Unusual spodumene pegmatites from the Late ...

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Ore Geology Reviews 9 (1994) 161-182

Unusual spodumene pegmatites from the Late Kibaran of southern Natal, South Africa R.J. T h o m a s a, D. B i i h m a n n b, W.D. Bullen a, A.J. S c o g i n g s c a n d D. De B r u i n b "Geological Survey, P. 0. Box 900, Pietermaritzburg, 3200 South Africa bGeological Survey, P. Bag X112, Pretoria, 0001 South Africa ¢507 Roper 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 Metamorphic 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 structural 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 yellowish-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 composition 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 / N a 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 low fH2o and low fo2. Consequently, the Lipegmatites 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 increasing 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 T h e p u r e white, sheet-like aplite a n d leuco-

p e g m a t i t e intrusions o f the H i g h b u r y area, s o m e 18 k m n o r t h o f P o r t Shepstone ( N a t a l ) , f o r m an integral p a r t o f the ~ 1 G a N a t a l M e t a m o r p h i c

0169-1368/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI0169-1368(93) E0032-5

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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 reported an abundance of large, rounded, optically continuous "patches" of poikilitic clinopyroxerie, thought to be diopside, on the farm The Corner. However, during routine mineralogical investigations by the senior author, the clinopyroxene 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 economic 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 albite-dominated, medium-grained, extremely leueocratic (Colour Index ~ 1 ), with saccharoidal texture and are best termed "aplite". However, 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 klippen which is bounded at its base by a zone of ductile thrusting. The pegmatitic and aplitic rocks under review here form a number of subconcordant 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 pegmatitic/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 synform, where it locally forms prominent 20-m-high white cliffs, on the farm The Corner, overlooking the Mzumbe River Valley (Fig. 2): The intermittent nature of the outcrops along consistent 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 expected to be elongated along the local S- to SWplunging stretching lineation direction, Samples of the pegmatite were also obtained from a stratigraphic 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 outcrops along a strike length of > 1 kin, centred approximately 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

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known that the pegmatites occur as multiple intrusions, though it was not possible to ascertain their precise number or thickness. The borehole was sited at an elevated location north of the axial 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 borehole. The ~ 2 5 0 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 approximately 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 intrusive. The contact zones tend to be enriched in garnet and graphite, suggesting that these minerals 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 appear 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 mineralization (Table 1 ). This group of sills, which occur from 113.5 m to 143.1 m depth, have a total 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 resuits obtained, are given in Table 1. Using a conservative, optically-determined spodumene: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 spodumene 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

represents >23% modal spodumene, which in turn equates with ~ 2% Li20. The subequal spodumene concentrations in the upper three pegmatite bodies suggests that they probably represent 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, plagioclase, microcline and spodumene are the major mineral species, along with accessory white mica, garnet and graphite. Other samples of coloured spodumene (yellow-green, light brown and pink varieties) were analysed and gave identical spectra. In one sample, minor chlorite was identified and the macroscopic identification of a single grain of beryl was confirmed. Mineral concentrates obtained from TC 3 were also analysed by XRD. The analysis showed the garnet to be a Mnrich 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-element geochemistry Whole-rock samples Major and trace-element analyses 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 analysed 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-

171

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 background and interference corrections from other spectral lines were made, while mass adsorption correction procedures were carried out for matrix 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-rock analyses 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 alumina (14.0-18.8%) reflects either a high feldspar or spodumene content. Trace-element abundances are uniformly low except for Rb ( ~ 500 to > 900 ppm). Rb values rise sympathetically with K20 and K/Rb ratios are extremely low (~20-50). One sample (MZ 34) contains anomalously high V, Cr and Ba. F concentrations 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 E1 Bouseily and E1 Sokkary ( 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 order to ascertain the Li20 content of the spodumene in the pegmatites. Two spodumene-rich

172

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Table 2 Major element analyses of outcrop samples of the Highbury Pegmatite Wt%

TC-1

'[C-2

TC-3

TC-4

T(-5

SiO2 TiO2 A1203 Fe203* MnO MgO CaO Na20 K20 Li20 P205 H20H20+ C02 Total

71.88 0.09 17.03 0.23 0.13 0.17 0.93 7.72 0.95 0.04 0.04 0.06 0.39 0.19 100.51

74.16 0. I 1 15 7 0 0.19 o.12 0.07 0.28 5.79 i,70 1.33 0.05 0.13 0.38 0.07 100,07

75.76 0.09 15.83 0.23 0.06 0.14 0. l 1 3.15 2.08 2.03 0.03 0.09 0.51 O. 13 100.25

74.68 0,10 14.81 0.10 7% LIE0. In addition to low-Fe pink s p o d u m e n e , which m a y be pure enough to satisfy the rigorous d e m a n d s o f the glass and ceramics industries, the Highbury Pegmatites could b e c o m e an i m p o r t a n t source o f Li-salts, with dim e n s i o n stone and feldspar as possible byproducts.

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