Types of Placer Deposit

Types of Placer Deposit
(a) Residual Placers: -

A residual placer is, in effect, a concentration of gold (or other heavy mineral) at or near its
points of release from the parent rock. Residual placers, which, when formed directly over
outcrops or on gentle slopes below them, are sometimes called "eluvial" placers.
(b) Eluvial Placers: -
Eluvial placers usually represent a transitional stage between a residual placer and a stream
placer. Where one type merges into another, they cannot be clearly distinguished. They are
characteristically found in the form of irregular sheets of surface detritus and soil mantling a
hillside below a vein or other source of valuable mineral. It should be noted that the parent
vein or lode may or may not outcrop at the actual ground surface. Eluvial placers differ from
residual placers in that surface creep slowly moves the gold and weathered detritus downhill,
allowing the lighter portions to be removed by rain wash and wind. The detrital mass
gravitates downhill, a rough stratification or concentration of values may develop but this is
rarely perfected to the degree found in stream placers. Eluvial placers are typically limited in
extent but there have been cases such as at Round Mountain, Nevada, (Vanderburg, 1936, pp,
133-145) Where this type of placer supported large-scale mining operations.
(c) Gulch Placer
When a stream start near the divide of a high steep mountain, its channel will be a narrow V-
shaped gulch. As there is very little drainage area above the stream, it will in most cases flow
only during the raining season or during summer cloud-bursts. Because of steep gradient, the
gravel accumulations are often thin and discontinuous.
(d) Creek placers: - Gulch streams empty into larger streams called creeks. The turbulent water
rushing into the somewhat quieter current has its velocity checked slightly: and if gold is
being carried, much of it is deposited.
(e) River deposits:-
River deposits are presented by the more extensive gravel flats in or adjacent to the beds of
present-day rivers and as a class; they have been our most important source of placer
minerals. They are generally similar to creek placers but the gold is usually finer, the gravel
well-rounded and large boulder fewer or absent. Although the overall deposit may be low-
grade, pay streak and bed rock capable of supporting dredging or other large scale mining
operation are not common.
(f) Gravel plain deposits
These are somewhat difficult to define as they may grade from river or bench deposits, into
flood-plain or delta-type deposits and they can be geologically old, or recent. Gravel plains
are found where a river canyon flattens and widens or more often, where it enters a wide, low
gradient valley. The contained placers are generally similar to those in river deposits except
or greater size and a more general distribution of gold. Because gravel plain deposits are built
by shifting stream channels, their gold is apt to have a wide lateral and vertical distribution
and because of stream flowing over flood plains, their placers are commonly made up of
smaller size gold compare with that found in the main stream deposits, any larger gold carried
by the main channel will likely be dropped closed to the upper edge of flood plain where the
stream's velocity decreases and transporting ability is reduced. Although subject to surface
wash and flood erosion, most gravel-plain deposits are relatively permanent.
(g) Bench placers
Bench placers are usually remainder part of deposits formed during an earlier stage of stream
development and left behind as the stream cuts downward. The abandoned segments,
particularly those on the hillsides, are commonly referred to as "bench" gravel. Frequently
there are two or more sets of benches in which case the miners refer to them as "high"
benches and "low" hunches. Bench deposits are formed by the river cutting itself a newer and
deeper channel in the bed-rock; rivers and streams may change their course and levels from
time to time, with the result that alluvial deposits may occur not only in the present river
beds, but also in older buried channels which form benches on the slopes of the existing
valleys.
(h) Beach Placers: -
By the time the waters of a river have reached the ocean, they are usually flowing very
slowly. Their ability to carry heavy particles has been reduced to such an extent that only the
finest sediment is transported. Flour gold, because of its relative size, is easily carried. Much
of this type of gold is dissolved by the sea water. Sea water will assay approximately 50 cents
per, ton in gold. To date no one has devised a method of extraction that can be applied at a
profit. If the mountains are steep immediately adjacent to the sea, the discharge of the creeds
will be turbulent and the deposited material coarse and unsorted. Gold will be in nuggets and
will be only slightly attacked by the sea water. Consequently, by the jiglike motion of the
waves a good concentration of the gold will take place, forming the rather scarce beach
placers.
(i) Glacial Deposits:
The mineral examiner working in the Western States may seldom encounter a placer directly
working in the west associated with glacial deposits but, on the other hand, it is not unusual
for a miner to assert that a particular deposit, particularly if its origin is obscure, is a "glacier"
placer. For this reason the engineer should known about glacial deposits as they relate to
placers.
(j) Lake-bed Placers:
In past geological ages, certain areas had more lakes than they have at the present time. In
some of these the old water-level marks may still be seen. Streams flowing into these lakes
brought with them gold from the mountains to be concentrated into placers similar to those
formed along present-day beaches.

Types of Placer Deposit

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Types of Placer Deposit

(a) Residual Placers: -

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