USGS Professional Paper 1697 - Alaska Resources Library

84 Metallogenesis and Tectonics of the Russian Far East, Alaska, and the Canadian Cordillera
1989). Alternatively, the mafic magmatism may have formed
in a short-lived mantle-plume setting similar to that in Java
(Richards and others, 1991; Lassiter and others, 1994).
Metallogenic Belt Formed During Early
Mesozoic Rifting? in Alaskan Passive
Continental-Margin Terranes
Farewell Metallogenic Belt of Gabbroic
Ni-Cu-PGE Deposits (Belt EAR), Western Alaska
The Farewell metallogenic belt of gabbroic Ni-Cu-PGE
deposits (fig. 32; tables 3, 4) is hosted in the the Dillnger,
Nixon Fork, and Mystic subterranes of the Farewell composite
terrane of Decker and others (1994) in western Alaska. The
belt contains the Farewell gabbroic Ni-Cu district (Foley and
others, 1997; Bundtzen and others, 2003a,b; Bundtzen, Sidorov,
and Chubarov, 2003) in the west-central Alaska Range.
The deposits in the district are hosted in the informally named
Farewell mafic-ultramafic suite that consists of differentiated,
tholeiitic, peridotite, clinopyroxenite, and gabbro sills and cogenetic
alkali-olivine basalt flows that intrude or overlie (1) silty
limestone and shale of the Cambrian to Ordovician Lyman Hills
Formation, and (2) calcareous sandstone and shale of the Permian-Pennsylvanian
Sheep Creek Formation. The mafic-ultramafic
suite are enstatite rich, orthopyroxene poor, and contain
Ti-chromitite. REE and other trace element data from the Farewell
suite suggests a magma mixing model with local crustal
contamination. 40 Ar/ 39 Ar isotopic ages for three sills range from
225.6 to 233.7 Ma (Norian). The Farewell district contains three
prospects at Gargaryah, Roberts, and Straight Creek.
Roberts PGM Prospect
The Roberts PGM prospect consists of disseminated to
semimassive pyrrhotite, chalcopyrite, pentlendite, speerylite,
and bravoite that occur in the lower part of an enstatite-rich
ultramafic sill that intrudes the Lyman Hills Formation. Surface
channel sampling yields grades as much as 16.9 g/t PGE,
1.48 g/t Au, 2.27 percent Ni, 1.31 percent Cu, and 0.14 percent
Co. A 5-meter-thick drill interval yields grades of as much as
4.13 g/t PGE, 0.67 percent Ni, 0.32 percent Cu, and 298 ppm
Co. The Straight Creek and Gargaryah River deposits, discovered
in 2001, consists of sills with as much as 1.59 g/t PGE,
1.00 percent Co, 0.87 percent Ni, and 250 ppm Co. Several
sills containing these PGE-Ni-Cu-Co prospects in the Farewell
district exhibit a strong magnetic signature with maximum
intensities of as much as 4,300 milligals. Trace element data
obtained for the sill intrusions hosting the prospects and PGE
element ratios (Pt, Pd, Ir, Rh, Ru, Os) are similar to those
reported from sulfide-bearing mafic intrusions in the Paxson-
Canwell Glacier in the Eastern Alaska Range and Kluane
Lake area in the Yukon Territory. These deposits are part of
the Eastern Alaska Range metallogenic belt, described below,
which is hosted in the Wrangellia superterrane.
Origin of and Tectonic Controls for Farewell Metallogenic Belt
The Farewell metallogenic belt is hosted in the Dillinger,
Mystic, and Nixon Fork passive continental margin terranes of
Nokleberg and others (1997c) (Dillinger, Mystic, and Nixon
Fork subterranes of the Farewell (composite) terrane of Decker
and others, 1994, and Bundtzen and others, 1997). North of the
Farewell district in the Dillinger terrane are similar, deformed
mafic-ultramafic sills in the Nixon Fork terrane at St. Johns
Hill (McGrath quadrangle) and in the Babybasket Hills (Medfra
quadrangle). Geological mapping and paleontological
data indicate that the Nixon Fork and Dillinger terranes were
coeval, continental margin platform sections that were overlain
by the Mystic terrane. These three terranes are interpreted as
having been rifted from the North Asian Craton Margin in the
Late Devonian and Early Mississippian (Blodgett and Brease,
1997; Blodgett, 1998; Fryda and Blodgett, 1998; Dumoulin
and others, 1998, 1999; Blodgett and Boucot, 1999) when the
North Asian and North American Cratons (and their margins)
are interpreted as having been adjacent (Nokleberg and others,
2000). The early to middle Paleozoic fauna in the Dillnger,
Nixon Fork, and Mystic terranes are typical of taxa that occur in
similar age units in the Kolyma region of the North Asian Craton
Margin (Verkhoyansk fold belt) in the Russian Northeast.
The tectonic origin of the Farewell metallogenic belt is
uncertain. The Late Triassic gabbroic Ni-Cu deposits of the
Farewell metallogenic belt and host rocks are similar to the
deposits and host rocks of the Late Triassic Eastern Alaska
Range metallogenic belt, now located a few hundred km to
the east in southern Alaska. However, available paleomagnetic
data (table 3 in Nokleberg and others (2000)), indicate that
the Wrangellia superterrane, which hosts the Eastern Alaskan
Range metallogenic belt, was within a few degrees of the
Late Triassic paleoequator. In contrast, the three subterranes
(Dillnger, Nixon Fork, and Mystic), constituting the Farewell
terrane, are interpreted as having been located several thousand
km away, near the North American Craton Margin (fig.
34) (Nokleberg and others, 2000). Additional work is needed
to determine the tectonic origin of the Farewell metallogenic
belt, contained deposits, and host rocks. Herein, the Farewell
metallogenic belt is interpreted as forming during incipient
Late Triassic rifting of Dillinger and adjacent passive-continental-margin
terranes
Metallogenic Belts Formed in Middle Mesozoic
Talkeetna-Bonzana Island Arc in Wrangellia
Superterrane
Kodiak Island and Border Ranges Metallogenic
Belt of Podiform Cr Deposits (Belt KOD), Southern
Coastal Alaska
The Kodiak Island and Border Ranges metallogenic belt
of podiform Cr deposits and one gabbroic Ni-Cu deposit (fig.