Vein (geology)

In geology, a vein is a finite volume within a rock, having a distinct shape, filled with crystals of one or more minerals, which were precipitated from an (aqueous) fluid. Veins are formed by fluids carrying mineral constituents into a rock mass as a consequence of some form of hydraulic flow within the rock. Usually this is the result of hydrothermal circulation.

Veins are classically thought of as being the result of growth of crystals on the walls of planar fractures in rocks, with the crystal growth occurring normal to the walls of the cavity, and the crystal protruding into open space.

This certainly is the method for the formation of some veins. However, it is rare in geology for significant open space to remain open in large volumes of rock, especially several kilometers below the surface. Thus, there are two main mechanisms considered likely for the formation of veins: open-space filling and crack-seal growth.

Open space filling

Open space filling is the hallmark of epithermal vein systems, such as a stockwork, in greisens or in certain skarn environments. For open space filling to take effect, the confining pressure is generally considered to be below 0.5 GPa, or less than 3-5 kilometres. Veins formed in this way may exhibit a colloform, agate-like habit, of sequential selvedges of minerals which radiate out from nucleation points on the vein walls and appear to fill up the available open space. Often evidence of fluid boiling is present. Vugs, cavities and geodes are all examples of open-space filling phenomenon in hydrothermal systems.

Alternatively, hydraulic fracturing may create a breccia which is filled with vein material. Such breccia vein systems may be quite extensive, and can form the shape of tabular dipping sheets, diatremes or laterally extensive mantles controlled by boundaries such as thrust faults, competent sedimentary layers, or cap rocks.

Crack-seal veins

When the confining pressure is too great, or when brittle-ductile rheological conditions predominate, vein formation occurs via crack-seal mechanisms.

Crack-seal veins are thought to form quite quickly during deformation by precipitation of minerals within incipient fractures. This happens swiftly by geologic standards, because pressures and deformation mean that large open spaces cannot be maintained; generally the space is in the order of millimetres or micrometres. Veins grow in thickness by reopening of the vein fracture and progressive deposition of minerals on the growth surface.

Tectonic implications

Veins generally need either hydraulic pressure in excess of hydrostatic pressure (to form hydraulic fractures or hydrofracture breccias) or they need open spaces or fractures, which requires a plane of extension within the rock mass.

In all cases except brecciation, therefore, a vein measures the plane of extension within the rock mass, give or take a sizeable bit of error. Measurement of enough veins will statistically form a plane of principal extension.

In ductilely deforming compressional regimes, this can in turn give information on the stresses active at the time of vein formation. In extensionally deforming regimes, the veins occur roughly normal to the axis of extension.

Mineralisation and veining

Veins are of prime importance to mineral deposits, because they are the source of mineralisation either in or proximal to the veins. Typical examples include gold lodes, as well as skarn mineralisation. Hydrofracture breccias are classic targets for ore exploration as there is plenty of fluid flow and open space to deposit ore minerals.

Ores related to hydrothermal mineralisation which are associated with vein material may be composed of vein material and/or the rock in which the vein is hosted.



Gold-bearing veins

In many of the gold mines exploited during the gold rushes of the 19th century, vein material alone was typically sought as ore material. In most modern mines, ore material is primarily composed of the veins and some component of the wall rocks which surrounds the veins.

The difference between 19th century and modern mining techniques and the type of ore sought is based on the grade of material being mined and the methods of mining which are used. Historically, hand-mining of gold ores permitted the miners to pick out the lode quartz or reef quartz, allowing the highest-grade portions of the lodes to be worked, without dilution from the unmineralised wall rocks.

Modern mining using larger machinery and equipment forces the miners to take low-grade waste rock in with the ore material, resulting in dilution of the grade.

However, modern mining and assaying allows the delineation of lower-grade bulk tonnage mineralisation, within which the gold is invisible to the naked eye. In these cases, veining is the subordinate host to mineralisation and may only be an indicator of the presence of metasomatism of the wall-rocks which contains the low-grade mineralisation.



For this reason, veins within hydrothermal gold deposits are no longer the exclusive target of mining, and in some cases gold mineralization is restricted entirely to the altered wall rocks within which entirely barren quartz veins are hosted.

From http://en.wikipedia.org/

Resource extraction

The related terms resource extraction and resource extraction industry both refer to the practice of locating, acquiring and selling any resource, but typically a natural resource.

Throughout most of the world it is an international and capitalist operation, with the sources for the material being extracted being far away from the possible markets.

Thus resources are often exploited in phases with regular periods of inactivity, as methods of resource extraction advance, the commodity becomes more valuable, or in the special case in which the resource is renewable. This secondary extraction may take the form of a further exploitation of a previously unaccessible source (such as modern mining techniques that are allowing access to heavily faulted seams) or a reprocessing of previously discarded material such as mine tailings.

This phased exploitation is often criticized for the effect it has on the workforce in the surrounding area, providing much opportunity for skilled and unskilled labor, followed by long periods of heavy unemployment, particularly with respect to logging in America's Pacific Northwest.

The specifics of resource extraction depend on the resource being exploited; for instance, the exploitation of timber is called logging, metal ore exploitation is called mining.

In recent times, environmental pressure groups have attempted to reduce the ecological impact of resource extraction, and there have been many successful redevelopment schemes such as reforestation of logging sites, and the large-scale successful conversion of British gravel pits into nature reserves.

From http://en.wikipedia.org/

Tailings

Tailings (also known as slimes, tailings pile, tails, leach residue, or slickens) are the materials left over after the process of separating the valuable fraction from the worthless fraction of an ore.

Tailings represent external costs of mining. As mining techniques and the price of minerals improve, it is not unusual for tailings to be reprocessed using new methods, or more thoroughly with old methods, to recover additional minerals. Yesterday's tails can be tomorrow's resource, as seen during the 1990s when the extensive tailings dumps of Kalgoorlie / Boulder in Western Australia were re-processed profitably by KalTails Mining.

In coal and oil sands mining, the word 'tailings' refers specifically to fine waste suspended in water.

Tailings composition

The composition of tailings is directly dependent on the composition of the ore and the process of mineral extraction used on the ore.

Certain types of extraction process, like heap leaching for example, may result in quantities of chemicals used to perform the leaching remaining in the material once leaching has been completed. Older forms of mineral extraction, such as those utilised during the early gold boom years of Australian gold mining, resulted in large heaps of fine tailings being left dotted around the landscape. These tailings dumps would continue to leach residual chemicals into the environment, and if weather conditions allowed it the finer fraction would become windborne, blowing around the townships surrounding the now-dormant mining areas.

Typically, the bulk quantity of a tailings product will be barren rock, crushed and ground to a fine size ranging from coarse sands down to a talcum powder consistency.

Tailings may contain trace quantities of metals found in the host ore, and they may contain minute amounts of added compounds used in the extraction process. Elements are rarely in elemental form, more often as complex compounds.

Common minerals and elements found in tailings include

* Arsenic - Found in association with gold ores
* Barite
* Calcite
* Fluorite
* Radioactive materials - Naturally present in many ores
* Mercury
* Sulfur - Forms many sulfide compounds / pyrites
* Cadmium
* Hydrocarbons - Introduced by mining and processing equipment (oils & greases)

Common additives found in tailings

* Cyanide - as both Sodium Cyanide (NaCN) and Hydrogen Cyanide (HCN). Leaching agent in extremely dilute quantities which readily volatize upon exposure to sunlight.
* SEX - Sodium Ethyl Xanthate. Flotation agent.
* PAX - Potassium Amyl Xanthate. Flotation agent.
* MIBC - Methyl Isobutyl Carbinol. Frothing agent.
* Sulfamic acid - Cleaning / descaling agent.
* Sulfuric acid - Used in large quantities in the PAL process (Pressure Acid Leaching).
* Activated Carbon - Used in CIP (Carbon In Pulp) and CIL (Carbon In Leach) processes.
* Calcium - Different compounds, introduced as lime to aid in pH control.

Tailings are generally a public relations challenge for the mining industry. While the global demand for mined products continues to rise the negative image of mine tailings grows in spite of a sound record of environmental stewardship with mine tailings in the past twenty years. There remain some operations that choose to not follow the very strict and readily available design and operating practices for tailings and these operations create the negative image for the entire mining industry. However, the non-compliant groups are becoming less common and increasing regulatory scrutiny that is global in extent and consistent in expectation is creating a very difficult environment for those who wish to operate with poor tailings management practices.

Tailings are not an option for a mine that has mills it ore. The option present is how to store those tailings for what needs to be considered perpetuity and the article below describes some of alternative storage methods available to the modern mining industry.

Environmental and social considerations

Tailings in general, are often pointed at by environmentally active groups as evidence of the destruction that mining operations can wreak upon the planet. For a very small percentage (less than 1%) of current operations, and unfortunately a much higher percentage of historic operations, these groups have a valid point. In the past, non-environmentally friendly methods (see "Storage Methods" below) were the method of the day. In today's modern mining environment, it is difficult to find mining operators continuing to engage in tailings storage methods that are not environmentally friendly, but these operations do exist. Unfortunately, the bulk of these operations tend to exist in developing nations where legislative requirements are more permissive than in industrialized countries as these developing countries want the same economic advantages the developed countries have (and obtained without the stringent environmental legislation that these developed countries want to impose on the developing countries). Some see this as having a global environmental responsibility while some developing nations see this as a hypocratic double standard.

There is, however, a strong push from the leading mining houses, their design consultants and global industry groups to support the cessation of unsustainable tailings disposal practices. Unsustainable, in the sense that continuing to potentially impact the local environment damages a mining company's social license to operate, and casts a negative view over the broader industry. Some advocates of globally consistent environmental protection argue that, it is in the mining industry's best interest to move away from tailings storage methods that damage the local environment.

Reprocessing of old tailings impoundments and dams has assisted in the cleaning up of legacy tailings dumps, with the reprocessed tailings being stored in a more effective method.

Tailings storage methods

Tailings Continuum

Traditionally, the only option for tailings storage was to deal with a tailings slurry. This slurry was a dilute stream of the tailings solids within water that was sent to the tailings storage area. The modern tailings designer has a range of tailings products to choose from depending upon how much water is removed from the slurry prior to discharge. The removal of water not only can create a better storage system in some cases (e.g. dry stacking, see below) but can also assist in water recovery which is a major issue as many mines are in arid regions.

The tailings continuum (link below) provides a summary of the range of tailings products and terminology associated with those products.

http://commons.wikimedia.org/wiki/Image:Tailingscontinuum.jpg#file

Pond storage

There are many different subsets of this method. Large earthen dams may be constructed and then filled with the tailings. Tailings may be deposited into natural topographical depressions. Exhausted open pit mines may be refilled with tailings. In all instances, due consideration must be made to contamination of the underlying water table, amongst other issues. Dewatering is an important part of pond storage, as the tailings are added to the storage facility the water is removed - usually by draining into decant tower structures. The water removed can thus be reused in the processing cycle. Once a storage facility is filled and completed, the surface can be covered with topsoil and revegetation commenced. However, unless a non-permeable capping method is used, water that infiltrates into the storage facility will have to be continually pumped out into the future.

Dry stacking

Tailings do not have to be stored in ponds or sent as slurries into oceans, rivers or streams. There is a growing use of the practice of dewatering tailings using vacuum or pressure filters so the tailings can then be stacked. This saves water, reduces the impacts on the environment in terms of space used, leaves the tailings in a dense and stable arrangement and eliminates the long-term liability that ponds leave after mining is finished.

Storage in underground workings

While disposal into exhausted open pits is generally a straightforward operation, disposal into underground voids is more complex. A common modern approach is to mix a certain quantity of tailings with waste aggregate and cement, creating a product that can be used to backfill underground voids and stopes. A common term for this is HDPF - High Density Paste Fill. HDPF is a more expensive method of tailings disposal than pond storage, however it has many other benefits – not just environmental but it can significantly increase the stability of underground excavations by providing a means for ground stress to be transmitted across voids - rather than having to pass around them – which can cause mining induced seismic events like that suffered previously at the Beaconsfield Mine Disaster

Riverine Tailings

Usually called RTD – Rivering Tailings Disposal. In most environments, not a particularly environmentally sound practice, it has seen significant utilisation in the past, leading to such spectacular environmental damage as done by the Mt Lyell Mining Company in Tasmania to the King River. It is still practised at some operations in the world, and while experts agree it is a feasible method for locations where the river is rapidly flowing and turbulent and the additional silt loading will not impact on the river quality, it is not generally favored and is seeing a gradual decline in use.

Submarine Tailings

Commonly referred to as STD (Submarine Tailings Disposal) or DSTD (Deep Sea Tailings Disposal). If a mine is located in close proximity to the coast, and the coast itself is not an excessive distance from a continental shelf, STD is conceptually an excellent method for the disposal of tailings. Tailings can be conveyed using a pipeline then discharged so as to eventually descend into the depths. Practically, it is not an ideal method, as the close proximity to off-shelf depths is rare. When STD is used, the depth of discharge is often what would be considered shallow, and extensive damage to the seafloor can result due to covering by the tailings product. It is also critical to control the density and temperature of the tailings product, to prevent it from travelling long distances, or even floating to the surface. The Solwara project being commenced in the Bismarck Sea by Nautilus Minerals proposes to use a modified STD method back down to depths below 1500 metres. Many countries specifically outlaw the use of STD methods which is based largely on poor politics and rhetoric than scientific arguments. Tailings are largely sand and silt which mimic natural river discharges but a significantly lower rates and can often very successfully be placed in a submarine environment with absolutely no impact on the receiving environment. Unfortunately, there are a number of mines that were using ocean (or lake) disposal systems and from political pressure changed to on-land storage which has ended creating far more environmental impact and legacy issues than the submarine system was even remotely creating.

It is likely when sound science and accurate comparison of actual environmental impacts are used as a basis for comparing projects that submarine (ocean and lake) tailings will see a revival in the future on projects where it makes the most environmental sense.

Phytostabilization

Phytostabilization is a form of phytoremediation that uses plants for long-term stabilization and containment of tailings, by sequestering pollutants in soil near the roots. The plant's presence can reduce wind erosion, or the plant's roots can prevent water erosion, immobilize metals by adsorption or accumulation, and provide a zone around the roots where the metals can precipitate and stabilize. Pollutants become less bioavailable and livestock, wildlife, and human exposure is reduced. This approach can be especially useful in dry environments, which are subject to wind and water dispersion. New work is also being done by Pan Pacific in the development of algal sequestration for plutonium and uranium tailings.

Different methods

Considerable effort and research continues to be made into discovering and refining better methods of tailings disposal. Research at the Porgera Gold Mine is focusing on developing a method of combining tailings products with coarse waste rock and waste muds to create a product that can be stored on the surface in generic-looking waste dumps or stockpiles. This would allow the current use of rivering disposal to cease. Considerable work remains to be done. However, co-disposal has been successfully implemented by several designers including AMEC at, for example, the Elkview Mine in British Columbia.

From http://en.wikipedia.org/

Prospecting

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

Prospecting is a small-scale form of mineral exploration which latter is an organised, large scale effort undertaken by mineral resource companies to find commercially viable ore deposits.

Prospecting is physical labor, involving traversing (traditionally on foot or on horseback), panning, sifting and outcrop investigation, looking for signs of mineralisation. A prospector must also make claims, meaning they must erect posts with the appropriate placards on all four corners of a desired land they wish to prospect and register this claim before they may take samples.

Historical methods

The traditional methods of prospecting involved 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 and 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. Although these are referred thought of as "old" prospecting methods, these techniques are still used today but usually coupled with more advanced techniques such as magnetic surveying and gravimetric analysis.

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 and California.

The Gold Rushes

In the United States and Canada prospectors were lured by the promise of gold, silver, and other precious metals. They travelled across the mountains of the American West, carrying picks, shovels and gold pans. The majority of early prospectors had no training and relied mainly on luck to discover deposits.

Other gold rushes occurred in Papua New Guinea, Australia at least four times, and in South Africa and South America. In all cases, the gold rush was sparked by idle prospecting for gold and minerals which, when the prospector was successful, generated 'gold fever' and saw a wave of prospectors comb the countryside.

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.

Metal detecting

Metal detectors are invaluable for gold prospectors, as they are quite effective at detecting gold nuggets within the soil down to perhaps 3 feet, depending on the acuity of the operator's hearing and skill.

Magnetic separators may be useful in separating the magnetic fraction of a heavy mineral sand from the nonmagnetic fraction, which may assist in the panning or sieving of gold from the soil or stream.

From http://en.wikipedia.org/

Overburden

Overburden is the term used in mining and archaeology to describe material that lies above the area of economic or scientific interest, e.g., the rock, soil and ecosystem that lies above the coal seam. Also known as 'waste'. Overburden is distinct from tailings, the material that remains after economically valuable components have been extracted from the generally finely milled ore. Overburden is removed during surface mining, but is typically not contaminated with toxic components and may be used to restore a mining site to a semblance of its appearance before mining began. Overburden may also be used as a term to describe all soil and ancillary material above the bedrock horizon in a given area.

A related term is interburden, meaning material that lies between two areas of economic interest, such as the material separating coal seams within strata.

By analogy, overburden is also used to describe the soil and other material that lies above a specific geologic feature, such as a buried astrobleme.

From http://en.wikipedia.org/