Thesis (Ph.D.) - University of Glasgow, 2007. / Ph.D. thesis submitted to the Faculty of Physical Sciences, Department of Geographical and Earth Sciences , University of Glasgow, 2007. Includes bibliographical references. Print version also available.

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Please read the abstract in the section 00front of this document / Dissertation (MSc (Chemistry))--University of Pretoria, 2003. / Chemistry / unrestricted

The supermature Middle-Late Ordovician Swan Peak quartz arenite was deposited on the western Laurentia passive margin and is very fine to fine grained, well-rounded, well-sorted, and silica-cemented. Laurentia was positioned over the equator during the Middle-Late Ordovician, suggesting that basement rock along the Transcontinental Arch was intensely eroded in a humid climate to produce this and other coeval quartz arenites. To determine provenance for the Swan Peak Quartzite, zircon grains were analyzed using LA-ICP-MS and the results were constrained within a sequence stratigraphic framework. Depositional environments of the Swan Peak Quartzite record an offshore-to-onshore transition with five facies (A-E). Facies A only occurs at the base of the Bear Lake section and may record an incised valley or localized embayment. It is the deepest water facies in the succession containing shale and quartz arenite interbeds. Facies B through E are interpreted as lower, middle, upper shoreface/foreshore depositional environments, respectively, based on primary sedimentary structures and bioturbation. Detrital zircon age spectra of the Swan Peak Quartzite have four distinct populations: the two main populations are at 1.8 - 2.0 Ga (Paleoproterozoic) and between 2.5 - 3.0 Ga (Archean), with a smaller, but persistent, population at 2.0 - 2.1 Ga, and a very minor 0.8 - 1.2 Ga (Mesoproterozoic) population occurring mainly in the tops of the measured sections. The base of each section has a larger Archean peak whereas the top of each section is predominantly Paleoproterozoic grains. Zircon data have overlap and similarity values ranging between 0.531 - 0.771 and 0.506 - 0.881, respectively, which indicates zircon age spectra of the Swan Peak Quartzite is similar to other Cordilleran Ordovician quartzites and that recycling of heterogeneous underlying sedimentary rocks was minimal. The Wyoming Craton (2.5 - 2.8 Ga) and the Trans-Hudson Orogen (1.8 - 2.0 Ga) provinces near the paleoequator likely provided the majority of zircons in the Swan Peak Quartzite. The source for the 2.0 - 2.1 Ga grains is currently unknown and the 0.8 - 1.2 Ga grains are interpreted to reflect Mesoproterozoic Laurentian tectonism. Sediment input varied in response to sea level fluctuations. Longshore transport was likely an important process in redistributing grains along the coastline during later deposition of the Swan Peak Quartzite.

The Trans-Suture Suite (TSS) of granitic plutons located in Northern Britain span the Iapetus Suture and represent a particularly enigmatic stage of post-Caledonian Devonian magmatism. Despite calc-alkaline affinities, proximity to the Iapetus Suture precludes a direct relationship to active subduction. Furthermore, the absence of inherited zircons distinguishes the TSS from plutons of a similar age throughout the Scottish Highlands, and is not easily reconciled with the abundance of peraluminous and S-type granites. Micro-analytical techniques are employed to analyse accessory zircon and apatite from three TSS plutons (Criffell, Fleet and Shap) in order to investigate connections between magmatic and tectonic processes within a continental suture zone. Accessory minerals contain a robust and accessible record of magma evolution. However, their trace element compositions are shown to document the final stages of pluton emplacement only, and are determined primarily by competitive crystallisation of other accessory phases at shallow crustal levels. By contrast, whole-rock compositions record an earlier stage of magma evolution that occurred in deeper and open-system crustal hot zones. The absence of inherited zircon in the final crystal assemblage reflects resorption during rapid and potentially adiabatic ascent of super-liquidus and water-rich magmas from the crustal hot zone. The concentrations of REE in apatite inclusions decrease with increasing crystallisation of other accessory minerals and in some samples have been distinguished on the basis of their host phase. In metaluminous granodiorites, no preferred crystallisation sequence is observed between host phases. In peraluminous samples, zircon-hosted apatite compositions appear more primitive compared to those hosted by other phases and reflects earlier saturation of zircon in these magmas. The isotopic record of zircon is shown to be biased towards earlier stages of magma evolution in peraluminous magmas and provides a means of assessing zircon isotope compositions in the context of often protracted histories during silicic magma evolution. The Mn content of apatite varies independently to whole-rock composition, correlating positively with decreasing oxygen fugacity and indices of increasing reduction. Apatite is proposed as a robust and effective redox proxy with application to magmatic, detrital and lunar studies of redox conditions. Zircon O-HfU- Th-Pb isotope compositions have identified increasing degrees of compositional heterogeneity in more silicic components of the TSS, including the involvement of more mafic magmas in the generation of the large S-type pluton of Fleet. Compositional trends between zircons from plutons emplaced on both sides of the Iapetus Suture are used to identify a common Avalonian component beneath the Southern Uplands and English Lake District related to Avalonian underthrusting beneath the Laurentian margin during the late Caledonian. New geochronological dating indicates that pluton emplacement occurred during periods of both pre- and post-Acadian transtension. The distinguishing characteristics of these plutons relative to other Caledonian plutons reflect their unique emplacement into the hydrated lithosphere of the Iapetus Suture Zone. Oxygen isotope disequilibrium between the magmatic compositions recorded by zircon and those of the whole-rock and some quartz crystals reflect hydrothermal alteration of the latter two archives. Hydrothermal alteration and exchange caused by 18O-rich magmatic fluids has elevated the 18O compositions of the whole-rock and some quartz crystals, with most disequilibrium observed around the outer margins of the plutons due to further fluid interaction with local 18O-rich sediments. The hydrothermal history of the TSS is markedly different from that of the British Tertiary Igneous Province where hydrothermal alteration resulted from circulation of meteoric water, reflecting significantly different magmatic and emplacement histories.

Micrometer-thin rims of hydrothermally altered zircon preserve significant geological information regarding the timing and nature of fluid infiltration. The research presented in this thesis details an investigation of the isotopic and geochemical composition zircon rims from deformed Archean meta-sedimentary rocks proximal to the world class Musselwhite gold deposit. A continuous ablation Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) U-Pb technique is proposed in conjunction with Secondary Ion Mass Spectrometry (SIMS) U-Pb and LA-ICP-MS REE depth profile methods. These techniques are capable of identifying and analyzing isotopic and chemical modification of zircon rims that are <5 μm thick by ablating directly into the surface of unpolished crystals. The age of analyzed rims is >100 m.y younger than crystal interiors and corresponds to periods of regional magmatism and hydrothermal activity. The measured thickness of rims is variable across single grains and between grain suites suggesting that they do not form as a uniform mantle around the crystal interior. Instead the interacting fluids exploit pre-existing structural weaknesses caused by sedimentary transport and the α-decay of uranium. A novel LA-ICP-MS single element grain 2-dimension mapping technique for unpolished zircon demonstrates preferential element mobility along fractures and in isolated patches. Pressure-temperature experiments were conducted on a zircon reference material under lower greenschist facies conditions to better understand the low temperature incorporation of REEs into the zircon structure. LA-ICP-MS element mapping of unpolished grains reveals that zircon are chemically modified under these conditions, as characterized by an increase in Ce and Y concentration (up to an order of magnitude enrichment) in CePO4-bearing and YPO4-bearing experiments, respectively, when compared with the concentrations of unaltered primary grains. The integration of these micro-analytical techniques on unpolished zircon can provide insight into the timing of low- to moderate-temperature history of supracrustal rocks which would remain otherwise undefined.

The Zeemal Heaton Lake metasedimentary assemblage comprises ~50% of the eastern limb of the North Caribou greenstone belt, which hosts the Musselwhite gold deposit (~ 5.4 Moz Au). The metasedimentary assemblage is divided into two based on their distribution: the Central-belt and North-rim of the belt. The Zeemal Heaton Lake metasedimentary rocks have been interpreted to represent a turbidite succession, consisting of pelite, arkose, wacke and conglomerate. Detrital U-Pb zircon age populations from this study indicate variable sources in the Central-belt metasediments with a maximum age of deposition of 2798.8 ± 29 Ma, while metasediments along the North portion of the limb display a maximum age of deposition of 2696 ± 39 Ma. This indicates the likelihood of two discrete basins with the Zeemal Heaton Lake assemblage. Neodymium isotopic values for the sedimentary rocks along the North-rim of the belt display εNd2680Ma ranging from -1.2 to -0.9. An overlapping but slightly more evolved signature shown in the Central portion with values of εNd2800Ma -1.6 to 0.1. This indicates detritus for the Central-belt was likely sourced from the older (~2850 Ma), (tonalite-granodiorite) surrounding intrusions. The North-rim sediments were sourced (in part) from the younger (~2700 Ma) granitic intrusions and rhyolites. The surrounding batholiths display εNdt overlapping with North-rim sedimentary rocks, however, indicate significant contribution beyond these immediate surrounding batholiths, which may include older mafic (2982 ± 0.8, 2870 ±2 Ma), younger rhyolitic unit (2723 ± 2 Ma) or more distal sources than the intrusions immediately adjacent to the belt. Peak metamorphic mineral assemblages indicate a minimum of amphibolite facies-garnet zone or greater across the Central-belt portion of the greenstone belt. The North-rim metasediments display evidence for aureole metamorphism which may overprint earlier regional metamorphism. Nitrogen abundance and δ15N values of biotite of the Zeemal Heaton Lake metasedimentary assemblage indicate variable values than those associated with mineralization at the Musselwhite Mine, which is reported by Isaacs (2008). Mineralizing fluids from the Musselwhite mine have been interpreted by Issacs (2008) to be related to both metamorphic and magmatic fluids. Fluids associated with the Eyapamikama and/or Zeemal Heaton Lake metasediments appear to be metamorphic with minor magmatic influence near regional shear zones, but not directly related to the auriferous fluids which formed the Musselwhite mine.

Oxidized hydrous intermediate composition magmas are responsible for porphyry copper (Cu ±Mo ±Au) deposits and epithermal Au ore deposits formed globally in the shallow crust (Sillitoe, 2010; Seedorff et al., 2005). Recently, zircon geochemistry has been used to characterize both productive and barren intrusions associated with porphyry Cu-Au ore deposits. Zircon composition differs slightly between the two intrusive groups, and researchers have proposed that zircon in productive intrusions has crystallized from a relatively more oxidized melt compared to barren intrusions (Ballard et al., 2002; Muñoz et al., 2012). Zircon rare earth elements record anomalies in Ce and Eu contents that allow estimation of the ratio of oxidized versus reduced species, i.e. Ce⁴⁺/Ce³⁺ (Ce[superscript IV]/Ce[superscript III]) and Eu³⁺/Eu²⁺ (Eu/Eu*)[subscript CN]. This study focuses on understanding the compositions of Eocene magmas associated with sediment hosted Carlin gold deposits and the gold-copper ores of the Battle Mountain porphyry Cu-Au-skarn district in northern Nevada. Zircon trace element composition was analyzed using LA-ICP-MS and SHRIMP-RG to determine differences between mineralizing and non-mineralizing intrusions in northern Nevada and to compare these compositions with known porphyry Cu-Au type magmas. These zircon and rock compositional data was then used to test the hypothesis of a magmatic origin of the Carlin type gold deposits (Muntean et al., 2011). Zircon U-Pb ages were calculated using multiple SHRIMP-RG spot analyses of each sample for two Carlin biotite porphyry dikes, two Battle Mountain porphyry dikes and the granodiorite of the Copper Canyon stock. The new U-Pb age dates for Carlin porphyry dikes are 38.7 ± 0.5 Ma and 38.8 ± 0.4 Ma. The age of the Copper Canyon stock is 38.0 ± 0.7 Ma, and the age of the Battle Mountain porphyry dikes are 40.2 ± 0.4 Ma and 41.3 ± 0.4 Ma. The Carlin dike ages are the same age, within uncertainty, with previous studies conducted (Mortensesn et al., 2000). The productive porphyry dikes from the Battle Mountain district have Ce(IV)/Ce(III) ratios of 500 to 10000 and a wide range of (Eu/Eu*)[subscript CN] values between 0.3 and 0.7 respectively. Carlin porphyry dikes have Ce(IV)/Ce(III) values between 100 and 1000, and a more limited (Eu/Eu*)[subscript CN] range of 0.5 to 0.7. Barren Eocene intrusions at Harrison Pass and Caetano have much lower Ce(IV)/Ce(III) ratios that range from 20 to 500, and have a very large span of (Eu/Eu*)[subscript CN] from 0.03 to 0.6. Calculated Ce(IV)/Ce(III) and (Eu/Eu*)[subscript CN] of zircon of this study illustrate a distinction between productive and barren intrusions in northern Nevada, and demonstrate a geochemical link between porphyry type magmas and dikes associated with Carlin type gold deposits. These ratios may provide a useful means of evaluating potentially economic geologic terranes and serving as a method to infer relative oxidation state of zircon bearing intrusive rocks. / Graduation date: 2013

The Alleghanian orogeny was a collision between the Gondwanan and Laurentian continents that produced the Pangean supercontinent. Mechanical and kinematic models of collisional orogens are believed to follow a critical taper geometry, where the tectonic imbrication of continental crust begins nearest to the edge of continental plate and advances toward the craton in a break- forward sequence. Studies of shear zones within the Alleghanian collisional orogen, however, suggest that most of the early deformation was translational. Propagation of craton-directed thrusts into the foreland did not occur until the latest Pennsylvanian in the southern Appalachians, and the middle-late Permian in the central Appalachians. Radiometric sedimentary provenance proxies have been applied to the late Mississippian-early Permian strata within the Appalachian foreland basin to determine the crustal composition and structural evolution of the orogen during the continental collision. U-Pb ages of detrital zircons from the early to middle Pennsylvanian sandstones suggest that most of the detritus within the Appalachian basin was recycled from Mesoproterozoic basement and Paleozoic strata of the Laurentian margin. The presence of Archean and late Paleoproterozoic age detrital zircons is cited as evidence of recycling of the Laurentian syn-rift and passive-margin sandstones. Detrital zircon ages from early-middle Permian-age sandstones of the Dunkard Group do not contain any Archean or Paleoproterozoic detrital-zircon ages, implying a source of sediment with a much more restricted age population, possibly the igneous and metamorphic internides or middle Paleozoic sandstones from the Appalachian basin. The persistance of 360-400 Ma K/Ar ages of detrital white mica suggest that the sediment was supplied from a source that was exhumed during the Devonian Acadian orogeny. Detrital-zircon and detrital-white-mica ages from Pennsylvanian-age sandstones indicate that the late Paleozoic orogen did not incorporate any significant synorogenic juvenile crust. The 87Sr/86Sr ratios of middle Pennsylvanian-early Permian lacustrine limestones within the Appalachian basin show a slight enrichment through time, suggesting that labile 87Sr-rich minerals in the Alleghanian hinterland are being exposed. Stable isotopic data from the lacustrine limestones also corroborates that the Appalachian basin became much more arid through time.

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