USGS Professional Paper 1697 - Alaska Resources Library

wim 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 (1997), Gray and others (1997),
and Miller and others (1989) all report that fluid inclusion data
from type 1 epithermal deposits in the Kuskokwim Mineral
Belt that 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 1998-99, 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 km 2 (Bundtzen
and Miller, 1997). The deposit occurs in a 4 by 2.5 km altered
area that contains abundant silica alteration and extensive
alunite, jarosite, and sericite alterations. Individual 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 are as much as 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 skarns at Nixon Fork (fig.
112), 45 km northeast of McGrath, have been developed and
mined since the 1920’s. The ore deposits occur in irregular and
rich, pipe-like, garnet-epidote-magnetite-gold-copper-skarns
that 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 6,210 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-magmatismrelated
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
Late Cretaceous and Early Tertiary Metallogenic Belts (84 to 52 Ma) (figs. 102, 103) 235
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 that intrude the
northern and eastern Yukon-Tanana terrane (table 4). Major
deposits are (1) a W skarn 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)
Garnet
Crystal
Granite porphyry dike
(Cretaceous)
Quartz monzonite-quartz
monzodiorite (Ordovician)
Skarn deposit with sulfide deposits;
names of individual bodies shown
Sandstone and siltstone
(Paleozoic)
Carbonate rocks, limestone
>dolomite (Ordovician)
Contact
Keen
Shaft
Pupinski
0 1,000 m
Figure 112. Nixon Fork Cu-Au skarn deposit, adjacent
prospects, and surrounding area, Kuskokwim
Mountains metallogenic belt, southwestern Alaska.
Schematic geologic map. Porphyry dikes, alteration,
and main stock all yield identical K-Ar ages of 69
Ma. Modified from Cutler (1994) and Newberry and
others (1997). See figure 103 and table 4 for location.