Abstract

Isotope analyses of galena from syngenetic barite-zinc-lead and epigenetic Zn-Pb- and Ag-rich deposits from the northern Canadian Cordillera are subdivided into four distinct fields and lines on 207 Pb/ 204 Pb and 208 Pb/ 204 Pb vs. 206 Pb/ 204 Pb plots. These lines, used in conjunction with constraints imposed by stratigraphy, by K-Ar isotopic dates, and by minor element analyses in sphalerite, define three metallogenic events dating about 0.52, 0.37, and 0.09 b.y., which are summarized as follows:1. "Old carbonate"-hosted Zn-Pb deposits formed in the Lower Cambrian (about 0.52 b.y. ago) define closely a maximum source age of 1.9 b.y. This is the first lead isotopic indication of Hudsonian basement in the northern Canadian Cordillera. Spatial association of these deposits to unconformities and generation of lead in an upper crustal but generally uranium-poor environment supports a karstic model for their origin.2. Shale-hosted Ba-Zn-Pb mineralization from Upper Devonian to Lower Mississippian (about 0.37 b.y.-old) rocks is indistinguishable in lead isotope composition from the majority of post-Lower Ordovician to Devonian young carbonate-hosted zinc-lead deposits. These deposits formed near 0.37 b.y. ago from brines dewatered from the Selwyn shale basin, where solutions were guided by faults, either within the basin (e.g., shale-hosted Ba-Zn-Pb deposits formed along grabens near hinge lines close to the margin of the basin), or peripheral to and away from the basin (e.g., carbonate-hosted Zn-Pb deposits). Lead for these deposits evolved in an upper crustal, uranium-rich, carbonaceous, shale basin environment from which lead relatively rich in radiogenic component was extracted.8. Although most Ag-rich deposits occur as veins not directly associated with plutons, some veins occur within, or are directly associated with, Cretaceous plutons, indicating a general age of mineralization of 0.09 b.y. An apparent Hudsonian source is indicated. Some highly radiogenic leads, characteristic of the silver deposits, were probably formed by preferential leaching of lead from uranium-rich minerals.

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