Isotopic evidence for complex Pb sources in the Ag–Pb–Zn–Au veins of the Kokanee Range, southeastern British Columbia

Abstract

In the Kokanee Range, more than 370 Ag–Pb–Zn–Au vein and replacement deposits are hosted by the Middle Jurassic Nelson batholith and surrounding Cambrian to Triassic metasedimentary rocks. The Kokanee Range forms the hanging wall of the Slocan Lake Fault, an Eocene, east-dipping, low-angle normal fault. The Pb isotopic compositions of galenas permit the deposits to be divided into four groups that form linear arrays in tridimensional Pb isotopic space, each group having a distinct geographic distribution that crosses geological boundaries. The Kokanee group Pb is derived from a mixture of local upper crustal country rocks. Ainsworth group Pb and Sandon group Pb plot along a mixing line between a lower crustal Pb reservoir and the upper crustal Pb reservoir. The Ainsworth group Pb isotopic signature is markedly lower crustal, whereas the Sandon group Pb is slightly lower crustal. The Bluebell group Pb plots along a mixing line between a depleted upper mantle Pb reservoir and the lower crustal Pb reservoir.The geographic distribution and the Pb isotopic composition of each group probably reflect deep structures that permitted mixing of lower crustal, upper crustal, and mantle Pb by hydrothermal fluids. Segments of, or fluids derived from, the lower crust and the upper mantle were leached by, or mixed with, evolved meteoric water convecting in the upper crust. Fracture permeability, hydrothermal fluid flow, and mineralization resulted from Eocene crustal extension in southeastern British Columbia.