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Surface geology at the Granduc Mine Davidson, Donald Alexander

Abstract

The Granduc Mine is near the British Columbia-Alaska boundary in rugged mountainous country about 36 miles northwest of Stewart B.C. The writer participated in detailed surface mapping in the vicinity of the mine during the field season of 1959. Rock specimens were collected in the field and examined microscopically in the laboratory. The results of the latter investigation form the greater part of this thesis. The oldest rocks in the area are north trending, steeply dipping and isoclinally folded metavolcanic and metasedimentary rocks that are believed to be correlative with the Hazelton group. These consist of a basal andesite complex which is overlain by a large thickness of metasedimentary rocks. The Hazelton group rocks have been regionally metamorphosed and synkinematically intruded by small subconcordant bodies of foliated diorite and hornblende granodiorite. The metasediments are believed to have formed from greywackes, marls and sandstones that have been progressively metamorphosed to form schists that can be classified in the quartz-albite-epidote-biotite subfacies of the greenschist facies. Almost identical mineral assemblages are found in the altered dioritic and metavolcanic rocks, and it is concluded that these initially high temperature assemblages have retrogressed during regional metamorphism to attain or approach equilibrium in the same metamorphic facies. At a late stage in the metamorphism strong differential movement was localized in a quartz-rich member of the metasedimentary rocks in a zone near the contact with metavolcanic rocks. All rocks in this zone have undergone retrogressive metamorphism, and have attained equilibrium in the quartz-albite-muscovite-chlorite subfacies of the greenschist facies. Drag folds show that this dislocation metamorphism was related to the formation of an anticlinal structure that lies to the east of the map area. Some of the major structural ore controls appear to have formed at this time. The strongly-developed isoclinal folding was later flexed during or following intrusion of the Coast Range batholith. Ore bearing solutions are believed to have been derived from batholithic emanations and these were channeled along crumpled and brecciated zones that formed during the earlier period of regional metamorphism. Two mineralized zones are present and these are essentially conformable with the metasediments and consist chiefly of chalcopyrite, pyrrhotite and sphalerite. Mineralization has replaced the host rock along favourable lithologic horizons, but appears most heavily concentrated in brecciated zones. The deposit is classified as Mesothermal Replacement.

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