Development of the diffusive gradients in thin films technique for geochemical exploration of gold

[Truncated] Gold, chemical symbol Au, is a precious metal that exists in most soils, sediments and natural waters at extremely low concentrations (<1 μg/kg). It has become increasingly difficult to find, especially in geological landscapes that feature regolith and transported overburden. This research modified and evaluated a passive sampling technique, diffusive gradients in thin films (DGT), for the geochemical exploration of gold. The DGT technique is used extensively for measuring trace metal concentrations in waters, sediments and soils; however, the technique has not been applied to geochemical exploration until now. This thesis details how the technique has been modified to detect Au and applied in a range of environments, including in groundwater surrounding an Au deposit, in an estuary containing trace quantities of dissolved Au, and in auriferous soils obtained from four Australian prospects.

The DGT technique was modified to detect Au by developing a new binding layer made with activated carbon, which was then evaluated under well-defined laboratory conditions. The performance of this modification was assessed using Au(III) in solution by: 1) determining the diffusion coefficient of Au(III) in hydrogels; 2) determining the uptake of Au(III) by the new activated carbon binding layer; 3) assessing the capacity of the activated carbon binding layer to adsorb Au; 4) determining the effect of pH and ionic strength on performance, and; 5) assessing the selectivity of the new binding layer for Au. The DGT technique was shown to behave predictably under all tested conditions (CDGT was between 92% and 109% of solution concentrations), so long as changes to Au speciation (and therefore the diffusion coefficient) are accounted for.

The second phase involved assessing the DGT technique in groundwater surrounding a known Au deposit. The DGT technique successfully detected signatures of Au mineralization in groundwater (between 2.0 ng/L and 213.4 ng/L), as well as associated pathfinder elements (As and Sb), which were statistically correlated with grab-sample Au. This study also evaluated DGT devices equipped with Purolite® A100/2412 binding gels for the detection of nanoparticles. Generally, the DGT technique demonstrated methodological improvement over grab sampling of groundwater for Au and pathfinders with respect to sensitivity (detection limit of 1.8 ng/L), replication and portability. In the field, Purolite DGT detected a considerably higher concentration of Au in two boreholes than measured by carbon DGT, suggesting the possible existence of Au nanoparticles in these waters; however, further investigation is required.