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

Au veln,and hots spring deposits at Red Devil, Kaiyah, Kagati Lake, Kolmakof, Snow Gulch-Donlin, Cinnabar Creek,
DeCourcey, and Gemuk Mountain; and felsic-plutonic U prospects at Wolf Creek Mountain andSicbu Creek.
The lode deposits in the occur in veins, stockworks, breccia plpes, skarns and replacement deposits which formed in
upper mesothermal to epithermal environments (Bundtzen and MUer, 1991; 1997; Gray and others, 1997). A plausibIe
metallogenic model suggests which most of tbese deposits are similar vertically-zoned hydrothermal systems which are exposed at
various erosional levels within the late Cretaceous and early Tertiary igneous complexes and wall rocks. Selected deposits and
environments include: (1) the Cirque, Tolstoi, Headwall, Mission Creek, Win, Won, and Bismarck Creek, greisen Sn-Cu-Ag-As
deposits (formed in upper mesothermal environment); (2) the Golden Horn, Chicken Mountain, and Von Frank porphyry Cu-Au
deposits (formed in lower mesothermal envirohment); and (3) the Red Devil, DeCoursey, Donlin, and Mountain Top Hg-Sb (Au)
deposits (epithermal deposits). The important Donlin Creek and Vinasale gold-polymetallic depasits resemble porphyry Au
deposits (Wilson and Keyser, 1988; Hollisier, 19921, but bck metallic, fluid inclusion, and wallrack alteration more typical of
porphyry Au systems (Bundtzen a d Miller. 1997; Ebert and others, 2000). Unusual, U-rich, REE deposits occur in ultra-potassic
felsic igneous rocks at Sischu Momtab and WoUCreek Mountain (Patton and othen, 1994; Bundtzw, 1998). The various Aupolymetallic
and other mineral deposhs in the are interpreted as forming in a distinctly higher, stFUchlFEil level thaa the deeper,
mesothennal systems in the East-Central Alaska metallogenic belt described below. Selected examples of important &posits in the
Kuskokwim Mineral Belt are described below.
The Southwestern Kuskokwirn Mountians metallogenic belt is hosted in the Kuskokwim Mountains sedimentary and
volcanic belt that consists of three major types of igneous complexes: (1) calc-alkalic, metaalumiwus granite and quartz
monzonite of Early and mid-cretaceous age (about 109 to 98 Ma); (2) peraluminous to rnetaaluminous, alkali-cakic to quartz
alkalic, volcanic-plutonic complexes which contain plutons ranging in composition from alkali gabbro to qwvtz syedte and
average about 70 Ma; and (3) peduminous granite porphyry sills and dikes which range from 48 to 71) Ma The various units of
the Kuskokwim Mountains sedimentary and volcanic belt mtrude and overlie several major bixhck units, including the
Proterozoic to Paleozoic Ruby terrane, the Paleozoic Nixon Fork terraae, tbe Late Pale~zoi~Uesozok lnnoko m e , the Triassic
to Early Cretaceous Gemuk Group, the Ordovician to Jurassic Goodnews terrane, the Late Cretaceous K~bkwirn Group, and
other basement rocks (Moll-Stalcup, 1994; Budtzen and Miller, 1-7).
Kuskokwim Mountains Sedimentary
and Volcanic Belt
The Kuskokwim Mountains sedimentary and volcanic belt which hosts the Kuskolswim metallogenic belt consists of
interlayered volcanic and sedimentary rocks which are intruded by coeval plutonic rocks. Geochemical and isotopic studies of the
igneous rocks of the belt reveal two types of igaaourr complexes (Bundtzea and Miller, 1997): (1) peraluminous to metaluminous,
alkall-calcic to quartz alkalic a d lesser calc-alkalic, volcanic-plutonic complexes composed of plutons rangiag in composition
from alkali gabbro, quartz djorite, gxaodiorite, monzonite, to syenite; and (2) peralurninous granite porphyry sills and dikes..
These two suites exhibit K-Ar crystallization ages ranging from 75 to 60 Ma (Late Cretaceous and early Tertiary).
The volcanic suite consists chiefly of rhyolite and &cite domes, flows, md tuff, and dndcite, andesite, and basalt flows
which exhibit K-Ar isotopic ages of 58 to 77 Ma (Bundtzen and Gilbert, 1983; Miller and Bundtzen, 1994; Moll-Stalcup, 1994;
Moll-Stalcup and others, 1994). They display moderate-K, calc-alkalic to shoshonitic composilional trends with andmite and
rhyolite being most common. REE patterns are variable, initial Sr ratios vary &om 0.704 to 0.708, and ggce element studies
suggest assimilation of small amounts of continental crust of the metamorphosed continental margin Rutzy terrane (Miller, 1994;
Moll-Stalcup, 1994; Moll-Stalcup and others, 1994). An unusually large range of incompatible cknmts occur in the igneous
rocks and their significance is not well understood (Moll-Stalcup, 1994).
Sedimentary rocks underlying the Kuskokwim Mountains igneous belt include the mid- and Eate Kuskokwim Ormp
(Cady and others, 1 955) and Cretdlceuus flysch of the Yukon-Koykuk basin (Patton and others, 1994). The Kuskokwim Group
consists mainly of conglomerate to coarse-grained sandstone tmbidites deposited in deep-marine conditions, and lesser sandstones
and conglomerates deposited in shallow-marine to nonrnarine conditions. The Kudsokwim Mountains igneous belt d the
Kuskokwim Group are generally mildly fokled and faulted, and, to tbe south, are interpreted to overlie the Wlna sedimentary
and volcanic assemblage (fig. 103). To the east. Ihe igneous belt locally overlies the Wmgellia supertermne. Bundtaed and Miller
(1997) and Patton and others (1994) provide more detailed geologic frameworks for both of these areas.
Origin of and Tectonic Setting for
Kuskokwim Mountains Metallogenic Belt
The Kuskokwim Mountains metallogenic beIt is hosted by the Kuskokwim Mountains sedimentary and vdcanic belt
which is interpreted as the back-arc part of the extensive, subduction-related Kluane igneous arc which occurred along the Late
Cretaceous and early Tertiary continenbal margin of southern and southeastern Alaska (Moll and Patton, 1982; Bundtzen and
Gilbert; 1983; Swanson and others, 1987; Ptafker and others, 1989; SzumigaIa, 1993; Miller and Bundtzen, 1994; Moll-Stalcup,
1994; Nokleberg and others, 19951; Bundlzen and Miller, 1997). Supporting data for this interpretation include: (1) the alkali-
calcic nature of the igneous rocks in the Kuskokwirn Mountains igneous belt which are more alkalic than the coeval Alaska
Range-Talkeetna Mountains igneous belt; (2) trans-tensional tectonim associated with the Kuskokwh Mountains igneous belt;