Groundwater flow in playa lake environments: impact on gold and pathfinder element distributions in groundwaters surrounding mesothermal gold deposits, St. Ives area, Eastern Goldfields, Western Australia

Abstract

Geochemical sampling of groundwater may be an effective tool in exploring for Au deposits in areas of transported overburden. However, to use hydrogeochemistry effectively, we need to understand which elements are useful pathfinders and how their distribution patterns are affected by groundwater flow and geochemistry, especially in areas that have hypersaline groundwater. A hydrogeochemical survey was completed over the strongly Au-endowed St. Ives area of the Eastern Goldfields of Western Australia. Extensive geochemical and hydrogeological data were measured from 80 holes drilled on a kilometre-spaced grid. A numerical model was developed to quantify the groundwater flow regime in the study area. Also, hydrogeological data in combination with the geochemistry of the waters were used to identify a complex flow regime adjacent to Lake Lefroy, a playa lake in the area. Regional brine flows towards the salt lake, evaporation occurs and hypersaline (playa) brines are formed that sink and flow away from Lake Lefroy and beneath the regional brines. Density-driven, convective mixing of these two brines is responsible for forming a zone of brines with transitional compositions located near the edge of Lake Lefroy. Knowledge of the groundwater flow regime was used to interpret the distribution of elements around bedrock Au mineralization. An Au-only groundwater anomaly (>10 ppt, maximum 52 ppt) is located downstream of the Junction Au ore body. No anomaly was identified in deep brines associated with the nearby Argo/Apollo Au ore body; however, shallow regional brines sampled in a previous survey showed elevated Au values overlying and downstream of the mineralization. The lack of elevated Au concentrations in deeper groundwater associated with Argo/Apollo is because the samples collected were hypersaline playa brines sourced from Lake Lefroy that had not flowed over the mineralization. Possible pathfinder elements, such as Sb, Bi and Te, were either present in concentrations too low to detect or showed no spatial pattern related to known mineralization; however, a strong correlation was noted between some of their concentrations and salinity. Evaluation of the groundwater anomalies observed in this study suggests that hydrogeochemical exploration is most effective at a resolution of 0.5–4 km spacing, and does not require fully cased and packed wells, at least in cases where no confined aquifers are intersected in drilling. The results of this study illustrate the importance of understanding groundwater flow regimes in planning and interpreting hydrogeochemical surveys.

A spectrophotometric study of aqueous Au(III) halide–hydroxide complexes at 25–80°C

“…This is much higher than commonly detected in groundwater associated with gold mineralization (e.g., up to 52 ppt; Gray, 2001;Carey et al, 2003), although up to approximately 40 ppb has been detected in groundwater from drill holes within pits of gold mines in the Yilgarn of Western Australia (the high values may be an artifact of the mining activity and consequent release of high levels of thiosulfate; Gray, 2001). The implications of this are that these groundwaters are well undersaturated with respect to native gold, even in areas of gold mineralization, and/or that the thermodynamic properties for native gold solubility reactions at low temperature overestimate the solubility and need to be refined.…”

“…The presence of [AuCl 4 ] À and other high-order gold halide complexes has potential implications for the formation of secondary supergene deposits, as the solubility of native gold decreases markedly with decreasing halide concentration (Fig. 13), important, for example, at the margins of playa systems where brines mix with less saline waters (e.g., Lake Lefroy in Western Australia;Carey et al, 2003). For many oxidized, saline waters and brines, [AuCl 4 ] À is likely to be the predominant gold complex, but for Cl/Br ratios such as in the Smackover Formation and seawater, mixed Au(III) bromide-chloride complexes are likely to predominate at acidic pH (e.g., less than 6; Fig.…”

“…Despite the research already published, as summarized above, there is a need to understand better the nature and properties of gold aqueous halide-hydroxide complexes, for applications such as using dissolved gold in groundwater for mineral exploration (e.g., Carey et al, 2003) and processing of gold ores (e.g., Senanayake, 2004). In this paper, we present the results of UV-Vis spectrophotometric experiments aimed at identifying the nature of Au(III) halide (Cl À , Br À and I À ) and hydroxide-halide aqueous complexes, and deriving their thermodynamic properties.…”

The immobilization of gold from gold (III) chloride by a halophilic sulphate-reducing bacterial consortium

“…Therefore, the occurrence of soluble gold must be considered dynamic since precipitation of elemental gold and formation of gold (III) chloride complexes can occur contemporaneously. It is important to note that gold concentrations have been measured in natural saline waters (Koide et al 1988) and in surficial, hypersaline groundwater located near mesothermal gold deposits (Carey et al 2003).…”

“…Understanding these biospheric contributions to gold immobilization has relevance because they provide information about fundamental processes that need to be incorporated into geochemical exploration programmes, creating additional means of vectoring geochemistry data to discover buried gold deposits and near-surface deposits containing gold. The Eastern Goldfields of Western Australia is one example of such an environment where soluble gold is transported as gold (III) chloride within groundwater systems and immobilized within calcrete (Carey et al 2003). Studies by Lintern et al (2009Lintern et al ( , 2011 indicated that, in semi-arid regions, soluble gold is derived from the leaching of underlying primary sources and transported to surficial environments.…”