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

- . ---
Origin of and Tectonic Controls for
Left Omolon Metallogenic Belt
The Left Omolon metallogenic blt occurs dong the northeast marginal of the Omolon crabnal terrane. Two possible
tectonic settings are interpreted for the Early Cretaceous granitic intrusions hosting this metallogenic belt: (1) The Early
Cretaceous granitic intrusions may be a transverse, neariy orthogonal branch of the Cretaceous Okhotsk-Chukotka volcanicplutonic
belt; or (2) the Early Cretaceous granitic intrusives, as interpreted herein, are part of the Oloy-Svyatoy Nos volcanic belt
that constitute part of the Early Cretaceous Oloy continental-margin magmatic arc which occurs to the norrheast of the Omolon
cratonal terrane (fig. 6 1) (Nokleberg and others, 1994~~ 1997b, c, 1998; 2000).
Western-Southeastern Alaska Metallogenic Belt of
Granitic-Magmatism-Related Deposits (Belt WSE)
Southeastern Alaska
The Western-southeastern Alaska metallogenic belt of granitic-magmatism-rehtd &posits (fig. 662; tables 3,4) oms in
Southeastern Alaska, and is hosted in Early to mid-Cretaceous pitoid plutons of the Muir-Chichagof plutonic belt which intrude
the Wrangellia superterrane (Nokleberg and others, 1995a). The significant deposits are in the Jumbo Cu-Au skarn district, and the
Bokan Mountain felsic plutonic U-REE deposit (table 4) (NokJeberg and others 1997a, b, 1998).
Jumbo Cu-Au Skarn Deposit
The Jumbo Cu-Au skarn district includes sigmf3cant deposit at Jumbo, several small deposits at Magnetite Cliff, Copper
Mountain, and Corbin, and lesser deposits at Late Magnetite, Oonnason, Houghton, Green Monster, and Hetta. All of the deposits
occur within a few kilometus of the Jumbo deposit. The Jumbo district contains an estimated 280,000 tow of ore grading 45%
Fe and 0.73% Cu, and has produced 4.6 million kg Cu, 220,000 g Au, and 2.73 million g Ag from 1 11,503 tomes of ore
(Kennedy, 1953; Herreid and olhers, 1978; Newbeny and others, 1997). The deposits are hosted in, or lie adjacent to early
Paleozoic marble and pelitic metasedimentary rocks of the Wales Grwp of the Alexander sequence where intruded by mid-
Cretaceous hornblende-biotite granodiorite which exhibit concordant hornblende and biotite K-Ar isotopic ages of 103 Ma. The
Jumbo deposit consists of chalcopyrite, magnetite, sphalerite, and molybdenite in skarn which occurs at the contact between
marble and a granodiorite stock. Gangue is mainly diopside and garnet. The deposit has k n
explored by more than 3.2 km of
underground workings and is the largest in the district. Tbc Magnetite Cliff deposit consists of a 25-m-thick shell of magnetite
which mantles granodiorite in contact with gamct-diopside skam. The skam contains 2% to 3% chalcopyrite, estimated reswrces
of 335,600 tonnes grading 46% Fe and 0.77% Cu. The Copper Mountain deposit consists of scattered chakopyrite and copper
carbonate in diopside endoskarn which conlains veins and masses of epidote, garnet, magnetite, and scapolite near podiorite.
This deposit has produced 101,800 kg Cu, 321,300 g Ag, and 4,5 10 g Ay and has about 410 m of tunnels and shafts.
Bokan Mountain Felsic plutonic &REE d-it
The Bokan Mountain felsic plutonic U-REE deposit (fig. 71) consists of disseminated U, Th, REE, and niobate minerals,
including uranothorite, uranoan thorianite, uraninite, xenotime, allanite. monazite, pyrite, galena, zircon, and fluorite which occur
in an irregular, steeply-dipping pipe of Jurassic peralkaline granite (Warner and Barker, 1989; Brew and others, 1991; Berg, 1984;
Foley and others, 1997; Thompson, 197). The deposit produced about 109,000 tonnes gradiog about 1% U3Qs; Th was not
recovered. Most of ore was produced hrn a crudely cigar-shaped upper part of the pluton. The central zone which contains the
deposit grades outward into oonnal granite. Associated pegmatite and vein REE, Nb, Th, and U deposits acwr in the outer parts
of granite or adjacent country rock, which consists of metamorphosed early Paleozoic gcanilic and sedimentary rocks of Alexander
sequence.
Origin of and Tectonlc Conbok for
Western-Southeastem Alaska MetaIlagenlc Ben
The Westem-Southeastern Alaska metallogenic belt is hosted in the Jurassic(?) and Early to mid-Cretaceous grmitoid
plutons of the Muir-Chichagof plutanic belt which are interprekd by Plafker and others (198%) as part of the granitoid roots of
the Late Jurassic to EarJy and mid-Cretaceous Gravina arc. The U-REE deposits in the belt, as at Bokan Mountaio, may have
formed in extensional setting (Goldfarb, 1997), possibly in a back-arc environment. The Western-Southeastern Alaska
metallogenic belt is interpreted as a northern extension of the Island metallogenic belt to the south, and as the southeastern
extension of the Eastem-Southem Alaska metallogenic belt. All three metallogenic belts are hosted in the Late Jurassic through
mid-Cretaceous Gravina island arc which was built on the Wrangellia superterrane. The Gravina island an: is interpreted as
forming on the northern, or leading edge of the Wrangellia island-arc terrane during migration towards North America (Nokleberg
and others, 1984, 1985,2000; NoMeberg and Lange, 1985a; Plafker and others, 1989; Plafker and Berg, 1994). The Gravina arc
and associated granitic magmatism deposits are tectonicaHy linked to the youngerpart of the McHugh Complex wbich Forms the
northern part of the Chugach subduction zone and accretionary wedge complex (Nokleberg and others, 2000).