Orogenesis, high-T thermal events, and gold vein formation within metamorphic rocks of the Alaskan Cordillera

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 6~SO values of 7%o-12%o and 6D values between -15%o and -35%o. Upper-crustal temperatures within the metamorphosed terranes reached at least 450-500 ~ 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.

Antimony quartz and antimony–gold quartz veins from northern Portugal

“…15 (Goldfarb et al, 1993). The localization of deposits adjacent to shear zones has been considered as evidence for a metamorphic source (e.g., Kyser and Kerrich, 1990;Murphy and Roberts, 1997).…”

Deformational controls on the dynamics of fluid flow in mesothermal gold systems

“…The most significant Palaeozoic deposits are hosted by turbidite sequences and occur in central Victoria in eastern Australia (Cox et al 1991, the Meguma Terrane of eastern Canada (Kontak et al 1990) and in the Welsh slate belt (Bottrell et al 1988). Mesozoic, and younger, lode gold systems occur particularly around the Pacific rim, including parts of Alaska (Goldfarb et al 1993), California (Knopf 1929;Bohlke & Kistler 1986) and New Zealand (McKeag & Craw 1989).…”

Fluid inclusion and stable isotope constraints on the genesis of the Jian copper deposit, Sanandaj–Sirjan metamorphic zone, Iran

“…10). The shift toward lower d 18 O fluid and dD values in type B veins may have been attained by increasing infiltration of cold circulating meteoric waters (Taylor 1974;Sheppard 1986;Dubinina et al 2011) at shallow levels during regional uplift (Goldfarb et al 1993). Anderson et al (2004) noted that meteoric water incursion into metamorphic complexes is common during the formation of many shallow-level metamorphic vein systems.…”