USGS Professional Paper 1697 - Alaska Resources Library

234 Metallogenesis and Tectonics of the Russian Far East, Alaska, and the Canadian Cordillera
zen and Miller, 1997). Recent industry exploration, including
soil surveys and geophysical and drilling programs (DiMarchi,
1993) indicate that the Vinasale Mountain is one of the most
important lode gold deposits in the Kuskokwim Mineral Belt.
Marshall District of Granitoid-Related Au Deposits,
West-Central Alaska
A suite of granitoid-related Au deposits of the Marshall
district occur at the Arnold prospect near the head of Willow
Creek in southwestern Alaska (Bundtzen and Miller, 1997).
The deposits consist of quartz-carbonate veins that occur in
or near sheared Late Cretaceous or early Tertiary alaskite sills
that intrude the Koyukuk island-arc terrane (Nokleberg and
others, 1994c, 1997c). The significant deposits and prospects
include the Arnold, Kako, and Stuyahok gold prospects.
The Arnold prospect consists of an approximate eastwest-trending
suite of low sulfide polymetallic veins that contain
gold in quartz and carbonate gangue. The sulfide minerals,
which are less than 1 percent of the deposit, are disseminated
chalcopyrite, molybdenite, galena, and tetrahedrite. The vein
system occurs in or near sheared alaskite sills. A distinctive
albite rhyolite dike or sill parallels the main vein system. The
sills intrude Neocomian (early Early Cretaceous) greenstone
derived from tholeiite metabasalt and metaandesite. The
deposit extends along strike for least 400 meters and ranges
from 0.5 to 2 m thick. Abundant carbonate alteration occurs
adjacent to the main polymetallic veins. Mo averages about 80
ppm in the veins, Mo anomalies occur in soils. Grab samples
of rocks contain as much as 97 g/t and 100 g/t Ag.
Kuskokwim River
41
61
55
80
Alder Creek
60
0 2 km
Other significant gold-polymetallic mineralization in the
Marshall district includes important lode and associated placer
gold occurrences at Kako Creek and Stuyahok, about 55 km
and 75 km east of the Arnold prospect, respectively (Miller and
others, 1998). Both of the latter prospects resemble the Donlin
Creek deposit described above and are spatially related to small,
Late Cretaceous sills and dikes that intrude the Koyukuk terrane.
Au-Ag-Hb-Sb Epithermal Deposits, Kuskokwim Mineral Belt
Epithermal systems of the Kuskokwim Mineral Belt are
subdivided into three major types (Bundtzen and Miller, 1997;
Gray and others, 1997—(1) structurally controlled (Au)-Hg-
Sb deposits related to altered olivine basalt dikes, (2) low
temperature Ay-Sb-Hg minerals in shear zones in high level
portions of intrusions, and (3) chalcedony breccias hosted in
subaerial volcanic piles, including stockwork veins adjacent
to volcanic calderas. Although generally poorly classified, the
three epithermal deposit types probably correspond to deposit
models the hot springs Au-Ag, Creede epithermal, and Sado
epithermal deposit models of Cox and Singer (1986).
The structurally controlled Au-Sb-Hg deposits (type 1)
are part of a well-studied Hg belt (Sainsbury and MacKevett,
1965) that extends nearly 500 km from the Red Top mine near
Bristol Bay to Mount Joaquin near McGrath. The principle
type 1 deposit is the Red Devil Hg-Sb mine near Sleetmute,
which produced more than 85 percent of the 1.5 million kg
of past Hg 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 Kuskok-
61
River gravel (Holocene)
Undifferentiated alluvium
(Quaternary)
Eolian silt/sand (Quaternary)
Approximate location of Central
Mineral Zone.
Granite porphyry (Early Tertiary
and Late Cretaceous)
Biotite monzonite and syenite
(Early Tertiary and Late Cretaceous)
Porphyritic quartz monzonite
and granite (Early Tertiary and
Late Cretaceous)
Kuskokwim Group clastic rocks
(Late Cretaceous)
Placer gold-scheelite
High-angle fault
Contact
Strike and dip of bedding
Figure 111. Vinasale Mountain granitoid-related (porphyry) Au deposit, Kuskokwim Mountains metallogenic
belt, southwestern Alaska. Schematic geologic map showing location of Central Mineral Zone. Adapted from
Bundtzen (1986), DiMarchi (1993), and Bundtzen and Miller (1997). See figure 103 and table 4 for location.