Redox Controlled Trace Element Attenuation In Pyrite: Observations From Natural & Experimental Studies

Abstract

The ubiquitous presence of pyrite in economic gold deposits is well established. It is understood that pyrite can host concentrations of gold as inclusions or substituted into the lattice structure. Pyrite can attenuate a wide gamut of elements over an extensive range of pressures, temperatures and (oxygen and sulfur) fugacities. Pyrite is often formed during every stage of ore development. This thesis investigates the possibility of using pyrite crystals to track and monitor changes in geochemistry during hydrothermal deposition, in particular Au-ore formation. Using LA-ICPMS and SHRIMP-SI to study pyrites from varied geologic deposits and experimental settings, the globally observed relationship between arsenic and gold will be investigated. The Au-deposit Wallaby (WA) is a major deposit which exhibits 5 generations of mineralised veins that each contain pyrite. Trace element concentrations demonstrate that pyrite displays progressive and systematic changes in geochemistry, particularly in respect to arsenic and nickel. Thus, pyrite can be used as a monitor of changing redox conditions. Argo (WA) is a mesothermal lode gold deposit which exhibits fluid-rock interaction culminating in 4 mineralised veins. All veins contain pyrite and the later stage veins contain pyrrhotite. Similarly to Wallaby, trace element analyses show changes in the pyrite geochemistry however, the appearance of pyrrhotite, a competing Fe-S mineral, causes these trends to differ. This study documents how co-precipitation of related minerals can greatly influence the concentrations of trace elements in pyrite and how partition coefficients may be successfully employed to better understand these changes and relate these to systematic changes relative to Au deposition. The S and I Type granites of the Lachlan Fold Belt are best known for classification systems based upon bulk chemistry. Due to their limited abundance, pyrites have not been investigated thoroughly in the LFB granites and as such present an excellent opportunity to use a single mineral approach in an attempt to delineate pyrites based on redox. The trends observed in the pyrites of the LFB granites suggest a wider control on partitioning of elements into sulfides and how this is linked to ore deposit genesis. At Wallaby, Argo and the Lachlan Fold Belt Granites, high Arsenic pyrites were related to Au in reduced conditions. Currently there is no clear understanding of what drives the mode or tenor of gold deposition over a range of redox conditions in the presence or absence of Arsenic. How Au oxidation states may be related to gold transport is also unclear. The experiments in this thesis aim to provide understanding into how a gold deposit bearing pyrite, may form (in particular Carlin type). Experiments are set over a range of oxygen fugacities and use CVT for crystal synthesis. Crystals were doped with As, Au0 and Au+. While As and Au (regardless of oxidation state) were shown to covary, although As and Au concentrations in pyrite increased systematically with reduction in experiments starting with As+Au0, in runs starting with As +Au+ concentrations decreased with reduction. This contrasting behaviour is controlled by the mode of gold deposition, i.e whether gold deposits as free gold or within the pyrite structure. The results demonstrate the usefulness of pyrite as a monitor of reduction over a range of geological settings. This implies a wider universal control on pyrite deposition which this thesis investigates. The trends observed in both the natural and experimental studies potentially explain why some magmatic rocks are more likely to host ore bodies and how factors influencing mineralisation can affect the tenor of differing ore deposit types. Show more