Many rocks and contained fluids have useful properties (e.g., quartz [abrasive], flint [tools], water) or can be used as an input to chemical processes (e.g., hematite [iron ore], or crude oil [fuels, plastics]). All such materials extracted from the earth (as opposed to grown) can be called ores or natural (earth) resources.
Most elements, even the 8+2 common elements, have to be concentrated above their overall abundance level to be usefully extracted, mainly because the purer form is cheaper and someone will sell it. Ores are mined in response to human needs/desires, and are sought in response to anticipated demand and profit.
A substance that is mined (or sought) is called a mineral ( mining sense )
Minerals (mining sense) are formed in a variety of geological settings, and usually involve unusually-high available concentrations of a particular substance or unusual physical or chemical state.
Ore a.k.a. Mineral ( mining sense ) A mineral ( geological sense ) or aggregate of minerals, more or less mixed with gangue, which can be won (extracted) at a profit or treated at a profit. (Treasure)
Gangue Non-valuable minerals associated with ore. (Trash, junk)
Country rock non-valuable minerals not associated with ore but surrounding it. (Backdrop)
Deposit (subjective) A concentration or occurrence of a substance in sufficient degree of concentration and of sufficient extent to warrant attention. Geologists look for mineral and oil deposits.
Reserve (Objective but conditional, has financial/legal implications) A deposit of a material that can be extracted legally and economically at the time of determination. This is primarily an economic/engineering term, meaning a deposit you can turn a profit by extracting the deposit. Mineable.
Reserves the sum total of all known reserve deposits
Resource (Conditional and subjective) A deposit that is currently or potentially feasible.
Resources the sum total of all known reserve deposits, and known and unknown resource deposits. (It includes all deposits that can be mined, some resources that cannot now be mined feasibly but could if prices or technology changed, and an estimate of undiscovered deposits based on theories about the occurrence of the commodity and the distribution of favorable environments.)
Supply (Objective and not conditional) total amount on the planet. Estimates are based on our understanding of the composition of the planet.
Proved : (=tested, measured) having reliable quantitative and qualitative estimates. (e.g. part of a gas field whose extent has been delimited by drilling and whose composition has been measured. (Bird in the hand)
Exploration : initial finding of a deposit (looking for bird in the bush)
A description of the formation and properties of a type of ore body is known as an exploration model , and is vital to exploration and production. It is often named after a particularly well-known or well-studied deposit, or after the process for generating the deposit
Development/Production/Management : detailed investigations carried out to help with exploitation
Vocabulary Diagrams Ore Gangue Banded Iron Formation Extracted
Mineral Economics in one over-simplified lesson
Increasing the value of a mineral usually increases its supply. Increasing the cost of a mineral usually leads to less being found and developed.
There is almost always a significant lag-time between starting a mining operation and initial delivery, so there are often dramatic short-term fluctuations in price super-imposed on an overall downward trend in price for most minerals.
Value of a mineral is related to:
Intrinsic copper is more valuable than sand. (a.k.a. unit value)
Time (the sooner, the better) Ore delivered next year is more valuable than ore delivered in 10 years. Because exploration must be paid for up-front, interest rates are often the limit on reserves
Place (closer to consumer the better) Limestone in Chicago is more valuable than limestone in Fairbury. For industrial minerals, the intrinsic value is usually very low compared to the place value
Costs are due to:
Exploration and Research
Production (salaries, equipment, fuel, supplies, processing costs)
Price of Oil vs. Time Petroleum Prices from BP's Statistical Review of World Energy 2008 at http://www.bp.com/productlanding.do?categoryId=6929&contentId=7044622 BP's Petroleum Price vs Time $0 $20 $40 $60 $80 $100 $120 1850 1875 1900 1925 1950 1975 2000 Year AD Price MoneyOfTheDay Price Inflation-adjusted_2007 Price
US Proven Oil Reserves for 20 th Century US Proved Oil Reserves are from US Dept. Energy’s Energy Information Agency, http://tonto.eia.doe.gov/dnav/pet/pet_crd_pres_dcu_NUS_a.htm Price of oil is from last year’s Statistical Review from BP http://www.bp.com/downloads.do?categoryId=9003093&contentId=7005944
Fuels are the only resources that are consumed, since it is their high-energy molecular state, and not the actual atoms, that are sought, and they will converted to lower energy states in use (burning)
The vast majority of energy consumed in the industrial world comes from fossil fuels. In the non-industrialized nations, fuel mainly comes from wood or other organic fuels. We currently have hundreds of years of coal reserves (Coal could be easily replaced for electricity with nuclear fission), decades of oil reserves and, depending on what you call potentially feasible, decades to hundreds of years of oil resources.
We have been “about to use the last” oil for over a hundred years so far, and yet it is still rather cheap, especially when normalized to inflation or labor. Technology will continue to improve and we will keep finding new reserves and extracting more from old ones, possibly at higher costs.
Because liquid fuels are rather convenient for vehicles, if we ever do replace crude oil, it will probably be with coal-generated synthoil or with biogenic fuels like corn or soybean oil or ethanol.
Metallic ores are desirable for the contained elements and usually require refining.
Mineral deposits often involve a high degree of concentration. This is often occurs due to sudden, localized changes in chemistry, especially phase changes like ex-solution, vaporization, and weathering.
Most metals occur in many different types of deposits. For example, gold can occur in veins, as explained above, or as placer deposits in stream sediments, where gold veins are weathered and the gold flecks are concentrated with coarse sand in river bed.
Industrial minerals are rocks that are not refined before use. They have low intrinsic value, and most are used in huge quantities. (This is a very vague classification.)
Mainly what separates them from metals are materials that are useful as is; the mineral/crystal/rock itself is used, rather than something obtained from it. Emery (a mixture of corundum and garnet) is sold because garnet and corundum are hard, not because of the contained aluminum silicon, and iron.
Industrial minerals include gravel (a size range), sand (size or composition), clays for ceramics, limestone, fluorite, quartz for electronics, phosphate, salt, and abrasives. These are used in HUGE quantities for building materials and in other industrial processes, usually without much additional processing.
Typically, industrial minerals are unusually high concentrations of some industrially useful mineral or rock in a good location with high purity.
Organic Remains Many ores are accumulated organic debris, maybe altered by geologic processes.
Reduced compounds (hydrocarbons, lipids, and such) can be preserved if the organic matter (dead organism or fecal matter or leaves…) is deposited under reducing conditions. If slightly more oxidizing conditions, reduced organic matter like sugars and lipids might be rapidly eaten, but other constituents, such as phosphate or nitrate, may accumulate. Under fully-oxygenated conditions, only indigestible constituents (calcite or silica shells…) are likely to accumulate. Organic-remains ores include phosphate rock and limestone, and fossil fuels.
Phosphates are mined from rocks, usually shales or limestones, that contain unusually high concentrations of the mineral apatite (Ca 5 (PO 4 ) 3 (F, OH, Cl, ½ CO 3 ) ). Mostly used for fertilizer, some chemical feedstock. Sometimes, this is nearly pure apatite, in which case it is called phosphorite, sometimes it is mixed with enough calcite or clay to be limestone or shale.
Phosphate accumulation is associated with oceanic upwelling (cold, oxygen and nutrient rich bottom waters coming to the surface, as happens off Peru). Under such conditions, there is a great profusion of life, and consequently death. Organic remains (soft-body parts, bones, fecal matter) sinks to the bottom. The great abundance of incoming organic matter may overwhelm the ability of bottom organisms to consume this rain of food, and some goes undigested. Under anaerobic conditions, the reduced organic matter remains. Under slightly more oxidizing conditions, the reduced organic matter gets consumed, but the phosphate remains. Under normal oxidizing conditions, the phosphate gets consumed or dissolved into seawater.
Limestones are accumulated shells of organisms. Limestone is quarried as cement-ore, for building material, and as road-bed material (filler). Surprisingly much mining is of limestone, gravel, and sand.
Diatomites and Chert are also accumulated shells of organisms. Diatomite is mainly mined for use as a filtering agent and for Kitty Litter. Chert was used mainly in making stone tools with a cutting edge; not important in most places anymore, but chert and obsidian used to be big business.
Coal is dead wood and leaves that accumulated in swamps (reducing conditions), underwent some organic decay and geologic alteration. Peat is nearly unaltered wood and leaves. With heating and compression, it alters (loses volatiles), becoming
brown coal->lignite->bituminous coal->anthracite->graphite (not coal)
Coal requires abundant plant growth, with remains accumulating in anoxic conditions (swamps).
Illinois coals accumulated in low, coastal swamps in a shallow sea. Rises and falls in sea level resulted in changing depths (and hence sediments) at a fixed location, so a series of cyclothems (cyclic layers of limestone, shale, and coal deposited close to sea level) accumulated. Some of these cyclothems can be traced from the East Coast to Iowa.
The steep, bright-red hills on I-55 before Joliet are tailings piles (gangue) from coal mining. They are barren due to high acid production from the weathering of pyrites associated with the other layers in the cyclothems.
Petroleum includes oil, natural gas, and tar sands. These are cooked, fluid remains of reduced organic matter. Because they are liquid, they tend to move and are only preserved if they are trapped by the permeability distribution in a reservoir.
Formation (accumulate organic matter, cook oil off it, migrate, and trap)
Organic matter accumulates in fine-grained, low-permeability sediments, either due to very high abundance of organic matter raining down overwhelming the supply of oxygen, or low initial oxygen concentrations. [Most organic matter doesn't get preserved] This organic mater progresses from dead organisms (fairly small molecules) to stuff very much like soil organic matter, and finally, with burial and heating, to kerogen (a type of organic matter found in rocks, HUGE molecules of various other biogenic molecules welded together, with most of the oxygen, sulfur, phosphorous, and nitrogen missing [eaten by early digesters]) At this point, you have a source bed , which may be an oil shale.
With continued burial and heating, chunks of the kerogen molecules are cook ed off. These are the constituents of oil. With continued heating, more stuff breaks off, and the oil molecules break down, and you get methane, or eventually, just carbon.
These oil droplets or gas bubbles somehow migrate into permeable sandstone or limestone conduit beds. If nothing stops them, they migrate right to the surface (due to their low density) and form oil seeps . Loss of volatiles and partial digestion by bacteria can result in a tar sand .
Certain irregularities in the permeability in conduit beds can act as traps , where oil accumulates in a reservoir . A trap is a place where the formation forms a downward facing concavity (otherwise, the oil would keep moving up) and an overlying cap rock (relatively impermeable, and completely impermeable to oil due to complicated capillary considerations) prevents further upward migration.
Distribution : oil is rather widely distributed, considering its origins. A lot is associated with rifting, due to good chances for anaerobic conditions and high heat flow. Compressive mountain ranges provide heating and structural trapping mechanisms, as do extensional zones.
Source (this is an oil shale) Trap Cook Migrate Seeps+ Tar
Petroleum includes oil, natural gas, and tar sands.
Oil, Crude oil (Black gold, Texas tea) is any liquid hydrocarbon, usually about the consistency of kerosene to olive oil. Carbon chains and rings with hydrogen, other elements in trace quantities. Some can be used out-of-the-ground as a fuel or lubricant, but most requires treatment at refineries.
Natural Gas is usually, methane, although it can include ethane, butane or propane, light “liquid” hydrocarbons, and impurities such as CO 2 or Nitrogen.
Tar (as in tar sands, a.k.a. "heavy oil") is crude oil that has lost volatiles by evaporation and digestion, and residually concentrated large molecules and non-volatiles, like vanadium and uranium. (i.e., tar is degraded oil.) Usually the result of a trap being breached or never having existed, and aerobic decomposition of crude.
Oil from a single field is usually pretty similar, but oil from different fields can be very different and require very different refining procedures, resulting in adjustments to price, sort of like the price penalties/bonuses on corn of differing water content (and delivery location), or on wheat based on water and protein content.
When you hear about the price of a "barrel of oil", that usually refers to either West Texas Intermediate or Brent Crude, these refer to hypothetical, desirable mixes of oil. Refiners pay for other oils as a percentage of these 'index' crudes, usually some lower amount. (The market price also specifies delivery location)
Occasionally, you see an oil well that is owned by one entity who is free to control the price, but that is rare. More often, oil wells are jointly owned by the landowner(s) (possibly a government), an oil company or consortium thereof, and operators, and this leads to the same problems as any other joint operation, of everyone wanting to maximize their profits and minimize costs, so they generally agree to sell the oil at market-price*adjustment, pay certain production costs, and proceeds are split according to some contract. [This is why oil companies don't control oil prices.]
Evaporites are sedimentary rocks that form in areas of high evaporation relative to influx of water. Economically important evaporites include gypsum/anhydrite (which may get reduced to elemental sulfur by microorganisms), halite/rock salt, sylvite (KCl), nitrates, barite, and borates.
Precipitates are sedimentary rocks that form where a mineral precipitates directly out of solution. (Note that evaporites are precipitates). In the early, reducing atmosphere, iron was soluble. When more free oxygen accumulated in the atmosphere and oceans, banded iron formation (thin beds of chert and hematite) formed.
Clastic sediments of economic importance include sand (sand for construction, source of amazingly pure silica, abrasive, …), gravel (for road material), and certain clays (ceramics, viscosity additive and sealant in drilling mud,…) Placer deposits
Placers: a dense, weathering-resistant mineral can be concentrated in coarse sediments. Gold, tin, gems
Primary mineral throughout rock: if the mineral is rare enough, like diamonds or really large crystals of quartz, the mere occurrence of a mineral may be significant enough to warrant mining.
Breaking rock apart is expensive.
Diamonds and gems and large crystals in pegmatites fall into this rare category.
Mineral Settling: In a fluid magma, early-formed crystals can settle out; dense, to bottom; light, to top. This can often occur in gabbroic intrusions. Chromite, magnetite, platinum.
As a magma cools, it may separate into two immiscible components.
In some mafic/ultramafic magmatic segregation deposits, one is siliceous and the other is sulfide or oxide rich. The denser sulfide or oxide fluid settles to the bottom of the intrusion and solidifies. Nickel, copper.
More often, an aqueous phase separates out, causing the freezing point of the silicic magma to drop suddenly (origin of most porphyries). The resulting metasomatic fluids may be highly enriched in ore-forming elements and can form hydrothermal deposits (below).
Hydrothermal deposits (from hot fluids) are associated with igneous or metamorphic activity. Typical minerals are sulfides, some oxides, and silicates.
Hydrothermal fluids are hot, ion-rich fluids.
Hydrothermal fluids can come from
the de-hydration of clays and other hydrous minerals during metamorphism,
the residual fluids left after a magma has crystallized,
ex-solution of an aqueous phase from a magma,
deep circulation of groundwater into hot regions,
water expelled from sediments at great depth (and temperature).
Hydrothermal fluids contain ions that did not go into minerals or that got leached from minerals: H + , metals, sulfide, sulfate, carbonate, … Hydrothermal fluids are generally hot, acidic, and very ion-rich. The acidity makes it very easy for feldspars and mafics to be hydrolyzed, resulting in big clay-altered zones (useful for exploration). The acids can leach any metal ions out of minerals, especially ones that don’t quite fit into their crystal structure (substitutions), extracting trace constituents from surrounding rocks.
Where the circulating fluids undergo a sudden change of pressure, temperature, or chemistry, precipitates often form as the mineral-stability changes with PT.
For example, where hydrothermal fluids are expelled into cold sea water, sulfides, that had been stable in solution, are suddenly over-saturated and precipitate, giving black smokers.
Gold deposits often occur in the upper part of hydrothermal circulation cells, where the metasomatic fluid reaches low pressure and boils: water and chlorine go into a vapor phase, and gold, which had been complexed with the chlorine in aqueous solution, precipitates out, along with quartz and other minerals.
Where hydrothermal fluids encounter limestone or dolomite, the acids react with the carbonates and this changes the chemistry of the water, and lots of weird minerals precipitate.
In disseminated deposits (typical with copper), the ore is distributed through a large volume of rock. In veins (typical of gold), the minerals are deposited in thin, tabular fractures (usually joints). Pegmatites are a special case of veins, where the crystals are unusually large, and often have high concentrations of very rare elements, such as beryllium.
In many hydrothermal systems, hydrothermal fluids discharge occurs below water, and a “ smoker ” deposit occurs, which is sulfide precipitating from metasomatic fluid where it hits cold seawater and forms a layered sulfide deposit on a seafloor.
In "recent" times (last billion years), plate tectonics has closely controlled the occurrence and distribution of geological environments favorable to deposition of many ores. Other ores, like bauxite, have little if any relationship to plate tectonics, and are instead controlled by weather.
The connection between plate tectonics and ores is indirect. Certain rock types or structural settings favor certain types of ore deposits, and these are often found associated with certain plate tectonic settings, but the relationships are non-unique. For example, copper is often associated with basalt, regardless of how it occurs. Granites have their associated ores (tin-tungsten, lead-zinc, silver, gold, molybdenum…).
In early stages, salt layers and other evaporites. Petroleum is common due to the thick accumulation of sediments, often anoxic (source bed); later heating from volcanic activity (cooking) and complicated stratigraphy and faulting (traps) [some early, some late]. In late stages, the passive margin accumulates the dead-stuff deposits. Igneous activity brings the standard suite of basalt-associated ores (mainly copper and zinc sulfides), and some molybdenum associated with granites if there's felsic magmas.
In oceanic setting, the volcanic activity causes hot springs, smokers, and mineral brines, often rich in copper. Chromite is found in lenses in gabbroic intrusives.
Oceanic Subduction (trench, island arc, volcano chain)
Wet partial melting of oceanic lithosphere and possibly mantle extracts gold, copper, silver, which occur in hydrothermal deposits, often associated with porphyries. [Circum-Pacific]
When these magmas pass through continental crust, the continental crust may also be leached resulting in tin-tungsten, molybdenum, and lead-zinc-silver deposits, all found landward of the arc.
Back-arc basins can also get all the divergent boundary deposits.
Continental collisions involve former divergent and ocean-continent convergent boundaries, and so can include those deposits, probably re-worked by metamorphism. Magmatism associated with the melting of continental crust gives rise to pegmatites, tin-tungsten, uranium, molybdenum, Cu-Pb-Zn, Ag, Au and others. The structural deformation and heating of sediments can produce extensive oil deposits.
Greenstone belts are some of the richest mining provinces in the world and have deposits associated with
felsic magmatism, including copper, Cu-Pb-Zn, gold, silver,
ultra-mafic magmatism (nickel sulfides), and
some strange sedimentary gold associated with iron formation.
Depending on the type of organization, an exploration program may take two forms
What mineral deposits and other resource would we find in PLACE? This is one of the things geological surveys do. Here, you start with an area, determine its geological settings through time, and figure out what types of mineral deposits might be found there. (like looking in the freezer)
Where would we find SUBSTANCE? This is typically the mode of an oil or mining company. Some engineer or economist/accountant figures out what minerals will be in demand and raises/assigns money to conduct the exploration. Geologists find exploration models of the desired substance, then regions to explore. (like deciding what store to go to)
In both cases, much of the work consists of examining airphotos, well logs, and geophysical maps, first to map the tectonic setting/regional geology, selecting favorable regions and mapping them in more detail, to narrow in on fields, then prospects, which, if confirmed become deposits. Then accountants re-appear. If they decide to go ahead, then engineers appear along with more geologists and detailed, site level exploration/development work begins, resulting in mines. Accountants keep re-appearing, often annoying and re-organizing the geologists and engineers (see www.dilbert.com ).
Industrial civilization requires huge amounts of materials.
Economic minerals include water, petroleum and other fossil fuels, industrial minerals, ores, and soils.
Economic minerals can thought of in terms of supply, resources, and reserves.
Reserves are legally allowed and economically viable at the time the reserve estimate. Reserves are objective but conditional.
Resources include reserves and various categories like undiscovered+profitable, discovered+unprofitable… Resources are subjective.
Supply (total) is objective and unconditional.
Most materials can form in several ways. Exploration models guide exploration and development.
Economic minerals form in a wide variety of geologic settings, from infiltration of rainwater into the ground and beach sand accumulating to explosive exsolution events forming copper and gold deposits.
Plate tectonics, through its control on tectonic environments and especially igneous activity, is one of the major constraints on the distribution of (economic) minerals.