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UBC Theses and Dissertations

Detailed geological mapping and interpretation of the Grand Forks-Eholt area, Boundary district, British Columbia. Reinsbakken, Arne


The Grand Forks-Eholt map area is underlain predominantly by a sequence of moderately deformed and slightly metamorphosed sedimentary and volcanic rocks, previously termed the "Anarchist Group" and ranging in age from Permian/or Earlier to Middle Jurassic. These rocks are divisible into two distinct sequences: (a) cherts (Knob Hill), quartzites, phyllites, and greenstones of Permian/or Earlier age. The term Anarchist Group is now restricted to this lower sequence which resembles the Cache Creek Assemblage (Penn.-Lower Triassic) widespread throughout southcentral B.C.; (b) Sharp-stone Conglomerate/Brooklyn Limestone sequence (Middle-Upper Triassic), and overlying Fragmental Andesites (Middle Jurassic) which correlates with the Takla-Hazelton Assemblage (Middle Triassic - Middle Jurassic) in north central B.C. This upper sequence rests unconformably on the lower and the Sharpstone Conglomerate forms the basal conglomerate separating the two sequences. The Grand Forks Group (pre-Cambrian/or Early Paleozoic), consisting of paragneisses, minor marble and amphibolites crops out east of the Granby River Fault. The fault forms approximately the eastern boundary of the map area. Latest Jurassic Nelson granodiorites; Latest Cretaceous quartz-diorite, quartz-monzonite porphyries, leuco-gabbros and diorites; and Eocene Coryell syenites and related alkalic rocks intrude the sediments and volcanics predominantly in the northern part of the map area. The Nelson granodiorites occur as large batholith-like masses and the younger intrusions form small irregular plugs, dykes and sills. A NNE to N trending nearly recumbent synclinal structure is outlined within the Sharpstone Conglomerate/Brooklyn Limestone/Fragmental Andesite sequence. It is transected by prominent NW trending shear/fault zones and has been broken by these into blocks that are downdropped and shifted to the southwest from north to south in the map area. The eastern part of the map area is transected by the NNE trending Granby River Fault which forms the northern projection of the Eastern Boundary Fault of the Republic Graben - a major structural element to the south in Washington State. Prominent NW and NE Late Cretaceous to Tertiary fractures are ubiquitous and often filled by sheared serpentinites and Tertiary pulaskite and diorite dykes. The Middle Jurassic and older sediments and volcanics have been regionally metamorphosed to the Greenschist Facies. The Grand Forks Group to the east has undergone metamorphism to the Almandine-Amphibolite Facies. Large hornfelsed metasedimentary aureoles surround the larger Nelson granodiorite masses. The Brooklyn Limestone has been thermally altered to marble and locally to chalcopyrite-magnetite bearing calc-silicate skarns, which are often of economic value. Thin contact thermal aureoles surround the Latest Cretaceous quartz-diorite and quartz-monzonite porphyry plugs and dykes, indicating high level intrusion.

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