Applied Geochemistry, Vol. 4, pp. 79-92, 1989 0883-2927/89 $3.00 + .00 Printed in Great Britain Pergamon Press plc

Salt sources and water-rock interaction on the Yilgarn Block, Australia: isotopic and major element tracers

J. M. MCARTHUR Department of Geological Sciences, University College London, London WC1E 6BT, U.K.

J. TURNER

Division of Water Resources Research, CSIRO, Private Bag, P.O. Wembley, W.A. 6014, Australia

W. B. LYONS Institute for the Study of the Earth, Ocean and Space, University of New Hampshire,

Durham, NH 03824, U.S.A.

and

M. F. THIRLWALL Department of Geology, Royal Holloway and Bedford New College,

Egham Hill, Egham, Surrey, TW20 0EX, U.K.

(Received 9 May 1988; accepted in revised forrn 7 November 1988)

Abstraet--Internal drainage basins in the south east of Western Australia are underlain by predominantly granitoid rocks. The regional shallow ground water are NaCI brines. Ratios of 87Sr/S6Sr in the brines are 0.7155 _+ 0.0015, which are close to the ratio of modern marine Sr (0.7092). A two-component isotopic mixing model, with end members of marine Sr and Sr from basement granitoid rocks, implies strongly that <~5% of the Sr is derived from rock weathering and/>95% is marine. The isotopic composition of S in gypsum crystallised from the brines is +19.8 + 0.39'o0 (CDT). This value is close to that of S in Recent evaporites (+21.5 + 1 CDT) and confirms that weathering of Archaean sulphides, with c~34S of 0 + 4%0 (CDT), has contributed insignificant amounts of S to the brines. The isotopic data are compatible with a late Tertiary to Recent age for the Sr and S and, by implication, for the salts as a whole. The formation of alunite has been a major control on brine composition while gypsum and calcrete formation have been minor influences. None of the ground waters is saturated with halite which suggests the brines are immature and have evolved their present chemistry within the past few tens of thousands rather than millions of years. Marine aerosols are the main source of salt. Water sequestered in internal drainage basins, by a post-Middle Miocene regression, possibly provided a subordinate fraction.

INTRODUCTION

SALINE ground water is common in continental interiors with arid to semi-arid climates. Predicting the location and quality of ground water in such environments is difficult unless an understanding is achieved of the source of salts in such water, and of the chemical processes that modify its composition through time. Salt sources, and the means of pro- ducing saline ground water, have been discussed by BETTENAY et al. (1964), WILLIAMS (1968), DREVER and SMITH (1978), EUGSTER and JONES (1979), MACUMBER (1983), LYONS (in press, 1988) and others. The principal sources of salts in any continental area are:

(1) Sea water stranded in internal basins by marine regressions.

(2) Connate water of marine origin released by weathering from the pores of marine sediments.

(3) Cyclic salt-marine aerosols formed at sea or the coastline and blown inshore.

(4) Rock weathering with evaporites, sulphides,

carbonates, igneous and metamorphic rocks as the major source, and clastic sediments as a minor source because they have passed through the weathering cycle at least once.

(5) Volcanic exhalatives and hydrothermal con- vection.

These mechanisms will contribute to different degrees at different times to the concentration of each element, and the contribution each makes will vary from point to point, so it is unlikely that indi- vidual contributions can be completely separated. Interpretation of data is made more difficult because element fractionation may occur during transport of aerosols to inland locations (HINGSTON and GAILITIS, 1976), biogenic contributions may increase inland, especially for S (GALLOWAY, 1985), and the dissolved load in soil and ground water may be fractionated during wet-dry cycles by precipitation of minerals in the phreatic zone (DREVER and SMITH, 1978). The fundamental question, then, concerning the origin of such salts in any continental area is "At any point on the mainland, what is the relative contribution of

79

80 J.M. MCArthur et al.

each of these mechanisms to the abundance of each element?" The question posed can be answered most effectively by closely examining salt origins in well defined environments where single controls are likely to dominate and then extending the lessons learned to more complex areas. This paper presents the results of such a study undertaken in Western Australia.

The compositions of saline ground waters on the southern Yilgarn Block, in the southern part of West- ern Australia, were reported by MANN (1983) who interpreted his major element data as showing that the dissolved solids were supplied by rainfall and that the brines formed by subsequent evaporation with little interaction occurring with bedrock. Element/Cl ratios of many of his samples are very different from marine ratios, however, many being much higher; this suggests that the brines may have other origins or might have been affected by unexplained processes. The samples MANN described were collected over a very large area across which salt sources, and the processes affecting brine chemistry, might vary sub- stantially. A more detailed study of a very much smaller area of the Yilgarn Block has therefore been undertaken in order to quantify the relative contri- butions of the processes mentioned above to the composition of the saline ground water. In this small area, 200-300 km from the southern coast (Fig. 1), marine influences should dominate the salt budget. Nevertheless, the widespread presence of acidic

ground water in southern Western Australia (pH < 3.5, MANN, 1983; MCARrnUR et al., 1988) suggests that rock weathering might produce a recognisable signal that could be quantified because the bedrock lithology is well known and distinctive. Greenstones and granitoids are present and the former contain massive sulphides as well as sulphides associated with Au-mineralisation (DOEPEL, 1973). The isotopic compositions of Sr and S have been used to quantify rock weathering, and major elements have been used to test further for marine influences.

GEOLOGY, PHYSIOGRAPHY AND CLIMATE

The investigation was conducted mostly between the latitudes of Norseman and Salmon Gums (Fig. 1). Sites at Lakes Tay, Hope, Johnston, Cowan, and Dundas were also sampled. In addition, sample sites 509, 510, 514, 513 and 393 of MANN (1983) were located and the ground water was resampled (our sites 1, 10, 43, 47 and 49, respectively).

The area is well described elsewhere (CLARKE et al., 1948; COCKBAIN, 1967; DOEPEL, 1973; OVERSBY, 1975; PLAYFORD et al., 1975a,b; GOWER and BtmTINC, 1976; MANN, 1983; MCARTHUR et al., 1988) so only brief details are given here.

The basement rocks are migmatites, granitoids and greenstones between 2.6 and 2.8 Ga in age. The eastern half of the area is underlain by greenstones,

( f "

O k m 50 i i L I i

j = j Basic Granuhtes

I P I Porphyrtttc Granites { ~ ' ] Mtgrnatlte

!7-,,/~J Greenst one

Wldg*emool tha Dyke Sui te

~ Syer~lte

121'

P

P ° ~ P

I. ~ J / J

FIG. 1. Map of the basement geology.

122 ~

Salt sources, Yilgarn Block, Australia 81

~ l 0 , , .m , , so

> ,

," \ \ ~ ~ ) i . . . . . . = ,o ~ow, , , ,

28 "

& i ? S ~ 9<~ ~,> +,¢/ i V" -

" ,--_.. . . . . . . < . , / -

Flo. 2. Location and numbering of sample sites.

porphyritic biotite granite and biotite adamellite. The western half is underlain mostly by migmatites within which biotite-granitoid masses are dispersed. Greenstones underlie Lakes Cowan and Kirk and parts of the Johnston Lakes and Lake Hope (Fig. 2). Outliers of Upper Eocene limestone, dolomite and spongolite of the Eundynie occur on the margins and floor of Lake Cowan, at the north end of Lake Dundas and 25 km east of Salmon Gums (CLARKE et al . , 1948; COCI<BAIN, 1967; DOEPEL, 1973). Gravel plains of ironstone pebbles in a sand or loam matrix cap the drainage divides, and calcrete and calcareous loam are widespread in the zone between the gravel plain and the saline lakes.

Claypans and playas, underlain by saline ground water, are a prominent feature of the area (Fig. 1). The large playas are termed "Lakes" on maps of the area despite the fact that they rarely contain surface water. The playas and claypans (not distinguished hereinafter) occupy broad valleys that in pluvial times carried rivers that flowed eastward towards the Eucla Basin. The valleys were cut prior to Cretaceous times, possibly by Permian glaciers (DOEPEL, 1973; PLAYFORD et al. , 1975a). The sediments of many playas contain centimeter-sized selenite crystals or, uncommonly, are overlain by a mush of microcrystal- line gypsum or bassanite up to 20 cm deep. Solid pavements of bladed selenite up to 20 cm thick occur on the north side of Lake Swann and the east side of Lake Gilmore. Such occurrences appear to be con-

AG 4 / 1 - F

fined to parts of the larger playas. Windblown gyp- sum dust (kopi) or, rarely, crystals of millimeter-sized seed gypsum, form prominent dunes on the east side of many playas and on the floor of a few. Halite was not seen.

The region is semi-arid with a yearly rainfall of 300 mm, most of which falls in winter. Many playas contain a few centimeters of ephemeral water after winter rain but surface flow of water along the main trunk valleys is a very rare event. The vegetation comprises eucalyptus woodland and low scrub (DoEPEL, 1973; GOWER and BUNTING, 1976).

SAMPLE COLLECTION

The locations of the sample sites are shoWn in Fig. 1. Site details (including grid references) are given in MCARTnUR et al. (1988). Shallow regional ground waters were obtained by digging holes with a shovel, or by augering. As a consequence of these methods water deeper than 4.3 m could not be sampled so most waters were taken at the margins of playas where the saturated zone approaches the surface. It is important to note that the samples described here were not characteristic of the main body of water beneath the playas, nor do they represent infiltrating surface water, although a few samples collected on traverses into Lakes Gilmore and Swann are influ- enced by surface water and saline wedges underlying

82 J.M. MCArthur et al.

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Salt sources, Yilgarn Block, Australia 83

Table 1. A comparison of salinity for surface water and ground water at sites where both were present

Salinity (%0)

Site No. surface water ground water

1 88 SeeTable2 2 108 98 3 33 96 4 28 120 5 60 175 6 18 81 8b 47 115

13 28 160 15(1) 24 117 15(2) 24 180 19 35 168 21 37 164 22 <1 43 24b 50 75 25 62 185 27 60 68 31 62 58 33 32 160 34 98 136 35 138 48 36 50 39 38 112 68 39 117 65 40 108 140 41 106 See Table 2 46 32 153

Salinity obtained by conversion of field conduc- tivity measurement.

the playas. Salinity data are given in Table 1 for ground water and surface water from sites where both were present. At all sites (except, perhaps, 27) field evidence and chemistry show that there was no hydraulic continuity between them.

In order to assess what sampling bias, if any, was caused by sampling mostly at playa edges sampling traverses were made across the margins of Lake Gilmore (Site 1, Fig. 3a) and Lake Swann (Site 41, Fig. 3b). The profiles started at the base of the dunes around the playa margins (i.e. as far from the playa proper as water could be obtained) and extended into the playa until the salinity of the water stopped increasing.

Samples were filtered in the field through 0.45/~ Millipore membrane filters and stored in plastic bottles. Measurements of pH were made downhole or upon collection using unfiltered samples. Solid samples of representative lithologies were collected from each excavation for mineralogical analysis.

ANALYTICAL METHODS

Samples were analysed by inductively coupled plasma- atomic emission spectroscopy for SO4, K, Ca, Mg and Sr, and by atomic absorption spectrometry for Na. Dilute solutions of Standard Sea Water (Institute of Oceano- graphic Science, Wormley, Surrey, U.K.) were used as standards. Chloride was determined by titration with AgNO3. Bicarbonate was determined by titration with

H2SO 4. Isotope data for Sr were obtained by standard methods of ion-exchange separation followed by analysis on a VG-354 five-collector mass spectrometer using a peak- jumping triple-collector algorithm. During the period of the analysis the ratio for NBS 987 was 0.71024 _+ 2 (2 o, N = 73). Details of the isotopic analysis of S are given by LYONS et al. (in press). Clay minerals were identified by XRD using oriented mounts of the <10 ~ fraction of the samples coupled with standard heat treatments and glycollation. Other minerals were identified by XRD using cavity mounts. A scan speed of l°/min 20, was used for all XRD analysis.

RESULTS

Table i presents a comparison of pH and salinity in surface and ground water from sites where both were present. The chemical composition of the samples is given in Table 2. Tables 2 and 3 present chemical profiles across the margins of Lakes Gilmore and Swann, the sample sites being shown in Fig. 3. Table 4 presents the results of an Sr isotope model used to assess the degree of rock weathering. Sample loca- tions and basement geology are shown in Figs 1 and 2. Profile sites at Lakes Gilmore and Swann are shown in Fig. 3a and b. Chemical data are presented graphically in Fig. 4. The results of major element modelling of the brine composition are shown in Fig. 5.

The sediment at most sites varied greatly in cotour, grain size, mineralogy and texture over distances of a few tens of centimetres, both vertically and laterally, often in response to irregularities in a decomposing shallow substrate of pallid zone. Most samples col- lected are mixtures of quartz and kaolinite with vari- able but minor amounts of illite or degraded mica. Dolomite is present 1.3 m below the surface of a small playa at Site 25. Alunite is a major mineral at Sites 1, 2, 3, 17, 18, 22, 24b, 41, 42b and 44. Not all lithologies at each site were sampled, so it is very likely that alunite was present at a greater number of sites than recorded here; according to MANN (1983) alunite is common in the area. Calcrete and gypsum are com- mon and widespread (DoEPEL, 1973; MANN, 1983). Gypsum is abundant as kopi dunes, as a major com- ponent of the soils, and as blankets and low dunes of seed gypsum. Calcareous loam is widespread and commonly contains abundant calcrete nodules.

The ground waters are NaC1 brines. Apart from bore water from Sites 48 and 42a, where perched lenses of fresher water were encountered, the minimum chlorinity (Tables 1-3) is about 26%0 at Sites 18, 22 and 1 (Hole 0).

No difference in chemical composition is seen between the brines on greenstone basement and those on granitoids (Table 2, Figs 4 and 5). Acidic samples (pH < 4.5) are generally less saline than others (Fig. 4), although there are exceptions to this generalisation.

The Sr isotope composition of the waters is very uniform at a value of 0.7155 + 0.0015 (Table 2).

84 J .M. MCArthur et al.

Table 2. Chemical composition of shallow ground water in the south east of western Australia

Site Hole Na K Ca Mg CI SO4 HCO3 Sr % Charge No. No. pH mM mM mM mM mM mM mM u M balance 87Sr/86Sr

1 0 3.3 659 4.6 3.8 52 762 19.5 0.00 33 -1.55 1 3.4 795 5.2 3.6 60 897 25.2 0.00 34 - 1.07 3 3.0 1214 6.5 4.4 98 1408 42.5 0.00 51 -2.31 4b 2.9 2245 12.3 11.3 174 2592 71.5 0.00 94 -2 .00 5 3.0 1777 10.7 8.9 142 2034 61.0 0.00 76 -1 .54 6(1) 2.9 2288 12.7 10.5 179 2601 70.2 0.00 86 -1 .13 6(2) 2.9 2306 12.9 10.5 181 2683 70.8 0.00 86 -2.24 8 3.5 3428 18.7 20.9 256 3893 104.1 0.00 156 -1.23

11 4.5 3197 15.1 23.1 222 3639 109.3 0.00 153 -2 .06 12b 4.6 3454 17.5 21.6 242 3836 112.5 0.00 194 -0 .79 14 5.3 2790 13.8 33.2 153 3159 79.3 0.00 178 -2 .18

2 3.2 2092 9.3 17.5 130 2358 71.5 0.00 81 -2 .13 3 2.8 1764 5.4 10.3 158 2054 75.1 0.00 53 -2.29 4 4.7 5 3.3 3402 12.2 16.2 264 3865 88.3 0.00 116 -0 .84 6 3.3 1467 8.2 23.5 78 1661 40.8 0.00 38 -1 .90 7 3.2

8a bore 6.4 1620 7.3 19.5 89 1805 45.1 7.02 61 -1 .72 9 6.1 3541 17.6 18.5 386 4288 120.8 0.41 138 - 1.82 0.71612 _+ 2

10 6.2 3061 7.8 15.8 337 3752 93.3 0.00 82 -2.12 0.71609 + 2 11 6.6 2079 5.7 21.0 170 2403 70.1 0.28 87 -1.56 0.71576 _+ 2 12 6.2 2044 12.6 42.2 149 2353 71.5 0.00 102 -1.14 13 5.9 2515 4.0 17.0 274 3018 75.4 0.00 176 - 1.07 0.71580 + 2 14 5.8 2568 8.3 21.7 222 3018 52.4 1.14 89 -0 .99 0.71482 + 2 15(1) 3.7 3271 30.7 23.9 303 3893 98.3 0.00 258 - 1.66 15(2) 3.2 2135 18.5 17.4 191 2536 66.7 0.00 153 -1.89 16 6.5 1965 21.6 32.4 170 2364 83.5 0.00 229 -2 .82 17 3.2 882 7.7 8.8 66 1013 31.4 0.00 55 -1.73 18 3.3 690 4.2 2.4 53 750 22.7 0.00 29 0.64 19 3.7 2694 22.5 16.5 292 3047 142.6 0.00 186 0.02 20 6.7 1913 11.9 29.4 153 2228 81.5 0.30 150 -2 .19 21 3.0 3183 23.3 11.1 265 3667 90.9 0.00 138 -1 .17 22 3.4 764 8.0 3.8 74 883 31.7 0.00 55 -1 .02 23 3.6 1559 18.9 27.9 142 1780 68.6 0.00 177 0.01 24b 3.6 1293 9.9 29.2 108 1495 63.6 0.00 123 -1.41 25 6.5 3393 14.1 23.2 258 3893 89.5 0.71 320 -1 .29 26 6.1 2969 11.2 9.8 349 3441 162.4 0.00 113 -0 .90 0.71579 + 2 27 7.4 28 3.8 29 6.2 4183 20.8 14.5 370 4711 128.1 0.44 194 0.06 0.71699 + 2 30 4.0 2965 10.7 18.6 239 3329 114.5 0.00 84 -0.95 31 3.2 1000 5.7 10.3 57 1126 27.5 0.00 51 -1.75 32 5.4 2904 11.1 20.8 220 3329 89.5 0.00 191 -1 .62 33 6.1 34 6.7 2402 14.0 19.9 153 2688 50.2 0.73 189 -0.53 35 3.3 908 5.4 2.0 82 1052 38.6 0.00 30 -2 .17 36 3.2 37 3.6 38 3.5 39 4.1 1105 9.5 8.1 102 1292 44.3 0.00 88 -1 .69 40 3.8 0 41 3.2 961 8.2 4.2 70 1086 24.3 0.00 74 -0 .74

3.4 1140 9.3 6.7 86 1300 29.2 0.00 89 -0 .90 3.3 1393 11.5 6.8 111 1622 38.8 0.00 104 -1 .75 3.1 2074 15.6 7.6 174 2386 59.9 0.00 127 -1 .09 3.0 2751 20.7 9.3 315 3300 123.9 0.00 164 -1 .84 6.4 4170 32.2 14.1 396 4767 138.5 1.75 172 -0.25 3.2 2467 17.1 11.0 218 2877 75.9 0.00 148 -1 .46 5.2 3707 32.0 12.3 465 4485 170.8 1.59 163 -1 .43 3.8 389 4.5 2.4 39 434 26.9 0.00 34 -1.31 5.9 1808 12.8 12.9 141 2034 62.9 0.70 53 -0 .77 3.9 1786 20.2 11.6 176 2093 94.3 0.00 94 -2 .24 3.5 913 11.5 16.3 93 1094 51.2 0.00 57 -2 .36 3.2 2367 30.3 8.6 223 2731 82.3 0.00 105 -0 .62 3.4 904 13.6 5.3 105 1069 56.1 0.00 68 -1 .83 3.7 3162 33.8 21.2 190 3357 112.5 0.00 339 0.51 7.3 72 1.1 0.7 6 83 4.6 2.74 3 -6 .34 3.5 1472 15.0 12.9 141 1760 40.9 0.00 79 -1.33

42abore 43 44 45 46 47(1) 47(2) 48 49

A C D E2 F2 5 6

11

0.71505 + 2

0.71619 + 2 0.71590 + 4 0.71402 + 2 0.71690 -+ 2

Salt sources, Yilgarn Block, Australia

Table 3. Profiles of salinity and pH across the margins of Lakes Gilmore and Swann

Site 1: Lake Gilmore

Hole

Site 41: Lake Swann

Salinity Salinity pH (%0) Hole pH (%0)

0 3.3 39 A 3.2 62 1 3.4 53 B 3.5 62 2 3.0 58 C 3.4 72 3 3.0 72 C2 3.3 75 4 3.0 73 D 3.3 92 4b 2.9 120 D2 3.2 90 5 3.0 103 E 3.2 120 6 2.9 162 E2 3.1 125 6b 2.9 82 F 3.1 171 7 3.0 207 F2 3.0 175 8 3.5 197 6 3.2 150 9 3.9 207 7 3.4 158

10 4.3 203 8 3.7 - - 11 4.5 200 9 3.5 115 12 4.2 203 10 3.6 115

+ 12b 4.6 208 11 t 5.2 200 13" 5.0 172 _+1 4.1 75 14" 5.3 16 -+2 3.2 160

• +3 3.5 170 +4 6.0 225 • +5~ 6.4 235

Surface water 5.9 88

* Influenced by surface water. t Influenced by water trapped in selenite pavement. Salinity %o by conversion of field conductivity measurement.

85

Table 4. Isotope mixing model for assessing the contribution of rock weathering to the pool of dissolved Sr. The model mixes Sr derived by rock weathering with marine Sr of Middle Miocene (0.07880) or modern (0.70918) age with a Sr concentration of 7.6/~g/ml (DEPAOLO and INGRAM, 1986; HESS et al., 1986). The isotopic ratio in the brines is 0.7155 _+ 0.0015 (Table 2). The percentage of the total Sr in solution that has been derived from rock weathering is shown as a function of the 87Sr/86Sr ratio in the basement rocks. ARRmNS (1971 ) and CHAPMAN et al. (1981) give average values shown in the table. The model incorporates

terms to account for the different Relative Atomic Masses of marine and rock Sr

% Sr derived from rock weathering

87Sr/86Sr ratio marine ratio marine ratio in rock 0.70880 0.70918

Arrien's basement average

Chapman's basement average

0.717 87.1 80.0 0.720 62.8 58.3 0.750 16.6 15.5 0.780 9.6 9.0 0.800 7.5 7.0 0.830 5.7 5.3

0.860 4.6 4.3 0.890 3.8 3.6 0.930 3.1 2.9 0.960 2.8 2.6

Table 2. Continued

The data here are calculated from concentrations in mg/1 rounded to 3 figures so the data here have not been rounded again.

Samples 6(1) and 6(2) were collected four days apart from Site 1, Hole 6. Sample 47(2) was collected from a re-excavation of the hole dug by MANN (1983). Sample 47(1) was collected from a site 150 m to the N of Mann's hole. Sample 15(1) was collected from a hole dug in the lake bed 50 m N of the southern shore. Sample 15(2) was taken on the southern margin of the lake.

86 J . M . MCArthur et al.

4-

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FIG. 4. Major e lement concentrat ions plotted against the concentrat ion of CI. Symbols signify: O = site 1, [] = site 41, A = all other sites. Symbols are filled if the sample pH ~< 4.5.

Salt sources, Yilgarn Block, Australia 87

40

g

co0 ~

• e c~ !

] • • /

50 100 150

SO, de f i c i t milli Molar

i i =

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~ 2 5 0 " =_- E

2 0 0 -

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Cq 100" +

d 50-

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150" "lO

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1:0 210 310 4~0 510 6 . 0

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FIG. 5. Deficit chemistry of the brines. The deficit of an element is calculated by subtracting the actual element concentration from the concentration that would be present in evaporated sea water of the same chlorinity, assuming no

precipitation of salt. Symbols as for Fig. 4.

Samples from greenstone and granitoid terrain show no difference in 87Sr/86Sr ratios, neither do samples from acidic (pH < 4.5) and less acidic (pH > 4.5) environments. Analysis of five samples of gypsum from the area of Lake Gilmore gave ~345 values of +19.8 + 0.3% (CDT).

The profiles across the margins of Lakes Swann and Gi lmore- -bo th very large playas--suggest that water of low salinity is moving toward the playas in response to meteoric infiltration, and is either evaporating or mixing with more saline playa water as it does so. Ground water discharge zones at the periphery of Lake Swann were clearly seen waxing and waning over intervals of 6-8 h in response to rainfall during the occupation of this site (MCARTnUR et al., 1988).

SOURCE OF SALTS

Only two samples have chlorinities of <19%o. Chemical evolution may have taken place at such low chlorinities and be seen now in a pronounced form because of evaporative concentration. Although this should be kept in mind throughout the ensuing dis- cussion it does not affect the conclusions presented. Phanerozoic volcanic activity, and its associated hydrothermal effects, are not known in the study area so this mechanism of salt supply can be dismissed.

Possible marine origin

The Na/C1 ratios of the brines are very close to that of sea water. Because these elements are geo- chemically conservative, and halite is not present in the study area, the ratios argue very strongly for a marine source for the salts as a whole. Ratios of C1 to Ca, Mg, K, Sr, and SO4 have values less than or equal to but never greater than the equivalent marine ratios (Fig. 4). For several samples many element/Cl ratios closely approach the marine ratios, e.g. at Sites 16, 23, 24, 43, and 45. Simple modelling of brine com- position (see below) suggests that the element/Cl ratios lower than the equivalent marine ratio result from the removal of alunite, gypsum and calcrete from the brines. The simplicity and coherence of these models further supports the suggestion that the brines are of marine origin.

Five analyses of the isotopic composition of S in gypsum from the Lake Gilmore area gave 634S values of + 19.8 +_ 0.3% (CDT) (LYoNs and CmvAs, unpub- lished data). The value for modern evaporites is +21.5 + 0.3% (CDT) and values for Miocene evapo- rites may reach 22.0 +_ 1% (CDT) (CLAYPOOL et al., 1980). The similarity of these (~34S values argue strongly for a marine origin for the sulphate and, by implication, for the brines as a whole.

Analyses of SVSr/S6Sr in 12 ground waters gave ratios of 0.7155 + 0.0015 (Table 2). The value for modern marine Sr is 0.70918 + 0.00003. The isotopic

88 J.M. MCArthur et al.

ratio for marine Sr decreases with age to a value of 0.70775 -+ 0.00005 in the late Eocene (DEPAOLO and INGRAM, 1986; HESS et al., 1986). The fact that isotopic ratios for modern marine Sr and Sr in the brines are so similar argues strongly for a marine source for the Sr. The only other possible source is from the weathering of basement rock, but weather- ing is shown below to contribute <5% of the Sr in the brines and to be a minor source of supply for most other elements.

Contributions f rom rock weathering

Weathering of basement rock is potentially a sig- nificant source of salts because the Yilgarn Block has been subaerially exposed since late Tertiary times at the latest. If weathering were important, then inter- element ratios for brines that overlie greenstone basement, around Norseman, would be different to those that overlie granitoid terrain, but they are not. Furthermore, element/C1 ratios in the basement granitoid rocks are ->1, and wt % ratios are -~1 for Na/K and >1.5 for Na/Ca (GOWER and BUNTING, 1976). Weathering of such material would give a solution composition very different to that seen in the area.

Nevertheless, many of the brines are acidic (Table 2; MANN, 1983; MCARTHUR et al., 1988) so some reaction with the basement must have occurred. The amount of rock weathering can be constrained using Sr isotope data to model the mixing of marine Sr with Sr derived from the basement rocks. The 87Sr/86Sr ratio in the brines is 0.7155 + 0.0015 (Table 2). This value is modelled with end members chosen for the S7Sr/86Sr ratio in the basement rock and in the possible marine source. Model runs were made with modern (0.70918) and Middle Miocene (0.7088) isotopic ratios for Sr end-members in order to represent the youngest and oldest possible sources of marine Sr (DEPAOLO and INGRAM, 1986; HESS et al., 1986). The model calculates the fraction of the Sr in the brines that is supplied by rock weathering, given inputs of 87Sr/86Sr for the basement end-member. An average S7Sr/86Sr ratio of 0.89 _+ 0.23 is given by ARRIENS (1971) for 83 granitoid rocks from the Yilgarn Block, southern Western Australia, with whole-rock ratios often >1.0. The average ratio, given by CHAPMAN et al. (1981), for 67 granitoids from the Diemals area, in the NW of our area, is 0.96 + 0.24 with ratios up to 1.7. Because the Gi lmore-Tay-Sharpe area is under- lain by biotite adamellites it is likely that the rocks in the area fall within the upper end of this range.

Using this model, the calculated percentages of Sr derived from weathering are shown in Table 4. The model is normalised to an original chlorinity of 19%o (that of sea water) and assumes that rock Sr has been added to an undepleted pool of marine St. In reality, Sr has been sequestered in precipitating gypsum and calcrete, so rock Sr has been added to a diminishing

pool of Sr, thus magnifying the change in the Sr isotope ratio. The figures in Table 4 are therefore overestimates of the Sr contributed by rock weather- ing. Furthermore, OVERSBY (1975) has shown that granitoids from Stennet Rocks, 2 km from the NW shore of Lake Gilmore, have suffered loss of U during Cenozoic time, even from relatively fresh- looking rocks. If preferential loss of Sr has also occurred the extent of weathering in Table 4 may be further overestimated. The Sr model confirms our view that very little of the Sr in the brine (probably ~5%) has been weathered from either basement or cover rocks in the area.

A 5% contribution to Sr by weathering represents the addition of =400/~g of Sr to each kilogram of original brine containing 7600~g/kg of Sr, the present marine concentration. The granitoid rocks of the area contain an average of 158 ppm Sr (CHAPMAN et al., 1981, average of 63 samples) to 185 ppm Sr (ARRIENS, 1971, average of 83 samples) and an aver- age of 4.3% Na20, 3.6% K20, 2.1% CaO, 0.62% MgO and <0.1% S (GowER and BUNTING, 1976). Assuming an Sr content of 170 ppm in the rocks, 400 pg of Sr would be contained in 2.35 g of rock, which, for the given brine composition, would contribute 0.7% of the Na, 8% of the Ca, 16% of the K, 0.7% of the Mg and 0.3 % of the SO4 present. Clearly, a minor but noticeable fraction of the Ca, and a small but significant part of the K, may have been supplied by rock weathering. In view of the arguments above, however, these estimates are the very upper limits to such additions, and contributions from rock weather- ing are probably inconsequential.

Gypsum precipitating from the brine is depleted in 34S relative to modern and Miocene evaporites by between 1.4 and 3.2%0. The isotopic difference is probably due to large scale crystallisation from the brine of alunite and also extraction of gypsum. This process would leave the residual SO4 depleted in 34S. If the removal of SO4 is a Rayleigh fractionation this degree of isotopic differentiation would result from the removal of 65% of the total dissolved SO4 from the brines if the fractionation on alunite formation is the same as that for gypsum (+1.65%o, THODE and MONSTER, 1965). The actual removal amounts to ~50% (Fig. 4).

It could be argued that the S-isotopic differences noted above may result from the weathering of sul- phides. Sulphides in the southern Yilgarn Block have values for (~345 ~0 -I- 4%o, although anomalous values as high as +8%0 have been recorded in sulphides at Kambalda, and values between +4 and -10%o have been found at Kalgoorlie (LAMBERT and DONNELLY, 1988; PHILLIPS et al., 1988). Because sulphide oxi- dation produces insignificant isotope fractionation the depletion of 34S in the brine would represent a weathering contribution to the SO4 of between 7 and 16%. Although the greenstones locally contain mas- sive sulphides and the felsic basement contains <0.1% S there is no difference in SO4/Ct ratios

Salt sources, Yilgarn Block, Australia 89

between samples overlying greenstones and those overlying granitoids. Thus weathering seems unlikely to be important as a source of S. This conclusion is supported by Sr isotope data which suggest strongly that little weathering has occurred (see above).

Release o f connate water f rom sediments

Eocene limestone, dolomite and spongelite occur in the study area (CLARKE et al., 1948; COCKBAIN, 1967; DOEPEL, 1973). Weathering of these rocks may have supplied the salts in the brines. Supply by weathering of limestone, with a porosity of 50%, would require the erosion of 350 m of rock. The Ca/CI ratio in the brines, before evolution, would have been >200 times the marine ratio. The Ca/C1 ratios in the brines do not support this scenario. Alternatively, only 350 m of clastic rock, with a porosity of 50% would supply the necessary salts, but this thickness (of carbonate or clastic rocks) seems improbable in an environment so marginal to the depocentres of the Eucla and Bremer Basins. The derivation of salts from connate water in marine sediments seems unlikely to be important.

Regressive sequestration vs cyclic supply

salt in the brines. The data of HIN~STON and GAILITIS (1976) on aeolian salt fluxes in Western Australia can be used to examine the validity of this assumption. These authors show that the present day flux of C1 to the area is between 10 and 20 kg/ha/a. They state that this amount may contain an unquantifiable contribu- tion of Cl recycled from playas. Although playas are common downwind of the study area none are under- lain by water that is sufficiently saturated with halite to cause much precipitation of halite (MANN, 1983) SO this contribution is assumed to be small. Neverthe- less, a lower limit is taken of 10 kg/ha/a for the present Cl flux. At this rate of supply the total amount of C1 calculated to be present in the area (see above for details) would accumulate in approximately 3.5 Ma. The major playas comprise about 15% of the study area and are evaporative sumps towards which ground water and salts move from the surrounding catchment. If all cyclic salt falling on the area con- centrates in the playas only 0.5 Ma would be needed for the accumulation. Furthermore, during several periods since the Miocene sea levels were higher than today so the flux of Cl would have been higher as the coast was closer to the area, e.g. during the early Pliocene when the Roe Calcarenite was being deposited in the Eucla Basin. In conjunction with the above calculation this consideration shows that cyclic salt could supply all of the salts in the area in <0.5 Ma.

The retreating Miocene sea might have left sea water sequestered within the internal basins of the area, the brines evolving subsequently in response to climatic changes. The route for inundation would presumably have been west and southwest, i.e. up the trunk valleys, from the northwestern and western sides of the marine embayment, represented now by the outcrop of Miocene limestone in the Eucla Basin. Whether this mechanism is feasible can be con- strained, if imprecisely, by a simple calculation in which assumptions are made that minimise the quan- tity of sea water needed to form the present brines. About 3000 km 2 of the study area is occupied by major playas (Tay, Sharpe, Gilmore, Dundas, Cowan, Hope and Johnston). The depth of sediment towards the centre of southern Lake Cowan is about 115 m (CAMPBELL, 1906) and similar sediment thick- nesses are found in the major palaeodrainage around Kalgoorlie (PLAYFORD, pers. comm., 1988). If it is assumed (1) that the average depth for all the lakes is about half this figure, 50 m, (2) the average chlorinity is 139%o (the maximum in the large playas; about 7 times that of sea water, Table 3), and (3) the average sediment porosity is 50%, then it follows that a minimum of 175 m of sea water would need to evaporate to give the present brine composition. This depth overtops all the present drainage divides by 125 m, so a salt contribution of more than 30% from this source seems unlikely.

The arguments presented above suggest, by elimin- ation, that cyclic salt has been the major source of the

TIMING OF SALT EMPLACEMENT

BOWLER (1976) has shown that subtropical humid environments persisted to the southern margin of Australia to late Tertiary times, after which aridity developed strongly around 7-10 Ma and was well advanced by 2.5 Ma. It seems likely that the main trunk valleys, now occupied by playas, carried rivers that flowed to the Lucia Basin as recently as late Miocene time (BuNXlNC et al., 1973) and possible that they did so into the Pliocene (BOWLER, 1976) although BUNTING et al. (1973) state that "since the Miocene little flow of water occurred in rivers drain- ing towards the Eucla Basin". PLAYFORD et al. (1975a) imply even more recent fluvial activity by stating that "widespread laterisation of much of Western Aus- tralia probably occurred during pluvial periods of Pleistocene, although some of the laterisation is prob- ably of Tertiary age". QUiLaV (1977) records an early Oligocene period of laterisation in the area. These conflicting views leave some doubt as to the time at which flushing of the catchments ceased and salt accumulation started. The observation of BuYnN6 et al. (1973) that none of the palaeodrainages on the Yilgarn Block continue across the Miocene limestone of the Nullarbor Plain may indicate that aridity was well developed when the Miocene sea retreated from the area.

The soluble weathering products of the periods of erosion mentioned above are clearly absent from the

90 J.M. MCArthur et al.

brines. Furthermore, none of the waters has reached halite saturation or undergone much chemical evol- ution and, most importantly, little weathering of basement rocks has occurred; all these facts suggest that the brines have not been resident in the area for long. Taken together with the statements above, and knowing that the present salt flux to the area is sufficient to supply all the salts within <0.5 Ma it seems reasonable to suggest that salt accumulation has occurred mostly in the last 2.5 Ma, during which period Australia's climatic pattern has resembled that of today (BOWLER, 1976).

It seems unlikely that a retreating Miocene sea could have stranded much salt in the internal basins of the area but the idea is worth further short con- sideration. The internal basins were certainly deep enough in Miocene time to have sequestered large volumes of sea water during a regression (about 115 m in Lake Cowan, west of Norseman and 100 m around Kalgoorlie: CAMPBELL, 1906; PLAYFORD, pers. comm., 1988). The regressing sea, backing out of the trunk valleys, may have left some of the salts in the present brines. On available evidence it is uncertain whether the entire lengths of the trunk valleys were at elevations sufficiently low to permit inundation, but the potential for this and subsequent salt seques- tration appears to be present. BUNI~NG et al. (1973) suggest, on the basis of topographic analysis, that in the NW of the Eucla Basin the Miocene sea extended little farther than the present limit of outcrop of Miocene strata. At latitude 32°S, however, the elev- ations of the f loors of Lakes Dundas and Gilmore would have been below the elevation of carbonate accumulation in the Eucla Basin if maximum depths for these lakes are equal to the depth of Lake Cowan. In summary, whether or not the Miocene sea supplied salt cannot be decided without a detailed knowledge of the basement topography of the trunk valleys. If it did so the amount has been shown above to have been <30% of the total now present. The Pleistocene inundation of the Eucla Basin that deposited the Roe Calcarenite did not reach the study area (PLAYFORD, 1975a; pers. comm., 1988).

PROCESSES OF BRINE EVOLUTION

The above results show that the brines have de- rived little of their dissolved load by weathering of basement rocks. They have, however, undergone chemical evolution by cycling of alunite, calcrete, and gypsum. The brines are depleted in K, Ca and SO4 relative to sea water or an evaporated equivalent. They are particularly depleted in K if depletions are considered in terms of the percentage removal of an element. In molar terms the depletion in SO4 is about twice the depletion in K (Fig. 5a). By assuming that the K depletion is due to abstraction of K into alunite (KAI3[SO412[OH]6, the only K-bearing mineral of note at many sites) an amount of SO4 equivalent to

the abstracted K can be calculated (SO4 = 2 × deficit in K mM). Returning this SOa to solution results in SO4/CI ratios that are close to the marine value for most samples (Fig. 5b). If it is assumed that the Ca depletion is the result of gypsum extraction alone and SO4 is returned to solution as an amount of SO4 equivalent to the Ca deficit (Ca predicted by SW evaporation line--actual Ca), then the 804/C1 ratios approach closer to the marine ratio than do uncor- rected ratios but they still show considerable scatter and SO4 depletion (compare Figs 4 and 5c). These calculations, and their diagrammatic representation in Fig. 5, suggest that alunite formation has been the major control on the contents of K and SO4 in the brines, and that gypsum formation has been a sub- ordinate control on the contents of Ca and SO4. The formation of calcrete may also play a subordinate role in controlling Ca concentrations because calcrete is widespread in the area. Alunite formation con- sumes H + ions whereas calcrete formation consumes alkalinity, so it seems possible that the formation of these minerals may be interactively driven in order to maintain charge balance and pH equilibria in sol- ution.

Processes similar to those discussed above seem to be influencing the chemistry of the ground water in the Belka Valley, south-central Western Australia (BE~ENAV et al., 1964). Although individual data are not given by these authors, plots of SO4 and K against CI show deficits of about 6 mM K and 12 mM SO4 for chlorinities of 800 mM (about 1.5 times sea water, the maximum chlorinity reported) whilst Ca/CI ratios are very variable.

The scatter in the data shown in Figs 4 and 5 shows that subordinate controls are influencing the brine composition. In particular, the K deficit shown in Fig. 5a has a positive intercept on the K axis, with more points plotting above the line, drawn for 2K = SO4, than plotting below it. This distribution is interpreted as reflecting the existence for K of a separate sink, which is minor in most samples but an important control in a few. Illitic clay is present at some sites and may be the sink for K.

The suggestion that calcrete formation may be a control on Ca concentrations in the brines conflicts with the interpretation of DOEPEL (1973) who con- sidered the calcareous loam of the region (and cal- cretes chemically reworked from them) to be aeolian and derived from the Eucla Basin to the east. He proposed this explanation because the calcareous loams of the area thicken to the east. The eastward thickening could result, however, from the removal from the study area, by westerly winds, of calcareous material that now thins westward as a feather-edge of eroded remnants of soils formed under different climatic conditions. The prevailing wind direction is now westerly and was so in the past when winds were stronger (BOWLER, 1976). This alternative accords with our data.

The processes described here should not remove

Salt sources, Yilgarn Block, Australia 91

much Mg from the ground water and Mg/C1 ratios are indeed much closer to the marine ratio than are the ratios to C1 of K, Ca, or SO4. The actual amount of Mg removal , in terms of moles/kg of brine is quite substantial, however , and far in excess of that which could be sequestered in calcrete, which is at most a minor sink for Mg. Nei ther palygorskite nor sepiolite have been identified in any sample and the main sink for Mg remains unknown.

The timing of brine evolut ion is impossible to assess on available evidence. BOWLER (1976) con- tends that playas in Western Australia were saline throughout the Pleistocene and experienced an episode of aridity between 15 ka and 20 ka that resulted in the building of gypsum/clay dunes on their eastern margins. The dunes remain a prominent fea- ture of the area but their huge size poses a possible quanti tat ive problem for the favoured explanation for the deplet ion of 34S in the brines. Figure 4 shows that no more than 50% of the SOa in the brines has been removed in precipitating minerals. Yet to pro- duce just the modest 1 m of kopi that presently floors the southern half of Lake Cowan would (assuming a kopi porosity of 75%) require the deposit ion of this much gypsum i.e. half that contained in about 60 m of brine. Field observat ion shows that vastly more gypsum surrounds southern Lake Cowan than the "modes t 1 m" ment ioned above. In addition, alunite, not gypsum, seems to be the major sink for SO4 (see section on processes). Were a major part of the kopi and seed-gypsum in the area to have an aeolian origin the difficulty might be resolved as to how abundant gypsum could co-exist with brines from which gypsum extraction has clearly been limited. Field evidence is lacking for anything other than minor gypsum for- mation at the present time. All that can be concluded in view of the arguments above, is that the period of brine evolut ion currently recorded in their compo- sition probably occurred during the last few tens of thousands, rather than millions, of years. Past episodes of brine concentrat ion and dilution in response to arid and pluvial climates may have been overprinted by the current evolut ionary cycle.

SUMMARY

Regional shallow ground waters in the south east of Western Australia are NaC1 brines that have evolved chemically from a solution of marine origin to their present chemical composit ion over (prob- ably) the past few tens of thousand of years. The formation of alunite has been a major control on brine composit ion with formation of gypsum and calcrete being subordinate controls. The isotopic composit ion of sulphate-S and Sr in the brines shows that rock weathering has contr ibuted < 5 % of the dissolved Sr, Na, C1, Mg, and SO4, < 8 % of the Ca and < 16% of the K in the brines. The minor source of salt may have been supplied by a marine inundation

during Miocene time. Marine aerosols have con- tributed a major component .

Acknowledgements The senior author expresses sincere thanks to The Royal Society and the Bureau of Mineral Resources of Australia for financial support for this work, without which it would not have been possible. We express thanks to Bruce Dickson (CSIRO, North Ryde) for the loan of field equipment and to Chris Harris (CSIRO, Perth) and Terry Donnelly (CSIRO, Canberra) for their valuable assistance during and after the execution of the fieldwork. J. N. Walsh provided facilities for plasma emission analysis. Gerry Ingram provided help and instruction with the Sr isotope work. The Sr Isotope Laboratory is partly supported by the University of London and the ICP Facility is partly supported by NERC. Tony Osborn gave extensive and invaluable help during the analysis of the samples. Janet Baker repeatedly redrew the ever-changing diagrams, with assistance from Colin Stuart, and Mike Gray provided the necessary photography.

Editorial handling: W. M. Edmunds.

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