USGS Professional Paper 1697 - Alaska Resources Library

chalcocite and covellite and also have minor enargite, bornite,
chalcopyrite, luzonite, and pyrite. Tennantite, sphalerite, and
galena are extremely rare. Local surface oxidation occurs with
alteration of sulfides to malachite and azurite. Sulfide minerals
occur mainly as large, irregular, massive, wedge-shaped
bodies, mainly in dolomitic parts of the Late Triassic Chitistone
or Nizina Limestone. The bodies are generally less than
100 m above a disconformity over the subjacent Middle and
(or) Late Triassic Nikolai Greenstone. The largest ore body
(Jumbo) consists of an almost pure chalcocite and covellite
mass, which is about 110 m high, as much as 18.5 m wide, and
extends 460 m along plunge. The Kennecott Cu mines were
among the largest group of mines in Alaska from 1911 until
1938, when the ore was mostly exhausted. The district contains
more than 96 km of underground workings. The major
mines in district are the Jumbo, Bonanza, Erie, Mother Lode,
and Green Butte. The district produced about 544 million kg
Cu and 280 million g Ag from 4.3 million tonnes of ore.
Origin of and Tectonic Controls for Wrangell Mountains
Metallogenic Belt
Throughout the Nikolai Greenstone and older rocks in the
Wrangellia terrane, quartz veins, which locally contain abundant
Cu sulfides, grade into clots of quartz, chlorite, actinolite,
and epidote (Nokleberg and others, 1985, 1994d). This relation
suggests that the Cu-Ag quartz vein deposits formed during
a period of lower greenschist facies regional metamorphism.
The age of metamorphism is interpreted as mid-Cretaceous
because Early Cretaceous and older units of the Wrangellia
sequence were affected. The formation of the Cu-Ag
quartz vein deposits and associated regional metamorphism
are herein interpreted as occurring during mid-Cretaceous
accretion and deformation of the Wrangellia superterrane to
the active margin North American Cordillera (Nokleberg and
others, 1985, 2000; Plafker and others, 1989).
The Kennecott Cu deposits are interpreted by Armstrong
and MacKevett (1982) and MacKevett and others (1997) as
forming by derivation of Cu from the Nikolai Greenstone, followed
by deposition from oxygenated groundwater in the lower
part of the overlying Chitistone Limestone along dolomitic sabkha
interfaces and as open-space fillings in fossil karsts. Armstrong
and MacKevett (1982) interpreted the age of deposition
as Late Triassic, but did not rule out possible later remobilization.
Subsequently, MacKevett and others (1997) interpreted the
hydrothermal event associated with formation of the Kennecott
Cu and Cu-Ag quartz vein deposits in the Wrangell Mountains
metallogenic belt as forming during nearby mid-Cretaceous
magmatism. In contrast, Nokleberg and others (1985, 1994d)
and Goldfarb (1997) suggested that lower greenschist facies
regional metamorphism of the Nikolai Greenstone, associated
with mid-Cretaceous accretion of the Wrangellia superterrane,
may have been the source of hydrothermal fluids that either
deposited or further concentrated the Cu-sulfides in the Kennecott
Cu deposits in the Kennicott district and in associated
Cu-Ag quartz vein deposits in southern Alaska.
Late Cretaceous and Early Tertiary Metallogenic Belts (84 to 52 Ma) (figs. 102, 103) 215
Late Cretaceous and Early Tertiary
Metallogenic Belts (84 to 52 Ma)
(figs. 102, 103)
Overview
The major Late Cretaceous and early Tertiary metallogenic
belts in the Russian Far East, Alaska, and the Canadian
Cordillera are summarized in table 3 and portrayed on figures
102 and 103. The major belts are as follows: (1) In the Russian
Southeast, continuing on from the early Late Cretaceous,
were the Kema (KM), Lower Amur (LA), Luzhkinsky (LZ),
Sergeevka (SG), and Taukha (TK) belts, which contain a large
array of granitic-magmatism-related deposits. The belts are
hosted in or near the East Sikhote-Aline volcanic-plutonic
belt and are interpreted as forming during subduction-related
granitic plutonism that formed the East Sikhote-Aline continental-margin
arc. (2) In the Russian Northeast, continuing
from the early Late Cretaceous are several zones of the Eastern
Asia belt, along with formation of several new zones. The
zones, which are all hosted in or near the Okhotsk-Chukotka
volcanic-plutonic belt, include the Adycha-Taryn (AT), Chaun
(CN), Chukotka (CH), Korkodon-Nayakhan (KN), Okhotsk
(OH), Omsukchan (OM), and Verkhne-Kolyma (VK),
Verkhne-Yudomsky (VY), and Verkhoyansk-Indigirka (VI)
zones. Together, these zones and belts of grantic-magmatismrelated
deposits are interpreted as forming during subductionrelated
granitic plutonism that formed the Okhotsk-Chukotka
continental-margin arc. (3) Also in the Russian Northeast,
continuing on from the early Late Cretaceous were the Koryak
Highlands (KH) belt, which contains zoned mafic-ultramafic
PGE and Cu massive sulfide deposits, and the Vatyn (VT) belt
that contains volcanogenic Mn and Fe deposits. Both belts
are hosted in the Olyutorka-Kamchatka island-arc terrane
and are interpreted as forming in different parts of the Olyutorka
island arc. (4) Also in the Russian Northeast was the
Iruneiskiy (IR) metallogenic belt of porphyry Cu deposits that
also formed in the Olyutorka island arc. (5) In Northwestern
Alaska, the Northwestern Koyukuk Basin (NWK), Seward
Peninsula (SP), and West-Central Alaska (WCA) belts, which
are hosted in the Alaska extension of the Okhotsk-Chukotka
volcanic-plutonic belt, are also interpreted as forming during
subduction-related granitic plutonism that formed the
Okhotsk-Chukotka continental-margin arc. (6) In southern
Alaska, the East-Central Alaska (younger part; ECA), Southern
Alaska (SA), and Kuskokwim Mountains (KM) belts,
which are hosted in the Kuskokwim Mountains sedimentary
and volcanic belt or the Alaska Range-Talkeetna Mountains
igneous belt, are interpreted as forming during subductionrelated
granitic plutonism that formed the Kluane continentalmargin
arc. (7) In Southern and southeastern Alaska, several
belts are interpreted as forming during oblique subduction of
the Kula-Farallon oceanic ridge under margin of Southern and
southeastern Alaska. In alphabetical order, these belts are the