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Alluvial Deposits

Where the bedrock has a higher magnetic intensity than the gravel, the position of magnetic lows may indicate the position of the pay streak channels. Electrical methods may also assist where there are conductivity or other differences between the bedrock and the basal alluvium.

Refraction seismic also can be used for profiling and outlining the notches or gutters containing pay streaks. The pay streak in gulches and youthful streams is generally easily located. It is commonly in or near the notch or gutter of the V-shaped valley or gulch on and just above bedrock.

As the valleys mature, widen out and become filled with sediments the streams shift their location from time to time, and the pay streaks may no longer be in the lowest depression or gutter. Furthermore, pay streaks long since buried may bear no relation to the present streams. Pay streaks in wide valleys, flood plains and deltas are extremely irregular in their distribution and are the product of an environment where shifting channels and migrating meanders hold sway. In most streams and rivers where placers are forming, gold collects along bars due to some obstruction, diversion or slackening of the water course in the slower moving water on the inside curves of fast-flowing meanders, in the main stream near the mouths of tributaries and elsewhere where slack water prevails. These guides are useful in the study of both recent and ancient watercourses. With respect to the latter, however, care must be taken to ascertain the direction of the paleogradients. For instance the Tertiary channels of the Sierra Nevada have been tilted by movements initiated by block faulting so that some of the gold channels now 'run up hill' as compared with the present gradient of the country.

Pay streaks may be reworked by subsequent streams in the same or different courses into exceedingly rich concentrations. In a broader sense the reworking of former placers may also be economically important in places.

Thus, the lean placer concentrations of gold and platinum formed during the Mesozoic and early Tertiary in the Urals, Russia were uplifted and crossed by a Pliocene-Quaternary drainage system in which rich new placers were concentrated.

Deltas may form in rivers, lakes, seas and oceans, but few gold placers in deltas in these locations have been described, probably because they are relatively rare. The principles governing the pay streaks in deltas are the least understood.

Deltas in rivers tend to concentrate gold in fairly well defined streaks just downstream from the mouths of the streams, mainly in topset and foreset beds if these are developed.

Deltas in lakes commonly receive only fine-grained to very fine-grained (flour) gold, and the metal may be dispersed more or less throughout the sedimentary pile. In some cases, however, the fine-grained gold may collect in streaks on the foreset beds, and these may have relatively high angles of inclination. Deltas in seas and oceans do not seem to develop well-defined streaks, probably because of the extremely fine subdivision of most of the gold reaching the oceanic environment. If streaks do develop, they are usually erratic and seem to be mainly located in the topset and foreset beds.

An unusual type of alluvial placer is located in Burma. This placer, near Tenasserim, consists of gold in mangrove swamp mud. The gold appears to be present in a colloidal form since repeated examinations of samples and concentrates under a high power microscope failed to show any free gold. From the description it would seem that this is a case of concentration of gold in a humic environment, perhaps by chelation or other organic bonding. The swamp mud averages about 0.285 ppm Au and is said to contain some £200 million worth of the metal.

At this point a few words should be said about alluvial placers originating from the reworking of glacial debris by glacial and later streams.

Valley glaciers give rise to vast terminal moraines and during wasting may clog the valleys with great accumulations of glacial till, clay and sand. If these glaciers have scoured valleys containing gold placers or the sides of valleys containing oxidized or unoxidized gold deposits, the glacial materials may be slightly auriferous. Reworking of these materials by meltwaters and subsequent streams may produce stream and river placers in the normal manner. Many of these placers are often buried, difficult to recognize, and their pay streaks are frequently erratic. Nevertheless, a number of these types of placers have been worked in the past particularly in New Zealand, and there are others in the Rocky Mountains of Canada and the United States.

The action of piedmont glaciers and ice sheets (continental glaciers) has generally been disruptive and destructive of preexisting placers. In places, however, there have been reconcentrations by glacial streams from the disrupted placers as in Yukon, British Columbia, Quebec and Siberia.

From the general considerations discussed above, some general conclusions about the concentration of gold and other heavy minerals in alluvial placers can be stated.
1. Coarser gold is generally deposited in the upper reaches of a placer gulch, stream or river, and the finer grained gold is generally deposited in the lower reaches of placer watercourses.
2. The richest and coarsest gold and nuggets are commonly deposited in the layers with comparatively coarse sediment, and the finer gold particles are commonly deposited in the finer, sandy fractions of the sediment.
3. Gold should normally occur in streaks with other heavy minerals such as magnetite, ilmenite, scheelite, barite, etc.
4. Gold found on a bottom with a steep gradient will commonly be coarser than that found on a flat-lying level bottom.
5. In narrow gulches and gorges mainly coarse gold will be deposited.
6. Little gold will lodge in places where scouring action is marked.
7. Pot holes do not generally form effective traps for gold mainly because of the centrifugal grinding action and ejection of fine gold from such sites.
8. On a favorable bottom the gold generally collects on the downstream side of natural riffles.
9. Zones of slack water in watercourses are favorable sites for the accumulation of gold particles.
10. Glaciation does not preclude the occurrence of placers.

Beach placers

Beach placers can be subdivided into two categories - modern and fossil. They are formed by the winnowing action of waves, undertow and alongshore currents along present and past shorelines bordering lakes, seas and oceans where a source of primary gold has been available for concentration.
Tidal movements and strong winds may accentuate the concentrating processes in some places. Beach placers tend to form predominantly along rectilinear shorelines; few are known along markedly broken and rocky shores. The source of the gold may be in the rocks along the shores; in sediments deposited by streams upon the coast; and in slightly auriferous stream or river terraces, sea terraces and gravel plains that border the coasts.

The constituents of beach placers are essentially the same as those in the alluvial category. Quartz pebbles and sand predominate, but on some beaches there may be considerable shingle composed of the pebbles and stones of the country rock of the adjacent region. Magnetite and ilmenite form the bulk of the heavy minerals in most deposits. Clay beds and hard pans are common in some beach deposits especially in those of a raised character. These may form false bottoms for pay streaks. Coquina accumulations may be present in some instances and may mark the lines along which the richest pay streaks occur in modem beach placers. In fossil beach placers coquina may be invaluable as marker horizons.

Most of the gold in beach placers is fine grained and generally of high fineness (greater than 900). The richest pay streaks usually follow the strand lines; most are a few centimeters to a few meters in thickness and generally only a few tens to several hundred meters in width. Their continuity along the strandline is frequently erratic, and numerous cutouts are encountered. Elevation or depression of the land leads respectively to the formation of raised or deeply buried beach placers.

[ 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
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Rafal Swiecki, geological engineer email contact

This document is in the public domain.

March, 2011