Special Volume, Vol. SV 46, No. 1995, 1995
The northern Cordillera of Canada and adjacent parts of Alaska host a variety of molybdenum, tungsten and tin deposits and occurrences, most of which are associated with granitoid intrusions. Skarn, porphyry and vein/breccia are the principal deposit types. Skarn tungsten deposits have been the most important economically and include one of the world's largest tungsten deposits (MacTung) as well as the largest single producer of tungsten concentrates in Canada (Cantung). Skarn tin deposits (e.g. JC) are small but may have some potential. Molybdenum and/or tungsten are also present in anumber of skarn copper deposits (e.g. Cowley Park). Porphyry molybdenum (e.g. Red Mountain, Adanac, Storie), tungsten-molybdenum (e.g. Logtung) and copper-molybdenum (e.g. Taurus, Casino) deposits are large, although none are currently economic. Veinlbreccia deposits of tin (e.g. EPD) and tungsten (e.g. Fiddler) are relatively small and, although high grade in places, appear to have limited economic potential.
The ages of molybenum, tungsten and tin deposits and occurrences, based for the most part on ages of associated intrusions, range from Devonian to Tertiary. Several periods of deposit formation
are evident, the peak periods being mid-Cretaceous (at 110 Ma and 90 Ma), Late Cretaceous (70 Ma) and early Tertiary (60 Ma) with minor periods during the Devonian, Mississippian, Early Jurassic and mid-Tertiary. Molybdenum deposits have the widest range of ages, having formed during all the main periods and most of the minor periods. Tungsten deposits on the other hand, are associated mainly with mid-Cretaceous and early Tertiary plutons and, to a minor extent, with Devonian plutons. Tin deposits are related mainly to intrusions of mid-Cretaceous age except for some occurrences associated with Late Cretaceous, early Tertiary and Mississippian intrusions.
The plutons associated with molybdenum, tungsten and tin deposits (excluding skarn copper and porphyry copper-molybdenum deposits) are composed predominantly of granite, as defined by Streckeisen (1976). The Red Mountain porphyry molybdenum deposit, which is related to granodiorite, is a notable exception. The intrusions can be distinguished, to some extent, on the basis of their major and minor element chemistry. For example, compared to other granites, the tin-related Seagull batholith is relatively enriched in Rb and depleted in Ba and Sr. At the MacTung and Cantung skarn tungsten deposits, the Logtung porphyry tungstenmolybdenum deposit and the Adanac and Storie porphyry molybdenum deposits, associated plutons consist of several phases that show significant variation in Rb, Ba and Sr contents.