Placer Examination - Principles and Practice
Technical Bulletin 4 Bureau of Land Management 1969
Table of Contents
Perhaps the best known schemes for classifying placer deposits are those by Jenkins (1946, p. 161) and Brooks (1913, pp. 25-32) II, the former being based on conditions of deposition and the latter on present position of the deposit. The field engineer should acquaint himself with these schemes, particularly that by Jenkins which has been developed in some detail. For the usual field investigation a somewhat simpler classification will be found suitable. This is:
- 1. Residual placers.
- 2. Eluvial placers.
- 3. Stream placers.
- 4. Bench placers.
- 5. Flood gold deposits.
- 6. Desert placers.
- 7. Tertiary gravels.
- 8. Miscellaneous types.
- a) beach placers.
- b) glacial deposits.
- c) eolian placers.
1. RESIDUAL PLACERS
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 an outside source. Residual placers may be rich but they are not likely to be large and as a class, they have been relatively unimportant. The "seam diggings" in EI Dorado County, Calif., (Clark and Carlson, 1956, p. 435) offer an example of residual gold placers.
2. 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 mayor 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 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, (Vanderhurg, 1936, pp. 133·145) where this type of placer supported large-scale mining operations.
3. STREAM PLACERS
Stream placers are the most widespread type in the Western States and, accordingly, are the type most frequently encountered in mineral examinations. lndividual deposits vary so much that few general statements can be made concerning them but for the purpose of this review, they can be conveniently divided into:
- a. Gulch placers.
- b. Creek placers.
- c. River deposits.
- d. Gravel-plain deposits.
- a. Gulch placers: Gulch placers are characteristically small in areal have steep gradients and are usually confined to minor drainages in which a permanent stream mayor 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 and discontinuous. Boulders are commonly found in quantities that preclude all but simple hand mining operations. The gold is likely to he coarse and well-concentrated on bedrock. Gulch placers were usually the first to be found by the early millers and because most can be worked with simple hand tools, unworked remnants of shallow gulch deposits are not likely to contain material that would yield a profit today. The early-day miner was generally well-schooled by experience, and a dilligent worker. Any pay gravel that he left was usually cleaned up by the patient Chinese who followed. This was particularly true in gulch placers.
b. Creek placers: In many districts creek placers have been important sources of gold hut like the gulch placers most were carefully prospected by the early miners and worked out, where worthwhile to do so many of the lower-grade remnants left by the early hand miners have since bccn exploited by some form of mechanized mining, notably by draglinc dredges during the depression years of the 1930's. Creek placers as a group no longer contain significant economic rescrves in the Western States but some in Alaska are mined with nonfloating washing plants and moveable sluices utilizing various combinations of hydraulic and mechanical excavation equipment.
c. 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 over-all deposit may be low-grade, pay streaks and bedrock concentrations capable of supporting dredging or other large-scale mining operations arc not uncommon. At many places in California, the early miners diverted rivers through tunnels or bypassed the water in flumes to permit mining the river bed. In this manner, many miles of the middle and upper reaches of the principle gold-bearing rivers were effectively cleaned out. The lower reaches of many of these streams were systematically dredged and, at one time, where conditions were favorable, gravels returning five cents per cubic yard were dredged at a profit. Needless to say, few important river deposits remain unknown in the United States.
d. 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 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 larger gold carried by the main channel will likely be dropped dose 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. Examples of this type of deposit are the dredging fields at Hammonton, and those near Folsom, California. Each produced gold valued at approximately $100,000,000.