Placer tin, in the form of cassiterite (Sn02) and (or) tinstone (fragments including cassiterite and some vein or rock material), is known or reported in deposits that have been prospected or mined for placer gold in four areas adjacent to the Yukon River in central Alaska, 120 to 240 miles west of Fairbanks. These areas are: the Morelock Creek area, on the north side of the Yukon River about 30 miles upstream from Tanana; the Moran Dome area, about 16 miles north of the Yukon River and 25 miles northwest of Tanana; the Mason Creek area, on the north side of the Yukon River about 36 miles west of Tanana; and the Ruby-Long area, on the south side of the Yukon River near Ruby and about 40 miles east of Galena. The only extensive placer mining in these areas has been in the Ruby-Long area. Other placer deposits including some cassiterite are known in central Alaska but are not discussed in this report.
Bedrock in these areas is predominantly schist of various types with some associated greenstone and other metamorphic rocks. Some granite is exposed in the Moran Dome and Ruby-Long areas and in areas close to Morelock and Mason Creeks. Barren, milky quartz veins and veinlets transecting the metamorphic rocks are common. No cassiterite was found in the bedrock, and no bedrock source of the tin has been reported. In the Moran Dome and Mason Creek areas, and in part of the Ruby-Long area, tourmaline is present in the rocks of the tin-bearing drainage basins, and apparently absent elsewhere in these areas.
The placer deposits are in both valley floor and bench alluvium, which are predominantly relatively thin, rarely exceeding a thickness of 30 feet. Most of the alluvium deposits are not perennially frozen.
In the Morelock Creek area tin-bearing deposits are 5 to 5? miles above the mouth of the creek, and meager evidence indicates that cassiterite and gold are present in Morelock Creek valley and some of the tributaries both upstream and downstream from these deposits. The concentrates recovered in samples average about 57 percent tin, and the gold averages about 922 fine. Prospecting indicates that the placer tin deposits are small and of relatively low grade, and that the greater part of the value of the deposits is the gold. In the Moran Dome area the known tin-bearing deposits are in the valley floor and bench gravels along upper Tozimoran Creek. Much of the alluvium is unfrozen, but the deeper portions of the bench gravels and the gravels some distance from the streams are in part frozen. Tin-bearing samples have been obtained from prospect pits and drill holes at a number of sites on Tozimoran Creek between its head and the confluence with Slate Creek. Gold recovered from some of these samples has a fineness of 835. The presence of cassiterite and gold on Ash Creek has been confirmed by sampling. Cassiterite and gold reportedly occur on upper Melozimoran Creek, and several other stream valleys in this area may be tin bearing.
In the Mason Creek area cassiterite has been reported in the valley floor and bench alluvial deposits on Mason Creek, but its presence could not be confirmed in the brief field examinations of creek and dump-pile gravel that were made. The limited number of pits and cuts available precluded a valid sampling without additional drilling, pitting, or trenching.
In the Ruby-Long area the valleys of Midnight, Birch, and Big Creeks are known to have appreciable concentrations of cassiterite in the gold-bearing placer deposits. The alluvial deposits in the valleys of Ruby, Glacier, Flint, Trail, Long, Fifth of July, Short, Flat, Greenstone, and Monument Creeks contain some cassiterite, but sufficient information could not be obtained to make an evaluation of these occurrences. Cassiterite concentrates, reportedly ranging from 52 to 70.24 percent tin, have been recovered in connection with gold mining operations on Midnight, Birch, and Big Creeks.