Ray River

Prospect, Active

Commodities and mineralogy

Main commodities Sn
Other commodities Au; Nb; REE; Ti; W; Zr
Ore minerals cassiterite; gold; ilmenite; monazite; wolframite; xenotime; zircon

Geographic location

Quadrangle map, 1:250,000-scale TN
Quadrangle map, 1:63,360-scale D-1
Latitude 65.9863
Longitude -150.5712
Nearby scientific data Find additional scientific data near this location
Location and accuracy This site represents an approximately 30-mile-long area of placer tin-REE on the Ray River drainage basin. The prospect area is accessible from the Dalton Highway only on the extreme eastern extent, or otherwise requires helicopter or winter trail access. This placer prospect is on and follows much of the Ray River and several tributaries and numerous terraces (Barker, 1991a,b). For this record, the site is at the confluence of an unnamed tributary from the north located at the east edge of NW¼NW¼ Section 5, T. 13 N., R. 14 W., of the Fairbanks Meridian. The site is about mid-way across the area of the prospect and accurate to less than 1,000 feet.

Geologic setting

Geologic description

The Ray River drainage is underlain at the headwaters by the Sithylemenkat pluton and flows eastward approximately 30 miles, dissecting the Ray River pluton and ultimately encountering the Ft. Hamlin Hills pluton on the east where it is forced by topography to flow south to join the Yukon River. Placer tin and REE occur along the entire length of the river within the Tanana Quadrangle. The granitic rocks are all considered part of the broad northeast-trending peraluminous Ruby Batholith of central Alaska (Chapman and others, 1982; Barker and Foley, 1986: Herreid, 1969). Generally the granitic rocks are coarse-grained equigranular to porphyritic K-spar-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. The granitic plutons, especially the Sithylemenkat pluton, feature alteration zones including chloritic greisen veins/bodies that are the apparent source of at least some of the detrital cassiterite and REE minerals. Minor placer gold is also consistently present in heavy mineral samples and though minor commodity it would constitute an additional value.
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 and 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 consequently the region is historically known for the Pleistocene mammals that survived there, as well as a multitude of placer gold camps; districts such as the Klondike and Fairbanks. In the Ruby batholith region, the on-going erosional and mineral concentration processes that can concentrate valuable heavy minerals, have apparently remained un-interrupted since the late Tertiary.
The heavy mineral concentration processes are further enhanced by multiple-stages of fluvial activity within the principal drainages that have subsequently reworked sediments in each drainage basin. Each episode of channel down-cutting leads to yet another phase of re-deposition at continually lower elevations; the process continues locally to rework the alluvial sheet deposits at several documented locations on the Ray River (Barker, 1991b). Each repeated down-cutting event removes and transport more of the lighter aggregate downstream, thus potentially up-grading the residual heavy mineral concentrations left behind. This is best exemplified along the middle and upper Ray River. Strikingly evident, the middle Ray River alluvium-filled basin is beginning yet another episode of down-cutting due to recent breaching of barrier highland topography by the Ray River in Section 1 and 2, T. 13 N., R. 14 W. Further downstream the basin-filling of a graben-type feature(s) continues and forms at least two, lower Ray River basins (Barker, 1991b).
Remnant terraces are evidence of ancient (late Tertiary?) alluvial gravel deposition that predictably contain REE and tin in heavy mineral fractions are now perched as high as 150m (500 ft) above present-day channels of the lower Ray River. Along the northern and western Fort Hamlin Hills the tin- and REE-bearing terrace deposits have been developed for construction aggregate for the Trans-Alaska Pipeline. Predictably the grade of contained tin and REE in the high terrace deposits is lower than the re-worked and re-concentrated active basin sediments.
Geologic map unit (, )
Mineral deposit model Placer Sn (Cox and Singer, 1986; model 39e).
Mineral deposit model number 39e
Age of mineralization In the Ruby batholith region, the on-going erosional and mineral concentration processes that can concentrate valuable heavy minerals, have apparently remained un-interrupted since the late Tertiary (Barker, 1991b).
Alteration of deposit Thermal alteration from the batholith is widespread. Locally extensive, tourmalization, potassic, carbonate, and argillic styles of advanced alteration can be mapped and generally are associated with regional-scale northeast trending faults. Such zones of altered and weakened rock give rise to locally intense large-scale disintegration of the bedrock granite which has released the contained heavy minerals. Source of the placer tin- and REE- minerals appears to be local areas of argillic alteration and greisen vein-like occurrences that are particularly evident in the better exposed Sithylemenkat; a few typically dark green colored greisen fragments can generally be found in creek float at any location throughout the region (Barker and Foley, 1986). Greisen samples will generally contain 100 ppm to as much as 2500 ppm Sn and contain elevated REE. Cassiterite has also been found associated with quartz veins at a few locations. The older fluvial deposits also exhibit a degree of residual (in-place) concentration that are characterized by feldspar grains altered variably to white clayey quartz-rich sediment that are seen to grade upward into unaltered cross-bedded fluvial gravel.

Production and reserves

Workings or exploration
The Ray River has been explored by the U.S. Bureau of Mines as part of the Critical and Strategic Minerals Program in the 1970s-1980s (Barker, 1983, 1991a, 1991b; Barker and Foley, 1986). Gravel samples have been collected and processed from 37 locations along the Ray River and from a series of 7 auger drill holes randomly located on river gravel bars of the Ray River. All samples were processed by gravity methods for the analysis of the heavy mineral content (Barker, 1991a).
Ucore Rare Metals, Inc., through their Alaska subsidiary Landmark Alaska, acquired the property in 2011 and in 2012 announced additional sample results for 98 samples which included Ray River drainage, No Name Creek, and some sites along the Kilolitna and its tributaries (Ucore Rare Metals Inc., 2012). Follow link to Excel data table.
Auger drill holes demonstrated higher heavy mineral values are encountered beginning about 5 to 7 ft. below the surface where gravel is more permanently clayey sand-bound as compared to the seasonally active gravel exposed in gravel bars near or at the surface. Holes were drilled to 15 to 21 ft deep. The sand fraction from a single hole in west-central area of the basin (Section 4), T. 13 N., R. 15 W., contained 897g/m3 REE+Sn while a second hole central in the basin in Section 6, T. 14 N., R. 14 W., reported 2,174g/m3 REE+Sn in the sand fraction. Surface samples from both a mile above and a mile below this location were mostly above the threshold value of 150g/m3 REE+Sn. Three auger drill holes on the east of and several miles beyond the upper basin reported similar threshold values (Barker, 1991a, Ucore, 2012).
Indication of production None
Reserve estimates A 'very preliminary' resource estimate of the tin resource was made by the U.S. Bureau of Mines 'for the purpose of land-use management and planning purposes' consisting of between 62- to as much as -172 million pounds-tin in 300 million cubic yards of gravel (Barker, 1991). Grade is estimated between 0.2 and 0.5 pounds-tin per cubic yard; higher grade values are mostly from No Name Creek, the northern fork of the Ray River (see ARDF number BT021). No estimate was made for the REE, gold, or other possible placer minerals.

Additional comments

The Ray River flows through lands on which the State of Alaska has filed ‘Priority Selection’ requests under the land entitlement provision of the 1959 Statehood Act. To date only a very small portion of the drainage (T. 13 N., R. 13 W.) has been transferred as ‘Tentatively Approved’ to the State from the federal government, Bureau of Land Management (BLM). The BLM will not issue any permits for surface disturbance activities including exploration under State Selection status.
Splits of most samples collected by the U.S. Bureau of Mines and referenced in this report have been archived at the Alaska Geologic Materials Center, Anchorage, Alaska and are available for review.

References