Angela M. Giblin

Geochemistry of acid brines : Lake Tyrrell, Victoria, Australia

Pore water, groundwater and sediments were collected from Lake Tyrrell to study the evolution of brines in an acidic environment. Lake Tyrrell is a 150-km2 inland salt playa. Acid-saline groundwater enters the playa through springs along the lake margin and is evapo-concentrated past halite precipitation. The brines were found to contain sea salts, but some solutes do not follow the seawater evaporation curve. Processes that occur in neutral-alkaline systems (e.g., gypsum formation) occur here, but precipitation of alunite (KAl3(SO4)2(OH)6), jarosite (KFe3(SO4)2(OH)6) and Fe-oxides also affect solute behavior. Processes that occur in the recharge area and along the groundwater flow path (K uptake, calcrete and gypcrete formation) affect the brines chemical signature. Regardless of the chemical variability found among the samples, solutes have evolved from a common source (sea salt). Acidification, which appears to occur early in the evolution of the groundwater, is related to pyrite oxidation and may be enhanced by precipitation of Fe-oxides, alunite and jarosite, and evaporation. Acidity is maintained in the system because of the lack of buffering capacity of aquifer minerals. We suggest that this acid brine from Australia is another end-member in the Eugster-Jones-Hardie model for the evolution of water by evaporation in closed basins.

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

Abstract Anomalous radioactivity highlighted the Lake Tyrrell area as a potential mineral exploration target and prompted a 5-year study of its groundwater geochemistry. The groundwaters are very acid (median pH=4.2) and saline (median ionic strength ( I ) = 1.24). Some samples contain unusually high concentrations of Cu (up to 3.5 mg l −1 ), Pb (up to 38.5 mg l −1 ) and Al (up to 155 mg l −1 ). Concentrations of most other trace elements exceed seawater values but are not different from those in groundwaters near two other randomly selected Australian salt lakes. Waters with elevated trace-element concentrations are spread at isolated sites across the area but some clustering of Cu-Pb-enriched waters occurs in the west and southeast. R and Q mode correspondence analysis identifies factors which relate to both trace elements and samples, and are interpreted as reflecting processes whereby trace elements are taken into groundwaters. Factor 1 involves Al, Cu, Fe, Mn and Zn whose concentrations relate directly to high acidity; factor 2 involves B and Sr and probably relates to local sediment enrichments of B and Sr; factor 3 is strongly weighted by Pb and can be explained by contact between groundwaters and locations where clays, organic solids and Fe-Mn-oxides are particularly enriched by heavy metals; factor 4 involves F, Y and I, and probably relates to residual mineral dissolution. Sample locations, grouped according to the influence of each factor on each groundwater composition, identify zones where a particular process is dominant. Laboratory leaching of Blanchetown Clay and Parilla Sand confirms that many of the trace elements could have entered the groundwater from local aquifer sediments. The potential of these waters as agents of present or future ore formation is also considered by comparing amounts of trace elements carried into the salt lake basin over the last 30 kyr with what persists in present-day lake brines. Results demonstrate that large tonnages of trace elements appear to have been deposited, but that sediment dilution mitigates against ore-grade concentrations.

The trace-metal geochemistry of the Lake Tyrrell system brines (Victoria, Australia)

Abstract We have analyzed the waters of the Lake Tyrrell system for the trace metals, Fe, Mn, Pb, Cu and Zn. In a number of samples both dissolved Fe T and Fe 2+ were also determined. Great care was exercized to minimize any contamination of the samples. Not surprisingly the highest metal concentrations occur in the low-pH, high-Eh regional groundwaters, with much lower values observed in the reflux brines and the neutral Wahpool-Timboram waters. It is thought that the source of these metals to the groundwaters is the dissolution of a suite of heavy minerals within the aquifer, the Pliocene Parilla Sand. As the acid groundwaters flow toward the lake some metals are removed near lake edge at depth. The metal concentrations in the spring zone region indicate an extremely dynamic system, where at different times of the year due to changes in the hydrologic regime metals can be either removed or solubilized.

An evaluation of methods for imputation of missing trace element data in groundwaters

Groundwater data-sets with pH and major cation–anion chemistry are widely available but data that include trace metals are much rarer. This paper examines two methods of data imputation to predict U concentrations using pH, major cations, anions and F in a data-set where some of the U concentrations are missing. The methods evaluated were self-organizing maps (SOM) and expectation maximization (EM). Evaluations were made using a groundwater data-set of 187 samples from NSW and Victoria, which contained a wide range of U concentrations up to 225 μg/l. Tests made by setting 25% and 50% of the U concentrations to missing showed that, at 25% missing, SOM gave reasonable estimates, identifying all the samples with higher U. EM did not clearly identify the higher samples. At 50% missing, neither method could accurately identify the higher U concentrations. Thus, imputation using samples with missing data included in the training data-set does not appear to be practical. However, a SOM pre-trained on a data-set with no missing U concentrations may be used to impute U concentrations for samples with 100% missing U data. Training using the original data-set and then imputing concentrations for a second set of 360 samples showed that the samples with higher measured U concentrations could generally be identified, but that other samples were also estimated to be U-rich. This method could substantially reduce the number of samples in a large data-set requiring further investigation. The performance of imputation for U reflects the complex interaction of water chemistry, geology and mineralogy that actually determines the U concentrations. Imputation is a useful method for improving estimates of data statistics. SOM, through its model-free approach, is a useful addition to the numerical analysis toolbox for geochemists.

Application of groundwater–mineral equilibrium calculationsto geochemical exploration for sediment-hosted uranium:observations from the Frome Embayment,South Australia

This paper considers the interpretation of uranium (U) ore–mineral equilibrium calculations in groundwater to help understand ore genesis and assist exploration. Groundwater samples collected on two recent field sampling trips in the Frome Embayment region of South Australia have been analysed for major and selected trace elements. These data together with previous published data have been used to construct geochemical models of the equilibrium state of the groundwaters. Species’ activities and mineral saturation states have been estimated. Calculated species’ activities are generally in the range reported in the previous modelling by Giblin involving groundwaters from similar local sources. Our models indicate that amorphous UO2, rather than the ore minerals coffinite and uraninite, is the solidU mineral phase that is the likely control on groundwater U concentration. The UO2(am) saturation index did not show a systematic variation with ore grade or proximity to mineralization. Its use for exploration at smaller scales, where the groundwater composition is similar between sample sites and U minerals are present at various concentrations, is doubtful. Inherent limitations on the information provided by mineral saturation indices restrict their exploration application,particularly in high salinity palaeochannel groundwater environments, such as those common in Australia.

The source of radium in anomalous accumulations near sandstone escarpments, Australia

Abstract The source of Ra has been determined in water samples from four areas in Australia where anomalously high surface concentrations of 226 Ra have accumulated from groundwaters. All four anomalies were located adjacent to sandstone formations, and the groundwaters, which were generally all acidic and low in dissolved salts, appeared to be meteoric water with short ground-residence times. Uranium, 226 Ra and 228 Ra concentrations of waters feeding the anomalous areas were comparable to those found in standing waters within the sandstones. The 226 Ra/ 228 Ra isotopic ratios were distributed about a median of 1.1 which suggests that the waters are in contact with rocks with near-normal U/Th ratios and, hence, that the Ra in the anomalies was derived from within the sandstones. The presence of the short-lived Ra isotopes, 223 Ra and 224 Ra, in high concentrations in most spring waters feeding these anomalies suggests that Ra enters groundwaters by recoil following alpha decay of a precursor parent radionuclide within mineral grains. Thus, although three of the areas were considered prospective for U, the radioactive anomalies studied appear to be due to natural transfer of Ra from the sandstones to the surface environment. In no case were the anomalies related to nearby known or undiscovered U deposits. Accordingly, a geochemical procedure, which includes Ra isotopic measurements, is recommended for evaluating radioactive anomalies for U exploration. This procedure should enable selection of only those anomalies with the highest potential for further exploration by more expensive techniques.

Uranium mobility in non-oxidizing brines: field and experimental evidence

Abstract Solution, transport and precipitation mechanisms for U which are dependent on the formation of soluble uranyl U 6+ ions and complexes under oxidizing conditions and of insoluble uranous U 4+ species under reducing conditions do not adequately explain U mobility under consistently reducing conditions. Uranium deposits formed under deep-seated metamorphic conditions and some hydrothermal deposits fall into this category. This paper presents field and experimental evidence supporting the mobility of U in strongly reduced polycationic brines. The Wollaston Group of Early Proterozoic sediments in northern Saskatchewan, Canada, locally contains series of associated semi-pelitic gneiss, calc-silicate gneiss and marble which have major and trace element characteristics and identify them as having formed in an evaporative environment. Associated with the evaporative metasediments are Na-rich and Cl-rich metasomes, albitite and scapolite calc-silicate rocks, respectively, which are the products of brine-rock reactions centred on the saliferous zones. Uranium and Mo appear to have been deposited syngenetically with the primary sediments but have been redistributed during metamorphism resulting in the formation of numerous mineralized zones which have attracted considerable prospecting attention. The present distribution of U can be related to the movement of the brines as revealed in the Na Ca Mg Cl-metasomes. Experiments were conducted at 60 and 200°C under stringently non-oxidizing conditions using solvents ranging from distilled water to a Ca Na K brine formulated to simulate the major element composition of the Salton Sea geothermal brines. The experiments were conducted on natural pitchblende (UO 2.67 ) and synthetic uraninite (UO 2.01 ). Natural pitchblende was more strongly dissolved than the synthetic uraninite, and the synthetic Salton Sea brine was a more potent solvent than distilled water, 1:4 diluted Salton Sea brine, or pure NaCl brine. Within analytical limits of detection the dissolved U is present in the uranous (U 4+ ) state. The evidence demonstrates empirically the mechanism of dissolution of naturally occurring U minerals in reduced brines and describes a geological case where this appears to have happened. Redeposition of the U presumably occurs in zones where the uraniferous brines are diluted or experience any changes in their cationic or anionic compositions which lessen U solubility factors.

Experiments to demonstrate mobility of metals in waters near base-metal sulphides

Abstract During the oxidation of a base-metal sulphide body, Cu, Pb and Fe are immobilised to varying degrees in sulphide residues and surrounding soils. However, Zn is relatively mobile and is a useful pathfinder element in groundwaters. Using material from the Woodlawn orebody, experiments were performed which simulated reactions in the intermediate subsurface ore zone where oxygen and carbon dioxide are Absent, and downward percolating waters contain metals leached in surface zones. Two simulation series were performed; one represented moderate percolation rates (0.7 cm/s) and the other slow percolation (0.04 cm/s). Results suggest that waters enriched in Cu, Pb and Zn from surface weathering of sulphides, seeping at the slow rate through a sulphide deposit out of contact with the atmosphere would lose these metals completely. If the waters were seeping at the moderate rate, only Cu would be completely lost, levels of Pb and Fe would be greatly reduced and Zn levels virtually unchanged. Metals in solution were exchanged with metals in the sulphide in the order Cu 2+ > Pb 2+ > Fe 2+ > Zn 2+ . Possible explanations regarding some aspects of gossan morphology and supergene enrichment are afforded by the experimental data.

Features and effects of some acid-saline groundwaters of southern Australia

Many salt lakes have formed in southem Australia where regional saline groundwaters form discharge zones. Some of these groundwaters are characterized by acidity and high salinity. These are commonly found where underlying Tertiary sediments are sulfide-rich. When waters from the resulting formations come to the surface or interact with oxidized meteoric water, acid groundwaters result from oxidation of the sulfide and/or the dissolved ferrous iron. In this paper examples of such waters around Lake Tyrrell, Victoria and Lake Maurice, South Australia, are reviewed. The acid-saline groundwaters typically have dissolved solids of 3060 mg/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 radium, a short-life daughter nuclide of uranium and thorium, by a process of cation exchange. Around salt-lakes, these acidic waters often emerge at the surface in marginal spring zones where the low density (s.g. -1.04) regional water flows out over the denser (s.g. -1.16) lake brines. In the spring zones examined, large amounts of iron are commonly precipitated. In a few places minerals of the alunite-jarosite family are formed which can trap many other metals, including radium which generates radiometric anomalies. Such anomalies may however be short-lived. The age of the present salt lakes is <40000 years but in that time very large amounts of many metals are estimated to have been deposited into the lakes but, apart from iron, this appears to be dispersed and to not have formed significant accumulations. 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 colunm within the porous sediments which is held in place by hydrostatic forces around the salt lakes. These brines are near neutral in pH.