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

Southwestern Akska Metrllogenic Belt of
Zoned Mafic-Ultramafk PGE Deposits (Belt SWA)
Southwestern Alaska
The Southwestern Alaska metallogenic belt of zoned mafic-ultrarnafic PGE deposits (fig. 49; hblm 3,4) arc hkd in th
Goodnews suMuctiun zane and Tagiak island-arc terranes (Nokleberg and others, 1994c, 195%). The significaut deposits in the
belt are: (1) a concealed Fe-Ti (PGE) deposit at Kemuk; and (2) a zoned mafic-ultramafic PGE occurrence at Rad Mmtrin (fig.
55) in ultramafic rocks at Goodnews Bay which is interpreted as tbe source of the extensive placer PGE deposits m the region
(table 4) (Southworth and Foky, 1986; Foley and others, 1997; Nokleberg and others 1997% b, 1998). The ultramafic plutono
which host tbc deposits io both areas are a part of a belt of similar zoned mafic to ultramafic plutons which intrude both the
Goodnews oceaslic and ibe adjacent Togiak island-arc terrane (Nokleberg and others. 1994~. 1997~).
0
Alluvium and colluvium
(Quaternary) &" Z.:
Hornblende gabbm m and tmmblsndfte
Figure 55. Red Mountain mafi~ultramafic PGE occurrence, Southwestern Alaska metallogenic belt, Southwestern Alaska.
Occurrence is hosted in Goodnews Bay rnafic-ultramafic complex. Schematic geologic map. Adapted from Southworth (1986) and
Foley and others (1997).
I Kemuk Mournin FaTP (PGE) Deposit
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The Kemuk Mountain Fe-Ti (PGE) deposit (Humble Oil and Refining Company, written cornmm, 1958; Eberlein and
others, 1977; C.C. Hawley, written commun., 1980; Foley and others, 1997) consists of a buried tibniferous magnetite deposit IS
crudely zoned pyroxenite which is interpreted as part of a zoned Alaskan-type ultramafic pluton. A steeply-dipping, hi@tempera-,
contact metamorphic zone occurs in adjacent Permian quartzlte and limestone. An aeromagnetic survey indicatcg the
concealed pluton is about 1,500 m thick, and underlies about 6 km2 area. Based on Humble Oil and Refining Compmy $rill data
(written comun., 1988; Bundtzen and others, 1994), the deposit is estimated to contain 2,200 million tonnes grading 15 to 17 %
Fe, and 2 to 3 % TiOz.
Ortgin ofand Tectorric Controls ibr
SwtWestern Alaska Metalloge& BeH
The ultramafic plutons which host the Kemuk Mountain and Red Mountain deposits are a part of a belt of similar zoned
mafic to u l t r ~ plutons c which intrude both the Goodnews oceanic terrane and the adjacent Togiak island-arc term. Thcst
plutonic rocks are interpreted as the oldest known remnants of the Togiak island arc which consists chiefly of two major
sequences (Box, 1985; Nokleberg and others, 1994c, 1997~): (1) a lower ophiolite sequence at the southwestern end of the t-e
which consists of Late Triassic midocean-ridge pillow basalt, diabase, gabbro, and ultramafic rocks; and (2) a coherent upper
sequence of Early Jurassic to Early Cretaceous marine volcaniclastic sandstone, conglomerate, shale, hrfficeous chert, minor
argillaceous limestone, and marine to nonmarine andesite and basalt flows, and flow breccia, and tuff. The Togiak island arc is