Hydrogeochemistry and geothermometry of thermal groundwaters from the Birdsville Track Ridge, Great Artesian Basin, South Australia

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1 Geothermics 33 (2004) Hydrogeochemistry and geothermometry of thermal groundwaters from the Birdsville Track Ridge, Great Artesian Basin, South Australia Mark C. Pirlo Department of Earth and Planetary Sciences, ARC GEMOC National Key Centre, Macquarie University, NSW 2109, Australia Received 29 May 2003; accepted 12 July 2004 Abstract The hydrogeochemistry of thermal artesian groundwaters flowing from 12 stock bores along the Birdsville Track Ridge in northeast South Australia has been examined. The Na-HCO3-Cl type groundwater composition has provided a basis for the application of chemical geothermometers to estimating aquifer temperatures and has allowed comparisons of various silica and cation geothermometers. Aquifer and bore penetration depth decrease between Birdsville and Marree from 1220 m to 170 m. A corresponding decrease in measured emergence temperature is also observed (94 31 C). Chalcedony geotemperature estimates ranging from 110 Cto41 C between Birdsville and Marree are considered the most accurate of the various geothermometers tested. Log(Q/K) versus T diagrams have also been evaluated to determine likely aquifer mineral assemblages and reservoir temperatures ( C). The Birdsville Track Ridge acts as a conduit for low salinity groundwater (total dissolved solids range from 640 mg/l at Birdsville to 1900 mg/l at Marree) in the Great Artesian Basin. Old, slowly migrating groundwater from the deeper basins on either side of the ridge is characterised by higher emergence temperature and higher total dissolved solids. This old groundwater is inferred to have mixed with the younger, lower temperature, lower salinity groundwater that is migrating relatively rapidly along the Birdsville Track Ridge axis. Since the various geothermometers provide distinct types of information, evidence for the mixing is provided by the interpretation of the different temperature estimates. Silica equilibration temperatures reflect aquifer temperatures along the ridge axis, whereas cation geotemperatures partly preserve higher temperatures from the deeper, Present address: CSIRO Exploration and Mining, P.O. Box 1130, Bentley 6102, WA, Australia. Fax: address: mark.pirlo@csiro.au /$ CNR. Published by Elsevier Ltd. All rights reserved. doi: /j.geothermics

2 744 M.C. Pirlo / Geothermics 33 (2004) flanking basins. This study demonstrates how the thermal regime and hydrodynamics of an area can be characterised using a sparse dataset, thus representing a novel and effective methodology for regions anomalous to this central Australian example CNR. Published by Elsevier Ltd. All rights reserved. Keywords: Hydrogeochemistry; Fluid mineral equilibria; Chemical geothermometers; Thermal water; Great Artesian Basin; Australia 1. Introduction The purpose of this study is to examine the hydrogeochemistry and geothermometry of groundwaters sourced from bores along the Birdsville Track Ridge (BTR) in a section of the Great Artesian Basin (GAB) in South Australia. The section represents a transect that starts in a relatively deep part of the basin and traverses to near where the main aquifer is exposed at the surface (Fig. 1). All groundwater samples were assumed to be sourced from the same hydrogeological unit, and they are all located along one of the inferred groundwater flow lines (Habermehl, 1980; Radke et al., 2000). Cased artesian stock bores provide sample sources that help eliminate mixing of groundwaters from separate aquifers and eliminate continued water rock interaction during ascent of the fluid from the aquifer to the surface. These problems have plagued application of geothermometers to natural sampling sites such as springs. Radke et al. (2000) have very briefly described geotemperature estimates (i.e., temperatures estimated on the basis of aqueous geothermometers) on GAB groundwaters obtained by calculating chalcedony saturation indices using the SOLMINEQ geochemical modeling code of Kharaka et al. (1988). However, no results for the region of the GAB around the Birdsville Track are reported. Much of the previous research on the hydrogeology and hydrochemistry of the GAB has been conducted by Habermehl (1980, 1983, 1986, 1996), Herczeg et al. (1988, 1991), Torgersen et al. (1991, 1992) and, more recently, by Radke et al. (2000). Armstrong (1990) has briefly examined the hydrogeology of the northeast desert region of South Australia, including that along the BTR. The work by Radke et al. (2000) represents perhaps the best and most recent overall discussion pertaining to the hydrochemistry and hydrodynamics in the most significant aquifers of the GAB. Much of this previous work has focused on the GAB on a much larger scale than the BTR. This study takes a more detailed look at the hydrogeochemistry in the region, focussing in particular on applications of aqueous geothermometers. 2. Location The GAB is the largest artesian basin system in Australia, underlying approximately km 2 (about 22%) of the Australian continent (Shepherd, 1978; Habermehl, 1980). The extent of the GAB system is shown in Fig. 1. The GAB hosts a confined groundwater aquifer system, consisting of extensive multilayer aquifers (Habermehl, 1980; Radke et al., 2000). A series of flowing artesian bores,

3 M.C. Pirlo / Geothermics 33 (2004) Fig. 1. (a) Map showing extent and location of the Great Artesian Basin (GAB) of Australia. (b) Location of the Birdsville Track Ridge (BTR) and bores considered in this study. The first number represents the sum of the straight-line distances between adjacent bores, with Birdsville being at 0 (km). The second number represents bore depth (m). Inferred groundwater ages are indicated using information from Radke et al. (2000). (c) Location of the BTR, exposed aquifer recharge beds and inferred groundwater flow lines after Habermehl (1980), heat flow provinces and related thermal parameters given below each province after Sass and Lachenbruch (1979); see text.

4 746 M.C. Pirlo / Geothermics 33 (2004) drilled for stock watering, are located beside the Birdsville Track between Birdsville and Marree. Stanley (1977) has described the development and maintenance of such bores. Twelve of these bores are considered in this study. The BTR represents an approximately north south tending sub-surface (basement) ridge in the Proterozoic basement that separates the Permian Pedirka Basin in the west from the Permian Cooper Basin in the east (Radke et al., 2000). The Birdsville Track is a dirt road linking Birdsville in Queensland in the north, and Marree in South Australia in the south. The Birdsville Track fortuitously follows the BTR; Fig. 1 shows the names, depths and relative locations of the artesian bores that have been considered. The Frome Creek bore in this figure is 5.8 km to the northeast of Marree. 3. Geology The GAB is a sedimentary basin that has been formed through alternating periods of marine and non-marine deposition (Shepherd, 1978). The artesian bores along the BTR considered in this study are predominantly drilled into, and thus produce groundwater from, the Jurassic Cadna-owie-Hooray and Algebuckina Sandstone units (Radke et al., 2000). Radke et al. (2000) describes the Cadna-owie-Hooray Aquifer as being a composite of several aquifers that are hydraulically connected over basement highs whilst merging in the western Eromanga Basin. The Algebuckina Sandstone in the western Eromanga Basin is described as an equivalent to the Cadna-owie-Hooray Aquifer, and it represents the main artesian aquifer in the South Australian portion of the GAB. The Algebuckina Sandstone is a fluvio-lacustrine sequence of Late Jurassic age that forms part of the J aquifer of the GAB described by Habermehl (1980). It has been described variously (Shepherd, 1978; Habermehl, 1980; Forbes, 1986; Radke et al., 2000) as consisting predominantly of a white, fine-to-medium grained kaolinitic quartzose sandstone, grading upwards into a coarse-grained conglomeritic sandstone composed of well-rounded quartz pebbles. It exhibits cross-bedding and plant fossils, and is thought to represent a continental, mainly fluvial (braided stream) environment. The Cadna-owie Formation is a fine, medium-brown quartz sandstone that may be calcareous (calcitic), feldspathic, silty and micaceous in parts. Pebbles and boulders of quartzite may also be present, and pyrite is commonly present as large concretions. The Cadna-owie Formation is indicative of shallow marine to fluvial conditions, and can be very difficult to distinguish from the Algebuckina Sandstone that lies beneath it. The Algebuckina Sandstone is reported to thicken towards the centre of the basin. The maximum known thickness reported by Radke et al. (2000) is 800 m and average thickness is 25 m. The Algebuckina Sandstone is a confined aquifer, with the Birkhead Formation (shale) and the Winton/Oodnadatta/Bulldog Shale formations below and above it, respectively (Shepherd, 1978). The porosity of the Algebuckina Sandstone, as with other aquifers in the GAB, is found to decrease with increasing depth of burial. Porosity of 23.5% at 1300 m and 15% at 1650 m has been indicated in a figure by Radke et al. (2000), based on unpublished data from other sources. He and Conaghan (1994) have examined the porosity and permeability of Jurassic and Lower Cretaceous Cadna-owie-Hooray sandstones in the NSW portion of the GAB. These findings are assumed to be applicable to the South Australian portions of the basin

5 M.C. Pirlo / Geothermics 33 (2004) that have similar lithology. The mineralogy of the Cadna-owie Formation and the Hooray Sandstone in the NSW portion of the GAB is summarised in Table Hydrogeology and groundwater ages Stable isotope studies involving deuterium and 18 O indicate that the groundwater in the GAB is derived exclusively from rainfall (Airey et al., 1979; Calf and Habermehl, 1984; Radke et al., 2000). Recharge in the GAB takes place mainly in eastern Queensland and New South Wales. There is, however, minor subsidiary intake along the western rim of the basin in the Northern Territory and Queensland (Habermehl, 1980). The extent of the outcropping recharge beds and hydrogeologically interpreted groundwater flow lines are displayed in Fig. 1c. Problems and assumptions involved with the 36 Cl groundwater dating technique (Bentley et al., 1986; Torgersen et al., 1991; Mazor, 1995; Radke et al., 2000) mean that 36 Cl can provide only limited information on groundwater flow rates and maximum groundwater ages. However, attempts to date groundwater in the GAB system with the 36 Cl method (Torgersen et al., 1991) suggest general agreement between hydrologic model ages derived from examination of potentiometric surfaces with application of Darcy s law, and the radio-isotopic model ages. The results indicate that the western portion of the basin, being geochemically distinct from the other parts of the basin, behaves as a separate flow system with its own recharge areas and geochemical evolution. The oldest groundwater in the basin tends to occur in the central region where the aquifer is at the greatest depth, rather than at the natural discharge points (mound springs) (Torgersen et al., 1991). Radke et al. (2000) have examined the ratio between 36 Cl and total Cl ( 36 Cl/ Cl) in groundwaters for nine transects within the GAB. This ratio has been used to offset problems caused by the variability experienced across the basin in the salt content of rainfall, near-surface evapotranspiration and mixing with surface salts. A progressive increase in alkalinity down potentiometric gradient tends to correspond to a decrease in the 36 Cl/ Cl ratio on some transects, particularly one that includes part of the BTR between Birdsville and Mungeranie. Along this transect, groundwater velocities have been estimated at approximately 2.4 m/a around the recharge areas in northeast Queensland. The age of groundwater at Birdsville has been estimated at 300 ka from recharge. The velocities decrease significantly down potentiometric gradient to around 1.25 m/a, where the groundwater age reaches approximately 500 ka. This occurs at Mungeranie, the upper limit of direct 36 Cl age estimates because of the dominance of in situ 36 Cl production. Further along the transect, as the groundwater becomes older still, the flow rate decreases further, to less than 1 m/a. The groundwaters that discharge from the Coward Springs mound spring complex (Fig. 1c) have been inferred to date between 1 Ma and 2.2 Ma (Radke et al., 2000). This in turn suggests average groundwater flow velocities of between 0.1 m/a and 0.2 m/a from Goyder s Lagoon (Fig. 1b) towards the south, since a significant change in the age versus alkalinity relation is observed from Goyder s Lagoon to Mungeranie relative to that from Birdsville to Goyder s Lagoon. Estimated flow rates are reduced if 36 Cl decay is considered as the only control on 36 Cl concentration. The commonly held view by most researchers is that the GAB exists as a series of interconnected aquifers that operate as a flow-through system. This view has been based

6 Table 1 Summary of mineralogy of the Cadna-owie Formation and Hooray Sandstone (after He and Conaghan, 1994) Formation N Quartz (%) Feldspar (%) Lithics (%) Authigenic minerals Micas (%) Cadna-owie Formation (volcano-lithic sandstone) Hooray Sandstone (predom-inantly quartzose) ± ± ± 17.8 Glauconite, pyrite, carbonates, smectite, zeolite, kaolinite Plagioclase (mainly andesine and oligoclase) K-feldspar (mainly sanidine and anorthoclase) 78.7% 21.3% ± ± ± 7.1 Chlorite, kaolinite, quartz over-growths 15.5 Includes degraded varieties of muscovite and biotite Includes degraded varieties of muscovite and biotite Plagioclase (mainly andesine and oligoclase) 58.2% 41.8% K-feldspar (mainly sanidine and anorthoclase) M.C. Pirlo / Geothermics 33 (2004) Mean proportions in QFL (%) ± 1S.D. The ranges have also been included, as have the mean percentages of micas. N = number of samples.

7 M.C. Pirlo / Geothermics 33 (2004) largely on evidence from 36 Cl and 4 He isotopic studies. An alternative view that questions the interconnectivity, recharge, discharge and groundwater flow in aquifers is called the stagnant aquifer concept (Mazor, 1995). Bethke et al. (1999) have recently re-examined the distribution of 4 He across the GAB with respect to currently assumed groundwater flow patterns and groundwater ages interpreted from 36 Cl. Mathematical simulations of groundwater flow in three dimensions can explain the 4 He distribution across the basin without the requirement of large or uneven helium fluxes from the crust, or the need for stagnant aquifer conditions (Bethke et al., 1999). For the purpose of this study, the GAB will therefore be considered as a flow-through system. 5. Geothermal regime Heat flow in the GAB has been estimated to range from 70 mw/m 2 to 80 mw/m 2 based on interpolation between traditional heat flow measurements and geothermal gradients (Sass and Lachenbruch, 1979; Cull, 1982; Cull and Conley, 1983). Virtually no heat flow measurements have been made within the actual confines of the GAB or the vicinity of the BTR reported in Cull (1982, 1991) or Cull and Denham (1979). However, Middleton (1979) has reported heat flow and geothermal gradient data for three gas fields in the area around Moomba/Big Lake/Toolachee in eastern South Australia, approximately 160 km east of the BTR at Mungeranie. The average geothermal gradient and heat flow for the gas fields is 52 C/km and 109 mw/m 2, respectively. These estimates were obtained through corrected bottom-hole temperature measurements. Heyl and Thomas (1964) considered approximately 3000 deep, flowing water bores in the GAB to estimate geothermal gradients. The measured surface water temperature from flowing bores was assumed to relate to the temperature of the sub-surface aquifer or to the depth of the bore, after a cooling correction and a near-surface temperature correction had been applied. Unfortunately a map was not provided with the publication nor has one been published by the authors since, making interpretation of the results difficult. The estimated gradients ranged from 30 C/km to 185 C/km. In a similar study in which bottom-hole temperatures of water bores were considered, Pollack and Horsfall (1979) reported that geothermal gradients measured in the GAB range from 15 C/km to C/km with a mean of 48 C/km. They consider geothermal gradients measured within the GAB to be higher than those measured outside it, to the extent that 75% of measured geothermal gradients in the GAB exceed the worldwide average of 33 C/km. No statistics are, however, provided to support this claim. Cull and Conley (1983) considered the apparent extreme variation in GAB geothermal gradients. They rejected bores that were within 100 km of Jurassic outcrops because they were assumed to tap steeply dipping aquifers that can dramatically affect measured temperatures. Narrow discharge zones can cause enhanced groundwater flow velocities and convective instability. Given the decreasing flow velocities of groundwater along the BTR to the south of Goyder s Lagoon, it is not clear whether this phenomenon is also prevalent in the BTR region of the GAB. Pollack and Horsfall (1979) produced a map of geothermal gradients in the GAB by compiling bottom-hole temperature data for 940 water bores that had been geophysically logged at least one year after drilling. The map covered seventy-four 1:250,000

8 750 M.C. Pirlo / Geothermics 33 (2004) map sheets; where more than one measurement is available for a map sheet, the available data were averaged. The map does not display any geothermal gradients for South Australia, despite the fact that the latter represents a significant part of the GAB. A geothermal gradient of 66.4 C per km (based on only one logged bore) is presented for Birdsville, placing it near the upper limit of geothermal gradients in the GAB. This tendency is, however, supported by other mean geothermal gradient estimates in adjacent map sheets. Data in Radke et al. (2000) show a clear and systematic decrease in groundwater temperature (at the surface) in the Jurassic aquifer from Birdsville (in the C range) through Kopperamanna (70 80 C) to near the edge of the basin at Frome Creek (30 40 C). Fig. 1c summarises the heat flow data for the two heat flow provinces in the GAB (the Central Shield and Eastern Australia) and includes the total heat flow (q 0 ), the reduced or mantle component of heat flow (q r ) and the crustal heat flow (A 0 D) components for each province. Total heat flow measured at the Earth s surface is the sum of the crustal and reduced heat flow. Crustal heat flow relates to the thickness of the crust (D) and the heat production within the crust (A 0 ). 6. Data sources Groundwater geochemistry data considered in this study come from three separate sources. Ten groundwater samples were collected from continuously flowing artesian bores located next to the Birdsville Track (Fig. 1b) as a part of this study in September In-field analysis of sample temperature, ph and conductivity/salinity was carried out at the well-head. A 500 ml sample from each bore was collected and analysed at Macquarie University and the CSIRO Division of Exploration and Mining (North Ryde) for major and trace elements, using procedures comparable to those described by Giblin (1994). Geochemical data for 21 samples (from 10 bores) along the BTR were provided by the Primary Industries and Resources of South Australia (PIRSA). Data for 34 samples (from 12 bores) were obtained from the CD-ROM database accompanying the report by Radke et al. (2000) that was prepared for the Bureau of Rural Sciences (BRS). Sample collection, preservation and measurement procedures are described in this report; they are considered comparable for the major elements/species considered in this work. Bores and data are lacking for the Simpson Desert and Pedirka Basins on either side of the BTR. A total of 12 sample localities have been considered along the BTR. All localities represent flowing artesian bores that are immediately adjacent to the Birdsville Track between Birdsville in the north and Marree in the south (Fig. 1b). 7. Geothermometers 7.1. Traditional geothermometers A series of geothermometers that have been developed over the past 30 years has been applied to estimate aquifer temperature. These traditional type geothermometers are based

9 M.C. Pirlo / Geothermics 33 (2004) Table 2 Traditional geothermometers considered in this study Name and reference Code Equation Quartz Truesdell (1976) S1 T SiO2 = log m Quartz Fournier (1977) S2 T SiO2 = 5.19 log m Quartz Rimstidt (1997) S3 T SiO2 = Quartz Verma and Santoyo (1997) Chalcedony Fournier (1977) S5 T SiO2 = Na/K Truesdell (1976) NK1 T Na/K = Na/K Fournier (1979) NK2 T Na/K = Na/K Arnorsson (1983) NK3 T Na/K = Na/K Giggenbach (1988) NK4 T Na/K = Na/K Verma and Santoyo (1997) NK5 T Na/K = Na/K/Ca Fournier and Truesdell (1973) NKC1 T Na/K/Ca = Na/Li Kharaka et al. (1982) NL1 T Na/Li = Na/Li Verma and Santoyo (1997) NL2 T Na/Li = Mg/Li Kharaka and Mariner (1989) ML1 T Li/Mg = K/Mg Giggenbach (1988) KM1 T K/Mg = S4 [( 1107 log m ) ] T SiO2 = ( m) + (( )m 2 ) + ( log m) log m log(na/k) log(na/k) log(na/k) log(na/k) log(na/k) log(na/k) + β[log( Ca/Na) ] log(na/li) log(na/li) log(li/ Mg) log(k/ Mg) β = 4/3 if T < 100 C, β = 1/3 if T > 100 C. All units are in mg/kg except for NL2 where molal units are used for Na and Li concentrations. on a relatively simple temperature-dependent fluid mineral equilibrium that occurs at depth in the aquifer. Information relating to this equilibration temperature is preserved in the chemical composition of the groundwater during its ascent to the surface. Table 2 lists the various traditional geothermometers considered in this study.

10 752 M.C. Pirlo / Geothermics 33 (2004) Multi-component geothermometers Equilibration between an assemblage of minerals constituting an aquifer and a groundwater is temperature-dependent. Formulating a numerical model to calculate the saturation state (log Q/K) of various likely aquifer minerals at a specified temperature in a particular groundwater allows us to gauge the degree of fluid mineral equilibria. By re-calculating over a range of temperatures, we can deduce saturation indices as a function of temperature. If the saturation index of a group of minerals converges to zero at a particular temperature, then the temperature most likely corresponds to the equilibration temperature, and the minerals represent the aquifer mineral assemblage (Crerar, 1975; Reed, 1982; Reed and Spycher, 1984; D Amore et al., 1987; Tole et al., 1993). This approach is called multi-component geothermometry. Pang and Reed (1998) refined the technique further, to overcome some problems associated with erroneous or absent Al analyses in geothermal waters. Their approach represents a modified log(q/k) versus T graph that eliminates problems with water analyses lacking Al or with erroneous Al data. The application of the technique in sedimentary basins is relatively rare, and the application to Australian examples has not been reported previously. Recently, Gemici and Tarcan (2002) and Xilai et al. (2002) have, however, applied a multi-component geothermometry technique in the Simav field in Anatolia, Turkey and the Guanzhong basin of China, respectively. 8. Results 8.1. Groundwater composition Geotemperature estimates obtained by modelling the equilibrium state of a groundwater with respect to temperature generally require analyses of major elements and silica; specific trace elements such as Al can, however, also be included. Table 3 shows the mean concentration of Na, K, Ca, Mg, Cl, SO 4, HCO 3, Al, ph and SiO 2 for sample localities along the BTR considered in this study. In most of these groundwaters, silica is a more dominant species than Ca, Mg and K. Groundwater compositions vary at particular localities, because the analyses come from different sources (sampling expeditions and laboratories), or because different sampling and storage techniques were used, or as a result of natural variations over time. Majorelement concentration plots, where concentration is expressed in milli-equivalents per litre (Schoeller Plots), are drawn for each sample locality (Fig. 2a l). Every available sample within a locality has been presented on a single plot. Also plotted is the mean of all PIRSA and BRS data from that locality. The hydrogeochemical data collected as part of this study fit well within the range of samples from other sources and laboratories, suggesting that sample collection, sample preservation and chemical analysis techniques are comparable. A number of apparently anomalous samples can be identified from the plots in Fig. 2. Most of the apparent anomalies exist in the lower ranges of the logarithmic equivalent major-element scale (for Ca, Mg and SO 4 ), and thus appear more pronounced (and deceptive).

11 M.C. Pirlo / Geothermics 33 (2004) Fig. 2. (a l) Major element concentration plots (Schoeller Plots) for groundwaters from each of the 12 sample localities along the BTR. Data collected during this study are indicated by black squares. The PIRSA and BRS data are indicated by grey crosses. The mean of the PIRSA and BRS data is indicated by black circles.

12 754 M.C. Pirlo / Geothermics 33 (2004) Table 3 Mean concentration of selected components in groundwater at 12 localities along the BTR Sample name Ca (mg/l) Mg (mg/l) Na (mg/l) K (mg/l) Cl (mg/l) SO 4 (mg/l) HCO 3 (mg/l) ph F (mg/l) SiO 2 (mg/l) Frome Creek n.d. Lake Harry Clayton 4 < Dulkaninna 4 < Cannawaukaninna Kopperamanna Mulka Mungeranie 2 < Mirra Mitta 2 < Mt. Gason 1 < Goyder s Lagoon 1 < n.d. Birdsville 2 < Al ( g/l) Mean data are from Primary Industries and Resources of South Australia (PIRSA), Bureau of Rural Sciences (BRS), and this study. Dissolved silica determined in groundwater samples collected during this study is generally lower than measurements from the other two data sources. There is, however, generally another dissolved silica determination from either PIRSA or the BRS that is within 5 ppm. The reason for this is unclear, but may be related to the analysis technique. The colorimetric molybdate-yellow method (visible spectrophotometry) was used to analyse the samples in this study. The analysis technique for silica used by the other two data sources was not provided, but was probably either the colorimetric molybdate method or the ICP AES analysis of Si, followed by calculation of maximum SiO 2. An ICP AES instrument will analyse the total Si content of an introduced solution, including that found in complexes, polymers and colloids. The molybdate-yellow method is only sensitive to the monomeric silica (H 4 SiO 4 ) component (Alexander, 1953). SiO 2(aq) determined using the colorimetric method could therefore be lower and may be better suited for geothermometry applications. The variability in groundwater compositions between the Birdsville Town Bore and the Frome Creek Bore (near Marree) is displayed in Figs. 3 and 4. The distances between bores on the transects represent the direct (straight-line) distance between adjacent bores. The transect appears to be approximately parallel to the groundwater flow line shown in Fig. 1c. The equivalent major-element concentration data indicate that the groundwaters along the BTR are of the Na-HCO 3 -Cl type. Bicarbonate is a more dominant anion in these waters than Cl (Table 3). Sodium is overwhelmingly the dominant cation. Calcium, Mg and K tend to be very close to detection limit in some samples. The total dissolved major element (TDM) concentration (a proxy for total dissolved solids (TDS)) increases fairly uniformly and regularly from Birdsville towards Marree (Fig. 4d) Measured temperatures By integrating bore depths, well-head temperatures and bore locations from all data sources, a transect/cross-section between Birdsville and Marree can be drawn. This cross-

13 M.C. Pirlo / Geothermics 33 (2004) Fig. 3. Major-element transects for cations (Ca, Mg, Na and K) along the BTR between Birdsville and (approximately) Marree. Error bars represent one standard deviation from the mean for each location. section is similar to that given by Armstrong (1990) and would translate into a map similar to that presented by Radke et al. (2000). The transect, presented here as Fig. 5, shows the depth variation in bores along the Birdsville Track. The depth of many of these bores can be used as a proxy for the top of the aquifer, since drilling of stock bores usually ceases when good quality water is first tapped, i.e. once the top of the Algebuckina Sandstone or Cadna-owie-Hooray aquifers is encountered. Petroleum exploration bores, however, tend to be completed in the basement at much greater depths, with the result that they may intersect a number of groundwater aquifers; there are none of these bores along the BTR. The high standard deviation in the Mungeranie temperature data (Fig. 5) is due to the measurement made during this study. The sample and measurements could not be made at the well-head because of access restrictions so the measurement was made on groundwater flowing continuously from a pipe approximately 300 m from the actual well-head. As a result, the measured temperature (48.5 C) would have been significantly lower than that measured at the actual well-head. If this measurement is excluded, and the measured temperature estimated from the two available BRS samples (temperatures of 83 C and 84 C), the shape of the measured temperature trace (Fig. 5) becomes much more regular. For all future calculations, the temperature measured during this study for Mungeranie will not be considered.

14 756 M.C. Pirlo / Geothermics 33 (2004) Fig. 4. Major-element transects for anions (Cl, SO 4, HCO 3 ) and total dissolved major elements (TDM) along the BTR between Birdsville and (approximately) Marree. TDM can be used as a proxy for total dissolved solids (TDS). Error bars represent one standard deviation from the mean for each location. Fig. 5. Variation in depth and measured borehead groundwater temperature for groundwater bores along the BTR between Birdsville and Marree. The y-axis error bars for measured borehead temperature represent one standard deviation from the mean.

15 Table 4 Mean aquifer temperatures calculated using traditional geothermometers, for each location considered in this study Sample name Distance a (km) Measured temperature ( C) S1A ( C) S2A ( C) S3A ( C) S4A ( C) S5A ( C) NK1A ( C) NK2A ( C) NK3A ( C) NK4A ( C) NK5A ( C) NKC1A ( C) Frome Creek Lake Harry Clayton Dulkaninna Cannawaukaninna Kopperamanna Mulka Mungeranie Mirra Mitta Mt. Gason Goyder s Lagoon Birdsville a Distance from Birdsville. NL1A ( C) NL2A ( C) ML1A ( C) KM1A ( C) M.C. Pirlo / Geothermics 33 (2004)

16 758 M.C. Pirlo / Geothermics 33 (2004) Fig. 6. Relationship between measured borehead temperature and silica geotemperature. Both individual measurements and the mean data for each particular locality have been included. Mean data are indicated by an A following the geothermometer code Geotemperatures Traditional geotemperatures The mean geotemperature calculated for each of the 15 geothermometers (Table 2) at each of the 12 localities along the BTR is presented in Table 4. Measured emergence temperature and cumulative distance from Birdsville have also been included. Figs. 6 8 show the various geotemperature estimates plotted against measured emergence temperature. Both individual samples and mean data for each locality (Table 4) have been included. Figs. 6 8 show the silica geotemperatures, the Na/K geotemperatures and the remaining cation geotemperatures (Na/K/Ca, Na/Li, Mg/Li and K/Mg), respectively. Table 2 lists the specific geothermometers that were used to calculate the geotemperatures used in these figures. Table 5 shows the results of a linear least squares regression analysis of the various data plotted in Figs The number of points considered, the correlation coefficients (r), and the slopes and intercepts are included Multi-component geotemperatures The REACT geochemical modeling code of Bethke (1996, 1998) has been used to construct a numerical model of fluid mineral equilibria in groundwaters from each of the suggested populations in order to predict a stable phase mineral assemblage. This assemblage is inferred to be an important control on the groundwater composition.

17 M.C. Pirlo / Geothermics 33 (2004) Fig. 7. Relationship between measured borehead temperature and Na/K geotemperature. Both individual measurements and the mean data for each particular locality have been included. Mean data are indicated by an A following the geothermometer code. Fig. 8. Relationship between measured borehead temperature and cation geotemperature. Both individual measurements and the mean data for each particular locality have been included. Mean data are indicated by an A following the geothermometer code.

18 760 M.C. Pirlo / Geothermics 33 (2004) Table 5 Summary of the correlations between traditional geotemperatures and measured emergence temperature for each of the traditional geothermometers considered in this study Geothermometer Number of points r Slope Intercept Individual samples S S S S S NK NK NK NK NK NKC NL NL ML KM Averaged samples by location S S S S S NK NK NK NK NK NKC NL NL ML KM The specified temperature range considered by the model was set between 20 C and 150 C. Only hydrogeochemical data collected as part of this study were used for multicomponent geothermometry. As already explained, these data are believed to represent bore water composition. Saturation index (S.I.) versus temperature plots [log 10 (Q/K) versus T] for each of the bores along the Birdsville Track have been calculated and interpreted. The Goyder s Lagoon and Kopperamanna bores were not sampled during this study. Fig. 9 shows the resulting plot for Mirra Mitta bore. This figure possibly corresponds to water with an original geothermal component that may have mixed with a cooler groundwater before possibly partially equilibrating. The range and trends of minerals plotting below the log 10 (Q/K) = 0 line within the C temperature range support this. The upper part of this temperature range may indicate the original

19 M.C. Pirlo / Geothermics 33 (2004) Fig. 9. log(q/k) versus T (multi-component geotemperature) plot for the Mirra Mitta artesian bore. The mean measured emergence temperature is 87 C. geothermal component, while the lower temperature values are more likely the result of partial equilibration. Interpretation of multi-component geotemperatures is based on determining the temperature at which the saturation indices for a relevant suite of minerals become zero. As a result, interpretation can be somewhat subjective, depending on the minerals chosen. Temperature estimates derived from a suite of likely minerals are shown in Table 6. The minerals considered by D Amore et al. (1987) served as a guide for the selection of a likely stable phase mineral assemblage, since low-temperature Na-HCO 3 -Cl type groundwaters were considered in our study. The temperature corresponding to S.I. = 0 was estimated. The numbers in parentheses in Table 6 represent the temperature range defined by S.I. ±0.25, to reflect the possible range in estimated geotemperature. The information in Table 6 is summarised in Table 7. Average geotemperatures and relative standard deviations (R.S.D.) have been calculated for: 1. All of the mineralogic species in Table All minerals excluding the smectites/montmorillonites (beidellites and saponites). 3. The beidellites. 4. The saponites. 5. All smectites/montmorillonites (beidellites and saponites).

20 762 M.C. Pirlo / Geothermics 33 (2004) Table 6 Geotemperature estimates and temperature ranges for different bores, derived from mineral saturation indices Mineral Estimated temperatures ( C) with temperature ranges in parentheses Birdsville Mt. Gason Mirra Mitta Mungeranie Mulka Albite 69 (61 79) 58 (53 66) 44 (39 49) 38 (24 45) 42 (38 47) Analcime 46 (40 55) 38 (32 44) 25 (<30) 25 (21 30) Calcite 119 ( ) 136 (>120) 139 (>124) 88 (106 70) Chalcedony 85 (61 115) 86 (61 116) 78 (55 106) 74 (52 102) 75 (53 103) Dolomite 115 ( ) 119 ( ) 129 ( ) 70 (78 62) Illite 79 (73 85) 68 (64 73) 56 (53 59) 60 (56 65) 45 (43 48) K-feldspar 100 (90 110) 91 (83 101) 76 (70 84) 72 (66 80) 65 (59 71) Kaolinite 79 (72 85) 71 (67 77) 63 (59 68) 78 (72 84) 45 (42 48) Laumonite 47 (44 52) 32 (30 35) 21 (<24) 23 (21 26) Microcline 100 (91 111) 92 (83 102) 76 (70 84) 73 (66 80) 65 (59 71) Muscovite 105 (97 114) 89 (83 95) 74 (70 78) 86 (81 92) 53 (51 56) Quartz 110 (82 144) 111 (83 145) 102 (76 134) 98 (73 130) 99 (73 130) Sanidine 70 (63 78) 64 (58 71) 53 (48 58) 49 (44 55) 45 (41 49) Beidellite-Ca 77 (71 82) 68 (64 72) 58 (55 61) 68 (64 72) 44 (42 46) Beidellite-K 69 (66 74) 64 (61 68) 56 (53 59) 64 (60 68) 42 (40 44) Beidellite-Mg 74 (70 80) 67 (63 71) 57 (54 60) 66 (62 71) 43 (41 46) Beidellite-Na 74 (69 79) 67 (63 71) 57 (55 61) 67 (63 70) 44 (41 46) Saponite-Ca 90 (93 87) 105 ( ) 125 ( ) 74 (77 71) Saponite-K 96 (99 93) 110 ( ) 131 ( ) 79 (82 76) Saponite-Mg 91 (94 88) 105 ( ) 125 ( ) 74 (77 72) Saponite-Na 92 (96 89) 107 ( ) 128 ( ) 76 (79 73) Cannawaukaninna Dulkaninna Clayton Lake Harry Frome Creek Albite 43 (39 48) 28 (24 31) 27 (23 30) 23 (<27) Analcime 28 (24 32) Calcite 37 (<64) 62 (84 38) 100 (117 83) 93 (110 75) 74 (92 54) Chalcedony 66 (45 91) 54 (36 77) 49 (31 70) 43 (26 63) 37 (21 56) Dolomite 32 (42 21) 74 (82 66) 64 (72 56) 30 (<39) Illite 40 (37 43) 28 (26 29) 31 (29 33) 26 (24 28) K-feldspar 63 (58 69) 45 (41 49) 41 (38 45) 32 (30 36) Kaolinite 34 (32 37) 26 (24 28) 36 (33 38) 31 (29 34) Laumonite 24 (22 26) Microcline 63 (58 69) 45 (41 49) 41 (38 45) 32 (30 36) Muscovite 45 (43 47) 32 (31 34) 39 (37 41) 32 (31 34) Quartz 88 (65 116) 75 (54 100) 69 (49 93) 62 (44 85) 55 (38 76) Sanidine 44 (40 48) 29 (26 32) 26 (23 29) Beidellite-Ca 34 (32 36) 24 (23 26) 31 (29 33) 27 (26 29) Beidellite-K 33 (31 35) 24 (22 25) 30 (28 31) 25 (24 27) Beidellite-Mg 34 (32 36) 24 (22 26) 31 (29 33) 27 (25 29) Beidellite-Na 35 (33 37) 25 (23 27) 32 (30 34) 28 (26 30) Saponite-Ca 47 (50 43) 92 (95 90) 89 (91 86) 68 (71 65) Saponite-K 51 (55 47) 98 (101 96) 96 (99 93) 74 (77 71) Saponite-Mg 47 (50 43) 92 (95 89) 88 (91 85) 67 (70 64) Saponite-Na 47 (50 43) 93 (96 91) 89 (92 86) 69 (72 65)

21 M.C. Pirlo / Geothermics 33 (2004) Table 7 Geotemperature estimates derived from average saturation indices for groups of minerals Bore location Average temperatures ( C) ± relative standard deviations All minerals Non Mont Beidellites Saponites Total Mont. Birdsville 85 ± ± ± 5 92 ± 3 83 ± 13 Mt. Gason 83 ± ± ± ± 2 87 ± 25 Mirra Mitta 80 ± ± ± ± 2 92 ± 41 Mungeranie 69 ± ± ± 3 66 ± 3 Mulka 58 ± ± ± 2 76 ± 3 60 ± 29 Cannawaukaninna 44 ± ± ± 2 34 ± 2 Dulkaninna 39 ± ± ± 2 48 ± 4 36 ± 35 Clayton 54 ± ± ± 3 94 ± 3 62 ± 54 Lake Harry 50 ± ± ± 5 91 ± 4 59 ± 58 Frome Creek 59 ± ± ± 5 70 ± 5 Mont. = montmorillonite. 9. Discussion Fig. 2 suggests that two populations may be distinguished in the major-element composition of groundwaters along the BTR. Between, and including Mulka and Goyder s Lagoon (Fig. 2g k), the equivalent SO 4 concentration is above 0.1 ( 5 ppm). The equivalent SO 4 concentrations in waters from bores to the south of Mulka (Kopperamanna to Frome Creek, Fig. 2a f) are mostly below 0.1. This is highlighted by the change in relative ratios from Na + K through Cl to SO 4, as the six bores closest to Marree show a change of inflection at Cl. The Birdsville bore (Fig. 21) does not fit this pattern because it is at the beginning of the transect, yet it shows a major-element trace that is more similar to bores from the southern end of the transect. One anomalous sample for the Cannawaukaninna bore (Fig. 1b) (obtained from the BRS data) has a significant effect on the average Mg and, to a lesser extent, on the average Ca and Na contents. A very large standard deviation reflects this for both Mg and Ca (Fig. 3). The slope of the measured temperature versus distance trace on Fig. 5 tends to change at Mulka. From Birdsville to Mulka (see bore locations in Fig. 1b) the slope generally exhibits a low gradient, but the gradient steepens significantly from Mulka to Frome Creek (see Fig. 1b). This point also seems to correspond to the change observed in the Schoeller Plots (Fig. 2). The measured temperature versus distance plot (Fig. 5) is almost an exact reflection of the depth versus distance plot in the same figure, suggesting that the bore/aquifer depth and temperature are closely related. 10. Traditional geothermometers Examining the correlation with measured emergence temperature has identified the most appropriate silica geothermometer for this section of the GAB. The linearity of the plots of geotemperature versus measured emergence temperature has been gauged with a correlation coefficient for both the plots of individual samples versus measured temperature and mean geotemperature versus mean measured temperature for a locality (Figs. 6 8).

22 764 M.C. Pirlo / Geothermics 33 (2004) A number of silica geothermometers give unrealistic estimates of sub-surface temperature. Geothermometers that estimate aquifer temperatures that are lower than the sampling temperature have been considered inappropriate, at least for this section of the GAB. The amorphous silica and the and cristobalite geothermometers fall into this category. The maximum steam loss quartz geothermometer is also considered inappropriate since this is not a high enthalpy geothermal system. The quartz geothermometers (S1 S4), as well as the chalcedony geothermometer (S5), are considered to give reasonable estimates of sub-surface temperature and are thus worthy of further consideration. In Fig. 6, S1, S2 and S4 return essentially the same geotemperatures over the temperature range. S3 returns the lowest geotemperatures of the four potential quartz geothermometers. S5 returns the lowest geotemperatures of the four potential silica geothermometers, and also the geotemperatures closest to a 1:1 ratio with measured temperature. If the measured emergence temperature is likely to give a reliable indication of aquifer temperature, a suitable geothermometer should give a slope and correlation coefficient close to 1 and an intercept close to 0. Since the measured temperatures have all been measured at the borehead, rather than bottomhole temperatures in the bore itself, it is reasonable to expect that the water may have cooled slightly on its ascent to the surface, even though this ascent is both direct and relatively rapid. An intercept that is slightly greater than 0 may therefore be reasonable, but the slope and correlation coefficient should both still remain close to 1. Of all the geothermometers that proved worthy of further consideration (including the cation geothermometers), the five silica geothermometers give the strongest linear correlation with measured temperature for both individual samples and mean data for localities (Table 5). The correlation coefficient (r) is statistically significant, with a two-tailed t-test at the 99% significance level. The slopes of the regression line are also close to 1, although the intercepts deviate significantly from 0 for all but S5. The chalcedony (S5) and quartz (S3) geothermometers return intercepts that are closest to 0 (3.4 and 18.8, respectively, based on the mean data). The chalcedony geothermometer in its present form is therefore probably the most appropriate for the silica geothermometers for the hydrogeochemical and thermal regime along the BTR. Both the Na/K and Na/K/Ca geothermometers also exhibit a significant correlation with measured temperature at the 99% significance level. The slope of the Na/K geothermometers regression line is also close to 1; it is, however, generally slightly higher (approximately 15%) than the slope returned by the silica geothermometers. Like the silica geothermometers, the various Na/K geothermometers display a range of intercepts, all greater than zero except for the mean NK1. The five variants of the Na/K geothermometer and the two variants of the Na/K/Ca geothermometer returned plausible estimates of sub-surface temperature and are therefore also worthy of further consideration; only the Na/K/Ca geothermometer that used β = 1/3 (NKC1) will, however, be considered, in accordance with the application procedure described in Fournier and Truesdell (1973). The Mg correction to the Na/K/Ca geothermometer is not considered important in this section of the GAB, because of the very low Mg content of the groundwater (all but 5 samples out of 65 have [Mg] < 1 ppm). The correction was devised so that erroneously high geotemperatures did not result from application of the Na/K/Ca geothermometer to waters that are relatively rich in Mg (Fournier and Potter, 1979).

23 M.C. Pirlo / Geothermics 33 (2004) The Na/Li and Mg/Li geothermometers have also been used to estimate plausible geotemperatures, though the variant of the Na/Li geothermometer (NL2) proposed by Verma and Santoyo (1997) estimates unreasonably low geotemperatures. The number of samples to which these geothermometers could be applied was significantly lower than the other cation and silica geothermometers, since only a few samples reported by the BRS had Li analyses. Li was used as an internal ICP MS standard in this study, so the samples collected as part of this study did not have an analysis of Li and could therefore not be used in geothermometry involving Li. The poor correlation coefficients, low slope and the high intercepts, together with the fact that both Mg and Li are present in very low concentrations in the groundwater, should preclude them from use as geothermometers in this section of the GAB. As was concluded by D Amore et al. (1987), in low-temperature, low salinity environments, with [Li] < 1 ppm, the Na/Li and Mg/Li geothermometers are not accurate because they have not been adequately calibrated for such conditions. The K/Mg geothermometer tends to estimate geotemperatures that are too low to be plausible. The plot of KM1 versus measured temperature (Fig. 8) does not even show a positive correlation. Both the slope and the correlation coefficient (Table 5) suggest a negative correlation, which is probably tied to the very low Mg content in the groundwaters. The clear relationship between bore depth and measured groundwater temperature along the BTR (Fig. 5) can also be observed from geotemperature estimates. Fig. 10 shows the variation in estimated geotemperature, measured temperature and position along the BTR. As suggested by the data in Table 5, the silica geotemperatures show a stronger relationship to measured temperatures than do the cation geotemperatures, since they closely reflect the measured temperature along the entire length of the transect. The cation geotemperatures tend to be higher than the silica geotemperatures, and a significant deviation is observed approximately 150 km from Birdsville. Here a significant increase in cation geotemperatures occurs relative to a decrease in both measured temperature and silica geotemperature. Possible reasons for these differences are discussed in the following section. 11. Multi-component geotemperatures A significant conclusion of D Amore et al. (1987) was that multi-component geotemperatures estimated in their study did not display as much scatter as temperatures calculated with empirical geothermometers in low-temperature systems, despite the fact that mixing phenomena may have interfered with the interpretation. In this research, mixing phenomena have been restricted through the use of cased bores. It has, however, not been eliminated entirely, as will be discussed later. The multi-component geotemperatures estimated in Tables 6 and 7 might help explain temperature and groundwater mixing trends in this region of the GAB. From Table 4 it can be seen that the local variation (difference) in geotemperatures estimated with NK4 and S5 is as great as 108 C at Mirra Mitta, and averages approximately 81 C over the length of the transect. This difference is also evident in Fig. 10. By comparison, the difference reported by D Amore et al. (1987) ranged from 69 C to190 C and the mean temperature difference for all four Na-HCO 3 springs

24 Fig. 10. Variation in traditional (a c), and multi-component (d) geotemperature estimates with distance along the BTR between Birdsville and Marree. The measured temperature profile in (d) differs slightly from that in (a c) because measured temperatures at two bores with no multi-component geotemperatures were not plotted. 766 M.C. Pirlo / Geothermics 33 (2004)

25 M.C. Pirlo / Geothermics 33 (2004) considered in that study was 80 C. The geotemperatures estimated from mineral phase equilibria could vary by as much as C depending on the spring; however, the maximum RSD was estimated as being only 11 C for the total group of minerals. The montmorillonite minerals suggested slightly higher temperatures than the other minerals, but the exact type of montmorillonite mineral involved was unclear. D Amore et al. (1987) interpreted this to mean that either the montmorillonite minerals were in disequilibrium with the other phases, or that they may be representative of a deeper groundwater circulation. Along the BTR, the montmorillonite minerals (Na, K, Ca and Mg beidellites and saponites) generally suggest temperatures on either side of the non-montmorillonite minerals. In Table 6, the range of return temperatures for quartz and chalcedony is large, up to C. Furthermore, the quartz and chalcedony geotemperatures calculated with the geothermometry equations do not correspond exactly to the temperature when log(q/k) = 0; indeed, a difference of as much as 25 C can exist. A similar difference, up to 11 C, can be observed in the limited results of D Amore et al. (1987). Table 6 lists three potassium feldspars that could control the K Al Si O system (Kfeldspar, microcline and sanidine). Only one (if indeed any) can control the concentration of a component in the system, in accordance with the phase rule. Both K-feldspar and microcline give geotemperature ranges that overlap. The sanidine geotemperature range tends to coincide with the geotemperature range for albite, the Na-feldspar end member. This is significant because ion exchange between Na- and K-feldspars is often used to explain the Na/K geothermometer (Fournier and Truesdell, 1973); assigning a specific mineral assemblage is, however, difficult because of the existence of polymorphs, e.g., high and low albite (Henley et al., 1984). The albite/sanidine temperature range tends to be below both the chalcedony and measured temperature for all bores along the transect. It is also significantly lower than any of the five Na/K geotemperature estimates. D Amore et al. (1987) also present results that show the albite/adularia temperature range to be significantly lower than the Na/K geotemperature estimates, so it is not clear what the albite/sanidine temperatures indicate. Fig. 10d shows the variation in modelled geotemperatures (listed in Tables 6 and 7) with distance from Birdsville. The chalcedony and quartz trends are very similar to those estimated from the chalcedony and quartz geothermometers (Fig. 6). The temperatures suggested by albite and sanidine equilibria are very similar to each other. They are also significantly lower than the geotemperature estimates from silica mineral equilibria. The mean multi-component geotemperature of all minerals in Table 7 is very close to the chalcedony geotemperatures for all but four bores, with the maximum difference being approximately 20 C. Since the chalcedony geotemperatures are the most closely related to measured temperatures, the mean multi-component geotemperature can generally be considered as a good estimate of groundwater temperature. The RSDs accompanying the mean multi-component geotemperature estimates in Table 7 are higher than those reported by D Amore et al. (1987). This is interpreted as being a consequence of considering mean geotemperatures derived from a greater number of mineral phases while also considering polymorphs. Nevertheless, the multi-component geotemperature estimates do not show as much scatter as those derived from traditional empirical geothermometers.

26 768 M.C. Pirlo / Geothermics 33 (2004) Groundwater evolution, flow and mixing The rate of addition of salts to the groundwater between Birdsville and Goyder s Lagoon is estimated at 1.7 mg/km, using data from Fig. 4. This rate is less than half the mean salt addition rate predicted over the whole transect (3.6 mg/km). The rate of salt addition clearly increases towards the southern end of the transect, although not at a linear rate (Fig. 4d). As a conservative estimate, if the addition rate of 1.7 mg/km is extrapolated some 650 km to the north or northeast of Birdsville, the inferred recharge area (Fig. 1), then the salt content of groundwater in the recharge zone is estimated as being unreasonably low. This of course assumes that the rate of salt addition remains constant over such a long distance, and is also based on groundwater in Birdsville having a mean total dissolved major ion concentration of 640 mg/l. Radke et al. (2000) show that groundwater on either side of the BTR is of higher salinity. Upwelling of older, higher salinity groundwater from the deep basins on either side of the BTR may therefore be invoked to explain the evolution of groundwater along the BTR, i.e., the salinity increase. A positive trend between measured emergence temperature and silica geotemperature definitely indicates an equilibrium in the reservoir. However, silica geothermometers may not reflect higher temperatures from deeper levels. Their rapid re-equilibration rate relative to Na/K and Na/K/Ca geothermometers at lower temperatures can help explain why silica geotemperatures can be significantly lower than Na/K and Na/K/Ca geotemperatures (Fig. 10). Consideration of silica geotemperatures alone can mask deeper and thus higher equilibration temperatures. Such a situation has been identified by Minissale and Duchi (1988) and it may be analogous to the BTR system. Deep groundwaters from the Cooper and Simpson/Pedirka Basins may be ascending from depth along the flanks of the BTR, while at the same time migrating to the southwest. The BTR itself strikes NE SW with the main groundwater flow interpreted as being along its axis. Some groundwater flow lines on either side of the BTR show a more E W orientation, towards the BTR, and maps presented by Radke et al. (2000) emphasise the E W flow components even more than Fig. 1c. Since the oldest groundwaters are found in the deepest parts of the basin, and permeability decreases with depth, groundwater flow rates along the BTR are higher than on either side of it. The BTR may therefore transmit a greater volume of groundwater (at least relative to its size) than the deeper, flanking basins. The greater volume and lower salinity of this BTR groundwater are expected to dilute any deep source, higher salinity groundwater with which it mixes. Na/K geotemperatures are less affected by groundwater mixing than silica geothermometers if the fluid with which they mix contains relatively low amounts of Na and K. If BTR groundwater mixes with the higher salinity groundwater from deeper sources then it may cause only relatively small changes in the estimated cation geotemperature of the deep groundwaters. The higher temperatures that are assumed to exist in these basins are thus partially preserved in the Na/K and Na/K/Ca geotemperatures, but not in the silica geotemperatures. The silica geotemperatures instead reflect the temperature on the BTR axis. A schematic diagram (Fig. 11) has been provided to help illustrate this. To determine whether the temperature difference between silica and Na/K geotemperature estimates reflects the upwelling and mixing of deeper groundwaters, the temperature difference between NK4 (the Na/K geothermometer that generally returns the highest cation

27 M.C. Pirlo / Geothermics 33 (2004) Fig. 11. Schematic figure of a section of the Birdsville Track Ridge (BTR). geotemperature) and S5 (the silica geothermometer that returns the lowest silica geotemperature) has been calculated and related to transect position and salinity in Fig. 12. The temperature differences between NK4 and MT and S5 and MT have also been plotted for comparison. The temperature differences have been calculated from the difference between mean geotemperatures for a locality (pooled) rather than the mean of geotemperature difference within a locality (paired). As in Fig. 10, it is shown in Fig. 12 that chalcedony geotemperatures are closely related to measured temperatures whereas the difference between Na/K and measured temperature is much greater. If the silica geotemperatures for bores along the BTR reflect equilibrium conditions in the aquifer on the BTR ridge axis, and the cation geotemperatures reflect temperatures in the deeper parts of the aquifer, i.e. on either side of the BTR, then the temperature difference between cation and silica geotemperatures might be expected to increase along the transect to the south. This difference increase might also be expected to relate to the increase in salinity, since increasing salinity is interpreted as an indicator of groundwater from the deeper side basins mixing with that on the BTR. Fig. 12 shows that the first three bores from Birdsville (Fig. 1b) (they are also the deepest three) return the lowest geotemperature differences on the transect, with the exception of