Unnamed Occurrences (ARDF - BT018; southeast of Sithylemenkat Lake), Koyukuk Mining District, Yukon-Koyukuk Census Area, Alaska, USAi

Regional Level Types
Unnamed Occurrences (ARDF - BT018; southeast of Sithylemenkat Lake)	- not defined -
Koyukuk Mining District	Mining District
Yukon-Koyukuk Census Area	Census Area
Alaska	State
USA	Country

Location: This record describes several lode and placer occurrences in an area approximately 9 to 10 miles southeast of the center of Sithylemenkat Lake. The occurrences are centered at an elevation of about 2,350 feet, about 0.3 mile east-northeast of the center of section 21, T. 14 N., R.17 W. of the Fairbanks Meridian. The occurrences include a small group of lode deposits at these coordinates; other lode occurrences are about 1 mile south and about 1.5 mile northeast. There are also several placer occurrences in streams draining the area. The location is accurate.
Geology: Tin-bearing rocks occur in the batholith of the Sithylemenkat granite and mineralized float is common in streams in the area (Barker and Foley, 1986). Tin minerals are also concentrated in the alluvial gravels of the upper Kanuti Kilolitna River. Cassiterite has been identified in chlorite-rich and locally magnetite-bearing greisen zones in the granitic rocks (Barker and Foley, 1986). Mineralized rock samples exhibit variable effects of greisenization, with tourmaline and magnetite sometimes present. Sericite-quartz veins and altered dikes contain abundant secondary chlorite and may contain up to several percent sulfides. The sulfide minerals include pyrite, arsenopyrite, galena, and molybdenite. The greisen range in color from light to dark green; the highest Sn values are in the darker varieties. The extent of the greisen is difficult to determine due to poor exposures. Some zones have been traced for as much as 1,200 feet, and one greisen zone is up to 10 to feet thick (Barker and Foley, 1986). Thin sections of the greisen show relict porphyritic texture; feldspar phenocrysts are replaced by intergrown quartz and sericite, which, in the more altered samples are replaced by chlorite and clay minerals. Greisen samples contain 25 to 2,300 ppm tin (Barker and Foley, 1986). The analyses also show up to 5,126 ppm arsenic, 326 ppm, bismuth, 253 ppm cesium, 1,808 ppm copper, 34,027 ppm lead, 1,156 rubidium, 135 ppm tungsten, and 4,044 ppm zinc. More recent rock samples also contain anomalous tin, zinc, copper, and lead, as well as up to 25.9 ppm silver, up to 75.6 ppm uranium, and up to 101 ppm thorium (Kurtak and others, 2002). Reconnaissance placer sampling shows alluvial concentrations of tin minerals in the upper part of the Kanuti Kilolitna River (Barker and Foley, 1986). Placer samples taken from near the surface contain up to 0.4 pounds of tin per cubic yard, with lesser tantalum, columbium, tungsten, and rare earth minerals. In addition to cassiterite, the placer concentrates also contained wolframite, pyrite, ilmenite, hematite, garnet, and monazite(?). The Sithylemenkat pluton is a composite batholith. Geologic mapping of its northern half has identified four textural phases, including porphyritic granite, granite porphyry, coarse-grained granite, and graphic granite (Barker and Foley, 1986). It is Early Cretaceous, based on K/Ar dating (Patton and Miller, 1973).
Workings: Only surface sampling.
Age: The Sithylemenkat pluton which is probably the source of the deposits is Early Cretaceous based on a K/Ar date of 106 +/- 3 my for biotite (Patton and Miller, 1973).
Alteration: Greisenization.

Commodities (Major) - Sn; (Minor) - As, Bi, Cs, Cu, Nb, Pb, Rb, REE, Ta, W
Development Status: None
Deposit Model: Sn greisen deposits (Cox and Singer, 1986; model 15c) and alluvial placer Sn (C

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Link to USGS - Alaska:	BT018

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References