USGS Professional Paper 1697 - Alaska Resources Library

as interpreted for the Rock Creek deposit (Apodaca, 1992)
and for the Mt. Distin and Bluff and deposits (Ford, 1990).
However, recent Ar-Ar isotopic studies indicate the vein mica
from quartz veins at the Bluff deposit formed at about 109
Ma, about 30 m.y. after metamorphic mica in the area reached
closure temperature (Ford and Snee, 1996).
Rock Creek Au Quartz Vein Deposit
The Rock Creek Au quartz vein deposit (Ted Eggelston
and R.V. Bailey, written commun., 1990-1991; Apodaca,
1992) consists of arsenopyrite, scheelite, galena, stibnite, and
pyrite that occur in a northeast-trending, sheeted, quartz vein
system. At the surface, the deposit extends for 1,200 meters
along strike, averages 70 meters width, and extends as much
as 150 meters in depth. The host rocks are phyllite and schist
of the Paleozoic Nome Group. Fluid inclusion studies indicate
ore deposition occurred in the mesothermal range (240
to 320°C). The ore minerals occur along selvages of quartz
vein-host rock contacts. Vein mica yields an Ar-Ar isotopic
age of 109 to 104 Ma (T.K. Bundtzen and P.W. Layer, written
commun., 1995). The deposit is interpreted as forming by
hydrofracturing and dewatering during the waning stage of a
mid-Cretaceous metamorphic event. The deposit contains an
estimated 10.2 million tonnes grading 2.4 g/t Au and about
0.43 percent W (Swainbank and Szumigala, 2000). At Mount
Distin, several similar enechelon Au quartz veins occur along
an east-west-trending thrust fault for at least 3 km.
Big Hurrah Au Quartz Vein Deposit
The Big Hurrah deposit (Collier and others, 1908; Cathcart,
1922; Asher, 1969; Mullen, 1984; Gamble and others, 1985;
Read, 1985; Read and Meinert, 1986) consists of four major
quartz veins and zones of ribbon quartz. The major veins and
ribbon zones range from 1 to 5 m thick and are a few hundred
meters long. The veins contain sparse gold, pyrite, and arsenopyrite
and minor scheelite, chalcopyrite, and sphalerite in a gangue
of quartz, carbonate, and feldspar. These veins are intermixed
with older, concordant, non-Au-bearing, metamorphic quartz
veins. The Au-bearing veins range from discordant tension veins
to discontinuous quartz lodes that occur in shear zones crossing
foliation. The Au-bearing veins range from 0.5 to 5 m wide
and extend to a depth of at least 90 m. Most of the veins are less
than 1 m wide. The veins and ribbon quartz zones are hosted
in quartz-rich, graphitic, quartz-mica schist or quartzite of the
Paleozoic Nome Group. The veins are interpreted as forming
during shearing and uplift associated with metamorphic dehydration
in the mid-Cretaceous. Mining occurred at the deposit from
1903 to 1909, and from 1953 to 1954. The deposit has produced
about 839 kg Au, averaging about 34.3 g/t Au (Reed and Meinhart,
1986). Recent assays range from 25 to 65 g/t Au.
Origin of and Tectonic Controls for Nome Metallogenic Belt
The Big Hurrah and Bluff deposits in the Solomon
district and the deposits in the Nome district exhibit several
Late Early Cretaceous Metallogenic Belts (120 to 100 Ma; figs. 61, 62) 167
similarities, including low-sulfide mineral concentration, fault
localization, and confinement to low-grade, greenschist facies
metamorphic rocks. In all the deposits, a post-metamorphic
fluid origin is suggested for the deposits by (Gamble and
others, 1985) (1) the discordance of the veins to metamorphic
foliations, (2) oxygen isotope and fluid inclusion data,
and (3) the lack of coeval intrusions in the belt. These relations
indicate that the Au deposits formed from fluids that
equilibrated with the sedimentary and (or) volcanic protoliths
of the Nome Group under greenschist facies regional metamorphism.
Subsequently, the fluids moved upward during a
later, post-kinematic event to deposit the vein minerals. As
in the southern Brooks Range, regional metamorphism of the
Seward metamorphosed continental-margin terrane occurred
during two stages. A relatively older blueschist facies event in
the Jurassic was followed by (1) thrusting of oceanic units of
the Angayucham terrane onto the passive continental-margin
arctic Alaska superterrane, and (2) subsequent Early to mid-
Cretaceous extensional, retrograde greenschist metamorphism
(Armstong and others, 1986; Moore and others, 1994; Miller
and Hudson, 1991; Till and Dumoulin, 1994).
The Nome metallogenic belt is interpreted as forming in
the waning stages of extension, which is defined by Early to
mid-Cretaceous greenschist facies metamorphism and companion
penetrative deformation (Miller and Hudson, 1991; Moore
and others, 1992; Nokleberg and others, 1994c, 1997c; Goldfarb,
1997). In a few areas, the Nome Group exhibits an older,
relict blueschist facies metamorphism that is interpreted as
forming in a Jurassic or older period of convergent deformation
and metamorphism (Armstrong and others, 1986; Moore and
others, 1994; Till and Dumoulin, 1994). The convergent deformation
and blueschist facies metamorphism is interpreted as
forming during Late Jurassic and Early Cretaceous subduction
of the Seward terrane under the oceanic Angayucham terrane,
resulting in formation of the Koyukuk island-arc terrane to the
south (present-day coordinates) (Moore and others, 1994; Patton
and others, 1994; Plafker and Berg, 1994; Nokleberg and
others, 2000). Incremental Ar isotopic studies suggest quartz
vein formation at about 109 to 104 Ma (Ford and Snee, 1996)
and that quartz vein formation was temporally related to a
thermal pulse. This thermal pulse is also interpreted as causing
high-grade metamorphism and anatectic plutons to the north
in the Kigluaik Mountains, and to the east in the Bendeleben
and Darby Mountains (Ford and Snee, 1996). The Au quartz
vein deposits of the Nome metallogenic belt are interpreted as
coeval with the Au quartz vein deposits in the Southern Brooks
Range metallogenic belt to the northeast.
Southern Brooks Range Metallogenic Belt of Au
Quartz Vein Deposits (Belt SBR), Northern Alaska
The Southern Brooks Range metallogenic belt of Aubearing
quartz vein deposits (fig. 62; tables 3, 4), which
includes the Chandalar district, lies in the southern Brooks
Range. The metallogenic belt is hosted in both the Hammond