[Truncated abstract] The Laverton Greenstone Belt (LGB), located in the northeastern part of the Eastern Goldfields Province (EGP) of the Yilgarn Craton, Western Australia, has a total contained gold endowment of over 690t. An important feature of the gold deposits in the LGB is their close spatial association with granitoids, with many gold deposits located adjacent to, or hosted by, granitoids. Recently-proposed genetic models for Archaean orogenic gold deposits have emphasised the role of granitoids in the formation of ore-deposits, but differ significantly in the nature of that role. Some models suggest that the granitoids are a source of ore-fluids and solutes, whereas others suggest that granitoids exert an important structural control on gold mineralisation. Such competing genetic models for gold mineralisation variably propose either a proximal-magmatic or distal-metamorphic, or less commonly distal-magmatic, source for goldbearing fluids, or mixing of fluids from multiple sources. Isotope geochemistry and geochronological studies are used to constrain the source and timing of auriferous fluids at nine gold deposits in the LGB in an attempt to differentiate between conflicting genetic models. To overcome the lack of detailed deposit-scale geological constraints inherent to any regional study, hypotheses generated from regional datasets are tested in a detailed case-study of the Wallaby gold deposit. The Pb-isotope compositions of ore-related sulphides from deposits in the LGB plot along the line representing crustal-Pb in the Norseman-Wiluna Belt of the EGP, with individual deposits clustering with other nearby deposits based on their geographic location. This trend is similar to that recorded in the Kalgoorlie-Norseman region in the southern EGP, and is consistent with a basement Pb reservoir for gold-bearing fluids. As such, data are consistent with a similar fluid source for all gold deposits. The Nd and Sr isotopic composition of goldrelated scheelite in the LGB clusters very tightly. The inferred ore-fluid composition has a slightly positive εNd, similar to ore fluids at other gold deposits in the EGP for which a proximal magmatic source is highly improbable. As such, Sr and Nd data are consistent with a similar fluid source for the gold deposits analysed in the LGB, but cannot unequivocally define that source. The median S, C and O isotopic compositions of ore minerals from all nine different gold deposits studied in the LGB fall in a very narrow range

Bibliography: leaves 233-260. / xxi, 260, [30] leaves : ill. (chiefly col.) ; 30 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Uses the late-orogenic, structurally-controlled Junction gold deposit near Kambalda, Western Australia, to examine the potential of soil-gas geochemistry as an exploration vector in an environment where mineralisation is present, but no ore-related trace elements are detectable in the near-surface regolith. / Thesis (Ph.D.)--University of Adelaide, Dept. of Geology and Geophysics 1999

[Formulae and special characters can only be approximated here. Please see the pdf version of the abstract for an accurate reproduction.] The formation of world-class Archean orogenic gold deposits in the Eastern Goldfields Province of Western Australia was the result of a critical combination of physical and chemical processes that modified a single and widespread ore-fluid along fluid pathways and at the sites of gold deposition. Increased gold endowment in these deposits is associated with efficient regional-scale fluid focusing mechanisms and the influence of multiple ore-depositional processes at the deposit-scale. Measurement of the complexity of geologic features, as displayed in high-quality geologic maps of uniform data density, can be used to highlight areas that influence regional-scale hydrothermal fluid flow. Useful measurements of geological complexity include fractal dimensions of map patterns, density and orientation of faults and lithologic contacts, and proportions of rock types. Fractal dimensions of map patterns of lithologic contacts and faults highlight complexity gradients. Steep complexity gradients, between domains of high and low fractal dimensions within a greenstone belt, correspond to district-scale regions that have the potential to focus the flow of large volumes of hydrothermal fluid, which is critical for the formation of significant orogenic gold mineralization. Steep complexity gradients commonly occur in greenstone belts where thick sedimentary units overly more complex patterns of lithologic contacts, associated with mafic intrusive and mafic volcanic units. The sedimentary units in these areas potentially acted as seals to the hydrothermal Mineral Systems, which resulted in fluid-pressure gradients and increased fluid flow. The largest gold deposits in the Kalgoorlie Terrane and the Laverton Tectonic Zone occur at steep complexity gradients adjacent to thick sedimentary units, indicating the significance of these structural settings to gold endowment. Complexity gradients, as displayed in surface map patterns, are an indication of three-dimensional connectivity along fluid pathways, between fluid source areas and deposit locations. Systematic changes in the orientation of crustal-scale shear zones are also significant and measurable map features. The largest gold deposits along the Bardoc Tectonic Zone and Boulder-Lefroy Shear Zone, in the Eastern Goldfields Province, occur where there are counter-clockwise changes in shear zone orientation, compared to the average orientation of the shear zone along its entire length. Sinistral movement along these shear zones resulted in the formation of district-scale dilational jogs and focused hydrothermal fluid-flow at the Golden Mile, New Celebration and Victory-Defiance deposits. Faults and lithologic contacts are the dominant fluid pathways in orogenic gold Mineral Systems, and measurements of the density of faults and contacts are also a method of quantifying the complexity of geologic map patterns on high-quality maps. Significantly higher densities of pathways in areas surrounding larger gold deposits are measurable within 20- and 5-kilometer search radii around them. Large variations in the sulfur isotopic composition of ore-related pyrites in orogenic gold deposits in the Eastern Goldfields Province are the result of different golddepositional mechanisms and the in-situ oxidation of a primary ore fluid in specific structural settings. Phase separation and wall-rock carbonation are potentially the most common mechanisms of ore-fluid oxidation and gold precipitation. The influence of multiple gold-depositional mechanisms increases the potential for significant ore-fluid oxidation, and more importantly, provides an effective means of increasing gold endowment. This explains the occurrence of negative δ34S values in ore-related pyrites in some world-class orogenic gold deposits. Sulfur isotopic compositions alone cannot uniquely define potential gold endowment. However, in combination with structural, hydrothermal alteration and fluid inclusion studies that also seek to identify multiple ore-forming processes, they can be a useful indicator. The structural setting of a deposit is also a potentially important factor controlling ore-fluid oxidation and the distribution of δ34S values in ore-related pyrites. At Victory-Defiance, the occurrence of negative δ34S(py) values in gently-dipping dilational structures, compared to more positive δ34S(py) values in steeply-dipping compressional structures, is potentially associated with different gold-depositional mechanisms that developed as a result of fluid-pressure fluctuations during different stages of the fault-valve cycle. During the pre-failure stage, when fluids are discharging from faults, fluid-rock interaction is the dominant gold-depositional mechanism. Phase separation and back-mixing of modified ore-fluid components are dominant during and immediately after faulting. Under appropriate conditions, any, or all, of these three mechanisms can oxidize orogenic gold fluids and cause gold deposition. The influence of multiple gold-depositional mechanisms during fault-valve cycles at dilational jogs, where fluid pressure fluctuations are interpreted to be most severe, can potentially explain both the large gold endowment of the giant to world-class Golden Mile, New Celebration and Victory-Defiance deposits along the Boulder-Lefroy Shear Zone, and the presence of gold-related pyrites with negative δ34S values in these deposits. This study highlights the interplay that exists between physical and chemical processes in orogenic gold Mineral Systems, during the transport of ore fluids in pathways from original fluid reservoirs to deposit sites. Potentially, a single and widespread orogenic ore-fluid could become oxidized, and lead to the formation of ore-related sulfides with variable sulfur isotopic compositions, depending on the nature and orientation of major fluid pathways, the nature of wall-rocks through which it circulates, and the precise ore-depositional processes that develop during fault-valve cycles.