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of the land represented by the Lower and Middle Triassic time interval must have been of the plateau-forming type, the land moving upward and downward, with an absence or minimum of tilting or deformation in the earth's crust. Possibly an oscillatory movement of the strand line due to variation in the ocean level might better fit the facts. At any rate, a stratigraphic discontinuity exists at this horizon not only along the Yukon but at other localities in Alaska, and this discontinuity appears to characterize the transition from the Paleozoic to the Mesozoic. In some parts of Alaska, particularly in southern Alaska, great extravasations of basaltic lava accompanied the regional uplift and subsequent depression. These eruptions attained their greatest development in the interval between the Permian and the Upper Triassic. Along the Yukon, however, no lavas of Pennsylvanian, Permian, or Triassic age have yet been recognized. At the end of the Triassic period this region was again extensively uplifted above sea level, and it appears to have remained so during all of Jurassic time. Some time during the Jurassic also occurred the great batholithic intrusions of granitic and related rocks, of which the batholith in the basin of the Charley River is a most striking example. The Lower Cretaceous conglomerates show little or no granitic material derived from such intrusive rocks, so that it is likely that the intrusives were injected late rather than early in the Jurassic and had not been uncovered to any large extent by erosion at the time of the formation of the Lower Cretaceous rocks. Concomitantly with these Jurassic granitic injections came the earliest period of mineralization of which there is any definite record in interior Alaska; and during this period were formed many of the gold-quartz veins of interior Alaska from which the present placers were subsequently derived.

By subsequent sinking of the land marine sedimentation was again begun in Lower Cretaceous time, resulting in the formation of the sandstone, slate, and conglomerate that now constitute the Kandik formation. After this epoch of marine sedimentation the land was again elevated, and a great series of fresh-water deposits, which constitute the assemblage of rocks here grouped as Upper Cretaceous and Eocene, was laid down. It is likely that this mid-Cretaceous regional elevation was accompanied by deformational movements that folded the Lower Cretaceous rocks prior to the deposition of the Upper Cretaceous rocks, thus resulting in an unconformity between these two groups of rocks. The structural relations in the Rampart district indicate that such an unconformity exists, but along the upper Yukon the two groups of rocks are not in contact, so that the hypothesis can not be absolutely proved. Nevertheless the unconformity is accepted as the probable condition.

As pointed out in the discussion of the Upper Cretaceous and Eocene sequence of rocks, there appears to have been no stratigraphic or botanic hiatus at the end of the Mesozoic era, so that as well as can be ascertained at present the Cretaceous deposition seems to have merged gradually into the Eocene through a transitional epoch. After Eocene time, however, the region was again uplifted, and it has not been subsequently depressed below sea level. Hence no Tertiary marine strata are present in interior Alaska. Along with this regional uplift in post-Eocene time came further intrusions of granitic rocks of the same general character as those injected in the Jurassic. These intrusives, however, appear to have consolidated fairly close to the surface and at some localities reached the surface, forming rhyolitic and dacitic lava flows, of which those at the head of the Charley River are examples. These post-Eocene granitic intrusions also gave rise to the second known period of mineralization in interior Alaska, forming the later gold quartz veins and cinnabar deposits and at some localities reopening and enriching the Jurassic vein systems.

The post-Eocene history of Alaska is recorded mainly in ancient fluviatile gravel, in the sediments deposited during the Pleistocene epoch, and in the Recent stream alluvium. The interpretation of this record in terms of elevations and depressions of the base level of erosion is largely a physiographic problem the solution of which bas hardly yet been begun. One fact in particular should be emphasized, namely, that interior Alaska, except in some isolated areas along the Yukon-Tanana divide, was not glaciated during the Pleistocene epoch. Yet it was bordered on the north, east, and south by great glaciers, and these, together with the climatic conditions which produced them, must have had a potent influence upon the nature of sedimentation in interior Alaska. Great bodies of black peaty silt were found deeply burying the pre-Pleistocene alluvial material, and such deposits, locally known as muck, themselves constitute an important problem in sedimentation and are as well one of the most hopeful sources of data bearing on Pleistocene conditions. The fine nature of these sediments shows that ordinary abrasion and movement of detritus by stream action was not the most typical process. Probably the mean temperature was low and precipitation also low, resulting in unusual conditions of erosion.

Although the Pleistocene is referred to as a glacial epoch, it is believed that interglacial stages formed a part of that epoch in Alaska, as in the States, but the glacial geology of interior Alaska has as yet received but little attention. For such study, as well as for post-Pleistocene glacial study, detailed topographic maps will be required, and little detailed topographic mapping has yet been done in Alaska.

ECONOMIC GEOLOGY

The principal source of exportable mineral wealth in this region is gold. The Eagle-Circle district and more particularly the Fortymile and Circle mining precincts, which lie to the south and west of the area here described, have long been known as sources of placer gold. Deposits were discovered at Franklin Gulch, in the Fortymile district, in the fall of 1887; at Birch Creek, in the Circle district, in the summer of 1893; and at American Creek, south of Eagle, shortly after the discovery on Birch Creek. All these old placer diggings are still producing gold.

No other mineral deposits are known in this region which give promise of possible commercial development in the near future. Some low-grade coal deposits exist, and attempts have been made to utilize these locally for fuel, but without success. Oil shale also is known at several localities, but this also does not seem possible of exploitation at the present time.

GOLD PLACERS

DISTRIBUTION

The accompanying map shows that all the creeks hereafter mentioned whose gravel has been mined for its content of gold lie southwest of the Yukon River, also that the granite intrusive bodies occur on the same side of the Yukon. The country northeast of the Yukon has not yet, to be sure, been much explored geologically, but the gravel from the streams draining that region does not indicate the presence there of any extensive granitic intrusives close to the river. The smaller degree of metamorphism seen in the rocks northeast of the Yukon also tends to confirm this idea. It is believed that all the placer gold in this region came originally from these granitic masses, although the proximate source may be gold quartz veins, mineralized shear zones, or ancient placer deposits. This distribution of gold placers is therefore only what might be expected from what is now known of the regional geology. This statement is not intended as a general proscription of all the region northeast of the Yukon as a possible seat of gold mineralization and gold placers, for practically all of the triangle between the Yukon and Porcupine Rivers and the international boundary is as yet unmapped geologically, and it may contain granitic bodies that have mineralized the surrounding rocks. But along the northeast side of the Yukon in the zone between Eagle and Circle the known geology does not encourage the hope for finding commercial gold deposits.

The two most productive gold placer camps in this region are the Fortymile precinct, lying south of Eagle, and the Circle precinct,

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

8

In a previous publication the writer 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 favor

8 Mertie, J. B., jr., The occurrence of metalliferous deposits in the Yukon and Kuskokwim regions: U. S. Geol. Survey Bull. 739, pp. 149-165, 1923.

able 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.

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

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