Monday, August 22, 2011

Common Geology, Mining and Prospecting Terms

Some geology, mining, rock, mineral and prospecting terms used in this field trip guide.
Adit: A horizontal mine tunnel (see also shaft).
Anticline:  A fold that is convex upward with the oldest rocks in the core.

Archean: see Precambrian.

Bioherm: A dome-like mass of calcareous rock built up by the remains of sedentary organisms such as corals, algle, and stromatoporoids).

Boxwork: Vuggy limonite residue left behind from the weathering of iron-rich minerals such as pyrite or siderite.

Dike: A narrow, linear, mass of igneous rock that often follows a prominent fracture in the earth's crust.

Fault: A fracture in which rocks have been displaced.  Some types of faults include reverse, normal, and thrust.  A thrust fault is a relatively low angle fracture in which older rocks have been displaced on top of  younger rocks.  A reverse fault is a high angle thrust fault. A normal fault has the opposite sense of movement of a reverse fault.

Gangue minerals: Gangue (pronounced "gang") minerals are the minerals associated with a vein or ore deposit that have no value.

Gossan: An iron-rich (rusty appearing) zone formed of limonite and related minerals found over many sulfide-rich (pyrite, chalcopyrite) mineral deposits.  Produced by oxidation and weathering.

Laramide orogeny: This was a major time of crustal deformation in the Rocky Mountains that culminated with the development of many of our present mountain ranges.  This episode of deformation extended from the late Cretaceous to the end of the Paleocene (about 100 million years ago to 55 million years ago).

Lode: In general, refers to a mineralized zone, quartz vein, or a mineral deposit occurring in a consolidated rock outcrop.

Metamorphism: The mineralogical and structural adjustment of solid rock to chemical and physical conditions imposed by elevated temperature and pressure at depth.

Mylonite: A compact chertlike rock produced by extreme granulation and shearing of rocks.

Ore:  A naturally occurring material from which minerals of sufficient value or quantity might be (or was in the past) mined at a profit.

Ore shoot: A large and unusually rich aggregation of ore in a vein or shear zone (lode). Sometimes referred to as a paystreak, although the latter more specifically describes rich mineralized gravel in a placer deposit.

Paraconglomerate: A conglomerate that is not a product of normal aqueous flow but deposited by such modes as mass transport (slides, turbidity flows, and glacier ice).

Phanerozoic: Rocks younger than the Precambrian.  Those that formed from about 570 million years ago to the present.

Placer: In general, refers to mineral deposits mechanically concentrated in streams and in stream-deposited terraces.

Pluton: A large mass of igneous rock.

Precambrian: An important division of geological time that is a general term for rocks older than the Phanerozoic.  Typically divided into the Archean Eon (3.8 billion to about 2.5 billion years ago) and the Proterozoic Eon (2.5 billion to 570 million years ago).

Proterozoic: see Precambrian.

Shaft: A vertical mine tunnel (see also adit).

Shear zone: A tabular zone of rock that has been crushed and brecciated with many parallel fractures.

Strategic mineral or metal:  Metals and minerals that our country does not mine in sufficient amounts to supply the needs of our domestic industries or the military in case of a national crisis, such as a war, or an embargo.  Nearly every metal found in nature is listed as strategic for the United States.

Sulfide: In reference to one of several metallic-sulfides such as pyrite.

Supergene enrichment:  Refers to the solution of a metal by surface waters from the upper part of an ore deposit and its redeposition below (generally at the water table).

Vein: A zone or belt of mineralized rock lying within clearly defined boundaries that separates it from the enclosing host rock.

Selected rock and mineral definitions

Amphibolite: A dark gray to black metamorphic rock formed almost entirely of the prismatic amphibole minerals. Amphibolites typically represent recrystallized basalts, gabbros, or greywackes.

Arsenopyrite (arsenic-iron-sulfide):  a silver-gray metallic sulfide that alters to a yellow green stain known as scorodite.  The mineral itself is of little value, although it often incorporates enough silver or gold in its crystal structure to make it an ore mineral.

Basalt: A fine-grained, dark-gray to black, mafic volcanic rock typically erupted onto the surface of the earth.

Beryl: Hexagonal (six-sided), green crystals typically found in pegmatites such as the Many Values pegmatite (Locality 57).  Rarely beryl occurs as a light blue, transparent to translucent gem known as aquamarine.  Beryl (beryllium oxide) is a common ore for the metal beryllium, which is a strategic metal used in the electronic and aerospace industries.

Chlorite: A greenish mica.

Chalcocite: A copper-sulfide ore mineral that is relatively heavy and generally earthy black.

Chalcopyrite: a bronze, metallic, copper ore mineral (copper-iron-sulfide).

Chrysocolla: A beautiful, glassy, azure blue copper silicate.

Columbite: A black mineral oxide that is the principal ore of the rare earth element niobium, and also a source of tantalum.  These high-tech metals have many important uses in alloys used in the electronics industry and aerospace industry.

Dolomite: A mineral, or a rock composed primarily of the mineral dolomite (calcium-magnesium-carbonate).

Euxenite: A rare earth oxide containing tantalum, niobium, and yttrium (see also columbite).  Yttrium, like the other rare earth elements is a high-tech metal used in the electronics industry, in lasers, and in superconductors.

Gabbro: A dark gray to black, medium- to coarse-grained, mafic igneous rock that is the subvolcanic and plutonic equivalent of basalt.

Garnet: There are a variety of garnets found in nature.  In the Medicine Bow Mountains, garnets are often found as translucent, reddish-brown, equidimensional crystals known as dodecahedrons.  They are common in schists, and are numerous in the stream beds as tiny red crystal grains.  Since garnets are relatively heavy, prospectors typically find many garnets in their black sand concentrates when panning for gold.

Gold: A valuable precious metal.  Native gold occurs as a warm, yellow, heavy, malleable metal.

Gneiss: A coarse-grained foliated metamorphic rock with alternating bands of light-colored and dark-colored minerals.

Granite: A coarse-grained, light-colored igneous rock composed primarily of pink and light-colored feldspar, gray quartz, and dark mica.

Greenstone: Refers to an igneous rock that is greenish in color.

Greywacke: A gray, micaceous sandstone.

Hematite (iron oxide): a soft, silver metallic to earthy red, non-magnetic mineral.

Huttonite (thorium-silicate): A colorless to pale-cream radioactive mineral that fluoresces white to pink under ultraviolet light.

Igneous:  "Fire rock".  A rock produced by the cooling of hot magma, either on the surface (volcanic) or at depth in the earth (plutonic).

Ilmenite (iron-titanium-oxide):  An important ore mineral for titanium (a strategic metal used in high performance civilian and military aircraft).

Limonite (hydrated iron oxide): A reddish-brown to yellowish-brown mineral formed as an weathering product of sulfide minerals.  Typically found in gossans.

Mafic: Refers to dark-colored igneous rocks typically containing relatively high amounts of magnesium in silicate minerals.

Magnetite (iron oxide):  Magnetite forms a black, equi-dimensional, highly magnetic octahedron.  Generally found in large amounts in placers and forms much of the black sands referred to by prospectors.

Malachite (copper carbonate):  Typically forms bright green stains, patina, and crusts on copper ores.  It is the alteration product of many primary copper minerals such as chalcopyrite.  May contain considerable amounts of silver in its crystal structure.

Mica (complex silicate): A platy (psuedo-hexagonal) mineral that forms in "books".  In pegmatites, crystals can obtain large dimensions of several inches across.  Can occur as white mica (muscovite), or very small white mica (sericite), black to brown mica (biotite), and dull green mica (chlorite).

Monazite: A wedge-shaped, yellowish to reddish-brown, generally weakly radioactive,  phosphate of the rare earth metals cerium and lanthanum.  Appreciable amounts of thorium and yttrium may substitute for the rare earth metals.  This is a heavy mineral found in some placers with the black sands.  The rare earth metals are also considered to be strategic metals and have many important commercial and military applications are are extensively used in superconductors.

Phyllite: A fine-grained foliated metamorphic rock of sedimentary origin.

Platinum: A malleable, heavy, silver, metallic, precious metal. This critical strategic metal has important uses in jewelry, electronics, metallurgy, and has widespread usage in catalytic converters.

Pluton: An igneous intrusion that formed below the earth's surface.  These rocks are typically coarse-grained (they cool and crystalize over relatively long periods of time) in contrast to volcanic rocks that erupt on the earth's surface and cool rapidly.

Pyrite (iron-sulfide):  Commonly known as fool's gold.  A brass-colored, metallic sulfide that is often mistaken for gold.  Pyrite, however, can incorporate large amounts of gold in its crystal structure.  As much as 60 ounces of gold per ton can be hidden in the crystal structure.  Generally, the fine-grained to massive pyrite carries some gold and the medium to coarse grained pyrite is barren of gold.

Quartz (silica):  A common rock-forming mineral that forms a large portion of most veins.

Quartz diorite: A medium-grained, gray igneous rock common in the Keystone distinct (Locality 43).

Quartzite: A hard, granular, metamorphic rock composed mostly of rounded quartz grains.  The metamorphosed equivalent of sandstone.

Rutile (titanium oxide):  A titanium ore mineral.

Schist: A distinctly foliated metamorphic rock.  Generally, mica forms large portions of schist.

Schorl: Black tourmaline.

Siderite (iron carbonate): A brownish, rhombohedral gangue mineral found in many veins.

Slate: A hard, black to dark gray, foliated metamorphic rock with the appearance of a chalkboard.  The metamorphosed equivalent of shale.

Sperrylite (platinum arsenide): A tin-white, bright, metallic ore mineral of platinum occuring in masses, less often in cubes, and rarely in octahedrons.

Sphene (calcium-titanium-oxide): An important titanium-bearing mineral with a relatively high specific gravity (relatively heavy) often found in black sands in placer deposits and in titaniferous black sandstone deposits (Locality 3).

Tantalite (an oxide of iron, manganese, niobium, and tantalum): A heavy mineral found in black sands in paystreaks of placer deposits and found in some titaniferous black sandstone deposits (Locality 3).  Tantalum is a strategic metal with several important uses, including the manufacture of surgical instruments.  Niobium is a strategic metal used in superconductors, and manganese is a critical strategic metal used in production of virtually all steels.

Tourmaline: A prismatic silicate mineral with distinct triangular to hexagonal cross sections.  In pegmatites such as the Many Values prospect (Locality 57), tourmaline occurs as long, narrow, jet black schorl in a white pegmatite matrix.

Ultramafic: An igneous rock with very high amounts of magnesium.  These rocks are interpreted to have originated from the earth's mantle where minerals are also rich in transition elements such as chromium and nickel.

Zircon (zirconium silicate): A relatively heavy, reddish brown, translucent, prismatic crystal often found in black sand concentrates and in titaniferous black sandstone deposits (Locality 4).  A primary ore mineral of the strategic metal zirconium. Zirconium is used chiefly in the facings of foundry molds, in refractory bricks for furnaces, and in the structural material in nuclear reactors.  Zircon also contains appreciable amounts of hafnium, which is used primarily in the control rods of the U.S. Navy's nuclear reactors.


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