USGS Professional Paper 1697 - Alaska Resources Library

Nokleberg and others, 1995a; Bundtzen and Miller, 1997)
consists of a stockwork of gold-quartz sulfide and sulfide
veinlets that occur in the cupola of the Chicken Mountain pluton.
The pluton is zoned—older, peripheral alkali gabbro and
wehrlite is successively intruded by monzonite, syenite, and
local quartz monzonite. A dumbbell-shaped alteration zone of
peripheral ankerite and sericite surrounds most of the known
parts of the deposit. Large dolomite replacement zones formed
synchronously with a sulfide mineral assemblage that includes
arsenopyrite, stibnite, cinnabar, scheelite, chalcopyrite,
molybdenite, various sulfosalts, and arsenopyrite. The sulfide
minerals rarely constitute more than 5 percent of veins in the
stockwork, and the veins typically are about 1 to 2 cm thick.
An apparent temperature-pressure zonation exists as determined
by mineral assemblage, alteration, and fluid inclusion data
(Bundtzen and others, 1992). A deuteric magmatic event that
contained elevated Ta and Sn values was followed by formation
of a black sulfide breccia rich in Cu and Mo and was in turn
Sulfide
fracture
86BT350e
500 ppm As
2 g/t Ag
0.1% Pb
0 30 m
86BT350f
0.10% Cu 1.4 g/t Au
0.15% Zn 500 g/t Ag
2.0% Pb 39 ppm Bi
1.0% As150
ppm Sn
0.2% Sb
Alluvium (Quaternary)
86BT350f Sample number
zone
Bedrock talus (Quaternary)
Altered basalt and tuff roof pendants
(Late Cretaceous and early Tertiary)
Porphyritic biotite-augite monzonite,
with locally diabasic texture
(Late Cretaceous and early Tertiary)
Zone with greater than 30% sulfides
86BT350c
50 ppm Sn
Tourmaline-sulfide zone with quartz-tourmaline-axinite
veinlets containing 2 to 20% sulfides and fluorite
Tourmaline felsite with 4-10% tourmaline
rosettes or bands by volume
Structurally incised bedrock drainage
with fault gouge and colluvium
Chip sample location
Fault
Contact
Late Cretaceous and Early Tertiary Metallogenic Belts (84 to 52 Ma) (figs. 102, 103) 231
Steep slope
followed by emplacement of As-W-Au veins and stockwork and
Pb-Au-Sb sulfosalts. A final Sb-Au (Hg) quartz vein episode
overprinted the older mineralizing events. K-Ar isotopic ages for
both plutonic rocks and mineral veins average about 69.5 Ma. A
secondary biotite age of 63.5 Ma from the Golden Horn deposit
(Bundtzen and Miller, 1997) may postdate the late-stage Sb (Hg)
epithermal vein mineralization. A drilling program conducted
in 1989 through 1990 suggests a reserve of 14.5 million tonnes
grading 1.2 g/t Au, 0.09 percent Cu, and 0.46 percent Sb to a
depth of about 200 m (Bundtzen and others, 1992).
Cirque, Tolstoi, Bismarck Creek, and Win Sn-Ag
Polymetallic Deposits
Sn-Ag-polymetallic deposits occur in the Beaver Mountains
(fig. 109) west of McGrath, and in other areas of the
Kuskokwim Mountains (Bundtzen and Laird, 1982; Miller
and Bundtzen, 1994; Nokleberg and others, 1995a; Burleigh,
B - Cirque Deposit
Steep cirque headwall
Tourmaline-fluorite-axinite
breccia zone with sulfides
81BT501c
21.05% Cu
0.08% Zn
884 g/t Ag
81BT501b
20.0% Cu
0.20% Zn
1105 g/t Ag
Steep slope
81BT501a
3.00% Cu
3.00% Pb
0.4% Zn
663 g/t Ag
86BT350g
340 ppm As
200 ppm Pb
380 ppm Zn
A - Tolstoi Prospect
Ridge top
Euhedral quartz and
oxidized chalcopyrite,
minor galena, and
sphalerite
Steep cirque
headwall
Figure 109. Beaver Mountains Ag-Sn polymetallic vein deposits, Kuskokwim Mountains metallogenic belt, southwestern Alaska. Two examples
of high-level, plutonic-related, B-enriched Ag-Sn polymetallic deposits or prospects: A. Tolstoi prospect, illustrating extensive tourmaline-sulfide
breccia zones; B. Cirque Cu-Ag (Au) deposit, showing fracture zone and tourmaline-axinite-fluorite breccia zone. Both deposits
occur just below roof pendants of basaltic andesite. Adapted from Bundtzen and Miller (1997). See figure 103 and table 4 for location.
156