Residual placers - Also known as "seam diggings." In order that gold may become released from its original source in bedrock, the encasing material must be broken down. This is most effectively done by long-continued surface weathering. Disintegration is accomplished by persistent and powerful geologic agents, which effect the mechanical breaking-down of the rock and the chemical decay of the minerals. The surface portion of a gold-bearing orebody will become enriched during this process of rock disintegration, because some of the softer and more soluble parts of the rock are carried away by erosion, leaving the remaining portion of higher tenor. The name residual placer is applied to this type of deposit. After the residual portion is mined away by comparatively inexpensive methods, the harder mineralized rock is encountered, and the mining methods must be changed to accommodate another type of deposit, i.e., the lode. The so-called "seam diggings" are in weathered, gold-bearing quartz stringers, occurring along fracture zones of disintegrated schists.

Eluvial placers - Also known as "hillside placers." After gold is released from its original bedrock encasement through agents of rock decay and weathering, the whole weathered mass may "creep" down the hillside (in some regions partly because of frost heaving) and may finally be washed down rivulets into gulches. Lindgren, a famous geologist states:

When the outcrops of gold-bearing veins are decomposed a gradual concentration of the gold follows, either directly over the primary deposits or on the gentle slopes immediately below. The vein when located on a hillside bends over and disintegration breaks up the rocks and the quartz, the latter as a rule yielding much more slowly than the rocks: the less resistant minerals weather into limonite, kaolin, and soluble salts. The volume is greatly reduced with accompanying gold concentration. The auriferous sulphides yield native gold, hydroxide of iron, and soluble salts. Some solution and redeposition of gold doubtless take place whenever the solutions contain free chlorine. The final result is a loose ferruginous detritus, easily washed and containing easily recovered gold. This gold consists of grains of rough irregular form and has a fineness but slightly greater than that of the gold in the primary vein.

On its way down the hillside, gold is sometimes concentrated in sufficient value to warrant mining. Such deposits are classified as eluvial placers. They are transitional between residual and stream, or alluvial, deposits.

There have been a number of residual and eluvial gold deposits mined in the United States.

Stream placers - By far the most important type of placer is the ordinary alluvial gravel or stream placer. So far, it has been the source of most of the placer gold mined; but now its supply is nearing depletion, save for values remaining in those ancient channels which lie deeply buried beneath a cover of lava or rock debris. Deposits by streams include those of both present and ancient times, whether they form well-defined channels or are left merely as benches. Stream placers consist of sands and gravels sorted by the action of running water. If they have undergone two or more periods of erosion, and have been re-sorted, the result will in all probability be a comparatively high degree of concentration of the heavier mineral grains. In order to understand thoroughly the subject of stream placers, streams themselves must be studied in regard to their habit, history, and character. The effects of existing and changing climates, the relation to surrounding geologic conditions, and the effect of movements of the earth must also be considered.

Glacial-stream placers - It is a frequent fallacy of the placer miner to attribute the deposition of gold-bearing gravels to the action of glaciers. Contrary to such belief, glaciers do not concentrate minerals; the streams issuing from melting ice, however, may be effective enough in sorting debris to cause placers to be formed under certain especially favorable conditions.

Bajada placers - Also known as "Alluvial fans." A bajada is a confluent alluvial fan along the base of a mountain range. B.N. Webber described the bajada type of placer deposit as such:

Bajada is the Spanish term for slope and is used locally in the Southwest to indicate the lower slope of a mountain range, the portion consisting of rock debris and standing at a much lower angle than the rock slope of the range proper....

The total production of gold from bajada placers in the southwestern United States is necessarily small, probably not over ten million dollars....

Most of all bajada placer gravels are Quaternary and the larger part are recent...

The genesis of a bajada placer is basically similar to that of a stream placer except as it is conditioned by the climate and topography of the arid region in which the placer occurs...

Erosion, transportation and deposition in a region of extreme aridity present some phenomena not encountered in more humid areas. Practically all the work of running water is strongly conditioned by aridity...

Rock-floored canyons through which rock fragments are moved by infrequent torrential floods should constitute excellent pebble mills for the further reduction of the material, but the amount of attrition accomplished seems to be slight, as fragments, large or small, on the bajada slope are decidedly angular and show little effect of attrition...

Probably a small percentage of the gold is freed during this phase of the movement of gravel. The gradient of these intermont drainage channels is too high to permit lodgment of the finer gravel. When a small amount of gravel is temporarily lodged in one of these channels, the deposit displays most of the characteristics of stream gravel. As debris reaches the bajada slope a rapid diminution in volume of water due to seepage and an extreme decrease in the grade of channel causes deposition of debris, and either (1) an alluvial fan or (2) a gravel-mantled pediment may be formed. If detritus is supplied to a bajada slope much faster than it can be removed, an alluvial fan is the result....If rock debris is supplied to the slope in considerable volume but not in excess of the quantity capable of transference to the center of the basin by the existing agencies, a gravel-mantle pediment results...

The bulk of the gold that has been released from its matrix on the journey from lode outcrop to bajada slope is deposited on the bajada slope close to the mountain range. The gold is dropped along the contact of the basin fill and bedrock; this is referred to hereafter as the lag line and is coincident with the line of contact of bajada gravels lying at a low angle and the rock slopes of the range standing at a high angle...

The heaviest deposition of gold is on bedrock at the lag line, and since the lag line is moving in the direction of the crest of the range, values on bedrock may be distributed over a large area of which the longest dimension is parallel to the foot of the range. Because bulk concentration does not operate as in a river channel, and a certain percentage of the gold is still locked in fragments of matrix, to be partly released by further disintegration on the bajada slope, there is a strong tendency for less gold to reach bedrock and for more to remain erratically distributed throughout the detritus than in the case of stream gravels.

Eolian Placers - Eolian placers are localized concentrations formed by winds blowing away the fine material. Wind action is responsible for the removal of large amounts of fine detritus in the desert. The process involved has been called deflation. It is quite likely that it will be found to play an important part in the surface concentration of desert placers. Although, no commercial eolian gold deposits such as those mined in Australia are known in the United States.

Beach Placers - Concentrations of heavy minerals occur in various places along the Pacific Coast as a result of the action of shore currents and waves, which tend to sort and distribute the materials broken down from the sea cliffs or washed into the sea by streams. Under special circumstances gold deposits can be formed by the action of the waves, winds, and currents on the sea shores. Beach placers are of two kinds, (a) present beaches and (b) ancient beaches. The beach placers of economic importance are those that have been reconcentrated over and over again.

Rob Allison © 2001-2002

Bryson Burke Diamond Corporation
© 2001 - 2003