Examples of alluvial deposits
The White Channel gravels vary in thickness from a few feet to 50 m. and in width from 30 to 800 m. or more. The deposit increases in volume descending the streams, and attains its greatest development near their mouths.
The deposits, unlike the creek and gulch gravels, appear to be destitute of vegetable and animal remains.
On Gold, Adams and other hills on Bonanza Creek the typical compact white variety of the White Channel deposit is replaced towards the sides of the old valley by flat rusty colored gravels, more loosely bedded and containing a smaller proportion of quartz than the ordinary white variety. These probably represent flood plain deposits. They have the appearance of overlying the white variety and were formerly considered to be younger. However, the long exposures, now available for study in the various hydraulic cuts, show that the two varieties pass gradually one into the other both horizontally and vertically and in places are interbedded, evidence of nearly contemporaneous deposition. The loose yellow variety seldom carries much gold.
The White Channel gravels were probably deposited by winding streams with easy grades and comparatively slack currents. The preponderance of vein quartz pebbles and boulders, the most resistant rock in the district, gives them the character of a residual deposit. They were built up slowly and in the long process the softer rocks were mostly destroyed and carried away.
The great length of the White Channel period is indicated by the enormous gold accumulations, all derived from the slow breaking up of auriferous quartz veins that took place in it. Since the close of the period the additions to the supply have been trifling, although a sufficient time has elapsed to enable small streams to excavate channels, mostly through hard schists from 100 to 200 m. in depth. Practically all the gold in the present low level valley flats is of secondary origin and derived from the partial distribution of the older gravels.
The age of the White Channel gravels has not been determined, but they must date back to the Pliocene at least. They were certainly deposited before the advent of the present severe climatic conditions, as the white coloration is largely due to the leaching out of the greater portion of the iron by circulating surface waters, and this must have taken place before they were permanently frozen.
Summarizing we can say that the gold in the Klondike placers has passed through at least four stages of concentration as follows:
Each of these stages has resulted in a successive enrichment, the last stage being phenomenally rich in some creeks.
The rich pay streaks in the White Channel gravels invariably occur in the first few feet above the bedrock or in the upper foot of the bedrock. False bottoms are rare. The remainder of the gravels is slightly auriferous carrying a few cents per m3. In the low level creek, gulch and river gravels and in the terrace gravels the pay streaks are commonly on the bedrock in the first few feet of gravel and weathered debris.
Where the bedrock is tight the thin stratum of gravel immediately above the bedrock is exceedingly rich. Where the bedrock is open the first meter of the bedrock and the first meter of the overlying gravels constitute pay material. In some places in a few creeks the shattered and frost wedged schist layers of the bedrock near the erosion surface contained all of the gold, the overlying gravels being practically barren. Most of the pay streaks in the Klondike were remarkably persistent in length, wide, some up to 300 m. or more, and exceedingly rich. The one in the low-level creek gravel of Eldorado Creek for instance ran some 200 oz per running meter for 7 km.
The gold of the pay streaks is variable both in appearance and fineness. Near the heads of streams and in the gulches and draws the gold in the low level gravels occurs as coarse, commonly angular grains, flakes, sprigs and wires, and as nuggets that are fairly large. Farther downstream the gold particles are more rounded, flattened, finer grained, and large nuggets are uncommon. The gold in the White Channel gravels ranges from fine-grained to relatively coarse, rough flakes, wires and particles. Nuggets are common but not generally very large. In both types of gravels, low and high level, the gold commonly has a crystallized character, and some of the nuggets enclose quartz or are partly enclosed by this mineral. The quartz is identical with that seen in the gold-bearing quartz veins, blows, etc.. Dendritic gold occurs in places on pebbles and stones in the placers, and crystals are not uncommon.
Spectrographic analyses of gold from a number of the creeks samples report Ag, Cu, Hg, Ti, Mg, Al and Fe, and some contain traces of Pb, As, Sb, V, Ba and Sn. The fineness of the Klondike gold is exceedingly variable not only on different creeks but also along different portions of the same creek. The lowest fineness recorded appears to be about 625 and the highest 890.
The minerals accompanying gold in the Klondike placers:
The most abundant suite in the low-level creek gravels included actinolite, barite, chlorite, epidote, hornblende, hypersthene, garnet, goethite, ilmenite, leucoxene, magnetite, pyrite, sphene, rutile, tremolite, tourmaline, zoisite and zircon; the less abundant to rare suite composed andalusite, anatase, apatite, biotite, cerussite, chromite, clinopyroxene, clinozoisite, dolomite, jarosite, micrometeorites, fluorite, kyanite, olivine, spinel, staurolite, topaz, monazite, scheelite, sphalerite and cassiterite. The heavy minerals in the White
Channel gravels are the same but slightly more rounded.
Highly resistant minerals such as garnet, magnetite and zircon are generally more plentiful than the softer minerals such as barite, goethite, pyrite and jarosite.
The origin of the gold in the Klondike placers has long been debated because few sizeable and rich primary gold-bearing deposits have ever been found in the district. There are a number of facts which support the view that most of the gold came from the quartz deposits, including the blows, stringers, etc. that are probably metamorphic secretion products and are younger than the quartz-barite and quartz sulphide veins mentioned above. These facts can be enumerated as follows:
1. All of the quartz-bearing deposits are auriferous. Some carry visible gold, occasionally in spectacular amounts.
2. The gold nuggets with quartz indicate that at least some of the gold came from quartz deposits. The quartz in the nuggets can be matched with that in the primary quartz deposits.
3. A number of the heavy minerals accompanying the gold in the placers evidently came from the gold-bearing veins. Barite is one of these minerals, and galena is another. Some of the pyrite in the placers matches that in the veins, and some came from the pyritic graphitic schists.
4. There is a general spatial relationship between the gold placers and the distribution of the Klondike Schist and the gold-bearing deposits it encloses. This suggests that the vein deposits, and probably also some of the rocks such as the pyritiferous graphitic schists, gave rise to the gold placers.
It seems probable, basing conclusions on these facts, that the gold came mainly from the quartz deposits enclosed within the Klondike Schist. The amount of quartz as a rough estimate in the Tertiary White Channel gravels as originally laid down probably exceeded 6 x l09 tons which at 0.3 g Au/ton of quartz according to our average analyses would yield about 60 million ounces of gold of which say one half, or 30 million ounces were released to accumulate in the modern gulch and stream placers. If we add gold which would have been released during Tertiary time from the weathered pyrite in the country rocks, especially the graphitic schists, the amount of gold for placer concentration in the White Channel gravels would be greatly augmented.
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.