USGS Professional Paper 1697 - Alaska Resources Library

Demin, and Krasilnikov, 1974; Nekrasova and Demin, 1977;
Sidorov, 1978; Layer, Ivanov, and Bundtzen, 1994) consists of
numerous adularia-quartz and adularia-carbonate-quartz veins
more than 200 m long and more than 0.2 m thick. The veins
are controlled by arcuate and linear faults that define and
crosscut a caldera filled with Late Cretaceous dacite, andesite-basalt,
and rhyolite. The Main deposit, which contains
about 80 to 90 percent of the reserves, is confined to few
major veins that are spatially related to a hypabyssal body cut
by circular faults and composed of andesite, andesitic dacite,
volcanic breccia of andesite-dacite composition, and rhyolite.
The most productive veins are associated with an altered zone
comprised of adularia-hydromica and quartz and explosion
and hydrothermal breccia bodies. A zone of kaolinite, alunite,
and quartz alteration occurs in higher parts of the ore deposit.
The ore minerals are pyrite, sphalerite, chalcopyrite, canfieldite,
freibergite, tennantite, naumannite (Ag 2Se), polybasite,
electrum, küstelite, native silver, and other less common
sulfides, selenides, sulfostannates, and sulfosalts of silver.
The Au:Ag ratio is 1:3 to 1:4 in the richest portions of the
Glavnaya vein. The veins form in clusters, which converge
at depth. The gold-canfieldite-freibergite-chalcopyrite and
gold-pyrite-sphalerite zones are the most productive; at depth
they are succeeded by a galena-canfieldite zone with tin-silver
minerals. The deposit is medium and is mostly mined.
The deposit was discovered in 1964 and produced 40 tonnes
Au from 1978 to 1992. The average grade was 100 to 129
g/t Au in 1978 and 16 to 18 g/t Au in 1992. An 40 Ar- 39 Ar age
isotopic study of adularia in Au-Ag vein yields an age of 79
Ma (Layer, Ivanov, and Bundtzen, 1994).
Julietta Au-Ag Epithermal Vein Deposit
The Julietta Au-Ag epithermal vein deposit (S.F. Struzhkov,
O.B. Ryzhov, and V.V. Aristov, written commun.,
1994; Struzhkov and others, 1994) consists of Au-Ag sulfidecarbonate-quartz
veins that occur inside a large Early Cretaceous
caldera. The volcanic rocks in the caldera and associated
subvolcanic intrusive rocks are comprised of andesite,
andesite-basalt, and andesite-dacite. The veins dip steeply
and vary from 200 to 500 m length and from 1 to 4 m wide.
The major ore minerals are native gold and silver, freibergite,
polybasite, galena, sphalerite, chalcopyrite, hessite, acantite,
cubanite, pyrrhotite, and naumannite. Associated minerals in
adjacent metasomatically altered volcanic rocks are ankerite,
calcite, chlorite, epidote, and hydromica. The ore minerals
formed in two stages, an older gold-polymetallic stage, and
a younger Au-Ag-sulphosalt stage. The deposit is medium
size with an average grade of 29 g/t Au, 325 g/t Ag. Proven
reserves as of 1994 were 18 tonnes Au and 200 tonnes Ag.
Estimated resources are 40 tonnes Au and about 1,000 tonnes
Ag. A Rb-Sr adularia isotopic age of 136 Ma (Struzhkov and
others, 1994) suggests that the Julietta deposit and host rocks
may be part of an undefined preaccretionary metallogenic
belt that would be hosted in the early and middle Mesozoic
Kony-Murgal island-arc terrane. Additional isotopic studies
Early Late Cretaceous Metallogenic Belts (100 to 84 Ma; figs. 79, 80) 199
are needed. The deposit is currently being developed by Bema
Gold Corporation and other partners.
Agat Au-Ag Epithermal Vein Deposit
The Agat Au-Ag epithermal vein deposit (V.I. Naborodin,
written commun., 1971, 1977) consists of several tens of
quartz, carbonate-quartz, and sulfide-quartz veins that occur in
sheets of propylitically-altered Cretaceous andesite. The veins
are generally simple and are controlled by northwest- to northsouth-trending
fissures. The ore bodies usually range from tens
to hundreds of meters long, but sometimes are as much as 2
km long; they average from 0.2 to 1 m thick, but some are 50
m thick. The veins are usually altered to hydromica, chlorite,
and silica; however, less eroded veins display weak adularia
alteration. The veins generally display a symmetrical crustification
banding and complex deformation structures. The main
ore minerals are galena, sphalerite, chalcopyrite, marcasite,
and pyrite. Locally present are arsenopyrite, pyrrhotite, tetrahedrite-tennantite,
tellurides, Ag-sulfosalts, and other minerals.
The main gangue minerals are quartz and carbonates, including
calcite, dolomite, siderite, Mn-rich siderite, rhodochrosite,
and kutnahorite. Barite, chalcedony, and opal occur near the
periphery of the deposit. Gold occurs as electrum. The veins
average 5 to 10 percent sulfide, but locally are as much as 20
to 30 percent sulfide. A gold-sphalerite-galena-quartz assemblage
is the most productive and is present in most veins. This
assemblage also contains chalcopyrite, tetrahedrite-tennantite,
Au- and Ag-tellurides, pyrargyrite, stephanite, and argentite.
The Au:Ag ratio varies from 5:1 to 1:100, and averages about
1:2 to 1:5. The deposit contains estimated reserves of 3.8
tonnes Au and 70 tonnes Ag. The deposit averages 6.5 to 11.8
g/t Au and 65 to 174 g/t Ag. Bonanza ores contain as much as
30 kg/t Au.
Eastern Asia-Arctic Metallogenic Belt:
Koni-Yablon Zone of Porphyry Cu-Mo and Cu-Mo
Skarn Deposits (Belt EAKY), Southern Part of
Russian Northeast
The Koni-Yablon zone of porphyry Cu-Mo and Cu-Mo
skarn deposits (fig. 79; tables 3, 4) occurs in the southern part
of the Russian Northeast. The zone occurs in the front of the
Okhotsk-Chukotka volcanic-plutonic belt along the northwest
margin of the Sea of Okhotsk and in plutons that intrude the
Koni-Murgal island-arc terrane. The significant deposits in
the zone are (table 4) (Nokleberg and others 1997a,b, 1998)
(1) porphyry Mo deposits at Guan-Ti (Arkhimed), Khakandya,
Molybdenitovy, Tikas, and Travka, (2) porphyry Cu-Mo
deposits at Etandzha, Gora Krassnaya, Ikrimun, Osennee,
Oksa, Usinskoe, Viking, (3) porphyry Cu deposits at Lora,
Nakhtandjin, and Yapon, (4) a granitoid-related Au deposit at
Tsirkovy, (5) Au-Ag epithermal vein deposits at Berezovogor,
Irgunei, Nyavlenga, Sergeev, and Serovskoe, (6) a Cu skarn
deposit at Maly Komui, (7) a Mo greisen and vein deposit at