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Late-Archaean deformation at Ora Banda 69km northwest of Kalgoorlie, Western Australia, resulted in upright folds (D2), ductile shear zones (D3), and a regional-scale brittle-ductile fault network (D4). Early low-angle faults (D', D1), documented in the surrounding Coolgardie, Kambalda and Boorara Domains are not developed in the Ora Banda Domain, and the fabrics reflect only the latest ENE-WSW shortening event. The western limb of the regional-scale ESE- plunging Kurrawang syncline (D2), is truncated by the Zuleika Shear Zone (D3), a within- greenstone ductile shear zone located 10km southeast of Ora Banda. The shear zone has a much greater strike length (250km) than depth extent, as seismic imagery reveals a sharp truncation against a mid-crustal decollement at a depth of 6km-depth below surface. The Zuleika Shear Zone is a NW-SE trending band of anastomosing S-C mylonite zones formed in conjugate sets of NW- SE trending sinistral and N-S trending dextral shear zones. Widely distributed flattening strains and more restricted zones of non-coaxial shear in the Zuleika Shear Zone, suggest deformation-path partitioning typical of a transpressional tectonic environment. Latetectonic brittle-ductile faults (D4) cross-cut the Zuleika Shear Zone and surrounding greenstones, and hence are not Riedel structures or other lower order faults genetically related to the ductile shearing. Gold mineralisation of the Zuleika Shear Zone began during the ductile deformation (D3), continued through peak metamorphism that postdates the shearing, and finally ceased after the brittle-ductile faulting event (D4). Gold deposits are primarily located where brittle-ductile faults intersect the Zuleika Shear Zone. / Brittle-ductile faults (D4), are developed in three principal structural orientations: N-S (dextral), NE-SW (dextral) and E-W (sinistral). These faults display mutual cross-cutting relationships and were formed synchronously during a single regional shortening event. The brittle-ductile fault network is developed unevenly over the region, being localised in packets of high fracture-density referred to as structural zones. The Ora Banda structural zone is an area of high density faulting in the vicinity of Ora Banda, composed of a network of interlinked faults in which alternating ductile and brittle conditions produced cataclasite, breccia and quartz vein systems overprinting mylonite and schistosity. Other areas of high fracture-density (eg. Grants Patch and Mount Pleasant structural zones), are located within the NW-SE trending Ora Banda mafic sequence and spaced at 10km intervals to the southeast of Ora Banda. This spatial periodicity of high fracturedensity within the mafic sequence may have developed as a result of layer-parallel extension during ENE- WSW regional shortening. Gold deposits are concentrated in the Ora Banda, Grants Patch and Mount Pleasant structural zones. Gold distribution within the Ora Banda structural zone traces out the distribution of brittle-ductile faults, indicating that the fault network was the major pathway for fluid flow during mineralisation. Hydrothermal minerals are integral components of fault fabrics within the structural zone, and textures indicate that the faults were formed under conditions of high fluid pressure and, for much of the deformation, may have been fluid-generated. / At Ora Banda the Enterprise gold deposit (40 tonnes Au) highlights the control of mesoscopic- scale fractures on gold distribution. On aeromagnetic imagery, the Enterprise fault zone appears as a narrow fault structure, but at a mesoscopic-scale, it is a broad zone of interlinked brittle-ductile faults and quartz veins. Fabrics developed in the layered, differentiated dolerite host rocks of the Enterprise fault zone, range from cataclasite to banded mylonite with a major component of net- veined breccia (mesofracturing). Kinematic analyses of fault slip lineations reveal an 055 directed (ENE-WSW) maximum shortening axis during brittle-ductile faulting. Microfabrics of the faults show extensive recrystallisation with significant post-deformation recovery that may be related to late to post - tectonic intrusion of the adjacent Lone Hand Monzogranite. Deformation mechanisms indicate that the D4 event occurred at a low-to-moderate temperature, in a low strain-rate enviromnent typical of mid to upper-greenschist facies crustal conditions. Gold mineralisation in the Enterprise deposit is controlled by faults with high-grade shoot development at the intersection of faults and host rock contacts that may represent gradients in tensile rock-strength. Although gold distribution indicates that faults are a major control on mineralisation, at a microscopic-scale, the control is by a linked network of microfractures that pervades the host rocks. / Fry analysis of gold deposits within the Ora Banda mafic sequence shows clustering into groups with about 10km spacing. Coincidence of high fracture-density zones and gold deposits in 1Okm spaced-corridors reveals the regional-scale nature of gold mineralisation within the brittle-ductile fault network. Fluid-pressure gradients generated by pressure release during high-density fracturing, may have effectively increased fluid-rock ratios by focussing of metamorphic fluids through these areas. The largest gold deposits in the Ora Banda mafic sequence are hosted by 060-090 trending brittle-ductile faults with dilational textures (hydraulic breccia), and minor evidence of slip with negligible offsets. The orientation of these structures is sub-parallel to the regional axis of maximum shortening, hence an environment of fluid overpressuring in the presence of a far-field stress system produced conditions where fluid pressure is greater than or equal to the combined minimum compressive stress and the tensile rock strength. Such conditions are conducive to multiple failure episodes with fluid-pressure cycling and transient permeability as a consequence of fault reactivation. Formation of the brittle-ductile fault network occurred as a result of a delicate balance between deviatoric stress and fluid pressure, hence incremental fault development contributed to, and was a consequence of, the gold mineralisation event. / The geometric relations of shear zones, brittle-ductile faults and gold mineralised zones are similar across all scales of observation from regional to microscopic and are therefore fractal. Fractal geometry indicates that deformation and gold mineralisation are temporally and genetically associated, and this combined with the textural relationships of the gold ores indicates that the sites of gold deposition were not structurally prepared prior to mineralisation. Development of early ductile to later brittle-ductile structures indicates changing conditions of deformation typical of decreasing crustal depth, or a variation of strain rate with time. The lack of a significant change in orientation of the maximum shortening direction and continuance of gold mineralisation throughout ductile and brittle deformation events, implies that deformation was progressive during a bulk shortening that accompanied uplift of the crust.

Several deposits in the Mt Isa - Cloncurry region have been studied, including those held by Australian Resources near Selwyn (Plume, Slate Ridge, Mobs Lease and Straight Eight),in particular, with respect to cobalt and gold mineralisation. Cobalt is associated with pyrite, pyrrhotite and arsenic sulfosalts. Other cobalt deposits in the Eastern Fold Belt of the Mt Isa Block were studied; these include the Queen Sally, Lorena and the Great Australia mine. Varying styles of Co-bearing mineralisation were encountered. In the Queen Sally mine a curious vanadium - substituted heterogenite has been found. This is only the world's second reported occurrence of this mineral of the halotrichite group. At the Great Australia, primary Co mineralisation has been shown to be confined to one generation of cobaltian pyrite. Several generations of pyrite are noted for this and other deposits. / Master of Science (Hons)

The Comet Gold Mine is in the Murchison mineral field which lies within the Yilgarn Craton of Western Australia. Several different geophysical methods were used in this study to define the geophysical signatures of sedimentary iron formations (SIF) and altered basalt associated with gold mineralisation. The geophysical surveys carried out at the Comet Gold Mine were gravity, sub-audio magnetics (SAM), transient electromagnetics (TEM) and downhole geophysical logging. Data from previous geophysical surveying were also used, and these included highresolution aeromagnetics and TEMPEST airborne electromagnetics. Other exploration information, such as geology and drillhole data, were integrated with geophysical results to study the geophysical responses and generate a geophysical interpretation map. / The main aim of this study was to generate an understanding of the various geophysical responses of geology and gold mineralisation in the Comet Mine area for future gold exploration in this region. Particularly, the study focused on the ability of the SAM method to map out geology and geophysical response for gold mineralisation. The response from SAM surveying has been investigated over an area of 13 sq kms. The SAM surveying was completed using a transmitter current of 5-8 Amp with a 50% duty cycle at 4 Hz frequency, which was considered the best setting for the Comet area. The SAM anomalies were compared to results from other geophysical methods. The results of all geophysical surveying suggested that the TEM method was also effective for identifying altered sulphide and magnetic altered rock associated with gold mineralisation. / Experiments were carried out using SAM surveying with electrodes in standard surface pits and pits placed directly into the gold mineralised structure. Both surveys showed very similar results, so in this area, surface electrode pits work well for current injection during SAM surveys. The similarities are probably due to the lack of conductive regolith cover in the Comet Mine area. / The SAM response was studied for survey grids using different electrode positions and directions. Experiments in changing SAM electrode position over the same area were carried out along and across geological strike to detect the different geological structure directions. The EQMMR response was different for electrodes oriented at 90º when surveys were repeated over the same area. SAM mainly measures conductive features running sub-parallel to the electrode direction, but the EQMMIP response was mostly the same, despite the difference in electrode direction. In addition, the EQMMIP result was very similar for rotated grids, with some distortion occurring around the main EQMMR anomaly near the Venus open cut pit. Therefore, SAM chargeability was not strongly polarised along the electrode direction like the EQMMR response. This is consistent with the theory of MIP that the method detects the effect of induced polarisation in the earth by virtue of the magnetic fields associated with current flow in polarisable bodies within the earth. / Gravity data were collected along in 4 transects 500 m apart and at 50 m station spacing. 3D gravity modelling using polygonal shapes was completed to a good fit with felsic and mafic rocks by having rock units dip to the SE. Euler depth solution calculations were applied to locate contacts and deep gravity sources. Gravity surveying has also proved to be a useful survey method for geological mapping and locating regional structures. / Ground TEM survey data were used at the Venus prospect to map out conductive zones at depths ranging from 30 to 90 m. All anomaly bodies were interpreted to have a SE dip. The modelled ground TEM results were compared to TEMPEST airborne electromagnetic conductivity depth slices. Both EM survey results showed reasonably similar patterns, but the ground TEM method provided more reliable conductor locations and depth estimations that correlated well with the drilling information and downhole conductivity logging. / Geophysical logs of natural gamma and inductive conductivity were surveyed in 5 drillholes that intersected gold alteration zones. The alteration zones associated with gold mineralised sediments, sulphide and magnetic minerals were identified in the downhole logs as increased conductivity, with a sight increase in the natural gamma response. Natural gamma was usually high above a background of host rock in the gold mineralised shear zones. This was likely due to K associated with the clay rich SIF units, and sericite and biotite from gold related alteration. During this study, drillhole CTRC028 was drilled into a modelled TEM anomaly, and gold mineralised SIF was intersected at the predicted location from the model. / Geophysical survey information (magnetics, gravity, SAM and TEM) and anomalies in the Comet area were found to be primarily controlled by the local structures and mineralisation along these structures. Modelled ground TEM results were compared to TEMPEST airborne, EM data and showed reasonably similar patterns. The geophysical survey data also highlights black shale units, which can produce a false target commonly running parallel to the sulphide altered fault zones and SIF units, because of graphite and sulphide in the black shale. / The TEMPEST data were a valuable guide to bedrock conductivity over the outline project area at Comet, and the follow–up ground TEM and SAM survey data was very useful for accurately pin-pointing anomalies for drill testing. / Geophysical and geological data analysed in this study was used to generate a geophysical interpretation map at 1:5,000 scale. The new interpretation of geological units and structures at Comet will provide geologists with a better understand about the geological and structural setting for mineralisation in the Comet area. For example, the Comet Fault represents a faulted limb of the Comet fold structure that has both limbs dipping to the SE, and plunges to the NE. Magnetic anomalies associated with SIF are considered to correlate with some gold bearing horizons and the location of the Comet Fault, that has become more siliceous and altered by sulphide minerals and magnetite minerals. / It is recommended that other prospect areas in the region should be surveyed using the SAM method in order to identify shallow gold bearing structures and improve geological interpretations ahead of drilling.

Over the last two decades, gold occurrences in the Palaeoproterozoic Pine Creek Orogen (PCO) have been cited as type-examples of high-temperature contact-metamorphic or thermal-aureole deposits associated with granitoid magmatism. Furthermore, spatial relationships between these gold occurrences and the granitoids have led to inclusion of these deposits in the intrusion-related gold deposit group. Research on the characteristics, distribution and timing of these gold deposits tests these classifications and supports an alternative interpretation. The deposits display many similarities to well-described ‘turbidite-hosted’ orogenic gold deposits described from several Palaeozoic orogens. As in most ‘turbidite-hosted’ orogenic deposits, the gold mineralisation is dominantly epigenetic, sediment-hosted (typically greywacke and siltstone) and fold-controlled. Most gold is hosted by concordant or discordant veins, with limited alteration halos in host rocks, except where they occur in silicate-facies BIF or other Fe-rich rocks. The domal culminations of major doubly-plunging anticlines, and/or fold-limb thrust-faults, are important structural controls at the camp- and deposit-scales. Many deposits are sited in parts of the lithostratigraphy where there is significant competency and/or chemical contrast between units or sequences. In particular, the complex interdigitated stratigraphy of euxinic and transitional high-energy sedimentary rocks of the c.1900-1880Ma South Alligator Group is important for the localisation of gold deposits. The distribution of deposits is influenced further by the location and shape of granitoids and their associated contact-metamorphic aureole. Approximately 90% of gold deposits lie within the ∼2.5km wide contact-aureole, and most of these are concentrated in, and just beyond, the biotite-albite-epidote zone (0.5-1.0km from granitoid), with few deposits located in the inner hornblende-hornfels zone. At the deposit scale, gold is commonly associated with arsenopyrite-loellengite and pyrite, native-Bi and Bi-bearing minerals, and is confined to a variety of extensional quartz-sulphide ± carbonate veins. Such veins formed typically at 180-320°?C and ∼1kbar from low- to moderate salinity, two-phase aqueous fluids. Isotopic studies of the deposits are equivocal in terms of the source of hydrothermal fluid. Most δD and δ18O values fall within the range defined for contact-metamorphic and magmatic fluids, and sulphur isotopes indicate that the fluids are within the range of most regional sources. Significantly, lead isotope ratios show that the goldbearing fluid does not have a felsic magmatic-source signature, but instead suggest a homogenous regional-scale lead source. Excluding a few outliers, the relative uniformity of deposit characteristics, including host rocks, structural style, alteration, sulphide paragenesis and fluid P-T-X conditions, suggests that most deposits represent a continuum of broadly coeval mineralisation that formed under similar geological conditions