The geology of the carbonate-hosted Blende Ag–Pb–Zn deposit, Wernecke Mountains, Yukon, Canada

The Ag–Zn–Pb Blende deposit is located in the Wernecke Mountains, Yukon and is hosted by the middle Proterozoic Gillespie Lake Group dolomitic siltstones. The sulphide mineralization is localized within the axial planar cleavage of a kilometre-scale anticline and is dominated by galena- and sphalerite-cemented mosaic, rubble and crackle breccias with minor pyrite, galena, sphalerite and dolomite veins. 206 Pb/ 204 Pb values from galena range from 16.355 to 16.600, 207 Pb/ 204 Pb from 15.430 to 15.461, and 208 Pb/ 204 Pb from 36.016 to 36.283, respectively, and yield model ages between 1,490 and 1,430 Ma. A hydrothermal alteration zone, which is younger than the mineralization, has a poorly constrained U–Pb monazite age of 1,307 ± 180 Ma, which suggests that the Blende deposit is Proterozoic in age. Dolomites associated with the main- and late-stage mineralization have δ 13 C values that range from −1.8 to 0.9 ‰ and δ 18 O values of 15.7 to 21.9 ‰. The total range of δ 34 S values from pyrite, galena and sphalerite is 9.4 to 58.1 ‰, indicating that the sulphur in the deposit was derived from reduction of seawater sulphate in a closed system. Strontium isotopes suggest there were three fluids involved in the Blende mineralizing system: Fluid 1 was derived from seawater and formed carbonate and quartz veins pre-mineralization; it has an 87 Sr/ 86 Sr ratio of 0.70948. Fluid 2 has a high 87 Sr/ 86 Sr ratio of 0.73866, and fluid 3 has a Sr isotopic ratio of 0.71602. Fluids 1 and 3 have similar isotopic compositions but different total Sr ion signals (a function of concentration). This suggests that fluids 1 and 3 may have ultimately been derived from Proterozoic seawater but have undergone different amounts of water–rock interaction. The isotopic and geochemical data suggest the mineralization formed when a H 2 S-rich fluid derived from seawater (fluid 3) mixed with a metal-bearing fluid (fluid 2) in the high permeability zones of the axial planar cleavage. The Blende deposit is an unusual carbonate-hosted massive sulphide deposit which has features of both Irish-type and clastic-dominated Zn–Pb deposits. The deposit formed in response to a change in tectonism from a compressional regime to extension and rifting.