Silver-lead-zinc veins and crustal hydrology during Eocene extension, southeastern British Columbia, Canada

Abstract

The Kokanee Range, southeastern British Columbia, Canada, contains more than 370 Ag-Pb-Zn vein and replacement deposits hosted by the Middle Jurassic Nelson batholith and surrounding Cambrian to Triassic metasedimentary rocks. All deposits are in the hanging wall of the Slocan Lake Fault, an east-dipping low-angle normal fault that reaches the Moho, and along which the Valhalla metamorphic core complex has been unroofed during the Eocene. Fluid inclusions in sphalerite, quartz, or siderite indicate a temperature and pressure of trapping of approximately 300°C and 0.17 GPa. The fluid inclusions display a large range of salinities from 0 to 18 wt% NaCl eq. Assuming lithostatic pressure at the time of mineralization, the depth of mineralization was 6 ± 3 km. δ 34 S sulfide values display a 20‰ range and a regional zonation that correlates with the distribution of sulfur isotopic compositions of host rocks. In contrast, gd 13 C siderite values are homogeneous ( −7.1 ± 0.5‰) over an area of 1200 km 2. It is concluded that S was derived from local country rocks whereas C was derived from mantle CO 2 degassing. The δ 18 O siderite and δ 18 O rock values display regional zonations revealing fluid flow paths of a large, ancient hydrothermal system. Three different fluids are identified from salinity, δ 18 O, and δD values: 1. (1) a high-salinity, deep-seated and isotopically crust-equilibrated fluid; 2. (2) a low-salinity, upper crustal fluid of evolved meteoric origin; 3. (3) meteoric water. Mixing between thermally similar fluids having different salinities is proposed to have been the dominant mineralizing process. Regional isotopic zonations are controlled by the Slocan Lake Fault which channelled deep-seated hydrothermal fluids and mantle CO 2 to higher crustal levels where mixing occurred with upper crustal fluids that leached local sulfur. Meteoric water invaded the fracture network in the waning stages of the hydrothermal system.