Rare earth element geochemistry of Archean metasedimentary rocks from Kambalda, Western Australia

GwchrrCu Y, Cowwchimicu Acru Vol. 44. pp. 639 to 648 0 Pergamon Press Ltd 1980.Printed in Great Britain

0016-7037/8O,Q501-0639$02,00/O

Rare earth element geochemistry of Archean metasedimentary rocks from Kambalda, Western Australia 0. A. BAVINTON* and S. R. TAnoR Research School of Earth Sciences, Australian National University, Canberra, ACT. Australia (Received

30 January

1919; accepted

in revisedform 18 December 1979)

Abstract-Archean sedimentary rocks of very limited lateral extent from horizons within basaltic and ultramafic volcanic sequences at Kambalda, Western Australia, are extremely variable in major elements, LIL and ferromagnesian trace element compositions. The REE patterns are uniform and do not have negative Eu anomalies. Two samples have very low total REE abundances and positive Eu anomalies attributed to a very much greater proportion of chemically deposited siliceous material. Apart from these two samples, the Kambalda data are similar to REE abundances and patterns from Archean sedimentary rocks from Kalgoorlie, Western Australia, and to average Archean sedimentary rock REE patterns. These show a fundamental distinction from post-Archean sedimentary rock REE patterns which have higher La/Yb ratios and a distinct negative Eu anomaly.

INTRODUCHON

THIS PAPER comprises a study of the rare earth element (REE) abundances in a suite of fine-grained Archean metasedimentary rocks from several interflow horizons within basaltic and ultramafic volcanics at Kambalda, Western Australia. It is a part of a more detailed study of these rocks (BAVINTON and KEAYS, 1978; BAVINTON,1979), and is also a part of a continuing investigation of REE in sedimentary rocks (NANCE and TAYLOR, 1976,1977; TAYLOR, 1977, 1979; TAYLOR and MCLENNAN, 1980; MCLENNAN et al., 1979). The objectives of this study are to provide data on REE abundances in well documented Archean sedimentary rocks, and to document the variation in sedimentary REE patterns within an ancient maficultramafic pile. Post-Archean sedimentary rocks have rather uniform and distinctive REE patterns with a characteristic europium depletion relative to chondrites (e.g. TAYLOR, 1979). (Typically, Eu/Eu* = 0.67 where Eu* is the interpolated Eu valne for no depletion.) In contrast, the (chondrite-normalized) REE patterns in Archean sedimentary rocks have lower REE abundances, less relative light REE enrichment, and lack the Eu depletion characteristic of postArchean sedimentary rocks (TAYLOR, 1979). Before broader conclusions can be based with confidence on these observations, more REE data are required from well-studied Archean environments such as Kambalda. REGIONAL

GEOLOGICAL

SETTING

The Kambalda district in the Yilgarn Block of Western Australia (PRIDER, 1965; GEE, 1975) (Fig. 1) is typical of * Present address: c/o Western Mining Corporation Ltd, P.O. Box 194, Glenside 5065, South Australia. 639

Archean granite-greenstone terrains, with greenstone belts (consisting of mafic-ultramafic and felsic volcanic and clastic associations) within gneissic and granitic rocks. It falls within the Kalgoorlie sub-province of the Eastern Goldfields Province of WILLIAM (1974, 1975). Mafic rocks in the district are dominantly low-K tholeiitic basalts (HALLBERG, 1972; HALLBERG and WILLIAMS, 1972) while ultramafic rocks cover a broad range from high-Mg basalts through to peridotites and dunites (NES BITT, 1971; Ross and HOPKIN$ 1975; NALDRETT and TURNER, 1977). Felsic volcanics of dacitic, rhyodacitic and rhyolitic compositions are abundant in some parts of the section, and felsic intrusions of both sodic and potassic types are also present (COMPSTONand TUREK, 1973). Thick sequences of elastic sedimentary rocks occur in the younger parts of the succession, and thin sulfide-rich sedimentary horizons are intercalated within the mafic and ultramafic sequences (WILLIAM$ 1975; Ross and HOPKIN$ 1975). Metamorphism in the Kambalda district was lowpressure medium grade (BINNSet al., 1976; BARRETTet al., 1977) with the maximum temperatures and pressures at Kambalda about 510°C and 2-3 kbar. (BAVINTON,1979). Major nickel sulfide deposits occur within the ultramaficmafic unit, generally at the base of the lower cycle of GEMUTSand THERON(1975). Geology of Kambalda WOODALL and TRAVIS (1969) showed that the geologic setting at Kambalda consists essentially of an ultramafic sequence some 2&800 m thick sandwiched between two basaltic sequences (Fig. 2). The lower basalt (known locally as the ‘footwall’ basalt, or FWB) is quite uniform, and tholeiitic in composition, while the upper, ‘hangingwall basalt (HWB) is chemically and texturally complex and consists of two formations separated by a sulfide-rich and commonly graphitic sedimentary horizon. The ultramafic sequence consists of a large number of submarine flow units most of which have a cumulativeolivine textured base and a spinifex-textured, less magnesium-rich upper section (Ross and HOPKINS 1975) analogous to the well-studied komatiitic extrusive sequence at Munro Township, Canada (PYKE et al., 1973; ARTH et al., 1977). More information on the ultramafic sequence, plus several other geological references. can be found in BAVINT~N and KEAYS (1978). Age determinations carried out by COMPSTON and

0. A. BAVINTDN

640

RfZGiMAL

GZtLOGYond

LOCALITY

and S. R. TAYLOR

MAP

Fig. 1. Generalized map of the Archean Yilgam Block, Western Australia. showing the major nickel deposits.

TUREK(1973), OVERSBY (1975) and RODDICK (1974) in the

Kambalda district indicate an absolute age of at least 2700-2800 Myr. THE SEDIMENTARY

ROCKS

Sedimentary horizons occur at Kambalda distinct stratigraphic situations:

in four

1. Within the footwali basalt sequence (called FWB sediments’). 2. At the very base of the ultramafic sequence, on the footwall basalt-ultramafic contact (called ‘contact sediments’). 3. Within the lower one-third of the ultramafic sequence at interflow boundaries (called ‘internal sediments’), and; 4. Within the hangingwall basalt sequence at the contact between the lower and upper formations (called ‘HWB sediments’). The relative age of the four sedimentary groups (above) from s~at~graphic evidence is 1, 2, 3, 4 with 1 the oldest. The time span from the youngest sedimentary horizon to the oldest is unlikely to be greater than l&20 Myr, and could well be less than l-5 Myr. Two important observations are: (i) the individual sedimentary horizons have little lateral persistence and cannot be correlated over distances exceeding X&500 m; (ii) localized variations within individual sedimentary horizons often cannot be correlated between adjacent drill cores. Thus the sedimentary rocks occur in localized lenses.

There are three main sedimentary rock types: a pale-grey to white siliceous variety; a dark-grey to black carbonaceous variety; and a dark-green chlorite-rich mafic variety. The majority of specimens are fine to medium grained, with no internal sedimentary structures, other than layering, minor contortions and some sulfide nodules. Generally, the sediments are sulfide rich, with pyrrhotite or pyrite layers alternating at various scales with the non-sulfide layers. Variations in the sulfide and non-sulphide proportions account for much of the observed major-element variation, although the non-sulfide fraction is itself variable, especially in Mg, Al and Si. The sulfide fraction consists of pyrite and pyrrhotite in all combinations, with minor chalcopyrite, sphalerite, galena and rarely pentalandite. The non-sulphide fraction generally consists of variable proportions of quartz, chlorite, tremolite, albitc, phengitic muscovite and biotite as well as accessories. A fine-medium grained fragmental texture is evident in thin-section with quartz and/or albite fragments, and fragments and patches of mafic minerals scattered throughout a fine grained matrix. There is no unequivocal textural evidence for a tuffaceous component, although the presence of flattened lentitular ‘clots’ of mat% minerals and the local high magnesium contents suggest a contribution from mafic-ultramafic igneous rocks. DONNELLY et ul. (1978) showed that the sulfide component was most likely magmatic in origin, with sulfur isotopic values occupying a narrow spread near zero per mil. These sediments are particularly enriched in Au, relative to other sediments and relative to other rock-types in the Kambalda district (BAWNTON and KEAYS, 1978). BAVINTON (1979) described the sediments in detail, and some further descriptive information is presented in BAVINTON and KEAYS (1978). BAVINT~N(1979) interpreted the origin of the interflow sediments at Kambalda as being complex. involving at least a mixture of: detrital material from distant felsic-granitic sources; some more or less locally derived mafic and ultramafic material; and a sulfur and chalcophile-rich exhalative component. There is some evidence for sulfur loss from the sediments into adjacent ultramafic rocks, and evidence for metasomatic introduction of K and CO2 into some locations.

Analyses for major elements, Rb and Sr were obtained by an X-ray fluorescence technique similar to that do scribed by NORRISH and HUTTON(1969) with loss on ignition being the total of H,O' , HZO-, C and COZ. Sodium was determined by flame photometry. The REE and Th, U. Zr. Nb, Hf and Ba were determined by an AEI MS7 spark source mass spectrograph (SSMS) using the method described by TAYLOR and GORTON (1971). Both precision and accuracy are better than _t51,,. Comparisons of the REE data obtained by this technique with those from other precise techniques, and the normalizing data for chondritic meteorites, the North

Geochemistry of Archean meta~~~ntary

rocks

641

DIACRMtUTIC STRATIGRAPHIC COLUMB:

ltAmmLmsECTIoN

WRXNWALL

generally a black, po-py rich slaty sedlsmt; some pale siliceoua zones; often intruded by felsic sills (l-&C& including typically ocelll-beerlng; frequent alternationoofpale feldspathic and magneaian sections;a complex and variable section

ULTRAMFIC

extrusive units with MgO um:all in the 19-32% range; splnifex development; interflow sediment very rarely present; nickel sul phide ore very rare (IOO-2Oti)

Ultramafics:a few (3-3) thick often >Zk;generallyU@-rfchw

ceous types;t

(Nickel ores and intrusive rocks omitted - not to scale)

Fig. 2. Generalized stratigraphic column showing the two basalts enclosing the central ultramatic sequence. Note that interflow sedimentary horizons occur in various areas, but are most abundant in the lower one-third of the sequence (Ross and HOPKINS,1975; BAVINTON and KEAYS,1978).

American Shale Composite (NASC) and the average PostArchean Australian Sediment (PAAS) are given by TAYLOR and GORTON(1977). Information on the mineral content of the samples was obtained by an X-ray diffraction technique described in HOOTON and GIORGETTA (1977). REE data were obtained

from 16 sedimentary rocks, from 12 separate stratigraphic horizons, covering an area of about 35 km* and a vertical stratigraphic section of about 800 m. Brief lithological descriptions and mineralogical assemblages of the samples analysed are given in the Appendix. Sample preparation was as described in BAMNTONand KEAYS (1978). The

0. A. BAUNTONand S. R. TAYLOR

642

major element data are presented in Table 1. The REE and Y data are given in Table 2, and other trace element data in Tables 3 and 4.

GEOCHEMISTRY

tionships basal&

Kambalda

ROCKS

Major element chemistry

Figure 3 presents various major element oxide rela-

Table

1

29.61 0.24

TiO*

A1203 LFeO

8790 2

IN

Km

INrnRNAL

a791

0792

6458

3

4

5

h

82.23

62.81

58.23

67.04

SEDmmiTS

7566

6462

most

rocks

from the

noteworthy

7

8

44.45

CONTACT SEDIMENZS

7643

a323

9

54.99

0.38

0.02

0.1R

0.ha

0.57

0.32

0.47

0.49

0.44

5.77

12.77

16.13

9.22

12.93

13.63

11.15

l0.4R

12.71

7.10

12.80

0.09

0.15

0.10

w

1.49

1.20

0.77

'0.95

CaO

2.55

9.20

1.95

Nap

0.09

1.84

1020

11

52.98

12.20

0.09

282

10

57.54

7.68 32.69

'2'5 J..O.I.

481

43.19

MnO

K2°

The

feature

of

sedimentary rocks. Nos I-16 refer to points plotted in Fig. 3. Data in wt% L.O.I. = TH20+, H,O-, C, COz

SEDIMENTS

2

district.

fields for data from

and granitic

1. Major element data for Kambalda

8789 SiO

rocks

both the plots in Fig. 3 is that the overall average of the main cluster of Kambalda sedimentary rocks falls roughly midway between the fields of the basalts and the granitic rocks. This could provide evidence for derivation of the sedimentary rocks from these two

OF THE

SEDIMENTARY

along with the general ultramafic

12

63.67

0.!.3

O.?')

87al

13

60.33

11.46

SEOl!II”J

7567

14

jl.93

1:.

-2

87a2

87P3

15

:7X

16

45.59

36.56

0.67

3.58

0.19

3.56

3.7'

13.71

12.61

6.79

12.Lk

1.31

1B.60

15.T

i.‘.:’

1.92

23.24

7.16

9.43

16.62

7.43

H.70

13.77

0.01r

0.02

0.03

0.06

0.05

0.05

0.15

2.08

c.1:

0.16

3.37

3.09

4.7

2.05

4.79

1.98

1.77

1.37

0.73

1.45

2.75

1.83

'i.?.

..-:',

1.55

1.61

1.62

2.52

12.28

5.06

2.88

4.09

3.61

4.53

4.57

3.64

3.7'

0.03

1.12

0.30

a.99

4.62

1.32

5.ak

5.33

0.29

2.79

3.5:

0.7%

1.95

1.13

2.44

1.42

0.07

0.15

2.73

0.05

0.03

1.45

1.16

1.2,

3.35

7.91

2.23

1.35

2.15

0.15

0.04

0.10

0.19

0.03

0.14

0.14

0.09

0.13

0.14

0.12

0.13

3.15

0.10

0.06

3.1

3.1-

10.37

15.11

5.03

10.25

4.93

0.85

4.13

5.39

2.01

4.24

4.9

7.0

4.73

i.li

L.h!

'.I

22.13

6.51

4.39

7.22

5.34

0.45

a.78

2.39

4.03

6.86

3.06

3.11

Ir.ii

5. 39

7.16

I ?.3G

Table 2. REE data for Kambalda sedimentary rocks: method SSMS. Data in ppm (wt) SEDIMENE

La Ce Pr

Nd Sm E" Gd Tb m Ho Er

nn

Yb (LU) L Fm Y

IREE+Y

LLREE LHrm? LLILH

Ia/Yb E&u”

m

IN

INmmNAL

SEDmENTs

CO!vmCT

SEDIrnNTS

SEDIMENTS

IN

a789

a790

a791

a792

481

6458

6462

7566

7643

a323

282

1020

7567

a7al

a7a?

I

2

3

4

5

6

7

8

9

10

11

12

13

14

15

1.47 2.93 0.38

4.93 9.61 1.15

0.33 0.17 0.34 0.06 0.47 0.09 0.29 0.29 a.27 3.27 11.54 6.56 1.54 4.26 5.07 1.65

1.26 0.a1 1.13 0.21 1.19 0.25 0.66 0.13 0.88 0.14 27.78 4.34 32.12 22.38 4.59 4.88 5.58 2.09

lo.26 17.26 22.49 37.05 3.14 3.64 lb.16 15.64 3.27 4.12 1.02 1.45 3.21 4.67 0.52 0.83 2.8 5.76 0.53 1.33 1.46 3.63 0.21 0.53 1.47 3.67 0.23 0.57 64.77 79.96 14.4 30.4 79.17 110.36 53.32 78.51 10.43 20.99 5.11 3.74 6.99 L.7 1.0 1.12

1.45

15.48 29.91 3.33

5.43

10.6 23.92 2.98

16.52 30.04 3.29

2.87 2.73 0.92 1.02 2.97 2.77 0.49 0.44 2.93 2.8 0.59 0.65 1.71 1.8 0.23 0.27 1.66 l.a4 0.24 0.29 76.71 84.92 19.05 22.44 96.76 107.36 64.97 73.04 lo.82 10.86 6.00 6.73 9.33 1.0

2.52 0.71 2.41 0.39 2.53 0.59 1.69 0.23 1.60 0.25 75.62 16.44 92.06 65.22 9.69 6.73

a.49 la.9

13.38 12.81 12.a5

20.27 40.71

2.06

4.57

a.5 19.55 2.24 4.05 0.74 1.26 2.54 3.91 0.45 0.61 2.54 3.65 0.57 0.83 1.6 2.37 0.2 0.34 1.34 2.37 0.22 0.37 50.39 104.86 17.42 22.66 67.81 127.52 40.19 89.15 9.46 14.45 4.25 6.17 6.34 a.57 1.0 1.0

a783 16

la.29 36.15

10.09 20.39

9.54 10.3 2O.lii 23.23

7.66 2.99 .3.9? I?.26

0.98 2.95 L.& 13.13

3.49 0.95 3.47 0.65 b.0 0.81 2.46 0.38 2.66 0.41 97.53 20.26 117.79 al.74 14.84 5.51 6.69 0.P

2.19 0.84 2.25 0.36 2.72 0.42 1.2 0.19 1.31 0.20 53.53 11.2~ 64.77 44.5'1 a.15 5.47 7.li 1."

2.58 r.ae 0.a3 0.9 2.66 2 0.44 0.U 2.75 2.G 0.55 0.56 1.51 1.56 0.2 0.23 I.!,;. 1.63 0.22 0.75 56.43 62.49 13.25 il.a5 69.6F 74.34 i5.85 51.66 9.75 9.93 1.7 Z:J, 6.6L ,."I,

1.26 0.41

4.6 19.21

2.23 9.6;

14 ‘i

3.23 8.2a

FniR

9.58 23.31

6.@7 15.@3

2.17

3.L6 1.04 3.53 0.29 0.5L 1.71 ?.!AL 0.37 0.65 1.06 1.90 0.16 0.26 1.10 I.Pi 0.17 O.?? 25.09 65.9). 7.35 lb.1 32.44 a0.01, 19.31 5T'.L3 6.111 l‘.L, 2.8L 1.95 5.?L I.02 0.9j

i.55

9.LR L.96 0.lF 1.77 2.37 ?.?L O.L?

I .,x

7.16 :.09 3.17 44.05 13.1 5h.15

36.21 5.96

k.,a; 5.‘.a 63. ?Z !.‘?I:

Table 3. LIL element data for Kambalda sedimentary rocks: method SSMS except XRF for Rb. Sr. Data in ppm (wt) SEDIMENTS 8789 1

rm Pb Kb sr Th 0

zr Hf Nb m/sr

ThiU ZrlHf zrim

473 45 38 61 4.21 1.67 60 1.99 3.2 0.62 2.5 30.2 18.8

IN

INTERNAL

HWB

8790

8791

8792

2

3

4

580 102 48 227 6.53 2.27 102 2.82 4.3 0.21 2.9 36.2 23.7

15 50 3 14 0.32 0.11 10 .14 0.7 0.21 2.9 71.3 14.3

42 65 8 93 2.46 0.73 47 1.59 3.2 0.09 3.4 29.6 14.3

I

SEDIHENTS

CONTACT SE,mlENTS

481

6458

6462

7566

7643

8323

5

6

7

8

9

10

7 32 rl 26 8.09 2.31 143 3.39 ‘ C.04 3.5 40.9 35.R

362 37 17 112 1.82 0.42 128 1.47 3.9 0.15 4.2 87.1 32.8

487 47 15 254 6.44 1.94 139 4.31 5 0.06 3.3 32.2 27.8

550 33 21 164 8.54 2.47 156 4.16 3.1 0.13 3.5 37.5 50.3

719 21 39 332 5.11 1.56 124 3.33 3.0 0.12 3.3 37.6 41.3

12 3
I

282

1020

7567

11

12

13

730 38 51 92 2.81 0.83 110 2.85 4 0.55 3.4 38.6 27.5

1043 27 66 219 2.85 0.9 100 3.21 3 0.30 3.2 31.2 33.3

530 17 67 91 2.1 0.68 96 2.45 3.3 0.74 3.1 39.2 29.1

SEDIMENTS

’

IN

FWB

8781

8782

8783

14

15

16

180 18 40 26 1.26 0.37 52 1.37 2.2 1.54 3.4 38.0 23.6

540 51 57 40 3.16 0.82 114 3.26 4 1.43 3.9 35.0 28.5

18 23 5 36 2.29 0.56 :4 1.88 2 0.14 4.1 39.4 37.0

Geochemistry of Archean metasedimentary Table 4. Ferromagnesian

trace element data for Kambalda sedimentary rocks: method XRF. Data in ppm (wt)

SErnHENTS IN awE

cr " Ni Co Mn C" 2n Ga CT/V VIM Nil& Ni/Cr

8790

8791

0792

I

2

3

4

64 135 127 360 384 3565 10 1.1 0.47 1.1 1.3

95 100 142 67 314 462 3975 20 1.0 0.7 2.1 1.5

65 42 17 6 446 68 300 16 1.5 2.5 2.8 .3

CONTACT sELlmENTs

INTERNAL SEDImNTS

8789

70

643

rocks

I

120 84 92 71 256 322 5060 12 1.4 0.9 1.3 .R

SEDINENTS IN em

481

6458

6462

7566

7643

8323

282

1020

7567

8781

0702

8783

5

6

7

8

9

10

11

12

13

14

15

16

129 47 61 12 139 54 345 1R 2.7 0.17 5.1 0.47

57 43 3290 118 190 1410 1510 13 1.3 0.01 27.9 57.7

183 76 125 4a 485 242 1410 17 2.4 0.61 2.8 0.61

208 77 100 45 350 260 2180 20 2.7 0.77 2.2 0.48

59 75 186 53 370 390 u-30 17 0.8 0.40 3.5 3.2

156 61 86 30 1160 288 780 17 2.6 0.71 2.8 0.55

128 121 74 59 620 456 1590 18 1.1 1.64 1.3 0.58

115 98 2000 117 990 715 905 18 1.1 0.05 17.1 17.4

113 62 73 61 1240 445 700 10 1.8 0.85 1.2 0.63

110 99 105 98 1190 570 3360 20 1.1 0.94 1.1 0.:5

70 63 196 131 520 1200 3060 16 1.1 0.34 1.5 2.8

362 114 268 61 310 265 1400 16 3.2 0.43 4.4 n.74

parental sources, but the data do not discriminate between a local and distant provenance. Two samples are distinct in composition. Sample 8791 (number 3 in Fig. 3) falls well outside the main

cluster in both triangles: in Fig. 3A, 8791 lies towards the CaO apex, while in Fig. 3B, it lies well towards the SiOz apex and with high (CaO and MgO): (Na20 and K20). Note that sample 8792 (number 4 in Fig. 3) falls well outside the main cluster, towards

the Si02 apex. Sample 8792 (number 4) is not so anomalous as sample 8791(3) since it falls within the main cluster in Fig. 3A, and has a (CaO and MgO):(Na20 and K20) ratio which is fairly typical of the set (Fig. 3B). The chemistry of these two samples does not suggest a major igneous source component, since their compositions plot well away from the igneous rock fields in Figs 3A and B.

CaO

A

Fig. 3A. Comparison between CaO, A&O, and MgO contents of Kambalda sedimentary rocks (this paper) and those of granites, basalts and ultramafic rocks from the Kambalda district. Granitic data from O’BEIRNE(1968), basaltic data from HALLBERG and WILLIAMS(1970) and ultramafic data from Ross and HOPKINS(1975). SiO&O

B

CaO+ MgO

Fig. 3B. Comparison between SiO 2, CaO + MgO, and Na10 + K20 contents of Kambalda sedimentary rocks with those of granites, basalts and ultramatic rocks from the Kambalda district. Data sources as above.

644

0. A. BAVIWON and S. R. TAVIM

La Ce Pr Nd REE

Fig. 4. Chondrite

RARE EARTH

normalized

Sm Eu Gd

Tb

Dy

Ho

Er

Tm Yb

PATTERNS FOR KAMBALDA SIZDIMNTS

REE patterns for the individual

ELEMENTS

The data are plotted, normalized to chondritic abundances, in Fig. 4. The two samples (8791 = 3 and 8792 = 4) which show positive Eu anomalies, will be considered separately. In the contact and internal sediments, (Fig. 4) there is very little variation among individual samples within the same stratigraphic location. Differences between individual samples are more marked within the basaltic environments (HWB and FWB, Fig. 4). There is little difference between the average REE patterns from the four different stratigraphic locations (Fig. 5). Although the patterns remain very nearly parallel. there is an apparent trend with increasing total REE abundance with time. i.e. the pattern shifts regularly to high concentration levels as the rocks become younger. In view of the variation within each of these four groupings, the small number of samples involved in each case, and the variable dilution of the REE-containing fraction by iron sulfides, too much significance cannot be placed on this apparent temporal trend. The close similarity in the shape of these average patterns suggests that the origin of the REEcontaining fraction of these sedimentary rocks was

sedimentary

rocks. Data from Table 2.

essentially the same throughout the time-span reflected by the different stratjgraphic horizons. The REE pattern of the ranges of the Kambalda sedimentary rocks and of those of similar age from Kalgoorlie, Western Australia (NANCE and TAYLOK, 1977) are compared in Fig. 6. Agreement is very close between the average of the Kambalda and Kalgoorlie sets, with essentially the same Gd-Yb values and slightly lower Eu and La/Sm in the Kambalda set compared with the Kalgoorlie set. Given the wide variation in the geologic and stratigraphic setting of the samples, this degree of similarity is a most significant result. It strongly suggests that the REE patterns are not greatly influenced by the local depositional environment. In this context. the REE patterns of the local igneous rocks have the following characteristics (BAVINTON. 1979). The ultramafic REE patterns have Rat heavy REE at about 3 x chondritic levJcls, with a LREE depletion to 1.5 x chondritic. The footwall basalt pattern is parallel to that of the ~itra~~cs. with heavy REE - 7 x chondritic and La about 6 x chondritic. ‘The lower hanging wall basalt has a Rat heavy RE,E pattern at 10 x chondritic and slight LREE enrichment (La = 20 x chondritic). The upper

Geochemistry

of Archean

KAMBAI COMPARISON

I 1

Fig. 5. Average

RELATIVE

AGE:

I

I

I

I

La

Co

Pr

Nd

chondrite

normalized

OF

metasedimentary

DA

’

1

*

POSITIONS

FWB + CON r/NT>

HWB

I

I

I

I

I

I

I

I

Eu

Gd

Tb

Dy

Ho

Er

Tm

Yb

REE patterns

hanging wall basalt has a REE pattern parallel to that of the sedimentary rocks. However, since it overlies them, it cannot be a source for the REE patterns. The other basalt patterns are too depleted to provide the sedimentary REE patterns, a conclusion reinforced by the major element compositions (Fig. 3). Figure 6 also shows average REE patterns for Archean and post-Archean sedimentary rocks, from TAYLOR (1979). Figure 6 shows that, compared to post-Archean average sedimentary rocks the average

L

SFDIMENTS

STRATIGRAPHIC

Sm

645

rocks

from the four different

groups

of sedimentary

rocks.

Kambalda sedimentary rock has lower REE abundances and lower La/Yb ratios. The negative Eu anomaly, characteristic of post-Archean sediments (NANCE and TAYLOR, 1976) is conspicuously absent in the Kambalda sediments, as it is in the Archean sedimentary rocks described by NANCE and TAYLOR (1977) and TAYLOR (1979). The uniformity of the REE patterns within this diverse set of sedimentary rocks is significant. The sediments are extremely varied in composition, and

J

1

.

PAAS

t

AAS

@

/

KALGOORLIE

I La

Ce

Pr

Nd

Sm

Eu

Gd

Tb

Dy

Ho

Er

Tm

Yb

Fig. 6. Comparison of the range in REE (chondrite normalized) patterns from Kambalda (this paper) and Kalgoorlie (NANCEand TAYLOR,1977). The average Archean sedimentary rock pattern (AAS) and the post-Archean sedimentary rock pattern (PAAS) from TAYLOR(1979) are shown for comparison.


44:5

H

646

0. A. BAV~NTON and S. R. TAYLOR

individual beds are of limited lateral extent. The REE show no significant correlations with the other trace elements. Thus Zr/La ratios range from 5.8 to 16, Ba/La ratios from 0.42 to 101. Laph ratios are a little less varied, from 2 to 4.7. Europium shows no correlation with Sr which is presumably associated with Ca in carbonate. This indicates that Eu is probably trivalent, since a correlation with divalent Sr would otherwise be predicted. REE patterns in the two HWB sedimentary samples (8791 and 8792) have anomalous majorelement chemistry and relatively low abundances of Th, U, Zr, Hf and Nb. Figure 4 shows these two samples to have anomalous REE patterns, with low REE abundances and a distinctly positive Eu anomaly (Eu/Eu* equals 1.65 and 2.09 respectively). There are at least four possible explanations for the positive Eu anomalies in these two samples: 1. They may be due to local accumulations of Euenriched igneous plagioclase detritus. 2. They may represent the addition of Eu-enriched solutions accompanying the widespread carbonate metasomatism in the region. 3. They may be due to chemical interactions with seawater accompanying prolonged submarine weathering. 4. They may reflect an origin from chemical deposits resulting from the precipitation from hydrothermal solutions. The first possibility, plagioclase accumulation, can be rejected on the basis that both AlzOs and feldspar are too low. The second possibility, dour supported by SUN and NESBITT’S(1978) work, which shows Eu can be mobile during the alteration of mafic-ultramafic piles, can also be rejected as inappropriate. Samples 8791 and 8792 are not excessively enriched in CaO or in CO* (L.O.I.). A normal sample (8790) has considerably higher CaO and L.O.I., but has no si~ificant positive Eu anomaly. Sample 8792 does not contain significant carbonate (Appendix) and has a normal (CaO + MgO):(Na,O -t- KzO) ratio. The third possibility, that their anomalous chemistry results from extreme sea-floor weathering is less easy to dismiss. But in this case, why have only two of the 16 been so affected? Since both samples came from sedimentary horizons between the two hangingwall basalt formations, it is possible that these samples were exposed to the seawater for a considerably longer period of time than those of other horizons. The difficulty with relying on prolonged weathering is that (with the exception of Th, U, Zr, Hf, Nb; all ‘immobile’ elements) the other trace element abundances and ratio’s are well within the range observed for the other 14 samples. These include such mobile elements as Ba, Rb, Mn, Cu and Zn. It seems extremely unlikely that weathering would be sufficiently intense and of such a character as to change the REE. yet not significantly affect the other trace-elements,

most of which are more mobile and reactive than the REE. GRAF (1978) has suggested that several variable factors may account for the observed REE patterns (and especially Eu variations) in Fe-rich sediments. He warns against simplistic interpretations of REE patterns in ancient chemical sedimentary rocks by showing that “the amount and type of precipitating and detrital phases, the REE pattern of the iron source solution, the REE pattern of the seawater into which the solution flows, and the mixing which takes place between input solution and seawater” may all affect the resultant sedimentary REE patterns (GRAF, 1978; see also FRYER,1977). The two anomalous sediments (8791 and 8792) from within the han~ngwall basalts differ from that of the other 14 samples in the following ways. Firstly, the proportion of detrital material was considerably less, as shown by the lower REE, Y, Zr, U, Th and Nb values. Secondly, the proportion of chemically deposited (probably partly exhalative) siliceous material was consi~rably higher, as indicated by the very high SiO, and low TiOz, A120, contents compared to the other samples. The abundance of the chalcophile elements in these two samples is not significantly different from those in the other 14 samples, suggesting that the degree and mechanism of sulfur accumulation (again probably exhalative) was constant. Thus a chemical origin appears the most likely to account for the positive Eu anomalies. LIL and ferromagnesiun consideration on these elements is in progress (BAMNTON, in only the salient points will be discussed here. In contrast to the uniformity of the REE abundance patterns, most of these elements show wide variations in abundances. This reflects the similar differences in major element concentrations.

NANCE

Post-Archean sedimentary rocks, compared to those of igneous rocks, is due to oxidation from U4+ to U6”, leaching and removal of uranium as soluble (UO# ‘-. The low values of the Kambalda sedimentary rocks are similar to those of modern island arc volcanic rocks (TAYLOR,1979), and indicate that oxidation and leaching of uranium has been minimal. The Zr/Hf ratios average 36 f 4 (1 sigma), not significantly

Geochemistry of Archean metas~imentary terns, are likewise distinct in having low values of Th, U, Zr, Hf and Nb, in comparison with the other

samples.

GEER. D. (1975) Regional geology of the Archean nucleii of the Western Australian Shield. In Economic Geology of Australia and Papua New Guinea. (ed. C. L. Knight), Australas. Inst. Min. Metall., Melbourne, Monogr. S(l), 43-55.

Conclusions

There is a close similarity in the REE patterns of 16 samples of Archean interIIow sedimentary rocks from 12 stratigraphic horizons. These come from an area of about 35 km’ and a stratigraphic thickness of about 800 m. All have light REE enrichment with the majority of the samples having La/Yb ratios in the range of 5-9. None of the samples has a si~i~~t negative Eu anomaly. The uniformity in REE abundance patterns is striking, in comparison with the variations in composition of the major, LIL and ferromagnesian trace elements. It is also remarkable in view of the laterally discontinuous nature of the sedimentary horizons. The average sedimentary REE pattern from Kambalda is indistinguishable from the average of Archean sediments from Kalgoorlie, and is very similar to other Archean sedimentary REE patterns from elsewhere in the world, (TAYLOR, 1979). The contrast with nest-Archean sedimentarv rock REE patterns (and &peciaUy in the absence oi a negative E; anomsly) is well demonstrated by these data. Acknowledgements-This study was commenced while OAB was on study leave from Western Mining Corporation undertaking doctoral studies at the Australian National University. The support of both organisations is gratefully acknowledged.

REFERENCES ARTH J. G., ARNDT N. T. and NALDREV A. J. (1977) Genesis of Archean komatiites from Munro Township, Ontario: trace element evidence. Geology 5, 590-594. BARRETTF. M., BANNSR. A., GROVESD. I., MARSTONR. J. and MCQUFENK. G. (1977) Structural history and metamorphic -m~i~~tion of Archean volcanic-type nickel deaosits. Yilaam Block, Western Australia. &on. Geot. 72: 1195-125. BAV~NTON 0. A. (1979) Interflow sedimentary rocks from the Kambalda ultramafic sequence: Their geochemistry, metamorphism and genesis. Ph.D. Thesis, Australian National University. BAVINTON0. A. and KEAYSR. R. (1978) Precious metal values from interflow sedimentary rocks from the komatiite sequence at Kambalda, Western Australia. Geockim. Cosmo&m.

647

rocks

Acta 42, 1151-l 163.

BINNS R. A.. GUNTHORPER. J. and GROVESD. I. (1976) Metamorphic patterns and development of greenstone belts in the Eastern Yilgarn Block, Western Australia. In The Early History of the Earth. (ed. B. F. Windley), pp. 303-313. Wiley. COMPSTON W. and T~REK A. (1973) Isotopic age limits for the provenance and deposition of the Kurrawang Beds, Coolgardie Goldfield, Western Australia. J. Geol. Sot.

GEMUTSI. and THERONA. (1975) The Archaean between Coolgardie and Nor~m~n-strati~aphy and mineralisation. In Economic Geology of Australia and Papua New Guinea. (ed. C. L. Knight), Australas. fnst. Min. Metali., Melbourne, Monogr. S(i), 66-14. GRAF J.L. (1978) Rare earth elements, iron formations and seawater. Geochim. Cosmochim. Acta 42, 1845-1850.

HALLRERGJ. A. (1972) Geochemistry of Archaean volcanic belts in the Eastern Goldfields region of Western Australia. f. Petrof. 13, 45-56. HALLBERGJ. A. and WILLIAMSD. A. C. (1972) Archean ma& and ultramafic rock associations in the Eastern Goldfields Region, Western Australia. Earth Planet. Sci. Lett. IS, 191-200. HCJ~TOND. H. and GIORGE~A N. E. (1977) Quantitative X-ray diffraction analysis by a direct calculation method. X-Ray Spectrom. 6(1X 2-5. MCLENNANS. M., FRYERB. J. and YOUNGG. M. (1979) Rare earth elements in Huronian (Lower. Proterozoic) sedimentary rocks: composition and evolution of the post-Kenoran upper crust. Geochim. Cosmochim. Acta 43. 375-388.

NALDRETI‘ A. J. and TURNERA. R. (1977) The geology and petrogenesis of a greenstone belt ‘and related ni&l sul-

fide minerali~tion

at Yakabindie, Western Australia.

Precamb. Res. 5.43-103.

NANCEW. B, and TAYLORS. R. (1976) Rare earth element patterns and crustal evolution-l. Australian postArchean sedimentary rocks. Geochim. Cosmochim. Acta 40,1539-1551.

NANCEW. B. and TAYLORS. R. (1977) Rare earth element patterns and crustal evolution-II. Archean sedimentary rocks from Kalgoorlie, Australia, Ibid. 41, 225-231. NESBITTR. W. (1971) Skeletal crystal forms in the ultramafit rocks of the Yilgarn Block, Western Australia: Evidence for an Archean ultramafic liquid. Geol. Sot. Aust. Spec. Publ. 3, 331-350.

NORRISHK. and HUTTONJ. T. (1969) An accurate X-ray spectrographic method for the’ analysis of a wide range of geological samples. Geochim. Cos~ehim. Acfa 33, 431-453.

O’BEIRNEW. R. (1968) The acid porphyries and porphyroid rocks of the Kalgoorlie area, W.A. Ph.D. Thesis, University of Western Australia. OVERSRYV. M. (1975) Lead isotopic systematics and ages of Archaean acid intrusives in the Kalgoorlie-Norseman area, Western Australia. Geochim Cosmochim Acta 39, 1107-l 125.

~RIDER R. 7. (1965) Geology and mineralization of the West Australian Shield. In Geoloav of Australiun Ore Deposits (2nd edn. 8th Publ.). -t?o&nonwealth Min. Metall. Conar. 1. 56-65.

PYKED. R., N~LL&TT A. J. and ECKSTRAND 0. P. (1973) Archean ultramafic flows in Munro Township, Ontario. Bull. Geol. Sot. Am. 84,955--9X. RODD~CKJ. C. M. (1974) Responses of strontium isotopes to some crustal processes. Ph.D. Thesis, Australian National University. Ross J. R. and HOPKINSG. h4. F. (1975) The nickel sulfide deposits of Kambalda, Western Australia. In Economic Aust. 20, 211-222. Geology of Australia and Papua New Guinea, (ed. C. L. DONNELLY T. H., LAMBERT I. B., OEHLERD. Z., HALLBERG Knight), Australas. Inst. Min. Metall., Melbourne, J. A., HUDSOND. R., S~mt I. W., BAMNT~N0. A. and Monogr. 5(l), 100-121. GOLD~NGL. Y. (1978) A reconnaissance study of stable isotope ratios in Archean rocks from the Yilgarn Block, SUN S. S. and NEsRin R. W. (1978) Petrogenesis of Western Australia. J. Geol. Sot. Aust. 24, 409-420. Archaean ultrabasic and basic volcanics: evidence from rare earth elements. Contrib. Mineral. Petrol. 65, FRYERB. J. (1977) Rare earth evidence in iron-formations 301-325. for changing Precambrian oxidation state. Geochim. Cosmochim. Acta 41, 361-367. TAYLORS. R. (1977) Island arc models and the composition

648

0. A. BAVINT~N and S. R. TAYLOK

of the continental crust. Am. Geophys. Union Monogr: Ewing Symp. 1, 325-335. TAYLOR S. R. (1979) Chemical composition and evolution of the continental crust: the rare earth element evidence. In The Earth: Its Origin, Structure and Eoolution. (ed. M. W. McElhinny), pp. 353-376. Academic Press. TAYLOR S. R. and GORTON M. P. (1977) Geochemical application of spark source mass spectrography-III. Element sensitivity, precision and accuracy. Geochim. Cosmochim. Acta 41, 1375.-1380. TAYL.OR S. R. and M~LENNAN S. M. (1980) The rare earth clement evidence in Precambrian sedimentary rocks: implications for crustal evolution. In Precambriun Platr Tectonics. (ed. A. KrGner), Elsevier (in press). WILLIAMS 1. R. (1974) Structural subdivision of the East Goldfields province, Yilgarn Block. Geol. Surr. M! Aust. Ann. Rep.

1973, 53-59.

Williams I. R. (1975) The geology of the Yilgarn Block, W.A. In The Geology of Western Australia (ed. A. F. Trendall), Geol. Surr. WA. Memoir, 2, 33-54. WOODALI. R. and TRAVIS G. A. (1969) The Kambalda nickel deposits, Western Australia. Commonwedrk Min. Meld/. Paper X, 17pp. (9th publ.).

Sample No. x7x9

.sedimenl samples

Lithological information grey ; brecciated; siliceous

black; slatey ; carbonaceous light grey; banded; siliceous black: slatey; carbonaceous

qtz-ab-py-po-chl-bio

481 6458 6462 7566 7643 8323

light-grey; sericitic white: siliceous slatey; po nodules greenish; glassy chert pinkish-grey; cherty grey; glassy: cherty

qtz-chl-must-py-po-cal ab-qtz-trem-(po) ab-po-trem-qtz-chl qtz-trem-ab-po-py ab-qtz-po-trem-kfs ab-qtz-po-trem-bio

282 1020 7567

white: siliceous light-grey: sericitic grey; siliceous

qtz-chl-ab-py-cal qtz-ab-py-must-bio-cal ab-qtz-trem-bio-po-cpy

8781 8782

black; massive black: slatey: carbonaceous grey; banded: siliceous

qtz-po-ab-bio-cal

8791 x792

8783

Mineral

rock

qtz-ab-py-po-cal-bio qtz-py-dol-po-(chl)

qtz-ab-po-bio-lrem-cal qtz-ab-po-chl-trem-cal

uhhrwiafions

ab, albite: qtz, quartz; chl, chlorite; trem, tremolite; bio. biotite: must. muscovite: kfs. K-felspar; cal, calcite: dol. dolomite: po, pyrrhotite; py, pyritk; cpy, chalcopyrite. Minerals in approximately decreasing abundance.

APPENDIX Kamhaldu

x790

LOCuriona

Mineral

assemblage

qtz-py-must-po-dol

8789 8792: within the hangingwall basalts; 4X1-8323: within the ultramafic sequence; 282-7567: ultramafic basalt contact: 8781-8783: within the footwall basalts.