USGS Professional Paper 1697 - Alaska Resources Library

150 Metallogenesis and Tectonics of the Russian Far East, Alaska, and the Canadian Cordillera
ore bodies. The ore minerals also include ascharite, kotoite,
datolite, harkerite, monticellite, fluoborite, clinohumite, calcite,
periclase, forsterite, diopside, vesuvianite, brucite, garnet,
axinite, tourmaline, biotite, phlogopite, serpentine, spinel,
hornblende, pyroxene, feldspar, quartz, and magnetite. Sn
occurs as an isomorphous admixture in ludwigite. Ludwigite
is often replaced by sulfide minerals (pyrrhotite, sphalerite,
pyrite, arsenopyrite, and chalcopyrite). Kotoite veins occur
along the margins of ludwigite bodies. The contact between
the intrusion and carbonate is highly irregular. Most of the
skarns occur where the contact forms embayments (pockets)
into the intrusion. The deposit covers a 3 by 6 km area, and is
of medium to major size. The average grades are 9.5 percent
B 2O 3; 0.3 percent Sn.
Chepak Granitoid-Related Au Deposit
The Chepak granitoid-related Au deposit (P.I. Skornyakov,
written commun., 1951; V.I. Shpikerman and N.A.
Goryachev, written commun., 1995) consists of steeply dipping,
quartz-sulfide veinlets, replacement veins, and associated
alteration zones that are both concordant with and cut
intensely contact-metamorphosed Late Triassic sandstone
and shale that overlie a buried granitoid pluton. The Au ore
bodies occur in zones of northeast-trending veins. The host
rocks are intruded by dikes of diorite porphyry, lamprophyre,
and dolerite and by small intrusive bodies of Late Jurassic
to Early Cretaceous granite porphyry, granodiorite porphyry,
and dacite. Disseminated veinlets also occur in the magmatic
rocks and in hornfels. The wall rocks are silicified, chloritized,
and sericitized. The veins are composed mainly of
quartz (30 to 60 percent), sericite, feldspar, chlorite, carbonate,
apatite, arsenopyrite, löellingite, scheelite, pyrrhotite,
and pyrite. Less common or rare are chalcopyrite, bismuth,
bismuthinite, marcasite, wolframite, magnetite, ilmenite,
rutile, sphene, tourmaline, epidote, and fluorite. Arsenopyrite
and löellingite make as much as 20 to 40 percent of the
veins. Most gold is finely dispersed in arsenopyrite, löellingite,
and pyrrhotite. The deposit is medium size. The Au
content ranges from 5 to 50 g/t Au, with values as high as
200 g/t Au. Proven reserves are 30 tonnes Au with an average
grade of 7 to 8 g/t Au.
Origin of and Tectonic Controls for Darpir
Metallogenic Belt
The Early Cretaceous granitic intrusions that host the
Darpir metallogenic belt are part of the Main part of the
Verkhoyansk collisional granite belt (fig. 61) that intrudes
Paleozoic and early Mesozoic bedrock of the Kolyma-Omolon
superterrane and the adjacent North Asian Craton Margin
(Nokleberg and others, 1994c, 1997c). The Main part of the
collisional granite belt is of Late Jurassic to early Neocomian
age. The Main part of the granite belt occurs along southwest
border of the Kolyma-Omolon superterrane and stitches the
superterrane to North Asian Craton Margin (Verkhoyansk fold
belt, unit NSV). The Main part of the granite belt occurs as
inclined, sheet-like plutons, as much as 200 km long, which
are generally conformable with major folds. Younger differentiates
are biotite, two-mica, and amphibole-biotite granitoid
rocks. Ar-Ar ages of granitoid rocks range from 134 to 144
Ma. The Main part of the Verkhoyansk collisional (anatectic)
granitic belt and associated Darpir metallogenic belt are
interpreted as forming during a period of anatectic granitic
magmatism that occurred immediately after the Late Jurassic
accretion of the Kolyma-Omolon superterrane to the North
Asian Craton Margin (Nokleberg and others, 1994c, 1997c).
Tompon Metallogenic Belt of Cu, W, Sn
Skarn, and Sn Quartz Vein Deposits (Belt TO),
West-Central Part of Eastern Siberia
The small Tompon metallogenic belt of Cu, W, and
Sn skarn deposits (fig. 61; tables 3, 4) occurs in the westcentral
part of eastern Siberia. The belt extends for about
150 km in the North Asian Craton margin (Verkhoyansk
fold belt, unit NSV; Nokleberg and others, 1994c, 1997c).
The deposits are hosted in altered Triassic limestone that is
interlayered with sandstone and shale. The major deposits
in the belt are the Khunkhada Sn-W skarn, Agylki W skarn,
and Erikag Sn quartz vein deposits (table 4) (Nokleberg and
others 1997a,b, 1998). The deposits generally occur above
the apical portions of unexposed granitoid intrusions. W
in the skarn deposits occurs as scheelite that is associated
with chalcopyrite. The deposits contain anomalous Bi. The
deposits are of small to medium size and are not economic.
Older K-Ar isotopic studies yield an Early Cretaceous age
of 125 to 130 Ma for the associated granitoid rocks. Newer
Ar-Ar isotopic ages of granitoid rocks range from 134 to
144 Ma (Layer and others, 1995).
The lode deposits of the Tompon metallogenic belt are
associated with intrusion of the Main part (Late Jurassic to
early Neocomian) of the Verkhoyansk collisional granite belt
(unit vk; Nokleberg and others, 1994c, 1997c). The Main collisional
granite belt extends for about 110 km along southwest
border of the Kolyma-Omolon superterrane and stitches the
superterrane to North Asian Craton margin. The granites in the
belt occur as inclined, sheet-like plutons, as much as 200 km
long, which are generally conformable with major folds. These
granitoid rocks are interpreted as forming immediately after
the Late Jurassic accretion of the Kolyma-Omolon superterrane
to the North Asian Craton Margin (Nokleberg and others,
1994c, 1997c).
Shamanikha Metallogenic Belt of Au Quartz
Vein and Cu-Ag Quartz Vein Deposits (Belt SH),
Central Part of the Russian Northeast
The Shamanikha metallogenic belt of Au quartz and
Cu-Ag quartz vein deposits (fig. 61; tables 3, 4) occurs in the
Shamanikha River Basin in the west-central part of the Rus-