Orogenesis, High-T Thermal Events, and Gold Vein Formation within Metamorphic Rocks of the Alaskan Cordillera

R. J. Goldfard, L. W. Snee and W. J. Pickthorn
U.S. Geological Survey, M.S. 973, Box 25046, Denver Federal Center, Denver, CO 80225, U.S.A.
U.S. Geological Survey, 345 Middlefield Rd., Menlo Park, CA 94025, U.S.A.

Abstract: Mesothermal, gold-bearing quartz veins are widespread within allochthonous terranes of Alaska that are composed dominantly of greenschist-facies metasedimentary rocks. The most productive lode deposits are concentrated in south-central and southeastern Alaska; small and generally nonproductive gold-bearing veins occur upstream from major placer deposits in interior and northern Alaska. Oreforming fluids in all areas are consistent with derivation from metamorphic devolatilisation reactions, and a close temporal relationship exists between high-T tectonic deformation, igneous activity, and gold mineralization. Ore fluids were of consistently low salinity, CO2-rich, and had δ18O values of 7‰–12‰ and δD values between −15‰ and −35‰. Upper-crustal temperatures within the metamorphosed terranes reached at least 450–500°C before onset of significant gold-forming hydrothermal activity. Within interior and northern Alaska, latest Paleozoic through Early Cretaceous contractional deformation was characterised by obduction of oceanic crust, low-T/high-P metamorphism, and a lack of gold vein formation. Mid-Cretaceous veining occurred some 50–100 m.y. later, during a subsequent high-T metamorphic/magmatic event, possibly related to extension and uplift. In southern Alaska, gold deposits formed during latter stages of Tertiary, subduction-related, collisional orogenesis and were often temporally coeval with calc-alkaline magmatism.

Keywords: mesothermal gold • Alaska • fluid inclusion • stable isotope • geochronology • orogensis

Mineralogical Magazine; September 1993 v. 57; no. 388; p. 375-394; DOI: 10.1180/minmag.1993.057.388.03
© 1993, The Mineralogical Society
Mineralogical Society (www.minersoc.org)