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Bureau of Mines / Minerals yearbook 1990
Year 1990, Volume 2 (1990)

Pittman, Tom L.
Alaska,   pp. [54]-70 ff. PDF (3.5 MB)


Page 63

 wide. Most of the gold and silver valuesALASKA—1990  63(WGM) (15
%),
of Toronto, Canada. Echo Bay is the project operator. The property is leased
from Alaska Electric Light and Power Co. and the City and Borough of Juneau
(CBJ), the owners. The 1990 program increased the proven and probable reserves
to over 100 million short tons containing about 3.3 million troy ounces of
gold, according to Echo Bay's 1990 Annual Report. The forecast milling grade
will be 0.052 troy ounces gold per St over a mine life of 12 years. Present
plans call for a flotation and gravity mill with a capacity of 22,500 
-tons of ore per day with regrind and cyanidation capacity to recover gold
and silver from the concentrates. Gold from the flotation and gravity concentrates
will be smelted to dore' bullion and shipped outside for refining. The new
mill, surface facilities, and haulage adit portal will be at Thane, about
4 miles south of Juneau. The 1990 exploration program completed about 83,000
feet of diamond drilling and a considerable footage of underground drifting,
crosscutting, and raising. This work increased the reserves about 50 % above
the 1989 figures. Other mineralization was listed by Echo Bay at about 2.
1 million troy ounces of gold in lower grade rock, a 300 % increase above
the 1989 estimate. 
-The Environmental Impact Statement (EIS) is being prepared by the Bureau
of Land Management (BLM). The scoping meetings began in April 1989 to obtain
public input and identify environmental and socioeconomic issues. A Preliminary
Draft EIS was released to government agencies in December 1989. After the
agency comments and public concerns were compiled, Echo Bay published a revised
project description in May 1990. The revised description proposed a site
for the surface facilities and mine access adit portal near Thane and proposed
using LPG fuel instead of diesel fuel for electrical power generation. Thane
is about 4 miles south of Juneau, practically free of snow avalanche danger,
has more favorable terrain, and is much closer to the preferred tailings
disposal site. The Final EIS should be released early in 1991. The deposit
is about 3 miles long and varies from about 50 feet to 600 feet 
are contained in ~ swarms of quartz stringers and veins that are from less
than I ~ inch wide to 15 feet wide and contain some pyrrhotite, lesser sphalerite,
small amounts of galena, and very small amounts of free gold. The veins average
about 6 inches in width and about 0.5 troy ounce in gold. The vein swarms
are localized along the Coast Range Megalineament and emplaced near the contact
between metadiorite and slate or phyllite hanging wall rocks and a green
schist footwall. The east-west Silverbow fault cuts the deposit near the
northerly end, dividing it into the north and south ore bodies. The fault
has about 1,800 feet of oblique left-lateral displacement. The north ore
body was developed by an internal shaft from the mill haulage level at an
elevation of about 460 feet above mean sea level (msl) to about 1,000 feet
below msl and partially mined to the surface. A long crosscut was started
from the 10th level of the shaft at about 530 feet - below msl and through
the fault toward the south ore body; it was only advanced about 750 feet
beyond the fault when the mine closed. The south section of the deposit was
partially mined above the old mill haulage level only and includes the ground
at the south end purchased by the Alaska Juneau Mining Co. from the Alaska
Gastineau Gold Mining Co. about 1934. 
 The Echo Bay-WGM joint venture also leases the Treadwell group of mines
on the east shore of Douglas Island, about 1 mile westerly across Gastineau
Channel. The Treadwell operations produced about 3 million troy ounces of
gold from 1881 to 1917 when a hanging wall cave-in flooded the mine with
sea water. Mining had progressed from the surface to 2,300 feet below msl
and exploration and development was progressing to the 2,800 foot level.
Echo Bay contracted with Tonto Drilling Services to diamond drill 
~ a directionally controlled hole from the east side of Gastineau Channel
to intersect the Treadwell deposit and then 
~ to drill three offset holes through the 
~ structure. Echo Bay's 1990 annual report 
~ stated: "The first three holes encountered the diorite formation that
hosts
the 
Treadwell zone, but showed scant gold. The fourth hole encountered 600 feet
of diorite, of which 186 feet graded 0.10 ounce/ton gold, including 49 feet
of 0.22 ounce/ton. " About 10,000 feet of hole was drilled. It may be
possible
to access the Treadwell ore zone by a decline from 
a shaft in the A-J workings. 
 The Kensington Mine is about 50 miles north of Juneau and about a mile east
of Lynn Canal. It is owned by a joint venture of Echo Bay (50%) and Coeur
d'Alene Mines Corp. (50%). Echo Bay is the operator. There was a small production
of gold and silver in the early 1900's from stopes near the outcrop and from
a short level at an elevation of about 2,400 feet. The 1990 annual report
by Echo Bay lists reserves at the end of 1990 at about 1 .9 million troy
ounces of gold 
I averaging 0. 148 troy ounce gold per st, 
~ an increase of about 40 % above the 1989 ~ reserves. The State survey lists
- this ~ reserve as contained in 12.8 million tons of ore. Drilling and other
work from the new 5,200-foot crosscut adit driven easterly from the foot
of the mountain at the 800-foot level has identified the Kensington stockwork
of quartz veins in diorite over a vertical range of 2,800 feet and along
strike for about 1 ,600 feet. The structure strikes about north and south
and dips steeply to the east. The average width is about 50 feet. Gold occurs
mainly as calaverite (a gold telluride) and native gold. There are small
amounts -of pyrite and minor chalcopyrite present. A raise connects the 800
level to the 2,000 level. Ramps provide access from the 800 level to levels
at elevations of 900 feet and 1 , 175 feet. The 800 level was driven as a
production sized opening although it would intersect the Kensington structure
about 600 feet below the lowest old drill holes. The 800 level crosscut the
Horrible and three other gold-bearing zones before reaching the Kensington.
The 2,000 level cut the Eureka zone just west of the Kensington and one small
zone to the east before intersecting and drifting north and south on the
Johnson zone. 
Planning in 1990 indicates rock and ore is competent enough to allow use
of modified longhole stoping techniques. Bulk sampling and milling tests
yielded 


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