The MacTung deposit is the largest of the economically important contact metasomatic deposits of the northeastern Canadian Cordillera which are characterized by the ore element assemblage W-Cu(Zn). The deposits occur within the thermal aureoles of Late Cretaceous felsic intrusions which were emplaced into a dominantly pelitic sequence along the eastern margin of the Selwyn basin. The mineralization occurs in skarn bodies which are comprised dominantly of pyroxene, amphibole, garnet, pyrrhotite, biotite, clinozoisite, plagioclase, and quartz, with economically important amounts of scheelite and chalcopyrite and minor sphalerite and ferberite. The skarn bodies tend to occur in relatively fiat-lying limestone or marble beds. Although the skarns are heterogeneous, there are clear patterns in the preferred associations and nonassociations of minerals on all scales, and an orderly sequence of alteration facies of skarn can be distinguished. The distribution, and compositional and mineralogical, relationships of alteration facies suggest that they formed as a result of the progressive reaction of an infiowing hydrothermal solution with a generally uniform limestone protolith, and that the dominant effect of this interaction was the progressive removal of Ca and addition of other constituents to the altered rocks. Scheelite was deposited in the zonal sequence where the product of the activity of the WO (super -2) 4 being added, and of the Ca (super +2) being extracted, exceeded the solubility product of scheelite. Concurrently with the alteration of limestone, interbedded pelites were progressively altered to Ca-rich assemblages and leached of sulfides and graphite in a broad zone surrounding the ore-bearing skarns. This raises the possibility that much of the material added to the limestones was derived from the pelites and, similarly, much of what was added to the pelites was derived from the limestones in the ore-forming process, although there is compelling evidence that the ore-forming fluids at least migrated through, and probably were partly derived from, the associated felsic intrusions.