Chechako No. 1; Eldorado; Westonvitch Mine, Fairbanks Mining District, Fairbanks North Star Borough, Alaska, USAi

Regional Level Types
Chechako No. 1; Eldorado; Westonvitch Mine	Mine
Fairbanks Mining District	Mining District
Fairbanks North Star Borough	Borough
Alaska	State
USA	Country

Location: Cobb (1972, MF-413), loc. 37; NW1/4, sec. 25, T. 3 N., R. 1 E., of the Fairbanks Meridian. This prospect is at the head of Cleary Creek about 1/4 mile west of the confluence with Willow Creek. Accuracy is within 2,000 feet.
Geology: The Chechako prospect contains two types of mineralization: lead, zinc and silver mineralization in flat-lying replacement bodies in marble and high angle shear-related mineralization which cuts the replacement bodies and is composed of complex sulfosalts containing antimony, arsenic and lead (Kent, 1985). The shear-hosted mineralization has been traced for 2800 feet along strike. Average grade of composite samples from this material was 14.4% lead, 2.06% zinc, 7.3% antimony, 13.7 ounces of silver per ton, 0.268 ounces of gold per ton, and variable arsenic (Kent, 1985). Geophysical surveys indicated the bedded sulfide horizon extended to the north and was down-dropped by the main shear zone indicating normal motion on the Chechako shear zone. Average grade of composite samples from the bedded replacement mineralization was 27% lead, 20% zinc, 1% antimony, 60 ounces of silver per ton, and 0.092 ounces of gold per ton (Kent, 1985). In July, 1980, approximately 186 dry tons of sorted ore was extracted from the oxidized portion of the Chechako shear zone; it averaged 0.07 ounces of gold per ton, 37.87 ounces of silver per ton, 18.98% Pb, 2.00% Zn, 2.61% Sb, and 1.83% As (Fairbanks Exploration Inc., unpublished report, 1985). Gold values appear to be higher in the unoxidized portion of the exposed lode than in oxidized ore shipped in 1980. Separation of sulfide versus oxide ore indicates the Chechako prospect carries average grades of oxidized ore of 0.07 ounces of gold per ton, 37.87 ounces of silver per ton, 18.98% Pb, 2.00% Zn and 2.61% Sb; unoxidized ore averages of 0.20 ounces of gold per ton, 25.20 ounces of silver per ton, 22.50% Pb, 3.77% Zn, and 7.80% Sb (Fairbanks Exploration Inc., unpublished report, 1985). Sampling of the exposed workings on the Chechako prospect was conducted by Fairbanks Exploration Inc. in 1986 (Fairbanks Exploration Inc., unpublished report, 1986). Silver values ranged up to 87 ounces per ton while gold values ranged up to 3400 ppb (0.1 ounces of gold per ton). Elevated arsenic (+1000 ppm) and antimony (+10000 ppm) values also were noted. Mineralization is most intense peripheral to high angle normal faults which cut the marble. Weak prograde skarn alteration in the form of pyroxene, diopside and biotite replacement is associated with blackjack sphalerite in marble units (Fairbanks Exploration Inc., unpublished report, 1985). In 1988 and 1989, Yukon Tanana Mining conducted extensive surface trenching on the Chechako and adjoining Tolovana mine prospects and outlined a surface mineable gold reserve of about 150,000 ounces (J. Blakestad, oral commun., 1991). In 1998, three holes were drilled in the Chechako area to invesitgate extremely high grade gold values that were found in soils (Freeman and others, 1998). On one 400 ppb soil anomaly, a drill hole intercepted 10 feet with 0.096 ounces of gold per ton and another 10 feet with 0.064 ounces of gold per ton. Another hole intercepted 20 feet with 0.037 ounces of gold per ton, 5 feet with 0.066 ounces of gold per ton, and another 5 feet with 0.041 ounces of gold per ton.
Workings: A 30-foot deep shaft was sunk on the prospect in 1910 but the prospect was inactive when visited in 1912 (Smith, 1913; B 525). By 1916, the prospect was being actively mined from an open cut and a 40-foot deep shaft (Mertie, 1918). The shaft was 45 feet deep with 30 feet of adjoining drifts and stopes by 1917 (Martin, 1919). By 1922, the prospect had two shafts which exposed a 3- to 5-foot-wide shear zone striking N 35 E and dipping 85 NW. (Stewart, 1922; Davis, 1922). The southwest shaft was 45 feet deep with a 30-foot drift to the southwest and a 45-foot drift to the northeast at the 45-foot level. From the 15- to the 40-foot station in the northeast drift, ore had been stoped out over a 12-foot height and 20-foot width. The northeast shaft is located 200 feet along strike from the southwest shaft and was sunk to a depth of 35 feet by 1922. A drift extends 45 feet to the southwest from the 35-foot level. Two adits had been driven an unknown length on the prospect between 1922 and 1931. These two adits and both previously described shafts were inaccessible in 1931 (Hill, 1933). The Chechako prospect was examined as a possible source of antimony in 1942 but no additional exploration was conducted due to low antimony values in the ore (Killeen and Mertie, 1951). In 1981, the Chechako prospect was leased to Placid Oil Company who conducted soil sampling, VLF and CEM ground geophysics and drilled 462 feet in three diamond drill holes (Porterfield and Croff, 1986). In 1998, three holes were drilled in the Chechako area to invesitgate extremely high grade gold values that were found in soils (Freeman and others, 1998).
Alteration: Oxidation and weak prograde skarn alteration in the marble as pyroxene, diopside and biotite replacement (Fairbanks Exploration Inc., unpublished report, 1985).
Production: Two shipments of ore from the prospect in 1916 totalled 24 tons and contained 3.67 tons of lead (15%) and 1,083 ounces of silver (45 ounces of silver per ton) (Smith, 1917; BMB 153). In July, 1980, approximately 186 dry tons of sorted ore from the prospect were shipped to the Asarco smelter at East Helena, Montana (Fairbanks Exploration Inc., unpublished report, 1985).
Reserves: In 1988 and 1989, Yukon Tanana Mining conducted extensive surface trenching on the Chechako and adjoining Tolovana mine prospects and outlined a surface mineable gold reserve of about 150,000 ounces (J. Blakestad, oral commun., 1991).

Commodities (Major) - Au; (Minor) - Ag, Cu, Pb, Sb, Zn
Development Status: Yes; small
Deposit Model: Polymetallic carbonate replacement.

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Link to USGS - Alaska:	LG107

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References