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

Lost River Sn-W Skarn and Sn Greisen Deposif
The classic Lost River Sn-W skarn and Sn greisen deposit (fig. 107) (Dobson, 1982; Hudson and Arth, 1%3; Reed, . -Y
Menzie, and others, 1989; Newbeny and others, 1997) consists of several prospects and one mine in veins, skarns, greisens, and
intrusion breccia formed above a shallow Late Cretaceous granite stock that intruded thick sequence of Early Ordovician
limestone and argillaceous limestone. Early-stage andradite-idoerase skarn and later fluorite-magnetite-idocrase vein skanns are
altered to chlorite-carbonate assemblages which are conlanporaneous with fonnatlon of cassiterite-bearing Sn greisen. The major
ore minerals in skarn and greisen are cassiterite and wol~te, and lesser stamite, galena, sphalerite, pyrite, chalcopyrite,
arsenopyrite, and molybdenite, plus a wide variety of other oonlect metasomatic and alteration minerals, The K-Ar isotopoic age
of the granite is 80.2 Ma. Production of 320 tomes Sn occurred mostly Gom 1951 to 1956 from underground workings a few
hundred meters deep along the Cassiterite dike, a near-vertical rhyolite dike which is utlensively altered to greisea over the buried
granite. Similar smaller deposits nearby incLu.de Sn greisen and veins near the Tin Creek Granite md various polyme=tallic veins
and skams near the Brooks Mountain Granite. A large beryUium &posit occurs peripheral to the skams and consists of Limestone
replaced by fluorite-white mica veins which cwkm diaspore, cbrysokryl. and tourmaline. The Be deposit is probably associated
with early stages of granite intrusion. Some plaeer Sn was recovered from creek below Lost River mine. A major exploratioa
program in early 1970's drilled several large Sn-W-fluorite-Bt are bodies. The dcpasit contains estimated reserves of 25 million
tonnes grading 0.15% Sn, 0.03% WO,, 16.3% CaF2 (WGM, Inc., written commun., 1975).
Felsic Plutonic U and Sandstone U deposHs
A complex, multi-phase, fe1si.c plutonic U depoait occurs at Eagle Creek and a sandstone U deposit occurs at Death
Valley, both in the eastern part of the S e d Peninsula. The felsic plutonic deposit consists of disseminated U-, Th-, and REEminerals,
which occur along tfbe margins of alkaline dikes hdod hto a Cretaceous granite pluton aod adjacent wall mb
(Miller, 1976; Miller and Bunker. 1976). The DeaU ValJey sandstone U deposit consists mainly of metaautunite in Paleocene
sandstone along the margin of a Tertiary sedimentary basin (Dickinson and Cunmghn, 1984; Dickinson and others, 1987). The
U in the sandstone probably was transporld by groundwater hrn Cretaceous granitoid plutons to the west (Dichson and
Cunningham, 1984).
Origin of and Tectonic Controk for
Seward Peninsula Metallogenic Belt
The felsic-magmatism-related deposits of Seward Peninsuh metdogcnic belt generally occur in, or adjacent to, moderate
or highly sil~clc granites of latest Cretacaous ape (Hubon and Arth, 1983; Nokleberg and others, 1995a). 'The porphyry Mo, felsic
plutonic U, and polymetallic vein deposits occur in shghtly older and slightly less silic~ous granites, whereas the Sn granite and
associated deposits occur in slightly younger and more siiicic deposits. The granites associated with both groups of deposits are
interpreted as forming during assimilation of the continental, Late Proterozoic and early and middle Paleozoic metmtsedimentary
rocks of the Seward metamorphosed continental margin &mane (Hudson and Mh, 1983; Swanson and OM, 1988).
Alternatively, on the basis of trace element data, the Sn granite and related deposits are interpreted as forming during crwtal
extension (Newbeny and others, 1997b).
On the basis of similar geochemistry, age, and spatial proximity, the host Late Cretaceous granitic rocks oa he Sewad
Peninsular are interpreted as part of the eastern edge of the Qkhotsk-Chukotka volcanic-plutonic belt which horn the Seward
Peninsula metallogenic belt extends for 3000 km along western margin of Sea of Okhotsk (Nokleberg and others, 2000). The
Seward Peninsula metallogenic belt and the nearby Northwestern Koyukuk Basin metallogenic belt of felsic plutonic U deposits,
described below, are interpreted herein as the eastern extension of the Eastern-Asian-Arctic metallogenic belt in the Russian Far
East (figs. 102, 103). The Seward Peninsula metallogenic belt with Sn granite and related deposits, is correlated with the Chsun
zone of the Eastern Asia metallogenic belt which contains similar deposits to the west in the Russian Northeast (fig. 102).
Northwestern Koyukuk Badn M-llogenk Belt of
Felsic Plutonic U and Manto-Replacement
(Polymetallic Pn-Zn, Au) Deposits (Belt NWK)
West-Central Alaska
The Northweslun Koyukuk Basin metallogenic belt of fefelsic plutonic U and manto-replacemmt @olymetdlic Pn-a Au)
deposits (fig. 103, tables 3,4) occurs in the Purcell district and adjacent area in the northwestern Koyukuk Basin in westanhal
Alaska. The metallogenic belt is hosted in the part of tbe region underlain by the Late Cretaceous Hog;atza plutonic belt (Miller,
1994). The significant f&ic plutonic U deposits in the belt arc? at Clear C~ek Wheeler Creek, aod h e klills (table 4) (Miller and
Elliott, 1969; Miller, 1976; Jones, 1977; Nokleberg and others 1997a, b, 1998). These felsic plutonic U deposits and bast plutonic
rocks are interpreted as the extreme northeastern end of the Okbolsk-Chukotka volcani~plutonic belt of the Russian Far hat
(Nokleberg and others, 1994c, 1997c, 2000). Also occwTing in the metallogenic belt is a plymetallic (Au-Pb-Zn) vein and manto
replacements(?) in the Illinois Creek area which produced 957 kg Au &om 1997 to 1999, and is isotopically dated at 1 I1 Ma,
about the same age as the nearby Khotol pluton (Flanigan, 1998).
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