USGS Professional Paper 1697 - Alaska Resources Library

158 Metallogenesis and Tectonics of the Russian Far East, Alaska, and the Canadian Cordillera
deposits are hosted in or lie adjacent to early Paleozoic marble
and pelitic metasedimentary rocks of the Wales Group of
the Alexander sequence where intruded by mid-Cretaceous
hornblende-biotite granodiorite that exhibits concordant hornblende
and biotite K-Ar isotopic ages of 103 Ma. The Jumbo
deposit consists of chalcopyrite, magnetite, sphalerite, and
molybdenite in skarn that occurs at the contact between marble
and a granodiorite stock. Gangue is mainly diopside and
garnet. The deposit has been explored by more than 3.2 km
of underground workings and is the largest in the district. The
Magnetite Cliff deposit consists of a 25-m-thick shell of magnetite
that mantles granodiorite in contact with garnet-diopside
skarn. The skarn contains 2 percent to 3 percent chalcopyrite,
and estimated resources of 335,600 tonnes grading 46 percent
Fe and 0.77 percent Cu. The Copper Mountain deposit
consists of scattered chalcopyrite and copper carbonate in
diopside endoskarn that contains veins and masses of epidote,
garnet, magnetite, and scapolite near granodiorite. This deposit
has produced 101,800 kg Cu, 321,300 g Ag, and 4,510 g Au,
and has about 410 m of tunnels and shafts.
Bokan Mountain Felsic plutonic U-REE deposit
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
that 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,
1997). The deposit produced about 109,000 tonnes grading
about 1 percent U3O8; Th was not recovered. Most of ore was
produced from a crudely cigar-shaped upper part of the pluton.
The central zone that contains the deposit grades outward into
normal granite. Associated pegmatite and vein REE, Nb, Th,
and U deposits occur in the outer parts of granite or adjacent
country rock, which consists of metamorphosed early Paleozoic
granitic and sedimentary rocks of Alexander sequence.
Origin of and Tectonic Controls for Western-Southeastern
Alaska Metallogenic Belt
The western-southeastern Alaska metallogenic belt is
hosted in the Jurassic(?) and Early to mid-Cretaceous granitoid
plutons of the Muir-Chichagof plutonic belt, which are interpreted
by Plafker and others (1989b) as part of the granitoid
roots of the Late Jurassic to Early and mid-Cretaceous Gravina
arc. The U-REE deposits in the belt, as at Bokan Mountain,
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 Eastern-Southern Alaska metallogenic
belt. All three metallogenic belts are hosted in the Late
Jurassic through mid-Cretaceous Gravina island arc that was
built on the Wrangellia superterrane. The Gravina island arc 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; Nokleberg
and Lange, 1985a; Plafker and others, 1989; Plafker and
Berg, 1994). The Gravina arc and associated granitic magmatism
deposits are tectonically linked to the younger part of
the McHugh Complex, which forms the northern part of the
Chugach subduction zone and accretionary-wedge complex
(Nokleberg and others, 2000).
Britannia Metallogenic Belt of Kuroko Cu-Zn
Massive Sulfide Deposits, Southern British
Columbia (Belt BR)
The Britannia metallogenic belt of kuroko Cu-Zn massive
sulfide deposits is hosted in the Late Jurassic to Early
Cretaceous Gravina-Nutzotin-Gambier volcanic-plutonicsedimentary
belt, which forms an overlap assemblage on the
inward margin of Wrangellia superterrane (fig. 63; tables 3,
4) (Nokleberg and others, 1997b, 1998, 2000). The significant
deposit is at Britannia. The metallogenic belt also includes the
Maggie, Northair, and Nifty deposits.
Britannia Kuroko Volcanogenic Cu-Zn Massive
Sulfide Deposit
The Britannia deposit consists of a series of kuroko Cu-
Zn (Ag-Au) ore bodies hosted a roof pendant of dacite tuff and
breccia. The massive sulfide bodies consists of massive pyrite,
chalcopyrite, sphalerite with minor galena, tennantite, tetrahedrite,
barite and fluorite that occur in numerous discrete,
concentrically zoned siliceous ore bodies (EMR Canada,
1989; Dawson and others, 1991; MINFILE, 2002). A U-Pb
zircon age for a feldspar porphyry dike that cuts the deposit
indicates a Middle Jurassic or older age for the deposit (Ray
and Webster, 1994). The host rocks are metamorphosed dacite
to andesite pyroclastic rocks. The massive sulfides consist
mainly of stringers that occur in the upper of two major mafic
to felsic metavolcanic layers, which are separated by and overlain
by metasedimentary rock. From 1905 to 1974, the mine
at the deposit produced 47,884,558 tonnes of ore from which
15.3 tonnes Au, 180.8 tonnes Ag, 517 tonnes Cu, 15.6 tonnes
Pb, and 125.3 tonnes Zn were recovered. The nearby Northair
deposits near Whistler are interpreted as volcanogenic massive
sulfide deposits remobilized during emplacement of adjacent
plutons (Miller and Sinclair, 1985). The Early Cretaceous
metavolcanic host rocks form pendants and screens within
granodiorite of the Coast Plutonic Complex. The deposit and
host rocks are thinned and partially remobilized by the northwest-trending
Britannia shear zone (Payne and others, 1980).
Origin of and Tectonic Controls for Britannia
Metallogenic Belt.
The Britannia metallogenic belt is hosted in the Middle
and Late Jurassic and Early Cretaceous sedimentary and calcalkaline
volcanic rock of the Gambier Group that is intruded