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

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(9) During subduction of the Cache Creek Ocean plate to form the Talkeetna, Bonanza, and Stikinia-Quesnellia arcs,
limestone blocks contalnlng mainly Permian Tethyan faunas were accreted, locally in thick and extensive blocks in the
subduction-zone complexes of the Chugach (CG) and Cache Creek (CC) terranes (Monger and Berg, 1987; Monger and
Nokleberg, 1996). Tethyan faunas are generally interpreted as being derived from the late Paleozoic and earIy Mesozoic Tetbys
Ocean, remnants of which occur in the present-day Mediterranean region, Middle East, Hunalayas, Southeast Asia, eastern China,
Russian Southeast, and Japan (Monger and Ross, 1971; Monger and others, 1972; Stevens and others, 1997).
(10) In the Wrangellia superterrane (WRA), back-arc rifting or hot-spot activity formed the widespread baedt fields of
the Nikolai Greenstone and Karmutsen Formation (Barker and others, 1989; Richards and others, 1991; Lassita and others, 1994).
The rnafic magmatism forming those rocks was fist interpreted as forming in a rift setting (Barker and otbers, 1989).
Alternatively, the mafic magmatism may have formed in a short-lived mantle-plume setting similar to which io Java (Richard9 and
others, 199 1; 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 Ddhgw, Nixon
Fork, and Mystic subterranes of the Farewell composite terrane of Decker and others (1994) in Western Alaska Tbe bell GO-s
the Farewell gabbroic Ni-Cu districi (Foley and others, 1997; Bundtzen and others, 2003a, b; Bundtzen, Sidorov, and Chubmv,
2003) in the west-central Alaska Range. The deposits in the district are hosted in the infody-named Farewell mafic-d-c
suite that consists of differentiated, tholeiitic, perkbtite, clinopyroxenite, and gabbro sills, and cogenetic rtlkali-olivine basalt
flows that intrude or overlie: (1) sdty Limestone and sbale of the Cambrian to Ordovician Lyman kiib Formation; and (2)
calcareous sandstone and shale of the Pamian-Pe~sylvanian Sheep Creek Formation. The md~c-ultnunafic suite are enstatite
rich, orthopyroxene poor, and contain Ti-chromitite. REE and other m e element data from tbe Farewell suite suggests a magma
mixing model with locd c d contamination. * ~.r/~~~r isotopic ages for three silk range fiam 225.640-233.7 Ma (Nodan). Tbe
Farewell district contains three prospects at Gargaryah, Roberts, and Straight Creek.
Roberts PGM Prospect
The Roberts PGM prospect consists of disseminated to semi massive pyxhothe, chalcopyrite, peotlendite, spwrylite, and
bravoite that occur in the lower part of an ewtatite-rich ulbamatic sill that intrudes the Lyman Hills Formation. SurFace chaMel
sampling yields grades of up to 16.9 g/t PGE, 1.48 g/t Au, 2.27?/a Ni, 13 1% Cu, and 0.14% Co. A 5-meter-thick drill interval
yields grades of up to 4.13 glt PGE, 0.67% Ni, 0.32% Cu, and 298 ppm Co. The Straigbt Creek and Gargaryah fiver deposits,
discovered in 2001, consists of rills with up to 1.59 g/t PGE, 1.008%~ Cct, 0.87% Ni, and 250 ppm Co. Scvd sills cantaming t&w
PGE-Ni-Cu-Co prospects in the Farewell district exhibit a strong magnetic signature with maximWn intensities of up to 4,300
milligals. Trace element data oblained for the sill intrusions hosting the prospects and PGE element ratios (Pt, Pd, Ir, Rh, Rn, 09)
are similar to those reported from fulfide-bearing matic intrusions in the Paxsm-Canwell Glacier in the Easam Alaska Range,
and Kluane Lake area in the Yukon Territory. These deposits are part of the Eastern Alaska Range metallogenic belt, dewr'bed
below, that is hosted in the WrangeUia superlerrane.
Origin of and Tectonic Conhots for
Farewell Metallogenic Belt
The Farewell rnetallogenio belt is hosted in the Dillinger, Mystic, and Nixon Fork pasgive continental margin terranes of
Nokleberg and others (1997~) millinger, Mystic, and Nixon Fork subterranes of the Farewell (composite) kmne of Decker and
others, 1994 and Bundtzen ad others, 1997). North of the Farewell district in the Dillinger Mrrane are similar, deformed maficultramafic
sills In the Nlxon Fork teme at St, Johns Hill (McGrath qwdraagle) and in the Babybasket Hitls (Mcdfm qwhngle).
Geological mapping and paleontological data indicate tbat the Nixon Fork and Dilhger tmanes were coeval, continental margin
platform sections that were ovcrlain by fhe Mystic terrane. These three terranes are interpreted as having been rifted &om the
North Asian Craton Mvgin in the Late Devonian and Early Mississippian (Bhdgett and Breese, 1997; Blodgert, 15%; Fryd~ and
Blodgett, 1998; Dumouh and others, 1998,1999; Blodgett and Bwcot, 1999) when the North Asian tn North American
Cratons (and their margins) are interpmted as having been adjacent (Nokleberg and others, 2000). Tne early to middle Fataaoic
fauna in the Dillnger, Nirm 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 tectonlc origin of the Farewell metallogenic belt is uncertain. The Late Triassic gabbroic Ni-Cu deposits ofthe
Farewell metallogenic belt and host rocks are similar to the deposits and host rocks of the Late Triassic Eastern Alaska Range