The distribution of uranium and thorium in the cape columbine granite from the southwestern cape province, South Africa

Ore Geology Reviews, 5 (1990) 223-246 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

223

The Distribution of Uranium and Thorium in the Cape Columbine Granite from the Southwestern Cape Province, South Africa A.E. SCHOCH and R. SCHEEPERS Department of Geology, University of the Orange Free State, Bloemfontein 9301 (South Africa) Department of Geology, University of Stellenbosch, Stellenbosch 7600 (South Africa) (Received December 6, 1988; revised and accepted September 21, 1989)

Abstract Schoch, A.E. and Scheepers, R., 1990. The distribution of uranium and thorium in the Cape Columbine granite from the southwestern Cape Province, South Africa. Ore Geol. Rev., 5: 223-246. The coastal outcrops of the Cape Columbine granite were mapped radiometrically on a 10 m grid by gamma-spectrometry. Numerous radioelement-enriched anomalies were found, each a few square metres in outcrop. Some of the anomalies, but by no means all, are associated with small bodies of dequartzified granite (episyenite), chloritized or fluoritized granite, evidently testifying to localized hydrothermal activity of considerable diversity. The uranium and thorium in the relatively unaltered main granite are present in a variety of host minerals: (a) rock-forming minerals; (b) accessory minerals, including U- and Th-minerals, (c) secondary minerals associated with hydrothermal activity; and (d) secondary minerals associated with weathering. Thin coatings of cryptocrystalline material in micro- fractures within rock-forming minerals may belong to either of the last-mentioned two groups. Variations in the relatively unaltered granite are described to both primary differences in the amount of accessory minerals and the percentage of uranium hosted by secondary minerals. The uranium is present, in order of decreasing abundance, in xenotime, thorite, along grain boundaries and within biotite and zircon, while the thorium occurs in monazite, thorite, xenotime, apatite and epidote, along grain boundaries and within zircon and biotite. The different types of alteration bodies are associated with wide variations in whole-rock radio-element distributions. Quantitative mineralogical study of one occurrence of fluoritized granite has made it possible to calculate the relative importance of local uranium host minerals: zircon (30% of the uranium), xenotime ( 13 % ), thorite ( 13% ), along grain boundaries (9%) and in biotite (9%) and other rock-forming minerals (4%). The thorium is present in zircon (28% of the thorium), monazite (20%), thorite (14%), biotite (9%), xenotime (2%) and along grain boundaries (2%). The uranium was apparently liberated from primary minerals during alteration and trapped in secondary minerals, causing a major radio-element redistribution. The redistributed uranium and thorium are more readily leachable during weathering, resulting in some variation in surface samples and outcrops. The demonstrated dekametre-scale anomalies are nevertheless considered to be a pristine feature of the rocks.

Introduction The occurrence of uranium in granite is a much discussed subject with a voluminous literature. Neuerberg (1955) described six possible modes of occurrence while Leonova and

0169-1368/90/$03.50

Renne (1964) indicated that half of the uranium and a third of the thorium are present in essential minerals, with little correlation between the amount of the radioelements. Stuckless et al. (1977) described the distribution of uranium in some major and accessory minerals

© 1990 Elsevier Science Publishers B.V.

224

A.E.SCHOCHANDR. SCHEEPERS

in a uraniferous granite, but Tieh and Ledger (1981) demonstrated that there are four distinct modes of occurrence, namely: (a) as essential constituent of certain accessory minerals; (b) along grain boundaries; (c) in microfractures; and (d) in quartz and feldspar grains. Refined fission-track techniques stimulated considerable progress in the study of uranium distribution in geologic materials (Brynard, 1983; Robb and Schoch, 1985; Robb et al., 1986). Cuney and Friedrich (!987) showed that the principal factors which control accessory mineral parageneses in granites are calcium activity, peralkalinity and silica activity. Cathelineau (1987) and Friedrich et al. (1987) pointed out that the primary accessory mineral parageneses in peraluminous granites are commonly considerably modified during subsolidus alteration. Owing to the importance of granite as possible provenance material for major uranium provinces, such as the Witwatersrand basin (Smits, 1984; Robb and Meyer, 1987; Hutchinson and Viljoen, 1988; Drennan, 1988) and the Karoo basin, it is necessary to investigate

the effect of alteration processes and weathering on the uranium distribution in these source rocks. Stuckless and Nkomo (1978) indicated that the uranium loss for surface samples could be as much as 80% and that primary uranium was possibly mobilised even to depths of 400 m. At Granite Mountain, Wyoming, Stuckless et al. (1981) interpreted the variations in hydrothermally affected leucogranite as lead-loss features rather than uranium mobility effects. However, identical distribution patterns in various comparable granites in Wyoming ( Stuckless, 1987) may reflect uranium loss in some cases and primary features in others. A detailed investigation of the radioetement distribution in the Cape Columbine granite was carried out by gamma-spectrometric surveying on a 10-m grid constructed by aid of geological base maps of 1:1000 scale. The gamma-spectrometer used was a Chemtron model Gl12 with a 50 m m thallium-activated NaI-crystal. Potassium values were derived from counts of 4°K at 1.461 MeV, uranium from '~14Bi at 1.764 MeV, and thorium from 2°ST1at 2.615 MeV. Calibration of the energy windows were made every two

Aylva E. Schoch was born in Nelspruit, South Africa. He graduated from the University of Stellenbosch, involved in a study of mylonite and of a granite batcholith. At present he is research professor at the University of the Orange Free State, Bloemfontein. His main fields of interest are the role of volatile components in geological processes (especially in Proterozoic mobile belts) and the diagnostic distribution of trace minerals in common igneous rocks.

Reyno Scheepers was born in Alexandria, South Africa. He graduated from Steltenbosch University and is presently lecturer in Petrology and Mineralogy at the University of Steltenbosch. His main field of interest is the geology, geochemistry and accessory mineral chemistry of granitic rocks.

U - T H IN CAPE COLUMBINE GRANITE

hours by means of a thorium source. Background was determined three times per day and all readings were adapted accordingly. The readings at a few selected grid points were later checked with a Geometrics GR104-A instrument. The radiometric survey was followed by mineral separation, autoradiography and chemical analysis on carefully selected samples. For the mineral separation large samples were crushed by aid of a jaw crusher and roller mill, sieved into suitable mesh fractions by means of a sieve stack, and then treated by means of a Frantz isodinamic magnetic separator and a superpanner. It is the aim of this paper to demonstrate that the distribution of radioelements in the Cape Columbine granite entails anomalous variations on metre to dekametre scale which are both primary and secondary in origin, and to present interpretations in terms of mineralogical variation.

225

field relations of the Cape Columbine granite were also described. The granite occurs as a dyke-like body exposed along the coast (Fig. 1 ), and a separate small pluton to the southwest of the town of Vredenburg. The data presented in this paper deal with the first-mentioned occurrence exclusively. The granite is exposed in a narrow coastal zone as continuous outcrops over a distance of 6.1 km, with a total area of 720,000 m 2 (Fig. 2 ). In an easterly direction, the granite is covered by sand as well as Cenozoic calcarenite of the Langebaan Member of the Bredasdorp Formation (Visser and Schoch, 1973). Numerous xenoliths of coarsely porphyritic Vredenburg adamellite are conspicuous in parts of the Cape Columbine granite. The Cape Columbine granite is medium evengrained with a subalkaline low-calcium char-

Geology The Cape Columbine granite is a younger leucogranite in the Saldanha batholith on the southwestern coast of South Africa (Fig. 1). The Saldanha batholith is probably connected with the adjacent Darling batholith under a thin cover of sediments (Schoch, 1975). Both batholits form part of the Cape Granites, a series of high-level intrusions, intrusive into Precambrian metamorphites of the Malmesbury Group (Potgieter, 1950; Hartnady, 1969; De Bruiyn et al., 1975; Rabie, 1975). The Saldanha batholith is compound and consists of petrochemically related granites and quartz-porphyries with zircon U / P b ages of 600 to 470 Ma {Schoch et al., 1975; Schoch and Burger, 1976), which agrees with R b / S r whole rock ages (Schoch et al., 1977b). The Cape Columbine granite is intrusive into the Vredenburg Adamellite, occupying the northern portion of the Saldanha batholith. In a regional study of the Vredenburg Adamellite (Siegfried, 1981 ), the petrography and

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Fig. 2. Map of the coastal outcrops of Cape Columbine granite, represented by the box in Fig. 1. Localities of localized alteration are shown and identified by three number sequences. The red coloured granite and white granite are not differentiated because they are, apart from colour, chemically and mineralogicallyidentical within observational error limits, thus both relatively unaltered. The box indicates an area shown in greater detail in Fig. 8. {Compare also Fig. 7. )

acter and envelops many small occurrences of aplogranite. Narrow aplite dykes are ubiquitous. Localized hydrothermal alteration of the granite is noticeable owing to colour variations or, in a few instances, the absence of quartz (Fig. 2).

Red feldspar granite An attractive red colouration affects extensive parts of the granite and increases in a southerly direction. Visser and Schoch (1973) ascribed this p h e n o m e n o n to hydrothermal activity which emanated from primary joint planes. Thin veinlets of alkali feldspar, one millimetre wide, are present in the joints under dis-

cussion. The first sign of red colouration in the adjacent granite is the development of red rims on potassium feldspar crystals. In the most evolved examples entire crystals are coloured. In spite of the conspicuous colour, the red Cape Columbine granite is chemically and mineralogically nearly identical to the normal Cape Columbine granite, within observational error limits, and is therefore regarded as unaltered relative to the other colour varieties. Henceforth the two types will be referred to as red unaltered granite and white unaltered granite.

Fluoritized granite Some altered granite is coloured purple to purplish-red owing to thin veinlets and dissem-

U-TH

IN CAPE COLUMBINE

227

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inated crystals of fluorite. Such fluoritized granite occurs in small areas (Figs. 2,8), of a few metres in diameter. No systematic variation in the presence of the fluoritized areas is apparent, but a slight increase towards the south is not excluded.

Chloritized granite Chloritized granite is rarely recognizable without the aid of gamma-spectrometry. Sometimes the associated feldspar exhibits a yellowish hue however, owing to minute veinlets of chlorite. The outcrops of chloritized granite (Fig. 2) are typically only a few metres in diameter.

Dequartzified granite The most dramatic effect of hydrothermal alteration is the dequartzified granite (episyenite; see Fig. 2). The contact zone between episyenite and granite is characterized by a gradual decrease in quartz content, accompanied by an increase in dark minerals. The episyenite differs texturally from the granite by the presence of potash-feldspar phenocrysts with surrounding biotite, hornblende and chlorite, exhibiting a sieve-like texture. All episyenites in the Cape Columbine granite are very localized (metre to dekametre scale).

as determined by petrographic study of thin sections, are summed up in Fig. 3 and will be discussed below in the sequence of fluoritized granite, chloritized granite and episyenite. The diagnostic mineral phase in the fluoritized granite, purple fluorite, is present as subhedral inclusions in plagioclase, microperthite and biotite and also occurs in microfractures. Primary biotite is largely replaced by fluorite with minute inclusions of radioactive opaque phases. The fluoritization was followed or accompanied by the formation of muscovite in the feldspar. Inclusions of tabular euhedral zircon are present in quartz, biotite and feldspar. In the chloritized granite, the chlorite occurs as an alteration product of biotite, and contains numerous veinlets and subhedral inclusions of iron-titanium-rich opaque phases. Veinlike patches of dark-green chlorite without included opaque phases surround the rock-forming minerals. The plagioclase in the chloritized granite

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228

A.E. S C H O C H A N D R S C H E E P E R S

shows intense saussuritization. ~ i d o t e is conspicuous in the altered ptagioclase, though usually very fine-grained, sometimes rimming individual crystals. Metamict zircon grains are commonly included in the chlorite. The characteristic features of the episyenite are the low quartz content and the network texture of fine-grained chlorite surrounding microperthite phenocrysts. Two chlorite types are distinguishable, namely a fine-grained network-type without opaque inclusions and a type with abundant inclusions. The two types differ in radioelement content. The biotite of the episyenite is iron-rich and includes sphene, which

occurs on the cleavage planes, and apatite. Epidote is present as an alteration product of plagioclase and sphene. Calcite is intimately associated with the sphene and epidote. The alteration and crystallization sequence in the episyenite is similar to that of the other alteration types with the difference that orthoclase formed as a secondary mineral. Quartz is represented only by relict grains testifying m the process which removed silica. This process was accompanied by chloritization involving the formation of orthoclase and dissociation of biotite, probably according to the reaction (Chayes, 1955):

'FABLE 1 Chemical analyses of the Cape Columbine granite and associated rocks (LOI = loss on ignition, 81 -S 1 and 81 -$2 are averages calculated from data of Siegfried, 1981 ) Fluoritized granite

81-10

81-39

Chloritized Episyenite granite

81-48

81-49

81-08

81-04

Unaltered Cape Columbine

granite (white)

81-07

81-09

81-40

81-45

81-WA

81-WD

81.,03

76.84 11.67 0.62 0.50 0.07 0.47 3.80 4.83 0.07 0.00 0.02 0.11 0.18

76.90 ll.94 0.36 ().83 0.00 0.54 3.88 4.86 0.05 0.00

76.19 11.97 0A0 {).76 0,(}2 0,52 3.96 4.78 0.05 0.0 i

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Major elements (wt%) SiOe A12Oa Fe2Oa FeO MgO CaO Na20 K20 TiO2 P20~ MnO COe H20TOTAL LOI

75.02 12.26 0.50 0.43 0.32 0.56 3.87 4.57 0.05 0.01 0.02 0.13 0,24

75.01 12.65 1.33 0.08 0.10 0.45 3.98 5.18 0.05 0,00 0,01 0,08 0.28

76.63 11.94 1.03 0.40 0.00 0.55 4.24 4.82 0.05 0.01 0.02 0.14 0.18

78.28 11.42 0.74 0.44 0.00 0.51 4.01 4.40 0.04 0.00 0.02 0.09 0.10

75.55 12.84 0.41 0.53 0.16 0.41 5.58 5.00 0.05 0.01 0.02 0.16 0.16

65.03 18.61 0.19 1.04 0.00 0.70 6.39 6.05 0.07 0.00 0.03 0.12 0.34

65.22 18.39 0.59 0.76 0.02 0.26 5.52 6.80 0.08 0,02 0.02 0.14 0,44

63.19 18.76 1.22 0.97 0.16 0.84 5.58 7.49 0.12 0.01 0.05 0.15 0.43

64.38 18.77 1.59 0.13 0.07 0.37 6.11 7.49 0.13 0.00 0.00 0.15 0.26

O. 16

97.98

99.20

100.05

100,10

100.88

98.57

98.26

98.97

99.45

99.18

99.67

0.67

0.77

0.75

0.52

0.48

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0.75

0.69

0.61

0.42

0.37

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Trace elements (ppm) Rb Sr Nb Zr Y Ba U Th Mo

638 33 68 107 210 17 13 71 0.7

759 6 87 105 150 246 11 115 2.0

647 20 99 102 208 225 25 187 1.0

597 6 45 81 154 199 14 137 2,0

647 2 148 113 138 252 17 65 2.0

645 19 67 158 206 262 8 51 0.7

793 14 94 167 288 219 10 53 0.7

814 35 92 164 331 402 14 56 0.7

903 8 155 166 150 257 9 60 1.0

275 7 48 125 154 0 8 30 3.0

698 13 84 132 185 261 8 56 1,7

698 13 84 132 185 280 8 56 1.7

627 3 56 110 161 273 8 54 ,t.0

U TH IN CAPECOLUMBINEGRANITE

229

biotite + quartz -- chlorite + orthoclase

Geochemical characteristics

The addition of potassium caused the crystallization of new orthoclase as rims on the microperthite crystals. In the case of plagioclase, fine-grained muscovite (sericite) was formed:

Major element analyses of the Cape Columbine granite and associated rocks are given in Table 1. A Phillips 1410 wavelength dispersive X-ray fluorescence spectrometer was used. The analyses were made on fusion discs by means of the technique of Norrish and Hutton (1969) except for Na, which was analyzed on pressed powder pellets. The target radiation for the elements from Na to Ti was CrKc~ and for Mg and Fe, white radiation from W. The trace element data for Rb, Sr, Nb, Zr and Ba (Table 1) were

plagioclase + K = muscovite + Ca The released calcium combined with iron liberated during chloritization of biotite to form epidote rims on plagioclase. Sphene was formed by a similar combination of titanium and calcium.

Table 1

(continued)

Unaltered Cape Columbine granite (red)

81-RA

81-RD

76.94 11.66 0.35 0.72 0.00 0.44 3.53 5.06 0.06 0.01 0.02 0.12 0.23

77.12 11.53 0.36 0.70 0.00 0.46 3.71 4.89 0.06 0.01 0.02 0.12 0.23

76.22 12.39 0.80 0.72 0.43 1.01 3.67 4.67 0.53

99.14 0.54

698 13 84 132 185 304 8 24 1.7

81-$2

81-42

81-4A

Aplogranite

Aplite

81-02

81-43

81-38

Vredenburg adamellite

81-44

81-41

81-01

81-S1

77.30 11.84 0.62 1.07 0.20 0.76 3.54 5.14 0.16 0.02 0.03 0.19 0.12

76.60 12.46 0.35 0.46 0.00 0.48 3.86 5.08 0.03 0.00 0.02 0.18 0.24

76.42 12.48 0.41 0.14 0.07 0.43 4.14 4.72 0.02 0.00 0.01 0.10 0.11

76.06 12.20 0.36 0.41 0.12 0.34 4.71 4.39 0.02 0.00 0.02 0.09 0.18

76.32 12.31 0.60 0.37 0.07 0.40 4.44 4.21 0.03 0.00 0.02 0.80 0.14

62.21 18.59 1.74 0.95 0.59 1.05 4,85 7,61 0,29 0.02 0.05 0.10 0.65

75.22 12.26 0.37 1.15 0.00 0.83 3.35 5.01 0.13 0.03 0.03 0.14 0.34

70.78 13.84 2.99

0.26

73.91 12.55 0.92 0.95 0.18 0.87 3.45 5.39 0.19 0.00 0.03 0.14 0.37

99.21

100.70

98.95

100.99

99.76

99.05

98.92

99.71

98.70

98.86

99.70

0.64

0.31

0.65

0.40

0.51

0.45

0.46

0.44

0.98

0.63

0.26

698 13 84 132 185 241 8 24 1.7

419 41 27 127 89 379 10 51 2.0

224 58 21 130 50 232 3 16 1.7

640 6 29 77 99 258 6 32 5.0

574 3 37 76 123 200 18 38 3.0

964 4 124 154 287 234 9 57

857 7 106 207 274 228 16 69 1.0

616 89 34 216 143 928 10 42 1.0

488 41 25 112 98 305 4 30

1.02 1.92 3.40 4.58 0.38 0.28 0.06 0.08 0.37

189 149 562

230

A.E.SCHOCHANDR. SCHEEPERS

obtained by aid of W-radiation and corrected for matrix absorption effects. The values for U, Th and Mo were obtained from neutron activation analysis. According to the chemical classification of De la Roche et al. (1980), the unaltered granite and episyenite form separated tight clusters in the alkali granite (alkali feldspar granite) and syenite fields, respectively. T h e fluoritized granite plots with the unaltered granite, but the chloritized granite is separated. One sample of altered porphyritic Vredenburg adamellite from a contact with the Cape Columbine granite, plots in the syenite field, together with episyenites derived from the Cape Columbine granite. The different granites and hydrothermalites are effectively clustered by eigenvectors calcuSecond Eiflenvector

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lated from covariance and correlation matrices for the analytical data set. The first eigenvectot, which can be regarded as a best-fit line in multidimensional space, accounts for most of. the variance. For the analyses of the Cape Columbine area the first two eigenvalues comprises 98,98% of the variance, which means that a two-dimensional representation can, in principle, display the real major element~ variation rather than a distorted projection of the multidimensional system. When the first and second eigenvectors are used as abscissa and ordinate, the analyses are effectively separated into clusters of comparable geochemical characteristics (Fig. 4). Although the data set is too small for accurate characterization of the chemical tendencies the clear distinction between granite and syenite is evident. The major variations in the unaltered Cape Columbine granite (Fig. 4} seems to be caused by biotite, while the fluctuations in the Vredenburg granite are possibly trending towards the point representing SiO2. As can be expected, the variations in the fluoritized granite are related to the CaO point. Variations in dark mineral and sodium content for both granite and syenite are indicated by variations along the second eigenvector. Chloritized granite plots farther away from the biotite point than the unaltered granite. The loss of calcium, owing to saussuritization, and of potassium, accompanying chloritization, probably enhances this trend. The eigenvector diagram evidently displays the difference in majorelement geochemistry between the different mappable hydrothermalites, in accordance with their petrographic characteristics. Comparison of the calculated aluminium coordination of chlorite and biotite from the Cape Columbine granite (Table 2, Fig. 5), with those from the St. Sylvestre massif (Turpin, 1984), reveals that the evolution of biotite {unaltered granite) to chlorite (chloritized granite) must have been similar. The alteration of the phyllosilicates of the Cape Columbine granite in-

231

U - T H IN CAPE COLUMBINE GRANITE

TABLE 2 Selected chemical analyses (electron microprobe) of chlorite and partly chloritized biotite from Cape Columbine granite; the structural formulas are based on 22 oxygen atoms (biotite) or 28 oxygen atoms (chlorite)

1 SiO2 A1203 FeO MgO CaO Na20 K20 TiO2

2

20.13 24.40 38.08 2.66 0.10 0.04 0.09 0.13

3

25.50 19.80 39.53 3.45 0.10 0.02 0.21 0.69

23.09 20.17 39.99 3.50 1.13 0.01 0.04 1.18

2.845 1.155

3.728 0.461 1.723 0.011

K Na Ca

0.013 0.009 0.013

AI(VI)/AI(IV)

0.954

0.798

1.0523

1.059

0.764

AIT

3.367

2.604

2.696

2.788

2.241

3.688 0.573 1.448 0.058 0.029 0.004 0.011

0.04

3.791 0.588 1.313 0.101

5.76

0.006 0.002 0.137

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St. Sylvestre chlorite [~

St. Sylvestre muscovite

Fig. 5. Aluminium coordination diagram on which analyses of chlorite and biotite from the Cape Columbine granite (given in Table 2 ) are compared with those of the St. Sylvestre granite, France (according to Turpin, 1984). (A1w=Total aluminium, A I ( V I ) / A I ( I V ) = calculated alum i n i u m coordination ratio ).

4.00

5.78

0.17

2.566 1.434

27.35 17.17 38.10 3.65 0.17 0.04 1.92 1.72

Fe Mg A1 Ti

5.92

2.617 1.382

22.47 20.71 42.87 3.32 0.03 0.01 0.03 0.11

2.563 1.644

,

4.00

5

Si A1

i

4.00

4

4.094 0.565 1.354 0.009 0.004 0.003 0.004

4.00

5.79

0.001

3.029 0.971 3.529 0.603 1.271 0.143 0.272 0.008 (}.020

4.00

5.55

0.30

volves an increase of aluminium, as is the case in the St. Sylvestre massif of the La Crouzille uranium district in France. In the former, however, the increase in the A1 {VI )/A1 { IV) ratio is only slight, which explains why muscovitization is not important in the rocks under discussion, in contrast to the French example. Grouping of the different Cape Columbine rock types is also apparent on an U - T h diagram {Fig. 6). The fluoritized granite has a high radioelement content and is relatively Th-enriched. The late stage fine-grained phases of the Cape Columbine granite are enriched in U relative to Th. The episyenite data points are situated on the best-fit line for the entire data set, indicating simultaneous enrichment of both radioelements during episyenitization in a U: Th ratio of roughly 1:6 while the corresponding

232

A.E. SCHOCHANDR. SCHEEPERS

• Episyenite

. ..fJf.~\

"¢~ Chloritized granite • Fluoritized granite D Unaltered Cape Columbine Gra nite(white~

150

/ i

Unaltered Cape o Columbine Granite(red~

aplogranite

/ /

/

/

/.

~@,'

/

. Th

/ /

• Vredenburg G r a n i t e /

o Aplite and

/

/

/

I00

/

/ID // 50 ///J

't 10

U

~o

30

Fig. 6. Uranium-thorium diagram for the various rock types from Cape Columbine. The values are in ppm. The best-fit line is also shown (r2=correlation coefficient ).

addition required for typical chloritized granite is relatively U-enriched. Radiometric

survey

A detailed radiometric survey was conducted by aid of a portable gamma-spectrometer with a Tl-activated NaI crystal. Readings for K, U and Th were taken on a 10 m grid on outcrops. The low relief of the coastal outcrops provides the ideal 2g-geometry for such a survey. The average spectrometric values out of a total of 3185 readings on the Cape Columbine granite and associated hydrothermal alteration products (+_2s/v~), is 14+5.4 ppm U and 44+8.2 ppm Th. Delayed neutron-activation analysis on selected samples (Table 1) confirmed the validity of the spectrometer read-

ings. In spite of the fact that analyses of' prepared representative powdered samples and analyses on outcrops (gamma-spectrometry) are not directly comparable, primarily owing to the inevitable difference in sample volume, the agreement was invariably better than 70%. Thus, the spectrometric values for uranium and thorium of 14 ppm and 44 ppm compare, respectively, with average neutron activation values (weighted for outcrop surface area) of 8.4 and 51.0. The determined average T h / U ratio (spectrometric data) for the Cape Columbia Granite of 3.1 _+0.6 is somewhat lower than the global 3.56 ratio for granites (Clark, 1982). Compared to the worldwide average values for U and Th, the average Cape Columbine granite could be considered to be an uranium-enriched leucogranite, although other plutons of evolved Cape granite in other batholiths are usually markedly thorium enriched (Theron. 1985; Scheepers and Schoch, 1988). This also becomes apparent when the distribution of parts of the granite with anomalous high values are taken into account (Fig. 7). Small areas with high radioelement concentration as well as large areas of low values are randomly distributed. The distribution of the uranium and thorium anomalies are strikingly nonuniform and also in most instances noncorresponding. The features discussed above are present at all scales as can be illustrated by a large-scale map of one of the anomalous areas (Fig. 8 ). The selective concentration of Th in fluoritized areas of the granite is demonstrated. The distribution of the higher uranium values do not show a particular preference for areas of alteration. Visibly weathered granite has a lower uranium content than usual, but the thorium remains unaffected. In general, the following conclusions can be made: ( 1 ) There is no relationship between the red colouration of the Cape Columbine granite and radioetement distribution. (2) Relatively high concentrations of ura-

U TH IN CAPE COLUMBINE GRANITE

233

Fig. 7. Map of the coastal outcrops of Cape Columbine granite, showing positive uranium and thorium anomalies obtained by detailed radiometric grid mapping by aid of a portable gamma spectrometer. The uranium and thorium anomalies were, respectively, defined by the 15 ppm and 50 ppm contour lines. (Compare Fig, 2.)

nium as well as thorium are associated with fluoritized Cape Columbine granite. The uranium- and thorium values for nine anomalies, representing fluoritized granite, are given in Table 3. The T h / U ratio of 8.3 is much higher than the average for the relatively unaltered granite of 3.1 although the uranium levels are only slightly higher, testifying to a preferential enrichment of thorium. (3) The chloritized granite has higher uranium and thorium values than the unaltered Cape Columbine granite of adjacent areas. One anomaly, however, showed uranium enrichment with no associated thorium increase. The average T h / U ratio of 3.8 (Table 3) is higher than for the average granite, but lower than in the fluoritized type discussed above. Anomalies associated with chloritized granite, thus, rep-

resent relative uranium enrichment. Uranium values higher than background are noticeable in unaltered granite surrounding the chloritized areas. (4) Uranium and thorium values and ratios associated with episyenite are highly erratic. The average uranium value is comparable with unaltered granite although T h / U ratios are highly variable, indicating sporadic enrichment of thorium. Unaltered granite surrounding episyenite typically shows higher uranium values than average granite, but this is not true of the thorium values. (5) Anomalies not associated with alteration are also present. This includes the higher uranium values associated with aplitic dykes and aplogranite, irrespective of the radioelement content of adjacent granite. There are also

234

A.E. SCHOCH AND R, SCHEEPERS

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e-

~ ' ~ o

2-

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0

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+

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~

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.....~.....

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¢..,

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-..5- +

~

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C.

+ T ~ C oCF~~

~g,

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U-TH IN CAPE COLUMBINEGRANITE

235

TABLE 3 Average u r a n i u m a n d t h o r i u m values ( p p m ) of selected anomalies (the n u m b e r sequence of the anomalies (An.) correspond to the numbers given on the map of Fig. 2; s = s t a n d a r d deviation; r = correlation coefficient) Fluorized granite

Chloritized a n d saussuritized granite

Episyenite

An.

U

Th

Th/U

An.

U

Th

Th/U

An.

U

Th

Th/U

1 2 3 4 5 6 7 8 9

13 11 14 14 19 25 20 17 23

71 115 137 66 113 187 118 124 151

5.5 10.5 9.8 4.7 5.9 7.5 5.9 7.3 6.6

1 2 3 4 5 6 7 8

20 24 25 20 24 21 17 25

48 75 92 66 101 84 65 103

2.4 3.1 3.7 3.3 4.2 4.0 3.8 4.1

1 2 3 4

8 10 14 9

51 53 56 60

6.4 5.3 4.0 6.7

2

15.8

127.5

8.3

:f

23.2

3.8

2

10.3

55.0

5.6

s r

7.0

s r

2.3

s r

3.3

61.6 0.87

83.4 19.2 0.76

10.2 0.02

Anomalies 1, 2, 3 a n d 6 of t h e fluoritized granite correspond, respectively, with samples 81-10, 81-39, 81-49 a n d 81-48 in Table 1, while anomaly 7 of the chloritized granited corresponds with sample 81-08. T h e episyenite sites 1, 2, 3 and 4 were, respectively, sampled as 81-04, 81-07, 81-09 a n d 81-40.

higher values associated with biotite concentrations. (6) The Vredenburg adamellite generally has lower U and Th values than the Cape Columbine granite, but xenoliths of the first-mentioned rock type has radioelement concentrations comparable to those in the enveloping granite. This indicates radioelement mobility during emplacement of the leucogranite.

Results of the autoradiographic study

Rock-forming minerals The gamma-spectrometer survey revealed differences in radioelement content between the relatively unaltered granite (white and red types) and hydrothermally affected granite. A few samples were selected for quantitative mineral separation. The large samples ( > 5 kg) were crushed by aid of a laboratory-scale jaw crusher and a roller mill and sieved. Accessory minerals were carefully concentrated by acom-

bination of magnetic separation with a Frantz isodynamic separator and density contrast by aid of a superpanner and a series of heavy liquids. The weights of the various concentrates of accessory minerals supplied a fair approximation of the relative abundances in the granite. Minerals were identified by their optical properties and, where necessary, by aid of an electron microprobe (EDAX). An autoradiographic investigation was conducted to study the mineralogical distribution as well as the effects of hydrothermal alteration on the radioelement content of rock-forming and accessory minerals. Alpha-sensitive cellulose-nitrate film (Kodak CA80-15) and high speed X-ray film with good resolution (Kodak Min-R) were used. The alpha sensitive film was sandwiched between the polished section and a glass slide. Exposure time varied from about 4 to 5 days for highly radioactive grains to 4 weeks for minerals with low radioelement content. The counting of the alpha-track density per cm 2, made it possible to determine the relative

236

A.E.SCHOCHANDR: SCHEEPERS

(A)

URANIUM MODEL

THORIUM MODEL

UNALTERED GRANITE

(B) X--18

"=

EPISYENITE Quartz a.d feldspar Biotite Chlorite Apa.tite, epidote / ~ Zircon

HI ~.~ ~ ~

Sphene

~

Thorogummite

Xenotime Monazite Thorianite

[ii~i~ Thorite/Orangeite ~ Secondary minerals ~ Unknown

Fig. 9. Sector diagrams illustrating the semiquantitative mineralogical distribution of uranium and thorium in: (a) the Cape Columbine granite (white variety) and (b) the episyenite derived from the Cape Columbine granite. Note that episvenitization involves the replacement of primary trace minerals such as xenotime and monazite by secondary, minerals such as thorianite and thorogummite as the principal hosts for radioelements.

variation in radioelement contents. Generally accepted average values for ionically substituted radioelements in the relevant minerals were taken from the literature (Frondel and Fleischer, 1955; Wedepohl, 1972; Cuney and Friedrich, 1987). In order to check on these values a few of the concentrates were analyzed for uranium and thorium by aid of neutron acti-

vation at the same time as the whole rock samples (Table 1), which confirmed that the selected values were acceptable on a first approximation basis. Models for the quantitative mineralogical distribution of uranium and thorium in various rock units were then calculated and are summed up in Figs. 9 and 10. The radioelement content of the rock-form-

U-TH IN CAPECOLUMBINEGRANITE

9'<:~7

URANIUM MODEL

THORIUM MODEL ~ = 1 2 7 ) IIII IIII

IIIII IIIII IIIIII

IIIIII IIIIIII IIIIIII IIIIIIII III

+++++~t~ +÷++÷~t,i ++~k++~ ++~++4t ++++~

~

Quartz,K-feldspar, Plagioclase Biotite Grain boundaries Unknown

Monazite Zircon ~-~ Xenotime Thorite

Fig. 10. Sector diagrams illustrating the quantitative mineralogical distribution of uranium and thorium in the fluoritized Cape Columbine granite. The numbers in the diagram rim represent percentages.

ing minerals, such as quartz, feldspar and biotite, excluding highly radioactive inclusions, is generally very low. Higher concentrations of alpha-tracks for these minerals prove to be related to submicroscopic fissures, indicated by the orientated character (Ranchin, 1968). Trapping of radioelements in crystal lattices

would cause uniformly distributed higher concentrations. Amongst the rock-forming minerals, the most important host for uranium and thorium proves to be biotite. As a matter of fact, 95% of the mineral activity attributable to rock-forming minerals, in the unaltered Cape Columbine granite, is due to biotite. However, if accessory and secondary mineral inclusions in the biotite are disregarded, the radioelement content'of the biotite is only slightly higher than those of the quartz and feldspar. In the unaltered Cape Columbine granite (Fig. 9), the quartz and feldspar accounts for 0.1% U and 0.4% Th, while the biotite acts as host for another 1.4% U and 1.7% Th. Similar values were determined for the episyenite (Fig. 9), namely, quartz and feldspar 0.3% U and 0.3% Th, with biotite accounting for 0.3% U and 1.1% Th. In the fluoritized granite, however, the biotite accounts for a remarkable 7.7% of the total U (Fig. 10) and a similar percentage of the Th, while the quartz and feldspar also has appreciably more uranium and thorium than in the unaltered granite and episyenite. It was found that most of the uranium and thorium reside in accessory minerals, such as zircon, sphene, epidote, xenotime, monazite, thorianite and fluorite, discussed below. The low calcium and very low phosphate contents (Table 1 ) explains the subdued role of apatite, but the paucity of uraninite is probably caused by oxidation. Only surface samples were used, which is in line with the aim to elucidate the distribution pattern determined by gamma spectrometry (Fig. 7 ). In a companion study of a leucogranite in the Darling batholith (Theron, 1985), for which a drill core was available, uraninite was a principal uranium host in samples from more than 10 m below surface but absent in all other specimens. The other radioelement host minerals were not affected by the oxidation, however. Zircon

Zircon is a major contributor to total ura-

238

nium {30% ) and thorium (28%) in the fluoritized granite {Fig. 10), as was also reported by other workers (Rimsaite, 1981). The interstitial space between groups of zircon crystals are sometimes filled with an orange/red isotropic radioactive product with an a-track concentration similar to that of the zircon (Fig. 11). In the zircons of the fluoritized Cape Columbine granite, minute opaque inclusions with extremely high alpha-activity, probably uraninite, are occasionally present. The dominance of zircon, as a radioetement host in the fluoritized granite, is not reflected in the unaltered granite and episyenite (Fig. 9). The episyenite contains at least two zircon populations conforming to the P4- and S3-type of Pupin (1980). The radioelement contents of the

A.E.SCHOCHANDR. SCHEEPERS

two types are comparable according to the alpha-track images. The alteration products chlorite, muscovite and opaque phases in the episyenite are higher in radioelement content, where they occur around zircon. This effect is illustrative of radioelement mobility with respect to zircon, during alteration in the episyenite. The radioelement distributions in zircon was much affected in rocks in which chloritization took place. Irregular zircon grain boundaries and lowered alpha-activities are indicative of a leaching process. The chlorite surrounding the zircon is highly radioactive. Characteristically, a red-brown unidentified isotropic mineral surrounds these zircons extending into fissures in the chlorite (Fig. 12 ). Electron microscopic in-

Fig. 11. Left: Photomicrograph (ordinary |ight ) showing thin zone of reddish-brown isotropic radioactive material which surrounds a zircon crystal, and which is also present in fractures in the enveloping biotite. The sample represents the chloritized and saussuritized Cape Columbine granite. Right: Autoradiogram illustratingalpha-activityassociated with the redbrown isotropic material as well as the zircon crystal:

u TH IN CAPECOLUMBINEGRANITE

239

to the presence of sphene (3% U and 11% Th). The high activity of sphene is attributed to the mobility of uranium during alteration of biotite and its conclusions. Sphene is of no importance, however, in the unaltered granite and fluoritized granite.

Epidote Group Radioactive epidote and allanite are present in the chloritized granite, episyenite and in minor amounts in unaltered granite. A characteristic of the epidote is a high alpha-activity where associated with sphene, but a low activity where associated with plagioclase.

Xenotime

Fig. 12. Photomicrograph (ordinary light) of veinlets of a dark radioactive material situated in biotite cleavage planes and derived from zircon crystals (centre). The sample represents the white coloured Cape Columbine granite.

vestigation (EDAX) indicated that the mineral has varying amounts of Ca, Zr, Hf an Y. Zircon and xenotime belong to the same space group and it is conceivable that the alteration processes affected Hf- and Y-enriched rims, while leaving the zircon cores relatively intact. Cuney (1978) described the effect of chloritemicrocline-albite-sericite alteration on the mobility of radioelements in the previously crystallized minerals during the deuteric stage of the Bois Noirs granite, and came to similar conclusions.

Sphene A significant proportion of the total amount of radioelements in the episyenite is attributed

The low phosphate of the Cape Columbine granite (Table 1), compared to other Cape granites, did not preclude the crystallization of xenotime and monazite. The yttrium and rareearth elements seem to have scavenged the available phosphorous. Unlike zircon, xenotime gives no indication of radioelement leaching associated with deuteric alteration. A possible explanation for this could be that xenotime crystallized after deuteric alteration, concentrating mobile radioelements. The alpha-activity of the mineral is usually two to three times higher than that of zircon. In the unaltered granite, it accounts for 54% of the total U and 14% of the T h (Fig. 9), but the respective corresponding figures for the fluoritized granite is 15% and 2% (Fig. 10). It is not important in the episyenite. The relative prominence of xenotime is corroborated by the available chemical analyses (Table 1 ); if it is assumed that all the Y is present in xenotime and all the Zr in zircon, the unaltered granite must contain 307 ppm xenotime on average, and only 256 ppm zircon. The relevant figures for the fluoritized granite are respectively 373 and 199 ppm. The Y in the episyenite must be hosted in some other mineral though.

240

A.E. SCHOCH AND R. SCHEEPERS

Monazite Monazite is present in unaltered granite and fluoritized granite, as well as in apIitic dykes. Alteration of the monazite is highlighted by a red-brown colour and the occurrence of opaque minerals in micro fissures. In fluoritized granite microfissures, in biotite-holding monazite, inclusions are occasionally filled with an orangebrown isotropic mineral with a high radioelement-content. Monazite contributes 33% to the total Th content in unaltered granite and 20% in fluoritized granite, but is not an important host for U.

Thorianite and thorogummite These minerals were only found in the epi-

syenite. Thorianite is present as inclusions in chlorite and microperthite, as finely disseminated grains along microfissures and on contacts between the two minerals {Fig. 13). A characteristic yellow halo surrounds the thorianite inclusions in chlorite (Fig. 14). According to Ranchin (1971) the yellow mineral eventually alters to sericite. The thorianite in the microfissures invariably showed a higher uranium-content than those present as inclusions, and may be classified as uranothorianite. Thorogummite occurs as rims around the uranothorianite and in microfissures within it. The thorogummite is characterized by the presence of Si, K, Fe, Pb, U and Th in variable quantities, according to EDAX spectra. A brown-green alteration product, inti-

Fig. 13. Left: Photomicrograph (ordinary light) showing thorogummite (dark streak, tg) associated with thorianite and a brownish-green anisotropic product (gaL The sample represents episyenite in the Cape Columbine granite, Right: Autoradiogram illustrating that the alpha-activity is chiefly associated with the thorogummite.

U-TH IN CAPECOLUMBINEGRANITE

241

Fig. 14. Left: Photomicrograph (ordinary light) of thorianite (black) enveloped by a characteristic yellow halo (light rim) and chlorite. The sample represents episyenite in Cape Columbine granite. The bar represents 250 ttm. Right: Autoradiogram illustrating alpha-activity associated with the thorianite and yellow halo. The bar represents 250 #m.

mately associated with chlorite and opaque minerals, surrounds the thorogummite. The alteration zone is characterized by an increase in Si, K, Ca, Fe and Mn with respect to the thorogummite. Thorianite contributes 30% U and 31% Th to the total radioelement-content of the episyenite, compared to values for thorogummite of 24% U and 33% Th (Fig. 9).

of iron and uranium. The high uranium content of thorite in the fluoritized granite causes this mineral to make a large contribution to the total uranium content (13% U; see Fig. 10). In the episyenite, the role of thorite is smaller (Fig. 9 ), but this is due to the relative scarcity of the mineral rather than a lower uranium content. Fluorite

Thorite

Thorite is present in unaltered granite as well as fluoritized and chloritized granite and the episyenite. The mineral is typically found as inclusions in feldspar, causing radiation damage to the feldspar lattice. Opaque haloes surround these thorite grains, with significant amounts

In the fluoritized granite, the introduction of secondary fluorite markedly redistributed the radioelements previously associated with primary minerals. During the deuteric stage of the consolidation of the Cape Columbine granite rock-forming minerals, especially biotite, lost a significant proportion of their radioelement-

242

A.E. SCHOCH AND R. SCHEEPERS

Fig. 15. Left: Photomicrograph (ordinarylight) of altered biotite envelopedby fluorite.The sample representsfluoritized Cape Columbinegranite. The bar represents250 pm. Right: Autoradiogramon the same scaleas the photomicrographillustrating alpha-activityassociatedwith the fluorite. content to fluorite. Five modes of alpha-activity are associated with the fluorite: (a)homogeneous distribution through t h e crystal (similar to the phenomena described by Tieh and Ledger, 1981 ); (b)enhanced activity in biotite enclosed by fluorite (Fig. 15); (c)activity in opaque iron-titanium inclusions in biotite in fluorite; (d)inclusions of uranium- and thorium-minerals in fluorite (Fig. 16); and (e) inclusions of an orange-red mineral with Ca, Pb, Na and U (possibly clarkeite) in fluorite.

Discussion Although biotite contributes relatively little

to the present total radioelement content of the different altered granites, it is a host for several primary accessory minerals which could have acted as source of uranium and thorium for secondary minerals during the deuteric s~age of the Cape Columbine granite. The uranium and thorium entrapped in opaque iron-titanium minerals in fluoritized and chloritized granite could have been derived in this way during alteration of biotite. Manning ( 1981 ) suggests that deuteric alteration may release fluorine from silicate phases to the vapour phase at an early stage during post-magmatic evolution. Fluorite is produced by the combination of fluorine, released from silicate phases, with calcium, introduced with the fluids. The fluorine in the fluid phase scavenges radioelements from biotite or accessory

U-TH IN CAPECOLUMBINEGRANITE

243

Fig. 16. Left: Photomicrograph of fluorite (re) filling a vein in feldspar (f) and with tiny inclusions of thorianite, in fluoritized Cape Columbine granite. The bar represents 250/tm. Right: Autoradiogram on the same scale as the photomicrograph, illustrating alpha-activity of the fluorite and enclosed thorianite.

minerals, forming uranium-fluorine complexes. When carbon dioxide was available uranium was probably also transported as uranyl carbonate, following release from zircon and monazite. Leroy and Poty (1969) described similar processes. The formation of highly radioactive sphene associated with the alteration of biotite in episyenite shows that the calcium level in the deuteric or hydrothermal fluid must have been significant. The high Th/U-ratio in fluoritized granite is explained by the fact that the thorite is mainly present as inclusions in rock-forming minerals, especially quartz and feldspar. Thus, thorite was protected against hydrothermal activity. The iron-rich haloes surrounding thorite in feldspar contains thorium, indicating the migration of

some thorium during deuteric alteration. Thorium loss from monazite during fiuoritization, is indicated by the highly radioactive orangebrown isotropic mineral (containing Th, P and Ca), which envelops the monazite in biotite/ chlorite. The processes discussed above were also active during episyenitization. The removal of silica, however, resulted in the destabilization of thorite so that thorogummite was formed, coating major minerals and filling micro-fissures. Towards the end of the quartz-removing process, chloritization resulted in the liberation of increasing quantities of uranium, stimulating the crystallization of uranothorianite coating thorogummite. Weathered granite suffered uranium loss, but

244

A.E. SCHOCH AND R. SCHEEPERS

U308(ppmJ 50

40

30

•

/ j

j

20 l(J

lJ

j

•

0,4

08

1,2

1,6

2,0

2,4

2,8

Fe203/FeO ThO2(ppm)

3oo! 24(

160

80

0,4

1,2

2,0 Fe203/FeO

2,8

Fig. 17. The uranium and thorium contents of the samples from Cape Columbine, expressed as a function of the oxidation ratio (Fe2OJFeO). A best-fit straight line is shown for uranium (r 2= correlation coefficient).

no significant loss in thorium. A positive correlation exists between uranium content and the Fe203/FeO-ratio, indicating higher uranium values with increasing oxidation (Fig. 17). Uranium was thus fixated in or on secondary Fe-oxides after liberation from the major uranium-bearing phases during surface weathering. Thorium shows no significant correlation with oxidation ratio, although it must be admitted that the data set is too small for final conclusions to be made {Fig. 17). Anomalies in unaltered granite outcrops are mainly associated with weathering products as well as minor variations in the content of xenotime, zircon and other accessory radioelement-bearing minerals.

Conclusions The Cape Columbine granite can be considered to be an uraniferous leucogranite. During the deuteric stage the granite suffered a major redistribution of radioelements in altered zones, making the uranium more leachable. Mild deuteric or hydrothermal alteration affected the entire granite pluton, leading to localized enrichment of uranium and thorium. The distri~ butional highs correspond to small areas showing the most intense alteration features. Biotite and accessory minerals suffered major radioelement losses during alteration, releasing radioelements which were fLxated in secondary minerals. Weathering resulted in the loss of

U - T H IN CAPE COLUMBINE GRANITE

uranium in some cases and in the formation of radioactive secondary iron-oxides in others. Acknowledgements The authors wish to express their thanks to the Atomic Energy Commission of South Africa for financial support and analyses for uranium and thorium. In particular we would like to express our gratituted to P.D. Toens and H.J. Brynard for encouragement. Major- and traceelement analyses were conducted by D.H. Cornell and A.M. Uttley of the Geology Department, University of Stellenbosch. The figures were prepared by J. Stallenberg and A. Felix, and the text was typed by P. Swart, both of the University of the Orange Free State. References Brynard, H.J., 1983. Fission-track studies of uranium distribution in geological samples. Spec. Publ. Geol. Soc. S. Afr., 7: 413-417. Cathelineau, M., 1987. U - T h - R E E mobility during albitization and quartz dissolution in granitoids: evidence from south-east French Massif Central. Bull. Mineral., 110: 249-259. Chayes, F., 1955. Potash feldspar as a by-product of the biotite-chlorite transformation. J. Geol., 63: 75-82. Clark, K.F., 1982. Mineral composition of rocks. In: R.S. Carmichael (Editor), Handbook of Physical Properties of Rocks, 1. CRC, Bocaraton, Flda, pp. 1-216. Cuney, M., 1978. Geologic environment, mineralogy and fluid inclusions of the Bois Noirs-Limouzat uranium vein, Forez, France. Econ. Geol., 73: 1567-1610. Cuney, M. and Friedrich, M., 1987. Physicochemical and crystal-chemical controls on accessory mineral paragenesis in granitoids: implications for uranium metallogenesis. Bull. Min6ral., 110: 235-247. Cuney, M., Leroy, J. and Pagel, M., 1980. Comportement de l'uranium et du thorium dans les granites uranif~res franqais. Sciences de la Terre, 13: 58-61. De Bruiyn, P.L., De Jager, F.S.J., De Swardt, A.M.J. and Rabie, L.P., 1975. Geological map of pre-Cape Beds in the Worcester-Swellendam mountain foreland. Ann. Univ. Stellenbosch, 49A (4). De la Roche, H., Leterrier, J., Grandclaude, P. and Marchal, M., 1980. A classification of volcanic and plutonic. rocks using the R1R2-diagram and major-element analyses - Its relationship with current nomenclature. Chem. Geol., 29: 183-210.

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