The Mount Isa and May Downs Faults are part of a network of significant faults that define, control, or partition deformation in the Early to mid-Proterozoic Mount Isa Inlier. The middle Proterozoic deformation history includes at least two extensional basin-forming events (Leichhardt Superbasin: ~1800 Ma to ~1700 Ma and Isa Superbasin: ~1700 Ma to ~ 1600 Ma) and a major protracted contractional orogenic event (Isan Orogeny: ~1585 Ma to ~ 1500 Ma). Uplift between the Mount Isa and May Downs Faults during the Isan Orogeny has exposed mid to upper amphibolite facies rocks of the structurally deeper levels of the early rift systems. Also exposed is the Sybella Granite, a composite batholith of variably deformed gneissic granite, which, at ~1660 Ma, is broadly coeval with inception of the Isan Superbasin basin. Two prevailing kinematic models had been proposed for the fault systems during Isan Superbasin formation. The traditionally accepted model involves episodic E-W or NW-SE extension with the N-S Mount Isa Fault, but Southgate et al (2000b) presented an alternative sinistral strike-slip model in which the May Downs Fault acted as a releasing bend fault associated with motion on the Mt Isa Fault. In the Southgate model, the Sybella Granite was interpreted as syn-tectonically filling the dilational releasing bend. This study provides a detailed structural analysis of the 100 km by 40 km area west of Mount Isa City lying between the Mount Isa and May Downs Faults. The aim was to resolve a number of outstanding issues, including those outlined above. The resultant 1:250 000 structural map of the area is based on: reconnaissance-scale mapping; aerial photography, satellite, magnetic and radiometric image interpretation; field observations at locations throughout the area; and local detailed mapping (1:12000 scale or less). The mapping and associated geometrical analysis of the area has shown that the Sybella Batholith consists of two granite sills and a more globular body of microgranite. The deepest, gneissic, sill is up to 5 km thick and was emplaced at about 15 km below the basal Mount Isa Group unconformity (palaeosurface). The other, less deformed, sill formed higher in the crust, and the microgranite intruded to within 1-2 km of the palaeosurface. The two sills are located between two major fault systems (Mount Isa and May Downs Faults) that developed from inherited basin margin faults. The fault systems dip toward each other and the rocks between them have been folded into a single large antiform and uplifted as a wedge. Previous interpretations of the area have suggested that the batholith consists of a single sill folded by tighter, shorter wavelength folds. A cross-sectional reconstruction of the study area suggests that thin-skinned processes dominated much of the Isan Orogeny, contrary to previous interpretations. A three-dimensional reconstruction of the area, evaluated by comparing the predicted strain and amount of shortening with measured strain and shortening estimates, suggests deformation was driven by a rigid block to the west of the May Downs Fault moving toward the northeast. In the restored pre-Isan geometry, both the margins of the lowermost gneissic granite sill and its immediate country rocks have a strong, horizontal, layer-parallel, shear foliation with top-to-the-east asymmetry. The fabrics are strongly constrictional and 2 Abstract the stretching lineation trends east-west. Field observations and thin sectional analysis of these fabrics provide positive evidence that the Sybella Batholith was syn-tectonically emplaced in a basin-forming environment. A kinematic model is presented to show that these features are consistent with granite emplacement into a dilational jog in a sub-horizontal shear zone with a top-to-the-east shear sense. A component of east-west directed horizontal simple shear across the dilating zone explains the strongly constrictional fabrics in the granite. Under these conditions significant north-south shortening in the deforming zone leads to the initiation of folds parallel to the stretching direction (as observed). The shear zone into which the granite was emplaced developed at about fifteen kilometres depth and was probably at or near the brittle-ductile transition. The consistent shear sense, very high strains and implied 30 km of translation required to accommodate the sill indicates that this was a major crustal structure, rather than a simple detachment at the brittle-ductile transition in a crustal pure shear extension. The results are consistent with the east-west extensional model for basin development and totally inconsistent with the sinistral strike-slip model.

The Mount Isa and May Downs Faults are part of a network of significant faults that define, control, or partition deformation in the Early to mid-Proterozoic Mount Isa Inlier. The middle Proterozoic deformation history includes at least two extensional basin-forming events (Leichhardt Superbasin: ~1800 Ma to ~1700 Ma and Isa Superbasin: ~1700 Ma to ~ 1600 Ma) and a major protracted contractional orogenic event (Isan Orogeny: ~1585 Ma to ~ 1500 Ma). Uplift between the Mount Isa and May Downs Faults during the Isan Orogeny has exposed mid to upper amphibolite facies rocks of the structurally deeper levels of the early rift systems. Also exposed is the Sybella Granite, a composite batholith of variably deformed gneissic granite, which, at ~1660 Ma, is broadly coeval with inception of the Isan Superbasin basin. Two prevailing kinematic models had been proposed for the fault systems during Isan Superbasin formation. The traditionally accepted model involves episodic E-W or NW-SE extension with the N-S Mount Isa Fault, but Southgate et al (2000b) presented an alternative sinistral strike-slip model in which the May Downs Fault acted as a releasing bend fault associated with motion on the Mt Isa Fault. In the Southgate model, the Sybella Granite was interpreted as syn-tectonically filling the dilational releasing bend. This study provides a detailed structural analysis of the 100 km by 40 km area west of Mount Isa City lying between the Mount Isa and May Downs Faults. The aim was to resolve a number of outstanding issues, including those outlined above. The resultant 1:250 000 structural map of the area is based on: reconnaissance-scale mapping; aerial photography, satellite, magnetic and radiometric image interpretation; field observations at locations throughout the area; and local detailed mapping (1:12000 scale or less). The mapping and associated geometrical analysis of the area has shown that the Sybella Batholith consists of two granite sills and a more globular body of microgranite. The deepest, gneissic, sill is up to 5 km thick and was emplaced at about 15 km below the basal Mount Isa Group unconformity (palaeosurface). The other, less deformed, sill formed higher in the crust, and the microgranite intruded to within 1-2 km of the palaeosurface. The two sills are located between two major fault systems (Mount Isa and May Downs Faults) that developed from inherited basin margin faults. The fault systems dip toward each other and the rocks between them have been folded into a single large antiform and uplifted as a wedge. Previous interpretations of the area have suggested that the batholith consists of a single sill folded by tighter, shorter wavelength folds. A cross-sectional reconstruction of the study area suggests that thin-skinned processes dominated much of the Isan Orogeny, contrary to previous interpretations. A three-dimensional reconstruction of the area, evaluated by comparing the predicted strain and amount of shortening with measured strain and shortening estimates, suggests deformation was driven by a rigid block to the west of the May Downs Fault moving toward the northeast. In the restored pre-Isan geometry, both the margins of the lowermost gneissic granite sill and its immediate country rocks have a strong, horizontal, layer-parallel, shear foliation with top-to-the-east asymmetry. The fabrics are strongly constrictional and 2 Abstract the stretching lineation trends east-west. Field observations and thin sectional analysis of these fabrics provide positive evidence that the Sybella Batholith was syn-tectonically emplaced in a basin-forming environment. A kinematic model is presented to show that these features are consistent with granite emplacement into a dilational jog in a sub-horizontal shear zone with a top-to-the-east shear sense. A component of east-west directed horizontal simple shear across the dilating zone explains the strongly constrictional fabrics in the granite. Under these conditions significant north-south shortening in the deforming zone leads to the initiation of folds parallel to the stretching direction (as observed). The shear zone into which the granite was emplaced developed at about fifteen kilometres depth and was probably at or near the brittle-ductile transition. The consistent shear sense, very high strains and implied 30 km of translation required to accommodate the sill indicates that this was a major crustal structure, rather than a simple detachment at the brittle-ductile transition in a crustal pure shear extension. The results are consistent with the east-west extensional model for basin development and totally inconsistent with the sinistral strike-slip model.

Includes copies of articles co-authored by author during the preparation of this thesis in back pocket. / Includes bibliographical references (leaves 113-124). / viii, 172 leaves : ill. (some col.), maps ; 30 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Focuses on the impact of change in the distribution of heat producing elements on lithospheric thermal regimes and on temperature dependent processes such as metamorphism, magmatism and deformation, with application to Proteozoic Australia (Mount Isa and Mount Painter inliers). / Thesis (Ph.D.)--Adelaide University, Dept. of Geology and Geophysics, 2001

The introduced mineral deposit model (MDM) is the product of a trans-disciplinary study, based on Complexity and General Systems Theory. Both investigate the abstract organization of phenomena, independent of their substance, type, or spatial or temporal scale of existence. The focus of the research has been on giant, hydrothermal mineral deposits. They constitute <0.001% of the total number of deposits yet contain 70-85% of the world's metal resources. Giants are the definitive exploration targets. They are more profitable to exploit and less susceptible to fluctuations of the market. Consensus has it that the same processes that generate small deposits also form giants but those processes are simply longer, vaster, and larger. Heat is the dominant factor in the genesis of giant mineral deposits. A paleothermal map shows where the vast heat required to generate a giant has been concentrated in a large space, and even allows us to deduce the duration of the process. To generate a paleothermal map acceptable to the scientific community requires reproducibility. Experimentation with various approaches to pattern recognition of geochemical data showed that the AUTOCLUST algorithm not only gave reproducibility but also gave the most consistent, most meaningful results. It automatically extracts boundaries based on Voronoi and Delaunay tessellations. The user does not specify parameters; however, the modeller does have tools to explore the data. This approach is near ideal in that it removes much of the human-generated bias. This algorithm reveals the radial, spatial distribution, of gold deposits in the Lachlan Fold Belt of southeastern Australia at two distinct scales – repeating patterns every ~80 km and ~230 km. Both scales of patterning are reflected in the geology. The ~80 km patterns are nested within the ~230 km patterns revealing a self-similar, geometrical relationship. It is proposed that these patterns originate from Rayleigh-Bénard convection in the mantle. At the Rayleigh Number appropriate for the mantle, the stable planform is the spoke pattern, where hot mantle material is moving upward near the centre of the pattern and outward along the radial arms. Discontinuities in the mantle, Rayleigh-Bénard convection in the mantle, and the spatial distribution of giant mineral deposits, are correlative. The discontinuities in the Earth are acting as platforms from which Rayleigh-Bénard convection can originate. Shallow discontinuities give rise to plumelets, which manifest at the crust as repeating patterns ranging, from ~100 to ~1,000 km in diameter. Deeper discontinuities give rise to plumes, which become apparent at the crust as repeating patterns ranging from >1,000 to ~4,000 km in diameter. The deepest discontinuities give rise to the superplumes, which become detectable at the crust as repeating patterns ranging from >4,000 to >10,000 km in diameter. Rayleigh-Bénard convection concentrates the reservoir of heat in the mantle into specific locations in the crust; thereby providing the vast heat requirements for the processes that generate giant, hydrothermal mineral deposits. The radial spatial distribution patterns observed for gold deposits are also present for base metal deposits. At the supergiant Broken Hill deposit in far western New South Wales, Australia, the higher temperature Broken Hill-type deposits occur in a radial pattern while the lower temperature deposits occur in concentric patterns. The supergiant Broken Hill deposit occurs at the very centre of the pattern. If the supergiant Broken Hill Deposit was buried beneath alluvium, water or younger rocks, it would now be possible to predict its location with accuracy measured in tens of square kilometres. This predictive accuracy is desired by every exploration manager of every exploration company. The giant deposits at Broken Hill, Olympic Dam, and Mount Isa all occur on the edge of an annulus. There are at least two ways of creating an annulus on the Earth's surface. One is through Rayleigh-Bénard convection and the other is through meteor impact. It is likely that only 'large' meteors (those >10 km in diameter) would have any permanent impact on the mantle. Lesser meteors would leave only a superficial scar that would be eroded away. The permanent scars in the mantle act as ‘accidental templates’ consisting of concentric and possibly radial fractures that impose those structures on any rocks that were subsequently laid down or emplaced over the mantle. In southeastern Australia, the proposed Deniliquin Impact structure has been an 'accidental template' providing a 'line-of-least-resistance' for the ascent of the ~2,000 km diameter, offshore, Cape Howe Plume. The western and northwestern radial arms of this plume have created the very geometry of the Lachlan Fold Belt, as well as giving rise to the spatial distribution of the granitic rocks in that belt and ultimately to the gold deposits. The interplay between the templating of the mantle by meteor impacts and the ascent of plumelets, plumes or superplumes from various discontinuities in the mantle is quite possibly the reason that mineral deposits occur where they do.

The introduced mineral deposit model (MDM) is the product of a trans-disciplinary study, based on Complexity and General Systems Theory. Both investigate the abstract organization of phenomena, independent of their substance, type, or spatial or temporal scale of existence. The focus of the research has been on giant, hydrothermal mineral deposits. They constitute <0.001% of the total number of deposits yet contain 70-85% of the world's metal resources. Giants are the definitive exploration targets. They are more profitable to exploit and less susceptible to fluctuations of the market. Consensus has it that the same processes that generate small deposits also form giants but those processes are simply longer, vaster, and larger. Heat is the dominant factor in the genesis of giant mineral deposits. A paleothermal map shows where the vast heat required to generate a giant has been concentrated in a large space, and even allows us to deduce the duration of the process. To generate a paleothermal map acceptable to the scientific community requires reproducibility. Experimentation with various approaches to pattern recognition of geochemical data showed that the AUTOCLUST algorithm not only gave reproducibility but also gave the most consistent, most meaningful results. It automatically extracts boundaries based on Voronoi and Delaunay tessellations. The user does not specify parameters; however, the modeller does have tools to explore the data. This approach is near ideal in that it removes much of the human-generated bias. This algorithm reveals the radial, spatial distribution, of gold deposits in the Lachlan Fold Belt of southeastern Australia at two distinct scales – repeating patterns every ~80 km and ~230 km. Both scales of patterning are reflected in the geology. The ~80 km patterns are nested within the ~230 km patterns revealing a self-similar, geometrical relationship. It is proposed that these patterns originate from Rayleigh-Bénard convection in the mantle. At the Rayleigh Number appropriate for the mantle, the stable planform is the spoke pattern, where hot mantle material is moving upward near the centre of the pattern and outward along the radial arms. Discontinuities in the mantle, Rayleigh-Bénard convection in the mantle, and the spatial distribution of giant mineral deposits, are correlative. The discontinuities in the Earth are acting as platforms from which Rayleigh-Bénard convection can originate. Shallow discontinuities give rise to plumelets, which manifest at the crust as repeating patterns ranging, from ~100 to ~1,000 km in diameter. Deeper discontinuities give rise to plumes, which become apparent at the crust as repeating patterns ranging from >1,000 to ~4,000 km in diameter. The deepest discontinuities give rise to the superplumes, which become detectable at the crust as repeating patterns ranging from >4,000 to >10,000 km in diameter. Rayleigh-Bénard convection concentrates the reservoir of heat in the mantle into specific locations in the crust; thereby providing the vast heat requirements for the processes that generate giant, hydrothermal mineral deposits. The radial spatial distribution patterns observed for gold deposits are also present for base metal deposits. At the supergiant Broken Hill deposit in far western New South Wales, Australia, the higher temperature Broken Hill-type deposits occur in a radial pattern while the lower temperature deposits occur in concentric patterns. The supergiant Broken Hill deposit occurs at the very centre of the pattern. If the supergiant Broken Hill Deposit was buried beneath alluvium, water or younger rocks, it would now be possible to predict its location with accuracy measured in tens of square kilometres. This predictive accuracy is desired by every exploration manager of every exploration company. The giant deposits at Broken Hill, Olympic Dam, and Mount Isa all occur on the edge of an annulus. There are at least two ways of creating an annulus on the Earth's surface. One is through Rayleigh-Bénard convection and the other is through meteor impact. It is likely that only 'large' meteors (those >10 km in diameter) would have any permanent impact on the mantle. Lesser meteors would leave only a superficial scar that would be eroded away. The permanent scars in the mantle act as ‘accidental templates’ consisting of concentric and possibly radial fractures that impose those structures on any rocks that were subsequently laid down or emplaced over the mantle. In southeastern Australia, the proposed Deniliquin Impact structure has been an 'accidental template' providing a 'line-of-least-resistance' for the ascent of the ~2,000 km diameter, offshore, Cape Howe Plume. The western and northwestern radial arms of this plume have created the very geometry of the Lachlan Fold Belt, as well as giving rise to the spatial distribution of the granitic rocks in that belt and ultimately to the gold deposits. The interplay between the templating of the mantle by meteor impacts and the ascent of plumelets, plumes or superplumes from various discontinuities in the mantle is quite possibly the reason that mineral deposits occur where they do.

The introduced mineral deposit model (MDM) is the product of a trans-disciplinary study, based on Complexity and General Systems Theory. Both investigate the abstract organization of phenomena, independent of their substance, type, or spatial or temporal scale of existence. The focus of the research has been on giant, hydrothermal mineral deposits. They constitute <0.001% of the total number of deposits yet contain 70-85% of the world's metal resources. Giants are the definitive exploration targets. They are more profitable to exploit and less susceptible to fluctuations of the market. Consensus has it that the same processes that generate small deposits also form giants but those processes are simply longer, vaster, and larger. Heat is the dominant factor in the genesis of giant mineral deposits. A paleothermal map shows where the vast heat required to generate a giant has been concentrated in a large space, and even allows us to deduce the duration of the process. To generate a paleothermal map acceptable to the scientific community requires reproducibility. Experimentation with various approaches to pattern recognition of geochemical data showed that the AUTOCLUST algorithm not only gave reproducibility but also gave the most consistent, most meaningful results. It automatically extracts boundaries based on Voronoi and Delaunay tessellations. The user does not specify parameters; however, the modeller does have tools to explore the data. This approach is near ideal in that it removes much of the human-generated bias. This algorithm reveals the radial, spatial distribution, of gold deposits in the Lachlan Fold Belt of southeastern Australia at two distinct scales – repeating patterns every ~80 km and ~230 km. Both scales of patterning are reflected in the geology. The ~80 km patterns are nested within the ~230 km patterns revealing a self-similar, geometrical relationship. It is proposed that these patterns originate from Rayleigh-Bénard convection in the mantle. At the Rayleigh Number appropriate for the mantle, the stable planform is the spoke pattern, where hot mantle material is moving upward near the centre of the pattern and outward along the radial arms. Discontinuities in the mantle, Rayleigh-Bénard convection in the mantle, and the spatial distribution of giant mineral deposits, are correlative. The discontinuities in the Earth are acting as platforms from which Rayleigh-Bénard convection can originate. Shallow discontinuities give rise to plumelets, which manifest at the crust as repeating patterns ranging, from ~100 to ~1,000 km in diameter. Deeper discontinuities give rise to plumes, which become apparent at the crust as repeating patterns ranging from >1,000 to ~4,000 km in diameter. The deepest discontinuities give rise to the superplumes, which become detectable at the crust as repeating patterns ranging from >4,000 to >10,000 km in diameter. Rayleigh-Bénard convection concentrates the reservoir of heat in the mantle into specific locations in the crust; thereby providing the vast heat requirements for the processes that generate giant, hydrothermal mineral deposits. The radial spatial distribution patterns observed for gold deposits are also present for base metal deposits. At the supergiant Broken Hill deposit in far western New South Wales, Australia, the higher temperature Broken Hill-type deposits occur in a radial pattern while the lower temperature deposits occur in concentric patterns. The supergiant Broken Hill deposit occurs at the very centre of the pattern. If the supergiant Broken Hill Deposit was buried beneath alluvium, water or younger rocks, it would now be possible to predict its location with accuracy measured in tens of square kilometres. This predictive accuracy is desired by every exploration manager of every exploration company. The giant deposits at Broken Hill, Olympic Dam, and Mount Isa all occur on the edge of an annulus. There are at least two ways of creating an annulus on the Earth's surface. One is through Rayleigh-Bénard convection and the other is through meteor impact. It is likely that only 'large' meteors (those >10 km in diameter) would have any permanent impact on the mantle. Lesser meteors would leave only a superficial scar that would be eroded away. The permanent scars in the mantle act as ‘accidental templates’ consisting of concentric and possibly radial fractures that impose those structures on any rocks that were subsequently laid down or emplaced over the mantle. In southeastern Australia, the proposed Deniliquin Impact structure has been an 'accidental template' providing a 'line-of-least-resistance' for the ascent of the ~2,000 km diameter, offshore, Cape Howe Plume. The western and northwestern radial arms of this plume have created the very geometry of the Lachlan Fold Belt, as well as giving rise to the spatial distribution of the granitic rocks in that belt and ultimately to the gold deposits. The interplay between the templating of the mantle by meteor impacts and the ascent of plumelets, plumes or superplumes from various discontinuities in the mantle is quite possibly the reason that mineral deposits occur where they do.