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Application of K-Ar and fission-track dating to the metallogeny of porphyry and related mineral deposits in the Canadian Cordillera Christopher, Peter Allen


This study evaluates the concept of metallogenic epochs as it applies to porphyry mineral deposits of the Canadian Cordillera, extends the study of the age of porphyry mineral deposits into northern British Columbia and the Yukon Territory, evaluates the usefulness of the fission-track dating method in determining the age and history of porphyry mineral deposits, and demonstrates the usefulness of an Ar⁴⁰ total vs %K isochron plot. Samples were obtained from six areas in the Canadian Cordillera: the Syenite Range and Burwash Landing area in the Yukon Territory and Cassiar area, Adanac Property, Granisle Mine, and Copper Mountain area in British Columbia. Apatite was separated from the samples for use in fission-track analysis, and co-genetic biotite or hornblende was separated in order to obtain K-Ar checks on the fission-track ages. A comparison of fifteen apatite fission-track ages with K-Ar ages demonstrates that the apatite fission-track method can be used to age date porphyry mineral deposits, however the K-Ar method is generally more suitable in terms of cost and reliability. Discordant apatite fission-track and biotite K-Ar ages obtained from the Copper Mountain area and Granisle Mine suggest that apparent apatite fission-track ages from highly altered rocks or thermally complex areas should be checked by using another dating method (e.g. K-Ar). Radiometric dating of the Cassiar Molybdenum, Adanac, Mt. Reed and Mt. Haskin porphyry mineral deposits in northern British Columbia suggests that the Early Tertiary metallogenic epoch- for porphyry deposits in central British Columbia and south-eastern Alaska, can be extended through northern British Columbia. Post-Eocene intermittent subduction of the Juan de Fuca plate below Vancouver Island and transverse motion along the Fairweather-Queen Charlotte-Shakwak-Denali Fault system with subduction into the Aleutian Trench are consistent with present plate-tectonic theory and the distribution of post-Eocene calc-alkaline igneous rocks in the Canadian Cordillera. If porphyry mineral deposits form in calc-alkaline igneous rocks above active subduction zones, then the youngest porphyry deposits in the Canadian Cordillera should occur west of the Fairweather-Queen Charlotte-Shakwak-Denali fault system, on Vancouver Island and in the Cascade Mountains. The relatively young 26.2 m.y. biotite K-Ar age determined for the Burwash Creek porphyry west of the Shakwak Trench in the Yukon Territory is consistent with the evolution of porphyry mineral deposits above an active subduction zone. Comparison of K-Ar ages obtained for this study with published K-Ar ages suggests that metallogenic epochs for porphyry mineral deposits in the Canadian Cordillera occurred at approximately 195 m.y. and 150 ± 10 m.y. for deposits of the plutonic and volcanic porphyry classes; and at approximately 100 m.y., 80 nwy., 65 m.y., 50 m.y., 35r40 m.y. and 26 m.y. for deposits of the phallic porphyry class.

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