Abstract

Gold-bearing quartz veins in the Klondike district of Yukon Territory are of earliest Cretaceous age and are hosted by Permian, middle to upper greenschist facies, metasedimentary and meta-igneous lithologies belonging to the Yukon-Tanana terrane. The veins occur in brittle, extensional structures, which may have developed during extensional deformation during uplift of the Klondike region in the Late Jurassic to Early to middle Cretaceous, following thrust imbrication in the Early Jurassic. The quartz veins are massive and unbanded, with localized carbonate flooding and/or pyritization of the wall rocks.Fluid inclusion studies have delineated significant, regional P-T-X trends in primary inclusion fluids from the Au quartz veins. The fluids have low salinities (<6 wt % NaCl equiv), but variable CO 2 concentrations, decreasing from approximately 0.13 to 0.18 mole fraction CO 2 in the southeastern Klondike, through variable concentrations in the central Klondike (X (sub CO 2 ) < 0.01 to [asymp] 1.00), to dominantly aqueous fluids (X (sub CO 2 ) < 0.01) in veins from the northwest portion of the region. Estimated trapping pressures decrease from 2,300 + or - 700 bars to between 300 to 900 bars, from southeast to northwest. Estimated fluid temperatures range from 300 degrees to 350 degrees C in central and southern lodes, to 200 degrees to 250 degrees C in the northwest Klondike. The P-X trends are interpreted as evidence for (1) effervescence of CO 2 from the mineralizing fluid, most likely in response to regional uplift during the life of the hydrothermal system, and (2) that the Au quartz vein system represents a section through a mesothermal vein system with deeper structural levels exposed in the southeastern Klondike. Convection of the hydrothermal fluid may have been driven by an elevated geothermal gradient resulting from the rapid uplift.The calculated isotopic composition of the Au quartz vein fluids (delta 18 O fluid 8.3 + or - 1.2ppmSMOW, delta D fluid [asymp] -150 + or - 18ppmSMOW) indicates that the mineralizing fluid was meteoric in origin but underwent considerable isotopic evolution prior to mineralization. Additionally, the delta D fluid values of extracted inclusion fluids (-166 + or - 11ppm) and from hydrothermal muscovites (-174 + or - 3ppm) from the Sheba vein system are in good agreement and indicate that the deuterium signature reflects that of the primary mesothermal fluid.

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