An interpretation of boron contents within a Palaeoproterozoic volcano-sedimentary succession: Pretoria Group, Transvaal Supergroup, South Africa

Pretumbrinn Resenrth ELSEVIER

Precambrian Research 78 (1996) 273-287

An interpretation of boron contents within a Palaeoproterozoic volcano-sedimentary succession: Pretoria Group, Transvaal Supergroup, South Africa Patrick G. Eriksson a, Boris F.F. Reczko a, David P. Piper a Department of Geology, University ofPretoria, Pretoria 0002, South Africa b British Geological Survey, Keyworth, Nottingham NG12 5GG, UK

Received 15 June 1995; revised version accepted 6 October 1995

Abstract

Boron contents of sedimentary rocks were measured in three profiles through the Palaeoproterozoic Pretoria Group. Enhanced boron values occur at a number of stratigraphic levels across much of the preserved basin, and only these anomalies are considered significant. Evaluation of the various possible factors controlling these high boron concentrations suggests that inherited boron, sedimentation rate, detrital tourmaline, silica and carbonate dilution, metamorphism and evaporitic borates were relatively unimportant. A major role is indicated for the preferential uptake of boron by clay minerals, and for both andesitic volcanic and related hydrothermal activity as a source of boron. The dilution of boron concentrations due to organic coating of clay minerals may also be important in the case of the Pretoria Group. Stratigraphically expressed boron anomalies which cannot be explained by these various factors, are inferred to represent enhanced palaeosalinity of the waters of deposition. The suggested marine transgression is supported by sedimentological data.

1. Introduction The absence o f fossils in most Precambrian sedimentary rocks greatly inhibits their palaeoenvironmental interpretation (for example, Long, 1978; Eriksson et al., 1995b). The discrimination o f epeiric marine settings and closed intracratonic basins is especially difficult for Precambrian successions (for example, Eriksson et al., 1993a). Tirsgaard (1993) has drawn attention to the inherent similarity of sedimentary processes and products from late Precambrian tidal and fluvial channel palaeoenviron-

ments; Eriksson et al. (1995a) have come to similar conclusions for the Palaeoproterozoic. In addition, the absence of vegetation within Precambrian continental environments probably enhanced aeolian removal of argillaceous sediment particles, and promoted the widespread formation of sandy braided fluvial deposits (Long, 1978; Dalrymple et al., 1985; Fuller, 1985; Aspler et al., 1994). For this reason, palaeoenvironmental interpretation of Precambrian sedimentary successions has to use all available techniques, including those considered less reliable in more m o d e m , fossiliferous strata.

0301-9268/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0301-9268(95)00079-8

274

P.G. Eriksson et a l . / Precambrian Research 78 (1996) 273-287

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P.G. Eriksson et al. / Precambrian Research 78 (1996) 273-287

The use of the boron content of a sedimentary rock to evaluate palaeosalinity of the waters of deposition is one such technique, and has been utilised with mixed success (for example, Ernst et al., 1958; Frederickson and Reynolds, 1960; Ernst and Wemer, 1964) and failure (for example, Eagar and Spears, 1966; Cody, 1970; Dewis et al., 1972; Baumann and Werner, 1973). This indicates that the relationship of boron and palaeosalinity is relatively complex, and the technique has fallen into general disrepute in most modem palaeoenvironmental studies. The factors affecting the boron content of a sedimentary rock are fairly numerous, but well known. This makes it difficult to evaluate the depositional palaeoenvironment of a single non-fossiliferous sedimentary unit by using boron. However, we here suggest that widespread (i.e., across much of a large preserved basin) variations in boron contents observed through a thick stratigraphic interval, allow significant boron anomalies to be observed. These would include those anomalies related to large shifts in palaeosalinity, such as are caused by marine transgressions into previously closed basin settings. As most of the other factors controlling boron contents within sedimentary rocks can also be evaluated by known parameters subject to stratigraphic variation, it should be possible to isolate widespread stratigraphic boron anomalies which appear to reflect major palaeosalinity variations. The Pretoria Group (Fig. 1) of the Transvaal Supergroup, provides the opportunity to test this suggestion, as it has significant variation in measured boron contents, in the stratigraphic expression of the factors controlling boron contents, as well as an inferred major marine transgression. We have analysed 275 samples for their boron content, which not only provides us with a reliable data base for this study, but also adds significantly to the known data base of sedimentary boron values, particularly those for Precambrian rocks.

275

2. Methods

Two hundred and seventy five samples, mainly of shales/siltstones and sandstones, with some volcanic tuffaceous lithologies, conglomerates and carbonate rocks, were taken in three profiles through the Pretoria Group (Fig. 1). The eastern and central profiles comprise surface samples and the western profile borehole material. To determine boron, powdered samples were covered by sodium carbonate, melted over a flame and the boron content of the solution was then read by use of an inductively coupled plasma. To avoid interference from high iron contents, peak heights were measured in preference to peak areas. The lower limit of detection was 1 ppm, and the precision of the analyses was ___6.4%. All samples were further analysed for major and trace elements with XRF, and 45 selected shale samples were analysed for REE using liquid chromatography. The mineralogy of the latter samples was determined by XRD.

3. Factors controlling the boron content of sedimentary rocks The boron content of a sedimentary rock reflects many factors. There is a general positive relationship between boron content and salinity of the waters of deposition (Goldschmidt and Peters, 1932; Landergren, 1945; Frederickson and Reynolds, 1960; Walker and Price, 1963). River waters are generally about 400 times lower in their boron contents than seawater (Harder, 1974c). Boron normally correlates strongly with total clay content (Landergren, 1958), and will tend to concentrate within poorly crystallised illite (Fleet, 1965; Porrenga, 1967). However, Harder (1974a) also reports the occurrence of very high boron contents in muscovite-sericite, glauconite, lepidotite, montmorillonite and serpenti-

Fig. 1. Map of the Pretoria Group showing the location of boreholes and surface traverses from which samples for boron analysis were taken.

276

P.G. Eriksson et al./Precambrian Research 78 (1996) 273 287

nite. The preferentia! uptake of boron by illite is well established, and boron contents of illite in shales are estimated on the basis of K 2 0 contents of the shale: boron in sample. 7.7 boron in illite =

wt.% K20

where the factor 7.7 is the theoretical K20 content of illite (Reynolds, 1965). The total boron and 'equivalent boron bound to illite' contents are thought to be related to the palaeosalinity of the depositional waters with defined fields for marine (110-200 ppm Bsample; > 300 ppm Billite), brackish (80-110 ppm Bsample, 200-300 ppm Billite) and freshwater (10-50 ppm Bsample , < 200 ppm Billite) depositional environments (Reynolds, 1965, 1972). However, Eagar and Spears (1966) concluded on the basis of a case study that a change in palaeosalinity is indicated by neither total boron contents nor boron/potassium ratios. In another case study, Cody (1970) questioned the preferential uptake of boron by illite, as samples containing large amounts of montmorillonite had similar total boron contents to samples with illite as the abundant clay mineral. Once boron has been adsorbed onto clay mineral particles, it becomes fixated by hydrogen bonding and incorporated into the tetrahedral sheet structures (Couch, 1971). Boron may thus be inherited from detrital clay particles (Eagar and Spears, 1966; Bouska, 1981), but will be protected from loss during low-grade metamorphism up to about greenschist facies (Harder, 1974e), or from loss during weathering and diagenesis (Nel, 1968; Fairchild et al., 1988). However, illite may be only one of the constituents of the clay mineral assemblage acting as a boron host, and thus the possibility of diagenetic loss of boron must be considered (Harder, 1974e). A slower rate of sedimentation will increase boron sorption, as will higher temperatures (Harder, 1963; Porrenga, 1967). Organic matter forms an inhibiting protective coveting of clay particles, decreasing boron adsorption (Landergren and Carvajal, 1969). Furthermore, organic and calcareous matter as well as silica can further lower the total boron content of a shale sample due to dilution (Eagar and Spears, 1966; Shaw and I3ugry, 1966). The effect of detrital tourmaline on boron content of a sediment may be

avoided by only examining the clay fraction, or by careful petrographic examination of coarser lithologies. However, as hydrothermal waters can contain high concentrations of boron (Harder, 1974c; Ellis, 1979; Weissberg et al., 1979), syn- a n d / o r post-depositional hydrothermal alteration processes can lead to the occurrence of authigenic tourmaline in the altered rock assemblage (Guilbert and Park, 1986), and, thereby, increase the boron content considerably. Post-depositional contamination by ground waters is thought to have led to a significant increase of boron contents in certain continental shales and an associated palaeosol (Cody, 1970). Evaporite deposits containing borate minerals may also lead to enhanced boron contents in both marine and nonmarine sedimentary rocks (Peng and Palmer, 1995). Recent work (Spivack et al., 1987; Morris et al., 1990; Bebout, 1991; Moran et al., 1991; Smith et al., 1991; Vengosh et al., 1991; You et al., 1991, 1993) has enabled a boron budget for the ocean basins to be proposed. From this it appears that the loss of boron due to alteration of oceanic crust and adsorption by clastic and carbonate sediments leads to an estimated excess boron sink of about 1.75-10 m m o l / y e a r . This large sink is thought to be balanced by boron obtained through volatilisation associated with island arc volcanism (You et al., 1993). This would imply that such explosive andesitic volcanism is, in fact, the most significant source of boron to oceanic sedimentary basins. Generally, it is assumed that the boron contents of sedimentary rocks tend to be increased in the presence of volcanogenic material (Bouska, 1981). Despite the problems involved in using boron as a palaeosalinity indicator, outlined above, it nevertheless seems to be the most suitable trace element for palaeosalinity interpretations (Bouska, 1981). The basin-wide and stratigraphic approach, adopted by us in this paper, follows the suggestion of Bouska (1981), that the boron concentrations of shales and their relative changes should be investigated on a basinal scale, instead of overemphasizing the results (i.e., absolute boron concentrations) of a local geochemical investigation. Eriksson (1992) reviewed the boron concentrations of Pretoria Group shales and siltstones from the Potchefstroom basin, sampled by B~Shmer (1977) from five widely spaced boreholes, 20-70 km apart. Eriksson (1992) was able to show

P.G. Eriksson et a l . / Precambrian Research 78 (1996) 273-287

Table 1 Average boron concentrations of sedimentary rocks after Harder (1974b, d) Type

n

Mean

S.D.

Range

Shale Sandstone Carbonate Andesitic tuff

2000 50 200 250

130 30 20 27

129 17 20 14

25-800 5 - 70 2 - 95 1-132

18 standard deviation (S.D.) is estimated by using the methods of Pearson and Stephens (1964) and Sachs (1992) (see text). Andesitic tuff average is calculated excluding the analyses of Lisitsyn and Khitrov (1962). The standard deviation of the andesitic tuffs is calculated by using the average range (Ravg = 66).

277

an average boron concentration of 65 ppm and a 16 standard deviation of _ 18 ppm (Shaw and Bugry, 1966). Cameron and Garrels (1980) report an average boron content of 25 ppm for their Average Canadian Proterozoic Shale. Andesitic lavas generally have boron contents between 20 and 40 ppm, with andesitic tuffs ranging between 7 and 132 ppm (see Table 1). However, in some cases, tuffs have reported B contents up to 2100 ppm (Lisitsyn and Khitrov, 1962).

5. The geology of the Pretoria Group that these variations in boron concentration are inphase stratigraphically, i.e., that boron increased or decreased at comparable stratigraphic levels in the different boreholes; these widespread variations were related to major changes in palaeosalinity of the waters of deposition.

4. Average boron contents of sedimentary and volcanic rocks Average boron contents of sedimentary and volcanic rocks are given by Harder (1974b, d) (Table 1). Unfortunately, Harder (1974b, d) omits to report quantitative measures of dispersion. The standard deviations listed in Table 1 were estimated by using the statistical methods described by Pearson and Stephens (1964) and Sachs (1992), after assuming a 90% (andesites and sandstones) and a 99% (shales and carbonates) probability of non-normality. These assumptions were made on the basis of the relationship of mean and range for shales, sandstones and carbonates, and, respectively, weighted mean and average range for the andesites. It should be noted that the method used here to estimate the standard deviations can be regarded only as a rough estimate of the real dispersions of the distributions described by Harder (1974b, d). Shaw and Bugry (1966) give the average boron concentration and the 16 standard deviation of various shales. The weighted mean of 168 analyses is 148 ppm with a weighted 16 standard deviation of + 9 9 ppm (recalculated from values of Shaw and Bugry, 1966). Twenty Proterozoic shale samples have

The Pretoria Group comprises a succession of shales, quartzose and arkosic sandstones, and andesitic volcanic rocks (Fig. 2), which occupy a central location within the Kaapvaal craton of southern Africa (Fig. 1). A combination of alluvial fans, rivers, fan-deltas, coastal processes and basinal turbiditic and suspension sedimentation systems is inferred to have deposited the sedimentary succession (Eriksson et al., 1993a, b, 1995a). Three andesitic volcanic units within the Pretoria Group (Fig. 2) are all widespread within the basin. The Bushy Bend lavas in the lower Timeball Hill Formation reach 90 m in the extreme south of the basin, with sporadic andesitic tufts at this level right around the basin (Eriksson et al., 1994). The Hekpoort Formation lavas are up to 800 m thick and also outcrop throughout the basin (Eriksson and Reczko, 1995). The Machadodorp Member pyroclastics and lavas in the Silverton Formation have a maximum thickness of 500 m, and a strike length of about 220 km (Button, 1973; Schreiber, 1990) in the eastern part of the basin; sporadic lavas and tufts occur at the same stratigraphic level in the centre and west of the basin (Eriksson et al., 1990). Thin tuffaceous shale beds are relatively common in the uppermost Vermont, Nederhorst and Houtenbeck Formations (Fig. 2) (Schreiber, 1990). Black shales are found at two stratigraphic levels, in the Timeball Hill and Silvenon Formations (Fig. 2). Those in the former, although locally carbonaceous, are largely coloured by iron minerals (Eriksson et al., 1994), whereas those in the Silverton Formation are strongly carbonaceous (Button, 1973;

278

P.G. Eriksson et al. / Precambrian Research 78 (1996) 273-287

TRANSVAAL BASIN Formations

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Upper mudrocks ±1700m & thin to north

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Total thickness +_600m

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130-200m, pebbly sandstones & mudrocks in tar west, thickens to west, locally A

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0-10m 0-18-250m 0-150m

Lower mudrocks 400-700rn, thin southwards tufts Only Bevets Member, 0-30m locally quartzite

PALAEOKARSTTOPOGRAPHY

Fig. 2. Lithostratigraphy and thickness of the Pretoria Group formations. Numbers refer to subdivisions shown in Figs. 3 and 4. Triangle symbol = upward-fining succession; inverted triangle = upward-coarsening succession.

P.G. Eriksson et al. / Precambrian Research 78 (1996) 2 7 3 - 2 8 7

279

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B ppm Fig. 3. Boron profiles of Pretoria Group sedimentary and volcanic rocks from the eastern, central and western parts (for location of profiles, see Fig. 1) of the Transvaal basin. Curves connect shale and siltstone samples, sandstone samples are displayed as asterisks, tuffs as stars. Vertical line at 110 ppm B marks the lower boron concentration thought to be indicative of marine affinity. Note logarithmic scale. Numbers refer to Fig. 2. Column at right-hand side of profiles depicts lithotype: shale and siltstone = dashed; sandstone = stippled; carbonate = blocks; volcanics = v. Volcanogenic material is further found at the base of the Timeball Hill Formation, and in the form of thin tuff beds throughout the Vermont Formation and, locally, in the Magaliesberg Formation. Thickness data corrected for post-depositional sills and dykes (P.G. Eriksson, unpubl, data).

280

P.G. Eriksson et al. / Precambrian Research 78 (1996) 273-287

Schreiber, 1990; Reczko, 1994). Immature arkosic sandstones, indicating relatively rapid rates of sedimentation, are only common in those units above the Magaliesberg Formation (Fig. 2) (Schreiber et al., 1991). The tectonic setting of the basin is difficult to interpret due to younger geological events, particularly the intrusion of the Bushveld Complex into the upper part of the Pretoria Group at about 2050 Ma (Eriksson et al., 1995a). However, most researchers suggest an overall intracratonic rift setting (Von Gruenewaldt and Harmer, 1993).

6. Boron content of the Pretoria Group rocks Table 2 summarizes the average boron contents of Pretoria Group sedimentary and volcanic rocks. The average boron content of Pretoria Group shales (96 ppm) is lower than shale averages of 130 ppm and 148 ppm quoted by Harder (1974d) and Shaw and Bugry (1966), although it must be noted that a simple comparison of the mean values must fail due to the large dispersions of all populations. The Pretoria Group shale average is enriched compared to both the Proterozoic shale averages given by Shaw and Bugry (1966), and by Cameron and Garrels (1980). However, the boron distribution of the Pretoria Group shale average has a median of 72 ppm and a geometric mean of 69 ppm. Both values thus compare favourably with the average shale estimate given by Shaw and Bugry (1966). The Pretoria Group sandstone average of 40 ppm is slightly higher than the 30 ppm quartzose sandstone average and the 35 ppm wacke average given by Harder (1974d). The elevated average boron content of Pretoria Group sandstones is explained by a Table 2 Average boron concentrations, 1 6 standard deviation and range of Pretoria Group sedimentary and volcanic rocks Type

n

Mean

S.D.

Range

Shale Siltstone Sandstone Conglomerate Carbonate Andesitic tuff

170 19 73 6 1 6

96 84 40 15 12 186

141 56 70 8 n.a. 105

4-1670 3 7 - 299 1- 96 7 - 29 n.a. 4 - 284

few samples with extraordinarily high boron contents (up to 563 ppm). If sandstone samples with a B content above 100 ppm are excluded from the calculation of the mean and standard deviation, the Pretoria Group sandstone average (n = 69; mean: 29 ppm; 1 6 standard deviation: ___27 ppm) closely resembles the quartzose sandstone average of Harder (1974d; Table 1). However, the boron distributions of both Pretoria Group sandstone averages (i.e., the average Pretoria Group sandstone reported in Table 2, and the average Pretoria Group sandstone without samples containing more than 100 ppm boron) show considerably larger dispersions than the sandstone average reported by Harder (1974d). The single dolomite sample analysed for boron compares well with the carbonate average given by Harder (1974d). The Pretoria Group andesitic tuff average is 186 ppm (Table 2), thus exceeding the average values given by Harder (1974b) significantly. However, their content is considerably lower than those reported by Lisitsyn and Khitrov (1962).

7. Stratigraphic variation of boron within the Pretoria Group The stratigraphic variation in boron contents for the three profiles measured through the rocks of the Pretoria Group is shown in Fig. 3. Boron values for shales above 110 ppm B in Fig. 3, a line marking the lower limit of inferred marine boron concentrations (Reynolds, 1965, 1972), are considered as anomalous in this paper. Anomalous boron contents are found in the lower Timeball Hill shales in the eastern and central Transvaal profiles, and very high values occur in the western Transvaal Hekpoort Formation tufts (Fig. 3). High boron values are again observed for shales in the Dwaalheuwel Sandstone Formation (eastern profile) and for subordinate shales in the stratigraphically higher Daspoort Sandstone Formation (eastern and central profiles) (Fig. 3). The overlying Silverton shales have anomalously high values for all three profiles, particularly the lower carbonaceous shales in this formation. High boron concentrations also occur in the uppermost part of the Magaliesberg Formation, in the lower Vermont Formation and in

P.G. Eriksson et a l . / Precambrian Research 78 (1996) 2 7 3 - 2 8 7

the Nederhorst Formation in the eastern profile (Fig. 3); the equivalent rocks in the other two profiles are less well developed (Fig. 2) and are only poorly preserved in the field.

EASTERN

281

8. Discussion By examining the stratigraphic variation of the factors controlling boron concentrations in sedimen-

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~0

'

,.%.-7: •. . , . . ''',,u

, 100

1 0 0 0

.~.__. ' --Z-.

.--_.

)

,-Z-. 010

100

1 0 0 0

Bml~ Fig. 4. Billite (see Eq. 1) profiles of Pretoria Group shales from the eastern, central and western parts of the Transvaal basin. Symbols as for Fig. 3. Note that Billite as used in this paper refers to the boron bound to clay minerals, expressed as a B / K 2 0 ratio, rather than to boron bound only to illite. Vertical line at 300 ppm Billite marks the lower Billite concentration thought to indicate marine affmity.

282

P.G. ErilcYson et al. / Precambrian Research 78 (1996) 273-287

tary rocks, it should be possible, by a process of elimination, to identify those anomalies probably reflecting major palaeosalinity changes. By only considering the significantly high anomalies given above, particularly if they are widespread across the three measured profiles (Fig. 3), the influence of factors such as diagenesis and post-depositional ground water contamination is reduced. The influence of the clay mineral content of the Pretoria sedimentary sequence on boron contents of its shales can best be examined by comparing the uncorrected B values (Fig. 3) with the Billite values (Fig. 4). It should be emphasised that the XRD analysis was unable to distinguish between illite and mica, and Fig. 4 thus illustrates a more general boron-clay mineral relationship; no expandable clay minerals were identified. Due to the Palaeoproterozoic age of the Pretoria sedimentary rocks, and the implicit diagenetic changes which must have affected the clay mineralogy, primary smectitic clay minerals may have played a role in the preferential adsorption of boron. However, Cody (1970) found that boron is not fractionated between smectite and illite. The more general boron-clay mineral relationship (i.e., Billite) shown in Fig. 4 is thus assumed to be a valid expression of the preferential uptake of boron by clay minerals, for the purposes of this study. In Fig. 4 the lower limit for Billite contents of possible inferred marine affinity is taken at 300 ppm (Reynolds, 1965, 1972). Values above 300 ppm Billite are considered as anomalous for this paper. Comparison of Figs. 3 and 4 reveals a reasonably good stratigraphic correlation of B and Billite; the internal correlation for the Pretoria shales is also good (Fig. 5). This probably largely reflects the clay mineralogy of the Pretoria Group shales, established by XRD analysis (Reczko, 1994). Reczko (1994) distinguished two mineralogical subgroups of shales: a kaolinite-mica-rich lower subgroup (up to Daspoort Formation) and a plagioclase-mica-rich upper subgroup. The mineralogy of the Pretoria shales thus seems to be, essentially, a constant factor in the variation of boron concentrations. However, Fig. 4 does reveal that the upper Magaliesberg-lower Vermont and Dwaalheuwel Formation boron anomalies identified in Fig. 3, are probably mainly due to an enhanced clay mineral content rather than to other factors. The Timeball Hill Formation also shows a

I0000

• marine

1000

l~w

100

10

oO

- ..............

•

,

~

I

•

•

--~,--~----

J brackish

&

freshwater I

f

' '''"qO

I

,

, ,,,,,,n

,

100

,,,,,,J

1000

,

, ,,,r,, I0000

Illite

Fig. 5. Billite (see Eq. 1) versus total B in Pretoria Group shale samples. Solid line: power regression of B in sample from Billite ([B ppm]=([Billite]°'66865)'2.141). Dashed lines: 'marine' and 'brackish and freshwater' fields for Billite and B ppm after Reynolds (1965).

general lowering of relative boron contents after correction for clay minerals, although the lower shales still have slightly raised values (Figs. 3 and 4). In contrast, the Daspoort, Silverton and Nederhorst anomalies identified in Fig. 3 are still very much in evidence in Fig. 4; they must thus reflect other factors than clay mineralogy. A number of factors do not seem to have played a major role in the observed boron contents of the Pretoria Group: inherited boron in detrital clay mineral structures, metamorphism, silica dilution, evaporitic borate deposits, and detrital tourmaline. The inferred granitic to basaltic source materials for the Pretoria Group sedimentary rocks appear to have been generally similar for the different stratigraphic units (Schreiber et al., 1991, 1992; Reczko, 1994). Variable T h / S c ratios suggest that sedimentary recycling was relatively unimportant during sedimentation, thus decreasing the significance of inherited boron. The major metamorphic event affecting the Pretoria Group was the intrusion of the Bushveld Complex into the upper Transvaal Supergroup at about 2050 Ma, mostly at a stratigraphic level above the Magaliesberg Formation and cross-cutting the post-Magaliesberg units (Eriksson et al., 1995a). Figs. 3 and 4 show no systematic upward decrease in boron contents with increasing stratigraphic height, as would be expected if metamorphic loss of boron had been significant. The silica-boron relationship is

P.G. Eriksson et al. / Precambrian Research 78 (1996) 273-287

not negative (Reczko, 1994), and thus contradicts a dilution by silica. Only one small and localised evaporitic playa lake deposit is interpreted in the Pretoria Group, associated with a palaeosol developed above the Hekpoort Andesite Formation (Martini, 1990). This inferred alkaline playa most likely derived sodium-rich salts from ground water leaching the underlying lavas, and no anomalous boron content is reported from these rocks (Martini, 1990). No detrital tourmaline was found in two extensive petrographic studies of Pretoria Group sedimentary rocks (Schreiber, 1990; Van der Neut, 1990). However, the occurrence of traces of authigenic tourmaline in the matrix of sandstones and shales cannot be excluded, although this is unlikely to cause regionally defined stratigraphic boron anomalies such as are observed in Figs. 3 and 4. Arkosic sandstones, indicating faster sedimentation rates, only become volumetrically important in the Pretoria Group above the Magaliesberg Formation (Schreiber et al., 1991). No concomitant significant decrease in boron contents of either sandstones or shales is noted at these stratigraphic levels (Figs. 3 and 4), thereby suggesting that this was not a major factor in the present study. Similarly, carbonate dilution of boron concentrations is thought to be relatively unimportant; carbonate rocks form only a minor part of the Pretoria Group stratigraphy, with thin lenticular units in the Silverton, Nederhorst and Houtenbek Formations (Fig. 2). The generally elevated boron values in the Silverton Formation and the Nederhorst boron anomaly (Figs. 3 and 4) suggest that carbonate dilution in these two units was ineffective. However, dilution may have played a role for the Houtenbek Formation, where low boron values are noted (Figs. 3 and 4). Andesitic lavas and pyroclastic rocks occur at three main levels in the Pretoria Group stratigraphy: thin lavas and widespread thin tuff beds at the base of the Timeball Hill Formation (Eriksson et al., 1994), the thick Hekpoort volcanics across the preserved basin (Harmer and von Gruenewaldt, 1991), and the thick Machadodorp lavas in the east of the basin (Button, 1973), with correlated tufts in the west of the basin (Eriksson et al., 1990) (Fig. 2). Thin tuff beds also occur in the Nederhorst Formation in the eastern Transvaal (Fig. 3). The highest boron values for any rock type in the Pretoria Group

283

are found within the Hekpoort tufts in the western profile (Fig. 3). The andesitic rocks also provide a plausible explanation for the anomalous boron contents of both the Silverton and Nederhorst Formations, as well as explaining the elevated concentrations found at the base of the Timeball Hill Formation in the eastem profile (Fig. 4). However, andesitic volcanic rocks cannot account for the high boron values observed in the Daspoort Formation. Organic matter (presumably from cyanobacteria) was probably responsible for the carbonaceous shales which characterise much of the Silverton Formation. It could thus be expected that this should have reduced higher boron values ascribed to andesitic volcanic activity in this unit. The fact that this does not appear to be the case (Fig. 4), suggests that either volcanic supply of boron was much greater than the impedance to its adsorption by clay particles due to organic matter, or that there was an additional factor counteracting the organic matter, perhaps a dramatically raised palaeosalinity of the waters of deposition. Raised palaeosalinity could also explain the boron anomaly observed for the Daspoort Formation (Fig. 4), which lies immediately below the Silverton Formation. It would be logical that an inferred marine transgression at this level could have raised boron concentrations within two succeeding stratigraphic units. However, the relative importance of andesitic volcanism, organic matter and palaeosalinity in determining eventual boron concentrations, must also take hydrothermal activity into account. Nesbitt and Young (1982) discuss the geochemistry of the Waterval Boven palaeosol on top of the Hekpoort Andesite Formation in the eastern Transvaal, and conclude significant geochemical changes due to diagenesis at this stratigraphic level. The correlation of high boron concentrations and volcanic material may thus be misleading, as diagenesis a n d / o r hydrothermal activity related to penecontemporaneous volcanism or later-stage magmatic activity may be the cause of the observed increase. Eriksson et al. (1994) investigated the geochemistry and mineralogy of the Timeball Hill Lower Shale Member, and concluded that syndepositional hydrothermal activity related to the extrusion of the thin interbedded Bushy Bend lavas and pyroclastic rocks has altered these shales. Fig. 6 shows a plot of the C e / C e * - a n o m a l y v e r s u s Billite

P.G. Eriksson et al. / Precambrian Research 78 (1996) 273-287

284 1.2

[] 1.0

i

t

i i i

r r i

0.8 ~-~

N O.6

0.4 0.2 •

0.0

r

4

i

,

,

,

,

*----_.___y

i

100

Illite Fig. 6. Bmi~e (see Eq. 1) versus C e - a n o m a l y in selected Pretoria G r o u p shale samples. Fields: N = ' n o r m a l ' shales; T = t u f f a c e o u s shales. D a s h e d lines: freshwater, brackish a n d marine values o f Bimte after R e y n o l d s (1965), see text. Symbols: open squares = Timeball Hill Formation, L o w e r Shale M e m b e r ; open d i a m o n d s = Timeball Hill Formation, U p p e r Shale Member; quartered diam o n d s = S t r u b e n k o p Formation; open stars = Silverton F o r m a tion; half-filled circles = V e r m o n t Formation.

(i.e., a boron/potassium ratio). The strongly negative Ce/Ce*-anomalies of the Pretoria Group shales are thought to be related to hydrothermal activity (Reczko et al., 1995). Tuffaceous shales from the Silverton Formation generally plot closely together and have somewhat elevated Billite contents. The highest values of BiHite are assigned to strongly negative Ce/Ce*-anomalies. The relationship of C e / C e *-anomalies and B ~l~itecan thus be interpreted as evidence for a complex influence of both volcanism and hydrothermal activity on the boron distribution of Pretoria Group sedimentary rocks. This could explain why the expected dilution of boron contents within the black shales of the Silverton Formation was not observed; volcanic sources of boron may well have been supplemented by hydrothermally derived boron for this unit. This would discount the enhanced palaeosalinity interpretation for the Silverton anomaly given above. However, the Daspoort Formation boron anomaly still remains unexplained, and, by a process of elimination, is thus inferred to represent a possible marine transgression and resultant increase in palaeosalinity of the waters of deposition. It is also possible that the similar boron anomaly in the overlying Silverton Formation reflects a combination of the positive

effects of andesitic volcanism, raised palaeosalinity, hydrothermal addition, and the dilution effects resulting from organic matter coating clay particles. There is sedimentological evidence to support a marine incursion into the Pretoria Group basin during deposition of the Daspoort, Silverton and succeeding Magaliesberg Formations. The Silverton Formation is a very thick, predominantly shaly unit, with a basal black shale, analogous to the transgressive systems tract of sequence stratigraphic affinity. The underlying Daspoort and overlying Magaliesberg sandstones are generally very clean and well sorted, being more mature than the arenites higher or lower in the Pretoria succession (Schreiber, 1990; Eriksson et al., 1993a, 1995b). In addition, bimodal palaeocurrent directions become important in the Daspoort and Magaliesberg sandstones (Eriksson et al., 1993a, 1995b), in contrast to the predominantly polymodal to unidirectional trends of the other formations in the Pretoria Group (Schreiber et al., 1991). The deposition of the Magaliesberg and Daspoort sandstones is generally ascribed to a combination of fluvial, braid-delta and shallow-marine sedimentation (Eriksson et al., 1993a, 1995b), thus probably representing the coastline to the transgressive Silverton basin (Eriksson et al., 1995a). The combination of both sedimentological and boron data thus supports the postulate of a marine transgression into the Pretoria Group basin during deposition of the Daspoort to Magaliesberg Formations.

9. Conclusions Boron concentrations in sedimentary rocks can provide useful information, if stratigraphic variations are examined on a basin-wide scale. If the stratigraphic expressions of the geological variables controlling boron contents are analysed individually, it is possible to isolate widespread boron anomalies which cannot be explained by factors other than palaeosalinity. If such stratigraphic units also have sedimentological evidence for marine deposition, then a palaeoenvironmental interpretation can be made with more confidence for ancient Precambrian rocks.

P.G. Eriksson et al. / Precarnbrian Research 78 (1996) 273-287

Using the present example of the Palaeoproterozoic Pretoria Group, some idea is gained of the relative importance of the various factors controlling boron concentrations in sedimentary rocks. Detailed petrographic studies have enabled us to dismiss inherited boron in detrital clay mineral structures, sedimentation rates and detrital tourmaline as being of minor importance. Similarly, geochemical data indicate a minor role for silica dilution. Stratigraphic trends in the Pretoria Group do not support a strong influence for thermal metamorphism by the very large intrusive Bushveld Complex, probably due to the protective nature of the clay mineral structures. Borates from evaporitic environments are also inferred to be unimportant in the case of the Pretoria Group. Limited data in this study indicate that carbonate dilution has also not played a major role. Preferential uptake of boron by clay minerals is important in the present case, as is evident from Figs. 3 and 4. Although the overall comparison of stratigraphic trends is similar, some anomalies and higher boron values disappeared when the correction for clay minerals (Fig. 4) was applied. Enhanced boron concentrations associated with andesitic volcanism a n d / o r hydrothermal activity related to the volcanism, appear to be very significant in the present study. The precise role of organic matter as a dilutant of boron contents is difficult to evaluate, as the relevant unit, the Silverton Formation, has also been subjected to strong volcanic influences. We find here that palaeosalinity of the waters of deposition was also a significant factor in forming some of the anomalous boron concentrations in the Pretoria Group.

Acknowledgements The authors acknowledge the Foundation for Research Development and the University of Pretoria for financial support, and Mrs. M. Geringer for her drafting skills. Roelf Venter of the Atomic Energy Corporation is thanked for the boron analyses, and the Council for Geoscience, Pretoria, for the REE analyses. The Geological Survey of Botswana kindly gave permission to sample the borehole core material.

285

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