Bonanza Creek Mine, Circle Mining District, Yukon-Koyukuk Census Area, Alaska, USAi

Regional Level Types
Bonanza Creek Mine	Mine
Circle Mining District	Mining District
Yukon-Koyukuk Census Area	Census Area
Alaska	State
USA	Country

Location: Bonanza Creek is a tributary to Porcupine Creek. The site of most placer mine activity extended approximately 1.5 miles upstream from the junction of Bonanza Creek and Porcupine Creek. This site includes both placer mining and mineralized schist.
Geology: Bonanza Creek, the main tributary of Porcupine Creek, is about 11 km long. Bedrock in the Bonanza Creek drainage is the Lower Schist unit described by Wiltse and others (1995) as a slightly calcareous quartz-muscovite schist, porphyroblastic-albite-quartz-chlorite-muscovite schist, and lesser amounts of quartzose porphyroblastic-albite-chlorite schist. This Bonanza Creek site includes both placer mining and mineralized schist. A pebble count of gravel taken 3 km above the mouth of Bonanza Creek, shows a composition of 55 percent quartz-mica schist, 36 percent quartzite, and 9 percent quartz (Yeend, 1991). Zircon, garnet, ilmenite, pyrolusite, pyrrhotite, pyrite, and galena are present in the heavy mineral concentrates (Mertie, 1932). Gravel is 3 to 6 feet thick beneath 2 to 8 feet of muck, with a 75 to 150 feet wide paystreak in the lower valley (Mertie, 1938). Coarse gold, with considerable intergrown quartz, is located on bedrock and in cracks in the top 4 to 5 feet of the bedrock (Mertie, 1938). In the upper part of the creek, 6 feet of gravel lies on weathered bedrock with a pay streak 150 to 200 feet wide (Menzie and others, 1983, p.39). The largest nugget reported here was 1 ounce (Menzie and others, 1983, p.39). Hydraulic mining with 2-inch nozzles was conducted in the 1930's (Mertie, 1938). Mining in the 1980's was confined to the middle and upper parts of Bonanza Creek. One to two meters of bedrock and one meter of overlying gravel was mined, and in places, the paystreak was over 60 meters wide. Exploratory drilling in the early 1980's in the gravel in the upstream part of the creek revealed sub-economic values (Yeend, 1991). Gold values in the gravel during the early mining years were 0.0375 to 0.05 ounces per cubic yard. The gold recovered was course and contained nuggets weighing as much as 10 ounces (Mertie, 1938). Fineness of gold varied little over a ten-year mining period (mean is 850 gold, 140 silver) (Mertie, 1938). In 1996, Paul and Company worked pay on Bonanza Creek, and Underwood Mining Co. tested a small paystreak at 114 Mile Steese Highway by washing about 50 cubic yards of gold-bearing gravel (Bundtzen and others, 1996). Metz (1984) describes stratabound sulfide mineralization in the placer workings on the lower 3.2 km of Bonanza Creek. The mineralized section strikes east-west and dips to the north at 15 to 45 degrees. Channel samples taken at right angles to the compositional layering indicated gold grades of 0.12 ounces per ton over 1.5 meters. Stratabound mineralization is cut by vertical quartz veinlets that range from less than a centimeter to 15 centimeters wide. The veinlets strike east-west and contain pyrite, arsenopyrite and gold. Gold wires up to 5 millimeters long and gold crystals up to 2 millimeters in diameter occur within vuggy quartz. Gold also occurs as inclusions in the arsenopyrite and along the sulfide grain boundaries (Metz, 1991, p. 108). Freeman and others (1988), conducted detailed alteration mapping and channel-sampling along lower Bonanza Creek. This mapping revealed sulfide mineralization that is largely restricted to graphitic and calcareous quartzites. Subequal amounts of pyrite and arsenopyrite dominate the sulfide mineralogy, with rare chalcopyrite and sphalerite. Local pale yellow stibiconite indicates the presence of antimony-bearing minerals. Anomalous gold, silver and arsenic were reported in a metatuff unit that included disseminated arsenopyrite and pyrite. The highest gold value reported was 489 ppb.
Workings: Hydraulic mining with 2-inch nozzles was conducted in the 1930's (Mertie, 1938). Mining in the 1980's was confined to the middle and upper parts of Bonanza Creek. One to two meters of bedrock and one meter of overlying gravel were mined, and in places the paystreak was over 60 meters wide. Exploratory drilling in the early 1980's in the gravel in the upstream part of the creek revealed sub-economic values (Yeend, 1991). Gold values in the gravel during the early mining years were 0.0375 to 0.05 ounces per cubic yard. The gold recovered was coarse and contained nuggets weighing as much as 10 ounces (Mertie, 1938). Fineness of gold varied little over a ten-year mining period (mean is 850 gold, 140 silver) (Mertie, 1938). In 1996, Paul and Company worked pay on Bonanza Creek, and Underwood Mining Co. tested a small paystreak at 114 Mile Steese Highway by washing about 50 cubic yards of gold-bearing gravel (Bundtzen and others, 1996). In 1988, Fairbanks Exploration, Inc., conducted detailed alteration mapping and channel-sampling of the mineralized schist along lower Bonanza Creek (Freeman and others, 1988).
Alteration: Sulfide mineralization in Bonanza Creek is largely restricted to graphitic and calcareous quartzites. The sample line along lower Bonanza Creek by Freeman and others (1988) indicates that sulfide minerals make up 10% of the rock. Sulfide mineralogy is dominated by subequal amount of pyrtie and arsenopyrite with rare chalcopyrite and sphalerite. Sulfide minerals occur as extremely fine-grained disseminations within the foliation of the schists. Late stage quartz veins commonly contain small amounts of pyrite or arsenopyrite. Iron oxides have formed after pyrite and pale green scorodite has formed after arsenopyrite. Local pale yellow stibiconite indicates the presence of antimony-bearing minerals (Freeman and others, 1988).
Production: Mining has been reported (see workings/exploration); however, amount of production is unknown.

Commodities (Major) - Au; (Minor) - As, Cu, Zn
Development Status: Yes; small
Deposit Model: Placer gold deposit (Cox and Singer, 1986; model 39a), Disseminated and strati

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Link to USGS - Alaska:	CI006

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