USGS Professional Paper 1697 - Alaska Resources Library

168 Metallogenesis and Tectonics of the Russian Far East, Alaska, and the Canadian Cordillera
terrane (Arctic Alaska superterrane) and the Coldfoot terrane
to the south (Nokleberg and others, 1994c, 1997c). The
significant deposits are at Little Squaw and Mikado in the
Chandalar district (table 4) (Nokleberg and others 1997a,b,
1998). Recent work on Au-bearing quartz vein deposits in the
Wiseman area is summarized by Eden (2000).
Mikado Au Quartz Vein Deposit
The Mikado Au quartz vein deposit (Chipp, 1970; DeYoung,
1978; Dillon, 1982; Ashworth, 1983; J.T. Dillon, oral commun.,
1986; Rose and others, 1988) consists of several quartz
veins as much as 3 m thick in a zone about 4.0 km long and 1.6
km wide. The veins contain scattered minor arsenopyrite, galena,
sphalerite, stibnite, and pyrite and sparse gold. The veins occur
along steeply dipping normal faults in Devonian or older quartzmuscovite
schist, phyllite, and quartzite. The Little Squaw Mining
Company drove more than 1,000 m of underground workings
from 1980 to 1983. Minor production and several episodes
of exploration activity have occurred, notably during the 1920’s
and 1960’s. The Mikado deposit contained an estimated 12,000
tonnes averaging 75 g/t Au and produced about 542 kg Au from
ore averaging about 30 g/t Au (Bundtzen and others, 1994). The
Mikado deposit and surrounding district contains an estimated
remaining 45,000 tonnes grading 30 g/t Au.
Origin of and Tectonic Controls for Southern Brooks
Range Metallogenic Belt
The Au quartz vein deposits occur along steeply dipping
normal faults in greenschist facies metasedimentary rocks that
are part of a structurally complex, polymetamorphosed, and
polydeformed assemblage of Devonian or older carbonate
rocks, including the Skajit Limestone, calc-schist, quartz-mica
schist, and quartzite, which are intruded by Proterozoic and
Late Devonian gneissic granitoid rocks. These early Paleozoic
metasedimentary rocks and Devonian metagranitoid rocks
form a major part of the Hammond passive continental-margin
terrane of the Arctic Alaska superterrane (Jones and others,
1987; Moore and others, 1992).
Field relations indicate the deposits, which were deposited
from hydrothermal fluids, were deposited during normal
faulting. The normal faulting may be associated with a period of
regional extension that was associated with the waning stages
of greenschist facies regional metamorphism and companion
penetrative deformation in the Early to mid-Cretaceous (Moore
and others, 1992; Nokleberg and others, 1994c, 1997c). This
period of regional metamorphism is interpreted as the last major
metamorphic event in the Hammond and Coldfoot terranes. In
a few areas, the Coldfoot and Hammond terranes exhibit relict
blueschist facies minerals that are interpreted as forming in a
Jurassic or older period of convergent deformation and metamorphism
(Moore and others, 1994). The convergent deformation
and blueschist facies metamorphism probably occurred during
Late Jurassic and Early Cretaceous subduction of the Coldfoot
and Hammond terranes under the Angayucham subduction-zone
terrane, and Koyukuk island-arc terrane to the south (Moore
and others, 1994; Patton and others, 1994; Plafker and Berg,
1994; Nokleberg and others, 2000). As in the Nome region, the
regional blueschist facies metamorphism was followed by (1)
thrusting on oceanic units of the Angayucham terrane onto the
Coldfoot and Hammond terranes and other parts of the passive
continental-margin arctic Alaska superterrane, and (2) extensional,
Early to mid-Cretaceous retrograde greenschist metamorphism
and formation of Au quartz vein deposits (Armstong and
others, 1986; Moore and others, 1994; Till and Dumoulin, 1994).
The Au quartz vein deposits of the Southern Brooks Range
metallogenic belt only occur in a small portion of the greenschist
facies metasedimentary rocks of the southern Brooks Range.
However, the more extensive placer Au deposits along the
Brooks Range may be derived from undiscovered or now totally
eroded Au quartz vein deposits. The Au quartz vein deposits in
the Southern Brooks Range metallogenic belt are interpreted as
coeval with the Nome metallogenic belt to the southwest.
Fish River Metallogenic Belt of Sedimentary P and
Fe Deposits (Belt FR), Northern Yukon Territory
The Fish River metallogenic belt of sedimentary P and
Fe deposits occurs in the northern Yukon Territory (fig. 62;
tables 3, 4) (Nokleberg and others, 1997b, 1998)and is hosted
in the Early Cretaceous Blow River Formation. The significant
deposits are at Big Fish River and Alto. The Fish River metallogenic
belt is herein tentatively interpreted as forming during
Late Mesozoic dextral movement along the Kaltag-Porcupine
fault system.
The Fish River (Big Fish, Boundary, Rapid) stratabound
Fe-P deposit consists of siderite and phosphatic ironstone that
occur in shale in an Early Cretaceous (Albian) clastic wedge
(Yukon Minfile, 1988; Butrenchuk, 1996). The ironstone
consists of phosphate-siderite pellets and granules in a matrix
of detrital quartz and mudstone. Rare phosphate minerals
occur in epigenetic fracture veins and to a lesser degree in
vugs, bedding plane partings, and fault breccia. The deposit
is the well-known type locality of lazulite, the official Yukon
gemstone. Estimated resources are more than 1 billion tonnes
grading 40 percent Fe.
The Alto oolitic magnetite iron formation is hosted in
black shale of the Jurassic and Cretaceous Kingak Formation
(Norris, 1976; Yukon Minfile, 1987). The deposit consists of
olitic magnetite that occurs in a 45-m-thick bed, which occurs
for a strike length of 350 m, and is part of a recessive-weathering
black shale that is about 50 m above the base of the
Kingak Formation. Estimated resources are 50 million tonnes
grading 55 percent Fe (Norris, 1976).
The Early Cretaceous Blow River Formation is part of
a thick clastic wedge deposited in the Blow Trough, which is
interpreted as forming during late Mesozoic, dextral movement
along the Kaltag-Porcupine Fault system (Norris and Yorath,
1981). The sideritic ironstone and phosphorites are interpreted
to be distal, reworked deep water pellet packstone deposits
within a thick silt and conglomerate unit with a westerly clastic