Fort Hamlin Hills Metalliferous Coal

Occurrence, Inactive

Commodities and mineralogy

Main commodities Ge; Pb; W
Other commodities Ag; Au; Cu; Ga; Mo; REE; Sb; U; Zr
Gangue minerals coal; coal ash

Geographic location

Quadrangle map, 1:250,000-scale BT
Quadrangle map, 1:63,360-scale A-1
Latitude 66
Longitude -150.3386
Nearby scientific data Find additional scientific data near this location
Location and accuracy The site location is the Ray River drainage where the river enters the Bettles quadrangle. The occurrences are found in a series of about six small topographic basins through which the Ray River meanders. Samples of mineralized occurrences begin on the west at 66.0240N, 150.2932W, and mineralized occurrences extend downstream for about twelve miles to at least 65.9331N, 150.0180W, where the occurrence area is covered by Quaternary gravel. It is possible that occurrences extend downstream to within a mile of the confluence with the Yukon River. Accuracy of the site and sample locations presented here is about 1,000 feet. The site location is in the SE¼, NE ¼ Section 32, T.14 N., R. 13 W., of the Fairbanks Meridian. An additional, geologically related occurrence location is reported approximately 10 miles farther upstream of the site report area and is described in the Tanana Quadrangle records (Sample No. 1, located at 65.9852, 150.5560).

Geologic setting

Geologic description

Metalliferous coal beds occur in small, semi-closed basins along the lower Ray River immediately west of the Fort Hamlin Hills pluton and east of the Ray River pluton (Barker, 2006). Both plutons are considered part of the broad, northeast-trending, peraluminous Ruby Batholith of central Alaska (Patton and Miller, 1973; Barker and Foley, 1986). 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 locally recognized. The granitic rocks cut Paleozoic schist, phyllite, quartzite, and lesser greenstone and limestone. The Ray River coal-bearing basins are part of a regional series of Tertiary basins that occur intermittently along the southeast margin of the batholith.
Along the lower Ray River the small, semi-closed basins, are mapped as graben features and range from several square miles in area to as little as 200 acres (Barker, 1991b, 2006). Metals are found highly concentrated in the calcium-rich coal ash derived from the coal. When prepared according to American Society for Testing Materials (ASTM, 1979) standards, ash samples will contain 0.01 to in excess of 1.0 percent each of germanium, lead, and tungsten. Ash samples also contain up to 0.5 percent total rare earth elements, and up to 0.1 percent antimony, copper, gallium, molybdenum, uranium, and zirconium. Gold and silver values are also present. All samples are variably mineralized within these limits (Barker, 2006). Tungsten in particular is consistently present with over half of the 21 samples analyzed found to contain in excess of 0.25 percent W.
The coal beds are poorly exposed and only several small rubble crops were located. However, during high water events the Ray River will rip coal fragments from the coal beds within the mudstone and conglomerate section that comprises the shallow bedrock under the river alluvium in this region. Most of the samples comprise random chip collections of fresh material found on gravel bars following spring breakup and the accompanying seasonal high water.
Geologic map unit (, )
Age of mineralization A sample of tephra from the Coal Creek-Dall River section was dated at 38.6 +/- 1.6 Ma (Barker, 1981). Albanese, 1987, reported an age of 30.59 m.y. +/- 0.92 Ma for the basalt flows at a location nearer the Dalton Highway and immediately east of the Ray River. Coal from the Ft. Hamlin Hills area is estimated to be Eocene in age.
Alteration of deposit Metal values in coal appear related to a northeast-trending series of geothermal springs that altered and mineralized the organic material during and/or after the coal-forming period, a process that was perhaps accentuated by overlying fissure basalt flows. Phosphoric alteration at the underlying contact of the basalt flows was mapped at an outcrop on the Ray River (Barker, 2006). Known geothermal springs extend about 65 miles N 60 E from Ishtalitna Creek on the southwest to the Dall River on the northeast and are assumed to correlate to a splay of the Porcupine-Kaltag fault.

Production and reserves

Workings or exploration
The site was explored by the U.S. Bureau of Mines as part of the Critical and Strategic Minerals Program in the 1970s-1980s (Barker, 1981, 1991b, 2006; Barker and Foley, 1986). All coal rank analyses and preparation of 21 coal ash samples were done by the University of Alaska, Mineral Industry Research Laboratory. Metal values were determined by certified commercial laboratories.
Coal in the Fort Hamlin Hills area, when fresh, will break with a conchoidal fracture and will have an apparent rank of high-volatile, lignite A to subbituminous B, generally with a 6-10 weight percent ash yield. Maceral composition is dominated by vitrinite. Higher metal values of the Fort Hamlin area coal beds occur in higher sulfur ( greater than 1.4 percent) coals underlying the Ray River valley as compared to nonmineralized low-sulfur coal in the Dall River valley to the northeast and elsewhere in interior Alaska, although mudstone beds stratigraphically above the Dall River coal beds are enriched in these same metals (ARDF number BV005).
Microprobe studies suggest the finely disseminated mineralization found in coal of the Ray River valley mostly occurred in peat bogs prior to, or concurrent with, diagenesis and coal formation (Barker, 2006). Mineralization is likely related to geothermal activity associated with rifting in a granitic terrane and volcanism. Water samples from six of the seven known nearby hot springs/warm springs were tested and determined to contain anomalous levels of tungsten. Mineralization processes affecting the original peat accumulations, or within the coal beds, were likely accentuated due to Oligocene fissure basalts that entrapped the geothermal waters in the coal-forming section. To the northeast mudstones in the Dall River-Coal Creek area contain elevated tungsten likely concentrated during formation of lacustrine deposits in shallow lakes fed by geothermal water and intermittently covered by ash falls; however, associated coal beds were not mineralized (Barker,1981, 2006).
Indication of production None

Additional comments

The Fort Hamlin Hills coal occurrences are on lands on which the State of Alaska has filed ‘Priority Selection’ under the land entitlement provision of the 1959 Statehood Act. Most of this occurrence lies within the ‘inner corridor’ of the Trans-Alaska Pipeline as defined by the Bureau of Land Management (BLM). Various temporary land withdrawals remain to be lifted by the BLM before these selections can be processed. The BLM will not issue any permits for surface disturbance activities including exploration.

References

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