Examples of alluvial deposits
The pyritic graphitic schists of the Klondike Schist are also feebly auriferous. They contain about 0.005 ppm Au, mostly in the pyrite, sample's of which contain up to 2 ppm Au.
The Klondike district is a thoroughly dissected upland, a part of the Yukon Plateau, marked by rounded hills and a multitude of small streams tributary to the main watercourses. The valleys are flat and wide in their lower reaches, but gradually narrow towards their heads into steep-sided narrow gulches ending abruptly in broad amphitheatres. Many of the lower slopes of the valleys are conspicuously terraced. The district has not been glaciated, but is underlain by permafrost, in places to a depth of 200 ft. This necessitates thawing the ground out before dredging operations can begin.
The placers of the Klondike owe their existence to a general uplift in late Tertiary time followed by deep secular weathering of the bedrock and the contained gold-bearing quartz bodies. V-shaped valleys were formed first, and these were gradually widened and filled at maturity with extensive gravel deposits. A later period of uplift, probably of Recent or Late Pleistocene age, has resulted in the streams channeling their valleys deeply, almost to bedrock in places, and leaving the characteristic terraces as benchmarks in their downward migration.
The auriferous Klondike gravels into three categories as follows:
Gravels at intermediate levels
The 'low-level creek gravels' are the most important gravels in the Klondike. These gravels floor the bottoms of all the valleys to a depth of from 1 to 3 m.. They rest on bedrock usually consisting of decomposed and broken schists and are overlaid by a sheet of black frozen muck ranging in thickness from 0.6 to 9 m. or more. They are local in origin and consist entirely of the schists and other rocks outcropping along the valleys. The schist pebbles are usually flat round-edged discs measuring 2 to 5 cm. in thickness and 5 to 15 cm. in length. They constitute the greater part of the deposit but are associated with a varying proportion of rounded and subangular quartz pebbles and boulders, and less frequently, with pebbles derived from the later eruptive rocks of the region. The pebbles are loosely stratified, are usually embedded in a matrix of coarse reddish sand and alternate in places with thin beds of sand and muck.
The 'low-level gulch gravels' occupy the upper portions of the main creek valleys and small tributary valleys. They differ from the creek gravels in being coarser and more angular. A considerable proportion of their material consists of almost unworn fragments of schist washed down from the adjacent slopes. They contain the same vegetable and animal remains as the creek gravels.
The only 'low-level river gravels' of the district proven, so far, to contain gold in paying quantities occur in the wide flats bordering the lower portion of the Klondike River below the mouth of Hunker Valley. The river gravels consist of quartzite, slate, chert, granite and diabase pebbles largely derived from the western slopes of the Ogilvie Range. They are harder and better rounded than the creek gravels, a result of the greater distance traveled.
The 'intermediate level terrace gravels' lie on rock terraces that occur at various points cut into the steep slopes of the present valleys. The terraces were produced during the deepening of the valleys and are simply remnants of former valley-bottoms. Most are small, seldom exceeding a few meters in width and a few hundred meters in length, irregular in distribution and occur at all elevations up to the bottoms of the old valleys. The beds of gravel supported by the terraces are usually from 2 to 5 m. in thickness, very similar to those in the creek bottoms but showing somewhat more wear. The terrace gravels, like the creek gravels, are overlain, as a rule, with muck, and at one point on Hunker Creek were found buried beneath 30 m. of this material.
The 'High-level river gravels' are extensively distributed along Bonanza and Hunker creeks and some of their tributaries and also occur on Eldorado, Bear, Quartz, Nine Mile and Allgold creeks. They consist, principally, of ancient creek deposits overlain near the mouths of some of the valleys by gravels laid down by the Klondike River when it ran at a much higher level than at present and occupied a somewhat wider valley.
Hunker creeks from the Klondike River terminate. They rest, in both places, on high-level creek gravels at an elevation of about 150 m. above the present valley bottoms. They have a thickness of from 50 to 60 m., and consist principally of well-rolled pebbles of quartzite, slate, chert, granite, diabase, and conglomerate embedded in a matrix of gray sand and derived, like those in the present stream, from the western part of the Ogilvie Range. The high-level river gravels are reported to contain gold in paying quantities at Acklens farm, a name given to a portion of the bench on the right limit of the Klondike, 3 km. above its mouth; they are, however, generally of little economic importance.
The 'high-level creek gravels' or 'White Channel gravels' are ancient creek deposits laid down in the wide, flat-bottomed valleys, which characterized the region before the last general uplift. After their deposition, the country was elevated 200 to 230 m., and the increased grades acquired by the streams enabled them to cut down through their old gravel beds into the bedrock beneath, and to excavate the steep-sided troughlike valleys in which they now run. The old gravels now occur on wide benches bordering the present valleys at elevations of from 50 to 100 m. above them, the elevation generally increasing downstream. Their distribution along the valleys is irregular, as a large portion of the deposit was destroyed during the deepening of the main valleys and the tributary valleys and gulches.
The general character of the White Channel gravels is remarkably similar in the various Klondike creeks but differs considerably from the ordinary type of stream deposits in other parts of Yukon. They consist of a compact matrix of small, clear, little-worn and often sharply angular grains of quartz and scales of sericite thickly packed with rounded quartz pebbles and rounded and subangular and wedge-shaped quartz boulders often 0.6 to 1 m. in diameter. Flat and subangular pebbles of sericite schist, the principal rock of the district, are also present but in much smaller numbers than the quartz constituents. The schist pebbles are usually decomposed and crumble rapidly when thawed out. The deposit is always stratified, but except in rare instances, there has been no sorting of the various constituents into separate beds, and the composition is very uniform throughout. The color is characteristically white or light gray due to the preponderance of the quartz constituents and the leaching out of the greater part of the iron. The color is darker and the sands are noticeably coarser towards the limit of the deposit on the upper part of the creeks.
Maps of alluvial gold deposits in: California, Western Canada, Eastern Canada, Russia, World
Maps of primary gold deposits in: Precambrian, Paleozoic, Mesozoic, Cenozoic Rocks
Rafal Swiecki, geological engineer email contact
This document is in the public domain.