Micas of the muscovite^lepidolite series from Karibib ...

Mineralogical Magazine, February 2007, Vol. 71(1), pp. 41–62

Micas of the muscovite ^lepidolite series from Karibib pegmatites, Namibia E. RODA1,*, P. KELLER2, A. PESQUERA1

AND

F. FONTAN3

1

Departamento de Mineralogıá y Petrologıá, Universidad Paı´s Vasco/EHU, Apdo. 644, E-48080 Bilbao, Spain Institut fu¨r Mineralogie und Kristallchemie, Universitat Stuttgart, Pfaffenwaldring 55, D-70569 Stuttgart, Germany 3 Laboratoire de Cristallographie et Mine´ralogie, URA-067-Universite´ Paul Sabatier de Toulouse, Alleés Jules-Guesde 39, F-31400, Toulouse, France 2

[Received 11 July 2006; Accepted 4 April 2007]

ABSTR ACT

Micas of the muscovite lepidolite series are main constituents of the evolved pegmatites from the Okatjimukuju-Kaliombo portion of the Karibib belt, Namibia. The compositional variations shown by the micas from the intermediate zones are mainly controlled by the Li3Al 1& 2 and SiLi2Al 2& 1 substitution schemes, whereas for the micas from the core margins and the replacement bodies, only the first of these two exchange vectors seems to operate. The chemical composition of the micas not only depends on the degree of pegmatite evolution, but also on the position in the internal zonation of the pegmatite. Micas from the core margins and the replacement units are generally richer in F, Li, Rb, Cs and Zn than those from the intermediate zones. In general, the contents of these elements increase with decreasing K/Rb ratio. However, some data departing from this general trend are also observed, which could be related to subsolidus processes. Some pegmatite bodies show a complete internal evolution, developed from the margins to the core zone, which is reflected in the chemical composition of the micas. The regional distribution of pegmatites does not define a zonation, because an overlapping of pegmatites with different degrees of evolution occurs. This could be due to the high level of evolution attained by most of the rare-element pegmatites, and to their topography with respect to a dome structure of the basement.

K EY WORDS : micas, granitic pegmatites, mineral chemistry, Karibib, Namibia.

Introduction IN the Karibib pegmatite belt, west-central Namibia, outcrops of many rare-element pegmatites occur (Keller, 1991), and micas of the muscovite lepidolite series are common in these pegmatites that have been mined in the past for several purposes. The physical properties, chemical composition and paragenetic relationships of micas can be useful in deciphering the degree of evolution and crystallization conditions of the pegmatite bodies (for an overview see Cˇerny and Burt, 1984; Foord et al., 1995; Wise, 1995; Jolliff et al., 1987, 1992; Henderson et al., 1989; Roda et al., 1995, 2006; Kile and Foord,

* E-mail: [email protected] DOI: 10.1180/minmag.2007.071.1.41

# 2007 The Mineralogical Society

1998; Brigatti et al., 2000). Moreover, trace elements in micas may provide some insights into fractionation trends and into the economic potential of their host pegmatites. This paper is concerned with micas of the main pegmatitic bodies from the Okatjimukuju-Kaliombo portion of the Karibib belt, which are mainly emplaced within dolomitic marbles. We discuss the paragenetic relationships, petrography and chemical composition in relation to the pegmatite evolution and subsolidus processes, and propose a model of pegmatite distribution in this area. Regional geological setting The geology of west-central Namibia is dominated by metasedimentary and igneous rocks that form the Pan-African Damaran orogen (Fig. 1). In

E. RODA ET AL.

Steven, 1993). Some zones are subdivided by other lineaments. For example, the Central Zone comprises the northern and the southern portions, separated by the magnetically defined Omaruru lineament (Corner, 1983). This lineament is a tectono-stratigraphic boundary across which significant stratigraphic, tectonic and magmatic differences are developed.

this region, a large number of pegmatites occur, some of which exhibit economic rare-metal mineralization (Miller 1983a, 1992). The Damara orogen has been divided into several zones on the basis of the stratigraphy, structure, grade of metamorphism, types of granitoids, and geochronology. Faults or major lineaments are boundaries between the zones (Miller, 1983b;

FIG. 1. Pegmatite belts of the Central Damara Province, Namibia.

42

PEGMATITE MICAS, NAMIBIA

evolved, Li, Cs, Be, Sn, Nb and Ta-rich pegmatites, which have been mined extensively. For comparison, a plot of K/Rb vs. Cs for K-feldspar from the Okatjimukuju-Kaliombo portion and the Winnipeg River district, Canada (Goad and Cˇerny, 1981) is given in Fig. 3. Thus, the pegmatites from the Okatjimukuju-Kaliombo area show a very large span from the barren through mineralized Rush lake pegmatites, to the field of the Tanco pegmatite. Details of the mineralogy, internal structure, size and mineralization of the pegmatites studied from the Okatjimukuju-Kaliombo portion are given in Table 1.

The micas under discussion occur in pegmatites from the southern portion of the Central Zone. It is a high-temperature/low-pressure terrane within the orogen, containing sillimanite-cordierite metamorphic assemblages that are intruded by numerous granitoids. In this southern portion, deep stratigraphic levels are exposed, floored by a 1700 2000 Ma granite-gneiss basement (Abbabis Formation). The basement is overlain by quartzite of the Late Proterozoic Nosib Group and by metamorphic rocks of the Swakop Group, mainly dolomitic marble (Karibib Formation) and metapelite or metagreywacke (Kuiseb Formation). A pronounced dome structure is developed for the granite-gneiss basement, predominantly in the southwestern area of the Southern Central Zone (Tack and Bowden, 1999). A high-strain zone occurring along the margin between the granitegneiss domes and the Damara cover that is interpreted as a late extensional mid-crustal detachment by Oliver (1995) can also be recognized in the area under investigation. The numerous occurrences of pegmatites in the Damara orogen can be grouped in various characteristic belts (Fig. 1): (1) the Northern, Central and Southern tin belts, also known as the Cap-Cross-Uis, Nainais-Kohero and SandamapNoord-Erongo pegmatite belts, respectively; (2) the Karibib pegmatite belt; and (3) the Okahandja pegmatite belt (Frommurze et al., 1942; Keller, 1991; Diehl, 1993; Steven, 1993). The Karibib belt is subdivided into the Okatjimukuju-(Kaliombo), Okongava, Etusis and Abbabis portions (Keller, 1991). The Okatjimukuju-Kaliombo portion is situated at the northeastern crest-line depression of the Abbabis inlier, where the basement is exposed in only a few small granite-gneiss domes, e.g. the Okakoara dome and the Okatjimukuju anticline (Fig. 2) (Gevers, 1963). At the flank of these domes and at the depression between them, most of the mineralized pegmatites are emplaced within dolomitic marbles of the Karibib formation. The Kuiseb mica-schists and the Abbabis basement are the host rock of only a few poorly mineralized pegmatites (Fig. 2). The pegmatites described here are mostly dykelike, ranging from 6 m64 m (OKAT 3.5 pegmatitic dyke) to 350 m6150 m (Karlsbrunn pegmatite) (Roering and Gevers, 1964). Most of the pegmatitic bodies show a well developed internal zonation, with more or less prominent quartz cores. The degree of evolution varies from a few nearly barren pegmatites up to highly

Sampling and analytical methods The mica samples were selected from the most representative mineralized pegmatites of the Okatjimukuju-Kaliombo portion (Karibib belt). A selection of samples was prepared by hand picking, and later examined with a binocular microscope to remove contaminated grains, and finally, ground in an automatic agate pulverizer. Major element analyses were performed on polished thin sections using a Camebax SX-50 electron microprobe, in the Laboratoire de Mine´ralogie et Cristallographie of the Universite´ Paul Sabatier, Toulouse, France. In total, >600 electron microprobe analyses of micas were performed. Operating conditions were: voltage 15 kV and beam current 10 nA. For internal standards: SiO2 (Si), MnTiO3 (Ti,Mn), wollastonite (Ca), corundum (Al), hematite (Fe), albite (Na), orthoclase (K), fluorite (F), periclase (Mg), synthetic glass (Rb2O = 1.11%, and Cs2O = 1.89%), BaTiO3 (Ba), sphalerite (Zn), and tugtupite (Cl) were used. For a selection of mica separates, the trace elements, including Cs, Ga, Nb, Ta, Sn, Be, Sr, Sc, V, Cr, Co, Ni, Cu and others were analysed by inductively coupled plasma-mass spectrometry, whereas the Li content was determined using atomic absorption spectroscopy (AAS). These analyses were performed by X-ray Assay Labs of Don Mills, Ontario. The Li contents of the microprobe analyses were estimated from the F content of the micas as described below. Given that the trace-element determinations are mean values, and the microprobe data are point analyses, the Li data correlate well and those of Cs and Zn are comparable. Samples were also analysed by means of an X-ray diffractometer with Si as the internal standard, by scanning over 5 70º2y using Cu-Ka 43

E. RODA ET AL.

FIG. 2. Locations of the different pegmatites in the Karibib belt of the Damara Orogen.

polished thin sections to make some back-scattered images of representative micas using a SEM (JEOL-6360 LV). Finally, some K-feldspar samples from representative pegmatitic bodies were analysed at the Institut fu¨r Mineralogie und Kristallchemie (Universitat Stuttgart, Germany). Separates of these feldspar samples were purified under the microscope and ground to a very fine-grained powder in a mechanical agate mill. To perform

radiation. Unit-cell dimensions were obtained using the LSUCRE program (Appleman and Evans, 1973), and the mica polytypes were identified using the data published by Bailey (1980). Infrared (IR) absorption spectra were collected with a Nicolet-740 FTIR spectrometer using 0.1 mg of sample as KBr pellets (total weight = 50 mg) over the range 3900 400 cm 1. The scanning conditions were: 1 cm 1 nominal resolution and 0.2 cm/s scan speed. We also used 44


FIG. 3. K/Rb vs. Cs for K-feldspar of: (a) individual Okatjimukuju-Kaliombo pegmatites and, (b) the OkatjimukujuKaliombo and Okongava pegmatites (Karibib, Namibia) and Canadian pegmatites (Osis Lake, Rush Lake and Tanko; after Goad and Cˇerny, 1981).

mixed with 1.1 g of ‘Mikrowachs C’ (Fa. Breitla¨nder), homogenized mechanically and, finally, compacted by a manual hydraulic press into briquettes 36 mm in diameter. The samples were measured with a Philips PW1404 spectrometer. Data correction was performed using the program ‘Super Q’ Version 3.0A, using 37 international standards.

the major element analyses, a mixture of 0.6 g of feldspar powder with 3.6 g of Li-tetraborate (Merck) was homogenized mechanically. For the bead technique, an automatic gas burner (Fusion Machine, Type VAA, HD Elektronik & Elektrotechnik GmbH, Kleve), was used to prepare beads of 30 mm diameter. For the trace element analyses, 8.8 g of feldspar powder were 45

TABLE 1. Main characteristics of representative pegmatites of the Okatjimukuju-Kaliombo portion and their associated micas. Pegmatite (size)

Internal zoning

————————————— Mineralogy ————————————— Contact zone Intermediate Core Replacement Wall zone Zone margin units

(1) Karslbrunn (350/150)

very complex exposed below the quartz core

Qtz, Ksp, mica, be, trip, triph, col, cass 3065610

Ksp, msc,Qtz clv, be, col-tan tour, pet, amb, ab Mn-apt, Li-msc 300660

Qtz, msc, Li-msc, tan, be, pet 120630

lpd, ab, Qtz, (ab dome) amb, tan, Mn-apt tpz, tour 180670

(2) Rickburg 2 12c**612c

well zoned

not exposed 2612

msc+Qtz, Ksp 268

Qtz be, tan 368

Ksp, ab, lpd,

poorly zoned

not exposed

Qtz+msc, Ksp, be, triph, allu, Li-msc 2620

Qtz

(4) Steens OKAT 7 30c63c

well zoned

aplitic, coarse Qtz +msc + Ksp, apt

Ksp, pet, Qtz col-tan

Qtz Li-msc, lpd

pyro

(5) Viljoon OKAT 10 4063

well zoned

aplitic, msc, Qtz 0.5640

Qtz + msc, Ksp, Li-msc, triph, col 2640

Qtz (loc)

ab, pet msc, Li-msc

(6) Clementine I OKAT 5 140620

well zoned, mainly below the core

aplitic (schlieren) triph (dent) 16?

Qtz+Ksp+msc Ksp, msc, ab, Li-msc, Qtz, col 4 56?

Qtz, Mn-apt triph (nod) 0 6610c

msc, ab, lpd, Qtz (coarse). ab dome: ab, Li-msc, Qtz 66?

(7) Clementine II OKAT 2 120630

well zoned, above the core

aplitic (schlieren) Qtz,Ksp,msc 1 36?

outer: Ksp, Qtz, msc, Li-msc, pho inter: Qtz-msc, col, Li-msc, pho, Mn-apt inner: Ksp, msc, pho, Li-msc 136?

Qtz, msc, Li-msc, lpd 8c615c

msc, Li-msc, lpd, Qtz, ab, pet 1 26?

(8) Fricke OKAT 1 90c640c

well zoned below the core

aplitic 0.56?

Qtz+ msc + Ksp tour, Li-msc be,col, pho 6 156?

Qtz 8 10c 40c

clv, msc, pet, Li-msc, pho, flr 1 36?

(9) Petalite OKAT 4 30c612c

zoned

not exp.

Ksp in Qtz+msc+ fsp, pho 3c6?

Qtz 5c6?

Ksp, pet, amb, clv, col 1c6?

(10) OKAT 3.3 48c68x

zoned

aplitic

Qtz+msc+fsp, Ksp

Qtz Li-msc, lpd

Li-msc

(11) OKAT 3.5 4c66c

?

not exp

fsp+Qtz+msc Ksp, be

not exp

not exp

(12) OKAT 13.a 8c612c

zoned

Qtz+msc+fsp 1 26?

Ksp, msc, Qtz, lpd, dent pho

not exp

pet, Li-msc amb

well zoned

aplitic fsp+Qtz+msc, Ksp 0.36?

Ksp, msc, Qtz Li-msc. ab, amb?, lpd, be, dent pho 1.56?

Qtz 16?

amb+tpz, ab, tan, Li-msc, pyro, ldp, wod, pet, tour

(3) OKAT 6 20?64c

(13) OKAT 9 20c64c

TABLE 1. (contd.)

Phosphates Fe/(Fe+Mn) 0.49 0.60

——— Ksp ——— K/Cs K/Rb

Texture

Mica K/Rb

Grain size*

Economic minerals

780 2370(5)

11 21(5)

msc as greyish bk, in a gr.int with Qtz, fb-t laths, or cv.sc to rnf Li-mica as pinkish finegrained scl or pale pinkish pnc

5 444

fine to medium

pet, amb, be, tan, Li-mica, cass qtz

420 2670(8)

24 31(8)

pinkish to white books

24

medium

be, ta

2100(2)

16 29(7)

dark grey books

7 16.5

fine to medium

be

100 270(6)

6 16(8)

msc as slightly curved pearly plates Li-mica as dark grey or pinkish bk, dark-purple mss with Qtz, or thin pnc

2.5 3.0

fine to medium

Li-mica, pet, pyro be, col-tan

0.58

200 970(4)

14 25(6)

greyish cv.scl; grey, strongly curved laths, and brownish bk of slightly deformed sheets

7 13

very fine to coarse

col, pet

dent 0.56 nod 0.35

330 1180 (3)

11 23 (5)

msc as pale greyish bk of fb t sheets, bk of colourless even plates, or very fine grained dense mss Li-mica as pinkish scl or dense mss

3 8

msc fine to coarse

col, be- Li-mica Qtz (pure) Li-mica very

dent 0.61 0.78 nod 0.49 0.76

150 5650 (29)

15 57 (50)

msc as greyish bk of fb-t, even plates, A-t.sh, or slightly curved Li-mica as deep-purple, pinkish or grey mss, pale pinkish pnc, cv.scl, or radiating intergrowth to rnf

5.5 14

msc fine to medium Li-mica very fine to medium

be, col, amb Li-mica

dent 0.64 0.66 nod 0.31

160 760 (15)

7 63 (15)

msc as pale greyish even bk, lath-like, gr.int with Qtz, or pale greyish A-t.sh Li-mica as dark grey, dense mss

12 35

msc fine to medium Li-mica fine

be, Li-mica, col-tan amb, Qtz, msc

dent 0.69 nod 0.65

260 900 (6)

8 33 (11)

slightly greyish scl and bks

fine

pet, amb

0.54 0.55

220 1560 (12)

8 47 (12)

msc as pale greyish, lath-like bk of fb-t sheets, gr.int with Qtz, Li-mica as pinkish mss of cv.scl

5.5 3.5

msc very fine to fine

0.52

1530 (1)

28 29 (2)

pale greenish bk of slightly deformed plates

19

medium to coarse

be

nod 0.65

80 (2)

7 (2)

pale greyish bk of A-t.sh

6 7

medium

pet, Li-mica amb

25 33

msc as brownish to dark brownish cv.scl, with rnf, or porous mss of pearly-white scl Li-mica as pinkish scl

3.5 229

msc very fine to fine Li-mica very fine

tan, Li-mica, amb

0.59

(2)

fine

Li-mica fine

In ‘Mineralogy’, the following abbreviations have been used: Qtz quartz; Ksp K-feldspar; msc muscovite; ab albite; clv cleavelandite; tour tourmaline; triph triphylite; pet petalite; pho phosphates; amb amblygonite; cass cassiterite; apt apatite; be beryl, tpz topaz; lpd lepidolite; pyro pyroclore; flr fluorite; col columbite; tan tantalite; trip triplite. In ‘Mica texture’, the following abbreviations have been used: book texture = bk; graphic intergrowth = gr.int; fishbone type = fb t; reniform texture = rnf; pencils = pnc; masses = mss; A type sheets = A.t sh; scales = scl; and, curved scales = cv.scl. * Grain size: very fine = 10 cm. ** c = estimated size. Sizes of pegmatites and units are given in metres as lengths6widths.

E. RODA ET AL.

(4) Curved scales of dark-greyish muscovite, (a) giving a reniform to glaskopf-like appearance, are typical for the outer part of the intermediate zone and for the contact zone (e.g. at Karlsbrunn, Viljoon and OKAT 9) (Table 1, Fig. 4d). (b) Fanshaped muscovite appears together with albite at the core margin of the Karlsbrunn and Viljoon pegmatites. (5) Very fine-grained scales of muscovite, typical of the aplitic contact and wall zones. (6) (a) Graphic intergrowth of quartz and muscovite, where the mica crystals exhibit small sizes (8 cm in diameter occurs widely (Fig. 4a), and it appears in all pegmatites under investigation, preferentially in the intermediate zone (Table 1). (2) ‘Fish-bone’ texture, with grain sizes generally smaller than those of the ‘book’ muscovite is less common and mainly observed in the intermediate zone, e.g. at Karlsbrunn, Clementine I and Clementine II (Table 1, Fig. 4b). (3) ‘A’-type shapes, where two groups of mica sheets wedge with a ~60º angle. This texture can be observed in the intermediate zone of, e.g. Clementine II, Fricke and OKAT 13 pegmatites (Table 1, Fig. 4c).

FIG. 4. Schematic representation of some of the textural characteristics of the micas studied.

48


result of subsolidus disequilibrium processes. Another disequilibrium texture is observed in the core margin of Karlsbrunn pegmatite, where micas frequently show symplectitic reaction borders between Li-mica and quartz, in addition to a well developed patchy zoning (Fig. 5d). Heterogeneity in micas is not only a result of replacement events, but it can also reflect a fractionation during crystallization. Such is the case of some darker rims that appear in book muscovites from OKAT 13, which are much richer in F than the white muscovite of the core (Fig. 5e). In general, all these textural heterogeneities are also reflected in the chemical composition of micas (see the geochemistry section).

core margin of Rickburg 2 and Steens pegmatites (Table 1, Fig. 4a). (2) ‘Pencil’ texture, where small hexagonal to rounded scales of Li-mica (0.5 to 1 cm in diameter) stacks, giving rise to ‘pencil’ forms up to 8 cm long, as well as ‘glaskopf’, often intergrown with albite (Table 1, Fig. 4f). This texture is common in the core margin of Karlsbrunn, Steens and Clementine II pegmatites. (3) Curved scales to reniform Li-mica is typical, e.g. of Karlsbrunn, Clementine II and OKAT 3.3 (Table 1, Fig. 4d). (4) Pinkish, fine-grained scales of Li-mica appear, e.g. in Karlsbrunn, Clementine II, and OKAT 9 pegmatites. (5) Massive, fine-grained Li-mica with different colours, partly intergrown with small amounts of quartz: (a) white-pinkish to dark purple, e.g. at Karlsbrunn, Steens, Clementine I, Clementine II, OKAT 3.3 and Fricke, as well as (b) grey crystals, as at Clementine I, Clementine II and Fricke pegmatites. (6) Fine-scaled Li-mica, intergrown with varying amounts of albite, typical of the layered bodies of evolved pegmatites, e.g. Karlsbrunn and Clementine I. Under the microscope, as well as in backscattered electron (BSE) images, micas show different textural characteristics. In general, book micas are the most homogeneous, mainly if they appear in the intermediate zones of the pegmatites. However, the micas are sometimes heterogeneous, with different replacement and disequilibrium textures. In many cases these replacement events could be favoured by deformation that affected micas after crystallization, as is supported by other textural evidence, such as the development of subgrains in plagioclases, the undulose extinction in quartz, and the kinked cleavages in the micas themselves. The most commonly observed replacement texture is a more or less well developed patchy zoning (Fig. 5a), though this zoning is much more common in the replacement units, it is also frequently observed in the intermediate units and core margins of some pegmatites (e.g. Karlsbrunn, OKAT 6, Clementine I, Clementine II and OKAT 9). In the intermediate units of Clementine II, a herring-bone texture is also observed (Fig. 5b), also typical of subsolidus replacement events. Frequently, in addition to the patchy zoning, other replacement textures are observed, such as the cellular texture observed in the replacement units of Clementine I (Fig. 5c), a

Results and discussion Structural characteristics The polytypes and structural data for some representative micas from the studied pegmatites are reported in Table 2. 2M1 is the sole polytype found in all the studied muscovites, as well as in some intermediate micas. Most of the Li-rich micas show mixed 1M and 2M1 polytypes in the same grain. This can be explained by the substitution of VILi by VIAl in the octahedral sites, which gives intermediate characteristics (mixed forms) in these Li-micas (Foster, 1960). The unit-cell dimensions of muscovite (Table 2) are very uniform, and do not show significant variations with the changes of their chemistry. The FTIR spectra of micas from the intermediate zone, and from the replacement units of the Clementine II pegmatite are illustrated in Fig. 6. The OH-stretching bands (~3629 cm 1) become less intense with the increase in Li and F contents (Robert et al., 1989, 1993). F enters preferentially the trioctahedral Li-bearing sites when it replaces OH, whereas hydroxyls are adjacent to the vacant octahedral sites. Thus, the OH-stretching bands of Li-bearing micas result from dioctahedral hydroxyls, and correspond to the same OH-stretch wavenumber as muscovite (~3629 cm 1 ) (Robert et al., 1989, 1993) (Fig. 6a d). In the case of trioctahedral Limica, no signal is observed in the OH-stretching region due to the high F content (Fig. 6e). Additional vibration bands occur in the range 1200 300 cm 1 (Robert et al., 1993). A band with a high wavenumber occurs in the antisymmetric wavenumber range (1132 cm 1) for the Li- and F-richest micas (Fig. 6d,e). This is the 49

E. RODA ET AL.

FIG. 5. BSE images of: (a) book mica from the core margin of the Karlsbrunn pegmatite, showing patchy-zoning; (b) herring-bone texture in a mica with a highly heterogeneous composition from the intermediate zone of the Clementine II pegmatite; (c) cellular texture in lepidolite from the replacement units of the Clementine I pegmatite; (d) symplectitic reaction borders between Li-mica and quartz from the core margin of Karlsbrunn pegmatite; and (e) book muscovite rimmed by Li-mica from the intermediate zone of the OKAT 13 pegmatite.

Geochemistry Major and trace elements of representative muscovite and Li-micas from the Okatjimukuju-

result of the deviation of the mica compositions toward a tetrasilicic trioctahedral mica (Robert et al. 1993). 50


Kaliombo pegmatites are reported in Table 2. A strong positive correlation has been found

between F data obtained by electron microprobe, and those of Li obtained by the AAS techniques for nine homogeneous micas, according to the equation Li2O = 0.7200F-0.6120 (R2 = 0.965, n = 9) (Fig. 7). Moreover, for the analytical data of Li-bearing micas from pegmatites of the Karibib belt given by Von Knorring (1985), a similar correlation between the F and Li contents is observed. Based on these correlations, the Li content of the studied micas has been estimated from their F content. Such a correlation is consistent with the empirical relationships proposed by Henderson et al. (1989), Tindle and Webb (1990), Tischendorf et al. (1997), Pesquera et al. (1999) and Roda et al. (2006). It has also been demonstrated experimentally in trioctahedral and partly dioctahedral Li micas by Monier and Robert (1986). The analysed F contents and the calculated Li contents, vary widely from nearly zero up to 10.7 and 7.0 wt.%, respectively and, as expected, are greater in the micas from the core margin and the replacement units, than in those from the intermediate zones (Table 2). Considering only the intermediate zones, micas from the Clementine II and Viljoon pegmatites have the greatest F contents with the following values of 1.9 9.5 (6.9) and 4.8 6.9 (5.9) wt.%, respectively, with the mean values given in brackets. Intermediate F contents have been analysed from OKAT 13, OKAT 9, Karlsbrunn and Clementine I with 1.0 10.3 (5.1), 2.3 6.7 (4.9), 2.5 6.4 (4.6) and 1.2 6.3 (4.3) wt.%, respectively. The lowest values occur at OKAT 6, Fricke, OKAT 3.3 and

FIG. 6. FTIR spectra of micas from the Clementine II pegmatite with variable Li and F contents.

FIG. 7. Plot of microprobe data of F vs. AAS data of Li2O (both in wt.%) showing the regression line.

51

45.39 0.03 27.69 0.00 4.81 0.00 1.12 0.00 3.69 0.51 9.63 0.00 0.00 0.00 5.97 2.52 1.49

45.21 0.13 32.70 2.51 0.24 0.40 0.18 0.01 1.07 0.46 10.23 0.00 0.34 0.08 2.33 0.98 3.28

98.18

SiO2 TiO2 Al2O3 FeO MnO MgO ZnO CaO Li2O* Na2O K2O BaO Rb2O Cs2O F O=F H2O*

Total*

1.009 2.991

2.619 1.381

2.754 1.246

3.706 0.294

4.412 0.000

99.36

51.71 0.00 21.74 0.00 0.13 0.00 1.87 0.00 6.96 0.27 10.19 0.00 0.40 0.00 10.51 4.43 0.00

Ricksburg core margin

F OH

98.55

49.07 0.00 25.94 0.00 0.42 0.00 0.00 0.00 5.64 0.35 9.95 0.00 1.80 0.03 8.68 3.66 0.33

Karlsbrunn replacement

6.862 1.138 8.000 2.262 0.000 0.000 0.014 0.000 3.715 0.184 6.175 0.000 0.070 1.724 0.034 0.000 1.828

98.05

46.22 0.01 28.97 0.77 1.13 0.00 0.26 0.03 3.97 0.46 9.84 0.00 1.36 0.00 6.36 2.68 1.36

Karlsbrunn core margin

Structural formula on the basis of 24 (O, OH, F) atoms Si 6.189 6.291 6.331 6.624 IV Al 1.811 1.709 1.669 1.376 (Z) 8.000 8.000 8.000 8.000 VI Al 3.464 2.813 3.007 2.752 Ti 0.013 0.003 0.001 0.000 0.288 0.000 0.089 0.000 Fe++ Mn 0.028 0.565 0.131 0.048 Mg 0.082 0.000 0.000 0.000 Li 0.588 2.058 2.185 3.062 Zn 0.018 0.114 0.026 0.000 (Y) 4.481 5.554 5.439 5.862 Ca 0.001 0.000 0.004 0.000 Na 0.122 0.137 0.122 0.091 K 1.787 1.703 1.719 1.714 Rb 0.030 0.000 0.120 0.156 Cs 0.004 0.000 0.000 0.002 (X) 1.944 1.840 1.965 1.962

97.82

Karlsbrunn intermediate

Karlsbrunn intermediate

0.592 3.408

6.085 1.915 8.000 3.679 0.016 0.012 0.187 0.059 0.207 0.076 4.235 0.000 0.215 1.723 0.003 0.000 1.941

97.87

44.55 0.15 34.75 0.11 1.62 0.29 0.75 0.00 0.38 0.81 9.89 0.00 0.04 0.00 1.37 0.58 3.74

OKAT 6 intermediate

2.325 1.675

6.405 1.595 8.000 2.869 0.000 0.540 0.052 0.000 1.788 0.028 5.276 0.000 0.068 1.770 0.123 0.000 1.961

98.30

46.05 0.00 27.24 4.64 0.44 0.00 0.27 0.00 3.20 0.25 9.98 0.00 1.38 0.00 5.29 2.23 1.81

OKAT 6 intermediate

2.282 1.718

6.509 1.491 8.000 3.223 0.000 0.000 0.004 0.000 1.756 0.000 4.983 0.000 0.079 1.674 0.269 0.036 2.059

99.20

47.89 0.00 29.43 0.00 0.03 0.00 0.00 0.00 3.21 0.30 9.66 0.00 3.08 0.61 5.31 2.24 1.90

Steens core margin

2.760 1.240

6.442 1.558 8.000 3.053 0.000 0.017 0.017 0.000 2.194 0.003 5.285 0.000 0.058 1.716 0.311 0.044 2.129

100.19

47.46 0.00 28.83 0.15 0.15 0.00 0.03 0.00 4.02 0.22 9.91 0.00 3.57 0.76 6.43 2.71 1.37

Steens core margin

4.305 0.000

6.597 1.403 8.000 2.392 0.000 0.050 0.076 0.000 3.609 0.000 6.127 0.000 0.068 1.659 0.309 0.025 2.061

99.75

48.65 0.00 23.75 0.44 0.66 0.00 0.00 0.00 6.62 0.26 9.59 0.00 3.54 0.43 10.04 4.23 0.00

Steens core margin

2.652 1.348

6.456 1.544 8.000 2.672 0.000 0.625 0.063 0.000 2.085 0.008 5.453 0.000 0.090 1.841 0.112 0.009 2.052

98.42

46.18 0.00 25.60 5.35 0.53 0.00 0.08 0.00 3.71 0.33 10.32 0.00 1.25 0.15 6.00 2.53 1.45

Viljoon intermediate

2.772 1.228

6.450 1.550 8.000 2.698 0.000 0.566 0.055 0.000 2.200 0.000 5.519 0.004 0.035 1.904 0.067 0.000 2.009

98.94

46.74 0.00 26.13 4.91 0.47 0.00 0.00 0.02 3.96 0.13 10.81 0.00 0.75 0.00 6.35 2.67 1.33

Viljoon replacement

1.278 2.722

6.077 1.923 8.000 3.460 0.000 0.297 0.022 0.006 0.838 0.006 4.629 0.000 0.176 1.762 0.100 0.005 2.043

99.72

44.80 0.00 33.67 2.62 0.19 0.03 0.06 0.00 1.54 0.67 10.18 0.00 1.15 0.08 2.98 1.25 3.01

2.764 1.236

6.245 1.755 8.000 2.818 0.000 0.464 0.076 0.000 2.190 0.023 5.570 0.000 0.038 1.793 0.150 0.007 1.987

98.78

44.99 0.00 27.96 4.00 0.65 0.00 0.22 0.00 3.92 0.14 10.13 0.00 1.68 0.11 6.30 2.65 1.34

Clement. I Clement. I interintermediate mediate

3.784 0.216

6.694 1.306 8.000 2.328 0.000 0.447 0.045 0.052 3.122 0.029 6.022 0.001 0.016 1.699 0.295 0.042 2.054

99.00

47.88 0.00 22.06 3.82 0.38 0.25 0.28 0.01 5.55 0.06 9.53 0.04 3.28 0.71 8.56 3.60 0.23

Clement. I core margin

TABLE 2. Chemical composition (major, minor and trace elements); and unit-cell parameters of representative micas from the pegmatites studied.

14

11

51

44.91 0.09 32.76 0.07 3.83 0.43 0.34 0.00 0.53 0.63 9.84 0.00 0.08 0.00 1.58 0.66 3.61

98.05

SiO2 TiO2 Al2O3 FeO MnO MgO ZnO CaO Li2O* Na2O K2O BaO Rb2O Cs2O F O=F H2O*

Total*

100.20

45.71 0.01 27.73 4.06 0.80 0.00 0.00 0.00 4.22 0.46 10.18 0.00 1.45 0.50 6.71 2.83 1.19

100.45

47.03 0.00 20.19 7.85 0.47 0.38 0.18 0.00 6.71 0.05 10.49 0.00 1.13 0.08 10.17 4.28 0.00 97.30

44.64 0.00 31.99 2.10 0.42 0.00 0.18 0.00 1.69 0.29 10.35 0.00 0.93 0.05 3.19 1.34 2.81 99.40

48.02 0.02 26.82 1.43 0.57 0.00 0.00 0.00 4.64 0.28 10.30 0.00 2.14 0.00 7.29 3.07 0.96

Clement. II Clement. II Clement. II Clement. II Clement. II interinterintercore core mediate mediate mediate margin margin

* Calculated

5.221(3) 9.061(7) 20.050(7) 95.85(6) 943.6(8) 2M1

a b c b v Polytype

95566

21 4 259 265 38 3 211 27 2

59

Be Ni Ga Nb Ta Zr Sn Tl Pb

K/Rb

98.55

45.20 0.06 34.28 2.74 0.00 0.00 0.00 0.04 0.37 0.53 10.07 0.00 0.64 0.08 1.36 0.57 3.75

Fricke intermediate

553

98.35

47.28 0.00 25.37 0.04 3.52 0.16 1.37 0.00 4.40 0.16 9.99 0.00 0.88 0.10 6.96 2.93 1.04

Fricke core margin

14

6

98.08

45.02 0.06 33.34 2.85 0.00 0.48 0.06 0.02 0.69 0.68 10.06 0.00 0.20 0.06 1.80 0.76 3.53

Okat 3.3 intermediate

15 10 223 180 312 0 393 149 4

6

5 18 4 138 133 161 2 284 96 4

99.58

48.22 0.01 27.46 0.00 0.31 0.00 0.25 0.00 4.78 0.15 10.52 0.00 2.31 0.34 7.49 3.15 0.89

Okat 3.3 core margin

99.56

45.32 0.00 34.45 3.45 0.03 0.14 0.00 0.00 0.00 0.38 10.71 0.02 0.55 0.04 0.70 0.29 4.09


2M1 + 1M 2M1 + 1M

15 2 276 168 264 1 414 138 5

16

98.49

44.41 0.03 33.84 2.10 0.17 0.09 0.21 0.01 1.09 0.53 9.69 0.06 1.44 0.26 2.36 0.99 3.25

Okat 13 intermediate

16 2 224 235 98 2 449 36 3

29

100.09

52.81 0.01 19.41 2.41 0.28 0.04 0.00 0.00 6.81 0.30 10.22 0.00 1.57 0.26 10.31 4.34 0.00


18 2 163 279 195 22 343 43 5

18

12 26 2 214 227 53 1 84 42 3

97.92

44.05 0.08 34.02 0.00 1.84 0.00 0.87 0.00 1.01 0.65 10.68 0.00 0.05 0.06 2.25 0.95 3.30

Okat 9 intermediate

98.32

44.12 0.00 33.48 0.00 3.12 0.00 0.69 0.00 1.42 0.47 10.28 0.00 0.07 0.00 2.81 1.19 3.04

Okat 9 intermediate

6

99.56

45.27 0.00 35.74 0.16 0.04 0.00 0.00 0.03 1.20 0.29 10.61 0.02 1.21 0.28 2.52 1.06 3.27

100.03

50.11 0.00 24.46 0.25 0.70 0.00 0.00 0.01 6.32 0.36 9.68 0.00 2.07 0.49 9.62 4.05 0.00

Okat 9 replacement

5.194(3) 9.0439(5) 20.055(5) 95.82(5) 937.2(6) 2M1

9 4 148 211 3030 1 15 107 2

Okat 9 replacement

5.186(3) 5.207(3) 9.018(3) 9.039(9) 20.005(7) 20.050(7) 95.70(6) 95.67(6) 931.0(7) 939.1(9) 2M1 2M1

23 3 225 237 42 1 80 33 2

2M1+1M

a b c b V Polytype

14

19 3 183 217 86 2 105 56 7

234

Be Ni Ga Nb Ta Zr Sn Tl Pb

K/Rb

5.209(3) 9.046(7) 20.040(8) 95.84(6) 939.5(8) 2M1

31 1 227 401 66 3 197 22 3

18

4.433 0.433

5.203(3) 9.051(8) 20.055(6) 95.71(5) 939.8(8) 2M1

28 3 196 211 70 1 91 43 4

22

1.400 2.600

5.174(4) 9.022(9) 20.19(8) 95.88(7) 929.7(9) 2M1

10

3.130 0.870

2.909 1.091

F OH

0.686 3.314

6.521 1.479 8.000 2.813 0.002 0.162 0.066 0.000 2.533 0.000 5.576 0.000 0.074 1.784 0.187 0.000 2.045

IV

Structural formula on the basis of 24 (O, OH, F) atoms Si 6.173 6.265 6.483 6.189 Al 1.827 1.735 1.517 1.811 (Z) 8.000 8.000 8.000 8.000 VI Al 3.480 2.743 1.763 3.417 Ti 0.010 0.001 0.000 0.000 0.008 0.465 0.905 0.243 Fe++ Mn 0.446 0.093 0.055 0.050 Mg 0.088 0.000 0.078 0.000 Li 0.291 2.327 3.721 0.942 Zn 0.035 0.000 0.018 0.018 (Y) 4.358 5.629 6.540 4.670 Ca 0.000 0.000 0.000 0.000 Na 0.169 0.123 0.013 0.078 K 1.726 1.779 1.844 1.831 Rb 0.007 0.128 0.100 0.083 Cs 0.000 0.029 0.005 0.003 (X) 1.903 2.059 1.963 1.995

Clement. II Clement. II Clement. II Clement. II Clement. II interinterintercore core mediate mediate mediate margin margin

TABLE 2 (contd.).

31

0.586 3.414

6.164 1.836 8.000 3.673 0.006 0.312 0.000 0.000 0.202 0.000 4.193 0.006 0.141 1.752 0.056 0.004 1.960

Fricke intermediate

23

3.039 0.961

6.528 1.472 8.000 2.654 0.000 0.004 0.412 0.033 2.444 0.139 5.687 0.000 0.044 1.760 0.078 0.006 1.888

Fricke core margin

20 4 140 163 270 1 218 85 4

9

3.202 0.798

6.518 1.482 8.000 2.893 0.001 0.000 0.036 0.000 2.600 0.025 5.556 0.001 0.040 1.813 0.201 0.019 2.074

Okat 3.3 core margin

22 3 189 446 42 6 186 13 2

39

0.301 3.699

6.153 1.847 8.000 3.665 0.000 0.392 0.003 0.028 0.000 0.000 4.088 0.000 0.100 1.855 0.048 0.002 2.005


5.197(4) 5.195(3) 9.042(9) 9.030(7) 20.000(7) 20.019(7) 95.78(6) 95.84(6) 935.0(9) 934.4(7) 2M1 2M1+1M 2M1

16 2 186 215 45 1 481 34 2

102

0.781 3.219

6.161 1.839 8.000 3.537 0.006 0.326 0.000 0.098 0.379 0.006 4.354 0.002 0.181 1.756 0.017 0.003 1.960


13

1.024 2.976

6.095 1.905 8.000 3.569 0.003 0.241 0.020 0.018 0.601 0.021 4.474 0.001 0.141 1.696 0.127 0.015 1.981


13

4.356 0.356

7.056 0.944 8.000 2.112 0.001 0.269 0.032 0.008 3.661 0.000 6.083 0.000 0.078 1.742 0.135 0.015 1.969


389

0.976 3.024

6.055 1.945 8.000 3.565 0.008 0.000 0.214 0.000 0.557 0.088 4.432 0.000 0.174 1.873 0.005 0.004 2.056

Okat 9 intermediate

300

1.220 2.780

6.049 1.951 8.000 3.459 0.000 0.000 0.362 0.000 0.781 0.070 4.672 0.000 0.124 1.798 0.006 0.000 1.928

Okat 9 intermediate

9

4.069 0.069

6.700 1.300 8.000 2.555 0.000 0.028 0.079 0.000 3.398 0.000 6.061 0.001 0.092 1.652 0.178 0.028 1.951

Okat 9 replacement

5.194(4) 9.048(8) 19.921(9) 95.73(8) 931.5(8) 2M1 + 1M 2M1

25 3 154 130 207 1 341 86 1

17

1.070 2.930

6.079 1.921 8.000 3.735 0.000 0.018 0.005 0.000 0.650 0.000 4.407 0.004 0.075 1.817 0.104 0.016 2.018

Okat 9 replacement


Clementine I, OKAT 9 and Viljoon pegmatites have the most heterogeneous F contents within a single thin section of 0.5 9.0, 1.0 9.0 and 0.1 6.4 wt.% of F, respectively. If all analysed samples from the core margin and the replacement units are considered, those from Riksburg 2 and OKAT 9 are the richest in F with contents of 9.1 10.5 and 7.1 9.6 wt.%, respectively. High to intermediate F contents are shown by micas from Steens, Karlsbrunn, Clementine II, OKAT 3.3, Clementine I and Fricke: 4.8 10.7 (8.3), 5.6 9.4 (7.4), 3.3 8.7 (7.1), 5.9 8.1 (7.0), 4.0 7.3 (5.9) and 2.1 6.7 (4.8) wt.%, while the smallest values are those of some mica samples from Viljoon 0.1 6.4 (2.5) wt.%. This chemical heterogeneity exhibited by many of the micas seems to be mainly related to subsolidus replacement processes, favoured by deformation events, as discussed above in the ‘mica occurrence and petrography’ section. In

OKAT 3.5: 0.5 6.4 (2.7), 1.4 2.4 (1.8), 1,5 2.4 (1.7) and 0.5 1.2 (0.7) wt.% of F, respectively. In general, a large variation in the chemical composition of micas, with respect to several major and trace elements has been observed (Figs 8 and 9). In addition, the chemical composition also changes over a wide range, within a single thin section, and even within single grains in some instances. The micas from the intermediate zones usually exhibit homogeneous F contents within a thin section, especially the ‘book’ muscovites with small amounts of F. Exceptions are samples from OKAT 13, Clementine II, OKAT 9 and OKAT 6 with wide ranges of F contents e.g. 1.0 10.3, 1.7 10.2, 2.3 7.9 and 0.5 6.4 wt.%, respectively. Compared with the intermediate zone, the Li-micas from the core margin and the replacement units generally display a broader variation in the F contents. Li-micas from the

FIG. 8. (a) Mg-Li vs. Fe+Mn+Ti-VIAl, and (b) Li vs. VIAl + VI& for the micas from the intermediate zone, the core margin and the replacement units. The arrows labelled with the exchange vectors represent the direction of the vector that reflects the evolutionary trend of the micas. (All data in a.p.f.u.)

55

E. RODA ET AL.

FIG. 9. (a) Plot of Li vs. K/Rb and (b) plot of Cs vs. K/Rb for some of the micas from the intermediate zone.

some intermediate units from pegmatites, ‘book’ muscovites sometimes have F-rich rims, as is the case for OKAT 13 (Fig. 5e), which are interpreted to be a result of fractionation during pegmatitic crystallization. The minor elements Rb, Cs, Fe, Mn and Zn also cover a wide range (Table 2), from virtually zero up to 4.33 (3.76) Rb2O wt.% (Steens, Clementine II and I), 1.38 (0.82) wt.% Cs2O, (Steens, OKAT 13, Clementine I and OKAT 9), 8.76 (5.50) wt.% FeO (Clementine II, Viljoon, OKAT 13, OKAT 6, and Clementine I), 4.39 (3.90) wt.% MnO (Karlsbrunn, OKAT 9, Clementine II, and Fricke), as well as 1.99 (1.88) ZnO wt.% (Riksburg, Clementine I, OKAT 9 and Karlsbrunn), where the mean value within a sample is given in brackets and the sequence of pegmatites is given from the largest to the smallest contents. An influence from the unusual country rock of these pegmatitic bodies could be expected, as dolomitic marbles are usually quite reactive. However, no contamination has been observed in the studied pegmatites, and the Mg or Ca contents are similar to those observed in other pegmatites with different host rocks.

Al0.5 1.5)O10(OH,F)2), but mostly with trilithionite (K(Li1.5Al1.5) (Si3Al)O10 (F,OH)2)–polylithionite (K(Li2Al)Si4O10(F,OH)2) components dominant (Fig. 8a). The majority of micas, however, are muscovite, slightly ferroan or manganoan. Intermediate Li-bearing micas are also present, in the sequence Karlsbrunn, Clementine I and II, OKAT 6 and Viljoon with increasing (Fe+Mn+Ti-AlY). Although assigned to the intermediate zone, some micas from OKAT 13 and Clementine II also appear in the lepidolite zinnwaldite field of composition. The micas from the core margin follow a well defined trend line (R2 = 0.87) from the muscovite to the polylithionite field of composition, along the polylithionite trilithionite lithian muscovite line (Fig. 8a). Most of them are lithian mica, but lepidolite also occurs frequently, e.g. at Riksburg, Steens, OKAT 9 and Karlsbrunn. A different behaviour is observed for the micas of the replacement units. They scatter over a larger area in between as well as overlapping those of the intermediate zone (Viljoon, Clementine I and II and Fricke), and the core margin (Karlsbrunn, Steens and OKAT 9) (Fig. 8a). Most of the samples consist of lithian mica, varying strongly in composition, even in the same thin section. Overall, the micas from the intermediate zones of an individual pegmatite plot, as might be expected, closer to the ideal muscovite point than do micas from the replacement units and core margins (Fig. 8a). However, it is remarkable that micas from the intermediate zone in Clementine II and OKAT 13 plot in the trilithionite–polylithionite–zinnwaldite field, and that there are micas

Chemical substitutions In the (Mg-Li) vs. (Fe+Mn+Ti-AlY) diagram, as introduced by Tischendorf et al. (2004), micas from the intermediate zone define a clear compositional trend (R2 = 0.78) from muscovite to an intermediate lepidolite-zinnwaldite ( K [ F e 21 .+5 0 . 5 L i 0 . 5 1 . 5 ( A l , F e 3 + ) ] ( S i 3 . 5 2 . 5 56


pegmatite-formation processes from primary crystallization to late-stage subsolidus reactions. The K/Rb ratio of micas, together with the Li or F content and some trace elements, such as Cs, Sn and Zn, have been used frequently as petrogenetic indicators of the degree of pegmatite evolution and that of associated rocks (e.g. Gaupp et al., 1984; Foord et al., 1995; Wise, 1995; Roda et al., 1995, 2006; Pesquera et al., 1999; Kile and Foord, 1998, Clarke and Bogutyn, 2003; Cˇerny, 2004). The proportions of these rare elements usually increase with decrease of the K/Rb ratio. Intermediate zones of the pegmatites are interpreted to have a primary origin from the crystallization of the pegmatite melt, and the core margins are supposed to be crystallized during more evolved stages, whereas replacement units are considered to be the result of metasomatic subsolidus replacement of pegmatitic units. The K/Rb ratio of micas from the Okatjimukuju-Kaliombo pegmatites changes considerably from one pegmatite to another. The lowest values for micas from the intermediate zone correspond to Clementine I, Clementine II, OKAT 13, OKAT 6, and Viljoon. Mean K/Rb ratios are shown by micas from Fricke, OKAT 3.5 and Karlsbrunn (Tables 1 and 2, Fig. 9). The highest K/Rb ratios were reported for micas from the intermediate units of Karlsbrunn, OKAT 6, OKAT 3.3 and OKAT 9. Relationships between K/Rb. vs. Li and K/Rb vs. Cs, as presented in Fig. 9a,b, are commonly used not only to characterize pegmatite evolution, but also for comparison with other pegmatite fields (e.g. Cˇerny, 2004; Morteani and Gaupp, 1986; Foord et al., 1995; Wise, 1995; Kile and Foord, 1998). According to these relationships, all pegmatites of the Okatjimukuju-Kaliombo portion studied here are moderately to highly evolved and are comparable with other rare-element pegmatites such as those of Harding, Tanco, Pikes Peak, La Fregeneda and Pinilla de Fermoselle (Morteani and Gaupp, 1986; Cˇerny et al., 1998; Cˇerny, 2004; Foord et al. 1995; Wise, 1995; Roda et al., 1995, 2005, 2006; Kile and Foord, 1998). The most evolved pegmatites would be Clementine II, OKAT 13, and Viljoon (Fig. 9a,b). Clementine I and OKAT 6 could be placed in a transitional group and, finally, Fricke, Karlsbrunn, OKAT 3.5 and OKAT 3.3 would belong to a less-evolved group. Micas from OKAT 9 show a different position in the ranking for Li and Cs (5 and 10, respectively). The Riksburg 2 and Steens

from the replacement units (Viljoon and OKAT 9), which are very poor in Li. Most of the micas belong to the K-Li-Al-Si mica composition plane. The VIAl contents show a clear negative correlation with Li (and so, with F) for all micas (Fig. 8b). The correlation coefficients are excellent for the micas from the intermediate zone (R2 = 0.94) as well as for those of the core margin and the replacement units (R2 = 0.89). This observation, together with the broad positive correlation between Si and Li for micas from the intermediate zone, and the good negative correlation between Li contents and (AlY + &Y) values (Fig. 8b), suggest that the substitution exchange vectors Li3Al 1& 2 and SiLi2Al 2& 1 were the dominant mechanisms of Li incorporation in the micas studied. However, these two substitution mechanisms did not operate to the same extent during all the pegmatite evolution and the subsolidus replacement events. In Fig. 8b it is evident that for the micas from the intermediate units, both mechanisms operated to the same extent, whereas for the micas from the core zones and from the replacement units, the first mechanism was much more effective. This could indicate that the SiLi2Al 2& 1 exchange vector played an important role from the first stages of pegmatitic crystallization, but during hydrothermal and subsolidus stages, this mechanism lost influence and the Li3Al 1& 2 vector mainly controlled the composition of the later micas. Some micas, commonly from the intermediate units, exhibit relatively large FeO, MnO or ZnO contents (Table 2), as mentioned above. A positive correlation between the Fe (Mn, Zn) and Li contents is generally observed, which suggests that the substitution Al 1 & 1 (Fe2+Mn2+Zn2+)Li may account for the incorporation of Li and (Fe-Mn-Zn) components in the octahedral site of such micas. No relationship has been observed between these enrichments and the Fe-Mn phosphates. Thus, Fe or Mn enrichments are not apparently related to the occurrence of FeMn phosphates in the pegmatite, nor with their Fe/(Fe+Mn) ratio. Similarly, the occurrence and chemistry of Fe-rich tourmaline do not seem to be directly related to this enrichment in micas. Petrogenetic implications Mica is a ubiquitous mineral in the pegmatites from the Okatjimukuju-Kaliombo portion. As it occurs in all pegmatite zones, it may document all 57

E. RODA ET AL.

subsolidus processes occurred in many of the micas studied, in some cases induced (or favoured) by deformation events. On the other hand, the incongruent trends shown by some micas from the pegmatites studied could also be the source of the disequilibrium reactions proven by the petrographic features. For the individual pegmatite bodies, a complete internal zonation is developed. In general, the micas from the core margins and the replacement units are enriched in F, Li, Rb and Cs, in comparison with the micas from the intermediate zones. This progressive enrichment in large-ionlithofile elements is consistent with the appearance of a fluid phase during pegmatitic crystallization; as for these elements the silicate melt/ fluid phase partition coefficients are not unity, and large-ion-lithofile elements partition into the fluid opposite to the high-field-strength elements (Bau, 1996). The presence of such liquids at the final stages of crystallization is indicated by melt inclusion studies (Thomas et al., 2000; Kamenetsky et al., 2004; Banadina et al., 2004). Moreover, the appearance of a fluid phase at the final stages of crystallization would be favoured by relatively low pressures, as it is suggested for the crystallization conditions of these pegmatites by the presence of petalite in many of them, whereas spodumene has never been observed. The fluids may become strongly enriched in incompatible elements, such as F, B, P, Be and Li (Veksler, 2004). This could be related to the main influence of the Li 3 Al 1 & 2 substitution mechanism for micas from the core margins and replacement units of the pegmatites, i.e. micas related to the last stages of pegmatite evolution. This exchange vector implies the incorporation of a larger amount of Li in the octahedral sites of micas than the Si2LiAl 3 vector, mainly operative during the crystallization of the intermediate units of these pegmatites. The internal evolution of the pegmatites is better understood in the largest bodies: Karlsbrunn (Roering and Gevers, 1964) and Clementine II (Keller and Von Knorring, 1989). In these pegmatites, a wide compositional range is observed for micas from the intermediate zone, the core margin as well as from the replacement units. The chemical evolution of micas from these pegmatitic bodies reflects the progressive enrichment of residual fluids in F and Li until the end of the crystallization history of these pegmatites. In the Karlsbrunn pegmatite, the mean F contents in micas range from 2.60 to 7.44 wt.% for the

pegmatites are not considered in this ranking, due to the lack of data from the intermediate zone. However, this ranking of pegmatite evolution does not fit with field and petrographic observations, as well as with other geochemical data. For example, the position of Karlsbrunn, a large and important producer of petalite, amblygonite and lepidolite, should be much higher in the ranking. The same could be expected for OKAT 9, a F-LiTa-rich pegmatite, and thus highly evolved due its lepidolite, amblygonite, topaz and microlite mineralization. Distinctly lower ranks as given in the sequence above would be envisaged for Clementine II, where only small quantities of Liminerals (lepidolite and amblygonite) have been mined. Likewise, Viljoon was only mined for small-scale columbite-tantalite, with no economic Li mineralization. Additional evidence for the degree of evolution can be provided by the aforementioned preliminary data on the K/Rb vs. Cs content for K-feldspar (Fig. 3). According to this diagram, the most evolved bodies in the Okatjimukuju-Kaliombo portion would be OKAT 13, Steens and OKAT 3.3. Viljoon, Fricke and Karlsbrunn show intermediate levels of differentiation and, finally, the higher K/Rb ratios for K-feldspar belong to OKAT 3.5, OKAT 6, Clementine I and Clementine II. These data are more consistent with the field and petrographic observations than those from micas. This lack of coherency between the mica data and the K-feldspar data, as well as between the mica geochemistry and petrographic and field evidence, bring into question the general usefulness of the K/Rb ratio in micas as an indicator of the degree of evolution attained by pegmatites. Subsolidus reactions may have affected the ranking of pegmatites on the basis of chemistry of micas, not only from the core margin and replacement units, but also from the intermediate zone. While the micas (from the intermediate zone!) with a virtually homogeneous composition relative to main and trace elements are not affected by subsolidus reactions, the opposite may be the case for micas with remarkable heterogeneities, like those from OKAT 13, Clementine II, OKAT 9 and OKAT 6 with respect to F or Li. It is worth noting the strong heterogeneity in the K/Rb ratios that appears in a number of mica samples of the intermediate zone, e.g. Karlsbrunn (83-209), Fricke (88-244) OKAT 6 (88-261), OKAT 9 (68-369), and Clementine II (75-819). Petrographic study, as well as the BSE images support the suggestion that important 58


members between polylithionite and zinnwaldite, i.e. between Li-Al and Li-Fe micas but, in general, closer to the polylithionite end-member. (2) Muscovites present the 2M1 polytype, whereas the Li-richest micas show mixed 1M and 2M1 polytypes. The substitution mechanisms proposed to incorporate Li into the octahedral sites of muscovite are Li3Al 1& 2, which would have been operative during all the stages of pegmatite evolution, and SiLi2Al 2& 1 mainly operative during the first stages of pegmatite crystallization. In the Fe-richest micas, the substitution Al 1& 1(Fe2+Mn 2+Zn2+ )Li may account for the incorporation of Li and (Fe-MnZn) components. (3) Most of the pegmatites of the Okatjimukuju-Kaliombo portion in the Karibib belt investigated exhibit a medium to high degree of evolution. (4) In general, the chemical composition of the micas changes according to its position in the pegmatitic body and to the degree of evolution of the pegmatites. In those cases, micas can be used in order to establish the degree of fractionation attained by the pegmatites. According to the K/Rb ratio of micas and its content in elements like Li, Rb, F, Cs and Zn, the most evolved pegmatites would be Clementine II, OKAT 13, and Viljoon. Clementine I, OKAT 6 and OKAT 9 could be placed in a transitional group and, finally, Fricke, Karlsbrunn, OKAT 3.5 and OKAT 3.3 would belong to a less evolved group.

intermediate zone and the replacement units, respectively. This pegmatite shows a series of lithian mica to lepidolite in the core zone, whereas the mica that occurs in the contact and in the intermediate zones is muscovite. This suggests an increasing fractionation of the pegmatitic melts from the margins to the core. Subsolidus processes gave rise to replacement units with a F-, Li-, Na-, P-, B- and Ta-enrichment, as is reflected in the mineralogical assemblages (Li-mica, albite, amblygonite-montebrasite, tantalite, cassiterite, topaz and tourmaline) (Table 1). In the Clementine II pegmatite, the F contents are also very variable, from 0.55 to 8.82 wt.% in the intermediate zone and replacement units, respectively. The FTIR spectra for micas from this pegmatite are consistent with such variations (Fig. 6). Despite these differences, a regional zonation in the Okatjimukuju-Kaliombo pegmatitic portion is not observed. The degree of evolution of the pegmatites apparently does not follow a defined direction; in any given area, an overlap of pegmatites with different degrees of evolution can be found. Such a lack of zonation could be due to: (1) the high degree of evolution attained by most of the pegmatites; (2) different levels of outcropping, which could give apparent differences in the degree of evolution reached by the pegmatitic bodies (Varlamov, 1958); (3) different ages and granite sources; and (4), most likely for the pegmatites of the Okatjimukuju-Kaliombo portion, the specific topography of the igneous basement the area under investigation is proposed to consist of different domes alternating with more depressed zones (Gevers, 1963; Oliver, 1995). Under such conditions, it could be expected that, at the same topographic level at the surface, over the domes the pegmatites would be less evolved than in the areas over the depressed basement (Fig. 10). This, together with the moderate to high degrees of evolution shown by most of the pegmatites, could explain the absence of an evident zonation in this pegmatitic portion. Conclusions Based on field relationships, internal structure, mineralogy and chemical composition, the following conclusions are drawn: (1) The micas of all these pegmatites belong to the muscovite lepidolite series, which are defined precisely as intermediate solid-solution

FIG. 10. Idealized scheme of the structures in domes of the basement, and the distribution of the pegmatites, with different degrees of evolution

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and I.M. Samson, editors). Short Course Notes, 17, Geological Association of Canada. Cˇerny, P. and Burt, D.M. (1984) Paragenesis, crystallochemical characteristics, and geochemical evolution of micas in granite pegmatites. Pp. 257 297 in: Micas (S.W. Bailey, editor). Reviews in Mineralogy, 13, Mineralogical Society of America, Washington, D.C. Cˇerny, P., Trueman, D.L., Ziehlke, D.V., Goad, B.E. and Paul, B.J. (1981) The Cat Lake-Winnipeg River and the Wekusko Lake pegmatite fields, Manitoba. Manitoba Mineral Research Division, Economic Geology Report, ER 80-1, 240 pp. Cˇerny, P., Ercit, T.S. and Vanstone, P.J. (1998) Mineralogy and petrology of the Tanco rare-element pegmatite deposit, Southeastern Manitoba. IMA 17th General Meeting (Toronto), Field Trip Guide Book. Clarke, D.B. and Bogutyn, P.A. (2003) Oscillatory epitactic-growth zoning in biotite and muscovite from the Lake Lewis leucogranite, South Mountain batholith, Nova Scotia, Canada. The Canadian Mineralogist, 41, 1027 1047. Corner, B. (1983) An interpretation of the aeromagnetic data covering the western portion of the Damara Orogen in South West Africa/Namibia. Special Publications Geological Society of South Africa, 58, 46 70. Diehl, (1993) Pegmatites of the Cape Cross-Uis pegmatite belt, Namibia: geology, mineralisation, rubidium-strontium characteristics and petrogenesis. Journal of African Earth Sciences, 17, 167 181. Foord, E.E., Cˇerny, P., Jackson, L.L., Sherman, D.M. and Eby, R.K. (1995) Mineralogical and geochemical evolution of micas from miarolitic pegmatites of the anorogenic Pikes Peak batholith, Colorado. Mineralogy and Petrology, 55, 1 26. Foster, M.D. (1960) Interpretation of the composition of Li-micas. US Geological Survey, Professional Paper, 354 E, M.A. 15 263. Frommurze, H.F., Gevers, T.W. and Rossouw, P.J. (1942) The geology and mineral deposits of the Karibib area, South West Africa. Explanatory Sheet, 79 (Karibib S.W.A.). Geological Survey of South Africa, 172 pp. Gaupp, R., Mo¨ller, P. and Morteani, G. (1984) Tantalpegmatite. Geologische, Petrologische und Geochemische Untersuchungen Monograph Series on Mineral Deposits, 23, Gebru¨der BorntraegerBerlin-Stuttgart, 124 pp. Gevers, T.W. (1963) Geology along the north-western margin of the Khomas Highlands between Otjimbingwe-Karibib and Okahandja, South West Africa. Transactions of the Geological Society of South Africa, 66, 199 251. Goad, B.E. and Cˇ erny, P. (1981) Peraluminous pegmatitic granites and their pegmatite aureoles in

(5) Differences between the K/Rb micas rank and the K/Rb K-feldspar rank as well as the field observations can be attributed to subsolidus processes related to late hydrothermal fluids, enriched in F, Li, Na, P, B and Ta, that affected micas, not only from the replacement units and the core margins of the pegmatites, but also from the intermediate units. (6) Some of the biggest pegmatitic bodies (Clementine II and Karlsbrunn) evolved from the margins to the core. (7) An apparent zonation in the OkatjimukujuKaliombo pegmatitic portion is not observed, which could be due to the high level of evolution attained by most of the bodies, and to the topography in dome structures of the basement. Acknowledgements The authors acknowledge the valuable comments and suggestions of one anonymous reviewer. We also thank Ph. De Parseval, who carried out the electron-microprobe analyses at the Universite´ Paul Sabatier, in Toulouse, France. References Appleman, D.E. and Evans, H.T., Jr. (1973) Job 9214: Indexing and least-squares refinement of powder diffraction data. US Geological Survey. The National Technical Information Service, PB2-16188. Bailey, S.W. (1980) Structures of layer silicates. Pp. 1 39 in: Crystal Structures of Clay Minerals and their X-ray Identification (G.W. Brindley and G. Brown, editors). Monograph 5, Mineralogical Society, London. Banadina, E.V., Veksler, I.V., Thomas, R., Syritso, L.F. and Trumbull, R.B. (2004) Magmatic evolution of Li-F, rare-metal granites: a case study of melt inclusions in the Khangilay complex, Eastern Transbaikalia (Russia). Chemical Geology, 210, 113 134. Bau, M. (1996) Controls on the fractionation of isovalent trace elements in magmatic and aqueous systems; evidence from Y/Ho, Zr/Hf, and lanthanide tetrad effect. Contributions to Mineralogy and Petrology, 123, 323 333. Brigatti, M.F., Lugli, C., Poppi, L., Foord, E.E. and Kile, D.E. (2000) Crystal chemical variations in Li- and Fe-rich micas from Pikes Peak Batholith (central Colorado). American Mineralogist, 85, 1275 1286. Cˇerny, P. (2004) The Tanco rare-element pegmatite deposit, Manitoba: regional context, internal anatomy, and global comparisons. Pp. 184 231 in: Rare Element Geochemistry and Ore Deposits (R.L. Linne

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