Ryan Noble

Naturally occurring gold nanoparticles and nanoplates

During the weathering of gold deposits, exceptionally pure, <200 nm diameter, nanoparticulate gold plates (6 nm thick) are formed. The particles display controlled growth of both size and shape and signs of assembly to form belts and sheets. The gold is associated and intergrown with minerals formed by evaporation and is interpreted to have been deposited rapidly from saline groundwater during a drying event. The size and morphology of the gold nanoparticles and nanoplates are identical to the products of experimentally manufactured gold colloids. This represents the fi rst direct observation of colloidal nanoparticulate gold in nature, confi rming this as an active mechanism of gold transport during the weathering of gold deposits.

Bacterial communities associated with a mineral weathering profile at a sulphidic mine tailings dump in arid Western Australia.

We investigated bacterial community assemblages and functions down a hill slope contaminated by tailings from a volcanogenic massive sulphide mine in arid Western Australia. Weathering of waste rock, high in S and Fe, had resulted in a varying elemental dispersal down a face of the tailings hill. Bacterial community assemblage, characterised by PCR-DGGE fingerprinting, was significantly associated with electrical conductivity (E.C.) (ρ = 0.664; P < 0.01). Analysis of mobile salts showed that E.C. values were driven by ionic S, Zn, Cl and Al. The bacterial community assemblage was directly characterised across an E.C. gradient using an oligonucleotide microarray (PhyloChip). The dominant taxa at the site were Proteobacteria, Actinobacteria and Firmicutes; however, 37 phyla were detected. The most responsive taxa to variation in E.C. was Acidobacteria (negative correlation). Patterns of heterotrophic processes (BioLog analysis) were also best explained by variation in E.C. (ρ = 0.53; P < 0.01), showing a link between primary mineral weathering by lithotrophic bacteria and abiotic processes, and secondary biogeochemical processes by heterotrophic taxa. These data significantly broaden our knowledge of the bacteria present in metallomorphic ecosystems, establish that mobile phase elements are key drivers of community structure, and that primary biogeochemical cycling is directly influencing other geochemical interactions in the samples.

Quantitative assessment of the success of geochemical exploration techniques using minimum probability methods

Hypergeometric statistics have been used to establish a quantitative measure of performance for geochemical exploration techniques over known mineral showings. An alternative and complementary measure of exploration performance is geochemical contrast, which determines how convincing or compelling a geochemical result is. Using the identical philosophy employed to assess exploration ‘accuracy’, the Students t distribution is used to create a quantitative measure of ‘geochemical contrast’. First, thresholds are selected to separate anomalous and background populations. Then, Students t test statistics for each of these sets of anomalous and background samples are calculated, and the Students t probability is determined for the highest test statistic. This probability describes the chance that the anomalous and background populations were derived from the same underlying distribution. If this probability is low, then the concentrations of the anomalous and background samples are very different, and high geochemical contrast exists. If this probability is high, the alternative is true. As a result, this approach can be used to quantitatively compare the geochemical contrast of competing exploration techniques. Furthermore, because both of these ‘accuracy’ and ‘geochemical contrast’ measures are probabilities that vary inversely with exploration performance, their joint probability (their product) can be used to collectively rate the performance of exploration techniques.

Metal migration at the North Miitel Ni sulphide deposit in the southern Yilgarn Craton: Part 3, gas and overview

Gases provide a known mechanism of metal migration through cover and a potential sampling medium to explore through cover that is under-utilised and under-studied. Understanding how metals move through transported cover and their link to buried deposits is critical information for successful mineral exploration in many regions of the world including the well-endowed Yilgarn Craton of Western Australia. Here we employ metal and hydrocarbon soil gas collection methods to successfully predict the location of the underlying North Miitel Ni ore body. Laboratory experiments to replicate soil moisture, hypergeometric evaluation and variable spacing tests were used to verify the gaseous Ni signature. Soil gas hydrocarbon analysis also reported an unqualified, but positive result. Integrating this study with previous research on soil, regolith, groundwater and vegetation chemistry in the study area enabled a model of anomaly formation to be derived explaining the observed results and the contributions of weathering, hydromorphic, biotic, aeolian and gaseous dispersion mechanisms operating at the North Miitel site. Weathering and hydromorphic dispersion are responsible for lateral and minor vertical Ni migration at depth, aeolian Ni is dispersed laterally near the road, whereas vegetation is cycling Ni in the shallow soils only. Results indicate a gaseous migration of Ni is responsible for vertical migration through cover at this site and provides a viable target for exploration through cover.

Optimizing geochemical threshold selection while evaluating exploration techniques using a minimum hypergeometric probability method

Hypergeometric statistics have recently been used to establish a quantitative measure of performance for geochemical exploration techniques over known mineral occurrences. Using this method, the effectiveness of new exploration techniques can be compared objectively with conventional approaches. Sample site classification is the basis on which survey results are compared. This performance measure requires prior knowledge of the location of mineralization and a model for element dispersion from the primary into the secondary environment. This information allows assignment of sample sites that should give ‘anomalous’ (e.g. those overlying mineralized zones) or ‘background’ results to be identified prior to the orientation survey. Previous application of hypergeometric statistics requires that a high contrast exists between ‘anomalous’ and ‘background’ subpopulations in the geochemical orientation data so that there is no uncertainty in the classification of samples. In this paper, a refinement is developed that allows consideration of geochemical variables that do not exhibit this required high level of geochemical contrast, that is, where ‘anomalous’ and ‘background’ subpopulations exhibit significant overlap. This refinement involves determining the hypergeometric probabilities of obtaining the same result at random (P(x)) for a range of geochemical thresholds, instead of only one threshold (i.e. the one used to classify anomalous and background samples in cases with high geochemical contrast). At each threshold, different numbers of samples will be classified as ‘anomalous’ and different numbers of ‘anomalous’ samples will occur at ‘anomalous’ sites. As a consequence, the resulting random hypergeometric probabilities will change with threshold level. Using a range of thresholds to classify the geochemical orientation survey results allows identification of the minimum hypergeometric probability (MHP) for the dataset. Using this threshold, the classification of anomalies will bear the least resemblance to what would be expected if the survey results were generated at random. Employing this refined MHP approach, one can simultaneously evaluate the effectiveness of an exploration method, and select the threshold that optimally classifies anomalous and background samples.

Regional scale hydrogeochemical mapping of the northern Yilgarn Craton, Western Australia: a new technology for exploration in arid Australia

The northern Yilgarn Craton, with an extensive mineral exploration history and relatively fresh and neutral groundwaters, was selected to test the utility of regional hydrogeochemical mapping in Australia. The assembled data of 2509 groundwater samples (generally at 4–8 km spacing) are relatively unbiased, allowing robust statistical analysis such as testing sample types (flowing v. ‘stagnant’), contamination, and lithological controls on groundwater characteristics. Lithological indicators were developed to map underlying bedrock through cover. Areas with discrepancies between groundwater results and previous geological mapping were identified. Where these are areas previously discounted as prospective for mineral commodities, they may now be re-considered on this basis. Even in well explored parts of this region, this study identified new areas which may have prospective rocks overlain with a thin (<50 m) veneer of granitic material. A large background data set was produced that has significant benefits for lithological discrimination, mineral exploration, guiding human and livestock drinking water supplies and environmental management (e.g. mine closure). Groundwater chemistry can effectively map large-scale lithological changes in these semi-arid environments, and in turn can reduce uncertainly about the prospectivity of areas within the northern Yilgarn Craton. This should reduce the drilling and associated costs required to delineate a target. The methods and interpretation developed in this study will enhance mineral exploration into covered environments as much of the northern two thirds of Australia has similar groundwater environments. This methodology can be expanded into covered arid terrains worldwide. Additionally, this can be used as background to improve interpretation of other small scale studies. Improving the exploration potential of other more difficult regions of Australia will encourage industry exploration in Australia, and provide potential future economic gains.

Metal migration at the North Miitel Ni sulphide deposit in the southern Yilgarn Craton: Part 1, regolith and groundwater

Weathering and groundwater dispersion were studied at the North Miitel komatiite-hosted Ni sulphide deposit to understand the near-surface signature of this economically important mineralization style. Soil chemistry, regolith chemistry and groundwater chemistry show the secondary Ni enrichment is weak, limited primarily to the residual parent material and dispersed a few hundred metres laterally and only a few metres vertically above the water table. Partial extractions of soils did not enhance the visibility of the mineralisation to geochemistry. However, a subtle Ni anomaly was observed in the interface of the carbonate/clay accumulation at c. 30–40  cm depth. The results show weathering and hydromorphic dispersion by groundwater are responsible for the large geochemical halo related to mineralisation at a depth of 10–20 m. A second mechanism involving gases or vegetation is proposed for the subtle near-surface anomaly. To successfully explore through transported cover it is essential to understand how surface anomalies form, what metals move and which sample media will best represent the buried ore signature. Here, groundwater provides the largest multielement target (Ni, Co, Pt and Pd), residual regolith is most accurately able to locate the mineralisation using Ni, and Ni in the soil clay/carbonate interface presents the easiest to collect but subtle surface geochemical signature of the primary ore which is buried 400 m below surface.

Traditional and novel geochemical extractions applied to a Cu–Zn soil anomaly: a quantitative comparison of exploration accuracy and precision

ABSTRACT Partial extractions have been employed and promoted as geochemical exploration tools, but assessment of performance is infrequently undertaken. Minimum probability statistics were used to quantitatively compare twelve extractions and document the level of exploration precision and accuracy. These methods were valuable in comparing the different digestion methods, identifying the best performance level and determining an appropriate geochemical threshold for future exploration. Eight ‘conventional’ reagents were tested, namely ‘four-acid’, aqua regia, bacterial leach (LocatOre®), Genalysis® proprietary leach (TL3), cold hydroxylamine hydrochloride, Mehlich I reagent, hydrogen peroxide, deionized water, together with four new digestible digests (Coke®, Pepsi®, Diet Coke® and a Tempranillo red wine). These tests involved analysis of thirty <63-μm soil samples from the Wartook region of Victoria, Australia, which exhibit Cu and Zn geochemical anomalies in six samples. All extractions successfully identified the anomalous zone; however, the digestible digests, Mehlich I, Genalysis® leach and the hydroxylamine hydrochloride leach performed the best in terms of both accuracy and precision. The new extractions were particularly effective for Cu, whereas the hydroxylamine hydrochloride leach was best for Zn. The Coke®, Pepsi® and Diet Coke® extractions exhibited some buffering effects. In contrast, the Tempranillo wine is exceptionally well buffered and the most robust of the new digestions. Results indicate that even over marginally anomalous soils, many partial extraction techniques will confidently identify the location of mineralization. The use of expensive and proprietary procedures may not produce any better result than using standard reagents or even common beverage solutions as soil extraction reagents.

Metal migration at the North Miitel Ni sulphide deposit in the southern Yilgarn Craton: Part 2, vegetation and organic soil

Transported regolith cover hinders exploration for economically important komatiite-hosted sulphide ore bodies in the Yilgarn Craton. Biogeochemistry offers a surface sampling technique to explore for these buried deposits that has not been extensively tested in this region. In this study, eucalypt trees and some other common plants were investigated at the North Miitel Ni deposit for their use in exploration targeting and to understand what role vegetation is taking in creating near surface anomalous geochemical signatures. Primary ore is deep (c. 400 m) with a thin, weakly-enriched (compared to primary mineralisation), in situ weathered zone 10–20 m below the surface, under a cover sequence of 5–10 m deep. This study investigated the efficacy of several sample media including leaves, bark, litter, and organic-rich soils in an orientation traverse and a more extensive (150 m - spaced, 9 km2) grid-based survey (footprint). The organic-rich soil and biogeochemistry were influenced by contamination from the haul road. Directly over mineralisation some elements showed anomalous concentrations but data were erratic, not statistically significant and, therefore, not useful for mineral exploration. High groundwater salinity and low pH, lack of supergene development, weak subsurface signature and aeolian contamination contribute to why Ni anomalies have not been readily indentified at the surface. Nickel is also essential to plant physiology and is actively absorbed in a controlled manner. Investigation of plant-soil-water interactions is valuable in understanding metal mobility in the environment. Here we refined the viable exploration techniques in this setting where Ni sulphide discoveries through cover are important for future economic and resource development.

The dynamics of gold in regolith change with differing environmental conditions over time

Significant Au discoveries are becoming less common because the remaining prospective, underexplored areas are obscured by transported cover. At Moolart Well (Western Australia), secondary Au deposits hosted in transported pisolitic ferricrete and saprolite are overlain by younger transported cover. Here, we show how Au has been, and is being, dispersed and concentrated in these deposits and the overlying younger transported cover and biota during the evolution of the landscape. We identified coarse (>400 µm), Ag-rich, primary, angular Au accumulated residually along with some precipitated, Ag-poor ( Acacia aneura ) and termite mounds indicates active dispersion.