The silver-lead-zinc veins of the Kokanee Range, British Columbia.

Abstract

In the Kokanee Range, southeastern British Columbia, more than 370 Ag-Pb-Zn vein and replacement deposits are hosted by the Middle Jurassic Nelson batholith and surrounding Cambrian to Triassic metasedimentary rocks in the hangingwall of the Slocan Lake Fault, an Eocene, east-dipping, low angle normal fault. K-Ar and step heating $\sp{40}$Ar/$\sp{39}$Ar analyses on hydrothermal vein and alteration muscovite indicate that hydrothermal fluids were forming vein and replacement deposits 58-59 Ma ago. A 100 Ma time interval is therefore documented between batholith emplacement and spatially associated mineralisation, ruling out any genetic link between the two. The Pb isotope compositions of galena permit the deposits to be divided into four groups that result from mixing Pb leached from three reservoirs located in the local, upper crustal country rocks, the lower crust, and the upper mantle. Sulfur was derived from local country rocks, and carbon was derived from mantle CO$\sb2$ degassing. The $\rm \delta\sp{34}S\sb{sulphide},$ $\rm \delta\sp O\sb{quartz},$ $\rm \delta\sp O\sb{siderite},$ and galena Pb isotopic ratios from veins display regional zonations revealing fluid flow paths of a large, fossil hydrothermal system. Regional isotopic zonations are controlled by deep fracture zones, such as the Slocan Lake Fault, which channelled lower crustal and mantle Pb, and mantle CO$\sb2$ to higher crustal levels where mixing occurred with upper crustal fluids which had leached local sulfur and upper crustal Pb. Three contrasting fluids are identified: (i) a high salinity, deep-seated and isotopically crust-equilibrated fluid; (ii) a low salinity upper crustal fluid of evolved meteoric origin; (iii) a late stage meteoric fluid. Silver-lead-zinc veins from a distinct type of vein deposit characterised by their mineralogy, form, and metal content. Silver-lead-zinc veins are late orogenic features of metamorphosed sedimentary basins within Pb-Zn metallogenic provinces and commonly occur near large crustal faults.