Abstract
The elements arsenic (As) and antimony (Sb) are commonly enriched in mesothermal gold deposits and predominantly occur as the sulfide minerals arsenopyrite and stibnite, respectively. In such deposits, these sulfides can be important carriers of nanoparticulate gold (Au). Arsenic and antimony are toxic at low levels (< 0.01 mg/L) so the stability of these minerals is of environmental importance. Furthermore, the geochemical processes that control the weathering, mobilisation and re-deposition of As and Sb in the supergene zone are also thought to control gold mobility. This thesis is the first study to consider the behaviour of As, Sb and Au together in the surficial environment through the application of SEM, XRD, fp-XRF and EBSD techniques.
A limitation of past studies of As and Sb behaviour in the surficial environment is that they focus solely on oxidation processes because these are the dominant reactions involved in the mobility and re-sequestration of these elements. This thesis provides new information on oxidation-related processes, and also describes newly-observed reductive micro-environments occurring within the surficial environment. These contrasting redox environments are described in a wide range of settings such as Cretaceous sedimentary rocks, Pleistocene gravels, and anthropogenic deposits (e.g., historic mine processing wastes). These redox processes occur over a wide range of time-scales, from decades in historic mine wastes, to millions of years in the Cretaceous Blue Spur Conglomerate. These observations suggest that reductive micro-environments are an important component of the supergene environment and are likely far more widespread than previously reported. As such, a new redox-dynamic model has been developed (see Graphical Abstract). In this newly-proposed redox-dynamic gradient model, the overall trend is towards oxidising conditions, but reductive micro-environments exist, due to highly localised fluctuations in Eh-pH. At least some of the redox processes described above have been facilitated by bacteria. Textural evidence presented in this thesis suggests that biological processes may be signifcant at the small-scale (nano- to mm-scale). However, at larger spatial scales and temporal scales (e.g., diagenesis of the Blue Spur Conglomerate) inorganic processes are dominant.
This thesis characterises two distinct sets of processes affecting gold in the supergene environment. In southern New Zealand, where groundwater is circum-neutral to alkaline, gold is predominantly mobilised by metastable thiosulfate ligands that form during the oxidative weathering of sulfides. In addition to redox-controlled (chemical) mobilisation, EBSD analysis has determined that the structure of alluvial gold grains is also affected by physical processes during erosion and transport. The principal deformation feature of this process are elongate smears on exterior surfaces of detrital Au. These smear structures are delicate and therefore an important intermediate stage in gold-particle shape change.
The work described within this thesis provides real-world geochemical constraints for future laboratory studies of supergene gold mobility. Experimental studies and field observations provide new insights into the redox pathways of arsenic and antimony with the aim of contributing to future mine remediation planning.