Metallogenesis and Tectonics of the Russian Far East, Alaska, and ...

production from the Kuskokwim Mineral Belt, and was the largest mercury mine in Alaska. At Red Devil, high-angle structures
cut flysch of the Cretaceous Kuskokwim Group and altered mafic dikes. The fluids tht deposited cinnabar and stibnite utilized
faulted mafic dikes as structural conduits; however no clear link exists between dikes and the deposit. Bundtzen and Miller (1 997),
Gray and others (1997), and Miller and others (1989) all report which fluid inclusion data from type 1 epithermal deposits in the
Kuskokwim Mineral Belt which indicate shallow depths of ore deposition at less than 1,500 bars pressure, and less than 200 C.
Gray and others (1997) report radiometric ages ranging from 72.5 to 76 Ma for several deposits. Type 2 Au-Sb-Hg vein deposits
include the Dishna River Au-Sb prospect and Glenn and Minnie Gulch prospects, which cut high level plutonic rocks in the Flat
area (Bundtzen and others, 1992; Bundtzen and Miller, 1997).
During 1998-1999, an important new epithermal Au-Ag deposit (type 3 above) was discovered at the Kaiyah prospect
about 125 km southwest of McGrath (Bundtzen and others, 2000). The deposit occurs about 2 km from a curvilinear, faulted
contact between the Early Cretaceous flysch of the Yukon-Koyukuk basin and andesite tuff, welded tuff, and rhyodacite domes of
the Late Cretaceous and early Tertiary Poison Creek caldera. The volcanic rocks have isotopic ages of 42.5 and 65.2 Ma, and the
caldera covers 75 km2 (Bundtzen and Miller, 1997). The deposit occurs in a 4 by 2.5 krn altered area which contains abundant
silica alteration, and extensive alunite, jarosite, and sericite alterations. lndividual Au-Ag veins average 30 m thick and extend
about 450 m along strike. Anomalous elements are Sb, Hg, As, Te, Pb, Cu, and Bi. Geological and geochemical data suggest that
the Kaiyah deposit is a high sulfidation epithermal Au-Ag deposit with a Ag:Au ratio of about 40: 1. Grades range up to 10.8 g/t
Au gold and 465 g/t Ag (Bundtzen and others, 2000).
Other Significant Deposits in the Kuskokwim Mineral Belt
The high grade Au-Bi-Cu skams at Nixon Fork (fig. 1 12), 45 krn, northeast of McGrath, have been developed and mined
since the 1920s. The ore deposits occur in irregular, and rich, pipe-like, garnet-epidote-magnetite-gold-copper-skarn which rim a
monzonite stock that has a K-Ar isotopic age of 68 Ma. The intrusion cuts Ordovician limestone of the Novi Mountain Formation
(Patton and others, 1994). From 1920 to 1999 Nixon Fork Mining Company and previous operators produced 6,2 10 kg Au, and
byproduct Bi and Cu from about 128,300 tonnes of ore (Bundtzen and others, 2000).
East-Central Alaska Metallogenic Belt of
Granitic Magmatism Deposits (Younger,
Late Cretaceous and Early Tertiary Part;
Belt ECA) East-Central Alaska and
Northern Canadian Cordillera
This major, long, extensive, and complex metallogenic belt of Late Cretaceous and early Tertiary granitic-magrnatism-
related deposits (belt ECA) (fig. 103; tables 3,4) occurs in east-central Alaska and the western Yukon Territory (Nokleberg and
others, 1995a). The significant deposits are mainly various W skarn, porphyry Cu-Mo deposits, polymetallic vein, Sn greisen and
vein, and Sb-Au vein deposits in the northern and eastern Yukon-Tanana terrane (table 4). Described below are the major Casino
and Taurus porphyry Cu-Mo deposits, and the Road Metal tourmaline-topaz-quartz sulfide greisen deposit.
The East-Central Alaska part of the belt contains a variety granitic magmatism deposits which are hosted in Late
Cretaceous and sparse early Tertiary granitoids which intrude the northern and eastern Yukon-Tanana terrane (Table 4). Major
deposits are: (1) a W skam deposit at Salcha River (possibly mid-Cretaceous); (2) porphyry Cu-Mo deposits at Mosquito, Asarco,
Taurus, and Casino; (3) a Sn greisen deposit at Ketchem Dome; (4) Sn greisen and Sn vein deposits at Ketchen Dome and Lime
Peak; and (5) a felsic-plutonic U deposit at Roy Creek (former Mount Prindle). For deposits in groups (1) to (4), the host rock of
the Yukon-Tanna terrane consists of metamorphosed and penetratively deformed, middle Paleozoic and older, quartz-
metasedimentary, sparse metavolcanic, and rare middle Paleozoic metagranitoid rocks (Foster and others, 1987). For the deposits
in group (5), the granitoid rocks hosting the felsic-plutonic U deposits intrude a sequence of weakly deformed, quartz-rich
sandstone, grit, shale, and slate, containing probable Early Cambrian fossils, which form part of the Wickersham passive
continental margin terrane (Jones and others, 1987). Yukon Territory part of the belt contains porphyry Cu-Mo deposits in the
Cannacks area.
Casino Porphyry Cu-Mo-Au Deposit
The large Casino (Patton Hill) porphyry Cu-Mo-Au deposit in the Carmacks area, Yukon Territory, is hosted in a breccia
pipe and associated porphyritic dacite and granite of the Casino Intrusive Complex which has a K-Ar isotopic age 7 1.3 Ma, which
intrudes the Early Jurassic Klotassin batholith. The deposit occurs in the center of the complex and consists of a central, conical
breccia pipe, fine-grained quartz monzonite, intrusion breccia, and plagioclase porphyry intrusions (EMR Canada, 1989; Mining
Review, 1992; Sinclair, 1986; Northern Miner, December 6,1993; Bower and others, 1995). An innermost zone of potassic
alteration is associated with hypogene sulfides, and contains superimposed phyllic and pyrite alteration, and weakly-developed,
peripheral propylitic alteration. Supergene minerals are chalcocite, digenite, and covellite which occur as replacements of
hypogene sulfides. A 70-m-thick, Au-bearing leached cap overlies a well developed supergene enrichment blanket. Estimated
reserves are: (1) for a combined Au-oxide zone in the leached cap and an copper-oxide supergene zone, 28 million tonnes grading
0.68 g/t Au, 0.11% Cu, and 0.024% Mo; (2) for a sulfide supergene zone, 86 million tonnes grading 0.41 g/t Au, 0.43% Cu, and