Geology, mineral equilibria, sulfur, rubidium-strontium and lead isotopes and intrusion chemistry of the McDame tungsten skarn prospect, north central British Columbia
Creators: name: Cooke, Bradford James
Publisher: name: University of British Columbia
Year: 2010

description: The McDame tungsten skarn prospect occurs in Hadrynian to Ordovician clastic and carbonate metasediments near Cretaceous felsic stocks. Three prograde skarn and ore facies and one retrograde facies are 1ithologically and structurally controlled. Prograde massive calcsilicate W-MO-Fe facies, characterized by garnet skarn with disseminated scheelite and molybdenite, has replaced graphitic marble in contact with biotite hornfels and is thicker near a northeast striking fault. Banded calcsilicate Fe facies, typified by quartz skarn, has replaced biotite hornfels as envelopes around fractures. Banded oxide W-MO-Fe facies, distinguished by magnetite skarn with laminations of molybdoscheelite, has replaced graphitic dolomite as veins within fractures. Retrograde massive sulfide Fe-Zn-Cu-W facies, identified by pyrrhOtite skarn with disseminated sphalerite, chalcopyrite and scheelite, has replaced the other skarn facies, and locally graphitic marble, in veins and pods. Lithostatic pressure during formation Of McDame skarns was in the order of 1500 bars. Temperature and mole fraction conditions Of the metasOmatic fluid are estimated for the different skarn facies by assuming an idealized iron-free system. Massive garnet skarn was stable to Tmax = 555°C and Xmax = 0.14; garnet-bearing quartz skarn formed below Tmax = 475°C and Xmax = 0.08; feldspar-bearing magnetite skarn formed above Tmin = 430°C and Xmin = 0.06; and massive pyrrhOtite skarn altered previously formed skarns at lower T and XCO₂. Calcsilicate mineral zoning resulted from dissolution, infiltration/diffusion and deposition Of SiO₂, CaO, Al₂O₃, MgO, H₂O and CO₂ in the skarn protoliths. Metallic mineral zoning, on the other hand, formed from W-, Mo-, O₂- and S₂-bearing magmatic fluids that reacted with country rocks to produce prograde facies, and mixed with Fe-, Zn-, and Cu-bearing formational waters to form retrograde skarn. Sulfur isotope data from McDame porphyry, skarn and hornfels form six discrete groups (skarn average δ³⁴S = +7.7) between ordinary magmatic sulfur (δ³⁴S = 0) and Cambrian sedimentary sulfate (δ³⁴S = +30). Their distribution can be explained by fractionation Of magmatic sulfur through reaction of metasomatic fluids with wall rocks to form porphyry pyrite, skarn pyrite and skarn pyrrhotite, and mixing with formational sulfur in connate waters to produce skarn pyrrhotite-sphalerite-chalcopyrite and porphyry pyrrhotite. Rubidium-strontium isotopes from Kuhn stock quartz feldspar porphyry define a 69 ± 2 Ma isochron with a high initial ratio of 0.712, indicating that the Kuhn stock had a sialic crustal component. Isotopic disequilibrium between mineral phases suggests progressive contamination of the granitic melt by continental crust during magma ascent and crystallization. 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 K₂O, K/Rb, U and U/Th and anomalous in W and Mo. Differentiation processes have concentrated these lithophile elements in magmatic fluids that produced the McDame tungsten skarn deposit.