Stable isotopic and fluid inclusion indications of large-scale hydrothermal paleoflow, boiling, and fluid mixing in the Keno Hill Ag-Pb-Zn district, Yukon Territory, Canada

Abstract

The Keno Hill vein system of the central Yukon is restricted predominantly to the highly fractured, graphitic Keno Hill Quartzite unit of Mississippian age. Hydrothermal mineral zoning is related spatially to a Cretaceous granitic pluton which intrudes the quartzite. During mineralization, the quartzite acted as a district-scale aquifer. Subsequent erosion has exposed a 40-km long vein system, from its plutonic roots, outward to polymetallic Ag-Pb-Zn veins, and further to assemblages of epithermal character. The δ 18O quartz values from veins near the pluton increase outwards from +10.6 to +20.1%. as a result of cooling of the hydrothermal fluids and exchange with the quartzite. Contours of isotope values outline broad paths of fluid movement within the quartzite. Proceeding further from the pluton, δ 18O quartz values decrease to +10.5%. at the outer edge of the system. The presence of meteoric water is indicated here, where late stage quartz has δ 18O SMOW values as low as −7.l%. The outward decreasing trend appears to have been established by mixing of isotopically light meteoric water with exchanged fluids that were in isotopic equilibrium with the quartzite. Fluid inclusions from quartz in the orebodies demonstrate an evolving H 2O-CO 2-NaCl-CH 4 system. Loss of CO 2 and CH 4 during water vaporization coincides with increasing salinity and decreasing temperature resulting from high enthalpy steam loss. Depressurization during active faulting is the principal mechanism. Late stage fluids are represented by dilute aqueous inclusions with lower homogenization temperatures. Quartz from silver-rich veins has been shifted to higher δ 18O values, by up to 4%. relative to adjacent silver-poor veins, the result of a minimum 10–25% adiabatic boiling and fractionation dominated by water vaporization and associated cooling. Graphite initially buffered the hydrothermal fluids to a high CO 2 content, with variable CH 4. Involvement of organic carbon from the host rocks is indicated by the negative δ 13 C PDB values for the carbonates, from −4.0 to −12.9%. Variations in the carbon isotopes result from fluctuating CO 2 CH 4 ratios, reflecting the contrasting volatility of the gas pair. Siderite formed as a late-stage product of the boiling event, and its formation coincides with a decreasing 18O trend in the water created by the equilibration of graphite and water in replacing exsolved CO 2. The formation of CH 4 during this stage had a reducing effect on the fluid, resulting in an increase in δ 13C siderite values in association with the decreasing δ 18O siderite values. A closed-system boiling model, together with calculations of water consumption during post-boiling CO 2 and CH 4 formation, indicates that greater than 50% of the original water in the ore fluid was removed. Relatively saline mineralizing fluids resulted.