Unnamed (east fork of Kilolitna River)

Prospect, Active

Commodities and mineralogy

Main commodities Sn; W
Other commodities Nb; REE; Ta; Ti; Zr
Ore minerals cassiterite; ilmenite; monazite; scheelite; wolframite (ferberite end-member); xenotime; zircon

Geographic location

Quadrangle map, 1:250,000-scale BT
Quadrangle map, 1:63,360-scale A-3
Latitude 66.0395
Longitude -151.0673
Nearby scientific data Find additional scientific data near this location
Location and accuracy The prospect is a series of tin, tungsten, and rare earth element placer prospects along the entire 10-mile length of the unnamed creek upstream of its confluence with the Kilolitna River. The site for this record is the confluence of the two principal upper tributaries. Accuracy of the location is less than 1,000 feet. The site is located in the SW¼, NW¼ Section 13, T. 14 N., R. 17 W., of the Fairbanks Meridian, but the prospect includes Sections 2, 7, 9, 10, 11, 13, 14, 15, 16, 17, 18, 23, and 24 of T. 14 N., R. 17 W., continuing west into T. 14 N., R. 18 W. Sections 12, 13, and 14. The prospect area is best accessible by helicopter from the Dalton Highway.

Geologic setting

Geologic description

The 10-mile long, unnamed creek, referred to by Ucore Rare Metals, Incorporated as the Caribou Heights prospect, drains about one-third of the known extent of the Sithylemenkat pluton (Ucore, 2012, 2014). Geological investigations recognized regional tin potential as early as 1969 (Herreid, 1969; Patton and Miller, 1970, 1973; Barker, 1983; and in early 1980s tin was specifically recognized in the valley of this record (Barker and Foley, 1986; Barker, 2012). Additional regional geochemical sampling by the Alaska Division of Geological and Geophysical Survey was reported in Bachmann, and others (2013).
The Sithylemenkat pluton is considered part of the broad northeast-trending peraluminous Ruby Batholith of central Alaska (Patton and Miller, 1970, 1973; Barker and Foley, 1986; Herreid, 1969). Generally, the granitic rocks are coarse-grained, equigranular to porphyritic, orthoclase-biotite-quartz monzonite varying to granite with subordinate phases of aplite, biotite aplite, tourmaline aplite, and fine-grained quartz monzonite. Tourmaline pegmatite phases have been recognized locally. The granitic rocks cut Paleozoic schist, phyllite, quartzite, and lesser greenstone and limestone. Mineralogical examination of heavy mineral concentrates from the bench channels and streambed samples confirmed abundant cassiterite and rare-earth minerals of monazite and xenotime (Barker and Foley, 1986).
Geologic map unit (, )
Mineral deposit model Placer tin (Cox and Singer, 1986; model 39e).
Mineral deposit model number 39e
Age of mineralization The prospect is Quaternary age. The Ruby batholith region is within the glacial ice-free Quaternary province of Beringia, defined as the non-glaciated intermontane region extending from eastern Siberia eastward across a then-dry Bering Sea floor, thence transecting interior Alaska, and extending into northwestern Canada. Across the Beringia region, the fluvial processes have generally continued since the Tertiary and the ongoing erosional and mineral concentration processes continue to concentrate valuable heavy minerals. A few small cirques are evident to the south in the higher elevations of the Ray Mountains, but evidence of glacial disruption of accumulating placer gravel deposits at this site was not seen.
Alteration of deposit
The granitic pluton features alteration zones, including a large central area of argillic alteration enveloping chloritic and hematic greisen veins/bodies that are the apparent source of at least some of the detrital cassiterite, wolframite, and REE minerals. A three-mile long north-northeast trend of these intermittent and cross-cutting greisen veins/zones occurs along the ridges in sections 11, 14, 21, and 22, T. 14 N., R. 17 W., immediately south of this creek (see Figures 6 and 7 in Barker and Foley, 1986).
Thermal alteration from the batholith is widespread and silicification extends well into the Paleozoic host rocks. Locally extensive, tourmalization, potassic, carbonate, and argillic styles of advanced alteration can be mapped and generally are associated with regional-scale faulting. Such zones of altered and weakened rock give rise to locally intense large-scale disintegration of the bedrock granite, which has released the interstitial or disseminated heavy minerals. The source of the placer tin and rare earth minerals appears to be widespread argillic alteration and greisen vein-like occurrences as described above, locally overprinted with chloritic and/or hematitic alteration (Herreid, 1969; Barker and Foley, 1986). Greisen samples will generally contain 100 ppm to as much as 2500 ppm tin and elevated rare earth elements.Cassiterite has also been found associated with quartz veins at a few locations in the batholith.

Production and reserves

Workings or exploration The prospect valley of the east fork of the Kilolitna River has been explored by the U.S. Bureau of Mines as part of the Critical and Strategic Minerals Program in the 1970s-1980s. In 1970s tin and tungsten potential of the areas adjacent to the Trans-Alaska pipeline were investigated and included 17 sample sites within this prospect area (Barker, 1983) and nearly all were anomalous; Barker and Foley, in 1986, sampled the creek for its placer potential, described the apparent placer prospect, and reported results for 30 concentrated samples. In 2011 Ucore Rare Metals, Incorporated, through their subsidiary Landmark Alaska, LLP, filed a claim block and reported more detailed sample results for 27 heavy mineral concentrates from several specific bench and active channel areas (see data table, Ucore, 2012, 2014). Bucket-size channel samples of an exposed bench deposit along the uppermost right limit of the southern tributary reported as much 9,700 g of rare earth elements plus tin per cubic meter of gravel. Generally anomalous results occur along the length of the creek though frozen ground makes sampling difficult. An additional well-defined mineralized terrace was located along the left limit of the creek in Section 13, T. 14 N., R. 18 W. (Ucore, 2012).
Indication of production None

Additional comments

The creek flows through lands on which the State of Alaska has filed ‘Priority Selection’ under the land entitlement provision of the 1959 Statehood Act. Various temporary land withdrawals remain to be lifted by the Bureau of Land Management (BLM) before these selections can be processed. The BLM will not issue any permits for surface disturbance activities including exploration under these withdrawals or State Selection status. Mining claims staked in the vicinity of this prospect are located according to the State of Alaska regulations for mineral locations on State Selected land.

References

Reporters J.C. Barker
Last report date 3/15/2016