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| Abutment | In coal mining, (1) the weight of the rocks above a narrow roadway is transferred to the solid coal along the sides, which act as abutments of the arch of strata spanning the roadway; and (2) the weight of the rocks over a longwall face is transferred to the front abutment, that is, the solid coal ahead of the face and the back abutment, that is, the settled packs behind the face. |
| Acid deposition or acid rain | Refers loosely to a mixture of wet and dry "deposition" (deposited material) from the atmosphere containing higher than "normal" amount of nitric and sulfuric acids. The precursors or chemical forerunners of acid rain formation result from both natural sources, such as volcanoes and decaying vegetation, and man-made sources, primarily emissions of sulfur and nitrogen oxides resulting from fossil fuel combustion. |
| Acid mine drainage | Run-off water from a mine generally with a pH between 2.0 and 4.5 produced by the oxidation of sulfide minerals within mine rock or tailings piles. Tailings ponds are designed to prevent acid mine drainage by submerging waste rock and thus preventing oxidation.
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| Acid mine water | Mine water that contains free sulfuric acid, mainly due to the weathering of iron pyrites. |
| Acidic rocks | A somewhat outdated term for igneous rocks containing a high proportion of silica. Formal definitions have varied over the years, including "rock with greater than 66% silica" and "rock with 10% or more free quartz". Now often used loosely to refer to any igneous rock composed mainly of light-colored minerals. Similar to "felsic" and opposite of "basic".
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| Active workings | Any place in a mine where miners are normally required to work or travel and which are ventilated and inspected regularly. |
| Adit | Entrance to a mine, generally a horizontal tunnel. |
| Advance | Mining in the same direction, or order of sequence; first mining as distinguished from retreat. |
| Aerial magnetometer | Aircraft-borne device used to measure the strength of the Earth's magnetic field. Mineral explorationists use this tool to look for magnetic anomalies caused by subsurface ore deposits. Metallic ores and basaltic rocks generally cause an increase in local magnetism, while granites and sedimentary rocks tend to produce reduced magnetic fields.
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| Aeromagnetic survey | Geophysical survey conducted with an aerial magnetometer.
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| Agglomerate | Generally, a rock composed of large, angular fragments of volcanic material. The term has had various formal definitions over the years. Also known as volcanic breccia.
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| Agglomeration | Concentrating valuable minerals or coal based on their adhesion properties. Includes sintering, pelletizing, and briquetting.
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| Agitation | Mechanical stirring or shaking. In metallurgy, agitation promotes the dissolution of ore minerals during processing.
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| Agitation leaching | Extraction of gold from mined rock accomplished by agitation in a cyanide solution.
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| Air split | The division of a current of air into two or more parts.
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| Airborne survey | Exploration work conducted from an aircraft. Includes aerial photographic surveys and geophysical surveys to measure magnetism or radioactivity.
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| Airway | Any passage through which air is carried. Also known as an air course. |
| Alloy | A mixture of two or more metals, often in combination with non-metals such as carbon. Common alloys include brass (copper and zinc) and bronze (copper and tin).
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| Alluvium | Clay, silt, sand, and gravel deposited by a body of running water during relatively recent geologic time. Includes flood plains, lake beds, and river deltas. Such deposits can contain heavy ores like gold, platinum, diamonds, and tin, which have been eroded and subsequently concentrated by the action of the river or stream.
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| Alpha meter | A device used by explorationists to measure positively charged particles given off by radioactive elements such as uranium and thorium.
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| Alpha ray | Positively charged particle given off by radioactive elements. Also known as an alpha particle.
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| Alteration | Change in a rock's mineralogic composition caused by heat, pressure, mechanical weathering by wind or water, or exposure to chemicals in hydrothermal solutions. Milder and more localized than metamorphism. Certain types of alteration can be indicative of nearby ore mineralization; for example, chlorite and sericite alteration often surround porphyry copper and gold deposits.
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| Amorphous | Describes rocks or minerals that lack crystalline structure or have an extremely irregular internal arrangement of atoms, such as amorphous silica.
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| Amortization | The gradual repayment of a liability -- such as loans for a mine start-up -- in regular installments over a specified period of time.
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| Amphibolite | Metamorphic rock composed mainly of the minerals amphibole and plagioclase with little or no quartz.
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| Anemometer | Instrument for measuring air velocity. |
| ANFO | Acronym for ammonium nitrate/fuel oil, an explosive mixture used as a blasting agent at many mines because of its ability to be pumped as a slurry.
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| Angle of dip | The angle at which strata or mineral deposits are inclined to the horizontal plane. Shallow-dipping mineralized horizons (i.e., those nearer to horizontal) are more favorable for mining, requiring less removal of overburden.
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| Angle of draw | In coal mine subsidence, this angle is assumed to bisect the angle between the vertical and the angle of repose of the material and is 20¡ for flat seams. For dipping seams, the angle of break increases, being 35.8¡ from the vertical for a 40¡ dip. The main break occurs over the seam at an angle from the vertical equal to half the dip. |
| Angle of repose | The maximum angle from horizontal at which a given material will rest on a given surface without sliding or rolling. |
| Annual Report | A document issued by corporations each fiscal year to fulfill disclosure obligations to shareholders. Such reports contain information on company finances, operations, and future directions.
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| Anode | A rectangular plate made of partially-refined metal such as blister copper. In electrolytic metallurgy, the anode is dissolved in an acidic solution, with atoms of the desired metal being recaptured at a cathode.
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| Anomaly | An observed or measured deviation from normal geologic conditions, generally used to describe features that may suggest an ore deposit. Geochemical anomalies such as high arsenic content, or geophysical anomalies such as high magnetism, may signal the presence of mineralization.
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| Anthracite | Coal of the highest metamorphic grade, containing 92-98% fixed carbon. Anthracite has the highest energy content of any coal.
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| Anticline | An upward fold or arch of rock strata. Petroleum or mineralized fluids can become trapped underneath anticlines.
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| Apex | The highest point of a vein relative to the surface. Under U.S. mining law, the holder of a claim containing the apex of a vein can mine the vein downdip, even if the mineralization eventually passes underneath an adjacent claim.
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| Aquifer | A water-bearing bed of porous rock, often sandstone. Often used to provide drinking water or process water for industrial operations.
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| Archean | The second oldest geologic era, lasting from 3.8 to 2.5 billion years ago. Most of the world's stable continental interiors, or shields, formed during the Archean, and these contain some of the largest known ore deposits. The Archean was also a time of prolific development of greenstone belts, which today are important targets for gold mineralization.
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| Arching | Fracture processes around a mine opening, leading to stabilization by an arching effect. |
| Area (of an airway) | Average width multiplied by average height of airway, expressed in square feet. |
| Arrastra | Mill, consisting of one or more large stones dragged around on a circular bed, used to grind ore. |
| Ash | Inorganic residue left after coal is burned. Also, volcanic particles less than 2 millimeters in diameter.
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| Assay | A measure of the amount of an economic metal within an ore sample. The act of collecting such a measure.
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| Assay foot (meter, inch, centimeter) | Value obtained by multiplying an assay grade by the number of feet, meters, inches, centimeters across which the assayed sample was taken.
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| Assay map | Schematic of an exploration property showing sampling locations and assay grades.
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| Assessment work | Approved activities that must be completed to retain the rights to claimed land. Generally refers to exploration mapping, sampling, or drilling. Mining laws often call for companies to spend a certain dollar amount yearly on assessment work.
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| Auger | A screw-shaped rotary drill used to penetrate, break, and then transport drilled material to surface. Suitable for soft materials such as coal or clay.
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| Authorized capital | See capital stock.
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| Autogenous grinding | An ore processing method where large pieces of mined rock are placed in a rotating cylinder, being crushed and ground as they contact each other. In the case of softer ores, steel balls must often be added to the cylinder, making the process semi-autogenous.
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| Auxiliary operations | All activities supportive of but not contributing directly to mining. |
| Auxiliary ventilation | Portion of main ventilating current directed to face of dead end entry by means of an auxiliary fan and tubing. |
| Azimuth | A surveying term that references the angle measured clockwise from any meridian (the established line of reference). The bearing is used to designate direction. The bearing of a line is the acute horizontal angle between the meridian and the line. |
