Paleoproterozoic to Mesoproterozoic evolution of Yukon Territory, Canada

The supercontinent Columbia existed from approximately 1.8 to 1.3 Ga. During this time, complex tectonic interactions occurred between northwestern Laurentia and Australia. This thesis concerns three geologic events on northwestern Laurentia. The first event is the development of a post-collisional sedimentary overlap assemblage at 1.6 Ga herein called the Wernecke-Ogilvie unlithified succession (WOUS). The second event is the formation of the Wernecke Breccia, a set of 1.60 Ga hydrothermal breccia zones. The third is the emplacement of the 1.38 Ga Hart River sills. The WOUS was a succession of sandstone and mudstone that was deposited after collision between Laurentia and Australia at ~1.6 Ga, and prior to the formation of the Wernecke Breccias at 1.60 Ga. The WOUS is contained as clasts within the Wernecke Breccia. On the basis of detrital zircon U-Pb ages, Lu-Hf model ages and neodymium isotope geochemistry, the WOUS is correlated to sedimentary units within the hydrothermal breccias of the Olympic Dam deposit on the Gawler Craton of Australia. The Wernecke Breccia occurs as zones that are metres to kilometres in size that are scattered over an area of 300 x 150 km. The breccia zones are post-orogenic and are hosted by the deformed and metamorphosed Wernecke Supergroup. The breccias formed from voluminous hydrothermal surges with a significant gaseous component. These surges vented, and overlying rock units foundered kilometres deep into the breccia zones. The Hart River sills extend from the eastern Ogilvie Mountains to the Wernecke Mountains. Individual sills are up to 200 km long and 500 m thick. The sills are mafic to intermediate, and tholeiitic. The sills are dated herein at 1382.15 ± 0.39 Ma and 1382.14 ± 0.36 Ma. The magma that formed the sills resulted from ~10% partial melting of spinel bearing mantle similar to a MORB source. Coeval and possibly related magmatism occurred farther south on Laurentia, and to the north on Siberia. These magmatic events occurred on a possible rift axis that extended along the western margin of Laurentia and may signify breakup of the supercontinent Columbia

Structural Setting of the Sixtymile Gold District, Yukon, Canada: Insights into Regional Deformation and Mineralization from Field Mapping and 3D Magnetic Inversion

The Sixtymile gold district, Yukon, Canada has been mined for placer gold since the late 19th century. However, increasing demand for gold has prompted exploration of new lode deposits. Previous studies in the nearby Klondike gold district have shown correlation between placer deposits and bedrock occurrences. Poor bedrock exposure and a complex deformation history, however, make it difficult to determine structural controls on gold mineralization. Through structural analysis involving mesoscopic-scale field observations of fractures, faults, foliation, and folds, and 3D geophysical inversion, the goal of this study was to determine the structural setting of the Sixtymile district to enhance discovery success. Structural measurements in the Glacier Creek, Miller Creek, Bedrock Creek, and Sixtymile River areas show the relationships among the orientations of foliation, fractures, and veins. In most localities, veins are found both parallel and at high angles to foliation, and there is generally a weaker correlation between fractures and veins compared to between foliation and veins. This correlation between foliation and veins is corroborated by inferred gold-bearing horizons from gold assay data. Outcrops of oblique reverse and strike-slip faults, possibly related to a larger-scale thrust-zone, and to the left-lateral Sixtymile-Pika Fault, respectively, were documented for the first time in this study. The results of the 3D probabilistic inversion of total magnetic intensity data for magnetic susceptibility show that magnetic susceptibility highs are preferentially associated with volcanics, but also point to possible intrusive bodies or hydrothermal alteration zones associated with mineralization. A geologic cross-section through the lithologies demonstrates highly variable deformation styles, including extensive folding, possibly indicative of a multiphase deformational history necessitating further, more detailed investigations of the area

Rifting of Western Laurentia at 1.38 Ga: The Hart River Sills of Yukon, Canada

The Hart River sills are a set of mafic to intermediate intrusions that occur in northern Yukon, Canada. The largest sills are over 500 m thick and over 200 km long. New U-Pb dates of 1382.15 ± 0.39 Ma and 1382.14 ± 0.36 Ma were obtained via chemical abrasion thermal ionization mass spectrometry on zircon. Whole rock initial neodymium isotopic compositions of the Hart River sills are juvenile and have εNdi from +1.5 to +4.0. The primary mineralogy of the Hart River sills is predominated by clinopyroxene and plagioclase. Geochemical modeling indicates that the Hart River sills lie on a common liquid line of descent defined by a fractionating assemblage of plagioclase, clinopyroxene and minor olivine. The Hart River sills have rare earth element and high field strength abundances similar to normal mid-ocean ridge basalts (N-MORB) but are enriched in large ion lithophile elements. The Sm/Yb and Dy/Zr ratios indicate \u3e8% partial melting of spinel-bearing mantle. During the emplacement of the Hart River sills, western Laurentia was juxtaposed with Australia and eastern Antarctica within the supercontinent Columbia. The degree of partial melting, similarity to N-MORB, and juvenile isotopic signature are consistent with an episode of rifting at 1.38 Ga. Coeval magmatism and intracontinental rift basins farther south on Laurentia provide additional evidence for rifting of supercontinent Columbia at 1.38 Ga