Добірка наукової літератури з теми "Geology, Structural Northern Territory Arunta Block"

AbstractNearly boron-free kornerupine is locally abundant in pods or lenses of coarse-grained, non-foliated, Mg- and Al-rich rocks that occur at high metamorphic grades in early Proterozoic metapelitic rocks from the Reynolds Range, Northern Territory, Australia. This is the third reported occurrence of boron-free kornerupine worldwide. The samples consist almost entirely of coarse-grained kornerupine and its breakdown products sapphirine, cordierite, and gedrite or orthopyroxene. The kornerupine contains only 0.45 wt.% B2O3, corresponding to 0.098 B atoms per 22 (O, OH), and closely approximates 11:10:11 in terms of molar ratios of (MgO + FeOtotal):Al2O3:SiO2, with XMg = Mg/(Mg + Fetotal) = 0.874. The unusual textures and bulk compositions of the rocks in the pods are interpreted to have resulted from metasomatism and high-grade metamorphism (750 to 800° and ∼ 4.5 kbar) of precursors that may have included sedimentary Mg-rich clays. Rocks containing boron-poor, and relatively boronrich kornerupine (2.18 wt.% B2O3; XMg = 0.892) are separated in outcrop by as little as 10 m of the foliated cordierite-quartzite country rock and other rock types, suggesting that the compositions or amounts of the metasomatic fluids varied on a local scale.

The Neoproterozoic to Late Paleozoic-aged Amadeus Basin is a large (~170 000 km2) east–west-trending basin, bounded to the south by the Musgrave Province and to the north by the Arunta Block of the Northern Territory. Commercial oil and gas production is established in the northern part of the basin but the southern part is still a frontier exploration area. Vintage and new seismic reflection data have been used with well data along the south-eastern Amadeus Basin to construct a new structural and depositional model. Three major phases of deformation controlling deposition have been identified. The first phase is characterised by a SW–NE trending structural fabric and is thought to be older than the deposition of the first sediments identified above basement (Heavitree and Bitter Springs formations). The second phase corresponds to the Petermann Orogeny (580–540 Ma) and trends in a NW–SE orientation. The third phase is the Alice Springs Orogeny (450–300 Ma) and is oriented W–E to WNW–ESE in this part of the basin. This tectono-stratigraphic model involving three distinct phases of deformation potentially explains several critical observations: the lack of Heavitree reservoir at Mt Kitty-1, limited salt movements before the Petermann Orogeny (~300 Ma after its deposition) and salt-involved structures that can be either capped by the Petermann Unconformity and overlying Cambrian to Devonian sediments, or can reach the present day surface. Finally, this model, along with availability of good quality seismic data, opens new perspectives for the hydrocarbon exploration of the Amadeus Basin. Each of the tectonic phases impacts the primary petroleum system and underpins play-based exploration.

Investigating openwork block accumulation has the potential to further our understanding of rock weathering, the control of geological structure on landforms, the production of substrates for biological colonisation and the impacts of climate change on landform development and dynamics. Various models for the development of these landforms have been proposed. This includes in situ weathering, frost heave and wedging. Furthermore, it has been suggested that cold-based ice has the potential to preserve these features rather than to obliterate them. Blocky deposits are also frequently used as proxy evidence for interpreting palaeoclimates. The morphology and processes acting on a blockfield located on the Northern Buttress of the Vesleskarvet Nunataks, Dronning Maud Land, Antarctica (2°W, 71°S) were investigated and characterised. Given block dimensions and orientations that closely resembled the parent material and only small differences in aspect related characteristics observed, the blockfield was found to be autochthonous with in situ block production and of a young (Holocene) age. Small differences in rock hardness measurements suggest some form of aspect control on rock weathering. South-facing sides of clasts were found to be the least weathered. In comparison, consistently low rock hardness rebound values for the north-facing aspects suggest that these are the most weathered sides. Additional indicators of weathering, such as flaking and pitting, support analyses conducted for rock hardness rebound values. Solar radiation received, slope gradients and snow cover were found to influence weathering of clasts across the study site. Furthermore, ambient temperatures and wind speed significantly influenced near-surface ground temperatures dynamics. However, the lack of a matrix and paucity of fine material in textural analyses suggest a limited weathering environment. It is suggested that the retreat of the Antarctic ice sheet during the last LGM led to unloading of the surface, causing dilatation and subsequent fracturing of the bedrock along pre-existing joints, leading to in situ clast supply. Subsequent weathering and erosion along other points or lines of weakness then yielded fines and slight edge rounding of clasts.

The Cathaysia Block, located in southeastern China, is characterized by a widespread magmatic belt, prominent NE-striking fault zones and numerous rifted basins filled by Cretaceous-Eocene sediments. The geology denotes a transition from an active to a passive margin, which led to rapid modifications of crustal stress configuration and reactivation of older faults in the Cathaysia Block. However, the timing and kinematics of the active-passive margin transition need to be better constrained. The SW Cathaysia Block, near the coastal area of the South China Sea, is selected for studying the transition. There are two major geological units in this region: the Nanling Range and the Yunkai Terrane. The Nanling Range is a magmatic belt composed of granitic plutons with formation ages ranging from Caledonian to Cretaceous. The Yunkai Terrane is a metamorphic terrane of Caledonian age. Thirty zircon fission-track (ZFT) data and thirty apatite fission-track (AFT) data were obtained from the granitic plutons in the SW Cathaysia Block. The distribution of ZFT ages shows two episodes of exhumation of the granitic plutons: the first episode is found to occur during 170 Ma – 120 Ma and affect the SW part of the Nanling Range; the second episode, a more regional exhumation event, occurred during 115 Ma- 70 Ma. The AFT dating results show a general cooling sequence from south to north during Late Cretaceous – middle Eocene, contrary to the conventional passive margin model. Numerical geodynamic modeling of subduction zone indicates that (1) high slab dip angle, high geothermal gradient of lithosphere and low convergence velocity favor the subduction process and the reversal of crustal stress state from compression to extension in the upper plate; (2) the late Mesozoic magmatism in South China was probably caused by a slab roll-back; and (3) crustal extension could have occurred prior to the cessation of plate subduction. The numerical model results of lithospheric deformation associated with subduction process reveal that granitic crust is easily deformed compared to gneissic crust. The results could be used to explain the observation that the granite-dominant Nanling Range was exhumed earlier than the gneiss-dominant Yunkai Terrane. In addition to the difference in geology between Yunkai and Nanling, the heating from Jurassic- Early Cretaceous magmatism in the Nanling Range may have softened the upper crust, causing the area to exhume more readily. On the other hand, the numerical models of crustal extension with pre-existing faults dipping ocean-ward demonstrate a trend of fault reactivation sequence from south to north. Assuming that granite exhumation in the Cathaysia Block was mainly a product of rifting and fault reactivation, the numerical models support the AFT results. The integrated FT dating and numerical model results suggest that roll-back of the subducting paleo-Pacific Plate slab during Late Cretaceous is likely to be the driving force of the transition from Mesozoic subduction to Cenozoic extension in the Cathaysia Block. The timing of the transition is suggested to have taken place at ~ 92 Ma, according to a rapid cooling as revealed by the thermal history modeling of AFT length data.
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Earth Sciences
Doctoral
Doctor of Philosophy

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

In northern British Columbia, the Sylvester Allochthon of the Slide Mountain terrane is the most inboard of Cordilleran suspect terranes, resting as a vast klippe upon miogeoclinal strata of the Cassiar Platform. The Sylvester is oceanic; it comprises gabbro, pillowed and massive basalt, banded chert, carbonate, argillite, ultramafics and minor arenite, which range in age from Late Devonian to Late Triassic. Internal structure in the Sylvester Allochthon is characterized as a stack of innumerable interleaved tectonic slices, bounded by subhorizontal, layer-parallel faults. These lithotectonic units are an order of magnitude smaller than the terrane itself and may consist of only a single or a few repeated rock types. The internal structure of the Sylvester is complex but not chaotic; small numbers of slices occur together in larger second-order packages which are also fault-bounded and lensoidal. However, tectonic juxtaposition of unrelated lithologies and older-over-younger faults are common. The "stratigraphy" of the Sylvester assemblage is thus tectonic. Sliver-bounding faulting within the Sylvester is known to have, at least in part, predated its post-Triassic, pre-mid Cretaceous emplacement. The Sylvester was emplaced onto North America as the roof thrust to a foreland-style duplex within underlying North American strata. vii viii The Sylvester Allochthon is the most inboard of accreted terranes, however it does not represent a simple marginal basin. New microfossil dating demonstrates that most rock types occur through the complete range of Sylvester ages. Coeval but depositionally incompatable lithologies must have accumulated in separate ocean floor paleoenvironments. Lithologies of the allochthon derive almost exclusively from layer 1, only the surface of oceanic crust. Thus, Sylvester slices are telescoped remnants detached from a vast area of ocean crust which ranged in age and width through the upper Paleozoic but which is now otherwise entirely consumed. Similarities of rock type, internal structure, age range, and regional tectonic setting have identified the Sylvester Allochthon as broadly correlative with a discontinuous series of terranes extending the length of the Cordillera. Together, these terranes may represent the remnants of what was once the late Paleozoic proto-Pacific ocean floor.

[Truncated abstract] The Tintina Gold Province (TGP) comprises numerous (<15) gold belts and districts throughout interior Alaska and Yukon that are associated with Cretaceous plutonic rocks. With a gold endowment of ∼70Moz, most districts are defined by their placer gold contributions, which comprise greater than 30 Moz, but four districts have experience significant increases in gold exploration with notable discoveries at Fort Knox (5.4 Moz), Donlin Creek (12.3 Moz), Pogo (5.8 Moz), True North (0.79 Moz), and Brewery Creek (0.85 Moz). All significant TGP gold deposits are spatially and temporally related to reduced (ilmenite-series) and radiogenic Cretaceous intrusive rocks that intrude (meta-) sedimentary strata. The similar characteristics that these deposits share are the foundation for the development of a reduced intrusion-related gold deposit model. Associated gold deposits have a wide variety of geological and geochemical features and are categorized as intrusion-centered (includes intrusion-hosted, skarns and replacements), shear-related, and epizonal. The TGP is characterized by vast, remote under-explored areas, unglaciated regions with variable oxidation depths and discontinuous permafrost, which, in combination with a still-evolving geological model, create significant exploration challenges. Twenty-five Early and mid-Cretaceous (145-90 Ma) plutonic suites and belts are defined across Alaska and Yukon on the basis of lithological, geochemical, isotopic, and geochronometric similarities. These features, when combined with aeromagnetic characteristics, magnetic susceptibility measurements, and whole-rock ferric:ferrous ratios define the distribution of magnetite- and ilmenite-series plutonic belts. Magnetite-series plutonic belts are dominantly associated with the older parts of the plutonic episode and comprise subduction-generated metaluminous plutons that are distributed preferentially in the more seaward localities dominated by primitive tectonic elements. Ilmenite-series plutonic belts comprise slightly-younger, slightly-peraluminous plutons in more landward localities in pericratonic to continental margin settings. They were likely initiated in response to crustal thickening following terrane collision. The youngest plutonic belt forms a small, but significant, magnetite-series belt in the farthest inboard position, associated with alkalic plutons that were emplaced during weak extension. Intrusion-related metallogenic provinces with distinctive metal associations are distributed, largely in accord with classical redox-sensitive granite-series. Copper, Au and Fe mineralisation are associated with magnetite-series plutons and tungsten mineralisation associated with ilmenite-series plutons. However, there are some notable deviations from expected associations, as intrusion-related Ag-Pb-Zn deposits are few, and significant tin mineralisation is rare. Most significantly, many gold deposits and occurrences are associated with ilmenite-series plutons which form the basis for the reduced intrusion-related gold deposit model

13 uranium mines operated in the South Alligator Valley of Australia’s Northern Territory between 1953 and 1963. At the end of operations the mines, and associated infrastructure, were simply abandoned. As this activity preceded environmental legislation by about 15 years there was neither any obligation, nor attempt, at remediation. In the 1980s it was decided that the whole area should become an extension of the adjacent World Heritage, Kakadu National Park. As a result the Commonwealth Government made an inventory of the abandoned mines and associated facilities in 1986. This established the size and scope of the liability and formed the framework for a possible future remediation project. The initial program for the reduction of physical and radiological hazards at each of the identified sites was formulated in 1989 and the works took place from 1990 to 1992. But even at this time, as throughout much of the valley’s history, little attention was being paid to the long term aspirations of traditional land owners. The traditional Aboriginal owners, the Gunlom Land Trust, were granted freehold Native Title to the area in 1996. They immediately leased the land back to the Commonwealth Government so it would remain a part of Kakadu National Park, but under joint management. One condition of the lease required that all evidence of former mining activity be remediated by 2015. The consultation, and subsequent planning processes, for a final remediation program began in 1997. A plan was agreed in 2003 and, after funding was granted in 2005, works implementation commenced in 2007. An earlier paper described the planning and consultation stages, experience involving the cleaning up of remant uranium mill tailings and other mining residues; and the successful implementation of the initial remediation works. This paper deals with the final planning and design processes to complete the remediation programme, which is due to occur in 2009. The issues of final containment design and long term stewardship are addressed in the paper as well as some comments on lessons learned through the life of the project.