ENERO%2031-GEOCIENCIAS-VOL%2012-2%20DIC-EN%20BAJA[1].pdf EARTH SCIENCES RESEARCH JOURNAL Earth Sci. Res. J. Vol. 12, No. 2 (December 2008): 213-234 MINERALIZATION POTENTIALS OFPEGMATITES IN THE NASARAWA AREA OF CENTRAL NIGERIA Akintola, O. F.1 and *Adekeye, J. I. D.2 1 Raw Materials Research and Development Council, P. M. B.232, Garki, Abuja, Nigeria Fax: 234 9 4136034, E-mail: akintolaolatunde@yahoo.com, Phone: 2348055239304 2 Geology and Mineral Sciences Department, University of Ilorin, P. M. B. 1515, Ilorin, Nigeria. Email: adekeye2001@yahoo.com, Phone: 2348033795444. * Corresponding author ABSTRACT Pegmatites in Nasarawa area of Central Nigeria lie within a fracture-controlled east-north-east trending rare metal pegmatitebeltcloselyassociatedwith latePanAfricanperaluminousgranites inNigeria.Traceelementsoffeldspars andmuscovitesofpegmatites inNasarawaareaofCentralNigeriawereanalyzedtodeterminetheraremetalsminer- alization potentials of the pegmatites. The minerals show fractionation even within units of complex mineralized pegmatites as indicated by the wide ranges of K/Rb in Na-feldspars (albites), muscovites and K-feldspars and the wide rangeofK/Tl in the latter twominerals.Cs,RbandTland the rareearthelements (REEs)Ce,La,Pr andYare enriched in theK-bearingmuscovitesandK-feldspars;K-feldsparshave thehighestaveragevaluesofTl,Ce,La,Pr andYwhile themuscoviteshave thehighest averagevaluesofSn,Nb,Ga,FandZn.AveragevaluesofTa,Sr,Ba, WandZrarehighest inthealbites,whichalsohavethelowestaverageNb/Taratio.Thisisanindicationthat theseel- ementsareenrichedalonglateNa-richraremetalmineralizingfluids.TherarealkalisRbandCsandTlarepositively correlatedwith the rare lithophile elements (Ta, REEs,Pb,Bi andY), inboth thepegmatiteK-feldspars andmusco- vites, stronglypositively correlated with Sn in themuscovites but negatively correlated with Nb in theminerals. In thepegmatiticalbites,NbhasverystrongpositivecorrelationswithTa(0.868)andZr(0.847),which is indicativeof the incorporations/substitutionof these elements in the crystal lattice of theoreminerals. There is a general enrich- mentoftherareelementstowardsthemiddleandinnerzonesofthecomplexmineralizedpegmatites.Fluorine,phos- phorusandboron-rich fluidsplayedsignificant roles in themagmatic complexation/fractionationandconcentration ofSn-Ta-Li-Cs-Beinthealbitizedzonesofthehighlyevolvedpagmatites.K/RbversusCsintheK-feldsparsclassify thepegmatites intobarren,Be-,Li-Be, andLi-Cs-Be-Ta types.Comparably lowNb/Ta ratios in the late albites and amblygonites indicate theparagenesisof thesemineralswith the tantalumores in themiddle/innerzonesof thecom- plex mineralized pegmatites. While the Be-, and Li-Be-types are highly prospective for gem tourmaline, beryl and 213 Manuscript receiver: October 22th, 2008. Accepted for publication: November 4 th , 2008. columbite; themostevolvedpegmatites in thisareaaregoodcandidates for tantalite, amblygonite-montebrasite,and probablypollucite prospecting/mining. Key words: Rare metals, Pegmatites, Muscovites, Feldspars, Mineralization, Correlations. RESUMEN PegmatitaseneláreadeNasarawa,Nigeriacentral, seencuentrandentrodeuncinturóndepegmatitasconmetales de tierras raras controlado por fracturas, con tendencia Este-Noreste, y estrechamente asociado con granitos peralumínicosdelPanAfricanotardíoenNigeria.Elementos trazasenfeldespatosymuscovitasde las pegmatitas delareadeNasarawa,Nigeriacentral, fueronanalizadosparadeterminarelpotencialdemineralizacióndemetales de tierras raras en las pegmatitas. Los minerales muestran fraccionamiento aun dentro de unidades en las pegmatitas complejamente mineralizadas, como lo indica el amplio rago K/Rb en feldespatos sódicos (albitas), muscovitas y feldespatos potásicos y el amplio rango de K/TI en estos dos últimos minerales. Las muscovitas potásicasy los feldespatospotásicos estánenriquecidosenCs,RbyTIyen los elementosde tierras raras (REEs) Ce,La,PreY;losfeldespatospotásicosposeenelmayorvalorpromediodeTI,Ce,La,PryYeYmientrasquelas muscovitas tienenlosmayoresvalorespromediosdeSn,Nb,Ga, laFyZn.LosvalorespromediosdeTa,Sr,Ba, la W y Zr son más altos en las albitas, las cuales poseen también el promedio más bajo de Nb/Ta. Esto indica que estos elementos son enriquecidos por fluidos mineralizantes tardíos ricos en sodio y metales de tierras raras. Los elementosalcalinosrarosRbandCsyTIsecorrelacionanpositivamenteconloselementoslitofilos(Ta,REEs,Pb, BiyY),en los feldespatospotásicosy lasmuscovitasde laspegmatitas,y tieneunafuertecorrelacionpositivacon Snen lamuscovitapero lacorrelacionesnegativaconNben losminerales.En lasalbiticaspegmatiticas,Nb tiene una correlacion positiva muy fuerte con Ta (0.868) y Zr (0.847), lo que indica la incorporación o sustitución de estos elementos en la estructura cristalina de los minerales de mena. Hay un enriquecimiento general de los elementos de tierras raras hacia la mitad y las partes mas profundas del complejo pegmatitico mineralizado. Fluidos ricos en flúor, fósforo y boro juegaron un papel importante en la complejización y fraccionamiento magmatico y en la concentración de Sn- Ta-Li-Cs-Be en las zonas albitizadas de las pegmatitas altamente evolucionadas.K/RbvsCsen feldespatospotásicosclasifican laspegmatitasen tiposestériles, deBe,deLi-Be,y de Li-Cs-Be-Ta. Relaciones de Nb/Ta comparativamente bajas en las albitas y ambligonitas tardias indican la paragenesis deestosminerales con lasmenasde tantalio en las zonasmedias e internasdel complejopegmatitico mineralizado.Mientras los tiposBeyLi-Besonaltamenteprospectivospara tourmalina,beriloycolumbitacomo gemas; las pegmatitas mas evolucionadas en esta area son buenas candidatas para la prospección minera de tantalita, ambligonita- montebrasita, y probablemente polucita. Palabras clave: Metales raros, Pegmatitas, Moscovitas, Feldospastos, Mineralización Correlaciones. Introduction Nigeria possesses a very large pegmatite environment. The pegmatites are widely distributed with a marked concentrationofmineralizedpegmatites inabroadbelt, which extends from Ago-Iwoye in the southwest to Bauchi in the northeast, an air distance of more than 400kilometers. Thousands of pegmatites occur in this belt, most of which have never been mapped or sam- pled ina systematic, scientificmanner.Potassiumfeld- spar, albiticplagioclase, quartz, and subordinate, if any muscovite and/or biotite constitute the major minerals while awide spread spectrumofminerals including le- pidolite, tourmaline(green,pink,blackandblue),beryl, chrysoberyl, apatite, amblygonite, monazite, lithiophyllite- triphyllite,ghanite,cassiterite,wodginite, nigerite, columbo-tantalite, tapiolite, microlite, bis- muthinite, bismuthite, scheelite, cholite, andalusite and 214 AKINTOLA, O. F. AND ADEKEYE, J. I. D. sillimanite occur as accessory minerals in the pegmatites,Bowden and Kinnaird (1984). The ages, mineralogy, and composition of these pegmatite units appear to be analogous to those of the pegmatites environment in Brazil, Canada and Australia. The pegmatitic belt and the orientation of the units within it appear to be re- lated to rotational stresses created by the Benue Trough.Fromamoreglobal perspective, this trend is probably the northern extension of the Brazilian pegmatite belt, which runs from Rio Grande del Sul toRioGrandedelNorte.Thepegmatite fieldof this study area is part of late Pan African, (Jacob- son and Webb, 1949; Wright, 1970), rare (spe- cialty) metals granitic pegmatites. The primary mineralizationof tantalum,niobium, tin, beryllium and lithium is hosted in quartz-feldspar-muscovite pegmatites (Kinnaird, 1984). The Nasarawa area lies between latitudes 8o 18’ N and 8o 30’ N and longitudes 7o 35’ E and 7o 50’ E, west of the southern boundary of Afu com- plex- the southern most unit of the Younger Gran- ites complexes (Figure1).Miningof tantalite from both pegmatites and the eluvials started in the area some years ago. The mining continues for the ma- jor periods of the year except during the very dry monthsofFebruary toearlyAprilwhen lackofwa- ter makes it difficult to mine and concentrate the minerals. Columbite and cassiterite are recovered as by-products of the tantalite mining, while other pegmatite minerals like quartz, feldspar, amblygo- nite-montebrasite, cookeiteandmicaare still being discarded in waste dumps. Area description, methods and materials studied Regional Geology CentralNigeria ispartofanUpperProterozoicmobile belt extending from Algiers across the southern Sa- hara into Nigeria, Benin, and the Cameroun. This Pan-African belt continues into Northeast Brazil where analogous rare-metal mineralized pegmatites are also known (Schuiling, 1967). Bordered to the west by the West African Craton (stabilized around 2 Ga) the Pan-African belt itself is made up of gneiss-migmatites,metasediments, andmetavolcanics thathavebeensubjected topolycyclicmetamorphism, and emplacement of igneous rocks. These rocks all constitute thePrecambarian toLowerPaleozoicBase- ment Complex rocks. The gneiss-migmatites bears imprints of the Liberian (ca. 2500Ma), Eburnean (ca 2000Ma)andPanAfrican(ca.600Ma) tectonicevents (Oversby, 1975; Turner, 1983). Within the sequence are domains of metasediments and metavolcanics in- truded by igneous rocks, which constitute the north/south trending schist belts. The schist belt lithologies which consist of fine grained clastics, pelitic schists, phyllites, banded iron-formations,marbleandamphibolitesareconsid- ered to be Upper Proterozoic assemblages (Turner, 1983). They host most of the economic minerals in theBasementComplex.During thePan-African epi- sode, the Proterozoic gneiss-migmatite-schist com- plex were intruded by various granitoids resulting from oceanic closure, subduction, oblique collision between the West African craton and the Hoggar – Nigeria shields (Black, 1984) andcrustal thickening. The Pan-African granitoids of Nigeria, which is col- lectively termed Older Granites, comprise gabbros, charnockites, diorites, granites, and syenites. Geochronological data frompreviousworks (Rb – Sr whole-rock and U-Pb zircon) of Pan-African granitoids intruding the reactivated Archean to LowerProterozoiccrustofcentralandsouth-western Nigeria show that intrusive magmatite activity in these areas lasted from at least 630 to 530 Ma (van Breemen et al., 1977; Rahaman et al., 1983; Dada et al., 1987; Matheis and Caen-Vachette, 1983; Umeji and Caen-Vachette, 1984; Akande and Reynolds, 1990). Results of the rock ages also show that pegmatities’ emplacement in the southwestern Nige- riaoccurredmainlyafter thepeakof thePan-African orogenic event in this area. The end of the Pan-African tectonic event is marked by a conjugate fracture system of the strike-slip faults (Ball, 1980). Fault directions have 215 MINERALIZATION POTENTIALS OFPEGMATITES IN THE NASARAWA AREA OF CENTRAL NIGERIA consistent trend and sense of displacement; i.e. a NE-SW (NNE-SSW) trending system having a dextral sense of movement and a NW-SE trending system a sinistral sense (McCurry, 1971;Wright, 1976;Holt et al., 1978; Ball, 1980). Both sets crosscut all the main Pan-African structures, including older N-S trending shear zones (mylonites) and late orogenic granites (Ball, 1980;Ajibade andWright, 1989;Kuster, 1990; Garba, 1992). Gold and pegmatities’ rare mineraliza- tion are closely associated with the fractures in the Pan-Africanbelt (Kuster, 1990;EkuemeandMatheis, 1995; Garba, 2002, 2003). About 100km north-east of the area of study at Wamba, rare metal pegmatites have also been geochemically linked to peraluminous late Pan-Afri- can tectonic granitoids, the emplacement of which have largely been controlled by the regional fractures (Kuster, 1990). Chemical data on granites, and gra- niticandpegmatiticmuscovites showthatRB,Cs,Sn, Nb, and Ta are enriched during both magmatic and postmagmatic evolution, with the highest contents of these elements occurring in early muscovites of the albitized and mineralized pegmatities (Kuster, 1990). Albite, K-feldspar, and quartz are the main pegmatitie-forming mineral; white mica is a typical but minor component. From a geochemical point of view, quartz is of no particular interest since it diadochially (substitutionally) hosts trace elements to a negligible extent. Feldspars and white micas are the most informative minerals because their element distributions reflect the traceelementscontents in ei- ther early pegmatite forming fluids or in the later metasomatizing solutions (Moller and Morteani, 1987). Feldspars and white micas of pegmatites in the Nasarawa area were sampled and analysed for traceelements todetermine themineralizationpoten- 216 AKINTOLA, O. F. AND ADEKEYE, J. I. D. Ilorin Ilesha Sokoto Gusau Kano Zaria Kaduna Abuja Study Area EnuguLagos Maiduguri Port-Harcourt Pan African Nigeria Belt 10°0° 10° 20° Atlantic Ocean Congo Craton0 5001000 50 100 150 Kilometres Kilometres Legend Faults Barren Pegmatites Newly metal pegmatites discovered rare- Known rare- pegmatites metal Major Cities / Towns Cretaceous- sedimentary cover Recent Mesozoic ring complexes [Younger Granites] Supracustal (Schist belts) Basement, Gneisses Migmatites and Pan - African (Older) Granites N Atlantic Ocean 6° 4° 8° 10° 12°N 6° 10° 16°E12°8°2° West African Craton 0° Figure 1: Geological Map of Nigeria Showing the Regional Fractures and Location of Areas of Rare-metal and Barren Pegmatites (after Garba, 2003) tials of the pegmatites. Data from this area is also compared with published data on well-studied pegmatities for comparison of the potentials. Analytical procedures are as stated in Akintola and Adekeye (2006). The chemical data on the peg- matitemicaand feldsparswere subjected tobivariate correlations after a lognormal transformation of the data. Bivariate correlation coefficient of the ele- ments, r, were interpreted to determine elemental geochemical associations and evolutionary trends in the pegmatites. Local Geology The geological setting of Nasarawa area is shown in Figure2. Fromfield evidence, theoldest rocks in the studyareaaremicaceouspeliticschists.Variations in the schist composition for instance, the mica versus thequartz–feldspathiccontentsareobservable in the field.Thisrockunithasundergoneapolyphasemeta- morphismandductiledeformationsevidencedby lo- cal variations in the strike and dips of the foliation. Like the other schist belt rocks in the country, the schistshaveageneralstrikeofnorth-south, andin the area, dips at gentle angle (10o – 30o) to the east. The schists were intruded by Older Granites, which out- crop as hills especially to the northwest, and west of the area. These Older Granites have a range of granodiorite to granite compositions. To theeastof theareaare rocksof theAfucom- plex of the Younger Granites. The Afu complex is made up of high-level anorogenic granites mainly bioticgranites andminorquartzporphyryemplaced within Precambrian Paleozoic Basement gneisses and Older Granites but exposed beneath the Creta- ceous-Recent sedimentarycoverof the lowerBenue Valley to the South. The geochemistry of the gran- ites and the mica schists hosting the pegmatites in this area are discussed in Akintola and Adekeye (2006). Two groups of pegmatites are noticeable in the area with minor muscovite in the area: (1) sim- ple, usually barren massive quartz – microcline pegmatites with minor muscovite and accessory tourmalines and (2) complex, albitized musco- vite-quartz-microline pegmatites, bearing the rare-metals Ta, Nb, Sn, Li and Be mineralization. The simple, barren quartz-feldspar (with mi- nor mica) pegmatites occur at the north western and western parts of the area and are spatially closelyassociatedwith themainphaseOlderGran- ites.Onanoutcrop scale, thepegmatites grade into patches of aplite. The complex rare-metal pegmatites are found far away from the granite plutons usually hosted by schists (exterior pegmatites). Wall-rock alteration in the simple pegmatites is negligible, but more pronounced in the complex pegmatites especially at contacts with the hanging walls (Jacobson and Webb, 1946). Al- though tourmalinization isby far themost common type of contact alteration, it is generally accompa- nied by silicification, albitization, greisenization and sometimes formation of apatites/fluorapatites which give rise to graded contacts. K-Feldspars Table 1 shows the trace element contents of the microcline,microperthites (K-feldspars) takenfrom different pegmatites in the study area. For example, one sample of K-feldspar has the highest Cs (3489ppm), Tl (73ppm), Bi (21ppm), REEs Ce (69ppm), La (166ppm), Pr (29ppm) and very high Rb (9474ppm). It has the lowest total Fe and Mn (Fe2O3, 0.13%; MnO, 0.002%), and K/Tl (1558), Rb/Tl (130), K/Cs (33) and K/Rb (12) ratios. These characteristic indicate that the feldspar crystallized from a highly fractionated melt and although this particular sample has low concentration of the ore elements, Sn-Ta-Nb, an eluvial concentrate from the mine had earlier yielded high values of Ta (1749ppm) with high Ta/Nb ratio of 5.45, NIMAMOP (1998). The average content of Ta in the K-feldspars is very low (2ppm). The K-feldspars have K/Rb range of12-35andameanof19.K/Rbhasaveryhighneg- ative correlation (-0.938) with Cs in the K-feldspars, Figure3. InFigure3, theCscontent is plottedversus the K/Rb ratio for K-feldspar together with the boundaries discriminating, according to Trueman 217 MINERALIZATION POTENTIALS OFPEGMATITES IN THE NASARAWA AREA OF CENTRAL NIGERIA and Cerny (1982), between barren, Be-, Li-Be-, and Li-Cs-Be-Ta mineralized pegmatites. From the fig- ure, the following pegmatites in the study area of which the K-feldspars were analyzed belong to the corresponding classes (Table 2). It is important tonotefromthisclassificationthat somepegmatites from the same location have differ- entdegreesofevolution–somearemoreevolvedbe- longing to the lithium-beryllium class and some belong to the beryllium class. The pegmatites in the beryllium and lithium-beryllium classes have high prospects for gem tourmaline and beryl mineraliza- tion, especially in the albitized zones of the pegmatites (Preinfalk et al., 2000). The Liberia and Onyelow Wazobia pegmatites mainly belong to the lithium-cesium-beryllium-tantalum class. It is also important to note here that a sample of the Liberia pegmatitesplots in the lithium-berylliumclass show- ing that the pegmatites must at least have evolved from the lithium-beryllium class to the lithium-ce- sium-beryllium-tantalum class. The Liberia pegma- tite therefore has potentials for bearing ore mineral characteristicsofboth the lithium-berylliumand lith- ium-cesium-beryllium-tantalumclasses in thediffer- ent zones of the pegmatite. Active mining of Sn-Nb-Ta ores and analysis of mica and other min- eral samples such as amblygonite and fluorapatite, confirmtheenrichmentof thepegmatitein theoresof Li-Be-B-Sn-Nb-Ta. Na – Feldspars The framework silicate samples (feldspars and feldspathoids) with low silica (SiO2 < 55.95%) and Na/K greater than 1 (except cookeite) were grouped together for convenience as Na-feldspars (Table 2). The true Na-feldspars according to Deer et al (1966) have the following average contents of ore and lithophile elements: P2O5 (1.17%), F (bdl), Ga (27ppm),Nb (145ppm),Sn (221ppm),Ta (195ppm), Rb (175ppm) and Cs (31ppm). The ore elements are finely disseminated in the albites. The aplitic footwall albite is probably sec- ondaryorat least lateprimaryincrystallization.Most of the albite in the pegmatites is found in the al- bite-rich aplitic zoneoftenoccurring in the footwalls of pegmatites (Jahn and Tuttle, 1963; Jahn and Burnham, 1969). Thesecondaryalbitehasvery lowNb/Taratioof 0.26, which is comparable to the low Nb/Ta ratio of 218 AKINTOLA, O. F. AND ADEKEYE, J. I. D. 25 \ Akpoku 30 Kama TudunWada 30 30 Endo NASARAWA 7°40’E7°35’E 8°30’E 3000 meters 0 3 6 Kilometers3 N Older granites Granodiorite tonalite gneiss Younger granites (afu complex) Mica schist Tourmalinite Basic dyke Pegmatite Strike and dip of foliation Geological boundary 30 Legend Roads 8°25’N 8°21’N 8°18’N 8°30’E 8°25’N 8°21’N 8°18’N 7°45’E 7°50’E GidanKwma Figure 2: Geological Map of Nasarawa Area. 219 MINERALIZATION POTENTIALS OFPEGMATITES IN THE NASARAWA AREA OF CENTRAL NIGERIA Table 1: Trace elements of the microline, microperthies (K-feldspars) of the pegmatites. Sample l3 l6 l7 l10a luz lu ls s2 k1 k3 w2a P (ppm) 2400 2461 2662 2579 2130 4774 5398 6642 2854 2138 3024 F 0 0 0 0 0 0 326 0 0 0 0 Ba 44 35 34 67 10 30 53 36 69 91 39 Bi 20 10 17 15 17 15 10 13 12 10 21 Cd 13 7 9 5 6 bdl bdl Bdl bdl bdl 16 Ce 40 32 32 12 52 21 0 19 35 0 69 Co 16 33 18 28 20 28 31 25 23 31 18 Cr 10 13 9 14 2 15 35 11 6 1 6 Cs 1722 1487 1540 1226 1482 602 111 160 844 692 3489 Cu 7 7 14 9 0 13 13 8 4 19 16 Ga 18 15 17 15 17 16 44 19 18 14 17 La 95 73 74 58 84 27 2 16 38 31 166 Nb 8 5 9 7 9 22 39 8 4 8 8 Ni 20 0 31 28 0 0 0 0 0 0 27 Pb 70 57 81 79 69 46 0 35 125 140 93 Pr 19 15 16 13 18 8 0 6 9 8 29 Rb 8546 6536 9534 8303 8420 5537 2593 3069 5440 4089 9474 Sn 21 13 187 15 14 28 24 28 9 8 24 Sr 38 144 58 44 51 22 30 183 64 70 32 Ta bdl 1 2 bdl bdl 9 6 Bdl bdl bdl bdl Tl 49 38 57 49 49 30 12 16 39 30 73 W 155 241 137 187 176 203 195 154 143 204 160 V 10 0 0 14 18 0 0 0 0 0 20 Y 30 18 20 0 19 24 12 11 0 14 24 Zn 44 0 0 0 0 0 40 0 0 0 0 K/Ba 2521 2906 3492 1694 11058 3628 1696 2841 1571 1165 2916 K/Rb 13 16 12 14 13 20 35 33 20 26 12 K/Cs 64 68 77 93 75 181 811 639 128 153 33 K/Tl 2264 2676 2083 2316 2257 3628 7493 6393 2780 3534 1558 Rb/Tl 174 172 167 169 172 185 216 192 139 136 130 0.26 average in the amblygonites. Obviously, both the secondary albite/mica and the amblygonites are in paragenesis with the tantalum minerals. This ob- servation is in agreementwith the findings of Moller and Morteani (1987). A sample with a very high (8.8%) L. O. I. was also identified (by XRD and ICP-OES) to be cookeite – a hydrated lithium aluminum silicate. It has Na/K ratio slightly less than 1. Cookeite crystal- lizes from late stage hydrothermal fluids evolved fromresidualalbiticmeltsonstabilizationof tourma- line(andthelossofother fluxingcomponentssuchas P, B, Li, F and H2O) from the pegmatite melts (Cerny, 1991b). Cookeite is known to occur with other low temperature minerals like adularia and zeolites in miarolitic cavities of the highly evolved complex rare metal pegmatites. White Micas (Muscovites) in the Pegmatites Theaveragechemicalcompositionof themuscovites is typicalof rareelementpegmatiteswithhigh,F,Cs, RbandLi (Deer et al., 1966;Gordiyenko,1971) (Ta- ble 3). The XRD diffractogram of selected samples of the micas show that the crystal structures of the muscovitesareclose to the idealdioctahedralandR2+ - free composition typical of muscovites from the rare-element pegmatites class (Cerny and Burt, 1984).Themuscoviteshaveawider rangeofRbval- ues (2659-10182ppm) and lower K/Rb ratios (8-31) than the K-feldspars’ (2593-9534ppm) and (12-35) respectively (Table 4). A late generation mica Ð taken from an inner zone of Liberia pegmatite has very high Cs (2353ppm), Rb (9910ppm), Sn (647ppm)andTa(103ppm)andcorrespondingly low K/Rb (8), Nb/Ta (0.66), and K/Tl (1600) ratios. These are comparable to average values of Cs (2294ppm), Rb (8978ppm), Sn (665ppm), and Ta (464ppm) and the correspondingly low average K/Rb (9), Nb/Ta (0.14), and K/Tl (1361) ratios in muscovites from highly mineralized pegmatites in the northern part of the area. Nb/Ta ratios for the pegmatitic muscovites range from 0.13 to 8.07 with a mean of 2.965. The K/Rbratios for themicas range from8and31witha mean of 16.20. Ta has a very high positive correla- tion with Cs (0.756) but a very high negative corre- lation with Nb (-0.856). Linnen (1998) and Morteani and Gaupp (1989) have observed that the differentbehaviourofTaandNbduringcrystalliza- tion is due to differences in solubility of Nb and Ta in Li-rich pegmatitic melts. The Cs vs. K/Rb plots of pegmatite muscovites from thearea (Figure 4) concentrate in the field typi- cal of rare-element class pegmatites as defined by Cerny and Burt (1984). Only one sample which was collected about 100meters south of the Liberia peg- matite plots in the muscovite class. The K/Rb ratios of the muscovites range between 7.58 and 30.94. These lowratiosofK/Rbaswell as thehighnegative correlation of K/Rb and Cs (-0.830) are typical of muscovites of the rare-metals class of pegmatites (Preinfalk et al., 2000) and have been used success- fully as exploration tools for the pegmatites. Correlation of the Trace Elements and Fractionation Trends K/Tl,K/Rb,K/Cs,Al/Ga,Zr/HfandNb/Taratiosgive fractionation indices in the granite-pegmatites suites. Some of these ratios are compared with the average valuesof the traceelements in thepegmatiticminerals (Table4).The table also shows that the rare earth ele- ments (REEs) Ce, La, Pr, and Y are enriched in the 220 AKINTOLA, O. F. AND ADEKEYE, J. I. D. barren 10p Be Li, Be Li, Cs, Be, Ta Cs 100001000100101 1 10 100 1000 K/Rb Figure 3. Classification of the Pegmatites Using the Plots of K/Rb Versus Cs of Their K-Feldspars. Note: The lines discriminating between barren pegmatites and pegmatites with significant Be; Li-Be-, Li-Be- Ta-mineralization are given according to Trueman and Cerny (1982). 221 MINERALIZATION POTENTIALS OFPEGMATITES IN THE NASARAWA AREA OF CENTRAL NIGERIA Table 2: Trace element content of the albitic pegmatite phases Phosphates Albites Cookeite Fluorapatite Amblygonites Sample lb6a lb8 sj3 rNa lc19 kj2fsp lb9 lb10 ch1 P2O5 (%) 0.481 0.487 2.169 1.763 0.243 0.017 24.97 49.973 43.25 F(ppm) 1240 0 0 0 0 294 11637 13897 14203 As 0 0 7 8 6 1 0 0 0 Ba 19 51 37 26 102 56 31 33 61 Bi 14 11 11 16 12 12 38 1 8 Ce 14 12 22 14 0 0 443 0 7 Cd 7 bdl bdl bdl bdl bdl 27 bdl bdl Co 115 47 25 31 37 5 16 8 6 Cr 1 0 0 11 0 15 1 2 1 Cs 851 23 19 74 9 49 1 28 6 Cu 0 0 24 16 4 0 0 16 535 Ga 76 24 21 22 39 58 14 21 18 Hf 2 3 0 3 3 2 0 3 2 La 48 9 0 3 3 0 204 0 0 Mo 0 5 1 1 0 4 0 1 0 Nb 88 75 62 326 115 33 190 26 25 Nd 3 11 9 8 2 0 144 5 7 Ni 14 5 3 4 0 3 9 4 3 Pb 25 10 7 0 0 0 74 9 0 Pr 10 2 1 2 1 0 65 0 1 Rb 4829 69 347 252 31 444 46 290 62 Sc 0 0 9 0 4 0 0 0 5 Sm 1 3 2 2 1 1 66 1 1 Sn 565 659 14 174 35 54 28 231 67 Sr 263 52 1037 305 51 8 64 11 188 Ta 345 109 67 305 297 37 21 107 86 Th 0 0 0 0 0 3 2 1 4 Tl 23 bdl bdl bdl bdl 5 bdl bdl bdl U 0 3 0 10 0 1 216 0 7 K-bearingminerals,K-feldsparsandmuscovites,with the highest concentrations of the element in the K-feldspars.Becauseof their similargeochemicalmi- gration patterns, K, Rb, Cs and Tl are concentrated, andhaveveryhighpositivecorrelations in theK-feld- spars andmuscovites (Table6).Themuscoviteshow- ever have higher fractionation indices compared with theK-feldsparas indicatedbytheir lowaverageK/Rb, K/Tl, and K/Cs ratios. The muscovites have high val- ues of Sn (383ppm average), Nb (155ppm average), Ta (96ppm average), Ga (196ppm average), F (2128ppm average), and Zn (379ppm average) and have higher Rb/Tl ratio than the K-feldspars. The muscoviteshavethehighestvaluesofGa(withamean valueof161ppm)andlowestAl/Garatioof theminer- als. This corroborates the observations of Cerny et al (1985) that tourmaline and muscovites are the main concentrators of Ga in pegmatites. Ba, Sr, W, Zr and Ta are highest in the Na-feld- spars with the lowest Nb/Ta ratios, which is an indi- cation that theseelementsare enrichedalongwithTa in the late mineralizing fluids. The following groups of elements (Table 5) have high positive correlation indices in theNa-feldspars:RbversusCs (0.922), Cs versus Zn (0.866), Cs versus Co (0.802), Co versus W (0.974), Ga versus Co (0.890), Ga versus W (0.851), Nb versus Zr (0.941), Nb versus Bi (0.850), TaversusBi(0.769),TaversusGa(0.638),Taversus Nb (0.629) and Ta versus W (0.538).The high posi- tive correlations of these elements reflect their asso- ciations in the formation of rare metal ores from the Na-rich late fluids. The pegmatitic K-feldspars have very high negative correlations of K/Tl and K/Rb with the rare lithophile elements such as Cs, Bi, Y, Rb, La, Pr, Pb (Table 7). Similarly, the muscovites have very high negative correlations of K/Tl and K/Rb with the rare lithophile elements as well as the ore elements Sn and Ta. Nb is negatively correlated with Rb, Cs, Tl, Ta, La, Pr and Sn but positively correlated with K and Ga in both the K-feldspars andmuscovites (Table7).NbandGahavepositive correlations in both muscovites and K-feldspars (0.704 and 0.472) respectively. Obviously, the 222 AKINTOLA, O. F. AND ADEKEYE, J. I. D. Sample lb6a lb8 sj3 rNa lc19 kj2fsp lb9 lb10 ch1 V 5 1 4 15 6 5 16 4 8 W 502 346 201 247 298 64 46 89 124 Y 6 1 0 0 1 0 1391 1 0 Zn 229 63 26 187 12 39 62 26 119 Zr 17 12 17 68 18 5 17 54 7 H2O 2.42 0.43 1.1 0.82 0.36 8.8 0.51 5.27 7.54 SUM 98.71 98.24 97.67 99.4 99.79 98.01 98.12 102.94 86.53 K 33457 1577 11540 3321 1079 9132 1494 3487 664 K/Rb 7 23 33 13 35 21 32 12 11 Mg(hx) 13 139 7 23 Li(hx) 225 685 16400 20750 Li(fusion) 227 2900 13366 17882 Na/K 1.17 43.74 5.81 19.80 72.80 <1 21.80 4.23 n.d Nb/Ta 0.26 0.69 0.93 1.07 0.39 0.89 9.05 0.24 0.29 223 MINERALIZATION POTENTIALS OFPEGMATITES IN THE NASARAWA AREA OF CENTRAL NIGERIA T a b le 3 . T ra c e e le m e n ts in th e p e g m a ti te m ic a s S a m p le (p p m ) le 7 le 8 le 1 4 le 1 3 le 1 8 l4 l1 0 b l8 a l9 a lz lu a ls 7 L c 1 9 lc 2 0 lc 2 0 a s1 k k a lc 2 3 lc 2 8 w w 2 R lc 3 0 lc 3 1 lc 3 2 lc 3 3 F 2 9 3 3 3 1 9 7 3 2 1 5 3 2 2 0 2 9 7 2 7 4 3 2 3 3 1 3 1 2 8 8 4 4 3 5 2 4 6 5 2 1 0 6 6 1 7 1 0 0 4 3 9 0 3 9 3 3 2 9 6 6 7 4 6 1 2 8 4 1 2 2 5 4 2 6 1 6 2 9 6 5 4 7 5 1 9 4 0 1 0 8 8 8 8 1 3 7 4 B a 1 5 6 0 3 1 2 7 2 4 1 2 3 0 3 5 2 0 3 1 2 3 5 3 7 6 9 3 5 1 9 1 3 2 8 1 5 4 2 2 5 1 0 0 7 4 3 3 1 6 2 3 4 4 1 8 B i 1 7 2 1 2 0 1 6 2 1 2 5 1 6 1 9 1 2 1 9 1 3 1 4 1 2 1 5 2 8 3 4 1 4 2 1 1 7 1 8 2 0 1 1 1 8 1 5 1 3 1 2 1 3 1 7 C e 1 2 2 9 5 2 0 2 6 1 3 0 1 1 0 0 0 0 9 3 2 3 9 3 7 1 8 2 6 1 4 1 2 9 0 2 7 0 2 2 0 2 3 C s 1 7 1 8 7 4 2 9 0 3 0 0 2 6 0 8 7 4 1 0 0 1 0 0 1 2 3 2 8 9 6 2 0 6 3 2 1 1 8 3 4 2 2 1 2 0 2 4 6 7 4 2 4 5 1 5 6 9 4 5 1 6 9 4 2 1 5 1 7 4 9 2 9 1 4 9 9 6 3 1 1 6 3 4 6 G a 1 6 7 1 6 3 1 6 3 1 5 3 1 6 7 1 6 8 1 5 8 1 6 4 1 6 2 1 7 4 1 5 5 2 0 3 2 0 0 1 6 6 1 5 6 1 6 4 1 5 9 1 4 1 1 7 6 1 7 4 1 9 7 1 5 4 1 4 5 1 6 2 9 7 1 4 7 1 6 1 1 3 8 L a 0 5 7 1 5 2 0 2 3 5 4 4 4 4 5 1 0 3 6 9 0 0 4 7 9 8 1 0 8 1 7 1 4 3 6 0 4 0 3 2 0 4 9 7 1 0 2 1 5 N b 1 8 1 1 1 6 1 7 8 1 6 5 1 8 2 1 3 1 1 1 9 1 1 3 1 8 5 1 4 4 1 7 8 2 1 8 2 5 6 1 8 7 7 5 5 5 1 4 6 1 5 1 1 6 0 2 1 1 1 4 3 1 9 3 2 0 0 1 2 7 6 4 2 2 3 1 9 0 1 3 5 N d 9 1 7 4 1 4 1 0 6 0 7 2 0 2 0 1 9 1 2 7 7 1 3 2 1 7 0 1 1 0 9 4 5 7 N i 7 2 7 1 7 1 8 2 1 2 3 2 4 2 4 2 1 2 3 1 3 8 9 1 8 2 7 3 1 1 8 2 1 1 6 1 1 2 0 1 0 6 1 7 1 1 1 6 1 8 2 4 P b 1 8 4 7 2 3 2 1 3 0 4 6 4 5 4 5 1 9 4 4 1 7 6 4 2 5 4 7 6 4 1 8 2 6 2 2 7 3 3 1 0 9 3 3 2 7 9 1 8 1 9 P r 3 1 5 6 8 8 1 4 1 2 1 2 6 1 1 6 1 3 1 1 1 9 2 2 6 6 9 2 1 1 7 3 1 1 1 3 2 4 5 R b 4 8 0 3 9 4 1 0 5 0 9 3 5 5 7 8 5 8 3 9 1 3 9 1 5 8 8 0 3 5 2 8 9 8 8 7 4 7 5 1 3 1 3 3 2 6 5 9 5 6 3 8 7 7 7 4 1 0 1 8 4 5 0 4 4 9 4 1 5 5 2 7 3 4 7 1 7 7 4 9 3 2 8 4 2 8 7 0 6 8 0 3 4 5 8 1 3 6 4 5 4 3 2 4 4 2 4 8 S n 2 1 7 4 7 1 2 7 5 3 3 6 3 5 7 5 2 5 5 3 3 5 9 7 2 7 4 6 8 1 2 3 9 6 1 2 6 6 3 9 7 6 8 1 6 4 9 2 9 5 3 6 4 3 9 4 8 7 4 3 7 1 3 9 1 1 8 9 0 2 5 3 9 1 4 7 2 7 1 2 1 9 S r 1 5 5 0 4 1 1 8 1 9 2 4 2 7 2 5 1 7 2 4 1 6 1 1 1 3 1 8 2 5 3 2 1 6 1 8 1 9 1 1 2 3 1 5 1 2 2 1 1 5 1 7 1 7 1 6 T a 4 4 5 8 4 5 5 0 6 3 5 3 5 9 7 1 7 1 7 1 4 1 2 7 4 6 7 2 5 0 2 4 2 5 7 5 8 5 1 1 5 3 1 5 1 6 2 6 4 1 2 0 1 8 3 3 9 3 1 1 4 0 T l 2 2 4 3 2 4 2 7 2 6 4 0 3 9 3 9 2 5 4 0 2 1 1 7 1 5 3 1 5 2 6 4 2 4 2 9 2 8 1 6 3 5 1 8 1 7 3 6 3 3 1 8 2 1 2 5 W 7 1 5 1 6 5 7 1 2 3 7 3 9 3 3 2 8 3 8 7 7 2 5 8 4 6 5 4 0 8 6 8 9 3 4 3 6 4 8 5 6 5 2 4 0 4 9 9 1 5 6 1 1 4 3 2 6 1 Z n 4 2 1 1 0 0 0 4 5 2 4 3 5 4 7 2 9 6 1 3 9 1 1 4 1 6 8 0 0 3 7 9 2 3 1 1 3 1 2 1 7 1 1 1 1 2 3 1 4 2 2 4 2 1 4 2 1 8 7 3 4 1 2 4 9 1 6 3 4 5 3 4 7 1 6 1 1 7 7 1 1 2 K /R b 1 7 9 1 6 1 5 1 4 9 9 9 1 5 9 1 7 2 7 3 1 1 4 1 0 8 1 8 1 6 1 5 2 3 1 0 2 6 2 8 1 2 1 7 2 3 1 8 1 9 M g (h x ) 6 1 2 0 6 9 6 9 7 6 2 2 2 8 2 2 5 2 2 1 7 4 8 1 1 2 9 7 5 7 0 6 6 2 0 5 0 3 1 8 9 6 6 9 1 1 0 7 8 5 6 6 8 3 3 6 4 4 N b /T a 4 .1 1 2 3 .9 6 3 .3 2 .8 9 2 .4 7 2 .0 2 1 .5 9 2 .6 1 2 .0 3 4 .3 4 8 .0 7 5 .5 7 2 .6 0 .1 5 0 .1 3 1 .9 5 1 .7 8 1 .3 9 6 .8 1 2 .8 3 .1 1 3 .1 3 1 .0 6 0 .3 5 5 .7 2 6 .1 3 0 .9 6 K /R b (h x ) 1 0 .9 5 6 .0 1 1 0 .6 6 9 .4 5 9 .6 1 6 .2 4 6 .3 0 6 .4 3 9 .6 0 6 .8 6 1 0 .5 8 1 5 .5 4 1 8 .3 5 9 .2 6 6 .9 5 5 .5 4 1 1 .2 9 1 0 1 0 .0 1 1 3 .4 8 7 .2 5 1 4 .2 4 1 5 .8 9 8 .2 1 9 .9 1 1 4 .0 5 1 1 .6 9 1 1 .1 4 M g /L i( h x ) 1 .3 0 .3 4 1 .3 5 1 .1 7 0 .8 4 0 .2 6 0 .1 7 0 .1 9 1 .2 4 0 .2 4 1 .5 7 1 6 .2 3 2 .2 5 2 .2 1 5 7 .3 3 1 0 1 .6 1 1 5 .5 1 1 .1 3 1 1 4 .5 5 4 .4 6 8 .5 2 2 5 .7 2 2 .2 5 1 1 L i- A A S (h x ) 4 7 5 8 5 1 5 9 9 0 8 6 1 6 8 1 1 5 4 2 8 8 4 7 5 4 3 4 1 4 9 2 3 1 2 8 6 2 0 2 4 1 0 3 1 4 .5 1 6 4 L i- A A S (f u s. ) 1 5 4 2 8 3 2 0 1 1 9 8 2 7 2 3 9 5 4 6 8 3 0 3 1 7 4 3 2 4 2 3 1 3 0 4 0 2 0 4 7 3 6 1 1 2 1 5 8 1 0 4 3 3 1 7 6 1 1 1 6 0 2 9 5 4 7 2 3 0 K -A A S (h x ) 5 8 0 5 4 1 8 0 5 9 1 8 4 6 8 0 0 0 7 2 2 5 4 2 2 5 4 8 1 5 5 0 5 0 2 8 4 5 4 0 7 5 4 3 1 0 3 5 1 5 5 3 0 5 4 2 5 0 7 4 5 5 5 0 5 5 4 9 3 0 4 1 3 0 5 1 6 5 4 5 5 5 3 8 1 5 5 9 7 0 6 0 8 0 4 2 9 0 R b -A A S (h x ) 5 3 0 6 9 5 5 5 5 4 9 5 9 0 0 8 7 5 8 9 5 1 1 2 4 4 4 0 4 4 5 5 3 2 5 1 5 5 4 4 0 6 2 0 6 3 5 4 7 0 4 2 5 7 4 5 3 7 5 6 8 0 2 9 0 3 2 5 5 5 5 3 8 5 4 2 5 5 2 0 3 8 5 -A A S (f u s. ) 2 8 1 9 5 2 9 9 3 7 0 5 3 6 0 4 6 8 6 1 0 6 3 4 3 6 0 3 8 0 8 2 5 1 2 2 3 0 0 3 9 3 4 4 2 3 1 6 8 8 6 3 2 5 3 3 0 8 3 4 1 5 0 2 7 8 3 4 2 9 1 1 4 4 4 2 1 8 3 4 4 3 5 3 0 7 7 2 2 0 7 3 2 5 8 3 5 8 0 224 AKINTOLA, O. F. AND ADEKEYE, J. I. D. Table 4: Average contents of trace elements (range given in brackets) in the K-feldspars, Na-feldspars and micas of the pegmatites K-fedlspar(n=11) Muscovite(n=29) Albitesn=4(true albites) Ba 46ppm(10-91) 31ppm (3-100) 54ppm (26-102) Cs 1214ppm (111-3489) 664ppm (32-2467) 31ppm (9-74) Ga 19ppm (14-44) 161ppm (97-203) 27ppm (21-39) La 60ppm (2-166) 33ppm (1-109) Bdl Nb 12ppm (4-39) 155ppm (55-256) 145ppm (62-326) Pb 73ppm (10-140) 27ppm (4-64) Bdl Pr 13ppm (1-29) 9ppm (1-22) Bdl Rb 6504ppm (2593-9534) 5930ppm (2659-10182) 175ppm (31-347) Sn 34ppm (8-187) 383ppm (61-902) 221ppm (14-659) Sr 67ppm (22-183) 22ppm (11-64) 361ppm (51-1037) F Bdl 2128ppm (329-4751) Bdl Ni 9ppm (0-31) 18ppm (6-31) Bdl Ta 2ppm (0-9) 96ppm (27-502) 195ppm (67-305) Ce 28ppm (0-69) 15ppm (0-47) 12ppm (0-22) Zn 3.64 379ppm (47-1023) 72ppm (12-187) Tl 40ppm (12-73) 30ppm (15-64) Bdl W 178ppm (137-241) 62ppm (25-237) 273ppm (201-346) Y 16ppm (0-30) 15ppm (0-39) 0.5ppm (0-1) K/Tl 3362 (1558-7493) 3096 (1206-5485) - Rb/Tl 168 (130-216) 198 (139-235) - K/Rb 19 (12-35) 16 (8-31) 26 (13-35) K/Cs 211 (33-811) 121 140 K/Ba 3226 (1165-11058) 2575 81 Al/Ga 5359 1197 3680 Nb/Ta 7.04 2.89 (0.15-8.07) 0.77 Nb-dominant minerals crystallize earlier than Ta and the positive correlations of Nb, K and Ga indi- cate thatNbenters thesilicate structuremoreeasily than Ta. This agrees with the observations of CernyandBurt (1989) thatNb/Ta ratios in silicates are rather high. Linnen (1998) has also observed that the different behaviour of Ta and Nb during crystallization is due to differences in solubility of NbandTa inLi-richpegmatiticmelts.Suchbehav- iour has also been observed in the Bruno and Xuxa pegmatites of the Aracuai pegmatite District of Brazil (Preinfalk et al., 2000). In themuscovites (Table7),Tahas averyhigh positive correlation (0.756) with Cs but lower cor- relations with Rb in both muscovites and the albites (0.428 and 0.213 respectively). Ta and K/Rb however have better correlations in the mus- covites and albites (-0.441 and -0.592 respec- tively)whencomparedwithTaandRb.The reason for this relationship between the elements may be explained by the fact that ideal positions for TaO 4 3- and TaO3- are lacking in the silicates. Therefore, Ta partitioning into mica will be small and TaO 4 3- which grow epitaxially at the mica faces account for much of the Ta contents in the micas (Moller and Morteani, 1987). Mg/Li has a high negative (-0.856) correlation withZinc; thehighpositive correlationsofZn,Li and F(³0.759) in themicasare traceable to thefact that the threeelementsoccupy theoctahedral layer in themica crystal structure. Li has negative correlation with Ta (-0.244),whichshowsthatLicontentof theearlypeg- matite micas are higher than the later ones; obviously increased concentration of Li through fractionation in the late fluidscrystallized to formdiscreteLi-minerals such as amblygonite-montebrasite rather than enter micaphyllosilicatestructure.This is inagreementwith the observation of Cerny et al (1985) that the buildup of Li concentration in a pegmatite melt that precedes the precipitation of Li minerals sensu stricto leads to increasedcontentsofLi inearly rock-formingmineral phases. Sn average value is highest in the muscovites; it also has much higher positive correlations with Cs (0.902),Rb(0.829),Tl (0.868),La (0.799) compared with the very low correlations it has with these ele- ments in the feldspars, which reflects the enrichment of tin with greisenization. UsingK/Rb,K/TlandCs, it is thereforepossible to recognize different generations of feldspars and micas within a complex-zoned mineralized pegma- tite (Table8).E.g. early and late,whichhave signifi- cant differences in the range, and mean contents of the rare elements. Generally, the rare alkali (Rb and Cs) and Tl concentrations in the muscovites and K-feldspars give a good reflection of the concentra- tion of the rare elements in the pegmatites. The Liberia (lb) Pegmatite Deposit In this section, the trace element contents of the K-feldspars and muscovites of pegmatites in the Nasarawaareaarecomparedwith thoseofwell-stud- ied pegmatites both within and outside Nigeria. The Liberia pegmatite in the study area is the most exposed of the studied pegmatites, because someblasting and drilling had been carried out on it. Twelve (12) muscovite samples of the pegmatite were analyzed. Extensive fractionation within the pegmatite is indicated by the wide range of K/Rb 225 MINERALIZATION POTENTIALS OFPEGMATITES IN THE NASARAWA AREA OF CENTRAL NIGERIA Rare-elements pegmatites’Class Muscovite Class Cs 10000 K/Rb 10001000 10 100 1000 Figure 4: Plots of K/Rb Versus Cs for The Pegmatites’ Muscovites. Note: The dash line represents the boundry between the muscovite and rare-metal classes (after Cerny and Burt, 1984).Thesolid lines represent thebest fit lines for theLCT type pegmatites. 226 AKINTOLA, O. F. AND ADEKEYE, J. I. D. T a b le 5 . C o rr e la ti o n m a tr ix o f so m e tr a c e e le m e n ts th a t h a v e si g n if ic a n t B iv a ri a te C o rr e la ti o n s in th e N a -F e ld sp a rs (n = 5 ) B a C s G a N b R b S n T a W Z n Z r K /R b N b /T a B a 1 .0 0 0 C s -0 .8 6 8 1 .0 0 0 G a -0 .2 3 7 0 .6 6 9 1 .0 0 0 N b -0 .1 8 3 0 .1 2 2 -0 .1 6 8 1 .0 0 0 R b -0 .9 0 0 0 .9 2 2 0 .5 6 5 -0 .0 8 7 1 .0 0 0 S n -0 .4 2 7 0 .6 1 3 0 .3 7 1 0 .1 3 2 0 .3 2 5 1 .0 0 0 T a -0 .1 4 9 0 .4 8 6 0 .6 3 8 0 .6 2 9 0 .2 1 3 0 .5 5 5 1 .0 0 0 W -0 .2 6 5 0 .6 7 7 0 .8 5 1 -0 .1 6 6 0 .4 6 5 0 .4 8 3 0 .5 3 8 1 .0 0 0 Z n -0 .8 9 1 0 .8 6 6 0 .2 7 4 0 .4 0 2 0 .7 1 8 0 .7 4 0 0 .3 9 3 0 .4 6 7 1 .0 0 0 Z r -0 .3 5 1 0 .1 6 0 -0 .2 7 5 0 .9 4 1 0 .0 6 9 0 .1 5 2 0 .4 5 1 -0 .3 5 5 0 .4 2 7 1 .0 0 0 K /R b 0 .8 3 4 -0 .9 7 3 -0 .6 0 8 -0 .3 0 1 -0 .4 7 6 -0 .8 1 8 -0 .5 9 2 -0 .6 8 5 -0 .9 2 8 -0 .2 9 0 1 .0 0 0 N b /T a -0 .0 1 6 -0 .4 6 1 -0 .9 6 1 -0 .3 1 5 -0 .1 3 1 -0 .3 5 4 -0 .5 4 1 -0 .8 3 6 -0 .0 4 5 -0 .3 9 6 0 .3 9 6 1 .0 0 0 227 MINERALIZATION POTENTIALS OFPEGMATITES IN THE NASARAWA AREA OF CENTRAL NIGERIA T a b le 6 . C o rr e la ti o n m a tr ix o f tr a c e e le m e n ts th a t h a v e si g n if ic a n t B iv a ri a te c o rr e la ti o n s in th e K -F e ld sp a rs (n = 1 1 ) C s G a L a N b P b P r R b S n T l W K /T l R b /T l K /R b K /C s C s 1 .0 0 0 G a -0 .6 6 7 1 .0 0 0 L a 0 .9 4 7 -0 .8 0 1 1 .0 0 0 N b -0 .5 6 6 0 .7 0 4 -0 .6 9 4 1 .0 0 0 P b 0 .7 3 8 -0 .8 5 2 0 .7 9 2 -0 .7 8 3 1 .0 0 0 P r 0 .9 2 3 -0 .8 1 2 0 .9 9 7 -0 .6 8 5 0 .7 7 7 1 .0 0 0 R b 0 .9 4 3 -0 .5 8 5 0 .9 0 3 -0 .4 5 4 0 .6 1 0 0 .8 8 8 1 .0 0 0 S n 0 .2 6 0 .1 4 7 0 .0 3 3 0 .2 9 2 -0 .2 1 6 0 .0 5 7 0 .2 3 6 1 .0 0 0 T l 0 .9 8 0 -0 .6 6 2 0 .9 4 0 -0 .5 6 4 0 .7 6 1 0 .9 2 1 0 .9 6 7 0 .1 1 4 1 .0 0 0 W -0 .1 3 3 -0 .0 3 3 -0 .2 1 9 0 .2 4 5 -0 .2 4 7 -0 .2 3 6 -0 .2 5 4 -0 .4 6 3 -0 .2 7 5 1 0 0 0 K /T l -0 .9 8 9 0 .6 3 6 -0 .9 3 4 0 .5 6 1 -0 .7 4 1 -0 .9 1 1 -0 .9 6 3 -0 .0 8 1 -0 .9 9 7 0 .2 3 8 1 .0 0 0 R b /T l -0 .6 5 0 0 .5 9 3 -0 .6 2 8 0 .6 3 7 -0 .8 6 7 -0 .6 0 2 -0 .4 3 2 0 .3 0 1 -0 .6 4 8 0 .2 1 5 0 .6 5 0 1 .0 0 0 K /R b -0 .9 3 8 0 .5 3 2 -0 .8 8 5 0 .4 2 8 -0 .5 6 1 -0 .8 6 8 -0 .9 9 6 -0 .2 4 3 -0 .9 5 4 0 .2 3 6 0 .9 5 5 0 .3 9 7 1 .0 0 0 K /C s -0 .9 9 9 0 .6 5 1 -0 .9 3 9 0 .5 6 3 -0 .7 2 3 -0 .9 1 3 -0 .9 3 3 -0 .0 0 3 -0 .9 7 1 0 .1 0 4 0 .9 8 3 0 .6 4 6 0 .9 3 1 1 .0 0 0 228 AKINTOLA, O. F. AND ADEKEYE, J. I. D. T a b le 7 . C o rr e la ti o n m a tr ix o f tr a c e e le m e n ts th a t h a v e si g n if ic a n t B iv a ri a te c o rr e la ti o n s in th e P e g m a ti te s’ M u sc o v it e s F C s G a L a N b R b S n T a T l Z n L i( x ) L i( fu s) R b (x ) R b (f u s ) K /R b N b /T a K /T l R b /T l M g /L i F 1 .0 0 0 C s -0 .1 1 6 1 .0 0 0 G a -0 .1 5 5 -0 .3 2 9 1 .0 0 0 L a -0 .0 0 2 0 .9 1 3 -0 .3 2 7 1 .0 0 0 N b 0 .2 1 7 -0 .8 7 5 0 .4 7 2 -0 .7 5 3 1 .0 0 0 R b 0 .1 8 0 0 .8 2 1 0 .0 1 2 0 .7 7 1 -0 .6 9 6 1 .0 0 0 S n -0 .0 3 6 0 .9 0 2 -0 .1 8 5 0 .7 9 9 -0 .7 0 0 0 .8 2 9 1 .0 0 0 T a -0 .5 0 7 0 .7 5 6 -0 .3 9 4 0 .6 7 5 -0 .7 9 6 0 .4 2 8 0 .5 9 5 1 .0 0 0 T l -0 .0 4 8 0 .9 2 2 -0 .1 5 6 0 .8 5 1 -0 .8 5 8 0 .9 3 9 0 .8 6 8 0 .6 8 2 1 .0 0 0 Z n 0 .7 5 9 0 .1 6 0 0 .3 4 8 0 .1 9 3 0 .0 7 5 0 .5 5 9 0 .2 8 1 -0 .3 7 5 0 .2 8 7 1 .0 0 0 L i( x ) 0 .6 6 6 0 .1 2 7 0 .0 4 4 0 .1 8 3 0 .0 5 6 0 .4 5 2 0 .2 3 2 -0 .2 4 4 0 .2 5 0 0 .8 1 4 1 .0 0 0 L i( fu s) 0 .6 3 0 0 .1 6 7 0 .0 0 5 0 .2 1 4 0 .0 0 5 0 .4 5 5 0 .2 7 5 -0 .1 9 4 0 .2 8 4 0 .7 7 3 0 .9 8 0 1 .0 0 0 R b (x ) 0 .2 7 9 0 .5 8 3 0 .0 7 2 0 .5 5 8 -0 .4 3 1 0 .8 5 9 0 .6 1 9 0 .1 8 2 0 .7 0 9 0 .6 0 8 0 .5 1 8 0 .4 4 5 1 .0 0 0 R b (f u s) 0 .1 1 6 0 .6 9 6 0 .1 2 9 0 .6 2 3 -0 .5 6 8 0 .9 1 9 0 .7 6 2 0 .3 3 3 0 .8 3 3 0 .5 1 8 0 .4 0 8 0 .4 3 1 0 .8 3 9 1 .0 0 0 K /R b -0 .1 6 4 -0 .8 3 0 0 .0 0 6 -0 .7 7 2 0 .7 0 8 -0 .9 9 8 -0 .8 4 0 -0 .4 4 1 -0 .9 4 3 -0 .5 3 4 -0 .4 4 1 -0 .4 4 6 -0 .8 5 8 -0 .9 2 1 1 .0 0 0 (8.0-17), K/Cs (66-406), K/Tl (1600-3764), and Nb/Ta (1.59-4.11) ratios in the micas (Table 9). Rb andCscontents andK/Rb,K/Csaswell asNb/Ta ra- tios in themicas show that there are at least two gen- erations of primary muscovites in the pegmatite, the early and the late. Table 8. Some trace elements distribution in the different muscovite generations in the Liberia Pegmatite Deposit Element Early generation Late generation Rb(%): Range 0.48-0.58 0.88-0.99 Mean 0.52 0.92 Cs(ppm):Range 171-300 874-2353 Mean 243 1168 Tl(ppm):Range 21-27 39-47 Mean 24.16 41.33 Ta(ppm):Range 41-71 53-103 Mean 52.33 69.16 Nb(ppm):Range 165-185 68-144 Mean 178 115 Sn(ppm):Range 217-357 471-681 Mean 283 575.67 K/Rb:Range 13.8-17 8.0-9.3 Mean 15.58 8.77 K/Cs:Range 242-406 66-81 Mean 301 74.6 K/Tl:Range 3007-3764 1600-2058 Mean 3375 1941 Nb/Ta:Range 2.61-4.11 1.59-2.47 Mean 3.54 2.02 Number of Samples 6 6 The Rb, Cs and Tl contents show enrichment in late muscovites by factors of 1.77, 4.81 and 1.71 re- spectively with a corresponding enrichment in the oreelementsSnandTabyfactorsof2.03and1.32re- spectively but a noticeable depletion of Nb by a fac- tor of 1.55 in the late muscovite. Thus, the enrich- ment of Ta and Sn in the late muscovites is accompanied by decrease in the K/Rb, K/Cs, K/Tl and Nb/Ta ratios. While there is also enrichment in Zn in the late muscovite, they are depleted in MgO and TiO2. Similar enrichments of the rare elements havebeenobserved in the lateprimarymuscovitesof well-studied pegmatites like Tanco, Noumas, Tip-top and Harding. Such late generation primary muscovitesare found in themiddleandcentral zones of the pegmatites where they occur in paragenesis with ores of the rare elements. InTable8,acomparisonismadeof theRb,Cs,Ta and Ga contents of muscovites of selected pegmatites in the study area, Wamba (100km north-east of the area), and some well studied pegmatites from other partsof theworld.ThemicashavehighRb(0.8978%) and Cs (2294ppm) which are comparable to those of the higly mineralized Tanco pegmatite of Manitoba, Canada as shown in Table 9. Pegmatites in the areahave awide rangeofmin- eralization potential from low Ta – pegmatites through medium Ta-pegmatites to high Ta-pegma- tite. From Table 9, it is evident that the Rb and Cs contents of the micas reflect Ta mineralization in pegmatites. Extremely low K/Rb (8.86 or less), and K/Cs (34 or less) characterize such highly mineral- ized pegmatites like Tanco and Liberia. The high Ta-mineralization potentials of the complex pegmatites in thisarea iscorroboratedbythehighav- erageTa2O5content (406ppm)andTa/Nbratio(3)of eighteen concentrate samples taken from different mines in the area (Okunlola, 1998). Muscovites of the mineralized pegmatites from Wambaareahave lowTa-potentialwith correspond- ingly low Cs (116ppm), Rb (0.3150%) and high K/Rb (25) and K/Cs (800) Nb/Ta (4.50) ratios. The Wamba pegmatites vary from barren, muscovite class to complex albitized, and Sn-mineralized pegmatites (Kuster, 1990). On the whole, it appears thepegmatities inNasarawa, thestudyarea, aremore fractionated and therefore have higher Ta-mineral- ization potential than those of Wamba and the other 229 MINERALIZATION POTENTIALS OFPEGMATITES IN THE NASARAWA AREA OF CENTRAL NIGERIA studied pegmatites in Nigeria (Matheis and Kuster, 2001, personal communication). Discussion and conclusion The very low Ta contents in the K-feldspars may be attributed to the fact thatTaO3 - cannot enter the fully polymerized (AlSi)O4 network of the K-feldspar (Moller and Morteani, 1987). Thus, the rare alkalis RbandCsarebetter indicatorsof theraremetalsmin- eralization potentials in the pegmatites’ K-feldspars than the ore elements Ta. The higher contents of the rare earth elements (REEs) Ce, La, Pr, and Y in the K-feldspar when compared with those of the other majorpegmatiteminerals shows that theREEs in the fluids from which the pegmatites crystallized were partitioned more into the K-feldspar than the other rock-forming minerals. This corroborates the obser- vations of Simmons and Heinrich (1980) that REE-bearing minerals occur in K-feldspar-rich parts ofpegmatites.ThehighnegativecorrelationofK/Rb versus Cs is characteristic of K-feldspars of rare metalpegmatitesandhasbeenusedalongwithK/Rb versusCs in themicasas reliableprospectionaids for rare metal pegmatites (Preinfalk et al., 2000). High positive correlations between Cs, Rb and the REEs: Ce, La, and Pr are notable in the K-feld- spars, compared with the muscovites. This may be due to selective complexing of the REEs in the re- sidual melt by the framework silicate feldspar. The same process of selective complexing by P and F must have enriched the fluorapatite with high con- centrations of the REEs. The low average Nb/Ta ra- tio for the muscovites (2.965) is still lower than the (4±0.7) upper limit diagnostic of Ta-pegmatites (Cerny, 1989). Higher Nb/Ta ratios (9.5±1.5) and high Nb-concentrations (³200ppm) are diagnostic of the less specialized Nb-rich columbite pegmatites. Beus (1966) determined that ³20ppm Ta concentrations are characteristic of columbo- tantalitepegmatites andGordiyenko (1971)alsode- termined that65-75ppmTaconcentrationsarechar- acteristic of the Ta-enriched pegmatite (Figure 5). SimultaneousenrichmentofRb,CsalongwithTa in the LCT granite pegmatite suites makes the alkalis reliable indicators of rare-metals mineralization in pegmatites. Thus K/Rb versus Cs, and Ta versus Cs plotsofprimarymuscoviteshavebeen reliablyused to determine the mineralization potentials of pegmatites. 230 AKINTOLA, O. F. AND ADEKEYE, J. I. D. Table 9. Geochemical characteristics of some well-studied pegmatites compared with Nasarawa Pegmatites K(%) Rb(%) Cs(ppm) Ta(ppm) Ga(ppm) K/Rb K/Cs No of Samples Liberia 8.0831 0.6972 556 57 163 12.6 198 11 Loc 20/20a 7.7997 0.8978 2294 464 160 8.86 30 2 K/Ka 8.2231 0.5234 605 100 159 15.75 119 2 W/W2 8.3145 0.3077 196 63 150 27.12 374 2 Tip Top, South Dakota (Low Ta) 8.52 0.355 222 56.0 175 24 384 13 Tanco, Manitoba (High Ta) 8.33 2.450 2420 240.9 433 3.4 34 19 Noumas, Namaqualand 8.21 0.357 566 74.2 92 23 145 2 Harding, New Mexico (Low Ta) 9.47 0.631 1917 64.0 123 15.0 49 21 Wamba, Central Nigeria n.d 0.3150 116 53 n.d 25 800 51 Matheis (1979) has earlier shown that mineral- ized pegmatites in Ijero and Egbe area of southwest- ernNigeriaarestronglydepleted inBa,Sr,andZrbut are enriched inRb,Li,Y,Be,Sn,Nb, andTa.Kuster (1990) has alsoobserved similargeochemical evolu- tion in latePanAfrican tectonicgranitesandmineral- ized pegmatites in Wamba area. The relative enrichment of Ba, Sr, W, Zr and Ta in the Na-feld- spars indicate that pegmatites inNasarawaarea crys- tallize from a more fluid and rare metals-enriched melt than those of Ijero and Egbe, as well as Wamba areas.These resultsagreewithobservationsofCerny et al. (1985) who noted that these elements are en- riched in late hydrothermal stages of pegmatite for- mation. High activity of P during the primary pegma- tite crystallization in this area resulted in the for- mation of the amblygonite subtype (Burt and London, 1982; London and Burt, 1982b; Cerny, 1991b) of the complex pegmatites. The exhaustion ofLi andFby thephosphates frompegmatiticmelt may account for the Li-, and F-poor micas with the highestTacontentswhichprobablycrystallizedaf- ter the crystallization of the amblygonites in the area. Tl, Rb, Cs and the REEs La, Ce, Pr fraction- ation in K-feldspars and white mica demonstrates congruent/similar trends. K/Rb, K/Tl and K/Cs ra- tios are lower in the white mica than the K-feld- spars. Ta and Cs have the highest positive correlation (0.756) in the micas, therefore low K/Rb (16),K/Tl (3096) andK/Cs (121) as found in the Nasarawa area indicate high rare metal Ta-Nb-Sn-Li-Be mineralization potentials as ob- served in the Nasarawa pegmatites. Acknowledgement FinancialAssistance for the fieldworkby theNige- rian Government through the Raw Materials Re- search and Development Council, RMRDC is gratefully acknowledged. The German Govern- ment through the German Academic Exchange Programme (DAAD) provided financial assistance for the rocks/minerals chemical analyses in the TechnicalUniversityofBerlin (TUB)which is also gratefully acknowledged. The authors are grateful to Dr. G. Matheis of theTechnicalUniversityofBerlin (TUB) forhis as- sistance on the research. 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