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Features of acid–saline systems of Southern Australia
Applied Geochemistry 24 (2009) 297–302
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Features of acid–saline systems of Southern Australia Bruce L. Dickson a,*, Angela M. Giblin b a b
Dickson Research Pty Ltd., 47 Amiens St., Gladesville, NSW 2111, Australia Careena Holdings Pty Ltd., 15 Sylvan St., Galston, NSW 2159, Australia
a r t i c l e
i n f o
Article history: Available online 24 November 2008
a b s t r a c t The discovery of layered, SO4-rich sediments on the Meridiani Planum on Mars has focused attention on understanding the formation of acid–saline lakes. Many salt lakes have formed in southern Australia where regional groundwaters are characterized by acidity and high salinity and show features that might be expected in the Meridiani sediments. Many (but not all) of the acid–saline Australian groundwaters are found where underlying Tertiary sediments are sulfide-rich. When waters from the formations come to the surface or interact with oxidised meteoric water, acid groundwaters result. In this paper examples of such waters around Lake Tyrrell, Victoria, and Lake Dey-Dey, South Australia, are reviewed. The acid– saline groundwaters typically have dissolved solids of 30–60 g/L and pH commonly <4.5. Many contain high concentrations of Fe and other metals, leached from local sediments. The combination of acidity and salinity also releases Ra. Around salt-lakes, these acidic waters often emerge at the surface in marginal spring zones where the low density (q 1.04) regional water flows out over the denser (q 1.16) lake brines. In the spring zones examined, large amounts of Fe are commonly precipitated. In a few places minerals of the alunite-jarosite family are formed which can trap many other metals, including Ra. The studied groundwater systems were discovered by U exploration programs following up radiometric anomalies related to this Ra. Evaporation concentrates the lesser soluble salts (gypsum and some halite) on the surface of the lakes. The lake brines contain most of the more soluble salts and form a column within the porous sediments which is held in place by hydrostatic forces around the salt-lake. These brines are near-neutral in pH. These observations are in contrast to the jarosite-bearing aeolianites found on the Meridiani Planum, Mars. These have an almost homogeneous distribution of Fe oxides and jarosite, with little separation of salts with different solubilities (CaSO4 and MgSO4) or differential separation of elements with differing solubility (K, Na, Ti, Cr). Thus, it is considered unlikely that groundwaters or evaporative salt-lake systems, as found on earth, were involved. Instead, these features point to a water-poor system with local alteration and very little mobilization of elements. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Areas underlain by acid–saline groundwaters of Australia are commonly found in the lower rainfall regions of Southern Australia. They can cover large areas but are not continuous, each groundwater being contained within its own drainage system. There are a number of styles of acid–saline groundwater in Australia, if the underlying geology and expression at the surface are taken as defining factors. The most common style is related to oxidation of pyrite contained in unconsolidated marine-lacustrine sediments. This can be expressed at the surface with salt-lakes containing surficial jarosite and large amounts of ironstone (Fe oxide indurated sands) and are the subject of this paper. Another style of acid–saline groundwater develops over granitic terrain (Mann, 1982) but is very similar in surface expression to those found with * Corresponding author. E-mail address: [email protected] (B.L. Dickson). 0883-2927/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.apgeochem.2008.11.011
unconsolidated sediments. A further type which is not considered in this paper and, which also may be granite-related, results in extensive deposits of alunite (NaAl3(SO4)2(OH)6) (McArthur et al., 1989). Much of the data described in this paper was collected 20 years ago although some more recent studies have been made in the vicinity of the study areas. The current interest in re-evaluating these systems derives from the models being proposed for the formation of layered sulfate-altered basaltic aeolianites (sometimes termed sandstones) which have been found on Meridiani Planum, Mars, by the rover Opportunity (Squyres et al., 2006). Features of these Martian materials include jarosite ((K, Na, H3O)(Fe3xAlx) (SO4)2(OH)6) in the outcrops, indications of mixed Ca and Mg sulfates, trace and major elements indicative of altered basalt, abundant small (3 mm) Fe2O3 spherules and only a small vertical variation in Ca/Mg found in the Endeavour crater (Clark et al., 2005; McLennan et al., 2005; Squyres et al., 2006). The proposed model to account for these altered aeolianites involves reworking
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of SO4-cemented sediments that formed within a desiccating playa lake and were, in turn, derived via chemical alteration of basaltic sources (McLennan et al., 2005). The rocks exhibit a variety of textural features which are interpreted to indicate an extended history of diagenetic alteration involving groundwater recharge and evaporative processes (Squyres et al., 2006; McLennan et al., 2005). However, a number of authors suggest that this model cannot account for the chemical features of these aeolianites (McCollom and Hynek, 2005; Kolb et al., 2006; Madden et al., 2004) and that the presence of jarosite, in particular, suggest in situ aqueous alteration of basaltic sands. Examining terrestrial acid–saline groundwaters and their related salt-lakes can assist in evaluating this model by identifying characteristic features of saline systems which do have associated jarosite. In this study two areas of acid–saline groundwaters and their associated discharge to salt-lakes are considered (Fig. 1). Lake Tyrrell is an example of a large salt-lake within an extensive regional basin (the Murray Basin) where the surface vegetation has been almost entirely removed due to the introduction of European agriculture. The subsequent rise in groundwater tables may have led to the current acidity of the groundwaters as evidence, discussed below, suggests some of the surface indications of the acidity are of recent formation. In contrast, Lake Dey-Dey is in an area unaffected by human habitation or exploitation with groundwaters confined to a Tertiary palaeochannel with the area of interest being a regional groundwater discharge zone. 2. Study areas 2.1. Location and climate Lake Tyrrell occurs in the western section of the Murray Basin (Fig. 1). The lake is one of three located in a depression within a basin formed following down-warping along a fault line 4 km to the west of Lake Tyrrell. The current climate of the area is irregular rainfall in the range of 200–300 mm/a during the winter but high annual evaporation above 2000 mm/a. Prior to introduction of European agriculture in the area (extensive wheat growing), the area was covered in mallee scrub, which is dominated by small multi-stemmed shrubs of the genus Eucalyptus. The second area of interest is a discharge zone covering around 160 km2 to the west of Lake Dey-Dey in the Great Victoria Desert in South Australia (Fig. 1), henceforth referred to as DDdz (Dey-Dey discharge zone). The area currently consists of sands of east-trend-
ing sief dunes with undifferentiated lake deposits of clay, silts, thin salt and gypsum crusts. Two large salt-lakes, Lakes Dey-Dey and Maurice are located east and SE of the area of interest but appear unrelated to the DDdz. The current climate is arid with irregular rainfall rarely exceeding 150 mm/a whereas evaporation exceeds 3000–4000 mm/a. Mallee scrub also occurs in this area along with spinifex on the dunes and grasses and mulga scrub (Acacia spp) on the interdune sand plains These have been untouched by European agriculture and the area is essentially uninhabited apart from a small Aboriginal community at Oak Valley some 30 km to the south. 2.2. Geology Lake Tyrrell occurs in the western section of the Murray Basin which is a shallow basin filled with up to 600 m of Tertiary sediments. The uppermost unit of the Basin is the 60 m thick Parilla sand, laid down in a marine transgression during the Late Pliocene. Underlying this is a dense 160 m clay unit, the Geera Clay which acts as a regional aquitard. Thus, lower units of the Murray Basin are not of interest in this study as the shallowest groundwaters are confined to the Parilla Sand unit. In places the Parilla sand is overlain by a clay unit, the Blanchetown clay, which was laid down in a freshwater lake during the Plio-Pleistiocene. This freshwater lake was present in the area for 1.5 Ma BP up till 0.7 Ma BP. The clay unit can be up to 20 m thick. The northern half of Lake Tyrrell has formed within the Blanchetown Clay but, to the south, the lake bed is cut into the Parilla Sand unit. Substantial movement of material from the lake floor took place during arid periods in the past 50 ka leading to the formation of a large dune to the east of the lake. The lake bed contains a complex, 6 m thick succession of clay-gypsum laminates, light massive to dark layered clay, dolomite, and some sand/silt lenses. The palaeoclimatic record of the lake shows considerable changes over 50 ka with possibly fresher water at 30 ka BP, a dry phase 30 ka BP followed by formation of the present saline system (Bowler and Teller, 1986). The DDdz occurs within the Noorina Palaeochannel which trends approximately north–south, draining water from the Musgrove Ranges some 200 km to the north. This palaeochannel formed during the Cretaceous and was filled with lacustrine sediments during various times in the Eocene-Miocene. Underlying the DDdz is around 10–20 m of Quaternary sands, including the Wintrena Formation which has clayey to silty sands and a pedogenic calcrete palaeosol. This is underlain by 5–10 m of the late Tertiary Woldra Formation with ferruginized sands and some interbedded silts and clays. Below this is 10–20 m of the Pidinga Formation which contains black pyritic/carbonaceous coated fine-medium quartz lignitic sands, with white, black and brown puggy clays which occasionally contain leaf imprints. Underlying this are 400 m of Eocene Rodda Bed equivalents which are mainly dolomitic silts and shales (Benbow, 1982). 2.3. Hydrogeology
Fig. 1. Location of Lake Tyrell and Dey-Dey in southern Australia.
The hydrology of Lake Tyrrell has been extensively studied (Macumber, 1992). With a surface area of 160 km2, it is the largest saline groundwater discharge lake in the Murray Basin. Regional water flows generally westwards but the flow pattern is disrupted by the formation of brine bodies under Lakes Timboran and Wahpool and the presence of the Tyrell Divide to the west of Lake Tyrell. Thus regional waters enter Lake Tyrell from the north, south and east of the lake. The cross-section (Fig. 2B) shows a brine body of density 1.16 that is formed under Lake Tyrrell. This body is held in position by hydrostatic forces and contains 98% of the salt (NaCl) in the Lake Tyrell. Lower density brines (q = 1.085) form
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aeochannel is from north to south with a gradient of 0.8 m/km. The two salt-lakes (Dey-Dey and Maurice) appear to be east of the palaeochannel. 2.4. Groundwater geochemistry
Fig. 2. (a) Regional water flows around Lake Tyrrell. Sample locations shown as circles. (b) Third dimension of waters flows for profile across line A – A’ in Fig. 2a (after Macumber, 1992).
under Lakes Timboran and Wahpool but are not stable and move westwards and also discharge into Lake Tyrell. Because of density differences, discharge into Lake Tyrell is along the margin of the lake, known as the spring zone. In winter, the lake contains 50 cm water with a salt content >250 g/L. In summer, the water evaporates, leaving a 7 cm thick halite crust and residual brines with salt concentrations >330 g/L. At springs along the eastern shore, neutral reflux brines emerge with salinities of 100 g/L grading into the hypersaline waters towards the centre of the lake. The hydrogeology of the DDdz is essentially unknown although, further south, the palaeochannel has been studied in efforts to locate water supplies for the Aboriginal community at Oak Valley. Potentiometric measurements indicate that flow through the pal-
The geochemistry of the regional, spring zone and lake waters of Lake Tyrrell have been extensively studied (Long et al., 1992; Giblin and Dickson, 1992). The chemical character of the regional waters is very similar to seawater particularly in Na, Mg, Cl and Br concentrations (Table 1). This reflects the derivation of the salt originally from marine aerosols over long periods of time (Herczeg et al., 2001). Evaporation concentrates the salt and interactions with the aquifer minerals leads to changes in minor (Ca, K) and trace elements. Many of the regional waters have a high acidity. As a result, concentrations of many metals are much higher than in seawater, particularly Al, Fe, Cu, Pb, Zn and Y (Table 1). In contrast to the regional waters, the lake brine is near-neutral and thus contains much lower concentrations of the metals and Ca. The Ca is lost from solution by precipitation on the lake surface as gypsum. Uranium is an exception to the higher concentrations of metals in the acidic waters. It is only slighter higher in the regional waters than the concentration found in seawater and must be lost from solution during precipitation of salts associated with formation of the brine body. The gamma-ray emitting daughter of U, 226Ra, is much higher in both Tyrell waters than in seawater. This reflects a rapid cation-exchange process where Ra in rock minerals is exchanged for Ca and Mg in solution (Dickson and Herczeg, 1992b). The spring zone on the western shoreline of Lake Tyrrell is the surface expression of the transition between the low-density (q = 1.03) regional groundwaters and the higher density (q = 1.16) brines underlying the lake (Fig. 3). Considerable studies have been made of the chemistry of this zone at one place, Daytrap Corner (Giblin and Dickson, 1992). Here the spring zone is about 250 m wide. Acid, metal-bearing groundwaters enter at the head of the zone but at 250 m onto the lake, the waters are near-neutral and highly saline but lower in metals. The zone is also important in that here the regional waters become completely oxidised by contact with air. Drilling has also revealed that in a small area beneath this zone the brines are also acidic. The small extent of acidic brines is an indicator that the acidity of the waters is a relatively recent event. Another indicator of this short period was indicated by levels of 14C in some mud from the spring zone and consideration of 210Pb/226Ra
Table 1 Average values of major and some trace elements in the Lake Tyrrell region, the Lake Tyrell brine and the range of values for DDdz groundwaters in comparison to seawater. Data for seawater from Lide (2005) (nd = not detected). Element pH I Na Mg Cl Br Ca K Al Fe Cu Pb Zn Y F U 226 Ra
Units mole/L mg/L mg/L mg/L mg/L mg/L mg/L lg/L lg/L lg/L lg/L lg/L lg/L mg/L lg/L Bq/L
Tyrrell regional
Tyrrell brine
Range DDdz groundwater
Seawater
3.93 0.78 11,080 1350 19,760 64 470 113 40,900 6300 670 2200 320 30
5.35 4.84 77,160 7100 130,960 347 514 660 1330 7600 60 230 220 100 0.09 0.26 4.5
3.6–5.8 0.92–4.60 12,200–670,000 1890–10,100 20,800–118,000 – 365–990 610–4300 – – nd–6100 300–26,000 50–10,800 – 0.2–4.2 nd–142 0.03–69.7
8.2 0.7 10,500 1300 19,000 65 412 38 2 2 0.25 0.03 4.9 0.013 1.3 3.2 0.0015
4.9 2.49
300
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Fig. 3. Chemical variations in and around the spring zone on the eastern shore of Lake Tyrrell (after Macumber, 1992).
and 228Ra/226Ra variations which together suggest recent (<100 a) formation of the anomaly at Daytrap corner (Dickson and Herczeg, 1992a). The waters contained in the DDdz are described in Giblin and Dickson (1984), Giblin (1987). They are generally acid (pH 3.6– 5.8) and saline (I 0.92–4.6 mol/L) and saturated with gypsum or anhydrite. As at Lake Tyrell, some of these waters carry high concentrations of base metals and Ra compared to seawater (Table 1). Some quite high values of U were recorded in the DDdz waters although, the median value was quite ordinary at 5.5 lg/L. The concentrations of base metals in some samples are particularly high. More complete analyses of Ra isotopes in these waters show some extraordinary activities of the short-lived isotopes 224Ra (t1/2 = 3.8 d, values up to 200 Bq/L) and 223Ra (t1/2 = 11 d, values up to 23 Bq/L) (Dickson et al., 1984). These activities are indicative of high exchange rates between Ra in the water and in the local material. Waters inside the palaeochannel but further south of the radioactive anomaly are reported to be still acidic, carrying high Fe and radioactivity whereas waters outside the channel are saline but neutral (Dodds, 1997).
quartz dunes. Salt lake margins have occasional outcrops of silcretes and ferricretes and gypsum crusts. Samples were collected at depths of around 4 m below the surface and contain minerals kaolinite, feldspar, muscovite, bassanite (2CaSO4.H2O), goethite, hematite, calcite, pyrite and jarosite As at Lake Tyrell, trapped radium within minerals at the surface generate an extensive (100 km2) radioactive anomaly.
2.5. Lake sediments
2FeS2 þ 7O2 þ 2H2 O ! 2Fe2þ þ 4SO4 þ 4Hþ
As described above, the spring zone on the eastern shoreline of Lake Tyrrell is an area of much chemical activity. Studies of the chemistry, mineralogy and radioactivity at Daytrap Corner (Dickson and Herczeg, 1992a; Giblin and Dickson, 1992) found a range of minerals including those expected in sediments (quartz, kaolinite, illite and feldspar) and minerals derived from chemical interactions in the spring zone surface (jarosite, alunite, goethite and akaganeite (Fe3+(O,OH,Cl)). Studies of the spatial distribution of the trace metals found that there was significant deposition of a wide varieties of trace metals but only to a depth of 20–30 cm (Dickson and Herczeg, 1992a). The Daytrap Corner locality is in fact a highly anomalous zone along the margin of Lake Tyrell to which attention was drawn by the deposition of Ra and the consequent generation of elevated radioactivity (Fig. 4). This enhanced radioactivity was noted in an aerial gamma-ray survey and was one reason for the interest in this section of the lake. Elsewhere, on the western side of the lake massive amounts of Fe oxides are deposited in various morphologies (plates, nodules) without trapping of Ra. Examination of Fe oxides from one such area of the lake zone found that jarosite was absent (Dickson and Herczeg, 1992a). Thus, it appeared that jarosite formation was the agent for precipitating many of the metals and 226Ra. Less information is available for the mineralogy at the surface of the DDdz. The surface consists of a veneer of ferruginous aeolian
Fig. 4. Spatial distribution of 226Ra in the surface sediments at Daytrap Corner (after Dickson and Herczeg, 1992a).
3. Discussion The acidification of groundwaters around Lake Tyrell probably results from infiltration of rainwater into the subsurface where the oxygenated waters can react with pyrite in the Parilla Sands. Clearance of the extensive mallee cover has accelerated this infiltration. The reactions can be summarized as
4Fe2þ þ O2 þ 4Hþ ! 4Fe3þ þ 2H2 O Fe3þ þ 3H2 O ! FeðOHÞ3 þ 3Hþ At the DDdz, the model for the generation of the acidic waters is more speculative. The authors believe a geological feature, possibly related to a major ridge in the regional magnetic image of the area, is forcing the waters in the palaeochannel close to surface. This regional water is highly reducing and contains dissolved Fe2+ from contact with pyrite in the Pidinga Formation. This water then reacts with O2 from the air as follows:
4Fe2þ þ O2 þ 4Hþ ! 4Fe3þ þ 2H2 O Fe3þ þ 3H2 O ! FeðOHÞ3 þ 3Hþ Thus, the acid groundwaters result from either oxidation of pyrite in unconsolidated sediments, or Fe2+ in reduced groundwaters. This acidity results in mobilization of many metals including Fe, Al, base metals and Ra. Processes force these waters to the surface leading to formation of gypsum beds when the water concentrates by evaporation. Also formed are Fe-indurated sands which form layers and jarosite which appears to be responsible for trapping base metals and Ra at the water/air interface. The hydrology at Lake Tyrrell shows the importance of considering the 3rd dimension – depth – when considering hydrology of saline waters in porous sediments. Evaporation leads to density changes, resulting in precipitation of the least soluble salt
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(gypsum) at the surface. The more dense, remaining brines will then sink back into the lower-salinity, lighter regional waters, leading to a separation of those elements associated with least soluble salts (Ca) from the most soluble (Na, Mg). Further those gypsum beds will be flat-lying. In addition to gypsum, the laminated sediments at Lake Tyrell contain clays (Bowler and Teller, 1986). The clays probably formed from alteration of minerals carried by surface waters into the lake. If salt-lakes were involved in the formation of the Meridiani sediments, some indications of similar flat-lying gypsum-rich sediments as well as clays may also be expected. To the authors’ knowledge these have not been seen. Instead, the evidence to date shows an almost homogeneous distribution of mixed Ca and Mg sulfates. The conditions under which these two sulfates crystallize from aqueous solutions are so different that they would not form at the same time from the same aqueous solution unless the solution is confined (i.e. the field equivalent of a laboratory beaker). But, the Meridiani sediments appear to be porous (as the proposal of groundwater movement (Squyres et al., 2006) also requires). Thus, the mixture of CaSO4 and MgSO4 alone suggests that saltlakes were not involved. Gypsum and kieserite have been located in separate areas by the OMEGA/Mars Express hyperspectral imager (Gendrin et al., 2005) which is more indicative that the separation of these salts could have occurred through evaporation in salt-lakes elsewhere on Mars. Comparison of trace elements ratios between the unaltered basalts and altered sediments on the Meridiani Planum show minimal variations (McCollom and Hynek, 2005) indicating there has been very little separation of elements with widely differing aqueous chemical properties (e.g. K, Na, Ti, Cr) in this same profile. This lack of separation suggests that the reactions took place in a water-poor system which did not allow the most soluble elements (K, Na, Mg, Br) to be mobilized away from the least soluble (Ti, Cr). This would also account for the crystallization of CaSO4 and MgSO4 within the same sediments. Further, the movement of Fe is extremely limited and Fe concentrations occur only in the small, 0.3 mm spherules. This again is indicative of local alteration with little mobilization in contrast to what would be expected if multiple episodes of groundwater flooding were involved. Such a model of local alteration has been previously suggested by others (McCollom and Hynek, 2005; Kolb et al., 2006; Madden et al., 2004). A basic tenet of groundwater geochemistry is that water is highly reactive and will try to reach chemical equilibrium with the minerals of the confining rocks. If some of those minerals are highly soluble, such as Mg sulfate, then the water would quickly either dissolve out all that is available or reach saturation if there is excess available. Either way, groundwaters in a formation such as that at Meridiani would have high concentrations of Mg sulfate. Such solutions have high viscosity and density. This aspect of proposed ground or surface waters in an area of high Mg sulfates appears to have been neglected when considering that ripple marks and festoons found in a few locations may indicate flowing surface waters (Grotzinger et al., 2005). The finding of jarosite in the Meridiani sediments (Squyres et al., 2006) places a strong restraint on the chemical conditions in the sediments. Jarosite stability requires both an acidic (pH 2– 4) and oxidizing environment (Fig. 5). At Lake Tyrell in the O2-rich terrestrial atmosphere these conditions are only met within 20– 30 cm of the lake surface in a very restricted area. Further jarosite shows little long-term stability so that substantial accumulations of jarosite do not appear to occur in the geological record in contrast to alunite. Thus somehow not only has jarosite formed in the Meridiani sediments but it has also been preserved. Since the occurrence of jarosite implies oxidizing conditions, there is a need for an oxidising agent. There is very little O2 in the atmosphere of Mars; 0.13%
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Fig. 5. Eh vs. pH diagram showing relationships among iron minerals in equilibrium with an evolved S-bearing brine and a CO2-bearing atmosphere at 0 °C and PT = 1.013 bar (after Knoll et al., 2005).
of a 6 mbar atmosphere compared to 8% of a 1000 mbar atmosphere on earth. Thus, another source of a strong oxidising agent is required. This is most probably related to either photochemical processes in the atmosphere, during the time the acidic gases spent in the atmosphere or subsequently on the surface after deposition of H2SO4 and basaltic material. Although oxidation of the SO2 in the volcanic gases is needed to form H2SO4, there is a need to add an oxidising agent to enable oxidation of Fe2+ after dissolution of minerals forming the basalt after acid dissolution. The agent could be another species such as H2O2, activated Fe oxide surfaces, possibly higher oxidation states of S (persulfates) or various radicals (SO4, O, etc). At this time the nature of the oxidising agents on the surface of Mars is the subject of ongoing research (Yen et al., 2000). 4. Conclusions Many (but not all) of the acid–saline Australian groundwaters are found where underlying Tertiary sediments are sulfide-rich. When waters from the resulting formations come to the surface or interact with oxidised meteoric water, acid groundwaters result from oxidation of the sulfide and/or the dissolved Fe(II). The acid– saline groundwaters typically have dissolved solids of 30–60 g/L and pH commonly <4.5. Many contain high concentrations of Fe and other metals, leached from local sediments. The combination of acidity and salinity also releases Ra, a short lived daughter nuclide of U and Th, but not U itself. Around salt-lakes, such as Lake Tyrell in Victoria, these acidic waters often emerge at the surface in marginal spring zones where the low density (q 1.04) regional water flows out over the denser (q 1.16) lake brines. In the spring zones examined, large amounts of Fe are commonly precipitated. In a few places minerals of the alunite-jarosite family are formed which can trap many other metals, including Ra. Evaporation concentrates the least soluble salts (gypsum and some halite) on the surface of the lakes. The lake brines contain most of the soluble salts and form a column within the porous sediments which is held in place by hydrostatic forces around the saltlake. These brines are near-neutral in pH.
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These observations can be applied to evaluating current models for the formation of jarosite-bearing aeolianites found on the Meridiani Planum, Mars. These sediments appear to have an almost homogeneous distribution of Fe oxides, jarosite, little separation of salts with different solubilities (CaSO4 and MgSO4), no differential separation of elements with differing solubility (K, Na, Ti, Cr). Thus, it is suggested that it is unlikely that groundwaters or salt-lakes systems as found on earth were involved. Instead, these features point to a water-poor system with local alteration and very little mobilization of elements. References Benbow, M.C., 1982. Stratigraphy of the Cambrian – Early Ordovician, St John’s Range NE Officier Basin, S.A. SADME Report Book No. 81/103 (unpublished). Bowler, J.M., Teller, J.T., 1986. Quaternary evaporites and hydrological changes, Lake Tyrell, north-west Victoria. Austral. J. Earth Sci. 33, 43–63. Clark, B.C., Morris, R.V., McLennan, S.M., Gellert, R., Jolliff, B., Knoll, A.H., Squyres, S.W., Lowenstein, T.K., Ming, D.W., Tosca, N.J., Yen, A., Christensen, P.R., Gorevan, S., Brückner, J., Calvin, W., Dreibus, G., Farrand, W., Klingelhoefer, G., Waenke, H., Zipfel, J.F., Bell III, J.F., Grotzinger, J., McSween, H.Y., Rieder, R., 2005. Chemistry and mineralogy of outcrops at Meridiani Planum. Earth Planet. Sci. Lett. 240, 73–94. Dickson, B.L., Herczeg, A.L., 1992a. Deposition of trace elements and radionuclides in the spring zone, Lake Tyrrell, Victoria. Chem. Geol. 96, 151–166. Dickson, B.L., Herczeg, A.L., 1992b. Naturally-occurring radionuclides in acid–saline groundwaters around Lake Tyrrell, Victoria, Australia. Chem. Geol. 96, 95–114. Dickson, B.L., Meakins, R.L., Giblin, A.M., 1984. Radium isotopic measurements in the search for Uranium in palaeodrainage channels. J. Geochem. Explor. 22, 363–365. Dodds, S., 1997. An assessment of the groundwater resources of the Lake Maurice Area, Northwest South Australia. For The Oak Valley Community, Maralinga Tjarutja Aboriginal Lands. Report Book 97/56. Gendrin, A., Mangold, N., Bibring, J., Langevin, Y., Gondet, B., Poulet, F., Bonello, G., Quantin, C., Mustard, J., Arvidson, R., LeMouélic, S., 2005. Sulfates in Martian layered terrains: the OMEGA/Mars express view. Science 307, 1587–1591. Giblin, A.M., 1987. Applications of groundwater geochemistry to genetic theories and exploration methods for early Tertiary sediment-hosted Uranium deposits in Australia. Uranium 3, 165–186. Giblin, A.M., Dickson, B.L., 1984. Hydrogeochemical interpretation of apparent anomalies in base metals and radium in groundwater near Lake Maurice in the Great Victoria Desert. J. Geochem. Explor. 22, 361–362. Giblin, A.M., Dickson, B.L., 1992. Source, distribution and economic significance of trace elements in groundwaters from Lake Tyrrell, Victoria, Australia. Chem. Geol. 96, 133–149.
Grotzinger, J.P., Arvidson, R.E., Bell III, J.F., Calvin, W., Clark, B.C., Fike, D.A., Golombek, M., Greeley, R., Haldemann, A., Herkenhoff, K.E., Jolliff, B.L., Knoll, A.H., Malin, M., McLennan, S.M., Parker, T., Soderblom, L., Sohl-Dickstein, J.N., Squyres, S.W., Tosca, N.J., Watters, W.A., 2005. Stratigraphy and sedimentology of a dry to wet eolian depositional system, Burns formation, Meridiani Planum, Mars. Earth Planet. Sci. Lett. 240, 11–72. Herczeg, A.L., Dogramaci, S.S., Leaney, F.W.J., 2001. Origin of dissolved salts in a large, semi-arid groundwater system: Murray Basin, Australia. Mar. Freshwater Res. 52, 41–52. Kolb, C., Martín-Fernández, J.A., Abart, R., Lammer, H., 2006. The chemical variability at the surface of Mars: implication for sediment formation and rock weathering. Icarus 183, 10–29. Knoll, A.H., Carr, M., Clark, B., Des Marais, D.J., Farmer, J.D., Fischer, W.W., Grotzinger, J.P., Hayes, A., McLennan, S.M., Malin, M., Schröder, C., Squyres, S., Tosca, N.J., Wdowiak, T., 2005. An astrobiological perspective on Meridiani Planum. Earth Planet. Sci. Lett. 240, 179–189. Lide, D.R. (Ed.), 2005. CRC Handbook of Chemistry and Physics, 85th ed. Section 14, Geophysics, Astronomy, and Acoustics; Abundance of Elements in the Earth’s Crust and in the Sea. CRC Press, Boca Raton, Fl, pp. 12–18. Long, D.T., Fegan, N.E., Lyons, W.B., Hines, M.E., Giblin, A.M., 1992. Geochemistry of acid brines: Lake Tyrrell, Victoria, Australia. Chem. Geol. 96, 33–52. Macumber, P.G., 1992. Hydrological processes in the Tyrrell Basin, southeastern Australia. Chem. Geol. 96, 1–18. Madden, M.E.E., Bodnar, R.J., Rimstidt, R.D., 2004. Jarosite as an indicator of waterlimited chemical weathering on Mars. Nature 431, 821–823. Mann, A.W., 1982. Hydrogeochemistry and weathering on the Yilgarn Block, Western Australia – ferrolysis and heavy metals in continental brines. Geochim. Cosmochim. Acta 47, 181–190. McArthur, J.M., Turner, J., Lyons, W.B., Thirlwall, M.F., 1989. Salt sources and waterrock interaction on the Yilgarn Block, Australia: isotopic and major element tracers. Appl. Geochem. 4, 79–92. McCollom, T.M., Hynek, B.M., 2005. A volcanic environment for bedrock diagenesis at Meridiani Planum on Mars. Nature 438, 1131–1139. McLennan, S.M., Bell III, J.F., Calvin, W.M., Christensen, P.R., Clark, B.C., de Souza, P.A., Farmer, J., Farrand, W.H., Fike, D.A., Gellert, R., Ghosh, A., Glotch, T.D., Grotzinger, J.P., Hahn, B., Herkenhoff, K.E., Hurowitz, J.A., Johnson, J.R., Johnson, S.S., Jolliff, B., Klingelhöfer, G., Knoll, A.H., Learner, Z., Malin, M.C., McSween, H.Y., Pocock, J., Ruff, S.W., Soderblom, L.A., Squyres, S.W., Tosca, N.J., Watters, W.A., Wyatt, M.B., Yen, A., 2005. Provenance and diagenesis of the evaporitebearing Burns formation, Meridiani Planum, Mars. Earth Planet. Sci. Lett. 240, 95–121. Squyres, S.W., Knoll, A.H., Arvidson, R.E., Clark, B.C., Grotzinger, J.P., Jolliff, B.L., McLennan, S.M., Tosca, N., Bell III, J.F., Calvin, W.M., Farrand, W.H., Glotch, T.D., Golombek, M.P., Herkenhoff, K.E., Johnson, J.R., Klingelhöfer, G., McSween, H.Y., Yen, A.S., 2006. Two years at Meridiani Planum: results from the opportunity rover. Science 313, 1403–1407. Yen, A.S., Kim, S.S., Hecht, M.H., Frant, M.S., Murray, B., 2000. Evidence that the reactivity of the Martian soil is due to superoxide ions. Science 289, 1909–1912.
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Features of acid–saline systems of Southern Australia Bruce L. Dickson a,*, Angela M. Giblin b a b
Dickson Research Pty Ltd., 47 Amiens St., Gladesville, NSW 2111, Australia Careena Holdings Pty Ltd., 15 Sylvan St., Galston, NSW 2159, Australia
a r t i c l e
i n f o
Article history: Available online 24 November 2008
a b s t r a c t The discovery of layered, SO4-rich sediments on the Meridiani Planum on Mars has focused attention on understanding the formation of acid–saline lakes. Many salt lakes have formed in southern Australia where regional groundwaters are characterized by acidity and high salinity and show features that might be expected in the Meridiani sediments. Many (but not all) of the acid–saline Australian groundwaters are found where underlying Tertiary sediments are sulfide-rich. When waters from the formations come to the surface or interact with oxidised meteoric water, acid groundwaters result. In this paper examples of such waters around Lake Tyrrell, Victoria, and Lake Dey-Dey, South Australia, are reviewed. The acid– saline groundwaters typically have dissolved solids of 30–60 g/L and pH commonly <4.5. Many contain high concentrations of Fe and other metals, leached from local sediments. The combination of acidity and salinity also releases Ra. Around salt-lakes, these acidic waters often emerge at the surface in marginal spring zones where the low density (q 1.04) regional water flows out over the denser (q 1.16) lake brines. In the spring zones examined, large amounts of Fe are commonly precipitated. In a few places minerals of the alunite-jarosite family are formed which can trap many other metals, including Ra. The studied groundwater systems were discovered by U exploration programs following up radiometric anomalies related to this Ra. Evaporation concentrates the lesser soluble salts (gypsum and some halite) on the surface of the lakes. The lake brines contain most of the more soluble salts and form a column within the porous sediments which is held in place by hydrostatic forces around the salt-lake. These brines are near-neutral in pH. These observations are in contrast to the jarosite-bearing aeolianites found on the Meridiani Planum, Mars. These have an almost homogeneous distribution of Fe oxides and jarosite, with little separation of salts with different solubilities (CaSO4 and MgSO4) or differential separation of elements with differing solubility (K, Na, Ti, Cr). Thus, it is considered unlikely that groundwaters or evaporative salt-lake systems, as found on earth, were involved. Instead, these features point to a water-poor system with local alteration and very little mobilization of elements. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Areas underlain by acid–saline groundwaters of Australia are commonly found in the lower rainfall regions of Southern Australia. They can cover large areas but are not continuous, each groundwater being contained within its own drainage system. There are a number of styles of acid–saline groundwater in Australia, if the underlying geology and expression at the surface are taken as defining factors. The most common style is related to oxidation of pyrite contained in unconsolidated marine-lacustrine sediments. This can be expressed at the surface with salt-lakes containing surficial jarosite and large amounts of ironstone (Fe oxide indurated sands) and are the subject of this paper. Another style of acid–saline groundwater develops over granitic terrain (Mann, 1982) but is very similar in surface expression to those found with * Corresponding author. E-mail address: [email protected] (B.L. Dickson). 0883-2927/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.apgeochem.2008.11.011
unconsolidated sediments. A further type which is not considered in this paper and, which also may be granite-related, results in extensive deposits of alunite (NaAl3(SO4)2(OH)6) (McArthur et al., 1989). Much of the data described in this paper was collected 20 years ago although some more recent studies have been made in the vicinity of the study areas. The current interest in re-evaluating these systems derives from the models being proposed for the formation of layered sulfate-altered basaltic aeolianites (sometimes termed sandstones) which have been found on Meridiani Planum, Mars, by the rover Opportunity (Squyres et al., 2006). Features of these Martian materials include jarosite ((K, Na, H3O)(Fe3xAlx) (SO4)2(OH)6) in the outcrops, indications of mixed Ca and Mg sulfates, trace and major elements indicative of altered basalt, abundant small (3 mm) Fe2O3 spherules and only a small vertical variation in Ca/Mg found in the Endeavour crater (Clark et al., 2005; McLennan et al., 2005; Squyres et al., 2006). The proposed model to account for these altered aeolianites involves reworking
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of SO4-cemented sediments that formed within a desiccating playa lake and were, in turn, derived via chemical alteration of basaltic sources (McLennan et al., 2005). The rocks exhibit a variety of textural features which are interpreted to indicate an extended history of diagenetic alteration involving groundwater recharge and evaporative processes (Squyres et al., 2006; McLennan et al., 2005). However, a number of authors suggest that this model cannot account for the chemical features of these aeolianites (McCollom and Hynek, 2005; Kolb et al., 2006; Madden et al., 2004) and that the presence of jarosite, in particular, suggest in situ aqueous alteration of basaltic sands. Examining terrestrial acid–saline groundwaters and their related salt-lakes can assist in evaluating this model by identifying characteristic features of saline systems which do have associated jarosite. In this study two areas of acid–saline groundwaters and their associated discharge to salt-lakes are considered (Fig. 1). Lake Tyrrell is an example of a large salt-lake within an extensive regional basin (the Murray Basin) where the surface vegetation has been almost entirely removed due to the introduction of European agriculture. The subsequent rise in groundwater tables may have led to the current acidity of the groundwaters as evidence, discussed below, suggests some of the surface indications of the acidity are of recent formation. In contrast, Lake Dey-Dey is in an area unaffected by human habitation or exploitation with groundwaters confined to a Tertiary palaeochannel with the area of interest being a regional groundwater discharge zone. 2. Study areas 2.1. Location and climate Lake Tyrrell occurs in the western section of the Murray Basin (Fig. 1). The lake is one of three located in a depression within a basin formed following down-warping along a fault line 4 km to the west of Lake Tyrrell. The current climate of the area is irregular rainfall in the range of 200–300 mm/a during the winter but high annual evaporation above 2000 mm/a. Prior to introduction of European agriculture in the area (extensive wheat growing), the area was covered in mallee scrub, which is dominated by small multi-stemmed shrubs of the genus Eucalyptus. The second area of interest is a discharge zone covering around 160 km2 to the west of Lake Dey-Dey in the Great Victoria Desert in South Australia (Fig. 1), henceforth referred to as DDdz (Dey-Dey discharge zone). The area currently consists of sands of east-trend-
ing sief dunes with undifferentiated lake deposits of clay, silts, thin salt and gypsum crusts. Two large salt-lakes, Lakes Dey-Dey and Maurice are located east and SE of the area of interest but appear unrelated to the DDdz. The current climate is arid with irregular rainfall rarely exceeding 150 mm/a whereas evaporation exceeds 3000–4000 mm/a. Mallee scrub also occurs in this area along with spinifex on the dunes and grasses and mulga scrub (Acacia spp) on the interdune sand plains These have been untouched by European agriculture and the area is essentially uninhabited apart from a small Aboriginal community at Oak Valley some 30 km to the south. 2.2. Geology Lake Tyrrell occurs in the western section of the Murray Basin which is a shallow basin filled with up to 600 m of Tertiary sediments. The uppermost unit of the Basin is the 60 m thick Parilla sand, laid down in a marine transgression during the Late Pliocene. Underlying this is a dense 160 m clay unit, the Geera Clay which acts as a regional aquitard. Thus, lower units of the Murray Basin are not of interest in this study as the shallowest groundwaters are confined to the Parilla Sand unit. In places the Parilla sand is overlain by a clay unit, the Blanchetown clay, which was laid down in a freshwater lake during the Plio-Pleistiocene. This freshwater lake was present in the area for 1.5 Ma BP up till 0.7 Ma BP. The clay unit can be up to 20 m thick. The northern half of Lake Tyrrell has formed within the Blanchetown Clay but, to the south, the lake bed is cut into the Parilla Sand unit. Substantial movement of material from the lake floor took place during arid periods in the past 50 ka leading to the formation of a large dune to the east of the lake. The lake bed contains a complex, 6 m thick succession of clay-gypsum laminates, light massive to dark layered clay, dolomite, and some sand/silt lenses. The palaeoclimatic record of the lake shows considerable changes over 50 ka with possibly fresher water at 30 ka BP, a dry phase 30 ka BP followed by formation of the present saline system (Bowler and Teller, 1986). The DDdz occurs within the Noorina Palaeochannel which trends approximately north–south, draining water from the Musgrove Ranges some 200 km to the north. This palaeochannel formed during the Cretaceous and was filled with lacustrine sediments during various times in the Eocene-Miocene. Underlying the DDdz is around 10–20 m of Quaternary sands, including the Wintrena Formation which has clayey to silty sands and a pedogenic calcrete palaeosol. This is underlain by 5–10 m of the late Tertiary Woldra Formation with ferruginized sands and some interbedded silts and clays. Below this is 10–20 m of the Pidinga Formation which contains black pyritic/carbonaceous coated fine-medium quartz lignitic sands, with white, black and brown puggy clays which occasionally contain leaf imprints. Underlying this are 400 m of Eocene Rodda Bed equivalents which are mainly dolomitic silts and shales (Benbow, 1982). 2.3. Hydrogeology
Fig. 1. Location of Lake Tyrell and Dey-Dey in southern Australia.
The hydrology of Lake Tyrrell has been extensively studied (Macumber, 1992). With a surface area of 160 km2, it is the largest saline groundwater discharge lake in the Murray Basin. Regional water flows generally westwards but the flow pattern is disrupted by the formation of brine bodies under Lakes Timboran and Wahpool and the presence of the Tyrell Divide to the west of Lake Tyrell. Thus regional waters enter Lake Tyrell from the north, south and east of the lake. The cross-section (Fig. 2B) shows a brine body of density 1.16 that is formed under Lake Tyrrell. This body is held in position by hydrostatic forces and contains 98% of the salt (NaCl) in the Lake Tyrell. Lower density brines (q = 1.085) form
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299
aeochannel is from north to south with a gradient of 0.8 m/km. The two salt-lakes (Dey-Dey and Maurice) appear to be east of the palaeochannel. 2.4. Groundwater geochemistry
Fig. 2. (a) Regional water flows around Lake Tyrrell. Sample locations shown as circles. (b) Third dimension of waters flows for profile across line A – A’ in Fig. 2a (after Macumber, 1992).
under Lakes Timboran and Wahpool but are not stable and move westwards and also discharge into Lake Tyrell. Because of density differences, discharge into Lake Tyrell is along the margin of the lake, known as the spring zone. In winter, the lake contains 50 cm water with a salt content >250 g/L. In summer, the water evaporates, leaving a 7 cm thick halite crust and residual brines with salt concentrations >330 g/L. At springs along the eastern shore, neutral reflux brines emerge with salinities of 100 g/L grading into the hypersaline waters towards the centre of the lake. The hydrogeology of the DDdz is essentially unknown although, further south, the palaeochannel has been studied in efforts to locate water supplies for the Aboriginal community at Oak Valley. Potentiometric measurements indicate that flow through the pal-
The geochemistry of the regional, spring zone and lake waters of Lake Tyrrell have been extensively studied (Long et al., 1992; Giblin and Dickson, 1992). The chemical character of the regional waters is very similar to seawater particularly in Na, Mg, Cl and Br concentrations (Table 1). This reflects the derivation of the salt originally from marine aerosols over long periods of time (Herczeg et al., 2001). Evaporation concentrates the salt and interactions with the aquifer minerals leads to changes in minor (Ca, K) and trace elements. Many of the regional waters have a high acidity. As a result, concentrations of many metals are much higher than in seawater, particularly Al, Fe, Cu, Pb, Zn and Y (Table 1). In contrast to the regional waters, the lake brine is near-neutral and thus contains much lower concentrations of the metals and Ca. The Ca is lost from solution by precipitation on the lake surface as gypsum. Uranium is an exception to the higher concentrations of metals in the acidic waters. It is only slighter higher in the regional waters than the concentration found in seawater and must be lost from solution during precipitation of salts associated with formation of the brine body. The gamma-ray emitting daughter of U, 226Ra, is much higher in both Tyrell waters than in seawater. This reflects a rapid cation-exchange process where Ra in rock minerals is exchanged for Ca and Mg in solution (Dickson and Herczeg, 1992b). The spring zone on the western shoreline of Lake Tyrrell is the surface expression of the transition between the low-density (q = 1.03) regional groundwaters and the higher density (q = 1.16) brines underlying the lake (Fig. 3). Considerable studies have been made of the chemistry of this zone at one place, Daytrap Corner (Giblin and Dickson, 1992). Here the spring zone is about 250 m wide. Acid, metal-bearing groundwaters enter at the head of the zone but at 250 m onto the lake, the waters are near-neutral and highly saline but lower in metals. The zone is also important in that here the regional waters become completely oxidised by contact with air. Drilling has also revealed that in a small area beneath this zone the brines are also acidic. The small extent of acidic brines is an indicator that the acidity of the waters is a relatively recent event. Another indicator of this short period was indicated by levels of 14C in some mud from the spring zone and consideration of 210Pb/226Ra
Table 1 Average values of major and some trace elements in the Lake Tyrrell region, the Lake Tyrell brine and the range of values for DDdz groundwaters in comparison to seawater. Data for seawater from Lide (2005) (nd = not detected). Element pH I Na Mg Cl Br Ca K Al Fe Cu Pb Zn Y F U 226 Ra
Units mole/L mg/L mg/L mg/L mg/L mg/L mg/L lg/L lg/L lg/L lg/L lg/L lg/L mg/L lg/L Bq/L
Tyrrell regional
Tyrrell brine
Range DDdz groundwater
Seawater
3.93 0.78 11,080 1350 19,760 64 470 113 40,900 6300 670 2200 320 30
5.35 4.84 77,160 7100 130,960 347 514 660 1330 7600 60 230 220 100 0.09 0.26 4.5
3.6–5.8 0.92–4.60 12,200–670,000 1890–10,100 20,800–118,000 – 365–990 610–4300 – – nd–6100 300–26,000 50–10,800 – 0.2–4.2 nd–142 0.03–69.7
8.2 0.7 10,500 1300 19,000 65 412 38 2 2 0.25 0.03 4.9 0.013 1.3 3.2 0.0015
4.9 2.49
300
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Fig. 3. Chemical variations in and around the spring zone on the eastern shore of Lake Tyrrell (after Macumber, 1992).
and 228Ra/226Ra variations which together suggest recent (<100 a) formation of the anomaly at Daytrap corner (Dickson and Herczeg, 1992a). The waters contained in the DDdz are described in Giblin and Dickson (1984), Giblin (1987). They are generally acid (pH 3.6– 5.8) and saline (I 0.92–4.6 mol/L) and saturated with gypsum or anhydrite. As at Lake Tyrell, some of these waters carry high concentrations of base metals and Ra compared to seawater (Table 1). Some quite high values of U were recorded in the DDdz waters although, the median value was quite ordinary at 5.5 lg/L. The concentrations of base metals in some samples are particularly high. More complete analyses of Ra isotopes in these waters show some extraordinary activities of the short-lived isotopes 224Ra (t1/2 = 3.8 d, values up to 200 Bq/L) and 223Ra (t1/2 = 11 d, values up to 23 Bq/L) (Dickson et al., 1984). These activities are indicative of high exchange rates between Ra in the water and in the local material. Waters inside the palaeochannel but further south of the radioactive anomaly are reported to be still acidic, carrying high Fe and radioactivity whereas waters outside the channel are saline but neutral (Dodds, 1997).
quartz dunes. Salt lake margins have occasional outcrops of silcretes and ferricretes and gypsum crusts. Samples were collected at depths of around 4 m below the surface and contain minerals kaolinite, feldspar, muscovite, bassanite (2CaSO4.H2O), goethite, hematite, calcite, pyrite and jarosite As at Lake Tyrell, trapped radium within minerals at the surface generate an extensive (100 km2) radioactive anomaly.
2.5. Lake sediments
2FeS2 þ 7O2 þ 2H2 O ! 2Fe2þ þ 4SO4 þ 4Hþ
As described above, the spring zone on the eastern shoreline of Lake Tyrrell is an area of much chemical activity. Studies of the chemistry, mineralogy and radioactivity at Daytrap Corner (Dickson and Herczeg, 1992a; Giblin and Dickson, 1992) found a range of minerals including those expected in sediments (quartz, kaolinite, illite and feldspar) and minerals derived from chemical interactions in the spring zone surface (jarosite, alunite, goethite and akaganeite (Fe3+(O,OH,Cl)). Studies of the spatial distribution of the trace metals found that there was significant deposition of a wide varieties of trace metals but only to a depth of 20–30 cm (Dickson and Herczeg, 1992a). The Daytrap Corner locality is in fact a highly anomalous zone along the margin of Lake Tyrell to which attention was drawn by the deposition of Ra and the consequent generation of elevated radioactivity (Fig. 4). This enhanced radioactivity was noted in an aerial gamma-ray survey and was one reason for the interest in this section of the lake. Elsewhere, on the western side of the lake massive amounts of Fe oxides are deposited in various morphologies (plates, nodules) without trapping of Ra. Examination of Fe oxides from one such area of the lake zone found that jarosite was absent (Dickson and Herczeg, 1992a). Thus, it appeared that jarosite formation was the agent for precipitating many of the metals and 226Ra. Less information is available for the mineralogy at the surface of the DDdz. The surface consists of a veneer of ferruginous aeolian
Fig. 4. Spatial distribution of 226Ra in the surface sediments at Daytrap Corner (after Dickson and Herczeg, 1992a).
3. Discussion The acidification of groundwaters around Lake Tyrell probably results from infiltration of rainwater into the subsurface where the oxygenated waters can react with pyrite in the Parilla Sands. Clearance of the extensive mallee cover has accelerated this infiltration. The reactions can be summarized as
4Fe2þ þ O2 þ 4Hþ ! 4Fe3þ þ 2H2 O Fe3þ þ 3H2 O ! FeðOHÞ3 þ 3Hþ At the DDdz, the model for the generation of the acidic waters is more speculative. The authors believe a geological feature, possibly related to a major ridge in the regional magnetic image of the area, is forcing the waters in the palaeochannel close to surface. This regional water is highly reducing and contains dissolved Fe2+ from contact with pyrite in the Pidinga Formation. This water then reacts with O2 from the air as follows:
4Fe2þ þ O2 þ 4Hþ ! 4Fe3þ þ 2H2 O Fe3þ þ 3H2 O ! FeðOHÞ3 þ 3Hþ Thus, the acid groundwaters result from either oxidation of pyrite in unconsolidated sediments, or Fe2+ in reduced groundwaters. This acidity results in mobilization of many metals including Fe, Al, base metals and Ra. Processes force these waters to the surface leading to formation of gypsum beds when the water concentrates by evaporation. Also formed are Fe-indurated sands which form layers and jarosite which appears to be responsible for trapping base metals and Ra at the water/air interface. The hydrology at Lake Tyrrell shows the importance of considering the 3rd dimension – depth – when considering hydrology of saline waters in porous sediments. Evaporation leads to density changes, resulting in precipitation of the least soluble salt
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(gypsum) at the surface. The more dense, remaining brines will then sink back into the lower-salinity, lighter regional waters, leading to a separation of those elements associated with least soluble salts (Ca) from the most soluble (Na, Mg). Further those gypsum beds will be flat-lying. In addition to gypsum, the laminated sediments at Lake Tyrell contain clays (Bowler and Teller, 1986). The clays probably formed from alteration of minerals carried by surface waters into the lake. If salt-lakes were involved in the formation of the Meridiani sediments, some indications of similar flat-lying gypsum-rich sediments as well as clays may also be expected. To the authors’ knowledge these have not been seen. Instead, the evidence to date shows an almost homogeneous distribution of mixed Ca and Mg sulfates. The conditions under which these two sulfates crystallize from aqueous solutions are so different that they would not form at the same time from the same aqueous solution unless the solution is confined (i.e. the field equivalent of a laboratory beaker). But, the Meridiani sediments appear to be porous (as the proposal of groundwater movement (Squyres et al., 2006) also requires). Thus, the mixture of CaSO4 and MgSO4 alone suggests that saltlakes were not involved. Gypsum and kieserite have been located in separate areas by the OMEGA/Mars Express hyperspectral imager (Gendrin et al., 2005) which is more indicative that the separation of these salts could have occurred through evaporation in salt-lakes elsewhere on Mars. Comparison of trace elements ratios between the unaltered basalts and altered sediments on the Meridiani Planum show minimal variations (McCollom and Hynek, 2005) indicating there has been very little separation of elements with widely differing aqueous chemical properties (e.g. K, Na, Ti, Cr) in this same profile. This lack of separation suggests that the reactions took place in a water-poor system which did not allow the most soluble elements (K, Na, Mg, Br) to be mobilized away from the least soluble (Ti, Cr). This would also account for the crystallization of CaSO4 and MgSO4 within the same sediments. Further, the movement of Fe is extremely limited and Fe concentrations occur only in the small, 0.3 mm spherules. This again is indicative of local alteration with little mobilization in contrast to what would be expected if multiple episodes of groundwater flooding were involved. Such a model of local alteration has been previously suggested by others (McCollom and Hynek, 2005; Kolb et al., 2006; Madden et al., 2004). A basic tenet of groundwater geochemistry is that water is highly reactive and will try to reach chemical equilibrium with the minerals of the confining rocks. If some of those minerals are highly soluble, such as Mg sulfate, then the water would quickly either dissolve out all that is available or reach saturation if there is excess available. Either way, groundwaters in a formation such as that at Meridiani would have high concentrations of Mg sulfate. Such solutions have high viscosity and density. This aspect of proposed ground or surface waters in an area of high Mg sulfates appears to have been neglected when considering that ripple marks and festoons found in a few locations may indicate flowing surface waters (Grotzinger et al., 2005). The finding of jarosite in the Meridiani sediments (Squyres et al., 2006) places a strong restraint on the chemical conditions in the sediments. Jarosite stability requires both an acidic (pH 2– 4) and oxidizing environment (Fig. 5). At Lake Tyrell in the O2-rich terrestrial atmosphere these conditions are only met within 20– 30 cm of the lake surface in a very restricted area. Further jarosite shows little long-term stability so that substantial accumulations of jarosite do not appear to occur in the geological record in contrast to alunite. Thus somehow not only has jarosite formed in the Meridiani sediments but it has also been preserved. Since the occurrence of jarosite implies oxidizing conditions, there is a need for an oxidising agent. There is very little O2 in the atmosphere of Mars; 0.13%
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Fig. 5. Eh vs. pH diagram showing relationships among iron minerals in equilibrium with an evolved S-bearing brine and a CO2-bearing atmosphere at 0 °C and PT = 1.013 bar (after Knoll et al., 2005).
of a 6 mbar atmosphere compared to 8% of a 1000 mbar atmosphere on earth. Thus, another source of a strong oxidising agent is required. This is most probably related to either photochemical processes in the atmosphere, during the time the acidic gases spent in the atmosphere or subsequently on the surface after deposition of H2SO4 and basaltic material. Although oxidation of the SO2 in the volcanic gases is needed to form H2SO4, there is a need to add an oxidising agent to enable oxidation of Fe2+ after dissolution of minerals forming the basalt after acid dissolution. The agent could be another species such as H2O2, activated Fe oxide surfaces, possibly higher oxidation states of S (persulfates) or various radicals (SO4, O, etc). At this time the nature of the oxidising agents on the surface of Mars is the subject of ongoing research (Yen et al., 2000). 4. Conclusions Many (but not all) of the acid–saline Australian groundwaters are found where underlying Tertiary sediments are sulfide-rich. When waters from the resulting formations come to the surface or interact with oxidised meteoric water, acid groundwaters result from oxidation of the sulfide and/or the dissolved Fe(II). The acid– saline groundwaters typically have dissolved solids of 30–60 g/L and pH commonly <4.5. Many contain high concentrations of Fe and other metals, leached from local sediments. The combination of acidity and salinity also releases Ra, a short lived daughter nuclide of U and Th, but not U itself. Around salt-lakes, such as Lake Tyrell in Victoria, these acidic waters often emerge at the surface in marginal spring zones where the low density (q 1.04) regional water flows out over the denser (q 1.16) lake brines. In the spring zones examined, large amounts of Fe are commonly precipitated. In a few places minerals of the alunite-jarosite family are formed which can trap many other metals, including Ra. Evaporation concentrates the least soluble salts (gypsum and some halite) on the surface of the lakes. The lake brines contain most of the soluble salts and form a column within the porous sediments which is held in place by hydrostatic forces around the saltlake. These brines are near-neutral in pH.
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