Transcript Geologic Resources - Illinois Wesleyan University

Geologic Resources
• 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.
v 0050 of 'Geologic Resources' by Greg Pouch at 2013-11-25 18:46:57
LastSavedBeforeThis 2011-04-12 09:43:10 22Resources.ppt
Geologic Resources
3 Amounts of Earth Resources Used Annually in US
4 Vocabulary
5 Vocabulary Diagrams
6 Mineral Economics in one over-simplified lesson
7 Reserve Depletion
Graphs ( 8 Chart of crude oil prices since 1861; 9 Crude oil prices since 1861 ; 10 Price of Oil vs. Time ;
11 US Proven Oil Reserves for 20th Century ;12 US Proven Oil Reserves for 20th Century ;13 Oil
reserves-to-production (R/P) ratios ; 14 Oil reserves-to-production (R/P) ratios )
15 Classification by Use and Economics
16 Classification by Geologic Origin
Sedimentary
17 Sedimentary Processes > Organic Remains
18 Sedimentary Processes > … > Fossil Fuels > Coal
19 Sedimentary Processes>…>Fossil Fuels > Petroleum
20 Sedimentary Processes>…>Fossil Fuels > Petroleum
21 Sedimentary Processes
22 Sedimentary Processes>Weathering
23 Igneous Processes
24 Metamorphic
25 Hydrothermal/Metasomatic Fluids
26 Hydrothermal Diagrams
27 Plate Tectonics
28 Plate Tectonics Map
29 Divergent Boundaries
30 Convergent Boundaries
31 Greenstone Belts
32 Mineral exploration
33 Summary
Amounts of Earth Resources Used Annually in US
• The overwhelming majority of solids is industrial minerals.
• This does not include fuels (about 8 tons) or water (about 1,000 tons per capita, or 243 tons excluding
industrial users and agriculture).
• Most of the “consumption” goes into building materials for roads and buildings.
Vocabulary
• 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. [Open to better term.] This is NOT "supply" in "supply and demand".
• 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, like Kidd Creek massive sulfides from black
smokers where hydrothermal fluids erupted underwater and precipitated the sulfides in strata, or
hydrothermal gold veins where boiling hydrothermal fluid precipitates gold at the zone of boiling, or
buried bedrock valley aquifers.
• Development/Production/Management: detailed investigations carried out to help with exploitation
(feeding and tending the bird in the hand)
Vocabulary Diagrams
Extracted
Geologic-total="Supply" is the total amount on Earth.
Resources are subset of Supply.
Banded Iron Formation
Reserves are subset of Resources.
Some stuff already extracted would not be profitable now, so
would not be reserves, maybe not even resources.
Mineral Economics in one over-simplified lesson
Increasing the value of a mineral usually increases its econ-supply (reserves). Increasing the cost of a
mineral usually leads to less being found and developed and decreases its econ-supply (reserves).
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)
–Failures (dry holes, empty workings,…)Transportation
–Capital
–Clean-up costs
–Taxes
–Much equipment and labor must be paid for before any production begins: this contributes to time value.
–Shipping costs go into place value.
Reserve
Depletion
There are several ways to deplete (reduce) reserves. (This assumes no recycling).
• Extract and don't replace (either no exploration or unsuccessful exploration)
• Make known reserves or resources illegal
–banning mining in national forests or public lands or offshore
• Make known reserves uneconomic
–declining prices/demand (alternatives, Depression)
– increasing extraction costs for labor or capital or shipping or regulatory compliance
• Make unknown resources unavailable
–banning exploration or making it more expensive
Generally, the opposites will increase reserves.
Improved exploration models, exploration techniques, and improved extraction techniques also increase
reserves.
See http://sun.iwu.edu/~gpouch/geo101/WorldAndUSOilReservesAndProduction.htm for examples of US
and World Oil Reserves 'decline'
Price of Oil vs. Time
$120
$100
Price
$80
MoneyOfTheDay Price
Inflation-adjusted 2012 Price
$60
$40
$20
Year AD
Petroleum Prices from BP's Statistical Review of World Energy 2013 at
http://www.bp.com/content/dam/bp/excel/Statistical-Review/statistical_review_of_world_energy_2013_workbook.xlsx
20
00
19
75
19
50
19
25
19
00
18
75
18
50
$0
Chart of crude oil prices since 1861
Crude oil prices since 1861
Price of Oil vs. Time
BP's Petroleum Price vs Time
$120
$100
Price
$80
MoneyOfTheDay Price
Inflation-adjusted_2007 Price
$60
$40
$20
$0
Year AD
Petroleum Prices from BP's Statistical Review of World Energy 2008 at http://www.bp.com/productlanding.do?categoryId=6929&contentId=7044622
US Proven Oil Reserves for 20th-21st Century
US Crude Oil Reserves
45,000
40
U.S. Crude Oil Proved Reserves (Million Barrels)
40,000
U.S. Crude Oil Estimated Production from Reserves (Million Barrels)
35
US Reserves to Production, Years
35,000
30
30,000
25,000
20
20,000
15
15,000
10
10,000
5
5,000
0
1900
1910
1920
1930
1940
1950
1960
1970
Year AD
Data from http://tonto.eia.doe.gov/dnav/pet/xls/PET_CRD_PRES_DCU_NUS_A.xls
1980
1990
2000
0
2010
Years
Millions of Barrels
25
US Proven Oil Reserves for 20th Century
50000
45000
Millions of Barrels
100.00
U.S. Crude Oil Proved Reserves (Million Barrels) from DOE>EIA
PricePerBarrel In 2003USD from BP
90.00
40000
80.00
35000
70.00
30000
60.00
25000
50.00
20000
40.00
15000
30.00
10000
20.00
5000
10.00
0
1900
0.00
1910
1920
1930
1940
1950
Date
1960
1970
1980
1990
2000
2010
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
Oil reserves-to-production
(R/P) ratios
Oil reserves-to-production (R/P) ratios
Classification
by
Use
and
Economics
• Water
• Soil
• 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 nonindustrialized 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, 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.
Classification by Geologic Origin
• Sedimentary
–Organic Remains
–Evaporites
–Precipitates
–Clastic Sediments
–Weathering Products
• Soil
• Groundwater
• Residuum
• Igneous
–Primary Minerals
–Mineral Settling
–Magmatic Segregation
–Hydrothermal
• Metamorphic
–Hydrothermal
–Minerals (like graphite and garnet)
Sedimentary Processes > Organic Remains
• 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 (Ca5(PO4)3(F, OH, Cl, ½ CO3) ). 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.
Sedimentary Processes > … > Fossil Fuels > Coal
• Organic Remains
–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 terrestrial and marine sandstones, clays, limestones, and coal, all 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.
Sedimentary Processes>…>Fossil Fuels > Petroleum
• 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 (organic matter accumulates, oil cooks off it, oil migrates to oil trap or out)
–Organic matter accumulates in fine-grained, low-permeability sediments, either
due to very high abundance of organic matter raining down overwhelming the
Source (this is an oil shale)
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
Cook
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 cooked
off. These are the constituents of oil. With continued heating, more stuff breaks
Migrate off, and the oil molecules break down, and you get methane, or eventually, just
carbon.
Seeps+ Tar
–These oil droplets or gas bubbles somehow migrate into permeable sandstone
or limestone conduit beds. If nothing stops them, they migrate right to the
Trap
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.
Sedimentary
Processes>…>Fossil Fuels > Petroleum
• 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 CO2 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.]
• In addition to the people who actually sell oil by finding and producing it and those who buy oil to refine or
burn, and brokers who deal with them, there are also financial speculators that buy and sell based on their
beliefs about upcoming shortages or surpluses: speculators can drive bubbles and crashes that have little if
anything to do with actual production and consumption, and are responsible for "fear premium" or
"instability premium"
Sedimentary Processes
• 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
Sedimentary Processes>Weathering
Weathering can produce ores, by what it removes or what it deposits nearby.
• Residuum: by removing other constituents in solution, some elements are concentrated Bauxite (laterite)
• Secondary enrichment: constituents are dissolved at the surface and deposited deeper (usually the water
table). Copper deposits often show this at the top.
• Soils
• Groundwater
Igneous Processes
• 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.
• Magmatic Segregation
–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 oreforming elements and can form hydrothermal deposits (below).
Metamorphic
• Certain (geologic) minerals occur in metamorphic rocks, like corundum (rubies, sapphires, abrasive), garnet
(gem, abrasive), and graphite (used in pencil "leads") that are worth mining.
• Contact metamorphism, especially with limestones and dolomites, often results in precipitation of minerals,
which may be ore.
• Finally, metamorphism often produces hydrothermal fluids that can cause hydrothermal ore deposits.
Hydrothermal/Metasomatic
Fluids
• 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
small, pervasive joints). In veins (typical of gold), the minerals are deposited in thin, tabular fractures
(usually scattered joints with filling). 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.
Hydrothermal Diagrams
Plate Tectonics
• 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 have little if any relationship to
plate tectonics, and are instead controlled by weather (e.g. laterites) or currents (e.g. phosphorites) or …
• 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…).
• Divergent boundaries (Rifting, passive margin, mid-ocean ridge)
–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.
Plate Tectonics Map
Divergent Boundaries
Convergent Boundaries
Greenstone Belts
Mineral exploration
• 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 ).
Economic Minerals
• 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.