School of Earth Sciences - Theses

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    Geologic expressions of faulting and earthquake strong ground motions in intraplate bedrock terrains
    King, Tamarah Rosellen ( 2019)
    Australian earthquakes offer unique opportunities to investigate environmental and landscape effects of reverse rupturing faults. All historic surface-rupturing earthquakes have occurred in arid, low-relief, bedrock dominated areas with little to no anthropogenic influence. Environmental earthquake effects identified following the 2016, reverse-mechanism, MW 6.1 Petermann earthquake in remote central Australia are categorised with the Environmental Seismic Intensity scale, the first application of this scale for an Australian earthquake. The intensity and distribution of environmental damage demonstrates strong asymmetry due to fault geometry, with damage increasing towards the surface rupture rather than epicentral region. The direction and distances of 1,437 co-seismically displaced rock fragments (chips) in the near-field of the Petermann earthquake provide a dense proxy-record of strong ground motions, both along- and across-rupture. Chips record preferred azimuths of displacement that are attributed to rupture fling effects. This unprecedented geological proxy-record of the distribution, directivity and intensity of strong ground motions has important implications for hazard analysis in the near-field of reverse earthquakes. Fine-scale mapping of the 2016 Peterman surface rupture and secondary fractures using field, drone-derived and remote-sensing datasets indicates surface rupture characteristics vary with changes in surface geology. Deformation zones are wider and less recognizable in granular materials (e.g. dunes, alluvium) compared with those in proximal bedrock. Kinematic analysis of bedrock fractures indicates sinistral-reverse faulting, consistent with published focal mechanisms, and a maximum compressive stress orientation generally consistent with the inferred regional SHMax orientation. Trenching and 10Be cosmogenic nuclide erosion rates provide preliminary evidence of absence for prior rupture on the Petermann faults within the last 200 to 400 kyrs. The 2016 earthquake is therefore hypothesized to be the first to rupture this fault in the near surface. Analyses of geological and geophysical data from ten moderate magnitude (MW 4.7 – 6.6) historical surface-rupturing earthquakes in cratonic Australia indicate that rupture likely propagated along pre-existing Precambrian bedrock structures. Six of seven events show evidence of multi-fault rupture across 2 to 6 discrete faults of greater than 1 km length, placing these events as some of the most structurally complex earthquake ruptures identified globally for this magnitude. No unambiguous geological evidence for preceding surface-rupturing earthquakes is present. This raises important questions regarding the recurrence behaviour of intraplate stable continental region faults, with implications for seismic hazard analysis. In summary, this thesis explores observational, seismic, and remote-sensing data of surface rupturing earthquakes in Australia to provide new (i) data regarding the recurrence patterns of Australian earthquakes (ii) insights into basement controls on these earthquakes (iii) and methods to quantify seismic directionality behaviour common to reverse earthquakes globally. These contribute to better understanding the why, what, when, where of intraplate earthquakes, and how seismic hazard varies across diverse tectonic and crustal environments.
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    Cryogenian iron formations: glaciation and oxygenation
    Lechte, Maxwell ( 2018)
    The Cryogenian Period (720–635 Ma) experienced extreme glaciations broadly coincident with a transformation of the Earth’s surface oxidation state, supercontinent breakup, and the evolution of complex animal multicellularity. However, the cause-and-effect relationships of these events are unresolved. The Cryogenian ice ages, known as ‘Snowball Earth' events, would have placed important constraints on the biosphere, and it remains unclear what role global refrigeration played in setting the stage for eukaryotic diversification and the origin of animals. The Cryogenian also experienced the deposition of iron-rich marine chemical sediments (iron formations), representing the first episode of global iron formation deposition in over one billion years. This shift in iron cycling highlights complexities in seawater chemistry and oxidation state during this time, and these iron formations offer valuable insights into Cryogenian palaeoenvironments. Iron formations from Cryogenian glacial successions in Namibia, USA and Australia were studied in order to investigate Cryogenian iron formation genesis and elucidate the relationships between glaciation, ocean chemistry, oxygenation and biotic evolution. In-depth sedimentology, stratigraphy and petrography reveals that these iron formations are intimately associated with Sturtian glacial sediments and are interpreted have been deposited in a range of glaciomarine environments. Geochemical analysis of these chemical sediments permits the reconstruction of Cryogenian ocean chemistry and the synglacial palaeoredox landscape. Multiple geochemical proxies, including rare earth element and iron isotope systematics, indicate widespread marine anoxia with increasing seawater oxidation with proximity to the ice shelf grounding line. A genetic model is proposed whereby the mixing of oxygenated glacial fluids with ferruginous seawater led to the deposition of iron formations in glacial successions during the Cryogenian. Atmospheric oxygen trapped in glacial ice was likely an important oxidant source, delivered to Cryogenian glaciomarine environments via subglacial meltwater outwash. This meltwater supply may have been crucial in establishing oxygenated marine habitats for eukaryotes, including early animals, during Snowball Earth. Multi-million-year oxidation of the oceans via this mechanism may have also set the stage for a Neoproterozoic marine oxygenation event.
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    Isotopic disequilibrium in granitic systems: the origins of heterogeneity in granites and implications for partial melting in the crust and petrogenetic models
    Iles, Kieran Anthony ( 2017)
    Unravelling the processes involved in granite magmatism is essential to our understanding of the continental crust, its formation and evolution. Radiogenic isotope systems are commonly employed to this end, but the behaviour of these systems may not be as simple as is often assumed. Understanding the causes of isotopic complexity in granite suites is the aim of this research. By modelling the disequilibrium partial melting of isotopically heterogeneous protoliths the different compositions of the melt, source and restite for a range of hypothetical scenarios have been calculated. Results demonstrate that the melt produced may have Sr, Nd and Hf isotope compositions distinct from both the protolith and restite. A corollary is that restite-bearing magmas may exhibit different isotope compositions than their melts, a feature which should be preserved as a difference between the Hf isotope compositions of bulk-rock samples and their magmatic zircon populations. The same modelling also suggests that a single source rock can produce melts with diverse isotope compositions. The predictions of this modelling have been tested by analysing S- and I-type granites from the Lachlan Fold Belt, southeastern Australia, including iconic examples of restite-bearing rocks. Comparisons of Hf isotope compositions between bulk-rocks and their magmatic zircons reveal discrepancies (ΔεHfbulk-zircon) ranging from -0.6 to +2.5 ε units for I-type granites. This intra-sample Hf isotopic heterogeneity is interpreted to represent disequilibrium between the melt and restite assemblage. The ΔεHfbulk-zircon values are consistent with calculated ΔεHfmagma-melt values (from -4.2 to +7.4) based on the disequilibrium amphibole dehydration melting of 0.5-1.0 Ga meta-igneous protoliths. S-type granites also record differences between their bulk-rock and magmatic zircon Hf isotope compositions; however, the disparity is more subtle. Both positive and small negative ΔεHfbulk-zircon values are observed, consistent with modelling the partial melting of isotopically heterogeneous meta-sedimentary protoliths. In addition to low-temperature granites, case studies of two high-temperature I-type granitoid suites (Boggy Plain and Wallundry) have also been conducted. Both display a weak coupling between geochemical parameters that have been interpreted previously to indicate the involvement of assimilation and fractional crystallisation (AFC) processes. Positive ΔεHfbulk-zircon values obtained in the Boggy Plain Suite support the existing petrogenetic model in which basaltic melt becomes variously contaminated by material derived from the continental crust. The positive value is explained by retention of earlier-crystallised, more radiogenic phases in isotopically evolved, more felsic samples. In contrast, the Wallundry Suite is characterised by negative ΔεHfbulk-zircon values caused by the presence of unmelted components of its contaminant. A complex interplay of contamination, crystallisation, melt segregation and interaction between magma batches is required to account for the Wallundry Suite isotope data. The results of this study indicate that disequilibrium partial melting can produce within-suite isotopic variability without recourse to assimilation or mixing processes (1) in mafic to felsic samples caused by the progressive separation of melt from its isotopically distinct restite assemblage; and (2) via the extraction of multiple batches of isotopically distinct melts produced from a single source as anatexis proceeds. Furthermore, the isotope variation resulting from restite unmixing may be distinguished from magma mixing by decoupling of the Rb-Sr, Sm-Nd and Lu-Hf isotope systems. Importantly, the isotopic discrepancy between bulk-rock granite samples and their magmatic zircon populations suggests that the most mafic bulk-rock granite samples of a given suite, not magmatic zircon, provide the most accurate estimate of source rock Hf isotope compositions. This raises concerns regarding the ubiquitous use of zircon Hf isotope data to constrain crustal growth models.
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    Aerosol contributions to speleothem geochemistry
    Dredge, Jonathan ( 2014)
    There is developing interest in cave aerosols due to the increasing awareness of their impacts on the cave environment and speleothems. This study presents the first multidisciplinary investigation into cave aerosols and their potential contribution to speleothem geochemistry. Aerosols are shown to be sourced from a variety of external emission processes, and transported into cave networks. Both natural (marine sea-spray, terrestrial dust) and anthropogenic (e.g. vehicle emissions) aerosol emissions are detected throughout caves. Internal cave aerosol production by human disruption has also been shown to be of importance in caves open to the public. Aerosols produced from floor sediment suspension and release from clothing causes short term high amplitude aerosol suspension events. Cave aerosol transport, distribution and deposition are highly variable depending on cave situation. Cave morphology, ventilation, and environmental conditions will influence how aerosols are distributed through cave networks. Aerosol deposition monitoring in Obir Cave, Austria has shown the significance of cave chamber size in aerosol transport, with large open chambers presenting higher levels of deposition. Modern monitoring of suspended aerosol concentrations, CO2 and temperature in Gough’s Cave, Cheddar Gorge have presented a strong relationship with cave ventilation processes. Temporal variations of aerosol levels have demonstrated the ability of aerosol monitoring to record seasonal ventilation shifts, beyond anthropogenic influences. Aerosol minima (based on 24 hours) provide a representation of natural aerosol baseline conditions without diurnal anthropogenic influences. Aerosols have shown a quicker recovery to natural background levels when compared to CO2 and T, making aerosols a sensitive and effective monitoring tool. When used in combination with more established monitoring methods, suspended aerosol monitoring is a beneficial addition to cave environmental studies. Theoretical modelling and calculations based on modern aerosol monitoring have established that aerosol contributions are highly variable. In some instances, modern aerosol supply is sufficient to account for speleothem geochemistry concentrations entirely. Aerosol contributions are of greatest significance under slow growth or hiatus scenarios and high aerosol deposition scenarios. Geochemical and stratigraphical analysis of a flowstone core from Gibraltar has highlighted the importance of hiatus events for future aerosol studies. Hiatus events provide a unique opportunity to investigate the type and amount of aerosol deposition and accumulation. Marine aerosol contributions have been quantified in the Gibraltar flowstone core and account for 18.5% of speleothem Sr. Sr isotopic analysis has confirmed the significance of marine aerosol contributions. Flowstone analysis has also demonstrated the ability of speleothems to record shifts in the supply of highly radiogenic terrestrial dust. Bio-aerosol deposits and bacterial colonisation have been identified as a potential source of trace element bioaccumulation and flowstone coloration in Yarrangobilly Caves, Australia. Bio-aerosols have shown to be deposited throughout cave networks. Inorganic aerosol deposition may provide a nutrient supply to cave surfaces allowing for, and sustaining microbial colonisation. 
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    The geology and geochemistry of the Agnew Intrusion: implications for the petrogenesis of early Huronian mafic igneous rocks in Central Ontario, Canada
    Vogel, Derek Christian ( 1996-07)
    The Early Proterozoic Agnew Intrusion is a well-preserved leucogabbronoritic to gabbronoritic layered intrusion that is a member of the East Bull Lake suite of layered intrusions (ca. 2490-2470 Ma) occurring in central Ontario. These intrusions are related to the development of the Huronian Rift Zone, which may be part of a much more widespread rifting event that involved the Fennoscandian Shield. Structural data suggest that these intrusions have been subjected to ductile deformation and are erosional remnants of one or more sill-like bodies originally emplaced along the contact between Archaean granitic rocks of the Superior Province and an Early Proterozoic Huronian continental flood basalt sequence in the Southern Province.
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    Geology and tectonothermal history of The Fishery Bay Region, Eyre Peninsula, South Australia
    Elliott, Andrew R. ( 1998)
    The Fishery Bay region, southern Eyre Peninsula, South Australia, consists of Archaean charnockitic and paragneissic sequences of the Sleaford Complex intruded by Palaeoproterozoic granitoids and two generations of mafic dykes. These rocks preserve the deformational and metamorphic effects of the Kimban Orogeny and the later Wartakan Event. Within the Fishery Bay area, five separate ductile deformation events (D1-D5) are recognised, the dominant of which (D2-D3) are associated with granulite facies metamorphism. The effects of the D3 event are pervasive throughout the Fishery Bay region, with D1 and D2 preserved only in regions of low-D3 strain. The overprinting nature or D3 is recognised in the reorientation of D2 structures. The dominant response of the area to D3 strain is a series of westerly-dipping dextral oblique reverse shears with west block-up movement. Much of the strain is localised within the paragneisses and along the margins of mafic dykes recognised in the development of a NNE-trending D3 high-strain zone termed the Cape Wiles Shear Zone. D3 observations from the Fishery Bay region correlate well with previous studies conducted on southern Eyre Peninsula which lead to the inference that D3 west block-up exhumation is responsible for the positive pressure gradient that exists from west to east across the Kalinjala Shear Zone. The pressure-temperature conditions preserved in the mineral assemblages of the paragneiss units and mafic dykes record two granulite facies metamorphic events, M2 and M3. Mineral assemblages associated with M2 and M3 are similar and passage from M2 to M3 did not result in reaction textures which indicates the proximity of the thermal conditions of these two metamorphic events. M2 corresponds to the second deformational event (D2) where peak metamorphic conditions reached pressures of 8.6±3.2 kbar at 750-900°C, The second thermobaric event correlates with the third deformation event (D3) and a metamorphic peak of 4.1±1.9 kbar at 750-850°C. The decompression of the Fishery Bay region during D3/M3 is synchronous with crustal thickening of the terrain east of the area.
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    Australian lineament tectonics: with an emphasis on northwestern Australia
    Elliott, Catherine I. ( 1994-08)
    Australia is transected by a network of systematic continental-scale lineaments that are considered to be zones of concentrated, aligned tectonic activity which have apparent continuity over vast distances. The influence of lineaments on the rock record can be identified in many types of data-sets, and existing data reveals previously undescribed basement influences. Several continental-scale lineaments can be traced offshore with apparent continuity for hundreds to thousands of kilometres, two of which are seen to cross the Tasman Sea in offshore eastern Australia. Geological and chronological evidence demonstrates that many of the lineaments have been zones of reactivation since at least the Early Proterozoic (- 1880 Ma) and that they appear to cross major terrane boundaries. Alternative models for their origin are a) a pre-existing lineament network maintained in an ancient basement underlying the entire continent; b) lateral propagation of crustal-scale structures; c) alignment of genetically unrelated lineaments giving the appearance of continuity. Australian deep-seismic profiles show that continental-scale lineaments are zones of crustal-scale structure which in some cases transect the crust-mantle boundary. Lineaments demonstrate many faulting styles, e.g. listric extensional (G3), planar moderate-angle thrusts (G2 l), and sub-vertical thrusts (G 17). In some cases the structural style varies laterally along the length of the lineament. (For complete abstract open document)
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    The low-temperature thermochronology of cratonic terranes
    Belton, David X. ( 2006)
    Cratonic terranes present many problems for geologists attempting to define those regions of the continental crust that are the core of today's continents. Inherent in the term is the great passage of time, and typically, the term defines consolidated Archaean or Proterozoic crust (Park and Jaroszewski, 1994). Cratons are further distinguished on the basis of tectonic activity. Marshak and his colleagues (1999) suggest that the lack of penetrative deformation or metamorphism is a useful definition but they further narrow this with the restriction of a Precambrian timeframe. Central to either view is the assumption of stability and perhaps senescence. And, since many aspects of geological research involve the detection of stratigraphic, structural or mineralogical change, stability implies a lack, or at least a minimum, of change. Thus the absence of these traditional markers of geological evolution or change, related to these processes, presents significant challenges in the study of cratons. This is particularly so in shield areas - those cratons with exposed basement rocks (Park and Jaroszewski, 1994). The extraordinary age of shield rocks and their apparent preservation at the surface, has encouraged research into the mechanics of landscape development and the individual evolution of landforms in the landscape, particularly by geomorphologists. From within this environment many thought provoking ideas have been proposed to explain the breadth of observations pertaining to almost every aspect of landscape process in these terranes. According to Summerfield (1991) the models of Davis, Penck, King and Budel have had the most impact in this field. The ideas presented by these workers remain a cornerstone in our understanding of the earth's surface, but in detail and in practice, the models have been shown to be antiquated. Nevertheless, landform evolution models such as those of L.C. King (1967) have held sway in southern Africa and elsewhere long after the underlying assumptions have been shown to lack validity. The purpose of this thesis is to investigate key aspects of landscape evolution in two regions, central Australia and southern Africa, where models have been espoused, arguing for extraordinary surface stability or alternatively a simple erosional history or pediplanation. Contemporary thermochronological techniques now permit us to investigate these regions in previously unavailable detail. The primary technique used in this work was apatite fission track analysis and an introduction to the fundamentals of the method is given in Chapter 1. The theoretical and practical aspects of the fission track method provided the basis for an innovative approach presented in Chapter 2. TASC is a scheme for analysing the raw fission track data so as to extract additional information about the rock's thermal history prior to undertaking traditional inverse modelling techniques. This method (recently described by the author in Ehlers et al., 2005) proved to be a powerful complement to the routine fission track analysis undertaken as part of the Australian and African case studies. Although first proposed for geological use in the 1960's, the fission track technique really only gained serious application with a number of technical and theoretical breakthroughs in the 1980's. Since then, growing understanding of the processes of annealing and how they might be modelled has allowed the technique continue developing. Chapter 3 is a discussion of this topic that expands on material previously published by the author and colleagues (Gleadow et al., 2002) and presents additional new work. Nevertheless, despite it's wide application in tectonic and basin studies amongst others, there remain many improvements to be made and problems to be solved. As part of this project, research into several areas presented the author with opportunities to contribute toward improvement in the apatite fission track technique, that have the potential to aid the study of cratonic terranes. The chlorine content of apatite has a profound influence on the sensitivity of the mineral for recording thermal events. Few current annealing models are capable of comprehensively addressing the variation of chlorine and other trace elements that appear to play a role in the annealing process. This issue is addressed in Chapter 4 where a universal annealing model is proposed to deal with the wide chemical variability observed in real apatites. For this theme, a fresh consideration of established empirical mathematical models was undertaken and all the current published annealing data was considered. Modern inverse modelling is based on a series of robust, but nonetheless empirical, equations that have withstood the test of time. However, with the aim of developing a more realistic and thus predictive model, Chapter 5 introduces an alternative, physicochemical to modelling the thermal annealing of fission tracks. This work attempts to draw firmer links between the processes of fission track formation, the mechanics of diffusion and the predicted response to variable temperature regimes. The first of the case studies is presented in Chapter 6 and is a comprehensive investigation of the long-term landscape evolution of the Davenport Ranges in the central Australian Craton. The study employs traditional petrographic methods as well as thermochronology and combines cosmogenic isotope analysis in an assessment of early landscape models. This chapter expands on work previously published by the author and co-workers (Belton et al., 2004) and has implications for our understanding of landscape evolution in the broader context of the Australian Craton. In order to maximise temperature sensitivity in slow cooled terranes, the relatively new thermochronological technique of (U-Th)/Helium analysis of apatite was tested on a suite of central Australian samples. The inconclusive results of this experiment prompted an investigation into the possible causes, and an important baseline study was conducted (Chapter 7). The study has implications for routine application of this new thermochronometer in cratonic and other terranes. More importantly the research identified a potential new thermochronometer with an even greater temperature sensitivity and near surface application for use in future landscape studies. Chapter 8 documents a larger, craton-wide study of the Mesozoic to recent landscape evolution of the Zimbabwe Craton. This work builds on material presented in earlier chapters and provides a broader view of the nature of crustal cooling, structural reactivation and landform development in the cratonic setting of southern Africa.
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    Deformation and the thermobaric history of the eastern coast of Williams Island
    Marks, Bianca ( 1997)
    Williams Island is located off the southern coast of the Eyre Peninsula of South Australia where the Palaeoproterozoic rocks of the Lincoln Batholith intrude a portion of an Archaean basement complex. The structures of the eastern coast of Williams Island are controlled by the rheological contrast between the mafic dykes and the felsic granite gneisses that comprise the batholith. Planes of rheological weakness exist at the dyke margins along which strain is localised. The plane of failure and the kinematics along it depends upon the orientation of the dyke with respect to the stress field. Displacements at cross-cutting dyke margins indicates the occurrence of three significant deformation events, D 1, D2 and D3. By comparison, the D1 is localised to a region of outcropping Jussieu Dykes, the D2 is pervasive and the D3 is confined to the discrete Northern and Southern Shear Zones. Associated with the latter two deformations is an increase in temperature and strain rate which controls the relative strength of the metabasic and the granite gneiss rocks. Brittle extensional structures, such as boudinage, form when the mafic dykes behave in a more competent manner relative to the host, whereas ductile extensional features, like pinch and swell, infer a greater homogeneity between the rock types. The rheological contrast is inverted with a preferential increase in strain resulting in granite boudinage. The D2 fabrics arc predominantly defined by a granulite two-pyroxene assemblage and the structural elements of D3 are characterised by minerals associated with amphibolisation. Average pressure calculations of representative assemblages give 7 ± 1 kbar for M2/D2 and 12 ± 2 kbar for M3/D3, which suggests crustal thickening over D2 - D3 time. Exhumation of the crustal block therefore occurred after peak D3.
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    Geochemistry and mineralisation of primary and secondary platinum-group elements in the ultramafic "Alaskan-type" Owendale complex and laterites in the Fifield Region, New South Wales, Australia
    Shi, Bielin ( 1995)
    The Owendale Complex belongs to a family of ultramafic-mafic intrusions that is characterised by a zonal, nonstratiform arrangement of the principal ultramafic units. The ultramafic rocks of the Owendale Complex are virtually identical to many of the Alaskan-type intrusions, however the associated gabbroic rocks (wehrlites) are K-rich and Si-undersaturated, in contrast to the tholeiitic gabbroic rocks of the Alaskan examples. The intrusion history of the Owendale Complex is thought to have involved emplacement of a gabbroic intrusion that was invaded by an ultrabasic magma, possibly while the former was still only partly solidified. Emplacement of both magmas probably occurred during Late Devonian tectonism and deformation synchronous with emplacement and crystallisation is necessary to explain the present non-stratiform arrangement of the rock units. The most obvious linkage factor between the two proposed parent magmas (gabbroic and ultrabasic) of the Owendale suites is their mutual affinity with tholeiitic basalt magmas and the similarities of their products with intrusions of alkalic basalt derivation. This suggests the possibility that the Owendale Complex rocks and those of other tholeiitic intrusions of the regions are comagmatic products of an ancestral magma that may have also produced the widespread assemblage of complexes. Viewed from this perspective, the ultramafic rocks of Owendale Complex would thus represent a very minor product of a period of regional magmatic activity. Most alloys, erlichmanite, cooperite and some grains with exclusion texture of Pt-Os-Ir-Pd-Rh are considered to represent a primary high-temperature paragenesis. Concentration of PGE in pegmatoidal units of dunite-wehrlite is explained by the accumulation of platinum-rich alloys that segregated directly from the melt at an early stage in the evolution of the complex. The high-temperature PGM segregate directly from a silicate melt and were not generated by exsolution from spinels or magmatic sulphides. These suggest that fS2 was generally low (subordinate sulphide formation) and, after some influence at the beginning, has given way to rising fO2 (chromite, olivine and Pt-Fe-Cu-Ni alloys formation). After lithification, the ultramafic rocks become subject to "reducing" conditions, i.e., conditions of lower O2 and S2 activities. Ni-Fe alloys, native Fe and Bi formed in cracks which filled the serpentine matrixes. The former PGM (erlichmanite, cooperite and Pt-Fe alloys) were exposed to the reducing conditions via cracks were desulphurated to form porous cooperite with Pt-Fe alloys and multiphase textural Os-Ir-Ni, Pt-Ir aggregates. It is plausible that the veinlets and aggregates of unnamed Rh-Sb-S, (Pt, Ir)2(Fc, Cu)3(S, Sb, AS)3 in the dunites may also have been formed by reduction of Ni-rich sulphides and erlichmanite, Pt-Fe alloys or cooperite. Late PGM are dominated by sperrylite-geversite solid solution resulting from the reaction of early PGM with a fluid phase. A hydrothermal origin is also indicated for native Fe, native Bi and awaruite (NiFe) and the base-metal sulphides (pentlandite, chalcopyrite, sphalerite, arsenopyrite, pyrite, pyrrhotite, and some Ni-Co-Fe sulfide). The cause of the reducing conditions may have been related to H2 production accompanying hydrous alteration of the dunites and clinopyroxenites. The laterites overlying the ultramafic complexes in the Fifield region are exceptionally well-developed and well-preserved weathering profiles. Field, textural and geochemical data all support a chemical weathering origin for the profiles and compatible with meteoric and ground water origins. Meteoric water with intermediate Eh and pH and negligible dissolved species sinks into the laterite where these parameters are modified. The Eh rises and pH decreases to the conditions typical of lateritic soils and the concentration of dissolved species increases. In this state the water is able to take PGE and Au into solution from a finely disseminated form in the bedrock as a part of the process of lateritisation. When the soil solution transports the PGE and Au towards a transitional interface must exist between the ferruginous and saprolite zones with lower Eh, neutral pH and lower concentration of dissolved salts. At this transitional region, deposition of the PGE and Au occurred. The presence of magnetic Pt-Fe-Cu-Ni alloys suggests that hydrothermal solutions play a later role in the Fifield region, and the alloys have grown in situ in a lateritic soil by a process involving laterite water solution in the high Eh, low pH conditions prevalent in such soil, followed by deposition when the conditions become less extreme. Some examples of the Pt-Fe alloys from such an environment become frequently strongly magnetic with larger size. It is assumed that the temperature of the hydrothermal solution is in the range of 300° - 500° C (Bowles, 1990). PGE mineralisation in the primary rocks and laterite in this region has demonstrated a good example of multi-stage process mineralisation including primary high temperature magmatic formation; low temperature postmagmatic hydrothermal alteration and residual lateritic enrichment.