Dissertations / Theses on the topic 'Calderas Geology Geology, Economic'

Master of Science
Geology
Matthew E. Brueseke
Previous K-Ar dating and petrography (Obradovich, 1978) have identified Pilot Knob as an ~3.4 (±0.06) Ma alkaline intrusive body. Bulk rock geochemistry obtained via XRF from four samples of Pilot Knob verifies the transitionally alkaline composition of the body, and new REE data also show enriched La, Ce, and Nd concentrations, consistent with rare earth element (REE) enrichment. Given the increased demand for REEs over the past ~30 years and China accounting for > 90% of global REE production (Kynicky, et al. 2012), it is important to evaluate new domestic REE sources. This includes those associated with alkaline intrusive complexes, because they are demonstrated to host high REE concentrations (Verplanck and Van Gosen, 2011). Such alkaline igneous occurrences show complex mineralization and consist of many minerals containing substitutional REEs (Mariano and Mariano, 2012). This study evaluates the petrogenesis and mineralogy of Pilot Knob (and a secondary field site, Wildcat Hill) and determines whether the intrusive body is consistent with an economically viable REE deposit. Additionally, given its geographic location and Pliocene faulting and magmatism (e.g., predating the earliest volcanism at Yellowstone), Pilot Knob may represent one of the earliest structural manifestations of the “arrival” of the Yellowstone hotspot at its current location under the North American lithosphere or magmatism associated with lithospheric extension to the south at the Leucite Hills, WY. Inspection of satellite imagery, which has been verified with field data, shows that other intrusive igneous bodies (e.g. - Wildcat Hill) exist, along an apparent normal fault zone along strike with a major extensionally related fault zone documented ~10 km northwest of Pilot Knob. Clinopyroxene geobarometry, coupled with Nd isotope data (εNd[subscript 3.5Ma] = -21.9), indicates that Pilot Knob formed via a multi-stage development history that initiated with melting of ancient lithospheric mantle, where crystallization occurred at a variety of depths. As an alkaline intrusive complex, Pilot Knob has been identified, based on mineral and chemical compositions, as a kersantite, and has been found to contain approximately 600 ppm total light-rare earth element ore lode with ~150 ppm Nd, ~175 ppm La, and ~338 ppm Ce enrichment. Apatite was found to be the primary REE-bearing mineral via petrography and electron microprobe analyses. Based on current technology and processing methods, REE concentrations were not found to be significant enough to denote an economically viable REE ore deposit at Pilot Knob.

Arsenic Ridge is located in the northwestern part of the Lost Horses batholith, Syenite Range, Yukon Territory. This area is within the Omineca Crystalline Belt of the Canadian Cordillera. North American miogeoclinal rocks of the upper Precambrian to Lower Cambrian Grit Unit were northwardly thrust onto Ordovician to Silurian Road River Formation during the Late Triassic to Early Jurassic arc-continent collision. Small mid-Cretaceous (83 to 100 Ma, K-Ar on biotite), epizonal felsic intrusions were emplaced in radiogenic Sr-enriched metasedimentary rocks of the ancient continental margin (pericratonic sedimentary prism). The mid-Cretaceous (syenite phase, 87+3 Ma, K-Ar on biotite; granite phase, 95+3 Ma, K-Ar on biotite) Lost Horses batholith is a circular, S-type, composite pluton within the Selwyn Basin. The Selwyn Basin is an epicontinental trough partially bounded on the west by the Cassiar Platform and on the east and northeast by carbonate rocks of the MacKenzie Platform. S-type plutons are a product of Hercynotype arc-continental collisional tectonics. The zoned intrusive rocks along Arsenic Ridge, from core to rim, have been classified as tourmaline orbicular granite, granite, quartz syenite and syenite. With increasing SiO₂ : (1) the major elements--Al₂O₃ , FeO, MgO, CaO, TiO₂, MnO and P₂O₅--tend to decrease,(2) trace elements--Zr, V, Sr, Ni, and Ba--are characterized by extreme depletions, and (3) the trace element, Rb, is slightly enriched. The syenite is alkaline whereas the quartz syenite, granite, granite dyke, and tourmaline orbicular granite are sub-alkaline. Magmatic differentiation of the pluton is demonstrated by a decreasing trend of (Na₂O + K₂O) and TiO₂ with increasing SiO₂, and by an increasing Rb content with a decrease in Ba and Sr. The estimated partial pressure of water during formation of the Lost Horses granite melt is broadly estimated as >10 kbar. Electron microprobe traverses across orthoclase megacryst cores and rims identified a concentration of albite lamellae in the rim and barium, strontium and calcite rich cores. The latter is indicative of a melt undergoing progressive depletion of barium by fractional crystallization. The age of the batholith is early Late Cretaceous, approximately 97 Ma. This was determined from early Late Cretaceous dates of 95+3 Ma from K-Ar on biotite and 88+4 Ma from K-Ar on a hornblende, and a late Early Cretaceous date of 101+6 Ma from a whole rock-mineral (biotite, hornblende, total feldspar) Rb-Sr isochron. Initial strontium ratios for the granitic rocks along Arsenic Ridge are about 0.712 suggesting that radiogenic strontium was derived mainly from melting and/or assimilation of old sialic crust during magma genesis. The model Rb-Sr age, TUR, for the granitic rocks on Arsenic Ridge is 238 Ma. This indicates that a dominantly upper mantle source is unlikely. Pb-Pb isotope ratios for the zoned plutonic rocks, the surrounding sediments and the ore mineral separates plot between the pericratonic and Bluebell curves (from 0 Ma to 140 Ma mixing lines) indicating that the lead is a mix of upper crust and lower crust sources. Lead sulphide analyses from the Zeta prospect, Tombstone Range and the Keno-Galena Hill areas are indistinguishable from the feldspar rock lead. This shows that the lead source for these vein deposits is the surrounding plutons and not the surrounding sedimentary rocks. The least radiogenic lead has a model age of about 100 Ma. Nd/Sm and Nd analyses indicate that Arsenic Ridge granitic rocks were derived from, or assimilated, old crustal rocks whose Sm/Nd had been lowered at the time of separation from CHUR. Nd ratios for the granite and the feldspar megacrysts are all very close to 0.51210. The model Sm-Nd age, TDM , for a granite along Arsenic Ridge is 1.26 Ga. Approximate percentages of continental crust and mantle incorporated in the melt were calculated. If the contamination is upper crustal in origin then there was a maximum of 30% mantle incorporated in the melt. No mantle component is needed if the contamination source is lower crust. However, since granite ¹⁴³Sm/¹⁴⁴Nd ratios are close to the average continental crust ratio, the origin is upper crust with a small mantle component. ¹⁴³Nd/¹⁴⁴Nd and ⁸⁷Sr/⁸⁶Sr ratios for the granitic rocks from the Lost Horses batholith plot in the Phanerozoic quadrant of Faure (1986) and are similar to values from the Sierra Nevada batholith. Epsilon values of Nd and Sr suggest the granite is S-type which agrees with the field, petrographic and chemical evidence. The granite plots within the field for miogeoclines as determined by Farmer and DePaolo (1983 ). The Zeta tin - silver greisen vein prospects lie in both the Ordovician - Silurian metasediments of the Road River Group at the northeastern contact, and in the zoned, mid-Cretaceous Lost Horses batholith. Mineralization on the property occurs in two forms: (1) cassiterite bearing greisen veins in hornfelsed quartzite, and (2) greisen veins (sulphide and quartz with minor tourmaline, and tourmaline and quartz with minor sulphide in granitic rocks). K-Ar muscovite dating of the sericitic cassiterite greisen (87.0+3.0 Ma), indistinguishable from the K-Ar biotite date for the syenite phase of the batholith (86.8+2.7 Ma), establishes a genetic relationship between the two. The following four-stage model describes the evolution of the Lost Horses batholith: stage 1, initial melting, stage II, melt accumulations and assimilation, stage III, diapiric rise and chemical differentiation (fractional crystallization), and stage IV, magmatic hydrothermal. This last stage generated tin-silver vein and greisen mineralization. The source for this lithophile mineralization and associated S-type granitic rock is dominantly from a sialic clastic wedge with upper crustal geochemical characteristics.
Science, Faculty of
Earth, Ocean and Atmospheric Sciences, Department of
Graduate

The contents of this dissertation fall into two broad areas: geology and history of geology. Although apparently unrelated, the two categories in fact parallel one another. The development of geological systems finds a mirror, on a shorter timescale, in the development of the human understanding of geological systems. The present state of a science - like the present state of an earth system - represents the concatenation of many subtle or evident processes and influences operating over time. Moreover, the events of the past condition the state of the present in science as well as in objects of scientific study. Thus, for instance, to understand why we now hold certain interpretations about the formation of sediment-hosted copper deposits, we must study not only the deposits themselves but the historical development and the philosophical concerns that guided and shaped modern thought about them. In this dissertation the geological and historical aspects are presented in sequence rather than juxtaposed. The geological section comes first, with three chapters detailing the formation and development of the Tenke-Fungurume Cu-Co district and the Central African Copperbelt, followed by another taking a broad view of the mineralogical, geochemical, and metallurgical implications of some of the geological features there. Then follows the history of geology: first two chapters on the role of Georgius Agricola in founding modern geology, and one on how it developed through the following centuries in tune with simultaneous developments in other sciences.

Residences in Warren County, Kentucky, are characterized by high levels of residential radon, which is one of the radioactive daughter products of uranium. According to the United States Environmental Protection Agency (US EPA), radon exposure causes approximately 22,000 lung cancer deaths in the United States per year. The City of Bowling Green, in Warren County, is underlain by karst, an easily soluble limestone subsurface, which allows radon gas to travel easily through cracks and fissures. Carbonate rocks under Bowling Green are underlain by the Devonian Chattanooga Shale, a low-grade uranium ore and a potential source of radon gas. A digital map of the Chattanooga Shale was created using Arc GIS. A 1.6 km by 1.6 km (one-mile by onemile) grid for Warren County was generated, and depth data from oil wells within each grid cell were averaged to render the elevation of the top surface of the Chattanooga Shale in a digital format. A socio-economic GIS of Warren County was created using US Census Bureau and Property Value Administration data. The Chattanooga Shale and the socio-economic layers were correlated to test points that have high residential radon measurements to determine whether proximity to the shale layer or home type is the better predictor for radon risk. Once risks have been determined, management decisionmaking is simplified and resources can be targeted towards high need areas. Although this study determined that home type, i.e., size of the home and whether there is a basement present, does have a significant effect on residential radon levels, proximity to the top surface of the Chattanooga Shale does not have a significant effect in Warren County, Kentucky. Due to this lack of a geologic pattern it is recommended that radon mitigation systems be included in all new home construction and design.

A number of ilmenite-rich heavy mineral deposits occur along the northern margin of the intracratonic Karoo basin, and are hosted by the fluvio-deltaic Middle Ecca Group. Coastal reworking of delta front sands within a regressive, microtidal shoreline may be considered as a depositional model for the development of the heavy mineral deposits. An economic analysis of titanium suggests long term positive demand, and sustained high prices for this commodity. An evaluation of the Dwarsfontein ilmenite-rich heavy mineral deposit, using available data, indicates how important it is for deposits of this type to be situated close to an upgrading plant or export harbour.

A microcomputer risk analysis model is developed and used in an exploration - delineation simulation to analyse in particular the effects of geological factors on exploration mining investment alternatives. Analysis of results indicate that geological parameters can have profound effects on such investment alternatives and that the role of the geologist in determining and evaluating the significance of the various geological factors is critical. Simulation examples highlight some of the key geological parameters and show how changes in these parameters influence both the expected mean results and the standard deviations of such means. The risk analysis model provides an ideal means of conveying the importance of the different geoiogical factors on exploration - delineation - mining investment alternatives and may be used as a geological education aid

The strained commodity price environment has triggered strong measures of cost containment and control by global and South African mining industries with workforce reductions, mine closures and shelved projects. Added to this, the South African mining industry is facing an unparalleled number of challenges, including an uncertain regulatory environment, infrastructure constraints, frequent industrial actions, rising costs and shortages of skills. The dynamism of discontinuous change has increased considerably and the South African mining industry is today facing an uncertain future with a blurred outlook. The results of the detailed analysis of future studies theory and practice in this research study support the argument that there is a strong need to fundamentally change the ways of planning for the future of the South African mining industry. The practice of developing new insight through the application of futures studies is central to this process. Today, collective decisions and strategies are progressively more founded on and informed by futures studies. The research study sought to develop insight regarding the future of the South African mining industry through the construction of four scenarios towards 2055: Divided We Fall, where a confident industry is threatened by social divisions as industry transformation is disregarded; Rock Bottom, where weak global economic conditions coincide with lacklustre industry innovation; Rising from Ashes, with similar economic conditions, but the industry responding positively through accelerated industry innovation; and Renaissance, set against positive global economic conditions with the South African mining industry adopting a collaborative, innovative approach to industry transformation. The research study further strived to uncover the preferred future for the South African mining industry as basis for the South African Mine of the Future Vision towards 2055. Throughout the research study, Inayatullah’s pillars of futures studies were applied as a guideline in mapping the present and future, deepening the future, broadening the future through the development of scenarios, and transforming the future by narrowing it down to the preferred. The study provides valuable insight into the driving forces relevant to the South African mining landscape. In addition, it provides insight on how to anticipate the changes these driving forces may bring about for the industry over the next 40 years from a decision-maker’s point of view. It is up to the mining industry to select the road to follow in terms of progress and sustainable development. Through an innovative approach, the creation of an environment of trust, the sharing of values, purposes and benefits, the South African Mine of the Future Vision is attainable. The South African mining industry must commit itself to working in collaborative partnerships with local communities, government, society and labour; stepping boldly into a world of social, environmental, technological and commercial innovation.

Com este trabalho procurou-se aprofundar a cartografia e a estrutura da área estudada. Considerou-se fundamental neste estudo o aprofundamento da informação geológica acerca das rochas com valor económico na região de Beheda, nomeadamente dos calcários e mármores aflorantes. Os resultados obtidos permitiem melhorar o detalhe da cartografia existente, marcando os afloramentos e os limite das rochas e unidades geológicas encontradas: calcários; mármores, peridotitos, epidotitos, argilitos e arenitos, conglomerados, etc. De marcante importância foram a descoberta de lavas em almofada próximo da zona de contacto entre rochas sedimentares com as rochas ígneas da Formação Aileu. A cartografia detalhada permitiu também ter uma melhor percepção da estrutura e do potencial para exploração económica quer dos mármores da região de Behau, quer dos calcários aflorantes na região de Beheda; ABSTRACT: With this work it is expected to deepen the cartography and structure of the studied area. It was considered crucial in this study the deepening of geological information about the rocks with economic value in the region, particularly the limestones and marbles. The results obtained allow to improve the details of the existing cartography, marking the outcrops and the limits of the rocks and the geological units found: calcareous; marbles, peridotite, epidotite, shales and sandstones, conglomerates, etc. Of striking importance was the discovery of Pillow lavas near the contact zone between sedimentary rocks with the igneous rocks of Aileu Formation. The detailed cartography allowed also to have a better perception of the structure and potential for economic exploitation of the Behau region marbles, and of the outcrops of the limestones of Beheda region.

In the central United States, undefined earthquake sources, long earthquake recurrence intervals and uncertain ground motion attenuation models have contributed to an overstatement of regional seismic hazard for the New Madrid Seismic Zone on the National Seismic Hazard Maps. This study examined concerns regarding scientific uncertainties, overly stringent seismic mitigation policies and depressed local economy in western Kentucky through a series of informal interviews with local businessmen, public officials, and other professionals in occupations associated with seismic mitigation. Scientific and relative economic analyses were then performed using scenario earthquake models developed with FEMA’s Hazus-MH software. Effects of the 2008 Wenchuan earthquake in central China and seismic mitigation policies in use there were considered for potential parallels and learning opportunities. Finally, suggestions for continued scientific research, additional educational opportunities for laymen and engineering professionals, and changes in the application of current earthquake science to public policy in the central United States were outlined with the goal of easing western Kentucky economic issues while maintaining acceptable public safety conditions.

Thesis (M.S.)--Northwest Missouri State University, 2006.
The full text of the thesis is included in the pdf file. Title from title screen of full text.pdf file (viewed on January 25, 2008) Includes bibliographical references.

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O projeto consiste na aplicação de métodos de caracterização de jazidas em um depósito mineral de matérias-primas para indústria cerâmica de pisos e revestimento. Este estudo demonstra que, ao contrário do que muitos pregavam anteriormente, este tipo de jazida possui variações significativas e que existe uma forte necessidade de utilização dos métodos quantitativos e qualitativos para o controle de qualidade das matérias-primas da massa cerâmica e dessa forma alavancar a competitividade dos produtos no mercado nacional e para exportação. Os principais resultados apresentados são a avaliação bidimensional das variações existentes em uma frente de lavra em operação, tanto do ponto de vista químico quanto físico e as variações das propriedades da matéria-prima ao longo do tempo, em diferentes pontos do processo produtivo até a sua influência relativa nas características dos produtos finais obtidos. Para tanto, foram utilizadas ferramentas da estatística uni e multivariada e também a análise variográfica temporal, neste caso uma ferramenta da geoestatística.
The project consists in the application of mine characterization methods in a mineral deposit of raw material for the ceramic tile industry. This study shows that in opposition of general opinion so far, this type of mine has significant variations and that is strongly recommended the use of quantitative and qualitative methods in order to improve the quality control of ceramic raw material, thus to enhance product competitivity in national and international markets. The most important results presented are the two dimensional variation existing in a open pit mine in operation, both by the chemistry and physics standpoint, and the variations of raw material properties along time, in different points of the productive process, until its relative influence in the end products characteristics obtained. For this study, tools of uni and multivarieted statistics were utilized and also temporal variografic analyses, the last one, a geostatistical tool.

Resumo: O projeto consiste na aplicação de métodos de caracterização de jazidas em um depósito mineral de matérias-primas para indústria cerâmica de pisos e revestimento. Este estudo demonstra que, ao contrário do que muitos pregavam anteriormente, este tipo de jazida possui variações significativas e que existe uma forte necessidade de utilização dos métodos quantitativos e qualitativos para o controle de qualidade das matérias-primas da massa cerâmica e dessa forma alavancar a competitividade dos produtos no mercado nacional e para exportação. Os principais resultados apresentados são a avaliação bidimensional das variações existentes em uma frente de lavra em operação, tanto do ponto de vista químico quanto físico e as variações das propriedades da matéria-prima ao longo do tempo, em diferentes pontos do processo produtivo até a sua influência relativa nas características dos produtos finais obtidos. Para tanto, foram utilizadas ferramentas da estatística uni e multivariada e também a análise variográfica temporal, neste caso uma ferramenta da geoestatística.
Abstract: The project consists in the application of mine characterization methods in a mineral deposit of raw material for the ceramic tile industry. This study shows that in opposition of general opinion so far, this type of mine has significant variations and that is strongly recommended the use of quantitative and qualitative methods in order to improve the quality control of ceramic raw material, thus to enhance product competitivity in national and international markets. The most important results presented are the two dimensional variation existing in a open pit mine in operation, both by the chemistry and physics standpoint, and the variations of raw material properties along time, in different points of the productive process, until its relative influence in the end products characteristics obtained. For this study, tools of uni and multivarieted statistics were utilized and also temporal variografic analyses, the last one, a geostatistical tool.
Orientador: Antenor Zanardo
Coorientador: José Ricardo Sturaro
Banca: Paulo Miranda de Figueiredo Filho
Banca: José Francisco Marciano Motta
Banca: Paulo Milton Barbosa Landim
Banca: Anselmo Ortega Boschi
Doutor

Understanding crystallisation in magma chambers is a key challenge for igneous petrology. It is particularly important to understand the origins of layering in peralkaline rocks, e.g. the kakortokite (nepheline syenite), Ilímaussaq Complex, S. Greenland, as these are commonly associated with high value multi-element economic deposits. The kakortokite is a spectacular example of macrorhythmic (>5 m) layering. Each unit consists of three layers comprising arfvedsonite-rich (sodic-amphibole) black kakortokite at the base, grading into eudialyte-rich (sodic-zirconosilicate) red kakortokite, then alkali feldspar- and nepheline-rich white kakortokite. Each unit is numbered -19 to +17 relative to a characteristic well-developed horizon (Unit 0), however there is little consensus on their development. This project applies a multidisciplinary approach through field observations combined with petrography, crystal size distributions (CSDs), mineral and whole rock chemistries on Units 0, -8 to -11 and a phonolite/micro-nephelinolite (“hybrid”) sequence that crosscuts the layered kakortokite. Textures and compositions are laterally consistent across outcrop and indicators of current activity are rare. CSDs indicate in situ crystallisation with gravitational settling as a minor process. Chemical discontinuities occur across unit boundaries. The layering developed through large-scale processes under exceptionally quiescent conditions. The discontinuities reflect open-system behaviour; units were formed by an influx of volatile-rich magma that initiated crystallisation in a bottom layer. Nucleation was initially suppressed by high volatile element concentrations, which decreased to allow for crystallisation of arfvedsonite, followed by eudialyte, then alkali feldspar and nepheline to form each tripartite unit. The chemistry of the hybrid indicates mixing between a primitive (sub-alkaline) magma and kakortokite. Thus injections of magmas of varying compositions occurred, indicating a complex plumbing system below current exposure. The lessons learned at Ilímaussaq, which is extremely well exposed and preserved, are relevant to understanding magma chamber dynamics in the more common instances of pervasively altered peralkaline rocks.

Nickel silicate laterite deposits developed on ultra-mafic rocks are similar in many general respects but they vary considerably in detail. The mineralogy of these surficial deposits is very complex and difficult to determine because of the fine grained nature and solid solution characteristics of the hydrous secondary minerals and because many of the phases are actually mineraloids that are poorly ordered or amorphous. To try some new approaches toward clarification of these phases, 24 samples from New Caledonia and Puerto Rico ranging from the ophiolite-ultramafic olivine-pyroxene-chromite-serpentine substrate rocks upward through intermediate phases of weathering to the final oxide -hydroxide iron cap phase were analyzed with the infrared spectrophotometer (IR -10) and with the automated X –ray diffractometer. Four limonite samples were also mineralogically analyzed. Goethite, secondary quartz, cryptomelane, hematite, chromite, talc, thuringite, and garnierite have been identified in various samples as weathering profile products.

Four multicyclic complex calderas and smaller ignimbrite shields located within the Altiplano Puna Volcanic Complex of the Central Andes (APVC) erupted 13000 km�� of magma within the last 11 Ma. One of the largest and most complex of these is the Cerro Guacha Caldera. Ar-Ar age determinations and paleomagnetic directions suggest that the Cerro Guacha Caldera was formed by two major eruptions, caldera collapse, resurgence cycles and several smaller eruptions. Two major ignimbrites (> 600 km��) are found with ������Ar-�����Ar from biotites and sanidines of 5.65 �� 0.01Ma for the 1300 km�� (magma volume) Guacha ignimbrite and 3.49 �� 0.01Ma for the 800 km�� Tara Ignimbrite. The last major eruption occurred on the western flank producing the 1.72 �� 0.02 Ma Puripica Chico Ignimbrite with a volume of approximately 10 km��. Characteristic remanent magnetization data (ChRM) for these ignimbrites show that the Guacha has reverse polarity, while the Tara is normally polarized and the magnetic fingerprints have allowed their current full extents to be identified. A conspicuous lineament of volcanic structures in the eastern part of the caldera, bordering a caldera moat, filled out welded ignimbrites and sedimentary lacustrine sequences suggest an earlier 60x40 km outer collapse associated with the Guacha explosive episode. A central graben formed on the Guacha welded ignimbrite is related to a first episode of resurgence. Evidence of a second 30 x15 km inner collapse includes offset of welded Guacha ignimbrites and alignment of lava domes associated with the Tara ignimbrite. A second resurgence episode is suggested by the presence of an uplifted central block consisting primarily of welded Tara ignimbrite. As a whole the three ignimbrites (Guacha, Tara and Puripica Chico) share the same petrological and geochemical characteristics: high-K series, compositional ranges from dacite to rhyolite, with andesitic members present as lavas (for the Guacha and Puripica Chico Ignimbrites) and as pumices (for the Tara Ignimbrite). Highest silica content is found in the Chajnantor dome. Rayleigh modeling for Ba, Rb and Sr suggests at least 60% of crystal fractionation to account for the compositional variation between the Guacha andesite and the Chajnantor dome. Dy/Hb ratio increases with time from the Guacha andesite to the Negreal andesite suggesting stabilization of garnet owing to crustal thickening. Fe-Ti exchange geothermometry for the Tara Ignimbrite yielded log fO��� values ranging from -13.06 to -13.38 and temperatures of 714�� to 801��C. Amphibole geobarometry yielded pressures ranging from 150 to 180 MPa equivalent to 5.3 and 6.4 km depth respectively for the Tara Ignimbrite; the pressures range between 133 to 242 MPa, equivalent to 5.0 to 9.2 km depth for the Guacha Ignimbrite. The zircon saturation method yielded saturation temperatures of 716�� and 705��C for the Guacha and Chajnantor dome respectively and 784��C for the Tara Ignimbrite. The zircon crystallization range for the magmas of the Cerro Guacha Caldera is 1.25 Ma for the Guacha Ignimbrite; 1.09 Ma for the Puripica Chico Ignimbrite and 0.95 Ma for the Tara Ignimbrite. Recycling of antecrystic zircons within the caldera magmas is continuos through time.
Graduation date: 2012

The eastern portion of the Namaqua-Natal Mobile Belt, the Natal Metamorphic Province is divided into four main tectonostratigraphic units. These units comprise two accreted island arcs: the Mzumbe and Margate Terranes; an imbricately thrust nappe zone consisting of four ophiolitic nappes in a hinterland-dipping duplex; and the highly deformed metavolcaniclastic/metagreywacke Mfongosi Group directly adjacent to the stable northern foreland of the Kaapvaal Craton. Theories of late-tectonic left-lateral movement in the southern island arcs are extrapolated northwards of the southern margin of the Kaapvaal Craton coincident with the Lilani-Matigulu Shear Zone. The relative timing and structural context of vein-hosted mineralization with respect to major recognized tectonic events is resolved in five separate areas, two in the Natal Nappe Zone and three in the Natal Thrust Front. The Madidima Nappe of the Natal Nappe Zone contains several north-northeast- to northeast-trending and northeast- to east-northeast trending quartzofeldspathic veined reefs considered to have formed in a late-tectonic left-lateral shear system (main shear and synthetic shear orientations, respectively). The northeast- to east-northeast-trending reef is duplicated due to infilling of normally-faulted steep structures in the semi-brittle, incremental normal faulting of the banded amphibolite component of the nappe. Later left-lateral movement has reactivated one of these steep structures along the southern margin of a regional F2-folded band of granite-gneiss in that a southwest extension of this structure may be responsible for sub-economic veining for a length of up to 9 km. The extensive flat-lying topography of the Mbongolwane Flats area, in which the reefs are situated, is accounted for by the accelerated weathering of rocks which underwent sustained late-tectonic metamorphism in the epidoteactinolite facies, accompanied by pervasive shearing and block rotation to the south of the southern limb of the regional F2 fold in the granite-gneiss. A large, kilometer-scale, open advective fluid system which provided fluid-mediated exchange between co-existing rocks existed at the time of vein formation. The fluid system was driven by early-tectonic intrusion of a granite gneiss and amphibole-rich granite. Two areas in the Mfongosi River valley, the northern and southern Mfongosi Valley areas, contain typical evidence of deformation at the leading edge of collision in a mobile belt. The southern Mfongosi Valley area, at the confluence of the Mfongosi and Tugela Rivers, contains veining which resulted from pressure solution of the host metavolcaniclastic/metagreywacke. Veining occupies predictable shear and tension fractures formed during the initial deformation of a foreland margin sequence, in addition to occupying those fractures formed by buckling on the layer-scale. The structural context of the northern Mfongosi Valley veining is defined by subsequent deformation and vein fragmentation such that the metavolcaniclastic/metagreywacke was reduced to a melange in which vein segments acted as competent clasts; a large-scale porphyroblast/matrix system. Formation of the Manyane Thrust to the south of the Mfongosi Group interrupted the normal retrograde metamorphism of the remainder of the Tugela Nappe and initiated a "hot iron effect" whereby a short-lived thermal pulse acted at the thrust plane, producing a reversed geothermal gradient in the underlying Mfongosi group. This reversed gradient would have been counteracted by a steepened normal geothermal gradient in the Mfongosi Group caused by overloading of the Natal Thrust Front by the Natal Nappe Zone. These geothermal gradients partly account for the concentration of veining in the areas of the Mfongosi Group which are directly adjacent to the Manyane Thrust, and directly adjacent to the Kaapvaal Craton, in the lower portions of the thrust front Stable isotope studies indicate fractionation between vein and wall rock under a short-lived, mainly rock-buffered, layer-scale fluid-movement system. Also forming part of the Mfongosi Group of the Natal Thrust Front, the Ngubevu area contains an apparently enigmatic distribution of veining accompanied by gold and base metal mineralization. The structural evolution of the Ngubevu area occurred during consistent left-lateral transpression into which has intruded early-tectonic veins, formed by pressure solution and having the same structural format as the early-tectonic veining in the southern Mfongosi Valley area. Subsequent deformation of the system was accompanied by 1900 -trending tension gashes which were continually ptygmatically-folded, sheared and offset to form occasionally mineralized quartzofeldspathic "blows" and along-strike stringers in the epidote- actinolite schist. Where veining cross-cuts narrow calcite - graphite - sericite - quartz - albite - tourmaline ± chlorite schist layers, gold mineralization occurred. The late-tectonic tension gashes, antitaxially filled by quartz and amorphous calcite, cross-cut the entire range of lithologies. The fluid system during vein deposition varied: during infilling of early-tectonic fractures a short-lived fluid-flow system dominated, with the emplacement of re crystallized wallrock occurring in a closed, non-advective regime under the influence of diffusion caused by pressure solution. The fluid system changed to a more open, advective, greater than layer-scale rock-buffered one with a decreasing contribution of material from immediate host rocks. An internal fluid source is implied for the entire period of vein emplacement, derived from structural analyses which indicates negative dilation across the Mfongosi Group in this area and by comparison of vein:wallrock δ180 values which indicate a lack of igneous-derived fluids. The Phoenix Mine, in the central portion of the Tugela Nappe, and the Ayres Reef, hosted in Manyane amphibolite adjacent to the Manyane Thrust, are grouped together on the basis of their cross-cutting nature and timing with respect to metamorphism and deformation of the host rock, and also due to their similarity in isotopic plots. Both vein sets occur in approximately east-west to east-northeast-trending zones which show evidence of late-tectonic left-lateral movement. Phoenix Mine veining occurs in weakly-metamorphosed meta-gabbro/meta-norite of the Tugela Rand Complex. The Manyane amphibolite demonstrates the amphibolite facies of metamorphism due to the short-lived thermal pulse at the Manyane Thrust. Both sets of veining display slickenlines which are indicative of their emplacement prior to the late-tectonic left-lateral movement. The unusually thick quartz veins of both deposits are the results of late- to post-Tugela Rand Complex fluids or the tapping of late-tectonic metamorphic fluid reservoirs. This caused silica metasomatism and redeposition of material in post-thrusting collapse features. A highly channelized, single-pass fluid system is proposed in the absence of intrusion-derived fluids. Whole rock geochemical data allow a distinction to be made between the Natal Thrust Front and the Natal Nappe Zone: the Foremost nappe of the nappe zone consists primarily of N-type mid-ocean ridge basalts/ocean-floor to within-plate basalts which were intruded prior to nappe emplacement by metaluminous orogenic volcanic arc granitiods. The thrust front displays a lateral variation in metabasite/metasediment ratio, with the ratio increasing from east to west in this inlier. In the east, in the Nkandlha area, melanged metagreywackes dominate and there is a marked paucity of associated metabasites. In the central portions of the thrust front, in the vicinity of the Mfongosi area, active continental margin/continental arc magmatogenic greywackes and arkoses are interlayered with calk-alkaline volcanic arc basalts (volcaniclastics). The greywacke geochemistry indicates little to no mafic/ultramafic influences in sediment contribution and the source of sediment is inferred to be the southern portions of the Kaapvaal Craton. The Nkandlha and Mfongosi area Mfongosi Group segments are considered to be in-situ or para-autochthonous. The western-most Ngubevu area predominantly hosts metabasites. The geochemistry of the metabasites indicates that they are N-type mid-ocean ridge basalts/ocean floor basalts from a destructive plate margin setting. The metabasites are interbanded with metapelitic/metacalcsilicate layers produced in a shallow water oxic environment, here inferred as a spatially-restricted shallow, marginal basin. The metabasites in the Ngubevu area are notably similar to those of the Madidima Nappe, indicating a similar provenance and pre-collisional mode of formation. It is proposed that the variation in the Natal Thrust Front was due to a north-east/south-west distribution of lithological proportions or mixing, with greywackes dominating in the northeast (in proximity to the Kaapvaal Craton) and metabasites dominating in the southwest. Left-lateral transpressional movement within the Mfongosi Group of the Natal Thrust Front, and the Natal Nappe Zone, was continuous throughout plate collision and obduction.
Thesis (Ph.D.)-University of Natal, Durban, 2000.