An early Proterozoic playa in the Pretoria Group, Transvaal, South Africa

Precarnbrian Research, 46 (1990): 341-351

341

Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

An Early Protcrozoic Playa in the Prctoria Group, Transvaal, South Africa J.E.J. MARTINI Geological Survey, Private Bag Xl12, Pretoria (South Africa) (Received November 23, 1988; revision accepted August 29, 1989)

Abstract Martini, J.E.J., 1990. An early Proterozoic playa in the Pretoria Group, Transvaal, South Africa. Precambrian Res., 46: 341-351. A 2.2-Ga-old occurrence of jasper-banded iron-formation forms a very thin layer of restricted extent, closely associated with a palaeosol developed on basaltic lava. Early diagenetic polygonal cracking, plastic microdeformation and quartz spherulites suggest a sodium silicate precursor for the cherty phase of the banded iron-formation. The same bed contains cyclic layers of well-preserved pseudomorphs after evaporites, forming radiating blades which were probably mirabflite. It seems that the sediments were deposited in an alkaline (presence of sodium silicate precursor) playa in a semi-arid but probably cool (presence of mirabilite) climate. The iron and silica of the banded iron-formation, as well as the salts of the evaporites, originated from ground water that leached lava. This occurrence provides further evidence that banded iron-formations had a sodium precursor.

Introduction

An alkaline playa generally results from evaporation of surface or ground water in a continental environment (Eugster, 1984, 1986). Sodium is the main cation in solution and originates from the leaching of silicates such as plagioclase. Na + ions are equilibratedb)CO~- and HCO~- supplied by atmospheric or biogenic carbon dioxide and the solution is strongly alkaline. Fossil alkaline playas can be identified by the presence of sodium carbonate minerals or by casts and pseudomorphs of the minerals. Moreover, chert with a morphology and texture characteristic of sodium silicate precursors has been proposed as further evidence for such an environment (Hay, 1968; Eugster and Ming Chou, 1973). Examples are common from the Jurassic to the present-day, such as, for in0301-9268/90/$03.50

stance, the famous soda deposit of the Green River Formation in the United States (Eugster and Hardie, 1975). In older rocks examples are scarcer; however, trona (?) casts and chert after magadiite have been reported in the Devonian of the Orkney Islands (Parnell, 1987), and, in the Cambrian of South Australia, trona(?), shortite or pirssonite casts and chert nodules with shrinkage cracks have been described by White and Youngs (1980). In late Proterozoic strata, gaylussite and thermonatrite pseudomorphs have been found in Namibia (Behr et al., 1983 ), as well as shortite and gaylussite (?) casts in South Australia (Rowland et al., 1980). In middle Proterozoic rocks of North Australia, Magadi-type chert has been described (Eugster, 1985). The wider concept that the chert in banded ironstone originated from a sodium silicate precursor would suggest that vast alkaline playas may have existed during the early Pro-

© 1990 Elsevier Science Publishers B.V.

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J.EJ. MARTINI

terozoic (Eugster and Ming Chou, 1973). This paper describes well-preserved evaporite pseudomorphs and chert after sodium silicate minerals deposited 2.2 Ga ago in a small inland playa, and now preserved in the Transvaal sequence of South Africa.

Geological setting The early Proterozoic Pretoria Group, Transvaal sequence, consists dominantly of shale, quartzite and lava, filling the Transvaal palaeobasin. These rocks underwent very little deformation and mainly thermal metamorphism due to the emplacement of the Bushveld igneous complex, which did not erase the original textures in most cases. The Hekpoort Basalt Formation is part of this group and consists of a few hundred metres of terrestrial flows which have yielded an age of 2224_+21 Ma (Burger and Coertze, 1973/1974). The topmost volcanics are affected by palaeoweathering over a thickness of up to 15 m. (Button,

1979; Button and Tyler, 1981; Retallack, 1986). A standard section (Fig. 1) of this palaeosol shows unaltered basalt at the base grading progessively upward into an iron-rich chloritic rock. After a sharp contact, the chloritic rock is overlain by a white sericite rock, probably originally kaolinitic and subsequently altered by diagenesis. Relicts of volcanic textures are common in the chloritic rock, indicating in-situ weathering, whereas the sericite represents more intensely leached material, probably transported (Retallack, 1986). The overlying stratum is a ferruginous quartzite marking the marine transgression at the base of the Strubenkop Shale Formation. The palaeosol forms a remarkably constant marker throughout the Transvaal palaeobasin. This regularity is only broken 35 km to the southeast of Pretoria where a thin bed of jasper-banded iron-formation is developed along a strike length of 13 kin. This bed was briefly reported by Kynaston (1929) and represents the playa deposit described in this article.

Grey shale '

.

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Ferruginous quartzite

White sericific shale

Jasper. banded-ironformation, siLicified siLfston

/./-/"

jf

/f

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Sericite rock

[hLoritized, sericitized and silicified basalt with

ferruginous noduLes UnaLtered basalt

Fig. I. (A) Stratigraphic column in the area where the playa deposit is developed and (B) usual Hekpoort palaeosol profile.

EARLY PROTEROZOIC PLAYA IN P R E T O R I A G R O U P

343

Legend Jasper-iron-formation

mmmm

v v

Rieffor 395 v v v

Z=Z=~ SiLveri'onshale

....

Mirabilite (7] casts

~ Daspoorfquartzite Pretoria rL--~-~Sfrubenkopshale Group

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~-- ~ - - ~ - - ~ - - - ~, .~. .

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2 km

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v v

Yzervarkfontein v 1% IR V v V v v v

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F i g . 2. G e o l o g i c a l m a p o f t h e a r e a s t u d i e d .

Group Group

~Chuniespoort

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~Archaean

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V V

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V

v v

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39.22 72.77 25.02 27.01 49.87 12.99 24.64 38.45

0.24 0.43 0.18 0.16 0.13 0.23 0.35 0.21

0.78 10.17 0.80 1.16 1.47 3.25 2.83 3.69

56.60 1.85 69.52 66.53 42.68 77.49 68.00 52.38

CaO

0.89 0.02 <0.1 <0.01 6.56 0.16 3.20 <0.01 0.90 0.02 0.91 <0.01 1 . 3 5 0.07 1.15 <0.01 2.73 0.04 1.01 <0.01 1.17 <0.01 0.33 <0.01 1.71 0:13 <0.01 <0.01 1.34 0.03 0.88 <0.01

MgO 0.10 0.17 0.11 0.09 0.07 0.15 0.12 0.08

<0.01 0.41 <0.01 <0.01 <0.01 <0.01 0.09 <0.01

Na20 K20 0.08 <0.01 <0.01 0.08 <0.01 0.22 0.09 <0.01

P20s 0.33 0.06 0.07 0.10 0.02 0.12 0.31 0.09

1.32 3.09 0.95 1.42 1.12 2.18 1.55 1.71

0.24 0.09 0.19 0.05 0.16 0.78 0.28 0.46

0.32 0.19 0.18 0.26 0.17 0.47 0.25 0.32

Total Ni

Cu

Zn

Au

0.05 100.19 < 5 50 8 86 0.01 99.16 213 119 119 115 0.03 98.88 < 5 67 <5 63 0.04 99.47 25 83 <5 98 0.02 99.49 <5 64 <5 83 0.07 99.47 22 850 14 308 0.03 100.38 14 113 <5 81 0.02 99.66 7 50 <5 107

Cr203 H20 + H20- CO2 S

~rhis material, with the exception of (2), has been oxidized by weathering. Analysis: Geological Survey Laboratory. Major elements in per cent. Traces in ppm, except Au in ppb. (1) =Replaced evaporite, Koffiespruit 197 IR; (2)=green jasper, Koffiespruit 197 IR; (3)=ferruginous red jasper, Tweefontein 552 JR; (4) =laminated iron-formation, eastern part of Knoppiesfontein 549 JR; (5) =banded iron-formation, central part of Knoppiesfontein 549 JR; (6) =iron-formation, 200 m to the NW of prospect on Knoppiesfontein 549 JR; (7)=iron-formation, prospect on Knoppiesfontein 549 JR; (8) = replaced evaporite, Onbekend 398 JR.

1 2 3 4 5 6 7 8

Sam- Si02 Ti02 Al20s Fe20s FeO M n O ple

Analyses of iron-formationand jasper~

TABLE 1

EARLY PROTEROZOIC PLAYA IN PRETORIA G R O U P

Proceeding from the southeast, the jasperiron-formation abruptly appears on the farm Koffiespruit 197 IR (Fig. 2). At this place the lava is chloritized,sericitizedand contains ferruginous nodules, but retains features such as amygdales and fluidal textures up to the very sharp contact with the overlying jasper. The latter is 1.5 m thick and can be subdivided into two parts. The lower part consists of banded red jasper, chert and fine-grained hematitemagnetite; the upper part is green jasper, also containing iron oxides, but richer in silt-sized detrital particles. Fine-grained chlorite is responsible for the green tinge. Occasional copper staining has been noticed and results from very fine chalcopyrite and chalcocite disseminated in the green jasper. Although these ferruginous rocks are generally finely laminated, there is evidence of agitation,e.g.'soft'clastsand chert oncoliths, suggesting a shallow environment of deposition. At thisplace poorly exposed sericite rock overliesthe green jasper. The next best exposure of this ferruginous layer occurs in the central part of Knoppiesfontein 549JR, where it consists of iron-formation at the base, grading upward into an Fe-rich chloritic rock; there is no red jasper present. The most complete exposure is located on the same farm but 2 km further toward the northwest, at the site of an old prospect (Fig. 2). From base to top the succession is palaeoweathered lava, 0.6 m of massive white sericite, 0.8 m of banded iron-formation, 1.2 m of white sericite shale and 4 m of diabase sill. The ferruginous quartzite crops out above the diabase after a 2 m gap. The last good exposures of banded iron-formation occur on the border between Knoppiesfontein 549JR and Onbekend 398JR (Fig. 2). At this location the red jasper is present. The strata above and below the iron-formation are, however, not exposed owing to a thick soil cover. The analyses presented in Table 1 give an idea of the chemistry of the iron-formation. As expected, iron and silica are dominant, a feature which characterizes Precambrian banded ironformation but not more recent ironstone

345

(Stanton, 1972) and indicates chemical sedimentation. Alumina is low except where detritals are relatively abundant, as in the green chert. Copper and gold are high; the latter averages 118 ppb, a figure which is similar to the gold content of banded iron-formation associated with greenstone belts (Saager et al.,1982 ) but which is much higher than for Proterozoic banded iron-formation not associated with volcanics. Evidence for a sodium precursor

Spectacular polygonal cracking (Fig. 3B) is c o m m o n on the surface of the cherty beds, especially in the lower part of the banded ironformation. It is well developed only near the border between Onbekend 398JR and Knoppiesfontein 549JR, and on Koffiespruit 197JR. The sizeof the polygons delineated by the cracks varies from a few millimetres to a few centimetres. The fillingof the cracks consists mainly of fibrous botryoidal chalcedony which was deposited firstand was then followed by some wellcrystallizedquartz. If these cracks are observed in vertical section, the chalcedony forms gash veins tapering both up- and downward (Fig. 3A). The upward tapering and the nature of the fillingclearly indicate that the cracking developed underground. This eliminates an originby subaerial desiccation because in this case the cracks would have been filledup by sediments from the overlying bed. The firstcracks to develop are generally wide and delineate large polygons. They may be followed by a second phase forming a finer network within the polygons. In some rare instances, secondary shrinkage may produce a peculiar concentric cracking (Fig. 3C). Shrinkage along the verticalaxis also produced cracks parallel to the bedding (Fig. 3A) demonstrating that the rock was already sufficiently rigid to prevent the closing up of the cavities.Certain horizons can even be brecciated (Fig. 3A), probably as a result of excessive shrinkage. It is possible that the cracking of the chert-

346

J.E.J. MARTINI

!i

Fig.3. (A) Polished specimen of banded iron-formation; silica(white) enhanced by H F etching,from Onbekend. Dark zones are due to differentialweathering or to diagenesis.Note the verticaland horizontalcracking due to shrinkage,and brecciation in the upper part. (B) Cracking network on bedding plane, same locality.(C) Secondary concentric cracking between large verticalprimary cracks, (white,filledwith chalcedony), from Koffiespruit. (D) Injectionof chert (former magadiite) across bedding and plastic deformation; sample from centre of Knoppiesfontein. (E) Quartz spherules after kenyaite (?), from Koffiespruit.

EARLY PROTEROZOIC PLAYA IN PRETORIA GROUP

jasper results from diagenetic transformation of a sodium silicate precursor, although there is not a complete similarity with the cracked chert nodules which are typical of magadiite replacement. The original sediments probably would have been rich in sodium silicate minerals, like magadiite, deposited in an alkaline playa. Magadiite occurs as a soft putty-like material. Shortly after sedimentation, magadiite would have been transformed into chert as a result of leaching by more dilute solutions or by pH decrease due to higher CO2 concentration (Surdam et al., 1972). This transformation into chert is usually accompanied by up to 25% shrinkage and by induration, causing extensive cracking in the host rock. In support of the presence of sodium silicate precursors, there is intense deformation of chert seams and microintrusions (Fig. 3D); this plastic behaviour seems to be characteristic of magadiite (Surdam et al., 1972). Chert sphemles (Fig. 3E) probably represent pseudomorphs after kenyaite, another sodium silicate mineral (Hay, 1968).

Evaporite pseudomorphs Palimpsest radiating crystals are observed in several places along the strike of the iron-formation bed (Fig.2). They were replaced by finegrained quartz-magnetite-hematite. Very fine needles of griinerite typically occur as inclusions in quartz. Careful examination of bedding planes (Fig. 4C-D), thin sections and polished slabs (Fig. 4A-B), indicates that the original crystals were very thin blades (a fraction of a millimetre thick) radiating horizontally (up to 10 cm) and upward (1-2 cm). This morphology was constant in all the samples. No faces were observed at the termination of the blades. These crystals form layers separated by thin laminae of sediment, suggesting cyclic, possibly seasonal deposition. There is evidence that the sediments were deposited on already formed crystals (Fig. 4B) and that the crystals did not grow at a later stage during diagenesis or meta-

347

morphism. The pseudomorphs are strongly suggestive of evaporite which has been replaced by silica and iron oxide during sedimentation and/or diagenesis (Arbey, 1980). Although it is possible to attribute these radiating crystalsto evaporitic minerals, itis more difficult to identify which mineral they represent. If the environment was alkaline, Ca and Mg sulphate minerals are unlikelyto have precipitated; deposition of these salts does not occur in these brines because the activity of CO32- is high and allows only a very low concentration of alkali-earth metals (Eugster, 1984). Several possibilities remain Rmong bladed radiating alkali salts. Trona is the most common sodium carbonate mineral and would have been a very likely candidate considering the alkaline nature of the environment. However, it is generally described as comprising rosettes of relatively thick, interpenetrating blades (Baker, 1958; Smith and Haines, 1964; Bradley and Eugster, 1969; Eugster and Hardie, 1975) which are different from the pseudomorphs described here. By evaporating a solution of sodium carbonate at low temperature, or by refrigerating such a solution saturated at room temperature, the author obtained radiating blades of natron bearing some similarities to the material from the banded iron-formation. However, the crystals were generally well formed, the blades were relatively thick and interpenetrating thereby forming box-works, a morphology that was not observed in the pseudomorphs. Another possibility would be mirabilite. The morphology of this mineral is variable, but thin, bladed needles and fibrous masses have been described (Cole, 1926; Palache et al., 1951 ). By refrigerating a solution saturated in sodium sulphate at room temperature, the author obtained crystals very similar to the pseudomorphs as far as morphology and size are concerned. Therefore it seems likely that the evaporite was mirabilite. Unfortunately, owing to the brittle nature of mirabilite, the lack of contrast between brine and mineral, and rapid

348

J.E.J. MARTINI

Fig. 4. (A) Polished specimens of siliceous iron-formation; silica (white) enhanced by HF etching, from Koffiespruit. Evaporite pseudomorphs, probably mirabilite, form radiating blades cut more-or-less perpendicular to their elongation except in the upper left corner. Note cyclic deposition of evaporite forming four main layers. (B) Detail at centre of (A) showing sediments deposited on an uneven crystal floor. (C) Evaporite pseudomorphs on bedding plane in siliceous iron-formation, from Onbekend. (D) As (C), same locality. Note bladed texture visible at centre.

dehydration into powdery thenardite, no good photograph could be obtained for comparison.

Origin of the playa and of the evaporites The jasper-banded iron-formation forms a very localizedoccurrence closelyassociatedwith a palaeosol. This favours deposition in a small continental playa or pan. Initialsedimentation

was dominantly chemical but became more detrital when bleached clay and silt were washed into the pan, filling up the depression. The latter may be compared with deflation basins occurring today in semi-arid regions where the soft part of the soil is removed by wind action, leaving abate rocky floor (Hugo, 1974). Such a firm floor is suggested on Koffiespruit 197IR where the banded iron-formation rests directly on lava

EARLY PROTEROZOIC PLAYA IN PRETORIA GROUP

which was somewhat weathered but lefttexturally undisturbed. If one accepts, however, a deflation origin,the isolatednature of the jasperbanded iron-formation occurrence remains to be explained, as pans are generally grouped in clusters,as for instance in some areas of South Africa today (De Bruiyn, 1972). At present, sodium silicateminerals are typicallyassociated with alkaline playas (Surdam et al.,1972). In theory, however, these minerals can precipitate from sodium chloride or sulphate solutions, saturated in amorphous silica, at a p H closer to neutrality than in alkaline playas (Bricker, 1969). In this case the problem is that during the formation of sodium silicate, H4SiO4 produces H + which will not be absorbed as efficientlyas in the case of a sodium carbonate solution. The resulting drop in p H should rapidly impede the precipitation of sodium silicatemineralsunless the solution was frequently renewed, as in a tidal basin for instance. In a continental playa, this could not have been the case and it is therefore safe to assume that in the studied occurrence the environment was alkaline. Adding the possible presence of mirabilite, one may conclude that the brine might have been of a sodium carbonate and sulphate type. The origin of the salts would be mainly lava which by leaching provided sodium from plagioclase and sulphate from the oxidation of disseminated sulphides. As the solubility of mirabilite is low at low temperature, it is typically a cool climate evaporite, crystallizingin winter (Cole, 1926; Dort and Dort, 1970; Hugo, 1974). If one accepts that the evaporite pseudomorphs are mirabilite,the banded iron-formation was deposited under cold conditions. Conditions were not extreme, however, because there are no permafrost structures in the palaeosol. Moreover, a cool climate is consistent with the presence of glacial beds below the Hekpoort Basalt Formation (Visser, 1971) and at the base of the Pretoria Group (Martini, 1979), and with the proximity of the magnetic pole during this period (Piper, 1976).

349

Deposition of iron and contribution to the genesis of banded iron-formations in general A model which comes naturally to the mind is that the iron has been leached from the palaeosol as Fe 2+ and precipitated as Fe 3+ in the pan after oxidation by the atmosphere. The bleached nature of the sericitelayer in the palaeosol suggests such leaching. At present this process occurs essentially in water-logged acid soil associated with peat bogs and swamps (Stanton, 1972). At present, iron-rich sediments are not commonly associated with evaporitic pans. Such an association might, however, have occurred during the early Proterozoic as the oxygen levelin the atmosphere was probably lower than now, making leaching under reducing conditions possible even under an arid climate without requiring organic matter (Martini, 1986). The possibility that the atmosphere was already oxidizing 2.2 Ga ago is suggested by the Fe 3+/Fe 2+ ratio in the Hekpoort palaeosol (Button, 1979) as well as from studies in other parts of the world (Schau and Henderson, 1983). It seems likelythat the gold in the banded iron-formation, averaging more than 100 ppb, has also been leached out of the lava and precipitated with the iron. A comparable process has been proposed to explain the high Au content of the ferruginous nodules in the Hekpoort palaeosol (Martini, 1986). It is of interest to evaluate the contribution of this ferruginous horizon to the understanding of the genesis of banded iron-formation in general. As the rocks described in this paper are typical of banded iron-formation, a genetic similarityisautomatically suggested. This adds weight to the model proposed by Eugster and Ming Chou (1973) that banded iron-formations have been deposited in playas and magadiite was a precursor of chert. The rock described here, however, contains evaporite palimpsests and an abundance of shrinkage textures: both are rare in banded iron-forma-

350

tions. In conclusion, it is probably premature at this stage to d r a w a strong parallel between the b a n d e d iron-formation in general a n d the thin b a n d e d iron-formation studied here.

Acknowledgements T h e author expresses his gratitude to Dr. C. Frick, chief director of the Geological Survey, for permitting the publication of this article a n d to J.H. W a r d w h o edited the manuscript critically. Mr. M . D . Kohler, Dr. C. M a c r a e a n d Mr. H.F.G. M o e n assisted with the photography.

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EARLY PROTEROZOIC PLAYA IN PRETORIA GROUP

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