Examples of alluvial deposits
The country rocks and the gold-bearing veins are deeply weathered, the latter down to at least 7 m. The stable minerals in the oxidized zones of the veins are mainly scorodite, limonite, residuals of pyrite and arsenopyrite and some anglesite and beudantite. Assays of the oxidized material are highly variable in gold content, ranging from 0.5 to 10 g Au/ton; the silver content is, likewise, highly variable averaging about 16 g Ag/ton. The gold in the oxidized zones consists of fine dust, small flakes and abundant wires. Some of the gold is closely associated with the scorodite and limonite and is probably in a chemically bound or adsorbed form.
Most of the gold in the oxidized zones is about 850 fine, although there are marked differences from vein to vein.
The area immediately around Dublin Gulch is beyond the limit of the last well-marked Pleistocene glaciation, but evidence of an early glacial incursion is present as witnessed by what seem to be remnants of weathered tills and also by the appearance in the placer concentrates of nodules of hematite (iron-formation), which probably came from the Snake River deposit in northern Mackenzie Mountains, Yukon. The streams in Dublin Gulch and Haggart Creek have entrenched parts of their courses in deep overburden forming terraces with old modified profiles. The exact nature of the original profile in the lower part of Dublin Gulch is difficult to ascertain because of the extensive stirring and digging since 1898. What appears to have been present at the surface was about 2 to 3 m. of gravels, schist particles, soil and granite boulders, probably mainly due to creep and slope wash. This was succeeded in places by 0.6 to 1 m. of bluish clay that overlay 1 to 1.3 m. of yellowish (limonite coated) gravels and weathered debris. The last materials represent the detritus arising from deep secular weathering of the bedrock of the area and the contained deposits during a period of uplift in late Tertiary time.
According to old reports fine colors of gold occurred in the surface materials where washed by the stream in the gulch. Most of the gold, however, is on the bedrock in the bottom foot or so of the lower gravels and weathered debris. The pay streak begins at Bawn Bay Gulch and continues with only a few breaks downstream. It is about 100 ft wide near the mouth of Dublin Gulch, narrowing gradually upstream. The grade is about 0.75 g Au/m3.
The gold is accompanied by scheelite (about 0.4 kg/m3), some woiframite and a little cassiterite. In addition there are a number of other heavy minerals, including magnetite, hematite nodules (iron-formation), arsenopyrite nodules, jamesonite nodules, bismuth, galenobismutite, rarely bismuth tellurides, pyrite, tourmaline and garnet. The gold is present as fine dust, scales, rough wires and sprigs, occasional crystals and small nuggets ranging from pea size up to about the size of hickory nuts. Most of the nuggets are worn and pitted. The principal minor and trace metals in the gold are Ag, Cu, Fe, Hg, Bi and Sn. All of these occur in the deposits from which the gold came. The average fineness of the Dublin Gulch gold is about 900.
The Dublin Gulch area provides an excellent example where gold can be traced from primary deposit to stream placer. The primary gold is mainly microscopic to submicroscopic and associated with pyrite, arsenopyrite and sulphosalts. Particles of gold greater than 100 µ are rare. On the oxidation of the sulphides and sulphosalts the gold was released and formed flour or mustard gold in some cases, but there was also a nucleation and growth of gold on fine rough wires, flake and small nuggets in the oxidized zones. All of these types of gold passed gradually into the eluvium where they probably accreted more gold on their journey to the gulch. Here, they passed slowly down gradient, some particles possibly accreting more gold but most being rolled along and hammered by the pebbles of the mills of nature. The fineness has changed during the journey from oxidized zone to placers in Haggart Creek a distance of some 5 km. In the oxidized zones the fineness averages about 890, in Dublin Gulch 900 and in Haggart Creek 925.
Stream and river placers
The main features of these placers are discussed in some detail above. Briefly their characteristics are:
(1) most occur in districts with a subdued topography marked by broad, often terraced, entrenched valleys developed in a terrain of rounded, deeply weathered hills;
(2) the present (and past) gradients of the streams are moderate to low;
(3) the pay streaks are generally not as rich as those in gulches and creeks, but they are longer, commonly wider and more uniform. Multiple pay streaks may occur at the same elevation or they may be stacked at different elevations. Most of the rich pay streaks are on the bedrock;
(4) the gold is generally finer grained than that in creek and gulch gravels. Large nuggets are relatively rare in stream and river placers;
(5) the fineness of the gold is usually higher than that in creek, gulch and eluvial placers;
(6) the overburden is commonly tens of meters to hundred of meters deep;
(7) most stream and river placers are amenable to large scale hydraulicking and dredging operations.
There are many famous districts where stream and river placers have been extensively worked. Some provide good Examples and will be described briefly.
Klondike district, Yukon
The stream and river placers of the Klondike district near Dawson, Yukon were discovered in 1896 or earlier and have yielded some 10 million ounces, more than half of the placer gold production of the Western Cordillera of Canada. Their heyday is past, although there are still some small hydraulic and bulldozer operations on some of the streams.
The most productive creeks were Bonanza, Hunker, Dominion, Gold Run, Sulphur and Quartz. The underlying bedrock is mainly the Klondike Schist (Klondike Series) comprising folded and faulted sedimentary quartz-mica schists, chlorite schists, sericite schists, quartzites, phyllites, pyritic graphitic schist and highly sheared quartz porphyry sills or flows sandwiched between the sediments.
The age of these rocks is not precisely known; they are probably Precambrian.
Throughout the Klondike Schist there are myriad primary quartz bodies, most carrying traces to small amounts of gold.
Briefly these bodies include:
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.