Understanding geochemical interfaces in the Georgina Basin. Applications for mineral exploration.

In-situ near-surface geological boundaries and redox transition zones are valuable geochemical interfaces for trapping and concentrating elements that can be indicators for deeper or distal mineralisation. As a result, they can be a more cost-effective and geochemically sensitive sampling target than in-situ alteration or mineralisation, yet this is often overlooked in traditional exploration and drilling campaigns. However, the geochemical processes that transport or trap these elements can differ markedly depending on many factors including lithology, weathering regime and hydrogeology. Thus, building an understanding of the local geochemical interface(s) and their provenance is needed when entering a new geological province (or basin), to best enable their use as a tool for mineral exploration.

As part of Geoscience Australia’s Exploring for the Future Program, our investigation focuses on the Georgina Basin. Using new whole-rock and isotope geochemistry from 9 drillholes in the central Northern Territory, we demonstrate an improved understanding of the processes affecting geochemical migration in this region. Our findings provide insights into Georgina Basin's sediment-hosted Zn-Pb and phosphate potential, while also having relevance for new clay-hosted rare-earth element (REE) prospectivity.

Portable XRF data of these drillholes provided the first indication of geochemically enriched interfaces in the Georgina Basin, with elevated Pb, Zn, Cu, P and REEs observed at consistent depths between drillholes. Follow-up whole-rock geochemistry (76 samples) and isotopes (Pb and Sr, 17 samples) is presented here, which targeted transects through these interfaces, as well as background lithostratigraphy.

Looking into the elemental and mineral associations across these interfaces, clear redox transitions are evident at two depths in the basin. The shallower geochemical interface (~5-20 m depth) is interpreted as a surface weathering profile, while the deeper interface, coined the water-intercept zone (~40-70 m depth), is found near the water-table of the regional aquifer. Supported by geochemistry, we discuss the mechanisms that have formed the water-intercept zone, covering the provenance, migration and trapping of geochemical signals. Finally, we showcase how the water-intercept zone can be practically used for mineral exploration, by integrating with the groundwater chemistry to validate and map radiogenic Pb isotope anomalies representative of the basin’s sediment-hosted Zn-Pb mineralisation.

Abstract presented at the 30th International Applied Geochemistry Symposium 2024 (IAGS 2024)