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Thomas_et_al_1994_Spodumene

The document summarizes unusual spodumene-bearing pegmatites found in the Late Kibaran region of southern Natal, South Africa. The pegmatites form part of a suite of sub-concordant sills intruded into high-grade gneisses. They contain quartz, albite, microcline and significant quantities of spodumene replacing primary petalite. Geochemical analyses indicate the pegmatites crystallized from relatively high-temperature, volatile-poor liquids. The spodumene occurrences are considered a promising resource for lithium and feldspar extraction.

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See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/232348244 Unusual spodumene pegmatites from the Late Kibaran of southern Natal, South Africa ARTICLE in ORE GEOLOGY REVIEWS · APRIL 1994 Impact Factor: 3.56 · DOI: 10.1016/0169-1368(94)90026-4 CITATIONS 5 READS 43 5 AUTHORS, INCLUDING: Bob Thomas Council for Geoscience 389 PUBLICATIONS 2,165 CITATIONS SEE PROFILE Deon De Bruin University of Western Australia 21 PUBLICATIONS 290 CITATIONS SEE PROFILE Available from: Bob Thomas Retrieved on: 13 January 2016
ELSEVIER Ore GeologyReviews 9 (1994) 161-182 ()RE ( ;E( )IL )( ,Y RI:XIFAVS Unusual spodumene pegmatites from the Late Kibaran of southern Natal, South Africa R.J. Thomas a, D. Biihmann b, W.D. Bullen a, A.J. Scogings c and D. De Bruin b "GeologicalSurvey, P.0. Box 900, Pietermaritzburg, 3200 South Africa bGeologicalSurvey, P. Bag X112, Pretoria, 0001 South Africa ¢507Roper St., Baileys Muckleneuk, Pretoria, 0181 South Africa (Received March 9, 1993;revised version accepted August 10, 1993) Abstract Significant quantities of spodumene have been identified from a number ofleucocratic pegmatoid bodies which crop out on The Corner, near Highbury, Port Shepstone District, in the southern part of the ~ 1 Ga Natal Meta- morphic Province. The mineralized rocks form part of a suite of sub-concordant, predominantly aplitic sills which intruded high-grade mafic gneisses of the Mucklebraes Formation. The entire package lies within a synformally folded klippen structure. The spodumene-bearing sills, which are up to 15 m thick, were emplaced at several struc- tural levels along the northern limb of the synform. These white, coarse-grained rocks are composed of quartz, albite, microcline and spodumene, with traces of white Li-mica, garnet, graphite and very rare beryl. Pale yellow- ish-green to pink spodumene, symplectically intergrown with quartz (SQI), forms irregular to ovoid poikilocrysts up to 40 cm across. This texture is characteristic of spodumene that has replaced primary petalite and the compo- sition of the SQI is comparable with published petalite analyses. This isochemical inversion model is supported by an observed volume change, evidenced by the radial cracks which commonly surround isolated SQI crysts. The P-T conditions of formation of SQI are consistent with near-isobaric cooling models which have been proposed for this part of the Natal belt. Geochemical analyses show that the Li-pegmatites are highly siliceous with variable K/Na ratios and low Fe. Trace-element abundances (including H20 and F) are low, with the exception of Rb. The largely anhydrous mineral assemblages (coupled with the absence of other mineral phases typically associated with Li-pegmatites), textural relationships and geochemistry suggests that the Highbury Pegmatites crystallized from relatively high-temperature, volatile-poor liquids under conditions of lowfH2o and low fo2. Consequently, the Li- pegmatites are considered to be late-stage differentiates of the characteristically anhydrous garnet leucogranites and charnockites of the Margate Suite, with which they share many mineralogical and geochemical similarities. The lithium may have been scavenged from country rock metasediments during anatexis. A simple gravimetric test shows that the spodumene can be efficiently beneficiated to a product with ~ 7% Li20. In view of the increas- ing economic importance of Li-minerals and the easily accessible position of the Highbury Pegmatite, coupled with feldspar and dimension stone as possible by-products, the prospects for future exploitation are considered favourable. 1. Introduction The pure white, sheet-like aplite and leuco- pegmatite intrusions of the Highbury area, some 18 km north of Port Shepstone (Natal), form an integral part of the ~ 1 Ga Natal Metamorphic 0169-1368/94/$07.00 © 1994Elsevier Science B.V. All rights reserved SSDI0169-1368(93) E0032-5
t 62 R.J. Thomas et al. / Ore Geology Reviews 9 (1994) 161-182 Province (Fig. 1). The rocks were first described by Thomas (1988), who referred to them as "alaskitic granite" and noted that, in addition to quartz, two feldspars and very sparse white mica, the rocks commonly contained garnet and graphite. In one area, Thomas (1988) also re- ported an abundance of large, rounded, optically continuous "patches" of poikilitic clinopyrox- erie, thought to be diopside, on the farm The Corner. However, during routine mineralogical investigations by the senior author, the clinopy- roxene was re-identified as spodumene. In view of the widespread interest currently being shown in spodumene, both by the glass industry (e.g., Kingsnorth, 1988) and as a source of lithium salts, a more detailed petrological study and eco- nomic evaluation of these rocks was initiated. This paper reports the results of that study and compares the Highbury rocks with other Late Kibaran pegmatites in southern Africa. 2. Nomenclature The term "alaskite", used by Thomas ( 1988 ), is now considered inappropriate, as it has now been established that K-feldspar is only locally the dominant feldspar (following Le Maitre, 1989). For the most part, the intrusions are al- bite-dominated, medium-grained, extremely leueocratic (Colour Index ~ 1), with saccharo- idal texture and are best termed "aplite". How- ever, this paper focuses on the coarse-grained spodumene-bearing phase, termed here the "Highbury Pegmatite", after Highbury farm which lies to the south of The Corner, where the majority of the Li-rich rocks crop out (Thomas et al., 1993). 3. Geological setting and field relationships Two discrete tectonostratigraphic terranes have been recognised in the southern part of the Natal Metamorphic Province (Thomas, 1989). The granulite-grade Margate terrane in the south is juxtaposed against the amphibolite-grade Mzumbe terrane to the north along the Melville Thrust (Fig. 1). In the Highbury area, north of the Melville thrust, an outlier of granulite facies marie rocks (the Mucklebraes Formation) has been identified. This outlier was interpreted by Thomas ( 1989 ) as a small Margate Terrane klip- pen which is bounded at its base by a zone of ductile thrusting. The pegmatitic and aplitic rocks under review here form a number of sub- concordant sill-like bodies that occur in a ~ 150 m thick zone within the mafic pyroxene gneisses and granulitic calc-silicates of the Mucklebraes Formation. After the intrusion of the pegmati- tic/aplitic sheets, the klippen was folded into an open, east-west-trending periclinal synforrn (Thomas, 1989). The foliation in the gneisses, and hence the contacts of the pegmatite sills, dips centripetally towards the axis of the synform at shallow angles (generally < 30 ° ). Consequently, the aplitic bodies now define a broadly arcuate outcrop, measuring ~ 8 km by 3 km, around the western, southern and eastern limbs of the syn-. form (Figs. 1, 2). They appear to be regionally transgressive, having been emplaced at a slightly lower structural level in the west. Outcrops of the pegmatite are poor except along the densely-bushed, NE limb of the syn- form, where it locally forms prominent 20-m-high white cliffs, on the farm The Corner, overlook- ing the Mzumbe River Valley (Fig. 2): The in- termittent nature of the outcrops along consis- tent structural levels suggests that the thickness of individual pegmatite sheets may vary along strike due to tectonic pinching and swelling. In this case, pegmatite mega-boudins may be ex- pected to be elongated along the local S- to SW- plunging stretching lineation direction, Samples of the pegmatite were also obtained from a stra- tigraphic borehole (the Mzumbe Borehole) lo~. cared some 300 m south of the outcrops on The Corner (Fig. 2 ). Correlation of the outcrops with the borehote core is good, suggesting that the pegmatite sheets dip at ~ 20-25 ° to the south in this region. Significant concentrations of spodumene have been discovered in this area, in pegmatite out- crops along a strike length of > 1kin, centred ap- proximately on the Mzumbe Borehole site (Fig. 3 ). The spodumene occurs throughout a number
R.J. Thomas et al. I Ore Geology Reviews 9 (1994) 161-182 r-~ CAPE FOLD BELT (-280 Ma) PAN AFRICAN OROGENS (-500 Ma) KIBARAN OROGENS (-1100 Ma) EBURNIAN PROVINCES (-2000 Ma) ARCHAEAN CRATONS AND MOBILE BELTS (>-2700 Ma) X NAMIBIA F~v~'~IMBABWE(t~'/ X ". 2 # / ~OTSWANA/'~--~ .~~ k rv/ I.~ ~ " - - ~ LESOTHO'L,/ X so.T,-, ~.. / 163 ~o• "'.. " / ~. ~..~n~T, i / + + + + + + + + + + + ÷ + + / "24°S ~'..':@//i] / // Ht::llU~U/r~ a + + + + + + + + + + + + + + /' T:'.,)'~ ~/ ~MAIN I+ + + + + + + + + + + + /.~ ~,.'~'~'Mxtl ? u~ l+ + + + + + Gaberone+ + + + + + / .~ I,~:/,.,':,',,,',. ' I +++~++++++'++++++ Preto,a +%÷++ -260S ~ {~'~%~ ~ , '~ ++ . . . . . . . Johannesburg +++ ~ "~ "~ "£-.7.."-~- L .,,(2 ..- "~. &",'+~. KAAPVAAL ++++++ ++++ / ~' R" 2 °S " + ~,.t,~:~-.,~",¢.~:~ + + + + + + + + + + - y,y-- w.. r.-, ~ .,->-x~,,,,'F<~ . . . . . . . + +~"*" P" <• .~_ ~ + + + + + + + + / ./ -30°S ~ ~ L ~ - - z z - - ~ /~/xI + + + + + . - * " / / O '.k-:-:---'~ "/4~Z ~+ + + +/ --I'/ " " ~ "-=-- - ~qO(z -"<-+->// ~F'~y,,~ Study area / a2°s "// - " ...... -1 -/ , o° . . . . . -- . . 34°S Cape Town ~ ~ ? 200km Elizabeth I j 16°E 20E 24°E 28°E i 32°E 36°E I I i i ~ I i / < 3 0 30 ° 35S Figure 2 [ ~ Cover rocks I -TAMORPHIC PROVINCE 3HBURY PEGMATITE :e tectonic plutons 040 - 1070 Ma) :umbe Terrane irgate Terrane [oO°°o° o-,1 Fig. 1. Regional geological setting and location of the Highbury Pegmatite.
164 R.J. Thomas et al. I Ore Geotog.: Reviews 9 (1994) 161-182 • " o i "~.... / / " - "" %)*'--'='--':'--- 30 °30'El + +~+ + ~,ol + + + + +1 / - t,34 ~ ( /oF" ".~ - ~ 1 t . It t + ± * ! / /-- ~.~ ~ ~°' /-~[,~ ~ - .-.,~, ----"':.-+_ +',, +/1+ + + +! . ..4. , : 7" _~-b~L------~ ,,w,.~ o,;......-..-..oe,,:,. 32 ~:3o~',,+ + + +1 / 32 , ,/.~...I---~,-I~,,7L~,~ , ~'. //,~I o a.'~. ~//,-,,~_ ° ot + + + | / / '~'_II~',L'/',~-%-';..."'.'-,':~;'z , ,UlP~ "" . ,I "Hlli-~;~'J~'~*l .~ ",o '. ',°1 , / qi~lr~::;~:, '-. - : , ~" ,v ; :MZq;T., "#:~ LX, i r~no~d~l ~. ~ ,_ ~-;wv.: ,-_ ~..,-,-,,7,~,~).z'_ "' . ". o- 5'' ! " /2o- : " .-"I -4 ° --- _ • / y -".'.........I ~'~" ~ "--- ..... "~ --" ~ 4n - - ~ " " I "- "~ "~ ~ "Z "~'~-- ~ ~ / / "" " ,/ / I MZ Mzumbe borehole site 0 2 km Alluvium Highbury pegmatite ( ,ann"" spodumenebearing) Garnet leucogranite (Margate Granite Suite) Mucklebraes Formation 4/24 Dip and strike of foliation Mzumbe gneiss (sheared) /4' Thrust zone Fig. 2. Geological map of the Highbury Pegrnatite and environs. Note the position of the Mzumbe Borehok: of sills and there is no evidence to suggest that the host bodies show any systematic vertical zonation in the fashion typical of lithium peg- matites (e.g., Norton, 1983). This does not, however, preclude the possibility of lateral zon- ation. A systematic search of the entire Highbury Pegmatite field failed to reveal further spodu- mene mineralization. The garnetiferous aplites constituting the vast bulk of the intrusions ap- pear to be barren, so the Li-pegmatites may have formed as a result of lateral fractionation within the intrusive system. In outcrop the rocks are typically granular, un- foliated to poorly foliated in texture. However, elongated trains of small garnets locally define a weak fabric parallel to intrusion margins and to the layering in the country rocks. The pegmatites are clearly intrusive into the gneisses of the Mucklebraes Formation, but were emplaced prior to the development of the regional synfor- mal structure. The upper and lower contacts ob- served in the Mzumbe Borehole are typically sheared. In the southern part of the synform, the rocks are aplitic and more strongly fotiated. These rocks closely resemble garnet-leucogranites of the Margate Suite, a voluminous syntectonic leuco- granite-charnockite association of the Margate Terrane (Thomas, 1988 ). Thus, despite the often unstrained appearance of the core areas of the
R.J. Thomas et al. I Ore Geology Reviews 9 (1994) 161-182 165 X S / 3 0 ° 30' E B ~ ' ~ / ~ ~ 0b~100 m / ~":~'-~'" "'~/ ~ ~ . . . . . . / v ........~...........-:~-- i .~..J ~"--0 J" • ~'~= ~.~.~ • ~.~,....~ . . . . (- , . ~ ~/vA-zuLu / % ~ ~..~ ~oo I o..../ ~ 120 ~" ~ / ~. o .J i/;" F ~ ,~---,~~ ~o- 1 A ~ o• ~,-XT...---"-- 160-- ~ - V ~-~' ~ MUCKLEBRAES/.~ I~'LL~.............. I / "....I . "~ j THECORNER ";-" TO1 :" ~o/ Contours at 40 m intervals• Mzumbe Borehole site /- • Spodumene pegmatite outcrop f Road ........... TrackSpodumene pegmatite • outcrop / boulders Farm boundary .:~i~' Interpretedextent of i '~:~ spodumene pegmatite • TC 2 Sample Site O Barren pegmatite / aplite A ~ B Line of section 200 100 A Mzumbe MUCKLEBRAES ~.:~ lorehole Outcrop of mainspodumene pegmatite zone CORNE~ Large bolders (outcrop?) ~:'~i::i:~ ~ _f ~' 4_ Spodumene pegmatite ~ . Intersectionsin borehole 250 Metres 5C)0 Bottom (2943m) Fig. 3. Detailed geological map of the spodumene-bearing pegmatites on The Corner and N-S section through the Mzumbe Borehole.
166 R.J. Thom tz~et al. / Ore Geology Reviews 9 (1994) 161-182 pegmatites, they clearly represent a syn- to late tectonic intrusive suite. 4. Petrographic characteristics In hand specimen, the extremely leucocratic pegmatites and aplites are strikingly white, with extremely low marie mineral contents (< 1%). The coarse-grained pegmatitic phases contain feldspar crystals up to 30 cm in length ( ~ 5 cm average). Aplitic phases are typically medium- grained (2-5 mm), saccharoidal, though inequi- granular. In addition to quartz and feldspar, small ( < 1 mm) flakes of graphite are locally conspic- uous, along with minute (< 1 mm), pale pink and brown garnet. In the Li-rich phase, spodu- mene occurs in equant, ovoid to irregular crys- tals, up to 40 cm across, in graphic and symplec- tic intergrowth with quartz (SQI, for Spodumene-Quartz-Intergrowth, modified after Burt et al., 1988). At their densest, these crysts are closely packed and can constitute up to 70% of the total rock. No pure spodumene crystals have been observed. The SQI crysts tend to be extremely inconspicuous and elusive in the field. but they form grey patches on weathered sure- faces (Fig. 4) and can be observed in glancing sunlight as large, optically continuous, poikilitic cleavage surfaces. On polished surfaces, the coarse-grained intergrowths typically resemble graphic textures, with regularly oriented, angular quartz grains enclosed within optically continu- ous spodumene (Fig. 5). The spodumene is white to pale yellow-green or pink, though light-brown shades have locally been observed. In certain lo- calities where the SQI crysts are relatively sparse and not in mutual contact, they are commonl) surrounded by radial cracks, suggesting that a significant volume change has taken place at some stage in their genesis. One 10 x 5 mm cry,s- tal of light-blue beryl was recorded in the spo~ dumene pegmatites. It is doubtless the mineral. ogical simplicity, coupled with the cryptic habit and unremarkable colour of the spodumene that has prevented the earlier discovery of the High- bury Li-pegmatites. The pegmatites, with ve~, low mica content~ Fig. 4. Field appearance of the Highbury Pegmatite, with grey patches of spodumene-quartz intergrowth, showing typ~c:finega- tive-weathering. The unremarkable appearance of the outcrops is probably why the spodumene was only recently recog¢fiscd.
R.J. Thomas et al. / Ore Geology Reviews 9 (1994) 161-182 167 Fig. 5. Graphic spodumene-quartzintergrowth (SQI) in cut surface through sample of the topmost mineralizedpegmatite in the Mzumbe borehole, with quartz (dark grey) and spodumene (pale grey) (Scale in centimetres). Fig. 6. Typical appearance of SQI in thin section. Field of view is 4 mm across. are thus typified by anhydrous mineral assem- blages. Despite a uniformly leucocratic appear- ance, the mineralogy is highly variable with re- spect to quartz and feldspar ratios. Quartz makes up between 18 and 50% of the rocks, while the plagioclase/K-feldspar ratio varies markedly. The majority of rocks are plagioclase-domi- nated, with up to 70% albite and as little as 2%
68 R.J. Thomas et aL / Ore Geology Revwws 9 (1994) 161-182 ig. 7. Photomicrograph of fibrous spodumene-quartz intergrowth at the edge of a poikilitic SQI crysL Field ot v~c ?,ross. Fig. 8. Photomicrograph of symplectic spodumene-quartz intergrowth. Field of view is 2 mm across.
R.J. Thomas et al. / Ore Geology Reviews 9 (1994) 161-182 169 microcline. However, there is a complete spec- trum of feldspar ratios through to rare K-feld- spar dominated rocks with up to 65% microcline and only ~ 5% albite. Grain size is also highly variable, with granular textures typical, but in many rocks an interlobate, amoeboid inter- growth is seen, where embayed grains of quartz and feldspar form a distinctive interlocking texture. The spodumene can only be distinguished op- tically from diopside on the basis of its lower ex- tinction angle ( ~ 22 ° ). As noted above, it only occurs in the form of poikilitic crystals, sym- plectically intergrown with quartz and modal analyses have shown that the spodumene: quartz ratio in these intergrowths is consistently ~ 60: 40 (Fig. 6). The quartz inclusions tend to be coarser- grained (up to 2 cm long) and sub-angular in the centres of large SQI crysts. However, these often grade into a symplectic intergrowth of minute, rounded quartz vermicles reminiscent of myr- mekite in spodumene at the contact with feld- spar grains. These myrmekitic textures com- monly grade into fibrous intergrowths at the margins (Fig. 7 ). Smaller SQI grains ( < 2 mm) are typically entirely symplectic (Fig. 8 ). Minor quantities of skeletal muscovite are seen in thin section, whilst accessory graphite (occurring as opaque flakes), garnet (forming small, rounded grains) and rare chlorite complete the accessory mineralogy. The rocks are characteristically unaltered, though some embayment and replacement of spodumene and feldspar by late-stage quartz is locally apparent. Xenomorphic and granular textures result from this alteration. However, spodumene is typically unaltered, except locally by albite, and no replacement by other Li-rich secondary phases such as eucryptite, lepidolite, etc., was observed (c.f. most spodumene depos- its). Microcline (perthitic in places) is either partially or wholly replaced by albite which em- bays and, in places, completely encloses K-feld- spar crystals, suggesting that extensive, late-stage sodium metasomatism has taken place. Evi- dence of post-crystallization strain is common, as shown by bent twin lamellae in plagioclase and undulose extinction in quartz. This supports the interpretation of the pegmatites as syn- to late tectonic. 5. The Mzumbe borehole This borehole was sunk with the idea of ob- taining a complete stratigraphic section through the Mucklebraes Formation to the base of the klippen structure, and to provide information on the Highbury Pegmatites. From outcrop it was Table 1 Modal percentage of spodumene in the Highbury Pegmatite in the Mzumbe borehole Depth Vertical True SQI* Spodumene (m) thickness thickness (%) (%X 0, 55 ) (m) (m) Appr.% Li20 113.5-128.9 15.4 14.3 44.9 24.7 2.0 132.3-135.0 2.7 2.5 41.9 23.0 1.9 136.2-143.1 6~9 6.4 42.5 23.4 1.9 181.1-191.3 10.2 9.5 22.1 12.1 1.0 192.4-196.2 3.8 3.5 9.7 5.3 0.4 216.8-225.6 8.8 8.2 < 1 << 1 - 229.3-241.5 12.2 11.3 < 1 << 1 - *Poikilitic spodumene percentages were determined from core samples in the following manner: A line, drawn from the top of each pegmatite intersection to the bottom, was subdivided into 12.5 mm increments. The mineral (spodumene or other) occur- ring at each increment margin was noted. For example, the uppermost pegmatite sheet was subdivided into ~ 1100 increments and the volume percentage of spodumene determined therefrom. These results represent the mean of two separate runs. True thicknesses were determined assuming a dip of 22 °.
70 R.J. Thomas et at. /Ore Geology Reviews 9 (1994) 161-182 O" r- £1- 03 0 © 0 % modal spodumene 0 50 1O0 0 - MZ 28 -ME 29 5 MZ 30 200 50. MZ 56 15 0 - MZ 34 0 - MZ 36 2 250 ~ Highbury pegmatite m Ii L_ ------] Mucklebraes Formation 6 E p = pegmatite > 15 cm thick }!'0 o 03 0 O3 09 C 2~ ~D 5 xz d) O3 q0 > known that the pegmatites occur as multiple in- trusions, though it was not possible to ascertain their precise number or thickness. The borehole was sited at an elevated location north of the ax- ial trace of the regional synform (Fig. 2 }. From mapping, however, it is known that at least one ~2-m-thick spodumene-rich sill occurs at a higher structural level than the top of the bore- hole. The ~250 m deep borehole penetrated seven separate sills of Highbury Pegmatite greater than 2 m in vertical thickness (maximum 15.4 m) and several minor sheets less than ! m thick (Fig. 9 ). The upper and lower contacts of the sills dip generally at ~ 22 ° to the south, indicating that the true thickness of the sills is approxi- mately 93% of the apparent thickness (Table 1 Contacts of the sills with the count,'3' rock gneisses are subconcordant with the gneissose layering, locally sheared, but often clearly intru- sive. The contact zones tend to be enriched in garnet and graphite, suggesting that these min- erals might represent contaminants from the country rocks. Spodumene-quartz intergrowths tend not to be developed in these marginal zones up to 50 cm from the contacts. Apart from the typically barren contact zones, there does not ap- pear to be any systematic mineral zonation throughout the sills, and SQI occurs randomly within the wide, central mineralized zone. The modal distribution of SQI in the thickest sills in the Mzumbe Borehole is shown graphically in Fig. 9. Of the seven sills with a thickness of more than 2 m intersected, the topmost set of three contains the most significant spodumene miner- alization (Table 1). This group of sills, which occur from 113.5 m to 143.1 m depth, have a to- tal thickness of 25 m and have, on average, > 40% modal SQI. The method of determination of modal SQI-content in the core, along with the re- suits obtained, are given in Table 1. Using a con- servative, optically-determined spodu- mene:quartz ratio of 55:45 for the SQI, this Fig. 9. Distribution of Highbury Pegmatite sills in the Mzumbe Borehole. The histograms show modal percent spo- dumene in the five main mineralized sills in the Mzumbe Borehole. Numbers prefaced by MZ refer to borehote samples,
R.J. Thomas et al. /Ore Geology Reviews 9 (1994) 161-182 171 represents >23% modal spodumene, which in turn equates with ~ 2% Li20. The subequal spo- dumene concentrations in the upper three peg- matite bodies suggests that they probably repre- sent a single, branching intrusion. Below this zone, spodumene grades are lower. A 10.2 m thick pegmatite at 181 m depth contains about half the SQI values of the upper three ( ~ 22%), whilst the intrusions below 200 m are effectively barren. 6. Mineralogy and geochemistry 6.1. XRD analysis A powdered sample of spodumene pegmatite (TC 3) was subjected to X-ray diffractometry (XRD) at the Geological Survey Laboratories in Pretoria. This confirmed that quartz, plagio- clase, microcline and spodumene are the major mineral species, along with accessorywhite mica, garnet and graphite. Other samples of coloured spodumene (yellow-green,light brown and pink varieties) were analysed and gave identical spec- tra. In one sample, minor chlorite was identified and the macroscopic identification of a single grain of beryl was confirmed. Mineral concen- trates obtained from TC 3 were also analysed by XRD. The analysis showed the garnet to be a Mn- rich almandine and the white muscovite mica was confirmed as a dioctahedral 2M1 mica. A systematic XRD search of a number of borehole samples for other Li-phases such as eucryptite or petalite proved fruitless. 6.2. Major and trace-elementgeochemistry Whole-rocksamples Major and trace-elementanalyses of five ( 1-5 kg) samples from outcrops of the pegmatite and six ( ~ 1 kg) borehole samples were made at the Geological Survey laboratories in Pretoria. For comparative purposes, the outcrop samples an- alysed included rocks with (TC 2 and 3) and without spodumene (TC 1, 4, 5). The majority of elements were analysed by X-ray fluorescence (XRF). Major element concentrations were ob- rained from fusion discs according to the method of Norrish and Hutton (1969). Trace elements were analysed by XRF on pressed powder pellets of the core samples. The appropriate back- ground and interference corrections from other spectral lines were made, while mass adsorption correction procedures were carried out for ma- trix effects. Li20 and F were determined by wet chemical analysis. A LECO analyser was used for the determination of structural H20, CO2 and S. The results of the eleven whole-rockanalyses are given in Tables 2 and 3. The great variation in feldspar content seen in thin section is naturally reflected in the Na20/ 1(20 ratio, ranging from 0.5 to 2.5. As expected, SiO2 is high (73.8-78.6%), and the high alu- mina (14.0-18.8%) reflects either a high feld- spar or spodumene content. Trace-element abundances are uniformly low except for Rb (~ 500 to > 900 ppm). Rb values rise sympa- thetically with K20 and K/Rb ratios are ex- tremely low (~20-50). One sample (MZ 34) contains anomalously high V, Cr and Ba. F con- centrations range from 100 to 350 ppm, lower than most typical granites (e.g., Middlemost, 1985), and reflecting the extremely low modal mica content of the pegmatite. When plotted on the Rb-Sr-Ba ternary diagram of E1Bouseily and E1Sokkary ( 1975), the Highbury Pegmatites plot within the field of "highly differentiated granites". Li20 in SQI and white mica separates In addition to whole-rock samples, separates of SQI and white mica were analysed for Li20 by wet chemical techniques. Spodumene-quartz concentrate from sample MZ 28 contained 4.67% Li20, corresponding to 58% spodumene, a figure consistent with the modal determinations. Wet chemical analyses of a hand-picked white mica concentrate from TC 3 returned an Li20 content of 0.51%, showing it to be a lithian muscovite according to the criteria of Newman and Brown (1987). Spodumene separates A simple beneficiation test was devised in or- der to ascertain the Li20 content of the spodu- mene in the pegmatites. Two spodumene-rich
172 R.J. Thomas et aL / Ore Geology Reviews 9 (1994) 161-182 Table 2 Major element analyses of outcrop samples of the Highbury Pegmatite Wt% TC-1 '[C-2 TC-3 TC-4 T(-5 SiO2 71.88 74.16 75.76 74.68 - 1,05 TiO2 0.09 0. I 1 0.09 0,10 0, ]i) A1203 17.03 1570 15.83 14.81 ~6,34 Fe203* 0.23 0.19 0.23 0.10 0.22 MnO 0.13 o.12 0.06 <0.0i 0t2 MgO 0.17 0.07 0.14 018 0.15 CaO 0.93 0.28 0. l 1 0,37 ~).52. Na20 7.72 5.79 3.15 5,83 ,~.6~, K20 0.95 i,70 2.08 3.44 .7 : Li20 0.04 1.33 2.03 < 0,01 0,05 P205 0.04 0.05 0.03 0.05 ~.).05 H20- 0.06 0.13 0.09 0. i i 0,02 H20+ 0.39 0.38 0.51 0.24 0.17 C02 0.19 0.07 O.13 0.35 ;}.13 Total 100.51 100,07 100.25 t00,21 98.98 *Total Fe as F%O3. Sample descriptions: TC 1: medium-grained albite aplite; TC2: albite-spodumene pegmatite; TC3: spodumene pegmatite: TC4: albite pegmatite; TC5: fine-grained aplite. Highbury Pegmatite samples were selected, TC 3 (fresh spodumene pegmatite with SQI from out- crop) and MZ 36 (SQI from the Mzumbe bore- hole). In addition, NS (spodumene from Nou- mas No I pegmatite, Namaqualand) was selected for comparative purposes. The samples were hand-crushed in a pestle and mortar. TC 3 was sieved to yield a - 1000/+ 500/t fraction, while MZ 36 was sieved to obtain - 1000/+ 500/t and - 500/+ 75 # fractions. The resultant fractions were then subjected to heavy liquid gravimetric separation using bromoform (S.G. = 2.9), washed in acetone and air-dried. Iron impurities in the sample were removed magnetically. Major element and Li20 analyses were undertaken on the separates by the techniques described previ- ously (Table 4 ). The spodumene samples from both sieve frac- tions of MZ 36 give Li20 values of ~ 7%, some- what higher than those from natural outcrop (TC 3, with 6.5% lithia). Li20 values from Highbury are higher than those obtained from the Noumas spodumene, but lower than the Durabees (Nor- rabees) spodumene (Bischoff and Bischoff, 1988). In general, major element compositions of the Highbury spodumene fall within the ranges quoted in Deer et al. (1974), with respect to m1203(~25-27%), CaO (<0.1%), and alkalis (0.1-2%). With regard to the latter, whilst K20 is always low in spodumene (<0.2%), Na20 is variable up to 2%, as seen in the Noumas sam- ple. The Highbury spodumene concentrates con- tain ~ 0.4% total Fe, similar to the typical value of ~0.5% (Deer et al., 1974). 6.3. Microprobeanalyses In order to obtain precise mineral chemical data from a number of spodumene grains and to test for compositional variability, seven grains from two samples of pale yellowish green spo- dumene from the Mzumbe Borehole (MZ 28 and MZ 36) were selected for microprobe analysis, along with three grains of a pale pink variety (TC 2). In addition, albite, microcline and mica analyses were made, to confirm the petrographic and chemical analyses. The analyses were made on a Jeo1733 Microprobe using wavelength tech- niques at the Geological Survey Laboratories in Pretoria (Table 5). The compositions of all grains analysed are very similar to the spodu- mene separates analysed by XRF (Table 4), with SiO2and A1203values of ~ 64% and ~ 18%, re- spectively. Furthermore. very little composi-
R.J. Thomas et al. / Ore GeologyReviews 9 (1994) 161-182 Table 3 Major and trace-element analyses of core samples of the Highbury Pegmatite from the Mzumbe Borehole 173 Wt% MZ 28 MZ 29 MZ 30 MZ 34 MZ 56 MZ 36 Petalite SiO2 74.69 74.13 71.56 73.05 72.07 77.63 78.00 TiO2 0.11 0.09 0.11 0.11 0.10 0.10 - A1203 15.05 17.37 16.80 15.60 15.80 16.61 17.00 Fe203* 0.59 0.48 0.26 0.40 0.34 0.26 0.04 MnO 0.13 0.07 0.03 0.04 0.08 0.07 0.00 MgO 0.14 0.21 0.03 0.19 0.11 0.13 0.00 CaO 0.41 0.17 0.18 0.53 0.30 0.04 0.00 NazO 4.22 2.67 3.10 3.20 6.45 0.30 0.07 K/O 2.63 1.78 7.45 4.96 4.03 0.12 0.05 Li20 1.49 2.47 1.01 0.60 0.04 4.59 4.74 P20~ 0.04 0.04 0.05 0.04 0.04 0.02 H20- 0.20 0.01 0.01 0.01 0.01 0.01 0.00 H:O + 0.10 0.19 0.64 0.18 0.14 0.04 CO2 - 0.08 0.10 0.43 0.16 0.14 S <0.01 <0.01 0.01 0.01 0.01 <0.01 Total 99.70 99.67 100.89 99.81 99.72 100.26 99.99 (Valu~ inppm) F 350 200 100 250 150 U 7 <6 <6 6 <6 Pb 15 14 25 19 17 Rb 674 477 > 900 > 900 642 Th 5 6 <5 <5 5 Y 8 <5 <5 <5 10 Nb 23 20 14 6 58 Sr 9 7 20 29 9 Zr 37 18 20 26 21 Mo 2 2 <1 1 1 Ga 38 30 37 31 35 Zn 32 67 39 14 69 Cr 7 112 61 268 53 V 7 9 8 264 9 Ba < 5 8 7 261 15 *Total Fe as Fe303. Sample descriptions: All samples are leuco-pegmatite with varying modal proportions of quartz, albite, microcline and spodu- mene, except MZ 56 which has no visible spodumene and MZ 36 which is SQI. Petalite analysis is from SW Finland, in Deer et al. (1974). tional variation throughout the seven grains is apparent. Significantly, Fe203 contents of the grains range from 0.3% to 0.5%, typical of spo- dumenes in general (Deer et al., 1974), confirm- ing the mineral separate XRF data. Fe203 values from twelve microprobe analyses of pale pink spodumene grains range from 0.14 to 0.32%, with a mean of 0.28%. These samples have concom- mitently higher MnO values which range from 0.22 to 0.33% (mean=0.28%). The low Fe and higher Mn content of these spodumene grains probably accounts for their pink colour. Anal- yses of the plagioclase and K-feldspars showed them to be almost pure albite and microcline re- spectively (Table 5). Micas from the two sam- ples are within the range of typical muscovites (e.g., Newman and Brown, 1987).
174 R.J. Thomas et al. / Ore Geology Revwws 9 (1994) 161-182 Table 4 Major element analyses of spodumene separates from the Highbury Pegmatite; analyses of two Namaqualand spodunienes are shown for comparison Wt% TC 3 MZ 36A MZ 36B NS ,~AB218,': SiO2 65.15 63.75 63.86 59.39 :440 TiO2 0.10 0.11 0.11 0. !t) ~,!!3 AI203 25.32 26.48 26.83 26.93 ' I(; Fe203* 0.35 0.39 0.39 (I.5~ i.~!2 MnO 0.26 0 12 0.14 0.1; i.!o MgO 0.21 0.18 0.17 0.28 } if) CaO 0.03 0.03 0.04 0. t i ~.()2 Na20 0.25 0.25 0.27 2.08 ~.2t) K20 0.09 0~08 0.10 15I) ~~2 Li20 6.50 7.06 7.03 5.49 "62 P205 0.30 0.02 0.02 0.07. ;.00 H20- 0.34 t).09 0.18 1 3; ).48 H20 + 0.84 0,3~ 0.44 2.3' (>~ CO2 0.18 0.13 0.17 0.18 S <0.01 -00i <0.01 0.01 Total 99.92 99.00 99.75 100.52 ~.).2i~ *Total Fe as Fe203. Sample descriptions: TC 3: pale yellowish-green spodumene separate (pegmatite outcrop); MZ 36A: pale yellow spodumenc separate, - 1000/+ 500/.t fraction (borehole core ); MZ 36B: pale yellow spodumene separate, - 500/+ 75 ~tfraction (borehote core); NS: large spodumene crystal, Noumas No. 1 pegmatite, Namaqualand; AAB2186: Spodumene, Durabees (Norrabees i, Namaqualand (Bischoffand Bischoff, 1988 ). 7. Petrogenesis Spodumene is one of the diagnostic minerals of complex Li- pegmatites, usually occurring in well-defined zones within intrusive bodies, typi- cally in association with other Li-phases such as petalite, eucryptite, lepidolite, amblygonite, etc. Stewart (1978 ) has shown that most Li-pegrna- tires have comparable Li-contents ( --, 1.5% Li20 ) and that the presence and distribution of spodu- mene (LiAISi206), petalite (LiAISiaOm) and eucryptite (LiAISiO4) is not controlled by com- positional differences, but rather by varying P/T conditions during crystallization. In the system albite-quartz-eucryptite-H20, petatite will crys- tallise at high temperatures and low pressures, whereas spodumene is the stable phase at lower temperatures and higher pressures, according to the reaction: l~talite-,spodumene + 2 quartz. Eu- cryptite is the stable phase at low temperatures and pressures and is commonly found as a low- temperature alteration product of spodumene. Further experimental work on the system eu- cryptite-silica-H20 by London (1984) generally confirmed Stewart's findings and provided a pe- trogenetic grid for Li-pegmatites at low to mod- erate pressures (0.5 to 6.0 kbar) at temperatures between 350 and 950°C. Spodumene usually forms large to extremely large, discrete crystals up to several metres in length or, less commonly, spodumene-quartz in- tergrowths. However, SQI is a relatively com- mon feature of Li-pegmatites, having been de- scribed, for example, from Zimbabwe (Cooper, 1964; Grubb, 1973), Dakota (Norton, 1982) and the Tanco pegmatite (Burt et al., 1988). However, SQI appears to occur in two distinct morphological habits, possibly depending upon the P- T conditions of formation: ( 1) Coarse-grained, graphic spodumene-quartz intergrowths: These textures have been recorded from Zimbabwean and South African pegma- tires, where they have been interpreted as the crystallization products of eutectic melts (e.g., Grubb, 1973; Bischoff and Bischoff, 1988). However, Stewart ( 1978 ) demonstrated that in
R.J. Thomas et al. / Ore Geology Reviews 9 (1994) 161 - 182 175 Table 5 Microprobe analyses of selected spodumene, feldspar and mica grains from the Highbury Pegmatite; note that the low totals reflect the 6-7% Li20 present in the spodumene MZ 28 (Spodumene) MZ 36 (Spodumene) Wt% A B C D A B C TC 2 (Spodumene) Mean* SiO2 65.44 65.22 65.34 65.75 65.21 65.14 65.22 65.82 A1203 28.47 28.26 28.25 28.35 28.50 28.62 28.49 28.73 Fe203 0.52 0.55 0.54 0.47 0.33 0.42 0.43 0.26 MnO 0.12 0.10 0.13 0.16 0.11 0.11 0.11 0.28 MgO nd nd nd nd 0.04 nd nd Na20 0.10 0.10 0.12 0.12 0.12 0.14 0.12 0.15 Total 94.65 94.25 94.49 94.85 94.43 94.45 94.37 95.24 MZ 28 MZ 36 Wt% Microcline Mica Albite Mica SiO2 63.76 45.88 68.27 45.71 TiO2 nd nd nd 0.11 A1203 19.63 39.48 20.48 34.35 Fe203 0.06 0.81 nd 3.62 MnO nd 0.04 nd 0.11 MgO nd 0.10 nd 0.98 CaO nd nd 0.17 nd Na20 0.65 0.38 11.02 0.34 K20 15.47 9.64 0.11 10.25 Total 99.57 96.33 100.05 95.47 Or 94.0 0.7 Ab 6.0 98.5 An - 0.8 N.B. nd= <0.04% (detection limit). Sample descriptions: MZ 28 = yellowish green spodumene; MZ 3628 = pale yellowish green spodumene; TC 2 = pink spodumene. *Mean of 12 analyses. N.B. Mica totals are low because they exclude Li20, H20 , F, etc. the system NaAISi3Os-SiO2-LiA1SiO4-H20, the eutectic growth of quartz and spodumene does not occur at magmatic temperatures in typical zoned complex pegmatites. Under magmatic conditions feldspar-quartz-spodumene assem- blages crystallise, and quartz-spodumene assem- blages will only form after the cessation of mag- matic crystallization by some process, such as sudden pressure loss (possibly caused by wall- rock fracturing), causing super-saturation in Li, leading to the precipitation of a spodumene- quartz eutectoid. Petalite is typically absent from such pegmatites, though late replacement of spo- dumene by eucryptite is found (e.g., Bischoffand Bischoff, 1988 ). (2) Fine-grainedsymplecticspodumene-quartz intergrowths:Cooper (1964), noting the similar LizO, SiOe and A1203 contents of acicular spo- dumene-quartz intergrowths and petalite from Bikita, suggested that both crystallized from the same liquid phase "under slightly different con- ditions". Subsequently, Norton (1982) has sug- gested from geochemical and textural data that fine-grained spodumene-quartz intergrowths in the pegmatites of the Black Hills, Dakota, repre- sent post-crystallization breakdown products of
176 R.J. Thomas et al. / Ore Geology Reviews 9 (1994) 161-182 originally magmatic petalite. In the Black Hills intergrowths, spodumene occurs as acicular crystals, typically only ~ 5 mm in length, inter- grown with quartz. Norton ( 1982 ) noted that the normative compositionsof SQI from Dakota plot in the petalite field on a feldspar-quartz-eucryp- tite diagram and show a close correspondence to petalite-albite-quartz rocks. He thus suggested that the SQI resulted from the breakdown of pri- mary petalite upon cooling. In support of this, pegmatites with fine-grained SQI of this type often contain petalite in other zones and petalite pseudomorphs or relicts are often present in the SQI (e.g. Bikita- Cooper, 1964; Tanco -- Burt et al., 1988). Similarly, London and Burr (1982a) ascribed coarse-grained symplectic quartz-spodumene intergrowths from the Tanco Pegmatite in Manitoba to the isochemical break- down of primary petalite. The spodumene-petal- ite inversion has important economic implica- tions for the glass ceramics industry where a low- iron, high lithia product is required. Although petalite has a lower Li-content than spodumene (theoretical maxima of ~ 4.9% and ~ 8.0%, re- spectively), it cannot accommodate as much iron in its lattice (~ 0.05% versus up to 2% in spo- dumene). The conversion of petalite to spodu- mene + quartz thus provides an efficient natural two-fold concentration of lithium, without the problems of high-Fe content that accompanies the crystallization of primary spodumene. The composition of SQI from the Highbury Pegmatite is almost identical to that of petalite (~78% SiO2, ~ 17% A1203, ~4.5% Li20, and little else, see Table 3). In addition, the symplec- tic-fibrous character of the SQI is comparable with those described from Tanco. Further field evidence in support of this model is provided by the radial cracks that have been observed to lo- cally surround SQI crysts. Clearly then, the phys- ico-chemical characteristics of the SQI in the Highbury Pegmatite points to an origin by the second (petalite transition) type. Furthermore, as the experimental work by Stewart ( 1978 ) has shown that the magmatic eutectic growth of quartz and spodumene is unlikely to occur at the relatively low temperatures associated with the Li-bearing zones of complex pegmatites, those intergrowths which have been described as eu- tectic textures (e.g., Grubb, 1973; Bischoff and Bischoff, 1988) should perhaps be re-inter- preted in the light ofthe petalite transition model. At very high temperatures fl-spodumene or virgilite will crystallise (London, 1984), though the latter has only been recorded in nature from high-temperature, silica-rich volcanic glasses (French et al., 1978). London (1984) con- firmed these results by means of additional ex- perimental data on the system LiAISiO4-SiO2- H20 and erected a complete petrogenetic grid for Li-rich pegmatites (Fig. 10). This grid is partic- ularly useful since, as noted earlier, the stability relations in Li-aluminosilicates are primarily a function of P and T and are largely independent of the compositions and proportions of the other phases present. Thermobarometric estimates from southern Natal, suggest peak P-T condi- tions of > 800 °C at ~ 4-6 kbar during the main regional tectogenesis (e.g., Thomas et al., 1992). Subsequent cooling from peak temperatures ap- pears to have been prolonged and near-isobaric (Grantham et al., 1993). This proposed P-T-t path is supported by the data from the Highbury Pegmatites. Field evidence suggests that the Highbury Pegmatite was emplaced after peak J~ J< n Spod + Qtz Ecr+/ Pet + Qtz Otz/ / B Spod 2;0 300 400 5;0 600 700 800 T (°C) Fig. 10. Suggested P-T crystallizationpath for the Highbury Pegmatite. Inferred paths for the Tanco and Bikita pegma- tites are shown for comparison (after London, 1984):
R.J. Thomas et al. / Ore Geology Reviews 9 (1994) 161-182 177 metamorphic temperatures and pressures. Un- der these conditions, the late-stage Li-enriched magma would crystallise within the field of pe- talite stability. Prolonged near-isobaric cooling would give rise to the breakdown of petalite to the observed symplectic SQI intergrowths. There is no textural evidence in the SQI to suggest that the low P and T reaction of spodumene to eu- cryptite+ quartz has occurred, possibly suggest- ing that pressures remained above 2 kbar at tem- peratures below 300°C. This is particularly relevant in situations of decreased fH20, which has the effect of enhancing the stability field of eucryptite+ quartz. Figure 10 shows a possible P- T crystallization path for the Highbury Pegma- rite, compared to that suggested for two major Li-pegmatites. The discovery of significant quantities of spo- dumene in the Highbury Pegmatite, suggested that they might represent the exposed part of a typical complex, zoned lithium pegmatite. How- ever, several important features militate against such an interpretation. Firstly, whilst a regional lateral zonation may be present throughout the pegmatite-aplite suite, no systematic vertical zonation has been recognized and no other com- plex pegmatite mineral assemblages were found. In addition, the extreme dearth of micas and the absence of typical complex Li-pegmatite para- geneses means that the Highbury Pegmatite can- not easily be fitted into any standard classifica- tion of pegmatitic rocks (e.g., Cameron et al., 1949; Cerny, 1991 ). This is in part due to the almost totally anhydrous mineralogy as a result of the low content of volatile elements, such as H20, F, etc. The anhydrous nature of the High- bury Pegmatite magma is further demonstrated by the lack of alteration to secondary Li-phases, seen commonly in volatile-rich Li-pegmatites (e.g., London and Burt, 1982b). The unusual presence of graphite suggests emplacement un- der conditions of low fo2. Similarly, apart from enrichment in Li20, the Highbury Pegmatite is not enriched in any of the expected trace ele- ments (except Rb) for which analyses are avail- able (e.g., P205, U, Th, Nb, Y). Nevertheless, the high Li20 and Rb content, and the very low K/Rb ratios testify to the highly fractionated na- ture of the Highbury Pegmatites. It has earlier been noted that the Highbury Pegmatites are petrographically similar to the voluminous garnet leucogranites of the Margate Suite, which crop out extensively in southern Natal. This suite is also characterized by anhy- drous mineral assemblages (quartz + feldspar + garnet _+minor biotite) and contains charnock- itic (quartz + feldspar + hypersthene) phases (Thomas et al., 1991). The Margate Granites are similarly characterized by high Rb, variable K/ Na ratios and low volatile contents and were em- placed under conditions of lowfo: (Grantham, 1988). It is thus reasonable to suggest that the Highbury Pegmatites might represent late-stage, volatile-poor differentiates of the Margate leu- cogranites that are enriched in lithia, silica and Rb. As to the source of the Li, Stewart (1978) pointed out that to achieve a LieO content of ~ 1.5%, it would be necessary to perfectly frac- tionate at least 70 times more parental magma with ~ 100 ppm Li than the resulting pegmatite. In the case of the Highbury Pegmatites, there is certainly enough parental garnet leucogranite magma, but it remains obscure as to why all the Li is concentrated in one place, without the cor- responding concentration of other incompatible phases, except Rb. More plausibly, Stewart (1978) suggested that Li was enriched in the source magma by anatectic melting and scaveng- ing of Li-rich metasediments (meta-evapo- rites? ). The presence of Li lowers the minimum melting temperature of the granite system by at least 75 °, allowing for heterogeneous Li-enrich- merits, that can then be further concentrated by fractionation. The immediate host rocks of the Highbury Pegmatites are mafic gneisses and granulites. However, farther south the country rocks to the Margate Suite granites are granulite facies metapelitic and calc-silicate gneisses of the Leisure Bay Formation and marbles and dolo- mites of the Marble Delta Formation. It is not inconceivable that at least some of these rocks might represent originally evaporitic deposits, modern-day examples of which are known to host large, low-grade Li-brines.
178 R.J, Thomas et a~, / Ore Geology Reviews 9 (1994) l O1-182 8. Comparison with other Li-pegmatites 8.1. General Lithium pegmatites are a common feature of many granitoid terranes worldwide, particularly those of Precambrian age and typically in rela- tively high-grade metamorphic environments. However, large bodies with sufficient Li-min- erals to be of economic value are scarce. The most important include Gwalia-Greenbushes (West- ern Australia), Tanco (Manitoba, Canada), Cy- prus Foote-Kings Mountain-Bessemer City (North Carolina, U.S.A) and Bikita (Zim- babwe) (Anstett et al., 1990 ). Economic lithium pegmatites are also known in Africa from Man- ono in Zaire (Guenther and Ngulube, 1992) and from the Karibib District of Namibia (the Pan- African Helikon-Rubikon pegmatite belt; Rich- ards, 1986). Lithium ores have also been ex- ploited from Kibaran pegmatites in Burundi, Rwanda and Uganda (e.g., Fransolet and Tack, 1990). These examples of economic Li-pegma- tites are often complexly zoned, with Li occur- ring in various mineral phases in different, well- defined zones. For example, the Bikita Pegma- tite contains spodumene, petalite, lepidolite, amblygonite, eucryptite, pollucite and bikitaite, complexly distributed through the intermediate and core zones of the pegmatite (Cooper, 1964; Wegener, 1981 ). Similarly, at Tanco, zones rich in spodumene occur in the central zones with in- termediate zones enriched in amblygonite, petal- ite, lepidolite and pollucite (Butt et al., 1988). The Li-minerals are consistently accompanied by other characteristic complex pegmatite minerals including tourmaline, beryl, columbite-tantalite, cassiterite, apatite etc., along with other rare- metal minerals. This association has been class- ified as being characteristic of "LCT'" (Li-Cs- Ta) pegmatites, derived from undepleted crustal lithologies undergoing their first anatectic event (Cerny, 1991 ). In such pegmatites, spodumene crystals of extreme size are common (up to 5 m ). 8.2. Late Kibaran Li-pegmatites of southern Africa A number of Li-pegmatites are known from southern Africa (see Keyser, 1976 ). Whilst some of these are probably Archaean in age (e.g., Bis- choff and Bischoff, 1988 ), many are associated with the ~ 1 Ga Namaqualand Metamorphic Province in the northwestern Cape, i.e. the west- ern continuation of the Natal belt (Fig. 1). The Late Kibaran pegmatites of the Namaquatand Metamorphic Province are largely confined to the "Pegmatite Belt", a narrow, E-W-trending tract of land up to 30 km in width and approximately 450 km in length (Hugo, 1986), (Fig. i I ). Lith- ium minerals, including spodumene, amblygon- ite, tepidolite, zinnwaldite, petalite and triphy.- tite-lithiophillite are found in complex. berylliferous pegmatites, mainly in the western portion of the belt. Individual bodies range from over 2 km long and 70 m wide to small veinlets and consist of coarse-grained quartz and feld- spar along with muscovite, tourmaline and gar- net (Von BackstrSm, 1976). In the western sector of the Pegmatite Belt spodumene has been extracted from the Noumas No 1, Spodumene Kop Nos 1 and 2, Koker- boomrand Nos 1 and 2, Groenhoekies and Nor- rabees Nos 1 and 2 pegmatites (Fig. l I ). Lith- ium-bearing pegmatites also occur in the Groendoorn River gorge (Fig. 11 ). However, the remoteness and inaccessibility of the gorge, cou-~ pled with the small size of the deposits, renders them uneconomic (Nel, 1968). The Noumas pegmatite has been worked intermittently since 1925 for a variety of commodities, including bis- muth minerals, beryl, tantalite-columbite, mus- covite, feldspar and for spodumene since t961 (Nel, 1968). The pegmatite ( 1 km long by l0 to 42 m wide) has been emplaced discordantly into foliated granodiorite (Hugo, 1986). Single spo- dumene crystals up to 1 m in length are charac. teristic of the 1 to 8 m wide intermediate zone and in places account for up to 50% of the min- eral assemblage. Production of spodumene, a to- tal of around 1000 t, from what is the largest known mineralized pegmatite in Namaqualand tapered off during the late 60's and early 70's owing to lowered market prices and increased operating costs. Grades have been calculated at approximately 5 t spodumene per 100 t of peg- matite material (Von Backstr6m, 1976l. An analysis of the Noumas spodumene is given in
R.J. Thomas et al. / Ore Geology Reviews 9 (1994) 161-182 179 ' I.19° NAMIBIA f::::::t::::-~.~.~.- ::::::::::::::::::::::::::::::: UPINGTON .,,~-.....,,,+..,-,, .... .- -, ,.- -', • • ~t • ,, ! .v....X.X.X.X.)X-X-X.X.:.X-X.:-X-X.X..~~ - : : ; ~ . •...............................~. ~ ~ : - - .'.',',',',','.'.'.','"~ ~ ~:.','.','.','.'.'.'.'.'.'.'.'.'."t: ............ ~ . . . . . . . . . . • ..... ".'. - .j ..... •........ ', A _ L z.4":'::':':':':':::':':':':':':':':':':':':'::':':~----:":::,;z ~;".:.:.:.:-:.:.:-:.:.:.:.:..', ~ t....:.:.:.:':-:':.:.:.:.:.:':':':':.:':':':':':':':':':':':~'~...-:-Y..:.:.:.:.:.~ "',:..~.:.:-:-:.:.:-:.:.:.%, ~ ~ . . ' . ' . I _ ~~ •v.v...'.. . . . . .... ,..v.v. ".'.'.....'.'.'.'.'.I EASTERN SECTOR .<,v.v.v.v.v...'-..-..,,, ....... : , ~ : . - . . : ! ...... ~:.:.:.:.:.:.:.....:.:............. t/....~ ~, .:':':~:'.2::2;~':,:;'.~':.,:'~,4.~..-~.~.:.:.:.:.:.:.:.:.:.:.:.:.:.:.S ",4":':':':':':':':':':':':':':':':':v. "<.~'o7~o "-~'~ ~ x,'.'.'.'.'.'.'.'.'.'.'.-I 1 NOUMAS No 1 W:'''v "~,'"'""'"'"'""~~'.'.'.'.'.'.'.'.'.'/ ~.'.'.'.'. +u~4 "-'.'.'.'.'.'.', ~. WESTERN SECTOR ",2-.~..~'," 2 SPODUMENE KOP No 1 ~:....:.:...lJ11))........,.....).~ 3 SPODUMENE KOP No 2 v.v.v....v.v.v,v.v. ~:i:~) PEGMATITE BELT --,,~. • LITHIUM DEPOSIT O SPRINGBOK o LITHIUM OCCURRENCE 4 KOKERBOOMRAND No 1 5 KOKERBOOMRAND No 2 6 GROENHOEKIES 7 NORRABEES No 1 8 NORRABEES No 2 9 SWARTBERG 10 STRAUSSHEIM No 1 11 STRAUSSHEIM No 2 12 ANGELIERSPAN 0 50 100 i I I SCALE km '~v.'.'.'.v.'.'.v.v.v 0 "'-~.,, "K"-X'X'X'X'X" 120 )/..x. "~ 4~.,., ,.-.- ,,,:.. ,..~ ~. SOUTH AFRICA 29 ° o 30 ° w Fig. 11. Distribution of lithium mineralizationin the Namaqualand Metamorphic Province. Table 6 A comparison of the main features of the Highbury Pegmatites with typical Li-pegmatites Highbury pegmatites Typical Li-pegmatites Multiple sheets, simple Unzoned Colour index ~ 1 Virtually anhydrous mineralogy (low H20, F, etc. ) Mineralogicallysimple (quartz + albite + microcline+ spodumene) White mica rare Small ( < 40 cm ) spodumene crysts Accessory garnet, graphite, no tourmaline, tantalite, cassiterite, etc. Usually dyke-like, complex Complexly zoned Colour index ~ 5 Hydrous phases common (high H20, F, etc. ) Mineralogicallycomplex--multiple Li-phases Abundant white mica Huge (up to 5 m) spodumene crystals Rare-metal phases characteristic Table 4. Pegmatites in the eastern sector of the Pegmatite Belt are largely devoid of lithium min- erals (Hugo, 1969). The Natal belt, in contrast to Namaqualand, is almost devoid of complex pegmatites. Although mineralogically simple aplitic and pegmatitic rocks are common (e.g., Evans, 1984; Thomas, 1988), spodumene has hitherto not been re- corded from Natal. Clearly the Highbury Peg- matite, with its simple largely anhydrous major mineralogy, unusual accessory phases (gar- net + graphite), absence of associated complex
180 R.J. Thomas et al. / Ore Geology Reviews 9 (1994) 161-182 pegmatite mineral phases and lack of vertical zonation is quite atypical of Li-pegmatites both worldwide and in the Late Kibaran of southern Africa (Table 6) and can be considered to rep- resent a unique form of Li-pegmatite. 9. Economic aspects The potential economic implications of the Highbury Pegmatites cannot be overlooked. Spodumene was last produced in South Africa from the Noumas Pegmatite No 1 (Fig. 11 ). Since then, South Africa has been obliged to im- port all its lithium requirements, either in min- eral form (mainly spodumene) or as a com- pound (lithium oxide, hydroxide or carbonate). From 1988 to 1991 the total value of these im- ports has approached $1 000 000 (South African Minerals Bureau records). Principal uses of the metal in South Africa are in the metallurgical, electronics, petrochemical, plastics and chemi- cal industries, while Fe-poor spodumene is used in glass and ceramics. Clearly, the presence of an economically viable lithium deposit in South Af- rica, particularly in an accessible position near a major industrialized centre, would be of great benefit to the local economy, particularly as worldwide demand for Li is expected to increase (Alexander et al., 1992 ). The results of the beneficiation analysis de- scribed previously (Table 7) confirm the spo- dumene: quartz ratio in SQI of approximately 60: 40 (sample MZ 36 ). The low Li20 values of the float ( < 0.21%) show that this simplebene- ficiation technique is an efficient means for be- neficiating spodumene from the Highbury Peg- matite. The results demonstrate further that this efficiency is not significantly enhanced by crush- ing and sieving finer than - 1000/+ 500/~. The Fe-content of the greenish-yellow spodumene ( ~ 0.4% FezO3) is higher than the pate pink va- rieties (Table 5), which contain ~0.2% Fe203. Iron content is an important factor in the glass and ceramics industries, so a detailed systematic geochemical study over the entire pegmatite field is necessary to establish the full range of varia- bility of the Fe-content in the spodumene, with the pink varieties constituting the most attrac- tive exploration targets. The size of the spodumene reserves is not proven, though outcrops of more than 35% SQI have been recorded intermittently over a t km strike length. The amount of potential structural disruption (by pinch-and-swell) has not been precisely ascertained, though good correlation with the Mzumbe Borehole and natural outcrops suggests that bodies may be largely laterally con- tinuous and that limited later faulting has oc- curred. The down-dip extent of the mineralized zone cannot be calculated with the present data and clearly a future drilling programme would be necessary to establish reliable estimates of ore reserves. Nevertheless, from that which is known, the Highbury Pegmatite appears to be a much larger body than the lithium pegmatites of Na- Table 7 Results of the spodumene beneficiation experiment with Li20 analyses Sample Sieve fraction Recovery Li20 Mineralogy (/t) ( wt% ) (wt%) TC 3C - 1000/ + 500 ,u 34.7 (sink) 6.50 TC 3F - 1000/+ 500/1 65.3 (float) 0.17 TC 3 - - 2.03 MZ 36A - 1000/+500# 61.1 (sink) 7.06 MZ 36C - 1000/+500# 38.9 (float) 0.21 MZ 36B - 500/+ 75 # 58.5 (sink) 7.03 Z 36D - 500/+ 75 # 41.5 (float) 0.15 MZ 36 - 4.59 Spodumene ( +garnet Quartz, feldspar, graphite Whole-rock (spodumene pegmatite) Spodumene Quartz, feldspar, graphite Spodumene Quartz, feldspar, graphite Whole rock (SQI)
R.J. Thomas et al. / Ore Geology Reviews 9 (1994) 161-182 181 maqualand and Transvaal and may be compa- rable in reserves to a medium-sized deposit such as Bikita. The easily accessible location of the pegmatites (,,-100 km south of Durban), cou- pled with their shallow-dipping geometry, are additional strongly favourable considerations. Furthermore, the uppermost sills are consis- tently the most spodumene-rich, clearly an ad- vantageous situation from a mining point of view. Finally, it should be noted that not only does the pegmatite have potential as a source of spo- dumene, but the extensive aplitic phases could also make an attractive dimension stone (Bullen et al., 1992 ), and the high albite content of much of the entire sill complex, may render feldspar a useful by-product of any future mining venture. Acknowledgements The analytical work was performed at the Geological Survey laboratories in Pretoria and thanks are due to the following: Mrs. H.C.C. Cloete, Dr. R. Edge, Mr. J.H. Elsenbroek, Mr. M. Lekotoko, Ms. A. Peense and Mr. G. Trojak. The authors would like to thank Mr K. Wilkinson for access to the pegmatite outcrops and for permit- ting the drilling operation on his farm in 1990. Thanks are due to Ben Peterse and Koos Venter of the Dep. Water Affairs in Howick for super- vising the drilling and to Ms. D. Breytenbach of the Minerals Bureau, Mr. P. van Eck of SAAR- CHEM and Mr. P. Gauch6, owner of Blesberg Mika (Pta) Ltd., for information about lithium production, imports and uses in South Africa. 10. Conclusions From the petrological study, it is suggested that the Highbury Pegmatite represents a volatile- poor, late-stage differentiate of the garnet leuco- granite phase of the largely anhydrous Margate Granite Suite. The source of the Li may be re- lated to anatectic melting of Li-enriched meta- sediments of the Margate Terrane. The pegma- tires crystallized primary petalite that isochemically reverted to symplectic spodu- mene + quartz intergrowths (SQI) on near-iso- baric cooling. Low-temperature replacement by eucryptite has not been observed, though late al- bitization is locally apparent. This is consistent with the proposed P-T evolution of the southern part of the Natal belt, which has been modelled in terms of near-isobaric cooling from high tem- peratures at moderate pressures (Grantham et al., 1993). From an economic point of view it has been shown that the spodumene can be efficiently be- neficiated from SQI by flotation, to give a prod- uct with > 7% LIE0. In addition to low-Fe pink spodumene, which may be pure enough to sat- isfy the rigorous demands of the glass and ce- ramics industries, the Highbury Pegmatites could become an important source of Li-salts, with di- mension stone and feldspar as possible by- products. References Alexander, J., McCracken, D.J. and Haigh, M., 1992. Lith- ium. Met. Miner. Annu. Rev., Min. J., London, pp. 88- 89. Anstett, T.F., Krauss, U.H., Ober, J.A. and Schmidt, H.W., 1990. International strategic minerals inventory sum- mary report -- Lithium. U.S. Geol. Surv., Denver, Inf. Circ. 930-1,28 pp. Bischoff,I. and Bischoff,A.A., 1988.A lithium-bearing peg- matite from the VredefortDome. S.Afr.J. Geol.,91: 550- 552. Bullen, W.D., Thomas, R.J. and Scogings,A.J., 1992. The geologicalsettingofindustrial mineral deposits in the Na- tal Metamorphic Province. S. Afr.J. Geol., 95: 51-59. Burt,R.O.,Hemming,J. Simard,R. and Vanstone,P.J., 1988. Tanco-- a newname in lowiron spodumene.Ind. Miner., (Jan.): 53-59. Cameron, E.N., Jahns, R.H., McNair, A.H. and Page, L.R. 1949.Internal structureofgraniticpegrnatites.Econ.Geol. Monogr. 2. Cerny,P., 1991.Fertile granitesofPrecambrian rare-element pegmatite fields: is geochemistry controlled by tectonic setting or source lithologies?Precambrian Res., 51: 429- 468. Cooper,D.G., 1964.The geologyofBikitapegmatite:In: S.H. Haughton (Editor), The Geologyof some Ore Deposits in Southern Africa.Geol. Soc.S.Afr., 2: 441-461. Deer, W.A.,Howie,R.A.and Zussman,J., 1974.RockForm- ing Minerals, 2: Chain Silicates. Longman, New York, N.Y., 379 pp. E1Bouseily,A.M. and E1Sokkary,A.A., 1975.The relation- shipbetweenRb-Baand Sr in graniticrocks.Chem.Geol., 16:207-219.
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