« PrejšnjaNaprej »
southwest of Circle. Both these camps lie without the area included in this report and therefore will not be described. The Eagle-Circle district, as the term is used in this report, includes mainly the area contiguous to the Yukon between the settlements of Eagle and Circle. Within this area gold placer mines are being worked on American Creek, on the Seventymile River and its tributaries, on Fourth of July Creek, and on Woodchopper and Coal Creeks.
SOURCES OF GOLD
In a previous publication the writer å has outlined the geologic occurrence of mineral deposits, including the gold lodes, in interior Alaska and has drawn therefrom such conclusions as may be useful for prospectors and miners in searching for lodes and placers in this region. The fundamental thesis advanced is that the gold ores have originated as a phase of granitic intrusion; and that therefore where bodies of granitic rocks are found the conditions in general are favorable for the occurrence of gold. It does not, of course, follow that all granitic intrusives have functioned as mineralizing agencies; but on the other hand it is of decided value to know that where such rocks or the quartz veins which resulted from their presence are absent the chances for discovering workable lodes or placers are poor.
One modifying factor in this working hypothesis should be mentioned. The magmatic fluids that carried in solution the metallic elements appear to have escaped from the molten granitic rocks at a late stage in their cooling and to have migrated upward to points at or near the apexes of the intrusive bodies before the metallic elements were precipitated to form the quartz veins and other types of mineralized zones. Now it will be observed that both large and small bodies of granitic rocks occur in the Yukon-Tanana region, but it is probable that if one could follow them downward to great depths some or all of them would be found to connect underground with one another. This simply means that the granitic rocks approached somewhat closer to the surface at some localities than at others and that in the process of their subsequent uncovering by erosion of the land surface some of the intrusive bodies that came closer to the surface have been more extensively eroded and exposed than others that lay at somewhat greater depths. Hence the occurrence of granitic rocks with large surficial extent suggests at once deep erosion into the lower parts of such intrusive bodies, with the attendant removal ages ago of the upper or apical zones. Hence, if the mineralization has been concentrated, as supposed, at or near these apical zones, such areas of large surficial exposure of granitic rocks are regarded as less favorable for the possible occurrence of metalliferous lodes. On the other hand, bodies of granitic rocks with small surficial area are in general interpreted as the apical parts of larger underlying granitic masses; and as their upper zones have not been removed by erosion, such bodies are regarded as more favorable sites for mineralization. Hence, it is believed that the best places to prospect for possible metalliferous lodes in the Yukon-Tanana region are at or near small outcrops of granitic rocks that are not more than 2 or 3 miles in diameter. Conversely, large granitic bodies like the one which covers nearly all of the Charley River Basin are not regarded so favorably as possible seats of mineralization.
8 Mertie, J. B., jr., The occurrence of metalliferous deposits in the Yukon and Kuskokwim regiong: U. S. Geol. Survey Bull. 739, pp. 149–165, 1923.
In much of interior Alaska, however, on account of the high costs of mining, metalliferous lodes have little present value unless they are of bonanza character; and such lodes are rare. Therefore the prospector is naturally more interested in placer deposits, especially in gold placers. Lodes must, of course, have existed at some time in order to produce the gold which subsequently became concentrated in the placers; hence the same generalizations that apply to prospecting for lodes apply also to prospecting for placers—that is, the streams best to prospect are those which drain areas where minerali. zation is known or may be expected to be present. In regions where extensive glaciation has taken place this generalization may not hold true, for the gold placers, if they were formed prior to the last advance of the glaciers, may have been destroyed and dissipated by the ice action. In the Yukon-Tanana region, however, practically no glaciation has occurred, so that this added complication does not enter into the matter.
In the Eagle-Circle district the gold of the placers on the Seventymile River and tributaries, Fourth of July Creek, and Woodchopper and Coal Creeks came originally from the granitic rocks that lie to the southwest. But the proximate source of some of this gold is in the coarse sandstones and conglomerates that extend from Eagle northwestward at least as far as Woodchopper Creek. This is not hard to understand when it is remembered that most of these granitic rocks were intruded during the Jurassic period, whereas the conglomeratic rocks were formed long afterward, in Upper Cretaceous or Eocene time. In other words, these conglomerates represent ancient gold placers, which originated so long ago that they have since become consolidated into rock. Moreover, it is probable that at the time of their formation the regional erosion level was much higher than at present and that only the upper parts of the big Charley River granitic body were then exposed to erosion. Hence the Charley River batholith, though now regarded as a not particularly favorable site for gold placers, was probably in Eocene time a more favorable area for lodes and for placer accumulation and probably did in fact supply the gold of the Upper Cretaceous and Eocene conglomerates. The recent streams in this area have therefore obtained a part of their gold from the destruction of the old fossil placers and have reconcentrated it in the present drainage channels. But the streams southwest of the conglomerate belt-for example, the tributaries of the Seventymile River above Barney Creek-have probably derived a part of their gold directly from the original gold lodes.
It does not follow, however, that all the streams that drain the Upper Cretaceous and Eocene belt are favorable sites for gold placers, for three reasons: (1) Some of these old conglomerates may consist of gravel laid down by streams that did not head far enough to the southwest to have tapped mineralized zones. (2) Even where the ancient streams drained out of bodies of mineralized country rock the gold was probably irregularly distributed in the old placer bodies, just as it is in many present-day gold placers. (3) The Upper Cretaceous and Eocene rocks have been greatly folded and otherwise disturbed from their original nearly horizontal attitude; this feature alone would make for irregular distribution of gold, even if the ancient streams had developed long, continuous pay streaks.
Therefore, although some of the gold which came originally out of the Charley River batholith may be preserved in these old conglomerates instead of being widely dissipated into larger stream valleys like the Yukon, yet the present irregular distribution of gold in the conglomerate belt adds an element of uncertainty to the problem of prospecting in the streams draining it that is not unlike the uncertainty that attends prospecting in recent stream gravel in glaciated areas. In other words, prospecting can be done more intelligently in the part of the area southwest of the Upper Cretaceous and Eocene belt, where, as in the Fortymile precinct, the gold placers have been derived for the most part directly from the original gold lodes rather than by reconcentration from earlier placer deposits.
No extensive study of the gold placer-mining operations on the streams between Eagle and Circle has been made by the writer, but most of the placer-mining plants were visited in 1925, in order to obtain a general idea of the character of the placers and of mining conditions. On American Creek the Upper Cretaceous and Eocene rocks extend from the mouth upstream for about 4 miles, but the placer-mining operations were confined to a zone beginning about 4 miles farther upstream, where the bedrock consists of schist, serpentine, and other metamorphic rocks. At this point American Creek is joined by Discovery Fork, which enters from the southeast. One operator, who owns Discovery claim at the forks and five claims below and six above Discovery claim, on American Creek, was preparing for hydraulic operations on a large scale. The stream gravel which was to be worked ranged in thickness from 3 to 10 feet. Three other men were operating small plants on Discovery Fork. The gold from American Creek is said to yield $17.25 to the ounce at the mint but is accepted at $16.70 commercially at Eagle.
On the Seventymile River the largest operating plant was at the upper end of Discovery claim on Crooked Creek. The bedrock on Crooked Creek belongs entirely to the Upper Cretaceous and Eocene sequence, and much or all of the gold in the present placers has probably been reconcentrated from the ancient conglomerates. The stream gravel that was being worked is from 5 to 6 feet thick in the center of the cut but becomes thicker toward the east valley wall. The pay streak, where worked in 1925, was 240 feet wide, but it is probably narrower upstream in the direction in which operations were being extended. The ground here averages about 13 cents to the square foot of bedrock, and the gold is of fairly high grade, yielding about $18 an ounce in Eagle. The largest nugget so far found was valued at $3.50. This ground was being mined by hydraulic operations, the gravel being moved by two 242-inch nozzles under a 100-foot head, and the tailings being stacked downstream by a 3-inch nozzle. The water for the nozzles was obtained from a ditch that taps Crooked Creek farther upstream, but as work progresses upstream a higher ditch will have to be dug in order to obtain the necessary head.
About 2 miles up the Seventymile River from Crooked Creek and on the same side of the river hydraulic placer-mining operations on a small scale were being carried on in 1925 at the mouth of a little creek called Broken Neck Creek, but this work proved unprofitable and was subsequently discontinued. The bedrock here, as on Crooked Creek, is the old conglomerate formation, and about 3 to 5 feet of gravel from 20 to 50 feet wide was being worked at the time of the writer's visit.
About 3 miles farther upstream, on the north side of the Seventymile River, another man was at work mining the river bars and prospecting. At this point the conglomerate formation lies to the northeast, and the country bedrock consists of quartzite schist, quartz-mica schist, amphibolite schist, and basalt and diabase of greenstone habit; but at one place along the river bank a piece of the conglomerate was seen faulted into the older schistose rocks. The Seventymile River at this point cuts through a short gorge in rapids which are known locally as “The Falls.” The valley of the river here has a number of well-developed benches, one of which, about 12 feet high, shows well on the north side of the river and slightly on the south side. On the north side, at a level 4 to 5 feet higher than the 12-foot bench, a great flat extends to the hills. On the south side another prominent bench occurs about 125 feet above the river level. These benches, particularly the lower ones, are being prospected by hand methods.
Barney Creek, about 6 miles farther upstream and also on the north side of the Seventymile River, is an old placer-mining site, but no mining operations were being carried on there in 1925. This creek also flows entirely through the Upper Cretaceous and Eocene rocks, and the gold in the placers probably came from that source.
Two other mining plants were being operated still farther up the Seventymile River, on the south side, the uppermost of which, on Alder Creek, had been worked intermittently for many years. This is a hydraulic plant working stream gravel, but as the bedrock is composed entirely of the old schistose rocks, the gold has doubtless. been derived directly from original mineralization in the valley of Alder Creek, produced by the granitic intrusives. The other plant, on Nugget Creek, was worked on a small scale, mainly by hand methods.
Farther down the Yukon, south of Nation, Fourth of July Creek is the site of another good-sized hydraulic plant, which had been in operation for several years. The lower part of Fourth of July Creek cuts across the Tahkandit (Permian) limestone, but in the upper part, where the mining operations were being carried on, the bedrock is the same Upper Cretaceous and Eocene conglomerate strata seen on the Seventymile River and on American Creek. The pay streak is about 500 feet wide and the gravel is 10 to 15 feet thick, covered by 2 to 7 feet of black muck. The gold lies in the lower foot of gravel and in the bedrock, it being necessary at places to mine 2 feet of bedrock in order to obtain all the gold. The gravel in the placers ranges in size from 2 to 10 inches, but a few boulders as much as 212 feet in diameter are encountered. The pebbles of the conglomerate bedrock are mainly flint and quartzite, with some vein quartz and rarely a piece of greenstone, and naturally the stream gravel is similar in character. Curiously enough, no granitic pebbles were seen at this locality either in the bedrock or in the overlying gravel. The gravel at this plant ranges in value from 20 to 30 cents to the square foot of bedrock.
The July Creek Placer Co. owns nearly 3 miles of claims on Fourth of July Creek, from claim 10 to claim 21 above Discovery, and plans to continue its mining operations for a number of years. Two ditches have been built, a lower one 21/2 miles long, which taps the upper part of Fourth of July Creek, and an upper one 914 miles long, which takes its water from the head of Washington Creek. The discharge from