Ray River 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 TN
Quadrangle map, 1:63,360-scale D-2
Latitude 65.9843
Longitude -150.589
Nearby scientific data Find additional scientific data near this location
Location and accuracy The site location is along the Ray River located immediately downstream of a prominent unnamed hot spring. The occurrence is in a topographic basin of the mid- to upper Ray River valley. Accuracy of the site and sample locations presented here is about 1,500 feet. The site location is in the SW¼, NE ¼ Section 6, T. 13 N., R. 14 W., of the Fairbanks Meridian. Similar mineral occurrence locations are reported approximately 10 miles farther downstream of this site and are described in the Bettles quadrangle ARDF site records.

Geologic setting

Geologic description

This site is similar to other metalliferous coal sites farther down river that also occur in small, semi-closed basins along the Ray River immediately west of the Fort Hamlin Hills 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 Ruby Batholith.
Along the Ray River these 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 generally 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).
The coal beds occur in soft sedimentary rock that is rarely found exposed; exposed coal beds are subject to destruction by forest fires. Sampling has relied on the fact that during high water events the Ray River will rip coal fragments from the coal beds within the mudstone and conglomerate section that comprise the bedrock under the river bed gravel in this region. Following high water events, random chips of fresh coal may be found on gravel bars, where they eventually oxidize and destruct.
Geologic map unit (, )
Age of mineralization A sample of tephra from the Coal Creek-Dall River section (about 30 miles to the northeast of this site) described above was dated at 38.6±1.6 Ma (Barker, 1981). Albanese, 1987, reported an age of 30.59±0.92 Ma for the basalt flows at a location nearer the Dalton Highway and immediately east of the Ray River. Coal mapped nearby is overlain by the basalt flows and therefore judged to be Eocene in age.
Alteration of deposit Metal values in the coal at this site appears related to a 65-mile-long, northeast-trending series of geothermal springs that likely altered and mineralized the organic material during and/or after the coal-forming period, possibly accentuated by overlying fissure basalt flows. An unnamed hot spring occurs about 200 meters upstream of the site, warm springs were found a few miles down river. Known geothermal springs extend about 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). Coal from this site was sampled, ash was prepared and analyzed as part of a group of metalliferous coal occurrences found at locations farther down river in the Bettles Quadrangle. Coal ash from this site contained 0.082 percent Ga, 1.22 percent Ge, 0.309 percent U, and 0.23 percent W. Ash constituted 14.68 percent of the coal.
All coal rank analyses and preparation of coal ash samples were done by the University of Alaska, Mineral Industry Research Laboratory. Metal values were determined by commercial laboratories.
Coal in the Ray River-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. Ray River coal consistently contains higher sulfur (>1.4 percent) as compared to non-mineralized low-sulfur coal in the Dall River valley to the northeast and most other coal in central Alaska, an apparent affiliation of the Ray River coal and elevated metal content. The stratigraphic relationship of mudstone overlying coal beds in the Ray River occurrence is similar to the Dall River occurrence, but at the Dall site coal does not contain the elevated metal values, though the mudstone beds do (see ARDF number BV005) (Barker, 1981, 2006).
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, including the spring immediately upstream of this site, were tested and determined to contain anomalous levels of tungsten. Mineralization processes affecting the original peat accumulations, or within coal beds, and were likely accentuated due to Oligocene fissure basalts that entrapped the geothermal waters in the coal-forming section. The mudstones in the Dall River-Coal Creek area contain elevated tungsten assumed concentrated during formation of lacustrine deposits in shallow lakes fed by the similar geothermal water and intermittently covered by ash falls (Barker, 1981, 2006).
Indication of production None

Additional comments

The Ray River 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. 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 until then.

References

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