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

comprises up to 70-80% in some 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 skams
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% B20;; 0.3% Sn.
Chepak Granitoid-Related Au Deposit
The Chepak granitoid-related Au deposit (P.I. Skomyakov, written cornmun., 1951; V.I. Shpikerman and N.A.
Goryachev, written commun., 1995) consists of steeply dipping, quartz-sulfide veinlets, replacement veins, and associated
alteration zones which are both concordant with, and cut intensely contact-metamorphosed Late Triassic sandstone and shale
which 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-Early Cretaceous granite
porphyry, granodiorite porphyry, and dacite. Disseminated veinlets are also occur in the magmatic rocks and in homfels. The wall
rocks are silicified, chloritized, and sericitized. The veins are composed mainly of quartz (30-60%), sericite, feldspar, chlorite,
carbonate, apatite, arsenopyrite, loellingite, scheelite, pyrrhotite, and pyrite. Less common or rare are chalcopyrite, bismuth,
bismuthinite, marcasite, wolframite, magnetite, ilmenite, rutile, sphene, tourmaline, epidote, and fluorite. Arsenopyrite and
loellingite make up to 20 to 40% of the veins. Most gold is finely dispersed in arsenopyrite, Ioellingite, 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 which host the Darpir metallogenic belt are part of the Main part of the
Verkhoyansk collisional granite belt (fig. 61) which intrudes Paleozoic and early Mesozoic bedrock of the Kolyma-Omolon
superterrane and the adjacent North Asian Craton Margin (Nokleberg and others, 1994c, 1997~). 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, up to 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 which occurred immediately after the Late Jurassic
accretion of the Kolyma-Omolon superterrane to the North Asian Craton Margin (Nokleberg and others, l994c, 1997~).
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 west-central 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, 1997~). The deposits are hosted in altered Triassic limestone which is interlayered with sandstone
and shale. The major deposits in the belt are the Khi~nkhada Sn-W skam, Agylki W skam, 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 skam deposits occurs as scheelite which 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-
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, 1997~). The Main collisional
granite belt extends for about 1 10 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, up to 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, 1997~).