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the upper ditch enters Ruby Creek, a headwater tributary of Fourth of July Creek, and helps to feed the lower ditch farther downstream. The pay streak, 500 feet wide, is mined in cuts 250 feet wide, working for a distance of 100 feet and then returning for the other half. The muck overlying the gravel is first ground-sluiced off, then three 21/2-inch nozzles under a head of 160 feet are used to move the gravel. A line of eight or ten 12-foot sluice boxes are used, with a steel shear board suspended longitudinally over them, thus making it possible to move gravel into the sluice boxes simultaneously from both sides of the boxes.
Little active mining is in progress on Woodchopper and Coal Creeks. The chief placer mining during the summer of 1925 was on Mineral Creek, a small right-hand tributary of Woodchopper Creek, about 5 miles from the Yukon. Here the baring of the bedrock by placer mining has revealed the contact between the lower Mississippian chert conglomerate and the Upper Cretaceous and Eocene conglomerate, the latter occurring upstream from the former. The pay streak is about 100 feet wide, and the gravel and muck are about 10 feet thick. Mining was being done by open cutting, aided by a small nozzle.
On Iron Creek, another eastern tributary of Woodchopper Creek, 2 or 3 miles above Mineral Creek, two men were at work in 1925, one on Discovery claim, at the mouth of Iron Creek, and another on claim No. 2 above Discovery. The work on Discovery claim was underground work done by winter drifting. The work on No. 2 above consisted of open cutting and shoveling in about 3 feet of gravel and 2 feet of muck, taking 8 feet on each side of the sluice boxes. The pay streak here is spotted and irregular, but the gold is coarse and of high grade, one sample sent to Seattle assaying $18.75 an ounce.
Three other men were at work farther up Woodchopper Creek, engaged in small-scale winter drifting and prospecting.
No placer-mining operations were in progress on Coal Creek at the time of the writer's visit in 1925, but this creek has been mined intermittently at a number of places in recent years. The bedrock at the mouth of Coal Creek and for 412 miles upstream is the Lower Cretaceous black slate, which is followed upstream for 312 miles by the lower Mississippian chert and chert conglomerate. Above this the Upper Cretaceous and Eocene rocks cross the creek. Two groups of claims on the creek are now held mainly by two men. The upper group lies at the lower end of the Upper Cretaceous and Eocene conglomerate, and the gold in these placers has probably been derived in part from the reconcentration of these ancient gold-bearing rocks. The lower claims lie mostly in the black-slate zone, and it seems likely
that the gold in these placers may have been derived from a mineralized zone in the creek itself, for this lower gold is brighter in color, coarser, and higher in grade than the gold from the upper claims. The pay streak on the lower claims is 100 feet wide and the gravel from 12 to 20 feet thick. The ground worked by the owner has not yielded less than 75 cents to the square foot of bedrock, although placers of lower grade than this are undoubtedly present. These two groups of claims, which together comprise 7 miles of placer ground on Coal Creek, should be thoroughly prospected and if possible should be mined as a unit. This should make a good hydraulic venture for some company, for Coal Creek always has plenty of water, and mining would therefore not be at a disadvantage, as it is on some of the smaller gold-bearing creeks tributary to the Yukon.
Location of the district..
crinoids in limestone of Silurian
Cairnes, D. D., fossils collected by... 53-55, 59
quoted... 21-22, 25–26, 34-37, 43, 112-113
Yukon River just north of....... pl. 4
Middle Cambrian limestone..
on Yukon River at mouth of.... pl. 7
below mouth of Shade Creek.... pl. 6
Dacite, occurrence of.....
Nation River formation....
Oil shale, results of distillation of..
Eagle, sulphur spring near, analysis of water
Foster, Margaret D., analysis by....
Geologic history of the region...
Paleozoic era, events of..
Woodchopper volcanics... pl. 11
rian) along Yukon River just
vonian limestone in......
Yukon Valley at international boundary,