Geology, mineral equilibria, and isotopic studies of the McDame tungsten skarn prospect, north-central British Columbia

The McDame prospect occurs in Hadrynian to Ordovician metasediments near Cretaceous felsic stocks. Three prograde skarn and ore facies and one retrograde facies are lithologically and structurally controlled. Lithostatic pressure during formation of McDame skarns was on the order of 1,500 bars. Temperature (T) and CO 2 mole fraction (X) conditions of the metasomatic fluid are estimated for the different skarn facies by assuming an idealized iron-free system. Sulfur isotope data from McDame porphyry, skarn, and hornfels form six discrete groups (skarn average delta 34 S = 7.7) between ordinary magmatic sulfur (delta 34 S = 0) and Cambrian sedimentary sulfate (delta 34 S = 30). Rubidium-strontium isotopes from Kuhn Stock quartz feldspar porphyry define a 69 + or - 2 m.y. isochron with a high initial ratio of 0.712, indicating that the Kuhn Stock had a sialic crustal component. Lead isotope data from vein and skarn deposits near Cassiar cluster around the upper crustal shale curve for the Canadian Cordillera, indicating an upper crustal source for the lead. Thus, anatexis and assimilation of continental crust produced granitic melts rich in lithophile elements. The Kuhn Stock is an oxidized granitoid of sedimentary origin, geochemically specialized in K 2 O, K/Rb, U, and U/Th and anomalous in W and Mo. Differentiation processes have concentrated these lithophile elements in magmatic fields that produced the McDame tungsten skarn deposit.--Modified journal abstract.