Abstract - Black shales in the Early Proterozoic Tsmeball Hill Formation exhibit a widespread dark grey colour due to disseminated iron minerals, pt~iominantly ...
Journal of African Earth Sciences, Vol. 18, No. 4, pp. 325-337, 1994 Copyright © 1994 Elsevier Science Ltd Printed in Great Britain. All fights reserved 0899-5362/94 $7.00 + 0.00
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Early Proterozoic black shales of the Timeball Hill Formation, South Africa: volcanogenic and palaeoenvironmental influences P.G. ERIKSSON1, B.EE RECZKO~, R.K.W. MERKLE~, U.M. SCHREmER~, J.P. ENGELBRECHT2, M. RES~ AND C.R SNYMAN~
l D e p t o f G e o l o g y , U n i v e r s i t y o f Pretoria, P r e t o r i a 0002, S o u t h A f r i c a 2 M i n e r a l o g y a n d P r o c e s s C h e m i s t r y Division, Mintek, P r i v a t e B a g X3015, R a n d b u r g 2125, S o u t h Africa 3 G e n g o l d Division, G e n m i n , c / o W i n k e l h a a k Mines, E v a n d e r 2280, S o u t h A f r i c a (Received 21 J a n u a r y 1994 : a c c e p t e d 1 July 1994) Abstract - Black shales in the Early Proterozoic Tsmeball Hill Formation exhibit a widespread dark grey colour due to disseminated iron minerals, pt~iominantly limonite after pyrite, with subordinate thin beds and laminae more intensely pigmented by finely disseminated flakes of carbonaceous materiaL Andesitic-basaltic volcanism is thought to have provided the s o m ~ of iron and sulphur for the ferruginous colouration in a basal and uppermost black shale facies. Sulpbate-reducing bacteria around volcanic vents possibly produced the organic matter for the darker beds. Turbiditic rhythmites and succeeding fluviodeltaic sandstones overlie the basal black shale facies and associated Bushy Bend lavas. In the south west of the basin both rhythmites and sandstones have black shales as either thin interbeds or matrix material suggesting the possibility of continued fumarolic emissions in this region. A repetition of the rhythmite facies again shows an association with subordinate thin black shale interbeds in the southwest of the basin and this facies is succeeded by a second occurrence of the black shale facies, underlying the Hekpoort Andesite Formation. The observed association of black shales with turbiditic rhythmites, lavas lacking pillow structures and fluviodeltaic sandsOonessuggests water depths which varied from shallow to a few hundreds of metres. Some potential for volcanogenic massive sulphide and sedimentary exhalative ore deposits exists in the black shale as there is volcanic rock associated at the base of the formation. R ~ u m ~ - Les schistes noirs du dC~butdu Prot6rozo'ique de la Formation de Ttmeball Hill poss&lent frt~quemment une couleur gris fonc~ due ~tdes min&'aux ferri~res diss~min~s: pour l'essentiel de la limonite provenant de la pyrite, aiusi que de fins bancs subordonn~s et des lamines plus intens~ment pigment~s par des paillettes de materiel carbon,s finement diss~min~. Le volcanisme basaltique et and~itique est probablement/k la souroe du fer et du soufre de la coloration ferrugineuse daus les fad~s schistes noirs de la base et du sommet de la sc~rie.Des bact~ries r~ductrices des sulfates pt~entes autour des chemin£,es volcaniques pourraient etre A l'origine de la matitTe organique des tits les plus fonc~s. Des rhythmites turbiditiques passant ~t des gr~s fluvio-deltaiques surmontent le facies schistes noirs basal et les laves associ~ de Bushy Bend; dans le sud-ouest du bassin, aussi bien les rhythmites que les g~s contiennent des schistes noirs en tant qu'intercalatious fines ou que matrice, ce qui sugg~re la possibitit~ d'une 6mission fumerollienne dans cette ~gion. La r~l~tition des facies rhythmiques montre/h nouveau une association avec de fines intercalations de schistes noirs dans le sud-ouest du bassinet ce facies est sulvi par une seconde occurrence d'tm facies ~tschistes noirs, sous-jascent ~ la formation and~sitique de Hekpoort. Les associations observers entre les schistes noirs et les rythmites turbiditiques, les laves sans structure en pillow et les gr~ fluvio-deltaiques sugg~ent des hauteurs d'eau variant de faible/~ quelques centaines de m~l~s. Des potentialit~s de gisements en sulfures massifs volcanog~niques et d'exbalaigon s&timentaire existent dans l'association schistes noirs comme il y ont des roches volcaniques a la base de la formation.
INTRODUCTION
are related to o x y g e n - p o o r settings w i t h i n either s e m i isolated m a r i n e basins, s u c h as t h e m o d e m Black Sea, the epeiric seas o f the geological past, o r stratified lakes, w i t h the o x y g e n deficiency b e i n g associated w i t h either an increase in oxygen consumption below high productivity zones or with a decrease in supply ( D e m a i s o n a n d M o o r e 1980). P a l a e o e n v i r o n m e n t s o f d e p o s i t i o n are n o r m a l l y i n f e r r e d to h a v e b e e n d e e p w a t e r b a s i n centres ( B o a r d m a n et al., 1984), b u t m a y also c o m p r i s e relatively s h a l l o w w a t e r coastal settings
T h e o r i g i n o f b l a c k o r g a n i c - r i c h m u d r o c k s , 'q3lack shales", h a s l o n g b e e n a subject o f d e b a t e , p a r t i c u l a r l y w i t h r e g a r d s to t h e i n f e r r e d d e p t h s o f f o r m a t i o n , w i t h e s t i m a t e s r a n g i n g f r o m s h a l l o w w a t e r to h u n d r e d s o f m e t r e s o f w a t e r (I-lallz~_ 1967; Leckie et al., 1990; Wignall 1991). T h e b l a c k p i g m e n t a t i o n is n o r m a l l y a s c r i b e d to either o r g a n i c m a t t e r o r d i s s e m i n a t e d f i n e - g r a i n e d iron s u l p h i d e s ( F r i e d m a n et al., 1992). Generally, black shales 325
326
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Figure 1. Map illustrating the location of the Timeball Hill and underlying Rooihoogte Formations within the Pretoria Group of the Transvaal Sequence. Note also small inset m a p showing the Transvaal and Griqualand West Sequence basirLs and the Vryburg Rise between them.
Table 1. Stratigraphy and major lithologies of the Transvaal Sequence (modified after Schreiber (1990)). SEQUENCE
GROUP
FORMATION
Rooibcrg Htmtenbek Steenkampsberg Nederhorst Lakenvlei Vermont Magaliesberg Silverton Pmtoria Daspoort Stmbenkop Dwaalbeuwel Hekpoort Boshoek
Transvaal
MAJOR LITHOLOGY Fclsite; pyroclastic rocks
Timebali Hill Rooihoogte
Mudrock; quanzitic sandstone Quartzitic sandstone Mudrock; arkose Quanzitic/arkosic sandstone Mudrock Quartzitic sandstone Mudrock; lavas of the Machadodorp Member Quanzific sandstone Mudrock Quartzitic/lithic sandstone Basaltic andesite Quartzific sandsto~; mudrock; conglomerate Mudrock; quartzRic sandstone Conglon~rat¢; bn:t~ia; quanzRic sandstone
regional
unconformity
Chuniespoort
Duitschland Penge Malmani (Subgroup)
Dolomite; mudrock; diamictite Iron-formation: mudrock Dolomite; chert
Black Reef
Quanzitic sandstone
Wolkh~
Mmlrock; quartzitic sandstone; basalt; conglomerate
EarlyProterozoicblackshalesof the TuneballHillFormation,SouthAfrica: volcanogenicand palaeoenvironmentalinfluences 327 below wave base (Leckie et al., 1990). In both cases they can be associated with the lowest portion of transgressive systems tracts (Leckie et al., 1990; Wignall 1991). In this paper we examine the black shales in the Early Proterozoic Tuneball Hill Formation (Pretoria Group, Transvaal Sequence) (Table 1; Fig. 1) of South Africa. Their age implies formation within geological environments that were essentially anoxygenic (Eriksson and Cheney 1992), rather than forming in reducing waters below a pycnocline (a zone where water density increases rapidly with depth with regard to temperature and salinity). These black shales form the base of the T'uneball Hill succession passing up into rhythmically interbedded mudrocks, siltstones and fine-grained sandstones, in turn overlain by recrystallized quartzitic sandstones. An uppermost member repeats the basal two argillaceous facies. The Tuneball Hill Formation has been subject to low grade contact metamorphism (hornfels facies) in most parts of the basin due to the intrusion of the 2050 Ma Bushveld Complex into the upper parts of the Transvaal Sequence. The black shales, especially the basal facies, show a large degree of resistance to thermal alteration (Button 1973). Most previous workers have interpreted the black shales as being deep water anoxic marine and prodeltaic suspension deposits, the rhythmite facies as representing delta front turbidities and the medial sandstones being prograding fluviodeltaic sediments (Visser 1969, 1971; Button 1973, 1986; Eriksson 1973; Eriksson and Clendenin 1990; Schreiber 1990; Van der Neut 1990; Eriksson et al., 1993). The Tlmeball Hill black shales have thus been considered analogous to the marine basin-centre model. Colouration is due to both p r e s e r v e d carbonaceous material and finely disseminated pyrite, much of the latter having been replaced by limonite in surface outcrops and in shallow borehole cores (Button 1973; Eriksson 1973). The inferred deep water marine TmaebaU Hill black shales would also fit into a transgressive systems tract model, as they succeed the alluvial fan deposits of the underlying Rooihoogte Formation (Eriksson 1988). In this paper we will demonstrate that the organic carbon content of the black shales is generally low and that they have a more complex association with the Timeball Hill lithofacies. In addition, a strong association of the black shales with volcanic rocks will be described, which suggests an alternative exhalative origin for the colouration, with organic activity possibly playing a role in the reduction of volcanic sulphate to produce disseminated pyrite. Water depths of black shale formation are estimated to have been relatively deep due to the observed association with turbidite deposits. METHODS OF STUDY
Major element geochemical analyses presented in this paper were determined using standard X-ray fluorescence techniques on an ARL 8420 spectrometer
at the University of Pretoria. The organic carbon values cited are from B6hmer (1977), who utilized acid digestion of carbonate minerals and loss on ignition to obtain his results. Fieldwork was carried out around the Tlmeball Hill basin with samples obtained where fresh material was found and borehole~cores and logs utilized where available. Opaque iron minerals were identified by reflected light microscopy. FACIES RELATIONSHIPS OF THE BLACK SHALES WITHIN THE TIMEBALL HILL FORMATION Stratigraphy and Facies A r c h i t e c t u r e of the Thneball Hill Formation Three major and two subordinate lithofacies are identified in the Timeball Hill Formation: black shales, rhythmically interbedded mudstones, siltstones and fine-grained sandstones (the rhythmite fades), and a sandstone facies, with minor occurrences of arkosic sandstones and a diamictite/immature wacke facies (~rmser1969; Button 1973,1986; Erikseon 1973; Eriksson and Clendenin 1990) (Fig. 2). With the exception of locally erosive conglomeratic sandstones in the sandstone facies, and erosive bases to the localized diamictites, wackes and arkosic sandstones in the two subordinate facies, contacts within the T'uneball Hill Formation are gradational. No regional unconformitybounded units are known and the facies architecture is essentially comprised of sheetlike units (Fig. 2). The widespread basal black shales grade up into the rhythmite facies, which in turn passes gradationally up into a medial sandstone facies. Throughout most of the basin these sandstones grade up into a second development of the rhythmic facies. The latter includes a lens of arkosic sandstone in the Potgietersrus region and, in the south-south-west of the basin, is overlain gradationally by a more localized black shale occurrence. A lens of black shale is found within the upper rhythmites at Penge, and another, succeeded by localized diamictites and wackes, occurs in the Pretoria region (Fig. 2). A thin layer of diamictite overlies the sandstone facies at Potchefstroom. In addition to the large scale distribution of black shales outlined above, this rock type also occurs as a subfacies of the sandstone, rhythmite and diamictite facies. At Potchefstroom, black shales form the mudstone element within the rhythmite facies, are present as either matrix material or as thin interbeds in the sandstone facies and form part of the matrix in the diamictite facies (Fig. 2). At Swartruggens (Fig. 2) black shales are interbedded with siltstones and fine-grained sandstones in the rhyttunite facies and at Gopane, they are found interbedded in the sandstone facies. Similar subordinate black shales within both rhythmite and sandstone facies are reported from eastern Botswana (Key 1983) in the area immediately to the west of Gopane. Elsewhere in the Th-neballHill basin, the black
328
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G. ER1KSSON,B. F. E RECZKO,R. K. W. MERKLE,U. M. SCHREIBER,J. E ENGELBRECFFI,M. RESand C. P. SNYMAN
shales are found as a discrete facies and not as a subfacies of the other lithotypes in the formation. This suggests that the formation of black shales was initially a basin-wide feature, forming the basal occurrence shown in Fig. 2, and that further black shale formation in the basin was restricted largely to the PretoriaGopane area.
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Figure 2. A fencediagram based on field profilesand boreholecores through the TimeballHill Formation illustrating sheetlikegeometry and vertical arrangement of lithofacies. Note the widespread basal black shale faciesand the more restricted repetition of this faciesin the upper part of the formation in the south-west of the basin.
Depositional Formation
Systems
in the Timeball
Hill
Previous workers are in agreement that the basal carbonaceous mudrocks are relatively deep water anoxic suspension deposits, and that the rhythmite facies p o s s e s s e s s e d i m e n t a r y structures (current lineations, load casts, flute and groove casts, mudclasts, localized r i p p l e m a r k s a n d cross-lamination) compatible with turbidity current resedimentation of distal delta deposits (Rust 1961; Kuenen 1963; Visser 1969, 1972; Button 1973; Eriksson 1973; Eriksson and Clendenin 1990). The upward-coarsening sandstone facies is interpreted as tidally reworked delta front sands (Visser 1969; Button 1973, 1986; Eriksson 1973; Eriksson et al., 1991), with localized oolitic ironstones having been deposited in a shallow water, offshore environment (Eriksson et al., 1993). The T'tmeball Hill palaeoenvironment comprised a relatively deep water basin, filled by fluviodeltaic complexes advancing from the north, north-west and north-east (Schreiber 1990; Eriksson et al., 1993). Only the more distal deltaic and basinal deposits of this system seem to have been preserved, with the exception of the arkosic sandstone lens in the Potgietersrus region (Fig. 2), which is inferred
to have been a more proximal fluvial s a n d s t o n e (Eriksson et al., 1991). A strong glacial influence in the Timeball Hill palaeoenvironment is postulated by Visser (1971) and Eriksson et al. (1991, 1993) due to the diamictite facies in the formation itself, which includes striated pebbles, and due to the varved shale and further striated pebble diamictites in the succeeding Boshoek Formation (Table 1). Visser (1971) correlates all of these lithologies with the Makganyene diamictites of the Griqualand West Sequence (a correlate of the Transvaal Sequence), which are of undoubted glacial origin. The diamictites within the Timeball Hill Formation, which are associated with slumped wackes and conglomerates, are thus seen as being reworked periglacial material, transported into the basin from a centre of glaciation located on the Vryburg Rise (a palaeohigh separating the Transvaal and Griqualand West basins) (Visser 1971; Eriksson et al., 1993; Fig. 1). There has been much debate in the literature whether the Timeball Hill basin was epeiric m a r i n e in n a t u r e or w h e t h e r it r e p r e s e n t e d an intracratonic meltwater depository (see Eriksson et al., 1991 for a summary of this debate). The discrimination of Early Proterozoic marine and freshwater basins is fraught with problems, but palaeosalinity trends (based on boron content) plotted against stratigraphic height for a number of boreholes in the south of the basin are similar and thus suggest a closed basin (Eriksson 1992). Volcanic R o c k s a s s o c i a t e d w i t h the B l a c k S h a l e s
Throughout much of the Timeball Hill basin, the basal black shales are associated with volcanic rocks, which reach their m a x i m u m d e v e l o p m e n t in the Potchefstroom region, which is also the part of the basin with the m a x i m u m d e v e l o p m e n t of black shales throughout the stratigraphy of the formation. Boreholes within this region have penetrated up to 39 m of lava over an area of about 30 km by 15 km. These lavas lie i m m e d i a t e l y a b o v e the R o o i h o o g t e F o r m a t i o n conglomerates and are in turn succeeded b y black pyritic shales of the Lower Ttmeball Hill Formation (Fig. 3). They are informally named the Bushy Bend Lavas and have chemical compositions (Eriksson et al., in press) consistent with them having been tholeiitic (Irvine and Baragar 1971) or andesitic basalts (De la Roche et al., 1980). Flow thicknesses vary between 0.3 m and 13 m and flows exhibit chilled and undulating bases; one flow exhibits pipe amygdales (Fig. 3). It would appear from Fig. 3 that a fairly complex history of volcanism p r e c e d e d d e p o s i t i o n of the black shales u n d e r discussion in this paper. Contact relationships with the lowermost black shales of the TimebaU Hill Formation are significant for this study: in borehole B, lava fragments are found in the lowermost shales, and in borehole C shale fragments occur within the uppermost crystalline lavas. This implies that the lowermost pyritic shales and the uppermost lavas are approximate time equivalents.
Early Proterozoicblack shales of the "I~meballHill Formation, South Africa: volcanogenicand palaeoenvirenmental influences 329
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Figure 3. Logs of three boreholeswhich penetrate the BushyBend Lavas in the Potchefstroomarea. Note the basal amygdaloidal lava flows are succeededby massivelocallyporphyritic lavas and uppermost brecciated siliceousvolcanicrocks. Immediately to the west of the area shown in Fig. 2, in eastern Botswana, Key (1983) reports a basal I m thick pyrite bed in the Lower Timeball Hili Formation, in which the sulphides are set in a variable matrix of grit, shale, chert, brecciated chert and feldspathic tuff; pyrite crystals are up to 8 cm 3in size and locally make up 20% of the mode. At Gopane (south-western Transvaal) (Fig. 2), chert interbedded in the basal pyritic T'tmeball Hill black shales contains possible shards of volcanic f r a g m e n t s (Klop 1978). In the Pretoria region, a mudclast conglomerate in the Rooihoogte Formation, which underlies the black shales of the T'tmeball Hill Formation (Table 1), contains very weathered clasts with preserved igneous textures and possible flow strucha~. These rocks have anomalously high Cr and Ni contents, as well as an enrichment in alumina and TiO2 (Van der Neut 1990), supporting a possible mafic or intermediate volcanic composition. X-ray diffraction analysis of these clasts indicates a predominant composition of quartz, kaolinite, muscovite and haematite. These could be weathering products of basaltic rocks with plagiocla~ producing the kaolinite and pyroxene weathering to form iron titanium oxides which led to higher I i O z values. It is thought that these weathered clasts in the conglomerate represent reworked volcanic rocks in the Pretoria region. In the eastern Transvaal, stretching northwards from Carolina (Fig. 2) for sixty kilometres within the basal black shales of the "Ftmeball Hill Formation, is a thin bed of weathered tuff (A.G. Bloomer pers. comm. 1993). This bed contains definite decomposed shards a n d has a maximum thickness of 0.5 m. An anomalous platinum
group metal content suggests a mafic composition but extreme weathering and eluviation of constituents in field outcrops makes chemical analysis pointless. Due to the possibility of economic mineralization, no b o r e h o l e cores for this area are a v a i l a b l e for examination. The more limited u p p e r m o s t black shales in the Thneball Hill Formation, those between Potchefstroom and Swartruggens and at Pretoria (Fig. 2), are spatially associated with the H e k p o o r t Formation basaltic andesites. At Swartruggens thin Boshoek Formation conglomerates and sandstones (Table 1) separate the Upper TLmeball Hill black shales from the Hekpoort lavas, but at Potchefstroom and Pretoria the Boshoek rocks are absent and the lavas directly overlie the black shales. In the Potchefstroom area the Hekpoort lavas i n c l u d e thin b e d s of t u f f a c e o u s b l a c k shales (Engelbrecht et al., 1986), and at Pretoria tuffaceous shales occur at the base of the Hekpoort succession (Visser 1969). Elsewhere in the basin, significant thicknesses of Boshoek sandstones and conglomerates (with subordinate Hi~mictites and shales) separate the Upper T'maeball Hill shales and the Hekpoort andesites (Schreiber et al., 1990). PETROLOGY OF THE BLACK SHALES
In thin section, these rocks are seen to be comprised of quartz and feldspar silt particles, d a y mineral grains, micas (mainly muscovite and minor phlogopite), opaque iron minerals and carbonaceous matter. Black
330
-P. G. ERIKSSON,B. E F. RECZKO,R. K. W. MERKLE,U. M. SCHREIBER,J. P. ENGELBRECHF,M. RES and C. E SNYMAN
colouration, mostly due to disseminated limonite, is w i d e s p r e a d a n d c ont r as t s with distinct l a m i n a e pigmented by very finely disseminated carbonaceous material (Fig. 4). The latter largely disappears after the heating of thin slices of black shale to 800 °C over four hours (Figs. 5 and 6), which also results in a pervasive red pigmentation of the slice. This is due to the limonite, which becomes dehydrated and converted to haematite on heating, providing the red colouration; some black coloured opaque iron minerals remain, however, due to t he p r e s e n c e of s u b o r d i n a t e i l m e n i t e . T he ferruginous minerals have anhedral forms in the south and the south west of the Timeball Hill basin, but limonite p s e u d o m o r p h s after pyrite with euhedral forms are very common in surface outcrops in the east of the basin (Button 1973). Pyritic black shales are also well developed locally in the southwest of the basin and in eastern Botswana (Martini 1975,1982; Klop 1978; Key 1983). The iron minerals have also been subject to secondary recrystallization as they commonly overlap onto silt and clay grain boundaries. The carbonaceous
material observed is in the form of very small flakes, which form a vein-like pattern within dark laminae. The black shales thus, have an overall dark colour due to disseminated opaque iron minerals, mainly limonite (commonly pseudomorphous after pyrite), with certain laminae being pigmented by carbonaceous material to give an additional and darker colouration (Fig. 4). In hand specimen the black shales are dark grey or black in colour, depending on the presence of only iron pigmentation or additional carbonaceous laminae. They are characteristically laminated, with thicknesses varying between less than a millimetre and 5 ram, an alternation of silty and clay laminae being common. Certain laminae are also notably micaceous. Under the microscope no grading of these laminae is seen, the only feature visible being a slaty cleavage at about 20o-45° to the laminations. The more silty laminae make up about 5-10 % of the rock in outcrop and exhibit planar crosslamination and small sinuous current ripple marks. Silty lenses, 4-6 cm thick and 30-80 cm long, occur locally, c o m m o n l y as a series of connected lenses
Figure 4. Photomicrographof black shale illustrating the pervasive colouration due to disseminated opaque iron minerals and a more intense pigmentation of d ~ lamina with finely disseminated flakesof carbonaceousmaterialflowerhalfof photograph).Note the presenceof opaque iron mineralsin the latterlamina as well. Length of field of view 6.2 mm.
Figure 5. Photomicrograph of two laminae of disseminated carbonaceousmaterialbeforethe heatingof a thin sliceofblackshale to 800 °C for four hours. Lengthof field of view 9.9 ram.
Early Proterozoicblack shales of the Tuneball Hill Formation,South Africa: volcanogenicand palaeoenvironmental influences 331
by the
w o r k of Schreiber and colleagues (Table 2; B6hmer 1977). However, when individual black shale samples from this southern region are studied their geochemistry shows very large variations in the major elements, particularly in SiO 2, A1203, total iron and to a lesser degree, in MgO contents (B6hmer 1977; Table 3). In addition, organic carbon contents are extremely variable, from 12% to zero, CO 2 is higher in samples enriched in MgO and total iron, and sulphur is generally low with only one sample out of nine having more than 1% (Table 3). The c o m b i n a t i o n of higher iron, magnesium and CO 2 values, together with low CaO values in samples L1/36 and L1/38, indicates the presence of siderite and Mg-rich siderite (sideroplesite), minerals which were not seen in thin sections, which is probably due to small grain sizes and the overall dark pigmentation of the black shales. The generally low sulphur and high total iron contents are compatible with the observed predominance of limonite over pyrite as the pigmenting mineral. When organic carbon contents (B6hmer 1977) are plotted against stratigraphic height for a number of boreholes in the south of the basin (Fig. 7), it can be seen that high carbon values are restricted to the basal black shales and to rhythmites overlying the medial sandstone member. Black colouration due to c a r b o n a c e o u s material is t h u s limited, a n d concentrated in specific stratigraphic horizons, with iron minerals being responsible for the general black p i g m e n t a t i o n seen t h r o u g h o u t the Timeball Hill Formation in this part of the basin.
Figure 6. Photomicrograph of same thin slice of black shale as in Fig. 5 after heating to 800 °C over four hours. Note that the upper, thicker lamina has largely disappeared but that the lower thinner one still retains much of the disseminated carbonaceousmaterial. The latter is thought to reflect differences in permeability and cementation within the slice. Length of field of view 9.9 ram. enclosed in dark coloured clay-rich laminae. This feature is widespread within the black shales of the Timeball Hill Formation (Visser 1969; Button 1973; Eriksson 1973; Eriksson and Clendenin 1990). Pyritic cone-in-cone structures, analogous to those described by Carstens (1985), are found locally. G E O C H E M I S T R Y OF THE B L A C K S H A L E S
When compared to average Proterozoic shales, the Pretoria Group exhibits small depletions in SiO 2, CaO and Na20, whereas Al20 s, total iron and to a lesser extent MgO are enriched (Table 2). When compared to the rest of the Pretoria Group, the Timeball Hill Formation has a further enrichment of Al203 and total iron and a depletion in both CaO and MgO (Schreiber 1990; Schreiber et ai.,1992; Table 2). T'uneball Hill shales from the Potchefstroom area in the south of the basin (Fig. 1), which are p r e d o m i n a n t l y black coloured (Eriksson 1973), exhibit the same geochemical trends in average values of the major elements as those shown
DISCUSSION Examination of thin sections of the black shales in the Tm~eball Hill Formation has demonstrated that their pigmentation is due to both carbonaceous material and to opaque iron minerals. We have further shown, that the carbonaceous material is very finely disseminated in discrete laminae and beds within the stratigraphy (Figs. 4 and 7), whereas the ferruginous colouration is pervasive. The latter is due largely to limonite, much of it pseudomorphs after pyrite, and to a lesser extent due to pyrite (partiollRrly in the southwest of the basin), and to minor ilmenite. These observations suggest that the black colour is due to a widespread development of ferruginous minerals in these mudrocks, much of w h i c h w a s primarily pyrite, before alteration to hydroxides and oxides took place. A more intense black p i g m e n t a t i o n o c c u r r e d either locally, or, in the Potchefstroom region, at two stratigraphic levels (Fig. 7), due to preserved organic carbonaceous material. The enrichment of alumina within the Tlmeball Hill shales, and also within the black shales (Tables 2 and 3), may be associated with a glacial source and generally cold conditions within the palaeoenvironment, as suggested by Visser (1971) and Eriksson et al. (1991, 1993). The chemical breakdown of feldspar under such conditions would be inhibited. The large variation in
332
P.G. E R I ~ , B. E E RECZKO,R. K. W. MERKLE,U. M. SCHREmER,J. E ENGELBRECHT,M. RESand C. 19.SNYMAN
Table 2. Average m a j o r e l e m e n t c o m p o s i t i o n s (given as the w e i g h t percent of oxides) of the Pretoria G r o u p shales, the T'maebaU Hill F o r m a t i o n shales, the black shales of the T'tmeball Hill F o r m a t i o n (all recalcttlated to 100 % volatile free basis) a n d a v e r a g e shales. (FeO t = total iron as FeO). 1
2
3
4
5
6
SiO 2
62.37
61.39
53.02
64.80
64.21
66.90
TiO 2
0.64
0.72
0.68
0.70
0.72
0.78
A1203
19.07
22.85
22.18
16.90
17.02
16.67
FeO t
7.68
9.29
10.45
5.66
6.71
5.87
MnO
0.05
0.05
0.07
0.06
0.50
0.06
MgO
3.10
0.86
1.18
2.86
2.70
2.59
CaO
2.71
0.26
0.59
3.63
3.44
0.53
Na20
0.56
0.63
0.35
I. 14
1.44
1.50
K20
3.58
3.56
1.38
3.97
3.58
4.97
P205
0.16
0.29
0.11
0.13
0.19
0.14
1 - Pretoria Groupa v e r a g e shale (n = 76) (Schreiber 1990) 2 - Ttmeball Hill F o r m a t i o n a v e r a g e shale (n = 21) (Schreiber 1990) 3 - Average Ttmeball Hill F o r m a t i o n shale f r o m the Potchefstroom region in the s o u t h of the basin; these shales are p r e d o m i n a n t l y black coloured (n = 204) (B6hmer 1977) 4 - N o r t h A m e r i c a n Shale C o m p o s i t e (Gromet et al., 1984) 5 - A v e r a g e shale (Clark 1924) 6 - Average C a n a d i a n Proterozoic shale ( C a m e r o n a n d Garrels 1980) Table 3. Major d e m e n t c o m p o s i t i o n s (weight percent) of individual black shale s a m p l e s in the Timeball Hill F o r m a t i o n (data f r o m B 6 h m e r (1977)). (FeO t = total iron as FeO). L1/36
L1/37
L1/38
L1/39
L1/42
L2/53
L2/54
L4/36
LA/37
SiO 2
47.83
55.25
28.55
69.17
69.00
58.64
60.07
63.58
62.78
TiO 2
0.67
0.51
0.16
0.57
0.5 ~
0.69
0.75
0.64
0.65
AI20 3
23.51
28.34
8.80
16.64
14.82
27.30
23.00
20.58
24.06
FeO t
12.41
5.45
33.30
4.29
6.26
4.05
7.30
6.93
5.26
MnO
0.06
0.16
0.20
0.02
0.14
0.03
0.04
0.30
0.02
MgO
2.00
0.85
3.10
0.72
0.73
0.62
0.87
0.72
0.56
CaO
0.32
0.14
0.44
0.58
0.10
0.49
0.39
0.44
0.19
Na20
0.20
0.27
0.13
0.13
0.23
0.22
0.13
0.18
0.16
K20
1.76
1.57
0.86
1.51
1.22
1.64
t .46
1.66
1.88
P205
0.13
0.03
0.00
0.04
0.04
0.12
0.09
0.05
0.05
C
0.58
0.35
12.00
0.00
0.00
0.83
0.37
0.70
0.01
CO2
4.56
1.42
9.50
1.83
1.64
0.00
0.98
0.31
0.07
0.17
0.30
0.08
0.24
1.58
0.10
0.07
0.04
0.04
H20
4.20
4.71
2.40
3.85
2.42
4.93
4.01
4.05
4.08
H20-
0.31
0.25
0.26
0.40
0.45
0.21
0.70
0.53
0.17
TOT.
98.71
99.46
99.78
100.0
99.14
99.87
99.71
100.45
99.99
S +
Early Proterozoicblack shales of the TimeballHill Formation, South Africa: volcanogenicand palaeoenvironmental influences 333
®
comm. 1993) reports a basal black shale-tuff-sulphide
I
=,.lO.S
26°40"S r~l Ce B A POTCHEFSTROOM
27"E
28°E
®
Irn) 25 !
o
N m
% organiccarbon 0
I
2
3
Sandstone facies
]
Rhythmite facies
I
Basal black shale facies
TlmebaU
Hill
~
Rooihoogte Formation (chert breccias)
Formation
Figure 7. Variation in organic carbon content against stratigraphic height for three bore_holesin the Potchefstroomarea. The organic carbon values are from B6hmer (1977).Note the high values at two stratigraphic intervals: within the basal blackshale faciesand within the rhythmite faciesabove the medial sandstones. SiO 2contents noted in the chemistry of the black shales is most likely a reflection of the proportion of silty material within the samples examined. The enrichment of iron in the black shales, the pyritic argillites found in the south west and south of the basin together with common limonite (pseudomorphous after pyrite) in the east of the depository, point to a possible fumarolic volcanic influence in the colouration of the black shales. This h y p o t h e s i s is s u p p o r t e d f u r t h e r b y the association of the basal black shale facies with volcanic rocks. Klop (1978) noted an association of pyritic black shales and possible shards in the southwest of the basin and there is evidence for the reworking of volcanic material below the black shales in the Pretoria region. Key (1983) reports a basal black shale-tuff-pyrite association in eastern Botswana and A.G. Bloomer (pers.
mineralization association over a strike length of 60 km in the eastern Transvaal. The most specific data on volcanic rocks immediately below or within the basal Tuneball Hill black shales is from the Potchefstroom area. Here tabular andesitic-basaltic lava flows are porphyritic locally and c o m m o n l y amygdaloidal, sometimes exhibiting pipe amygdales (Fig. 3); this suggests possible subaerial eruption. This suggestion is compatible with the chilled bases of individual lava flows and the lack of pillow lavas. The uppermost brecciated lavas on the contact with the overlying pyritic black shales (Fig. 3) may reflect more rapid s u b a q u e o u s cooling of eruptive magma. It is thus suggested that the transition from the subaerial alluvial fans a n d s h a l l o w l a c u s t r i n e c o n d i t i o n s of the Rooihoogte Formation (Eriksson 1988), to the black shales of the Lower Timeball Hill Formation, was accompanied by fumarolic basaltic-andesitic volcanism; the latter may have been subaerial locally or took place within the Timeball Hill basin, then starting to form. Such volcanism could have provided a source of both iron and sulphur to form the features observed in the ferruginous black pigmentation of the shales, and would most likely have been associated with sulphatereducing bacterial organisms which led to the formation of localized carbonaceous black shales. The uppermost black shales of the Timeball Hill Formation in the Potchefstroom and Pretoria areas (Fig. 2) are overlain directly by the Hekpoort lavas, which include tuffaceous shales in their lower portions (Visser 1969; Engelbrecht et al., 1986). A similar volcanic source of black shale colouration may thus be postulated for these rocks. The lenficular black shales at Penge, and those overlying the diamictites at Pretoria (Fig. 2), show no direct association with volcanic rocks although they may represent the resedimentation of black coloured mudrocks from the basal portion of the Ttrneball Hill stratigraphy. E n o r m o u s quantities of iron s u l p h i d e s can be g e n e r a t e d b y basaltic a n d a n d e s i t i c v o l c a n i s m (Maynard 1983). One day after the eruption of the andesitic Augustine Volcano in Alaska ceased in 1986, emission rates of 24 000 tons per day of sulphur dioxide were recorded (Yount et al., 1987). Similarl~ pyrite is the d o m i n a n t ore mineral forming along basaltic spreading zones in the eastern Pacific Rise today (Dill et al., 1992). Although Martini (1982) reports small quantities of chalcopyrite in the Ttmeball Hill black shales, the black shales being studied here are almost exclusively associated with iron sulphides. Such iron sulphide-dominated volcano-sedimentary associations are well known throughout the world (Maynard 1983). It is thus suggested that fumarolic emission of iron and sulphur dioxide provided a source for the ferruginous pigmentation which characterizes the Timeball Hill black shales. H y d r o t h e r m a l vents associated with mid-ocean ridges in the present day oceans which emit sulphur
334
P.G. ERIKSSON,B. F. E RECZKO,R. K. W. MERKLE,U. M. SCHREIBER,J. P. ENGELBRECH'I,M. IRESa n d C. R SNYMA~N
are noted as oases of benthonic life, which sustain 10 000 to 100 000 times the amount of living matter in normal deep ocean settings (Gross 1990). Sulphatereducing bacteria play a pivotal role in the development of such vent communities with the bacteria protecting other organisms from sulphur poisoning and feeding off waste material derived from more organised life forms (Gross 1990). We suggest therefore that sulphatereducing bacteria may have led to the formation of pyrite in the early Timeball Hill basin. This led to a pervasive pyritic, black colourafion of the clays settling out of suspension in the basin and to discrete beds of carbonaceous material where decaying organic matter accumulated and was covered by the ferruginous suspension deposits. The w i d e s p r e a d association of the basal and uppermost black shales in the Ttmeball Hill Formation with volcanic rocks is thus t h o u g h t to reflect a palaeoenvironment in which fumarolic volcanism accompanied suspension sedimentation. Black shales within the upper rhythmite facies at Swartruggens, the s a n d s t o n e facies at G o p a n e and the diamictite, rhythmite and sandstone facies at Potchefstroom, may reflect the reworking of the basal shales. Alternatively, it may be speculated that fumarolic volcanism was relatively continuous in the south-western portion of the basin, resulting in black pigmentation of suspension muds, turbidites and delta front sands, which are the three main d e p o s i t i o n a l s y s t e m s within the palaeoenvironment of this formation. Water depths related to the formation of the Timeball Hill black shales are open to debate. The lack of pillow structures in both the Bushy Bend (Eriksson et al., in press) and Hekpoort (Sharpe et al., 1983) lavas is more c o m p a t i b l e w i t h a s h a l l o w w a t e r setting. The interbedded thin black shales in the rhythmite facies at Swartruggens, Gopane and in Botswana (Fig. 2) appear to be part of a turbidite depositional sequence (Eriksson et al., 1991, 1993). The minimum depths for turbidite formation along the California coast are a few hundreds of metres (Gorsline and Emery 1959; Moore 1969), < 200-250 m in fjords along the coast of British Columbia (Prior and Bornhold 1988) and considerably less in lacustrine settings, sometimes being less than 100 m (Rhine and Smith 1988) or even 60 m (Ashley 1975). Kersey and Hsii (1976) found turbidites forming in lakes Zurich and Walenstadt in Switzerland at depths of about 130 m. It is thus possible that the Ttmeball Hill black shales formed in depths of about 100 m or less as t u r b i d i t e s can be created artificially in f l u m e experiments at those depths (D.A. Leckie pers. comm. 1993). Shallow water black shale generation is also supported by the association of black mudrocks with the sandstone facies at Potchefstroom, Gopane and Swartruggens (Fig. 2) either as thin interbeds within the arenites or even as matrix material. The sandstone facies exhibits localized mudcracks and adhesion ripple marks in both the western part of the basin (Engelbrecht 1986) and in the Pretoria region, s u p p o r t i n g the
generally held view that these sandstones represent delta front sand deposits (Button 1973; Eriksson 1973; Schreiber 1990; Eriksson et al., 1991). It may thus be speculated that the Timeball Hill black shales formed at depths varying from shallow to some hundreds of metres. The widespread association of pyritic black shales and andesitic-basaltic volcanism at the base of the Timeball Hill Formation is of potential economic interest, particularly for volcanogenic massive sulphide and sedimentary exhalative ore deposits. The alluvial fan deposits, which are inferred to have characterized the underlying Rooihoogte Formation indicate syn-rift sedimentation (Eriksson 1988), with the deeper water Timeball Hill basin probably representing a post-rift setting. The change from a syn-rift to a post-rift tectonic setting is commonly associated with volcanism, as is the case here with the Bushy Bend lavas, and often forms a promising location for potential mineral d e p o s i t s . A zone t r e n d i n g n o r t h w e s t from Potchefstroom towards Gopane (Fig. 2) appears to have the best d e v e l o p e d a s s o c i a t i o n b e t w e e n welldeveloped, pyritic, black shales and volcanic rocks and w o u l d be a p r i m e target area. H o w e v e r , the predominance of iron sulphides and the general lack of observed copper and zinc sulphide mineralization in these black shales (Martini 1982) mitigates against the discovery of economically significant deposits. In addition, the absence of hydrothermal alteration, such as carbonization, chloritization and silicification within the black shales, supports such a cautious view. The platinum group element anomaly, associated with the basal Timeball Hill black shales in the east of the basin, must unfortunately also be considered as uneconomic, due to the small thickness of the tuff bed (< 0.5 m). CONCLUSIONS The black shales in the Timeball Hill Formation are p r e d o m i n a n t l y d a r k grey in colour, reflecting d i s s e m i n a t e d limonite with s u b o r d i n a t e pyrite. Localized laminae and beds within these dark grey shales display a more intense black pigmentation due to very finely disseminated flakes of carbonaceous material (Fig. 4). The w i d e s p r e a d f e r r u g i n o u s colouration is thought to have resulted from fumarolic volcanic emissions into the I"tmeball Hill basin, which were rich in both iron and sulphur. Sulphate-reducing bacteria p r o m o t e d the formation of pyrite, which produced the disseminated dark grey pigmentation of muds within the depository. Diagenesis subsequently altered much of the pyrite to limonite. The shallow water basin which developed in early Timeball Hill times was characterized by prodeltaic muds settling out of suspension with widespread volcanic activity, which included deposition of lava, pyroclastic eruptions and fumarolic emissions, resulting
EarlyProterozoicblackshalesof the TuneballHillFormation,SouthAfrica: volcanogenicand palaeoenvironmentalinfluences 335 in a basal black shale facies throughout most of the basin. As fluviodeltaic systems, reworking glaciallyderived detritus, advanced further into the basin, shallow water delta-slope turbidites covered much of the floor of the depository; producing a rhythmite facies of interbedded fine-grained sands, silts and muds. The medial sandstone fades was laid down across the basin (Fig. 2) as the deltas reached their m a x i m u m development. A continued glacial influence, largely restricted tosource areas lying to the west of the basin on the Vryburg Rise (Fig. 1), is evident from localized diamictite lenses above the medial sandstone facies (Fig. 2). As the delta systems retreated, the Ttmeball Hill basin was subject once more to turbiditic deposition of the rhythmite facies, with an uppermost black shale facies being laid down within the southwest of the basin (Fig. 2), reflecting volcanic activity related tO the Hekpoort lavas. The widespread basal black shale facies is associated with the development of the'Iimeball Hill basin, which succeeded the alluvial fan systems of the underlying Rooihoogte Formation, and may thus be considered analogous to a transgressive systems tract. However, the colouration of these mudrocks was probably due largely to volcanism in early T'unebaU Hill times and not to reducing conditions below a pycnocline as the transgressive basin deepened and enlarged, thereby preserving organic debris over a large area. The Timeball Hill Formation thus indicates that black pigmentation of shales can be the result of fumarolic andesitic to basaltic volcanism. In addition, it appears that water depths for black shale formation under these conditions varied from relatively shallow, to depths of a few hundreds of metres. Finally, there is limited potential for economic volcano-sedimentary base metal deposits in the basal portion of the Timeball Hill Formation.
Acknowledgements The authors wish to thank both the Foundation for Research and Development and the University of Pretoria for financial assistance. Dr M.R. Sharpe is acknowledged for XRF analyses and Mr M.Mahlangu for thin sections. Mr E.L. Brand, Consulting Geologist of Gengold, is thanked for permission to u~ili~ borehole data used in this paper, and the Buffelsfontein Gold Mining Company is acknowledged for providing cores which penetrated the Bushy Bend Lavas. We sincerely thank ProL I.W. H/ilbich and Drs A.C. Kendall and D.A. Leckie for their constructive criticism of an earlier draft of this paper, which enabled us to i m p r o v e the manuscript appreciably. Mrs IL Kuschke is thanked for typing and Mrs M. Geringer for drafting.
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