ELUVIAL-ALLUVIAL

Placer Deposits

There are about 50 pans (18 in. diameter) per yard. Coarse gold (nuggets) is usually considered to be that which remains on a 10-mesh screen (>l.5mm); medium gold will pass a 10-mesh screen and be held on a 20-mesh screen; fine gold passes a 20-mesh screen and is held on a 40-mesh screen (value I color per cent); and very fine gold passes a 40-mesh screen. Flour gold is very much finer than the last; in some places 500 to 1000 colors or more are required to equal the value of a cent!

In general it can be said that the farther that gold has traveled from its source the smaller in size the particles become. Near the source the gold particles are usually rough and relatively large; with distance the particles become smooth, much rounded, flattened and smaller. The particles also apparently lose mass with distance from the source. It was noted that particles of the Snake River gold, near its source below the outcrops of auriferous conglomerates in Jackson Hole, with diameters approximating 0.125 mm, averaged l0mg in weight, whereas near Twin Falls, Idaho, some 300 km downstream, particles of the same diameter weighed only half as much. The cause of the mass loss is not exactly known.

To summarize this introduction we can say that for the development of placers of any type, four requisites are necessary:
1. The occurrence of gold in deposits, in widespread quartz veins and blows or in a disseminated form in pyritic shales or other country rocks.
2. A fairly long period of deep secular chemical and mechanical weathering on a surface of submature to mature topography, during which time the gold is set free from the deposits or country rocks.
3. Concentration of the gold by some agency, generally water.
4. Absence of extensive glaciation: glaciation does not entirely preclude the occurrence of placers since both eluvial and alluvial types of placers may be overridden and little disturbed by the glaciers in some cases and buried by their deposits of till, clay, etc. in others. Such placers occur in British Columbia, Alaska, Yukon, Quebec and the Lena district of Russia.

Country kindly to the occurrence of extensive placers is readily recognized. The topography is subdued and marked by broad, often terraced entrenched valleys and rounded deeply weathered hills commonly with nearly accordant summit levels. Few extensive placers are found in terrains marked by sharp alpine features and high gradient V-shaped valleys; similarly, excessively flat terrains far from mountain systems and their foothills yield productive placers only in tropical terrains where chemical reactions are a predominant agents of transport. Like all generalities in geomorphology and geology there are some exceptions to these observations.

It need hardly be emphasized here that the processes involved in the concentration of gold in placers are extremely complex, and the conditions that control them so variable that even under the most favorable circumstances observable, it is generally not possible to estimate the potential of a placer even roughly without detailed sampling, either by pitting, test holes, or deep cuts or trenches.

The distribution of the principal eluvial and alluvial auriferous placer districts of the world is shown in attached map. Many of the districts have long been exhausted, some before the beginning of the Christian era.

[ Placer Deposits 1 2 3 Eluvial 5 6 Alluvial 8 9 10 Examples 12 13 14 15 ]

Maps of alluvial gold deposits in: California, Western Canada, Eastern Canada, Russia, World
Maps of primary gold deposits in: Precambrian, Paleozoic, Mesozoic, Cenozoic Rocks
Diamonds: Large and Famous Properties Geology and Mining Diamond Cutting

Rafal Swiecki, geological engineer email contact
February, 2008

This document is in the public domain.