Porcupine Creek Mine, Skagway District, Juneau District, Haines Borough, Alaska, USA
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Geology: According to Hoekzema and others (1986), Porcupine Creek is a steep, rapidly downcutting drainage with an average gradient of 350 feet per mile. Three classes of placer deposits exist: 1) abandoned channel and bench deposits, 2) recent stream gravels, and 3) an alluvial fan. The abandoned channel and bench deposits have the highest grade. Gold fineness ranges from 841 to 909 and averages 866 (Hoekzema, 1986). Wright (1904 [B 225 and B 236]) reported gravels in the lower portion of creek to be 40 feet thick. The bottom 2- to 3-foot layer of gravel on top of bedrock contained high gold values and the uppermost 15 to 20 feet of gravel carried good values as well. Bundtzen (1986) notes that Pleistocene ice advances and readvances resulted in at least three, possibly four, bedrock incised channels or terrace levels. The most likely bedrock sources for placer gold are crosscutting auriferous quartz-sulfide veins associated with altered mafic dikes cutting Porcupine Slate in the McKinley and Cahoon Creek drainages (SK042) (Hoekzema and others, 1986; Wright, 1904 [B 225 and B 236]). This zone of quartz-sulfide veins and altered mafic dikes is part of a larger area of less intensive quartz-sulfide veining that Eakin describes as extending in a northwesterly direction from the Salmon (Tsirku) River, through Porcupine, Glacier, and Jarvis Creeks to the mountain mass north of the Jarvis Glacier. Wright (1904 [B 225 and B 236]) refers to three types of gravel deposits in the Porcupine area: creek gravels, side benches and high benches. The creek gravels and side benches are are related to present and recent past fluvial activity. The high benches are older and were deposited during earlier interglacial periods. High benches occur up to 200 feet above the current stream level as on McKinley Creek (SK045). Stream gravels consist of fragments and slabs of slate and boulders of diorite and some greenstone up to 2 or 3 feet in diameter. Hoekzema and others (1986) add alluvial fan gravels at the mouths of Porcupine Creek and Glacier Creek (SK065) to the gravel deposit types recognized by Wright (1904 [B 225 and B 236]).
Workings: Several flumes and tunnels were built in early 1900's to divert the creek to allow mining of the creek bed and to supply water for hydraulic methods; Hand methods, sluices, rockers, and a trolley lift were used in early 1900's; A resurgence of activity in 1970's and 1980's saw use of mechanized placer methods (Hoekzema and others, 1986).
Age: Quaternary placers.
Production: The estimated total production from the Porcupine Mining Area from 1898 to 1985, including Christmas (SK062), Nugget (SK048), Porcupine (SK041), Cahoon (SK044), and McKinley (SK046) Creeks, is 79,650 troy ounces of gold. From 1898 to 1906, there were small manual operations producing as much as 9,000 troy ounces per year that were destroyed by flooding in 1906. From 1907 to 1915, mining by the Porcupine Gold Company was conducted using a flume constructed one mile below the junction with McKinley Creek. Production averaged 3,000 troy ounces of gold per year, until the flume was destroyed by a flood in 1915. From 1916 to 1918, the old flume was repaired and a new flume was built. Over 6,000 troy ounces of gold was produced during this period. The flumes were destroyed in the flood of September, 1918. Porcupine Gold Mines, which later became Alaska Sunshine Gold Mining Company, took over in 1926. In 1928, they completed a flume that originated on Porcupine Creek 0.5 miles above the junction with McKinley Creek and bridged McKinley Creek. Mining commenced in 1929 but closed at the end of the season due to poor returns. After much additional exploration, mining began again in 1935. Work continued into 1936 when the bridge over McKinley Creek was destroyed by a rock slide. As of 1986, there had been only minor, sporadic production since World War II. During times of high gold prices in the 1970's and early 1980's, some mechanized mining was conducted (Hoekzema and others, 1986).
Reserves: The following is summarized from Hoekzema and others (1986). Three classes of placer deposits exist: 1) abandoned channel and bench deposits, 2) recent stream gravels and, 3) an alluvial fan. Abandoned channel and bench deposits are the highest grade. Five resource areas are estimated to contain a total of 152,000 cubic yards of gravel with grades of 0.0106 or more ounces of gold per cubic yard. Stream channel gravels in lower Porcupine Creek are estimated to contain at least 500,000 cubic yards of material of unknown grade. The alluvial fan consists of 12 to 15 feet of recent stream gravels overlying an unknown thickness of older gravels. The alluvial fan is estimated to contain 6,000,000 cubic yards of material, but much is probably uneconomic. Alluvial fan samples contained from a trace to 0.11 troy ounces of gold per cubic yard. There is potential for older high grade gold bearing channels beneath the alluvial fan.
Commodities (Major) - Au; (Minor) - Ag
Development Status: Yes; small
Deposit Model: Modern stream, paleo-channel and alluvial fan, gold placers (Cox and Singer, 19
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