Ch 6 Economic Geology

Alteration

Any change in the mineral composition of a rock caused by physical or chemical means, especially by hydrothermal fluids. _______ is milder and more localized than metamorphism.

Assay

An analysis of the proportions of metal in an ore. Composition, purity, weight, or other properties of economic interest are tested.

Average Grade

The average quantity or percentage of ore mineral content in an ore body

Cut off grade

Lowest grade that can be mined economically; if ore is below this value it is not used in calculation of average grade

Base metal

A common and chemically active metal (it oxidizes or corrodes easily) - lead or copper for example.

Coalification

The process that produces coal of increasing rank

Density

Mass per unit volume

Dispersion

The process by which elements move away from the enriched ore zone

Dolomitization

The process by which limestone is converted to dolomite by the replacement of calcium carbonate by magnesium carbonate

Eh

Oxidation potential; the potential of a half-cell, measured against the standard hydrogen half-cell

Epigenetic

An ore deposit that originates later than the enclosing host rocks; low T and low P conditions involving heated aqueous fluids not associated with igneous activity

Epithermal

Shallow hydrothermal

Exhalite

The chemical product of exhalation; a deposit formed by the interaction of volcanically derived water and sea water with subjacent rocks

Gangue

Valueless rock or mineral aggregates in an ore; economically undesirable

Giant Field

A petroleum field containing 500 million barrels or more of recoverable oil; a natural gas field which has a minimum of 3.5 trillion cubic feet of recoverable gas.

Gossan

An iron-bearing weathered product overlying a sulfide deposit

grade

A coal classification based on degree of purity

Hydrothermal

Pertaining to hot water, the action of hot water, or the products of hot water. Term is generally used for all hot water whether or not the water is of magmatic origin

Metasomatism

The process of practically simultaneous solution and deposition of a new mineral in an old mineral by means of interstitial fluids. The replacement occurs at constant volume with little disturbance of structural or textural features.

Pathfinder

A relatively mobile element whose geochemical properties are used to more easily find a deposit of greater importance or value

pH

the negative logarithm of the hydrogen ion concentration; measure of the acidity or basicity of the solution

Porphyry

An igneous rock that contains phenocrysts in a fine-grained matrix

Precious Metal

Gold, silver or minerals of the platinum group

Pyritization

Introduction of, or replacement by, pyrite, a common process of hydrothermal alteration.

Rank

The degree of progressive diagenesis or metamorphism of peat (coalification)

Replacement

a solid-state change in composition of a mineral accomplished by diffusion of new material in and old material out

Reserves

Deposits that are economically available and may be extracted profitably with existing technology; a subset of resources

Resources

Naturally occurring deposits in such amounts or concentrations to be minable now or in the future, including reserves

Silicification

The introduction of, or replacement by, silica

Skarn

Silicate rocks of complex metamorphic-metasomatic mineralogy formed in carbonate rocks in a contact metamorphic aureole

Specific Gravity

The ratio between the density of a substance and the density of water

Stoping

Extraction of ore in an underground mine by working laterally in a series of levels in the plane of the vein

Stratabound

mineral deposit confined to a single stratigraphic unit

Stratiform

A special type of stratabound deposit in which the ore constitutes a layer or layers in the rock

Supergene enrichment

Special case of weathering; a near-surface process of mineral deposition in which metals are leached by acidic solutions, carried downward, and re-precipitated, enriching the sulfide minerals already present.

Syngenetic

a mineral deposit formed contemporaneously with, and by the same processes as, the enclosing rocks

Ore deposits

Naturally occurring materials from which minerals of economic value may be mined at a profic

magmatic segregation deposits

________ are produced by differentiation or direct crystallization of magmas. They form in magma chambers and are deep seated intrusive bodies.

mafic magmatic segregation deposits

______ are lodged in cratons or continental crust undergoing rifting or protorifting. Formation temperatures are from 700 degrees C to 1500 degrees C and pressure is very high

Layered Mafic Intrusions (LMI)

Example of a mafic ore deposit; formed by crystal settling of melt; major sources of chromium, nickel, copper, platinum, titanium, iron, vanadium, tin, and sulfur

Anorthosites

Type of mafic ore deposit; Precambrian in age and occur in close relationship to deep faults along old rift zones; contain titanium rich ores composed of titanium bearing magnetite, hematite, rutile, or ilmenite

Kimberlites

Type of mafic ore deposit; diamond-bearing, volatile-rich, potassic, ultra mafic igneous rocks dominated by olivine

Carbonatite

Type of mafic ore deposit; closely related to kimberlites, igneous rock composed of at least half of flow banded calcite, dolomite, or siderite; exploited for rare earths; young deposits appear related to rift zones

Ophiolite - peridotite terrane

Where does most of the world's chrysotile asbestos come from?

Cordilleran vein deposits

______________________ form at intermediate to shallow depths as open space fillings or replacement veins in a convergent margin setting. Associated with Calc alkaline igneous activity in space and time. Many deposits are of major economic significance . (type of ore deposit)

Porphyry deposits

Deposits are gigantic hydrothermal systems; have a porphyritic texture

Pegmatites

Igneous rocks that are made of extremely large grained minerals; are the late stage differentiates of a water-rich, siliceous igneous melt of intermediate to felsic composition; formed at upper end of temperature range 250 - 700 C and high pressures

Deposits associated with Felsic-Intermediate Intrusions

Consist of the base and precious metals associated with calc alkaline orogenic belts; Over 40% of the total metal output of the world comes from this deposit

Subduction; young; Mesozoic or cenozoic

Most of the major ore deposits of the American Cordilleras formed while _____ was progressing below them. Mountain belts with Calc-alkaline rocks are generally geologically ______. Most porphyry systems and Calc-alkaline rock formed during _____ or ____ orogenies.

Deposits related to Solution-Remobilization

Deposits form from circulating fluids; ore components are extracted from rocks due to an area wide thermal metamorphic gradient, rather than an intrusion; precipitated in shear zones or fractures; quartz carbonate gold veins of the Mother Lode in California have been explained using this theory

1. Mississippi Valley deposits (MVDs) of lead-zinc

2. Western states uranium - roll-front, humate, and salt-wash

3. Athabascan-type unconformity-related uranium

What are the major types of epigenetic deposits?

Mississippi Valley Deposits (MVDs)

Type of epigenetic deposit; ______ occur in structurally passive areas such as the mid continent craton of the US. Form when warm brines containing metals migrate from basins into carbonate sediments at the margins of the basin. Principle of lead-zinc deposits.

Western States Uranium - roll-front, humate, and salt wash

Epigenetic uranium deposits that form at 25 - 75 degrees Celsius and low pressures; account for 40% of worlds uranium resources and 90% of U.S uranium production. Form when U+4 ions in rocks undergoing weathering are oxidized to U+6 ions which are soluble in water. Groundwater encounters a reducing environment and uranium is precipitated.

Athabascan-type in conformity related uranium

Type of epigenetic deposit; formed in a similar way to U.S uranium deposits but at a slightly higher temperature (120 to 220 C). Uranium was leached from permeable arkosic sandstones above an unconformity and dissolved in groundwater to form U+6 ion. Ion reduced by by methane rich fluids moving along fault zones underlying unconformity; uranium precipitated out

Subaerial volcanic deposits

Deposits that are formed by the eruption of intermediate to silicic volcanic rocks; most deposits are epithermal - result of volcanic-related hydrothermal activity at shallow depths and low temps; emplaced predominantly as open spaced fillings and many rocks show replacement texture; occur along convergent plate boundaries and related to orogenic belts - found near areas of tertiary volcanism; famous examples include gold-silver mines of the Comstock Lode - Virginia City District, antimony deposits in China, and "invisible" Carlin type gold deposits in Nevada

Volcanogenic Deposits

Stratsbound deposits formed underwater by volcanic processes and activities of thermal springs

Volcanogenic

Term used to refer to oxides, carbonates, and sulfates that have been formed in environments located distally to the volcanic source as well as proximal massive sulfur ore bodies

Near hydrothermal source; in the sea near "black smoker" vents

Where are massive sulfide deposits formed as volcanogenic deposits?

They are best developed in Archaean and Proterozoic systems at least 2 billion years before plate tectonics had developed.

What time period are volcanogenic massive sulfide deposits best developed?

Exhalites

Deposits formed from the interaction of volcanically heated water, connecting seawater, and subaqueous igneous rocks; ore components are derived from sea-floor igneous rocks, converted by seawater, and driven back into the underlying rocks and circulated through, heated up and jetted through fractures. The exhaled fluids then chemically precipitate into sediments.

black shale-hosted ores

low T, may be biologically influenced; evidence of forming in continental side of back-arc basin with distant volcanic source or formation directly over a vent in an intracratonic rift basin

Black shales

A substantial proportion of the worlds lead-zinc reserves are found in _____. The deposits are characteristically stratiform and strata bound and composed of lens or wedge shaped beds of galena-sphalerite-pyrite.

Regional metamorphic deposits

Deposits that form under high temperatures and pressures; U, gems, industrial rocks and minerals: graphite, garnet, micas, asbestos redistribution of mineral components.

Sedimentary ores

Formed by sedimentary processes or diagenesis at the same time as the enclosing rocks (syngenetic deposits); include those formed by chemical precipitates and mechanical accumulations (placers).

Chemical sedimentation deposits

Common characteristic of the genesis of this type of deposit is a solubility contrast in low temp aqueous solutions; ore component must be soluble, transported in solution or as a colloid to an environment where chemical conditions promote a reaction and precipitation of an insoluble compound

Sulfide deposits

Type of chemical sedimentation deposit; ______ form when metal ions are supplied into a deposition basin where reducing conditions exist from decaying organic debris

Black and phosphatic shales

Type of chemical sedimentation deposit; ______ form in low lying stable areas associated with minor unconformities - metals are enriched in the black carbonaceous marine shales

Siderite

Type of chemical sedimentation deposit; Black banded ores that are widespread and have iron ore

Limonites

Type of chemical sedimentation deposit; iron ore which is produced from the biochemical precipitation of iron minerals in a bog

Phosphorite

Chemical sedimentation deposit; forms on continental shelves where deep, cold sea currents upwell and mix with shallow, warm longshore currents; calcium phosphate becomes less soluble and precipitates this rock

Marine evaporites

Chemical sedimentation deposit; form in arid climates in rift zones at or near continental margins; provide the worlds major sources of gypsum,salt, and anhydrite

Lacustrine deposits

Chemical sedimentation deposit; form in extensional fault blocks at high structural levels or in topographic lows in the desert

Oolitic iron ores; nearshore environment

What are the most economically important sedimentary iron deposit and where do they form? This is a type of chemical sedimentation deposit.

Placer deposits

Clastic sedimentation deposit formed by mechanical concentration of heavy mineral particles which are chemically stable or metastable in the weathered zone of earth

Placers formed by stream and marine action

Which environment are placers found to be most economically significant?

Gold and platinum; gold; Cenozoic

What are the most common and abundant placers? Most valuable placer? What is the age of most of the exploited placers?

Laterites

A red, highly leached soil type found in the tropics that is rich in oxides of iron and aluminum

They form over serpentine and ultra mafic rocks

Where do nickel laterites form?

They form over ferromagnesian rocks deficient in silica.

Where do iron rich laterites form?

bauxite

laterite rich in aluminum and low in silica and iron

Form in humid tropical or subtropical environments where the underlying rocks are also rich in aluminum and low in iron and silica

Where do bauxites form?

Copper ore

What is the principal ore redistributed by supergene enrichment?

Induced polarization and electromagnetism

What are the two most useful geophysical methods in mineral exploration?

rare earth elements

the elements with atomic numbers 39, 57, and 58-71

1/4th

U.S has _____ of the world's coal reserves

Acid mine drainage

Environmental and health problem associated with mining and burning coal; results from the oxidation of sulfur in pyrite associated with coal

Coal

Resultant product of the decay, burial, diagenesis and finally metamorphosis of plants growing in either fresh or brackish water swamps; high humidity and high stagnant water table necessary for growth of plant matter

Ash

Part of coal that doesnt burn

Melting point temperature

The ___ of the ash determines the design of coal furnace.

methane gas

During the coalification process, what gas is stored within coal beds and later released when mining?

Increasing rank means coal becomes progressively harder, contains less water and volatiles, becomes blacker and more massive, breaks less readily, and becomes a cleaner more efficient fuel

How does coal change with increasing rank?

As coal becomes more deeply buried, rank will increase because temperature and pressure will increase. In general, higher rank coals are older.

How does rank correlate to burial depth of coal?

No

Is coal grade used in making resource estimates?

Coal deposits must be reliably and accurately mapped, of a specified thickness based upon rank, and shallower than 6000'. Minimum thickness for rank is 14" or more for anthracite and bituminous coal and 30" or more for subbituminous coal and lignite

What are the qualifications for coal deposits to be classified as identified resources?

33

Coal beds containing _______ percent ash are not included in resource calculation

Surficial

____ mining methods produce higher recovery

Surficial; underground

___ coal mining dominates west of the Mississippi River, whereas ____ coal mining dominates East of the Mississippi River

Bench

Narrow, strip of land cut into the side of an open pit mine

Room-and-pillar mining and long wall mining

What are the 2 underground mining methods for coal?

room and pillar mining

Common method of subsurface mining in which tunnels are dug underground