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

198 1). The mine at the deposit produced an estimated 622,000 g ALL, and 93,300 g Ag The host rocks are an altered Mesozoic
diorite pluton and an amphibolite mass within the pluton. The pluton intrudes late Paleozoic low-grade pelitic and intermediate
volcanic rocks. Minor sulfides occur in the altered diorite wall rocks. The deposit also contains disseminations, veidets, and small
masses of scheelite. The vein system is symmetrical around a ve~lical fault which bisects the deposit.
Origin of and Tectonic Controls for
Baranof Metallogenic Belt
The Au quartz vein deposits of the Baranof melallogenic belt occur mainly in the Late Cretaceous flysch of the Sitka
Graywacke @art of the Valdez Group) where these units are metamorphosed to greenschist facies. The Au-quartz vein deposits
also occur in early Tertiary granitoid plutoas (with isotopic ages of about 5 1 to 52 Ma) which inhvde the Chugach terrane;
hydrothermal muscovite from Au-bearing veins has been dated at about 52 Ma (Taylor and others, 1994). As described above for
the origin of the Chugach Mountains metallogenic belt, tbeAu quartz veins of the Baranof metallogenic belt are interpreted as
forming in response to subduction of the spreading Kula-Farallon Ridge beneath the southern Alaska continental margin (Plafker
and others, 1989; Bradley and others, 1993; Haeussler and Nelson, 1993; Haeussler and others, 1995; Goldfarb and others, 1995;
1997; Goldfarb, 1997).
Juneau Metallogenic Belt of
Au Quartz Vein Deposits (Belt JU)
Southeastern Alaska
The Juneau metallogenic belt (also refmed to as the Juneau gold belt) of Au quartz vein deposits (fig. 103; tables 3,4)
(Twenhofel and others, 1949; Twenhofel, 1952; Goldfarb and others, 1997, 1998) occurs in the Coast Mountains of Southeastern
Alaska. The belt, which was first defined by Spencer (1906) and redefined by Brew (1993), occurs in two areas. The northan part
of the belt contains significant deposits at Alaska Juneau, Jualin, Riverside, Sumdum, and Treadwell. The southern part of the belt
contains smaller deposits.The belt occurs along strike for about 250 km and is hosted in the Yukon-Tanana and Stiba terranes,
the Gravina-Nutzotin Gambier overlap assemblage, and younger, early Tertiary granitoid plutonic rocks. The significant deposits
in the belt are at Alaska-Juneau, Gold Standard (Helm Bay), Goldstream, Jualin, Kensington, Riverside, Sea Level, Swndum Chief,
and Treadwell (table 4) (Nokleberg aad others 1997a, b, 1998). Most of he deposits occur in the western, greenschist hies part
of a belt of inverted, regional-grade metamorphic that occurs to the west of, and underneath of a extensive, foliated tonalite sill
which intruded along the western edge of the Yukon-Tanana teme and the Wrangellia superterrane (Brew, 1994; Gehrels and
Berg, 1994).
Alaska-Juneau Au Quartz Vein Deposit
The Au-quartz vein deposit consists of a network of lenticular quarlz veins a few centimeters to 1 rn thick that contain
sparse scattered masses of gold, pyrite, pyrrhotite, arsenopyrite, galena, with minor sphalerite, chalcopyrite, and silver (Goldfarb
and others, 1986, 1988, 1991, 1997; Newbeny and Brew, 1987,1988; Light and others, 1989; Brew and others, 1991; Miller and
others, 1992). The Alaska-Juneau mine produced 108 t Au, 59 t Ag, and 2 1,800 t Cu from 80.3 miIlion toms of ore mined
between 1893 and 1944. Reserves of 61.6 million tonne$ grading 1.8 g/t Au remain (Alaska Mineral Industry, 1993, p. l3).The
vein lode system is about 5.6 km long and 600 m wide. The deposit consists of a series of parallel quartz stringers that are hosted
in several units in: (1) phyllite and schist near the contact between the Late Triassic Perseverance Slate; (2) mphibolite derived
from late(?) Mesozoic (meta)gabbro dikes and sills; and (3) the informally named Gastineau volcanics of Permian and (or) Late
Triassic age. Most of ore occurs in quartz veins; some in adjacent altered metamorphic rocks.
Jualin Au Quartz Vein Deposit
The Jualin Au-quartz vein deposit consists of four or five major quartz fissure veins and pipe-like stockworks which
contain minor gold, and considerable pyrite, chalcopyrite, galena, minor sphalerite, and secondary copper minerals (Bundtzen and
others, 1990; Goldfarb and others, 199 1, 1997; Brew and others, 199 1 ; Swainbank and others, 199 1). Pynte is the dominant
sulfide. Gold associated with pyrite occurs as minute blebs in goethik rims and frecture fillings in corroded crystals. The gangue
consists of quartz and lesser ankerite, chlorite, and sericite. The mine produced about t .5 million g Au, and reserves are estimated
at 907,000 tonnes of ore grading 10.5 g/t Au. The age of mineralization is interpreted as 55 Ma. The deposit contains more than
5,500 m of horizontal working~.The deposit is hosted in Crebceous waltz diorite which exhibits proximal ankerite, quartz, and
sericite alteration adjacent to veins, and more widespread propylitic alteration. The quartz diorite intrudes Late Triassic
greenstone, graywacke, and argillite of Alexander sequence of the WrangeUia supertenane.
Riverside Au Quartz Vein Deposlt
The Riverside Au quartz vein deposit consists primarily of disseminated galena, pyrite, tettahedrite, pyrrbotite,
chalcopyrite, sphalerite, gold, and scheelite in two large quark veins, but in the Lindeberg lode, part of the deposit is a combined
quartz vein and epigenetic replacement deposit (Byers and Sainsbury, 1956; Smith, 1977). The mine at the deposit produced about