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

Pogo Granitoid-Related Au Quartz Vein Deposit
The Pogo deposit (Liese Zone) consists of at least two, subparallel, gently dipping tabular quartz bodies which 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-dipp~ng quartz bodies at L1, L2, and L3.. The quartz bodies, none of the bodies crop out at
the surface, vary from 1 up to 2 1 meters 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 21 5 m apart, range from 1 to over 20 m thick (average 7
m thick), are roughly parallel, and dip from 25 to 30 degrees. 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, IoeIlingite, arsenopyrite,
chalcopyrite, bismuthinite, various Ag-Pb-Bi+S minerals, maldonite, native bismuth, galena and tetradymite. Native gold typically
ranges from to -25 microns in diameter, and locally up to 100 microns. Gold is often rimmed by native bismuth. The deposit grade
ranges up to more than 75 g/t Au over widths of several meters. High Au values are is 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, while 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 grn 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
that average 8.6 glt Au for over 75 meters. This area may be a possible feeder zone. The quartz is saccharoidal to polygonal.
Biotite alteration envelopes up to 0.5 meters wide occur adjacent to veins, and are overprinted by younger, widespread, quartzsericite
stockwork, and sericite-dolomite alteration.
The host rocks consist of highly deformed, amphibolite-grade, mainly metasedimentary which rocks are intruded by
diorite, granodiorite pegmatite and aplite dikes and sills and a granodiorite intrusion. The deposit is younger than a granite dike
which exhibits a preliminary U-Pb monazite isotopic age of 107 Ma, and is older than a cross-cutting 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 plutonic-related
Au system (D. McCoy, personal communication, 1999).
Kantishna District
At least seventy 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, 198 1; 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 poly-deformed 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. 39~r-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
(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 to larger than 300 krn2. 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 which 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 with high-temperature and high-pressure metamorphism;
(2) a slightly younger period of extension associated with lower greenschist facies metamorphism; and (3) intrusion of anatectic
granitoid plutons and relatively younger Au quartz veins. The mid-Cretaceous granitoid rocks and relatively younger quartz veins