Metal migration at the DeGrussa Cu-Au sulphide deposit, Western Australia: Soil, vegetation and groundwater studies

Abstract

Transported overburden is a persistent problem for mineral exploration in Australia and elsewhere, with explorers not confident in the current understanding of where, when and how geochemical anomalies form (or don't form) in surficial materials and how to make the critical interpretive link to buried deposits. The recent discovery of the high-grade VHMS Cu-Au DeGrussa deposit on the margins of the Yilgarn Craton in the Bryah Basin of Western Australia highlights the value of effective exploration in these covered terrains. We investigated this deposit to understand the near surface geochemical signature from weathering and groundwater dispersion. Multiple sample media (soil, regolith, groundwater and vegetation) and analytical methods were used to examine metal migration. The DeGrussa (Cu-Au) ore body was successfully identified using organic soil, vegetation and groundwater with multi-element anomalies associated with the buried ore. Primarily, the anomalies are caused by weathering and vertical dispersion through thin (2 – 10 m) transported cover and vegetation cycling of Au and Cu located a few metres below the surface. Aqua regia extractions of soils and vegetation, and MMI-M extraction of soils were effective by providing strong, coherent multi-element anomalies at relatively close spacing (50 m). Conversely, portable XRF analysis was less effective at this site as the elemental concentrations were too low. Gold, As and Cu in groundwater were present in anomalous concentrations, but inconsistently so at larger sample spacing (100 s of metres). When compared to regional hydrogeochemistry (e.g. >1000 m spacing), Au, As, Cu, Pd and Pt element concentrations in the DeGrussa groundwater were elevated in a broad, general area along strike potentially identifying additional targets. Weathering, hydraulic lift by vegetation and hydromorphic dispersion are the mechanisms responsible for metal migration at the DeGrussa site.