Metamorphic Origin of Ore-Forming Fluids for Orogenic Gold-Bearing Quartz Vein Systems in the North American Cordillera: Constraints from a Reconnaissance Study of delta~(15)N, delta D, and delta~(18)O

摘要

The western North American Cordillera hosts a large number of gold-bearing quartz vein systems from the Mother Lode of southern California, through counterparts in British Columbia and southeastern Alaska, to the Klondike district in central Yukon. These vein systems are structurally controlled by major fault zones, which are often reactivated terrane-bounding sutures that formed in orogens guilt during accretion and subduction of terranes along the continental margin of North America. Mineralization ages span mid-Jurassic to early Tertiary and encompass much of the evolution of the Cordilleran orogen. Nitrogen contents and delta~(15)N values of hydrothermal micas from veins are between 130 and 3,500 ppm and 1.7 to 5.5 per mil, respectively These values are consistent with fluids derived from metamorphic dehydration reactions within the Phanerozoic accretion-subduction complexes, which have delta~(15)N values of 1 to 6 per mil. The delta~(18)O values of gold-bearing vein quartz from different locations in the Cordillera are between 14.6 and 22.2 per mil but are uniform for individual vein systems. The delta D values of hydrothermal micas are between -110 and -60 per mil. Ore fluids have calculated delta~(18)O values of 8 to 16 per mil and delta D values of -65 to -10 per mil at an estimated temperature of 300 deg C; delta D values of ore fluids do not show any latitudinal control. These results indicate a deep crustal source for the ore-forming fluids, most likely of metamorphic origin. Low delta D_(H_2O) values of-120 to -130 per mil for a hydrous muscovite from the Sheha vein in the Klondike district reflect secondary exchange between recrystallizing mica and meteoric waters. Collectively, the N, H, and O isotope compositions of ore-related hydrothermal minerals indicate that the formation of these gold-hearing veins involved dilute, aqueous carbonic, and nitrogen-bearing fluids that were generated from metamorphic dehydration reactions at deep crustal levels. These data are not consistent with either mantle-derived fluids or granitoid-related magmatic fluids, nor do they support a model involving deeply circulated meteoric water.