Gold exploration using groundwater in Western Australia

Abstract

The use of groundwater geochemistry to explore for Au mineralization has faced some challenges. For this type of mineralization, the best pathfinder element is Au itself. However, the analytical technique for precious metals including Au and Ag has a relatively low sensitivity, in particular for brackish to saline waters. Medium- to high-salinity waters need to be diluted before analyses, leading to even lower Au concentrations and more samples below detection thresholds. Here, we present methods where a pre-concentration step is added by using activated carbon sachets that adsorb and concentrate the precious metals of interest from the water sample. This approach was used during a regional survey in the Northern Yilgarn Craton, as well as on several smaller case studies, to determine the potential of groundwater geochemistry in identifying Au mineralization at depth. The Northern Yilgarn Craton is a particularly well-suited area for groundwater hydrogeochemistry surveys as it has an abundance of sampling sites at windmills and bores. Groundwater analysis of the Northern Yilgarn Craton was performed on >5000 samples following a semi-regular grid of 4–10 km spacing. This study covers an area of c. 315 000 km 2 including numerous minesites and buried ore deposits. The regional study revealed many widespread groundwater Au anomalies when samples were collected <5 km of a Au deposit. The primary pathfinder elements are As and Ag, and the oxy-anions Mo and W are shown to be closely related to Au mineralization and are effective pathfinders. Each of the pathfinder elements shows potential but cannot be used as a single element to reveal the mines and deposits with accuracy. This is mostly related to the diversity of orebody geochemistry. However, these pathfinder elements can highlight areas of interest and have value when used in combination with other elements and within a regional context. Yet, the oxy-anions Mo and W stand out. When detected in groundwater these elements are closely related to Au mineralization and are effective pathfinders. The multi-element index AuMin can solve some of these individual elements’ limitations and be used to highlight areas of interest from the groundwater hydrogeochemistry. AuMin anomalies are more widespread than single Au anomalies but it is useful to focus the exploration effort. The ‘background’ baseline data provided by the regional sampling are crucial when interpreting the data acquired around minesites and deposits. It revealed differences between east and west Northern Yilgarn Craton greenstone hosted samples. In the eastern area we focused on two case studies on different style deposits: orogenic Au at Agnew and secondary and orogenic Au at Garden Well. Groundwater geochemistry at Agnew returned a high concentration of many pathfinder elements (As, Ag, Co, Sb, W), which strongly reflects the pathfinder assemblages found in individual orebodies along the mineralized corridor. In contrast, groundwater sampled at Garden Well had only a few samples with detectable or significant concentrations in pathfinder elements (Ag and minor As). We showed that Au in groundwater is a powerful technique to identify areas of Au mineralization undercover, in particular when detected in association with pathfinder elements. Individual pathfinder elements in isolation are challenging to interpret as they can be associated with other mineral systems (e.g. Ni or volcanic hosted massive sulfides deposits). Overall, the application of groundwater geochemistry to Au exploration in the Northern Yilgarn Craton is promising, as long as the data are put into a regional geo-environmental context and used for large-scale exploration such as tenement selection rather than deposit delineation. Supplementary material: Additional site data and geological background legends are available at https://doi.org/10.6084/m9.figshare.c.6454122 Thematic collection: This article is part of the Hydrochemistry related to exploration and environmental issues collection available at: https://www.lyellcollection.org/topic/collections/hydrochemistry-related-to-exploration-and-environmental-issues