Bruce Dickson

Recognition of aeolian soils of the Blayney district, NSW: implications for mineral exploration

Aeolian soils in the Blayney district are characterised by having approximately 80% of their mass finer than 63 μm, Ti/Zr ratios of approximately 12, radioelement contents of 0.7% K, 2 ppm eU and 11 ppm eTh and a position high in the landscape. Their Ti/Zr ratio and abundant quartz content distinguishes them from soils derived from Tertiary basalts, which share some of the other characteristics. Many soils in the district have a partial aeolian contribution, which can be recognised by a Ti/Zr ratio that differs from that of the underlying rock. Abundance of an aeolian contribution suggests that airborne radiometric surveys should be very carefully interpreted. Ground studies suggest that high Th content in soils contrasted with low Th in adjacent rocks can be indicative of an aeolian contribution in soils. A substantial amount of fine material in a soil can also significantly dilute geochemical signatures if −63 μm (<80 mesh) samples are used for soil surveys during exploration.

Interpreting aerial gamma-ray surveys utilising geomorphological and weathering models

Abstract Applying weathering and geomorphological data can improve the interpretation of aerial gamma-ray surveys. Data from two potentially mineralised areas in eastern Australia are used to demonstrate the new approach. At Develin Creek, NW of Rockhampton in Queensland, Permian andesitic basalts host VMS-style Cu-Zn deposits. Topographic and SPOT data were used to generate a digital elevation model (DEM) over an area of approx. 125 km 2 . The Universal Soil Loss Equation (USLE) was then applied to model soil erosion and deposition. Surface estimates of the distribution of clay, iron-oxides and vegetation were derived from Landsat TM data. Multi-variate linear regression and analysis of residuals (i.e. the difference between predicted and observed maps for each radioelement) were then used to look for anomalies that may indicate near-surface mineralisation. This analysis indicated no surface indicators of undiscovered mineralisation, a finding in accord with extensive ground geophysics and mapping. At Barry, SW of Bathurst, NSW, Ordovician basalts potentially host porphyry-related gold deposits. These deposits are most probably seen in aerial gamma-ray survey data through elevated K in alteration haloes. A DEM was created over a study area of approx. 180 km 2 and the soil erosion/deposition characteristics modelled using the USLE. Laboratory analyses of rock and soil samples showed that weathering of the basalt (median radioelement contents of 1.4% K, 0.7 ppm eU, 1.6 ppm eTh) produced a soil with decreased K (0.8%) but increased eU (1.5 ppm) and eTh (7.0 ppm). These values were used along with the erosion model to predict the surface K distribution, which was then compared to the observed distribution using a linear regression model. Areas with high K residuals are considered prospective for Au mineralisation.

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.

Evaluation of radioactive anomalies using radium isotopes in ground waters

ABSTRACT Dickson, B.L., Meakins, R.L. and Bland, C.J., 1983. Evaluation of radioactive anomalies using radium isotopes in ground waters. In: G.R. Parslow (Editor), Geochemical Exploration 1982. J. Geochem. Explor., 19: 195–205. An empirical scheme based on the concentrations of uranium and the three alpha-emitting radium isotopes 226 Ra, 224 Ra and 223 Ra is proposed for rating the significance of ground waters with respect to uranium exploration. The scheme has been developed from the results for over 200 water samples from the vicinity of known uranium deposits and radium anomalies in areas of Australia with climates varying from arid to tropical. The scheme uses relative levels of the four factors to rate the potential of a sample as good, possible or poor. An example of the use of the system in ground-water exploration in the Frome Embayment, South Australia, is presented to illustrate the value of the scheme in rejecting falsely anomalous samples with high uranium concentrations whilst detecting nearby uranium mineralization from drill holes not intersecting mineralization.

Hydrogeochemical interpretations of apparent anomalies in base metals and radium in groundwater near Lake Maurice in the Great Victoria Desert

Abstract Apparently anomalous levels of Cu, Pb, Zn (up to 6.1, 26.0 and 10.8 mg 1 −1 respectively) and Ra (2000 pg 1 −1 ) have been noted in groundwaters from 28 drill holes within a 20 km × 20 km zone centred about a 10 times background airborne radiometric anomaly near Lake Maurice in the Great Victoria Desert in South Australia. Within 6 km of the anomaly centre the water table depth is generally less than 10 m, increasing to approximately 30 m in the drill holes furthest from the anomaly centre. All waters are very acid (pH 3.6 to 5.8) and deficient in carbonate species (all −1 ) but saturated with respect to calcium sulphate minerals. XRD traces of drill hole cuttings show the presence of quartz and halite at every sample site, pyrite at 75% of sites, variable amounts of kaolinite and muscovite at all sites, and variable amounts of feldspar, jarosite, calcium sulphate minerals, hydrated iron oxides, siderite, chlorite and calcite at certain locations only. Salinity of waters is very high ranging from that approximating sea water (Ionic strength ( I ) = 0.93 and Cl − = 19 g 1 −1 ) to approximately six times sea water salinity ( I = 4.61 and Cl − = 120 g 1 −1 ).

Evaluation of lead isotopic methods for uranium exploration, Koongarra Area, Northern Territory, Australia

Abstract Analysis of radioactive ( 210 Pb) and stable lead isotopes in near-surface samples has been tested as a method of uranium exploration in the Pine Creek Geosyncline, Northern Territory, Australia. The lead isotopes were extracted from the samples by a mild leaching agent and were measured by alpha spectrometry for 210 Pb and by mass spectrometry for stable lead isotopes. The results are compared with those obtained by conventional methods utilizing measurements of radioactivity and radon (Track Etch) in situ and 226 Ra, 228 Ra and U contents of soils. The major problems addressed were whether the lead isotopic methods are more sensitive than the conventional methods and whether they can discriminate “real” anomalies from the common barren anomalies found in black soils and swamps which contain radium in excess of the uranium present. Four test areas, representing a range of exploration problems, were chosen in the vicinity of the Koongarra uranium deposits and 25 samples from each area were analyzed. Most samples have more 226 Ra than uranium. Radium analyses of several water samples show the source of this radium to be non-uraniferous rocks within the Kombolgie sandstone. The results for soil 226 Ra, radon, scintillometry and 219 Pb were generally closely correlated, and as a result, the 210 Pb method was not considered to have any advantages over the conventional methods. At the Koongarra X prospect, which has a weak surface expression, the 206 Pb 294 Pb ratio gave the strongest indication of the underlying uranium mineralization with an anomaly to background ratio of 12.5. However, this ratio is correlated with uranium content and does not offer any particular advantages over uranium analyses alone. More subtle indications of uranium mineralization were found by relating the radiogenic lead ( 206 Pb) and the thorium-derived lead ( 208 Pb) to the common lead content ( 204 Pb). A plot of 208 Pb 204 Pb versus 296 Pb 284 Pb (horizontal axis) is linear for country rock samples, irrespective of the amount of more recently introduced 226 Ra. Samples above uranium mineralization lie off this trend, along a line of near-zero slope. By the use of this plot, indications were found of the Koongarra No. 2 orebody, which is concealed by about 40 m of barren overburden; none of the other techniques detected this mineralization.

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-series disequilibrium in Australian soils and its effect on aerial gamma-ray surveys

Measurements of U and 226Ra in 445 soil samples collected from areas throughout Australia show that these soils have a mean U content of 2.0 mg kg−1 and activity ratio (U226Ra) of 1.09. There is a wide spread in individual values but, overall, more soils are U-rich than 226Ra-rich. Some indications of regional disequilibrium were found, with 226Ra-rich soils being noted in the Southern Cross-Kambalda area (WA), in south-eastern Eyre Peninsula (SA) and in an area north of Cobar (NSW). Uranium-rich soils were found in two Carboniferous volcanic terrains in north-eastern Queensland. The spread in activity ratio values indicates that disequilibrium will contribute to the noise in U channel data obtained by aerial gamma-ray surveying. The combination of statistical noise from low count rates (reflecting the low U concentrations in soils) and U-226Ra disequilibrium combine to give variances of 40–60% in typical U channel data. Data appearance can be improved to some extent by appropriate filtering.

Further assessment of stable lead isotope measurements for uranium exploration, Pine Creek Geosyncline, Northern Territory, Australia

Abstract Soil samples from the vicinity of Mount Brockman in the Pine Creek Geosyncline, Northern Territory, Australia, have been analyzed for stable Pb isotopes and two methods for displaying the data to reveal subtle indications of U mineralization have been investigated. The first, a plot of 208 Pb/ 204 Pb vs. 206 Pb/ 204 Pb, relates the U-derived lead ( 206 Pb) and the Th-derived lead ( 208 Pb) to the common lead content ( 204 Pb). For country rocks, the two ratios are linearly related, irrespective of the amount of recently introduced 226 Ra, whereas samples from near U mineralization lie off this trend along a line of near-zero slope. The second, a plot of 207 Pb/ 206 Pb vs. 204 Pb/ 206 Pb, gives a linear data array that may be interpreted either as indicating an apparent age of 1445 ± 20 Ma for the group of samples or as a mixing of the lead from mineralized and barren (country) rocks. In the latter interpretation, a threshold can be selected, with values below the threshold indicating U mineralization. Whereas the first method can be used in searching for any U deposit with a high U/Th ratio, the second plot is specific for a particular area. The value of both plotting methods is illustrated for the Mount Brockman area of the Pine Creek Geosyncline. Not only has the stable Pb isotope method been able to show that known radiometric anomalies are unrelated to U mineralization in this area, it has also been successful in identifying a previously unknown surface indicator of the Koongarra No. 1 deposit.

Radium isotopic measurements in the search for uranium in palaeodrainage channels

Abstract Analyses have been made of many groundwater samples, some of which were collected from the vicinity of uranium deposits and others from unmineralized areas, for dissolved uranium and for the four naturally occurring isotopes of radium: 226Ra (238U decay series, t 1 2 = 1600 y), 228Ra and 224Ra (232Th decay series, t 1 2 = 5.75 y and 3.8 d) and 223Ra (235U decay series, t 1 2 = 11 d). The radium isotopes 226Ra, 224Ra and 223Ra, are measured by alpha-spectrometry after extraction from a water sample soon after collection and 228Ra at a later time by determining the amount of ingrown 212Po.