USGS Professional Paper 1697 - Alaska Resources Library

The deposit consists of disseminated chalcopyrite, pyrite, and
molybdenum, accompanied by minor to trace galena, sphalerite,
and arsenopyrite, in a stockwork vein system. The deposit
contains an inferred reserve of 430 million tonnes grading 0.35
percent Cu, 0.4 g/t Au, and 0.015 percent Mo. Recent data
indicates 1.0 billion tonnes grading 0.61 percent Cu equivalent,
or 0.4 percent Cu and 0.30 g/t Au, 0.015 percent Mo (Northern
Dynasty news release, September 25, 2003). The deposit
is hosted in a mid-Cretaceous granodiorite porphyry and its
adjacent hornfels aureole. The sulfide minerals formed during
a late-stage intense hydrofracturing that followed potassic,
silicic, and sericitic alteration events. Tourmaline breccias also
exist locally. K-Ar ages for hydrothermal sericite and igneous
K-feldspar are 90 and 97 Ma, respectively. The granodiorite
hosting the deposit is part of a larger, composite 40 km 2
volcanic-plutonic complex that also includes pyroxenite, alkali
gabbro, and granite, and overlying dacite volcanic rocks. The
volcanic and plutonic rocks are alkalic-calcic and quartz alkalic
in composition. The granodiorite porphyry intrudes the Late
GT
ID-GT
Garnet skarn
GT
MT
SUL
GT
GT-PYX
MT-SERP
GT-PYX Garnet-pyroxene skarn
PYX Pyroxene skarn
PYX
Late Jurassic Metallogenic Belts (163 to 144 Ma; figs. 48, 49) 127
Jurassic and Early Cretaceous Kahiltna overlap assemblage and
is overlain by Tertiary volcanic rocks.
Orange Hill and Bond Creek Porphyry Cu-Mo Deposits
The Orange Hill and Bond Creek porphyry Cu-Mo deposits
(fig. 57) (Van Alstine and Black, 1946; Richter and others,
1975a,b; Nokleberg and others, 1995a) occur in the northern
Wrangell Mountains. The deposits consist of pyrite, chalcopyrite,
and minor molybdenite that occur in quartz veins that contain
K-feldspar and sericite, and as disseminations in the Cretaceous
Nabesna pluton. The pluton, which has K-Ar isotopic ages of
112 to 114 Ma, forms a complex intrusion of granodiorite and
quartz diorite intruded by granite porphyry. The deposits exhibit
abundant biotite-quartz, quartz-sericite, and chlorite-sericiteepidote
alteration, and late-stage anhydrite veins (R.J. Newberry,
written commun., 1985). Widespread, late-stage chlorite-sericiteepidote
alteration also occurs in the Nabesna pluton. The altered
areas associated with the deposit have dimensions of about 1.0
by 3.0 km at Orange Hill and 2.0 by 3.0 km at Bond Creek.
WNW ESE Colluvium (Holocene)
Wrangell Lava (Quaternary)
Nabesna Stock (mid-Cretaceous)
Argillite
Thin-bedded limestone
metamorphased
to calc-silicate hornfels
Massive limestone
metamorphosed to marble
Algae marker bed
Massive dolostone
metamorphased to marble
Nikolai Greenstone
Contact
0 100 m
ID-GT Idocrase-garnet skarn
MT-SERP Magnetite-serpentine skarn
MT Magnetite bodies
SUL Sulfide ore
Late
Triassic
Figure 56. Nabesna Fe-Au skarn mine, eastern-southern Alaska metallogenic belt, southern Alaska. Schematic cross
section showing sulfide-magnetite-skarn relations. Magnetite-rich ore replaces dolomite. Gold-rich ores form small
marble front replacements. The chief ore mineral is chalcopyrite in garnet-pyroxene skarn. Adapted from Wayland
(1953), Weglarz (1991), and Newberry and others (1997). See figure 49 and table 4 for location.