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

Nome Metallogenic Belt of Au
Quartz Vein Deposits (Belt NO)
Seward Peninsula
The Nome metallogenic belt of Au-bearing quartz vein deposits (fig. 62; tables 3,4) forms a 200 lan long, east-westtrending
belt along the southern par1 of the Seward Peninsula. The metallogenic belt occurs along the sou^ margin of the
Seward metamorphosed continental margin terrane (Nokleberg and others, 1994c, 1997~). The most favorable mineTalized areas
are associated with upper greenschist facies metamorphic rocks. Two major concentrations of deposits occur at (table 4)
(Nokleberg and others 1997a. b, 1998): (I) Bluff and Big Hurrah in the Solomon District; and (2) Rock Creek, Mount Distin, and
Sophia Gulch in the Nome District. In both areas, the deposits consist mainly of mesothermal sulfide-poor, Au-quartz depasits in
individual high grade veins or in zones of multiple, more-or-less en-echelon, sheeted veins which generally contain lower Au
grades. The quartz veins, which typically contain minor carbonate, albite, and oligoclase, cut shallowdipping metamorphic
foliation. The best studied deposits in the Nome district are at Rock Creek and Mount Distin, both immediately north of Nome
(Gamble and others, 1985; Apodaca, 1992). These newly evaluated lodes may be examples for the rich placer Au deposits mined
in the Nome District.
In the Nome district, Bundtzen and others (1994) recognize three major mineral deposit types: (I) edy-stage
chalcopyrite-sphalerite-gold-quartz-carbonate veins, which appear as boudins which are rolled around early fold axes (Banger
deposit); (2) Au-quartz plysulfide veins which crosscul schistosity at low angles modine, McDufQ, and Twin Mountain
deposits); and (3) brittle, Au-polysulfide-quartz-albite-carbonate veins which crosscut schistosity at high angles (Rd Creek,
Sliscovish, Sophies Gulch deposits). The Au deposits are thought to have formed during various stages of dewatering of a
metamorphic pile during Barmvian-type, greenschists facies regional metamorphism and associated plutonism, 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 hm quartz veins at the Bluff deposit formed at about 109 Ma, about 30 my. after
metamorphic mica in the urea reached closure temperature Pord 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 which 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 up to 150 meters in depth.
The host rocks are phyllite and schist of the Paleozoic Nome Group. Fluid inclusion studies indicate on: deposition occurred in the
mesothermal range (240°C-320°C). The ore minertils 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 dqwit is interpreted as
forming by hydrofracturing and dewatering during the waning stage of a midCretaceous rnetamoiphk event. The deposit contains
an estimated 10.2 million tomes grading 2.4 g/t Au and about 0.43% W (Swainbank and Szumigala, 2000). At Mount Distin,
several similar, en-echelon 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 othm, 1984;
Read, 1985; Read and Meinert, 1986) consists of four major quartz veins, and zones of ribbon quartz. The major veins and ribboo
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 intenmixed with older,
concordant, non-Au-bearing, metamorphic q u a veins. The Au-bearing veins range from discordant tension veias to
discontinuous quartz lodes which occur in shear zones crossing foliation. The Au-bearing veins range fcom 0.5 to 5 rn 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 cturing
shearing and uplift associated with metamorphic &hydralion in the mid-Cretaceous. Mining occurred at the deposit fiom 1903-
1909, and from 1953- 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 Contmls for
Nome Metallogenic Belt
The Big Hurrah and Bluff deposits in the Solomon district and the deposits in the Nome district exhibit several
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 o th,
1985): (1) the discordance of the veins to metamorphic foliations; (2) oxygen isotope and fluid inclusion data; and (5) the lack of
coeval intrusions in the belt. These relations indicate that the Au deposits formed from fluids which equilibrated witb the
sedimentary and (or) volcanic protoliths of the Nome Group under greenschist facies regional metamorphism. Subsqueally, the
fluids moved upward during a later, post-hnematic event to deposit the vein minerals. As in the southern Brooks Range, regiollal