High Grade Au Deposits: Processes to Prediction

This report documents progress made over the life of M410 in understanding the processes that controlled high grade gold deposition and application to targeting in the brown-fields environment. The three modules of MERIWA M410 were organised around a series of site campaigns and off site studies coupled with an on-site embedded researcher. The modules focused on macro-scale 4D alteration and chemical gradient mapping (Module 1), micro-scale gold characterization, transport and deposition (Module 2) and study of the very high grade Wattle Dam deposit (Module 3).

Field campaigns conducted on the Argo-Apollo- Athena-Hamlet-Yorick (AAAHY) corridor, St Ives and Wattle Dam deposit created integrated pXRF, hyperspectral, stable isotopic and minor mineral data sets that have been used to define the lithological architecture and map the critical chemical gradients controlling Au deposition. Variations in stable isotope ratios (δ¹³Ccarbonate and δ³⁴Ssulfide) were used to map redox gradients (chapter 2) and the zoning in the minor Ti-phases (titanate, ilmenite, rutile) used to identify volatile-rich zones (ƒCO₂(g) + ƒCH₄(g)) with low water activity (chapter 3). Gradients in pH and sulphur activity appear to be related to redox and water activity gradients.

Gold deposition occurs at high pH and volatilepressure (ƒCO₂(g) + ƒCH₄(g)) and low water activity but across a range of redox and sulphur activity conditions. These constraints provide a basis for mineral-systems targeting of high grade Au resources. Proxies developed from the multi-element and spectral data allow mapping of reduced and oxidized pathways at a range of scales. More than 10 target areas have been identified on the basis of coincident reduced-alkaline domains and oxidized, S-rich fluid domains in suitable architectural settings (chapter 2).

The field work was supported by detailed SEM (Scanning Electron Microscopy) mapping of minor mineralogy, paragenetic studies and trace-metal mapping with the Australian synchrotron. The outcomes of these detailed studies are summarized in chapters 3, 4, 5 and 7. We are of the view that the tools developed in this project to map redox gradients and gradients in anhydrous volatilepressure can be adapted for routine use in brownfields exploration. Hyper-spectral proxies for isotopic mapping could accelerate mapping of redox gradients.