USGS Professional Paper 1697 - Alaska Resources Library

246 Metallogenesis and Tectonics of the Russian Far East, Alaska, and the Canadian Cordillera
Bohemia Basin Gabbroic Ni-Cu Deposit
The Bohemia Basin gabbroic Ni-Cu deposit consists of
magmatic sulfide segregations, chiefly consisting of pyrrhotite,
pentlandite, and chalcopyrite (Himmelberg and others, 1987,
Still, 1988; Brew and others, 1991; Berg, 1984; Foley and others,
1997). Estimated resources are19 million tonnes grading 0.33
percent Ni, 0.21 percent Cu, and 0.01 percent Co. The deposit
occurs in a trough-like body, about 45 m thick, near the base of a
basin-shaped, composite norite of a Tertiary lopolith. The norite,
which locally grades into gabbro and diorite, intrudes metagraywacke,
phyllite, and greenschist of the Cretaceous and Cretaceous(?)
Kelp Bay Group of the Chugach terrane.
S
A
SW
0 20 m
B
0 40 m
Mine
workings
Brady Glacier Gabbroic Ni-Cu Deposit
The Brady Glacier gabbroic Ni-Cu deposit (fig. 117)
consists of disseminated and lensoid pyrrhotite, pentlandite,
and chalcopyrite, with rare pyrite. The sulfide minerals occur
near the base of a large, layered Tertiary lopolith that consists
mainly of gabbro and sparse peridotite, all part of the La
Perouse layered gabbro (Czamanske and Calk, 1981; Himmelberg
and Loney, 1981; Foley and others, 1997). The gabbro
intrudes metagraywacke and phyllite of the Cretaceous Sitka
Graywacke that is part of the Chugach terrane. The mafic and
ultramafic rocks locally contain as much as 10 percent disseminated
sulfides. Estimated resources are 82 to 91 million
N
NE
+
+
+ +
+
+ + + +
+
Diorite (Early Tertiary)
Stockwork zone
Mafic lava flows
Tuff and
hyaloclastite
Massive sulfide
Volcanic Sequence
Vein and disseminated sulfide,
and breccia containing
< 50% sulfide matrix
Contact
Massive and pillowed
lavas
Pillow lava, pillow lava
breccia, and hyaloclastite
Mixed volcanic and chert
breccia
Sulfide Deposits
Orca Group
(Early Tertiary)
Massive sulfide deposited on seafloor
Massive sulfide blocks and
quartz flooding in breccia
Abundant disseminated and
vein controlled sulfide
Arrows indicating interpreted
pathway for hydrothermal fluids.
Contact
Orca Group
(Early Tertiary)
Figure 116. Copper Bullion (Rua Cove) Cyprus massive sulfide deposit, Prince William Sound metallogenic belt, southern Alaska. A,
Schematic geologic map of 370 level. B, Interpretive model. Modified from Koski and others (1985) and Crowe and others (1992). See
figure 103 and table 4 for location.