Foley and Barker produced the BM Information Circular 9103 in 1986. In 1980, the Bureau of Mines investigated the northern Darby Mountains on the Seward Peninsula, AK, for radioactive mineral deposits. Uranium is concentrated in silicified shear zones in biotite quartz monzonite of the Darby pluton. The shear zones are best exposed in steep, narrow, avalanche gullies; they are characterized by radiometric anomalies and by uranium and thorium geochemical anomalies. Radiometric and geochemical anomalies are also found over altered biotite quartz monzonite where there are no visible shear zones. The radiometric and uranium anomalies are often spatially, but apparently not genetically, associated with alkaline mafic and felsic dikes.
Similar geochemical and radiometric anomalies over discolored altered zones and geochemical anomalies in sediments downstream from the altered zones indicate that additional areas may also be favorable targets for uranium deposits. Analyses of panned, heavy mineral concentrates show no significant concentrations of uranium or thorium in resistant heavy minerals. This indicates that radioactive elements in the Darby pluton are concentrated in soluble, nonresistant minerals that may constitute a recoverable uranium resource.
The authors concluded that investigations in the Darby Mountains by government and industry (with university geologists from Rice University) targeted the Darby pluton as a potential host for deposits of uranium and thorium. During the summer of 1980, work by the Bureau of Mines personnel were curious about the claim staking undertaken by the Omega Energy Corporation and elsewhere by Houston Oil and Minerals (and subsequent owners) substantiated the earlier conclusions by locating vein-type uranium deposits and uraniferous zones in altered quartz monzonite in the northern Darby Mountains. Where exposed in steep mountainside gullies, the vein deposits and altered shear zones typically yield above-background gamma radiation measurements, and anomalously high geochemical uranium values.
Geochemical sampling of altered zones in the pluton, overlying soils, and sediments from streams draining the pluton, indicates additional areas may contain similar uranium occurrences. Leached and weathered bedrock, and scree and soil cover on gentler slopes, ridges, and saddles generally mask radiometric and geochemical evidence of radioactive minerals. Analyses of panned, heavy mineral concentrates show no significant concentrations of uranium or thorium in resistant heavy minerals. The negative results of the pan concentrate survey and the discovery of vein-type uranium deposits indicate that uranium in the Darby pluton is concentrated in soluble, nonresistant minerals and may be recoverable. For the database of the mapping and sampling for this investigations, see (more).
Additional mineral exploration is warranted in the Darby Mountains. Future exploration would be expected to reveal more uranium vein deposits, particularly in the west Vulcan Creek-Rock Creek area. Evaluation of possible low grade, large tonnage uranium deposits indicated by the concentration of discolored altered zones in cirques along the northwest side of the divide between upper west Vulcan Creek-Rock Creek is recommended.
Results of geochemical analyses indicate that uranium is associated with lead, zinc, and to a lesser extent, silver and molybdenum. These elements may serve as geochemical pathfinders during future exploration. Mineral occurrences other than uranium are known, or are indicated in the northern Darby Mountains. Silver, lead, molybdenum, copper, gold, tin, zinc, and coal deposits are reported. Also, graphite lenses were observed in the Vulcan Creek Valley. Analyses of panned heavy mineral concentrates indicate that tin, tungsten, and columbium deposits may also exist.
[No mention was made by BM personnel about the roll-front deposit known at the time or about the potential of the large basin as a potential new roll-front uranium district. See below].
[Also see the earlier work conducted by Omega, which was subsequently published in a 2018 comprehensive review of the data developed during the mid-1970s and since by I2M Consulting, LLC, Houston: http://www.i2mconsultants.com/downloads/corp/JGG-2-023.pdf ].
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