RB/SR,S, AND O ISOTOPIC ANALYSES INDICATING SOURCE AND DATE OF CONTACT METASOMATIC COPPER DEPOSITS, CRAIGMONT, BRITISH COLUMBIA, CANADA

Rubidium-strontium dating of chalcopyrite-bearing potassium feldspar veins from the Craigmont ore body gives an initial Srs7/SrS6 ratio of 0.7037 ± 0.0002 and an isochron of 200 ± ;2 m.y., in good agreement with Northcote's 198 ± 8 m.y. K/At date for the presumed parent, the copper-rich Guichon batholith. The low initial SrST/Sr s6 ratios imply that the batholithic rocks, the K feldspar gangue and associated Craigmont mineralization were derived from a mantle source similar to that of recent basalt. Sulfur in the main ore body ranges in 8 values from +0,.5%0 to - 9.8%0 and averages -3.3%. Disseminated sulfides in the Guichon batholith have 8 S a4 values dose to mantle sulfur (- 0.3% to +3.0%) whereas disseminated sulfides in sparsely mineralized wall rocks have 8 S a4 values averaging - 11.3%. Sulfur isotope data suggest that early in mineralization, light country rock sulfur was dominant (sulfurization) and during the late period of potash veining and chalcopyrite mineralization magmatic sulfur (meteoritic values) was emplaced. Carbon and oxygen determinations on host rock skarn and marble agree with this thesis and indicate that while little or no magmatic carbon was added to these rocks during the early period of skarn formation, late calcite associated with mineralizafion has 80 ±s values ranging from - 18.6%o to - 23.5%0 and 8C xs values that average - 4.2%o, typical values for hydrothermal calcites and carbonatites, suggesting a non-limestone source for a large part of the carbonate associated with late mineralization. © 1969 Society of Economic Geologists, Inc.