Science of Minerals South African Journal of Science 95, September 1999 381

to classical placers set in fluvial, marine Characteristics of in and aeolian environments. The heavy mineral deposits include gamet, zircon, placer deposits along the pyroxene, and other opaques as major constituents with amphiboles, west coast of South Africa and kyanite present in minor or trace amounts. Exploration for heavy minerals during the 1980s resulted in the discovery of several deposits of which c. Philander, A. Rozendaala' and R.J. de Meljerb one, the Namakwa deposit at Graauwduinen (Fig. I), proved economi­ cally viable. Namakwa Sands has re­ ining along the west coast of South try and radiometric signature support this sources of an estimated 500 Mt at a grade Africa is dominated by the exploita­ observation. This variation depends on the of 10 % total heavy minerals and mines Mtion of onshore and offshore dia­ composition of the source rock and stage 10 Mt per annum. Today, it is a significant of sedimentological evol uti on of the sands, mond deposits. The relatively recent producer of high quality zircon, ilmenite discovery of vast resources of heavy miner­ A high degree of heterogeneity of the zircon als in the area has resulted in the establish­ population will adversely affect benefici­ and rutile.! ment of a major related Industry. Today, ation of these minerai deposits. As a result, Zircon, which has the general chemical Namakwa Sands is a 1o-million-ton-per­ its quantification Is required to optimize formula ZrSi041 usually contains a range year operation and significant producer of mineral recovery. of elements in subordinate or trace ilmenite, zircon and rutile by world stan­ amounts substituting for Zr and Si in the dards. Heavy minerals are widely distrib­ The discovery of at the begin­ crystal lattice. In most instances these uted along the entire west coast and are ning of the century drew interest to the trace elements are considered impurities mainly concentrated In Mesozoic fluvial, Namaqualand coast and, since then, and decrease the quality of the zircon con­ Cainozoic marine and Recent aeolian un­ consolidated placers. Basement rocks along the west coast of South centrate and therefore its nett worth. In form part of the middle to late Proterozoic Africa has been dominated by the success­ addition, mineral, fluid and gas inclu­ Namaqualand Metamorphic Complex, ful exploitation of onshore and offshore sions contribute to the heterogeneity of Garlep Group and Palaeozoic Cape deposits. However, in recent this mineral. As a result, variability in the Supergroup. Their diverse lithologies are years the emphasis has shifted from ex­ physico-chemical characteristics of zircon the source of the younger heavy-mineral clusive diamond mining and exploration as well as its alteration products have an concentrations. to include exploration for heavy minerals. adverse influence on the recoverability of

. The present study focuses on the charac­

) The coastal plain of the west coast of the mineral in conventional separation

0 teristics of zircon (ZrSiOJ, the mineral with

1 the highest Intrinsic value of the entire South Africa is marked by widespread plants. 0

2 heavy mineral deposits. These are related Although several studies have been

heavy-mineral suite. Both the heavy­ d mineral fraction and zircon concentrates of e t a representative suite of samples along the a d west coast were investigated using analyti­ (

r cal techniques that Included: transmit­ e ted/reflected light microscopy, electron N h s i microprobe mineral analysis, ICP for rare Namibia l b earth element analyses, PIXE for single­ u grain analyses and cathodoluminescence. P

e Their radiometric characteristics were de­ h

t termined using a hyper-pure germanium

y detector. Buffsls b All samples studied comprised a hetero­ Klelnzee \ ~ • Sprlngbok d e geneous population of zircon grains with t frtJnQl6S River n diverse physlco-chemlcal properties. This a

r was expressed by large differences in g

colour and trace element chemistry of Hondeklip B e c single grains. The percentage of grains n

e hosting inclusions, such as ilmenite, mag­ c i netite, , and fluids, varied SOUl RIver l

r for each sample. Zoned, metamict and Graauwdulnen e

d grains with overgrowths and replacement n textures contributed to the diverse charac­ u terlstlca of zircon samples. Concentrations y a of the various grain types differed among w samples and contributed to the unique e t character of each population. Bulk chern is- a G

t "Department of , University of Stellenbosch, e Private Bag X01, Matieland, 7602 South Africa. n i E.mall: [email protected] 50 100 150 km

b o "Nuclear Geophysics Division, Kernfysisch Versneller i a

S Instituut, Rijksuniversiteit Gronlngen, Zemikelaan 25, 9747 AA Groningen, The Netherlands. Melkbosch81rand y

b E·mall: [email protected]

d "Author for correspondence. E-mail: [email protected] Fig. 1. Locality map of the study area along the west coast of South Africa. e c u d o r p e R 382 South African Journal of Science 95, September 1999 Science of Minerals

conducted on the characteristics and min­ minerals. Several marine and aeolian nium scintillator crystal. 15.16 REE analyses eralogy of the heavy-mineral fraction of cycles of sediment reworking have con­ for U, Th, La, Ce, Pr, Nd, Sm, Eu, Gd, Dy, west coast deposits, limited attention was centrated the heavy mineral population.2 Ho, Er and Yb were carried out by devoted to the zircon fraction.1-4 This in­ ICP-AES at the Physics Department, Uni­ vestigation demonstrates the diversity of Methods versity of Stellenbosch. The ICP equip­ zircon concentrates in placer deposits. A A representative suite of samples of the ment was operated at 1.0 kW forward multi-disciplinary approach was fol­ various sedimentological environments power and a plasma gas flow rate of lowed using a variety of physical and along the coast was collected from 15.0 I min-1and 1.51 min-1for the auxiliary chemical methods, which included trans­ Melkboschstrand in the south to Oranje­ gas. mitted/reflected light microscopy, elec­ mund in the north. The suite consisted of tron microprobe analysis, inductively four basic groups, which included Results coupled plasma-atomic emission spectro­ palaeo-fluvial (Group A), present-day metry (ICP-AES) for rare earth element beach (Group B), aeolian sands (Group C) Physical characteristics (REE) analyses, radiometric analysis, and present-fluvial (Group D). Samples All four sample groups contained zir­ particle-induced X-ray emission (PIXE) were first washed in de-ionized water cons that varied in size from 75-180 J.Lm; for single-grain analyses and cathodo­ and the heavy mineral fraction separated larger grains of up to 250 J.Lm occurred luminescence spectroscopy. from the 63-250-J.Lm sieve fraction in in Group A. Their colour was extremely bromoform (relative 2.90). A variable. Geological seHlng relatively uncontaminated zircon fraction Although the colourless to pink variety The west coast of southern Africa be­ was obtained using a Frantz isodynamic dominated, some displayed shades of tween Saldanha Bay and Oranjemund is separator with a side slope of 25° and yellow to brown and also orange to rocky and generally straight with a few forward tilt of 15°. A series of settings purple. Some grains were frosted, a local embayments and prominent head­ ultimately exceeding 1.5 A was used to feature caused by abrasion and etching lands (Fig. 1). It is backed by the 'Great remove the magnetically susceptible frac­ during fluvial and marine transport. Escarpment,' a topographical feature that tion and produced a zircon-rutile concen­ Metamict in particular showed a runs for 100 km parallel to the coast and a trate. Rutile and other constituents were range of colours from light grey to yellow 3-6-km-wide coastal plain that rises removed by subsequent panning. Finally, and brown. The zircons of Group A were gently to a maximum height of 100 m.5 single grains were handpicked under a mainly light pink to colourless with This semiarid, Namaqualand coastal binocular microscope to obtain an un­ yellow and metamict varieties present in plain is drained by numerous , of mixed zircon fraction. This was a source of minor to trace amounts. Colourless zir­ which only the seasonal Orange, and samples for polarized-light microscopy, cons dominated the populations of

. Krom, a tributary of the Olifants, cross the microprobe analyses, PIXE analyses, Group B (present-day beach) and Group )

0 escarpment. SEM-EDS studies and cathodolumi­ C (aeolian). By contrast, the zircons from 1

0 The regional geology of the Namaqua­ nescence (CL). Another fraction was pow­ the riverine environment (Group D) 2 land coast consists of Cainozoic to Recent dered, treated with 8 N HCl and dissolved showed a wide variety of colours with d e t sediments deposited mainly on Precam­ for REE determination. Radiometric yellow to yellow-brown dominating. a brian basement.6-9 The latter includes Fielding17 suggested that strong colours d analyses were performed on the total ( diverse lithologies from the mid-Protero­ concentrate. Monazite was removed by are associated with the presence of U and r e zoic Namaqualand Metamorphic Com­ diluted HC!. supports the conclusions of Matumura h s 18 i plex, late-Proterozoic Gariep Group and Selected carbon-coated grains were and Koga that colour centres are related l b Palaeozoic Cape Supergroup. Early examined by combined SEM-EDS to to Zr+ produced by radiation-induced re­ u

P Miocene, clay-filled fluvial channels duction of Zr4+.

semi-quantify geochemistry and verify e form along the coast and are locally asso­ optical observations. The SEM also pro­ Most zircons appeared as rounded to h t ciated with high concentrations of vided a means of studying texture within spherical grains more numerous than y 10 12 b

diamonds. - Pleistocene to Pliocene single grains. Back-scattered electron idiomorphic crystals. Distinctly zoned d

e diamondiferous marine deposits overlie (BSE) images allowed location of distinct grains with complex internal structures t

n the coastal channels and occur as a num­ chemical zones within grains that other­ were also present. Group D zircons were a

r ber of wave-cut, raised marine terraces wise displayed a homogeneous optical mainly distinctive euhedral crystals com­ g consisting of basal gravels, overlain by a character. Luminescent images were prising a variety of crystal forms and e c succession of marine and aeolian acquired using an Oxford Instruments habit, whereas Groups A, Band C con­ n e sands. 13,14 c MonoCL system attached to a Leitz tained more rounded grains, with poorly i l

Heavy minerals, defined as the mineral Analytical S440 SEM with an electron­ crystalline faces, Irregularly shaped, an­ r e fraction of sands with a density >2.9, are beam energy of 15 kV and beam current gular fragments were more common in d n concentrated in semi-consolidated sands of 2.1 nA. Zircons were also studied in a Group D. u of palaeo- and recent strandlines and Cameca Camebax electron microprobe Cracks radiating from the centre had y a overlying dunefields. The combined microanalyser at the Department of developed in many grains and were a w e effects of littoral drift, wave action and GeochemiStry, University of Cape Town. typical feature of metamict zircons, par­ t a sections of J-shaped bays controlled the Accelerating voltage was 15 kV with a ticularly common in Group D. Speer19 G suggested that this feature arises from the t anomalous local concentration of heavy beam current of 40 nA measured at the e minerals along the coast. In some areas, Faraday cup, Radiometric analysis was radioactivity of Th and U present as sub­ n i

b these minerals constitute up to 90 %of the performed at the Kernfysisch Versneller stitution elements in the zircon crystal a

S total in these sands. High-grade meta­ Instituut, Groningen, the Netherlands. lattice. Opaque spots are believed to form

y morphosed basement rocks are consid­ The equipment uses a high-sensitivity when atoms are forced from the crystal by b radioactive emissions from U and Th,

d ered the most important source of these gamma-ray detector using a pure germa- e c u d o r p e R Science of Minerals South African Joumal of Science 95, September 1999 383

leaving vacant positions in the crystal lattice that subsequently leads to crystal shattering. Inclusions were common, comprising spherical and tabular gaslfluid types, transparent crystals, opaque phases and their combinations. Tabular inclusions were often elongated parallel to crystallo­ graphic axes whereas others showed no preferred orientation. 50lid inclusions confirmed by ED5 included ilmenite, rutile, biotite, muscovite, titanite, apatite, monazite, xenotime, REE-silicate (alla­ nite), AI-silicate (sillimanite), feldspar, quartz and several unidentified phases. Inclusions of small, euhedral zircon crys­ tals, often orientated parallel to the c-axis, were noted and confirmed by ED5. The larger zircons hosted the most inclusions, of which Ilmenite formeq the largest and most common, closely ~ollowed by apatite.

Cathodoluminescence Cathodoluminescence displayed by zir­ cons can be primarily attributed to transi­ tions of D)?+ ions, defects in the zircon crystal lattice (specifically those localized

on 5i04 tetrahedra), and also by the presence of small amounts of Nd, Mn, Ho, Tm, Yb and LU.20-23 Other elements such .

) as Fe, Hf, Y, P and U act as quenchers and 0 suppress luminescence.24.25 Thus, the rela­ 1

0 tive intensity of light emission depends 2

d on the quantity and types of activators e 26 t and quenchers. a d AIthough most zircons appear struc­ (

r tureless in plane-polarized or reflected e Fig. 2. Cathodoluminescence images of selected west coast zircons (see text for description of h light, CL spectroscopy revealed complex types). s i l internal structures (Fig. 2). A variety of b

u types were identified in the experimental prints the surrounding oscillatory phases of growth. A bright rim over­ P sample: zones. Occasionally the different os­ growth was present. e h I. Zircon that was either homoge­ cillatory zones replaced each other. A The zircons displayed complex CL spec­ t

y neously weak or brightly luminescent thin, very luminescent rim over­ tra demonstrating their variable trace b and sometimes passed into a periph­ growth was observed. element chemistry and protracted evolu­ d e eral overgrowth with contrasting V. Grains similar to type IV but in addi­ tion. Few grains displayed similar charac­ t n luminescence. tion contained secondary growth teristics across the entire population a r II. Grains that showed a patchy distribu­ zones with differing luminescence sampled. g

e tion of nonuniform luminescence that surrounded and replaced the os­ c

n throughout the crystal. A thin, bright cillatory zones from the outside of the Geochemistry e c overgrowth marked the rim. crystal. A well-defined bright over­ The chemical formula of zircon is i l

r Ill. Grains that displayed narrowly growth marked the rim. The cores Zr5i04o' but a range of trace elements can e and growth zones were marked by a be incorporated in the crystal lattice d spaced, oscillatory growth zones. n More than one region of such growth well-preserved resorption surface. through coupled substitution. Zr4+ is u commonly replaced by Hf4+, U4+, Th4+, y was commonly present and revealed VI. Grains with a prominent core, sur­

a 3 s by a difference in luminescence. The rounded by one or more concentric yJ+, REE + (La -7 Lu), Nbs+, Ta +, Ti4+, Pb4+, w 2 3 2 2 4 e Pb +, Fe +, Fe +, Ca +, Na+ and K+ and 5i + t overall luminescence of the different growth zones of contrasting lumines­ a regions decreased towards the rim. cence and, if present, a brighter rim. by AI3+, Ps+ and 56+ .19 It is also possible that G

t Cores were rarely observed and rims These zircons had a distinct boundary these trace elements are present as inclu­ e

n were marked by a bright overgrowth. between the core and subsequent sions of separate mineral phases. i b Cracks radiating from the core indi­ overgrowth; each region gave a Microprobe analyses were performed a

S cated metamict zircons. distinctive CL emission. on a representative suite of zircons for 5i, y rv: Zircon similar to type III, but a well­ YD. Zircon similar to type VI but the core Zr, HE, AI, Fe, K, Ca, Y, P, Th and U. The re­ b

d defined core was present that over- replaced the surrounding brighter sults are presented in Table 1 as averages e c u d o r p e R 384 South African Journal of Science 95, September 1999 Science of Minerals

Table 1. Average chemistry of zircons in the various groups.

Si Zr Hf Y P Th U Ca Mg Fe K Total

Group A (n = 33) x 32.35 65.06 1.30 0.15 0.04 0.03 0.01 0.01 0.01 0.02 0.01 99.01 s.d. 0.48 0.95 0.13 0.12 0.04 0.03 0.01 0.01 0.01 0.01 0.01 0.90 Group B (n = 54) x 32.39 65.78 1.29 0.04 0.02 0.01 0.00 0.00 0.00 0.03 0.01 99.59 s.d. 0.15 0.61 0.05 0.01 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.73 Group C (n = 55) x 31 .55 64.96 1.90 0.12 0.31 0.02 0.01 0.01 0.01 0.03 0.01 99.11 s.d. 1.36 2.09 0.35 0.28 0.71 0.01 0.01 0.01 0.01 0.01 0.01 1.94 Group D (n = 33) x 32.07 66.36 1.51 0.21 0.08 0.00 0.00 0.00 0.00 0.06 0.00 100.35 s.d. 0.43 0.78 0.16 0.12 0.06 0.00 0.00 0.00 0.00 0.02 0.00 0.74

Detection limit = ±0.05 wt %.

Table 2. REE data for zircons In Groups A, Band C.

Sample La Ce Pr Nd Sm Eu Gd Dy Ho Er Yb mEE

A1 504 833 74 179 26 3 46 116 40 165 223 2210 A2 200 345 35 111 23 4 58 132 47 170 215 1340 A3 280 429 41 120 20 4 41 124 41 173 235 1506 B1 62 106 7 37 13 3 35 106 35 144 263 811 B2 36 60 3 19 13 5 70 217 68 288 328 1106 B3 43 75 3 21 13 5 70 216 67 278 321 1110 B4 69 108 4 28 16 4 68 229 70 304 328 1229 C1 34 58 6 20 11 3 37 110 40 161 316 796 C2 35 58 6 20 12 4 39 128 43 180 334 858 C3 48 82 10 36 21 7 59 167 52 210 380 1072 Normalized data

Sample La Ce Pr Nd Sm Eu Gd Dy Ho Er Yb La/Yb

. A1 2059 1306 767 378 171 223 458 705 995 1351 1.25 ) 60

0 A2 818 541 363 234 149 74 286 520 829 1023 1300 0.53 1

0 A3 1144 672 421 253 130 67 199 486 729 1044 1421 0.68 2 B1 252 166 68 78 84 59 171 416 615 868 1595 0.18 d e B2 147 95 27 40 82 82 343 854 1200 1733 1986 0.06 t a B3 176 117 29 44 86 80 344 848 1174 1674 1943 0.08 d

( B4 282 44 102 1984 169 60 76 335 902 1235 1833 0.10 r

e C1 137 90 62 43 73 59 181 433 699 972 1912 0.10 h 142 s C2 90 64 43 80 65 193 503 751 1082 2020 0.09 i l C3 195 128 99 77 134 126 291 655 919 1266 2301 0.12 b u P

e 3.0 h t for each group. The compositional differ- ° Group A (paIaeor1V&r)

y ences among zircons from the different • " Group B (present-day beach) b • • Group C (aeolian) groups were slight. Those from Groups A d 2.5 <) Group 0 (fluvial) e • t and B had on average 32.35 wt % Si, • • n 65.06 wt Zr and 1.31 wt Hf. Zircons • • • • • • a % % • • r

g from Group Chad Zr values of 64.96 wt % •

2.0 .... e and a relatively elevated Hf content of • .#Ji. c • .& ...... - ... n 1.90 wt %. Group 0 displayed a slightly • R c<)cJ?a • e ;j!. c i lower Hf content. 0" c <) \,~~~ •• l ! 1.5 <) oc~8 ~:J. ti <) r The Hf-Zr relationships among zircons I ° 0 e 0 B c a • ° cP' rg,0 Q>o~ ~... <) g d of the various groups are illustrated in o c ~ g D ($)0 n g D C Bc u Fig. 3. Minor elements include Y, P, Th and 1.0 c c D D y Fe and are not particular to any specific a c

w group. Most of the concentrations were e t very close to the detection limit of the 0.5 a

G instrument. Semi-quantitative SEM data, t

e supported by PIXE analyses, showed that n

i the strongly coloured zircons were rela- 0.0 b 62.0 63.0 64.0 65.0 66.0 67.0 68.0 69.0 70.0

a tively enriched in Fe and AI, particularly S

in Group D. Trace amounts of K, Ca and Zr(wt%) y b Mg were also detected. Within the con- Fig. 3. Binary diagram of hafnium/ relationship of zircons from the various groups along d straints of the electron microprobe, no the west coast. e c u d o r p e R Science of Minerals South African Journal of Science 95, September 1999 385

1~0,------, -0- Group A (palaeoriver) the other groups. -0- Group B (present-

1 The La/Yb ratio is calculated as series were measured radiometrically. • A variety of analytical techniques have 0 2

The results for the different groups are shown that the zircon population in La +Ce+Pr+ Nd+ Sm d

e depicted in Fig. 5, in which the activity of each group is heterogeneous with re­ t Gd+ Dy+ Ho+ Er+ Yb a Th and Bi is shown. Zircons from Group B gard to physical and chemical proper­ d

( The total REE content of Group A zir­ plot in a very tight field that is marked by ties. r e cons ranged from 1340-2210 ppm. The the lowest CB; and Crh values of all the • The zircons can be discriminated by h s chondrite-normalized patterns had a typ­ groups. Group C zircons clustered close size, colour, and morphological features i l

b ical 'birdwing' distribution, symmetrical to Group B, but Groups A and D had such as crystal habit and form. u about Eu. These zircons were character­ distinctly different characteristics. The • Generally, west coast zircons have P

e ized by prominent LREE- and HREE­ latter's elevated radiometric signatures stoichiometric proportions of Zr and Si. h t enriched profiles, well-defined Eu anom­ reflect greater V and Th content. They Minor to trace amoUnts of Hf, Fe, Al, Fe,

y aly and the absence of a Ce anomaly. also had very different REE profiles from Y, P, V, Th and REE substitute for Zr and b

d La/Yb values showed that, except for one e 3~0~------~~~~~r=~ t case, the zircons of Group A were rela­ o Group A (palaeortver) n o Group B (present-day beaCh) a tively enriched in the HREE with respect .. Group (aeolian) r C ~ g • Group 0 ("uvial)

to the LREE. The HREE profiles were no­

e • c tably smooth with a minor change of n e slope at Dy. The LREE pattern resembles a 2500 c i

l steep, straight line. • r Group B zircons had a total REE content e 1- CT ~ d considerably lower than those in Group !E. n u A. Generally, these zircons had a steep

J y HREE and a moderate LREE-enriched 1500 a

w profile, slight Eu anomaly and a negative 8 e t anomaly at Nd. The La/Yb ratios were a o o G much lower than those in Group A. t

e The total REE content of zircons in n ~ ~ ~~ i Group C was slightly lower but compara­ b 2~ ~ 4000 ~ 6000 7~ BOOO 9000 a ble with Group B zircons. It displayed S similar normalized profiles, but instead of Ca. (Bq kg-') y b

a Nd anomaly, a negative anomaly at Pr Fig. 5. Radiometric activity plotot c",against c,,; torGroupA(n=4), B (n= 5), C(n= 5) and 0 (n= 2) d zircons. e was present. La/Yb values were similar c u d o r p e R 386 South African Journal of Science 95, September 1999 Science of Minerals

Si in the crystal lattice. In addition, of recovery. 12. Siesser WG. and Dingle R.VD. (1981). Ter­ mineral inclusions also contribute • With prior knowledge of zircon proper­ tiary sea-level movements around south­ ern Africa. f. Geol. 89, 83-%. small, but significant amounts of impu­ ties in a placer deposit, plant conditions 13. Dingle R. V, Siesser WG. and Newton AR. rities to the composition. can be adjusted to yield maximum (1983). Mesozoic and Tertiary Geology of • CL spectra demonstrated the great com­ recovery of a product required to meet Southern Africa. Balkema, Rotterdam. plexity of single zircons. The several particular specifications. 14. Pether J. (1994). The , palaeon­ types of zoning have significance with • REE and radiometric analyses of zircon tology and stratigraphy of coastal-plain depos­ respect to Hf, U and Th distribution in its at Hondeklip Bay, Namaqualand, South could prove useful in provenance Africa. M.Sc. thesis, University of Cape the crystal lattice. discrimination as they demonstrate Town. • Although major element chemistry in­ differences that can be related to the 15. Greenfield M.B., De Meijer R.J., Put L.W, dicates only subtle differences among nature of the sedimentary source. Wiersma J.F. and Donahue J.F. (1989). the four groups studied, contrasting Monitoring beach sand transport by use of trace element chemistry, REE and radio­ We thank R. 0' Brien for REE analysis and D. radiometric heavy minerals. Nuclear Ge0- physics 3, 231-244. metric characteristics indicate that the Gumeycke for SEM and cathodoluminescence studies. Microprobe analyses were performed 16. De Meijer R.J., Lesscher H.M.E., Schuling west coast zircons from particular geo­ by D. Rickard. R.D. and Elburg M.E. (1990). Estimate of logical environments are distinct. the heavy mineral content in sand and its Group A is associated with a palaeo­ provenance by radiometric methods. Nu­ fluvial environment, Group B with a 1. Palmer G.L. (1994). The discovery and de­ clear Geophysics 4, 455-460. present-day beach, Group C with an lineation of heavy mineral sand orebodies 17. Fielding P.E. (1970). The distribution of at Graauwduinen, Namaqualand, Repub­ , rare earths and colour centres in aeolian placer and Group D with a con­ lic of South Africa. Exploration in Mining a crystal of natural zircon. American Miner­ temporary river. Geology 3, 399-405. alogist 55,428-440. • Samples dominated by strongly col­ 2. Macdonald WG. and Rozendaal A (1995). 18. Matumara O. and Koga H (1%2). On colour oured zircons show high Bi-Th activities The Geelwal Karoo heavy mineral de­ centres in ZrSiO •. J. Phys. Soc. Jap. 17,409. and were enriched in U, Th, Fe, Hf, Y posit: a modem day beach placer. African 19. Speer J.A. (1980). In Orthosilicates, ed. P.H. Journal of Earth Science 21, 187-200. and AI, whereas colourless zircons had Ribbe. Mineral. Soc. Am., Rev. Mineral. 5, 3. Macdonald WG., Rozendaal A and De 67-112. reduced Bi-Th activities and possessed a Meijer R.J. (1997). Radiometriccharacteris­ 20. Mariano AN. (1988). Some further geo­ notably lower trace element concentra­ tics of heavy mineral-deposits along the logic applications of cathodolumi­ tion. west coast of South Africa. Mineralium nescence. In Cathodoluminescence of • Zircons from the Richards Bay Minerals Deposita 32, 371-381. Geologic Materials, pp. 94-123, ed. D.J. Mar­ (RBM) deposit in northern KwaZulu­ 4. De Meijer R.J., Stapel C, Jones D.G., Rob­ shall. Unwin Hyman, Boston. erts P.D., Rozendaal A and Macdonald Natal displayed similar major element 21. Mariano A.N. (1989). Cathodolumi­ WG. (1997). Improved and new uses of nescence emission spectra of rare earth . ) chemistry but had U, Th, Hf, REE and natural radioactivity in mineral explora­ element activators in minerals. In Geo­ 0

1 Bi-Th concentrations several orders of tion and processing. Exploration in Mining chemistry and Mineralogy of Rare Earth Ele­ 0 Geology 6, 105-117. 2 magnitude higher than those from the ments, eds B.R. Lipin and G.A. McKay.

d west coast. RBM zircons are generally 5. Heydorn AE.F. and Tinley K.L. (1980). Es­ Mineral. Soc. Am., Rev. Mineral. 21,339-348. e t colourless to pink, but metamict and tuaries of the Cape: Part I: synopsis of the 22. Yang B., Luff B.J. and Townsend P.D. a Cape coast, natural features, dynamics (1992). Cathodoluminescence of natural d yellow varieties make up a considerable (

and utilization. CSIR Research Report 380, zircons. f. Phys. Condens. Matter 4, 5617- r component. Abundant inclusions with e Pretoria. 5624. h a diverse chemistry are present in 6. De Villiers J. and Sohnge P.G. (1959). Geol­ s 23. Vasconcellos M.A.Z., Chemale L.'I and i l nearly all zircons.27 These differences in­ ogy of the Richtersveld. Mem. Geol. Surv. S. Steele LM. (1996). Zircon zonation: an ex­ b Afr. 48, 1-295. perimental study using electron probe u dicate contrasting provenance terrains.

P 7. Hallam CD. (1964). The geology of the microanalysis, cathodoluminescence • Heterogeneity of the zircon population e coastal diamond deposits of southern spectroscopy and synchrotron X-ray fluo­ h from the various locations and geologi­ t Africa (1959). In The Geology of Some Ore De­ rescence. International Conference on

y cal environments influences its recov­ posits in Southern Africa, ed. S.H. Haugh­ Cathodoluminescence and Related Techniques b ery in conventional mineral separation ton. Geological Society of South Africa, pp. in Geosciences and Geomaterials. d e 671-728. 24. Hanchar J.M. and Miller CF. (1993). Zircon

t plants. Zircons that are relatively en­ n riched in Hf, U, Th and REE together 8. Pether J. (1986). Late Tertiary and early zonation patterns as revealed by cathodo­ a

r Quaternary marine deposits of the Nama­ luminescence and back-scattered electron with those that contain abundant g

qualand coast, Cape Province: new per­ images: implications for interpretation of e ilmenite inclusions have increased elec­ c spectives. S. Afr. f. Sci. 82,464-470. complex crustal histories. Chem. Geol. 110, n tromagnetic and conductive suscepti­ 9. Woodborne M.W (1986). The Seafloor 1-13. e c

i bility compared to a homogeneous, Geology of the Namaqualand Inner Shelf 25. Hanchar J.M. and Rudnick R.L. (1995). Re­ l between White Point and Stompneus Bay vealing hidden structures: the application r purer zircon. Impure zircons will be e (Diamond Concession Area No.4). Rep. of cathodoluminescence and back­ d removed early in the electromagnetic

n Geol. Surv. S. Afr. 1986-0077,1-21 pp. scattered electron imaging to dating zir­ and conductive cycles and conse­ u

10. Carrington AJ. and Kensley B.F. (1969). cons from lower crustal xenoliths. Lithos y quently could amount to a considerable Pleistocene molluscs from the Namaqua­ 36, 289-303. a loss in the separation process. This loss w land coast. Ann. S. Afr. Mus. 52, 189-223. 26. Koschek G. (1993). Origin and significance e t of zircon could adversely affect the 11. Rogers J., Pether J., Moleneux R., Genis G., of the SEM cathodoluminescence from a economic viability of mining opera­ Kilham J.L.V, Cooper G. and Corbett LB. zircon. f. Microscopy 171, 223-232. G (1990). Cenozoic geology and mineral de­ 27. Pietersen K.J. (1993). Richards Bay zircon. t tions. e posits along the west coast of South Africa M.5c. thesis, University of Nal.ll. n • It i is therefore necessary to quantify the and the Sperrgebiet. In Guidebook Geo­ 28. Evensen M.N. (1978). Rare-earth abun­ b zircon populations in both the plant­ a congress '90, Geological Society of South dances in chondritic meteorites. Geochem. S

feed and tailings to have better control Africa, Johannesburg. Cosmochim. Acta 42, 1203. y b d e c u d o r p e R