Schroder, Ivan1; de Caritat, Patrice1
1Geoscience Australia, Canberra, Australia
With the increasing need to extend mineral exploration under cover, new approaches are required to better understand concealed geology, and to narrow the mineral prospectivity search-space. Hydrogeochemistry is a non-invasive exploration technique based on the premise that groundwater interacting with a deposit or supergene alteration can cause anomalous elemental and isotopic signatures down-gradient. Water chemistry can reflect mineralisation directly, but can also reveal other key components of a mineral system, including fluid-flow pathways (e.g. fault/fracture zones), evidence for mineral system traps (e.g. evaporites, shales), or metal sources (e.g. mafic rocks).
The Northern Australia Hydrogeochemical Survey (NAHS) was a multiyear regional groundwater sampling program that aimed to understand the regional mineral potential within the Tennant Creek to Mt Isa area (Schroder et al. 2020). This presentation will explore the application of NAHS for investigating mineral potential of a region and present a workflow for establishing spatial or lithological baselines to evaluate hydrogeochemical anomalies.
The Georgina Basin is well known for its phosphate potential, with several >1 Mt deposits discovered in recent years such as Amaroo and Wonarah; however, the basin has been largely unmapped in terms of phosphate distribution under cover. This work focuses on a subset of 162 NAHS samples collected within two predominant aquifers of the Cambrian Georgina Basin (and time equivalents in the Wiso Basin). This focus restricts us to samples which experience a similar climate, recharge conditions, and aquifer compositions, reducing the hydrogeochemical variation that can mask intra-aquifer anomalies.
Elevated concentrations of P (used as a proxy for PO43- and normalised to HCO3– or Cl–) is observed in the groundwater on the eastern margin of the Georgina Basin. This region is known for Cambrian phosphorite deposits, with sampled bores proximal to a number of near-surface Georgina Basin phosphorite deposits. Additionally, several other subgroups within the Georgina Basin were identified with elevated P, corresponding to areas of greater cover and without known phosphorite deposits nearby.
Because the solubility of phosphate minerals depends on many factors — including groundwater physicochemical parameters (pH, T etc.), groundwater chemical composition, and phosphate mineral composition (such as degree of CO32- substitution) — a high (or low) P concentration is not a definitive indicator of the presence (or scarcity) of phosphates in the aquifer or recharge area. Thus, our data analysis focused on minor (i.e. F) and trace (i.e. U, V) elements commonly substituting in phosphate mineral structures or typically enriched in phosphorites, to evaluate these elements’ relationships with P concentration as a tool for recognising dissolution of phosphate-bearing minerals. When normalised to Cl– on log-log scatterplots, clear linear relationships between selected minor/trace elements and P concentrations become apparent in several of the identified subgroups that cannot be attributed to other water-rock interactions. This observation supports the implication that there is further undiscovered regions of the Georgina Basin with potential for hosting phosphorites.
Schroder, I.F., Caritat, P. de, Wallace, L., et al., 2020. Northern Australia Hydrogeochemical Survey: Final Data Release and Hydrogeochemical Atlas for EFTF. Geoscience Australia, Canberra. http://dx.doi.org/10.11636/Record.2020.015
Ivan is a geochemist within the Mineral Potential of Australia Section of Geoscience Australia. His focus in recent years has been on hydrogeochemistry and it’s application to mineral exploration. He has been an instrumental part of the Northern Australia Hydrogeochemistry Survey project which he will present on today.