USGS Professional Paper 1697 - Alaska Resources Library

212 Metallogenesis and Tectonics of the Russian Far East, Alaska, and the Canadian Cordillera
Richardson lineament. Free gold occurs as interlocking alloys
of native silver and gold that average 67 percent Au and 33
percent Ag. Silver sulfosalts are abundant locally and exhibit
very high grades of as much as 66,000 g/t Ag. In 1989, a pilot
mill produced 88,000 tonnes grading 2.2 g/t Au and 5.0 g/t
Ag. The deposit is interpreted as forming during high-level
emplacement of a Late Cretaceous granite porphyry along the
Richardson lineament.
Pogo Granitoid-Related Au Quartz Vein Deposit
The Pogo deposit (Liese Zone) consists of at least two,
subparallel, gently dipping tabular quartz bodies that are
hosted by Proterozoic to early Paleozoic paragneiss and minor
orthogneiss of the Yukon-Tanana Terrane (Lefebure and
Cathro, 1999; Smith, 1999; Smith and others, 1999, 2000).
The deposit contains both vein and replacements quartz bodies
and consists mainly of three or more large, tabular, gently
dipping quartz bodies at L1, L2, and L3. The quartz bodies,
none of the bodies crop out at the surface, vary from 1 to 21 m
thick, and the largest at L1 extends at least 1,300 meters along
strike. The L2 quartz body underlies the eastern half of the L1
body. The quartz bodies lie from 1 to 215 m apart, range from
1 to 20 m thick (average 7 m thick), are roughly parallel, and
dip from 25° to 30°. The bodies cut the foliation in the host
rocks and are offset by high angle faults.
The quartz bodies contain three percent sulfide minerals.
The ore minerals are pyrrhotite, pyrite, loellingite,
arsenopyrite, chalcopyrite, bismuthinite, various Ag-Pb-Bi±S
minerals, maldonite, native bismuth, galena, and tetradymite.
Native gold typically ranges from 5 to -25 microns in
diameter and locally as much as 100 microns. Gold is often
rimmed by native bismuth. The deposit grade ranges as much
as more than 75 g/t Au over widths of several meters. High
Au values are associated with high values of Bi, Te, W, As,
and possibly Sn. Both white and gray quartz occur; the former
is associated pyrrhotite and loellingite and is interpreted
as early stage, whereas the latter is associated with arsenopyrite
and pyrite and is interpreted as late stage. As of 1999, the
L1 and L2 zone contained an estimated 9.05 millions tonnes
grading 17.8 g/t for a total of 147,386,400 gm Au at a 3.42
g/t cutoff. Median grade for Ag in the mineralized zones is 2
g/t (Smith and others, 1999, 2000). One drill hole along steep
shears and quartz-veins averaged 8.6 g/t Au for over 75 m.
This area may be a possible feeder zone. The quartz is saccharoidal
to polygonal. Biotite alteration envelopes as much
as 0.5 m wide occur adjacent to veins and are overprinted by
younger, widespread quartz-sericite stockwork and sericitedolomite
alteration.
The host rocks consist of highly deformed, amphibolite
grade, mainly metasedimentary rocks that are intruded by diorite,
granodiorite, pegmatite, and aplite dikes sills, and a granodiorite
intrusion. The deposit is younger than a granite dike that
exhibits a preliminary U-Pb monazite isotopic age of 107 Ma,
and is older than a crosscutting diorite that has a preliminary
U-Pb zircon isotopic age of 94 Ma. The Pogo deposit shares a
number of characteristics with plutonic-related quartz veins in
the Fairbanks district and Yukon, including a similar geological
setting, a close association with Cretaceous granitoid rocks,
low sulfide content, and similar geochemistry. The Pogo deposit
may represent a deeper, higher-temperature part of a plutonicrelated
Au system (D. McCoy, personal commun., 1999).
Kantishna District
At least 70 polymetallic and Sb-Au vein deposits occur
in the Kantishna district in the western part of the southern
Yukon-Tanana terrane in the northern Alaska Range (Bundtzen,
1981; McCoy and others (1997). The significant deposits
are Sb-Au vein deposits at Caribou, Eagles Den, Slate Creek,
and Stampede, and polymetallic vein deposits at Banjo,
Quigley Ridge, and Spruce Creek (table 4) (Nokleberg and
others 1997a,b, 1998). The polymetallic vein deposits occur in
middle Paleozoic or older, polymetamorphosed and polydeformed
submarine metavolcanic and metasedimentary rocks of
the Yukon-Tanana terrane, which also hosts an extensive belt
of middle Paleozoic kuroko massive sulfide deposits described
previously (Aleinikoff and Nokleberg, 1985; Nokleberg and
Aleinikoff, 1985). Most of the polymetallic vein deposits
occur as crosscutting quartz-carbonate-sulfide veins and are
confined to a 60-km-long, northeast-trending fault zone, which
extends from Slate Creek to Stampede (Bundtzen, 1981). Mineralization
occurred before, during, and after fault-zone movement
as illustrated by both crushed and undeformed ore shoots
in the same vein system. 39Ar-40Ar isotopic ages indicate vein
formation was from 91 to 88 Ma (McCoy and others, 1997).
Origin of and Tectonic Controls for East-Central Alaska
Metallogenic Belt (mid-Cretaceous part)
The mid-Cretaceous granitoid rocks of the older part of
the Yukon-Tanana igneous belt (Moll-Stalcup, 1994), which
host the older part of the East-Central Alaska metallogenic
belt, occurs in large batholiths and small, isolated granitoid
plutons (Foster and others, 1987; Miller, 1994). The plutons
range in area from smaller than 1 km2 to larger than 300 km2 .
The irregular-shaped plutons were intruded after a period of
intense metamorphism and deformation in the mid-Cretaceous.
The plutonic rocks exhibit K-Ar mineral, Rb-Sr whole
rock, and U-Pb zircon ages that range from about 100 to 90
Ma (Wilson and others, 1994; Smith and others, 1999, 2000).
The polymetallic, Sb-Au vein, and granitoid-related Au
deposits of the mid-Cretaceous part of the East-Central Alaska
metallogenic belt, and similar deposits in the Tombstone metallogenic
belt to the east, are generally hosted in or near mid-Cretaceous
granitoid plutons (Nokleberg and others, 1995a; McCoy
and others, 1997; Smith, 1999, 2000) and are herein interpreted
as forming during the waning stages of a major mid-Cretaceous
collision of the Wrangellia superterrane with the previously
accreted Yukon-Tanana terrane (Stanley and others, 1990; Dusel
Bacon and others, 1993; Pavlis and others, 1993; Nokleberg
and others, 2000). The collision and associated metamorphism
and deformation is interpreted as forming in three phases—(1)
a relatively older period of collision and thrusting, associated