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Ore-bearing potential of granitic rocks from the Jos—Bukuru Complex, northern Nigeria
Chemical Geology, 28 (1980) 69--77 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
69
ORE-BEARING POTENTIAL OF GRANITIC ROCKS FROM THE JOS-BUKURU COMPLEX, NORTHERN NIGERIA
EBO GAB. IMEOKPARIA Department of Geology, University of Ibadan, Ibadan (Nigeria) (Received May 7, 1979; accepted for publication August 16, 1979)
ABSTRACT
Imeokparia, E.G., 1980. Ore-bearing potential of granitic rocks from the Jos--Bukuru Complex, northern Nigeria. Chem. Geol., 28: 69--77. Geochemical analyses of fresh bedrock samples indicate that the Jos--Bukuru biotite granites are characterized by enhanced values of a suite of trace elements, including Sn, Zn, Rb, Li, Nb, Th and F, and depletion in Ba and Sr. The abundance and interrelationships of these elements are most useful as indicators of mineralizing processes and orebearing potential. The Sn-bearing rocks are characterized by low K/Rb, Ba/Rb, and Mg/Li and high Li/Zn and (Li × 1000)/K ratios. These results, as applied to the Jos--Bukuru Complex, indicate that Sabon Gida and Rayfield have the best ore-bearing potential, while that of Bukuru and N'gell is only limited. The Jos and Kuru phase could be regarded as barren.
INTRODUCTION Nigeria r a n k s a m o n g t h e w o r l d ' s leading p r o d u c e r s o f tin a n d c o l u m b i t e , o f w h i c h t h e m a i n sources are r e c e n t alluvial a n d p l a c e r deposits. I t is believed t h a t these d e p o s i t s are derived f r o m t h e Y o u n g e r G r a n i t e P r o v i n c e intrusives o f n o r t h e r n Nigeria. I t is also k n o w n t h a t s o m e p r i m a r y cassiterite a n d c o l u m b i t e are dispersed t h r o u g h o u t t h e granitic m a s s i f a n d at p r e s e n t m i n e d f r o m d e c o m p o s e d p a r e n t m a t e r i a l and greisenized (zones) veins a n d q u a r t z stringers. H o w e v e r , t h e r e is a m a r k e d p a u c i t y o f l o d e Sn d e p o s i t s in t h e Y o u n g e r G r a n i t e Province. As t h e s e easily w o r k e d alluvial d e p o s i t s b e c o m e e x h a u s t e d , c o n s i d e r a b l e a t t e n t i o n is being f o c u s s e d o n p r o s p e c t i n g f o r large d i s s e m i n a t e d d e p o s i t s t h a t c o u l d b e n e f i t f r o m large-scale o p e n - p i t m i n i n g m e t h o d s similar to t h o s e o f p o r p h y r y Cu deposits. T h e J o s - B u k u r u C o m p l e x is t h e largest individual c o m p l e x w i t h i n t h e Y o u n g e r G r a n i t e region a n d it c o n t a i n s t h e richest a n d m o s t a b u n d a n t alluvial d e p o s i t s o f Sn and N b in Nigeria. T h e d i s t r i b u t i o n o f alluvial w o r k i n g is c o n f i n e d t o t h e b i o t i t e granite p h a s e s o f t h e c o m p l e x a n d t h e genetic r e l a t i o n s h i p b e t w e e n t i n - - c o l u m b i t e m i n e r a l i z a t i o n a n d b i o t i t e granites is
70
well known (Kloosterman, 1967, 1970). Equally important is the fact that not all the biotite granite phases are equally rich in these economic deposits, which may reflect on their ore-bearing potential. To determine if the Jos--Bukuru Complex has some geochemical features indicative of its ore-bearing potential, 80 fresh bedrock samples were analysed for various elements by X-ray fluorescence, atomic absorption, emission spectrography and ion-selective electrode. Detailed descriptions of analytical procedures are presented elsewhere (Imeokparia, 1977). GEOLOGICAL SETTING
The Jos--Bukuru Complex lies at the focal point of the Younger granitic activity and occupies the north-central region of the Jos Plateau in the main mining area. It covers an area of nearly 430 km 2 defined by longitudes 8°45 '- 9~E and latitude 9°40'--10~N. The massif is elliptical in surface form and except for a deep embayment in the south, shows smooth and regular margins with the surrounding basement complex (Fig. 1). Ft.
Jos . u . u . u
v o u . o c . GR^N, TE c o . ~ E x .
] Biotite microgranite unclassified IT~ Sabon-Gida North Biotite granite ] Sabon-Gida South Biotite granite [;~]. Bukuru Biotite granite ] Shenhornblende-fayalite granite
91~.Sl
]
Detimi Biotite granite
] ] ] ] ] ]
RayfieldGonaBiotite
granite
Kuru Stock Biotite granite
N'gellBiotite
granite
Jos Biotite granite Naraguta quartz pyroxene fayalite porphyry
VomHornblende
Biotite granite
]
Neils valley Granite porphyry
] ]
Rhyolites Bassement complex
Numbers indicate location of analysed samples After Jacobson et al (1958)
, 0
~ ~ ~ ~miles 2
a
_..2
Fig. 1. Geological map of Jos--Bukuru Younger Granite Complex.
6
B kilometres
71 It is a composite b o d y of Jurassic age (dated as 165+ 3 Ma ago) made up of granite porphyries and biotite granites. The complex exhibits an extremely intricate pattern of separate granite intrusions and it is believed that largescale ring-faulting and cauldron subsidence operated during the initial stages of granite emplacement, which have been modified by piecemeal stoping and superposition of later cauldron structures (Mackay, 1949; Macleod, 1954). Detailed descriptions of the constituent rock types have been presented by Macleod (1954). Two major cycles of igneous activity have been recognised, and the sequence of emplacement of the principal members of the igneous suite is summarised in Table I (Macleod, 1954). TABLEI Sequence of emplacement of the rocks of the Jos--Bukuru Complex Second intrusive cycle:
(b) Upper granite
Delimi biotite granite Sabon Gida biotite granite Vom Road microgranite Bukuru biotite granite Rayfield biotite granite
(a) Upper granite porphyry
Shen--Kuru porphyry
First intrusive cycle:
(b) Lower granite
Kuru biotite granite N'gell biotite granite Jos biotite granite
(a) Lower granite porphyry
Nell's Valley porphyry Naraguta porphyry rhyolites
After Macleod (1954). Igneous activity started in this complex with emplacement of volcanic rocks, mainly rhyolites. This was followed by a series of plutonic intrusions. The earlier intrusive cycle is represented by the granite porphyries and quartz porphyries associated with ring structures and block subsidence. This was followed by three distinct phases of biotite granites. The last intrusive cycle was initiated by granite p o r p h y r y followed by five separate phases of biotite granites. These granites occur mainly as sheeted bodies that have only been partially unroofed at the present level of erosion. PETROLOGY OF BIOTITE GRANITES The predominant rock t y p e in the complex is a variety of biotite granites which have been distinguished from each other by grain size and texture. Textural variation depends on the relative order of crystallization of felsic and mafic minerals and the degree of late- and post-magmatic recrystalliza-
72 tion. Three types of biotite granites have been distinguished in the complex: (1) Jos type which is coarse-grained; (2) N'gell type, medium-grained pinkish granite grading into (3) white finer-grained Rayfield type, with biotite growing in separate flakes rather than in clusters and with small areas of hematite staining visible hand specimens. Structurally the biotite granites are more often discordant with the early ring dykes. Evidence of this is their irregular shapes and shallow outwarddipping contacts, which contrast with the regular circular or elliptical ring dykes with their steep contacts. In many cases the biotite granite actually cuts across and partly obliterate early ring structures. Despite the textural variations that mark the different phases, the biotite granites are composed mainly of quartz, K-feldspar, albite and biotite. Fluorite, zircon and Fe oxides are the most c o m m o n accessories. Cassiterite, columbite, xenotine and thorite are also present. They contain between 28% and 35% of modal quartz aggregated into groups of arcuate trains between the large feldspars, 50--56% K-feldspar, 5--10% albite, 3% biotite and 2% accessory minerals. Orthoclase microperthite is c o m m o n in the coarse-grained granite, b u t microcline-microperthite is dominant in the finer-textured granites together with small discrete crystals of albite and patchy perthitic replacement intergrowths of albite. Microcline occurs as interstitial grains or replacement rims around orthoclase probably of metasomatic origin. Late deuteric albitization is a common feature in the Rayfield and Sabon Gida phases. Biotite shows variable habit in these rocks. In the coarse-grained granites (Jos type) the biotite is generally pleochroic between deep-brown and strawyellow but in the pink to white, medium-grained and finer-textured granites biotite is distinctly paler green in colour. Two generations of quartz have been recognized in the granites: primary (early formed) and secondary (metasomatic) varieties. The primary quartz is large, clear and anhedral. It occurs in clusters and as graphic intergrowths with K-feldspar. Secondary quartz is smaller in size and occurs as replacement grains or inclusions in feldspar and biotites, occasionally forming mosaic and amoeboid structures. GEOCHEMISTRY The geochemistry of granitic plutons has been used successfully in the search for economic mineral deposits. It is a well-known concept that granitic rocks genetically related to Sn mineralization may be characterised by anomalous contents of Sn and other related elements (Sainbury and Hamilton, 1967; Beus and Sitnin, 1968; Beus and Gregorian, 1975). The nature of orebearing processes may also be indicated by trace-element studies of mineralized granites (Krauskopf, 1967).
73 MAJOR-ELEMENT CHEMISTRY OF JOS--BUKURU ROCKS Chemically the biotite granites are peraluminous, containing normative c o r u n d u m. Major-element differences (Table II) are shown by FeO exceeding Fe203 and slightly higher A1203 than the associated rocks. The biotite granites associated with mineralization are n o t a b l y low in MgO, CaO and FeO. The ratio of soda to potash varies considerably and those biotite granites which have suffered late deuteric albitization usually have excess soda over potash. T h e data in Table II, however, suggest that the major-element com posi t i on of the host rocks is not a sufficient criterion of Sn mineralization, since m a n y granites which have the required composition are not associated with Sn deposits (e.g., Kuru biotite granite and Jos biotite granite). TABLE II Average major-element composition Jos--Bukuru biotite granites
SiO2 TiO~ A120F%O 3 FeO MgO CaO Na~O K20 P20s
Jos
Kuru
N'gell
Ray field
Bukuru
Sabon Gida
74.80 0.25 12.40 0.78 1.65 0.20 0.90 3.57 4.51 0.03
75.80 0.07 12.60 0.64 0.86 0.14 0.30 4.40 4.93 0.05
76.00 0.07 12.87 0.42 0.38 0.10 0.25 4.50 5.05 tr.
76.00 0.04 13.51 0.43 0.61 0.08 0.30 4.23 4.61 tr.
75.50 0.04 12.95 0.22 0.70 0.08 0.27 4.50 4.90 tr.
75.69 0.03 12.97 0.17 0.47 0.06 0.20 4.60 4.98 tr.
tr. = trace. TRACE-ELEMENT GEOCHEMISTRY It has been emphasized t hat ore-bearing granites are characterized by an irregular distribution of rare elements and c o n c e n t r a t i o n in the late phases o f intrusion (Tauson et al., 1968; Smith and Turek, 1976). Since elements (such as Li, Be, Nb, Ta, Sn, U, Th, W, Zr) and the rare earths are preferentially c o n c e n t r a t e d in the residual fluids high in volatiles, it has been argued t hat an evaluation o f the primary dispersion patterns of these elements m ay help to estimate the Sn ore-bearing potential (Smith and Turek, 1976). The importance of volatiles (mineralizers) in the t ransport at i on of Sn, especially F and C1, has also been n o t e d by various workers ( G o t m a n and Rub, 1961; Lyakhovich, 1965; Hosking, 1967; Rub, 1972; T i s c h e n d o r f et al., 1972). Table III shows the trace-element c o n c e n t r a t i o n of the biotite granite phases of the J o s - B u k u r u Complex com pa r ed with average values for granites by Vinogradov (1962).
74 TABLE III Average trace-element contents (ppm)and elemental ratios of the Jos--Bukuru Complex
Jos biotite granite Kuru biotite granite N'gell biotite granite Rayfield biotite granite Bukuru biotite granite Sabon Gida biotite granite Average values for the complex Average granite *~ Average granite .2
Rb
Ba
Sr
433 413 583 657 687 704 563 170 200
285 24 129 5 77 7 84 9 73 7 63 6 276 7 840 100 600 285
Pb
Zn
Li
Be
18 13 20 19 26 29 24 19 20
150 50 5 240 50 5 164 71 14 183 106 8 162 100 8 190 100 16 175 76 9 39 40 3 60 40 5.5
Sn
F
10 4 18 21 18 31 12 3 3.0
3,404 2,485 2,270 3,512 3,535 4,732 3,384 850 800
*~From Turekian and Wedepohl (1963); *2from Vinogradov (1962). The Li, Rb, Sn and Zn values are above the average values for granites while the Ba, Sr and Be values are lower than average. These values indicate t hat these granites may be mineralized. T he mean value of Sn for the biotite granite is 20 ppm, which is more t han three times the value for barren granites in the Younger Granite Province and six times the world average of 3 p p m for low-Ca granites (Turekian and Wedepohl, 1961}. This enhanced value m ay reflect the abundance of disseminated cassiterite in the biotite granite and is indicative of the ore-bearing potential of the rocks. Only the granite t ype s associated with alluvial Sn workings have mean Sn c o n t en ts significantly greater than the background value of 12 ppm. More than 80% o f the Sabon Gida biotite granite samples contain m ore than 12 p p m Sn, corresponding figures for the Rayfield, Bukuru and N'gell are 73%, 78% and 73%, respectively. The Bukuru and N'gell phases have mean Sn c o n t en ts of 18 ppm, which is significantly greater than values for the nonstanniferous granites of the J o s - B u k u r u Complex (average 3 ppm) and also comparable with values r e p o r t e d for m any Sn granites from elsewhere (Ivanova, 1963; Rattigan, 1963; Lyakhovich, 1965; Mulligan, 1966; Ivano and Narnov, 1970; Neiva, 1972; Sheraton and Black, 1973; Smith and Turek, 1976). The Rayfield and Sabon Gida phases have rather higher Sn cont ent s (average 21 and 31 ppm, respectively). However, Jos and Kuru are particularly p o o r in their Sn c o n t e n t (Table III). F r e q u e n c y distribution of Sn in the biotite granite (Fig. 2) is multi-modal and shows a wide dispersion which agrees with th e observations of Sheraton and Black (1973), and Tauson and Kozlov (1973) t hat considerable variation in Sn values (Table III) characterize Sn-bearing granites. This is also reflected in a high coefficient variance. Nb is an i m p o r t a n t e c o n o m i c mineral in the Jos--Bukuru Complex. Enhanced values are recorded in all the biotite granite phases which reflects the abundance o f disseminated columbite in the biotite granite.
75
-
Nb
K/Rb
Ba/Rb
Li/Zn
Tn
Mg/Li
(Li x 1000)/K
81 159 177 150 133 126 121 21
105 103 69 64 62 60 72 250 165
0.87 0.38 0.13 0.11 0.10 0.09 0.49 4.94 3.0
0.33 0.21 0.51 0.64 0.65 0.74 0.43 1.02 0.66
42 33 42 46 40 63 39 17 ....
30 15 10 6 6 5 14 40
1.22 1.24 1.87 2.35 2.44 2.46 1.87 0.98 1.20
-
N=89 24
2C
16
Cr 8
4
lo
Th
20
30
40
so
&~ [~7o
~o
~o ~1oo
Sn in rock (ppm)
Fig. 2. Histogram showing the distribution of Sn in rocks The c o n c e n t r a t i o n s o f R b a n d Li are generally c o n s i d e r e d as g o o d indicators of ore-bearing p o t e n t i a l (Hosking, 1 9 6 7 ; Beus a n d Sitnin, 1 9 6 8 ; Flinter, 1 9 7 0 ; Flinter et al., 1 9 7 2 ; T a u s o n and Kozlov, 1 9 7 3 ) . R b a n d Li in t h e J o s - - B u k u r u b i o t i t e granites s h o w e n h a n c e d c o n c e n t r a t i o n relative t o b a r r e n b i o t i t e granites (Table III). O f p a r t i c u l a r interest are the Rayfield, B u k u r u , S a b o n Gida a n d N'gell biotite granites. This reflects t h e close association o f Sn m i n e r a l i z a t i o n with e l e m e n t s t h a t have s t r o n g a f f i n i t y f o r the volatile phase o f m a g m a t i c r e s i d u m ( T a u s o n , 1 9 6 7 ; T a u s o n a n d Kozlov, 1973). Li a n d R b s h o w positive c o r r e l a t i o n with Sn (r = 0.47 and 0.59, respectively).
76
The K / R b ratios of the biotite granites are very low (65) which indicates a highly fractionated granite with potential for mineralization (Beus and Gregorian, 1975). K/Rb, Ba/Rb, Mg/Li, Li/Zn are low in ore-bearing granites while (Li X 1000)/K is relatively high (Tauson and Kozlov, 1973). These ratios suggest that the Rayfield, Sabon Gida and Bukuru biotite granites have the best ore-bearing potential in the complex. Both water-extractable and total-extractable F show considerable increase in the biotite granites (Table III). Enhanced values are obtained for Rayfield, Bukuru and Sabon Gida phases which reflects the presence of appreciable amounts of fluorite in these rocks. The enhanced F and Sn contents of these rocks tends to suggest that they may be ore-bearing. CONCLUSION
The geochemical data obtained suggest that mineralization was closely associated with the biotite granites which are potentially ore-bearing. The biotite granites are enriched in a suite of ore and potential pathfinder elements. The ore-forming fluids were derived from the Sn-rich biotitegranite magma by fractional crystallization, but the absence of any major lode in the complex may be attributed to the absence of major fractures in the consolidated granite. This has led to the formation of a diffused type of greisenization, dissemination of cassiterite in the granite and extensive albitization of the host rock; which in turn have contributed to the alluvial concentration of Sn and columbite. In regional reconnaissance surveys for Sn deposits in the Younger Granite Province, it is r e c o m m e n d e d that biotite granites that are potentially ore bearing m a y be identified by enhanced values of Sn, Nb, Zn, Rb, Li and F and depletion in Ba, and Sr. Of critical importance is the Sn content of the host rock, which serves as the best indicator of Sn mineralization. ACKNOWLEDGEMENTS
The author is indebted to Prof. M.O. O y a w o y e who provided the atmosphere and encouragement for this study. This paper forms part of an M. Phil. dissertation completed at the University of Ibadan.
REFERENCES Beus, A. and Gregorian, S.V., 1975. Geochemical Exploration Methods for Mineral Deposits. Applied Publishing, pp. 53--71. Beus, A.A. and Sitnin, A.A., 1968. Geochemical specialization of magmatic complexes as criteria for the exploration of hidden deposits. Rep. 23rd Int. Geol. Congr. Soc., 6: 101--105. Flinter, B.H., 1970. Tin in acid granitoids; the search for a geochemical scheme for mineral exploration. Can. Inst. Min. Metall., Spec. Vol. II, pp. 323--330.
77 Flinter, B.H., Hesp, W.R. and Rigby, D., 1972. Selected geochemical mineralogical and petrological features of granitoids of the New England complex, Australia, and their relation to Sn, W, Mo, and Cu mineralization. Econ. Geol., 6 7 : 1 2 4 1 - - 1 2 6 2 . Gotman, Y.D. and Rub, M.G., 1961. Comparative characteristics of tin bearing granitoids of different ages of the South Primor'ye and certain other tin-bearing areas. Int. Geol. Rev., 3: 878--884. Hosking, K.F.G., 1967. The relationship between primary tin deposits and granitic rocks. Tech. Conf. Tin Counc., pp. 269--306. Imeokparia, E.G., 1977. Bedrock geochemistry of Jos--Bukuru complex in relation to Sn--Nb mineralization. M. Phil. Thesis, University of Ibadan, Ibadan (unpublished). Ivano, V.S. and Narnov, G.A., 1970. On the behaviour of tin in the granitoid intrusives of northeastern U.S.S.R. Geochem. Int., 7: 424--431. Ivano-¢a, G.F., 1963. Content of tin, tungsten and molybdenium in granites enclosing tin--tungsten deposits. Geochemistry, 5: 492--500. Kloosterman, J.B., 1967. A tin province of the Nigerian type in southern Amazonia. Tech. Conf. on Tin, London, 2: 388--398. Kloosterman, J.B., 1970. A two-fold analogy between the Nigerian and the Amazonia tin provinces. 2nd Tech. Conf. on Tin, Bangkok, pp. 193--221. Krauskopf, K.B., 1967. Source rock for metal bearing fluids. In: H.L. Barnes (Editor), Geochemistry of Hydrothermal Ore Deposits, Holt, Rinehard and Winston, New York, N.Y., pp. 1--33. Lyakhovich, V.V., 1965. Characteristics of distribution of tin and boron in granitoids. Geochem. Int., 2: 10--15. Mackay, R.A., 1949. The geology of the plateau tin folds, Resurvey 1945--48. Bull. Geol. Surv. Nigeria, No. 19. Macleod, W.N., 1954. The geology of the Jos--Bukuru Younger Granite Complex with particular reference to the distribution of columbite. Rec. Geol. Surv., Nigeria, pp. 7--34. Mulligan, R., 1966. Geology of Canadian tin occurrence. Geol. Surv. Can., Pap. 64-54, 22 pp. Neiva, J.M., 1972. Tin--tungsten deposits and granites from northern Portugal. 24th Int. Geol. Congr. Sect. 4, pp. 282--288. Rattigan, J.H., 1963. Geochemical ore guides and techniques in exploration for tin. Proc. Australas. Inst. Min. Metall., 207: 137--151. Rub, M.G., 1972. The role of the gaseous phase during the formation of ore-bearing magmatic complexes. Chem. Geol., 10: 89--98. Sainbury, C. and Hamilton, J.C., 1967. The geology of lode tin deposits. Tech. Conf. on Tin, London, 1: 267--306. Sheraton, J.W. and Black, L.D., 1973. Geochemistry of mineralized granitic rocks of northeast Queensland. J. Geochem. Explor., 2: 331--348. Smith, T.E. and Turek, A., 1976. Tin-bearing potential of some Devonian granitic rocks in S.W. Nova Scotia. Mineral. Deposita, 2: 234--245. Tauson, L.V., 1967. Distribution regularities of trace elements in granitoid intrusives of the batholith and hypabyssal types. In: H. Ahrens (Editor), Origin and Distribution of the Elements, pp. 62--639. Tauson, L.V. and Kozlov, V.D., 1973. Distribution functions and ratios of trace element concentrations as estimates of the ore bearing potential of granites. In: Geochemical Exploration 1972, Inst. Min. Metall. London, pp. 37--44. Tauson, K.L., Kozlov, V.D. and Kuz'min, M.I., 1968. Geochemical criteria of potential ore-bearing in granite intrusions. Rep. 23rd Int. Geol. Congr. Sect. 6, pp. 126--129. Tischendorf, G., Lachelt, S., Lange, H., Palchen, W. and Meinel, G., 1972. Geochemical specialization of granitoids in the territory of German Democratic Republic, 24th Int. Geol. Congr. Sect. 4, pp. 266--275. Turekian, K.L. and Wedepohl, K.H., 1961. Distribution of the elements in some major units of earth crust. Geol. Soc. Am. Bull., 72: 641---664.
69
ORE-BEARING POTENTIAL OF GRANITIC ROCKS FROM THE JOS-BUKURU COMPLEX, NORTHERN NIGERIA
EBO GAB. IMEOKPARIA Department of Geology, University of Ibadan, Ibadan (Nigeria) (Received May 7, 1979; accepted for publication August 16, 1979)
ABSTRACT
Imeokparia, E.G., 1980. Ore-bearing potential of granitic rocks from the Jos--Bukuru Complex, northern Nigeria. Chem. Geol., 28: 69--77. Geochemical analyses of fresh bedrock samples indicate that the Jos--Bukuru biotite granites are characterized by enhanced values of a suite of trace elements, including Sn, Zn, Rb, Li, Nb, Th and F, and depletion in Ba and Sr. The abundance and interrelationships of these elements are most useful as indicators of mineralizing processes and orebearing potential. The Sn-bearing rocks are characterized by low K/Rb, Ba/Rb, and Mg/Li and high Li/Zn and (Li × 1000)/K ratios. These results, as applied to the Jos--Bukuru Complex, indicate that Sabon Gida and Rayfield have the best ore-bearing potential, while that of Bukuru and N'gell is only limited. The Jos and Kuru phase could be regarded as barren.
INTRODUCTION Nigeria r a n k s a m o n g t h e w o r l d ' s leading p r o d u c e r s o f tin a n d c o l u m b i t e , o f w h i c h t h e m a i n sources are r e c e n t alluvial a n d p l a c e r deposits. I t is believed t h a t these d e p o s i t s are derived f r o m t h e Y o u n g e r G r a n i t e P r o v i n c e intrusives o f n o r t h e r n Nigeria. I t is also k n o w n t h a t s o m e p r i m a r y cassiterite a n d c o l u m b i t e are dispersed t h r o u g h o u t t h e granitic m a s s i f a n d at p r e s e n t m i n e d f r o m d e c o m p o s e d p a r e n t m a t e r i a l and greisenized (zones) veins a n d q u a r t z stringers. H o w e v e r , t h e r e is a m a r k e d p a u c i t y o f l o d e Sn d e p o s i t s in t h e Y o u n g e r G r a n i t e Province. As t h e s e easily w o r k e d alluvial d e p o s i t s b e c o m e e x h a u s t e d , c o n s i d e r a b l e a t t e n t i o n is being f o c u s s e d o n p r o s p e c t i n g f o r large d i s s e m i n a t e d d e p o s i t s t h a t c o u l d b e n e f i t f r o m large-scale o p e n - p i t m i n i n g m e t h o d s similar to t h o s e o f p o r p h y r y Cu deposits. T h e J o s - B u k u r u C o m p l e x is t h e largest individual c o m p l e x w i t h i n t h e Y o u n g e r G r a n i t e region a n d it c o n t a i n s t h e richest a n d m o s t a b u n d a n t alluvial d e p o s i t s o f Sn and N b in Nigeria. T h e d i s t r i b u t i o n o f alluvial w o r k i n g is c o n f i n e d t o t h e b i o t i t e granite p h a s e s o f t h e c o m p l e x a n d t h e genetic r e l a t i o n s h i p b e t w e e n t i n - - c o l u m b i t e m i n e r a l i z a t i o n a n d b i o t i t e granites is
70
well known (Kloosterman, 1967, 1970). Equally important is the fact that not all the biotite granite phases are equally rich in these economic deposits, which may reflect on their ore-bearing potential. To determine if the Jos--Bukuru Complex has some geochemical features indicative of its ore-bearing potential, 80 fresh bedrock samples were analysed for various elements by X-ray fluorescence, atomic absorption, emission spectrography and ion-selective electrode. Detailed descriptions of analytical procedures are presented elsewhere (Imeokparia, 1977). GEOLOGICAL SETTING
The Jos--Bukuru Complex lies at the focal point of the Younger granitic activity and occupies the north-central region of the Jos Plateau in the main mining area. It covers an area of nearly 430 km 2 defined by longitudes 8°45 '- 9~E and latitude 9°40'--10~N. The massif is elliptical in surface form and except for a deep embayment in the south, shows smooth and regular margins with the surrounding basement complex (Fig. 1). Ft.
Jos . u . u . u
v o u . o c . GR^N, TE c o . ~ E x .
] Biotite microgranite unclassified IT~ Sabon-Gida North Biotite granite ] Sabon-Gida South Biotite granite [;~]. Bukuru Biotite granite ] Shenhornblende-fayalite granite
91~.Sl
]
Detimi Biotite granite
] ] ] ] ] ]
RayfieldGonaBiotite
granite
Kuru Stock Biotite granite
N'gellBiotite
granite
Jos Biotite granite Naraguta quartz pyroxene fayalite porphyry
VomHornblende
Biotite granite
]
Neils valley Granite porphyry
] ]
Rhyolites Bassement complex
Numbers indicate location of analysed samples After Jacobson et al (1958)
, 0
~ ~ ~ ~miles 2
a
_..2
Fig. 1. Geological map of Jos--Bukuru Younger Granite Complex.
6
B kilometres
71 It is a composite b o d y of Jurassic age (dated as 165+ 3 Ma ago) made up of granite porphyries and biotite granites. The complex exhibits an extremely intricate pattern of separate granite intrusions and it is believed that largescale ring-faulting and cauldron subsidence operated during the initial stages of granite emplacement, which have been modified by piecemeal stoping and superposition of later cauldron structures (Mackay, 1949; Macleod, 1954). Detailed descriptions of the constituent rock types have been presented by Macleod (1954). Two major cycles of igneous activity have been recognised, and the sequence of emplacement of the principal members of the igneous suite is summarised in Table I (Macleod, 1954). TABLEI Sequence of emplacement of the rocks of the Jos--Bukuru Complex Second intrusive cycle:
(b) Upper granite
Delimi biotite granite Sabon Gida biotite granite Vom Road microgranite Bukuru biotite granite Rayfield biotite granite
(a) Upper granite porphyry
Shen--Kuru porphyry
First intrusive cycle:
(b) Lower granite
Kuru biotite granite N'gell biotite granite Jos biotite granite
(a) Lower granite porphyry
Nell's Valley porphyry Naraguta porphyry rhyolites
After Macleod (1954). Igneous activity started in this complex with emplacement of volcanic rocks, mainly rhyolites. This was followed by a series of plutonic intrusions. The earlier intrusive cycle is represented by the granite porphyries and quartz porphyries associated with ring structures and block subsidence. This was followed by three distinct phases of biotite granites. The last intrusive cycle was initiated by granite p o r p h y r y followed by five separate phases of biotite granites. These granites occur mainly as sheeted bodies that have only been partially unroofed at the present level of erosion. PETROLOGY OF BIOTITE GRANITES The predominant rock t y p e in the complex is a variety of biotite granites which have been distinguished from each other by grain size and texture. Textural variation depends on the relative order of crystallization of felsic and mafic minerals and the degree of late- and post-magmatic recrystalliza-
72 tion. Three types of biotite granites have been distinguished in the complex: (1) Jos type which is coarse-grained; (2) N'gell type, medium-grained pinkish granite grading into (3) white finer-grained Rayfield type, with biotite growing in separate flakes rather than in clusters and with small areas of hematite staining visible hand specimens. Structurally the biotite granites are more often discordant with the early ring dykes. Evidence of this is their irregular shapes and shallow outwarddipping contacts, which contrast with the regular circular or elliptical ring dykes with their steep contacts. In many cases the biotite granite actually cuts across and partly obliterate early ring structures. Despite the textural variations that mark the different phases, the biotite granites are composed mainly of quartz, K-feldspar, albite and biotite. Fluorite, zircon and Fe oxides are the most c o m m o n accessories. Cassiterite, columbite, xenotine and thorite are also present. They contain between 28% and 35% of modal quartz aggregated into groups of arcuate trains between the large feldspars, 50--56% K-feldspar, 5--10% albite, 3% biotite and 2% accessory minerals. Orthoclase microperthite is c o m m o n in the coarse-grained granite, b u t microcline-microperthite is dominant in the finer-textured granites together with small discrete crystals of albite and patchy perthitic replacement intergrowths of albite. Microcline occurs as interstitial grains or replacement rims around orthoclase probably of metasomatic origin. Late deuteric albitization is a common feature in the Rayfield and Sabon Gida phases. Biotite shows variable habit in these rocks. In the coarse-grained granites (Jos type) the biotite is generally pleochroic between deep-brown and strawyellow but in the pink to white, medium-grained and finer-textured granites biotite is distinctly paler green in colour. Two generations of quartz have been recognized in the granites: primary (early formed) and secondary (metasomatic) varieties. The primary quartz is large, clear and anhedral. It occurs in clusters and as graphic intergrowths with K-feldspar. Secondary quartz is smaller in size and occurs as replacement grains or inclusions in feldspar and biotites, occasionally forming mosaic and amoeboid structures. GEOCHEMISTRY The geochemistry of granitic plutons has been used successfully in the search for economic mineral deposits. It is a well-known concept that granitic rocks genetically related to Sn mineralization may be characterised by anomalous contents of Sn and other related elements (Sainbury and Hamilton, 1967; Beus and Sitnin, 1968; Beus and Gregorian, 1975). The nature of orebearing processes may also be indicated by trace-element studies of mineralized granites (Krauskopf, 1967).
73 MAJOR-ELEMENT CHEMISTRY OF JOS--BUKURU ROCKS Chemically the biotite granites are peraluminous, containing normative c o r u n d u m. Major-element differences (Table II) are shown by FeO exceeding Fe203 and slightly higher A1203 than the associated rocks. The biotite granites associated with mineralization are n o t a b l y low in MgO, CaO and FeO. The ratio of soda to potash varies considerably and those biotite granites which have suffered late deuteric albitization usually have excess soda over potash. T h e data in Table II, however, suggest that the major-element com posi t i on of the host rocks is not a sufficient criterion of Sn mineralization, since m a n y granites which have the required composition are not associated with Sn deposits (e.g., Kuru biotite granite and Jos biotite granite). TABLE II Average major-element composition Jos--Bukuru biotite granites
SiO2 TiO~ A120F%O 3 FeO MgO CaO Na~O K20 P20s
Jos
Kuru
N'gell
Ray field
Bukuru
Sabon Gida
74.80 0.25 12.40 0.78 1.65 0.20 0.90 3.57 4.51 0.03
75.80 0.07 12.60 0.64 0.86 0.14 0.30 4.40 4.93 0.05
76.00 0.07 12.87 0.42 0.38 0.10 0.25 4.50 5.05 tr.
76.00 0.04 13.51 0.43 0.61 0.08 0.30 4.23 4.61 tr.
75.50 0.04 12.95 0.22 0.70 0.08 0.27 4.50 4.90 tr.
75.69 0.03 12.97 0.17 0.47 0.06 0.20 4.60 4.98 tr.
tr. = trace. TRACE-ELEMENT GEOCHEMISTRY It has been emphasized t hat ore-bearing granites are characterized by an irregular distribution of rare elements and c o n c e n t r a t i o n in the late phases o f intrusion (Tauson et al., 1968; Smith and Turek, 1976). Since elements (such as Li, Be, Nb, Ta, Sn, U, Th, W, Zr) and the rare earths are preferentially c o n c e n t r a t e d in the residual fluids high in volatiles, it has been argued t hat an evaluation o f the primary dispersion patterns of these elements m ay help to estimate the Sn ore-bearing potential (Smith and Turek, 1976). The importance of volatiles (mineralizers) in the t ransport at i on of Sn, especially F and C1, has also been n o t e d by various workers ( G o t m a n and Rub, 1961; Lyakhovich, 1965; Hosking, 1967; Rub, 1972; T i s c h e n d o r f et al., 1972). Table III shows the trace-element c o n c e n t r a t i o n of the biotite granite phases of the J o s - B u k u r u Complex com pa r ed with average values for granites by Vinogradov (1962).
74 TABLE III Average trace-element contents (ppm)and elemental ratios of the Jos--Bukuru Complex
Jos biotite granite Kuru biotite granite N'gell biotite granite Rayfield biotite granite Bukuru biotite granite Sabon Gida biotite granite Average values for the complex Average granite *~ Average granite .2
Rb
Ba
Sr
433 413 583 657 687 704 563 170 200
285 24 129 5 77 7 84 9 73 7 63 6 276 7 840 100 600 285
Pb
Zn
Li
Be
18 13 20 19 26 29 24 19 20
150 50 5 240 50 5 164 71 14 183 106 8 162 100 8 190 100 16 175 76 9 39 40 3 60 40 5.5
Sn
F
10 4 18 21 18 31 12 3 3.0
3,404 2,485 2,270 3,512 3,535 4,732 3,384 850 800
*~From Turekian and Wedepohl (1963); *2from Vinogradov (1962). The Li, Rb, Sn and Zn values are above the average values for granites while the Ba, Sr and Be values are lower than average. These values indicate t hat these granites may be mineralized. T he mean value of Sn for the biotite granite is 20 ppm, which is more t han three times the value for barren granites in the Younger Granite Province and six times the world average of 3 p p m for low-Ca granites (Turekian and Wedepohl, 1961}. This enhanced value m ay reflect the abundance of disseminated cassiterite in the biotite granite and is indicative of the ore-bearing potential of the rocks. Only the granite t ype s associated with alluvial Sn workings have mean Sn c o n t en ts significantly greater than the background value of 12 ppm. More than 80% o f the Sabon Gida biotite granite samples contain m ore than 12 p p m Sn, corresponding figures for the Rayfield, Bukuru and N'gell are 73%, 78% and 73%, respectively. The Bukuru and N'gell phases have mean Sn c o n t en ts of 18 ppm, which is significantly greater than values for the nonstanniferous granites of the J o s - B u k u r u Complex (average 3 ppm) and also comparable with values r e p o r t e d for m any Sn granites from elsewhere (Ivanova, 1963; Rattigan, 1963; Lyakhovich, 1965; Mulligan, 1966; Ivano and Narnov, 1970; Neiva, 1972; Sheraton and Black, 1973; Smith and Turek, 1976). The Rayfield and Sabon Gida phases have rather higher Sn cont ent s (average 21 and 31 ppm, respectively). However, Jos and Kuru are particularly p o o r in their Sn c o n t e n t (Table III). F r e q u e n c y distribution of Sn in the biotite granite (Fig. 2) is multi-modal and shows a wide dispersion which agrees with th e observations of Sheraton and Black (1973), and Tauson and Kozlov (1973) t hat considerable variation in Sn values (Table III) characterize Sn-bearing granites. This is also reflected in a high coefficient variance. Nb is an i m p o r t a n t e c o n o m i c mineral in the Jos--Bukuru Complex. Enhanced values are recorded in all the biotite granite phases which reflects the abundance o f disseminated columbite in the biotite granite.
75
-
Nb
K/Rb
Ba/Rb
Li/Zn
Tn
Mg/Li
(Li x 1000)/K
81 159 177 150 133 126 121 21
105 103 69 64 62 60 72 250 165
0.87 0.38 0.13 0.11 0.10 0.09 0.49 4.94 3.0
0.33 0.21 0.51 0.64 0.65 0.74 0.43 1.02 0.66
42 33 42 46 40 63 39 17 ....
30 15 10 6 6 5 14 40
1.22 1.24 1.87 2.35 2.44 2.46 1.87 0.98 1.20
-
N=89 24
2C
16
Cr 8
4
lo
Th
20
30
40
so
&~ [~7o
~o
~o ~1oo
Sn in rock (ppm)
Fig. 2. Histogram showing the distribution of Sn in rocks The c o n c e n t r a t i o n s o f R b a n d Li are generally c o n s i d e r e d as g o o d indicators of ore-bearing p o t e n t i a l (Hosking, 1 9 6 7 ; Beus a n d Sitnin, 1 9 6 8 ; Flinter, 1 9 7 0 ; Flinter et al., 1 9 7 2 ; T a u s o n and Kozlov, 1 9 7 3 ) . R b a n d Li in t h e J o s - - B u k u r u b i o t i t e granites s h o w e n h a n c e d c o n c e n t r a t i o n relative t o b a r r e n b i o t i t e granites (Table III). O f p a r t i c u l a r interest are the Rayfield, B u k u r u , S a b o n Gida a n d N'gell biotite granites. This reflects t h e close association o f Sn m i n e r a l i z a t i o n with e l e m e n t s t h a t have s t r o n g a f f i n i t y f o r the volatile phase o f m a g m a t i c r e s i d u m ( T a u s o n , 1 9 6 7 ; T a u s o n a n d Kozlov, 1973). Li a n d R b s h o w positive c o r r e l a t i o n with Sn (r = 0.47 and 0.59, respectively).
76
The K / R b ratios of the biotite granites are very low (65) which indicates a highly fractionated granite with potential for mineralization (Beus and Gregorian, 1975). K/Rb, Ba/Rb, Mg/Li, Li/Zn are low in ore-bearing granites while (Li X 1000)/K is relatively high (Tauson and Kozlov, 1973). These ratios suggest that the Rayfield, Sabon Gida and Bukuru biotite granites have the best ore-bearing potential in the complex. Both water-extractable and total-extractable F show considerable increase in the biotite granites (Table III). Enhanced values are obtained for Rayfield, Bukuru and Sabon Gida phases which reflects the presence of appreciable amounts of fluorite in these rocks. The enhanced F and Sn contents of these rocks tends to suggest that they may be ore-bearing. CONCLUSION
The geochemical data obtained suggest that mineralization was closely associated with the biotite granites which are potentially ore-bearing. The biotite granites are enriched in a suite of ore and potential pathfinder elements. The ore-forming fluids were derived from the Sn-rich biotitegranite magma by fractional crystallization, but the absence of any major lode in the complex may be attributed to the absence of major fractures in the consolidated granite. This has led to the formation of a diffused type of greisenization, dissemination of cassiterite in the granite and extensive albitization of the host rock; which in turn have contributed to the alluvial concentration of Sn and columbite. In regional reconnaissance surveys for Sn deposits in the Younger Granite Province, it is r e c o m m e n d e d that biotite granites that are potentially ore bearing m a y be identified by enhanced values of Sn, Nb, Zn, Rb, Li and F and depletion in Ba, and Sr. Of critical importance is the Sn content of the host rock, which serves as the best indicator of Sn mineralization. ACKNOWLEDGEMENTS
The author is indebted to Prof. M.O. O y a w o y e who provided the atmosphere and encouragement for this study. This paper forms part of an M. Phil. dissertation completed at the University of Ibadan.
REFERENCES Beus, A. and Gregorian, S.V., 1975. Geochemical Exploration Methods for Mineral Deposits. Applied Publishing, pp. 53--71. Beus, A.A. and Sitnin, A.A., 1968. Geochemical specialization of magmatic complexes as criteria for the exploration of hidden deposits. Rep. 23rd Int. Geol. Congr. Soc., 6: 101--105. Flinter, B.H., 1970. Tin in acid granitoids; the search for a geochemical scheme for mineral exploration. Can. Inst. Min. Metall., Spec. Vol. II, pp. 323--330.
77 Flinter, B.H., Hesp, W.R. and Rigby, D., 1972. Selected geochemical mineralogical and petrological features of granitoids of the New England complex, Australia, and their relation to Sn, W, Mo, and Cu mineralization. Econ. Geol., 6 7 : 1 2 4 1 - - 1 2 6 2 . Gotman, Y.D. and Rub, M.G., 1961. Comparative characteristics of tin bearing granitoids of different ages of the South Primor'ye and certain other tin-bearing areas. Int. Geol. Rev., 3: 878--884. Hosking, K.F.G., 1967. The relationship between primary tin deposits and granitic rocks. Tech. Conf. Tin Counc., pp. 269--306. Imeokparia, E.G., 1977. Bedrock geochemistry of Jos--Bukuru complex in relation to Sn--Nb mineralization. M. Phil. Thesis, University of Ibadan, Ibadan (unpublished). Ivano, V.S. and Narnov, G.A., 1970. On the behaviour of tin in the granitoid intrusives of northeastern U.S.S.R. Geochem. Int., 7: 424--431. Ivano-¢a, G.F., 1963. Content of tin, tungsten and molybdenium in granites enclosing tin--tungsten deposits. Geochemistry, 5: 492--500. Kloosterman, J.B., 1967. A tin province of the Nigerian type in southern Amazonia. Tech. Conf. on Tin, London, 2: 388--398. Kloosterman, J.B., 1970. A two-fold analogy between the Nigerian and the Amazonia tin provinces. 2nd Tech. Conf. on Tin, Bangkok, pp. 193--221. Krauskopf, K.B., 1967. Source rock for metal bearing fluids. In: H.L. Barnes (Editor), Geochemistry of Hydrothermal Ore Deposits, Holt, Rinehard and Winston, New York, N.Y., pp. 1--33. Lyakhovich, V.V., 1965. Characteristics of distribution of tin and boron in granitoids. Geochem. Int., 2: 10--15. Mackay, R.A., 1949. The geology of the plateau tin folds, Resurvey 1945--48. Bull. Geol. Surv. Nigeria, No. 19. Macleod, W.N., 1954. The geology of the Jos--Bukuru Younger Granite Complex with particular reference to the distribution of columbite. Rec. Geol. Surv., Nigeria, pp. 7--34. Mulligan, R., 1966. Geology of Canadian tin occurrence. Geol. Surv. Can., Pap. 64-54, 22 pp. Neiva, J.M., 1972. Tin--tungsten deposits and granites from northern Portugal. 24th Int. Geol. Congr. Sect. 4, pp. 282--288. Rattigan, J.H., 1963. Geochemical ore guides and techniques in exploration for tin. Proc. Australas. Inst. Min. Metall., 207: 137--151. Rub, M.G., 1972. The role of the gaseous phase during the formation of ore-bearing magmatic complexes. Chem. Geol., 10: 89--98. Sainbury, C. and Hamilton, J.C., 1967. The geology of lode tin deposits. Tech. Conf. on Tin, London, 1: 267--306. Sheraton, J.W. and Black, L.D., 1973. Geochemistry of mineralized granitic rocks of northeast Queensland. J. Geochem. Explor., 2: 331--348. Smith, T.E. and Turek, A., 1976. Tin-bearing potential of some Devonian granitic rocks in S.W. Nova Scotia. Mineral. Deposita, 2: 234--245. Tauson, L.V., 1967. Distribution regularities of trace elements in granitoid intrusives of the batholith and hypabyssal types. In: H. Ahrens (Editor), Origin and Distribution of the Elements, pp. 62--639. Tauson, L.V. and Kozlov, V.D., 1973. Distribution functions and ratios of trace element concentrations as estimates of the ore bearing potential of granites. In: Geochemical Exploration 1972, Inst. Min. Metall. London, pp. 37--44. Tauson, K.L., Kozlov, V.D. and Kuz'min, M.I., 1968. Geochemical criteria of potential ore-bearing in granite intrusions. Rep. 23rd Int. Geol. Congr. Sect. 6, pp. 126--129. Tischendorf, G., Lachelt, S., Lange, H., Palchen, W. and Meinel, G., 1972. Geochemical specialization of granitoids in the territory of German Democratic Republic, 24th Int. Geol. Congr. Sect. 4, pp. 266--275. Turekian, K.L. and Wedepohl, K.H., 1961. Distribution of the elements in some major units of earth crust. Geol. Soc. Am. Bull., 72: 641---664.