution (Smith, 1990; Turner, 1978) leached from intercalated volcanics tuffs, as well as granitic basement rocks in the catchment. The Karoo mineralisation is rich in primary U ores, especially below the water table. Primary U ores have not been identified in the Zona anomaly as of date. This, coupled with the small amount of organic debris in the Zona area, suggest that the Zona anomaly is not likely to continue at depth. This assumption is however subject to confirmation upon drilling of the Zona anomaly, which has not been done. Such spotty near-surface anomalies are likely to be encountered at several different locations within the Bima Sandstone throughout the Upper
study
of the Zona
uranium
anomaly
Benue Trough and should be treated with caution during exploration.
ACKNOWLEDGEMENTS Fieldwork was sponsored by a Senate Postgraduate Research Grant of the Abubakar Tafawa Balewa University, Bauchi, to C.E.S. The Department of Geology, Obafemi Awolowo University, Ile-lfe, assisted with the milling and X-ray diffractometry analyses of the samples. The authors are grateful to both institut#ions. The authors thank the Director of the Gleological Survey of Nigeria and NUMCO for granting access to the study area. This paper benefitted from critical reviews by Dr. J. P. le Roux and an anonymous reviewer, to whom the authors are greatly indebted. Editorial Handling - I? G. Eriksson.
REFERENCES Benkhelil, J. 1989. The origin and evolution of the Cretaceous Benue Trough (Nigeria). JournalAfrican Earth Sciences 8, 251-282.
Binks, R. M. and Fairhead, J. D. 1992. A plate tectonic setting for Mesozoic rifts of West and Central Africa. Tectonophysics
213,
141-152.
Charlet, J.-M. 1992. Discovery of uraniferous occurrences in the eastern extremity of the Maladeta granitic massif (Central Pyrenees, Spain): Geologic and metallogenic context. Comptes Rendu Academic Sciences, Sbrie /I 314, 683-690.
Colin, F., Alarcon, C. and Vieillard, P. 1993. Zircon: an immobile index in soils? Chemical Geology 107, 273-276. Dickinson, W. W. and Milliken, K. L. 1995. The diagenetic role of brittle deformation in compaction and pressure solution, Etjo sandstone, Namibia. Journal Geology 103, 339-348.
Dunn, C. E. and Ramaekers, P. 1978. Surface geochemical patterns associated with uranium in and beneath the Athabasca sandstone, Saskatchewan, Canada. In: Geochemical Seventh
Exploration
19 78.
In terna tional Geochemical
Proceedings Exploration
of
the
Symposium
(Edited by Watterson, J. R. and Theobald, P. K.) pp9-15. Association of Exploration Geochemists, Ontonio. Funtua, I. I. and Okujeni, C. D. 1996. Element distribution patterns in the uranium occurrence at Mika, Northeastern Nigeria. Chemie Erde 56, 245-260. Guiraud, M. 1990. Tectono-sedimentary framework of the early Cretaceous Continental Bima Formation (Upper Benue Trough, N.E. Nigeria). JournalAfrican Earth Sciences 10, 341-353. Guiraud, M. and Maurin, J. C. 1992. Early Cretaceous rifts of Western and Central Africa: an overview. Tectonophysics 213, 153-168. Kreuser, T., Wopfnen, H., Kaaya, C. 2.. Markwort, S., Semkiwa, P. M. and Asdanids, P. 1990. Depositional evolution of Permo-Triassic Karoo Basins in Tanzania with reference to their economic potential. JournalAfrican Earth Sciences 10, 151-168. Le Roux, J. P. 1993. Genesis of stratiform U-MO deposits in the Karoo Basin of South Africa. Ore Geology Review 7, 485-509.
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Maurin, J. C. and Lancelot, J. R. 1990. Structural setting and U-Pb dating of uranium mineralisation from the northeastern part of Nigeria (Upper Benue Region). Journal African Earth Sciences 10, 421-433. Ogunleye, P. 0. and Okujeni, C. D. 1993. The geology and geochemistry of the Zona uranium occurrence, Upper Benue Trough; N.E. Nigeria. Journal Mining Geology 2, 175-l 82. Ramdohr, P. 1980. The Ore Minerals and their Intergrowth, 2nd Edition. Vol. 2, 1075~. international Series in Earth Sciences, Academic Press, New York. Rebelle, M. 1990. The marine transgression in the Benue Trough (N.E. Nigeria): a palaeogeographic interpretation of the Gongila Formation. Journal African Earth Sciences 10, 643-656. Riese, W. C., Lee, M. J., Brookins, D. G. and Della Valle, R. 1978. Application of trace element geochemistry to prospecting for sandstone-type uranium deposits. In: Geochemical Exploration 19 78. Proceedings of the Seventh International Geochemical Exploration Symposium (Edited by Watterson, J. R. and Theobald, P. K.) ~~47-64. Association of Exploration Geochemists, Ontonio. Smith, R. M. H. 1990. A review of the stratigraphy and sedimentary environments of the Karoo Basin of South Africa. Journal African Earth Sciences 10, 1 17-l 38.
140 Journal of African Earth Sciences
Suh, C. E. and Dada, S. S. In press. Fault rocks and differential reactivity of minerals in the Kanawa Vioiaine uraniferous vein, N.E. Nigeria. Journal Structural Geology. Tejan Kella, M. S. 1991. Scanning electron microscope study of zircons and rutiles from a podzol chronosequence at Coloola, Queensland, Australia. Catena 18, 1 l-30. Tada, R. and Sievers, R. 1989. Pressure solution during diagenesis. Annual Reviews, Earth Planetary Sciences, 17, 89-1 18. Tauchid, M. 1993. Nuclear raw materials: developing resources through technical cooperation. lntemational Atomic Energy Agency Bulletin 35, 14-l 7. Tullis, J. and Yund, R. A. 1980. Hydrolytic weakening of experimentally deformed Westerly granite and Hale albite rock. Journal Strucrural Geology 2, 439-45 1. Turner, B. R. 1978. Sedimentary patterns of uranium mineralisation in the Beaufort Group of the Southern Karoo (Gondwana) Basin, South Africa. In: FluvialSedimentology (Edited by Miall, A. D.) Memoir Canadian Association Petroleum Geologists 5. 859p. Underhill, D. H. and Muller-Kahle, E. 1993. World uranium supply and demand: the changing market. International Atomic Energy Agency Bulletin 35, 8-13. Zhang, J., Wong, J. F. and Davis, D. M. 1990. Micromechanics of pressure-induced grain crushing in porous rocks. Journal Geophysical Research 95, 341-352.
Vol.
Earth Sciences.
All rights
Pll:SO899-5382(98)00051-7
27, No. 1, pp. 129-l 40, 1998 o 1998 Elsaviar Science Ltd reserved. Printed in Great Britain 0899~5362/98 $19.00 + 0.00
COMMUNICATION
Integrated structural and mineral alteration study of the Zona uranium anomaly, northeast Nigeria C. E. SUH,’ S. S. DADA,’ T. R. AJAY12 and G. MATHEIS3 ‘Geology Programme, Abubakar Tafawa Balewa University, Bauchi, Nigeria 2Department of Geology, Obafemi Awolowo University, Ile-lfe, Nigeria 3Technische Universitat, Berlin, Serkr, BH4, D-l 0587, Berlin, Germany
Abstract-The Zona U anomaly is a sandstone-hosted anomaly which is structurally controlled. Three main zones of mineral alteration are recognised, namely a silicified zone; a red-brown ferruginised (hematite) zone and a brownish/yellowish-brown ferruginised (goethite) zone. These zones are typified by brecciation and kaolinitisation. The U mineralisation is epigenetic in origin and post-dates the main tectonic deformations. Uranium was leached from the Basement Complex granites and syngenetically disseminated in the sandstones. The ore was subsequently concentrated by percolating groundwater accompanied by the formation of kaolinite and goethite at low temperatures. The main U mineral is autunite. o 1998 Elsevier Science Limited. Resume-L’anomalie en U de Zona est incluse dans des gres et contr8lee par la structure. Trois zones principales d’alteration ont bte reconnues: une zone silicifiee, une zone ferrugineuse brun-rouge (hematite) et une zone ferrugineuse brun-jaune (gcethite). Ces zones sont brechifiees et kaolinisees. La mineralisation dpigenetique d’U est posterieure aux phases tectoniques majeures. L’uranium a Bte lessive des granites du Complexe de Base et dissemine syngenetiquement dans les gres. Le minerai s’est ensuite concentre par percolation d’eau profonde accompagnee de la formation de kaolinite et de gosthite a basse temperature. Le mineral majeur porteur d’U est I’autunite. o 1998 Elsevier Science Limited. (Received 19 June 1996:
revised version received 16 September
1997)
INTRODUCTION As the power
principal plants,
of energy
fuel
in many
With the relaxation Europe
and
stockpiles
for the
U hasebecome countries of tension
Eastern
Europe,
of U on the world
environmental
concerns
the price
remained
very low. However,
not all that and
Muller-Kahle
cumulative
aggregate
the
1993-2010
period
whereas planned
basis,
(Tauchid,
1993).
567,000
tonnes
of U is needed.
supply
is expected
between the
Western
presence
market
of
and global the
nuclear scale
the future
has
for U is
existing
stockpiles,
occurrences mapped
this optimistic
that
the
non-producing
U requirement
for
tonnes,
production capacity from existing and centres over this same period is only
short
countries
term
to come
as welil as from
but in the long term,
new
be sought,
and
forecast
explored
It is perhaps
that has led to a renewed
in U exploration, countries
especially
in currently
with high future energy
requirements like Nigeria. The exploration for U in Nigeria, in the period establishment
additional
This additional
out ready for production.
interest
is 1,242,950
must
a cumulative
in the
from Eastern European
Underhill
projected
world
supply and demand
on a worldwide
For example,
(1993)
To balance
source
of U on a global
pessimistic.
tonnes.
nuclear
around
industry,
676,000
world’s a valuable
which
peaked
between 1976-1983 with the of the Nigerian Uranium Mining
Journal
of African
Earth
Sciences
129
??
.
1
Yolde
3asement
vith in&be&~
Formation
Bima
Lower
-----
Formation
Bima
Mid
----_
Formation
Bima
Upper
Formation
y
Figure 1. Geological map and stratigraphical sequence of the Upper Benue Trough, Nigeria (from Rebelle, 19901. insert shows the location of the study area.
??
??
.-
BASIN
.......... . il.
.
.
P‘ir@a
! Fika I Shale 1. 6-n. jeiii
i
Kerri - Kerri Formation Gombe Sandstones
__jktiary Bass%
AGE
-APTIAN
NEOCOMIAN
--_?--_
---
ALBIAN
:ENOMANIAN
TURONIAN
SANTONIAN CONIACIAN
CAMPANIAN
dAASTRlCHTlA
PALEOCENE
MIOCENE
Integrated
strut Ural and mineral alteration study of the Zona uranium
Company (NUMCO), is on a steady decline for several reasons, key amongst which was the failure to find an economic deposit during the 1976-I 983 period. During the post 1983 years, U research has been rather scanty and to a large extent confined to universities. However, with the creation of a government Ministry of Solid Minerals in 1995, with an arm charged with the responsibility of radioactive minerals exploration, the future for U research and exploration in Nigeria, now and in the years ahead is bright. Although some significant U occurrences were noted during the 1976-I 983 search period, e.g. the Mika and Ghumchi anomalies (Maurin and Lancelot, 1990; Funtua and Okujeni, 19961, they were mostly confined to granitic bodies. The hope of finding significant sandstone type deposits, though still elusive, is not lost however. The Precambrian basement/Cretaceous sediment unconformity is a possible secondary U mineralisation zone and it is the focus of a new Uranium Research Programme in Nigerian institutions. The Zona uraniferous anomaly is arguably the most significant sandstone-hosted U anomaly in northeast Nigeria. Zona lies within the well known Benue Trough (Fig. 11, which is a major intracontinental basin in the west African subregion. The host rocks are fluvial arkosic sandstones (Bima Sandstone Formation) interbedded in places with silty lenses and mudstones and occupying a more or less N65E trending structural basin. This basin around Zona is bound by a very extensive fault which separates the Precambrian Complex from the overlying Bima Sandstone sedimentary cover (Fig. 2). The Bima Sandstone Formation is Cretaceous (Upper Aptian-Lower Albian) in age and comprises a lower sequence of massively bedded, poorly-sorted conglomeratic and medium- to coarse-grained sandstones, passing upwards into an upper sequence of moderately sorted sandstones. Pebble density decreases from bottom to top and trough cross-lamination is a prevalent primary structure. It is certain from field studies that the Zona anomaly, which occurs on a gently sloping broad-top ridge, is structurally controlled by fractures and faults trending in three main directions (Figs 2 and 3). The mineralisation is also associated with different mineral alterations. Previous geochemical studies show that the ore zone has enhanced Fe3+ and Mn2+ levels (Ogunleye and Okujeni, 1993). This paper examines the overall mesoscopic and microstructural setting of the Zona
anomaly
mineralisation. It also describes the associated mineral alteration zones and demonstrates that the U mineralisation is epigenetic in origin and that it postdates the deformation episodes. The mineralisation occurred at low surface temperatures accompanied by kaolinitisation and goethite formation, which are bot:h surface hydrolysis reactions.
GEOLOGICAL CONTEXT Regional aspects of Bima Sandstone Formation The Bima Sandstone Formation is the most extensively outcropping formation of the Upper Benue Trough. The Benue Trough forms part of the West African Rift subsystem, which extends from the Gulf of Guinea northwestwards for over 1000 km in a northeast-southwest direction, along strike-slip wrench faults (Binks and Fairhead, 1992). On a very broad scale, the infilling of the upper part of the Benue Trough is characterised by a succession of continental to marine deposits ranging from Aptian to Palaeocene in age (Rebelle, 1990). The Bima Formation rests unconformably on the PanAfrican basement (Fig. I) and forms the basal succession. of the Cretaceous part Sedimentological studies (Guiraud, 1990) show that the continental Bima Sandstone Formation can be divided into three siliciclastic members, namely the Lower, Middle and Upper Bima Members, designated as B,, B, and B, respectively. B, represents alluvial fan deposits, B, represents high energy braided river deposits and B, represents shallow water sand barlacustrine deposits. The Lower Bima (B,) basement-derived comprises mainly fanglomerates with granite boulders supported by a rather scanty muddy to sandy matrix. This sequence is succeeded by B, which consists of gravels to coarse-grained sandstones passing upwards into the homogeneous, well-sorted, medium- to fine-grained sandstones of the Upper Bima (B,) Member. These lithological subunits of the Bima Sandstone are generally conformable and have gradational boundaries. B, is rich in clasts of basalt, rhyolite and rhyolitic to basaltic volcanic lenses, indicating contemporaneous volcanic activity (Guiraud, 1990; Guiraud and Maurin, 1992). The palaeosols of B, are associated with silicified wood, which ranges in size from complete tree trunks to tiny fragments. Palaeocurent direction studies indicate that the detrital deposits were transported along varying directions from uplifted basement areas towards neighbouring ‘rhomb grabens’ or ‘pull-apart’ sub-
Journal of African Earth Sciences 13 1
C. E. SW
et al.
-
L \-
-
-
KubukuGaru
- -
-
n.--
21
,-
;Y--
3
--
+
-
ll >-Fig.
Yimirdllang
50’
_
3 ‘O
+
+
/<
\I+
*
!I+
“/I
+
Jauro
.
+
+
Yaya
/
/
LO0
Peta +&%I
10Vl’N
+
2 Km.
0 EXPLANATION
_-_-
Granite
gneisses
Granites
( undifferentiated
Fractures
Z ++
and
migmatitic
1
C 3
0
0
0:
1 Bima
Iu
Sandstone
-
_T-
Dip
and
strike
of
bedding
Uranitim
mineralisation
I foliation
Figure 2. (Al Map of Nigeria showing the location of study area (shaded portion). iBI Generalised geology and structural setting of the Mona lJ anomaly and its surroundings. (Cl Rose diagram showing the main fracture directions in the Zona area.
132 Journal of African Earrh Sciences
Integrated structural and mineral alteration study of the Zona uranium anomaly
0
200 m
??
Fresh,
unbrccciated,
cootse
-*
Brccciatcd,
red
-
Brecciated,
siliciticd
a a
Brccciatcd,
fcrruginiscd
brown
Fault / Fractures Uranium
m
Facics
boundary
sandstone
zone
zone I yellowish
(
minerallsatlon :
-graincd
m
(
brown
vtsiblc
Topographic
radiometric
ferruginiscd
zone
) contour
line
)
Figure 3. Structural deformation pattern and alteration zones of the Zona U anomaly.
basins (Benkhelil, 1989) characteristic of the Benue Trough. The sandstones around the Zona anomaly represent transitional B,/B, sediments. In the Zona area, a northwest-southeast current direction is constant and the source of the sediments is the adjoining granitic basement (Fig. 1). The sedimentology of the Bima Sandstone around Zona indicates derivation of coarse detrital materials from erosion of the granitic basement and deposition in the medial part of an alluvial fan. This was followed by a fluviatile episode with very strong variations of energy levels resulting in alternating sandstone beds and silty-shaly lenses. Towards the top of the sequence, the sediments become thinner, low energy deposits (NUMCO, pers. comm., 1994) with fossilised remains of vegetation, probably of lacustrine type, in which the streams would have been blocked up by a fluviatile bar and would have made flood plains which have been filled with fine-grained sediments. Such confined environments rich in carbonaceous material are chemically reducing and favourable for U precipitation. Mesoscopic features of the Zona anomaly The fresh Bima Sandstone in this area is generally whitish to buff in colour, poorly-sorted and medium- to coarse-grained with excellent permeability. Although organic debris (petrified wood) is prevalent in the Bima Sandstone, only
a few silicified wood fragments are known to occur within the vicinity of this anomaly. Three main structural trends are present in the Zona area (Figs 2 and 31, namely: i) An inherited north-south trend related to the late reactivation of the Pan-African ‘basement complex. This trend is very prominent in the lineaments of northeast Nigeria, especially on the Kaltungo inlier (Fig. 1). ii) A general N65OE trend which is subparallel to the Benue Trough and defines the basement/ sediment boundary. This is the main mineralised trend and has a predominant sinistral shear displacement. iii) Numerous but less extensive N40°WN60°W fractures and faults. These faults offset the N65OE trends and the zones of intersection of these two trends yield relatively higher radiometric readings. The N40°W-N60°W fractures are low angle sinistral and dextral shearing surfaces which are curviplanar in a few cases. There is evidence of widespread cataclasis and mineral alterations associated with these tectonic trends. The Zona anomaly shows a te’xtural and mineralogical variation from the undeformed peripheral sandstone towards a highly brecciated centre. Three main alteration zones are recognised (Fig. 3). These are: i) Silicified zone: This zone is surrounded by fresh unbrecciated sandstone. The rocks in this zone are whitish, indurated and generally yield
Journal of African Earrh Sciences 7 33
C. E. SUH et al.
Table 1. Summary of radiometric from the Zona U anomaly 1 Radiometry (counts oer second)
60-90
and mean spectrometric
2
3
80- 140 1000-2000
readings
4 > 2500
Spectrometry U
0.83
0.73
1.83
Th
0.44
1.07
0.34
17.59 2.10
K
5.11
0.15
-0.11
-0.14
Spectrometric readings corrected for background. 1: fresh unbrecciated sandstone; 2: silicified zone; 3: red-brown ferruginised (hematite) zone; 4: brownish/yellowish-brown ferruginised (goethite) zone.
only background radiometric readings (80-l 40 cps; SPP2 Scintillometer). The quartz and feldspar grains are held together by microcrystalline and cherty silica. ii) Red-brown ferruginised (hematite) zone: Texturally, the sandstones in this zone are fine-
grained due to intensive brecciation. The rocks are reddish and indicative of oxidation reactions. Hematite, though not detected in hand specimen, is a major alteration product, as shown by X-ray diffractometry. Radiometric readings in this zone range from 1000 cps to 2000 cps.
Figure 4. Ial Quartz crystals (white) with intragranular cracks due to brittle deformation. Scale bar= 1 mm. (bJ Brecciated sandstone with quartz crystals (lower left half of figure) showing 120° grain boundary angles. Scale bar = 1 mm. (cl Kaolinite (white) haloes around disintegrating feldspar crystals (grey rounded grains; centre of figure). Left half of the figure shows kaolinite (white) surrounded b y autunite coating (white portions of veinlet) traversing the groundmass. Scale bar= 1 mm. Id) Ferruginous veinlet (opaque) in a fine-grained brecciated sandstone sample. To the left of the veinlet is a mass of kaolinite (pure white) and autunite (light greyl. Scale bar= 1 mm.
Figure 5. Diffractograms of whole rock samples from the different alteration zones of the Zona U anomaly. (e) From a silicified/ferruginised zone con tact; (bJ silicified zone; (cJ red ferruginised zone; Id) brownish/yellowish-brown zone: mineralised. Cu Ka radiation.
134 Journal of African Earth Sciences
1
I
56
I
60
30
I
,
I
56
,
,
11
18
t
20
50
52
,
22
I1
(tow)
24
54
26
1.64&7 Quartz
28
4.210 Kaolinite
,
,
-
44
11
16
ze-
46
28
16
42
1C
,
”
40
12
,
38
10
,
6.9700 Kaolinite (naCritC)
,
’
36
8
,
“1 34
6
,
*
32
,
2
30
f
0
1
.
51
57
55
I
53
51
I1
47
.
h
49
47
1,8953 Kaolinitc
19
65
I
43
43
2e-
45
2e-
1.6679 Kaolinite
I I ‘.:?e!: _ II .
1.7039 Quartz
53
I”
1.7565
~~,,‘I~,‘,‘,‘l’l”““““““r
Godhitt
I
55
1.681
57
1.700 Ouartz
11
39
41
39
2.2758
2.2758 Quartz
41
II
37
37
-..-‘-
I
.._A.
33
35
33
I.%#“PA, A
35
I
31
J’
31
I
29
n
29
I
27
C. E. SUH et al.
iii) Brownish/yellowish-brown ferruginised (goethite) zone: This zone has the highest radiometric reading (>2500 cps), is rather discontinous and occurs as enclaves within the central axis of the hematite zone (Fig. 3). Boulders from this zone are fine- to mediumgrained and when split open reveal bright yellowish flakes of secondary U minerals associated with brownish goethite. A summary of the radiometric and spectrometric readings ,for the alteration zones of the Zona anomaly is given in Table 1. Although the radiometric readings are generally low, they define a U related trend. Microscopic and X-ray diffractometry studies Laboratory procedures Representative samples from the zones described above were thin sectioned for petrographic analysis, All the samples were cut normal to the apparent foliation and parallel to the weakly developed lineation defined by seams of finegrained clayey, goethite and opaque mineral aggregates. A fraction of each sample was crushed and thoroughly milled into a fine powder in a carbidecoated mill. The powders were then analysed using a Ni filtered Cu Ka radiation X-ray diffractometer at the Department of Geology, Obafemi Awolowo University, Ile-lfe. Both the unorientated and orientated pressed powder mounts were analysed. Further analyses were carried out on heated and glycol-treated replicate samples (Fig. 5).
RESULTS AND INTERPRETATIONS Under the microscope,‘the fresh sandstone is very rich in quartz (>70%), K-feldspar (15%), muscovite (I 0%) and plagioclase feldspars (5%). The quartz content of the brecciated samples is still high (-60%) with some resistant plagioclase clasts and interstitial primary muscovite. Kfeldspars are rare or totally absent. Accessory zircon with a distorted tetrahedral to rounded morphology is a common feature. The effects of brecciation on a microscopic scale are very evident. In these samples, most of the quartz crystals show transgranular fracturing (Fig. 4a), whereas the feldspars are fractured along their cleavage planes. In the silicified samples these microcracks are filled by winding seams of fine-grained quartz (silica). Such intergranular authigenic quartz represents a significant sink for silica in sandstones in response to brittle deformation at shallow depths
136 Journal of African Earth Sciences
(Dickinson and Milliken, 1995). Both the resistant quartz and feldspar clasts have arcuate and interpenetrative contacts. Although concavoconvex interpenetrative contacts commonly result from pressure solution due to compaction, the presence of quartz aggregates with individual crystals making 120° contact with each other (Fig. 4b) represents deformation rather than diagenetic compaction. By contrast, in the ferruginised samples most of the K-feldspars have disintegrated into a clayey mass which cannot be distinguished under the microscope (Fig. 4~). The few resistant quartz clasts are rimmed by veinlets of earthy reddish/brownish ferruginous compounds criss-crossing randomly, although a local alignment is observed in some of the samples (Fig. 4b). The yellowish flakes of the U ore occur as coatings around these ferruginised seams, as well as in areas of feldspar disintegration (Fig. 4c, d). This association suggests a metallogenetic link between the secondary Fe compounds and the disintegration of the feldspars in response to the invading U bearing fluids. Examples of the X-ray diffractograms are shown in Fig. 5a-d. Glycolated peaks of the kaolinite group of clays do not differ significantly from the orientated mounts peaks, indicating the absence of halloysite. No illite was detected, confirming the continental nature of the sandstones. In the brecciated samples Kfeldspars have also not been detected from the diffractograms and this tallies with the hand specimen and petrographic examinations. Quartz, kaolinite and muscovite are the minerals with significant peaks (Fig. 5a-d). A goethite peak is observed on the diffractogram of the sample from the brownish/yellowish-brown zone (Fig. 5d). This particular sample, as well as others from this zone, are mineralised and therefore confirm the link between goethite formation and U precipitation. Hematite is prevalent in the samples from the ferruginised zone (Fig. 5~). Because of its disorganised nature, it could not be distinguished under the microscope. An autunite peak (Fig. 6) was detected in diffractograms obtained from the non-glycolated, unorientated powder of a sample from the goethite zone. In the hand specimen of this sample, the U mineral is yellowish, flaky to micaceous and occurs as crusts and powdery aggregates coating the goethite-stained quartz crystals. These physical properties agree with those of autunite. No other U ore mineral was detected.
Integrated structural and mineral alteration study of the Zona uranium anomaly
3.339 Quartz
1.661
3.58 Au tunite
Goethite
Figure 6. Diffractogram from the brownish/yellowish-brown zone) with a prominent autunite peak. Cu Ku radiation.
DISCUSSION
AND CONCLUSIONS
The nature of the fracturing of the quartz grains as described here, assuming normal lithostatic and hydrostatic pressures, corresponds to deformation at very shallow depths. Tada and Sievers (I 989) and Zhang et a/. (1990) attributed such features to brittle deformation at < 1 km depth of burial (about 1 O-l 00 bars pressure) and 20-60°C, which are minimum conditions for pressure solution in quartzose rocks. Generally, K-feldspars at surface temperatures disintegrate more rapidly than plagioclase feldspars (Tullis and Yund, 1980) and it is also the case here. The petrographic and X-ray diffractogram results of this study indicate that most of the K-feldspars in the brecciated host rock have been altered to kaolinite. Kaolinitisation is a near-surface low pressure/ low temperature reaction. This could indicate that the U ores were precipitated from a rather low temperature fluid; probably groundwater percolating after the main deformation episodes. The absence of neo-crystallised mineral phases such as micas and chlorites also supports this argument. Feldspar alteration at much higher temperatures would normally lead to the formation of secondary micas, epidotes and/or chlorite associations. This secondary mineral association typifies the Central Pyrenees U anomaly, for example, which is hosted by hornfels formed at much higher temperatures (Charlet, 1992). Under the microscope, as in hand specimen, the U ore is concentrated in the goethite alteration zone. Brownish goethite forms as a result of water reacting with Fe bearing minerals,
zone (goethite
commonly at surface temperature conditions (Ramdohr, 1980). Hematite in the Zona sandstones readily provided the reactant for the formation of this goethite. Fe,O, + H,O -+ 2FeO(OH) (Hematite) (Goethite) Paragenetic studies (Fig. 7) clearly show that kaolinite and goethite are secondary minerals typical of the ore stage. The U ore occurs in areas of K-feldspar alteration to kaolinite and as veinlets and/or haloes around rims of goethite and kaolinite. This therefore suggests that the genesis of goethite and kaolinite is synchronous to that of the U ore. Because these secondary minerals (goethite and kaolinite) form from groundwater reactions with primary minerals, it is postulated that the U ore must have been precipitated from this fluid. This supports the epigenetic origin of this anomaly. Initially, the U was leached from the Pan-African basement complex granites, followed by syngenetic dissemination in the sandstones and subsequent epigenetic concentration by groundwater. The numerous fractures and faults in the vicinity acted as good conduits along which the mineralising fluids were channelled. Organic debris probably aided the precipitation of the U by reduction. Epigenetic ore deposits often show evidence of leaching and element remobilisation. Although the geochemical data so far obtained on the Zona anomaly are incomplete, two elements, Fe and Zr, clearly show that chemical leaching and mass transfer were significant processes in the genesis of this anomaly (Table
Journal of African Earth Sciences 137
C. E. SlJH et al.
Post-Ore
Ore - stage
Pre - ore stage
stage
Quartz K -feldspar Ptagioclase
Wdspars
Muscovite
Coethite
--
--
Kaotinite
--
-
-
------
-
I
Autunite Quartz
--
----
Hematite
(Chert
I Silica
,-,__-
)
-_
-ate
Figure 7. Paragenetic sequence of the Zona lJ anomaly, northeastern Nigeria.
Table 2. Mean concentration alteration
of selected
2 (r-r= 6)
1 (n=lO)
Si02 (wt%) Fe203r (wt%) U (ppm)
elements
of the various
zones of the Zona anomaly
3 (n=5)
4 (n=6)
66.53
79.40
72.81
87.03
0.34
2.81
10.69
4.72 548
19
192
269
Th (ppm)
13.9
10.7
4.3
3.4
W (ppm)
140
177
513
608
Zr (ppm)
549
395
130
193
1: fresh unbrecciated sandstone; 2: silicified zone; 3: red-brown ferruginised (hematite) zone; 4: brownish/yellowish-brown ferruginised (goethite) zone.
2). The concentration of Zr decreases progressively from the fresh unbrecciated sandstone to the extensively altered and mineralised central sandstone (Table 2). Zirconium in sedimentary formations is found principally in sandstones that have igneous rocks in their provenance and its behaviour and distribution in these sediments is determined by the high weathering resistance of zircon, the principal source of resistate Zr (Riese et a/., 1978). Because the concentration of Zr in the alteration zones of the Zona anomaly shows a general decrease from the fresh sandstone to the mineralised and brecciated centre, Zr must have been leached from this central goethite zone. Zircon dissolves under acid pH with significant chloride concentrations (Colin ef al., 1993) and
138 Journal of African Earth Sciences
in the presence of organic matter (Tejan Kelia, 1991) leading to the release of Zr. The mobility of Zr in the Zona anomaly may therefore indicate that such surficial acid conditions prevailed during the ore concentration process. It is therefore proposed that an epigenetic model for the Zona anomaly is used in which U is derived from granitic fragments in the sandstone, remobilised by groundwater and precipitated in the brecciated zone. The finegrained nature of the sandstone in this zone, coupled with probable organic debris, provided a suitable geochemical substrate for the precipitation of U. The fine-grained sandstones presumably offered a greater surface area for adsorption of ions, notably U. In sandstone type
Integrated
structural
and mineral
alteration
U deposits, evidence for formation and/or meteoric waters having transported U comes from the presence of U in clay seams (Dunn and Ramaekers, 1978) or fine-grained materials, as in the Zona anomaly. The association of U with Fe (Table 2) indicates that the ferromanganese component does play a significant role in scavenging U in this environment (Dunn and Ramaekers, 1978). Although the volcanics associated with the Lower 8ima Member are widely assumed to be the source of the U which was leached and subsequently concentrated around the Zona fracture zone (see Ogunleye and Okujeni, 19921, no evidence of U enrichment in these volcanics has been documented. However, vein type U mineralisation does exist within the Pan-African basement, e.g. the Kanawa Violaine uraniferous vein (Sub and Dada, in press) which lies southwest of Zona. Mindful of the fact that this area is the major provenance of the Bima Sandstone around Zona, debris from such mineralisation are the most plausible source of primary U ores, syngenetically disseminated in the Bima Sandstone and subsequently remobilised by groundwater. In the Karoo Basin, for example, sandstones of the Abrahams Kraal Formation (South Africa) and K7/K8 formations of the Beaufort Group (Tanzania; Kreuser et a/., 19901 are enriched in uraninite and coffinite, concentrated mainly within high sinuosity multistoried channel sandstones. However, le Roux (1993) showed that most of the U bearing sandstones throughout the Karoo Basin were deposited by distal braided rivers, although anastomosing river and lacustrine delta deposits also host U orebodies in these formations. These formations are exceptionally rich in organic matter, which acted as oxidising agents precipitating U from mildly reducing groundwater carrying uranylcarbonate complexes in solution (Smith, 1990; Turner, 1978) leached from intercalated volcanics tuffs, as well as granitic basement rocks in the catchment. The Karoo mineralisation is rich in primary U ores, especially below the water table. Primary U ores have not been identified in the Zona anomaly as of date. This, coupled with the small amount of organic debris in the Zona area, suggest that the Zona anomaly is not likely to continue at depth. This assumption is however subject to confirmation upon drilling of the Zona anomaly, which has not been done. Such spotty near-surface anomalies are likely to be encountered at several different locations within the Bima Sandstone throughout the Upper
study
of the Zona
uranium
anomaly
Benue Trough and should be treated with caution during exploration.
ACKNOWLEDGEMENTS Fieldwork was sponsored by a Senate Postgraduate Research Grant of the Abubakar Tafawa Balewa University, Bauchi, to C.E.S. The Department of Geology, Obafemi Awolowo University, Ile-lfe, assisted with the milling and X-ray diffractometry analyses of the samples. The authors are grateful to both institut#ions. The authors thank the Director of the Gleological Survey of Nigeria and NUMCO for granting access to the study area. This paper benefitted from critical reviews by Dr. J. P. le Roux and an anonymous reviewer, to whom the authors are greatly indebted. Editorial Handling - I? G. Eriksson.
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