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.