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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.

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Geochemical orientation surveys were completed over covered Carlin-type gold deposits in the Cortez mine area with the expressed aim of identifying and evaluating exploration methods to discover Au ore under transported alluvial cover in Nevada. Orientation tests were designed to assess the utility of geochemical applications at various scales of exploration, both drill targets at the deposit scale and "footprints" associated with deposits at the district scale. Detailed surveys were completed over the covered Gap deposit, located adjacent to the giant Pipeline deposit. Both Carlin-type gold mineralization and earlier, spatially associated, base metal skarn mineralization at Gap were located by soils, soil gas, and vegetation. Loam soils at 6-to 12-cm depth provided a consistent and uniformly available sample medium. Gold ore under 10 m of cover in the northern portion of the Gap deposit was readily detected by analysis of Au by fire assay and ultra trace aqua regia methods in the-80 mesh fraction of these loam soils. Arsenic anomalies occur over the northern end and over the main ore zone at Gap, where there is 25 to 50 m of alluvial cover. Zinc concentrations in soil show the most coherent spatial relationship with underlying Au ore. Tests of MMI-B and Enzyme Leach selective leaches did not result in significant enhancement of the anomalies relative to aqua regia. CO2 and O2 in soil gas indicate faults and underlying mineralized carbonates at Gap, where weathering reactions likely generated CO2 from acid reaction with carbonate. Elevated Au and As in mixed sagebrush and shadscale occur over most ore zones, with the highest Au concentrations over the main ore zone rather than the shallowly buried northern zone. Like soils, elevated Zn in vegetation shows the most coherent spatial relationship with underlying ore. Assays from 1,859 drill holes show a 4-to 5-km 2 "footprint" at the basin gravel-bedrock unconformity centered on the Pipeline Carlin-type gold deposit, where samples of basal alluvium provide a large, coherent >50 ppb Au anomaly. Enriched As, Tl, K, and F in alkaline groundwater sampled from monitoring wells surrounding the Pipeline open pit provide a ≥5-km 2 hydrogeochemical footprint. Higher As and Tl concentrations occur down gradient from Pipeline indicating Carlin-type Au mineralization is the source of the enrichment. The surface metal anomalies are consistent with upward migration of metals through fractured alluvial cover. Likely metal transport mechanisms include barometric pumping of gases or seismic pumping of groundwater. Upward diffusion of metals through the thick vadose zones in northern Nevada is not a viable process. Surface anomalies over Gap and other deposits appear to be mature and may have developed over millions of years. In such mature anomalies, much of the metal that migrated from underlying bedrock is probably hosted in resistant secondary minerals, which are more readily dissolved by aqua regia than by various selective leaches. Sampling of soil gas and soils is appropriate ...

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