Lead-isotope and potassium-argon studies in the East Kootenay District of British Columbia

The structural history of part of the Purcell Mountains of British Columbia is interpreted in the light of the discovery of the Precambrian age of the Hellroaring Creek stock. This stock, the first Precambrian granitic intrusion recognized in situ in the Canadian Cordillera, cuts Moyie (Purcell) Intrusions and verifies their Precambrian age. The Sullivan ore body, one of the world's largest deposits of Pb and Zn, is younger than a low-grade regional metamorphism of its host metasedimentary rocks but, on the basis of K-Ar dating of a lamprophyre, is probably no younger than 765 million years. The Pb-isotope content of the Sullivan deposit is homogeneous and indicates a single mineralization. Deposits of the E. Kootenay district contain 2 main types of Pb; a relatively nonradiogenic variety and a markedly radiogenic type. Their distribution is unrelated to geography and to major structural trends, but there is an apparent relation between isotopic composition and specific combinations of structure and host rock. Deposits that are structurally conformable with strata of the Aldridge Formation probably belong to the relatively nonradiogenic Pb group, although there is no relation between isotopic compositions and occurrences in the Aldridge Formation in general. Deposits believed to be genetically related to the Moyie Intrusions in which they occur are probably characterized also by Pb of the relatively nonradiogenic group, a correlation that is in harmony with their proposed origin. Another correlation is between isotopic composition and size: small deposits, with the significant exception of those probably related to Moyie Intrusions, are likely to be of the markedly radiogenic type, whereas the commercial Pb production of the district has come from deposits of the relatively nonradiogenic type. If any single Pb was common to all the deposits of the district its presence in many of them, especially the smaller ones, has been masked by additions of more radiogenic Pb.