Distribution of yttrium and rare-earth elements in the Penge and Kuruman iron-formations, Transvaal Supergroup, South Africa

Premmbrinn Resenrth ELSEVIER

PrecambrianResearch 79 (1996) 37-55

Distribution of yttrium and rare-earth elements in the Penge and Kuruman iron-formations, Transvaal Supergroup, South Africa Michael Bau, Peter Dulski GeoForschungsZentrum Potsdam, PB 4.3 Lagerst~ttenbildung, Telegrafenberg A-50, D-14473 Potsdam, Germany

Received 12 January 1993; revised version accepted 1 December 1995

Abstract

Shale-normalized rare-earths and yttrium (REYsN; Y inserted between Dy and Ho) patterns for detritus-free samples from both the Kuruman and Penge Iron-Formations (IFs) in the Late-Archaean to Early-Palaeoproterozoic Transvaal Supergroup display pronounced heavy rare-earth element (REE) enrichment, and positive anomalies of LasN, EUsN, GdsN, YSN, and ErsN, but neither positive nor negative CeSN anomalies. Excepting CesN and EUsN anomalies, the Transvaal IFs yield all the features that are typical of the REY distribution in Modem seawater. (Eu/Eu*)SN ratios in the Kuruman IF correspond to ratios observed in other IFs of similar age, whereas the Penge IF is characterized by distinctly higher ratios. Within a sequence of eleven adjacent samples (each comprising less than ten microbands) from the Kuruman IF, (Eu/Eu*)SN ratios were found to vary significantly. Positive EusN anomalies reveal the presence of a high-temperature hydrothermal component in Transvaal seawater. The absence of positive CesN anomalies rules out the existence of an alkaline 'soda-ocean' with pH considerably above the Recent value of 8.2. Small-scale variation of (Eu/Eu*)SN ratios within the Kuruman IF as well as alternation of iron- and silica-dominated layers cannot be due to post-depositional modification of initially homogeneous material showing homogeneous REY distribution, because neither diagenetic nor metamorphic conditions were suitable for decoupling of Eu from the other REY. The observed small-scale variation may indicate short-term variability of (Eu/Eu*)SN ratios of Transvaal seawater, probably resulting from temporal variation of the activity of high-temperature venting at the seafloor. Preservation of this feature in IF microbands and the presence of positive Ysr~ anomalies suggest that IF precipitation from upwelling marine bottom waters in an oxygenated shelf environment occurred very rapidly. Hence, REY adsorbed on the surface of iron-oxyhydroxide particles that eventually became Fe-rich IF microbands, were not in exchange equilibrium with REY dissolved in ambient seawater. Higher (Eu/Eu*)SN ratios in the Penge IF compared to the Kuruman IF suggest significantly more important REY input from high-temperature solutions to the REY budget of bottom waters in the Eastern Transvaal than in the Griqualand West sub-basin. The REY distribution in Penge and Kuruman IFs is compatible with a palaeogeographic setting which invokes the existence of a rather small basin in the northeast (the Eastern Transvaal sub-basin) in which spreading-related high-temperature fluid-rock interaction occurred. The basin widened towards the southwest (the Griqualand West sub-basin) where it was connected to the open ocean.

0301-9268/96//$15.00 © 1996 Elsevier Science B.V. All rights reserved SSD! 0301-9268(95)00087-9

38

M. Bau, P. Dulski / Precambrian Research 79 (1996) 37-55

1. Introduction

Studies on the distribution of rare-earth elements (REEs) in Precambrian iron-formations (IFs) have provided valuable insight into the composition of contemporaneous seawater and the evolution of the terrestrial atmosphere-hydrosphere-lithosphere system. Thereby, they considerably enlarged our knowledge of processes having operated during the earliest periods of geological history. Thorough studies of REE and Nd-isotopic systematics lead to the development of general models of IF genesis and revealed the origin of the individual components constituting the multicomponent system of Precambrian IFs (e.g., Barrett et al., 1988; Dymek and Klein, 1988; Jacobsen and Pimentel-Klose, 1988; Derry and Jacobsen, 1990; Danielson et al., 1992; Alibert and McCulloch, 1993; Bau and M~Sller, 1993; and references in these papers; for a general review covering most aspects of Precambrian IFs see Kimberley, 1989). The widespread occurrence of IFs within the Late-Archaean to Early- Palaeoproterozoic Transvaal Supergroup in South Africa has always attracted much attention (e.g., Beukes, 1983; Beukes and Klein, 1990; and references therein). The REEs, however, have only recently been studied in greater detail in the Kuruman and Griquatown IFs occurring in the Griqualand West sub-basin in the northern Cape Province (Klein and Beukes, 1989; Beukes and Klein, 1990). No REE data had hitherto been available for the Penge IF which is thought to be a contemporaneous equivalent in the Eastern Transvaal sub-basin to the Kuruman and Griquatown IFs in Griqualand West (e.g., Beukes, 1983). Thus, one aim of this contribution is to report and discuss REE data for the Penge IF. Furthermore, without need to further expand the data base for the Griqualand West IFs in general, we focus on the fine-scale REE distribution in the Kuruman IF. Finally, we present a comparative discussion of the Kuruman IF in the Griqualand West sub-basin and the Penge IF in the Eastern Transvaal sub-basin which is based on these and previously reported (Klein and Beukes, 1989; Beukes and Klein, 1990) REE data. Recent studies (Kawabe et al., 1991; Bau and Dulski, 1994, 1995; Zhang et al., 1994; Bau et ai., 1995, 1996) have shown the value of comparative studies of Y and REE behaviours in aqueous solu-

tions and their precipitates. Thus, instead of concentrating on the REEs proper, we discuss the distribution of Rare-Earths and Yttrium (REY) and present normalized REY patterns in which Y is inserted between Dy and Ho according its ionic radius. The closest similarity exists between Y and Ho and hence anomalous behaviour of Y with respect to the REEs will be evaluated by considering Y / H o ratios. Every presentation and discussion of the REE distribution in IFs faces the question whether abundances should be normalized to chondrite or to shale. Although shale-normalization (suffix 'SN') is more frequently used in the literature, chondrite-normalization (suffix 'CN') has the advantage of illustrating the loss of the positive EucN anomaly at the Archaean/Proterozoic boundary. Furthermore, the REE distribution in shales is rather constant only in Post-Archaean shales, whereas significant variation and evolution of E u / Sm and G d / Y b ratios occur in Archaean epiclastic sediments (Taylor and McLennan, 1985; McLennan, 1989). The relative abundance of Y in REY patterns is not affected, since chondrites and shales display similar Y / H o ratios. Hence, in this contribution we present figures showing shale-normalized REY data (Post-Archaean Australian Shale, PAAS, from McLennan, 1989), but will occasionally refer to chondrite-normalized ratios (Cl-chondrite from Anders and Grevesse, 1989). Due to anomalous abundances of Gd, EUsN anomalies (EUsN/EU~N) have been quantified as EUsN/(0.67SmsN +0.33TbsN). F e 2 0 3 / P r ratios refer to Fe203 in wt% divided by Pr in ppm.

2. REE systematics of IFs

Independent from their provenance, age, and metamorphic grade, Precambrian IFs free from clastic contaminators display a similar REE signature: ( L a / S m ) c n > 1, (Sm/Yb)sN < 1, and (Eu/Sm)sN > 1 (Bau and M/511er, 1993), although their E u / S m ratios and the size of the positive Eusn anomaly become smaller with decreasing age of deposition (Derry and Jacobsen, 1990; Danielson et al., 1992). From a study of the REE distribution in early Precambrian ( > 2.3 Ga) IFs, Bau and M~511er (1993) extended previous models (e.g., Barrett et al., 1988; Dymek and Klein, 1988; Jacobsen and Pimentel-

39

M. Bau, P. Dulski / Precambrian Research 79 (1996) 37-55

Klose, 1988; Derry and Jacobsen, 1990) and described the chemical composition of these sediments as a result of mixing in a multicomponent system: at lower Po2 in the early Precambrian atmosphere in comparison to that of today, there existed a chemocline in the oceans which separated reducing (with respect to the Fe(III)/Fe(II) redox couple), slightly

[ ~ Transvaal S

acidic marine bottom water from oxidizing, less acidic marine surface water. High-temperature ( > 250°C) and low-temperature ( < 250°C) hydrotherreal fluids, yielding (Eu/Eu*)cN > 1 and (Eu/ Eu* )CN = 1, respectively, which received their REE signatures during alteration of ocean-floor basalts and/or komatiites both contributed to the REE bud-

u p e ~

[Griqualan~ ~ x ~

I

AD-

23 °

Adelaide

5

(--- ~

~h_lapitsi

m 24 °

-26 °

fo~d belt ,+]

28 °-

ZC30'- MF-2

~-~.:+~

rz~.30 ,

X

W B - 98

÷

25"

25: ao" 0 I

Z0 I

30"30, 40km I

Sco/e

Bushveld Complex Younger Proterozoic sequences Kuruman IF Penge IF

Campbellrand /

~

SchmidtsdrifSubgroup Malmani Subgroup

Archaean Basement

Fig. 1. Simplified geological maps showing distribution of the Transvaal Supergroup (simplified after Beukes, 1983; Miyano 1987; Kleinand Beukes, 1989) and locations of boreholes from which samples are discussed in the text.

and

Beukes,

40

M. Bau, P. Dulski / Precambrian Research 79 (1996) 37-55

get of the marine bottom water, where REEs together with Fe e+ and Mn2+accumulated. Above the chemocline, the REE distribution was controlled by fluvial input of dissolved REEs, similar to the situation today. Along the chemocline limited REE exchange between these two different reservoirs occurred via redox cycling of Fe in which the REEs were involved due to their semiquantitative scavenging onto precipitating iron-oxyhydroxides above and desorption due to dissolution of this carrier phase below the chemocline. Migration of bottom water into shallow-water environments lead to formation of colloidal iron-oxyhydroxide which after coagulation precipitated to form a gelous layer which eventually became oxide-facies IF. Transport into less oxygenrich environments resulted in precipitation of siderite (carbonate-facies IF). This chemically precipitated component may have been contaminated to various extends by eolian a n d / o r turbiditic epi- a n d / o r pyroclastic detritus. The important difference between the scenario outlined above and previous ones is the incorporation of a low-temperature hydrothermal REE source into the model. Low-temperature hydrothermal solutions, such as those only speculated upon by Bau and

MSller (1993), have recently been reported from Teahitia volcano, Society Islands (Michard et al., 1993). Similar to high-temperature basalt alteration fluids, these solutions yield positive eNd values that match closely with those of the basalts. In contrast to high-temperature solutions, however, they do not display positive EucN anomalies, as was suggested by Bau and MSller (1993). Today, REE input into seawater by high- and low-temperature hydrothermal solutions is negligible, due to immediate scavenging of hydrothermal REEs by precipitating Fe-Mnoxyhydroxides. In reducing early Precambrian marine bottom water, however, this removal mechanism did not operate, and thus, there existed a hydrothermal REE flux from the oceanic crust into the Precambrian ocean, which comprised both a high-temperature and a low-temperature component.

3. Regional geology and origin of samples The regional geology of the area under investigation is discussed in detail elsewhere in this special issue and is here only briefly summarized. Strata of

G R I Q U A L A N D - W E S T SUB - BASIN ONGELUK

ANDESrrE

EASTERN TRANSVAAL SUB - BASIN I

~ ~

DIAMICTITE GRIQUADANIELSKUIL MEM.

I FORMATION I

[

HEIGO(X)RTANDESITE

BosHoE~ FO~TIOS

~

ROOIHOOOTE FORMATION ~

KOEGAS

TOWN IRON-

DUITSCHLAND FORMATION

O ASBESHEUWELS

R/RIES MEM. SUBGROUP ICORUMAN GROENWATER IRONMEM. FROMATION KL1PHUISMEM. TSINENGMEM. GAMOHAAN FORMATION CAMPBELLRAND SUBGROUP

VRYBURG FORMATION

"~

~~

~ ~g~

PENGE IRON-FORMATION

°

eL *e~

c~

7.

MALMANI SUBGROUP

r~

SCHMIDTSDRIF SUBGROUP

BLACK REEF FORMATION ~ ~ Z ~

~

SADOWA FORMATION

~ ~

MOB1N FORMATION

~:

SELATI FORMATION

Fig. 2. Simplified correlation of stratigraphy of the Transvaal Supergroup in the Griqualand West and Eastern Transvaal sub-basins, South Africa (after SACS, 1980; Beukes and Klein, 1990; Eriksson et al., 1993).

M. Bau, P. Dulski / Precambrian Research 79 (1996) 37-55

the Transvaal Supergroup, deposited on the Kaapvaal craton, are preserved in two structural basins (Fig. 1) which, following Button (1986), are referred to as the Eastern Transvaal (or northeastern) sub-basin and the Griqualand West (or southwestern) sub-basin. Whereas the Penge IF is confined to the former, its contemporaneous equivalents, the Kuruman and Griquatown IFs (Fig. 2), occur within the latter. The age of the Transvaal IFs used to be rather poorly defined. As best available age bracket, Pb-Pb whole-rock isochron data were usually cited (e.g., Klein and Beukes, 1989) which set 2357 + 53 Ma and 2239 + 90 Ma (Armstrong, 1987; Walraven et al., 1990) as upper and lower limit, respectively. However, a stilpnomelane-rich band (thought to have

41

been derived from pyroclastic precursors) from the base of the Griquatown IF, yielded a U - P b zircon age of 2432 + 31 Ga (Trendall et al., 1990), whereas new data reported by Sumner and Bowring (1996) for a volcanic ash bed occurring in the uppermost Campbellrand Subgroup about 40 to 50 m below the base of the Kuruman IF gave an U - P b zircon age of 2 5 2 1 _ 3 Ma. The latter age is supported by an U - P b zircon age of 2552 + 11 Ma (Barton et al., 1994) for a tuff occurring in the Nauga Formation of the Campbellrand Subgroup. Hence, deposition of the Kuruman IF (and presumably the Penge IF) commenced as early as the latest Archaean and continued over the Archaean/Proterozoic boundary into the Siderian of the Early-Palaeoproterozoic.

Table 1 Abundances of selected elements in samples from the Kuruman IF, Transvaal Supergroup, South Africa Kuruman Iron-Formation

Fe203 Sc Cr Rb Sr Y Zr Cs Ba La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Pb Th U

KK-11

KK-12

KK-X3

AD-1

AD-2

AD-2a

AD-2b

66.5 0.16 <5 0.9 3.8 6.10 < 5 0.54 21 1.90 3.20 0.39 1.71 0.35 0.14 0.49 0.08 0.55 0.14 0.50 0.07 0.48 0.08 < 0.1 1 0.05 0.01

61.70 0.35 <5 1.0 3.6 12.6 < 5 0.38 < 5 2.21 3.50 0.42 1.88 0.42 0.19 0.67 0.12 0.82 0.22 0.81 0.12 0.78 0.13 < 0.1 2 0.09 0.02

23.17 < 0.5 9 1.2 6.0 4.66 5 0.11 6 0.78 1.53 0.21 0.96 0.27 0.09 0.36 0.06 0.43 0.I0 0.35 0.06 0.45 0.08 0.11 1 0.10 0.03

83.66 < 0.5 13 1.2 16 5.70 29 0.33 13 2.71 4.30 0.46 1.83 0.36 0.15 0.43 0.07 0.53 0.13 0.43 0.07 0.50 0.09 0.60 3 0.48 0.22

23.31 0.5 <5 3.8 38 10.3 17 0.47 16 3.09 5.20 0.63 2.79 0.63 0.42 0.82 0.14 0.95 0.24 0.76 0.12 0.81 0.15 0.42 7 0.74 0.07

35.89 0.5 5 1.3 20 14.6 16 0.21 6 4.31 7.00 0.66 4.23 0.91 0.36 1.22 0.19 1.27 0.30 0.88 0.12 0.75 0.13 0.37 1 0.27 0.07

25.31 76.79 < 0.5 0.5 <5 12 1.2 0.7 6 19 3.67 7.07 11 20 0.21 0.12 5 14 1.17 2.19 1.70 3.87 0.19 0.47 0.79 1.98 0.16 0.42 0.10 0.17 0.23 0.53 0.04 0.09 0.28 0.65 0.07 0.17 0.24 0.59 0.04 0.10 0.27 0.69 0.05 0.12 0.22 0.47 11 3 0.10 0.39 0.05 0.13

AD-3

AD-4

AD-5

AD-6

AD-7

AD-8

AD-9

16.59 < 0.5 <5 0.6 3 1.21 5 0.15 < 5 0.21 0.40 0.05 0.20 0.05 0.02 0.07 0.01 0.09 0.03 0.09 0.02 0.11 0.02 0.10 <1 0.05 0.04

12.87 < 0.5 <2 0.4 4 1,43 < 5 0.10 < 5 0.32 0.54 0.07 0.28 0.07 0.03 0.09 0.02 0.12 0.03 0.11 0.02 0.12 0.02 < 0.1 <1 0.03 0.02

69.64 0.5 12 0.7 21 9.68 25 0.13 16 2.64 4.75 0.58 2.54 0.55 0.22 0.71 0.12 0.85 0.22 0.72 0.11 0.76 0.15 0.57 3 0.34 0.09

15.73 < 0.5 <2 0.7 4 2.10 < 5 0.13 < 5 0.47 0.83 0.10 0.46 0.11 0.04 0.14 0.03 0.18 0.05 0.15 0.02 0.18 0.03 < 0.1 < 1 0.03 0.02

75.79 0.7 10 1.4 14 7.70 41 0.13 38 2.62 4.82 0.61 2.58 0.57 0.15 0.68 0.12 0.76 0.19 0.60 0.10 0.71 0.13 0.96 7 0.55 0.14

23.02 < 0.5 3 1.2 2 2.01 6 0.25 7 0.57 0.97 0.11 0.46 0.11 0.03 0.13 0.03 0.17 0.05 0.16 0.03 0.20 0.04 0.12 13 0.08 0.03

Concentrations in ppm, except Fe203 in %. All data obtained by ICP-MS, except Fe203, Sc, and Cr by INAA. All samples comprise less than 10 microbands, except KK-11, -12, and -X3 which comprise ca. 150 microbands.

M. Bau, P. Dulski/ Precambrian Research 79 (1996) 37-55

42

3.1. Kuruman IF (Griqualand West sub-basin) K u r u m a n IF s a m p l e s f o r w h i c h R E Y data are

g e n e t i c - m e t a m o r p h i c o v e r p r i n t w a s low with m a x i m u m t e m p e r a t u r e s b e t w e e n 1 10 ° and 170°C at pressures o f less than 2 kbar. H o w e v e r , intrusion o f

p r e s e n t e d here originate f r o m core A D - 5 drilled at A d e l a i d e 189-13 f a r m w e s t o f P o m f r e t a s b e s t o s m i n e (Fig. 1). T h e y b e l o n g to the G r o e n w a t e r M e m b e r

m a f i c sills c a u s e d l o c a l i z e d c o n t a c t m e t a m o r p h i s m l e a d i n g to f o r m a t i o n o f m i n n e s o t a i t e and m a g n e t i t e

(Fig. 2) w h i c h y i e l d s r h y t h m i c alternations o f siderite,

b l a g e in a d j a c e n t IF. A l o n g the c o n t a c t s o f sill and IF

h e m a t i t e , a n d m a g n e t i t e l a m i n a e and o c c a s i o n a l l y

the c h e m i c a l c o m p o s i t i o n o f b o t h l i t h o l o g i e s has

i n t e r c a l a t e d s t i l p n o m e l a n e . In certain units o f the

b e e n m o d i f i e d b y h y d r o t h e r m a l f l u i d - r o c k interac-

K u r u m a n IF o x i d e - m i n e r a l s a p p e a r to b e o f seco n d a r y origin and to h a v e f o r m e d f r o m c a r b o n a t e

tion. T h e s e p h e n o m e n a h a v e b e e n d i s c u s s e d e l s e w h e r e (Bau, 1993; K a u f m a n , 1996), and such s a m -

p r e c u r s o r s (N,

p l e s are not i n c l u d e d in the data a d d r e s s e d here.

Beukes,

pers. c o m m u n . ,

1994;

J.

from a previous s i d e r i t e - h e m a t i t e - m a g n e t i t e assem-

K a u f m a n , pers. c o m m u n . , 1995). T h e 320 m thick K u r u m a n IF s e q u e n c e i n t e r s e c t e d b y the drill core is virtually u n d e f o r m e d a n d d i p s slightly ( < 5 °) to the

3.2. Penge IF (Eastern Transvaal sub-basin)

w e s t . M i y a n o and B e u k e s (1984) s t u d i e d the m e t a -

P e n g e IF s a m p l e s d i s c u s s e d in this p a p e r originate

m o r p h i c m i n e r a l a s s e m b l a g e and c o n c l u d e d f r o m the

f r o m core M F - 2 , drilled at M a f e f e , - 30 k m northw e s t o f P e n g e (Fig. 1). The m i n e r a l o g y o f the P e n g e

o c c u r r e n c e o f s t i l p n o m e l a n e and greenalite that dia-

Table 2 Abundances of selected elements in samples from the Penge IF, Transvaal Supergroup. 2outh Africa

Penge Iron-Formation Fe20 3 Sc Cr Rb Sr Y Zr Cs Ba La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Pb Th U

PE-I 1

PE-21

PE-31

PE-41

PE-42

PE-51

PE-61

PE-71

PE-81

PE-91

21.6 16.4 169 444 23.9 21.3 114 52.1 205 23.3 47.1 5.51 20.2 4.20 1.08 3.74 0.60 3.45 0.72 2.27 0.29 2.29 0.34 3.29 15.9 7.48 1.96

59.7 0.08 < 5 2.4 2.2 7.08 <5 0.74 < 5 1.90 2.40 0.28 1.13 0.24 0.13 0.41 0.065 0.49 0.13 0.45 0.06 0.42 0.07 < 0.1 4 0.04 0.02

22.9 l 6.8 169 464 18.0 21.4 112 54.6 148 22.8 45.4 5.19 20.1 3.91 1.01 3.51 0.64 3.51 0.70 2.15 0.31 2.34 0.34 3.45 6.2 6.48 1.69

54.1 4.57 71 139 10.0 9.74 37.9 50.3 98.1 4.07 7.73 0.91 3.49 0.66 0.20 0.78 0.13 0.98 0.26 0.92 0.14 0.99 0.16 0.98 4.65 1.59 0.29

44.4 0.26 < 5 14.3 6.8 3.39 < 5 5.07 13.0 1.57 2.67 0.29 1.07 0.17 0.06 0.18 0.03 0.25 0.07 0.26 0.04 0.31 0.06 < 0.1 2.7 0.15 0.08

51.7 0.17 < 5 2.9 8.8 3.94 <5 1.07 8.79 1.17 1.50 0.16 0.69 0.145 0.07 0.23 0.04 0.28 0.08 0.26 0.04 0.24 0.04 < 0.1 1.9 0.03 0.04

65.0 0.43 <5 3.2 4.3 7.32 < 5 1.08 7.19 2.78 3.47 0.39 1.70 0.33 0.14 0.50 0.07 0.51 0.135 0.44 0.06 0.40 0.07 < 0.1 1.7 0.1 0.025

54.5 0.09 <5 4.3 4.2 I 1.6 < 5 1.76 <5 3.14 3.78 0.42 1.78 0.35 0.20 0.61 0.10 0.76 0.21 0.70 0.10 0.63 0.11 < 0.1 2 0.05 0.025

57.7 0.33 <5 4.1 4.7 3.73 <5 1.55 13.0 1.46 1.84 0.20 0.83 0.17 0.08 0.23 0.04 0.28 0.07 0.23 0.03 0.21 0.035 < 0.1 2. I 0.09 0.04

54.8 3.14 33 25.6 23.3 12.5 20.1 9.32 50.3 6.40 11.3 1.36 5.62 1.15 0.38 1.33 0.19 1.14 0.26 0.81 0.1 l 0.71 0.12 0.46 4. I 1 0.94 0.16

Concentrations in ppm, except Fe203 in %. All data obtained by ICP-MS, except Fe203, Sc, and Cr by INAA.

M. Bau, P. Dulski / Precambrian Research 79 (1996) 37-55

43

some shale layers were sampled as well. Since the basal part of the Penge IF (Fig. 2) was intruded by mafic sills which caused further local contact metamorphic overprint of the regional contact metamorphic assemblages (resulting in the formation of hornblende, clinopyroxene and fayalite in adjacent IF layers), sampling was restricted to the upper part of the Penge IF in the drillcore section.

IF was severely modified at peak temperatures between 500 ° and 550°C (Sharpe and Chadwick, 1982) by the intrusion of the Bushveld Complex, resulting in formation of magnetite, grunerite, quartz, biotite, ankerite, calcite, K-feldspar, and minor sulfides, riebeckite, and ferriannite (Miyano et al., 1987). However, care was taken to confine sampling to pure magnetite-quartz layers, although for comparison

sample/PAAS 1E+0( IIE-01 sample/PAAS

1E+O(1E-Ol

1E-02

V •

1E'~ I

KK-AD2a (~ KK-X3

(a) 1E-04

;

;

;

~' KK-AD2

~ ;

:

La Ce Pr Nd

:

;

"

;

:

;

:

:

~) KK-AD2b

(b)

SW ;

;

;

;

Sm Eu Gd Tb Dy (Y) Ho Er Tm Yb Lu 1E+00

1E-O3

X KK-AD3

La Ce Pr Nd

Sm Eu Od To Dy (Y) Ho Er Tm Yb Lu

sample/PAAS

1E-01

1E.~ 4- KK-AD7 ~) KK-AD8 ntr KK-AD9 ; ; -" ; : I I ; ; ; ; ; : ; ; ; La C,e Pr Nd Sm Eu Gd Tb Dy (Y) Ho Er Tm Yb Lu (C)

1E-03

Fig. 3. REYsN patterns of samples from the Groenwater Member of the Kurumma IF (drillcore AD-5 near Pomfre0. REY are arranged in order of decreasing effective ionic radii in trivalent oxidation state and octahedral coordination. In (a), average (1000 to 2000 m water depth) South Pacific seawater ( × 10 s) from above the East Pacific Rise is shown for comparison (data from Ban et al., 1996). Note that samples in (a) comprise about 150 microbands each, whereas samples in (b) and (c) comprise less than 10 microbands. For further explanation see text.

44

M. Bau, P. Dulski / Precambrian Research 79 (1996) 37-55

4. Analytical procedures

10:

~ La (ppm) Sm/Yb

Chemical data presented in this contribution have been obtained by INAA and ICP-MS (Tables 1 and 2). The analytical procedures are similar to those described in detail by Dulski (1992, 1994). Analytical precision and accuracy of determinations of REY abundances and ratios were checked by multiple analyses of the IF-geostandards IF-G and FeR-1FeR-4, and were found to be usually much better than + 10% (for details see Dulski, 1992). Sample decomposition for ICP-MS analyses was done by pressure digestion employing HF/HC104, and HC1. Determined REY data have been corrected for interferences of BaO on Eu and NdO on Gd and Tb (for details see Dulski, 1994).

5. Results and discussion: Kuruman IF From studies by Klein and Beukes (1989) and Beukes and Klein (1990), the REE distribution in the Kuruman IF and the succeeding Griquatown IF is comparatively well known. Thus, we placed emphasis on specific problems such as the possible occurrence of negative CeSN anomalies, the behaviour of Y with respect to the REEs, and the bearing of diagenesis and low-grade metamorphism, sample size, and small-scale variability on the REY distribution in the Kuruman IF. In general, we focus on IF material proper, i.e. stilpnomelane-rich layers, possibly derived from pyroclastic precursors (e.g., Klein and Beukes, 1989), or riebeckite-bearing layers, possibly recording metamorphic overprint (e.g., Alibert and McCulloch, 1993), are not included in the sample set (Table 1) discussed here. 5.1. REY in detritus-free Kuruman IF

In agreement with the results of previous studies (Klein and Beukes, 1989; Beukes and Klein, 1990), the majority of detritus-free IF samples from the Kuruman IF not affected by hydrothermal overprint display the same characteristics as do other IFs of similar age: (La/Sm)cN > 1, (Sm/Yb)sN << 1, and (Eu/Eu*)sN > 1 (Fig. 3). The latter ratio reveals that similar to other Archaean and Palaeoproterozoic IFs, the Kuruman IF precipitated from seawater

(Eu/Eu*)sn

o.1

Fe Qz Fe/Qz Qz Fe 2a 2b . . 3. At~-1 2'

Qz . 4

Qz 5

Fe ~

Fig. 4. Variability o f La abundances and S m / Y b

Qz 7'

Fe Qz ~ AD-9 '

and ( E u / E u " )sN

ratios within drillcore section KK-AD. Fe and Qz denote Femineral- and chert/quartz-dominated samples, respectively. Note lack of any correlation between La, S m / Y b , and (Eu/Eu*)SN'

which had been fertilized by high-temperature hydrothermal solutions. However, without any e Nd data it is impossible yet to decide whether REY were (partly) derived from high-temperature alteration of oceanic crust or from leaching of sediments, although by analogy to other Precambrian IFs (Jacobsen and Pimentel-Klose, 1988; Derry and Jacobsen, 1990; Alibert and McCulloch, 1993; for a line of arguments favouring leaching of sediments see Holland, 1984) the former seems more likely. Complete ICP-MS determined REYsN patterns for samples from the Kuruman IF (Fig. 3) reveal that there exist neither negative nor positive CesN anomalies and show the presence of positive anomalies of LasN, YSN, and ErsN. Although less clear, due to the anomalous Eu abundances, there occur small positive GdsN anomalies as shown by (Gd/Gd*)sN [calculated as GdsN/(0.33SmsN + 0.67TbsN)] above unity in all but two samples. Close inspection of REY abundances and (Eu/Eu*)sN ratios especially in drillcore section KK-AD (Table I, Fig. 4) reveals surprisingly large scatter. Positive ErsN anomalies in REYsN patterns determined by ICP-MS may theoretically result from interferences of oxides and hydroxides of Nd and Sm with Er. However, even a very conservative estimate based on determined oxide and hydroxide yields suggests that only less than 0.2% of the bulk intensity measured for 167Er result from oxide and hydroxide interferences (NdO: 0.002%; NdOH: 0.12%; SmO: 0.002%; SmOH: 0.04%). Thus, the small positive ErsN anomalies presented here do not represent

M. Bau, P. Dulski / Precambrian Research 79 (1996)37-55

analytical artifacts caused by interelemental interferences. Furthermore, they are in agreement with observations that have recently been made for the Hamersley IFs, western Australia (Alibert and McCulloch, 1993). Complete ICP-MS determined REYsN patterns reveal that the positive Er N anomalies present in IDMS determined pattems (see Alibert and McCulloch, 1993) more likely form part of a 'humped pattern' between Tb and Yb rather than being an independent ErsN anomaly. Despite that, for clarity this feature is here still referred to as a positive ErsN anomaly. The 'humped pattern' or the ErsN anomaly is most probably a primary feature of REEsr~ patterns of detritus-free marine precipitates, because it is a reflection of a similar feature in seawater (MSller et al., 1994), where its occurrence is ascribed to the tetrad effect (for further discussion of REE tetrad effects in seawater see, for example, de Baar et al., 1985). Hence, Alibert and McCulloch (1993) suggested that the presence of positive ErsN anomalies may be used to recognize primary REEsN patterns. Comparison of E u / S m ratios of the Kuruman IF in the northern part of the Griqualand West sub-basin (drillcore AD-5; data from Klein and Beukes, 1989; and this work) with the Kuruman IF further south (drillcores DI-1 and WB-98; data from Klein and Beukes, 1989, and CN-109; data from Beukes and Klein, 1990) is hampered by the fact that IF strata at the four locations are not strictly contemporaneous. Southwards decreasing thickness of the Kuruman IF suggests that IF deposition occurred earlier in the north than in the south (Klein and Beukes, 1989). Following the stratigraphic correlation proposed by Klein and Beukes (1989), there remain four IF samples from drillcore WB-98 and none from drillcore DI-1 which can be compared with data from core AD-5. Despite limited data, the average E u / S m ratio of 0.40 for WB-98 is in good agreement with that for AD-5. Within drillcore AD-5 differences between the Kliphuis Member and the succeeding Groenwater Member are analytically insignificant with an average E u / S m ratio of 0.38 (n = 7) for the former (Klein and Beukes, 1989) and 0.42 (n = 14) for the latter. Further upwards in the succession the average E u / S m ratio is 0.33 (n = 13) for the Riries and Ouplaas Members in drillcore CN-109 (Beukes and Klein, 1990) and 0.39 (n = 9) for the Daniel-

45

skuil Member of the Griquatown IF (Beukes and Klein, 1990). The observed differences may in part represent analytical artifacts, so that average E u / S m ratios for the different units within the Griqualand West IFs may be rather similar. However, it is important to note that the average E u / S m ratios for all units within the Griqualand West IFs are significantly lower than the average E u / S m ratio for the Penge IF which will be addressed later.

5.2. Effects of diagenesis and low-grade metamorphism Effects of post-depositional processes such as diagenesis and metamorphism on the REE distribution in IFs have been discussed elsewhere (Ban, 1993) and have been found to be of only minor importance in most cases. This is supported by the presence of positive ErsN anomalies in samples from the Kuruman IF (see above). Moreover, the IF samples display positive LasN, GdsN, and YSN anomalies similar to those of Recent seawater (Ban et al., 1995). This indicates that distributions of Y and REEs in the samples discussed here reflect the distribution in the primary marine precipitates. Nevertheless, before interpreting the variability of (Eu/Eu*)SN ratios, we have to make sure that this does not result from small-scale mobility of Eu. Decoupling of Eu from the other REEs under diagenetic conditions has been reported from Pleistocene muds of the Amazon deep-sea fan, and has been attributed to preferential Eu mobility due to diagenetic reduction of Eu(III) (MacRae et al., 1992). However, this mechanism is unlikely to have operated in the Kuruman IF, since anomalous migration of Eu(II) would have been accompanied by mobility of the smaller Fe(II) ion, which in turn would have resulted in the destruction of the well-defined rhythmic altemation of Fe-rich and Si-rich microbands. Furthermore, formation of significant quantities of divalent Eu is restricted to extremely reducing conditions or to elevated temperatures (Sverjensky, 1984; Bilal, 1991). However, the presence of Fe 3+ in the Kurnman IF reveals that fo2 was always close to the hematite/magnetite buffer. Even a conservative estimate suggests that under such conditions temperature must have been above 200°C to stabilize significant amounts of Eu 2+. Since peak metamorphic temperatures for the Kuruman IF

46

M. Bau, P. Dulski / Precambrian Research 79 (1996) 37-55

have never exceeded 110° to 170°C (Miyano and Beukes, 1984), decoupling of divalent Eu from the other still trivalent REEs can be ruled out. Thus, variable (Eu/Eu*)sN ratios are unlikely to result from REE mobility during diagenesis and low-grade metamorphism but must be a primary feature of seawater preserved in the Kumman IF.

5.3. Effects of sample size Considering the overall similarity of the REY distribution in early Precambrian IFs, the variability observed in drillcore section KK-AD is rather surprising. Comparison of the range of E u / S m ratios of our samples (0.26-0.67) with data for the Kliphuis Member (0.32-0.43) or the Riries Member (0.280.39) of the Kuruman IF (data from Klein and Beukes, 1989, and Beukes and Klein, 1990, respectively), suggests that this phenomenon only shows up in our sample set. In their studies, N. Beukes and C. Klein analysed drillcore samples about 50 mm in length (N. Beukes, pers. commun., 1991). Considering an average thickness of 0.3 mm for each individual microband (Klein and Beukes, 1989), their chemical analyses represent averages derived from about 170 individual layers. In our sampling we concentrated on drillcore sections showing relatively thick microbands of up to 12 mm and we took drillcore samples only 5 to 25 mm in length. Thus, our analyses represent averages derived from less than 10 individual layers. Whereas in average data for 170 microbands minima and maxima are effectively smoothed out, they may have a pronounced impact on average data derived from only 10 microbands. Since each individual microband is considered to be the result of one specific precipitation event, its composition reflects the composition of seawater at a specific period of time. Thus, short-term variations of the REY distribution in Kuruman seawater can be recognized in our sample set (Fig. 4), but may be hidden in the data of Klein and Beukes (1989) and Beukes and Klein (1990).

5.4. Short-term variability of EusN anomalies The short-term variability of the EUsN anomaly is of particular interest, because it cannot result from variable mixing ratios between marine deep water

and surface water alone. Since early Precambrian oceanic deep water masses were characterized by higher (Eu/Eu* )SN and S m / Y b ratios than surface waters, simple two component mixing would have resulted in a positive correlation between ( E u / Eu* )SN and S m / Y b . However, there exists no such correlation, and moreover, S m / Y b ratios show less scatter than (Eu/Eu*)sN ratios (Fig. 4). This suggests that the (Eu/Eu*)SN ratio of the deep water component itself was variable, possibly due to changing mixing ratios between high-temperature ( > 250°C) and low-temperature ( < 250°C) hydrothermal solutions expelled at the seafloor. This may have been caused by temporal variation of the activity of high-temperature venting or by subtle changes in seafloor topography which temporarily restricted mixing between expelled high-temperature solutions and surrounding deep water masses. Accepting a deep-ocean source of the Fe and REY in the Transvaal IFs allows for at least two scenarios for their deposition: (i) precipitation occurred due to changing Eh-pH conditions in a shallow-water environment, i.e. Fe- and REY-rich waters accumulated in a reducing, slightly acidic depositional environment which later became slightly oxidizing and alkaline (for example, due to seasonal variation of biological activity); (ii) precipitation occurred due to upwelling of reducing, slightly acidic bottom water into an already more oxidizing, slightly alkaline shallow-water environment, i.e. Eh-pH conditions at the site of deposition remained rather constant but Fe- and REY-rich deep water intruded a different physicochemical environment. The preservation of short-term variations in IF microbands suggests that each microband reflects one upwelling event and that precipitation occurred rapidly and shortly after bottom water masses with a specific REY signature had entered the depositional environment (accumulation of REY from several upwelling events would have averaged out pronounced minima and maxima). The suggestion that precipitation occurred very rapidly is further substantiated by the positive YSNanomalies (Fig. 3). Slowly growing Modem marine hydrogenetic ferromanganese crusts are close to the exchange equilibrium between adsorbed and dissolved Y and REEs, and thus yield REYsNpatterns with negative YSN anomalies (Ban and Dulski, 1994; Bau et al., 1996). In

M. Bau, P. Dulski / Precambrian Research 79 (1996) 37-55

contrast, rapidly precipitated Modem hydrothermal ferromanganese precipitates scavenge REY without significant fractionation and display positive YSN anomalies (German et al., 1990; Ban and Dulski, 1994). If processes have operated in a similar way in the Precambrian oceans, this implied that REY of Precambrian IFs which show positive YSN anomalies were not in equilibrium with REY of ambient seawater. Hence, it is not because of quantitative removal of REY from the water column that REYsN patterns of these IFs reflect the seawater pattem, but because, similar to the situation during scavenging of REEs at Modem hydrothermal vent sites (e.g., German et al., 1990), REY in these IFs have been removed from the water column too rapidly to allow for equilibration and significant REY fractionation. Rapid precipitation, however, strongly favours scenario (ii), because in case of (i), water masses in a large depositional environment such as the Transvaal shelf would have displayed temporally less variable REY distributions. Thus, the recorded small-scale variability of (Eu/Eu*)SN ratios and the presence of positive YSN anomalies are compatible with and support models argueing that even in Late-Archaean/EarlyPalaeoproterozoic times there existed regions where enough oxygen for formation and preservation of Fe(III)-oxyhydroxides had generally been available. Furthermore, the significant differences between REY abundances and distribution in chert/quartzdominated and in Fe-mineral-dominated microbands (Fig. 4) show that their rhythmic alternation is of primary sedimentary origin. Post-depositional formation of these layers from mixed iron- and silica-precipitates would have resulted in similar ( E u / E u * )SN ratios in both an Fe-mineral and its associated chert/ quartz layer. Thus, a diagenetic origin of the rhythmic layering in Precambrian banded IFs--similar to the model for the genesis of chert-shale couplets proposed by Murray et al. (1992)--can be ruled out. 5.5. Cerium anomalies

Excepting Eu, Ce is the only REE for which anomalies that are due to redox reactions are observed in aqueous solutions and their precipitates. The anomaly results from oxidation of trivalent Ce to Ce 4+ and subsequent decoupling of Ce from the other REEs due to formation of less soluble Ce(IV)

47

species a n d / o r preferential adsorption of Ce(IV) species on particle surfaces. These processes lead to a pronounced negative CesN anomaly in Recent seawater which is mirrored by a strong positive anomaly in most Recent ferromanganese nodules and crusts (e.g., Elderfield, 1988). Development of CesN anomalies appears to be restricted to oxidizing environments of high complex-forming capacity, and therefore, occurrence of negative Cesn anomalies in the ~ 3.8 Ga old Isua IF, Greenland (Dymek and Klein, 1988), and the Kuruman and Griquatown IFs (Klein and Beukes, 1989; Beukes and Klein, 1990), was used to argue in favour of the presence of oxidized early Precambrian surface waters (e.g., Towe, 1991). Unfortunately, interpretation of CeSN anomalies in seawater and marine precipitates is complicated by possibly anomalous abundances of La. Recent seawater, for example, displays higher La abundances than back-extrapolation from Sm over Nd and Pr suggests (e.g., de Baar et al., 1991). Similar observations have been made for Ancient and Recent marine metalliferous sediments (e.g., Barrett et al., 1988, Fralick et al., 1989; Bau et al., 1996) and for some high-temperature hydrothermal fluids at midocean ridges (see data in Klinkhammer et al., 1994). Anomalous abundances of Ce, as suggested by ( C e / Ce*)SN ratios calculated as Cess/(0.5LasN + 0.5Prsn), may therefore result from anomalous La enrichment and are not necessarily a consequence of anomalous Ce behaviour. An escape from this dilemma offers the calculation of ( P r / P r * )SN ratios [(Pr/Pr*)sN = PrsN/(0.5CesN + 0-5NdsN)]- Provided there exist neither PrSN nor NdsN anomalies (which we can safely assume, since there is a priori no chemical reason for the existence of PrsN and NdsN anomalies, and furthermore, such anomalies have never been reported from primary REEsN patterns of chemical sediments), a negative CesN anomaly inevitably results in ( P r / P r * ) s N > 1, whereas a positive Cess anomaly generates ( P r / Pr* )sN < 1. The combination of ( C e / C e * )SN < 1 and ( P r / P r * )SN ~ 1, however, indicates a positive LasN anomaly (field IIa in Fig. 5). In this study we employed chemical analyses by ICP-MS, because this technique allows determination of the full set of REEs, and is thus more reliable than INAA in revealing anomalous behaviour of an

48

M. Bau, P. Dulski / Precambrian Research 79 (1996) 37-55

12. (Ce/Ce*)sn

lib

Ilia

1.1

1

II

0.9

O 0 KK

0.8

IIIb

,=o 0.7

.... 0.7

I .... 0.8

! . . I. . . ., . . .I 0.9 1

Fig. 5. Samples f r o m the K u r u m a n IF

(PE)

in a graph o f ( C e / C e * ) s s

(Pr/Pr*)sn

(KK)

: .... 1.1

: 1.2

and the Penge IF

vs ( P r / P r * )ss. Field I: neither

CeSN nor LaSN anomaly; field Ila: positive Lass anomaly, no CeSN anomaly; field llb: negative Lass anomaly, no Cess anomaly; field Ilia: positive CeSN anomaly; field IIIb: negative CeSN anomaly. Note that excepting two shale samples from the Penge IF, all samples display positive Lass anomalies but no analytically significant negative Cess anomalies. For further explanation see text.

individual REE. However, although we even analysed samples from the same drillcore AD-5 which was sampled and analysed by Klein and Beukes (1989), we could not detect analytically significant CesN anomalies (Fig. 3). (Ce/Ce*)SN ratios below unity result from the presence of positive LasN anomalies, as is indicated by (Pr/Pr*)sN ratios close to unity (Fig. 5) and smooth shape of the patterns between CesN and SmsN (Fig. 3). Furthermore, studies on the REE distribution in the Isua IF carried out prior to (Appel, 1983, 1987) and after (Shimizu et al., 1990) the study by Dymek and Klein (1988) did not report negative CesN anomalies. As mentioned previously (Dulski, 1992; Bau and MSller, 1993), determinations of Ce by INAA (which was used in the studies suggesting occurrence of negative CeSN anomalies) may yield lower Ce abundances than actually prevail, because at F e / C e ratios above 104 there occurs considerable overlap of the photopeak of Fe (142 keV) on the photopeak of Ce (145 keV). Correcting for this interference may eventually suggest Ce abundances which are too low.

A general discussion of the significance of CeSN anomalies in Precambrian IFs is beyond the scope of this paper. However, the absence of positive CesN anomalies from the Kuruman IF bears some information on the pH of the Transvaal ocean. By comparison with lakes such as Lake Van, Turkey, and the East-African rift lakes, Kempe and Degens (1985) suggested that Precambrian so-called 'soda-oceans' might have been characterized by an alkaline pH above 9. But in contrast to Precambrian IFs and hence seawater, alkaline waters from Lake Van (pH = 9.6) display a conspicuous positive Ce s anomaly (MSller and Ban, 1993) which is probably due to stabilization of polycarbonato-Ce(IV) complexes in solution. The absence of this positive anomaly from the Kuruman IF argues against a pH of Transvaal seawater that was considerably higher than the pH of Modem oceans which is 8.2. The absence of negative CesN anomalies from Transvaal marine surface waters suggests that at the Archaean/Proterozoic boundary Ce(III) oxidation during supergene processes was minor to negligible. The oxygen content of the Early-Palaeoproterozoic atmosphere apparently was high enough for formation of Fe(III)oxyhydroxides but too low for stabilization of significant quantities of Ce(IV) compounds.

6. Results and discussion: Penge IF

6.1. Penge shales Shales (samples PE-11 and PE-31) from the Penge IF are ferruginous ( F e 2 0 3 : ~ 21%), indicating that shale deposition occurred in iron-rich waters. Hence, these shales may result from events which caused transport of epiclastic material into the otherwise clastic-sediment-starving, chemical-sediments-precipitating environment. Nevertheless, since the shales yield the highest trace element abundances of all samples analysed (Table 2), their FeEO3/Pr ratios are low ( ~ 4). Ratios considering 'immobile' elements, such as T h / H f (2.3, 1.9), C r / T h (23, 26), T h / S c (0.5, 0.4), and L a / S c (1.4, 1.4), are equal or very similar in both samples. REYsN patterns (Fig. 6a) are parallel to each other, but slightly depleted in LREEs, and show small, but analytically significant positive EusN anomalies. Mixing of 80% Penge

M. Bau, P. Dulski / Precambrian Research 79 (1996) 37-55 1E+01

sample/PAAS

,~ PE-41

0 PE-,~ 4- PE-91 1E+O

1E-01

(a) 1E-O2,

:

:

:

:

La C,e Pr Nd 1E+O(

1E-Ol

:

:

:

:

•

:

:

:

:

I

I

I

Sm Eu Gd Tb Dy (Y) Ho Er Tm Yb Lu

sample/PAAS

~

1E-O~

~

PE-51

•~t PE-61 PE-71

(b) 1E-03 ,

O

PE-81

: ; ; : ; : : : : ; ; I I I I I La Ce Pr Nd $m Eu Gd Tb Dy (Y) Ho Er Tm Yb Lu

Fig. 6. REYsN patterns of (a) shales (PE-II, PE-31) and IF samples contaminated to various degrees by delrital aluminosilicates, and (b) pure IF samples from the Penge IF. (a) Note positive EusN anomaliesin the Penge shales, and decreasing sizes of all anomalies with increasing REY abundances, i.e. with increasing amounts of aluminosilicatesin the chemical sediment. (b) Note close similarity of REYsN patterns of pure Penge and Kuruman IF (Fig. 3) samples.

chemical IF-sediment with 20% PAAS gives the correct E u / S m ratio but results in a L a / S m ratio which is too high and REE abundances which are considerably too low (only 30% of those required). Hence, these patterns cannot result from mixing of chemical sediment (with positive EUsN anomaly) and pelitic material showing PAAS-Iike REE distribution

49

alone. Rather, the positive EUsN anomalies are a source-related signature, indicating that the REE distribution in the Penge shales is intermediate between that of typical Archaean and post-Archaean shales. This once more highlights the problems that may result from the commonly applied normalization of REE data for Late-Archaean to Early-Palaeoproterozoic marine chemical sediments to shale data such as PAAS or the North-American-Shale-Composite (NASC). Although analyses of only two samples does not allow thorough interpretation, comparison of the trace element composition of Penge shales with data for other formations in the Transvaal sequence reveals interesting features. In general, T h / S c and L a / S c ratios are similar to those typically observed in Archaean mudstones and lower than those of Palaeoproterozoic fine-grained epiclastics (Taylor and McLennan, 1985; Condie, 1993). Wronkiewicz and Condie (1990) list average trace element data for pelites from the Selati Formation of the Wolkberg Group, the Black Reef Formation inbetween the Wolkberg and the Chuniespoort Group, and the Timeball Hill Formation of the Pretoria Group. C r / Th, La/Sc, Th/Hf, and T h / S c ratios of shales from the Penge IF of the Chuniespoort Group fall inbetween those of the Black Reef and the Timeball Hill Formations but show more affinity to the former than to the latter. This is supported by L a / S m , La/Yb, and (Eu/Eu*)SN ratios which all are similar to average Black Reef Formation. Thus, the trace element composition of shales from the Penge IF suggests that source composition and area did not change significantly from late Wolkberg to Chuniespoort times.

6.2. Penge IF samples Low abundances of trace elements typically related to clastic detritus (e.g., Dymek and Klein, 1988; Bau, 1993) such as Sc (<0.43 ppm), Th ( < 0.1 ppm), and Hf (< 0.1 ppm) reveal that the Penge IF samples (PE-21, PE-51, PE-61, PE-71, PE-81) are very pure chemical sediments (Table 2). Only samples PE-41 and PE-91 (and to a smaller degree PE-42) appear to be contaminated with detrital aluminosilicates as is indicated by elevated abundances of Th, Hf, Zr, Rb, and Cs (Table 2), and by

50

M. Bau. P. Dulski / Precambrian Research 79 (1996)37-55

F e 2 0 3 / P r ratios (59 and 40, respectively) which fall between those of Penge shales ( ~ 4) and pure IF (130 to 317). REYsN patterns of pure Penge IF samples (Fig. 6b) all display the same characteristics, i.e. HREE enrichment and positive anomalies of LaSN, EUsN, GdsN, YSN, and ErsN. There exist no CesN anomalies (Fig. 5) and the patterns are subparallel to those of the Kuruman IF. However, the average E u / S m ratio (0.5; n = 5) is significantly higher than that of any unit within the Kuruman IF discussed earlier, which leads to positive Eu anomalies even in chondrite-normalized patterns. Elevated REY abundances and smaller E u / S m ratios in samples PE-41, PE-42, and PE-91 result from various degrees of clastic contamination.

6.3. Effects of regional contact metamorphism The Transvaal Supergroup in the Eastern Transvaal sub-basin was subjected to severe regional contact metamorphism due to emplacement of the Bushveld Complex, leading to the formation of magnetite-quartz assemblages in pure Penge IF samples. Although typical of Archaean IFs, positive EUcN anomalies are uncommon in Palaeoproterozoic IFs unaffected by hydrothermal alteration or metasomatism (Danielson et al., 1992; Bau, 1993; Bau and MNler, 1993). May this feature which clearly distinguishes Penge IF and Kuruman IF, therefore, be the result of contact metamorphic overprint? Theoretical considerations (Ban, 1991, 1993) suggest that Eu may potentially be more mobile than its REE neighbours during metamorphism under reducing conditions, provided peak temperatures exceed ~ 250°C. Under such conditions, divalent Eu dominates over Eu 3+ (Sverjensky, 1984; Bilal, 1991), resulting in decoupling of REEs 3+ and Eu 2+ due to larger ionic radius of the latter. The predominance of magnetite in the Penge IF indicates that the environment during contact metamorphism at ~ 500°C (Sharpe and Chadwick, 1982) was reducing with fo2 below the hematite/magnetite buffer. The Eu 2+ ion, however, is rejected from incorporation into the crystal lattice of magnetite, because of its considerably larger size compared to Fe 2+ and Fe 3÷ ('mineralogical control' of Morgan and Wandless, 1980). Thus, Eu becomes concentrated along grain-boundaries and is easily

accessible for mobilization during fluid-rock interaction. In case the IF acted as an open-system (with respect to the REY) during or after metamorphism, this would have generated preferential loss of Eu. Thus, if the size of the EusN anomaly has been modified in the Penge IF, the discrepancy between Kuruman and Penge IFs was even larger prior to this hypothetical Eu loss. However, evidence for only insignificant REY and especially Eu mobility in the Penge IF comes from detritus-rich sample PE-41 and the immediately adjacent detritus-poor IF sample PE-42. Despite severe regional contact metamorphism, both samples retained their specific features, i.e. ( E u / E u * )SN and Y / H o ratios close to those of the Penge shales in detritus-rich sample PE-41 and similar to those of the IF samples in detritus-poor PE-42. Severe infiltration metasomatism by metamorphogenic fluids typically yielding positive EusN anomalies (Bau and M/511er, 1992) would have overprinted the REY signatures of both the detritus-rich and the detritus-poor IF sample, and would have eliminated these differences. Moreover, despite having been subjected to very different metamorphic conditions, REYsN patterns of Penge IF samples are almost parallel to those of samples from the Kuruman IF, and still display typical seawater signals such as positive anomalies of LasN, GdsN, YSN, and ErsN. Thus, we conclude that the REY distribution in the Penge IF samples still reflects that of the primary precipitates, and that regional contact metamorphic overprint of the Transvaal sequence is neither responsible for the positive EUcN anomalies ubiquitous in detritus-free Penge IF nor for the different size of the positive EUsN anomalies in the Penge IF as compared to the Kumman IFs.

7. Comparison of Kuruman IF and Penge IF

Comparison of the chemical composition of IF samples from the Kuruman and from the Penge IF requires that only samples of similar sample size (for reasons mentioned earlier) and similar purity are considered. Hence, data for drillcore section KK-AD must be excluded from the following discussion. Comparison of their REY signatures reveals that the Penge IF in the Eastern Transvaal sub-basin and the Kuruman IF in the Griqualand West sub-basin dis-

M. Bau, P. Dulski / Precambrian Research 79 (1996) 37-55 2.2 SngYb

+ KK (pure) •

0

0

PE (pure)

0 PE(cont.)

1A

* BIC90 (pure)

0.,

*

*

0

~**0+

t

***+ • •

ee

(Eu/Eu*)sn 0.-"

; 1.2

.

I 1.4

I 1.6

I 1.8

,

i

2

,

: 2.2

,

: 2.4

Fig. 7. Pure IF samples from the Groenwater Member of the Kuruman IF (KK-pure) and the Penge IF (PE-pure) together with shale and contaminated IF samples from the Penge IF (PE-cont.) in a graph of S m / Y b vs (Eu/Eu*)sN. Samples BK'90 from the Ouplaas and the Rifles Members of the Kumman IF (data from Beukes and Klein, 1990) are shown for comparison. Note that only samples that comprise similar numbers of microbands are plotted. While S m / Y b ratios are similar, (Eu/Eu*)sn ratios of pure Penge IF samples are significantly higher than those of pure Kumman IF samples. For further explanation see text.

play rather similar S m / Y b , but significantly different (Eu/Eu*)SN ratios (Fig. 7). HREE enrichment and positive YSN anomalies in Precambrian IFs are signals inherited from the marine surface water. Similar S m / Y b and Y / H o ratios in the Penge IF and the Kuruman IF, therefore, suggest a rather uniform REY distribution in the surface water masses covering the Kaapvaal craton during Transvaal times. Since the REY distribution in seawater is depth-dependent (e.g., Elderfield, 1988), this similarity may indicate comparable water depths in the northeastern and southwestern depositional sites. Positive EusN anomalies in Precambrian IFs are a signal inherited from the marine bottom water. Since higher ( E u / E u * )SN ratios in the Penge IF relative to the Kuruman IF are not accompanied by higher S m / Y b ratios, increased mixing ratios of marine bottom water (with positive EucN anomaly and HREE depletion) and marine surface water (without positive EucN anomaly and with HREE enrichmen0 in the Eastern Transvaal sub-basin cannot account

51

for the higher (Eu/Eu*)SN ratios of the Penge IF. As suggested earlier, the REY distribution in the Archaean and Palaeoproterozoic marine bottom water may have been controlled by high- and low-temperature hydrothermal solutions which received their specific REY signatures during alteration of ocean floor basalts or komatiites (Bau and Mtller, 1993). If this assumption is correct, the mixing ratio between these two components controlled the ( E u / E u * ) s n ratio of the marine bottom water. Higher ( E u / Eu* )SN ratios of the Penge IF relative to the Kuruman IF indicate higher (Eu/Eu*)SN ratios in bottom waters of the Eastern Transvaal sub-basin compared to the Griqualand West sub-basin, suggesting more important REY input from high-temperature solutions in the northeast than in the southwest. More important contributions from high-temperature fluids most likely resulted from enhanced activity of spreading-related high-temperature hydrothermal venting. With respect to the palaeogeographic setting during the deposition of chemical sediments in Campbellrand-Malmani and Kuruman-Penge times there exist controversial models, although most authors apparently agree that these formations represent parts of one depositional system and that the banded and granular IFs were deposited below and above wave base, respectively. Beukes and co-workers (e.g., Beukes, 1983; Beukes et al., 1990) suggest a carbonate ramp-deep shelf model and postulate the existence of a deep (back-arc?) basin to the north and west of the submerged platform of the Kaapvaal craton, that had some access to open ocean waters. Others (e.g., Clendenin et al., 1988a, b; Eriksson and Clendenin, 1990; Eriksson et al., 1993) favour a carbonate ramp-steepened ramp model. In the latter, syndepositional tectonics resulted in mainly northnortheasterly directed transgression of the GhaapChuniespoort sea. During a first stage of development it was largely confined to a Griqualand West compartment, but following an episode of regression the basin covered by the Ghaap-Chuniespoort sea expanded during a second stage, and reached the Eastern Transvaal compartment. In contrast to Beukes and co-workers, Clendenin and co-workers postulate a deep basin to the south of the Kaapvaal craton. Palaeocurrent measurements suggest that source regions for the Black Reef Formation, underlying the

52

M. Bau, P. Dulski / Precambrian Research 79 (1996) 37-55

Chuniespoort Group, lay to the north and to the east (Button, 1973, 1986). Considering the close affinity of the chemical composition of shales from the Penge IF to shales from the Black Reef Formation, it appears rather unlikely that the former have been derived from different source regions. Hence, the idea that a deep basin existed north of the Penge IF's depositional site can only be maintained if it is assumed that fine-grained epiclastic material was transported southwards across the basin axis. With respect to water depth during IF deposition, the REY distribution does not allow to distinguish between the deep shelf setting favoured by Clendenin and co-workers and a more basinal setting favoured by Beukes and co-workers (as a matter of fact, this difference may be rather marginal). However, the presence of positive EUsN and YSN anomalies in both the Griqualand West and the Eastern Transvaal IFs give evidence against the model of H~ilbich and co-workers (H~ilbich and Altermann, 1991; H~ilbich et al., 1992, 1993) who suggest that the Transvaal IFs precipitated in a shallow intracratonic basin from brackish or even fresh water. Lack of significant positive EUcN anomalies (or the size of positive EusN anomalies) in bottom waters of the Griqualand West area is in agreement with the REY distribution observed in the contemporaneous Hamersley IF in western Australia (Danielson et al., 1992; Alibert and McCulloch, 1993) and fits into the general pattern of declining importance of high-temperature hydrothermal solutions for the REY budget of Precambrian oceans (Derry and Jacobsen, 1990; Bau and MSller, 1993). Thus, there is no evidence from the REY distribution that 'additional' high-temperature venting occurred close to the Griqualand West depositional environment, strengthening Beukes et al.'s (1990) statement that exchange between open ocean and water masses in the Griqualand West region was possible. In contrast, the positive EUcN anomalies (or considerably larger positive EusN anomalies) in bottom waters in the Eastern Transvaal region which are 'anomalous' for Early- Palaeoproterozoic seawater, suggest either the presence of additional volumes of high-temperature solutions, or decreased availability of low-temperature solutions. Whatever the reasons have been, exchange between open ocean waters and waters in the Eastern Transvaal sub-basin must have been

effectively restricted to allow preservation of the anomalous REY distribution. However, if there existed no close connection between the Eastern Transvaal sea and the open ocean, high-temperature alteration fluids must have been generated in the former itself. This suggests that active spreading took place in the Eastern Transvaal sub-basin, which provided pathways and source material for high-temperature fluid-rock interaction. Few microbands yielding anomalously high (Eu/Eu*)sN ratios have been recognized in the Griqualand West area as well (Fig. 4), which may indicate that 'fertilized' bottom waters at least occasionally reached the northern parts of the Kuruman IF's depositional site. Thus, if the Penge and the Kuruman IFs were deposited more or less contemporaneously and on one large submerged platform (e.g., Beukes, 1983, 1986), their REY distribution can be best explained assuming that the associated basin was rather small in the northeast and widened to the south-southwest, where it was connected to the open ocean. Whether or not basin development was somehow related to subduction may in principle be decided from thorough interpretation of stilpnomelane-bands which are presumed to be derived from former pyroclastic intercalations within the chemical sediments (e.g., Beukes and Klein, 1990). However, this approach is severely hampered by the extreme alteration of the material. Moreover, when lapilli or remains of glass shards are lacking, it is often impossible to tell between pyroclastic and epiclastic precursors, because of the close similarity of the chemical compositions. Z r / T i O 2 ratios of stilpnomelanebands in the Kuruman IF range from 0.02 to 0.1 (M. Bau, unpubl, data), either suggesting an andesitic to dacitic composition of the pyroclastic precursors or good homogenization of pyroclastic input from both basic and felsic sources. However, Z r / T i O 2 ratios of 0.2 for shales from the Penge IF are rather similar. High F%O 3 abundances between 26.4 and 43.1% for the Kuruman stilpnomelane-bands and 21.7 and 22.6% for the Penge shales show that each of these samples is a mixture between Fe-rich chemical sediment and clastic sediment. Even the presence of small negative Nb a n d / o r Ta anomalies in Peircetype spidergrams (Peirce, 1982) does not indicate subduction-related volcanism, since these anomalies are shown by both pelites and stilpnomelane-bands

M. Bau, P. Dulski / Precambrian Research 79 (1996) 3 7 - 5 5

(M. Bau, unpubl, data). Thus, the latter are only of very limited use in reconstructions of the palaeogeotectonic situation.

8. Conclusions REYsN patterns for detritus-free samples from the Kuruman and Penge IFs are almost parallel to each other and characterized by HREE enrichment and positive anomalies of LasN, EUsN, Gdss, YSN, and ErsN. These features are in close agreement with the REY distribution in the Hamersley IFs, western Australia, and in Recent seawater, and suggest that postdepositional modification of the REY distribution in the IF samples is almost negligible. Redox-related anomalies occur for Eu (positive EusN anomalies) but there exist neither positive nor negative CesN anomalies. The absence of positive C e s N anomalies discard models promoting the existence of an Early-Palaeoproterozoic 'soda-ocean' with pH values considerably above the Recent value of 8.2. Lack of negative CesN anomalies in pure IF samples suggests that Po2 levels in supergene environments at the Archaean/Proterozoic boundary were high enough for Fe(II) oxidation, but too low to stabilize significant amounts of Ce(IV). The presence of positive EUsN anomalies reveals contributions from high-temperature hydrothermal solutions to the REY budget of Transvaal seawater. Small-scale variation of ( E u / E u * )SN ratios within a sequence of eleven adjacent samples (each comprising less than ten microbands) from the Kuruman IF may reflect short-term variations in the intensity of hydrothermal high-temperature venting at the seafloor close to the Kuruman IF's depositional site. The preservation of these variations in the IF samples and the presence of positive YsN anomalies suggest that IF precipitation occurred very rapidly and immediately after reducing, slightly acidic marine bottom waters reached a more oxidizing and more alkaline shallow-water environment. Furthermore, the positive YSNanomalies reveal that REY in these IFs have not been in equilibrium with ambient seawater. The variable (Eu/Eu*)SN ratios between adjacent silica- and iron-dominated layers suggest that the alternation of Si- and Fe-dominated layers in banded IFs is of primary origin.

53

Higher ( E u / E u * )SN ratios in the Penge IF relative to the Kuruman IF indicate considerably more important REY input from high-temperature hydrothermal solutions in the Eastern Transvaal than in the Griqualand West sub-basin. The REY distribution in the Penge and Kuruman IFs is compatible with a palaeogeographic setting which invokes the existence of a small basin in the northeast (the Eastern Transvaal compartmen0 in which spreadingrelated high-temperature fluid-rock interaction occurred. This basin widened towards the southwest (the Griqualand West compartmen0 where it was connected to the open ocean.

Acknowledgements Thanks are due to Nic Beukes for numerous discussions and for giving access to drillcores AD-5 and MF-2, and especially to Antje Danielson for comments and kind hospitality in Johannesburg. Thanks to Peter MSller for stimulating the interest of M.B. in rare-earth element geochemistry. This paper benefited greatly from comments by two anonymous journal reviewers and by the Special Issue's editors Antje Danielson and Jay Kaufman. The study presented here was completed in cooperation with IGCP 318 'Genesis and Correlation of Marine Polymetallic Oxides'.

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