Mineral occurrences, prospects

These classifications of mineral occurrences are generally the least important and least economic. They included are all known occurrences of minerals of economic interest, including obviously uneconomic outcrops and manifestations. However, these are often mentioned in a company prospectus because of "proximity"; a concept that something valuable may be found near these occurrences because it has been in the past due to a similar geological environment. Often, such occurrences of mineralisation are the peripheral manifestations of nearby ore deposits. "Ore deposit" is a term applied specifically to those economic mineral occurrences which could be mined at a profit after consideration of all factors impacting a mining operation.

Mineral resources

Mineral resources are those economic mineral concentrations which have undergone enough scrutiny to quantify their contained metal to a certain degree. None of these resources are ore, because the economics of the mineral deposit may not have been fully evaluated.

Indicated resources are simply economic mineral occurrences which have been sampled (usually by drilling) to a point at which an estimate of their contained metal and grade has been made. Generally this is very approximate and subject to sampling errors and uncertainties.

Measured resources are indicated resources which have undergone enough further sampling that a 'competent person' (which is defined by the norms of the relevant mining code; usually a geologist) has declared them to be an acceptable estimate of the grade, tonnage and occurrence of the mineral occurrence.

Resources may also be portions of a mineral occurrence which are attached to reserves, but are not

• sufficiently drilled out to qualify as Reserve status
• too deep to economically extract
• too deep to technically extract
• too low of grade to economically or technically extract
• contaminated, or refractory in nature
• have not yet met all of the criteria for Reserve status

Mineral reserves

Mineral reserves are the resources which are known to be economically feasible for extraction. Reserves are either 'Probable Reserves or Proven Reserves, in a similar manner to resources, above. Generally the conversion of resources into reserves requires;

• knowledge of the geology of the deposit sufficient that it is predictable and verifiable
• consideration of metallurgy, including plans for extraction, mineral processing
• calculation and design of open pit or underground mine plans based on ore models
• quantification of geotechnical risk; basically, managing the geological faults, joints and fractures in the ground so the mine does not collapse
• consideration of technical risk; essentially, statistical and variography to ensure the ore is sampled properly
• scrutiny of the assay data to ensure the accuracy of the information supplied by the laboratory

This is required because ore reserves are bankable. Essentially, once a deposit is brought up to reserve status, it is an economic entity and an asset upon which loans and equity can be drawn - generally in order to pay for its extraction (hopefully, at a profit).

PROSPECTING

Geologists are involved in the study of ore deposits, which includes the study of ore genesis and the processes within the Earth's crust which form and concentrate ore minerals into economically viable quantities.

Study of metallic ore deposits involves the use of structural geology, geochemistry, the study of metamorphism and its processes, as well as understanding metasomatism and other processes related to ore genesis.

Ore deposits are delineated by mineral exploration, which utilises geochemistry prospecting, drilling and resource estimation via geostatistics to quantify economic ore bodies. The ultimate aim of this process is mining.

Prospecting is the physical search for minerals, fossils, precious metals or mineral specimens, and is also known as fossicking.

Prospecting is synonymous in some ways with mineral exploration which is an organised, large scale and at least semi-scientific effort undertaken by mineral resource companies to find commercially viable ore deposits, however prospecting is increasingly restricted to describe the activities of the amateur and hobbyist who search for small quantities of ore or mineralisation.

Prospecting is increasingly a hobby or vocation undertaken as a form of relaxation and diversionary activity by modern people however in the past prospecting was the only way new mineral deposits were found.

Prospecting is quite intensive physical labor, involving a considerable amount of traversing (traditionally on foot or on horseback), panning, sifting and outcrop investigation, looking for tell-tale signs of mineralisation.

Old prospecting methods

The traditional methods of prospecting involved carefully and intensely combing through the countryside, often through creek beds and along ridgelines and hilltops, often on hands and knees looking for signs of mineralisation in the outcrop. In the case of gold, all streams in an area would be panned at the appropriate trap sites looking for a show of 'colour' or gold in the tail.

Once a small occurrence or show was found, it was then necessary to intensively work the area with pick and shovel, and often via the addition of some simple machinery such as a sluice box, races and winnows, to work the loose soil and rock looking for the appropriate materials (in this case, gold). For most base metal shows, the rock would have been mined by hand and crushed on site,the ore separated from the gangue by hand.

Often, these shows were short-lived, exhausted abandoned quite soon, requiring the prospector to move onwards to the next and hopefully bigger and better show. Occasionally, though, the prospector would strike it rich and be joined by other prospectors and larger-scale mining would take place.

In most countries in the 19th and early 20th century, it was very unlikely that a prospector would retire rich even if he was the one who found the greatest of lodes. For instance Patrick (Paddy) Hannan, who discovered the Golden Mile, Kalgoorlie, died without receiving anywhere near a fraction of the value of the gold contained in the lodes, the same story repeated at Bendigo, Ballarat,Klondike, California.

Modern prospecting

Modern prospectors today rely on training, the study of geology, and prospecting technology.

Knowledge of previous prospecting in an area helps in determining location of new prospective areas. Prospecting includes geological mapping, rock assay analysis, and sometimes the intuition of the prospector.

· Procedure

Steps of process

1. Prospecting to locate ore
2. Exploration to find and then define the extent and value of ore where it is located ("ore body")
3. Conduct resource estimate to mathematically estimate the extent and grade of the deposit
4. Conduct mine planning to evaluate the economically recoverable portion of the deposit (and including reclamation planning)
5. Conduct a feasibility study to evaluate the total project and make a decision as whether to develop or walk away from a proposed mine project. This includes a cradle to grave analysis of the possible mine, from the initial excavation all the way through to reclamation.
6. Development to create access to an ore body
7. Exploitation to extract ore on a large scale
8. Reclamation to make land where a mine had been suitable for future use

· Techniques

Surface mining is a type of mining in which soil and rock overlying the mineral deposit are removed. It is the opposite of underground mining, in which the overlying rock is left in place, and the mineral removed through shafts or tunnels.

Surface mining is used when deposits of commercially useful minerals or rock are found near the surface; that is, where the overburden (surface material covering the valuable deposit) is relatively thin or the material of interest is structurally unsuitable for tunneling (as would usually be the case for sand, cinder, and gravel). Where minerals occur deep below the surface—where the overburden is thick or the mineral occurs as veins in hard rock— underground mining methods are used to extract the valued material. Surface mines are typically enlarged until either the mineral deposit is exhausted, or the cost of removing larger volumes of overburden makes further mining uneconomic.

In most forms of surface mining, heavy equipment, such as earthmovers, first remove the overburden - the soil and rock above the deposit. Next, huge machines, such as dragline excavators, extract the mineral.

Types of surface mining

There are four main forms of surface mining, detailed below.

Strip mining

Strip mining is the practice of mining a seam of mineral by first removing a long strip of overlying soil and rock (the overburden). It is most commonly used to mine coal (especially lignite coal) or tar sand. Strip mining is only practical when the ore body to be excavated is relatively near the surface. This type of mining uses some of the largest machines on earth, including bucket-wheel excavators which can move as much as 12,000 cubic meters of earth per hour.

There are two forms of strip mining. The first, and more common one is area stripping, which is used on fairly flat terrain, to extract deposits over a large area. As each long strip is excavated, the overburden is placed in the excavation produced by the previous strip.

Contour stripping involves removing the overburden above the mineral seam near the outcrop in hilly terrain, where the mineral outcrop usually follows the contour of the land. Contour stripping is often followed by auger mining into the hillside, to remove more of the mineral. This method commonly leaves behind terraces in mountainsides.

Among others, strip mining is used to extract the oil-impregnated sand in the Athabasca Tar Sands in Alberta. It is also common in coal mining. Bucket excavators are widely used for this purpose, however, they are prone to damages and require many millions of dollars to repair.

Open-pit mining

Open-pit mining refers to a method of extracting rock or minerals from the earth by their removal from an open pit or borrow. Although open-pit mining is sometimes mistakenly referred to as "strip mining", the two methods are different.

Mountaintop removal

Mountaintop removal (MTR) is a relatively new form of coal mining that involves the mass restructuring of earth in order to reach the coal seam as deep as 1,000 feet below the surface. It is used where a coal seam outcrops all the way around a mountain top. All the rock and soil above the coal seam are removed and the spoil placed in adjacent lows such as hollows or ravines. Mountaintop removal replaces previously steep topography with a relatively level surface.

The technique has been used increasingly in recent years in the Appalachian coal fields of West Virginia, Kentucky, and Tennessee in the United States. The profound changes in topography and disturbance of pre-existing ecosystems have made mountaintop removal highly controversial.

Advocates of mountaintop removal point out that once the areas are reclaimed as mandated by law, the technique provides premium flat land suitable for many uses in a region where flat land is at a premium. They also maintain that the new growth on reclaimed mountaintop mined areas is better able to support populations of game animals.

Dredging

Dredging is a method often used to bring up underwater mineral deposits. Although dredging is usually employed to clear or enlarge waterways for boats, it can also recover significant amounts of underwater minerals relatively efficiently and cheaply.

Sub-surface mining or underground mining refers to a group of techniques used for the extraction of coal and other valuable minerals or other geological materials from the earth. In contrast to the other main type of excavation, surface mining, sub-surface mining requires equipment and/or manpower to operate under the surface of the earth.

In underground coal mines, another major environmental risk is fires. Hundreds of coal mines smolder in the United States, China, Russia, India, South Africa, and Europe. The inaccessibility and size of these fires make many impossible to extinguish or control.

Methods

• Longwall mining: Longwall mining machines consist of multiple coal shearers mounted on a series of self-advancing hydraulic ceiling supports. The entire process is mechanized. Longwall mining machines are about 800 feet (240 meters) in width and 5 to 10 feet (1.5 to 3 meters) tall. Longwall miners extract "panels" - rectangular blocks of coal as wide as the mining machinery and as long as 12,000 feet (3,650 meters). Massive shearers cut coal from a wall face, which falls onto a conveyor belt for removal. As a longwall miner advances along a panel, the roof behind the miner's path is allowed to collapse.

• Room-and-pillar mining: Room and pillar mining is commonly done in flat or gently dipping bedded ores. Pillars are left in place in a regular pattern while the rooms are mined out. In many room and pillar mines, the pillars are taken out, starting at the farthest point from the mine haulage exit, retreating, and letting the roof come down upon the floor. Room and pillar methods are well adapted to mechanization, and are used in deposits such as coal, potash, phosphate, salt, oil shale, and bedded uranium ores.         

GOLD MINING

 

Idiomatic use

In colloquial English, the term gold mine is used to describe a very profitable economic venture. For example, a business or investment opportunity that provides (or could potentially provide) a considerable amount of money for the owners or shareholders as net earnings could be considered a gold mine. (A profitable physical gold mine could also be called a gold mine due to its profitable nature.)

An example sentence using the term gold mine in the idiomatic sense could be: "This oil deposit is a regular gold mine. We make more and more money every year!"

As a verb, to gold mine can mean to date or marry someone rich (normally much older) in the hopes of inheriting their money. One who does this is called a gold digger.

Gold mining consists of the processes and techniques employed in the removal of gold from the ground. There are several techniques by which gold may be extracted from earth and rock.

// Placer (Sediment) mining

Panning

Gold panning is a mostly manual technique of sorting gold. Wide, shallow pans are filled with sand and gravel that may contain gold. Water is added and the pans are shaken, sorting the gold from the rock and other material. Gold being much denser than rock, quickly settles to the bottom of the pan. The silt is usually removed from streambeds, often at a bend in the stream, or resting on the bedrock bed of the stream, where the weight of gold causes it to separate out of the water flow. This type of gold found in streams or dry streams are called placer deposits.

Gold panning is the easiest technique for searching for gold, but is not commercially viable for extracting gold from large deposits, except where labor costs are very low and/or gold traces are very substantial. It is often marketed as a tourist attraction on former goldfields. Before production methods can be used, a new source must be identified and panning is a good way to identify placer gold deposits so that they may be evaluated for commercial viability.

Sluicing

Using a sluice box to extract gold from placer deposits has been a common practice in prospecting and small-scale mining throughout history to the modern day. A sluice box is essentially a man-made channel with riffles set in the bottom. The riffles are designed to create dead zones in the current to allow gold to drop out of suspension. The box is placed in the stream to catch water-flow and gold bearing material is placed at the top of the box. The material is carried by water through the box where gold and other heavy material settles out behind the riffles. Lighter material flows out of the box as tailings.

Dredging

A mostly historical, 'industrial' way of gold mining, this involves dredging up large amounts of gold-bearing sediments for later processing in various ways. Dredges were for example used strongly during various New Zealand gold rushes. Unlike the panning method, it required capital investment, and was thus unavailable for most individual prospectors, though under the correct circumstances, it offered rich yields. Very large dredges (often known as "bucket line dredges") were used in many places during the early gold rush periods in the U.S. and elsewhere. They consisted of a barge-like floating structure, with a "digging ladder" of linked buckets that would excavate the pay gravels from below the waterline. The excavated materials would then be washed in a trommel or shaking screen, and further concentrated through sluices or jigs. Their use was discontinued many years ago but a few of them still exist and are on display as tourist attractions. One example is the Jack Wade Dredge, visible from the Taylor Highway in central Alaska. Other bucket-line dredges may be found in Nome.

 

Currently, some dredging is done by small scale miners using suction dredges. These are small machines floating on the water and are usually operated by one or two people. Unlike the old bucket line dredges, modern suction dredges have little to no detrimental impact on the area being mined. These machines are much more efficient at extracting smaller gold than the old "bucket line" ever was. This means there is a better chance of you finding gold than ever. There are some large suction dredges (100 hp+ 10 inch)used in commercial production throughout the world. Smaller ones with 2 to 4 inch suction tubes are used to sample the areas behind boulders and along the potential pay streaks, until color (gold) first appears. Again the best way to begin any prospecting venture is with the good old pan, sluice and shovel, and a GPS See more on suction dredging and techniques at Dave McCracken's gold prospecting site.

Hydraulic mining

Hydraulic mining is a type of placer mining used in areas where large amounts of loose gravel and sand or soil are poorly packed and may be washed away with a heavy stream of water. Fire hoses (Water cannons) are sometimes used to strip away entire hills of loose gravel, which are then run through a sluice (a wooden trough with ripples). Gold, being heavier, does not move as easily as other material in the sluice. This technique can damage the environment, causing mud in streams below the mining site and erosion damage at the site itself.

Hard rock mining

Hard rock gold mining is done when the gold is encased in rock, rather than as particles in loose sediment. Sometimes open-pit mining is used, such as the Ft. Knox Mine in central Alaska. Barrick Gold Corporation has one of the largest open-pit gold mines in North America, located on its Goldstrike property in northeastern Nevada. Other gold mines use underground mining, where the ore is extracted through tunnels or shafts. Hard rock mining produces most of the world's gold.

Byproduct gold mining

Gold is also produced by mining in which it is not the principal product. Large copper mines, such as the Bingham Canyon mine in Utah, often recover considerable amounts of gold and other metals along with the copper. Some sand and gravel pits, such as those around Denver, Colorado, may recover small amounts of gold in their washing operations.

Extraction of Gold Ore

Gold extraction or recovery from its ores may require a combination of comminution, mineral processing, hydrometallurgical, and pyrometallurgical processes to be performed on the ore. Gold mining from alluvium ores was once achieved by techniques associated with placer mining such as simple gold panning and sluicing, resulting in direct recovery of small gold nuggets and flakes. Placer mining techniques in the present day are generally the practice of artisan miners. Hard rock ores form the basis of the majority of commercial gold recovery operations, and are mined using open pit or sub-surface mining techniques.

Once the ore is mined it can be treated as a whole ore using a dump leaching or heap leaching processes. This is typical of low-grade, oxide deposits. Normally, the ore is crushed and agglomerated prior to heap leaching. High grade ores and ores resistant to cyanide leaching at coarse particle sizes, require further processing in order to recover the gold values. The processing techniques can include grinding, concentration, roasting, and pressure oxidation prior to cyanidation.

Concentration

Gravity concentration or froth flotation processes may be used to concentrate the gold. In some cases, particularly when the gold is present in the ore as discrete coarse particles, a gravity concentrate can be directly smelted to form gold bars. In other cases, particularly when the gold is present in the ore as fine particles or is not sufficiently liberated from the host rock, the concentrates are subjected to cyanidation leaching, followed by recovery from the leach solution. Recovery from solution typically involves by adsorption on activated carbon followed by solution concentration or stripping and or electrowinning.

Froth flotation is usually applied when the gold present in an ore is closely associated with sulfide minerals such as pyrite or arsenopyrite, and when such sulfides are present in large quantities in the ore. In this case, concentration of the sulfides results in concentration of gold values. Generally, recovery of the gold from the sulfide concentrates requires further processing, usually by roasting or wet pressure oxidation. These pyrometallurgical or hydrometallurgical treatments are themselves usually followed by cyanidation and carbon adsorption techniques for final recovery of the gold.

Sometimes gold is present as a minor constituent in a base metal (e.g. copper) concentrate, and is recovered as a by-product during production of the base metal. For example, it can be recovered in the anode slime during the electrorefining process.

Leaching

If the gold can not be concentrated for smelting, then it is leached by an aqueous solution:

1. The cyanide process is the industry standard.
2. Thiosulfate leaching has been proven to be effective on ores with high soluble copper values or ores which experience pregrobbing by carbonaceous components.

Refractory Gold Processes

A "refractory" gold ore is an ore that is naturally resistant to recovery by standard cyanidation and carbon adsorption processes. These refractory ores require pre-treatment in order for cyanidation to be effective in recovery of the gold. A refractory ore generally contains sulfide minerals, organic carbon, or both. Sulfide minerals often trap or occlude gold particles, making it difficult for the leach solution to complex with the gold. Organic carbon present in gold ore may adsorb dissolved gold-cyanide complexes in much the same way as activated carbon. This so-called "preg-robbing" carbon is washed away because it is significantly finer than the carbon recovery screens typically used to recover activated carbon.

Pre-treatment options for refractory ores include:

1. Roasting
2. Bio-oxidation
3. Pressure oxidation
4. Ultrafine grinding

The refractory ore treatment processes may be preceded by concentration (usually sulfide flotation). Roasting is used to oxidize both the sulfur and organic carbon at high temperatures using air and/or oxygen. Bio-oxidation involves the use of bacteria that promote oxidation reactions in an aqueous environment. Pressure oxidation an aqueous process for sulfur removal carried out in a continuous autoclave, operating at high pressures and somewhat elevated temperatures. Ultrafine grinding may be used when liberation of gold particles from the surrounding mineral matrix is the primary refractory characteristic of the ore.

 Gold Smelting

· Mercury removal

Mercury is a health hazard, especially when in gas form. To remove this hazard, before smelting, gold precipitates from electrowinning or Merrill-Crowe processes are usually heated in a retort to recover any mercury present, that would otherwise cause health and environmental problems due to its release (volatilization) during smelting. The mercury present is not usually from the mercury amalgamation process that is no longer used by formal gold mining companies, but from mercury in the ore that has followed gold through the leaching and precipitation processes.

In the event that there are high levels of copper or silver present, leaching of the precipitate using nitric or sulfuric acids may be required.

· Iron removal

Nitric acid or forced air oven oxidation can also be used to dissolve iron from the electrowinning cathodes before smelting. Gravity concentrates can often contain high grinding steel contents, and so their removal using shaking tables or magnets is used before smelting. During smelting iron can be oxidized using nitre. Excessive use of nitre will corrode the smelting pot, increasing both maintenance costs and the risk of catastrophic leaks (known as run-aways, or holes in the pot through which the molten charge is lost).

 

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