Bureau of Mines / Minerals yearbook 1990
Year 1990, Volume 2 (1990)
Pittman, Tom L.
Alaska, pp. -70 ff. PDF (3.5 MB)
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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