The Transvaal-Griqualand West banded iron formation: geology, genesis, iron exploitation

Journal of African Earth Sciences, Vol. 16, No. 1/2, pp. 63-120, 1993. Printed in Great Britain

0899-5362/93 $6.00+0.00 © 1993 Pergamon Press Ltd

The Transvaal-Griqualand West banded iron formation: geology, genesis, iron exploitation I. W. HALmCH,R. SCHEEPERS, D. LAMPRECHT, J. L. VANDEVENTER*and N. J. DE KOCK** Geology Department, University of Stellenbosh, Stellenbosh 7600, South Africa *Thabazimbi Iron Ore Mine, Thabazimbi 0380, South Africa **Sishen Iron Ore Mine, Sishen 8445, South Africa A b s t r a c t - Much research has been conducted on these banded iron formations (BIF) over the last 15-20 years. This contribution s e e k s to provide an overview o fold and new facts and critical discussion on the latest ideas regarding the origin of these sediments in the early Proterozoic. The recently suggested stratified ocean water model and a new stratified lake water model are compared using new evidence of a stratigraphic, major and trace element, stable isotope and REE nature. It appears that any hypothesis on the genesis of these Transvaal Supergroup rocks will have to satisfactorily account for at least the following: i) A constant supply of enough Fe and Si over at least several hundred thousand years. ii) A macro- and a micro-cyclicity. The latter is the most basic building stone and any hypothesis that can not explain these phenomena must be considered unrealistic. iii) The stratigraphic and isotopic evidence for heterogeneity of the waterbody. iv) The REE and trace element evidence for contributions from different sources. v) The distribution of organic carbon in rock facies and minerals. vi) The difference between Proterozoic and present day atmospheres and surface waters. vii) The fossil record of the early Proterozoic, and coupled to this the role that chelation, complex formation, stable colloids and co-precipitation played in weathering, transportation and deposition of Fe. viii) Factors influencing or controlling cyclicity. ix) The role that atmospheric and crugt-mantle evolution plays in producing most large BIF-deposits over a time span of about 500 Ma from the late Archaean into the early Proterozoic. x) The concomitant evidence provided by early Proterozoic paleosols. xi) The fact that several large Proterozoic BIF deposits are immediately preceded by platform carbonates. Finally, the two important ore districts, Sishen in the Northern Cape Province and Thabazimbi in the Central Transvaal, are dealt with. The general geology, mineralogy and genesis of these very high-grade major deposits are presented. New information on ore morphology and new evidence on multiple epigenetic enrichment o f BIF-protore are presented and discussed. The modern and specialised ore-mining, -processing and blending techniques at Sishen are explained.

important question these rocks and their counterp a r t s in A s i a , A u s t r a l i a a n d A m e r i c a p o s e o n t h e d e v e l o p m e n t of t h e g l o b a l a t m o s p h e r e a n d h y d r o sphere.

INTRODUCTION

The BIF of the Transvaal Supergroup have been t h e o b j e c t o f m a n y i n v e s t i g a t i o n s in t h e p a s t a n d the literature on this subject has become volumin o u s , q u i t e b e y o n d t h e s c o p e of full i n d i v i d u a l r e c o g n i t i o n in a n o v e r v i e w p a p e r s u c h a s t h i s one. However, the most important contributions with abundant references to previous titles are cited here. In South Africa these very Early Proterozoic BIF c a r r y i r o n ( h e m a t i t e ) a n d a s b e s t o s (crocidolite a n d a m o s i t e ) o r e s o f m a j o r i m p o r t a n c e to t h e c o u n t r y and of some significance on a global scale. This c o n t r i b u t i o n will d e a l w i t h t h a t o f t h e m u c h m o r e r e s t r i c t e d i r o n m i n e r a l s of e c o n o m i c i m p o r t a n c e . S h o r t c o m i n g s in o u r k n o w l e d g e will b e p o i n t e d o u t a n d f u t u r e r e s e a r c h t a r g e t s will b e a d d r e s s e d . S o m e n e w a p p r o a c h e s will b e i n d i c a t e d to t h e v e r y

DISTRIBUTION

- CRATONIC

SETTING

- AGE

T h e o u t c r o p s of t h e s e r o c k s o n t h e K a a p - V a a l craton are severely limited, a fact that does not always receive due consideration during genetic a n d e n v i r o n m e n t a l m o d e l l i n g . Five o u t c r o p a r e a s a r e k n o w n , t h e f o u r s m a l l e r o n e s viz. t h e P e n g e , Thabazimbi, Crocodile River Fragment and Z e e r u s t a r e a s o c c u r r i n g e a s t of a l o n g - l i v e d a x i s of i n t e r m i t t e n t u p w h a r p in t h e K a a p - V a a l c r a t o n w h e r e a s the N o r t h e r n C a p e ( G r i q u a l a n d West) a r e a lies w e s t o f t h i s u p w h a r p (Figs 1 a n d 2). These four areas are essentially erosional remnants truncated by a pre-Pretoria Group (Fig. 2 a a n d l e g e n d of Fig. 4) c h e r t f r a g m e n t lag

63

I.W. HXLBICH,R. SCHEEPERS,D. LAMPRECHT,J. L. VANDEVENTEKand N. J. DE I(OCK

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Fig. 1. The Kaap-Vaal craton: Structures pertinent to BIF deposition and deformation. All early Proterozoic (+ 2400 Ma) BIF outcrops shown. c o n g l o m e r a t e (Bevets conglomerate) p r o d u c e d b y a wide, s u b c r a t o n i c p e n e p l a n a t i o n p h a s e during w h i c h p a r t of t h e underlying i r o n s t o n e s a n d carb o n a t e s w e r e slowly eroded east a n d n o r t h of t h e u p w h a r p . The position a n d s h a p e of this zone of epeirogenic m o v e m e n t are revealed b y several long k n o w n features: a) The c r a t o n b a s e m e n t s t r u c t u r e a n d s e g m e n t ation as outlined by linear greenstone belts c h a n g e s along t h e N-S t r e n d i n g a r m of this upw h a r p from a N-S grain in t h e w e s t to a n ENE-WSW

t r e n d in the e a s t (Fig. 1). b) An early post-BIF erosion s u r f a c e h a s not developed to t h e w e s t of this axis w h e r e (apart from local effects on t h e M a r e m a n e d o m e s o u t h w e s t of K u r u m a n (Fig. 7)), a c o m f o r m a b l e stratigraphic s e q u e n c e h a s b e e n d e p o s i t e d u p to s o m e 2 0 0 Ma later w h e n glacial e r o s i o n o c c u r r e d , m a r k e d b y t h e M a k g a n y e n e tfllite (Table 1) (Beukes, 1987 p. 381). O b v i o u s l y this a r e a s t a y e d at or n e a r s e a level for a m u c h longer time t h a n t h e central a n d e a s t e r n T r a n s v a a l after BIF deposition.

The Transvaal-Griqualand West banded iron formation: geology, genesis, iron exploitation c) The BIF-stratigraphy west of the axis differs somewhat from that of the best known and most preserved record east of it (Beukes, 1987, 1980a; Dreyer 1982 and this paper). d) The trend of conspicuous linear magnetic anomalies (Comer, 1991) that m a y represent large s h e a r zones in the b a s e m e n t because no known surface p h e n o m e n a correlate with t h e m (Fig. 1). c) Several of these s t r u c t u r e s mentioned u n d e r a) and d) above are duplicated a n d / o r e n h a n c e d as weak zones by later crustal movements affecting cover rocks of the craton, such as the Mohlapitsi fold t r e n d in the E a s t e r n Transvaal and the fault and mineralization t r e n d s of Central-Northern Transvaal (the Thabazimbi-Rooiberg area - Fig. 1). In the western area (Griqualand West) the outcrop trend of the BIF and the Black Ridge t h r u s t zone (Beukes and Smit, 1987) duplicate the N-S basem e n t trend. Related t h r u s t s lately diseovered in the BIF themselves (Altermann and Htilbich, 1990, 1991) to some extent duplicate the already mentioned t r e n d of b a s e m e n t lineaments in this area (Figs 1 and 7). I} Pre-BIF and pre-Transvaal upwharp along a very similar N-S axis is indicated by the erosion of large parts of the u p p e r Ventersdorp strata (Pniel succession - Winter, 1976) in the W e s t e m Free State, N o r t h e r n Cape Province a n d W e s t e r n Transvaal, i.e., along and to the west of this axis. g) Although m a x i m u m thicknesses of 750 m m e a s u r e d by Dreyer (1986) for the Penge area, and of 300 m (Hartzer, 1989) for the Crocodile River F r a g m e n t are almost certainly tectonic thicknesses (Fig. 2a), It seems very likely that the BIF sequence east of the axis (in the Transvaal) thins against the u p p e r erosional contact from N to S and from E to W (Fig. 2b). The unconformity b e n e a t h the Bevets conglomerate m a r k s the first regionally developed positive epeirogenic m o v e m e n t s after BIf-deposition in the Transvaal. The original continuity of these ironrich deposits east of the warp axis is presently not doubted, b u t unproven. Therefore, s o m e w h a t loosely, a n d w i t h o u t a detailed stratigraphic analysis of these outcrops at Zeerust and in the Crocodile River Fragment, all these rocks are designated as Penge Iron Formation (see explanation of Fig. 4), as compared to the stratigraphically s o m e w h a t b e t t e r k n o w n A s b e s h e u w e l s Subgroup of the western part of the craton. The correlation across the 150 k m gap between the closest outcrops of the Penge Iron Formation and the Asbesheuwels Subgroup in Botswana along the w h a r p axis (Fig. 1) relies on the fact that both BIF-sequences conformably overly thick platform carbonates (Beukes, 1978, 1980; Tankard et al., 1982) of t h e M a l m a n i S u b g r o u p in the Transvaal and the Ghaap Subgroup in Griqualand

65

West respectively (Table 11. Outcrops of the latter two sequences are only separated by some 50 k m in Botswana (Fig. I). They both have a partially preserved and conformably overlying fine-grained clastic to partly c h e m i c a l s e q u e n c e , viz. the Duitschland (see subscript and legend of Fig. 41 and Koegas Formations (Table 1) respectively. After that, there was extensive erosion in the Transvaal with deposition of the Bevets conglomerate whereas the Koegas Formation and the Asbesheuwels Subgroup were unconformably covered by the Makganyene glacial conglomerate (Table 1). Locally the latter is disconformably overlain by widespread, partly s u b a q u e o u s , lava flows of the Ongeluk Formation dated at 2239 + 90/-92 Ma (Armstrong, 1987). In the Transvaal a mainly conformable succession of shales and s a n d s t o n e s of the Pretoria Group follows on Bevets conglomerate, but is interrupted by the Hekpoort lavas (age: 2224 +_21 Ma, Tankard et al., 1981), very similar to the Ongeluk Formation in chemistry and age. The higher up in the stratigraphy (Table 1 and legend of Fig. 4) one proceeds, the less direct a correlation is possible and it becomes obvious that the two sub-basins have definitely developed separately during the latter part of their histories. An age for the Asbesheuwels Subgroup (Fig. 7, Table 1) has been determined by dating single zircons from ash beds at 2432 + 31 Ma (Trendall et al., 1990). By the same m e t h o d the ages of graded lapilli tufts of the u p p e r part of the Nouga Formation conformably underlying the lower K u r u m a n BIF n e a r Prieska in Griqualand West (Figs 1 and 7) have been preliminary dated at 2550 _+ 10 Ma (W. Altermann, and C. Smith - personal communication). The Penge Iron Formation has apparently not been dated yet. Carbonates from the Schmidtsdrif Subgroup (Table I) have been dated at 2556 + 49 Ma by J a h n et al. (1991). Accordingly the sequence of c o n t i n u o u s deposition plus the upper hiatus in the Asbesheuwels Subgroup and the Koegas Subgroup could represent a m i n i m u m of about 60 Ma (2400-2340). This m u s t be seen in the light of calculations done by "Prendall (1983) on the a u t o c h t h o n o u s BIF of the Hamersley Group in Australia. He a s s u m e s that a microcycle pair, on average about 0.7 m m thick, represents a year. Then several h u n d r e d metres of a u t o c h t h o n o u s K u r u m a n BIF would scarcely need a few h u n d r e d t h o u s a n d years for their deposition. The basic unit would need to represent a sunspot cycle or a Milankovitch cycle if the time needed for deposition is a few million years as is suggested by a calculated rate of deposition of 3-4 m / M a for the chemical sediments of the Hamersley Group (Amdt et al., 1991). Should the cyclicity in a u t o c h t h o n o u s BIF be entirely of chemical or biochemical origin and controlled by environmental conditions, then

66

I. W. HALmC~,R. SCHEEPEP,S, D. LAMPRECHT,J. L. VAN DEVEWrm~and N. J. DE KocK Table 1. Stratigraphy of the Transvaal Supergroup in Grlqualand West, Northern Cape Province (after Beukes and Smit, 1987) as amended.

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Laterally into

KLIPPAN PAPKUIL

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Dolomite

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MONTEVILLE

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250

The Transvaal-Griqualand West banded iron formation: geology, genesis, iron exploitation a volcanic a s h layer w h i c h i n t e r r u p t s this cycle r e p r e s e n t a time interval ( n e c e s s a r y to r e e s t a b l i s h a c h e m l c a l b a l a n c e in t h e environment) m u c h longer t h a n t h e cycle itself. C o u n t l e s s larger a n d s m a l l e r ashfalls are r e c o r d e d in the K u r u m a n BIF of G r i q u a l a n d West. The time interval for deposition of t h e o r t h o c h e m i c a l - a l l o c h e m i c a l (Beukes. 1978) G r i q u a t o w n BIF t h a t follows c o n f o r m a b l y on t h e K u r u m a n BIF c a n not b e e s t i m a t e d b e c a u s e it c o n s i s t s essentially of u n c o u n t a b l e r e d e p o s i t e d e n d o c l a s t l c lenticular u n i t s s e p a r a t e d b y as m a n y h i a t u s e s of u n k n o w n length. REGIONAL GEOLOGY The Zeerust-area W e s t of Z e e r u s t in t h e Marico district, a maxim u m t h i c k n e s s of 60 m of Penge Iron F o r m a t i o n is r e p o r t e d to w e d g e o u t a g a i n s t a fault w e s t w a r d s to b e c o m e v e r y t h i n n e a r the b o r d e r of the Republic of S o u t h Africa with B o t s w a n a , ( H u m p h r e y , 1910 a n d W a l v a r e n , 1981). T h e y a r e p a r t of t h e L i n o k a n a Hills a n d c o n s i s t of clastic a s well a s c h e m i c a l c o m p o n e n t s , b u t a detailed s t u d y w a s a p p a r e n t l y never done. F r o m this very general d e s c r i p t i o n it w o u l d s e e m t h a t t h e y p o s s i b l y res e m b l e t h e G r i q u a t o w n F o r m a t i o n in G r i q u a l a n d West r a t h e r t h a n a u t o c h t h o n o u s BIF. U p p e r a n d lower c o n t a c t s w h e r e s e e n are r e p o r t e d to have s h a r p a s well a s g r a d a t i o n a l c o n t a c t s to contig u o u s s e q u e n c e s above a s well a s below. An overlying D u i t s c h l a n d F o r m a t i o n (see legend s u b s c r i p t of Fig. 4) or e q u i v a l e n t s are not reported a n d t h e s e q u e n c e is a p p a r e n t l y conformable, with the Bevets c o n g l o m e r a t e only a p p e a r i n g at t h e b a s e of t h e Pretoria G r o u p f u r t h e r east, t o w a r d s R u s t e n b u r g . No detailed m a p s or s e c t i o n s of t h e s e o u t c r o p s have b e e n p u b l i s h e d . It is therefore u n k n o w n w h e t h e r this s e q u e n c e r e p r e s e n t s t h e whole or only p a r t of t h e Penge Iron F o r m a t i o n a n d if so, w h i c h part. It c a n be s p e c u l a t e d that the u n f a u l t e d p a r t r e p r e s e n t s the total original sedim e n t d e p o s i t e d at very shallow w a t e r depth, w h e r e c h e m i c a l d e p o s i t i o n w a s followed shortly afterw a r d s b y p a r a - a u t o c h t h o n o u s redeposition. The BIF episode w a s p r o b a b l y short-lived a n d the site w a s , in a c c o r d a n c e with e a s t w a r d a n d n o r t h w a r d thickening of t h e s e BIF, r a t h e r close to t h e edge of the d e p o s i t o r y (Figs 2a a n d 2b). No exploration or mining activities of a n y import a n c e s e e m to have b e e n a t t e m p t e d on t h e s e outcrops, a n d it m u s t be a s s u m e d that significant iron a n d / o r a s b e s t o s m i n e r a l i z a t i o n h a s not b e e n o b s e r v e d in spite of r e p o r t e d faulting (Humphrey, 1910).

67

T h e Crocodile River F r a g m e n t Several t e n s of k i l o m e t r e s of BIF o u t c r o p s in this tectonic inlier of the B u s h v e l d I g n e o u s Complex have b e e n briefly m e n t i o n e d b y Verwoerd (1963) w h o s t a t e s t h a t the t h i c k n e s s e s v a r y from 2 3 0 m in one place to 100 m in general. T h e s e t h i c k n e s s e s are certainly tectonic b e c a u s e t h e r o c k s in t h e Crocodile River F r a g m e n t are highly d e f o r m e d during three fold p h a s e s . According to Verwoerd (1963) t h e y have also b e e n m e t a m o r p h o s e d a n d a fourfold m e g a cycle o c c u r s c o n s i s t i n g of m a c r o cycles changing from g r u n e r i t e - m i c r o b a n d e d chert n e a r the b o t t o m to g r u n e r i t e - m a g n e t i t e with occasional riebeckite n e a r t h e top. The Penge Iron F o r m a t i o n a s a whole b e c o m e s m o r e enriched in Fe-oxides u p w a r d s in the stratigraphy. The expos u r e s at this locality s e e m to b e m o r e c o m p a r a b l e to a u t o c h t h o n o u s I. F. a n d to t h e lower p a r t s of this formation at Penge, t h e locality in t h e North E a s t e r n T r a n s v a a l (see below). In the Crocodile River F r a g m e n t t h e u p p e r p a r t of the iron formation is certainly e r o d e d a n d overlain b y a b a s a l c o n g l o m e r a t e of the Rooihoogte F o r m a tion of the Pretoria G r o u p w h i c h a p p a r e n t l y is not morphologically similar to the Bevets conglomerate f o u n d in this stratigraphic position elsewhere in the Transvaal. The u p w a r d g r a d a t i o n of BIF to clastic r o c k s of the D u i t s c h l a n d F o r m a t i o n s e e n in the Penge area h a s n e v e r o c c u r r e d or this formation w a s later eroded. Some r e m a r k s are w a r r a n t e d o n the environm e n t a l c o n d i t i o n s obtaining during the position of the Frisco Formation, the y o u n g e s t c a r b o n a t e t h a t c o n f o r m a b l y u n d e r l i e s t h e Penge Iron F o r m a t i o n in this area. According to H a r t z e r (1989) it c o n s i s t s of c l a s t i c - l a m i n a t e d dolomite n e a r the b a s e followed u p w a r d s b y l a m i n a t e d fenestral dolomite t h a t is occasionally c r o s s - b e d d e d a n d i n t e r b e d d e d with c a l c a r e n i t e s n e a r its lower a n d central portions. A c h e r t shale m a r k e r - b r e c c i a n e a r t h e b a s e also testifies to occasional erosion. S o m e giant d o m a l s t r o m a t o l i t e s are also found. This f o r m a t i o n s e e m s to have b e e n d e p o s i t e d m a i n l y in a n intertidal to su pratidal e n v i r o n m e n t c o m p a r e d to the next lower Eccles F o r m a t i o n w h i c h c o n s i s t s m a i n l y of d o m a l s t r o m a t o l i t e s pointing to a n interdital s u b t i d a l regime. C o n t r a r y to the e n v i r o n m e n t a l interpretation given b y Hartzer, fenestral calcarenites a n d c r o s s - b e d d i n g point to u p w a r d shoaling (Tucker a n d Wright, 1990, p. 143-144) for the Frisco seq u e n c e in this area. If the a u t o c h t h o n o u s Penge Iron F o r m a t i o n t h a t follows s t r a t i g r a p h i c a l l y u p w a r d s , is a fairly deep w a t e r facies below the tidal range a n d the c h e m o c l i n e (Klein a n d B e u k e s , 1989), t h e n a large a n d very fast t r a n s g r e s s i o n h a s to be p o s t u l a t e d , for w h i c h t h e r e is no facies indication in the c a r b o n a t e s here.

I.W. I~LBICH,R. SCHEEPERS,D. LAMFRECHT,J. L. VANDEVENTERand N. J. DE KocK

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i~ / / .L.L% "~SP-1/F • Prieska

f f Overlayn by Makganyene glacials '~ via Koegas Subgroup south of F - F oo Overlayn by Bevets conglomerate AIIochthonous BIF estimated AIIochthonous BIF measured utochthonous BIF measured Basal contact to carbonate

iA

Fig. 2a. Kaap-Vaal craton with BIF-megafacies and total BIF-thicknesses as m e a s u r e d at 12 localities. W-2 = Weterberg BH; Sp- 1 = Spioenkop BH; D- 1 - Derby BH; MA-2 = Matlipani BH; WB-98 = Whitebank BH; PF = Pomfret BH AD-1; TA = Thabazimbi profile; ZE = Zeerust profile; KR = Crocodile River Fragment profile; CP = Chuniespoort-Malipsdrift BH M8; MO = Mohlapitsi BH MF2; PE = Penge BH PA 26.

W e c o n c l u d e t h a t t h e a v a i l a b l e e v i d e n c e in t h i s area need not mean that the Penge Iron Formation w a s n e c e s s a r i l y d e p o s i t e d in a d e e p e n i n g w a t e r body. S o m e v e r y l i m i t e d a n d l o c a l i z e d i r o n ore m i n i n g a c t i v i t y i n t h e u p p e r m o s t p a r t s of t h e P e n g e F o r m a t i o n is r e p o r t e d (Verwoerd, p e r s o n a l c o m m u n i c a t i o n , 1992).

The

Thabazimbi

Area

At T h a b a z i m b i in t h e n o r t h w e s t e r n T r a n s v a a l {Fig. i) t h e s t r a t i g r a p h i c s u c c e s s i o n f r o m t h e b a s e of t h e C h u n i e s p o o r t G r o u p u p to t h e P e n g e I r o n F o r m a t i o n (P. I. F.) (see l e g e n d , Fig. 4} is t h e s a m e a s for t h e E. T r a n s v a a l e x c e p t t h a t t h e r e t h e Duitschland Formation intervenes below the un-

The Transvaal-Griqualand West banded iron formation: geology, genesis, iron exploitation

250

69

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TRANSPORT DIRECTIONS High energy Low energy ? Rim of basin unknown

Fig. 2b. Kaap-Vaal craton with wharp axis and syn- to post-BIF zones of uplift and deposition. c o n f o r m i t y at t h e b a s e of the Rooihoogte F o r m a t i o n a n d t h e P. I. F. O t h e r differences are t h a t at Thabazimbi the Boshoek Formation and the D w a a l h e u w e l Q u a r t z i t e are a b s e n t f r o m t h e Pretoria Group. Also, the Wolkberg G r o u p below t h e Pretoria G r o u p is n o t developed a n d the Black Reef Formation directly overlies Archaean g n e i s s e s a n d g r a n i t e s of t h e K a a p v a a l Craton. The iron ore deposits o c c u r at or n e a r the base of the f e r r u g i n o u s s e d i m e n t s of the P. I. F. Van Deventer et aL (1986) reported the occurr e n c e of iron f o r m a t i o n s for a b o u t 100 k m along EW strike in a s o u t h e r n o u t c r o p belt a n d for a b o u t 40 k m along a n o r t h e r n tectonic duplication. The Frisco F o r m a t i o n , w h i c h u n d e r l i e s t h e oreb e a r i n g Penge F o r m a t i o n , w a s divided by Fourie (1984) into t h r e e zones viz. a b a s a l m a s s i v e dolomite u n i t , a middle algal l a m i n a t e d dolomite u n i t a n d a n u p p e r horizontally layered shale-d01omite unit. A b o u t 130 m from t h e top of t h i s formation, a m a r k e r b a n d of chert, overlain by a zone of discoidal dolomite, is developed. However, w h e r e

iron ore deposits occur, s o l u t i o n of t h e u p p e r m o s t dolomites c a u s e s the b a s e of t h e o r e b o d y to be a n y t h i n g from 100 m to 5 m above t h i s m a r k e r . The Penge F o r m a t i o n evidently h a s collapsed into solution d e p r e s s i o n s with c o n c o m i t a n t brecciation. In a typical profile (Fig. 3) the b a s e of t h e Penge F o r m a t i o n c o n s i s t s of s o m e 7 m to 10 m of a l t e m a t ing c h e r t a n d s h a l e b a n d s . The c h e r t b a n d s are either m a s s i v e , or i n t e r b e d d e d with c o n t o r t e d siderite m i c r o b a n d s . In the s h a l e b a n d s , pyrite n o d u l e s are often developed on the b e d d i n g planes. Where iron ore h a s developed, the b a s a l u n i t is brecciated a n d varies in t h i c k n e s s from 5 to 30 m. This r e s i d u a l breccia c o n s i s t s of c h e r t a n d shale f r a g m e n t s set in a w a d a n d s h a l e y matrix. The c h e r t - s h a l e u n i t is s u c c e e d e d b y a n oxide facies r h y t h m i t e zone with a t h i c k n e s s of 80 m. Chert m e s o b a n d s are well developed in t h e m o r e siliceous b a s a l p a r t of t h e zone, w h e r e a s t h e u p p e r part is d o m i n a t e d b y s h a l e b a n d s . The iron oxide zone is overlain b y a n iron silicate r h y t h m i t e zone with a t h i c k n e s s of 115 m a n d

I. W. HALBICH,R. SCHEEPERS,D. LAMPRECHT,J. L. VANDEVENTERand N. J. DE KocK

70

GROUP

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Fig. 3. Stratigraphic section of the Penge Iron Formation at Thabazimbi. o c c a s i o n a l crocidoIite m e s o b a n d s . D u e to c o n t a c t m e t a m o r p h i c effects of t h e B u s h v e l d I g n e o u s C o m p l e x o n t h e P. I. F., t h e iron silicates (stilpnom e l a n e a n d m i n n e s o t a i t e ) w er e a l t e r e d to biotite

a n d g r u n e r i t e - c u m m i n g t o n i t e (Beukes, 1978). T h e n follows a n iron oxide r h y t h m i t e z o n e with a t h i c k n e s s of 115 m m a r k e d by p i n c h a n d swell s t r u c t u r e s . Two m a s s i v e c h e r t m a c r o b a n d s with

The Transvaal-Griqualand West banded iron formation: geology, genesis, iron exploitation

71

DOORNHOEK 318KQ ROSSEAUSPOORT 319KQ SPffKOP 346KQ

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Fig. 4. Geological m a p of the T h a b a z l m b i a r e a a n d s t r a t i g r a p h i c c o l u m n for t h e T r a n s v a a l S u p e r g r o u p i n t h e Transvaal. Note t h a t in t h e E a s t e r n T r a n s v a a l t h e D u i t s c h l a n d F o r m a t i o n i n t e r v e n e s b e t w e e n t h e P e n g e Iron F o r m a t i o n a n d t h e Rooihoogte F o r m a t i o n below t h e d i s c o r d a n c y . N a m e s of M i n e s are: A = D o n k e r p o o r t - W e s t , B = Donkerpoort,-West, C = E a s t Mine, D = K w a g g a s h o e k - E a s t .

GROUP

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co

~

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disseminated iron oxide minerals are developed near the middle of it. At the top of the Penge Formation there is a zone of alternating well developed carbonate, chert and shale meso- and macrobands. This zone is discordantly overlain by elastic sediments of the Pretoria Group. The Thabazimbi area is situated within the Thabazimbi-Murchinson Lineament and is structurally complex. A number of east-west striking faults duplicate the stratigraphy and cause the southerly dips. Du Plessis and Clendenin (1988) suggested that the duplication resulted from wrench faulting. Subsequent differential erosion produced two prominent mountain ranges (Fig. 4).

0

~,x3io

,.I

T I M E B A L L HILL

re" I'-

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pe.Ge

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Z

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Quartzite Dolomite ~evets conglomerate Iron formation Chert poor dolomite Chert rich dolomite Chert poor dolomite Chert rich dolomite Dark coloured dolomite

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~.

(Posl-Karoo)

DIp & strike

I. W. H~LBICH,R. SCHEEPERS,D. LAMPRECHT,J. L. VANDEVENTERand N. J. DE KOCK

72

The N o r t h - E a s t e r n T r a n s v a a l In this a r e a b e t w e e n Penge in the s o u t h , the locally d e v e l o p e d ENE-trending Mohlapitsi fold zone at Mafefe in t h e n o r t h a n d Malipsdrfft a n d C h u n i e s p o o r t in t h e w e s t (Fig. 5), a n d following t h e s h a p e of t h e n o r t h - e a s t e m o u t c r o p of t h e B u s h v e l d I g n e o u s Complex, t h e P e n g e a n d Mafefe Iron F o r m a t i o n s are e x p o s e d below a n e r o s i o n s u r f a c e over a strike length of s o m e 120 k m at t h e b a s e of t h e Rooihoogte Shale F o r m a t i o n of the Pretoria Group. Fig. 6 gives t h e s t r a t i g r a p h i c s u b d i v i s i o n s of the iron f o r m a t i o n s a n d their r e l a t i o n s h i p s to t h e u n d e r - a n d overlying u n i t s after D r e y e r (1982). Stratigraphy The contact between the chert-free Frisco D o l o m i t e F o r m a t i o n (Fig. 6) a n d t h e M a l i p s M e m b e r of t h e Penge Iron F o r m a t i o n (P. I. F.) is conformable. Below t h e Frisco F o r m a t i o n the u p p e r p a r t of t h e c h e r t y Eccles F o r m a t i o n for at least several k i l o m e t r e s to t h e s o u t h - e a s t of Penge is i n t e n s e l y d i s t u r b e d over 2 0 - 3 0 m of stratigraphic t h i c k n e s s . The c h e r t y b e d s are i n t e n s e l y folded into r e c u m b e n t s a s well a s d i s r u p t e d a n d brecciated. If t h e s e c h e r t s are of diagenetic origin, as h a s b e e n a c c e p t e d b y T a n k a r d et al. (1982), t h e n this

d i s t u r b a n c e m a y b e i n t e r p r e t e d a s of tectonic origin, a n d it i n d i c a t e s t h r u s t i n g in t h e M a l m a n i S u b g r o u p . If, o n t h e o t h e r h a n d , t h e c h e r t s have originated as gel layers, t h e dislocation zone c o u l d also b e of s y n s e d i m e n t a r y origin, a n d r e p r e s e n t s gravity i n d u c e d s l u m p i n g b e c a u s e of b a s i n tectonics. The m e a s u r a b l e fold axes w i t h i n this zone c o n f o r m to t h o s e of t h e regional F 1 - p h a s e in the Penge a r e a (Auman, 1986) with a x e s trending s o u t h - e a s t e r l y . P o r p h y r o b l a s t s of chlorite a n d p r o b a b l y scapolite, grow a s long p r i s m a t i c minerals with preferred orientation a n d r u p t u r e a c r o s s b o u d i n a g e d c h e r t s a n d therefore c o u l d be syntectonic to F 1. This m u s t be s t r e s s e d here, b e c a u s e it m e a n s t h a t this d e f o r m a t i o n is n o t linked to t h a t of s e d i m e n t a r y t e c t o n i c s p r o d u c i n g t h e ENEtrending Selati T r o u g h in t h e Wolkberg G r o u p of the T r a n s v a a l S u p e r g r o u p . (Fig. 5). It is also u n related to the later Mohlapitsi folding, b o t h being a l m o s t at right angles to F1. Instead, a genetic c o n n e c t i o n to t h e B u s h v e l d intrusive is likely. All evidence therefore, s p e a k s for a tectonic origin of B u s h v e l d age, with t h e i n t r u s i o n p u s h i n g in a n o r t h - e a s t e r l y direction in the Penge area, a n d p r o d u c i n g a n o r t h - e a s t facing t h r u s t zone in the sediments. The lowest unit in t h e P. I. F. (Fig. 6) are t h e m e s o a n d m a c r o - b a n d e d c h e r t y iron f o r m a t i o n rhythm i t e s followed u p w a r d s b y d a r k grey to b l a c k car-

30°E

,,,lull,, 7//, 7// •

24 $ ,

/.

•

.. ........

o

evets

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conglomerate at base• Duitschland Formation where thick line. F/////A

Malmani Subgroup (dolomite)

Fig. 5. Geological map of the Penge-Malips drift area in the N. Eastem Transvaal.

73

The Transvaal-Griqualand West banded iron formation: geology, genesis, iron exploitation b o n a c e o u s a n d pyritic s h a l e s carrying dissemin a t e d p o r p h y r o b l a s t s of small a l m a n d i n e g a r n e t s and magnetite. These constitute the Malips M e m b e r (Dreyer, 1986). It is followed b y t h e S t r e a t h a m M e m b e r m a d e u p of several m a c r o cycles c o n s i s t i n g of gruneritic biotite fels - g r u n e ritic m i c r o b a n d e d c h e r t - gruneritic siderite b a n d r h y t h m i t e - gruneritic m a g n e t i t e - biotite r i b b o n a n d b a n d - r h y t h m i t e s . G r u n e r i t e a n d biotite are m e t a m o r p h i c m i n e r a l s f o r m e d m a i n l y from greenalite a n d stflpnomelane. This M e m b e r is overlain b y a n 8-10 m t h i c k pyritic m a r k e r - s h a l e with chert n o d u l e s a n d b a n d s . Next follow t h e E t o n M e m b e r of the P. I. F. with a s t r u c t u r e a n d c o m p o s i t i o n v e r y r e m i n i s c e n t of t h e S t r e a t h a m Member. In this c a s e however, a m o s i t e mineralization is f o u n d t o w a r d s the top. The Mohlapitsi M e m b e r of the Mafefe I. F. (Dreyer, 1986) h a s z o n e s of b r o a d gruneritic c h e r t b a n d s alternating with biotite - m a g n e t i t e - g r u n e r i t e lutites a n d fels. T h e y are crocidolite a n d a m o s i t e b e a r i n g in t h e lower parts. Both c h e r t s a n d fels t h i c k e n u p w a r d s into the Nice M e m b e r of t h e Mafefe I. F. a n d finally grade into m a s s i v e grey c h e r t s with t h i n intercalations of shales. In t h e Mohlapitsi folded zone t h e Mafefe I. F. overlies t h e Penge I. F. c o n f o r m a b l y (Dreyer, 1986) a n d t h e total (tectonics?) t h i c k n e s s is a b o u t 8 5 0 m e t r e s (Fig. 6}. P a s t Malips Drift a n d t o w a r d s C h u n i e s p o o r t in the w e s t a s well a s t o w a r d s Penge in t h e s o u t h (Fig. 5), the D u i t s c h l a n d F o r m a t i o n a n d t h e Mafefe I. F. h a v e b e e n eroded a n d the Bevets c o n g l o m e r a t e c u t s d o w n w a r d s , in t h e strat i g r a p h y until eventually it r e s t s directly on eroded M a l m a n i Dolomite to the s o u t h of Penge (Fig. 6).

W

The erosional r e m n a n t s of t h e only u p to 15 m t h i c k D u i t s c h l a n d F o r m a t i o n (Figs 5 a n d 6) f o u n d b e t w e e n t h e P. I. F. a n d t h e Rooihoogte F o r m a t i o n at Penge, c o n s i s t of f e r r u g i n o u s m i c r o - c r o s s l a m i n a t e d s h a l e s a n d sfltstones, c h e r t y F e - r h y t h m i t e s , occasional c a r b o n a t e a n d quartzite l e n s e s a n d a t h i n i m p e r s i s t e n t b a s a l c o n g l o m e r a t e . The overlying Bevets c o n g l o m e r a t e h a s two c o m p o n e n t s . One c o n s i s t s of m a t r i x s u p p o r t e d , D u i t s c h l a n d F. a n d B. I. F.-derived small p e b b l e s in a s h a l e y matrix. The o t h e r is c o n s t i t u t e d of a regionally developed, clast s u p p o r t e d a n g u l a r c h e r t - c o b b l e c o n g l o m e r a t e derived from e r o d e d c h e r t y dolomite f u r t h e r afield. The m a t r i x of t h e locally derived m a t e r i a l is m e t a m o r p h o s e d , c a r r y i n g dissemin a t e d g r u n e r i t e r o s e t t e s exactly a s f o u n d in t h e s h a l e s a n d fels of the D u i t s c h l a n d F. a n d the P. I. F. This proves t h a t all t h e s e m e t a m o r p h i c effects p o s t date t h e deposition of t h e Pretoria G r o u p a n d are therefore of B u s h v e l d age. Locally, listric t h r u s t s with small d i s p l a c e m e n t s f o u n d in m a n y p a r t s of the P. I. F. of the Penge area, also d i s t u r b the c o n t a c t with the Pretoria Group. Structure

The n a r r o w E N E - t r e n d i n g intractonic Mohlapitsi fold - s h e a r zone (Button, 1973) overlies t h e Selati " T r o u g h " ("Basin" P o t g i e t e r , 1988) in t h e T r a n s v a a l S u p e r g r o u p . The Mafefe a n d Penge I. F. s t r a t a are intensely folded with large v a r i a t i o n s in t h i c k n e s s e s . Axial P l a n e s are vertical or dipping steeply to the SSE. According to D r e y e r (1982) t h e folding effect is rapidly lost u p w a r d s into t h e Roihoogte shales. It is s u g g e s t e d t h a t a c o m p e n s a -

N

Malips Drift Area (Chuniespoort)

S

Mohlapitsi Fold Belt Area

M8

Penge Area

MF2

PA26

KP2

Orn

l

500

u 1000 t"

0

v

,"

25

~km 50

Fig. 6. S t r a t i g r a p h l c r e l a t i o n s h i p s in t h e P e n g e - M a f e f e - M a l i p s d r l f t a r e a N. E a s t e r n T r a n s v a a l . Mafefe I. F. = Nice member and M o h l a p i t s i M e m b e r . P e n g e I. F. = E t o n p l u s S t r e a t h a m p l u s M a l l p s M e m b e r . FM = Pyritic footwall s h a l e ; B1 = B r o a d c h e r t b a n d lutite; MM = M a i n m a r k e r s h a l e ; BC = B e v e t s c o n g l o m e r a t e ; D F = D u i t s c h l a n d F o r m a t i o n ; B o r e h o l e s = M8, MF2, PA26 a n d KP2.

74

I. W. H~BICH,R. SCHEEPERS,D. LAMPRECHT,J. L. VANDEVENTERand N. J. DE KocK

tion by cleavage m a y be the reason. On the other be studied over a distance of 450 km. Large parts h a n d the deformation along the Mohlapitsi belt, of this tract are fair to good exposure, (Fig. 7}. which is said to be of pre-Bushveld intrusion age Contrary to this the east-west outcrop width rarely (Potgieter, 1988), m a y h a v e t a k e n place in exceeds 50 kin. Therefore, stratigraphic informaincrements with the last increment only affecting tion, lateral correlation and environmental models the post-Bevets conglomerate s e d i m e n t s (post are all severely c o n s t r a i n e d b y t h i s o u t c r o p h u m o u s folding}. This is also suggested by two pattern, a fact easily overlooked when, for excontradictory observations, i.e., that diabase sheets ample, paleo-basin margins are estimated for the in the area d i s c u s s e d are folded together with the A s b e s h e u w e l s Subgroup. Thrusting recently disiron formations, w h e r e a s Miyano et al. (1987) covered in this S u b g r o u p (Altermann and Htilbich, produce good evidence that two diabase sheets at 1990 and 1991) along the s o u t h e r n to southPenge are of early post-Bushveld age. central outcrops h a s also cast d o u b t on earlier long A y o u n g e r set of N-S trending and more gentle distance stratigraphic correlation (Beukes, 1978 folds also exists in the Malipsdrift - Mafefe area and and 1980a). they m a y correlate with the D2-phase at Penge (see Apart from being covered up b y y o u n g e r strata to below). the west of the present outcrop for u n k n o w n In the Penge area, A u m a n n and Htilbich (1986) distances (possibly up to the present (tectonic) recognised three p h a s e s of deformation. East edge of the Kaap-Vaal Craton which c a n only be vergent asymmetric F 1-folds trend NW-SE and derived by geophysical means), this BIF-distribudevelop on meso- to macro-scale. Late D 1 reverse tion of course only represents r e m n a n t s left by faults on a mesoscale post-date the growth of younger erosion cycles. The original distribution of amosite mineralisation, b u t pre-date the ubiqui- these BIF of early Proterozoic age can only be t o u s growth of grunerite rosettes in all the BIF. roughly g u e s s e d at from meagre indirect evidence, This latter m e t a m o r p h i s m is related to the p e a k of as will be shown below. Bushveld intrusion (Miyano et al., 1987). The s t r a t a - b o u n d fold and breccia horizon found at the S t r a t i g r a p h y top of the Eccles Formation in the Penge area (as d i s c u s s e d above) s e e m s to be a syn-Bushveld The s t r a t i g r a p h y of the A s b e s h e u w e l s w a s s t r u c t u r e with affinity to D 1 and unrelated to the worked out in some detail by B e u k e s (1978 and s y n s e d i m e n t a r y s l u m p s reported to be so c o m m o n 1980a, 1983) who u s e d a n o m e n c l a t u r e and a in the Malmani Dolomite (Martin et al., 1988) and facies classification devised by B e u k e s (1978) and due to b a s i n tectonics. F2 macro-scale b u c k l e s b a s e d on that of chemical carbonates. tend NNE to NE in the Penge area. Fibre drag and This enabled him to produce a unique stratibedding plane slip towards the ESE produced by graphic subdivision b a s e d on the cyclicity of these this phase are very obvious. These m o v e m e n t s are rocks that occurs on meso- macro- and megacomplimented b y steep reverse faults verging ESE, scales. Very often these very low-grade (anchiand duplicating some a s b e s t o s fibre seams. Open to epi-zonal) rocks have a micro-lamination conF3 - folds trending E-W are only locally developed. sisting of two alternating c o m p o n e n t s which seems A set of vertical E-W trending D 1-compressive to be primary and could display on a n n u a l climatic shear fracture zones m a y represent areas of in- cycle (Garrels, 1987; Trendall, 1983) b u t m a y also tense hydraulic fracturing. They are partly follow- represent a m u c h larger one, e.g. a s u n s p o t cycle ed b y diabase s e a m s and F 1 - folds are intensified of several years or Milankovitch cycles. Schwarznear them. D2- faults a b u t against t h e m and acher and Fischer (1982) have shown that the a s b e s t o s fibre s e a m s fade out in their vicinity. ratios of quasiperiodicities found in carbonates, duplicate those of k n o w n orbital p e r t u b a t i o n s of THE N O R T H E R N CAPE PROVINCE the Earth at 413 000, 43 000 and 19-23 000 years. Dominant b e d s seem to equate with the shortest Distribution period, i.e. the Earth's precession. Before 1978 there existed a stratigraphic subThe a u t o c h t h o n o u s (Ferhythmite-dominated) division b a s e d on m a r k e r b e d s and which w a s (and K u r u m a n Iron Formation and the conformably still is) mainly u s e d by the geologists of the crocioverlying orthochemical to allochemical (endo- dolite a s b e s t o s mining industry in Griqualand clastic) G r i q u a t o w n I r o n F o r m a t i o n of t h e West (Genis, 1961; Cilliers, 1961; Hanekom, 1966 A s b e s h e u w e l s S u b g r o u p (Beukes, 1978, 1980b) and Fockema, 1967; as cited b y Beukes, 1980a). o u t c r o p in G r i q u a l a n d West, N o r t h e r n Cape Beukes' subdivisions are sub-parallel or parallel to Province, South Africa. From Pomfret against the these units. This a u t h o r (op cit), admits that B o t s w a n a b o r d e r in the north to Prieska and macro-cycles can seldom be correlated from one Koegas (Westerberg} in the south, these rocks can borehole to the next separated b y distances u s u a l -

TheTransvaal-GriqualandWestbandedironformation:geology,genesis,ironexploitation Recent + Tertiary Olifantshoek Group

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I. W. I-~LBICH,R. SCHEEPERS,D. LAMPRECHT,J. L. VANDEVENTERand N. J. DE KOCK

76

ly from 50-60 km, b u t occasionally up to as m u c h 100 k m or more; b u t mega-cycles, c o n s i s t i n g of macro-cycles, a p p a r e n t l y can. Both m a c r o - a n d mega-cycle b o u n d a r i e s are c o m m o n l y m a r k e d by p r o m i n e n t m e s o - b a n d s or z o n e s of m i c r o - a n d / o r m e s o - b a n d s o f s t f l p n o m e l a n e lutite (ash falls), a n d t h e s e are u s e d in lateral correlation if a s s o c i a t e d with o t h e r c o n s p i c u o u s f e a t u r e s o f p r l r n a r y origin,

e.g., m e s o - b a n d s or z o n e s o f m e s o - b a n d s ofriebeckite a n d / o r crocidolite, or of disclutites a n d grainstones. S u c h c h a r a c t e r i s t i c s , c o m b i n e d with t h e fairly r e g u l a r vertical a l t e r n a t i o n of s e t s of m o r e or less complete F e - c a r b o n a t e - silicate- oxide macrocycles, e n a b l e d B e u k e s (1980a) to correlate megacycles from hole to hole (Fig. 8). Considering the i n f o r m a t i o n available t h e n this

LITHOLOGY

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The Transvaal-Griqualand West banded iron formation: geology, genesis, iron exploitation

77

procedure seems to have worked well, but it m u s t Bank profile, this Member is dominated by at least be pointed out here t h a t there are clearly difficul- 5 fully developed macrocycles of comparable thickties with s u c h a n approach, some of these are of a ness, all of which carry riebeckite. To the north of technical n a t u r e and others are based on recently Matllpani the Pomfret profile is m a r k e d by three discovered geological factors the importance of internally very incomplete cycles (no silicates, no which should not be overlooked. carbonates} of which two are very thick and riebeca) There is not a single d o c u m e n t e d case where kite and crocidolite are absent. It is therefore, not a megacycle or a macrocycle h a s been actually clear how lateral correlation is justified with so m a p p e d out in the field, to prove that they are really little data verification over s u c h large distances. Another example is the Elgan M e m b e r j u s t below continuous. Successfully mapping out macrocycles is curbed by the following factors (i) Exten- the Derby Member (Fig. 9). In the Matlipani profile sive differential weathering that does not allow the this m e m b e r consists of about 7 macrocycles which details of these cycles (the basic building stones become progressively less complete towards the according to Beukes (1980a, see also Fig. 8) to be top with a total thickness of 66 m. A megacycle followed out laterally everywhere. (ii) Frequent subdivision for the Elgan and Derby Members in displacements along at least two prominent NE- the Matllpani section has not been attempted by and NW-trending sets of normal and scissor faults Beukes (1980a) and indeed seems impossible. Yet with rapidly varying separations ranging from zero the Elgan Member in the Whitebank profile is to several tens of metres. Basic dykes commonly subdivided into one and a half megacycles with a follow these faults. (iii) Monoclinal folding on a thickness of only 36 m, also consisting of 7 m u c h macroscale. (iv) Narrow zones of intense macro- t h i n n e r macrocycles of which the top three are folding and reserve faulting. (v) Low angle t h r u s t fully developed up to the oxide facies and with zones recently discovered (Altermann and Httlbich, riebeckite. At Pomfret the Elgan Member is 53 m 1990, 1991) t h a t occur south of Griquatown where thick, built up of one megacycle that consist of the t h i c k n e s s e s of the BIF have for some time now about 5 macro-units of which only the fourth one been k n o w n to quadruplicate (Beukes, 1980a) from the bottom is fully developed. The attempt to relative to the area north of that town (Fig. 7). These correlate through all three boreholes in this case two areas are separated by a zone of disturbance seems to hinge on the appearance of a low in the that is very poorly exposed and has been called the graph n e a r the middle of the Member. However, Griquatown Fault. Surface correlation across this even this does not appear to be at the same position zone is complicated because the transition from relative to the macrocycle subdivision of this carbonates of the Campbellrand Subgroup to the Member. Mesostflpnomelane b a n d s or zones of BIF of the overlyingAsbesheuwels Subgroup (Table t h i n n e r stilpno-bands in the Elgan Member are not I) is not exposed for the first 150 km south of this correlatable through these holes. The entire corfault. relation should therefore be considered rather b) When attempting correlation, apparent lateral tentative and not reliable at all until it c a n b e tested facies c h a n g e s recorded by Beukes, (1980a, p.79). in the field. complicate m a t t e r s . Taking into a c c o u n t the Trying to correlate the Matlipani Member in m o n o t o n o u s repetition ofmacrocycles observed in Fig. 9, one is faced with very similar difficulties. A deep boreholes it is clear that lateral correlation of more detailed correlation using 16 profiles (Fig. 10) these units over m a n y tens of kilometres can only from the transition zone of Campbellrand carbonbe a rough first approximation that certainly needs ates into BIF of the Asbesheuwels Subgroup recorroboraUon by mapping before it is used as a veals that even over these shorter distances and for a critical stratigraphlc section, the lateralvariatlon basis for regional environmental analysis. is great because of the lenticular n a t u r e of the c) Some of the published lateral megacycle units or rapid lateral facies changes (Htilbich et aL, boundaries (Beukes, 1980a) are not unique (Fig. 9) 1992). A zone of transition of varying width (here which m e a n s t h a t these correlations are uncalled the Finsch formation b e c a u s e of its very certain. If one megacycle correlation is uncertain good outcrops in Finsch Mine) is all that can be the internal lateral continuity of the entire subgroup becomes shaky. defined. Correlation problems of a n a t u r a l and artificial The graphic record of cyclicity in Fig. 8 based on a full macrQcycle, becomes the basis of comparison n a t u r e are revealed w h e n drill core surface data for cyclicity also on a megascale. The Derby Member separated by only 6 kin but compiled by different in the Matllpanl profile of Fig. 9 for example begins authors, are compared as is true with White Bank and ends with thick carbonate facies in four in- borehole WB98 a n d the s u r f a c e o u t c r o p s at complete macrocycles of greatly differing thick- K u r u m a n k o p (Figs 7 a n d 10). Comparing the two nesses which are neither riebeckite nor crocidolite profiles from Alphen and Spitskop (Htilbich et aL, bearing. In the next profile to the south, the White 1992) which are only one or two kilometres from

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The Transvaal-Griqualand West banded iron formation: geology, genesis, iron exploitation

79

each other (Fig. 10) produces similar uncertain- (Van Wyk, 1987). Several generations of carties. Rlebeckite fills early diagenetlc as well as late bonates have developed, but disequilibrium of cartectonic dllatational wedges in buckle folds. Quartz bonates h a s not yet b e e n tested against age of carpseudomorphically replaces crocidolite in tiger's bonates. Some contact m e t a m o r p h i s m occurs next eye. In the m o s t intensely d y n a m o m e t a m o r p h o s e d to diabase intrusions. The effects of this on the terrain n e a r the Orange River in the south (Fig. 7) mineralogy and carbonate isotope distribution has a well developed preferred orientation of riebeckite also been studied by Beukes e t al. (1990). a n d acmlte crystals grows In decimetre thick blastomylonites in several tens of metres thick Metamorphism s h e a r zones in BIF. The m a i n effects of m e t a m o r p h i s m in autochthoIt seems that riebeckite occurs very early in the history of these rocks and recrystallises repeated- nous iron formation are: (Van Wyk, 1987, where ly. This Na-rich amphibole m a y be part of a tecto- not otherwise stated). (i) The primary microlamination is disturbed by nically driven h y d r o t h e r m a l system of partly endogenetic a n d partly exogenic derivation. It is most minnesotaite a n d / o r magnetite growing in layers. prominently developed where shearing is most The size of the metamorphic minerals increases evident. Therefore this mineral and its fibrous with the rise in temperature. Magnetite m a y also variety crocldollte should not be considered a safe grow as disseminated e u h e d r a across all other mineral phases. criterium in the identification of cyclity. (ii) In borehole Sp- 1 (Figs 7 a n d 1 I) and in Finsch Lithostratigraphtc and cyclostratigraphic lateral correlation within the Asbesheuwels Subgroup is Mine, (Fig. 7 and Fig. 12) thin sections, chert still full of uncertainties. Environmental models r e c r y s t a l l i s e s into m o r t a r - t e x t u r e d i r r e g u l a r presently based on s u c h correlations are not very zones, lenses and entire layers of quartz or fibre quartz in which individual grains are up to 0.5 m m well founded. in diameter. This h a p p e n s along zones of beddingparallel slip movement (Lamprecht, 1993, and own Mineralogy observations - first author). (iii) All carbonates except calcite are c o n s u m e d Van Wyk (1987) describes the petrography of various lithofacles from the K u r u m a n and Griqua- during metamorphism. The latter mineral appears town Iron Formation in great detail. She also looks only in higher grades occasionally as very coarse at the bulk and mineral chemistry of these rocks crystals. (iv) Late ferrotremolite-actinolite occur together and discusses genetic models as well as diagenetic and m e t a m o r p h i c effects. This very thorough treat- with calcite but hornblende is very rare because of m e n t h a s not yet received the attention it deserves. a dearth of AI. The following parageneses form in orthochemical Therefore, it is treated in somewhat greater detail (silicate lutite) BIF: In low grade zones: Non pleohere. The minerals mainly found in these iron-rich chroic greenalite + brown ferristilpnomelane + siliceous rocks are: (i) Chert and quartz (ii) The magnetite + zoned carbonate + minnesotaite, in phyllosflicates: greenalite, minnesotaite, stilpno- this order of crystallisation. At higher grades blue-green hornblende appears melane (two varieties), ferri-annite, thuringite (iii) The amphiboles: riebeckite, tremolite-actinolite and in a matrix of yellow-brown stflpnomelane and hornblende-pargasite (iv) Sphene (v) The carbo- magnetite. Latergrowth ofminnesotaite and spheric nate group: siderite, ankerite, calcite (vi) The trace occurs in quartz rich phases. Metamorphism has increased the CO 2 content minerals: zircon and apatite. Metamorphic biotite replaces stilpnomelane and acmite m a y occur and decreased H20, whereas traces of Pb decrease together with riebeckite in syntectonically recrys- with increasing temperature. Trace elements like tallised BIF of s h e a r zones (Altermann arid Htilbich, Cu, Y, Zn, Sr, Th0 Co, Rb, Nb, U, Ni, Sr and Mo have 1990).The mineralogy of this BIF bears primary, not changed significantly. diagenetic and metamorphic features (van Wyk, 1987). Regional m e t a m o r p h i s m depends on burial Diagenesis depth and Miyano and Beukes (1984) came to the This is m a r k e d by the following features in the conclusion t h a t m a x i m u m t e m p e r a t u r e s did not exceed 150°-170°C for the bulk of the low grade K u r u m a n Iron Formation, noted b y v a n Wyk (1987), BIF. In these rocks minnesotaite is the main indi- Lamprecht (1993) and own observations mainly c a t o r m i n e r a l t o g e t h e r with the p a r a g e n e s i s from borehole SP- 1 and Finsch mine, unless othergreenalite-magnetite-quartz-hematite. Disequili- wise indicated: (i) Early magnetite serves as a n u c l e u s for b r i u m conditions are commonly revealed by zoned carbonates and by two phases of ferrotremolite massive riebeckite sheaths. This occurs mainly on

I.W. H~LBICH, R. SCHEEPERS)D. LAMPRECHT,J. L. VAN DEVENTERand N. J. DE KOCK

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The Transvaal-Griqualand West banded iron formation: geology, genesis, iron exploitation the silicate facies. (ii) Later riebeckite needles of varying length grow radially o u t w a r d s from t h e s e s h e a t h s . Where complex zoned r o m b o h e d r a of ankerite grow in close proximity to these needles their outer zones Include a higher concentration of radiating riebeckite needles. This Indicates that ankerite grew last, probably w h e n m e t a m o r p h i c grades were highest. (iii) Riebeckite engulfs carbonate-silicate laminates until only composite globules consisting of a chert matrix, with magnetite phenocrysts and partly resorbed ankerite rombohedra drift in this m a s s of riebeckite. If compared to (ii) this shows that riebeckite moves and recrystallises several tUnes. Both (ii) and (iii) indicate t h a t riebeckite is a diagenetic a n d / o r metamorphic alteration product of early, mainly siliceous p h a s e s and possibly even derived from exogenic alkaline solutions. This was also suggested by Miyano and Beukes (1984). Acid to alkaline and progressively becoming more i n t e r m e d i a t e v o l c a n i s m (as revealed by t h e c h a n g i n g composition of stilpnomelane lutite m e s o b a n d s representing ashfalls) m a y have been the ultimate source of K and Na in these rocks (van Wyk, 1984 and Lamprecht, 1993). On the other h a n d the e n r i c h m e n t s of riebeckite south of the Griquatown lineament, where the BIF are markedly tectonised and metamorphic grades increase, also point to remobilization with a possible hydro-

81

thermal input of the alkalis during deformation. Chert is seen as mainly dlagenetlcally redistrib u t e d into mesobands, ribbons and mlcrolaminae either clear or with dispersed Inclusions of reddish, t r a n s l u c e n t hematite d u s t or hexagonal specularite platelets. In other cases greenalite, stilpnomelane a n d / o r magnetite are arranged In chert along parallel micro laminae. Ferri-annite, quartz or crocidolite m a y grow as dilatation fibre across the bedding commonly In contact with magnetite. Chert recrystallises dlagenetlcally In lenticles or laminae at larger grainsizes t h a n w h e n considered primary. With so m u c h very fine microlamination present in chert b a n d s and ribbons In a u t o c h t h o n o u s iron formation and chert forming massive, several m e t r e s thick units in the orthochemical to allochemical Griquatown Iron Formation it is hard to believe the commonly accepted suggestion that most of the chert banding is of diagenetic origin. What did it replace? Why did it move and where has the replaced material gone? These questions have never been dealt with from a chemical m a s s transport and equilibrium angle. Primary deposition of chert into layers as they are found today with some local exchange and replacement where bedding is disturbed, is m u c h more acceptable until proven wrong. Microrhythmically precipitated siderite and chert are so c o m m o n and so readily explained by seasonal evaporation in a closed or semiclosed basin (e.g., Garrels, 1987)

Fig. 10. S t r a t i g r a p h i c c o r r e l a t i o n a c r o s s t h e b a s a l t r a n s i t i o n zone of the K u r u m a n Iron F o r m a t i o n a t F i s c h Mine (Northern C a p e Province) [from Hgtlbich et al., 1992). C o l u m n s m a r k e d N (New) a n d R (Repeated} were recorded by H~ilbich eta/., 1992. C o l u m n s m a r k e d B are from B e u k e s 1980a + b). T h o s e m a r k e d K a r e from B e u k e s eta/., 1990. Legend: TSI = T s i n e n g Member; KL = Kliphuis Member; MTL = Matlipani M e m b e r STFB = S t o t b a k k i e s Member. T h e s e n a m e s a n d n u m b e r e d z o n e s a s i n t r o d u c e d by B e u k e s ( 1 9 8 0 a + b). 1) C o m p l e t e (siderite - Fe-silicate - m a g n e t i t e - hematite) BIF cycles not individually m e a s u r e d . 2) c o m p l e t e individually m e a s u r e d macrocycles, c o m m o n l y s t a r t i n g w i t h s t i l p n o m e l a n e lutite (tuffite) followed b y siderite - Fe silicate - m a g n e t i t e - h e m a t i t e b a n d r h y t h m i t e . 3) As 2) b u t riebeekite t o w a r d s top w i t h occasional t h i n l e n s e s a n d b a n d s of crocodilite. 4) M a c r o - b r e c c i a of BIF in 2} r e s t i n g o n t h i n - m a g n e t i c b l a c k c h e r t s i n t e r c a l a t e d with t h i n f e r r u g i n o u s s h a l e s a n d cryptalgal a n d a r e n i t i c limestone. 5} Top: M u d s t o n e - c h e r t rues-cycles. Bottom: L i m e s t o n e - c h e r t mesocycles. 6} L a m i n a t e d , occasionally g r a d e d chert, CC = grey blue to w h i t e chert; FeC - limonitic chert; CH = h e m a t i t i c chert. 7} Like 6) b u t AC; ankeritic; CS = sideritic. 8} Like 6) but: sucessively s l u m p e d a n d / o r folded; occasionally brecciated; highly f e r r u g i n o u s l a m i n a e = FE. 9} Like 6) w i t h iron-silicate b a n d s in lower p a r t a n d repitition of s e q u e n c e by oblique slip ( u p p e r part). 10} Top: b r e c c i a of tectonic derivation. Bottom: m e s o b a n d s of b l a c k or g r e e n volcaniclastics. 11) Recrystallized l i m e s t o n e a n d / o r c h e r t = jaspilite. 12) L e n s e s of c h e r t a n d s h a l e m a i n l y in c a r b o n a t e matrix. 13) V a r i o u s s h a l e s a n d m u d s t o n e s (clay stones), g r e e n i s h - g r e y in lower p a r t s of sections, b l a c k a n d c o m m o n l y pyritic in u p p e r p a r t s of sections. 14) D i s p e r s e d f e r r u g i n o u s pisolites - originally pyritic. 15) Single layers of f e r r u g i n o u s pisolites - originally pyritic. 16) S h a l e s w i t h S 2 - cleavage c u t t i n g a c r o s s b e d d i n g fissility. 17) Cryptalgal m a t limestone. 18) C a r b o n a c e o u s c h e r t s . 19) V a r i o u s l a m i n a t e d algal m a t l i m e s t o n e s a n d i n t r a m i c sparites; dolomite in lower part; if f e r r u g i n o u s = Fe. 20) Algal g r a i n s t o n e s . 21) C o n t o r t e d , f e r r u g i n o u s shale. 22) D o m a l stromatolite. 23) F e n e s t r a l s t r u c t u r e s a n d c o l u m n a r s t r u c t u r e s in c a r b o n a t e s . 24} L e n s e s of cryptalgal limestone. 25} Algal m a t l i m e s t o n e w i t h d i s p e r s e d f e r r u g i n o u s pisolite - i n t e n s e l y contorted.

82

I-I~LBICH,R. SCttEEPERS,D. LAMPRECHT,J. L. VA~ DEVENTERand N. J. DE KocK

I.W.

~" °=

~ 44,50m

LITHOLOGY {with disseminated magnetite throughout) -

m

~

Thick greenish-gray massive sideritegreenalile Jut|re zones wilh ribbons pods and pillows of chert and riebeckitic chert (R)

~-----t

~ CL

(t)ST(25OiRT?m-(4o)sr-

too

i~~

Z Ribbons of magnelite and dispersed slilpnomelane (ST) near base. Breccia (BR)dipping 30" on magne~c ar e - u l e wl occasional R.-ribbons. Thick massive R-cherts at base. Greenish-black Fe-lulite with fragments of

12(X)|Fe~

dislocated siderilic chert. melane lutite bands,

tVtO)RtSOIR-

Ferristilpno-

Dislurbed bedding

dips up to 45 °. Thick R-chert bands up to 150 cm.

z

(3001Fe~ Be--

14ootni2m (t51S[= (15|R--~ R-200 R--

Shear zone:-eyes of R-chert in Fe-lutite. Dark Fe-lutite with chert lenses.

162m

Microlaminaled greenalile-lutite alternating with black silales and stllpnomelane lutite. Cycles of microlaminaled meso bands of cherlsidedte-greenalite-magnelile*slilpnomelane rhythmites. Occasionally disturbed and bcecclated in decimetre Ibick zones, Chert bands become magnetic near sheet DIABASE SHEET

(2)ST-(IIS~ (3)ST--: (sls~_~

Regularly bedded complete and incomplete cycles of microlaminated siderite-greenalileminnesotaile-s lilpnomelane meso bands

o7>

(2|ST--I (3)S/--I 13)S[...~ 151S~] sional massive chert ribbons. R,QIz in dilatalional spaces o! folds.

~

m

As above bul horizontally bedded (t)ST --3001120}-~-

R (goiR I

~ s a ore u o e , myomlc w e l l ~ coarse carbonale Siderite-magnetite ribbon rhylhmite with carbonate blasts. nale blasts. R-cherl pillows wilh micr~am. of sider.- greenal.-magnelile. Distorted if ma_.maqnel.-R-ribh wilh crocidol, shear fibre. ttorizontal microlaminaled gmenalitemagnetite meso rhythmites. Occasionally massive greenalite chert ribbons and bands.

~

Z

X ::T o ¢D

Dips 30 ° where R and disturbed. 400

0

-416m_. R 415m--"

z Bands of s i d e r i t e - ~ i t e

P¥ n PY R PY (20)ST-'~-~ R py R PY R--PY (20)np~ R 500m PY

Very regularly micmlaminaled sideritegreenalite- minnesotaile - stilpnomelane rhythm|re, with occasional R~ ST mesobands (I-2cm). Euhedral PY in microbands (1-2mm)

As above but R-enriched matrix with normal size-grading of quarlz-carbonate-magnel, globules

Fig. 1 l. S t r a t t g r a p h l c r e c o r d a n d cyclicity o f b o r e h o l e S p - 1 o n f a r m K o u p o o r t , s o m e 4 5 k m s o u t h o f G r i q u a t o w n .

The Transvaal-Griqualand West banded iron formation: geology, genesis, iron exploitation t h a t it is s u r p r i s i n g to see this fundanaental uniformttarlan mechanism being either rejected (e.g., Klein a n d B e u k e s , 1989) or conveniently overlooked b y s o m e w r i t e r s on this subject. Chert a n d BIF r h y t h m l t e s of t h e K u r u m a n I. F. a s well a s c a r b o n a t e s of t h e u p p e r m o s t C a m p b e l lrand S u b g r o u p are c o m p a r a b l e in their very low t r a c e e l e m e n t c o n t e n t (Lamprecht, 1993). It c a n b e s h o w n t h a t t h e stratigraphlc position of c h e r t s cod e t e r m i n e s their t r a c e e l e m e n t c o n c e n t r a t i o n (e.g., w h e n t h e y are i n t e r b e d d e d with m u d s t o n e s at t h e b a s e of t h e K u r u m a n Iron F o r m a t i o n - L a m p r e c h t , 1993). This m e a n s t h a t t h e c h e r t w a s precipitated w h e n e v e r t h e s a t u r a t i o n level for SiO 2 w a s exceeded (Garrels, 1987) even d u r i n g brief i n t e r l u d e s b e t w e e n c a r b o n a t e p r e c i p i t a t i o n a s in u n i t c (Fig. 12). Therefore fresh w a t e r - b r a c k i s h w a t e r interaction a n d s e a s o n a l e v a p o r a t i o n are strongly i n d i c a t e d a s t h e s i m p l e s t p r o c e s s controlling microlamination.

Primary Deposition The original m i n e r a l a s s e m b l a g e s are very difficult to e s t a b l i s h w i t h o u t d o u b t in the a u t o c h t h o n o u s , very fine-grained b u t often well l a m i n a t e d facies. Original, chemically precipitated m i n e r a l s w o u l d o c c u r in the finest grain sizes f o u n d here, i.e., chert, h e m a t i t e , siderite, greenalite?, stilpnom e l a n e a n d pyrite in nodules. C a r b o n , thuringite a n d ferrl-annlte m a y classify as clastic c o m p o n e n t s (van Wyk, 1987, p. 151). Rare r o u n d e d m o n a z i t e a n d zircon grains are trace minerals.

Chemistry Typical a n a l y s e s of v a r i o u s lithofacies are given b y W y k (1987) a n d B e u k e s e t al. (i 990). L a m p r e c h t (1992) a n d H o r s t m a n n a n d Htilbich (1993) have a n a l y s e d m e s o b a n d s of v a r i o u s l i t h o l o g i e s . Averages of all t h e s e a n a l y s e s are given in Tables 2 a n d 3. Mineral a n a l y s e s (van Wyk, 1987) reveal that some minerals undergo systematic changes relative to the stratigraphy. T h u s , t h e F e / F e + Mg a n d t h e MnO c o n t e n t of siderite a n d of Fe-dolom i t e - a n k e r i t e i n c r e a s e s stratigraphically u p w a r d s . According to Garrels (1987, p. 98 + 99 a n d Fig. 7) t h i s w o u l d m e a n t h a t t h e c o n c e n t r a t i o n of Fe 2÷ in h i s overflowing e v a p o r a t i o n m o d e l p r o d u c i n g m i c r o - l a m i n a e of iron c a r b o n a t e a n d c h e r t , d e c r e a s e s relative to Mg 2÷ in time a n d therefore the e v a p o r a t i o n c o n c e n t r a t i o n factor or degree of e v a p o r a t i o n of his m o d e l w o u l d d e c r e a s e stratigraphically u p w a r d s . This is only p o s s i b l e ff t h e v o l u m e of f e e d w a t e r with a certain ionic composition i n c r e a s e s or the overflow d e c r e a s e s annually. C o n s e q u e n t l y the b a s i n will have to grow a n d / o r deepen. Inter alia this m a y m e a n t h a t climate

83

b e c a m e increasingly h u m i d wtth time. Within single m e s o b a n d s however, t h e composition of c a r b o n a t e s a s well a s of silicate m i n e r a l s s t a y s relatively c o n s t a n t . For s t i l p n o m e l a n e t h e F e / F e + Mg d o e s not c h a n g e s y s t e m a t i c a l l y relative to s t r a t i g r a p h y , b u t Na20 a n d A120 z d e c r e a s e u p w a r d s from 0.7% to 0.2% a n d from 5.2% to 3.8% respectively, w h e r e a s I ~ O i n c r e a s e s from 1.5% to 3%. The A1203 a n d K20 (but n o t Fe/Fe+Mg) of t h e almost ubiquitous metamorphic mineral minnesotaite c h a n g e s with t h e c o m p o s i t i o n of the facies in w h i c h it occurs. Rlebeckite, ferrl-annlte a n d thuringlte have no s y s t e m a t i c variation in c o m p o sition. L a m p r e c h t [1993) f o u n d t h a t the c a r b o n a t e s in c a r b o n a t e - c h e r t m e s o b a n d p a i r s over s e v e r a l t e n s of m e t r e s in the t r a n s i t i o n zone b e t w e e n C a m p b e l l r a n d f e r r o - m a g n e s i a n l i m e s t o n e s a n d BIF have a very low b u t steadily increasing s o d i u m c o n t e n t stratigraphically u p w a r d s . This agrees well with r e p o r t e d i n c r e a s e s in Na a s c a r b o n a t e s b e c o m e m o r e evaporitic a n d less marine.

Chemical Mass Calculation W h e n the c h e m i s t r y of v a r i o u s lithofacies in t h e t r a n s i t i o n from c a r b o n a t e s to overlying iron formations as e x p o s e d in the F i n s c h o p e n c a s t m i n e w a s d e t e r m i n e d , it a p p e a r e d t h a t t h e t h r e e m u d s t o n e chert u n i t s t h a t s e p a r a t e t h e first c a r b o n a t e - o x i d e BIF at the b a s e of the K u r u m a n Iron F o r m a t i o n from the c o n t i n u o u s BIF s e q u e n c e higher u p in the s t r a t i g r a p h y (Figs 10 a n d 12), are extremely enriched in iron. A m a s s c a l c u l a t i o n for t h e entire profile w a s therefore applied, in w h i c h e a c h facies w a s length a n d d e n s i t y w e i g h t e d to d e t e r m i n e its average m a s s percentage. The result (Table 4) reveals that t h e r e is only a b o u t 3.7% m o r e SiO 2 a n d 6.6% m o r e Fe203 a n d s o m e w h a t m o r e CaO in BIF t h a n in m u d s t o n e . The latter also have a b o u t four t i m e s a s m u c h AI203 a n d TiO 2, 20 t i m e s as m u c h K20 a n d 7 times a s m u c h MnO t h a n the BIF, w h e r e a s Na20 s t a y s r a t h e r c o n s t a n t . More K20, A1203. TiO 2 are p r o b a b l y i n t r o d u c e d with the i n c r e a s e d clastic c o m p o n e n t . More MnO a n d less CaO indicate t h a t m o r e fresh w a t e r with m o r e dissolved CO 2 w a s available d u r i n g m u d s t o n e deposition. I n c a s e t h e b a s i n w a s closed during BIF a n d m u d s t o n e t i m e s c o n s t a n t Na20 m a y m e a n that the e v a p o r a t i o n factor did not c h a n g e significantly. The c o m p a r a t i v e l y small difference in Fe + Si m a y m e a n t h a t t h e s e e l e m e n t s were d e p o s i t e d in the s a m e e n v i r o n m e n t at a n almost c o n s t a n t rate even t h o u g h m o r e m u d w a s i n t r o d u c e d to the e n v i r o n m e n t from l a n d (by river water) during m u d s t o n e deposition. This c a n only m e a n t h a t the w a t e r b o d y w a s a l r e a d y receiving a s u b s t a n t i a l c o n t r i b u t i o n from rivers all the time

84

I.W.

H~LBICH,R. SCHEEPERS,D. LAMPRECHT,J. L. VAN DEVENTER and N. J. DE KOCK

(a)

(b)

ENE

SSE

FINSCH MINE

"1"1

0

°o

1(

2(

15.00m: Two Incomplete BIF cycles at base followed by 6 complete stilpnomelane lutite slderlte - Fesilicate - hematite - magnetite rlebeckite - crocldollte cycles. Continuing upwards. Disc lutltes may occur at top of mature as well as Immature cycles.

40

~

~

0

0

~,

¢.o

--" 0 i~

•~ -2

0

0

-2

0

2 -2

0

2 "1

2

J

4.05m: Four mudstone - chert stderltic chert cycles with stilpnomelane lutite mesobands at base. 7.40m: 4.5 pink to red mudstone massive chert - pale slderltlc chert cycles.

30

0

0

m

0

0"~--

m

ro

4.05m: Cream coloured well bedded mudstones, prominent chert bands towards too.

8,67m: 150 mesocycles of brown mudstone and bluish massive chert. Three stllpnomelane bands near base.

h+i

~3..... o

g-

0 ~

~

-

t2J

f.~

g

o~

'~"':5

-8 -4 0 r-T-t--t-

o 8.51m: 5 simple cycles (proto BIF) of stilpnomelane lutite - siderite magnetite - magneUte siderite magnetite hematite bandrhythmite.

5O I1"1

1,28m: Ankerlte - banded chert, 4.08m: 58 cycles of limestone massive and ankeritlc chert mesobands. 1.94m: Massive black limestone with a few very thin shale bands.

10.37m: 138 cycles of dark algal mat limestone and black pyritic shale. Three conoform stromatolite beds occur.

g

e

VI tD

C

b

I

I n = I I -8 -4 0 -2

a 0

2-2

0

2-2

0

.i 2

The Transvaal-Griqualand West banded iron formation: geology, genesis, iron exploitation t h a t BIF w e r e deposited. It is also evident t h a t t h e Fe + SI precipitation w a s a d o m i n a n t , overruling effect u p h e l d b y a long t e r m p r o c e s s p o s s i b l y of global i m p o r t a n c e , a n d n o t easily s t o p p e d or dist u r b e d b y local s h o r t t e r m i n f l u e n c e s a s for exa m p l e f l u c t u a t i o n s In r u n - o f f rate. E x a m p l e s of long t e r m p r o c e s s e s are e p e i r o g e n e s i s , m a j o r climatic f l u c t u a t i o n s , f e e d - b a c k controlled a t m o s p h e r i c cycles, etc. A long t e r m epeirogenic control t h a t k e e p s a w a y s u b s t a n t i a l clastlc c o n t r i b u t i o n s to c h e m i c a l / b i o c h e m i c a l platform c a r b o n a t e s s u c h a s t h o s e of the C a m p b e l l r a n d a n d M a l m a n i S u b g r o u p s m u s t also b e p o s t u l a t e d for t h e d e p o s i t i o n of t h e BIF s e q u e n c e t h a t follows i m m e d i a t e l y on the c a r b o n ates. W h a t did c h a n g e however, w a s the type of e n v i r o n m e n t , w h i c h m e a n s t h a t the p H / E h conditions a n d e v a p o r a t i o n control m a y have c h a n g e d drastically, p r o d u c i n g a different kind of waterbody. If this e n v i r o n m e n t initially w a s a platform o p e n to t h e s e a d u r i n g c a r b o n a t e precipitation, it m a y well have b e e n a m o r e sheltered, at least partly or t e m p o r a r i l y closed cratonic b a s i n in BIF-times. This m a t t e r is d i s c u s s e d below.

Stable Isotope Investigations The effects of p o s t depositional alteration on c a r b o n a t e m i n e r a l s a n d organic c a r b o n in v a r i o u s lithofacies of t h e t r a n s i t i o n zone from Lhe C a m p bellrand S u b g r o u p to t h e BIF of t h e K u r u m a n Iron F o r m a t i o n h a v e b e e n investigated b y B e u k e s et al. (1990). I s o t o p e s of oxygen, c a r b o n a t e c a r b o n a n d organic c a r b o n are e v a l u a t e d together with knowledge a b o u t t h e geological setting a n d t h e petrography. The d a t a are s u m m a r i z e d in Table 5. Their m a i n c o n c l u s i o n b a s e d on t h e s e d a t a are: (i) A ~aC depletion in oxide iron formation a n d s h a l e s w h i c h c o n t a i n a b u n d a n t e u h e d r a l ankerite or f e r r o a n dolomite b l a s t s respectively, r e s u l t s from a fixation of light organic c a r b o n in t h e s e m i n e r a l s during deep b u r i a l diagenesis. (ii) Siderite a n d c h e r t recrystallise in BIF to m i n n e s o t a i t e a n d this is a m e t a m o r p h i c effect. (iii) Apart from d i s s e m i n a t e d fine-grained primary magnetite, early diagenetic m a g n e t i t e formed during r e d u c t i o n of p r i m a r y h e m a t i t e t h r o u g h p r o c e s s (i). Magnetite m a y also be a m e t a m o r p h i c m i n e r a l t h a t grows at t h e e x p e n s e of siderite b y oxidation, b u t this p r o c e s s is not dealt with b y B e u k e s et al. (1990). (iv) Sideritic iron f o r m a t i o n could not have deve-

85

loped o u t of oxide Iron f o r m a t i o n b y r e a c t i o n with organic c a r b o n a s a r e d u c t a n t for v a r i o u s r e a s o n s w h i c h are of textural, isotopic a n d g e o c h e m i c a l nature. (v) Limestone a n d siderite o c c u r t n t e r b e d d e d a s m i c r o s p a r i t e s . Both b e a r m a n y t e x t u r a l signs of a n unaltered primary precipitate from the same h o m o g e n e o u s w a t e r b o d y . Therefore, according to t h e r m o d y n a m i c theory, t h e siderite is e x p e c t e d to b e lsotopically heavier (Golyshev et a t , 1981). However, it is f o u n d to be e n r i c h e d in t h e lighter isotope b y 4% relative to limestone. T h u s t h e w a t e r b o d y c o u l d n o t have b e e n h o m o g e n e o u s . (vii The chemical c o m p o s i t i o n of c a r b o n a t e s in Iron f o r m a t i o n is n o t diagnostic of their s e d i m e n tary or diagenetlc origin, b u t their p a r a g e n e t l c relationship is. The a u t h o r s ( B e u k e s et aL, 1990) c o n c l u d e t h a t the l i m e s t o n e s a n d s h a l e s (which have m u c h m o r e in c o m m o n chemically t h a n the BIF a n d t h e shales, a l t h o u g h the latter two are stratigraphically m o r e closely associated) were d e p o s i t e d "from an essentially iron-depleted shallow w a t e r m a s s close to s h o r e in a r e a s of high organic p r o d u c t i v i t y a n d m i n o r siliciclastic input: in c o n t r a s t , t h e ironf o r m a t i o n s were d e p o s i t e d in a n iron enriched d e e p e r o c e a n w a t e r m a s s , r e m o v e d from high organic productivity a n d a n y t e r r i g e n o u s detritus." (p. 682). The off-shore oxides a n d siderites were d e p o s i t e d from h y d r o t h e r m a l w a t e r s d e p l e t e d In ~3C b e c a u s e "there m a y j u s t not h a v e b e e n e n o u g h organic matter in the depositional environment' ( B e u k e s et aL, 1990, p. 685). The l i m e s t o n e s a n d s h a l e s o n t h e contrary, derived their light c a r b o n s i g n a t u r e from material p r o d u c e d in a n d t r a n s p o r t e d into the shallow w a t e r b o d y . B e c a u s e AL2Oa. Th a n d Ba all correlate positively with C-organic in the order oxide iron f o r m a t i o n - c a r b o n a t e iron-informationlimestone a n d dolomite-shale this s e e m s to agree with m o d e m o c e a n w a t e r s where, e.g., Ba increas e s with organic productivity. Light c a r b o n is a s s u m e d to have b e e n contrib u t e d b y deep s e a h y d r o t h e r m a l w a t e r s that mix with n o r m a l o c e a n w a t e r to p r o d u c e a depleted ~ac profile during t r a n s g r e s s i o n (deepening of the waterbody). This s e e m s to fit a m o d e l of a stratified o c e a n on a c o n t i n e n t a l platform w h i c h w a s derived from an investigation of REE a n d t r a c e - e l e m e n t c o m p o s i t i o n (Klein a n d B e u k e s , 1989) w h e r e t h e Eu a n d Ce s i g n a t u r e s indicate t h a t h y d r o t h e r m a l a d m i x t u r e could have occurred.

Fig. 12a. Finsch Open Pit. About I km in diameter and 350 m deep, looking ESE from rim of pit. Bar scale=400 m. Access road descends from right to left. a = Kimberlite. b= carbonates of CampbeUrand (units "a '° and "b" of Fig. 2b). c = lower cherty horizons and proto BIF (Units c, d and e of Fig. 2b). Oblique signature = karsted areas. Dumps on horizon. Short black vertical bars = samples subsections above access road level. Fig. 12b. Strastigraphic section: Detail of transition zone between the Campbell Subgroup below and the Kuruman I. F. above with important chemical parameters.

0.08-1.22

260-14.14

TiO 2

Am203

1.24-4.74

0.15- 0.74

0.09 0.12 0.15 008

0.13-32.72

0.03- 0.30

<001 - 060

011 -019

<0.01-043

CaO

Na20

K20

P205

Cr203

1270

<1 - 15

3-8

2- 60

<1-11

110- 142

St

U

Rb

Th

<1-13

53- 144

Cu

Ni

22- 108

Co

W

<1-6

Mo

Ba

V

O

7- 134

Zn

Ga

I°b

5.5

9- 50

Y

770

13

1062

3.0

48.3

19.8

35

0.3

24.3

74- 115

36- 190

<1-6

0- 23

121 - 171

<1-7

19 - 98

<1 -7

<1 -44

6- 23

56-111

1105

56- 194

Zr

4-7

0.08- 0.32

0.43- 2.21

6.24-10.98

9.2

5- 17

10.28

Nb

Totals

0 38-9 68

8.25

647-15.06

002-094

MgO

0.27

30.34-4433

0 02-0 63

MnO

34.84-61.75

0.09-0.22

0.43 8.33

36.95

1848-50.50

3434

Fet

FeO

17.76-46.86

•~O 2

Min - Max

987

83.7

0.7

4.1

142,4

1.3

50.3

39

110

130

745

5.3

0.17

1.14

0.32

341

8.87

0.47

46.10

2.05

0.15

33.40

AM

n=10

n=7 AM

lutite

claystones

M i n - Max

Stilpnomelane

T~uringitic

77- 110

42- 57

146- 180

<1-9

187 - 983

5-9

5-20

3-43

53- 135

6-9

<0.01 - 0.16

<0.01

2.53 - 1007

0.07-0.31

0.10- 1.22

3.19-803

0.06-0.54

37.20-48.34

1.42-6.62

0.01-0.11

32.05-45.49

M i n - Max

n=6

933

49 8

155.2

4.2

596.8

6.5

17.0

15.8

87.8

7.0

0.04

6.30

0.17

0.64

5.48

0.18

43.5

3.60

0.05

40.31

Av

Ferri-annite-lutite

-

13

12- 48

<1 -4

<1 - 25


24- 92

<1 - t0

3-11

<1-7



20-37

5-7

Traces

<001-0.06

<0 01 - 004

0.02 - 0.08

O67- 6.01

1.66-3.48

0.12-027

695-26.94

67.45-8389

M i n - Max

n=4

chert

205

13

110

08

508

2.5

6.3

20

43

3.0

288

58

002

002

005

247

250

0.21

16.57

77.76

Av

57- 93

23- 43

74-140

3-4

<1 - 5

<1-3

2-7

35-40

4-6

<001

<0.01-0.25

<0.01

<0.01-0.05

152- 3.35

3.73-4.20

0.19-0.54

30.58-40.83

<0.01

0.01-O02

5201-6259

Min - M a x

n=3

76 3

33.7

108.3

3.7

2.0

1.3

5.0

37.7

5.0

0.15

0.03

1.85

3.97

0.31

34.58

0.013

58.62

Av

c a r b o n a t e facies

92- 109

48- 51

<1 -6

170- 179

3-5

8- 15

7-8

4-8

45-46

4-7

<0.01

0.11-0.26

<0.01

<0.01 - 0.04

1.56-3.81

1.68-3.63

0.06-0.46

47.00-52.66

<0.01

0.01-0.02

3924-4514

Min - M a x

n=3

102.0

50.0

2.0

174.0

4.0

12.3

7.3

5.3

45.7

5.7

017

0.02

2.96

2.91

0.22

49.38

0.013

45.20

Av

oxide facies

Table 2. Average major and trace element analyses of BIF lithofacies from drill core in Griqualand West (after v a n Wyk, 1987).

96 - 123

45- 68

<1

140- 199

63 - 388

3 - 10

5 - 28

1-9

44-60

5-7

<0.01-0.04

<0.01

1.28-4.28

<0.01 - 0.33

0.16 - 1.81

1.82-4.41

0.08-0.73

43.34-85.32

008-1.77

<0.01-0.08

3.18-48.97

Min - Max

n=11

lutite

1104

55.2

160.1

172.7

6.0

18.0

3.3

48.3

5.6

0.02

2.38

0.17

0.63

2.69

0.58

54.30

0.60

0.03

38.23

Av

colour siderite

m

.z

t::/ m

z

.r-'

>

P

ta3

,.-,

o~

oo

<0.01-O02 <0.10

TiO:~

AI203

49.13

2.12

0.17-1.64 201-222

MnO

MgO

0.13 0.07

<0,01-0.24

0.01-0.19

P2Ofi

Cr203

* Fet = ( F e 2 0 3 ) t

Co

Mo

Ba

V

Or

1060

99-111

= 1.114 FeO + F e 2 0 3

98- 111

50-61

107.0

59-94

24-104

43- 45

Ni

44.0

<1-9

<1-22

CO

57.8

163.5

<1-13 <1

158-171

t07-514

Zn 80

144.0

67.0

<1-4

6-16

4-15

58-207

6-12

<0.01-0.13

0.06-0.11

2.89-6.41

0.20-0.38

0.01-0.37

6.52-10.82

0.07-0.29

31.06-39.76

2.08-8.53

t03-t49

Ga

134 - 150

44-I15

8.3

23.5

1.3

42.42-47.54 0,04-0.22

Pb

177

6-12

11-44

<1.2

46.8

5.5

0.013

1.07

0.26

0.83

1.93

0.39

50.13

0.22

0.04

44.86

Min - M a x

<1 - 16

14-22

Rb

43

29.7

2.7

46-48

5-6

<0.01-0.02

<0.01

0.77-1,55

0.20- 033

0.39-1.54

1.21-2.37

0.27-0.49

48.71-52.01

014-0.39

<0.01-0.05

43.43-46,39

Av

n=9

0.02

0.07

5.28

0.28

0.12

8.50

0.15

35.79

4.85

0.09

44.92

Av

82.0

41.6

2.9

2.1

80.2 1.2

<1-5

50.3

6.0

143.3

4.7

5.3

17.7

4.2

41.2

5.7

0.01

0.09

1.097

5.45

302

0.49

45.74

45.16

Av

60-106

27-76

<1-16

104-217

1-9

<1-13

0-33

<1-7

36-48

5-7

<0.01 -0.03

<0.05-0.20

<0.01

0.06.0.19

0.54-921

0.16-2.52

0.15-0.77

31.34-69.54

<0.01

<001

20.65-67.33

Min - M a x

g r a i n lutite

TABLE 2 (CONTINUED)

123.2

7.0

418.0

2.0

94

9.8

109.2

7.4

Traces

lutite

Stilpnomelane

Th

15- 52 3-6

<1 -4

Y

U

39- 42

Zr

Sr

5

Nb 40.7

0.26

009-037

K20

Totals

1.95 0.33

0.95- 2.82 009.0.52

COO

Na?O

1.80

42.16-47.78 4491

0.02

Fet

FeO

47.68-51.87

~o~

Min - Max

n=3

Av

mesobands n = 4

laminate

Min - Max

Stilpnomelane lutite w i t h c h e r t

c o l o u r siderite

106-121

57.66

<1-15

164-i81

2-6

3-8

<1-2

40-42

5-6

<0.01

0.06-0.13

<0.01-0.02

<0.01-0.09

0.43- 070

2.73-3.34

0.34-0.66

49.95-60.38

<0.01

<0.01

35.24-45.32

Min - M a x

111.7

62.0

5.0

171.0

4.0

5.3

1.3

40.7

5.7

0.09

0.01

0.06

0.55

3.03

0.50

55.71

39.67

Av

greenalitic lutite

96-109

43-65

<1-9

133-145

5-I0

2-5

<1-2

2-3

39-44

5-6

<0.01

0.06-0.07

<0.01-0.03

0.06

0.37 - 0.88

3.46~.55

0.08-0.40

42.93-60.27

<0.01

<0.01

31.22-51.46

Min - M a x

lutite

103.0

52.3

4.0

138.0

7.7

3.3

0.7

2.3

41.3

5.3

0.06

0.01

0.61

4.87

0.21

49.31

41,.34

Av

siderite-hematite

o

oo "~

g

o c~

0Q

o o o~

.=.

5~

3

c;r

f-

i C~

<

1.05

5.66

540

1.65

0.99

0.06

0.58

3.48

2.69

0.87

0.99

0.04

~O

~o

~O

N~O

K20

P?O5

4

Ba

W

Co

Mo

6

13

6

V

14

38

nd

Ga

Zn

38

0

22

nd

Pb

Cr

0

nd

Th

0

0

21

Rb

29

0

110

nd

U

25

39

14

Sr

nd

6

4

Y

Ni

7

4

Cu

0

nd

Nb

Zr

Totals

Cr20,3

27.68

22.29

Fe~O}

0.64

27,49

0.21

19.56

007

FeO

0.05

TiO~

63.91

Max

AI;O:)

48,73

Av

so~

0

6

38

0

21

9

0

0

0

2

0

7

3

2

0

0.02

0.99

0.11

0.40

1.48

0.29

1985

10.48

0.05

006

40.95

Min

Av

26

6

34

nd

21

11

2

5

4

46

nd

32

10

6

2

0.28

0.56

0.63

2.08

303

0.21

26.61

22.13

039

0.04

45.49

0.12 0.02 0.01

2.26 2.09 1.43

2 4 4 0

2 16 38 0

5

2

9

0

2

5

9

3

99

0

0 166

34

4

105

34

3

2

31

23

2

0.24

4.81

2

1.99

0.04

0.51 4.85

10.18

14.53

0.07

0.02

31.44

Min

41.78

36.23

1.90

0.05

61.86

Max

Av

12

8

15

0

26

18

4

5

4

54

5

44

19

7

0

0.26

0.56

2.13

6.43

4.82

0,91

30.14

35.66

0.53

0.02

64.96

Max

T A B L E 3a

6

4

15

nd

16

11

4

5

4

15

5

26

9

4

nd

Traces

0.12

0.22

0.77

3.35

3.97

0.58

13.65

29.98

0.20

0.02

43.70

n=14

0

3

15

0

6

6

4

5

4

4

5

12

6

2

0

0.02

0.12

0.14

1.62

2.87

0.34

6.81

20.37

0.08

0.02

31.10

Min

n=12

n=6 chert

m a g n e t i t e carbonate

magnetite c h e r t

magnetite c h e r t

DRILL

Griquatown I.F.

Kuruman I.F.

CORE

Griquatown I.F.

DRILL

Av

Kuruman I.F.

14

5

nd

nd

24

10

2

7

nd

19

nd

25

7

4

nd

0.11

0.25

1.16

2.16

3.57

0.42

18.93

25.90

0.21

nd

46.03

n=21

42

7

0

0

69

19

2

7

0

68

0

56

11

10

0

0.23

0.92

4.07

4.36

7.50

0.80

38.33

40.58

0.95

0.05

55.90

Max

chert

0

3

0

0

4

5

2

7

0

3

0

2

3

2

0

0.02

0.02

0.12

0.20

2.15

0.03

608

12.71

0.04

0.05

31.19

Min

magnetite carbonate

CORE

Av

18 30

11 30

11

6

6

11

5

5

42

22

11

10

16

22 9

5

2

0.23

0.07

2

0.60

0.23

0.11 0.60

13.40

2.75

10.53

1.10

0,18 1.96

60.13

9.51

1.99

0.09

76.50

Max

30

4

11

5

5

2

4

5

2

2

0,01

0.60

0.07

0.06

0.05

0.01

31.53

1.27

0.01

0.00

17.12

Min

n = 10

45,16

4.02

0.24

0.02

50.76

chert

magnetite carbonate

Kuruman I.F.

FINSCH MINE

Table 3a. C o m p a r i s o n b e t w e e n average m a j o r a n d t r a c e e l e m e n t a n a l y s e s of v a r i o u s BIF mesollthofacles of drill core from t h e A s b e s h e u w e l s S u b g r o u p ( H o r s t r n a n a n d H~Ibich, 1993}, a n d t h e b a s a l K u r u m a n I. F. t r a n s i t i o n zone a t R n s e h Mine (Lamprecht, 1993). T r a c e s i n ppm.

8

U r~

.z

e~

m

< > z

5

t>"

~0

v-w

0.05

0.27

22.92

17.73

0.40

3.24

2.17

2.02

0.36

0.07

Ti02

AI~O3

FeO

Fe203

IVlnO

Iv~O

CaO

Na20

K?O

P205

12

20

Zn

OJ

W

Co

Mo

8

nd

Ga

32

nd

Pb

Ba

3

Th

V

37

15

0

nd

U

Rb

28

5

26

Sr

C~

0

5

Y

Ni

77

4

Zr

164

16

59

15

48

33

0

240

8

19

nd

0

4

8

15

6

4

0

0

2

3

0

4

2

2

0

0.01

0.18

0

0.01

0.22

2.43

0.14

4.40

2.19

0.03

10.55

7.39

6.63

0.98

27.04

12.79

0.06

1.54

37.22

0.02

41.67

Min

0.15

61.69

Nb

Totals

Cr70,3

51.57

,902

Max

30

5

8

nd

15

10

nd

11

2

22

nd

48

6

3

nd

0.15

0.25

1.60

3.62

2.91,

0.35

20.42

19.42

0.13

0.02

50.20

Av

177

6

8

0

28

22

0

19

2

71

0

152

16

8

0

0.92

060

5,63

12.09

5.81

0,71

28.08

36.52

0.72

0.02

68.29

Max

DRILL CORE

nd

nd

nd

nd

nd

15

nd

0

0

0

0

0

20

0

0

0

nd nd

5

4

0

6

4

4

0

Traces

0.03

0.00

0.14

4.49

5.46

0.53

14.87

41.70

0.15

0.00

46.18

0

0

0

0

0

12

0

0

0

3

0

6

3

2

0

0.02

0.00

0.11

0.45

5.15

0.29

9.28

34.69

0.06

000

37.84

Min

TABLE 3a (CONTINUED)

0

4

8

0

3

2

0

5

2

2

nd

6

4

3

nd

0.02

nd

0.12

1.87

527

0,41

12.52

38.35

0.10

nd

41.38

Max

153

6

15

nd

5

19

nd

nd

8

100

nd

20

3

17

8

0.07

1.05

1.28

1.03

4.17

0.31

13.04

25.26

1.47

0.05

52.96

Av

508

7

15

0

5

33

0

0

20

225

0

35

5

52

8

0.19

1.74

1.81

3.00

5.69

0.56

25.06

38.28

4.67

0.08

73.89

Max

15

5

15

0

5

6

0

0

4

3

0

12

2

3

8

-0.02

0.74

0.15

0.16

2.62

0.10

8.34

11.31

0.14

0.03

42.11

Min

n=6

n=3 Av

stilpnomelane lutite

4

0

DRILL

Griquatown I.F

iron silicate c h e r t

Griquatown I.F

3

2

0

0.02

0.02

0.28

0.28

158

002

11.11

11.12

0.04

002

3425

Min

c h e r t n = 26

chert n = 18

Av

riebeckite carbonate

riebeckite carbonate

CORE

Kuruman I.Fo

DRILL

Griquatown I.F. Kuruman I.F.

130

I4

21

17

25

28

5

13

12

162

5

21

7

36

4

0.11

1.58

1.16

2.30

6.64

0.44

11.73

33.03

2.39

0.12

43.69

Av

474

32

29

77

79

90

6

32

21

541

6

71

13

146

7

0.40

3.64

2.37

10,03

11.44

1.07

24.04

62.30

7.31

0,40

66.89

Max

n=19

5

5

13

4

6

5

5

4

4

4

4

5

3

2

1

0.01

0.02

0.10

0.07

2.93

0.06

3.45

15.52

0.11

0.04

26.09

Min

stilpnomelane lutite

CORE

224

15

34

25

10

22

6

6

22

22

19

15

87

9

0.11

0.21

0.13

2.40

5.69

0.43

34.75

7.09

7.61

0.14

39.16

Av

579

41

133

76

43

36

10

20

32

85

56

22

176

16

0.20

0.65

0.30

5.33

11.17

1.37

5908

22.99

15.62

0.20

52.37

Max

n=9

33

4

10

8

5

7

4

5

3

3

4

10

26

2

0.03

0.02

0.06

0.82

0.79

0.07

15.79

0.00

2.21

0.06

22.24

Min

sUlpnomelane lutite

Kuruman I.F.

FINSCH MINE

~O

~o

o

o

o~

~m

5~

~ o

=_.

¢#3

!Nb Zr iY St U Rb Th PIo Ga Zn Cu Ni Or V Ba

(9) 8.64 86.96 14.83 18.68 0.000 21.62 17.18 506 5.89 1731 9.67 24.78 33.61 13.29 224.25

(9) 1.36 12.25 11.98 13.02 1.69 7.87 044 11.46 2.89 3.44 8.00 10.44 1.00 8.36 11.71

0.279

Corg

4.564 0.040 0,956 13.517 0.802 13.138 26.290 0.187 0.097 0.025 0.000 0.000 36.947 0.260 98,823

Carbonate n=9

10.923

31.457 0.121 6.234 38.621 0.389 4.329 2.256 0.105 0.115 0.0861 0.000 0.000 6.539 0.000 99.759

Stilpnomelane lutite n = 10

i Ctot

S~O2 TiO2 AI20 3 Fe203 IV~O MgO i CaO Na20 K20 920 5 Or203 NiO LOI H20" Total

Wt%

i

12.84 179.10 12.17 22.30 5.22 193,13 23.59 34.00 16.67 18.00 74.67 86.67 120.38 79.67 361.24

(3)

0.000

3.312

n=6

n=3 55723 0.602 15.494 ! 2.239 0.116 2.389 2.936 0.153 9.984 0.108 0.000 0.000 1.742 0.525 98.012

Mudstone

Shale

(11) 7.89 68.15 41.25 28.83 3.34 60.57 1038 12.12 9.36 44.82 25.91 102.82 78.37 128.50 574.92

38.955 0.559 11.079 34.079 0.378 0.936 1.412 0.126 3.397 0.086 0.000 0.000 5.498 2.419 99.412

n=4 17.429 0.069 1.422 64.868 2.412 0.822 1.171 0.101 0.241 0.704 0.000 0.000 8.407 1.614 98.259

Mudstone

Chert+

n=4

Chert

0.18 4.04 2.01: 1.00 0.00 030 0.00 4.81 2.00 2.00 3.75 3.50 4.29 0.88 17.50

(4)

90.670 0.000 0.145 1.755 0.093 1.076 2.414 0.055 0.000 0.00( 0.00( 0.000 3.646 0.098 99.952

Fe-chert n=12

0.00 609 8.68 2,44 0.00 0.00 0+00 2.12 350 4.50 17.50 7.50 0.00 32,48 57.37

(2)

77.510 0.0(30 0.003 20.843 0.249 0.114 0.026 0.075 0.000 0.948 0.000 0.000 1.970 0.397 101.235

n=12

(2) 0.00 1.82 10.43 11.66 0.00 2.23 0.00 0.00 3.50 37.00 81.00 5.001 0.00 0.00 518

43.461 0.011 0.074 29.091 0.181 4.498 7.898 0.129 0.014 0.057 0.000 0.000 17.297 0.352 103.071

Anker, chert

Table 3b. Average major and trace element composition of all rocks encountered in the Finsch mine transition profile at the base of the K u r u m a n I. F. (Lamprecht, 1993). Traces in ppm. N = n u m b e r of samples.

I

I

n=lO

0.19 4.88 8.80 5.65 0.00 5.21 0.47 2.84 0.00 0.05 0.00 0.00 1.09 2.22 3.05

(io)

48.206 0.012 0.182 41.092 0.136 1.560 2.240 0.101 0.042 0.057 0.000 0.000 6.193 0.362 100.183

Rhythmite

:Z

c~

m

,< > z

>

==

.=

o

The Transvaal-Griqualand West banded iron formation: geology, genesis, iron exploitation

91

Table 4. Mass distribution of various lithofacies of the transition zone exposed In the Flnsch open cast mine. Refer to Fig. 12 for a descripUon of the various stratlgraphic units.

Unit

j

SiO2 TiO2 AI203

h+i

46.04 0.06 1.31 38.71 0.19 2.15 2.611 0.211 0.08 0.08 7.53 1.84 100.81

Fe203 MnO MgO CaO Na20 K20 P205 LOI H20Tot.

61.67 0.12 2.41 27.9 0.36 0.7 0.991

o.o81 0.08 0.07 4.4 2.57 101.35

g

f

e

e+j/2

64.54 0.6 12.89 7.93 0.04 1.43 0.46 0.12 5.92! 0.05 3.9 1.4 99.28

18.01 0.07 1.62 64.06 2.36 0.91 1.18 0.1 0.24 0.68 8.39 1.77 99.39

57.57 nd

Aver. f t o j

50.2 0.04 0.93 38.02 0.15! 1.63 1.98 0.11 0.05 0.07 5.85 1.34 100.37

0.26 36.81 0.03 0.71 1.67 0.09 nd 0.07 2.87 0.46 100.53

46.46 0.17 3.56 38.12 1.03 0.87 0.98 0.1 0.94 0.29 5.76 2.12 100.4

Table 5. Averages of specific chemical analyses and stable isotope data on the carbonate - BIF transition at the base of the Kuruman I. F. (after Beukes et aL, 1990). LIMESTONE ~-vg

5t. dev I

DOLOMITE No.

~,vg

St. dev

SHALE No.

Mlalyses

Avg

SIDERITE IRON-FORMATION klo.

St. dev

imal),ses

~vg

OXIDE IRON-FORMATION

No.

St. dev

Inalyses

Av 9

SI. dev

No.

analyses

anallfses

WI% Cor~anic.

0.8!

079;

8!

0.521

022

4

3.91

1.24

1C

0.0~

0 04

16

0 01

0003

10

AI?O 3

2 0(

1.40

8i

1.731

0.68

4

9 5.=

3 11

1C

0.1

0.06

16

007

0 05

10

FPO?

2.7(

087

8

3,13

058

4

1(

248]

573

16

38 01

5 83

10

0.01

004

8

0.06

0,02

4

148! i 0 08!

10.31

P?O~

0.0;

1(

0.0(

003

16

0 11

008

10

S

0.51

0.491

8

0.41

0.1~

4

4.2~

6.61

1(

0.0~

OJ~

16

0.01

0 01

10

145.4~

74.1~

Ppm Ba

2055

8'

21.75

12.97

4

2061

1.47

7

1.31

0.62

4

11.07i

4 2;

1[

0.1(

0.1C

1C

007

-37.47

2.53

8

-34.03

1 06

4

-36 58

1.48

1(

-331:

2.9£

IE

-1.30

0.73

8

-0.78

0.88

4

-3 54

1.02

8;

-6.291

1 41

1E

r5

37.58

258

8

34.42

1.49

4

34 29

1.72

8

28.79;

2.4~

8n

-970

059

8

-5.90

2.05

4

-1090

109

8

-10.40

08[

Th,

358;

110(

<20

10 005

5

-26 12

1 17

10

-9 18

2 85

9

1E

173~

302

9

1(

-12 C

1 46

g

ppt4 ann

1 St dev = slandard deviation ( cs x) 2 Total Fe as FeO 3 Open space means value POt determined 4 Ppl = parts per lhnusand 5 ~= [( 8ca . 1.0OO)/(~co + 1.(300) - 111.000

S o m e a s p e c t s of this m o d e l are not c o n s i s t e n t with t h e stratigraphic facts. B e u k e s e t al. (1990) t h e m s e l v e s s t a t e t h a t stratigraphically t h e s h a l e s a n d BIF are m o r e closely a s s o c i a t e d t h a n t h e s h a l e s a n d t h e limestone. This fact is not a c c o u n t ed for b y t h e model. A very similar s i t u a t i o n w a s f o u n d in t h e F i n s c h t r a n s i t i o n profile (Fig. 12) as d e s c r i b e d b y H f i l b i c h e t al. (1992) a n d b y L a m p r e c h t (1993). Here f e r r u g i n o u s m u d s t o n e s a n d c h e r t - b a n d e d m u d s t o n e s actually intervene b e t w e e n two o x i d e - c a r b o n a t e iron formation units. S e c o n d l y t h e m o d e l of Klein a n d B e u k e s (1989)

a n d B e u k e s et aL (1990) explains t h e origin of m a c r o - u n l t s a n d w h e n t h e y s a y ( B e u k e s e t aL, 1990, p. 685) t h a t their BIF-micro-cyclicity c a n be genetically c o m p a r e d to t h a t of Garrels (1987) b e c a u s e siderite m a y precipitate b y a similar seas o n a l f l u c t u a t i o n along a c h e m o c l i n e in the w a t e r c o l u m n of their model, t h e n t h e y s e e m to forget that t h e y p o s t u l a t e a n o p e n s e a m o d e l w h e r e e v a p o r a t i o n p l a y s no p a r t a n d w h e r e s u c h a proc e s s h a s n o t b e e n d e m o n s t r a t e d to exist. B u t i n t h e Black Sea a n d in w a t e r b o d i e s s u c h a s Lake Malawi (Mflller a n d FSrstner, 1973) a n d Lake Kivu (Degens

92

I. W. HALBICH,R. SCHEEPER$,D. LAMPRECHT,J. L. VANDEVENTERand N. J. DE KOCK

a n d Stoffers, 1977 a n d D e g e n s eta/., 1981) w h e r e f l u c t u a t i o n s of t h e picnocline or t h e t h e r m o c l i n e are i n f l u e n c e d b y e v a p o r a t i o n a n d river influx, FeMn oxidising - r e d u c i n g cycles are r e c o r d e d t h a t last a b o u t 1 0 0 y e a r s i n t h e c a s e of Lake Kivu. I n t h e s e d i m e n t s of Lake Van, Turkey, however, (Degens a n d K u r t m a n n , 1978) long d i s t a n c e correlatable m i c r o c y c l e s have b e e n discovered. Thirdly, Klein a n d B e u k e s (op. cit.) find t h a t higher B a a n d Th c o n c e n t r a t i o n s in t h e s e a n d limestone lithofacies are indicative of n e a r s h o r e c o n d i t i o n s b e c a u s e t h e s e e l e m e n t s correlate positively with t h e organic c a r b o n c o n t e n t w h i c h is h i g h e s t in s h a l e s a n d l i m e s t o n e s of n e a r shore, m a r i n e facies. L a m p r e c h t (1993) a n d H/~Ibich a n d L a m p r e c h t (in preparation) f o u n d Ba c o n c e n t r a t i o n s a s high a s 1700 p p m in several s a m p l e s of f e r r u g i n o u s m u d s t o n e s intercalated with c h e r t s on a m e s o - s c a l e . This unit, w h i c h is 24 m e t r e s t h i c k (Fig. 12), i n t e r v e n e s b e t w e e n two microl a m i n a t e d c a r b o n a t e - o x i d e BIF. Similar o n e s o c c u r in several o t h e r t r a n s i t i o n profiles of t h e K u r u m a n I. F. (Fig. 10). O n a c c o u n t of t h e i r oxidised n a t u r e a n d a s revealed in t h i n s e c t i o n t h e s e m u d s t o n e s b e a r no free c a r b o n at all. This m e a n s t h a t the Bac o n t e n t n e e d n o t have a n y relationship to organic c a r b o n in a BIF t r a n s i t i o n zone. It is only a n indicator of detrital c o n t r i b u t i o n to t h e s e d i m e n t a r y record. Moreover, a s s h o w n above u n d e r " m a s s calculations" n e i t h e r the w a t e r e n v i r o n m e n t n o r t h e d e p t h n e e d have c h a n g e d from BIF to m u d s t o n e t i m e s in the F i n s c h t r a n s i t i o n s e q u e n c e (Figs 10 a n d 12) b e c a u s e the rate of Fe a n d Si precipitation h a s b a r e l y c h a n g e d . Therefore, H~Ibich a n d L a m p r e c h t (in p r e p a r a tion) confirm t h a t t h e ferro-dolomites of the Finsch profile a n d t h e b l a c k c a r b o n a c e o u s s h a l e s so closely a s s o c i a t e d with t h e m were d e p o s i t e d a s s u g g e s t e d b y B e u k e s et al. (I 990) in a shallow o p e n m a r i n e a n d n e a r s h o r e e n v i r o n m e n t w h i c h w a s to s o m e extent m i x e d with river water. The BIF a n d t h e intercalated m u d s t o n e - c h e r t s e q u e n c e were laid d o w n in a closed fresh to b r a c k i s h w a t e r b a s i n from w h i c h Fe a n d Si were r h y t h m i c a l l y d e p o s i t e d a s chert, i r o n - c a r b o n a t e a n d iron-oxide facies b y e v a p o r a t i o n a n d pH-control to p r o d u c e s e a s o n a l m i c r o - r h y t h m i t e s a s t h e b a s i c building s t o n e s of t h e s e a u t o c h t h o n o u s BIF. There w a s e n o u g h ~3C depleted c a r b o n available b e c a u s e of a proliferation of life f o r m s at all stages. Therefore it is not n e c e s s a r y to introduce isotopically light carbon a n d Fe f r o m u n k n o w n hydrothermal sources that contaminate fluctuating d e e p s e a upwellings. The Fe is s u p p l i e d b y river w a t e r a s e x p l a i n e d u n d e r "Discussion" below. As the pH i n c r e a s e s a n n u a l l y during t h e d r y s e a s o n , t h e picnocline is lowered a n d c h e l a t e d Fe a n d c a r b o n are precipitated as siderite. As t h e pH d e c r e a s e s during t h e wet sea-

s o n a n d the picnocline rises, m o r e or less ferrugin o u s c h e r t is precipitated. T h u s t h e b a s i c microcycle is p r o d u c e d a s a p r i m a r y b a n d i n g , controlled b y s e a s o n a l v a r i a t i o n s in climate. If influx e x c e e d s e v a p o r a t i o n on a n a n n u a l b a s i s for m a n y y e a r s t h e c l o s e d - o f f b a s i n will grow, d e e p e n a n d f r e s h e n with time. HtUbich et aL (1992), L a m p r e c h t (1993) a n d Htilbich a n d L a m p r e c h t (in preparation) s h o w t h a t the c a r b o n a t e c a r b o n isotopic s i g n a t u r e of t h e ferroan l i m e s t o n e s or ferro-dolomites in t h e Finsch t r a n s i t i o n zone b e c o m e s lighter u p w a r d s in t h e s t r a t i g r a p h y from u n i t s b to d in Fig. 13. Keith a n d W e b e r (1964) a n a l y s e d the isotopic c o m p o s i t i o n of over 500 l i m e s t o n e s of v a r i o u s a g e s a n d of well k n o w n fresh w a t e r a n d m a r i n e origin a n d f o u n d t h a t t h e y c o u l d b e s e p a r a t e d . Most m a r i n e c a r b o n a t e s h a v e ~3 C g r e a t e r t h a n - 2, w h e r e a s a l m o s t all fresh w a t e r c a r b o n a t e s are ~3C depleted. Fig. 13 reveals t h a t the w a t e r b e c a m e fresh in u n i t c (see also Figs 10 a n d 12). This agrees v e r y w e U with the m o d e l of a n u p w a r d f r e s h e n i n g a n d g r a d u a l l y sealed-off b a s i n with a n n u a l rain w a t e r influx exceeding evaporation. The Finsch profile also u n d e r g o e s a stratigraphically u p w a r d depletion of 180 in c a r b o n a t e s . In the e v a p o r a t i o n m o d e l s u g g e s t e d here this simply m e a n s t h a t m o r e light, organically p r o d u c e d toxic oxygen w a s b o u n d to Fe a s it b e c a m e m o r e a n d m o r e c o n c e n t r a t e d in the closed fresh w a t e r basin.

The Fossil Record and its Meaning In 1919 H a r d e r (as m e n t i o n e d b y Walter a n d H o f m a n in TrendaU a n d Morris, 1983) w a s p e r h a p s the first p e r s o n to report possible r e m a i n s of microfossfls from iron formation. C l o u d a n d Licari in 1968 (as r e p o r t e d b y Walter a n d H o f m a n in Trendall a n d Morris, 1983) a n d Licari a n d Cloud (1967) a p p a r e n t l y d e s c r i b e d the first microfossil r e m a i n s from the K u r u m a n Iron Formation. La Berge (1973) f o u n d fossil like spherical f e a t u r e s in f e r r u g i n o u s c h e r t s from G r i q u a l a n d W e s t a n d t h e Transvaal. K l e m m (1979) depicted spherical a n d elongated b u t perforated siliceous n a n o s t r u c t u r e s from t h e c h e r t y m a t r i x of t h e K u r u m a n Iron F o r m a t i o n w h i c h c o u l d r e p r e s e n t organic remains. S o m e of t h e fossils r e p o r t e d b y Walter a n d H o f m a n (1983) from t h e Gunflint Iron F o r m a t i o n at Lake S u p e r i o r strongly r e s e m b l e m o d e m soil microbes, a n d o t h e r s depicted b y La Berge (1973, Fig. 10) r e s e m b l e multiple s h e a t h s t r u c t u r e s . Very similar f e a t u r e s were recently discovered b y t h e first a u t h o r (in preparation) a s definitely prediagenetic c a r b o n a t e g l o b u l e s in t h e c h e r t y m a t r i x of a thick ankeritic c h e r t zone only 1 m e t r e below the first full fledged c a r b o n a t e oxide cycles of t h e K u r u m a n Iron F o r m a t i o n (unit d in Figs 12 a n d 13)

The Transvaal-Griqualand West banded iron formation: geology, genesis, iron exploitation

KURUMAN

GAMOHAAN FORMATION

IRON FORMATION

For subdivisions a to e see Fig 12 UNIT a

[

I

I

I

I

I

I

I

I

UNITc UNITb I •.I I II tt I

r-~ 4l

TO~LC

/

/

/

t

:

I.

I I l

t

II

: :

ORGANIC C

I I I I I

..,~ I

: : : :

I I , ~ ,," ' ' @

nLU

o ",O

~

MARINE ENVIRONMENT

:E

12

F'I, ]

I I

il

II II II

1 1 l

tl II

A

-J

I I I

-8

U

l

I

l 1 1

•

-lo_ Z <:

nr

1 l

o-Jk

o ..J

o

I.-

.I

-1

-I

-2

Ill ACCORDING TO

- 4 II

"~

(KEITH AND WEBER 1964)

-3

-2

I: l

rn Ci

UNITe

.I :1 !

I t

UNIT d

I

• !1 I

•

93

-0

t

II

l

U I

l l l

-5 - ~r FRESH WATER ENVIRONMENT

•

-7

I

I1

-17

-13

BOTTOM .......

>

I

I

-1

-9

DISTANCE IN MM

b 13 C PER MIL (PDB)

TOTAL C

I

I I

I

0

I 3 (X 1 000)

> TOP " ORGANIC C

Fig. 13. ~3C per rail. (PDB) of carbonate carbon and carbonate carbon as well as organic carbon content plotted against lithology in the Finsch transition zone. For stratigraphic units compare Fig. 12b. in t h e F i n s c h o p e n c a s t mine. T h e y strongly resemble modern lagoonal cyanophytes from L a g u n a G r a n d e (Harodyski a n d V o n d e r Haar, 1975) in B a y a California. Multiple s h e a t h s t r u c t u r e s w o u l d proliferate m o r e easily b e c a u s e t k e y s t a n d a b e t t e r c h a n c e of resisting u l t r a violet radiation or o t h e r stringent climatic effects.

T h e r e f o r e t h e r e is little d o u b t t h a t m i c r o o r g a n i s m s were p r e s e n t in t h e S o u t h African early Proterozoic h y d r o s p h e r e a n d p r o b a b l y even m o r e so in s o i l s (for r e a s o n s m e n t i o n e d u n d e r "DISCUSSION" below) b u t it r e m a i n s u n c e r t a i n w h i c h p a r t t h e y played (directly or indirectly) in t h e d i s s o l u t i o n a n d t r a n s p o r t a t i o n of Fe a n d Si. '

94

I. W. H~LBICH,R. SCHEEVERS,D. LAMPRECHT,J. L. VANDEVENTERand N. J. DE KocK

The REE-Contents and its Interpretation The rare e a r t h e l e m e n t d i s t r i b u t i o n in s e d i m e n t s is influenced b y m a n y factors s u c h a s fluid to r o c k ratio, p r i m a r y d i s t r i b u t i o n in the fresh rocks, climatic conditions (temperature a n d rainfall), o p e n or closed w a t e r s y s t e m s , partitioning b e h a v i o u r b e t w e e n m i n e r a l p h a s e s a n d c o m p l e x forming ( H u m p h r i s , 1984). The R E E - c o n c e n t r a t i o n in t o d a y ' s river w a t e r s , m e t a s o m a t i c w a t e r (hydrot h e r m a l ) a n d s e a w a t e r differs c o n s i d e r a b l y (Humphris, 1984; Fleet, 1984). Differences t h a t m a y have o c c u r r e d in P r e c a m b r i a n t i m e s c a n only b e g u e s s e d at m a i n l y b e c a u s e t h e a t m o s p h e r e a n d climate m a y have b e e n different (Holland, 1984). A s i n g u l a r a b u n d a n c e of prokaryotic life forms in early Proterozoic t i m e s m a y have c o n t r i b u t e d to special w e a t h e r i n g a n d t r a n s p o r t i n g conditions a s m e n t i o n e d earlier in this p a p e r a n d spelt out in m o r e detail u n d e r "DISCUSSION" below. Very little information is available on the REEd i s t r i b u t i o n in fresh w a t e r d e p o s i t e d s e d i m e n t s , l a k e s a n d evaporitic e n v i r o n m e n t s . Most investig a t i o n s c o n c e n t r a t e on s e a w a t e r a n d d e p o s i t s laid d o w n in s e a water. S e a w a t e r is a n o p e n s y s t e m t h a t t o d a y maint a i n s a fairly c o n s t a n t R E E - c o m p o s i t i o n o n s u r f a c e a n d far from the s h o r e s (Fig. 15). Most c h a r a c t e r i s t i c (when c h o n d r i t e normalised) are p r o n o u n c e d negative Eu- a n d Ce-anomalies, a n d a m a r k e d l y increasing e n r i c h m e n t of LREE with d e c r e a s i n g atomic n u m b e r is m a i n t a i n e d . Deep s e a w a t e r differs in t h a t the c o n c e n t r a t i o n s are higher (which p o i n t s to c o n t r i b u t i o n s from s e a b o t t o m a n d r e m a i n s from micro organisms, t h a t are k n o w n to c o n c e n t r a t e H R E E in their s k e l e t o n s above t h e photic zone). This h e l p s to k e e p shallow s e a w a t e r at a c o n s t a n t REE profile. (Humphris, 1984; Fleet, 1984). R e s i d e n c e t i m e s for REE in s e a w a t e r are short, a n d s h o r t e s t for Ce b e c a u s e in today's O-rich a t m o s p h e r e this e l e m e n t a t t a i n s a higher oxidation s t a t e in w h i c h it is readily r e m o v e d from s u r f a c e s e a w a t e r s to leave a negative anomaly. Therfore all o c e a n w a t e r s t h a t are r e d u c i n g s h o u l d n o t develop s u c h a Ce anomaly. However, this c h a n g e s w h e n w a t e r s b e c o m e acidic (Burkov a n d Podporina, 1967) b e c a u s e t h e n the LREE a n d Ce b e c o m e particularly mobile. In an early Proterozoic CO2-rich a n d oxygen p o o r a t m o s p h e r e , acid rain a n d s u r f a c e w a t e r s w o u l d be the n o r m in h u m i d , w a r m c l i m a t e s with relatively fast run-off. W h e n r u n - o f f is slow, a s with low t o p o g r a p h y a n d w h e n this r u n - o f f c r o s s e s a k a r s t i c c a r b o n a t e l a n d s c a p e very close to g r o u n d w a t e r level, Ce w o u l d b e f r a c t i o n a t e d relative to o t h e r LREE, leaving a negative a n o m a l y in the s u s p e n d e d clay m i n e r a l s a n d in solution. It is also possible t h a t acid rain

w a t e r in t h o s e d a y s w a s m u c h closer to hydrot h e r m a l w a t e r s of t o d a y a n d t h e n it m a y have h a d a negative C e - a n o m a l y to begin with. According to H u m p h r i s (1984) still very little is k n o w n a b o u t t h e c h e m i c a l a n d b i o c h e m i c a l beh a v i o u r of R E E in w a t e r s . Co-precipitation, complexing a n d chelation are little dealt with in literature. T h e s e p r o c e s s e s m a y h o w e v e r b e of great i m p o r t a n c e in removing REE from solution. The REE c o m p o s i t i o n of s e a w a t e r is n o t t h e s a m e a s t h a t of precipitates t h a t form in it w h e t h e r of chemical or b i o c h e m i c a l n a t u r e (Humphris, 1984; Fleet, 1984). This will b e so for all s t a n d i n g w a t e r s in w h i c h biological s y s t e m s play a n i m p o r t a n t role. If two t y p e s of w a t e r s mix this d o e s n o t m e a n t h a t precipitates eventually forming from this mix will reflect t h e c o m p o s i t i o n of either of t h e w a t e r s or the mix. The r e a s o n is t h a t m a n y w a t e r s (esp. s e a water) are u n d e r s a t u r a t e d with r e s p e c t to their l a n t h a n i d e content. In t o d a y ' s river w a t e r s t h e c h o n d r i t e n o r m a l i s e d REE are enriched with d e c r e a s i n g atomic n u m b e r t h r o u g h o u t the profile (Fig. 15). Their d i s t r i b u t i o n is very similar to t h a t of s h a l e s a n d clay material in s u s p e n s i o n (Humphris, 1984). A similar relationship m a y not have held for Proterozoic s u r f a c e w a t e r s a n d clays t r a n s p o r t e d a n d d e p o s i t e d b y them, b e c a u s e the partitioning b e t w e e n solution a n d s u s p e n s i o n load d e p e n d s on the pH. A cons p i c u o u s negative E u - a n o m a l y exists in t o d a y ' s s u r f a c e w a t e r s b u t no C e - a n o m a l y (Fig. 15). Eu in the r e d u c e d state is partitioned into Ca-rich minerals a n d t h u s effectively r e m o v e d from solution. This a c c o u n t s for the E u - a n o m a l y in m o d e m fresh a n d s e a w a t e r alike, w h e r e C a - m i n e r a l s precipitate. River w a t e r s t o d a y have a n o r d e r of magnit u d e higher l a n t a n i d e c o n t e n t s t h a n s e a w a t e r (Fig. 15) a n d t r a n s p r o t a t i o n p r o b a b l y t a k e s place m a i n l y in c o m p l e x e d form (Fleet, 1984). This effect w o u l d have b e e n even g r e a t e r in m o r e acidic Proterozoic s u r f a c e w a t e r s raining onto silicates a n d flowing over them. It m e a n s t h a t m o s t of t h e s e REE do not p a r t a k e in c h e m i c a l p r o c e s s e s en r o u t e to a p e r m a n e n t b a s e level of deposition u n l e s s t h e s e w a t e r s flow over c a r b o n a t e s at a slow rate. F e r r o - m a g n e s i a n oxihydrates, all Ca-precipitates, Fe s u l p h i d e s a n d B a S O 4 r e m o v e the l a n t a n i d e s effectively from p r e s e n t d a y w a t e r s . The 4 m u d s t o n e s of figure 12 have t h e h i g h e s t Ba c o n c e n t r a tions a n d the h i g h e s t l a n t a n i d e c o n c e n t r a t i o n s (Fig. 14) of all r o c k s a n a l y s e d . Fleet (1984) s t a t e s t h a t preferential mobilization of REE a n d of LREE over H R E E o c c u r s from r o c k s during w e a t h e r i n g u n d e r h u m i d , w a r m climates a n d u n d e r acidic c o n d i t i o n s , w h e r e a s u n d e r n e u t r a l to alkaline c o n d i t i o n s t h e y r e m a i n fixed. Other s e d i m e n t s are r e p o r t e d to have similar REE profiles for s h a l e s b u t the a b s o l u t e v a l u e s

The Transvaal-Griqualand West banded iron formation: geology, genesis, iron exploitation

transition zone as reported b y H o r s t m a n n et ca. (in preparation) b u t can not be fully evaluated against the transition zone data by Klein and B e u k e s (1989) b e c a u s e Pr and Nd have not b e e n reported by these authors. These anomalies are reminiscent of sea water (Fig. 15) in which the removal of the excess occurs into precipitates. (Humphris, 1984; Fleet, 1984). Therefore, the REE distribution of sea water does not resemble its chemical precipitates. The two anomalies in the Finsch case simply reflect a loss of both elements relative to the full suite. In the case of Eu this could be associated with the general slight deficiency of Ca (Lamprecht, 1993) in this transition zone as c o m p a r e d to other Asbesheuwel S u b g r o u p BIF. In the case of Ce the possible "hydrothermal" character of the surface w a t e r s in early Proterozoic times due to the singular CO2-rich atmosphere, and the transport conditions explained above, m a y be the reason for low concentrations of Ce in its precipitates.

decrease in the order graywackes, s a n d s t o n e s , limestones. Volcanic input however, can change the REE-distrtbution of shales considerably. In Proterozoic shales Eu is found to be enriched relative to shales of other ages. This h a s apparently not b e e n explained so far. Dlagenesls m a y leach REE from sediments as pH lowers and Eu would suffer m o s t from this effect. These rather general findings can now be u s e d to look at the distribution of the lantanides in BIF of the Transvaal Supergroup and in c a r b o n a t e s and shales associated with t h e m in transition zone profiles (Figs 14 to 19, and Tables 5, 6 and 7). There is good agreement within facies although t h e y are s o m e w h a t differently defined by the various a u t h o r s (Figs 14, 15, 17, 18 and 19), b u t m a r k e d differences exist, between different facies. However, the Finsch transition profiles have more distinct negative Eu- and Ce-anomalies which are real ff c o m p a r e d with the data away from the

FINSCH TRANSITION ZONE (5)M - IVludstone (ferruginous + nonferrug) (2)Scp = Shale (carbonaceous and pyritic) KLEIN + BEUKES (1989) 3 Transitionzones 16)Sc (carbonaceous) (2)Sf (ferruginous) (2)Sp (piritic) ~ HydrothermalFe-oxide " " ' ~ ' " Hydrothermalclays

200100-

~,,~,. ~ , , ~ ~-~ ' ~ "~.,~ "~ ' ~ ~ \ ~

NASC

=~','~,~k~~

\

95

\.'.',.I\"

X

\

10-

\

. . . .

~L~_

~o-. . . . . .

~

s,(2)

_.I-.

~

Scp(2)

---C>- . . . . . . . . . .

-I--i

Sp(2)

.-~ 4 . . . . . . . . . . .

.4, ,''b

\

\

X \

w

%% \

0.5I

I

i

I

La Ce Pr Nd

i

I

I

I

i

I

i

i

I

I

Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Fig. 14. Averages ofchondrite-normalised REE of 17 m u d s t o n e s a n d Shales from the basal K u r u m a n I. F. transition zone compared with hydrothermal Fe-oxides and hydrothermal clays, after Klein a n d Beukes, (1989) a n d to NASC.

I. W. HALmC., R. SCHEEPERS,D. LAMPRECHT,J. L. VANDEVEN'rERand N. J. DE KOCK

96

The F i n s c h b l a c k s h a l e s and m u d s t o n e s have very similar trends to t h o s e of black s h a l e s reported from other transition profiles by Klein a n d B e u k e s (1989). However, the Finsch m u d s t o n e s (Fig. 14) have a l m o s t double the c o n c e n t r a t i o n of the richest black shales. This m a y indicate their closer relationship to river water (Fig. 15) a n d s h a l e s in general (see NASC-profile on Fig. 14) a n d m a y be the result of co-precipitation with Ba that is also high in t h e s e rocks (Lamprecht, 1993). They are also the s h a l e s with the h i g h e s t organic carbon c o n t e n t (Klein a n d B e u k e s , 1989). Stratigraphically b o t h Klein a n d B e u k e s (1989) as well as Lamprecht (1993) a n d Htilbich et at_ (1992) find the black s h a l e s m o s t intimately associated with carb o n a t e s (Fig. I0), w h e r e a s the ferruginous m u d s t o n e s of the F i n s c h profile (Fig. 12) and others s u c h a s t h o s e at G l a d s t o n e , G a k a r o s a a n d Spitskop (Fig. 10) are intercalated with BIF. These m u d s t o n e s are m u c h more related to the carbonates and s h a l e s with respect to one or more of Mn, Al, Ti a n d K of w h i c h the last three are typically land derived w h e r e a s their Fe and Mg c o n t e n t

c o m p a r e s better with that of BIF (Table 3b). The traces are very similar to t h o s e of stflpnomelane lutites (ashfalls), s h a l e s and c a r b o n a t e s (Table 3b) a n d therefore of land derived material. The LREE of all m u d s t o n e s a n d s h a l e s alike are not related to pelagic hydrothermal Fe-oxides a n d the entire profile of marine hydrothermal clays is far removed from that of the BIF transitional pelites (Fig. 14). The BIF intercalated m u d s t o n e s therefore, are a peculiar m i x t u r e of land derived material with BIF affinities a n d a possible volcanic c o m p o n e n t . If a hydrothermal c o m p o n e n t is present, it is m a i n l y revealed by the two negative REE - anomalies. With the exception of a negative C e - a n o m a l y all t h e s e s h a l e s a n d m u d s t o n e s , b u t m o s t closely the latter, have a n REE-profile very similar to that of NASC (Fig. 14) a n d to averages of Ventersdrop a n d Transvaal S u p e r g r o u p s h a l e s (Wronkiewicz a n d Condie, 1990). On Ni/Cr-plots they correlate m o s t closely with late A r c h a e a n pelites of Taylor and McLennan (1985) a n d with pelites of the Transvaal Supergroup. On Rb/K-plots however, they com-

Table 6. REE-averages (in ppm) for various lithofacies in the carbonate-BIF transition zone at Finsch Mine after Lain )recht, 1993). Facies 1 MHR CSR CSSR ST MD* AC MFeD SCP CDL N2 La

1.19i

5.4~

2.66

6.01(2.0:

38.95(7.7)

5.80(0.6'

4. 59(0.4)

1424(3.2)

2.29

Ce Pr

1 3! 0.36

9.6~, 1.8~

2.73 043

IO09(23: 1.90(0 9'

42.43(27,41 5.67(3.11

933(1.1: 1 85(0.E

6.71(0.2 0.86(0.2)

23.41(4.3) 353(0.7)

2 1.23

Nd

0.93

5,9~

141

5.76(06:

19.34(10.61

4.81(0.71

3.02(0.4)

1242(1.4)

2 33

Sm Eu

0.34 0.07

1.41 0.2

0.23 0.1

3.20(0.3: 0.21(0.0:

3.20(1 3]

0.61(0.01

0.53(0,2)

222(0.2

0.61

0.71 (0.3]

0.16(0.0:

0.12(0.0)

052(0,1 )

0.09

Gd

0.41

1.6z

0.6E

1,34(02)~

3.28(1.0}

1.3e(0.5:

0.88(0.0)

2.31(02)

0.76

Dy

0.16

1.4Z

0.2

0.41(0.3]

3.21(0.7]

1.25(0.71

0.48(0.1)

2.31(0.3)I

0.89

HE

0.06

0.37

0.03

0.31(0.1',

0.69(0.1]

0,28(O1:

0,10(0,0)

0,46(00):

0,16

Er

0.13

1.0£

0.16

1.03(02:

2.30(0.5]

0.80(05:

0.27(0,1)

1 34(0 1)

047

Yb

0.14

0,8£

0.29

0,81(02:

1.79(061

0,58(0.4:

0.23(0,1)

1 01(0,0)

027

1 MHR = Magnetite - hematite ribbon rhythmite; CSR = Colour - siderite rhythmite: CSSR = Colour siderite + siderite rhythmite; ST = Stilpnomelane lutite; MD = Mudstones, cherty; AC = Ankeritic chert + chert; MFeD = Carbonaceous micritic ferruginous dolomite; SCP = Carbonaceous black shale, pyritic; CDL = Chip and disc lutite 2 Number of samples analysed in duplicate, sometimes in triplicate. *One mudstone analysis of FIN 136-3 was considered aberrant, because it differs too much from the rest It is not included In brackets are standard deviations 8 n-1

Table 7. REE-average (in ppm) of drill core from various lithofacies of the Griquatown I. F. = (G. I. F.) and the Kuruman I. F. = (K. I. F.) from Griqualand West. (after Horstmann et at, 1993). MAGNE"IITE CHERT BIF

Griquatown IF. n " 2

, .I.a :Ce LNd Sm Eu Gd b__~._ 7 HE Fr Tm Yb LU

Av 2 2(} 1 70 0.28 1.00 0.18 0G6 020 0.03 0.22 0.06 020 0.03 018 003

Max 3.84 2,84 0.38 147 027 007 0.25 0.04 0.27 0.07 0+28 0.04 0.26 004

Min 0.88 0.88 0,14 0.53 0,10 0.04 0.14 002 0,17 0.04 0.12 0.02 0 10 0.02

MAGNETITE CARBONATE CHERT BIF

Kuruman I.F, n = 5 Av 3.88 635 0,82 2,53 0.45 0.10 0.89 0.08 0.51 0.13 0.42 0.06 0.38 0.06

Max 8.94 17,63 2.01 5.31 0.91 0,17 1,09 0.15 0.M 0.21 0.74 0.11 0.83 0.09

Kuruman I.F. n = 2

Min 0.99 1.20 0,14 0.54 0.C0 0,02 0.12 0.02 9.1(~ 0.03 0.10 0.01

Av 3.15 0.22 0.57 1.67 0.37 0,10 0.37 0.06 0.39 0,09 0.28 004

o.o~

o.23

0.01

004

Max 5.46 9.17 0.97 3,30 0.80 0.18 0,68 0,09 0.57 0,12 0.3~ 0.~3 0.31 0.05

Min 0.63 1.26 0.15 0.64 0.13 0.04 0.17 0.03 0.20 006 0.16 0.02 0.16 0 02

STILPNOMELANE LUTITE Gdqualc~cn I F n = 3 Av 4.70 1.25 0.77 2.48 0.43 008 042 0.07 046 0.12 040 008 0.34 005

Max 6.07 13.81 1,40 4 47 0,76 0.11 0.82 0,11 088 0.17 060 0 09 0.55 0.(~

Min 1.19 2.11 0.26 1.00 0.21 006 0,24 0.04 0.28 0.06 0.18 0.02 013 0,02

RIEBECKfTE CARBONATE CHERT 011=

Kuruman I.F n = 5

Av 10.68 19.70 2.02 6.38 1,02 0.14 0.76 0.12 0.75 0.17 061 008 0.50 0,06

Max 21.37 41.85 4.38 13.44 193 0.22 1.06 0.17 101 0.24 0.79 0.11 069 0.10

Min 4.17 7.32 0.50 1.59 0.29 0.06 0.38 0.07 0,53 0,12 0,34 0.06 0.35 006

Griqualown I.F. n = 3

Av 072 1,07 0.14 054 0.11 0.03 0.14 0.02 0.18 0.04 0.14 0.02 0,12 0.02

Max 0.EA 1.32 0.18 0.88 0,14 0.04 O10 0.03 0.21 005 0.16 0.02 0.16 0.03

Min 0.69 0.79 0.12 0.45 0.10 0.03 0.11 0.02 0.14 0.03 0.11 0.01 0.08 001

Kuruman IF. n t 8

Av 147 2.11 025 100 021 0.08 0.26 0.88 0~ O.0g 029 004 0.26 0.04

Max Min 2A7 0.46 3.69 0.70 0,40 0,10 2.02 , 048 0.44 ~ 0.09 0.19 0.03 0.58 0.13 0.10 002 0.71 0.17 0.16 0,04 0.49 0.14 0.0e 0.02 0.37 0,|9 OOa

0.02

The Transvaal-Griqualand West banded iron formation: geology, genesis, iron exploitation

oxides {Fig. 16) w h i c h have a very fiat profile except for La (without d a t a for Pr a n d Nd) a n d the BIF-REE are m o r e e n r i c h e d u p to a n o r d e r of m a g n i t u d e . Stflpnomelane lutites, m e t a m o r p h o s e d (vanWyk, 1986) m i n n e s o t air e-rich BIF a n d c h e r t s from Flnsch Mine have profiles t h a t c o m p a r e closely with pres e n t d a y shallow o c e a n w a t e r (away from shores) (Fig. 15). The t e m p t a t i o n to mix river w a t e r a n d s e a w a t e r p l u s h y d r o t h e r m a l w a t e r to eventually prod u c e a c c u r a t e r e p r o d u c t i o n s of s u c h l a n t a n i d e profiles of d e p o s i t s s h o u l d b e r e s i s t e d however,

p a r e b e t t e r with V e n t e r s d o r p , lying above t h e average c r u s t a l K / R b of 230. V a r i o u s BIF facies are c o m p a r e d in Figs 15 a n d 16 to a field o c c u p i e d b y two s u i t e s of riebeckitic BIF from t h e G r i q u a t o w n a n d K u r u m a n I. F o r m a tions. T h e y all h a v e very similar trends. It s e e m s t h a t riebeckitisation diffuses t h e H R E E c o n c e n t r a t i o n s b u t d o e s n o t m u c h affect t h e LREE slope. D u r i n g d e p o s i t i o n of v a r i o u s BIF t h e LREE are f r a c t i o n a t e d into t h e p r e c i p a t e in a m a n n e r t h a t is n o t c o m p a r a b l e with h y d r o t h e r m a l m a r i n e Fe-

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KLEIN and B E U K E S ( 1 9 8 9 ) (16) Sideritic BIF (1) Minesotaite-magn.-siderite BIF (1) Magnetite-siderite BIF H O R S T M A N N et al (submitted) Field delineated by averages of two suites of riebeckite-magn.-siderite cherts (BIF) FROM FINSCH MINE (3) Stilpnomelane Lutites

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Fig. 15. Averages of chondrite-normalised REE distributions of riebeckitic BIF (Horstmann et al., in preparation) and various BIF (Klein and Beukes, 1989) from the Asbesheuwels Subgroup in Griqualand West, Northern Cape Province, South Africa. Also: average sea water and average river water as well as BIF, chert and stilpnomelane lutite from Finsch Mine.

98

I. W. H~LBICH,R. SCHEEPERS,D. LAMPRECHT,J. L. VANDEVENTERand N. J. DE KOCK

b e c a u s e t h e r e is n o t m u c h logical r e a s o n i n g t h a t s u p p o r t s s u c h a step. W h a t precipitates o u t of certain w a t e r s certainly n e e d not have the c o m p o sition of t h e s o l u t e in t h e water. It is clear from Fig. 15 t h a t relatively c l e a n c h e r t s a n d the s t i l p n o m e l a n e m e s o b a n d s are n o t closely related to t h e u n m e t a m o r p h o s e d BIF w h e t h e r riebeckitic or not. The m i n n e s o t a i t e a n d siderite d o m i n a t e d BIF at F i n s c h reveal large c o n c e n t r a t i o n d i f f e r e n c e s (Fig. 17). If t h e sideritic micrite of t h e t r a n s i t i o n s m e a s u r e d b y Klein a n d B e u k e s (1989) is not affected b y recrystallisation this m a y m e a n t h a t t h e possible m e t a m o r p h i c alteration of the F i n s c h m i n n e s o t a i t e BIF merely p r o d u c e d a deficiency in Ce a n d Eu. The siderite d o m i n a t e d BIF from F i n s c h a n d o t h e r t r a n s i t i o n s c o m p a r e well with m a g n e t i t e b e a r i n g o n e s (Fig. 17). All t h e s e BIF a n d t h e Fec a r b o n a t e t r e n d s are r a t h e r different from pelagic Fe-oxide precipitates. The s t i l p n o m e l a n e lutites from Finsch and from

200-

the two iron f o r m a t i o n s are very similar except for the negative C e - a n o m a l y of t h e f o r m e r (Fig. 18). With t h e exception of a l m o s t 10 t i m e s higher c o n c e n t r a t i o n s of all l a n t a n i d e s t h e s t i l p n o m e l a n e lutite t r e n d s c o m p a r e fairly well with t h e hydrot h e r m a l pelagic clays. This m a y i n d i c a t e t h e c o m m o n volcanic derivation a n d / or it m a y point to the very m u c h m o r e "hydrothermal" c h a r a c t e r of the w a t e r s in w h i c h t h e s e a s h b e d s w e r e laid d o w n 2 5 0 0 Ma ago. S u c h w a t e r w o u l d h a v e to be relatively tranquil a n d shallow to p r e s e r v e t h e c h a r a c t e r of t h e a s h falls in t h e s e BIF. A closed w a t e r b o d y of b r a c k i s h or fresh n a t u r e w o u l d b e m u c h closer to "hydrothermal" in the Proterozoic CO2-rich a t m o s p h e r e t h a n a n o p e n sea. The c a r b o n a c e o u s ferro dolomites of F i n s c h have a l a n t a n i d e t r e n d similar to t h a t of l i m e s t o n e s a n d dolomites from o t h e r t r a n s i t i o n profiles in t h e K u r u m a n I. F. (Fig. 19) except for the strong negative E u - a n o m a l y at F i n s c h w h i c h in this c a s e c a n not be explained b y Ca-loss (see general

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Fig. 16. Averages of chondrite-normalised REE distributions of various types of oxidic BIF compared with riebeckitic BIF from the Asbesheuwels Subgroup after Horstmann et al. (in preparation). The pelagic hydrothermal Fe-oxide distribution is added for comparison.

The Transvaal-Griqualand West banded iron formation: geology, genesis, iron exploitation discussion on Figs 14 to 19 above). The best g u e s s is again the action of hydrothermal waters that m a y have affected these marine rocks that were deposited J u s t before a closed basin developed with enrichment of"hydrothermal" surface waters. The very strong effect of percolating ground waters and spring w a t e r s in lakes is revealed b y East African lake s t u d i e s b y Mtaller and FOrstner (1973). The very slight negative Ce-anomaly in the Finsch c a r b o n a t e s of Fig. 19 as c o m p a r e d to other Finsch rocks (Figs 14, 17 and 18) could indicate that the change over from open shallow water and euxinic marine conditions during carbonate precipitation to a closed shallow fresh water b a s i n e n h a n c e s Ceoxidation. In the largely oxygen deprived early Proterozoic atmosphere, the water would have to become rather shallow for some oxidation to take place. Klein and B e u k e s (1989) suggest that the behaviour of NASC-normalised REE patterns in the K u r u m a n I. F. transition zone points to an admixture o f h y d r o t h e r m a l waters which also contribute Fe and Si to the formation of BIF. They feel that this

99

is s u p p o r t e d by O + Nt + Cu a b u n d a n c e s as plotted against total REE w h e n m a n y of their analyses fall within the present day shallow pelagic hydrothermal water field b u t very close to its lower metal c o n t e n t b o u n d a r y . The G a l a p a g o s - F a m o u s m e a s u r e m e n t s which define this field come from water depths of 2000 m or more which could hardly apply to the envisaged comparatively shallowwater BIF environment. That periodic upweMngs, which

do occur near shores, carry enough hydrothermal Fe and Si to precipitate regular microcycles of BIF for h u n d r e d s of years after excessive dilution, h a s still to be proved. To m a k e this model work, it will also have to be shown w h y this m e c h a n i s m preferably applies to late Archaean and early Proterozoic times and does not more regularly lead to BIF precipitation whenever oceans open up. A regular supply of enough Fe and Si in complexed form b y rivers even today, u n d e r a highly oxidising atmosphere, can not be doubted. The only serious problem with land derived Si and Fe to form Proterozoic BIF is the separation of elastic load from "dissolved" metal content and deposit both

FINSCH TRANSITION (1) Minnesotaite-siderite BIF (1) Siderite + siderite-minnesotaite BIF

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Fig. 18. Averages of condrlte-normalised REE patterns of 3 stilpomelane lutites from the basal transition zone of the Kuruman I. F. at Finsch Mine compared with two suites from the rest of the Kuruman I. F. (Horstmann et al. in preparation) and with pelagic hydrothermal Fe-oxides and clays (Klein and Beukes, 1989). Number of analyses in brackets. s e p a r a t l y m o s t of t h e time. To explain a preference for t h i s e n v i r o n m e n t a l b e h a v i o u r at a c e r t a i n stage of t h e p l a n e t ' s history, c r a t o n i c evolution m u s t be t r a c e d relative to t h e evolution of t h e planet, a n d planetary weather patterns.

DISCUSSION Dissolving and Transporting Iron in the Early Proterozoic Atmosphere Garrels a n d Crist (1965) a n d D a n i e l s o n (1989) s h o w t h a t a n oxygen deficient a t m o s p h e r e s u c h as is s u g g e s t e d b y Holland a n d o t h e r s (1984) at pO 2 = 10 .3 for t h e b e g i n n i n g of t h e Proterozoic, r e s u l t s in a n Eh = 0.7 at pH = 7. In t h i s case all Fe will be in t h e trivalent state. Oxygen partial p r e s s u r e s of n e a r lO4°would be n e e d e d to m a k e s u r f a c e w a t e r s

reducing. Simple s o l u t i o n of Fe *÷in s u r f a c e w a t e r s at t h a t time c a n therefore be excluded. Even t o d a y however, the t r a n s i t i o n e l e m e n t s Fe a n d Mn are carried by s u r f a c e w a t e r s at pH = 6.5 in the form of colloids a n d organo-metallic complexes, or incorporated into o r g a n i s m s by chelation. These p r o c e s s e s are i n d e p e n d e n t of t h e oxygen partial p r e s s u r e s t o d a y a n d d u r i n g t h e e a r l y Proterozoic a t m o s p h e r e in n e a r - s u r f a c e waters. However, for the Proterozoic t h e y d e p e n d on w h e t h e r e n o u g h Fe a n d Mg c a n be e x t r a c t e d from the rocks b y w e a t h e r i n g u n d e r c o n d i t i o n s t h a t applied in t h o s e days. To find a n a n s w e r we h a v e to look at t h e evidence supplied by paleosols of t h e early Proterozoic and late A r c h a e a n , m a k i n g s u r e t h a t t h e profiles u s e d really have p h y s i c a l c h a r a c t e r i s t i c s of paleosols (Palmer e t al., 1989). T h e s e paleosols m u s t re-

The Transvaal-Griqualand West banded iron formation: geology, genesis, iron exploitation

lOl

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KLEIN + BEUKES (1989), Three transition zones "~" (8) Limestones "~o-'" (9) Dolomites ~, ~"-=-" (1) Sideritic micrites tk~ . Hydrothermal Fe-oxide ~, K~.'-~ ~--~ ' " Hydrothermal clays

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Fig. 19. Averages of chondrite-normalised REE patterd of limestones, dolomites and Fe-dolomites of the Finsch transition and three other transitions (Klein and Beukes, 1989) at the base of the Kurumann I. F., compared with pelagic hydrothermal Fe-oxides and clays (Klein and Beukes, 1989). p r e s e n t sites close to p e r m a n e n t b a s e level of deposition on l a n d a n d followed b y deposition of s e d i m e n t s o n top of t h e w e a t h e r i n g p:'offle s o o n afterwards, o t h e r w i s e t h e y w o u l d not have formed in t h e first place a n d t h e n b e e n p r e s e r v e d until today. E n v i r o n m e n t s with s u c h c h a r a c t e r i s t i c s are p r o b a b l y t o p o g r a p h i c a l l y very low lying areas, waterlogged m o s t of the time b u t with a freeflowing w a t e r t a b l e to b e able to p r o d u c e the deep a n d t h o r o u g h l y w e a t h e r e d profiles t h a t are indeed f o u n d (Wright, 1986). The d r a b c o l o u r of m o s t early Proterozoic paleosols i n d i c a t e s Fe removal. This especially so w h e n t h e y h a v e developed on top of basic, iron enriched rocks. Here, Fe h a s b e e n lost c o m p a r e d to t h o s e on F e - p o o r r o c k s w h e r e iron h a s b e e n c o n c e n t r a t e d (Wright, 1986) (Retallack, 1986). At P O 2 / p C O 21.3 + 0.5 a r o u n d 2.5 Ga, c o m p a r e d to PO2/pCO2= 600 of t o d a y a s a s s u m e d b y Holland (1984), this

p h e n o m e n o n c a n be explained. There is not e n o u g h oxygen in t h e a t m o s p h e r e to i m m e d i a t e l y oxidize all Fe 2, r e l e a s e d in Fe-rich r o c k s d u r i n g w e a t h e r ing. If a lot of acid leaching in r o c k s t a k e s place b e c a u s e of high pCO 2 partial p r e s s u r e s , m u c h Fe 2, c a n be r e m o v e d from the site b y flowing w a t e r s albeit not in t h e form of simple solution. In Fe-poor r o c k s on t h e c o n t r a r y all Fe 2+ is oxidized effectively u n d e r t h e s e conditions. Ferruginized p a l e 0 s o l s are m o r e c o m m o n in y o u n g e r P r e c a m b r i a n r o c k s (Wright, 1986) a n d it seems that terrestrial weathering processes drastically c h a n g e d only after 2 1 0 0 Ma ago, bec a u s e only from t h e n on calcrete p a l e o s o l s are found, w h i c h m e a n s t h a t t h e a t m o s p h e r e h a d lower pCO 2 p r e s s u r e s t h a n in the early Proterozoic a n d the acidity of rain w a t e r d e c r e a s e d . Yariv a n d C r o s s (1979) a r g u e t h a t the oxidationr e d u c t i o n as well a s the Iron e x c h a n g e a n d hydrolysis of f e r r o - m a g n e s i a n m i n e r a l s d u r i n g chemical

102

I. W. H~LBICH,R. SCHEEPERS,D. LAMPRECHT,J. L. VANDEVENTERand N. J. DE KocK

w e a t h e r i n g of r o c k s In t h e Proterozoic will not have differed m u c h from w h a t h a p p e n s today, for t h e s a m e r e a s o n t h a t t h e Fe z. w a s n o t c a r r i e d in s o l u t i o n in t h o s e days. This m e a n s that, given t h e evidence from paleosols, t h e effect of t h e only biological activity t h a t w a s available at t h a t time m u s t b e c o n s i d e r e d . This m a y have b e e n a d e n s e m a t of algae a n d m i c r o b e s , t h a t existed on a m a t u r e l a n d s c a p e during w a r m , h u m i d c l i m a t e s (Wright, 1986). To b e able to lead this a r g u m e n t convincingly it m u s t first be s h o w n t h a t life in soils w a s indeed a likelihood. P e r h a p s soils were even t h e cradle of life on Earth. This a n d t h e s u b s e q u e n t proliferation m a y h a v e b e e n g r e a t e r in this e n v i r o n m e n t t h a n in water, for t h e following r e a s o n s : (Wright, 1986, p. 14) (i) Soils are far less h o m o g e n e o u s t h a n water. (ii) Soils are t h e principal sites of clay formation, b u t clay s e r v e s a s a n excellent c a t a l y s t for c o m p l e x molecules. (iii) The r e d u c e d mobility c o m p a r e d to w a t e r c a u s e s e n z y m e s to be less s u b j e c t e d to competition. This e n h a n c e s t h e c h a n c e s for the proliferation of primitive life in soils. (iv) The p r o t e c t i o n from ultra violet light is g r e a t e r in soils. A primordial sludge in low lying a r e a s on land m a y therefore have served a s an excellent s o u r c e (better t h a n t h e sea) for life f o r m s t h a t controlled t h e organic complexity a n d colloidal state in run-off, a n d t h u s kept Fe reduced. The 2 2 0 0 Ma old a n d 5.5 m t h i c k paleosol from Waterval O n d e r in the D w a a l h e u w e l F o r m a t i o n of the T r a n s v a a l S u p e r g r o u p is well leached a n d s e e m s to b e a r evidence of microbial action in its t o p - m o s t p a r t (Retallack, 1986}. Yariv a n d Cross, (1979) d e s c r i b e d t h e p r o c e s s of incorporation of ions freed b y c h e m i c a l w e a t h e r i n g from r o c k s into microbial s y s t e m s b y c h e l a t i o n (forming of HCNring s t r u c t u r e s ) a n d into clay m i n e r a l s b y ion s u b s t i t u t i o n , a s i m p o r t a n t p r o c e s s e s that leach stable hydrophilic colloids a n d soluble organometallic c o m p l e x e s o u t of t h e r o c k s a n d into percolating w a t e r s of t h e w e a t h e r i n g zone. This type of t r a n s p o r t a t i o n o c c u r s in m a j o r s rivers of the U. S. A. today. I f a s u b s t a n t i a l p a r t of t h e s e m e t a l s is Fe, t h e n the r e q u i r e d 7 p p m (Holland, 1984) n e e d e d to u p h o l d a S u p e r i o r type BIF-formation in Proterozoic w a t e r s , d o e s not s e e m to be a problem. Yariv a n d C r o s s {1979) also p o i n t e d o u t t h a t Fe 3÷ forms even stronger complexes t h a n Fe 2÷ (see also Ahrens, 1983) with v a r i o u s organic s u b s t a n c e s . For Mn, Yariv a n d C r o s s (1979) r e p o r t that 13 mg/1 is f o u n d in soil s o l u t i o n s at pH = 7 today. Of this 84 - 99% are complexed. T h u s there is every r e a s o n to believe t h a t sufficient a m o u n t s of Mn a n d Fe could have b e e n t r a n s p o r t e d to p e r m a n e n t sites o f d e p o -

sition in Proterozoic times. The control of environm e n t a l pH w a s m a i n l y b y b a c t e r i a (Ivarson a n d Heringa, 1972 as q u o t e d b y Yariv a n d Cross, 1979). However, b a s e d on the available geologic evidence, one will have to s h o w w h y weathering, t r a n s p o r t i n g a n d depositing c o n d i t i o n s o n t h e p l a n e t were especially c o n d u c i v e to t h e formation of t h i c k BIF in very late A r c h a e a n a n d early Proterozoic t i m e s (i. e., a time s p a n of a b o u t 5 0 0 Ma) a n d not to t h e s a m e degree before or after t h a t stage. It will have to b e a s s u m e d t h a t t h e proliferation of primitive life f o r m s a n d t h e evolution of t h e a t m o s p h e r e w e n t h a n d in h a n d , a n d t h a t d u r i n g the early Proterozoic, a critical stage w a s at least periodically r e a c h e d in the the p l a n e t ' s life cycle. This m a y have b e e n c o u p l e d to internal evolvement of the mantle, the c r u s t a n d volcanic activity, as well as to external (solar) d e v e l o p m e n t in a m a n n e r that is far from k n o w n today, a n d is b e y o n d t h e scope of this p a p e r (but see D e g e n s etaL, 1981 for example). The role of multiple s h e a t h m i c r o b e s r e s e m b l i n g c y a n o p h y t e s h a s a l r e a d y b e e n dealt with above. In Finsch r o c k s f e r r u g i n o u s l a m i n a e display n u m e r o u s circular s t r u c t u r e s r e s e m b l i n g the Huronios p o r a depicted b y Walter a n d H o f m a n n (1983) from the Gunflint Formation. Even fungi m a y have played an i m p o r t a n t role in t h o s e days. H a l l b a u e r a n d Warmelo (1974) r e p o r t e d very late A r c h a e a n fossil fungi from t h e W i t w a t e r s r a n d S u p e r g r o u p , that existed on land. Berthelin et al. (1987) have investigated t h e role of b a c t e r i a a n d fungi in t h e cycling of elements. Berthelin (1988, a n d p e r s o n a l c o m m u n i c a t i o n ) discovered t h a t fungi in softs are very active a g e n t s of U - p r e c o n c e n t r a t i o n u n d e r n a t u r a l a n d l a b o r a t o r y conditions. This effect h a s not b e e n t e s t e d for Fe or Mg, b u t n e e d s u r g e n t attention. All t h e s e findings s e e m to indicate t h a t in the early Proterozoie a low lying land s u r f a c e close to the w a t e r table c o u l d indeed have b e e n extensively inhabited at times b y micro-bial life forms. The deeply l e a c h e d early Proterozoic paleosols therefore s e e m to be indicative of p a s t biotic lands c a p e s r e p r e s e n t i n g land s u r f a c e s stable for several millions of years. As certain evidence indicates (Wright, 1986) sea w a t e r a n d t h e a t m o s p h e r e could have b e e n m o r e r e d u c i n g t h a n the c o n t e m p o r a n e o u s soils in w h i c h c a s e c h e l a t i o n controlled t h e removal of Fe from r o c k s m o r e t h a n did the oxygen level of the a t m o s p h e r e . Although this idea h a s not yet b e e n explored, it r e m a i n s a p r o c e s s t h a t could explain w h y m o r e BIF formed at a time in the E a r t h ' s history w h e n biogenic c h e l a t i o n w a s at its p e a k s o m e 2 5 0 0 Ma ago.

The Transvaal-Griqualand West banded iron formation: geology, genesis, iron exploitation A possible Model for the BIF o f the Transvaal Supergroup We have s e e n t h a t it s e e m s unrealistic to a s s u m e t h a t p r e s e n t s u r f a c e w a t e r s a n d w e a t h e r i n g condition s h o u l d b e c o m p a r e d with t h o s e of the early Proterozoic. This is also revealed b y excessive removal of Fe from very deep w e a t h e r i n g profiles over H e k p o o r t lava on the K a a p v a a l c r a t o n In t h o s e d a y s (Button, 1979; Reimer, 1987), leaving A120ae n r i c h e d r e s i d u a l s over large areas. The latter m a y t h e n b e t r a n s p o r t e d to a b a s e level of deposition s o m e w h a t later or intermittently, d e p e n d i n g on t h e tectonic c o n d i t i o n s t h a t prevail. It Is also possible to d e m o n s t r a t e t h a t in t h o s e d a y s e n o u g h Fe w a s i n d e e d t r a n s p o r t e d into f e r r u g i n o u s oolitic iron-formations slightly younger than these w e a t h e r i n g s u r f a c e s (Reimer, 1987). A similar p r o c e s s m a y have b e e n r e s p o n s i b l e for the origin of B I F j u s t s o m e w h a t earlier in time a n d u n d e r m o r e stable epeirogenic conditions. The m o s t likely source in this c a s e are the maInly b a s i c to andesitic V e n t e r s d o r p l a v a s w h i c h at t h a t t i m e c o u l d certaInly have b e e n s u b j e c t to erosion at least along t h e L o b a t s e Arch a n d its e x t e n s i o n to t h e s o u t h (Figs 2a a n d 2b) a s d i s c u s s e d above u n d e r "Distribution - cratonic settIng - age". When, u n d e r t h e c o n d i t i o n s m e n t i o n e d earlier, c h e m i c a l w e a t h e r i n g on the Kaapvaal c r a t o n bec a m e of p a r a m o u n t i m p o r t a n c e at least over of several h u n d r e d t h o u s a n d s of y e a r s a n d affectIng a very m a t u r e l a n d s c a p e , the evaporation m o d e l of Garrels (1987) (for the very s a m e r e a s o n it is rejected b y KleIn a n d B e u k e s (1989)) b e c o m e s very probable. The latter a u t h o r s claim that a n n u a l (micro-scale) varves, w h i c h are a f u n d a m e n t a l buildIng s t o n e of m a n y a u t o c h t h o n o u s Proterozoic BIF, c a n b e f o r m e d in a n o p e n s e a d o u b l e layer deep w a t e r m o d e l as p r o p o s e d b y t h e m b e c a u s e of f l u c t u a t i o n s In a n n u a l t u r n o v e r w h i c h alternatively f a v o u r s Fe- a n d Si-precipitation. S u c h a p r o c e s s h a s not b e e n d e m o s t r a t e d , b u t it d o e s exist for closed b a s I n s even on t h e millimetre scale as s h o w n b y D e g e n s a n d K u r t m a n (1978) for Lake V a n (the volumetrically fourth largest lake on Earth) w h e r e m i c r o c y c l e s of alternatIng white (winter) a n d d a r k (clastic c o n t a m i n a t e d ) s u m m e r layers h a v e b e e n deposited. Half of t h e 4 0 0 m of s e d i m e n t s t u d i e d In t h e fault controlled 60 0 0 0 y e a r old lake-fill Is of a primary, c h e m i c a l n a t u r e in spite of t h e fact t h a t t h e area is s u r r o u n d e d b y high m o u n t a i n s In a tectonically a n d volcanically u n stable area of Turkey. The average chemical precipitation p e r y e a r varies from 0.4 to 0.9 m m . The s h o r t e s t t e r m cyclicity of the lake w a t e r level d e p e n d s strictly on t h e 11 y e a r s u n s p o t cycle a n d t h e y c o n c l u d e t h a t a plcnocline only develops in this lake over p a r t of the year. In o t h e r l a k e s of

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w a r m e r climates In Africa [e.g., lake Klvu or lake Malawi, D e g e n s a n d Stoffers, 1977) plcnoclines a n d t h e r m o c l I n e s are m a i n t a i n e d at d e p t h s a s low a s 70 m e t r e s . The a r g u m e n t u s e d b y KleIn a n d B e u k e s (1989) a n d on w h i c h g r o u n d s t h e y reject G a r r e r s model, c o n c e r n s t h e rate of deposition. However, this Is influenced b y so m a n y factors, n o t t h e least beIng lake m o r p h o l o g y a s s h o w n convIncingly b y D e g e n s a n d K u r t m a n (1978) t h a t R c a n n o t be c o n s i d e r e d valid. Mfiller a n d F 6 r s t n e r (1973) c o n c l u d e d from their s t u d i e s of c h e m i c a l lake s e d i m e n t s , t h a t layers of n o n t r o m t e (FeSiO 3) form b e l o w t h e picnocllne a s t h i n c r u s t s on elastic s e d i m e n t s . T h e y held climate controlled s u d d e n lowerIngs of t h e plcnocllne r e s p o n s i b l e for this precipitation b e c a u s e Fe a n d Mn c o n c e n t r a t e at or very n e a r this s u r f a c e cons t a n t l y e v o M n g from the dissolved to t h e precltated state until a larger, s u d d e n d o w n w a r d shift o c c u r s (Degens a n d Stoffers, 1977J w h i c h precipit a t e s a n e w layer of Fe + Mn w h e r e t h e plcnoclIne o c c u r s close to lake b o t t o m . Longer oxldtslngr e d u c i n g cycles of + 100 y e a r s were o b s e r v e d b y D e g e n s et aL (1981) in lake Klvu to have b e e n m a i n t a i n e d over several t h o u s a n d s of years. Klein a n d B e u k e s (1989) a r g u e d convincingly t h a t the organic c a r b o n s u p p l y to the deposltlonal e n v i r o n m e n t is of great i m p o r t a n c e to t h e formation of m a c r o cyclicity in BIF a s silicate, c a r b o n a t e a n d oxide facies are precipitated. T h e y a t t r i b u t e It to t r a n s g r e s s i o n a n d r e g r e s s i o n c o u p l e d to organic carbon production and Fe-supply by hydrot h e r m a l l y loaded a n d upwelling s e a w a t e r in a d o u b l e layer o c e a n w a t e r model. The very low organic c a r b o n c o n t e n t of oxidic BIF m e a n s that no c a r b o n w a s available or it w a s u s e d u p In t h e p r o c e s s of forming siderite closer to C-procluctlon sites (i.e., closer to s h o r e a n d to t h e photlc zone). A photic zone ancl a c h e m o c l t n e m u s t also have existed In Proterozoic l a k e s a n d b e c a u s e of t h e oxygen deficiency the chemoclIne o c c u r r e d at m u c h lower d e p t h t h a n in p r e s e n t lakes. The d e p t h of t h e photic zone d e p e n d s on factors s u c h a s solar radiation intensity a n d t h i c k n e s s of t h e atmosphere. The f l u c t u a t i o n s of the c h e m o c l i n e d e p e n d on climatic conditions a n d t h e s h o r t e s t cycle Is t h e a n n u a l one (Degens a n d K u r t m a n , 1978) p r o d u c ing v a r v e s in lake Van. Larger cycles are t h o s e t h a t control the general rising or falling w a t e r level of the lake over tens, h u n d r e d s or t h o u s a n d s of y e a r s (Degens et aL, 1981). Volcanic e r u p t i o n s m a y i n t e r r u p t the r e g u l a r cycles at irregular intervals. Tectonic a n d t h e r m a l long t e r m effects control t h e growth of t h e b a s i n a n d t h e type of influx. For t h e T r a n s v a a l S u p e r g r o u p BIF t h e u n d e r lying c a r b o n a t e s were already d e p o s i t e d on a r a t h e r

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I. W. HALBICH,R. SCHEEPERS,D. LAMPRECHT,J. L. VANDEVENTERand N. J. DE KOCK

stable platform over very wide areas, with m i n i m a l clastic input. Therefore, a v e r y s u b d u e d topog r a p h y of t h e h i n t e r l a n d h a d existed for a long time. D u r i n g t h e e n s u i n g BIF-deposition it looks as f f t w o c o n t e m p o r a n e o u s s u b b a s i n s develop on t h e craton, the Transvaal and the Griqualand West one, with no direct correlation of Bff subdivisions. Fe a n d Si were delivered b y m a n y small, sluggish flowing s t r e a m s from deeply w e a t h e r e d lava a n d T r a n s v a a l c a r b o n a t e regosols w e s t w a r d s into t h e Griqualand West subbasin and eastwards and n o r t h w a r d s into t h e T r a n s v a a l s u b b a s i n (Fig. 2b). This c h a n g e - o v e r from o p e n to closed b a s i n n a t u r a l l y is m a r k e d b y instability in t h e c h e m i s t r y of t h e e n v i r o n m e n t . T h u s it h a p p e n s t h a t m u d s t o n e s with a terrestrial signature(Figs 10 a n d 11 a n d T a b l e s 3b a n d 4, b u t see also Klein a n d B e u k e s , (Fig. 3) a n d B e u k e s etal., 1990 (Fig. 2)) are i n t e r c a l a t e d with typical m i c r o b a n d e d BIF. The m u d s t o n e s at F i n s c h Mine however, were depo•sited in a w a t e r b o d y t h a t precipitated a l m o s t a s m u c h iron a n d silica a s d u r i n g BIF precipitation.This s h o w s t h a t no d e p t h f l u c t u a t i o n occurred, only clay m a t t e r w a s s u d d e n l y i n t r o d u c e d from l a n d b e c a u s e of a macrocyclic to megacyclic event (4 m u d s t o n e u n i t s t o g e t h e r s o m e 24 m e t r e s thick, Fig. 11) in t r a n s i t i o n zones. In a c l o s e d - b a s i n - stratified - w a t e r - c o l u m n m o d e l a slight shift i n t h e c o u r s e s of rivers p e r h a p s c a u s e d b y w h a r p i n g or a slight unplift during faulting, c o u l d have b e e n r e s p o n s i b l e for this effect. Depth, pH a n d t e m p e r a t u r e of the waterb o d y therefore, did not c h a n g e s u b s t a n t i a l l y , b u t t h e microcyclicity w a s d e s t r o y e d a l t h o u g h t h e pres e n c e of m e s o - c h e r t b a n d s proves t h a t the next longer cyclic c o n t r o l w a s not i n t e r r u p t e d (units f, g, h, i, Figs 10 a n d 11). A factor t h a t m a y have b e e n crucial in t r a p p i n g m o s t clastics is the g r a d u a l e x p o s u r e of a j u s t p r e v i o u s l y f o r m e d c a r b o n a t e platform to chemical erosion. The k a r s t s y s t e m t h a t develops allows s u b a e r i a l a n d g r o u n d w a t e r s to flow t o w a r d s t h e b a s i n a n d c a r r y only c o n s t i t u e n t s in t r u e s o l u t i o n or complexed. Even t r u e s o l u t i o n s of Fe c a n be t r a n s p o r t e d u n d e r s u c h c i r c u m s t a n c e s in a n MnFe-CO2-S-H20 s y s t e m (Hem, 1972). This is exactly w h a t one c a n expect from n e a r s u r f a c e a n d s u r f a c e w a t e r s u n d e r a CO2-rich a t m o s p h e r e . The depletion of organic m a t t e r during oxide BIF deposition in t h e fresh w a t e r m o d e l s u g g e s t e d here, c a n be explained in a similar m a n n e r a s for t h e stratified o c e a n w a t e r m o d e l of Klein a n d B e u k e s (1989) a n d B e u k e s et al. (1990). Organic m a t t e r is in s h o r t e r s u p p l y as s o o n a s the w a t e r level (and therefore t h e c h e m o c l i n e a n d photic zone) rises (probably for at least several metres) for longer time intervals (Degens et al., 1981) to preferentially precipitate Fe-oxide, whereas the

a n n u a l shifts c a u s e d b y s e a s o n a l e v a p o r a t i o n are r e s p o n s i b l e for m i c r o c y c l e s once a c e r t a i n w a t e r b a l a n c e is achieved. An iron c a r b o n a t e - or ironoxide-chert cyclicity is t h u s p r o d u c e d o n a s u b millimetre scale m a i n l y b e c a u s e of c h a n g e s in pH, a n d salinity, less so b e c a u s e of c h a n g e s in t e m p e r a t u r e w h i c h are t a k e n to b e m i n i m a l at t h e prevailing s e a s o n a l l y h u m i d a n d w a r m climate. ORES T h a b a z i m b i Iron Ore I n t r o d u c t i o n T h a b a z i m b i Iron Mine is s i t u a t e d in the n o r t h w e s t e m T r a n s v a a l (Figs 1 a n d 4) a n d p r o d u c e s iron ore for Iscor's steelworks. The m i n e w a s e s t a b l i s h e d in 1931 a n d p r o d u c t i o n s t a r t e d in 1933. Up to 1958 this m i n e w a s t h e leading p r o d u c e r of iron ore in S o u t h Africa, w h e n it lost this position to the S i s h e n deposits. Since 1933 s o m e 112.8 Mt of high-grade ore h a s b e e n produced. At p r e s e n t T h a b a z i m b i p r o d u c e s 0.9 Mt run-of-mine from an underground mine per a n n u m , while 2.1 Mt is p r o d u c e d from a n open-pit section. The c o m p o s i t i o n of t h e ore from v a r i o u s ore drill core s e c t i o n s a n d the average c o m p o s i t i o n are given in Table 8. A n u m b e r of s e c o n d a r i l y - e n r i c h e d h e m a t i t e oreb o d i e s o c c u r on t h e f a r m s D o n k e r p o o r t 3 4 4 KQ, W a c h t e e n b i e t j i e s d r a a i 3 5 0 KQ, K w a g g a s h o e k 3 4 5 KQ, a n d B u f f e l s h o e k 351 KQ (Fig. 4).The s u p e r g e n e - e n r i c h e d iron oxide o c c u r s in the b a s a l 80 m t h i c k iron oxide facies r h y t h m i t e s (Fig. 3). Ore grade material h a s over 60 p e r c e n t iron b y weight a n d less t h a n 15 p e r c e n t SiO 2 a n d o c c u r s a s irregular, t a b u l a r b o d i e s for a total strike length of 12 k m (Fig. 4). T h i c k n e s s e s in t h e s e w e d g e s h a p e d b o d i e s v a r y b e t w e e n 2 a n d 60 m, with a n average of a b o u t 20 m. The dolomites, iron-formation, a n d orebodies dip to t h e s o u t h at a b o u t 50 °. L e n s e s of p r i m a r y i r o n - f o r m a t i o n h a v e r e m a i n e d within s u p e r g e n e - e n r i c h e d material, a n d r a n g e from a few millimetres to a few m e t r e s in t h i c k n e s s . The u p p e r c o n t a c t of the ore with overlying iron-formation is gradational. The o r e b o d i e s are affected b y d i a b a s e a n d dolerite intrusions, faults a n d paleo-sinkhole s t r u c t u res. Two p r o m i n e n t o r t h o g o n a l j o i n t s y s t e m s w h i c h p o s t d a t e t h e m a i n period of ferruginization have developed t h r o u g h o u t t h e dolomite f o r m a t i o n s a n d the ores. The deposits The m a i n ore d e p o s i t s o c c u r in the N o r t h e m R a n g e a n d are k n o w n a s the K w a g g a s h o e k - E a s t , E a s t Mine, D o n k e r p o o r t a n d D o n k e r p o o r t - W e s t o r e b o d i e s (Fig. 4). The specific geological setting,

The Transvaal-Griqualand West banded iron formation: geology, genesis, iron exploitation lithology, and geometry of each deposit are briefly d i s c u s s e d below. East Mine - The East Mine region was subjected to brecclation before the formation of iron-ore bodies (van Deventer, 1985). Therefore, m o s t of the iron ore is brecciated, and consists of primary hematite fragments set within a secondary hematite matrix. The solid ore h a s a steel grey to blue grey colour, a metallic lustre and a dense compact texture. The secondary hematite is fine-grained and steel grey to black-grey. Hard massive ore is better developed in the u p p e r portions of the orebody (Fig. 20) that plunges west with an u n d u l a t i n g irregular basal contact. Soft ores in the middle and western parts of the orebody are closely associated with paleo-sinkholes in the underlying dolomites. The u p p e r contact between the orebody and the overlying iron-formation is irregular and also in places gradational, with a two to three metre low-grade transitional zone. In m o s t c a s e s however, the ore is in direct contact with the unbrecciated, unaltered iron-formation. Down dip the orebody grades into brecciated talc-hematite s h e a t h s that often engulf carbonate-hematite rock. Hematite fragments drift in a talc- or carbonate rich ferruginous matrix (van Deventer e t al., 1986). The intensity of brecciation increases near the contact with ore. At depth a stratified alternating s e q u e n c e of m e s o b a n d s of hematite carbonate occur. The latter takes the place of chert in oxide facies rhythmites. Lenses of primary iron formation are a b s e n t from the talc-carbonate complex, the latter m a y fill karst depressions or rest on evenly b e d d e d shales on top of dolomite. Van Deventer (1985) suggests that these rocks were formed by t h e r m o d y n a m i c m e t a m o r p h i s m of the iron-formations and the underlying dolomites. According to Winlder (1967) a reaction between dolomite and quartz at about 400°C and partial

105

p r e s u r e s of b e t w e e n 200 and 4 0 0 MPa c a n produce calcite and talc. Talc normally forms first In zones a r o u n d the calcite as a result of lower t e m p e r a t u r e in the o u t e r m o s t portions of areas affected b y thermodynamic m e t a m o r p h i s m {Winkler, 1967). The model is consistent with the observed gradation of iron ores Into c a r b o n a t e - h e m a t i t e rocks through a talc-hematite contact zone. This orebody pinches out down dip and in areas where paleo-sinkholes are developed the u p p e r part of the orebody also t e r m i n a t e s In an east-west direction (Van Deventer, 1985}. Along strike it lnterfingers a g a i n s t u n a l t e r e d p r i m a r y Ironformation. D o n k e r p o o r t W e s t - The Donkerpoort West opencast presently (1992) is the m a i n source of ore at Thabazimbi. Two orebodies, separated b y a diabase sill and some iron-formation, have developed (Fig. 2 i). The lower orebody directly overlies the shale unit, dips at 30 ° SSW and h a s developed in the basal iron oxide facies rhythmites. This ore is a hard, massive breccia with other (primary) hematite fragments set in a y o u n g e r hematite matrix. The shape and position of the basal orebody w a s controlled by karst topography, the intensity of which decreases from north to south. The dolomite underlying the s o u t h e m m o s t parts of the orebody is moderately folded along NNE-SSW striking fold axis, most probably c a u s e d b y the intrusion of the Bushveld Igneous Complex, while the younger karst s t r u c t u r e s only developed during the Mokolian Erathem (1200 - 2100 Ma) (van Deventer, 1985). The thicker north western part of the lower orebody (average 25-30 m) extends up to the diabase sill. T h e u p p e r overbody (Fig. 21) is a soft, friable breccia of fragments of primary hematite in a platy hematite matrix and diluted with shale fragments.

Table 8. Chemical composition (weight per cent) of the iron ore at Thabazimbi (Van Deventer, 1985). Profile No.

Fe20 3

SiO;~

AI20 3

MnO

K20

TIO 2

CaO

MgO

P205

S

Na20

Total

w11

86.8

7.17

1,04

1,01

0,07

0,07

0,36

0.12

0,029

0.014

0,02

96,5

w15

86,2

9,09

0,68

0,30

0.03

0.05

0,28

0,13

0.046

0,012

0.02

96,8

w19

88.1

9,07

t,04

0,25

0,02

0,08

0.31

0,22

0.018

0.011

0.02

99.1

w23

69,9

4.31

0,92

0.60

0,06

0,05

0,24

0.14

0,039

0.010

0,01

96,3

w26

87,4

7,86

0.46

0.47

0.21

0,07

1.02

0,11

0.032

0,009

0,01

97.7

w33

91.5

3.49

0.78

0.48

0.25

0,06

0,65

0,30

0,042

0,011

0,01

97,6

A3

89.9

1,76

0,65

0,16

0,05

0,07

1,77

1,10

0.000

0,015

0.01

95,5

A8

91.5

3.89

1.48

0.16

0,06

0.04

0,45

1.63

0,095

0.006

0,01

99,3

A13

89.9

2,66

0.96

0,30

0,05

0,05

1.73

3,93

0,059

0,001

0,01

99.7

A18

89,4

3.27

0,70

0.27

0,02

0.04

1.61

1,73

0,048

0,001

0,01

97,1

B3

91.5

3.12

0.99

0.75

0.06

0,00

0,67

0.94

0,097

0.012

0,03

98,2

Weightedaverage

89.3

4,85

0.76

0,35

0.09

0,06

0.85

1,00

0,046

0.009

0,01

97.6

I . W . I-I~LB[CH,R. SCHEEPERS,D. LAMPRECHT,.1. L. VAN DEVENTERand N. J. DE KOCK

106

~

Diabase

~

lronOre

" ~ Shale ~ ~

Dolomite

Talc-HematiteRock Carbonate-Hematite Rock J

Dolomite

~

"~-~.~"~I~

~x, /

Dolomite 20O

0

180

Fig. 20. Isometric profiles t h r o u g h E a s t Mine ore body, (Fig. 4} T h a b a z i m b i iron ore mine.

~

Diabase

~

lronOre

~

Shale

Dolomite

Dolomite

Dolomite

Fig. 2 I. Isometric profiles through Donkerpoort West ore body, (Fig. 4) T h a b a z i m b i iron ore mine.

The Transvaal-Griqualand West banded iron formation: geology, genesis, iron exploitation This ore w a s derived from t h e u p p e r shale-rich iron oxide facies r h y t h m i t e , a n d p e t e r s o u t in all directions. It h a s t h e s h a p e of a s a u c e r a n d is y o u n g e r t h a n t h e b a s a l o r e b o d y (van Deventer, 1985). Kwaggashoek - E s s e n t i a l l y this is a soft, friable b r e c c i a of f r a g m e n t s of p r i m a r y h e m a t i t e set in a m a t r i x of s e c o n d a r y platy hematite. At d e p t h it g r a d e s into a h a r d , m a s s i v e ore. Dips are at 45 ° to t h e s o u t h a n d t h e ore t e r m i n a t e s a b r u p t l y a g a i n s t a dolerite dyke in t h e w e s t (Fig. 22). It s e e m s likely t h a t t h e dolorite a c t e d a s a b a r r i e r to the percolation of iron-rich g r o u n d w a t e r s t h r o u g h t h e p o r o u s b r e c c i a t e d zone t h a t w a s t h u s s u b j e c t e d to epigenetic ferruginization. This o r e b o d y directly overlies t h e b a s a l s h a l e unit, is wider in the w e s t a n d e a s t w a r d s splits into t h r e e lenses. Iron-formation dilutes t h e ore in t h e middle a n d at the b a s e . The u p p e r c o n t a c t with t h e overlying iron-formation is m a i n l y s h a r p a n d the s h a p e is irregular a n d locally dips are slightly to t h e east. In one s e c t i o n (Fig. 22) c a r b o n a t e - a n d talch e m a t i t e r o c k s have developed n e a r the dolerite dykes.

D o n k e t p o o ~ - The D o n k e r p o o r t o r e b o d y h a s a strike length of 1.5 km, a n average width of 30 m a n d a n average dip of 45 ° to the s o u t h (Fig. 23). At d e p t h it s t e e p e n s to over 60 ° a n d peters o u t laterally to the w e s t a n d east.

~

Diabase

~

Carbonate-Hematiterock

Nearer to s u r f a c e a m a s s i v e ore b r e c c i a overlies the shale u n i t a n d g r a d e s into a soft, friable breccia at depth. The internal f e a t u r e s are similar to t h o s e d e s c r i b e d for t h e E a s t Mine. The average p h o s p h o r o u s c o n t e n t of this o r e b o d y is higher t h a n t h a t of all the others. Apatite is the d o m i n a n t p h o s p h a t e mineral f o u n d in z o n e s directly above t h e shale unit. A dolerite striking NNW-SSE follows a fault t h a t d i s p l a c e s a d i a b a s e sill a n d s o m e of t h e ore. To t h e east of t h e dyke t h e b o d y a p p e a r s to b e t h i c k e r a n d b e t t e r developed t h a n to t h e w e s t of it (Fig. 23). V a n Deventer, (1985) c o n c l u d e d t h a t a s e c o n d period of ferruginization o c c u r r e d to the e a s t of the dyke a n d that the dyke t h u s a c t e d a s a n i m p e r m e a b l e b a r rier. The ore east of the dyke m a y b e of t h e s a m e age a s t h a t of K w a g g a s h o e k East, while t h e ore w e s t of the dyke is older.

Mineralogy Minerals forming the iron ore are c h e r t a n d v a r i o u s iron oxides. Grain sizes within t h e c h e r t s increase from a b o u t 0.01 to 0.3 m m a s a result of c o n t a c t m e t a m o r p h i s m , p r o b a b l y related to intrusion of the B u s h v e l d C o m p l e x (Beukes, 1978). Almost everywhere goethite r e p l a c e s chert. The end p r o d u c t is a m a r t i t e - h e m a t i t e rock. Average v a l u e s of m a j o r e l e m e n t s in the ore are c o n s i s t e n t with t h o s e of an iron f o r m a t i o n from w h i c h SiO 2

' - ~ Talc-HematiteRock Dolomite ~-/'~ Shale

Dolomite

Dolerite

Dolomite Dolerite 250 m

0 Dolerite

I07

loo~ J

Fig. 22. Isometric profiles through Kwaggashoek ore body, (Fig. 4) Thabazlmbl iron mine.

108

I. W. HJd.,BICH,R. SCHEEPERS,D. LAMPRECHT,J. L. VANDEV~r~ and N. J. DE KocK

h a s b e e n removed and to which only iron h a s b e e n added (Table 8) (vanDeventer, 1985). T h l s w a s also found b y Morris (1980) to be the case in the Hamersley iron ore deposits of Australia. Friable ore exhibits f e a t u r e s of a porous, bedded, massive hematite that contains micro- and macrob a n d s . These b a n d s are m a d e up of clusters of hematite. Individual c l u s t e r s (about 0.07 m m diameter) consist of several interlocking anhedral hematite grains (generally less t h a n 0.02 m m diameter) held together b y fine specularite. Recrystallisation of hematite to specularite forms a m e s h of randomly oriented grains that m a y grow across a p o r o u s layer between microbands, or fill cavities t h a t result from microscale fracturing and possibly also from dissolution of primary minerals. The fragments of massive ore consist of individual m i c r o b a n d s of variable t h i c k n e s s (from 0.10.25 mm) which are intensely fractured and contain cavities. Hematite grains of the m i c r o b a n d s are anhedral, interlocking and 0.0075 to 0.015 m m in size. The matrix however, consists of a loose p o r o u s m e s h of more elongated grains of hematite of very similar size. Genesis

Various theories on deposition of the primary iron-formations are d i s c u s s e d elsewhere in this paper. An overview of the historical development of theories on the genesis of the secondary enriched

~

~

Diabase lronOre

~

Shale

~

Dolerite

iron ore is given b y van Deventer eta/. (1986). They range from introduction of exogenic mineralizing solutions derived from the B u s h v e l d Igneous Complex over reconstitution b y tectonically driven endogenic g r o u n d w a t e r s to supergene enrichment in more t h a n one phase. Two periods of m e t a m o r p h i s m have effected the iron-formations which primarily contain a b o u t 30%-40% of Fe203 on average in the oxide facies. The contact m e t a m o r p h i s m associated with intrusion of the Bushveld Complex to the s o u t h of the S o u t h e r n Range in Fig. 4 w a s followed by thermodynamic m e t a m o r p h i s m resulting from tectonism (Strauss, 1964). Grain size in the chert m e s o b a n d s increased from 0.01 to 0.3 m m as a result of contact metamorphism. This recrystallization w a s controlled b y iron oxide, c a r b o n a t e and silicate impurities (Beukes, 1973). In addition stilpnomelane and minnesotaite were altered to biotite and grunerite - cummingtonite (Beukes, 1978). It is thought that the t h e r m o d y n a m i c metamorphism of the iron-formations and underlying dolomites led to the development of the talc-hematite and carbonate-hematite rocks. According to v a n Deventer (1985) the iron oxide facies rhythmites as well as talc- and carbonate-hematite rocks t h e n b e c a m e supergenetically enriched to form iron ore. Supergene enrichment explains the lenses of iron-formation present in the ore, the c o m m o n mineral associations found in both wall rocks and ores, the absence of wall rock alteration, orebodies b o u n d e d by shale b e d s and the presence of mineral

%

%

'ZO0~

Dolomite

ro

80

Fig. 23. Isometric profiles through Donkerpoort ore body, Thabazimbi iron ore mine.

The Transvaal-Griqualand West banded iron formation: geology, genesis, iron exploitation p s e u d o m o r p h s like m a g n e t i t e - m a r t i t e (van Deventer, 1985). An increase in pH leads to elevated solubility of silica a n d a l u m i n a (Goldich, 1938), except for quartz grains larger t h a n 1 m m (Door, 1964). Because quartz grains in these chert m e s o b a n d s are smaller they m a y therefore provide up to 6 m g / l of dissolved Si at 25°C (Hem, 1970). Silica does not normally reprecipitate easily and c a n readily be removed from the system by groundwater (Dorr, 1964). At Thabazimbi this is borne out by very scarce silicification within the mining area (Van Deventer, 1985). The hematitisation of these c o u n t r y rocks m a y have t a k e n place as follows: Primary chert, Fesflicates and c a r b o n a t e s in the iron-formations, talc-hematite a n d carbonate-hematite rocks, were d i s s o l v e d or r e p l a c e d by m a r t i t e - h e m a t i t e / goethite (Van Deventer, 1985). Primary hematite r e m a i n e d stable, while magnetite was oxidized to martite. According to Morris (1980) the secondary hematite could have formed in three f u n d a m e n t a l steps: Firstlychert, silicates, and carbonates of the primary rock types were replaced by hydrated iron oxides; this was followed by the growth of hematite plates and goethite from the hydrated iron oxides, a process that stopped before all the iron oxides h a d been altered. Finally, removal of the remaining goethite led to high grade hematite ores. The mineralogy and textures of the ores at Thabazimbi support this model (Van Deventer, 1985). At Thabazimbi all orebodies pinch out at depth. Sinkholes are filled with overlying strata and not with iron ore. Lenses of iron formation occur inside these iron orebodies. The conglomerate of the Pretoria Group (Bevets conglomerate), which u n c o n f o r m a b l y overlies the Penge Formation, carries no clasts of iron ore. Some of the iron deposits are b o u n d by doleritedykes (Kwaggaskhoek East), while the same type of dyke cuts through the orebody at Donkerpoort. Faults associated with the dykes, and with the same orientation, terminate the upper orebody at Donkerpoort-West and displace ore at Donkerpoort and East Mine. S u m m i n g up, the history of the iron ores at Thabazimbi was as follows: After the southward tilting of the dolomites and iron-formation through intrusion of the Bushveld Complex, wrench faulting duplicated the stratigraphy. As a result of the faulting talc-hematite a n d c a r b o n a t e - h e m a t i t e rocks were,formed. A long period of weathering began during which the slow solution of dolomites below the iron-formations led to the collapse and brecciation of the overlying iron-formations, talc and carbonate-hematite rocks. Sinkholes formed locally and became filled with these breccias. This karst activity took place above the groundwater

109

table and the porous brecciated zone was t h u s subjected to slow supergene replacement of chert, calcite a n d talc by goethite and hematite. The deformed basal shales acted as barrier to percolating iron-rich groundwaters. Some dykes and faults evidently displaced existing orebodies b u t others controlled their shape and extent. Therefore two m a i n periods o f s u p e r g e n e e n r i c h m e n t seem to have occurred: one was controlled by early prefault karsting (prefaulting) while the other was controlled by faults and dolerite dykes. SISHEN IRON O R E Introduction

The S i s h e n ore deposit is s i t u a t e d in the Northern Cape, approximately 200 k m west of Kimberley and occurs in the rocks of the Griqualand West Sequence (Fig. 7). The deposit consists of hard, high-grade hematite ore a n d is m i n e d by Iscor Ltd. After beneficiation most of the ore is railed to Saldanha Bay for export while the rest is sent to Iscor's steelworks at Vanderbijlpark, Newcastle and Pretoria. The high Fe-content and, in particular, the hardness of the ore m a k e this orebody r a t h e r unique in comparison with other iron ore bodies in the world. The h a r d n e s s provides desirable strength in blast furnace loads which m a k e s it a good blending ore if mixed with other iron ores. Opencast mining m e t h o d s are used for the exploitation of the highgrade hematite. Regional Geology

Sishen iron ore mine is situated n e a r the northwestern edge of the Maremane Anticline which has a core of dolomite of the Ghaap Plateau Formation, the lithotypes of which are e n c o u n t e r e d both to the west an to the east of the K u r u m a n Hills (Fig. 7). These are underlain by the b a n d e d iron formation and jaspilite of the Asbesheuwels Subgroup. The latter is e n c o u n t e r e d within a n extensive area bording the Maremane to the east. Lava of the Ongeluk Andesite Formation o u t c r o p s in the Dimoten and Ongeluk-Witwater Synclines plunging N and S respectively. On the Maremane Anticline and along the G a m a g a r a r a n d to the west ofit there are several scattered outcrops of Chert Breccia a n d / o r Banded Iron Formation (BIF) called the Wolhaarkop Breccia and the Manganore Iron Formation (Beukes, 1978). West of the line Kathu-Postmasburg (Fig. 7) the succession dips westwards. Its u p p e r part comprises the lithofacies succesively belonging to the Gamagara Shale, Makganyene Diamictite, Ongeluk Andesite and V6elwater J a s p e r Forma-

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I. W. I-IXLBICH,R. SCHEEPERS,D. LAMPRECHT,J. L. VANDEVENTERand N. J. DE KOCK

tions of the Griqualand West Sequence. These are succeeded by the Mapedi Shale, Lucknow Quartzite a n d Hartley Andesite F o r m a t i o n s of the Olffantshoek Sequence. A succession of conglomerate, clay a n d calcrete of the Kalahari Formation of Tertiary age a n d recent windblown sand cover the e a r l y Proterozoic rocks of the Sishen area to a consider-able extent. Below the Kalahari sedim e n t s a n d between t h e m and the rocks of the Olffantshoek Sequence there exist patchy suboutcrops of tillite and shale of the Dwyka Formation which are not shown on Fig. 7. Figs 24 and 25 illustrate the stratigraphic position of the ore relative to the other rock types in the Sishen mining area. The iron ore occurs in the G a m a g a r a a n d the Asbesheuwels Subgroups. The Gamagara Subgroup consists of a sequence of shale, ferruginous flagstone, grit, conglomerate, quartzite a n d hematite rock. The latter as well as very iron rich varieties of grit and conglomerate, constitute the high- to low-grade iron ore. Shale intercalations within the ore bodies are considered waste. The Asbesheuwels Subgroup here consists of b a n d e d iron-formation,jaspilite and shale, which are ferruginised in patches to form irregular lentic u l a r bodies of high- to low-grade so-called Thabazimbi type ore, which has been n a m e d after the secondary enriched hematite ore at Thabazimbi in the Transvaal. The stratigraphic subdivision in the Sishen iron ore mine is seen differently by van Schalkwyk and Beukes (1986). They correlated the ore zone with parts of the Manganore I. F. which, according to them, is the equivalent of the Griquatown I. F. and the K u r u m a n I. F. of the Asbesheuwels Subgroup, that unconformably overlies the cherty carbonates of the various Formations of the Campbellrand Subgroup on the eroded Maremane Dome (Fig. 7). They defined a Gamagara Formation t h a t is part of the Olifantshoek Group (Fig. 7 and Table 1) and starts at the top with the Paling Shale {Fig. 24) (absent from sections of Fig. 25) and continues progressively with the Marthaspoort Quartzite, Sishen Shale Member (apparently including the '~l"ech" shale and ferruginous quartzite in Fig. 24). This is t h e n underlain by the D oornfont ein Conglomerate Member t h a t is very thin in the mine but apparently thickens considerably westwards from the open pit. The correlation of this unit with units in Fig. 24 is uncertain. The Manganore I. F. mentioned above unconformably underlies this basal conglomerate of their Gamagara Formation. In a more regional context Beukes and Smit (1987) have shown that the west dipping t h r u s t that passes j u s t west of Sishen Mine (Fig. 26) and locally seems to follow the unconformity below the

Makganyene glacials (Fig. 24) is part of the major Black Ridge T h r u s t Zone t h a t duplicates the stratigraphy from the Koegas Subgroup upwards (Fig. 7). This m e a n s that the G a m a g a r a Formation of Beukes and Smit (1987) is part of the Olifantshoek Group (Fig. 7) and not part of the Postmasburg Group (Fig. 24). The resulting duplication is reflected in Table 1. According to van Schalkwyk and Beukes (1986) and Beukes and Smit (1987), the iron a n d manganese of the Manganore Formation as found on top of the Maremane Dome, is part of the Asbesheuwels Subgroup (dated at 2432 + 31 Ma, TrendaU et al., 1990) and derived from it probably by several cycles of secondary e n r i c h m e n t through the action of ground water and karstification along unconformities (Fig. 24) both overlying and underlying these ores (S6hnge, 1977). The comparatively thin b a n d e d iron formations and intercalated stratiform Kalaharifield m a n g a n e s e deposits of the Voelwater Formation (Fig. 7) are m u c h younger because they post-date the basal Gamagara (Olifantshoek) unconformity a n d the Ongeluk lava dated at 2239 + 90/-92 Ma (Armstrong, 1987). The Sishen orebody strikes n o r t h - s o u t h on the western flank of the Maremane Anticline and h a s a regional dip of approximately 10 ° to the west. Local variations in the direction and magnitude of the dip delineates domes a n d depressions. Northsouth trending graben and horst s t r u c t u r e s occur, but NW-SE striking faults are also present. This brings about very complex s t r u c t u r a l relationships in the mine that require extensive exploration drilling. THE VARIOUS TYPES OF ORES C o n g l o m e r a t i c Ore

This type of ore, which forms part of the Gamagara Formation {Figs 24 and 25), occurs as lenses intercalated with shale. It is composed of angular to s u b a n g u l a r pebbles of laminated and massive hematite varying in size from 1 to 20 cm. Pebbles of chert and b a n d e d iron formation also occur in lower grade ore and increase in frequence higher up in the succession as the conglomerate reaches subgrade quality. The matrix of this poorly-sorted ore varies from hematite-rich to a ferruginous, aluminous shale (van Schalkwyk, 1984). Interbedded lenses of shale and gritstone (representing a finer grained facies of the conglomeratic ore) from upward fining sequences. These conglomeratic and gritty ores are erosional products of laminated and massive ore. Minor impurities are chert, quartz and muscovite.

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The Transvaal-Griqualand West banded iron formation: geology, genesis, iron exploitation

Laminated Ore

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Ore G e n e s i s

According to Page (1992) the b a n d e d iron formation (BIF) represents a chemically precipitated sediment deposited in relatively deep water over an extensive area in the Northern Cape during early Proterozoic times. It forms the proto-ore of the Thabazimbi type ore. Paleosinkhole formation due to extensive solution of the underlying dolomite, c a u s e d the BIF to slump into the dolomite over a large area from P o s t m a s b u r g in the South to Sishen in the North. Consequently the BIF was fractured and brecciated. Through the open c h a n n e l s silica (chert) w a s leached out of the BIF b y alkaline ground w a t e r s and deposition of s e c o n d a r y hematite occurred in the resulting openings. These t h e n formed high grade l a m i n a t e d a n d b r e c c i a t e d supergene enriched iron ore bodies (Page, 1992). After the formation of the Thabazimbi type ore, a period of gentle regional folding w a s followed b y uplift with development of k a r s t a n d g r a b e n structures. Consequently, differential erosion of dolomite, Manganese Marker (Fig. 24), BIF and Thabazimbi type ore occurred. S i m u l t a n e o u s l y t h e solution of dolomite below the BIF probably continued with s u b s i d e n c e of the Thaba-type ore and BIF in the grabens (Page, 1992). Massive Ore Then an unconformity developed, followed b y the deposition of Gamagara laminated and massive The massive ore occurs in more t h a n one strati- ore. These rocks originally were ferruginous m u d graphic position and, according to van Schalkwyk and chemically precipitated sludge. The iron is (1984), two types are distinguished. The first type p r o b a b l y locally derived from the redeposited consists of a b a c k g r o u n d ofprlmary platy hematite underlying BIF and Thaba-ore. The finely lamiwith high porosity. Secondary chert with inclu- nated even structureless, massive nature of the sions of anhedral hematite are present in some of deposit indicate slow deposition in calm water over the voids. The second type contains scattered an extended period (Page, 1992). A further period of regional uplifting a n d local, quartz grains in a very fine grained hematite-rich matrix. Minor impurities in the ore are mainly gravity induced deformation followed. Sinkhole development in the dolomite continued and the muscovite and to lesser extent quartz. graben s t r u c t u r e s deepened. The upliftment caused erosion of the s u r r o u n d i n g laminated ore Thabazimbi Type bodies, and provided material for the conglomeraThis ore is found as lenticular and irregular tic and gritty ore. b o d i e s in t h e b a n d e d i r o n - f o r m a t i o n of the MINING OPERATION A s b e s h e u w e l s Subgroup. It is similar to the iron ore at the Thabazimbi mine, in that it occupies the s a m e stratigraphic horizon. The Thabazimbi type Mine planning of ore is massive in places (ferruginised jaspilite), The main purpose of long and short term planb u t it m a y also be laminated (ferruginised b a n d e d iron-formation and shale) and in places it is brec- ning is to e n s u r e that ore of acceptable quality is ciated. Impurities are mainly chert and to a lesser continously provided for the internal and foreign extent quartz a n d various forms of mica. Except for m a r k e t s at a m a r k e t related price. Open pits at Sishen (Fig. 26) are designed with pit their stratigraphic position it proved to be extremely difficult even microscopically, to distinguish be- slopes of 20 to 55 ° inclination, d e p e n d e n t on rock tween the different s u b - t y p e s of Thabazimbi ore and structures, with b e n c h heights of 12.5 m. Mineable ore blocks have more t h a n 6 m of ferruand the G a m a g a r a laminated and massive ore. ginous rock (midbench) with an iron content of

Laminated ore and massive ore (see below) occur at the b a s e of the G a m a g a r a Formation. Mesoscopically two types of laminated ore c a n be distinguished, namely thinly and thickly laminated ore (van Schalkwyk, 1984). Thinly laminated ore consists of very thin m i c r o b a n d e d hematite with a high lustre, altemating with equally thin earthy porous hematite bands. The latter contains a vuggy network of hematite platelets (van Schalkwyk and Beukes, 1986). The finely laminated ore often contains interbedded clay-rich laminae. Some finely laminated ore is highly ferruginized and consist of tightly packed m i c r o b a n d e d aggregates of hematite platelets with minor chert. It overlies thickly laminated ore with a gradational contact. The latter ore type consists of alternating m a s s i v e porous hematite m e s o b a n d s and dull, as well as bright m i c r o b a n d e d hematite m e s o b a n d s between 2 and 15 m m wide. The former consists ofanhedral to s u b h e d r a l platy hematite intergrown with secondary chert against a b a c k g r o u n d ofmanyvoids. The finer-grained microbanded m e s o b a n d s consists of density packed very fine grained and laminated aggregates of platy hematite (van Schalkwyk and Beukes, 1986).

114

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The Transvaal-Griqualand West banded iron formation: geology, genesis, iron exploitation

115

m o r e t h a n 60%. G r a d e p l a n s for m i n e p l a n n i n g are b a s e d on geological profiles a n d geological level p l a n s a s I n t e r p r e t e d from exploration boreholes. An i n t e g r a t e d p l a n n i n g s y s t e m is e m p l o y e d to link all d e p a r t m e n t s a n d f u n c t i o n s , starting from t h e "life of m i n e plan" right d o w n to w e e k l y scheduling. Mining s o m e t i m e deviates from t h e original planning d u e to c h a n g i n g c o n d i t i o n s of a geological, mining or g r a d e n a t u r e . This flexibility is m a d e possible b y t h e high degree of c o m p u t e r i z a t i o n of m i n e p l a n n i n g t a s k s . S u r m i n (a geological, mining-modelling a n d planning system) a n d Whittle 3D (a pit-optimization program) are utilized. T h e s e c o m p u t e r p r o g r a m m e s are designed to m a k e provision for f a c t o r s t h a t influence planning, like c h a n g i n g m i n i n g costs, ore prices a n d strip ratios. B e c a u s e of Its size, the S i s h e n mining area h a s to be divided into geological modelling a r e a s A, B a n d C (Fig. 26). The following m o d e l s are g e n e r a t e d for each area: (i) Rocktype-outline-model o f m i d b e n c h contours. All r o c k t y p e s are divided into 12 different groups. (ti) O r e - q u a l i t y - m o d e l (Fe, I~O a n d P) for highgrade ore a s well a s 2 low-grade ore t y p e s on a 25 X 25 m grid. (lli) Outline-of-pit-slopes-model in which specific s l o p e s are to be u s e d in v a r i o u s p a r t s of the mine a s p r e s c r i b e d b y t h e m i n e ' s geotechnical services department. F r o m the m i n i n g a n d geological m o d e l s economical b l o c k m o d e l s are g e n e r a t e d w h i c h are u s e d for pit-optimization. A v a l u e is allocated to each b l o c k in the model, w h i c h is b a s e d on i n c o m e a n d costs, a n d is positive for ore a n d negative for w a s t e rocks. The m o d e l s are f u r t h e r divided into s u b - a r e a s in w h i c h different pit s l o p e s a n d c o s t s are allocated to v a r i o u s r o c k types. The pit-optimization p r o c e s s provides a final pit l a y o u t w h i c h defines economical m i n e a b l e blocks.

halves. The one portion ls w a s h e d . The w a s h e d fraction of each m e t r e ls logged a n d b o t h s a m p l e s are s e n t to t h e l a b o r a t o r y for a n a l y s e s of Fe, 8102, A120 a, I~O a n d P a n d the d e n s i t y of e a c h type of r o c k Is d e t e r m i n e d . F r o m t h e a n a l y s e s a n d logging of t h e boreholes, the b l o c k to b e b l a s t e d is divided into f o u r different quality m a t e r i a l s n a m e l y A, B, C a n d D, b a s e d on Iron content. The m a t e r i a l t y p e s are depicted on t h e b l a s t i n g - b l o c k p l a n s b y m e a n s of c o l o u r codes. The different quality m a t e r i a l s are defined a s follows: A- material is ore c o n t a i n i n g above 60% Fe. B- material is ferruglnlzed w a s t e with a Fec o n t e n t b e t w e e n 50% a n d 60%. C - material is w a s t e with a F e - c o n t e n t b e t w e e n 3 5 % a n d 50%. D- material is w a s t e with a F e - c o n t e n t lower t h a n 35%. After the b l o c k h a s b e e n blasted, white lime lines are p a i n t e d on t h e m a t e r i a l c o r r e s p o n d i n g to t h e divisions on the b l a s t i n g b l o c k plans. The shovel o p e r a t o r is t h e n able to d i s t i n g u i s h b e t w e e n the four grade categories a n d direct t h e m a t e r i a l s to their correct destinations. A c o m p u t e r b a s e d syst e m (Dispatch) is u s e d to allocate t r u c k s to the different material t y p e s (ore a n d waste). A detailed record is k e p t of t h e r a w ore feed of each b l a s t i n g b l o c k to the plant, a n d s a m p l i n g of the ore is done h o u r l y in t h e benefication plant while a c o m p o s i t e s a m p l e is s e n t to t h e l a b o r a t o r y every 8 h o u r s . F e e d - b a c k from the p l a n t a n a l y s e s e n a b l e s the d e p a r t m e n t r e s p o n s i b l e for quality control to re-direct shovels loading t h e ore. T h u s , b y c o m b i n i n g the material from different locations in the mine, it is possible to m e e t the required specifications.

Quality Control

Preparation

In o r d e r to m e e t ore quality specifications, a quality control s y s t e m utilizing limited blending, is applied. The c h e m i c a l quality of the final p r o d u c t is partly controlled b y providing t h e cc~rrect mixt u r e of raw ore to the p r i m a r y c r u s h e r . To determ i n e w h i c h m a t e r i a l in a specific blasting b l o c k c a n b e s e n t to the p l a n t a n d onto w h i c h w a s t e d u m p t h e r e m a i n i n g material m u s t b e delivered, factors like t h e beneficiatlon f e a t u r e s of the rawmaterial with r e s p e c t to t h e iron, silica, alumina, p o t a s s i u m a n d p h o s p h o r o u s c o n t e n t are considered. A b l a s t b l o c k is d e m a r c a t e d a n d every s e c o n d borehole is s a m p l e d during b l a s t drilling b y collecting drill c h i p s for every m e t r e drilled. The s a m p l e s are t h e n p r e p a r e d a n d divided into equal

The ore is r e m o v e d from t h e o p e n pit b y m e a n s of t r u c k s a n d t r a n s p o r t e d to the p r i m a r y rotating c r u s h e r (Fig. 27). F r o m t h e p r i m a r y c r u s h e r the ore is t r a n s p o r t e d via c o n v e y o r belts to two conical c r u s h e r s to p r o d u c e a n o m i n a l p r o d u c t o f - 1 6 0 mm. F o u r conical tertiary c r u s h e r s r e d u c e the size to -90 m m w h i c h is t h e n s t a c k e d on a p r i m a r y stockpile.

ORE TREATMENT

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The Transvaal-Griqualand West banded iron formation: geology, genesis, iron exploitation Five p r o d u c t s a r e o b t a i n e d f r o m t h e w a s h i n g plant: (i) C o a r s e ore o f - 9 0 + 25 m m (ii) L u m p ore of -25 + 8 m m (iii) M e d i u m ore of -8 + 5 m m (iv) F in e ore of -5 + 0.2 m m (v) Silts b elo w 0.2 m m T h e silt a n d o t h e r v e r y fine light p a r t i c l e s are p u m p e d to t h e t h i c k e n e r s a n d s l i m es d a m s o u t s i d e t h e p l a n t ar ea. T h e o t h e r f o u r ore f r a c t i o n s are e a c h d u m p e d i n d i v i d u a l l y o n a h e a v y - m e d i u m feeds t ock p ile p r i o r to f u r t h e r p r o c e s s i n g .

Heavy-medium beneflciation Four h e a v y - m e d i u m benificiation installations a r e u s e d to s e p a r a t e t h e w a s t e f r o m t h e ore (Fig. 27). H e a v y - m e d i u m s e p a r a t i o n b y m e a n s of a s t a t i c b a t h at t h e d r u m p l a n t a r e u s e d for t h e two c o a r s e r ore f r a c t i o n s . A c o n t i n u o u s feed of ferrosilicon a n d w a t e r i n c l u d i n g ore f r o m t h e c o a r s e f r a c t i o n s t o c k piles, is s u p p l i e d to t h e d r u m s e p a rator. T h e m i x t u r e is k e p t in s u s p e n s i o n b y t h e r o t a t i n g d r u m . T h e m a t e r i a l is s e p a r a t e d o n a w a s h i n g s c r e e n as w a s t e , while all t h e d e s c e n d i n g m a t e r i a l is r e r o u t e d to a n o t h e r w a s h i n g s c r e e n as a final b e n e f i c i a t i o n p r o d u c t . B o t h t h e ore a n d t h e w a s t e m a t e r i a l are w a s h e d to r e m o v e t h e r e m a i n ing ferrosilicon, w h i c h is t h e n r ecover e d. T h e two fi n er f r a c t i o n s ar e p u t t h r o u g h h e a v y - m e d i u m c y cles w h i c h s e p a r a t e h e a v y m a t e r i a l (with a high relative d e n s i t y = RD) f r o m m a t e r i a l with a lower RD. T h i s is d o n e b y m i x i n g w a t e r with ferrosilicon. S e p a r a t i o n of t h e two finer f r a c t i o n s f rom t h e c y c l o n e o n t h e w a s h i n g s c r e e n is t he s a m e as for the coarser fractions. T h e w a s t e m a t e r i a l is t r a n s p o r t e d via a c o n v e y o r belt to t h e d u m p . T h e c o n c e n t r a t e of l u m p ore f r o m t h e d r u m p l a n t a n d t h e fine ore f r o m t h e cycl one p l a n t is r o u t e d directly to t h e b l e n d i n g plant, a n d classified as l u m p ore o f - 2 5 + 8 m m a n d fine ore of -5 m m . T h e c o n c e n t r a t e f r o m t h e c o a r s e d r u m p l a n t is d u m p e d on a b u f f e r stockpile bef o re b e i n g fed to t h e q u a t e r n a r y c r u s h e r , w h e r e it is c r u s h e d a n d s c r e e n e d to a n ore p r o d u c t o f - 8 + 5 m m (Direct r e d u c t i o n o r DR ore).

.Blending and s h i p m e n t T h e two final p r o d u c t s f r o m t h e q u a t e r n a r y s c r e e n i n g p l a n t (DR-fraction), t o g e t h e r with t h e p r o d u c t s f r o m t h e l u m p ore d r u m p l a n t a n d t h e fine ore c y c l o n e p l a n t a r e piled o n t o t he b l e n d i n g b e d s b y m e a n s of a s t a c k e r (Fig. 27). A d r u m type r e c l a i m e r r e c o v e r s t h e ore f r o m t h e ore bed, w h e r e a f t e r it is c o n v e y e d to t h e l oadi ng s t a t i o n s . T h i s m e t h o d of piling a n d r e c o v e r i n g e n s u r e s t h a t a n y d e v i a t i o n s in ore qua l i t y is e l i m i n a t e d a n d a

117

p r o d u c t of u n i f o r m q u a l i t y is d i s p a t c h e d . T h e a u t o m a t i c l o a d i n g s t a t i o n s l oad t h e different sizes of ore f r a c t i o n s o n t o rai l w ay t r u c k s for d i s p a t c h to S a l d a n h a B a y a n d to local steel w o r k s at Newcastle, V a n d e r b i j l p a r k a n d Pretoria.

Acknowledgements - I. W. Hfllbich wishes to thank the following : P. Eriksson for suggesting the challenging subject treated here and for providing Fig. 4; two coauthors for their very important contributions on the two well known South African iron ore districts; Mrs. S. Smit for meticulous word processing; J. Swart for setting the tables and preparing the final manuscript; my family for shear endless patience; the University of StelIenbosch for financial aid in drafting text figures of high quality; the C. S. I. R. for providing research funds through the F. R. D. program. J. van Deventer and N. J. de Kock express their gratitude to YSKOR for logistic assistance and for allowing certain information to be published. All the authors are grateful for the constructive criticism of two referees which helped to improve the readability and clarity of this text.

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