Prospecting for placer gold, except perhaps in the case of buried placers, is the simplest form of prospecting.
Gold, platinum, and tin are the principal metallic minerals won from placers, but gold (alloyed with varying percentages of silver) is the only metal that has been recovered in commercially important quantities from placers in the United States. The following brief discussion applies to gold placers.
The most important type of placer deposit results from the weathering and disintegration of lodes and of rock formations containing gold disseminated through them, the erosion and transportation of these rocks by running water accompanied by further disintegration and liberation of the gold from the enclosing rock minerals, and the concentration of the gold in places where the velocity of the transporting medium (flowing water) is such that the gold and other heavy minerals come to rest but the lighter and finer material is carried away.
Another type of placer is the residual placer, consisting of disintegrated gold-bearing lode material in or very near its original position with only a relatively small amount of barren material removed by erosion; in some of these a considerable part of the soluble constituents may have been removed by chemical action, resulting in enrichment by gold.
Transported placers have been classified, upon the basis of their present position relative to that of streams and other waters, by Brooks as follows:
Creek placers, beach placers, hillside placers, river-bar placers, gravel-plain placers, sea-beach placers, ancient beach placers, and lake-bed placers. The positions of the most productive types are shown in figure 1.
Placers consist for the most part of unconsolidated alluvium, rounded pebbles, and boulders; from the above brief discussion of their origin it is apparent that prospecting for them should begin along stream beds (existing or ancient) and bars, from which they may extend to benches and hillsides.
Ancient placers sometimes have been buried under considerable thicknesses of later glacial or stream deposits or lava flows; in such instances their discovery is more difficult.
For more comprehensive discussions of prospecting for placer gold the reader is referred to a few of the more recent publications dealing with this subject; only the most important principles are summarized here.
Since placers are the result of transportation and concentration of gold particles by running waters, prospecting begins along streams. The most favorable streams are those originating in and flowing through areas of igneous rocks, which are the chief source of lode gold. If these rocks are known to contain gold the chances for the formation of placers along streams flowing through them obviously are enhanced. In actual prospecting, it more often happens that gold is found first in placers and the lode source of the gold is then sought, rather than the reverse. It should be noted, however, that in many instances valuable placers have been found and worked without finding the source of their gold, and many rich gold lodes have been worked in areas where no placers result from their erosion.
Placer gold, then, is usually first and most easily found along streams and their bars, even though the richest deposits may lie in benches or on hillsides above. Since the heaviest particles of gold are deposited nearest their source in the host rock, prospectors should work upstream from the original discovery, especially if the gold is fine. At some point upstream the gold may disappear, and it is then apparent that the source of the gold has been passed.
In prospecting along a stream, the prospector pans the gravel at various points, selecting particularly places that show concentrations of heavy minerals (“black sands”) with which placer gold is commonly associated. These usually will be found on and just above bedrock and in cracks and crevices therein. Depressions in the bedrock are
favorable places for concentration. Excavating may be necessary to reach bedrock in the stream bed and its bars and along its banks.
In small-scale operations, prospecting and actual mining are often concurrent operations, the gold being recovered by panning, rocking, long toms, or small sluices. The gold pan is an indispensable part of the prospector’s equipment.
If gold is found in and along the stream beds and their bars, it may be possible to trace it upstream and to old river benches above the present channel; prospecting of these benches should not be neglected.
Exploration of large placer deposits with a view to large-scale operation will be touched upon later.
Although placer deposits are comparatively simple from a geological standpoint, accurate sampling and interpretation of sampling results are often difficult. Erroneous conclusions often have resulted from incomplete, inaccurate, or poorly interpreted sampling and have led to unprofitable operations.
Placer deposits are commonly explored by means of test pits or small shafts, by drifts, or by drilling. Drifts may be employed when the pay gravel is concentrated within a few feet of bedrock, is covered by thick overburden, or is exposed along the face of a bank or cut. Test pitting and drilling are the methods most generally employed, especially where the deposit must be sampled from surface to bedrock or to the pay horizon, either because the method of mining proposed involves handling all of the gravel or because the gold occurs throughout the entire depth.
Where test pits can be employed they are satisfactory because of the large samples obtained and the opportunity to see the gravel as it occurs in the bank. Where a large volume of water is met in sinking this method becomes impracticable, not only because of the difficulty and cost of sinking but because the samples are salted by material that caves or washes in from the sides of the pit.
The drilling method employed is the drive-pipe method. Briefly, this method involves driving a pipe or casing into the gravel to a measured depth, removing the material inside the pipe (which constitutes a sectional sample of the material over that depth), driving again, and removing the material for another sample, and so on. Sometimes the material in the pipe may be loose enough so that it may be removed by simple bailing, but in other instances it may have to be broken up first by drilling inside the pipe with a churn drill.
However the samples may be obtained—by test pits, drifts, or drilling—the gold is washed from each sample by panning or rocking and is weighed. From the known or calculated volume of material in place, represented by the sample, and the weight of gold recovered from the sample, the weight and value per cubic yard may be estimated. Assays of the samples are not made (except, perhaps, occasionally as a check to determine the proportion of total gold in the sample recoverable by washing), and the gold actually recovered is used as a basis for estimating the value per cubic yard of the deposit.
It is not always a simple matter to determine accurately the amount of material in place represented by the sample or to be sure that salting or dilution has not taken place.
As in other types of deposits, large samples are more likely to be representative than small ones, and for this reason accuracy may often be improved by washing all the material excavated from test pits, shafts, or drifts rather than smaller samples cut from the sides. When such large samples are employed, a rocker ordinarily is used for washing, final clean-up being made with a pan.
Careful consideration should be given to the arrangement and spacing of test holes, which should depend on the characteristics of the deposit in each instance. In the case of channel deposits, long in one dimension and short in the other, it is customary to drill rows of holes across the channel normal to the long dimension, with the rows spaced at regular intervals. For blanket deposits, broad in both dimensions, the holes should be spaced at the corners of equal squares, checkerboard fashion.
The distance between rows and the holes in the rows in channel deposits and likewise the size of the squares in blanket deposits will depend on the continuity of the pay gravel and its degree of uniformity in grade. Several channel deposits have been drilled with holes 150 feet apart in rows 1,000 to 1,500 feet apart. This distance between rows would seem to be too great for erratic deposits.
Engineers of the Reconstruction Finance Corporation, which has granted loans for dredging operations in the western United States, like to have drill holes in narrow deposits not more than 100 feet apart in rows not more than 1,000 feet apart and in bench or blanket deposits on coordinates not less than 400 feet apart.
The best engineering practice probably would be to space the first holes or shafts at about these maximums, with later check-holes between, spaced as indicated to be necessary by the distribution of the gold and the shape of the deposit as shown by the earlier holes.
Tests should be carried down to and into bedrock a short distance, since coarse gold is often concentrated in crevices that commonly occur in the top of the bedrock.
The formula for calculating the value per cubic yard represented by each drill sample is as follows:
C x M/A x D x 27 = theoretical value per cubic yard
where,
27 = cubic feet in a cubic yard,
C= value of gold in cents per milligram,
A = area of drive shoe, square feet,
D = length of sample in feet,
M=milligrams of gold recovered from sample.
The average value for each hole is obtained by averaging the values of the individual samples, weighted when not representing equal yardages; and the average for the entire deposit likewise is determined according to mathematical principles by averaging the weighted values of all the holes. The yardage is determined from the depths of the holes and the areas that each represents. In combining the values and estimating the yardages, experience and good judgment are required. The estimator must consider the type of the deposit, the distribution of the gold, and the method of mining to be employed and its limitations with respect to the degree to which sample recoveries can be duplicated in actual mining practice. He will have to make allowances dictated by his judgment and experience.
Placer Mining
Placer mining is the mining and treatment of alluvial deposits for the recovery, of their valuable minerals. The method has been used principally for mining gold, but a large proportion of the world’s production of tin, platinum, and diamonds and other gem stones and minor quantities of other heavy minerals have been won in this manner. In the United States, as in the world at large, gold has been the principal mineral obtained by placer mining. Minor quantities of metals of the platinum group are recovered with the gold in some localities. Important quantities of sapphires have been produced at placer mines in Montana, and tungsten minerals have been obtained on a commercial scale from placer deposits in California and Colorado. Other heavy minerals or gem stones, however, have not been mined to any important extent by this method in the United States.
The search for placer gold and the working of the deposits when found have had much to do with the early development of the West, placer mining has been gradually overtaken and surpassed in importance by lode-gold mining, until in 1932 less than a quarter of the country’s total gold production was from placers or about an eighth, excluding Alaska. In 1932 about 76 percent of the placer gold produced in the United States was recovered by dredging. Although other forms of placer mining still are important, they have been declining for many years, as the richest and most readily mined deposits of gravel along the stream courses have been exhausted. During 1931 and 1932 there was a revival of small-scale mining, but few new deposits were discovered.
California has ranked first in the production of placer gold since the discovery of gold on Sutter Creek in 1848. In 1932 the relative importance of the other Western States in gold production by placer mining was as follows: Oregon, Idaho, Nevadi, Montana, Arizona, Colorado, New Mexico, South Dakota, Washington, Utah, and Wyoming.
This paper deals with the history of placer mining and production of placer gold, geology of placer deposits, location of placer claims on public lands, sampling and estimation of gold placers, and the classification of placer-mining methods, together with discussions of hand mining and ground sluicing.
Two subsequent papers deal with other phases of placer mining. All phases of placer mining are discussed in the three papers and current practices are illustrated in descriptions of individual mines.
General Geology of Placer-Gold Deposits
Placer-gold deposits result from the weathering and erosion of gold-bearing rocks. Change of temperature, water, and natural solvents disintegrate the rock and partly free the gold from its gangue. Running water transports the products of weathering seaward, meanwhile grinding them to smaller and smaller sizes and thus setting free more gold. Because of its high specific gravity the gold settles through the moving mass of silt, sand, and gravel being carried by streams or floods, and most of it is left behind as the lighter material is carried onward. It comes to rest when the velocity of the stream is insufficient to carry it farther and usually becomes concentrated on bedrock.
The formation of rich placers is favored by the peneplanation or baseleveling of an area, which results in very deep disintegration of the rock surface. When this is followed by uplift and renewed rapid erosion, minerals such as gold or platinum in the great masses of rock are concentrated in the stream channels. This has been the geologic history of many important placer districts.
Gold placer deposits generally are found in districts where lode gold deposits occur. Typically the lodes comprise numerous thin quartz veins with small but rich ore shoots; some of the gold is coarse and free-milling, that is, not intimately combined with other metallic minerals. The lode deposits may be too small or too low-grade to be of commercial value; in some places they may have been removed entirely by erosion.
The discovery of gold in a present stream bed is followed logically by searching for bench gravels, that is, remnants of early stream gravels now lying at relatively higher elevations because of the deepening of the stream bed. This point was made by Mertie with reference to interior Alaska but it applies to many other districts, such as the Sierra Nevada, where some of the deposits laid down during an earlier (Tertiary) cycle of erosion have been exceedingly rich and productive.
Placer deposits usually are not found in the extreme upper portions of streams where the gradient is steep, but under exceptional conditions enough coarse gold might concentrate even there to form valuable deposits. Generally extensive placer-gold deposits are formed just below the steeper grades, where the streams emerge into the lower relief of the foothills. In the Idaho (Boise) Basin, for instance, the placers are found in the flat, lower stretched of Boise River and its tributaries where the stream-bed grades range from 25 to 50 feet per mile rather than in the headwaters where the grades increase rapidly to 100 or 200 feet or more per mile.
Characteristics of Placer Gold
Placer gold occurs as particles ranging in size from minute grains to nuggets weighing 100 or 200 pounds. Pieces worth more than 5 or 10 cents are spoken of as nuggets; smaller ones are “colors.” A scale of sizes, quoted from C. F. Hoffman by Lindgren, is as follows;
Coarse gold, plus 10-mesh.
Medium gold, minus 10-plus 20-mesh.
Fine gold, minus 20-plus 40-mesh.
Powder (flour) gold, minus 40-mesh.
Here, the medium gold averaged 2,200 colors per ounce, or, if pure and valued at $35, about 2/3 of a color to a cent; the fine gold, 12,000 colors per ounce or 3 colors to a cent; and the powder, 40,000 colors per ounce or 10 colors to a cent. Most beach gold and some river gold, such as that of the Snake and Green Rivers, is much finer, ranging from 200 to 1,000 colors to a cent,
Colors and even nuggets almost always are flattened to some extent, Some placer gold occurs as thin flakes, which makes recovery more difficult as the flakes are not separated readily by water action from the more compact rounded grains of heavy minerals such as magnetite or garnet.
Placer gold occurs universally as an alloy with silver. Ordinarily it ranges in fineness from 700 to 950 parts of pure gold to 1,000 parts of the natural alloy, the remainder being chiefly silver. However, lower and higher degrees of fineness are common. Lindgren cites the Folsom dredging field, Sacramento County, Calif., where the gold ranges from 974 to 978 fine. Yale states that some gold from a drift mine near Vallecito, Calif,, was 993 fine, or $20.52 per ounce (at $20.67), and that the gold from this property never fell below 955 fine. In a single small district the fineness of the gold is fairly uniform for any one channel. Some miners, consider the fineness a distinguishing feature of a channel in districts where several channels are being explored or mined. This rule, however, is subject to exceptions, because varied sources of gold may contribute to a placer deposit and because the gold appears to lose part of its silver content and hence increases in fineness as it travels farther from its source. According to several authorities this is due to dissolving of the silver by surface waters, an action that would have relatively more effect on fine particles than on large nuggets. Fine or flour gold usually is of relatively high purity.