A significant content of radioactive material was recognized in a few placer concentrates from Sweepstakes and Rube Creeks in the northeastern part of the Seward Peninsula, Alaska, when old collections were scanned for radioactivity in the spring of 1945. Subsequent field investigations with a Geiger-Mueller counter were made of the creek gravels and the placer-gold paystreak on the bench ground of Sweepstakes Creek and its tributaries, the syenite stock of Granite Mountain to the north of Sweepstakes Creek, and the creek gravels of Rube and Anzac Creeks which are tributaries of the Peace River east of the syenite stock.
A reconnaissance conducted in 1946 in areas adjacent to highways in the Tanana and upper Copper River valleys was the first time that field radioactivity tests were made in eastern interior Alaska. Before 1946 the scanning of the Alaskan placer-concentrate collection of the Geological Survey for radioactivity had indicated that radioactive minerals occurred in a few localities in the Fairbanks and Livengood areas. The objectives of the 1946 reconnaissance were to determine whether significantly radioactive lodes occurred in the vicinity of the placers from which the radioactive concentrates had been obtained and to conduct a routine survey for radioactive deposits in accessible areas adjacent to highways in eastern interior Alaska. The techniques used in the survey consisted of the direct testing of outcrops for radioactivity with a portable survey meter and the collection of rock samples and placer concentrates for equivalent uranium analysis in the laboratory.
Antimony-bear ing ores in the Fairbanks district, Alaska, are found principally in two areas, the extremities of which are at points 10 miles west and £3 miles northeast of Fairbanks; and one of two minor areas lies along this same trend 90 miles farther to the northeast* These areas are probably only local manifestations of minerallzatiom whfcefc affected a much broader area and formed antimony-bearing deposits in neighboring districts, the closest of which is BO miles away* The ores were exposed largely as a result of lode gold mining, but at two periods in the past, high prices for antimony ore warranted an independent production and about 2500 tons of stibnite ore was shipped* The sulfide deposits occupy the same fractures along which a goldquartz mineralization of greater economic importance occurred; aa4 both are probably genetically related to igneous rooks which intrude the schistose country rock* The sulf ide is in part contemporaneous with some late-stage quartz feesMSjsjsa in which it occurs at disseminated crystals; and in part the latest filling in the mineralized zones where it forms kidney-shaped masses of essentially solid sulfids* One extremely long mass must hays contained nearly 100 tons of ore, but the average of the larger kidneys is closer to several tons. Much of the ore is stibnitej with quarts as a minor impurity, and assays show the tenor to Tary from 40 to 65 percent antimony* Sulphantlmonites are less abundant but likewise occur as disseminated crystals and as kidney shaped bodies* Antimony ox!4es appear on the weathered surface and along fractures within the sulfide ore* Deposits containing either stibnite or sulphantimonite are known at more than 50 localities, but only eighteen have produced ore and the bulk of this asms from £ mines. The geology of the deposit, and the mature, extent, and period of the workings are covered in the detailed description* of individual occurrences.Several geologic and economic faetors, which greatly affect prospecting and mining for stibnite ore in the area, are outlined. The principal available ore and reserves are considered to be ore* earlier mined but never shippad, ore minable from near-surface deposits, and ores recoverable as a by-product of future gold mining. The outlook for stibnite production In the district is very uncertain. Apparently the greater portion of stibnite ore has already beam recovered and present operations will strip the two principal areas of the district. This conclusion is based on the scanty discoveries since the last war and the fact that the areas are so pock-marked with prospects that there is little likelihood that any other large near-surface bodies remain to be discovered. Future prospecting would essentially be limited to attempts to seek the continuation of lodes previously having high yields of stibnite.
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