Technically speaking, placers are considered to be minerals that are not veins in place. Although the location, size, and shape of a placer will reflect the regional forces of erosion, transportation, and deposition which created it, its final form will be controlled or modified by purely local conditions. As a result, each placer deposit can be expected to be unique in one or more ways.
All placers begin with weathering and disintegration of lodes or rocks containing one or more heavy, resistant minerals such as gold, platinum, magnetite, garnet, zircon, etc. The end richness and size of a placer deposit will depend more on there being an abundant supply of source material, and on conditions favorable for their concentration, than on the actual richness of the primary source.
Although placers are commonly found in lode mining districts, history has shown that there is no fixed relationship between the richness of the parent lode and the richness or size of the resultant placers. Some of the most noted gold mining districts such as Goldfield, Nevada, contain no significant placers. On the other hand, some highly productive placer areas such as the Klondike region, are not associated with valuable lodes. In some cases, the lode source may have been completely removed by erosion, while in others, the gold or valuable mineral was not derived from a single source, but instead, from many small mineralized seams or zones scattered through the bedrock. Although individually unimportant, these smaller sources can collectively furnish substantial amounts of gold.
A residual placer is, in effect, a concentration of gold (or other heavy mineral) at or near its point of release from the parent rock. In this type of placer the enrichment results from the elimination of valueless material rather than from concentration of values brought in from another source. Residual placers may be rich but they are not likely to be large and as a class, they have been relatively unimportant.
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 down hill, allowing the lighter portions to be removed by rain wash and wind. As the detrital mass moves 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 where this type of placer has supported large-scale mining operations.
Stream placers are the most widespread type and, accordingly, are the type most frequently encountered in mineral examinations. Stream placers can be divided into four main categories: a.) Gulch placers, b.) Creek placers, c.) River deposits, and d.) Gravel-plain deposits.
Gulch placers: Gulch placers are typically small in area, have steep gradients and are usually confined to minor drainages in which a permanent stream may or may not exist. This type of placer is, as a rule, made up of a mixture of poorly sorted gravel and detritus from adjacent hillsides. Because of steep gradient, the gravel accumulations are often thin discontinuous. Boulders are commonly found in quantities that preclude all but simple hand mining operations. The gold is likely to be coarse and well-concentrated on bedrock. Gulch placers were usually the first to be found by early miners and because most can be worked by simple hand tools, unworked remnants of shallow gulch deposits are not likely to contain material enough to yield a profit today. Any pay gravel that he left was usually cleaned up by the patient Chinese who followed.
Creek placers: In many areas creek placers have been important sources of gold but like the gulch placers most were carefully prospected by the early miners and worked out. Many of the lower-grade remnants left by the early hand miners have since been exploited by some form of mechanized mining, notably by dragline dredges during the depression years of the 1930’s. Creek placers as a group no longer contain significant economic reserves, although, some in Alaska and the Yukon are mined with nonfloating wash plants and moveable sluices utilizing varying combinations of hydraulic and mechanical excavation equipment.
River deposits: River deposits are represented 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 boulders fewer or absent. Although the overall deposit may be low-grade, pay streaks and bedrock concentrations to support large-scale mining operations are not uncommon.
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 for 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 the relatively low velocities of streams flowing over flood plains, their placers are commonly made up of smaller-size gold compared with that found in the main stream deposits. Any large gold carried by the main stream will likely be dropped close to the upper edge of the flood plain where the stream’s velocity decreases and its transporting ability is reduced. Although subject to surface wash and flood erosion, most gravel-plain deposits are relatively permanent.
Bench placers are usually remnants 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” gravels. Frequently there are two or more sets of benches in which case the miners refer to them as “high” benches and “low” benches. Some of these benches were worked by primitive forms of underground mining in order to reach the rich bedrock, which were referred to as “hill diggings.” Many of the larger bench deposits were worked by hydraulicking and the smaller ones by sluicing.
Flood Gold Deposits
As a rule, finely-divided gold travels long distances under flood conditions. This gold which can be best described as “flood gold”, consists mostly of minute particles so small that it may take 1,000 to 5,000 colors to be worth 1 cent. With few exceptions, such gold has proven to be economically unimportant. The miner should recognize the true nature of flood gold deposits so that he can guard against being misled by their seemingly-rich surface concentrations.
As a stream sweeps around a curve, the water is subject to tangential forces which cause a relative increase in velocity along the outer radius of the curve with a corresponding decrease along the inside radius. The bottom layer of water is retarded by friction and as a result, it has a tendency to flow sideways along the bottom toward the inner bank. This, in turn, causes sand and small gravel to accumulate in the form of an accretion bar along the inside bank of the curve and where flood-borne particles are being carried down the stream, some will be deposited near the upper point of such bars, as shown in Figure 1.
The foregoing is an oversimplification of a complex stream process but the fact is, in streams draining a gold-bearing region, seemingly rich deposits of fine-size gold may be concentrated near the upper point of the inside bars, between the high and low water marks. Good surface showings of fine-size gold are not uncommon and although they may appear to be valuable, experience has shown that in most cases the gravel a few inches beneath these surface concentrations is nearly worthless.
In many gold-producing areas, particularly in South America, river bars have been skimmed by natives year after year from time immemorial. These are not permanently exhausted because floods deposit a new supply of gold and the renewal will continue indefinitely. These are called Charcas de Oro which literally translates “Gold Farms.” Here they are call “skim bars” and have been intermittently been worked since about 1860. One such example here in British Columbia, was the famous Hill’s Bar gold. Enormous amounts of fine flour gold was skimmed from this bar. It was inevitable that some would proceed to install dredges or other large washing plants and launch ambitious mining schemes on the strength of surface showings. Needless to say, following the exhaustion of superficial pay streaks, most of these large-scale mining schemes proved unprofitable, and in many cases went bankrupt.