Source, distribution and economic significance of trace elements in groundwaters from Lake Tyrrell, Victoria, Australia

Chemical Geology, 96 (1992) 133-149 Elsevier Science Publishers B.V., Amsterdam

133

Source, distribution and economic significance of trace elements in groundwaters from Lake Tyrrell, Victoria, Australia A.M. Giblin and B.L. Dickson CSIRO, Division of Exploration Geoscience, North Ryde, P.O. Box 136, North Ryde, N.S. W. 2113, A ustralia (Accepted for publication August 30, 1991 )

ABSTRACT Giblin, A.M. and Dickson, B.L., 1992. Source, distribution and economic significance of trace elements in groundwaters from Lake Tyrrell, Victoria, Australia. In: W.B. Lyons, D.T. Long, A.L. Herczeg and M.E. Hines (Guest-Editors), The Geochemistry of Acid Groundwater Systems. Chem. Geol., 96:133-149_ Anomalous radioactivity highlighted the Lake Tyrrell area as a potential mineral exploration target and prompted a 5year study of its groundwater geochemistry. The groundwaters are very acid (median p H = 4 . 2 ) and saline (median ionic strength (1) = 1.24). Some samples contain unusually high concentrations of Cu (up to 3.5 mg 1- J ), Pb (up to 38,5 mg 1 -~ ) and AI (up to 155 mg I - ~). Concentrations of most other trace elements exceed seawater values but are not different from those in groundwaters near two other randomly selected Australian salt lakes. Waters with elevated trace-element concentrations are spread at isolated sites across the area but some clustering of Cu-Pb-enriched waters occurs in the west and southeast. R and Q mode correspondence analysis identifies factors which relate to both trace elements and samples, and are interpreted as reflecting processes whereby trace elements are taken into groundwaters. Factor I involves AI, Cu, Fe, Mn and Zn whose concentrations relate directly to high acidity; factor 2 involves B and Sr and probably relates to local sediment enrichments of B and St;, factor 3 is strongly weighted by Pb and can be explained by contact between groundwaters and locations where clays, organic solids and Fe-Mn-oxides are particularly enriched by heavy metals; factor 4 involves F, Y and I, and probably relates to residual mineral dissolution. Sample locations, grouped according to the influence of each factor on each groundwater composition, identify zones where a particular process is dominant. Laboratory leaching of Blanchetown Clay and Parilla Sand confirms that many of the trace elements could have entered the groundwater from local aquifer sediments. The potential of these waters as agents of present or future ore formation is also considered by comparing amounts of trace elements carried into the salt lake basin over the last 30 kyr with what persists in present-day lake brines. Results demonstrate that large tonnages of trace elements appear to have been deposited, but that sediment dilution mitigates against ore-grade concentrations.

1. Introduction Groundwaters at Lake Tyrrell, a salt lake in northeastern Victoria, Australia (Fig. 1), became of interest for mineral exploration because a significant air-borne radiometric anomaly located at the northwest margin of the lake could have resulted from groundwatersediment interaction. During investigations of this anomaly, enhanced trace-element concentrations in the local groundwaters stimulated 000%2541/92/$05.00

further interest in the area, suggesting it had potential as a base-metal exploration target. This paper reports and discusses the distribution of some economically important trace elements in groundwaters in the Lake Tyrrell region, assesses the likely sources for noneconomic abundances of these elements and considers the economic significance of the region where apparent enrichments are evident. Lake Tyrrell is located near the centre of the Murray Basin where Quaternary sands overlie

© 1992 Elsevier Science Publishers B.V. All fights reserved.

134

A.M GIBLINAND B.L. DICKSON

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Fig. 1. Map of Lake Tyrrell area with i n s e r t of Australia. a 300-m sequence of Tertiary marine sands, clays and limestones (Lawrence, 1966). The lake has formed within the freshwater Blanchetown Clay, which has developed over the marine Tertiary Parilla Sand. Sediments in the area are prospective for heavy minerals which have accumulated in palaeostrand lines during basin formation (Mclntyre, 1989). Lake Tyrrell groundwaters are generally highly saline (total dissolved solids up to 300 g 1-] ) and acid (pH-values between 3 and 6.5 ). Similar highly saline, acidic groundwater systems have been located at sites within a 2300-kmwide belt across the southern half of Australia, from Lake Tyrrell, west to the Darling Ranges east of Perth. Groundwaters from all these areas are noteworthy for the extraordinarily high concentrations (up to tens of mg 1- J ) of base metals (Cu, Pb and Zn) and U, and for high activities of Ra. Some studies have been reported on these systems; in Western Australian waters (Mann, 1983; Dickson, 1985), a

comparative study of four U prospects in South Australia related to such waters (Giblin, 1987), and a brief survey of waters near Lake Maurice, South Australia (Giblin and Dickson, 1984). The present-day hydrology of the Lake Tyrrell area has been described by Macumber (1984). Regional groundwaters flowing into Lake Tyrrell travel approximately westwards through Parilla Sand in an unconfined regional aquifer. Underlying the Parilla Sand is the Geera Clay which forms an effective base to the aquifer. The Parilla Sand waters have a major-element composition resembling that of seawater but have pH between 3 and 5. The floor of Lake Tyrrell intersects the regional water table and groundwater flows into the lake in a narrow zone around its margin (known as the spring zone). Lakes Timboran and Wahpool lie up flow to the east of Lake Tyrrell (Fig. 1 ) and regional waters flowing from that direction are concentrated by evaporation and neu-

TRACE ELEMENTS IN GROUNDWATERS FROM LAKE TYRRELL

tralised within those lakes. These waters then flow into Lake Tyrrell (Macumber, 1984). Acidic regional groundwaters constitute only a small part of the total water input to Lake Tyrrell (other components being surface flow and rainfall) and the regional waters are neutralised within the lake to pH ~ 6.5. Concentrations of conservative trace elements in the inflowing regional groundwaters increase with salinity and reflect the geochemistry of all lithologies upflow from the sampling point. However, most trace elements are likely to be removed by processes including adsorption, mineral formation and organic complexations as a groundwater flows through different lithologies. Consequently, detectable concentrations generally relate only to lithologies close to the sampling point. The trace-element contribution to a groundwater from leaching local rock-forming minerals contributes the trace-element "background" which would probably be exceeded if the aquifer also contained ore minerals.

2. Sampling, analytical methods and leaching studies Groundwater samples were collected in 198 l, 1985 and 1987 from an area which extends 10 km to the west and 50 km to the east of Lake Tyrrell, using boreholes drilled by the Victorian Department of Water Resources (Macumber, 1984). Samples were classified as regional groundwaters, Lake Tyrrell brines or Lake Wahpool/Lake Timboran brines. From the 1985 and 1987 collections, 76 samples (locations in Fig. 1 ) were selected for this study. In addition 15 shallow piezometer holes were sampled which were set out in two lines (PZ and NPZ) across the spring zone at Daytrap Corner (Fig. 2). This particular section of the spring zone is of interest because it is the location of a major radiometric anomaly which has resulted from the interaction between regional groundwaters and lake brines. Samples were collected using a flow-through

135

0

PZ lin,

Fig_ 2. Map of sampling locations at Daytrap Corner.

sampler, with some sites such as the piezometers being resampled during each collection program. Measurements ofpH, Eh, conductivity and reduced Fe were made as soon as each sample was collected. Because overall salinity is involved in the processes whereby trace elements are added to and removed from groundwaters, the major-element compositions were also measured and are reported as ionic strength (I). Samples for analyses of major and trace elements were returned to the laboratory where solids were removed by centrifugation, and the separated solution acidified to pH ~ 2 with high-purity HNO3. In addition, an extra set collected in 1987 were pressure-filtered (0.45-/zm filter) in the field, immediately after collection, and also acidified with high-purity concentrated HNO3. The acidified samples were analysed for major cations, trace elements (other than F) and radionuclides (Dickson and Herczeg, 1992a in this issue). All samples were analysed by inductively coupled plasma-atomic emission spectroscopy (ICP-AES) for major (Ca, Na, Mg) and some minor elements (Zn, A1, total Fe, B, Mn, P, Y, Ag, Ba, St). Analyses using ICP-AES on these highly saline waters required successive

1985 1985 1985 1985 1985 1985 1985 1985 1985 1985 1985 1985 1985 1985 1985 1985 1985 1985 1985 1985 1985 1985 1985 1985 1985 1985 1985 1987 1987 1987 1987

BRK2 BRK3 BRK5 B RK I 4 BRK8002 B R K8 00 5 BRK8006 55 55A 55B 57 58 59B 64 76 82 84A 84D 6032 6518 6519 6520 6026 T23 T24 T25 T26 54A 54B 61 62

5.80 5.46 1.51 4.43 3.46 5.12 1.83 0.90 0.86 2.17 0.35 1.21 0.85 0.92 0.70 1.03 3.27 2,91 0.64 0.01 0.16 1.26 0.44 0.67 0.05 0,70 0.68 1.26 1.52 0.72 0.73

Year 1

Name

6.2 6.9 6.4 5.5 8.1 5.5 4.3 4.2 6.5 5.9 6.9 4.1 3.9 6.8 3.6 3.8 6,5 7.5 3.8 7.7 6.8 6.1 5.0 6.4 9.9 3.8 5.8 3.0 2.0 2,9 3.3

pH 1.3 <0.01 <0.01 <0.01 <0.01 <0.01 1.2 67.1 0.13 0.09 <0.01 18.7 19.2 0.16 1.3 107 0,10 0.10 109 0.71 <0.01 0.12 1.5 0.06 <0.01 0.25 0.10 56 50 98 155

AI 0.20 0.06 0.06 0.08 0.08 0.07 0.06 0.04 0.03 0.05 <0.01 0.08 0.07 0.06 0.03 0.07 0,07 0.09 0.04 <0.01 <0.01 0.07 0.06 0.03 <0,01 0.03 0.04 n.m. n.m. n.m. n.m.

Ag 1.3 1.9 1.0 2.3 2,8 1.6 4.5 2.3 1.4 1.5 2.3 4.4 5.0 4.8 4.3 1.5 2,6 2.2 2.3 0.05 0.12 2.4 3.1 4.3 0,39 4.2 4.4 1.6 1.8 3.5 1.1

B

Cd

0.012 0.08 0.015 0.O5 0.07 0.045 0.010 0.06 0,05 0.006 0.05 0.010 0.030 0.04 0.13 0.250 0.21 0.010 0.06 0.050 0.44 0,050 0.05 0,100 0.080 0.04 0.09 <0.005 0.04 0.060 0.08 0.070 0,06 0,200 0.06 <0.005 0.02 0.070 0.02 <0.005 0.07 <0.005 0.04 <0.005 0.06 0.015 0.03 0,120 0,01 0,012 0.04 0,100 0.03 <0.005 0.24 0.022 0.09 0.005 0.09 <0.005 0.12 0.04

Ba <0,010 <0.010 < 0,010 0.020 <0,010 < 0,010 0.035 0.990 0.030 < 0.010 0,050 0,040 0,150 <0.010 0.175 3.1 < 0,010 < 0,010 3.4 <0.010 0.012 0.023 0.500 < 0.010 0.190 0,140 < 0,010 3.5 0.07 0.66 0.80

Cu

Fe

<0.1 0_76 0.1 1.1 <0.1 209 0.09 0,1 0.13 0.3 0.12 0,2 0.11 0.7 0,34 1.0 0,05 0.2 8.3 <0.1 0.04 0.8 2.5 0.10 4,7 0.04 0.04 0,8 0.58 1.3 2.2 0.8 0.12 0.1 0.11 <0,1 1.7 2.7 1.4 0.3 <0.1 <0.01 <0.1 1.7 0.7 0.09 0.9 0.03 0.9 <0.01 0,3 0.72 0.5 <0.01 0.4 0.67 0.2 <0.01 2.5 68 1.5 27

F

P <0.01 <0.01 0.3 <0.01 <0.01 <0.01 <0.01 0.38 0.51 0.2 1.5 0.74 0.90 1.2 1.0 0,42 <0.01 <0.01 0.49 <0,01 1.1 <0.01 0.81 1.7 0.23 0.73 0.99 n,m, n.m. n.m. n_m.

Mn 0.21 0.21 17.9 0.51 0.71 0.14 0.48 0.34 0.01 5.8 0.2 0.31 0.12 0.33 0.22 0.60 0.28 1.2 3.3 <0.01 0.03 0.65 3.4 0.02 <0.01 0,17 0.13 0,51 0.28 3.1 0.94

Trace-element concentrations (mg 1- ~), pH and I for groundwaters from the vicinity of Lake Tyrrell

TABLE 1

0.020 0.010 0.008 0.060 <0.005 <0,005 0.055 0.350 <0 .0 5 0 0.025 <0.005 0.045 0,230 <0,005 0.155 0.910 <0.005 0.012 38.5 0.040 0,011 0.053 0.285 <0.005 0.011 0.045 <0_005 1.1 0,045 0.22 1.2

Pb 6,3 6.0 11.7 9.2 8.9 8.1 11.3 3.1 4.1 3.9 4.1 18.9 11.6 11.9 9,1 3.7 8.4 17.0 5.7 0.20 1.6 11,1 6.9 9.0 0.3 9.4 8.4 6.7 7.9 8.5 4,4

Sr

<0.0005 0.0005 0,0005 0.0005 0.0005 0.0005 0.0005 <0.0005 <0.0005 <0.0005 <0.0005 0.019 0.022 0,066 0.0001 0.0001 0.0005 0,011 0.0005 0.0001 0.0001 0.0005 0.0001 0.0001 0.0001 0.0001 0.0001 0.0003 0.0053 0.0014 0.0036

U

0.03 <0.05 <0 .0 5 < 0.05 <0.05 <0.05 0.06 0.03 <0 .0 5 <0.05 <0.05 0.15 <0-05 < 0.05 0.10 0.06 <0,05 <0.05 0.11 <0.05 <0.05 < 0.05 0.06 <0.05 <0,05 <0.05 <0.05 0.05 0.05 0.11 0.11

Y

0.21 0.14 0.12 0.23 0.10 0.04 0,16 0.40 0.09 0.08 0.07 0.77 0.23 0.03 0.13 0,59 0.15 0.03 1.1 0.07 0.02 0.06 0.28 0,08 <0.01 0.11 0.09 1.3 0.16 0.56 0.5

Zn

7~

7 t~

t"

k~ O~

1987 1987 1987 1987 1987 1987 1987 1987 1987 1987 1987 1987 1987 1987 1987 1987 1987 1987 1987 1987 1987 1987 1987 1987 1987 1987 1987 1987 1987 1987

0.74 0.87 1,50 0.78 4.11 2.42 2.66 2.95 1,04 0.70 0.76 0.73 0.65 0.72 0.55 0.59 0,44 0.46 1.00 3.76 2.37 4.12 2.00 1.64 2.09 2.05 2.34 0,87 0.88 0.87

n,m. = n o t measured.

63 65 69 74 78 78A 84 84B 86 87 88 90 91 92 96 97 98-C 99 IC5 IC9 MORI BIM4 BIM5 BIM6 83R 83L-C 83B-C SWO-AB SWO-C-C SWO-A-C

3,2 42 5.6 0.98 4,5 1.1 3.8 0.66 3.3 1.7 3,4 12.3 4.5 1.6 4.3 0.87 3.3 47 3.0 59 7.3 0.45 4.8 0.74 2,7 77 3.0 76 2.5 115 3.3 29 5.7 0.37 5.2 0.41 5.5 1.0 4,5 0.80 5,1 0.99 4.7 1.0 3.2 36 3.0 48 5.3 1.3 4.4 l.l 4.6 0.95 3.1 10.6 3.2 11 3.1 18 rum. n.m. n,m. n.m. n.m. n.m. n.m. n.m. n.m. n.m. n.m. n.m. n.m. n.m. n.m. n.m. n.m. n.m. n.m. n.m. n.m. n.m. n.m. n,m. n.m. n.m. n.m. n.m. n.m. n.m.

3.4 4.4 3,5 5.3 2.2 1.3 3.8 3.6 1.4 1.5 2.3 4.5 3.9 4.6 3.2 3.8 1.3 3.2 5.9 3.9 4.2 2.7 1.9 1.7 4.1 2.9 4.2 5.l 5,3 5,6

0,09 0.09 0.07 0.06 0,14 0.15 0,08 0.09 0.13 0.09 0.10 0.16 0.08 0.06 0,05 0.10 0.14 0.27 0.04 0.06 0.06 0.12 0.10 0.06 0.09 0.07 0.08 0.08 0.06 0,06

<0.005 <0.005 <0.005 <0.005 <0,005 <0.016 <0.005 0,23 0,01 <0.005 <0.005 <0.005 <0.005 <0.005 <0.005 <0.005 <0.005 <0.005 <0,005 <0.005 <0.005 <0.005 <0.005 <0.005 <0.005 <0.005 <0.005 <0,005 <0.005 <0.005

0.43 0.07 0.06 0.30 0.020 0,025 0.025 <0.01 0.200 0.285 0.010 0.045 0,300 0.020 1.2 2.4 0.02 0.O4 0.02 <0.01 0,025 0,05 0,04 0.03 0.02 0.01 0.02 0.05 0,01 0.06

2_1 0.5 0.4 <0.1 <0.1 <0.1 <0.l <0.1 1.2 1.1 <0.1 <0.1 2.6 2.3 2.4 2.2 0.4 1,0 0.4 <0.1 0.2 <0,1 <0.1 0.2 <0.1 <0.1 <0.1 6.1 5.0 6,1

0.34 0.03 0.16 34 67 12.4 1.3 0.08 0.53 1,0 33 <0.01 0.52 0.50 3.6 0.59 <0.01 O.05 0.11 22 12.8 42 18,7 7.3 <0.01 <0.01 <0.01 0.28 0.O7 5.0

1.3 0.03 0.13 1.0 0.56 0.18 0.52 0.42 0.29 0.52 1.9 0.67 0.15 0.12 0.44 0.76 0.09 0.34 0.08 0.40 0.30 0.43 0.39 0.29 0.17 0.09 1.1 0.19 0.12 0.13

n.m. n.m. n.m. n.m. n.m. n.m. n,m. n.m. n.m. n.m. n,m. n.m. n.m. n.m, n.m. n.m. n.m. n.m. n.m. n.m. n.m. n.m. n.m. n,m. n.m. n.m. n.m, n.m. n.m. n.m.

0.20 0.010 0.010 0.050 0.50 0.065 0.30 <0.005 12.7 0.24 0,030 0.005 0.13 0_01 0.60 0.60 <0.005 <0.005 <0.005 <0 ,0 0 5 0.008 0.007 0.40 0.055 0.015 0.025 <0.005 0.085 0.025 0.020

4.2 18,7 27 10.7 8.6 5.1 16.5 12.3 2.9 2_5 3.3 12 9.3 11.7 7.4 8.4 5,9 7.1 34 14.3 19,7 9.8 8.9 8.0 19,1 19.5 16.5 12.6 12.7 13,3

0.28 0.02 0.03 0.2 0.14 0.07 0.03

0.03 0.11 0,1 0.04 0.01 0.03 0.1 0.13 0.13 0.01 0.01 0,01 0.01 0.01 0.03 0.05 0.04 0.04 0.03 0,02 0.27 0.29 0.33

0,012 0.036 0.044 0.0009 <0.0005 0.0065 0.0048

<0.0005 0.0030 0.025 <0.0005 <0.0005 0.015 0.0038 0.012 0.0071 0.0036 0.017 0.034 <0.0005 < 0.0005 <0,0005 0.0039 <0.0005 0.0046 0.0023 <0.0005 0.0068 0.0024 0.0036

0,35 0.53 0.06 0.15 0.12 0.19 0.62 0.61 0,02 0.1 0,11 0.06 O, 1 0.15 0.47 0.28 0.06 0.07 0.07 O, 1 I 0.07 0.08

0.04

0.31 0.09 0.1 0.09 0.54 0.37 0.12

-.....I

138 dilutions to read different trace elements. Prior tests were made with waters of different salinities and it was found that dilution to < 104 p p m Na was sufficient to read the traces listed but not all traces required that level of dilution. Checks were also made on optical peak shapes and intensities of backgrounds to determine effects of interferences. C1 and S were analysed by ion chromatography, total carbonate (TCO3) with a thermal conductivity analyser, K by atomic absorption spectrophotometry (AAS) and F with an ion selective electrode using a buffer to compensate for the presence of high levels of dissolved AI in many of the samples. Cd, Cu and Pb were analysed by voltammetry, and U by laser fluorescence spectrometry. Because the near-surface lithologies, Parilla Sand and Blanchetown Clay, provide an immediate source for trace elements in Lake Tyrrell groundwaters, the effect of acid-saline solutions on these two lithologies was investigated to determine what level of trace elements could be released with short leaching times. Samples of Parilla Sand and Blanchetown Clay (20 g) were placed in a 100-ml solution of synthetic regional water and regularly shaken over a period of 5 months. Details of the samples sources, mineralogy and trace-element composition are given by Dickson and Herczeg ( 1992b in this issue). Three such tests were run for each lithology with pH adjusted initially with H2504 to 3.0, 4.0 and 5.0, respectively. The solutions removed from the six leaching experiments were analysed for trace elements by ICP-AES. 3. Results and discussion

Trace-element, pH and salinity (I) data for the regional groundwaters and lake brines are listed in Table 1. The results are summarised by median values of each parameter for each groundwater class in Table 2. Results for waters from spring zones are listed in Table 3. Most samples are acid (median p H = 4 . 2 )

A.M GIBLIN AND B.L. DICKSON

and saline (median I = 1.24). Regions of lower acidity occur between Lake Tyrrell and Lake Wahpool and around the SW edge of Lake Tyrrell. Salinity generally decreases with distance of sample location from the lakes. Resampling of some wells through the period of 1981-1987 confirmed the persistence of groundwater acidity, salinity and trace-element abundances. Table 4 illustrates this persistence for a selection of elements at three sample sites. The major variation evident was a higher pHvalue in the 1982 sample from well 86, together with higher TCO3 and lower A1 contents. An increase in Fe in the 1987 sample from well 62 indicated that samples from previous collections which were not filtered and acidified in the field, may have lost significant Fe before analysis. Otherwise, results for both trace and major elements in the two 1987 sample sets showed close agrement.

3.1. Trace-element content of Lake Tyrrell groundwaters Trace-element contents of groundwaters range from very high (tens of mg 1-1 for base metals) down to undetected (Tables 1 and 3). No regular spatial variation of concentrations is apparent, apart from waters from Parilla Sand aquifers in the west (samples 6023, 82, 61 and 56) and southeast (samples 96 and 97) which have very high Cu and Pb, and a small number of samples from south of the lake which are high in F. By contrast, a group of waters from north of Lake Tyrrell have lower Ca, A1 and F than most other samples. Samples exhibit a broad spectrum of majorelement compositions ranging from those which are similar in composition to seawater, to the almost halite-saturated Lake Tyrrell brines. Median concentrations of B, U and Sr are very similar to seawater concentrations (Table 2 ), but elements such as Cu and Pb, for which acidity increases solubility, are present at much higher concentrations in acidic Lake

TRACE ELEMENTSIN GROUNDWATERS FROM LAKETYRRELL

139

TABLE 2 Mean and median element concentrations (mg 1- ~) in lake brines, regional groundwaters in Padlla Sand aquifers and in seawater Water

n~

Tyrrell 6

Element

mean

pH I F Cu Pb Zn U AI Fe B Ba Sr Y Mn Cd P

5.35 4.84 0.09 0.06 0.23 0.22 0.00026 1.33 18.5 2.00 0.08 8.00 0_03 0.34 0.02 1.5

Wahpool 13 median 5.50 4.78 0.08 0.02 0.04 0.18 0.00028 1.30 0.93 2.05 0.07 8.35 0.02 0.32 0.01 1.5

Parilla 29

seawater

mean

median

mean

median

5.33 2_ 11 0.22 0.03 0.09 0_09 0.0170 0.86 2_83 3.83 0.07 18_9 0.02 0.37 0.03 0.73

5,10 2.09 0.05 0.02 0_01 0.07 0.00484 0.98 0.08 3.90 0.07 18.7 0.02 0.30 0.005 0,73

3.93 0.78 2_02 0.67 2.20 0.32 0_00492 40_9 6.48 3_62 0.07 8.82 0.10 0.79 0.049 0_77

3.60 0.73 1.80 0.18 0.13 0.19 0.00140 18.7 0_51 3.90 0.06 9.00 0.10 0.34 0.01 0.74

8.2 0_7 1.3 0.0005 0_00003 0.002 0.0045 0_002 0.002 4.5 0.002 8.0 0.000001 0.0002 0.00005 0.05

n = number of samples.

Tyrrell groundwaters than in alkaline (pH = 8.2) seawater. Acidity in Lake Tyrrell groundwaters is readily explained by observations of pyrite in reducing layers of the Parilla Sand (Macumber, 1983). Interaction between pyritic sediments and oxidising groundwaters in a contact zone produces acid and SO4 by the reaction: 2FeS2 + 7 0 2 + 2 H 2 0 ~ 2Fe2+ +4SO 2- + 4 H +

(l)

In the dynamic sediment-groundwater system at Lake Tyrrell cycles of sulphide formation during anaerobic burial of S-containing waters and solids, followed by restoration of contact with O2-rich waters, would provide a steady-state source for groundwater acidity effectively driven by cyclic contact with atmospheric 02. Any trace elements, particularly base metals incorporated into sulphides would also cycle in and out of the groundwater with the SO4.

3.1.1. Laboratory investigation of near-surface lithologies as sources for groundwater trace elements. For the solutions removed from the six leaching experiments the average results for each lithology (Table 5 ) demonstrate that both the Parilla Sand and the Blanchetown Clay are feasible sources for Zn, AI, Mn, Ba and Sr, that the Parilla Sand is a better source for Zn, A1 and Fe, and the only source for Y, whilst Cu, B and P are only derived from Blanchetown Clay. The pH of the solutions at the end of each testrun (Table 5) showed that both lithologies were buffering the system pH to values between 3.6 and 4.0, which are close to the average pH of the regional water 3.2. Sources of trace elements in Lake Tyrrell groundwaters Chemical processes involved in leaching of aquifer sediments may be both a short-term, as demonstrated by our leaching experiments, or a longer-term aspect of salt lake generation.

140

A.M GIBLINAND B.L. D1CKSON

TABLE 3 Trace-element concentrations (rag l - ~) from piezometers across the spring zone (a) NPZ Line NPZ I

NPZ 2

NPZ 3

NPZ 5

NPZ 6

pH I AI B Ba FeT Mn U F Zn Pb Cu Sr

3.30 1.15 52 1.40 0.10 1.40 0.16 3_57 0.40 0.11 0_09 0.035 4.90

3.30 1.37 52 1.70 0.09 1.50 0.29 3.23 0_40 0.25 0_19 0_02 7.30

3.60 2.60 13.10 2.30 0.11 33 0.50 2.38 0.05 0.45 1.55 0.07 10.30

n.d. 5.18 88 5.60 0.15 23 1.0 13.16 0_05 0_37 0_33 0.02 18.50

2.85 6.13 81 7.20 0.17 34.0 1.30 12_50 0.05 3.70 1.55 < 0.01 12.50

PZ l

PZ 2

PZ 3

PZ 4

PZ 5

PZ 6

PZ 7

PZ 8

3_40 0_99 79 1.70 0.10 11.7 0.18 3.03 0.40 0.23 0.105 0.03 3.90

3.40 1.07 81 0.10 0.08 0_31 2_40 6.25 0.40 0.13 0.035 0.02 3.90

3.25 1.61 55 1.70 0.08 13_6 0_23 5.71 0_40 0.18 0.025 0.01 5.20

3.33 1.83 55 1.80 0.08 15.5 0.25 6. 0.40 0.19 0.03 0.05 5.50

3.20 2.56 41 2_00 0.10 20 0.32 6.90 0.20 0.26 0_04 0_04 6.50

2.60 3.60 56 2.50 0.12 7.3 0.51 5.56 0.05 0.41 0.042 0.025 7.50

2.82 5.21 26 4.30 0_15 75 1.10 0.01 0.05 0.47 0.045 0.155 8.50

3.05 5_56 3.50 3.10 0.17 33 1.40 0.01 0.05 0.33 0.095 0.17 4.50

(b) PZ Line

pH I AI B Ba Fe Mn U F Zn Pb Cu Sr

PZ 9 6.20 5.80 0.65 4.30 0.17 1.00 1.60 0_01 0.05 0.15 0.025 0.03 7.30

n.d = not determined.

Examples of short-term leaching processes are: ( 1 ) Exchange of major cations and hydrogen ions from waters for trace elements such as Cu and Pb previously precipitated or adsorbed onto clays or Fe-oxides. During formation of sediments in the Murray Basin, palaeoclimatic conditions, and hence groundwater compositions could have favoured trace-element enrichment of sediments, a process reversed by present-day saline waters. (2) Dissolution ofFe-, Al- and possibly Mnoxides by the acidic, chloride-rich groundwaters would be the principal control on concentrations of Al, Fe and Mn.

Some longer-term processes which contribute to the present-day trace-element content of Lake Tyrrell groundwaters are: ( 1 ) Evaporative concentration (associated with local formation of saline waters) of the typically low concentrations of trace elements in average low-salinity groundwaters up to levels now present at Lake Tyrrell. This process would contribute to varying extents to presentday concentrations of B, Sr, Cd, Ag and Y. (2) Dissolution by acid-saline waters of otherwise resistant minerals which may have accumulated in palaeostrand lines in the Parilla Sand. X-ray diffraction of a sample of Par-

141

TRACE ELEMENTS IN GROUNDWATERS FROM LAKETYRRELL TABLE 4

Concentrations (rag 1 - ~) of selected elements in three wells which were sampled three times during a 5-yr period Well

pH

F

Ca

Mg

Na

K

rCO3

62

1987A 1987C 1985C 1982C 1982C

3.3 3.0 3.0 3.9 3.8

1.5 1.5 1.1 1_6 1.6

255 295 86 135 136

1,600 1,650 1,585 1,700 1,700

ll,400 ll,200 ll,000 12,500 12,000

99 99 94 78 78

1.0 1.0 2.0 0.5 0.5

86

1987A 1987C 1985C 1982C

3.3

1.2

97

1,000

16,300

175

4.5 6.7

1.6 0.6

122 255

1,030 1,050

16,000 17,000

180 185

1.0 1.0 10.0 425

1987A 1987C 1985C 1982C 1982C

3.0 3.0 3.7 4.4 4.2

0.4 0.4 0.4 0.6 0.6

275 280 232 300 320

2,400 2,350 2,335 2,400 2,400

22,800 22,100 22,100 23,500 22,000

222 222 220 230 230

1.0 1.0 2_0 20.0 0.5

54

AI

Fe

155 135 224 215 217 47.0 47.0 20_8 0.16

B

Ba

Sr

Zn

27.0 1.5 8.0 7.2 5.0

l_l 1.1 1.1 1.3 1.3

0.12 0.14 0_05 0.08 0.06

4.40 4.40 3.40 4.10 4_20

0_50 0.45 0.52 3.75 5.20

0.53 <0.02 1.6 0.02

1.4 1.3 1.3 1.3

0.13 0.13 0.09 0.16

2.90 2.80 2.70 3.00

0.35 0.03 0.24 0.06

0_67 0.13 0.51 0.12 0.09

1.6 1.6 1.6 1.6 1.5

0.24 0.11 0.05 0.09 0.10

6.70 2.70 6.60 6.5 6.0

1.30 1.10 0_46 4.35 6.50

56.0 57.0 63.1 45_1 9.0

A = field filtered and acidified; C = no field treatment, centrifuged prior to analysis in laboratory.

TABLE 5

Trace-element concentrations (rag 1- 1 ) in laboratory leachates from Parilla Sand and Blanchetown Clay pH

Cu

Zn

AI

Fe

Mn

B

P

Ag

Ba

Cd

Sr

Y

Parilla Sand: 3 4 5

0.01 0.01 0.01

0.41 0.44 0.64

13.6 13.6 13_6

0.04 0.03 0.03

0.50 0.50 0.51

0.08 0.08 0.08

<0.01 <0.01 <0.01

0.002 <0.002 0.005

0.046 0.046 0.046

<0.01 <0.01 0.01

0.28 0.28 0_28

0.10 0.10 0.10

Mean

0.01

0.5

13.6

0.03

0_50

0.08

<0_01

0.002

0.046

0.01

0.28

0_10

0.01 0.01 <0.01

0.32 0_31 0.31

0.33 0.32 0.27

0.09 0.13 0_14

<0.002 <0.002 <0.002

0_030 0.031 0.027

0.005 0.004 <0.001

1.70 1.70 1.70

0.014 0.014 0.014

0_31

0.31

0.12

<0.002

0.029

0.003

1_70

0.014

Blanchetown Clay: 3 4 5

0_18 0.22 0.21

0.17 0.16 0.28

0.91 0.71 0.59

Mean

0.20

0.20

0.74

0.007

ilia Sand identified quartz, muscovite, rutile, ilmenite, maghemite and tourmaline (Dickson and Herczeg, 1992b in this issue). Mineral exploration has identified monazite and xenotime in major quantities within the Parilla Sand south of Lake Tyrrell (Mclntyre, 1989). This process would contribute to varying extents to present-day concentrations in Lake Tyrrell groundwaters of Y, B, F, Mn and Fe.

Whatever the leaching process, each trace element in Lake Tyrrell groundwaters was originally derived from aquifer minerals, and remains in solution until prevailing chemical conditions cause it to be incorporated back into an aquifer mineral. Sources and controls on specific trace-element concentrations in Lake Tyrrell groundwaters include: ( 1 ) Aluminium. Because AI occurs in most

142

silicate minerals in aquifers feeding Lake Tyrrell, it is available for dissolution at all stages of groundwater development. However, as the solubility of AI is pH-dependent, both pH-values of > 10.6 and <6.0 are required for concentrations at the median value ( 1 mg 1-1 ). With one exception (BRK2), AI concentrations in groundwaters in this study in excess of 1 mg 1-1 occur in samples with pH-values of <6.0. (2) Barium and strontium. Association of Ba and Sr in minerals is not reflected in natural water abundances as natural water Ba concentrations, in contrast to Sr, are controlled by the low solubility of the c o m m o n mineral barite (BaSO4) rather than the Ba mineral source. Barite is saturated in all samples in this study (Dickson et al., 1988) keeping Ba concentrations below 0.5 mg 1-~. Abundance of Sr in Lake Tyrrell groundwaters directly correlates with concentrations of Ca in accord with the common Ca-Sr association in both igneous and chemically precipitated minerals. ( 3 ) Boron. B concentrations in Lake Tyrrell groundwaters (median value 2.75 mg 1- ~) reflect contact with rock-forming minerals such as biotite (Edmunds et al., 1984 ) during earlier stages of groundwater evolution, and more recently, equilibration with B incorporated into authigenic clays (Dewis et al., 1972). Although tourmaline, identified in the Parilla Sand (Dickson and Herczeg, 1992b in this issue) is normally resistant to low-temperature leaching, some degree of dissolution could occur during long-term contact with the acid-saline groundwaters. Concentrations of B in the lake brines are generally less than in the regional waters (Table 4), suggesting that as salinity increases some degree of back-incorporation of B into clays may be occurring. (4) Cadmium, copper, silver, lead and zinc. Median concentrations of these chalcophile elements in Lake Tyrrell groundwaters greatly exceed values in seawater (Table 2) and in published compilations of natural water concentrations such as in Rose et al. (1979). De-

A.M GIBLIN AND B.L. DICKSON

tectable concentrations of Ag, and elevated concentrations of Cu and Pb in some Lake Tyrrell groundwaters could have exploration significance, but they can also be explained by non-economic aquifer enrichments, because the host groundwaters are very acid, and may be leaching locally enriched aquifer clays, Feor Mn-oxides, or organic solids. Such enrichments could date from adsorption or coprecipitation of these elements from waters during the original formation of Murray Basin sediments. The absence of Zn concentrations in presentday waters comparable to the occasional very high Cu and Pb values, implies that Lake Tyrrell sediments were deposited under water conditions which while less acid than exists at the present day, were nevertheless more acid than would have allowed Zn sediment enrichment comparable with that of Cu and Pb. In some parts of the study area, Cd concentrations are unusually high considering the unremarkable levels of groundwater Zn, the universal associate of Cd in rock minerals ( Z n / Cd,~ 500:1 ). Local enrichments of Cu, Pb, Cd and Ag may also relate to their incorporation into cycles of formation and subsequent oxidative dissolution of pyrite and other metalenriched sulfides. In most natural waters Ag is only slightly mobile, but in highly saline waters such as those in this study Ag dispersion is increased by formation of chloro-complexes. (5) Fluorine. F concentrations are commonly controlled by fluorite (CaF2) solubility and hence dissolved Ca, which in turn is controlled by equilibria with sulphate and carbonate solutes. Formation of complex ions with the high concentrations of A1 in these waters also stabilises dissolved F so that an equilibrium involving A1, SO4, CO3, pH and Ca controls the actual F concentration in each groundwater sample. (6) Iron and manganese. Fe and Mn are abundant and widespread constituents of Lake Tyrrell aquifer minerals. These include Fe- and Mn-oxides, sulphides and mafic minerals which survive as residual minerals in the Par-

| 43

TRACE ELEMENTS IN GROUNDWATERS FROM LAKE TYRRELL

ilia Sands. Groundwater dissolution and dispersion of Fe is favoured by acidity, reducing conditions and high concentrations of chloride with which it forms stable complexes. These conditions combine to produce the high Fe concentrations in groundwaters in this study (median value ~ 1 mg 1-t ). Although Fe and Mn solution chemistries are similar, Fe would have been more easily precipitated than Mn onto solids during Murray Basin sediment formation, so that present-day groundwaters have less Mn (median value ~0.5 mg 1-1 ) than Fe available for dissolution from aquifer minerals. (7) Phosphorus. Marine sediments in the Murray Basin would include many mineral sources for present-day groundwaters to leach P, but adsorption onto clays and other minerals, and formation of insoluble phosphates (e.g., Ca) restricts development of high concentrations in Lake Tyrrell groundwaters. ( 8 ) Yttrium. Y together with La and the rareearth group of elements (lanthanides) occur in mature sediments at comparable levels to average igneous rocks. Therefore, Murray Basin sediments, in particular the numerous palaeostrand lines in the Parilla Sand (Macumber, 1983), constitute adequate source material for groundwater Y. Lanthanide minerals are principally phosphates and carbonates which are generally poorly soluble in natural waters, but are dissolved to some extent by acid-saline waters, which explains the presence of detectable Y in Lake Tyrrell groundwaters. (9) Uranium. Median value of 4.8/zg l corresponds with the concentration of U in seawater. U in excess of 10/zg 1- I occurs in 14 samples, including a group between Lakes Tyrrell and Wahpool which range from 34 to 66/zg 1- 1. These values are low by comparison with other Australian saline groundwater systems. For example, groundwaters which are leaching felsic igneous rocks carry U in the range 0.1100 /tg 1- 1, depending upon the pH-redox conditions and the availability of U complexing species. Where water-rock contact is ex-

tended such as in arid regions, or U-bearing minerals are present in aquifer rocks, groundwater U abundances are enhanced, > 10 mg U 1- i having been reported in groundwaters in Finland (Asikainen and Kahlos, 1979) and Australia (Giblin, 1987). Clearly aquifers in contact with Lake Tyrrell groundwaters are not particularly enriched in U.

3.3. Factor analysis of trace-element data Factor analysis of the trace-element data distinguishes multivariate trends. These can be related to the processes described in the previous section which control trace elements in Lake Tyrrell groundwaters. Correspondence R and Q mode factor analysis is of particular value as it calculates factors which relate to both samples and data variables allowing their mutual influence on the factor, and hence on the process to be assessed. Applied to the Lake Tyrrell data, computed factors which describe the distribution of trace elements support the concept that a few discrete enrichment processes have been responsible for the presentday trace-element load. Sample locations, grouped according to the influence of each factor on each groundwater composition identifies zones where a particular process is dominant. Factors were extracted from Lake Tyrrell trace-element data using the correspondence analysis procedure described by Grunsky (1986). The data set comprised samples in Table 1 and used 11 variables (Cu, Pb, Zn, total Fe, Mn, F, Y, B, Sr, A1 and ionic strength) for which most samples had concentrations above the detection limit. The first 4 factors obtained accounted for ~ 92% of the overall data variance. Factor 1 is dominated by A1 (38%) and total Fe (50%) and accounts for significant proportions of the total data variability of Cu (63%), Zn (51%), A1 (81%), total Fe (69%) and Mn (82%). This factor is interpreted as relating to acidity of groundwaters and resulting trace-metal enrichments. Factor

144

2 accounts for 92%, 88% and 37% of the total variability of Sr, B and ionic strength, respectively. It is interpreted as relating to evaporative concentration as a process of trace-element enrichment. Factor 3 is dominated by Pb (89%) and accounts for 85%, 31% and 11% of the total variability of Pb, Cu and Mn, respectively. This factor is believed to account for Pb, and to a lesser extent Cu and Mn released by acid groundwaters from previously enriched clays, organic solids or Fe- or Mn-oxides. Factor 4 is dominated by ionic strength ( 55%) and F (36%) and accounts for 82%, 77% and 52% of the total variability of Y, F and ionic strength, respectively. It is interpreted as relating to trace elements released from certain residual minerals by long-term contact with very acid-saline waters. Scatter plots of factor scores I vs. 2 and 2 vs. 4 (Fig. 3) delineate sample groupings which principally separate regional groundwaters from a group of waters which have higher concentrations of B and Sr. This latter group includes regional waters both west and southeast of the lake and the brines located between Lakes Tyrrell and Wahpool. A c o m m o n feature of this group is lower acidity (mean pH = 5.7 ) than other waters in the sample set which fits well with their factor analysis separation from waters in which acidity is the dominant feature. It is not clear whether the lower acidity reflects neutralisation by surface water inflow, or denotes a local aquifer environment in which the source of acidity is absent. A small group related to F and Y directly south of Lake Tyrrell is evident in both plots, and a group of waters at the BRK site with ionic strength values of > 4.4 is evident in the second plot. Factor 1 separates out an F e - M n group which is spread across the study area making it difficult to interpret in terms of particular groundwater sources, but which perhaps relates to locally low redox conditions which favour dissolution of Fe and Mn in their reduced states. Unpub-

A.M GIBLIN A N D B . L D I C K S O N

1.5

(a) • MI

B'S,rl

1.0

¢4 0.5

F I

0

Y

Zn

0 0.0

Mn -0.5 z

Fe -1.0

i,i,,,,

,,i,ii,,,,,ii

-0.5

-1.0

,,,, ,,i, , I , , i i , , , , , i , , , , , ,

0.0

0.5

Factor

,,, I

1.0

1.5

1

(b) 0.7

Y Pb

•,~

"B

0.2

Fe •

ArCW•

Sr •

"

0

-0.3

.z

-0.8 I -1.~

llll,i, - I .0

,rlp~Jll,ljll,,,,lll,,ll,,l,,l,l[i -0.5

0.0

Factor

0.5

I0

. . . . . . . I'.'5

2

Fig. 3. Correspondence analysis of sample concentrations of Cu, Pb, Zn, Fe, Mn, F, Y, B, Sr, AI and ionic strength (I). Factor scores of samples ( × ) and chemical components: (a) factor I vs. factor 2; and (b) factor 2 vs. factor 4.

lished measurements (A.M. Giblin, 1 9 8 5 ) o f reduced Fe made during sample collection support this conclusion.

145

TRACE ELEMENTS IN GROUNDWATERS FROM LAKE TYRRELL

3.4. Trace elements in Lake Tyrrell groundwaters compared with those from other Australian salt lake environments

A perspective on the trace-element concentrations in Lake Tyrrell groundwaters may be gained by comparison with groundwater data from other Australian salt lake environments. Two examples used here are Lake Gilles and Pernatty Lagoon, locations of which are shown in Fig 1. These lakes are both located in regions which have attracted mineral exploration, and in which some local groundwaters contain apparently enhanced trace-element concentrations (A.M. Giblin, unpublished data, 1980). Both lakes are in areas climatically similar to the Lake Tyrrell region, though in completely different geological provinces in southern Australia. Lake Gilles has formed over Early Proterozoic metasediments which form the regional basement of the Gawler Block (Thompson, 1980). Pernatty Lagoon has formed over Late Proterozoic metasediments of the Stuart Shelf (Thompson, 1980). On the basis of the medians and 95% confidence intervals of the three trace-element data sets, F, Cu, Pb, Zn, Al, Mn, B, Ba, Cd, Sr and Y occur at concentrations in Lake Tyrrell groundwaters which are not significantly different to the concentration range in one or both of the other salt lake region groundwaters. This suggests that the common factor among salt-lake groundwater systems which leads to enhanced trace-element concentrations relates more to the leaching power of salt-lake brines during interactions with recent, generally unconsolidated sands and clays, than to any specific, perhaps economically significant local lithologies. Perhaps more related to the local Lake Tyrrell geology is the groundwater concentrations of U, P and Ag which are significantly below the levels in the other two sample sets, and Fe which is significantly higher.

3.5. Trace elements in Lake Tyrrell spring zone waters The waters of the spring zone represent the interface of the low-density regional waters and the high-density brines. Density differences between inflowing regional water and the lake brine cause the regional water to flow up on the heavier brine and thence into the lake around its margins. Consequently, between the lake surface and aquifer base there is a mixing zone which has the potential to be an area of deposition for a wide range of trace elements. Waters in the mixing zone were sampled along the PZ and NPZ piezometer lines set across the lake margin 1 km east of Daytrap Corner (Fig. 2 ) through an area of high surface radioactivity (Dickson and Herczeg, 1992b in this issue). Waters along both lines show a steady increase in salinity from the shore into the lake (Table 3). The PZ line pH-values are generally a little over 3 but dip below 3 at PZ6 and PZT, then increase through PZ8 to 6.2 at PZ9, the sample furthest into the lake. Along the NPZ line pH-values are also a little above 3 in waters from the two sites closest to the shore, rise to 3.7 at NPZ3, then decrease below 3 in waters towards the lake interior. Differences in pH trends between the PZ and NPZ imply that the PZ line passes through a zone of current water-sediment interaction into a more stable zone where some degree of water-sediment equilibrium has been reached, whereas the NPZ line represents only the reactive zone. Samples collected along both piezometer lines, have essentially unchanging Sr and Ba concentrations indicating saturation of these waters with precipitating minerals, probably sulphates as SO4 concentrations increase from the shore to the lake interior. B and Mn concentrations increase with increasing salinity. Along the PZ line groundwater concentrations ofPb, Cu, Zn, Y and Fe are fairly low and change inversely with pH, to peak values where pH drops below 3 and then decrease towards the lake interior. The Pb peak is offset, sug-

146

gesting that increase in complexing anions such as C1 may also be involved in the concentration increase. A1 concentrations in waters on the PZ line are high near the shore, suggesting a more easily leached source, with a general steady decrease into the less acid lake interior. Concentrations of U and F in PZ line samples are low near the shore and not detectable further into the lake. Along the NPZ line pH is the dominant control on concentrations of AI, Y, U, Pb, Cu, Zn and Fe. Superimposed on the overall trend of concentrations varying inversely with pH, is a large Pb anomaly at NPZ3 accompanied by lesser but against the pH trend (pH rises to 3.7 ) anomalies in Cu, Zn and Fe. Variations in F concentrations on the NPZ line are the same as the PZ line. Most notable differences in trace-element concentrations between the two lines are the higher Pb and Zn values on NPZ line, and the higher Cu values on PZ line. These differences probably reflect local scale variations in concentrations in the surface sediments ofCu, Pb and Zn. Minor concentrations of Cu and Zn in surface sediments have been found by Dickson and Herczeg ( 1992b in this issue ) at certain locations along the N P Z line.

3.6. Lake Tyrrell groundwaters as agents of ore deposition Processes have been described to explain trace-element enrichments in Lake Tyrrell groundwaters. The question then arises as to whether these waters are part of a present-day ore-forming system. Central to this possibility are feasible processes whereby the groundwater trace elements could be deposited into sediments. Analyses presented here show that significant trace element losses have occurred during concentration of regional waters to form the present-day brines, making the mixing zone where these waters meet a potential location for ore formation. One method of determining the extent of metal deposition in the Lake Tyrrell region is

A.M GIBLIN AND B.L. DICKSON

to estimate the mass balance of various metals in the groundwater system. This may be done by comparing the amount of metal stored in the two brine bodies, associated with Lakes Wahpool and Timboran and Lake Tyrrell, with the amount added to those lakes by inflow of regional waters. Such a calculation requires either that the time over which the brine has accumulated is known or that one or more elements can be assumed to be conservative and so used as a time indicator. Macumber ( 1983 ) presented a mass-balance calculation for the Lake Tyrrell system, assuming Na is conservative and obtained a time span of 25 kyr for brine accumulation. This time scale agrees with that proposed by Bowler and Teller (1986) whose studies, based on 14C measurements showed lake level fluctuations dating back possibly to 36 kyr ago. The calculation described by Macumber (1983) is extended here to a wide variety of elements. Using data from Macumber (1983 ) the mass of the Lake Tyrrell brine was calculated from a volume of 5.04-l09 m 3 and a specific gravity of 1.16. The volume of brine flowing from the Lakes Timboran-Wahpool is 3' 105 m 3 yr- t and has a specific gravity of 1.12. The flow of regional water from the Parilla Sand aquifer into Lake Tyrrell is estimated at 4.105 m 3 yr-t. This has a specific density of 1.04. Macumber (1983) demonstrated that salt inflow into Lake Tyrrell is almost entirely from the groundwater inflow and is minimal from surface water flows or aerial transport. Thus if the lake brine has been closed to any losses from the brine to regional aquifer, the time of closure, t, may be obtained from the mass-balance relationship: VTPT CT =

Rr tpr Cr+ Rw tpw Cw

(2)

where V=volume; p=density; c=concentration; and subscripts r, W and T refer regional water, Lake Wahpool brine and Lake Tyrrell brine, respectively. Using median concentrations for Na in the three water bodies (Table 2), the period during which the Lake

TRACE ELEMENTSIN GROUNDWATERSFROM LAKETYRRELL

Tyrrell brine has accumulated is 27 kyr. This period was then applied to calculations of mass balances for the other elements. The estimated differences between the calculated inflow of metals to Lake Tyrrell and the present content of the Lake Tyrrell brine (Table 6 ) indicate a considerable loss of Ca and SO4 from the waters which accords with the predominance of gypsum in sediments in and around Lake Tyrrell (Teller et al., 1982). Of the trace metals, only 10% of the Fe has been lost from solution whereas AI shows a 98% loss. Similarly high losses are calculated for base metals (Cu, Pb and Zn), as well as trace elements such as B, Sr, F and U. The high losses are not surprising given that these trace elements do not increase in concentration between the regional waters and the Lake Tyrrell brine (Table 4 ). K occurs in the brines at a higher concentration than can be accounted for by influx of regional waters. Interaction between lake clays and the brines could release K into solution by proTABLE6 Mass balance of Lake Tyrrell system for 27 kyr, based on median concentrations of brines in Lakes Timboran and Tyrrell and of the regional groundwater Element

F Cu Pb Zn Ca Mg Na CI K SO4 U A1 Fe B Ba Sr Y Mn

Mass difference (t)" l 5,000 260 - 53 2,700 1.02-107

0.99" 107 5.83-107 7.46' 107 -95,000

2.89" 107 83,400 266,000 2,600 68,900 1,200 246,000 1,400 4,400

"1 t = 1 metric tonne= 103 kg.

% difference in mass 98 23 - 1.6 75 85 16 10 8.1 -2.2 21 99 98 30 84 72 83 95 63

147

cesses including cation exchange and clay alteration. A number of interacting processes underlie the differences between the element content of inflow waters and lake brines. These include element loss by chemical precipitation into the lake sediments, aeolian removal of evaporated brine from the lake surface and outflow of waters away from the brine formation zone. If the precipitation is within the sediments of the mixing zone between the regional waters and the lake brines, the volume of this zone is large enough for the deposition of the amounts of trace metals calculated here to raise concentrations in the host sands by at most a few ppm. If the deposition takes place only at the surface in the spring zones then measurable concentrations should be observed, Studies to locate such trace-element enrichments are described by Dickson and Herczeg ( 1992b in this issue) but only minor increases in some elements were found. However, studies to age-date the accumulation of radionuclides in the sediments indicated that these are very recent ( < 100 yr). This suggests that the acidity and consequent high concentrations of radionuclides and trace metals in the Lake Tyrrell groundwaters developed following rises in water tables following clearing the land for agriculture (Peck, 1978 ). In this case, for t < 100 yr, accumulations of these elements into the sediments is limited by associated groundwater acidity. 4. Conclusions

Analyses of groundwaters from the vicinity of Lake Tyrrell confirmed that they were generally acid, due to oxidation of abundant pyrite in local sediments, and saline, with salinities ranging from seawater to halite saturation. Some groundwaters were also enriched with a range of trace elements including A1, Cu, Fe, Mn, Ag, Y and to a less extent U. In addition to generalised trace-element enrichment related to evaporative concentration, samples with anomalously high concentrations of trace

148

elements came from discrete locations, indicating local aquifer sources for trace elements. Laboratory leaching of local sediments (Blanchetown Clay and Parilla Sand) with synthetic regional groundwaters confirmed them as adequate trace-element sources. Processes driven by acidity and salinity which would extract trace elements from these materials include cation and H ÷ exchange for ions adsorbed on aquifer solids, and dissolution of trace-element enriched Fe- or Mn-oxides and possibly sulphides. Residual heavy minerals, although insoluble in most groundwaters, may also be dissolved by very acid-saline waters. Although similarities were apparent between major-element compositions of the regional groundwaters and seawater, only B, U and Sr of the trace elements were comparable with seawater concentrations. Differences in concentrations of most other trace elements relate to alkalinity of seawater ( p H = 8 . 2 ) compared with acidity of the regional groundwaters Statistical comparisons indicate that traceelement concentrations of Lake Tyrrell groundwaters are not significantly different to the concentrations in groundwaters near salt lakes elsewhere in Australia. This implies that a common property (probably the leaching power of very saline waters) of salt lakegroundwaters controls trace-element concentrations rather than compositions of local lithologies. Application of R and Q mode correspondence analysis identified four factors which could each be related to one of the processes which control the trace-element composition of samples. Factors were identified which: (1) involved A1, Cu, Fe, Mn and Zn whose concentrations related directly to high acidity; ( 2 ) involved B and Sr (probably related to locally enriched clays); ( 3 ) i n v o l v e d Pb (probably related to heavy metals released from clays, organics and Fe-Mn-oxides); (4) involved F, Y and I (probably related to residual mineral dissolution). Projections of both trace ele-

A.MGIBLINANDB.L.DICKSON

ments and samples onto factor axes separated samples into groups, on the basis of the influence each process had on sample compositions. The first group incorporated samples with trace-element concentrations (AI, Cu, Pb, Zn, Fe and Mn ), concentrations of which were enhanced because of groundwater acidity. Samples in a second group are related by their B and Sr values, and in a third group samples are related by Y and F concentrations and come from a cluster of sites south of Lake Tyrrell. Samples in the fourth group have extremely high salinities. Differences in trace-element concentrations between lake brines and inflowing regional waters suggested that deposition of metals could be occurring within the Lake Tyrrell basin. Mass-balance calculations show that significant tonnages of many trace elements should have accumulated during the past 30 kyr. However, unless these were confined into restricted zones, economic accumulations are unlikely.

Acknowledgments The authors would like to thank all those who contributed to this study. In particular, at CSIRO, John Eames and Lesley Dotter for their perseverance in carrying out the ICP analyses on so many saline samples, Sarah Kulakoff for the AAS and ion chromatography results, Aisling Kenny for patiently preparing and analysing waters and sediments and Keith Scott for interpretation of XRD traces. Thanks are also due to Eric Grunsky for advice and for generously providing computer programs for the correspondence analysis.

References Asikainen, M. and Kahlos, H., 1979. Anomalously high concentrations of uranium, radium and radon in water from drilled wells in the Helsinki region. Geochim Cosmochim. Acta, 4 3 1681-1686. Bowler, J.M. and Teller, J_T., 1982. Quaternary evapo-

TRACE ELEMENTS IN GROUNDWATERS FROM LAKE TYRRELL

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