Abstract

The Blende zinc-lead-silver deposit, 60 km northeast of Keno City, Yukon Territory, is spatially associated with a mid-Proterozoic fault zone that crosscuts stromatolitic dolostones of the Middle Proterozoic, upper Gillespie Lake Group. It is the largest known strata-bound, carbonate-hosted, zinc-lead deposit in the Yukon Territory. Mineralization, largely epigenetic, consists of sphalerite and galena, with lesser pyrite and chalcopyrite, and trace friebergite in quartz-carbonate veins and veinlets. Veining is zoned from copper- and silver-rich mineralization at the base of the deposit, through lead- and zinc-rich, to zinc-dominated at the top. Detailed cross-section mapping indicates that controls on the deposit are both stratigraphic and structural. Many of the stromatolite beds are mineralized and are especially rich in lead and zinc close to normal faults. Fluid inclusion studies indicate that mineralizing fluids were about 285 degrees C during main-stage mineralization. Sulfur isotope analyses of unmineralized whole-rock samples cluster tightly around an average delta 34 S value of 23.6 + or - 0.4 per mil. The source of sulfur is apparently seawater sulfate, probably from minor anhydrite in the host dolostone. Sulfide sulfur isotope ratios define three populations. Local blebs of pyrite associated with stromatolites have markedly negative ratios (- 15.1%) that are probably bacteriogenic in origin. Vein sulfides in tight dololutites have ratios between 19.0 and 26.3 per mil, with an average delta 34 S value of 23.2 PM 0.9 per mil. This is virtually identical to the whole-rock sulfur isotope ratio. Veins that crosscut stromatolitic beds have sulfur isotope ratios between 18.3 and 7.4 per mil with a mean of 15.0 + or - 1.2 per mil. Intermediate sulfur isotope ratios apparently reflect a mixed sulfur source of both whole-rock sulfate and biogenic sulfide. Carbon and oxygen isotopes in host rocks and ore-stage dolomite spar are related genetically; both are related to ancient seawater. The calculated isotopes for mineralizing fluids are characteristic of basinal brines...

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