Notes Accompanying Original Release of Maps - Geoscience BC

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Metallogenesis of the Iskut River Area, Northwestern B.C. --- Regional maps, Lewis Rhyolite flows, breccias, and tuffs in the Eskay Creek area are assigned to the Eskay Rhyolite Member of the Salmon River Formation. Although this rhyolite is lithologically similar to some exposures of the Bruce Glacier Member, particularly in the Virginia Lake region, it can be distinguished geochemically on the basis of an Al:Ti ratio of greater than 100. At Eskay Creek, the member forms a distinct mappable unit overlying the Bruce Glacier Member and underlying the John Peaks Member, with thicknesses of up to 250 m. Age: Age constraints for the Salmon River Formation include U-Pb zircon ages from the Bruce Glacier Member and fossil collections from intercalated sedimentary sections assigned to the Troy Ridge Member. Because of the interfingering relationships of the different members these determinations are interpreted as being representative of the entire formation. U-Pb zircon dates obtained from the Bruce Glacier Member bracket the age of the unit to around 172-178 Ma. At Bruce Glacier, a U-Pb age of 176.2 ± 2.2 Ma has been obtained from flow-banded dacites near the base of the section. Stratigraphically equivalent flows across the Unuk River valley have yielded a U- Pb age of 173.6 Ma + 5.6/-0.5 Ma (Childe, 1994). In the Snippaker Creek area, two U- Pb ages fall within this same range: 172.3 ± 1.0 Ma, and 178.2 ± 5.0 Ma. Fossil collections from within the Troy Ridge Member are consistent with U-Pb age determinations of adjacent rocks, but are problematic when compared to biochronological constraints from the underlying Betty Creek Formation. Fossil collections at Eskay Creek indicate a middle Bajocian age for the unit, while slightly older Upper Toarcian ammonites have been collected from the Julian Lakes area. This Toarcian age is older than Upper Aalenian ammonites from the Treaty Ridge Member at Treaty Ridge, indicating that either formations are diachronous across the map area, or that units at Julian Lakes should be reassigned to lower positions. Mount Dilworth Formation: The term “Mount Dilworth Formation”, as defined by Alldrick (1991) has been applied to felsic volcanic portions of the Hazelton Group throughout the Iskut River area. Recognition that the Hazelton Group contains multiple, temporally distinct felsic volcanic intervals casts many of these assignments in doubt. One objective of the MDRU research program has been to obtain conclusive age data for both the type Mount Dilworth Formation and for mappable felsic intervals to the north, and if possible, to establish map continuity between the type section and more northerly areas. Despite several attempts, no wellconstrained age has been determined for the formation’s type area, and map continuity is lacking. In order to avoid possible erroneous correlations, the term “Mount Dilworth Formation” has been dropped from the MDRU maps, and the stratigraphic nomenclature used is based on type sections within the area mapped as part of the study. Felsic intervals which might have previously been included in the Mount Dilworth Formation are now assigned to either the Brucejack Lake Member of the Betty Creek Formation, or the Eskay Rhyolite or Bruce Glacier Member of the Salmon River Formation, depending on age and stratigraphic position. REFERENCES Alldrick, D.J., 1991, Geology and ore deposits of the Stewart mining camp, British Columbia: Unpublished Ph.D. thesis, Vancouver, B.C., University of British Columbia, 347 p. Alldrick, D.J. and Britton, J.M., 1991, Sulphurets area geology: British Columbia Ministry of Energy, Mines and Petroleum Resources Open File Map 1991-21. ---------- 1992, Unuk area geology: British Columbia Ministry of Energy, Mines and Petroleum Resources Open File Map 1992-22. Anderson, R.G., 1993, A Mesozoic stratigraphic and plutonic framework for northwestern Stikinia (Iskut River area), northwestern British Columbia, in Dunne, G. and McDougall, K., eds., Mesozoic paleogeography of the western United States, II: Los Angeles, CA, Society of Economic Paleontologists and Mineralogists, Pacific Section, p. 477-494. Childe, F.C., 1994, Radiogenic isotopic investigations of the Eskay Creek volcanic hosted massive sulphide deposit, British Columbia, Canada [abs.]: United States Geological Survey Circular 1107, p. 58. Childe, F.C., Barrett, T.J. and McGuigan, P.J., 1994, The Granduc VMS deposit, northwestern British Columbia: U-Pb ages and Pb isotope relations [abs.]: Geological Society of America Abstracts with Programs, v. 26; 7, p. 381. 82
Metallogenesis of the Iskut River Area, Northwestern B.C. --- Regional maps, Lewis Grove, E.W., 1986, Geology and mineral deposits of the Unuk River-Salmon River-Anyox area: British Columbia Ministry of Energy, Mines and Petroleum Resources Bulletin 63, 152 p. Henderson, J.R., Kirkham, R.V., Henderson, M.N., Payne, J.G., Wright, T.O. and Wright, R.I., 1992, Stratigraphy and structure of the Sulphurets area, British Columbia, (104 B/8, 9): Canada Geological Survey Paper 92-1A, p. 323-332. Lewis, P.D., 1992, Structural geology of the Prout Plateau region, Iskut River map area, British Columbia: British Columbia Ministry of Energy, Mines and Petroleum Resources Paper 1992-1, p. 521-528. ---------- 1993, Stratigraphic and structural setting of the Iskut River area, in Metallogenesis of the Iskut River area, northwestern British Columbia: Unpublished Annual Technical Report - Year 3, Mineral Deposit Research Unit, University of British Columbia. Lewis, P.D., Macdonald, A.J. and Bartsch, R.D., (this volume), Hazelton Group/Bowser Lake Group stratigraphy in the Iskut River area: progress and problems, in Metallogenesis of the Iskut River area, northwestern British Columbia: Mineral Deposit Research Unit Special Publication Number 1. Macdonald, A.J., (this volume, a), Eskay Porphyry, in Metallogenesis of the Iskut River area, northwestern British Columbia: Mineral Deposit Research Unit Special Publication Number 1. ---------- (this volume, b), Kerr-Sulphurets camp and Brucejack Lake, in Metallogenesis of the Iskut River area, northwestern British Columbia: Mineral Deposit Research Unit Special Publication Number 1. Macdonald, A.J., van der Heyden, P., Lefebure, D.V. and Alldrick, D.J., 1992, Geochronometry of the Iskut River area - an update: British Columbia Ministry of Energy, Mines and Petroleum Resources Paper 1992-1, p. 495-502. Nadaraju, G., 1993, Triassic-Jurassic biochronology of the eastern Iskut River map area, northwestern British Columbia: Unpublished M.Sc. thesis, Vancouver, B.C., University of British Columbia, 223 p. 83
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Notes Accompanying Original Release of Maps - Geoscience BC

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