Mineralisation in northern Nigeria

Journal of African Earth Sciences, Vol. 3, No. 1/2, pp. 185-222, 1985 0731-7247/85 $3.00 + 0.00 Printed in Great Britain © 1985 Pergamon Press Ltd.

Mineralogy, geochemistry and mineralization of the Ririwai complex, northern Nigeria

J. A. KINNAIRD, P. BOWDEN

Department of Geology, University of St Andrews, Fife KY 16 9ST. U.K.

R. A. IXER

Department of Geological and Mineral Sciences, University of Aston, Gosta Green, Birmingham B4 7ET, U.K.

and

N. W. A. ODLING

Department of Geology, University of Hobart, Tasmania, Australia

(Received 27 September 1984)

Abstract- -The Ririwai complex represents the eroded roots of an alkaline volcano developed as part of a sequential chain of anorogenic centres in early Jurassic times. An outer ring-dyke fracture which formed a volcanic feeder is filled with quartz porphyry, and granite porphyry surrounds and partly encloses a caldera- collapsed volcanic pile into which peralkaline granite and biotite granite have been emplaced. The volcanic rocks are dominantly rhyolitic ignimbrites with minor basalts, showing petrological and geochemical features of magmatic crystallization and subsolidus re-equilibrium. The volcanic feeder intrusions are partly quenched and partly degassed representatives of the original granite magma but petrological and geochemical data testify to the limited interaction of an alkaline residual fluid phase. The effects of the fluid phase are seen as a series of metasomatic reactions generating peralkaline granites, biotite granites and their mineralization. The interactions between crystal and fluids have been monitored by XRD, XRF, INAA, wet chemical analyses and fluid inclusion studies. Ore mineralogy confirms the paragenetic columbite (pyrochlore)-cassiterite-sphalerite evolution. The metasomatic reactions commence with Na + metasomatism followed by K + , then H + and finally Si a~ . The latter reactions are best displayed in the Ririwai lode where particle track studies have delineated the relative mobility of U and Th. Mineralization in biotite granite can be grouped according to the dominant metasomatic process. Columbite, minor cassiterite and sphalerite can be equated with albitite formation. Potash metasomatism generated columbite, wolframite, cassiterite and sphalerite deposition, which continued into greisen formation as H + metasomatism developed. This was accompanied by molybdenite, chalcopyrite and galena deposition during silica metasomatism. According to isotopic data the source of the ore metals and the granite magma was probably the Pan-African continental crust with contributions from the mantle.

INTRODUCTION

DURING the International conference on Alkaline Ring Complexes in Africa, the Ririwai complex was visited by delegates as part of a field trip to the Ririwai underground tin mine. This paper is presented as a geological record of recent research completed on the Ririwai complex and is intended to complement the field guide compiled by Ike et al. (1983).

The geology, petrology, geochemistry and mineralization of the Ririwai complex have been extensively studied. It is now one of the best known complexes in Africa. The first description of the geology by Jacobson (1947) reluctantly accepted the interpretation as a ring structure. It was not until 1958 that Jacobson et al. (1963) described in some detail the general geology of the massif, with more substantial information provided by Jacobson and MacLeod (1977).

Ririwai provides a relatively simple but complete sequence of evolution of oversaturated alkaline magmatism in which the volcanic, subvolcanic and postmagmatic crystallization processes are not influenced by adjacent or overlapping centres. Furthermore, there is

extensive Nb and Sn-Zn mineralization both dispersed in the cupola zones of the granites and concentrated along zones of vein-controlled mineralization, the best known of which is the Ririwai lode. With the decline of the alluvial tin reserves in Nigeria, the primary concentration of tin and other economically important metals takes on a greater significance.

GEOLOGICAL SETTING OF THE RIRIWAI COMPLEX

Ririwai was one of the first ring complexes to be identified in the Niger-Nigeria province which consti- tutes one of the world's best examples of mid-plate magmatism (Fig. 1). Each ring complex is emplaced into the Precambrian to early Palaeozoic basement. The ring complexes are chemically distinct from suites of caic- alkaline and subalkaline granitoids intruded at the close of the Pan-African event 600 Ma ago. The two suites are often referred to as "Older" and 'Younger" granites.

The Younger Granite ring complexes of Nigeria are situated in the southern part of a 200 km wide zone,

185

86 J . A . KINNAIRD et al.

llaN

O°N

9°N

~0~W

Kano

G.,..,..ooo

FAGAM

~ , , ~o,~,~ ~ 1 7 1

. . . . . ~ 7 o . . . . ~ _

(~ ~MOf~l ~ T O N G O L 0

> < . ...... , . . . . L / ~ ' - " . . . . , ...... o ~- - .

..... ,,,o J ' . . . . . . . ii := ~ ~ ~ ....... 16

. , k~.:.:~ ~ ,/

MAOA h

1 4 1 ~ .... AFU -8 : [

8enue Trough

] Ec3ge of sedimentary cover [ ] Volcanic rocks

] A l k a l i granites and syenograndes [ ] Ring fraclure

[ ] .................... [ ] ........

~ ] Syeo,tes

Fig. I. Geological map of the Nigerian anorogenic province drawn from a 1 : 50(I,000 scale map and compiled by Kinnaird (1981). Figures indicate age dates in millions of years.

Mineralogy, geochemistry and mineralization of Ririwai complex 187

KEY

Vein controlled Zn-Sn minerallsatton Arfvedsomte granite porphyry

@I Arfvedsonlle alblte gran,,e

•nn Granlteporphyry with calc~an arfvedsonlte (rIMno, ‘e.y.l,,* *Il.5 h~d*“b.rg,t.,

lzl Blotbte m~crogran,te m Quartz fayallte porphyry with ferrohedenbergite

and ferrorichterrte

I3 Blollfe gramte

cl Agglomerates. rhyolltic I locally Intercalated with p agqhync basalts P

mmbrites and lavas

Arfvedsonlte aeglrlne granw Basement

Fig. 2. Simplified geological sketch map of the Ririwi complex

along the 9th meridian and extending 1250 km from the northern-most part of the Niger Republic southwards to the margin of the Benue Trough in Nigeria. The most northerly complex in Niger is Adrar Bous which is Ordovician in age, the most southerly in Nigeria is Afu, which is late Jurassic. Within the apparently southerly progression of ages of the anorogenic centres, aeromagnetic anomalies suggest that a series of buried north-east-south-west lineaments of incipient rifts (Ajakaiye et al. 1983) controIIed the disposition of the individual centres. Each anorogenic centre represents the exposed roots of an eroded volcano formed by successive periods of migrating magmatism.

The Ririwai complex (Fig. 2) occupies an area of approximately 180 km2 in the southern corner of the

and is situated between latitudes 8.41~8.48”E and longi- tudes 10.41-10.48”N. It is enclosed within an outer ring-dyke of fayalite granite porphyry, rising steeply from the level Pan-African Plain. Intrusions of peralkaline granites with a central biotite granite are separated from the outer ring-dyke by a broad, hilly tract of volcanic rocks. It is principally within the biotite granite that the ore deposits of cassiterite, columbite, wolframite and base metal sulphides are found (Kinnaird 1984). Except for the columbite and a little dispersed cassiterite the ore minerals occur within a braided quartz-greisen system termed the Ririwai lode.

The pattern of mineralization, which differs between the various granites, can be related to small but significant changes in bulk rock chemistry (Bowden and Kin-

Kano province. It is oval in shape, measures 17 x 16 km naird 1978) and amount of rock-fluid interaction. New

188 J .A. KINNAIRD et al.

data on uranium distribution and polished ores, coupled with mineralogical and chemical data have clarified various post-magmatic and mineralization stages.

STRUCTURAL EVOLUTION AND MAGMATIC EVOLUTION OF THE RIRIWAI COMPLEX

The structural evolution of the Ririwai complex is similar to the adjacent massif of Tibchi described by Ike (1983). It appears that the earliest igneous activity at Ririwai was violent and eruptive, dominated by pyro- clastic products and culminating in a central shield volcano built on an updomed terrain. At a later stage, a master cone fracture was formed, which was inward dipping at depth, but steepening to almost vertical towards the surface. This ring fracture provided access to the surface for a fluidized granite magma. The sudden emptying of the magma chamber caused surface caul- dron subsidence of the volcanic edifice and the concomitant extrusion of intracaldera ignimbrites. The feeders to the ignimbrites were frozen in tile ring fractures as quartz fayalite porphyry (Fig. 2). The granite porphyry in the ring dyke corresponds to the gas-poor magma which partially displaced the quartz porphyry in the ring-dyke structure by stoping and marking the transition to the subvolcanic (plutonic) phase of activity.

The phase of granite porphyry emplacement was quiescent and fracture-controlled. The granite emplacement was by means of piecemeal stoping through the collapsed central block of basement. It is clear that the granites, particularly the biotite granite magma, stoped to high levels into the volcanic pile causing local contact metamorphism and hydrothermal fluid retention at the contact, resulting in recrystallization and mineralization.

There are two distinct types of mineralized granitic rocks at Ririwai, consisting of:

(i) peralkaline granites which plot close to the Q-A join in the Streckeisen Q-A-P modal diagram (Fig. 3);

(ii) biotite granites which plot in the alkaline granite field (Fig. 3) and extend marginally into the syenogranite field.

Magmatic evolution

The magmatic evolution of the Ririwai massif was considered by earlier workers to represent sequentially fractionated pulses of granitic magma. Each mineralogical variant was believed to display the petrological and geochemical characteristics inherited from magmatic processes. Considerable difficulty was encountered in explaining how fluctuating aluminous or peralkaline granites could be generated as part of an evolutionary series. More recently it has been advocated that the alkaline granite magma of petrogeny's residual system can crystallize and recrystallize in the subsolidus under the influence of residual fluids. Following the experi-

+ a l b i t i s e d D e r a l k a l i n e g r a n i t e x p e r a l k a l i n e g r a n i t e s

• a l k a l i n e s y e n i t e s

• b i o t i t e g r a n i t e s

,~ f a y a l i t e g r a n i t e s , h o r n b l e n d e g r a n i t e s

D s y e n i t e s

~' m o n z o n i t e

, g a b b r o s

o h y b r i d c o m p o s i t i o n s , i g n e o u s b r e c c i a s

Q

lo \ \ P

Fig. 3. Representative modal data for Ririwai and granite rocks from the Nigerian Younger Granite province plotted in the Streckeisen

QAP diagram.

mental work of Mustart (1972), Taylor et al. (1980) postulated that crystallizing alkali granite magmas exsolve a peralkaline fluid phase on cooling. The effect of alkali loss or retention, which is well documented for peralkaline volcanic series, has also played a major role at subvolcanic levels, Furthermore, with boiling fluids, the compositions of vapour and liquid can drastically change during cooling and consolidation of the volcanic pile or the underlying subvolcanic intrusions. Such processes can be demonstrated in the volcanic successions as well as in the peraikaline and biotite granites. Whilst these modification processes can be seen in samples collected at the surface they are most obvious in samples from a drill core which penetrated the biotite granite in the vicinity of the lode. These processes have been examined from the mineralogical, geochemical and mineralization viewpoints following the geological sequence in the legend of Fig. 2.

As well as noting felsic modal variations of the biotite and peralkaline granites displayed in the Streckeisen QAP diagram, the major element data are also presented in the Streckeisen and LeMaitre (1979) Q'-ANOR plot (Fig. 4). This diagram is valuable for observing the CIPW salic constituents expressed as appropriate mineralogical parameters. Furthermore, rare-earth data and both LIL and HFS trace elements are included to demonstrate elemental behaviour during subsolidus processes of rock fluid interaction, particularly in mineralized biotite and peralkaline granites.


50

Q'

20

ANOR ANOR

Fig. 4. Q ' - A N O R normative diagram (after Streckeisen and LeMaitre 1979). Closed circles, biotite granites; open circles, drill core L13; inclined cross, arfvedsonite granite; straight cross, arfvedsonite albite granite. Analytical data used to define Q' and A N O R are presented in

Tables 4, 5 and 8. Q' = A N O R = Off(Or ~ An) x 10(I.

MINERALOGY, GEOCHEMISTRY AND MINERALIZATION OF THE PRINCIPAL

ROCK TYPES

Volcanic rocks

Volcanic successions are well exposed in the northern and southern parts of the complex. In the north, successions of basic and acid volcanic rocks partially flound- ered into the caldera. Towards the centre of the complex, in contrast, the volcanic rocks were partially uplifted by the subvolcanic intrusions. Hydrothermal fluids escaping from the subvolcanic rocks, combined with the redistribution of the fluid phase in the extrusives, have affected the whole volcanic pile.

Near the base of the volcanic succession is a series of basaltic lavas, some with plagioclase phenocrysts, interpreted as a fractionation series from alkali olivine basalt through hawaiite to mugearite. The volume is small and the original mineralogy is largely destroyed. One of the characteristic features of the ignimbritic volcanic successions at Ririwai is the occurrence of peralkaline extrusives (Jacobson et al. 1963). Jacobson et al. (1963) referred to the rocks as comendite, based on descriptions from the type area of Commende, San Pietro, Sardinia. In reality, the acid volcanic succession consists of a series of tufts, with varying degrees of welding and peralka- linity. The peralkaline variations are associated with the development of aegirine and alkali amphibole (arfvedsonite) crystallized within zones of compaction welding. Although many workers have assumed these features to represent primary magmatic characteristics, it can be proved that peralkaline assemblages of subsolidus mineralogy must be related to the evolution and partial retention of the fluid phase (cf. Taylor et al. 1980). Within the dominantly fragmental and welded volcanic

E

100

La Ce Nd

I I l l l I I I ! I l l I

Pr Sm Gd Dy Er Yb Eu Tb Ho Tm Lu

Fig. 5. Chondrite-normalized rare-earth patterns for aegirine crystal tuff (82-69) compared with volcanic feeder quartz porphyry (85-69). Note HREE enrichment in peralkaline volcanic rock. Samples were

collected from northern part of Fig. 2.

succession are occasional outcrops of aphyric units interpreted as lavas. However, these may be extreme examples of welding by compaction.

Analyses of comenditic ignimbrites with characteristic peralkaline subsolidus mineralogy are presented in Table 1. Rare-earth data are tabulated in Table 2 and presented diagrammatically in Fig. 5. The effect of the peralkaline imprint can be seen in the appearance of ac in the CIPW norms and the distinctive HREE enrkh- ment of the chondrite normalized curves.

To the south of Ririwai, a similar volcanic successi'~n outcrops, but erosion has revealed a satellite parasitic centre at Dutsen Shetu. Here, in addition to the volcanic succession, there is an important volcanic feeder intrusion of quartz fayalite porphyry exposed. This feeder shows little subsolidus mineralogical modification and is compositionally similar to marginal zones of the oval ring-dyke fracture, the centre of which is occupied by granite porphyry.

Volcanic feeder intrusions

Quartz fayalite porphyry and granite porphyry. The emplacement of quartz fayalite porphyry in the Dutsen Shetu vent complex is considered to be a close representative of the degassed granitic magma that rose up the ring-dyke fracture as the caldera collapsed (Martin and Bowden 1981). This degassed magma crystallized as granite porphyry. Thus the mineralogy and geochemistry of the porphyries provide important information on the dominantly magmatic features of anorogenic magmatism with only a partial imprint of subsolidus reactions.

The quartz porphyries are characterized by irregularly shaped phenocrysts of alkali feldspar and quartz, some of which show obvious signs of fragmentation. Others have typical arrested growth embayments of "quench" crystallization and, occasionally, olivine and hedenbergite may be enclosed by feldspar. The groundmass may have originally been dominantly glass but this has now been devitrified and partially re-equilibrated with a mildly peralkaline residual fluid phase.

190 J. A. KINNAIRD et al.

Table 1. Analyses and C1PW norms of volcanic rocks, including volcanic feeders and quartz porphyries for Ririwai

0i02

TIO 2

AI203

Fe203 FeO

MnO

M90

CaO

Ba20

KzO

P20

B20~

H20-

CO 2

ZrO 2 CI

F

S

Less 0

N81-69 X572 N82-69 X580 X584 X1531 X669 X1363 1185-69 X592 GP-N96 N96 co l end i t i c comendit ic eegi r ine comandit ic v o l c l n i c volcanic volcanic volcanic quezt l g z l n i t e g z a n i t l g z l n i t a i g n i a b r i t e i gn imbr i te crys tml i gn imbr i t e feeder feeder feeder feeder pozphyry porphyry porphyry porphyzy

t u f f

74.10 74.04 73.97 72.13 74.66 73.01 71.85 72.49 69.63 71.68 72.40 71.39

0.20 0.17 0.35 0.16 0.17 0.06 0.08 0.35 0.45 0.30 0,28 0.28

10.88 11.95 11.96 10.57 11.50 11.13 12.12 12,51 13,97 12.04 12,35 13.39

3.24 2.08 0.74 3.50 1.70 2.69 3.58 2.60 3.62 2.28 1.58 1.50

1.15 1.72 3.47 3.31 1.94 4.26 1.98 1.43 0.69 1.89 2.30 2.30

0.08 0.02 tr 0.46 0.02 0.35 0.23 0.07 0.05 0.13 0.09 0.15

tr 0.02 0.08 0.35 0.03 0.18 0.14 0.19 0.22 0.29 0.05 0.04

0.26 0.35 0.14 0.42 0.17 0.28 0.42 0.58 0.58 0.79 0.76 0.74

4.43 4.46 4.48 4.64 3.16 3.49 3.84 3.91 3.62 4.36 4.44 4.02

4.09 4.76 4.51 4.14 5.30 3.98 4.68 5.13 5.42 5.12 4.87 5.17

tr 0.05 0.01 0.02 0.06 0.02 0.04 0.05 0.14 0.07 0.04 0.04

0.31 0.65 0.41 0.29 0.52 0.29 0.43 0.25 0.70 0.61 0,24 0.24

0.II 0.20 0.09 0.21 0.18 0.04 0.05 0.04 0.19 0.16 0.13 0.13

t r 0.06 0.03 t r 0.02 0.08 0.09

0.27 0.30 0.Ii

tr 0.03 tr 0.03 0.03 0.03 0.02

0.ii 0.07 0.II 0.12 0.26 0.03 0.12

0.02 0.01 0.07 0.04 0.01 0.01

98.85 99.87 100.19 100.37 99.92 99.97 99.76 99.75 99.28 I00.06 99.29 99.47

0.06 0.04 0.08 0,08 0.12 0.02 0.05

98.85 99.81 100.19 100.33 99.84 99.89 99.64 99.73 99.28 i00.01 99.29 99.47

Fe20 0.74

Be

Cu

Zn

Rb

5r

Zr

Pb

U 8.9

Th 40

Q 33.25

0r 24.16

Ab 33.19

An

£

Ac 3.77

~o 0.25

Di 0.62

Ny Mg 2.81 Hm

IL 0.38

4p

wmter 0.42

fluor

0.68 0.18 0.51 0.47 0.39 0.64 0.65 0.84 0.55 0.41 0.41

24

5

135

i75

125

13

830

3O

7.3 4

37 21

29.51 28.13 27.67 35.10 34.26 30.97 29.02 26.67 25.53 26.06 25.50

28.12 26.65 24.46 31.31 23.51 27.65 30.31 32.02 30.25 28.77 30.55

34.96 36.41 31.32 26.73 29.52 32.48 33.07 30.62 33.42 36.41 34.00

0.39 1.44 1.96 3.23

0.57 0.94 0.74 1.43

2.43 1.31 5.98

3.04 1.01

0,57 0.55 1.28 0.73 2.26 3.12 0.15

1.64 5.46 5.98 1.98 6.60 1.32 0.17 0.55 1.77 1.40 2.75

1.80 0.42 1.57 2.46 3.90 5.19 3.77 1.08 1.78 1.79 2.29

2.87

0.32 0,66 0.30 0.32 0.ii 0.15 0.66 0.85 0.57 0.53 0.53

O. 12 0.02 0.05 0.14 0.05 0.09 0.12 O. 33 0.17 O. 09 O. 09

0.85 0.50 0.50 0.70 0.33 0.48 0.29 0.89 0.77 0.37 0.37

0.22 0.14 0.13 0.24 0.51 0.05 0.23 0.00

Table 2. Rare-earth data for various rock types from the Ririwai com )lex

volcanic rocks volcanic arfvedsonite granite Cgnimbrites_~ feeder

N81-69 N82-69 N85-69 N83 N86 N89 (c@mendite) (aegirine

t u f f )

La 125 85.8 125

Ce 189 156 30

Pr (20) (25) (25)

Nd 102 170 91

Sm 37.5 40.3 23.5

Eu 1,15 1.49 2.54

5d (41.4) 41.3 23.6

Tb ii.0 8.7 3.6

Dy 62 (51) (17.1)

Ho (11.7) (i0.I) (3.5)

[ r (29.3) (27) (9.0)

Tm (3.0) (3.4) (1.2)

Yb 20.1 22.7 8.0

tu 4.47 1.88 1.64

REE 658 645 636

ta/Sm n 2.0 1.3 3.3

Sm/£u n 86 71 24

Le/Lu n 2.9 4.7 7.9

130 72.4 90

287 362 393

(25) (36.9) (49.2)

151 157 243

38.0 46.8 68.9

1.40 0.79 1.31

57.3 32.4 67.1

16.1 7.9 15.0

(86) (58) (93)

(21) (11.7) (23.4}

(45) (27) (67.5)

(5.8) (3.7) (10.2)

33.5 25.6 f12.0

5.3 3.7 15.3

910 856 1219

2.2 0.78 0.80

71 190 139

2.7 2.0 0.6

Biotite granite

N78 Ngl

i10 47.5

312 113

(25) (9.8)

115 (37.0)

17.9 8.4

0.27 0.ii

21.7 10.0

5.8 3.6

(31) (21)

( 6 . 6 ) (4.3)

( l f l ) ( 10 . i )

(2.7) (1.7)

19 .6 9 . 9

4.0 2.1

690 279

3.8 3 . 5

175 201

2.8 2 . 3

Ririwai lode

N58A N588 N580

55.2 89.8 59.1

93.3 190 110

(8.6) (3.1) (9.0)

33.2 49.9 35.1

8.0 15.2 12.9

0.18 0.24 0.23

7 . 5 17.6 19.8

1.6 3.4 3.2

(B.6) (5.8) (20.6)

(2.0) (1.3) (4.7)

(6.0) (3.6) (10.1)

(0.9) (0.6) {2,0)

6.1 17.3 13.5

1.01 2.84 2.23

232 401 303

4.3 3.6 2.8

117 167 148

5.6 3.3 2.7

Values in parentheses are interpolated values from rare-earth curves: all ratios from chondrite-normalized data. Analyst: J. E. Whitley, S U R R C , East Kilbride: instrumental neutron activation analysis.

Mineralogy, geochemistry and mineralization of Ririwai complex

Table 3. Microprobe analyses of opaque minerals in granite porphyry (N96)

lllmenite Titanomagnetite

OP OR OP OP OP2 OR9 OPlO OP8 12 13 core rim

15 16

5i02 0.35 0.48 0.35 0.26 0.65 1.04 0.61 0.64

TiO 2 50.64 50.16 50.64 54.75 18.33 17.53 17.a6 18.23

A1203 0.22 0.28 0.22 0.14 0.49 0.45 0.31 0.32

FeO 46.67 ~4.B6 46.67 41.61 77.47 77.19 78.47 76.38

MnO 1.fl5 3.08 1.88 2.16 1.89 2.25 1.97 1.99

NgO 0.26 0.26 0.26 0.21 0.24 0.20 0.24 0 . 3 3

CaO 0 0.12 0 0.13 0.17 0.28 0.21 0.15

K20 0 0.07 0 0.06 O.11 O.lO 0.08 0.07

ZnO 0 0.71 0 0.68 0.82 0.86 0.83 0.79

99.99 59.28 99.99 100.00 100.17 i00.00 100.00 98.90

191

Si O.O1 O.O1 0.01 O.O1 0.03 0.04 0.02 0.03

Ti 0.97 0.96 0.90 1.03 0.57 0.54 0.44 0.57

A1 0.01 O.O1 O.O1 - 0.02 0.02 O.O1 0.02

Fe 2+ 0.99 0.96 1.09 0.87 2.67 2.67 2.98 2.67

Mn 0.03 0.05 0.04 0.04 0.05 0.06 0.06 0.06

Mg 0.01 0.01 0 Ol 0.01 O.O1 0.02 0.01 0.02

Ca O.O1 O.O1 0.01 O.O1

K O.O1 0.01 O.O1 -

Zn 0.02 0,01 0.01 0.02 O.O1 O.O1 0.02

Formula calculated to 3 oxygens

OP12 Fe0.99Ti0.9703

OP13 Fe0.967i0.9~03

OF18 Fe1.09Ti0.9[03

OPl6 Feo. BTTil.0203

Formula calculated to 4 oxygens

OP2 Fe2.67Ti0,5704

g Fe2.67Ti0.5404

10 Fe2.98Tio.4404

8 Fe2.67Ti0.5704

Apart from quartz and alkali feldspar in the groundmass, there are rounded crystals of fayalite and ferrohedenbergite partially destabilized to iron-titanium oxides, sodic-calcic amphibole (ferrorichterite), aenigmatite and aegirine-hedenbergite. A typical major element analysis of quartz porphyry is given in Table 1, with a chondrite normalized rare-earth spectrum in Fig. 5. Since this rock type is the most primitive crystallized equivalent of the Ririwai granite magma it will be used as a reference for illustrating the superimposed postmagmatic effects of hydrothermal metasomatism.

The granite porphyry represents a more crystallized equivalent of the quartz porphyries, with a coarsening of groundmass texture. A representative major element analysis is given in Table 1. Detailed mineralogical studies on the ordering of alkali feldspar as phenocrysts and in the groundmass of the porphyries by Martin and Bowden (1981) have revealed some interesting observations. The feldspar phenocrysts are structurally intact as disordered orthoclase with the only obvious signs of alteration along small linear zones of turbidity following thermal shock features traversing twin planes. In contrast, the groundmass feldspar has undergone ordering in response to residual fluids. A typical sample exhibiting these features is N96.

Sample N96, from the ring-dyke west of Zenabi, contains microphenocrysts of partially destabilized fayalite, with hedenbergitic clinopyroxene commonly enclosed in orthoclase cryptoperthite, or isolated in the

AES 3 : 1 / 2 - M

groundmass. Surrounding green, iron-rich, rimmed hedenbergite, are a series of feather-like subsolidus growths of sodic or sodic-calcic amphiboles. In the groundmass these form the dominant mafic mineral assemblage and range in composition from ferrowinchite in the core to ferrorichterite then arfvedsonite in the rim.

There are two series of iron-titanium oxides: (i) ilmenite, with up to 10 mol% haematite, forms

euhedral or skeletal crystals, considered a primary phase; (ii) later blebs of titanomagnetite, possibly forming as

a result of the destabilization of ferromagnesian minerals at subsolidus temperatures.

The earliest generation of ilmenite crystallized before the clinopyroxenes (ferroaugite and ferrohedenbergite). Titanomagnetite is often surrounded by the development of sodic and sodic-calcic amphibole which postdate clinopyroxene crystallization. Microprobe traverses from core to tim of ilmenite grains (Table 3) have shown an increase in the molecular proportion of haematite (1.2 mol% haematite in core OP12 and 4.3 mol% in rim OP13). Titanium always decreases from the centre of the crystal outwards whilst Fe generally increases from core to rim. The content of Mn is variable and shows no consistent trend. Variations from core to rim are from almost pure ilmenite to around 10 mol% haematite (Fig. 6).

The titanomagnetite is more variable in composition. The percentage of ulvospinel appears to vary between 20 and 30 mol% (Fig. 6).

192 J. A. KINYAIRD et al.

Rutile TiO 2

Ilmeno-ruti FeTi205

Wustite, FeO

Ilmenite FeTiO,~/

Ulvospinel Fe3TiO4 /

Magnetite Fe304

~ ' , ~ P s e u d o b r o okit e

~ \ Haematite Fe203

Maghernite ~'Fe203

Fig, 6. Compositional wtriations in iron-titanium oxides from sample N96 granite porphyry, Ririwai. Average composition of Fe Ti oxides found in igneous rocks (shown shaded), from Buddington and Lindsley (1964).

Geochemistry. Chemically, the fayalite granite porphyry is similar to the earlier quartz porphyry of the Dutsen Shetu vent complex. Both are chemically slightly alkaline and plot close to the vertical line ofNa + K = AI in a triangular plot SiO2-AI203.3SiOe-(Na20 + K20).3SiO2 (Martin and Bowden 1981), suggesting that the granitic magma may have been characterized by Na + K + (Ca) = AI, indicating that the granite magma was neither peralkaline nor peraluminous in nature.

The major element data for volcanic feeders, including quartz porphyries (described in earlier texts as in- trusive rhyolite), together with those for granite porphyries are shown in Table 1. The quartz porphyries are regarded as the most primitive crystallized equivalent of the Ririwai granite magma. Sample 85-69 has been used as a reference chondrite-normalized spectrum in Fig. 5. There is a substantial variation in the volcanic rock compositions as shown by the Q-Ab-Or normative plot, which spans a similar zone to the volcanic feeders. The major part of this variation can be attributed to a combi- nation of magmatic and subsolidus processes.

Arfvedsonite granites and their albitic variants

The peralkaline granites are located in the south-east of the complex (Fig. 2) and are rather restricted in areal outcrop. They comprise three units:

(i) The arfvedsonite granite porphyry intrudes the peripheral ring-dyke in the south-east of the complex and the contact dips to the east at a low angle. It has phenocrysts of pink microperthite up to 1 cm in length and quartz, up to 5 mm across, set in a fine grained vuggy ground mass. Small amounts of albite occur, whereas

the arfvedsonite forms strongly pleochroic prisms up to 2 mm long. The quartz and feldspar in the ground mass often form graphic intergrowths (Jacobson and Mac- Leod 1977).

(ii) The arfvedsonite aegirine granite has the largest outcrop area of 9 km 2. In hand specimen it is medium to coarse grained, greyish to yellowish and consists of quartz, microcline microperthite, albite and large ragged plates of dark blue arfvedsonite intergrown with green aegirine. Astrophyllite is a constant accessory and is locally abundant in the contact zone as small lustrous black plates, often in rosettes. The accessory minerals are zircon, fluorite and dark red aenigmatite. Thin sections show that the arfvedsonite and aegirine, in roughly equal proportions, form a spongy network between quartz and perthite crystals, to give a typical subsolidus crystallization texture.

(iii) The Kaffo Valley arfvedsonite albite granite has an outcrop area of only 1.3 km 2 and lies between the arfvedsonite aegirine granite to the east and the biotite granite to the west. In hand specimen the granite is fine to medium grained with a distinctive sugary texture and fresh appearance--unusual in Nigerian granites. In thin section, abundant well-twinned laths of albite-oligo- clase, averaging 0.3 mm in length, are sporadically distributed throughout the recrystallized granite, enclosed in microcline and quartz. Aegirine, which is grass-green in colour is subordinate to lithian arfvedsonite which usually encloses it, however, the reverse relationship has also been described (Beer 1952). The lithian arfvedsonite forms deep blue needles up to 5 mm in length, which have well developed prismatic cleavage. It is intensely pleochroic from X = dark blue, Y =


Q

2

,~b Or

Fig. 7. CIPW normative Q - A b - O r values plotted in triangular diagram to show variations in the salic constituents between v, volcanic rocks; open triangles, volcanic feeders/fayalite granite porphyry; inclined cross arfvedsonite granite, perpendicular cross, arfvedsonite-albite

granite.

greenish blue to Z = brownish yellow. Throughout the outcrop there is a marked variation, not only in the essential minerals, but also in the accessories which include cryolite, thomsenolite, astrophyllite, zircon, topaz and uraniferous pyrochlore which is clearly visible as honey-coloured octahedra up to 1 mm across.

G e o c h e m i s t r y . The distribution of the samples collected for analysis is shown in Fig. 7. Chemically, the six specimens of arfvedsonite granite analysed in Table 4 show a silica content similar to that of the biotite granites with a range of 76.10 to 78.5% which is slightly higher than average for the province as a whole. In contrast, the silica content of the albitized granite is much lower with a mean of 72.8% and CIPW normative quartz falling to as little as 20% (Table 4). There is a corresponding increase in alumina from a mean of 11% in the arfved-

sonite granite to an average of 12.4% in the albite-rich facies.

The granites are peralkaline in character owing to a deficiency in alumina rather than to particularly high alkalis. In the arfvedsonite granite the soda content is always lower than the potash, whilst in the a[bite-rich facies the soda content may exceed 6% with a corresponding decrease in the potash which has a maximum of 4.5%.

The iron content is variable although ferric oxide tends to be consistently higher than in the biotite granites. In the arfvedsonite granite both the ferrous to ferric ratio and concentration vary, whilst in the albite rich granite, ferric oxide exceeds ferrous iron but also shows a varying ratio. The heterogeneous distribution of aegirine, arfvedsonite, astrophyllite and biotite account for the variation in iron oxide content. Where biotite occurs it is a distinctive red-brown colour and is more annitic in composition than the mica from biotite gtanite. This red-brown biotite has an FeO content which may exceed 34% in contrast with less than 29% in biotite from biotite granite. No other ferromagnesian mineral is present.

The chemistry of the granites is reflected in the norms. For peralkaline granites as a whole ac and ns are characteristic and whereas for the Ririwai rocks ac consistently appears in the norm, ns is rarer. There is, however, an important increase in ns and ab associated with pyrochlore mineralization.

The salic normative data has been converted into equivalent felsic minerals for plotting in the Streckeisen and LeMaitre diagram (1979) of Fig. 4.

Peralkaline granites with alkali amphibole in the compositional range ferrorichterite-arfvedsonite plot close to the alkali granite/alkali quartz syenite boundary along the Q - A join. From their modal proportions therefore (Fig. 3), they should be regarded as quartz-rich alkali syenites. In contrast, granites with calcic amphiboles in the compositional range ferroedenite-ferroactinoli te plot clearly in the zone straddling the syenogranite- alkali granite field.

Bulk chemical analyses of arfvedsonite granite and arfvedsonite albite granite plotted in the R I - R 2 de la

Fig. 8. Location map of sample sites in the central part of the Ririwai complex (see Fig. 2 for details).


3000

R2 6 C a.I- 2 M g-I- AI

R1 4Si-11{Na+K)-2{Fe- I -T i )

in m i l l i ca t i ons

R i r iwa i

2000 y '

R 2 / " ' i

/ ' " ' " . . ' i"

. , . :. '.. 1000 (. ..

• ,. . .

i i " l J

1000 2000 3000 R~

Fig. 9. RI-R 2 de la Roche cationic diagram. Dotted zone represents volcanic rocks for the Niger-Nigeria anorogenic province. Dashed zone represents metasomatic compositions of originally magmatic rocks altered by postmagmatic fluids. Straight crosses, arfvedsonite albite granite; inclined crosses, arfvedsonite granite; closed circles, biotite granite, from the Ririwai complex. Analyses tabulated in

Tables 4 and 5.

=)

o

0

0 r r

I0,000 i./......

100(

100

t % i •

I t " - ~

i

i I

•'.=,;;.:..=.:-.

Rb Th Nb La Ce Hf Zr Y

Roche cationic diagram (Fig. 9), confirm that albite granites do not lie on the volcanic (magmatic) trend shown as the dotted zone. Instead, the samples are desilicated variants which plot in the zone of metasomatic syenites enriched in ordered albite or microline.

The trace element patterns of the arfvedsonite granites (Table 4) show similarities to those of the biotite granites particularly for Sn, Nb and Zn. There is a slight enrichment in Be, Rb, Sr, Zr, Ce, Hf and Pb. With the exception of Cu, Sr and Ba which remain consistently low, there is a very marked enrichment in all the trace element concentrations including Th, Sb, Mo, Cs, Cd, and particularly Rb and Zn in the albitic facies. The La concentration is similar in all peralkaline and peraluminous phases.

A selection of trace element data is shown in Table 4. The trace element populations grouped according to their compatibility and distribution coefficients (Fig. 10) show distinctive trends unrelated to crystal-liquid partitioning. Instead, the peralkaline granites have distinctive enrichments in the most incompatible elements and the high field strength elements such as Zr and Hr.

Mineralization within the peralkaline granites. The disseminated pyrochlore mineralization is related to a phase of sodic metasomatism. A similar process is responsible for the introduction of columbite into biotite granite cupolas (Kinnaird 1984). Sodic metasomatism in the peralkaline granites is characterized by a significant increase in normative albite, accompanied by a decrease in normative quartz. It is characterized by the development of albite, aegirine and alkali amphiboles in the compositional range riebeckite to lithian arfvedsonite accompanied by pyrochlore, cryolite, amblygonite, astrophyllite and possibly narsarsukite and chevkinite.

Fig. 10. Chondrite-normalized trace elements arranged to show fluid- rock "partitioning". Elements to the left of the diagram have inferred bulk KDS partitioned to the rock; elements towards the right of the diagram have elements partitioned to the fluid phase• Horizontal lined shading, arfvedsonite albite granite, Ririwai; white zone between two continuous lines, biotite granite, Ririwai; dotted zone, localized potash metasomatism on Ririwai biotite granite; inverted closed triangles,

greisen sample from Ririwai lode.

Cryolite, abundant as intergrowths within the albite laths, has an RI of 1.340 and is sometimes weakly birefringent (Jacobson and MacLeod 1977). Ambly- gonite forms large interstitial crystals showing good cleavage. The honey-coloured octahedra of pyrochlore have been analysed by Beer (1952). The Nb205 values range from 45.0 to 51.3% and Ta205 values are 2.8 and 3.5%. The U308 content of the pyrochlore may be over 5%. The most radiogenic pyrochlore is found in the west near the biotite granite contact where normative ab values are also at a maximum. The pyrochlore becomes less radioactive towards the arfvedsonite granite as normative ab also decreases.

The dispersed mineralization related to the peralkaline granites is not of economic significance. In the immediate post-war years a great deal of attention was paid to the uraniferous pyrochlore and associated mineralization. Despite localized high U308 concentrations the heterogeneous distribution of the pyrochlore combined with the dispersed nature of mineralization over such a small area proved too difficult and expensive for extraction to be of economic interest.

Biotite granites

The central stock of biotite granite covers an area of about 30 km ~ and has low angled outward dipping contacts. There are several facies, with smaller micro- granitic bodies within the main biotite granite.

Mineralogy, geochemistry and mineralization of Ririwai complex

T a b l e 4. A n a l y s e s a n d C I P W n o r m s o f a r f v e d s o n i t e g r a n i t e s a n d a r f v e d s o n i t e a l b i t e g r a n i t e s , R i r i w a i

c o m p l e x

195

SiO 2

TIO 2

A1203

Fe203

FeO

MnO

MgO

CaO

Na20

K20

P205

820+

820-

F

C1

O=F

arfvedsonite granites

N80 N81 N82 N83 N84 N85 76.30 77.10 76.10 76,5 77.20 78.5

O. 18 O. 10 O. 14 O. 14 O. 15 O. 05

10.41 11.46 10.88 11.09 11.60 11.10

1.49 1.31 2.45 1.63 2.62 0.25

0.74 1.02 0.86 0.80 0.01 1.55

0.05 0.03 0.05 0.05 0.03 0.02

0.03 0.05 0.06 0,01 0.03 0.00

0.35 0.12 0.12 0.25 0.06 0.20

3.68 4.00 3.96 3.77 2.70 4.11

5 . 1 8 4 . 3 6 4 . 5 6 4 . 9 7 4 . 5 9 4 . 1 2

0 .00 0 .01 0 .01 0 .01 0 .01 0 .01

0 . 3 4 0 . 3 2 0 . 5 6 0 . 2 8 0 . 5 2 0 . 2 0

0.22 0 .08 0 .35 0 .20 0 .31

0 .24 0 .16 0 .10 0 .22 0 .05 0 .14

.40

- 0 . 1 0 - 0 . 0 7 - 0 . 0 4 - 0 . 0 9 - 0 . 0 2 - 0 . 0 6

T o t a l 99 .12 100,06 100.16 99 .83 99 .86 100.09

LI

Be

V

Cu

Zn

8b

9 r

Y

Z r

Nb

8n

Ba

La

Ce

Hf

Pb

Th

Co

Sc

Ta

Cs

Sb

Q

Or

Ab

C

Ac

8y

MS

Ap

Ha

I1

90 112 93 83 nd

27 18 8 11

9

117 126 117

235 340 500 478

298 386 396 411 421

9 3 5 8 3

114 43 234 590 99

63

17

8 6 5 6 1 5 1 1 0 3 8 1 6 8 8 3

158 2 0 2 4 3 4 2 4 5 2 8 8

25 25 50 17 4 0 8

40 1 12 81

111 33 28 139 14

2 1 5 181 70 2 8 6 2 2

20 19 41 26 31

70 60 89 55 82

111 66 156 61 87

1 . 4 0 . 1

0 . 2 0 . 2

31 34

1 1 .3

O. 04

3 6 . 5 1 3 6 . 7 8 3 6 . 1 4 3 5 . 8 8 4 3 . 9 0 3 7 . 7 3

3 0 . 6 0 2 5 . 7 6 2 6 . 9 4 2 9 . 3 6 2 7 . 1 2 2 4 . 3 4

24.71 33 .83 3 0 . 5 7 29.37 22 .84 34 .16

O. 16 2 .19

4 .31 2 .57 2 .21 0 .53

1 ,22 0 .80 0 .30 0 .68 0 ,07 2 ,76

1 .90 2 .26 1.25 0 .10

0 . 0 2 0 . 0 2 0 . 0 2 0 , 0 2 0 . 0 2

2 . 6 2

0 . 3 4 0 . 1 9 0 . 2 7 0 . 2 7 0 . 0 9 0 . 0 9

Na met 0 . 3 6

W a t e r 0 . 5 6 0 . 4 0 0 . 9 1 0 . 4 8 0 . 8 3 0 . 2 0

F l u o r 0 . 4 7 0 . 1 5 0 . 1 5 0 . 3 3 0 . 0 7 0 . 2 ~

T o t a l 9 9 . 1 1 1 0 0 . 0 0 1 0 0 . 1 4 9 9 . 7 9 9 9 . 7 4 1 0 0 . 1 8

arfvedsonite albite granites

N86 H87 N88 N89 N90

71.6 74.10 74.50 74.S0 70.00

0.23 0.14 0.36 0.13 0.27

13.54 12.19 10.45 11.47 14.50

1.85 1.79 2.14 1.89 1.63

1.05 1.15 0.66 0.83 0.39

0.07 0.07 0.09 0.01 0.13

0.01 0.01 0.01 0.08 0.13

0.01 0 . 0 4 0.02 0.32 0.14

6.79 5 .56 6 .04 5 .28 5.97

3 .79 4 .56 4 .55 4 .38 3.91

0 .01 0 .01 0 .01 0 .02 0 .09

0 . 3 7 0 .40 0 .48 0 .43 0 .83

0.01 0.01 0.07 0.08 0.17

0.94 0 .50 0 .53 0 .81

- 0 . 3 9 - 0 . 2 1 - 0 . 2 2 - 0 . 3 4 - 0 . 3 9

99 .88 100.32 99 .69 99 .69 99 .00

400 435 369 376

46 75 49 31

0

110 114 108 103

766 988 900 968

1 2 2 6 1303 1442 1450

0

144 549 154 414

3071 2 2 2 4 5552 3073

1604 854 1388 1494

50 50 67 58

0

70 65 64 83

232 197 221 260

103 89 268 127

103 311 254 485

126 217 128 318

0 . 7 0 . 0 4

0 .2 0 .1

312 282

6 .7 14.3

0 .6 2 .85

19.85 26.95 32.18 29.75 20.14

22 .39 26.94 26 .88 25.88 23 .10

48.54 37.31 28.42 34.61 50 .49

O. 45

5.35 5 .18 6 .19 5 .47

1 .80 2 .03 0 .80 1 .53 0 .32

O. 90

0 . 0 6 0 . 0 2 0 . 0 2 0 . 0 5 0 . 2 1

1.01

0.44 0.27 0 .68 0.25 0.51

0.65 0.89 3.64 0.89

0.38 0 .41 0 .55 U.51 1.00

0 . 0 4 0 . 0 1 0 . 4 1 0 . 4 5

99 37 100.04 99,38 99.34 98 .59


100

10

0 0 .C :

r r 0

a)

I 1 I I I I 1 I I I I I I I

La Pr Sm Gd Dy Er Yb Ce Nd Eu Tb Ho Tm Lu

c 0 0 0 - ~

r r 0

J( b ] N89 i ooI l o 1

I i i i i I i I i l I I Y|D I La Pr Sm Gd Dy Er

Ce Nd Eu Tb Ho Tm Lu

Fig. l l (a) . Chondrite-normalized rare-earth spectrums for arfvedsonite granite, Ririwai. (b) Chondrite-normalized rare-earth patterns

for arfvedsonite-albite granites. Analytical data in Table 2.

The main biotite granite is medium grained, pinkish in colour, with clustered biotite, although towards the eastern part there is a whitish variety which is more even textured with individual flakes of biotite. Textures may vary from coarse grained aphyric, with large feldspar crystals, to porphyritic, medium and fine grained aphyric variants. The biotite contains radioactive haloes around enclosed zircon, whilst fluorite which is common, may also be enclosed by biotite. Other accessories include columbite, xenotime and thorite. Iron oxides occur as secondary minerals formed during the breakdown of biotite. There is no primary ilmenite or titanomagnetite.

Sample N94 (Fig. 12) is the most pristine specimen of biotite granite and represents a chilled facies of granite magma in the roof. It is porphyritic with paramorphs of /3-quartz, mica-containing zircon with pleochroic haloes, and pale greenish grey euhedral perthite, up to 8 mm across, containing orthoclase and albite. The orthoclase component is the most disordered feldspar at Ririwai. In contrast, the whitish, slightly haematite-stained, ground mass of N94 is mineralogically quite different from the phenocryst assemblage. It consists of equigranular perthite, anhedral quartz and biotite. The feldspars are not braided but show a patch pattern with coalesced domains of more turbid K-feldspar and fresh albite. The structural contrast between phenocryst and groundmass K-feldspar reflects the importance of grain size in sub-

A L K A L I F E L D S P A R R I R I W A I

- - - 0

o

©

©

94

75

77

91

sur face

samples

A

A

A

A

-10

-95

-295

-441

core

LI3

P J

0 0 2

Q)- . . . . . . . . . 4~ 66g

0 4 0 6 0 8 1 0

K l ines

K + Na lines

Biotite granite

Fig. 12. Simplified XRD data for alkali feldspars from the Ririwai complex, Nigeria, expressed as number of diffraction lines for K and Na feldspar (adapted from Martin and Bowden 1981 ). Closed circles, phenocryst feldspar; open circles, matrix feldspar for surface biotite granite samples; Open star, feldspar composition in reddened wall

rock of Ririwai lode; Closed triangles, feldspar from core L13.

solidus re-equilibration. The finer grained feldspar in the ground mass has become more ordered by solution and redeposition in a fluid medium in response to rock- fluid interaction (Martin and Bowden 1981).

The other analysed samples of biotite granite (Fig. 12) are less peraluminous, devoid of orthoclase but contain perthitic intermediate untwinned microcline in which the original perthitic texture is highly disturbed. All samples are biotite-bearing (annite-zinnwaldite) and columbite is the most abundant opaque mineral.

Mineralization. The roof of the subvolcanic biotite granite shows a series of facies variations which were induced both by late stage magmatic crystallization and postmagmatic alteration by rock-fluid interaction. A sequence of subsolidus processes has mineralogically modified the original biotite granite. These processes have been important economically for the introduction of dispersed ore metals and lode-forming ores. The types of processes are similar to those associated with porphyry copper formation and have been covered in more detail for the Nigerian province as a whole in Kinnaird (1985). The major processes are sodic, potash-, H+-metasomatism, hydration and silicification with minor argillic and chloritic alteration. Each of these processes may be superimposed upon an earlier phase in a dispersed fashion. Along the central east-west axis of


R i r i w a i l o d e I r ;- ; ' : : : : . . . . . . . . . . . . . . . " ° ' " - N 9 1 N 5 8 N 6 6 N 7 5 " " . . . . . . . - . . . .

Fig. 13. Schematic diagram of relationships in a cross-section of biotite granite, Ririwai, showing location of mining levels, relative sampling

position of drill-core LI3, and surface sample locations.

the cupola, however, the fluids responsible for the processes of alteration have been channelled along a major mineralized fracture, known as the Ririwai lode. Drill cores taken to depth near the lode (Fig. 13) have shown all the postmagmatic modifications. Observations from core L13, taken to a depth of 445 m, have been crucial to the understanding of the alteration processes.

Geochemistry. The major element data displayed in Table 5 together with the CIPW norms indicate that the surface collection of samples contains material which has already begun to show indications of subsolidus fluid reactions. One very characteristic feature in hand specimen is the development of small "blotches" of haematite, often formed by the oxidation of opaque minerals and biotite. This development is verified by hm in the norm and by relatively high Fe3+/Fe 2+ ratios. The granites are characterized by normative c which is absent in the peralkaline facies. Generally there is a decrease in an and a concomitant increase in c with the intensity of mineralization.

The recast CIPW norms as felsic minerals are plotted into the Streckeisen and LeMaitre diagram (Fig. 4). The mineralized biotite granites shown as closed circles plot in the alkaline granite field at relatively higher Q levels (SIO2%) than other granite types at Ririwai. In contrast the L13 core samples, shown as open circles are depleted in quartz to give alkaline syenite compositions.

The trace element populations (Fig. 10) indicate that

100

rr ! 10 0 0 0 J: :

1

. • N78 _

I I I I I i I i I I I [ [ i


Fig. 14. Chondrite-normalized rare-earth patterns fl)r biotite granite, Ririwai complex.

subsolidus crystal-fluid reactions have been substantial in the surface samples at Ririwai. In particular, the mineralized roof zone is characterized by high trace element values of Zn, Sn, W, Nb, U, Li, Rb, Th, Y and F and a depletion in Sr and Ba relative to barren rocks of similar composition and compared to the average values for low calcium granites compiled by Turekian and Wedepohl (1961).

The rare-earth chondrite-normalized abundance patterns (Fig. 14) show significant depletions in both heavy and light rare-earths compared with the peralkaline granites (e.g. Fig. ll(b)). Crystal-fluid partitioning suggests that the rare-earths have been depleted due to rock-fluid interaction.

PROCESSES OF ALTERATION AFFECTING THE BIOTITE GRANITES

The alteration processes can be monitored by the subsolidus mineral reactions of the feldspars, micas and opaque minerals. The micas whose compositions (Table 6) lie within the annite-siderophyllite-zinnwaldite spectrum (Fig. 15) show gradational and abrupt compositional changes which can be linked to periods of disseminated, followed by vein controlled, mineralization. Occasionally, the original magmatic mica, or compositions near annite, break down to secondary ilmenite and futile assemblages which surround an essentially Ti-free magnetite.

Sodic metasomatism (albitization )

Sodic metasomatism is concentrated in the apical region of the biotite granite cupola and is the earliest alteration process. It is characterized by the development of new mica in the compositional range Li-annite through protolithionite to zinnwaldite, destruction of original Ti-Fe oxides, enrichment in uranium and rare- earths and introduction of columbite with minor cassiterite, thorite, xenotime, Th-rich monazite and Hf-rich zircon. There is a dramatic increase in normative albite balanced by a decrease in normative quartz.

198 J . A . K I N N A I R D e t a l .

T a b l e 5. A n a l y s e s a n d C I P W n o r m s of b io t i te g r a n i t e Ri r iwai c o m p l e x . N ige r i a

N75 N77 N78 N79 M91 N92 S I 0 2 7 5 . 9 0 7 6 . 8 0 7 7 . 9 0 7 6 . 3 0 7 5 . 9 0 7 6 . 3 0

TIO 2 0 . 1 1 0 . 0 7 0 . 0 4 0 . 0 7 0 . 1 0 0 . 1 2

A I 2 0 3 1 2 . 8 5 1 1 . 9 9 1 1 . 8 6 1 2 . 2 9 1 2 . 7 1 1 2 . 5 4

F e 2 0 3 0 . 3 3 0 . 4 7 0 . 1 8 0 . 5 5 0 . 3 9 0 . 2 4

FeO 1.05 0.97 1.17 1.02 1.12 1.21

I /n0 0 . 0 5 0 . 0 3 - 0 . 0 3 0 . 0 3 0 . 0 3

W80 0 . 0 2 O. 0 1 0 . 0 2 0 . 0 2 O. 01 0 . 0 1

CaO 0 . 2 4 O. 24 0 . 4 4 0 . 3 2 0 . 1 6 0 . 2 2

Na20 3 . 9 1 3 . 9 2 3 , 9 5 3 . 1 8 3 . 7 2 3 . 7 4

K20 4 . 3 1 4 . 3 1 4 . 1 9 4 . 5 0 4 . 5 0 4 . 3 3

P205 0.01 0.01 0.01 0.01 0.01 0.01

H20+ 0.46 0.17 0.24 0.33 0.24 0.32

H20 0 . 0 6 0 . 0 5 - 0 . 0 8 0 . 0 9 0 . 1 5

F 0 . 3 6 0 . 1 8 n d 0 . 2 5 0 . 3 5 0 . 2 8

O=F -0.15 -0.08 -0.10 -0.15 -0.12

T o t a l 9 9 . 5 1 9 9 . 1 4 1 0 0 . 0 0 9 8 . 8 5 9 9 . 1 8 9 9 . 3 8

L1 391 177 163 2 0 9 1 6 0 2 0 2

Be 7 8 7 7 3 5

V 0 0 0 0 5

Cu 1 2 0 1 2 0 0 1 1 0 1 1 5

Zn 376 2 7 0 2 4 8 2 9 2 4 5 0

Rb 9 7 9 6 0 2 6 5 1 7 1 7 6 2 8 7 5 1

S r 15 0 4 0 0 0

Y 6 9 6 1 3 9 152 75 1 1 3

Z r 3 9 9 195 2 4 9 230 2 5 8

Nb 2 1 4 1 9 9 2 1 6 1 8 3 1 7 9

Sn 4 0 I I 0 2 5 2 5 5 0

Ba 1 0 9 0 0 0 0

L a 2 3 4 6 5 5 0 39 92

Ce 296 98 95 7 3 1 4 2

H f 2 8 9 14 11 14

Pb 56 2 3 31 21 18

Th 1 1 1 6 2 7 3 78 7 3

N94 7 5 . 6 0

0 . 0 1

1 3 . 9 2

0 . 4 3

1 , 0 7

0 . 0 5

0 . 0 1

0 . 2 6

3 . 8 4

4 . 4 1

0 . 0 1

0 . 2 7

O. 10

0 . 4 2

- 0 . 1 8

1 0 0 . 2 2

2 7 8

8

5

0

3 1 0

9 1 6

0

147

2 5 5

181

2 5

0

5 3

1 4 9

14

26

71

Q 3 5 . 9 4 3 6 . 9 0 3 7 . 0 8 3 9 . 9 5 3 6 . 3 0 3 7 . 1 2 3 5 . 6 2

Or 2 5 . 4 6 2 5 . 4 6 2 4 . 7 6 2 6 . 5 9 2 6 . 5 9 2 5 . 5 8 2 6 . 0 6

Ab 3 3 . 0 7 3 3 . 15 3 3 . 4 1 2 6 . 9 0 3 1 . 4 6 3 1 . 6 3 3 2 . 4 8

C 1 . 7 6 0 . 8 8 0 . 0 5 2 . 1 9 1 . 7 2 1 . 7 0 2 . 8 3

Hy ~ En 0 . 0 5 0 . 0 2 0 . 0 5 0 , 0 5 0 . 0 2 0 . 0 2 0 . 0 2

Hy % F s 1 . 5 7 1 . 3 3 1 . 9 3 1 . 3 6 1 . 6 2 1 . 8 8 1 . 6 9

Ma 8 0 . 4 8 O . 6 8 O . 2 6 0 . 8 0 0 . 5 7 0 . 3 5 0 . 6 2

I I 0 . 2 1 0 . 1 3 0 . 0 8 0 . 1 3 0 . 1 9 0 . 2 3 0 . 0 2

Ap 0 . 0 2 0 . 0 2 0 . 0 2 0 . 0 2 0 . 0 2 0 . OZ 0 . 0 2

W a t e r 0 . 5 2 0 . 2 2 0 . 2 4 0 . 4 1 0 . 3 3 0 . 4 7 0 . 3 7

F l u o r 0 . 3 2 O. 32 - 0 . 4 3 0 . 2 0 0 . 2 9 0 . 3 4

T o t a l 9 9 . 3 9 9 9 . 1 3 i 0 0 , 0 0 9 8 . 8 2 9 9 . 0 4 9 9 . 3 0 IOO. 08

N D , no t d e t e r m i n e d . T r a c e e l e m e n t s d e t e r m i n e d by X R F (R. B a t c h e l o r ) .


Table 6. Microprobe analyses of mica from biotite granite, surface collection

N75 N77 Ngl N94 (Greisen mica)

core rim core rim

SiO 2 43.84 44.91 36.56 43.13 40.74 39.04 42.23

TiO 2 0.60 0.57 1.97 3.16 3.17 1.52 0.34

AI203 12.33 14.34 ii.05 10.75 ll.16 13.01 21.54

FeO 30.84 24.67 33.45 26.91 28.27 32.66 18.90 (T)

MnO 0.98 0.88 1.66 1.12 L 27 1.02 0.87

MgO 0.22 0.16 0.73 0.32 0.27 0.25 0.16

CaO 0.31 0.16 0.ii 0.18 0.16 0.16 0.20

Na20 0.32 0.36 0.49 0.34 0.36 0.42 0.36

K20 8.82 9.07 8.65 8.82 8.76 8.70 9.43

98.26 95.14 94.67 94.73 94.15 96.78 94.04

5i 7.03 7.18 6.37 7.09 6.84 6.51 6.64

Ti 0.07 0.07 0.26 0.39 0.40 0.19 0.04

A1 2.33 2.70 2.27 2.08 2.21 2.56 3.99

Fe2 4.14 3.30 4.88 3.70 3.97 4.56 2.49

Mn 0.11 O.lO 0.20 0.13 0.15 0.12 0.09

Mg O.OS 0.04 0.19 0.08 0.07 0.06 0.04

Ca 0.05 0.03 0.02 0.03 0.03 0.03 0.03

Na O.iEi 0.ii 0.17 O.11 0.12 0.14 0.ii

K 1.8[ 1.85 1.92 1.B5 1.88 1.B5 1.89

Structural formula calculated to 23 oxygens.

Samples taken from the surface, i.e. near the roof of the original intrusion, show some original perthite being replaced by aibite. In samples from core L13, through the granite, there is a diminishing proportion of albite within replacement perthite as depth increases (Fig. 12) until, at a depth of 295 m, the biotite granite consists of quartz, annitic mica and microcline perthite with little trace of albitization. At about 400 m, the core shows an abrupt change in texture and mineralogy until in sample L13-441,

from 441 m depth, an almost monomineralic albite-rich rock is encountered (Fig. 13), which is interpreted as the roof facies of a later granite at depth. The feldspar grains consist almost entirely of chequered albite-twinned domains. This albitite has been examined for the ore petrology related to sodic metasomatism (Fig. 16).

Mineralization associated with sodic rnetasornatisrn. Associated with sodic metasomatism is the development

M I C A R I R I W A I

. . ~ • Sid Z n w

77 " ' " ~ " Ann 2 3 4 5

F e 2. B i O t i t e g r a n i t e

Fig. 15. Simplified plot of mica compositions for surface samples of biotite granite Ririwai. showing variations from annite to zinnwaldite.

200 J .A . KINNAIRD et al.

Fig. 16. altered

Monazite Zircon

Ilmenite Cassiterite Wolffamite Columbite

TiO 2 Rutile Molybdenite

Sphalerite Stannite Pyrite Marcasite chalcapyrite Cubanite Pyrrhotite Mackinawite

Bismuth Bismuthinite

Galena Haematite Chalcocite Covellite

Albitite AheredWallrock QuartzVein -Greison

m

R

- 4

w

I

Blaubleibonder- Co~llim

Schematic paragenetic ore sequence of deposition for albitite, wall rock-greisen and quartz vein of the Ririwai lode. Note

ihnenite represents ilmenite and magne t i t e

of a dispersed mineralization. Early zircon and ilmenite followed by columbite, which is the dominant ore, are disseminated throughout the roof zone (Fig. 16).

The columbite is iron-rich and tantalum-poor. Six microprobe analyses are shown in Table 7 taken from samples N94 and N75. The average Nb205 value is 74.5% whilst Ta2Os at around 3% gives a very high and constant N b : T a ratio. In thin section, the columbite is seen as opaque, ragged skeletal prisms which may be occasionally reddish and translucent particularly near

grain margins. The columbite is often associated with clusters of pale mica in the compositional range protolithionite to zinnwaldite. Opaque iron-titanium oxides are small, limited in distribution and appear to have formed as a breakdown product of earlier formed bio- tites. Xenotime occurs as colourless, perfect prismatic crystals. Coarse grained haematite laths are common.

The episode of soda metasomatism and its ac- companying mineralization appears to be spatially, chemically and genetically independent of those associated with the formation of the Ririwai lode.

Erosion of the albitized roof zone, which is now exposed at the surface, is believed to be responsible for the alluvial and eluvial ore deposits of these minerals which have been worked for centuries.

Geochemistry. The geochemical data (Table 8) indicates that the early exsolved fluid responsible for albitization, in addition to enrichment in soda must have contained important concentrations of Fe combined with U, Th, Zr, Nb, and HREE. There is also enrichment in Zn and particularly Sn (M. Jones personal communication).

Norm calculations show increasing ab in Table 8 from 32 to 69%. In Fig. 17 the open circles represent samples from the drill core L13, taken at different depths with the albitite L13-414 and 440 occurring closest to the albite pole and shown as circles within a cross.

Potash metasomatism

Potash metasomatism is more restricted than the earlier process of albitization. The effects are most noticeable within the Ririwai lode and in localized alteration pods in the cupola zone of the biotite granite, immediately beneath the volcanic pile at Uwar Gida (Fig. 2).

Tablc 7. Microprobc analyses ot cohuubitc

Sample N94 N94 N75 N75 N75 N75

Point 12 13 8 9 lO Ii

Nb205 76.10 74.75 74.28 73.62 74.49 74.02

FeO 18.10 18.21 18.24 18.04 18.18 18.38

Ta205 3.60 3.12 3.14 3.33 3.16 3.14

MnO 3.13 2.41 2.50 2.40 2.59 2.45

TiO 2 0.26 1.38 0.55 0.52 0.71 0.66

5nO 2 [).07 0.05 0.13 0.15

ZnO 0.03 0.03

181.29 99.92 98.74 98.05 99.28 98.65

N54 N94 N75 N75 N75 N75

Nb 1.934 1.913 1 . 9 3 2 1 . 9 3 0 1 . 9 2 6 1.926

Fe [.851 0.862 0.877 0.875 0.869 0.885

Ta C.055 0.048 0.049 0.053 0.049 0.049

Mn 0.149 0.i16 0.122 0.118 0.125 0.119

Ti O.Oll 0.059 0.024 0.023 0.031 0.029

5n 0.002 0.001 0.000 0.003 0.003 O.O00

Zn 0.001 0.000 0.001 0.000 0.000 O.O00

Structural formula calculated to six oxygens

Ta

ble

8.

An

aly

ses

an

d C

IPW

n

orm

s o

f c

ore

L I

3 i

n b

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te g

ran

ite

, R

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co

mp

lex

, N

ige

ria

SiO

2

TiO

AI2

~ 3

F

e

0 3

MnO

M

gO

CaO

P2

0

H20~

H2

0-

F O=

F

Li

Be

Cu

Zn

Rb

Sr

Y

Zr

Pb

Th

U

Q

Or

Ab

An

C

Ac

Di

%

~o

D

i %

E

n

Di

% F

s

Hy

% E

n

Hy

% F

s

Mag

I1

Na met

Ap

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ter

Flu

or

To

tal

lOm

26

75

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35.93

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1.04

0.53

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1.75

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K 7~

5-

ffQ

:r"

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3 p~

5 O 3 ..,

-j

x

ND

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de

tcrm

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An

aly

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[. A

ba

a

(19

76

).

t,o


Q

1 •

2 0 3 ~

4<>

Ab 0~"

Fig. 17. CIPW normative salic constituents for all the Ririwai rock types described in the text, plotted in Q-Ab-Or triangular diagram. (1) Biotite granite surface samples, (2) albitised biotite granite, (3) albitite below 400 m, (4) microcline-rich wall-rock to Ririwai lode, (5) pervasive greisen assemblage Ginshi Hill, (6) quartz vein-greisen

assemblage, Ririwai lode.

Potash metasomatism is characterized by the generation of intermediate to ordered microcline which is red in colour (due to expulsion of haematite laths 1-2/zm long from the feldspar lattice), mica ranging from original annite to siderophyllite, an increase in K20, Rb, Li and Zn, loss of Na20 and the introduction of a disseminated oxide mineralization dominated by cassiterite and wolframite.

In thin section, the microcline feldspar generated during potash metasomatism appears as a low birefringent mineral without the characteristic cross-hatch twin- ning. XRD studies by Martia and Bowden (1981) sum- marized in Fig. 12, for sample N66, indicate that compared with the host biotite granite (open circle), the feldspar generated during potash metasomatism (open star) is more fully ordered and compositionally microcline. It is accompanied by mica which is usually pleochroic from deep green to brown and rimmed by ferrous siderophyllite. As potash metasomatism continued to lower temperatures mica compositions changed to lithian siderophyllite accompanied by the breakdown of alkali feldspar to give a topaz-sericite assemblage. Such compositional variations are represented by the zone between N75 and the greisen mica (g) on Fig. 15.

The potash metasomatism in the roof zone has resulted in the formation of microclinitic cavities containing microcline (Fig. 18). The reddened wall rock of the Ririwai lode which is also produced by potash metasomatism is described in more detail in the section on the lode.

Vesicular, pink quartz-free rocks, that resemble the albitites described earlier are found near the alteration aureole associated with a tangential series of braided quartz veins, south-east of the Adit (Fig. 2). Martin and Bowden (1981) describe a sample from this area which consists of a well-ordered microcline predominating over albite, with cassiterite and fluorite in the cavities.

The dominance of microcline in the wall rock alteration zone of the lode and the appearance of microclinites suggests that the earlier trend of Na for K replacement during albitization was locally reversed, probably as a result of pressure release (Fournier 1976).

Mineralization associated with potash metasomatism. Associated with the potash metasomatism, there is the development of a disseminated oxide assemblage of ore minerals. This assemblage, dominated by cassiterite and wolframite, has only made a small contribution to the mineralization within the biotite granite as a whole, since the process of potash metasomatism is less widespread than that of soda metasomatism and not as well developed as the process of greisenization. Cassiterite and wolframite are accompanied by tabular crystals of monazite, clusters of highly zoned euhedral zircons and columbite crystals rimmed by rutile. The detailed ore mineralogy of this red-coloured, quartz-microcline wall rock is described in detail in the section on the lode. The paragenetic diagram of Fig. 16 gives a simplified idea of the relative amounts and paragenetic position of the various ore minerals.

Geochemistry. Chemically the potash metasomatism is characterized by an increase in K 2 0 , Rb, Li, Zn and a loss of Na20 (Table 9) and the introduction of an oxide ore assemblage. Norm calculations show a dramatic increase in normative or balanced by a decrease in ab so that in the Q-Ab-Or diagram (Fig. 17) the salic compositions plot away from the central biotite granite field towards the Q-Or join and are represented by open diamonds.

Trace element populations are further depleted during potash metasomatism of biotite granite, as shown in Fig. 10. This effect is demonstrated by the dotted zone confined to the lower concentration levels for Rb-Y.

Similar depletions are recorded for the whole rare- earth spectrum. In Fig. 19, which is a summary diagram showing the chondrite-normalized curves for various rock types already discussed, the potash-metasomatized wall rocks (shown as open diamonds) are significantly the most depleted. The lower temperature process of potash metasomatism is characterized by the release of most rare-earths from the various host minerals to the pervading fluids. The overall decrease is quite dramatic and must be related to the destabilization of rare-earth host minerals in the sodic-metasomatized rocks.

H ~ metasomatism (greisenization) and hydration

The petrological characteristics and the extent of greisen development depends upon the extent and intensity of earlier processes. Thus H* metasomatism may affect perthite granite not affected by processes of sodic or potash metasomatism, or may be superimposed on an already altered assemblage.

The H + metasomatism may be a pervasive process within the roof zone, occasionally producing pockets associated with microclinite, or it may be concentrated


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0 r -

rr 0 1

" ~ Q ~ ' F ' ~ + ' " + ' - - - - - - - - - - - - + ~ '

• i i I i i J i i i i l I I


Fig. 19. Summary chondrite-normalized rare-earth patterns for Ririwai compared with "parental" syenite from Kila Warji, Nigeria. (1) Straight crosses, arfvedsonite albite granite; (2) "parental" syenite;

(3) biotite granite; (4) potash metasomatized wall rock.

along fissures and fractures. The most important H + metasomatism has resulted in the formation of the Ririwai lode.

H + metasomatism is characterized by the destabilization of the granitic minerals. Sericite and topaz are generated from alkali feldspar, whereas albitized feldspar breaks down to fluorite, cryolite, and topaz with some montmorillonite. Micaceous aggregates, chlorite, or more rarely kaolinite may form at the expense of microcline. Mica compositions range from green siderophyllite, appearing initially as rims to the more annitic brown mica, through protolithionite to zinnwaldite (Fig. 15). There is a major deposition of an oxide assemblage of ores again dominated by cassiterite and wolframite (Fig. 16).

In the roof zone in the region of LIwar Gida (Fig. 2), pervasive H + metasomatism has utilized incipient micro- fractures in the host rock to penetrate and transform the original mineral assemblage. The resulting greisenization (samples 6A, 6B, 6C, located in Fig. 8) is superimposed on remnant albitization and partial microclinization of the porphyritic biotite granite adjacent to the volcanic cover.

Mineralization associated with H + metasomatism. Associated with the H + metasomatism, there is the development of an oxide assemblage of ore minerals similar to that associated with potash metasomatism. The assemblage is important economically, particularly for the abundant cassiterite. Erosion of the roof zone in the centre of the complex has undoubtedly added some cassiterite to the alluvial deposits which was originally introduced into the roof by pervasive greisenization. However, the vein-forming greisens are now being considered as a primary source of tin. Figure 16 gives a simplified idea of the ores associated with this alteration process and the relative paragenetic position of each.

Geochemistry. Chemically the development of a greisen shows a marked decrease in potash and alumina (Table 9) due to feldspar breakdown. There is a comp- lementary increase in Li content due to the development of Li rich micas. There is a substantial enrichment in Sn,

o o o J :

1 0 0

10

N58B

1 i i I I i i I I I I i I i


Fig. 20. Chondrite-normalized rare-earth patterns for potash metasomatized wall rock, Ririwai lode (N58A) compared with greisen

and quartz vein (N58B).

Pb, Zn and Fe-- the latter element occurring within both the micas and ore minerals--and an increase in the U/Th ratio.

Norm calculations (Table 9) show a distinct decrease in Or with a substantial decrease in Ab and concomitant increase in Q. In the Q - A b - O r plot of Fig. 17 they are represented by closed inverted triangles which lie on the Q-Or join towards the Q pole. In contrast, the pervasive greisenization recorded at Uwar Gida, represented by the RS6 and RS31/3 series of samples in Table 9 (and as inverted triangles in Fig. 17) is superimposed on remnant albitization and partial microclinization of the porphyritic facies of the biotite granite adjacent to the volcanic cover. Norm calculations on the RS6 and RS31/3 series reflect the more intense albitization and plot towards the Ab-Q join close to the Q pole of Fig. 17.

The inverted closed triangles in Fig. 10 represent the trace element populations of the greisen compared with the other rock types discussed earlier. It shows that there is some enhancement in Th, Ce and Y relative to the potash metasomatized rocks.

Similar enhancements for the chondrite-normalized rare-earth spectrum for 58B compared with 58C are observed in Fig. 20. In particular, there is partitioning of the LREE to the greisen mineral assemblage (development of monazite), a slight enrichment in Eu coupled with increasing Yb and Lu (possibly related to fluorite and xenotime), with the interpolated HREE spectra of Dy, Ho, Er and Tm partitioned slightly to the fluid. Thus the lower temperature acid fluids responsible for greisenization, deposit a selection of REEs as accessory minerals in the greisen and the destabilized wall rocks.

Silica metasomatism (silicification )

Throughout the granite outcrop, minor silicification is present in the form of deposition of quartz into vugs created by the earlier potassic alteration or greisenization. Often this quartz is accompanied by small cassiterite or sphalerite crystals. Like the earlier process of greisenization, pervasive silicification is most noticeable in the roof zone of the cupola in the region of Uwar Gida. However, most of the siticification is associated


CROSS CUT

///'./~JJJJJJJ.,~. qua r l z ve i n w i t h sDhaPe r l l e - -

. . . . . . . T ~ ~ ' / / ~ - - g . . . . . .

m= r qne ua r O 0 0 0 O 0 0 wa l l t ock asbemb lage - -~ ; * 0 0 O 0 0

NO I W! ST

o 1 I I I

ME TPES

13 1h=1 g re l sens

TO RAMP

WES ' [ RAMP

--1I " " ][ I I -:. ~ No 2 l e v e l

No 5 l e v e l

Bas i c dyke

0 100 2 0 0 3 0 0 I I I I

METRES

EAST RAMP

Fig. 21. Sections showing (A) mineralogical variations across the lode at the No. 1 level intersection with the east ramp (taken from a field report compiled by J. N. Bennett 1975); (B) the extent of underground mining operations in the Ririwai lode (cross-section (B) adapted from the Ririwai project report RIR80-4 (Nigerian Mining Corporation)). Small asterisk below east ramp is location of cross-section (A). No. 1 level is equivalent to the 100 level of Fig. 13, No. 2 to the 200 level, etc.

with quartz veining which has its greatest development in the region of the Ririwai lode. Along the lode also, there is a superimposition of silicification on the vuggy, red wall rock with deposition of quartz in cavities.

The quartz veins of the lode, Makota path and other areas are up to 75 cm in width and are steeply dipping. In the lode, a series of en-chelon veins strike east-west. Elsewhere they trend north-east-south-west. The narrower veins contain glassy, idiomorphic long-prismatic quartz crystals, perpendicular to the walls of the fracture zone, whereas in the larger veins the quartz is generally massive and milky in colour.

Mineralization. The silicification process is economically important for the introduction of a series of mainly sulphide ore minerals, dominated by sphalerite (Fig. 16). Cassiterite is the first ore mineral to be deposited and occurs as large crystals at the vein margins. It may also be intergrown with sphalerite. The sphalerite forms large crystals and masses up to 1 m across in the Ririwai lode. The crystallization of cassiterite and sphalerite is followed by chalcopyrite and later galena. Minor quantities of stannite, pyrrhotite and mackinawite accompanied by traces of cubanite, native bismuth and bismuthinite occur as inclusions in the major sulphides.

RIRIWAI LODE

The east-west-trending lode cuts across the centre of the elliptical biotite granite stock (Fig. 2), exploiting the major east-west joints and can be traced for more than 5 km. The lode dips to the south at 85 ° and extends downwards for more than 400 m.

The lode comprises a series of parallel to subparallel or braided quartz veins, separated by zones of grey greisen, grading outwards into a reddened microclinized

wall rock--occasionally through a buff-coloured zone - - into the normal pale pink biotite granite. Projections from the surface have shown that although the lode system widens initially with the first 10 m from the surface, it remains constant in thickness with depth. Only the proportion of quartz vein to greisen vein varies. At 100 m the greisen is more dominant than at 30 m or at the surface where small individual quartz stringers are numerous. The maximum surface width of the lode, including the reddened wall rock is 8 m (Fig. 21).

The western end of the lode is well exposed at the surface in the region of the Adit (Fig. 2). The lode narrows in width eastwards and in the Uwar Gida region can be traced as a series of thin quartz-rich stringers which extend for a short distance into the volcanic cover.

The lode is extensively mineralized. An oxide assemblage of ore minerals, dominated by cassiterite, occurs within the greisen and wall rock. A sulphide assemblage of ores, dominated by sphalerite is introduced with the quartz veins. Although sphalerite is the most abundant ore, the lode has only been worked for cassiterite and wolframite. About 50 tons of wolframite were extracted from the west of the lode during the 1939-1945 war and small scale cassiterite extraction has taken place over several decades. During the 1970s the Nigerian Mining Corporation opened up the lode with two ramps leading to levels at 30 m intervals to a depth of 170 m. The estimated production figures suggest that when in production the mine could produce 1600 tonnes of tin metal a year and around 6000 tonnes of zinc metal.

Despite localized concentrations of certain ores, e.g. galena at the Adit and wolframite in the west, there is no obvious zonation of the ores, either along strike or at depth. However, since sphalerite is rapidly oxidized at the surface, there is a marked increase at depth.

The processes of K +, H + and Si 4+ metasomatism have been important in lode formation. The formation of a


microcline-quartz wall rock assemblage produced by K + metasomatism, has been followed by greisen and quartz vein formation. The characteristics of each facies is distinctive both in feldspar and mica compositions and in its ore assemblage.

Microcline-quartz wall rock assemblages

A distinct microcline-quartz (mica) wall rock, usually brick red in colour, and known colloquially as reddened wall rock, grades into the grey-green greisen (Fig. 22). Texturally it may appear similar to the equigranular unaltered pink perthitic granite into which it grades, sometimes through a zone of buff-coloured granite (Fig. 23G). Chemically the microcline-quartz wall rock con- trasts with outer zones and the greisen zones of the Ririwai lode (Figs 22-24).

Hand specimens show abundant coarse, clustered quartz grains, 4 mm in size, feldspar--partially altered to sericite or chlorite--and scattered flakes of biotite, which may also have been chloritized.

In thin section there is a rapid transition from remnant perthitic biotite granite through reddened feldspar- quartz-mica-sericite (topaz) to quartz-mica greisen. The pink, medium-grained biotite granite contains an assemblage of high purity intermediate microcline (riO -- 0.91-0.70) and fully ordered pure albite in subequal amounts. On entering the red zone, the perthitic feldspars become very turbid, the brick-red feldspar is structurally intact and is microcline close to the ordered configuration (t10 = 0.95-0.93) (Martin and Bowden 198l).

Vuggy, pink, quartz-free rocks have been found locally. In these, well-ordered microcline (tlO = 0.92- 0.89) predominates over albite and cassiterite and fluorite occur in some of the cavities. In the area of the Adit a sphalerite microclinite showed complete K for Na exchange with the reddish microcline almost fully ordered (rio = 0.97-0.95). The dominance of microcline in the wall rock alteration zone and the appearance of ordered microclinite suggests that the earlier Na for K exchange associated with albitization of perthitic feldspars was locally reversed during cassiterite and sphalerite mineralization.

Transition zones. Between the red microcline-quartz wall rock and pink perthitic equigranular granite host rock a zone of buff-coloured granite is sometimes developed. X-ray studies of the feldspar show that the microclinization process is only partial and also that there has been a very minor degree of chloritization and kaolinization. The development of kaolinite may be a peripheral effect during falling temperature combined with increasing H ÷ concentration in the hydrothermal fluid. Sample R1/20A, represented by a dot-centred diamond on the Q-Ab-Or diagram (Fig. 17) was collected from this transition zone.

An inner transition zone also occurs where microcline becomes unstable, probably under the influence of

increasing H + in the hydrothermal fluids to give the greisen assemblage. There is a gradation from red wall- rock through a chloritized microcline zone to the grey- green greisen assemblage.

Greisen zone. Texturally the greisen shows a granitic texture peppered with abundant tiny, microclinitic cavities which may be filled with quartz, cassiterite, colourless fluorite or pale coloured sphalerite. The greisen matrix consists of abundant anhedral unstrained quartz grains accompanied by pale blue-green lithium siderophyllite and zinnwaldite mica. Original feldspars have been totally destroyed and replaced first by chlorite and subsequently by lithium siderophyllite with overgrowths of pale coioured zinnwaldite and fluorite along mica cleavages. In addition, small rounded grains of topaz, up to 1 mm in diameter are developed from the feldspar breakdown. Prismatic needles of Th-rich monazite are associated with fluorite and lithium siderophyllite.

Quartz veins. The quartz veins vary from 5 mm to 75 cm in thickness. They are at their widest towards the centre of the lode, As many as ten separate quartz veins can occur over a horizontal distance of 7 m. Generally the narrower veins contain glassy, prismatic crystals perpendicular to the walls of the fracture zone. The larger veins are usually massive, milky coloured quartz.

Some large cavities, up to 100 cm in size may be infilled with kaolinite and twinned euhedral cassiterite crystals up to 1 cm in size. Others show an assemblage of quartz crystals with long prism faces and short pyramids, extending into the cavity with cassiterite and sphalerite. Massive sphalerite is the dominant ore of the massive milky quartz. Near the Adit, the proportion of galena increases. As the lode narrows rapidly, further to the west, the only important ore is manganoan wolframite. By contrast, at the eastern end of the lode there is no wolframite and towards Kerigateri hill the quartz veins appear to contain only disseminated sulphides.

Ore mineralogy of the lode and wall rock

Polished ore studies of samples taken from the lode show that there is an early phase of oxide mineralization associated with mica growth in the greisen and microclinized wall rock. This is followed by a later sulphide dominated mineralization associated with the fissure- filling quartz vein formation. Thus there is a paragenetic sequence from early monazite and zircon, through cassiterite, wolframite and TiO2 minerals to later sphalerite, galena and chalcopyrite (Fig. 16). Early formed minerals are consistently enclosed by later phases. The early formed phases are small in size, up to 1 mm commonly, and are disseminated throughout the greisen and wall rock. The size of many of the minerals within the greisen increases towards the cross-cutting quartz vein and is at a maximum at the contact between the quartz vein and greisen.

M i n e r a l o g y , g e o c h e m i s t r y and mine ra l i za t ion of Rir iwai complex 207

Fig. 18. Microclinite generated adjacent to the Ririwai lode by potash metasomatism. Field width 2.5 cm.

208 J .A. K1NNAIRD etal.

(a) C B A G

(b)

SiO 2

85%

80

75

B A G

AI203

~ 10%

5

C B A G

2000

°i ° C B A G

Fig. 22, Cut-section of specimen N58 showing zones A, B, C and G used for chemical, trace and rare earth analysis (a). Elemental variation diagram (b).

C arid A = reddened microcline-~quartz wall-rock assemblage formed by K + metasomat ism, B = greisen zone formed by H + metasomat ism, G = pink coloured biotite granite.

Major and trace clement data used to construct variation diagrams are tabulated in Table 9.


(a) C B A G

• --___ SiO 2

C B A

85%

80

75

G

AI203

10%

C B A G

5000ppm

Zn 4000

3000

~ 2000

/ \ \,000

\o C B A G

Fig. 23. Cut-section of specimen R1/20 (a) showing chemical variation zones A, B, C and G in (b). Complete analyses m Table 9.

C = greisen zone, B = reddened microcline~luartz wall-rock with chlorite and miarolitic cavities, A = buffcoloured transition zone between red coloured microclinc~quartz wall-rock and pink biotite granite, G = pink biotite granite.


(a) A B C

A B

85%

75

10%

N a 2 0 , 0

20000 ppm

4000

3000

2000

~ ~ looo

Pb

0 A B C

Fig. 24. Cut-section of specimen R1/23 (a) with significant chemical variations (b) shown as zones A, B and C. Note that this sample does not display marginally argillic alteration. Analyses taken from Table 9.

A and C = greisen zones, B = mierocline-quartz wall-rock.


(b microcline Quartz

chlorite (quartz)

I -ZONE 1

-I I ZONE 2-_1 OZONE 31

Fig. 27. Sample slice (a) across a portion of the Ririwai lode (first level) showing mineralogical variations, and zones (b) used to assess particle track variations and element concentrations (taken from MacKenzie et al. 1984).


Cassiterite, which is the first mineral to be deposited at the quartz vein stage forms large crystals at the junction between greisen and quartz vein and is intergrown with, or rimmed by, sphalerite. Sphalerite is by far the most abundant ore mineral of the quartz vein and of the lode as a whole. The suiphides are often very coarse grained with sphalerite and some galena, up to 1 m across at the junction between the quartz and greisen. Smaller amounts of chalcopyrite and traces of cubanite, native bismuth and bismuthinite occur as inclusions in the major sulphides.

The paragenetic sequence in Fig. 16 gives a simplified presentation of the relative amount of each ore mineral and the comparative order of appearance. However, several minerals form more than one phase, cassiterite and sphalerite have at least two important generations of deposition and there are three distinct phases of chalcopyrite inclusion in sphalerite.

The following descriptions of the ore minerals therefore attempt to discuss the sequence of ore formation and the characteristics of each of the ore generations.

Monazite occurs in minor amounts as 100 x 10 to 300 x 60 /~m yellow lath-shaped crystals, which are associated with cassiterite and molybdenite within the micas of the greisen and wall rock. Often it is haematitic- ally stained or contains traces of enclosed chalcopyrite and pyrite 10-20/zm.

llrnenite is rare forming 20 x 2 to 120 × l0 p~m laths together with traces of magnetite crystals up to 60 ~m in diameter. Usually it is associated with laths of orange TiO2 within the wall rock. Both ilmenite and magnetite are altered to fine grained haematite and TiO~.

Zircon forms euhedral, highly zoned crystals 20-100 #m in length within mica, or up to 200 p.m between quartz crystals, in both the wall rock and greisen. Zircon is common but is especially concentrated at the junction between the greisen and the quartz vein. Collections of between 5 and 30 zircon crystals form aggregates up to 400 /~.m in diameter. The zoning has different reflec- tances related to compositional differences, and some zones carry haematite dust within them. Coarser grained haematite and pyrite is associated with zircon as inclusions, 5-15 ~m in size or rims, 5-10 ~m in width. Zircon itself forms the nucleus to aggregates of cassiterite, TiO2 minerals, columbite or wolframite crystals.

Early generations of cassiterite occur in the wall rock as small 10-150 # m euhedral, light coloured, zoned and simply twinned crystals; or as larger darker coloured crystals (50-800 p.m in diameter) within the micas of the greisen. Collections of smaller cassiterite crystals up to 300/zm in diameter, often surround zircon or monazite, whereas cassiterite itself is often surrounded by, or enclosed within, euhedral TiO2 minerals, columbite, wolframite, molybdenite and sphalerite crystals (40-100 ~m). Many cassiterites carry abundant 2-6 /~m inclusions of columbite and TiO2. This generation of cassiterite is replaced by mica and quartz. A later, darker-coloured and coarser grained generation of cassiterite with crystals up to 1 cm across occurs within the central quartz-sulphide-cassiterite veins. Here cassiter-

AES 3:1/2-N

ite is the first mineral to precipitate and hence forms the margins to the quartz veins which later carry sulphides. However , intergrowths of cassiterite and sulphides have been observed and some sphalerite is enclosed within 40-300 p~m cassiterite rims. More commonly, however, cassiterite crystals are replaced by sphalerite and galena.

Wolfram#e, like cassiterite, has at least three generations. The earliest consists of small lath-shaped crystals 30 × 5 to 350 x 20 p,m within the greisen micas. Wolframite encloses both zircon and cassiterite but itself is enclosed in TiO2. This is followed by a later generation which consists of larger tabular crystals up to 0.2 cm in length which are twinned, slightly zoned and are within the quartz of the greisen. These crystals are extensively replaced by quartz, so giving irregular aggregates of "smaller crystals". The latest phase of wolframite formation is associated with the quartz veins. Large lath- shaped crystals, several centimetres in size, of twinned manganoan-rich wolframite occur between prismatic quartz crystals.

There are at least three generations of TiO 2 minerals and all occur only within the micas of the greisen or wallrock. The earliest is a minor generation that comprises orange, poorly-polished subparallel laths 100 × 10 to 300 × 20 p~m in size, which are associated with smaller ilmenite or wolframite laths, This generation of TiO 2 is cemented by the main TiO2 generation. This generation forms euhedral equant crystals 20-60 gm, laths up to 200 × 20 ~m or even prismatic aggregates up to 500 x 100 p~m. Elongated crystals normally have their long axis parallel to the cleavage of the enclosing mica. White, yellow, brown, blue or green internal reflections are abundant as are polysynthetic or sector twins. TiO2 encloses and cements all earlier phases but typically is radially overgrown on 5-25 ~m cores of columbite. Large crystals of columbite are uncommon but reach 300 x 150/xm in size and enclose cassiterite.

Minor amounts of molybdenite occur in the wallrock but most is found within or about the micas of the greisen. It is concentrated at the greisen-quartz junction, or occurs between quartz crystals. Within mica it forms fibres 20 x 1/~m, and laths 30 x 5 but up to 250 × 40 p,m which surround all earlier non-sulphide ores. However, much molybdenite forms radiating clusters (80-200 /xm in diameter) of fibrous crystals between quartz or mica crystals. Only very minor amounts are intergrown with other sulphides, mainly sphalerite and galena.

Sphalerite is found in small amounts in the wall rock or within the micas of the greisen but mainly as the major sulphide of the quartz vein. In the wall rock and greisen, smaller anhedral crystals 40-200 ~m in size have white, blue-green or commonly citrone-yeliow internal reflections. They are inclusion-free, enclose earlier non- sulphides, or chalcopyrite and stannite inclusions (2-10 p.m). Larger sphalerite crystals 150-800 p.m have dark orange to brown internal reflections, often with lighter margins and carry rounded chalcopyrite or stannite inclusions, usually within their cores. This sphalerite is intergrown with galena as 40-100 p.m inclusion-free

214 J. A. KINNAIRD et al.

rims around galena. Sphalerite in the quartz vein is coarse grained up to 1 m or more in size, and has dark internal reflections. It is the earliest sulphide of the veins and hence is intergrown with, or forms rims around, coarse cassiterite. Slighly later sphalerite has complex intergrowths with chalcopyrite, which are usually accompanied by a thin 2-10 txm stannite rim between them. Here sphalerite occurs as 10-150 p.m anhedral crystals or 5-20 txm sphalerite stars within chalcopyrite, but mostly chalcopyrite occurs as inclusions that range in size from l -2 p~m to 0.1 cm within sphalerite. Other inclusions within sphalerite are stannite, pyrrhotite, galena, plus rare pyrite and molybdenite. The sphalerite is extensively fractured and replacement by galena veins, 2-10 txm wide, along sphalerite cleavage and fractures is extensive. Along many fractures, and around some large chalcopyrite inclusions, a 10-30 p.m wide zone of light coloured sphalerite containing abundant crystallo- graphically oriented chalcopyrite inclusions (1-2 p.m) in a herring-bone pattern is present.

Stannite occurs within sphalerite as small inclusions 2-10 p.m in diameter, which are either randomly orientated or occur along its growth zones, accompanied by similar sized chalcopyrite blebs. Some sphalerite rims also have very fine (0.1-1.0 txm) crystallographicatly oriented stannite inclusions. Most stannite is, however, associated with chalcopyrite which itself is within sphalerite. Here it forms 2-20 /~m wide rims between the chalcopyrite and sphalerite and replaces the sphalerite. This stannite displays slight optical zoning with lighter reflectance values away from the sphalerite towards the chalcopyrite. Much stannite is found as inclusions within chalcopyrite, it occurs as 10--40 but up to 100 tzm diameter stars (accompanied by sphalerite stars), small irregular blebs 5-20 txm, or as myrmekitic intergrowths with chalcopyrite up to 250 x 50 txm in diameter. Traces of stannite are found as inclusions within galena. Larger stannite crystals (60-100 Ixm) are zoned and contain very fine grained exsolved chalcopyrite within their cores.

Trace amounts of chalcopyriw occur within the micas of the greisens or as coarse inclusion-free crystals up to 0.5 cm in the quartz veins. Most is, however, intimately intergrown with sphalerite, either as coarse grained crystals up to cms in size or more commonly as smaller inclusions in sphalerite. At least three distinct generations of chalcopyrite inclusions occur. Subhedral to euhedral inclusions of chalcopyrite 10-20 p.m in diameter themselves contain inclusions of pyrrhotite, cubanite and mackinawite; large irregular chalcopyrite 100 txm-0.1 cm masses with stannite rims and inclusions, enclosed in a rim of light coloured sphalerite with orientated chalcopyrite dust; and 5-150 p~m irregular, inclusion-free chalcopyrite which may or may not be enclosed in a 10-30 m rim of light coloured sphalerite with herring-bone orientated chalcopyrite dust. The first two generations of chalcopyrite inclusions are quite discrete and are not seen together within one sphalerite. Minor amounts of chalcopyrite 40-100 Ixm in size occur as a discontinuous rim between sphalerite and galena in

some quartz veins; as rare inclusions in galena 10-100 p,m; or associated with native bismuth and bismuthinite in

quartz veins.

Pyrrhotite-cubanite, mackinawite. The relative abun- dances of these three phases within chalcopyrite is mackinawite >> pyrrhotite > cubanite. Both cubanite and mackinawite only occur within chalcopyrite but pyrrhotite also forms discrete inclusions within sphalerite, 20-40/xm in diameter or thin laths up to 200 x 2 p~m along sphalerite cleavage. Within chalcopyrite, cubanite laths 20 x 1 to 60 x 15 p.m enclose small 5-10 /~m pyrrhotite. Pyrrhotite grains in adjacent chalcopyrite inclusions are often in optical continuity. Mackinawite forms as irregular flames and ribbons 5 x 1 to 30 x 5 p.m in size growing out from pyrrhotite and replacing both cubanitc and pyrrhotite.

Pyrite is more common than marcasite but both only occur in minor amounts. There are a number of generations. An early generation is associated with the main sulphide mineralization. Here pyrite cubes 5-125/xm in diameter, anhedral marcasite 20-40 p~m, or fine grained intergrowths of the two, to 40 txm, occur within chalcopyrite inclusions. Larger pyrite crystals, 100-150/xm in diameter, occur within quartz, they contain marcasite inclusions 10-2(t p.m, and are replaced by chalcopyrite. A later generation of poorly crystalline pyrite and marcasite, associated with late TiO2 form aggregates 60--400 /xm in diameter which replace mica, initially along its

cleavage. Native bismuth, accompanied by mosaics of bis-

muthinite crystals both up to 150 txm in diameter are associated with chalcopyrite in veinlets cutting quartz and sphalerite. Smaller crystals of native bismuth also occur as acicular crystals 1-10 p~m in length within galena or within bismuthinite. A little bismuthinite, 150 x 30 p.m in size, is associated with wolframite.

Minor amounts of galena occur as euhedral crystals 20-100 txm in diameter within quartz and larger crystals 100-400 p~m occur within the micas of the greisen and are concentrated at the junction between the greisen and quartz veins. Often this galena has a sphalerite rim. The majority of the galena, however, is the last sulphide to be deposited in the central quartz veins where it occurs as discrete crystals up to 1 m in size, that contain a variety of inclusions including native bismuth (1-5 p~m), stannite, pyrite and chalcopyrite 5-20 /xm in diameter, together with a number of unidentified grey anisotropic phases 20-30 # m in diameter, some of which are argen- tian. Much galena replaces earlier sphalerite and hence occurs as anhedral inclusions 10-100 p.m; veinlets 2-10 txm wide that cross-cut both sphalerite and its enclosed chalcopyrite; and rims 10-100 /xm in width about sphalerite crystals. Galena similarly extensively replaces vein cassiterite along its crystal edges and fractures. A minor amount of chalcopyrite is intimately intergrown with galena.

Supergene alteration

Very little supergene alteration is present. In addition


to the alterations already described, some wolframite has altered to haematite; pyrite and marcasite have oxidized to limonite and j arosite. All the major sulphides are altered along fractures, cleavage or grain boundaries to successive rims, 5-20 ~m wide, of covellite and blaubleibender covellite together with minor amounts of blue copper sulphide, chalcocite. Trace amounts of bornite/idaite (40 p.m) and native copper, 15/~m in size, are found associated with chalcopyrite and 20 ~m wide cerussite rims occur around galena.

OTHER MINERALIZED AREAS

Although the Ririwai lode is the major mineralized zone of the complex it is not the only area that has been hydrothermally altered.

To the south-west of the main lode a series of com- paratively short, narrow mineralized greisens occur along the Makota path (Fig. 2). They consist of quartz with a little topaz and greenish brown mica and form three series of north-west-south-east-trending veins, the eastern one of which has largely been worked out. They cut biotite granite and grade into yellowish or reddish coloured wall rock. The marked brick-red zone characteristic of the main lode is not developed although like the main lode, the wall-rock is very vuggy. The veins follow the dominant joint directions which tend to be north-west-south-east away from the immediate vicinity of the lode.

The most easterly of the three localities consists of two parallel north-west-orientated greisen veins, 2 m apart, separated by thin fine grained light grey greisen veinlets. The two major veins are 30 and 45 cm in width with central quartz veins of 2.5 and 18 cm, respectively. The vuggy quartz veins show a patchy distribution of cassiterite and a little coarse molybdenite while the light grey, medium grained greisen into which it grades contains galena, chalcopyrite, sphalerite, pyrite and cassiterite. The veins are vertical and are exposed for about 200 m, pinching out rapidly southwards. The middle of the three vein systems is very similar in mineralogy but does not show the same degree of mineralization at the surface. At depth it may be very similar to the eastern vein and they may even coalesce.

The western mineralized greisen vein has been extensively worked for cassiterite to a depth of 3 m in places and extends over 400 m towards the outer arc of volcanics. From the remaining channel it is assumed that the greisen was vertical and also with a north-west strike. Excavated boulders indicate a quartz vein up to 7.5 cm wide containing cassiterite grading into a 60 cm wide greisen. There is no wall rock reddening.

To the south-east of the lode mineralized veins cut fine grained biotite granite at Kerigateri Hill. The granite is unusual--i t is light in colour with biotite in a peculiar prismatic habit. The siliceous mineralized veins which consist of quartz, reddish brown mica and abundant topaz grade into vesicular reddened quartz, then into cassiterite-rich altered wall rock. Disseminated sphaler-

ite, chalcopyrite, pyrite and galena may be found in the veins together with rare traces of genthelvite and molybdenite. Coarse, hexagonal crystals of molybdenite up to 3 cm across have been found in the altered wall rock.

To the east of the lode similar north-east-trending mineralized veins have been found on the slopes of Ginshi Hill (Fig. 2). These cut medium grained biotite granite and extend a short way into the volcanics. Air photos show prominent joints controlling the distribution of the veins.

At Uwar Gida, on the south-west shoulder of Dutsen Ginshi, a saucer-shaped milky quartz reef up to 3 m thick lies between the biotite granite and the overlying ignimbrite. The reef is almost entirely quartz with occasional specks of ferromagnesian minerals and crystals of cassiterite but there are occasional pockets of a soft white clay mineral which is probably decomposed feldspar. Pits sunk below the reef have failed to find any feeder system but 30 m below the reef 1 cm wide vertical greisen veins trending 210 ° cut unaltered pink biotite perthite granite and contain galena, chlorite, chalcopyrite and sphalerite. The veins cannot be traced into the quartz reef as the granite gradually becomes decomposed towards the roof, grading from a pink medium grained biotite granite with perthite, albite, quartz and biotite into a fine grained granite rich in genthelvite which, within 30 m, grades into a soft, mottled, reddened material consisting of quartz and kaolinite.

At the hilltop quarry, to the west of Dutsen Ginshi, east-west-trending quartz-chlorite veins and quartzo- feldspathic pegmatites have been extensively worked for cassiterite but have been long since abandoned. Although of geological interest none of these areas is likely to be of economic importance because of their small size.

FLUID INCLUSION STUDIES

Fluid inclusions have been studied in all rock types for their size, morphology, l iquid-solid-vapour content and abundance. In addition, microthermometric measure- ments have been undertaken on selected samples, utilis- ing a Linkam TH600 programmable heating/freezing stage described by Shepherd (1981). In the Ririwai samples, only quartz proved suitable for study. Most samples of quartz contained more than one generation of inclusions and planes of secondary inclusions are common. The six major types of inclusions described for Nigeria (Kinnaird 1985) can also be found at Ririwai. Sketches of these types found in various samples are shown in Table 10.

Type 1 inclusions are filled with an aqueous solution with a gas bubble occupying 10-30% of the volume of the inclusion. Inclusions may be up to 50/xm in size in vein quartz and are extremely abundant.

Type 2 inclusions are gas-rich and contain a large gas bubble forming more than 60% of the inclusion and often more than 90% at room temperature. They contain

to

o%

Sample No

Locality

Makota

pa

th

RS6

Uw

ar

Gid

a

NB5

Ka

ffo

Valley

Adit in

the lode

R1/44 Ririwai lode

R514C Ririwai

lode-surface

Mineral

quartz

quartz

qu

art

z

qu

art

z

quartz

quartz

Tab

le 1

0. F

luid

in

clu

sio

n c

har

acte

rist

ics

Mineral

Typ

e o

f Inclusion

Primary

Assemblage

metasomatism

Typ

e

or

Secondary

quartz-microeline

pervasive potash

wallrock

quart~ (albite), microcline

mica, cassiterite, sulphides

quartz, arfvedsonitetalbite

quartz, microcline,chlorite

quartz

~+ spalerite

+ galena

+ chalcopyrite etc)

quartz ~

galena +

spalerite +

wolframite)

pervasive sodicpthen

potash, then H+ metasomatism

pervasive sadie

potash

Silica:- in

Fissure Filling quartz

vein

Silica:- in

fissure filling

quartz vein

1,2,3

1,2,6

1,2,3B,4,5

1,2

1,2,6

1,2,6

P P,5

P,S

P

P,S

P,5

Size

in

pm

30

10

20

i0-i00

80

Mea

n temp or

range

360

360-380

260-360

460

360-380

34

0

Salinity in

wt 9~ N

at1

5-8

6-11

> 7~

7: > 2

"9='~

0

m;,i..~

=

$

0o/

0 (~

k ,~

Ty

pe

1

Ty

pe

2

Ty

pe

3

a

Ty

pe

3b

T

yp

e

4 T

yp

e

5 T

yp

e

6

Sket

ch o

f ty

pica

l fl

uid

incl

usio

ns.


a liquid phase but no solid phase has been noted. Size is variable from >4 to <40 ~m. Mixed populations of types 1 and 2 inclusions have been noted in some sub- surface samples where vapour-filled inclusions coexist with liquid rich inclusions. This is taken as an indication of boiling.

Type 3 inclusions have formed from saline fluids and contain in addition to a liquid and gas phase, one or more solid phases. They are usually small in size and can be seen in granitic quartz, or in the microcline-rich wall rock of the lode.

Type 4 inclusions contain melt inclusions now present as devitrified glass in addition to fluid. These have only been identified so far in the arfvedsonite albite granites.

Type 5 inclusions contain CO2 in addition to the water-rich fluid. The "double-bubble" effect is rare and CO2 presence is usually detected by clathrate development during cooling experiments. This type of inclusion occurs in albitized granites.

Type 6 monophase liquid inclusions are common at high levels within the biotite granite pluton, indicating hydrothermal activity continued down to <70°C. They occur as secondary inclusions in granitic, greisen and vein quartz and may occur as primary inclusions in milky quartz overgrowths on clear prismatic quartz in fissure- filling veins from the Adit area (Fig. 2).

METASOMATISM AND FLUID INCLUSION POPULATIONS

Sodic metasomatism is characterized by the variety of inclusions and particularly by type 5 CO2-bearing inclusions. Loss of CO2 seems to have been important for deposition of uranium in the lattice of minerals such as monazite, pyrochlore, thorite and zircon. Homogeniza- tion temperatures are in the temperature range 260- 460°C.

Potash metasomatism is characterized by very small abundant saline, type 3 inclusions, which may be accompanied by vapour and liquid-rich inclusions. Homogenization temperatures are in the range 360-460°C although above 420°C decrepitation of inclusions is common.

Pervasive H + metasomatism is characterized by the coexistence of liquid and gas-rich inclusions. Both gas and liquid inclusions homogenize in the same temperature range. This indicates that the low salinity hydrothermal fluids were boiling at temperatures of 360-380°C. Since the fluids were boiling at the time the inclusions were trapped, depth of formation can be interpreted from the data of Haas (1971) which suggests that for a 10 wt% salinity inclusion homogenizing at this temperature range, the depth of formation was at least 1416 m, with a pressure of around 120 bars. This seems a reason- able estimate since field evidence suggests an overlying volcanic pile of 1000 m.

Vein-controlled H + metasomatism is characterized by small but abundant inclusions in the greisens from Makota path and the Ririwai lode. Primary inclusions

are invariably types 1 or 2 with secondary monophase liquid, type 6 inclusions forming trails along healed fractures in the quartz. Homogenization temperatures show a considerable overlap between those found in fluids related to potash and silica metasomatism and are in the range 360-380°C. This range is similar to that obtained for pervasive H + metasomatism.

Silica metasomatism is characterized by inclusions of large dimensions, generally between 20 and 50 /~m, dominantly type 1, liquid-rich. A detailed study has been undertaken on sample R1/44 collected from a depth of 30 m in the quartz vein of the main lode. Homogenization temperatures on 38 inclusions, with two exceptions, were within the range 360-380°C, with a mean of 367°C. The two exceptions homogenized at 321 and 269°C. Both these inclusions had lower than average salinities and were therefore considered to be trapped at a later stage. Clearly the quartz of the lode has been formed over a range of temperatures since fluid inclusions in vein quartz containing galena and wolframite in sample RS14C, collected at the Adit, have slightly lower salinities and lower homogenization temperatures than R1/44. Salinity varied little at around 7.8 eq. wt% NaCI with the majority of the inclusions giving homogenization temperatures between 338 and 342°C. Also in the area of the Adit, two generations of quartz can occasionally be distinguished in hand specimen. The earlier phase forms clear glassy, prismatic crystals coated by a rim of later milky quartz. Inclusions in this later phase are commonly large, monophase inclusions. Such inclusions indicate that quartz deposition continued to temperatures of less than 70°C and that these lower temperature fluids were probably also responsible for adularia formation or limited kaolinization. Analysis of fluid inclusions using a pulsed laser microprobe (Bennett and Grant 1980) showed that inclusions in the quartz vein of the lode contained K, AI, Ca, Cu and W. They did not detect Ti, Mn, B and Fe although where there is abundant mineralization Fe is often enriched.

TRACE ELEMENT AND ISOTOPIC STUDIES

Rubidium-strontium

Rb-Sr isotope studies on Ririwai rocks have been carried out in Britain (van Breemen et al. 1975) and France (Bonin et al. 1979). Results are almost identical. The arfvedsonite aegirine granite plots on a concordant isochron with the granite porphyry (containing calcian arfvedsonite and pseudomorphs of fayalite and ferrohedenbergite) to yield an isochron of 170 _+ 5 Ma (0.708 _+ 0.015). The Rb-Sr values are typically magmatic, controlled by fractionation of alkali feldspar. For the small stock of albite arfvedsonite granite, enrichment in Rb reaches 1600 p.p.m, whilst Sr remains low (Bonin etal. 1979). The isochron of 172 + 5 Ma (0.752 + 0.021) is essentially controlled by two points rich in Sr for the initial ratio and by three points, poor in Sr. Rb values for the central biotite granite are variable between 600


Ririwoi; Nigeria 400

300

laag

Ibg

~fg i !

100 a g e m . a .

0760

0"750

875 r

86Sr

0'720

0.710

Fig. 25. Initial Sr isotopic variations in samples for the Ririwai complex, Nigeria. fg, Fayalite granite; bg, biotite granite: aag arfvedsonite

albite granite (adapted from Bowden and Kinnaird 1984).

and 1000 p.p.m, for the main granite, reaching 1200 p.p.m, at the margins of the main lode which itself contains less than 200 p.p.m. Strontium values are always low. The isochron published by van Breemen et

al. (1975) for the biotite granite gave 167 + 2 Ma with an initial ratio of 0.729 + 0.009. Data produced by Bonin et

al. (1979) suggests that the lode and its margins has an age of 168 _+ 2 Ma and STSr/S%r:0.728 _+ 0.007. If one sample rich in Sr is eliminated, the differences between biotite granite and the lode are not significant. This suggests that the Rb-Sr system remained open until the circulation of mineralizing hydrothermal fluids had ceased and that the ages calculated for biotite granites throughout the province are essentially the ages at which mineralization ceased. The relationship in Sr isotopic ratios for biotite granite, arfvedsonite albite granite and fayalite granite are shown in Fig. 25. Bonin et al. (1979) suggest that the proportion of Rb-Sr in anorogenic granites was controlled by fractional crystallization of alkali feldspars in the magma chamber. After the emplacement of the granites, hydrothermal fluids mobilized Rb relative to Sr and therefore modified the relative proportions of Rb-Sr without any notable modification of the ~TSr/86Sr ratio.

It is well known that Rb is enriched in many Nigerian anorogenic granites and in contrast the amount of potash does not vary widely. Bowden and Turner (1974), suggested that Nigerian granites containing high Rb and low K-Rb ratios have undergone substantial recrystallization in the subsolidus during albitization. This process may be recognized by a negative correlation of K against Rb. Thus the trend of K-Rb ratios to low values in certain Ririwai granites is believed to represent the degree of postmagmatic readjustment in response to mineralizing albite-rich fluids. It is significant that the granites plot within Shaw's pegmatitic hydrothermal field (Shaw 1968) emphasizing the importance of certain

Th ppm

300

200

x

I 0 0 -

e x

0

+ 1

2

• 3

• 4

o 5

o 6

0

oo

o

I

4"

0

I I I

100 200

U ppm

Fig. 26. Diagrammatic representat ion of U and Th variations in samples from the Ririwai complex. Data tabulated in Table 11. l = arfvedsonite albite granite, 2 - arfvedsonite granite, 3 = biotite granites, 4 = greisen vein, 5 = microcline quartz wall-rock,

6 = albitised biotite granite.

geochemical readjustments in response to fluids, as the granite cools and consolidates. It is also clear that the Rb-Sr system remained open until after the vein system formed and that rehomogenization within the Rb-Sr system continued down to 340°C or below. The Rb-Sr data further suggests that the age dates on similar rock types from other complexes, represent the age of mineralization and not necessarily the age of emplacement. Initial Sr isotopic ratios cannot be used in these circumstances to suggest the source region of the granitic magmas.

L e a d isotopes

It is worth recalling that it was a pioneering pilot study of U-Pb and Pb-Pb in material from Ririwai and elsewhere (Jacobson et al. 1963) that first suggested, a considerable age difference and similarities between the foliated "basement" granites termed 'Older" granites and the younger alkaline intrusives. While U-Pb ages gave Jurassic dates, Pb-Pb studies, including galena from the Ririwai lode suggested Upper Precambrian- Cambrian ages. A later study by Tugarinov et al. (1968) confirmed the inherent old lead ages in Ririwai arfvedsonite albite granite and in the lode of the biotite granite. Such data was used by Bowden (1970) to infer that a crustal source input was an essential feature of anorogenic magmatism.

More recently, the Pb isotopes have been re-examined by Dickin et al. (1983) from the same series of samples used by van Breemen et al. (1975) for Rb-Sr dating. The most important conclusion is that all the Ririwai samples show lead contributions from both the older Eburnean (lower) crust and from the Pan-African (upper) crust.


Table 11. U-Th concentrations in samples from the Ririwai complex, Nigeria

Sample No Description O Th Th/O ppm ppm

N 83 arfvedsonite granite 14 51 3.6 N 80 arfvedsonite granite lO 120 12

N 87 ~rfvedeonite albite granite 60 190 3.2

N 88 arfvedeonite albite granite 108 lO0 0.95

N 89 arfvedeonite albite granite 105 225 2.1

N 86 ~rfvedsonite albite granite 145 350 2.4

N 77 miotite granite 7.5 41 5.5



N T5 oiotite granite lO.O 83 8.3

Core L 13

L 13-10 biotite granite from lO m 30 25 0.83 Jepth

L 13-205 biotite granite from 205 m 53 51 0.96 Jepth

L 13-310 oiotite granite from 310 m 5B 56 0.97 Jepth

L 13-411 albitite from 411 m depth B1 73 0.90

L 13-440 albitite from 440 m depth 66 69 1.05

R 1-18 red microcline-quartz 12.8 41.7 3.25 ~allrock adjacent to lode from 30 m depth

R 1-35 red microcline-quartz 10.2 3B.8 3.BO wallrock adjacent to lode from 30 m depth

R 1-14 greisen vein from 30 m 1B.9 97.5 5.16 depth

Analyses by INAA: J. Whitlcy and S. 1. Abaa.

The inference is that most of the ore metals like Sn, Zn, Pb, etc. are also derived from reworking and fluid interaction of crustal sources.

URANIUM, THORIUM AND PARTICLE TRACK DISTRIBUTION

Since uranium and thorium have distribution coefficients of less than one in the major rock forming minerals (Rogers and Adams 1978) they should concentrate in residual liquids provided early crystallization of substantial amounts of accessory minerals, such as zircon and monazite, does not occur. In the peralkaline acid en- vironment, these minerals are inhibited from early crystallization. However, under the more aluminous con- ditions of biotite granites they may crystallize early. There are enriched levels of uranium and thorium (Fig. 26) in both the peralkaline and aluminous granites, with evidence of considerable mobility of both during periods of rock-fluid interaction (Bowden et al. 1981).

Loss of uranium relative to thorium occurred during the leaching of the pile of rhyolitic ignimbrites. Similarly, surface samples of both peralkaline and aluminous granites shows progressive loss of uranium relative to thorium. Samples N77, N91, N92 and N75 (Table l l) show the U- Th variation for surface samples of biotite granite, which is interpreted as the result of uranium mobilization to higher levels which have subsequently been eroded with Th retained in accessory minerals (Bowden et al. 1981). Surface values of both U and Th

are lower than those at depth which may be due to element mobility at lower temperatures (Chatterjee 1980). Samples from drill core L13 show progressive increases in U and Th between 10 and 310 m depth in the biotite granite. Highest concentrations are encountered in samples from 411 and 440 m depth which come from the albitic roof zone of the later biotite granite at depth. High levels of U and Th are therefore clearly related to the albitization process and the T h - U ratio is significantly decreased from the average for biotite granites. During the ensuing potash metasomatism, the uranium introduced during soda metasomatism is remobilised and the Th therefore preferentially concentrated, resulting in an increased Th -U ratio. This reaches a maximum in the greisen so the selective removal of uranium relative to thorium must continue during hydrogen metasomatism.

Thus hydrothermal processes have clearly modified the concentrations of U and Th depending on the location within the cupola, the relative concentration of U and Th, the intensity of fluid interaction, the composition of the fluid and type of fluid interaction, e.g. albitization, microclinization, etc. Hence the Th -U ratio may vary in Ririwai samples from 0.83 to 12 suggesting relative mobilization of U in the subvolcanic roof zone.

In an attempt to discover how the thorium and uranium were distributed and mobilized during these processes, combined neutron activation and particle track analysis was undertaken on a rock slice from the Ririwai lode (MacKenzie et al. 1984).


A rock slice taken across the Ririwai lode from the No. 1 level (Fig. 27) can be divided into three distinctive mineralogical zones based on the identification of minerals in hand specimen:

Zone 1 consists mainly of microcline feldspar (opaque grey in Fig. 27) and chlorite (small dark aggregates) with mica as dark spots within the feldspar in addition to translucent grey quartz. There is a high proportion of fission tracks in this zone. Some of the induced radioactivity is emanating from miarolitic cavities (small ovoid dark holes). This suggests that some uranium minerals have been deposited from the vapour phase of a hydrothermal fluid. Some of the uranium sites can be correlated with quartz grain boundaries, the development of chlorite and outer zones of lithium micas. A few sites of fission track activity may also be related to the distribution of zircons but thin section studies to confirm this observation have yet to be completed. All the alpha tracks are confined to the quartz-rich translucent areas, but the alpha track activity is sparse compared with the proportion of fission tracks. No particle tracks have been recorded in the feldspar.

Zone 2 consists of microcline feldspar (opaque grey), quartz (translucent grey) and mica (dark black speck- les). Chlorite is not obvious in hand specimen. The fission track distribution is concentrated in patches but is not as intensive as in zone 1. The same particle track- crystal correlations found in zone 1 also apply in zone 2. Between zones 1 and 2 there is a narrow band developed from the destabilization of feldspar to yield the mica- quartz aggregates of the greisen. The position of this band of greisen is indicated on Fig. 27~ It can be observed in this figure that the overall density of fission tracks is diminished within this greisen band. In contrast, the alpha track activity is more locally concentrated in the small greisen zone.

Zone 3 is represented by a larger band of greisen marked by a conspicuous absence of microcline feldspar and characterized by large patches of translucent grey quartz and aggregates of dark mica. The fission track activity is considerably reduced in this zone and is confined to selected sites. Some of the activity is emanating from small concentrations of ore minerals such as sphalerite and cassiterite. It appears that in the greisen zones, fission track activity reflects the ore distribution pattern. There are also more alpha tracks per unit area in the greisen zone than elsewhere in the slice. There is some overlap of track density with the fission sites, but also in other quartz-rich areas particularly towards the margin of the specimen. Chemically these alpha tracks can be correlated with an increase in thorium and caesium from the element scans suggesting that thorium-rich monazite may be present at a submicroscopic scale in the quartz- rich aggregates. Alpha track distribution on the poly- acetate sheeting represents the natural radioactivity from 232Th and 238U sites. The alpha track activity is concentrated in zones of greisenization and can be correlated with ore mineral distribution. The major part of the activity can be related to geochemical concentrations of Th, Ce, H R E E , Sc and Cs, and MacKenzie et al.

(1984) suggested that minerals such as thorite, Th-rich monazite, xenotime and cassiterite are the sites of most alpha emissions. In contrast, the sites of induced fission, represent activity of 235U and these are very much concentrated in an older part of the lode. This means that the fission tracks are related to other minerals than those responsible for alpha track distribution and substantiate the interpretation of the U and Th values, that the uranium was introduced at an earlier stage than the main lode-forming process of hydrogen ion metasomatism (greisenization). Fission track activity can be related to geochemical concentrations of Rb and, to a lesser extent, with Hf, La and Ta. It is likely that minerals such as lithian micas, Hf-rich zircon and allanite are the sites of most fission emissions.

The element analyses and particle track distribution confirm the relative fixation of Th and the relative mobility of U during periods of rock-fluid interaction.

SUMMARY AND CONCLUSIONS

The Ririwai complex is an isolated ring structure within the Nigerian anorogenic province. It provides a unique but complete sequence of rock types related to oversaturated alkaline magmatism and subsequent postmagmatic metasomatism.

The volcanic rocks, dominated by rhyolitic ignimbrites with minor basalts, are preserved within the con- fines of the ring fracture. At the base of the volcanic pile compositions range from olivine basalt through hawaiite to mugearite, indicating a fractionating alkaline trend. The acid volcanic rocks show a variety of petrographic and geochemical features which illustrate both the ter- minal stages of magmatic evolution as well as the effects of an alkaline fluid phase separation. The comenditic ignimbrites provide petrological and geochemical criteria for important subsolidus re-equilibration and crystal-fluid reactions.

The volcanic feeders represent examples of granite magma which have partially quenched and crystallized in the ring fractures. Quartz porphyries are part of the fluidized system which fed the ignimbrite pile above, while granite porphyries are a more substantially degassed equivalent which has crystallized more thoroughly. Again petrological and geochemical parameters such as the ordering of alkali feldspars, the phenocryst-matrix mineralogy, the major and trace data--including the REEs- -a re used to differentiate magmatic and postmagmatic processes.

Peralkaline granites at Ririwai are classic examples where fluid reactions have continued to destabilize earlier magmatic minerals like fayalite and hedenbergite to form alkali amphibole and aegirine. Where the fluid has been retained, albite-rich granites containing pyrochlore result from intense sodic metasomatism. Major element and trace element chemistry confirm the dominant role of sodic metasomatism. This effect is to desilicate the host granite, partially removing quartz to yield alkaline syenite compositions. Rare-earths as well as Rb, Th, Zr,


Hf and the ore elements Zn, Cu, Nb and Sn are considerably enriched in arfvedsonite albite granites testifying to the widespread fluid to crystal partitioning at subsolidus temperatures. Fluid inclusion studies in the peralkaline granites monitor the complete spectrum of fluid evolution from silicate melt/saline separation through to aqueous liquid and gas (CO2, H20). The varied populations with contrasting homogenization temperatures indicate the wide range of re-equilibration in the subsolidus.

Biotite granites at Ririwai contain trioctahedral micas ranging in composition from annite through siderophyllite to zinnwaldite. The alkali feldspars are intermediate to ordered microcline as patch perthites with irregular domains of ordered aibite. Quartz shows evidence of recrystallization and, in some samples, rich in dispersed cassiterite-columbite mineralization, quartz is stained with a haematite coating as mica destabilized. Haematite is also found associated with the ordering of microcline, colouring it red. The biotite granite which forms a central pluton intruded into the overlying volcanic pile contains an extremely wide range of textures and fluid reactions. Sodic metasomatism is not so intensive as in the peralkaline granites suggesting that part of the peralkaline fluid phase has been lost on cooling. This is substantiated by a diminished number of fluid inclusion types with dominantly liquid + vapour inclusions and some liquid + vapour + solid inclusions. These populations may be related to potash metasomatism which overprinted and partially removed the features of sodic metasomatism.

The biotite granite at the surface and to a depth of 400 m is pervasively mineralized. However, intense potash metasomatism is restricted to the marginal changes associated with the Ririwai lode stockwork. Locally pockets of microclinite occur, indicating that initially potash metasomatism is a desilication process.

The development of the lode and the associated K +, H + and silica metasomatism in biotite granite is probably related to the fluid reactions at 400-500 m where albitite has been recorded in drill cores. One particular core length--L13--provides an indication of the mineralogical and geochemical changes below the current exposed surface.

Petrological and geochemical data substantiate the fluid reactions with the rare-earths providing abundant evidence for element loss to the interacting fluid phase during potash metasomatism. Subsequent breakdown of the alkali feldspars to yield greisens or, less commonly, kaolinite, is accompanied by distinct ore mineral assemblages and characteristic aqueous fluid inclusion populations.

Within the biotite granite the earliest stage of mineralization is equated with the albitite containing zircon and ilmenite followed by columbite, minor cassiterite and sphalerite. Potash metasomatism developed along the Ririwai lode, generated wolframite, cassiterite and sphalerite accompanied by columbite, molybdenite, chalcopyrite and galena. This association is continued into greisen and quartz vein formation. Minor amounts

of secondary copper and bismuth mineralization are attributed to later lower temperature stages which continued to below 70°C.

Evidence for considerable element mobility during K +, H + and Si 4+ metasomatism is provided by combined neutron activation and particle track studies. The distribution of sodium clearly shows that it is mobile during H + metasomatism. The mobility of Rb parallels the behaviour of K, and Fe is released from the microcline lattice during destabilization. Sc seems to offer the best monitor of element mobility and concentration during the greisen process. The behaviour of Cs, Yb, and Th parallels that for Sc. In general, most of the trace elements studied in the Ririwai complex show varying degrees of mobility to fixation and concentration within ore minerals, lithian micas and associated minerals during greisenization. In particular U is depleted in biotite granites and their mineralized variants, with Th fixed and concentrated by greisenization. This leads to high Th/U ratios.

Sr, Nd and Pb isotopic studies on granite samples from Ririwai suggest that the source(s) of the magma(s) and, indirectly, the ore mineralization lies partly in the Pre- cambrian (Pan-African) continental crust and partly in the mantle.

The Ririwai complex provides one of the most complete volcanic, magmatic, postmagmatic and ore assemblages in Nigeria. It is an isolated centre unaf- fected by adjacent or overlapping complexes and therefore provides an important reference for the anorogenic province as a whole.

Acknowledgements--The authors would like to thank the Mining Corporation of Nigeria, particularly S. Ford and U. Turaki. The managemen t of Gold and Base, particularly L. Gibb, are acknowledged for allowing free access to the mine and providing accommo- dation. Messrs Bray, Park, Hannaford and Clive and others at the mine are thanked for their help and hospitality. Thanks also to everyone in Nigeria who helped us either logistically or socially. J .A.K. would like to thank members of the Fluid Inclusion Laboratory at CRPG. Nancy, particularly B. Poty, J. Leterrier and M. Pagel for their help. J .A.K. and P.B. would like to thank all members of the Laboratoirc dc Pctrologie at the Universit6 de Paris VI for assistance and microprobe facilities, particularly J. Lameyre and A. Giret. P.B. would like to thank the staff of B R G M at Orleans for help with microprobe facilities, particularly L. Burnol. The authors are grateful to the SURRC, isotope laboratory and radiochemistry section, East Kilbride for providing data, particularly A. B. Mackenzie, J. Whitley, A. Halliday, A. Dickin, and also O. van Breemen. P.B. and J .A.K. would like to thank all members of the Younger Granite research project, particularly R. F. Martin, J. N. Bennet t , C. A. Abernethy , E. C. lke and S. I. Abaa. Members of the Technical Staff of St. Andrews arc acknowledged for extensive help, particularly J. Allan, R. A. Batchelor, A. Mackie and C. Finlay. Professor Walton has provided unfailing support. Finally, thanks to Allan Kinnaird for help with computing techniques. P.B. and J .A.K. were financed by the Overseas Development Ministry Research Scheme R2679.

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Mineralisation in northern Nigeria

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