how are placer deposits mined

Where is placer mined?

Placer mining continues in many areas of the world as a source of diamonds, industrial minerals and metals, gems (in Myanmar and Sri Lanka ), platinum, and of gold (in Yukon, Alaska and British Columbia ).

What is placer mining?

Unsourced material may be challenged and removed. Placer mining / ˈplæsər / is the mining of stream bed ( alluvial) deposits for minerals. This may be done by open-pit (also called open-cast mining) or by various surface excavating equipment or tunneling equipment. Placer mining is frequently used for precious metal deposits (particularly gold) ...

What is the metal that accumulates in placer deposits?

The metal or gemstones, having been moved by stream flow from an original source such as a vein, are typically only a minuscule portion of the total deposit. Since gems and heavy metals like gold are considerably denser than sand, they tend to accumulate at the base of placer deposits.

What type of gold is the richest?

Alluvial. Alluvial or eluvial deposits are the most common type of placer gold, and are often the richest. They contain pieces of gold that have been washed away from the lode by the force of water, and have been deposited in sediment in or near watercourses or former watercourses.

Where does the word "placer" come from?

Etymology. The word placer derives from the Spanish placer, meaning shoal or alluvial/sand deposit, from plassa (place) from Medieval Latin placea (place) the origin word for "place" and "plaza" in English. The word in Spanish is thus derived from placea and refers directly to an alluvial or glacial deposit of sand or gravel.

How to extract gold from placer ore?

The simplest technique to extract gold from placer ore is panning. This technique has been dated back to at least the Roman Empire. In panning , some mined ore is placed in a large metal or plastic pan, combined with a generous amount of water, and agitated so that the gold particles, being of higher density than the other material, settle to the bottom of the pan. The lighter gangue material such as sand, mud and gravel are then washed over the side of the pan, leaving the gold behind. Once a placer deposit is located by gold panning, the miner usually shifts to equipment that can treat volumes of sand and gravel more quickly and efficiently. Gold panning was commonly used on its own during the California gold rush, however it is now rarely used for profit since even an expert gold prospector can only process approximately one cubic yard of material for every 10 hours of work.

How old are placer deposits?

Placer deposits can be as young as a few years old, such as the Canadian Queen Charlotte beach gold placer deposits, or billions of years old like the Elliott Lake uranium paleoplacer within the Huronian Supergroup in Canada.

How do eluvial placers form?

Eluvial placers form on hillslopes from weathered deposits. They are not acted on by streams but by rainfall and wind, which carry away the light materials; thus they may be considered intermediate in the formation of stream placers. Examples include the earlier worked gold deposits of Australia and the cassiterite placers of Malaysia.

What are the different types of placer deposits?

There are several varieties of placer deposits: stream, or alluvial, placers; eluvial placers; beach placers; and eolian placers. Stream placers, by far the most important, have yielded the most placer gold, cassiterite, platinum, and gemstones.

Why do stream placers depend on water?

Because the ability to transport solid material varies approximately as the square of the velocity, the flow rate plays an important part ; thus, where the velocity decreases, heavy minerals are deposited much more quickly than the light ones .

What are some examples of placers?

Among the examples of beach placers are the gold deposits of Nome, Alaska; the zircon sands of Brazil and Australia; the black sands (magnetite) of Oregon and California; and the diamond-bearing marine gravels of Namaqualand, South Africa. Get a Britannica Premium subscription and gain access to exclusive content.

Where do Eolian placers form?

Eolian placers may form in arid areas where wind, not water, acts as the concentrating agent, removing fine particles of the lighter dross. The gold deposits of some parts of the Australian desert are examples.

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Where are phosphate deposits found?

Large submarine deposits of phosphate rock (phosphorite) are found in the Pacific off the coasts of Peru, eastern Australia, and California and on the Chatham Rise east of New Zealand. Smaller deposits also occur in lagoons of some of the Pacific Islands.

What are placers made of?

Placers are unconsolidated deposits of detrital material containing valuable minerals. The natural processes by which they form range from chemical weathering to stream, marine, and wind action. Typical minerals recovered in placers are gold, tin, platinum, diamonds, titaniferous and ferrous iron sands, gemstones…

What is placer mining?

Placer mining, ancient method of using water to excavate, transport, concentrate, and recover heavy minerals from alluvial or placer deposits. Examples of deposits mined by means of this technique are the gold-bearing sands and gravel that settle out from rapidly moving streams and rivers at points.

What is a placer?

Placers are unconsolidated deposits of detrital material containing valuable minerals. The natural processes by which they form range from... An improvement over the pan was the rocker, or cradle, named for its resemblance to a child’s cradle. As it was rocked, it sifted large quantities of ore.

What is a placer deposit?

placer deposit. Placer deposit, natural concentration of heavy minerals caused by the effect of gravity on moving particles. When heavy, stable minerals are freed from their matrix by weathering processes, they are slowly washed downslope into streams that quickly winnow the lighter matrix.

What is the ancient method of mining?

Placer mining, ancient method of using water to excavate, transport, concentrate, and recover heavy minerals from alluvial or placer deposits. Examples of deposits mined by means of this technique are the gold-bearing sands and gravel that settle out from rapidly moving streams and rivers at points. Placer mining, ancient method of using water ...

What is the most important method of mining placer deposits?

Early in the 20th century, dredgingbecame the most important method of mining placer deposits. In particular, bucket-ladder dredging, which is characterized by a continuous chain of buckets that rotate around a rigid adjustable frame called the ladder, is used worldwide. A later method known as paddock dredging allows placer deposits to be mined even when they are not adjacentto a river. In this method the dredge floats in its own pond, which is continuously extended by digging at one end while simultaneously being filled at the other end with waste, or tailings.

Why is placer mining important?

Placer miningtakes advantage of gold’s high density, which causes it to sink more rapidly from moving water than the lighter siliceous materials with which it is found. Though the basic principles of placer mining have not altered since early times, methods have improved considerably.

What is a placer deposit?

In geology, a placer deposit or placer is an accumulation of valuable minerals formed by gravity separation from a specific source rock during sedimentary processes. The name is from the Spanish word placer, meaning " alluvial sand". Placer mining is an important source of gold, and was the main technique used in the early years ...

What are the elements in placers?

Substances commercially mined from placer deposits include: 1 Diamonds 2 Gold 3 Garnet 4 Iron, from ironsands containing high concentrations of magnetite 5 Platinum group metals 6 Rare earth elements, from the mineral monazite 7 Ruby 8 Sapphire 9 Thorium, from the mineral monazite 10 Tin, in the mineral cassiterite 11 Titanium, from the minerals ilmenite and rutile 12 Uranium, from Precambrian paleoplacers 13 Zirconium, from the mineral zircon

How are alluvial placers formed?

Alluvial placers are formed by the deposition of dense particles at a site where water velocity remains below that required to transport them further. To form a placer deposit, the particles desired must show a marked density contrast with the gangue material, which is able to be transported away from the trap site.

Why is chemical stability important for placer minerals?

Chemical stability is important for a placer mineral to resist chemical breakdown such as oxidation, Minerals lacking chemical stability will form less desirable alteration when subject to chemical breakdown.

What is the purpose of hardness in placer minerals?

Hardness. Hardness allows a placer minerals to resist mechanical breakdown and loss of mass from abrasion during transport. A hardness greater than quartz is desired, however substances such as gold typically deform and create irregularly shaped nuggets when subject to mechanical stress.

What is the difference between placer and quartz?

The separation of the valuable minerals from the most common non-economic mineral, quartz, depends on the difference in specific gravity / density .The weathering process allows for the accumulation of placer minerals, while less dense material such as quartz are swept ...

What are gold bearing placers?

Gold bearing beach placers consist of large strips of black sands and are typically constantly changing as a result of storms or sporadic wave action. Typically beach placers are populated by ilmenite and magnetite, however gold, platinum and chromite are found in varying amounts.

What is roughing gold?

Roughing is the upgrading of the ore (referred to as feed in the concentration process) to produce either a low-grade, preliminary concentrate, or to reject tailings that contain no valuable material at an early stage. The equipment used in this application are referred to as roughers. Roughers may produce a large amount of concentrate, permit the recovery of a very high percentage of feed gold, produce clean tailings, or produce a combination of the above. Roughers include jigs, Reichert cones, sluices, and dry washers.

What is the purpose of concentration of placer gold?

The concentration of placer gold ore consists of a combination of the following three stages: roughing, cleaning, and scavenging. The object of concentration is to separate the raw ore into two products. Ideally, in placer gold recovery, all the gold will be in the concentrate, while all other material will be in the tailings. Unfortunately, such separations are never perfect, and in practice some waste material is included in the concentrates and some gold remains in the tailings. Middlings, particles that belong in either the concentrate or the tailings, are also produced, further complicating the situation.

What is the most expensive phase of placer mining?

With placer mining, recovery of the gold from the ore is usually the most expensive phase of the mining operation and can be the most difficult to implement properly. The value of gold deposits is based on the amount of gold that can be recovered by existing technology. Failure to recover a high percentage of the gold contained in ...

How does concentrate grade affect recovery?

Usually, the higher the concentrate grade, the lower the total recovery. Some valuable material is lost in producing a high grade concentrate. In such cases, the higher grade concentrate is easier to refine than a lower grade concentrate , reducing refinery costs. The savings in refining costs is usually greater than the cost of recovering the small amount of remaining gold from the tailings. For each mining operation, a carefully determined combination of grade and recovery must be achieved to yield maximum profitability. The best recovery systems will collect a maximum amount of placer gold in a minimum amount of concentrate.

How is placer gold recovered?

First, the valuable material is separated from the valueless waste through concentration. The final concentrate, usually obtained by repeated processing, is smelted or otherwise refined into the final product. This report focuses on the equipment and methods used for initial processing, or concentration. As in other processing applications, many specialized terms are used to describe the phases of mineral concentration. Although these terms are described herein as they relate to the processing of placer gold ores, most of the terms identified apply to mineral processing in general.

What information is presented in placer mining?

Recreational and independent miners will find information on available equipment and designs with some suggestions for improving recovery. Those intending to mine small- to medium-sized placer deposits will find detailed descriptions of suitable equipment and recovery methods. Finally, those interested in byproduct gold recovery from sand and gravel operations and other large placer deposits will find descriptions of appropriate equipment and byproduct recovery installations.

How much gold did miners recover in 1945?

Early miners recovered no more than 60% of assayed gold values, and as late as 1945 recovery of free gold averaged only 70-75% (Spiller, 1983). Moreover, it is likely that most remaining placer deposits have a higher percentage of fine gold than placers worked during the gold rush.

Where is placer gold found?

Annotated references to all reports that contain information about individual deposits are given for each district. Most of the placer gold found in Arizona was derived from systems of small gold-quartz veinlets and stringers scattered throughout the bedrock of the adjacent mountain ranges.

Where was gold found in the 1858 era?

The most productive placer mining era was 1858-80, when rich deposits of placer gold were found in the southwestern and central parts of Arizona and hundreds of individuals worked the rich surface accumulations ...

Overview

Deposits

Equipment

History

Etymology

Results

Composition

Terminology

Environmental Effects

Industry