Placer Examination - Principles and Practice
Technical Bulletin 4 Bureau of Land Management 1969
Table of Contents
Many of the following terms have universal definitions, that is, they have definitions common to all branches of the mineral industry. On the other hand some are unique to the placer industry or at least they have placer-related meanings different from those in general use. For example: The term FLOTATION, as used in the general mining industry, relates to a mineral separation process. But in placer mining, the term FLOTATION is applied to the minimum water depth needed to move an operating dredge.
It should also be noted that the definitions given here are intended to be descriptive rather than legal, and they should be used accordingly.
Names in parentheses refer to sources as follows:
AGI. Dictionary of Geological Terms. Prepared under the direction of the American Geological Institute. Doubleday & Company, Garden City, N.Y. 1962.
Brooks. Brooks, A. H., The Gold Placers of Parts of Seward Peninsula, Alaska. U.S. Geol. Survey Bull. 328, pp. 114-145, 1908.
Dunn, E. J. Dunn, E. ]., Geology of Gold. Charles Griffin & Co., London. 1929.
Dunn, R. L. Dunn, Russell 1., Drift Mining in California. In Eighth Report of the (Calif.) State Mineralogist, pp. 736-770. 1888.
Fay. Fay, Albert H., A Glossary of the Mining and Mineral Industry. U.S. Bureau of Mines Bull. 95. 1920.
McKinstry. McKinstry, Hugh E., Mining Geology. Prentice-Hall, New York. 1948.
Placer Terms A-E
Skip to: Placer Terms F-N Placer Terms O-Z
ACCRETION BAR A low-level deposit of sand and gravel formed in a stream by gradual addition of new material. Accretion bars are typically formed along the short, or inside radius of curves. See - Skim bar.
ADJUSTED VALUE A sample value that has been increased or decreased by an amount deemed necessary to offset known variables or other factors that may cause discrepancies in the initially indicated value. In placer drilling, the adjusted value is also known as a CORRECTED VALUE. To be valid, such adjustments must be based on careful diagnosis of sampling problems, and must reflect sound judgement. See - Indicated value.
AINLAY BOWL A wet, gravity concentrator used for the recovery of gold and other heavy minerals from alluvial materials. It consists essentially of a bowl-shaped vessel, rotated about its vertical axis and provided with circular riffles. Feed entering at the center is carried upward and outward by the flow of water and centrifugal force. Tailings overflow the rim while gold and other heavy minerals are retained by the riffles. A somewhat similar bowl-shaped concentrator is known as the KNUDSEN BOWL.
AIRPLANE DRILL A compact, engine-powered placer drill designed for use in areas of difficult access. The term AIRPLANE DRILL is actually a trade name which through common use, has become part 6f the placer vernacular.
ALLUVIAL 1. Deposited by a stream. 2. Relating to deposits made by flowing water. (Fay)
ALLUVIAL FAN A cone-shaped deposit of alluvium made by a stream where it runs out onto a level plain or meets a slower stream. The fans generally form where streams issue from mountains upon the lowland. (AGI)
ALLUVIAL GOLD Gold found in assocation with water-worn material.(Fay)
ALLUVIAL PLAIN 1. Flood plains produced by the filling of a valley bottom are alluvial plains and consist of fine mud, sand, or gravel. 2. A plain resulting from the deposition of alluvium by water. (AGI)
ALLUVIUM A general term for all detrital deposits resulting from the operations of modern rivers, thus including the sediments laid down in river beds, flood plains, lakes, fans at the foot of mountain slopes, and estuaries. (AGI)
AMALGAM An alloy of mercury with gold or another metal. In the case of placer gold, a "dry" amalgam, that is, one from which all excess mercury has been removed by squeezing through chamois leather will contain nearly equal proportions of gold and mercury.
AMALGAMATION The extraction of the precious metals from their ores by treatment with mercury.
ANCIENT BEACH PLACER Deposits found on the coastal plain along a line of elevated beaches. (Brooks)
ANCIENT CHANNEL See Tertiary channel.
ANNUAL LABOR See Assessment work.
ASSAY (verb) To determine the amount of metal contained In an ore. (McKinstry)
- 1. The act of making such a determination
- 2. The result of such a determination
- See - Fire assay
ASSAY VALUE The amount of gold or silver, contained in an ore or other material, as shown by assay of any given sample.
ASSESSMENT WORK The annual work upon an unpatented mining claim on the public domain necessary under the United States law for the maintenance of the possessory title thereto. Same as ANNUAL LABOR. (Fay)
AURIFEROUS Containing gold.
BAJADA PLACER Placers found in confluent alluvial fans along the base of a mountain range or in a mantle of rock debris along the lower slope of a mountain range, in arid regions. The deposits are mainly residual detritus and poorly sorted alluvium found in gulches and on slopes that are subject to occasional torrential rain wash. Bajada is the Spanish term for slope. This term has not found general use in placer mining, most bajada placers being referred to collectively as "Desert" placers.
BANK-MEASURE The measurement of material in place, such as gravel in a deposit before excavation. In placer work, values are normally reported as cents per cubic yard and unless specified otherwise, this means a cubic yard in place, or bank-measure.
BANK WATER See By-Wash.
BANKA DRILL A placer drill consisting essentially of a flush-jointed casing equipped with a serrated cutting shoe. The casing is rotated by means of a man or animal-powered sweep attached to the upper section. Men standing on an attached platform, chop up the drill core and remove it from the casing by means of hand-powered tools. Also known as an EMPIRE DRILL.
BAR A deposit of alluvial material above or below the water line of present streams. Bars may form where the current slackens or changes direction. See - Accretion bar.
BATEA A wide and shallow, cone-shaped vessel, usually of wood, used for panning gold. The batea is in common use in Mexico, Central and South America, and Asia.
BEACH PLACER See Sea-beach placers.
BED LOAD Soil, rock particles, or other debris rolled along the bottom of a stream by the moving water, as contrasted with the "silt load" carried in suspension. (AGI)
BEDROCK The solid rock underlying auriferous gravel, sand, clay, etc., and upon which the alluvial gold rests. (Fay) In placer use, the term bedrock may be generally applied to any consolidated formation underlying the gold-bearing gravel. Bedrock may be composed of igneous, metamorphic or sedimentary rock. See - False bedrock.
BENCH PLACER Gravel deposits in ancient stream channels and flood plains which stand from 50 to several hundred feet above the present streams. (Brooks)
BLACK GOLD Alluvial gold coated by black oxide of manganese. (Dunn, E. J.)
BLACK SAND Heavy grains of various minerals which have a dark color, and are usually found accompanying gold in alluvial deposits. (Fay) The heavy minerals may consist largely of magnetite, ilmenite and hematite associated with other minerals such as garnet, rutile, zircon, chromite, amphiboles, and pyroxenes. In Western gold placers, the black sand content is commonly between 5 and 20 pounds per cubic yard of bank-run gravel.
BLUE GRAVEL Some of the deeper, water-saturated gravels found in California's Tertiary channels have a distinctive bluish-gray color and for this reason early miners referred to them as "blue gravel" or more commonly, as the "blue lead". At one time they were believed to represent a separate gravel flow, distinct [rom the overlaying red gravels. Actually, these blue gravels represent unoxidized portions of the gravel channels whereas the red gravels represent the oxidized portions of the same material.
BLUE LEAD (pronounced leed) See - Blue gravel.
BOOMING A variation of ground sluicing in which water is stored in a reservoir and suddenly released to provide a rush of water, in a large volume, which erodes and transports the gravel. Booming is generally employed where water is scarce. In California the contrivances for collecting and discharging water are termed SELF-SHOOTERS. See - Ground sluicing.
BRAIDED STREAMS 1. A braided stream is one flowing in scveral divided and reuniting channels resembling thc strands of a braid, the cause of division being the obstruction by sediment deposited by the stream. 2. Where more sediment is being brought into any part of a stream than it can remove, the building of bars becomes excessive, and the stream develops an intricate network of interlacing channels, and is said to be braided. (AGI) 3. Conditions which cause braiding are common in glacial areas where much sediment is addeu by the melting ice and in semiarid regions where the transporting power of streams is reduced by seepage and evaporation. In general, such conditions are not conducive to the formation of placers.
BREAKOUT A point where a ravine or canyon cuts into, but not through, a channel. (Dunn, R. L.) Usually applied to buried Tertiary channels. Compare with Outlet, and with Inlet.
BREAST The working face of a prospect drift on the pay lead; the face of a gangway being mined. (Dunn, R.L.)
BUCKET-ELEVATOR DREDGE See - Bucket-Line dredge.
BUCKET-LINE DREDGE A dredge in which the material excavated is lifted by an endless chain of buckets. (Fay) Also known as Connected-bucket dredge. The type of bucket-line ureuge generally employed in placer mining is a self-contained digging, washing and disposal unit, operating in a pond and capable of digging, in some cases, more than 100 feet below water. Its machinery is mounted on a shallow-draft hull and the dredge backfills its working pit (pond) as it advances. The capacity of individual buckets is used as a measure of dredge size. For example, an "18-foot" dredge is equipped with buckets having a struck capacity of 18 cubic feet each. Compare with Dragline dredge; also Suction dredge.
BULLION Unrefined gold that has been melted and cast into a bar. In placer mining, the gold sponge obtained by retorting amalgam, is commonly melted with borax or other fluxes, then poured into a bullion bar. See - Sponge.
BURIED PLACER Old placer deposits which have become buried beneath lava flows or other strata. (Fay) See - Tertiary Channel.
BY-WASH In many cases, hydraulic giants are capable of cutting more material from the bank, than can be swept into the slucies by means of the giants alone. In such cases supplemental water may be brought into the pit by means of a ditch, to assist carrying the material to the sluices. This is locally called BY-WASH, BY-WATER or BANK WATER.
BY-WATER See - By-Wash. CABLE DRILL. See - Churn drill.
CABLEWAY SCRAPER See - Slackline scraper.
CAISSON A metal cylinder used to sink prospect shafts in loose ground or in the presence of a large quantity of water. Caissons are usually provided in sets of 4 or more telescoping units.
CALICHE A brown or white material commonly found as a subsoil deposit in arid or semi-arid climates, and which is composed largely of calcuium carbonate. It is commonly encountered in desert placers where its cementing effect adversely affects the mining and washing processes.
CANNON CONCENTRATOR See - pinched sluice.
CAPPING Volcanic flow materials or agglomerates that cover and in some cases, conceal underlying auriferous gravels. Commonly found associated with Tertiary channels in California's Sierra Nevada region. Also called CAP ROCK.
CASING Steel tubing or pipe used to case a drill hole. In placer sampling it is usually driven into the formation ahead of the drill bit and when so used, is commonly called a "drive pipe".
CASING FACTOR The depth to which a churn drill casing must be driven to take in a sample volume of 1 cubic yard. For example, a standard 6-inch drive pipe equipped with a new, 7 1/2-inch drive shoe would be driven 88 feet to cut out a theoretical volume of 1 cubic yard. This is sometimes called PIPE FACTOR, but it is most commonly known as the DRIVE SHOE FACTOR. See - Radford factor.
CEMENT The Material that binds together the sand and gravel particles in an indurated placer or other formation. The cementing material can be calcareous, silicious or ferruginous. Also used when referring to the hardened formations as a whole. Cemented gravels must, in some cases, be milled to release their gold content.
CEMENT CHANNEL A channel depression completely filled with lava, no auriferous gravel. (Dunn, R. L.)
CHALK Volcanic tuff or ash, largely rhyolitic in composition, is commonly found as intraformational strata or masses in Tertiary channels of California's Sierra Nevada region. The whiter, fine-grained and homogeneous beds are locally called "Chalk ".
CHANNEL A stream-eroded depression in the bedrock, ordinarily filled with gravel. See - Tertiary channel.
CHURN DRILL A portable drilling machine arranged to successively raise and drop a heavy string of tools suspended from a drill line. By means of the successive blows the formation is chopped up and the hole deepened. The type of churn drill designcd for placer sampling is often referred to as a "Keystone" drill or "placer" drill. A hand-powered type, used extensively in South America, is known as a "Ward" drill.
CLAIM See - Mining claim.
CLEAN-UP 1. The operation of collecting the gold or other valuable material from the recovery system of a dredge, hydraulic mine or other placer operation. 2. The valuable material resulting from a clean-up.
COARSE GOLD The word "coarse", when applied to gold, is relative and is not uniformly applied. Some operators consider coarse gold to be that which remains on a 10-mesh screen. Others consider individual particles weighing 10 milligrams or more to be coarse gold. Some apply the term "coarse gold" to any particle that is relatively thick as compared to its diameter and can be easily picked up with the fingers.
COBBLE A smoothly rounded stone, larger than a pebble and smaller than a boulder. (Fay)
COCOA MATTING A heavy, coarse-woven fabric made of jute-like material and commonly placed on the bottom of a sluice to aid in saving fine gold.
COLLOIDAL GOLD Gold in an extreme state of subdivision. In a true colloid, the individual particles are of almost molecular dimensions.
COLLUVIAL Consisting of alluvium in part and also containing angular fragments of the original rocks. (Fay).
COLOR A particle of metallic gold found in the prospector's pan after a sample of earth has been washed. Prospectors say, "The dirt gave me so many colors to the pan". (Fay)
CONCENTRATE (verb) To separate a metal or mineral from its ore or from less valuable material. (noun) The product of concentration.
CONCENTRATION The removal by mechanical means of the lighter and less valuable portions of ore. (Fay)
CONFLUENCEM A junction or flowing together of streams; the place where streams meet. (Fay)
CONGLOMERATE Rounded waterworn fragments of rock of pebbles, cemented together by another mineral substance. (AGI)
CORE See - Drill core.
CORE FACTOR In churn drilling, when the casing is driven downward ahead of the drill bit, it should take in a cylinder of gravel having a diameter equal to the effective diameter (cutting edge) of the drive shoe. If the effective diameter of the shoe were the same as the inside diameter of the casing, a I-foot drive would produce a 1-foot core core rise inside the casing. But this is not so. Take for example a standard 6-inch casing equipped with a new, 71/2-inchdrive shoe. The effective area of the shoe is 44.17 square inches while that of the casing is about 26 square inches. As a result, when driven, the core should rise 44.17/26 = 1.7 or, in other words, there should be a 1.7-foot core rise inside the casing for each foot of drive. Here, the CORE FACTOR is 1.7. The core factor will, of course, vary according to the combination of casing and drive shoe used, and it will vary with the amount of wear on a given shoe. The core rise per fool of drive is less commonly referred to as the SHOE FACTOR, but to do so, invites risk of confusing it with other factors or terminology. See - Drive shoe factor; Pipe factor; Casing factor; Drill factor; Radford factor.
CORE RISE The measured length of the cylinder of gravel entering a churn drill casing as it is driven downward. For example, a standard 6-inch casing fitted with a 71/2-inch drive shoe should produce a core rise of 1.7 feet per foot of drive. The difference between the actual core rise and the theoretical rise is sometimes used as a factor for adjusting drill hole sample values.
CORRECTED VALUE See - Adjusted value.
CRADLE See - Rocker.
CREEK PLACER Gravel deposits in the beds and intermediate flood plains of small streams. (Brooks)
CREVICING A small-scale mining method in which the miner removes detrital material from cracks and crevices in the bedrock, usually by means of pry bars and long-handled spoons, and washes the material to recover its gold content.
CRIBBING Close timbering, as the lining of a shaft. (Fay) In placer work cribbing may be needed to support the walls of shaft or test pit put down in loose or wet ground.
DEBRIS The tailings from hydraulic mines.
DEEP LEAD A gold-bearing alluvial deposit buried below a considerable thickness of soil, lava or other barren material. See - Tertiary channel.
DESERT PLACER See - Dry placer.
DETRITUS A general name for incoherent sediments, produced by the wear and tear of rocks through the various geological agencies. The name is from the Latin for "worn" rock waste. (Fay) A deposit of such material.
DIP BOX A modification of the sluice box used for small-scale mining where water is scarce. It generally consists of a short sluice made of 1 x 12-inch lumber, and standing on legs arranged to provide a steep slope. The gold-bearing material is washed in batches by first shoveling it into the upper end of the dip box and then pouring water over it, usually from a dipper.
DIRT A miner's term for auriferous gravel or for the material being worked. See - Pay dirt.
DISCHARGE HEAD The vertical distance from the center of a pump to the center of the discharge outlet where the water is delivered, to which must be added the loss due to friction of the water in the discharge pipe.
DISCOVERY The finding of a valuable mineral deposit in place upon a mining claim. Although "discovery" and "valuable", as they relate to mining claims, have not been defined by statute, a long history of court decisions have held that in order for a location to be valid, there must be a discovery of mineral within the limits of the claim and the discovery must be such as would justify a person of ordinary prudence in the further expenditure of time and money, with reasonable prospect of success in developing a profitable mine. In some decisions the word "valuable" is interchanged with "profitable"
DISCOVERY CLAIM (Alaska) A claim covering the initial discovery on a creek. Subsequent claims are commonly designated as one above, two above, three above; one below, two below, etc., depending on their position in relation to the discovery claim.
DOODLEBUG 1. Miners' term for a dragline dredge. 2. A divining rod or similar device supposedly useful for locating gold or other valuable minerals. See - Dragline dredge.
DRAGLINE A power shovel equipped with a long boom and a heavy digging bucket that is suspended from a hositing line and is pulled toward the machine by means of a "drag" line. By manipulating the two lines (wire ropes), the bucket can be caused to dig, carry, or dump the excavated material. Such a machine is more properly called a dragline excavator. See - Dragline dredge.
DRAGLINE DREDGE A dragline dredge consists of two units; a self-propelled power shovel equipped with a dragline bucket, and a floating washing plant which is similar to, but usually smaller than that of a bucket-line dredge. The washing unit contains a hopper for receiving gravel dug by the dragline; a revolving screen; riffled sluices or other gold-saving equipment, and a tailings stacker. Dragline dredges are generally employed to mine relatively small, shallow deposits that are too small to amortize a bucket-line dredge.
DREDGE A machine, operated by power, and usually mounted on a flat-bottomed hull provided with the equipment necessary to dig, process, and dispose of alluvial or other unconsolidated materials of a type found at the bottom of rivers or in certain terrestrial and offshore deposits. See - Bucklet-line dredge; Dragline dredge; Jet dredge; Suction dredge.
DREDGE SECTION The depth of gravel, or a particular vertical section within a placer deposit, that will pay to mine by dredging.
DRIFT (geol.) Any rock material, such as boulders, till, gravel, sand, or clay, transported by a glacier and deposited by or from the ice or by or in water derived from the melting of the ice. (Fay)
DRIFT (mining) 1. A sub-tunnel running from the main tunnel to prospect for the pay lead; 2. A sub-tunnel run from the main tunnel across the pay lead to block out the ground and to facilitate its working; 3. Generally, a sub-tunnel. (Dunn, R. 1.)
DRIFT MINING A method of mining gold-bearing gravel by means of drifts, shafts or other underground openings, as distinguished from surface methods for placer mining
DRILL See - Chum drill.
DRILL CORE A cylindrical core of sand and gravel forced upward into the drill casing as the casing or "drive pipe" is forced into the deposit, usually ahead of the drill bit. See - Core rise.
DRILL FACTOR A figure used to designate the effective area of a drive shoe used in placer sampling. For example: A new, 71/2-inch drive shoe has an open area of 0.306 sq. ft. but to allow for wear and other variables, some engineers use a lesser figure (commonly 0.27) in their value calculations. The figure so used is referred to as the DRILL FACTOR. See - Radford Factor; Core F actor; Volume Factor; Drive shoe factor.
DRILL LOG The record of a drill hole, usually recorded on a prepared form as the work progresses. The usual placer log, in addition to showing the drilling progress, type of material penetrated, its mineral content, etc., will also show the type and size of equipment used, personnel employed, cause of delays, and other details of the work. A complete log will also show the essential calculations and all factors used in arriving at the reported value.
DRIVE PIPE See - Casing.
DRIVE SHOE A hardened steel protective shoe attached to the lower end of a drive pipe or casing. The drive shoe is usually slightly larger in diameter than the casing and is provided with a beveled cutting edge. See - Casing.
DRIVE SHOE FACTOR The depth to which a churn drill casing must be driven to take in a sample volume of 1 cubic yard. For example, a standard 6-inch drive pipe equipped with a new, 71/2-inch drive shoe would be driven 88 feet to cut out a theoretical sample volume of 1 cubic yard. This is less commonly called the PIPE FACTOR, or CASING FACTOR. See - Radford factor.
DRY DIGGINGS In the 1850's, placers in or along the banks of California's rivers were known as "Wet diggings", and those in the dry ravines adjacent to the rivers were referred to as "Dry diggings". Compare with DRY PLACER.
DRY PLACERS Placers in arid or semiarid regions, or generally where surface water is not available.
DRY WASHER A device for recovering gold or other heavy minerals from dry alluvial material without the use of water. The typical dry washer is a small, hand-powered machine employing a sloping riffle board and a bellows or blower arrangement. The bottom of the riffle board is made of some porous material such as heavy cloth. Puffs of air forced up through the bottom by the bellows or blower, cause the lighter materials to hop over the riffles and work their way through the machine, while the gold or other heavy materials lodge behind the riffle bars.
DRY WASHING The extraction of gold or other minerals from dry sand and gravel by the use of machines in which air is employed as a separating medium.
DRYLAND DREDGE A mechanical washing plant, sometimes of appreciable size, designed to follow a dragline, or other excavator, as the mining cut advances. Some are equipped with trommel-type revolving screens and rock stackers, and are mounted on crawler-type tracks.
DUMP 1. The fall immediately below a hydraulic mine outlet and in particular, the area available for tailings storage. 2. A specially prepared place outside of a drift mine, usually near the portal, where the pay gravel is deposited preparatory to washing. 3. A pile or heap of material, usually waste material, extracted from a mine.
DUST See - Gold dust.
DUTY 1. A measure of the effectiveness of water employed in hydraulic mining, usually expressed as the number of cubic yards of gravel washed per miners' inch per day (M.LD.). The duty varies with the coarseness of gravel, height of bank, grade, available head, etc., usually varying from 1 to 7 cubic yards per miners' inch per 24 hours. 2. The effectiveness of water generally.
ELECTROSTATIC SEPARATOR A device employing charged fields with little or no current flow, and used to extract or separate the component minerals of sands or heavy mineral concentrates. Speaking generally, electrostatic separators do not make sharp separations and they are sensitive to humidity, temperature, and other variables. Electrostatic separators have not found wide application in placer mining. Compare with High Tension separator.
ELEVATED SLUICE See - Trestle sluice.
ELEVATOR A device for ejecting gravel or tailings from a hydraulic mine pit. See - Hydraulic elevator; Rubel elevator.
ELUVIAL DEPOSIT See - Eluvium.
ELUVIUM Loose material resulting from decomposition of rock. Eluvial material may have slumped or washed downhill for a short distance but it has not been transported by a stream.
EMPIRE DRILL See - Banka drill.
EOCENE One of the earliest of the epochs into which the Tertiary period is divided; also the series of strata (and auriferous gravels) deposited at that time. Specifically, an epoch of the Tertiary between the Paleocene and Oligocene.
EROSION CYCLES The Earth's erosional land forms develop in successive stages which can be divided into three broad categories - youth, maturity, and old age. Youthful land forms in the erosion cycle are featured by steep, narrow, V-shaped canyons and fast-cutting streams. In time, as the valleys deepen, they become wider and have gentler slopes. In early maturity they are roughly U-shaped instead of V-shaped. In late maturity down cutting has become slow, and conspicious flats develop. Old age is marked by wide flat valleys or peneplains, over which sluggish streams follow meandering courses. The mature stage is most favorable for the development of extensive placers.
EXPANDED METAL (Expaned-metal lath) A type of punched-metal screen. The style commonly used in placer mining, for saving fine gold, consists of a latticework of diamond-shaped openings (about 3/4" x 11/2") separated by raised metal strands that have a decided slope in one direction. When installed as riffles, with this slope leaning downstream, eddies form beneath the overhangs, thus creating conditions well-suited for the saving of fine gold. When used as riffles, expanded metal is generally placed over cocoa matting or similar material. A flat-lying style of expanded metal (without overhangs) is less-suited for this use.
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