GEO-113: ENVIRONMENTAL GEOLOGY

MINERAL AND ROCK RESOURCES

LECTURE NOTES

I. Introduction

Eight elements (O, Si, Al, Fe, Mg, Ca, Na, and K) make up 98% of the earth's crust. Many important elements are found, on average, in the crust in very small amounts (ppm ≠ ppb) Finding these elements in high concentrations is the exception and requires unusual geologic processes.

Ore: Rock where the concentration of a needed element is sufficiently high enough to make the material economically worth recovering (resource is part of the economic reserve). Value of the contained element is greater than the cost of mining, processing, and marketing the material.

Typically, a high concentration factor (multiple of average crustal value needed to make it economic) of the element is required. Varies for different elements. The higher the average value of the element, the lower the concentration factor needed.

Fe, Al: 4-5 times the average crustal values (average is relatively high)

Cu: 100 times the average crustal values (average is relatively low)

Au: 2000 times the average crustal values (average value is extremely low, but material has great value - $300/ounce)

Hg: 25000 times the average crustal values (average value is extremely low and material is relatively low in value)

Economic considerations (market value versus production costs) often dictate whether a deposit is an ore or not. These change over the short term.

Mineral resources are not distributed evenly around the globe.

U.S.: 60% of the world's Mo, 40% of the world's Pb, 0% of the world's Al (use 40 %), Cr, Co, Mn, Ni, Sn and many others.

Zaire and Zambia: 50% of the world's Co

South Africa: 50% of the world's Au, 75% of the world's Cr, and 40% of the world's Pt (former Soviet Union has 50%).

This inequitable distribution has important geopolitical ramifications. Forced us to deal with South Africa for many years despite their social system of apartheid. We had no other place to get the resources we needed. The Soviet Union wasn't going to give them to us. As soon as the Soviet Union collapsed and was willing to sell us these resources, South Africa had to change its social and political systems or be shut out of our markets.

II. Types of Mineral Deposits

Ores form in a variety of ways, some igneous, some sedimentary, some metamorphic.

1) Igneous Processes: Produced by igneous activity. Often associated with volcanic activity.

Magmatic deposits

A) Incompatible elements: Some igneous rocks enriched in incompatible elements (Be, Li, Rare Earth Elements, U, Th, Rb, Au) ≠ elements that donπt want to go into the mineral structures that are crystallizing from the magma. As magma crystallizes, these elements remain in the magma and their concentration increases. Eventually this incompatible-rich magma crystallizes, forming incompatible-rich rocks. Often, these rocks may be very coarse-grained. Known as pegmatite. Large grain size (30 inches and longer) is due to high the water (an incompatible compound) content of the magma. Tourmaline, beryl, aquamarine, and emerald are examples of the unusual minerals often found in pegmatite.

B) Crystal settling: gravity causes early-crystallizing dense minerals to sink to bottom of magma chamber (may also float to top if they are less dense than magma). Process may produce monomineralic layers. Chromite, magnetite, and platinum-group mineral deposits often formed in this way.

C) Diamonds: Form at very high pressures, Usually requires 100-200 km depth. Brought rapidly to the surface by Kimberlite magmas. These are gas-rich magmas that explode their way to the surface. May erupt at very high velocities. Only need a few diamonds per ton of rock to make the rock an ore.

Hydrothermal deposits

Hot waters percolating through the rocks surrounding an intrusion leach elements from them. Later deposits these elements in minerals that crystallize when the fluids cool as they approach the surface. Often this process forms veins rich in sulfide minerals that contain metals such as Cu, Pb, Zn, Au, Ag, Pt, and U.

Some of the circulating fluids originate as water that percolates down from the surface. Other fluids may be expelled directly from the crystallizing magma. Intrusion acts as a heat engine that pump water through the surrounding rocks.

Process is occurring today beneath the mid ocean ridges. Produce "black smokers" where hot fluids are expelled into the cold waters of the ocean bottom. Minerals immediately precipitate turning the fluid a smoky gray or black. Sulfide minerals provide the energy for sulfur reducing bacteria. These then act as a food source for more complex organisms. A whole habitat is powered by chemosynthesis. No sunlight or photosynthesis is required. May be how life originated on earth and perhaps on other planetary bodies, such as Europa.

Most igneous processes are volcanogenic. That is associated with volcanic provinces at either convergent and divergent plate boundaries. Look at presently active ones or at old, extinct ones for new deposits of ores produced by igneous processes. For example, copper and molybdenum deposits in North and South America found at present or recent convergent boundaries. Look at other similar geologic environments to find new sources of copper and molybdenum.

2) Sedimentary Deposits: Processes associated with the formation of sedimentary rocks.

Chemical Deposits

A) Banded iron ores: Early in earthπs history atmosphere was reducing (oxygen poor). In reducing environments iron is soluble in water. However, world's oceans contained more oxygen due to the action of once-celled plants, such as algae. Therefore, the oceans were oxidizing. In oxidizing environments iron precipitates out of solution. So as iron-rich waters entered the oceans the iron precipitated out forming layers of iron-rich sediment and eventually rock.

B) Evaporites: As seawater is evaporated in hot, dry climates dissolved minerals, such as calcite, gypsum and halite, crystallize. Form sediment layers (and rocks) rich in these minerals. Occurs naturally and also in man-made evaporation ponds.

C) Weathering: Intense chemical weathering leaches out most elements. Material left behind is enriched in Al and Fe. Forms bauxite (the ore for aluminum) in lateritic soils.

Clastic deposits

Dense, heavy minerals fall out of suspension when water velocity decreases. Can concentrate a specific mineral in a confined area. Often occurs in streams or in coastal environments. Minerals form in one place by another process and then are transported, sorted, and concentrated by the water. Form a placer deposit. Many gold, tin, and diamond deposits form in this way.

3) Metamorphic deposits: Increasing pressure and temperature creates new minerals, sometimes in great abundance if the composition is right. Generally, metamorphism produces concentrations of useful minerals, rather than a concentration of a specific element. Examples include graphite, asbestos, and garnet (abrasive).

III. Uses of Mineral Resources

1) Metals

Fe: Fundamental to society. Many different iron products, the most important being steel (an alloy of iron and another metal(s). Most important sources are banded irons, laterites, and magmatic magnetite.

Al: Light, strong, and corrosion resistant. Has replaced steel in many applications. It is the third most common element in crust. However, very hard to extract from silicate minerals. Need bauxite ore. Still a very energy intensive process to refine. Need lots of electricity.

Sulfide ore deposits: Usually form from hydrothermal and magmatic processes. Source of Cu, Pb, Zn, Ni, Co, Ag, Au, and Pt. Many of these are used in making various kinds of steels. They also have military uses. Known as strategic minerals.

Cu: electrical applications, pipes.

Pb: batteries, solders, paints, ceramics.

Zn: alloys (bronze), tires, galvanizing, and tin cans

Au: jewelry and electronics

Ag: photographic uses, electronics, and jewelry

Pt: jewelry and catalysts. Used in auto emission controls and the petroleum and chemical industries.

Some of these may be the by-product of refining a more abundant element.

2) Nonmetals

S: Derived from volcanic deposits, sulfide minerals, petroleum, and evaporates. Used to make sulfuric acid.

Na and Cl: Derived from halite in evaporite deposits.

Gypsum: Used in construction materials (cement, plaster, and dry wall)

Other evaporite minerals provide phosphorus and potassium needed for fertilizers.

Clay minerals: Very diverse group derived from sedimentary deposits. Used in ceramics, construction materials, and drilling muds.

3) Rock resources

In the U.S., 900 million tons/year of sand and gravel are used in making cement and concrete. 1.21 billion tons/year of crushed rock is used for road fill and other construction needs. 50 million tons/year of clean sand is used for glass making and abrasives.

IV. Mineral Resources Supply and Demand

U.S. uses much more than it produces. We are very reliant on foreign sources, particularly for some important strategic minerals, such as Cr, Mn, U, Ti, and Pt.

Demand has been increasing exponentially worldwide 2-10%/year except during the late 70s and early 80s when a worldwide recession due to an OPEC oil embargo occurred.

Worldwide, we have >20 to hundreds of years of reserves based on present demand. U.S. generally has 30 years or less for many materials. However, as present reserves become scarcer price will go up, making presently non-economic deposits worth mining. Reserves will not be depleted as quickly as rate of consumption would suggest. On the other hand, new resource discoveries will not keep up. Makes estimates or time to depletion minimum values. Future availability may depend on technological developments and disregard of greater environmental impact as we mine lower grade (more dispersed) ore.

V. Sources of New Mineral Resources

More resources are needed to raise standard of living in rapidly growing underdeveloped countries. Where will they come from?

1) New exploration techniques: Utilize new technologies, such as gravity, magnetics, and conductivity, to discover previously unknown reserves. Gets less likely with time.

2) Geochemical prospecting: Surface water chemistry controlled by chemistry of underlying rocks. Can cover a large area rapidly and relatively cheaply.

3) Satellite photography and remote Sensing: Can map and analyze rocks over hundreds of square miles in a single photograph. May indicate particular areas of interest that can then be investigated in detail on the ground.

4) Improved geological understanding: Leads to looking for new resources in previously overlooked areas. Molybdenum deposits now known to be associated with subduction zones. Now look around the world at old subduction zones for new Mo deposits.

5) Marine resources: Now flooded placer deposits (sand, gravel, diamonds, and gold) formed on continental shelf when sea level was lower 20,000 years ago. Hydrothermal deposits of sulfide-rich minerals Cu, Pb, Zn, Mn, and Ag found at vents along the mid-ocean ridges. Problem of exploiting them and destruction of unique habitats. Manganese nodules rich in Fe, Mn, Zn, and Cu litter thousands of square miles of the world's oceans. Who owns them? Who controls their mining and enforces environmental regulations?

6) Conservation of existing resources: Can be done in two major ways:

1) Substitution: Substitute something we have a lot of for something we don't. For example, plastics for metals and copper for silver.

2) Recycling: Reduces the amount of new resources needed. Commonly done in the U.S. Works well for some metals, such as Cu, Al, Pt, and Pb, but not for others (Ti). May also save energy of refining new raw ore. Particularly the case for Al.

VI. Mining Impacts

Underground mining usually only disturbs a relatively small area. Abandoned mines may collapse causing subsidence problems if near the surface. This type of mining can be very expensive and less is being done.

Surface mining usually done as an open pit (quarry) or by strip mining. Open pits often leave a very large hole in the ground. May be used as a lake. Strip mining can destroy large areas of vegetation. Both types lead to increased erosion and chemical weathering. May produce acid mine drainage and pollution of local water systems. Land reclamation now required. This may make ore not economic to mine. Marginal ores may not be mined, decreasing available resource.

Mine tailings are the unused portion of the ore. It's whatπs left after the resource has been extracted. May contain toxic elements. Ore refining process may release pollution and cause acid rain. In Sudbury, Ontario, hundreds of square miles denuded of vegetation.