Unnamed Occurrences (ARDF - TN006; north of Kanuti Kilolitna River), Koyukuk Mining District, Yukon-Koyukuk Census Area, Alaska, USAi

Regional Level Types
Unnamed Occurrences (ARDF - TN006; north of Kanuti Kilolitna River)	- not defined -
Koyukuk Mining District	Mining District
Yukon-Koyukuk Census Area	Census Area
Alaska	State
USA	Country

Location: This record represents occurrences of anomalous rock samples in a mile-long area on the north valley wall of the Kanuti Kilolitna River. The site is at an elevation of 1,850 feet, at the west boundary of section 35, T. 14 N., R. 20 W., of the Fairbanks Meridian. The location is accurate within 1,000 feet.
Geology: The Kanuti River region straddles the southeastern boundary of the Yukon-Koyukuk Basin and includes sedimentary and volcanic rocks of the basin sequence as well as metamorphic and plutonic rocks of the adjoining Kokrines-Hodzana Highlands (Patton and Miller, 1970). The Sithylemenkat Pluton, a 170-square mile body of mid-Cretaceous granitic rocks, intrudes metasedimentary rocks on the south side of the Kanuti River. Dikes of pegmatite are locally abundant on the perimeter of the predominantly porphyritic, biotite quartz monzonite pluton. Overlying and intruding the metasedimentary rocks along the northwest flank of the Kokrines-Hodzana Highlands is an ophiolitelike assemblage of Permian to Jurassic, altered pillow basalt, diabase, and gabbro; serpentinized peridotite and dunite; and bedded chert (Patton and Miller, 1970). Six of the ultramafic bodies extend for 65 miles, from Caribou Mountain in the northeast to the upper Melozitna River in the southwest. Numerous smaller bodies also occur in this belt. The ultramafic rocks are crudely layered, tabular bodies dipping gently to steeply northwest. The Holonada body is about 2,500 feet thick. Layering in the Kilolitna body is less well defined, but the width of its outcrops suggests that it is at least as thick. The lower contact of the ultramafic bodies is sharply defined, possibly by a fault, with little evidence of thermal alteration of the underlying rocks. The ultramafic rocks are composed almost entirely of serpentinized peridotite, chiefly harzburgite, and serpentinized dunite. They are cut by veinlets and irregular masses of chalcedony and drusy quartz. Patton and Miller (1970) found one small mass of colloform magnesite in the northern part of the Kilolitna body (in the Bettles quadrangle). Ultramafic rocks in the north Kanuti-Kilolitna area consist of harzburgite with lenses of dunite, hobnail peridotite with magmatic segregations of dunite, and garnet peridotite (J.Y. Foley, written commun., 2004). In the late 1960s, the U.S. Geological Survey collected composite rock samples at several locations across the Kilolitna and lower Kanuti ultramafic bodies (Patton and Miller, 1970). Nine samples, each weighing approximately 3 pounds, were analyzed. The range of analytical values for these samples was: 2,400 to 3,000 parts per million (ppm) chromium, 1,900 to 2,400 ppm nickel, les than 0.010 ppm platinum, less than 0.005 ppm rhenium, and less than 0.004 ppm to 0.008 ppm palladium. Chromium content ranged as high as 9 percent in selected grab samples of dunite streaked with grains of chrome spinel. The U.S. Bureau of Mines Alaska Field Operation Center investigated chromite deposits in Alaska between 1979 and 1984 as part of the Bureau's critical and strategic minerals program (Foley and others, 1985). Several hand-sorted, chromite-rich samples collected in the Kilolitna River area contained an average of 46.7 percent Cr2O3 (Foley and others, 1985). Seven samples of chromite-rich dunite were collected by the U.S. Bureau of Mines during mineral investigations in the north Kanuti-Kilolitna area (Foley, written commun., 2004). The samples contained 1,800 to 4,800 parts per million (ppm) chromium, 1,900 to 2400 ppm nickel, and no detectable platinum or palladium.
Workings: In the late 1960s, the U.S. Geological Survey collected composite rock samples at several locations across the Kilolitna and lower Kanuti ultramafic bodies (Patton and Miller, 1970). Nine samples, each weighing approximately 3 pounds, were analyzed. The range of analytical values for these samples was: 2,400 to 3,000 parts per million (ppm) chromium, 1,900 to 2,400 ppm nickel, les than 0.010 ppm platinum, less than 0.005 ppm rhenium, and less than 0.004 ppm to 0.008 ppm palladium. Chromium content ranged as high as 9 percent in selected grab samples of dunite streaked with grains of chrome spinel. The U.S. Bureau of Mines Alaska Field Operation Center investigated chromite deposits in Alaska between 1979 and 1984 as part of the Bureau's critical and strategic minerals program (Foley and others, 1985). Several hand-sorted, chromite-rich samples collected in the Kilolitna River area contained an average of 46.7 percent Cr2O3 (Foley and others, 1985). Several samples of chromite-rich dunite were collected by U.S. Bureau of Mines geologists and submitted for mineralogical and beneficiation tests at the Bureau's Albany Research Center (J.Y. Foley, written commun., 2004). Sample PB19670 is iron-stained, highly fractured and partly altered, tan dunite containing light green pyroxene crystals and veins and lenses of serpentine along fractures and replacing olivine. Euhedral to subhedral chromite crystals are densely to sparsely disseminated in the dunite. Sample PB19671 is similar, but the serpentine is accompanied by chlorite and the dunite also contains nearly massive aggregates of disseminated chromite crystals (Foley, written commun., 2004). Chromite analyzed by the Albany Research Center was classified as high-iron, high-aluminum chromite and high-chromium chromite, respectively, for the 2 samples. Neither sample contained detectable platinum or palladium, but the minus- 65 mesh concentrate of sample PB19670 contained 0.160 ounce of gold per ton, and the minus-65 mesh concentrate contained 0.002 ounce of gold per ton (Foley, written commun., 2004). Seven samples of chromite-rich dunite were collected by the U.S. Bureau of Mines during mineral investigations in the north Kanuti-Kilolitna area (Foley, written commun., 2004). The samples contained 1,800 to 4,800 parts per million (ppm) chromium, 1,900 to 2400 ppm nickel, and no detectable platinum or palladium.
Age: Permian to Jurassic, coeval with the presumed age range of the mafic and ultramafic hostrocks.

Commodities (Major) - Cr, Ni
Development Status: None
Deposit Model: Podiform chromite (Cox and Singer, 1986; model 8a).

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Link to USGS - Alaska:	TN006

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References