Abstract Perhaps a dominant theme that has been expressed in the last few years concerning the petrogenesis and metallogeny of granitoids in western North America concerns the relative proportions and composition of various types of crustal material that have been incorporated in batches of magma (Keith et al., 1985a; Barton, 1987; Farmer and DePaolo, 1983; Ague and Brimhall, 1985, 1986, 1987; Christiansen et al., 1986; Stein and Hannah, 1985; Newberry and Swanson, 1986). Many workers suggest that these processes of crustal melting or contamination by crustal components exert one of the strongest controls on the character or grade of mineralization that ultimately may be produced by that magma. However, broad distinctions between classes of mineralization (i.e. porphyry Cu versus W skarn) may largely be a function of depth of crystallization and correlative water content of the magma (Einaudi et al., 1981; Burt et al., 1982; Barton et al., 1988; Newberry and Swanson, 1986). The controversy over how metal ratios in a deposit are related to granitoid composition is particularly applicable to W, Mo, and Sn deposits because of their close spatial association to granitoids and strong evidence that the dominant portion of the metals and the hydrothermal fluid are derived from the magma. This being the case, the question that must be asked is which magmatic characteristics are set (or modified) during assimilation of crustal material, that have some control on the metal ratios or grade of mineralization, versus those which are "inert" in terms of effecting mineralization. Perhaps the