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

Yiefei Jia

Department of Geological Sciences, University of Saskatchewan, 114 Science Place, Saskatoon, Saskatchewan, Canada S7N 5E2, and Research School of Earth Sciences, Australian National University, Canberra ACT 0200, Australia

Robert Kerrich

Department of Geological Sciences, University of Saskatchewan, 114 Science Place, Saskatoon, Saskatchewan, Canada S7N 5E2

Richard Goldfarb

U.S. Geological Survey, Mineral Resources Program, Denver Federal Central, Box 25046, Mail Stop 964, Denver, Colorado 80225

Corresponding author: email,yiefei.jia{at}anu.edu.au

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 built 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 ¹⁵N 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 ¹⁵N values of 1 to 6 per mil. The ¹⁸O 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 D values of hydrothermal micas are between –110 and –60 per mil. Ore fluids have calculated ¹⁸O values of 8 to 16 per mil and D values of –65 to –10 per mil at an estimated temperature of 300°C; 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 D_H2O values of –120 to –130 per mil for a hydrous muscovite from the Sheba 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-bearing 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.

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