Economic Geology
EAS-100-51: Introduction to Earth Science

The term "earth resources" may be defined as: "any supplies needed to sustain society, derived from the earth, including air, water, energy, and minerals." Food, water, and some forms of energy (solar, geothermal) are renewable, that is, can be replenished in a relatively short time by natural processes. Mineral resources , however, cannot be renewed quickly. Our need for mineral resources has grown faster than we can find new reserves. Knowing something about the distribution of the earth's elements, and the geological processes which can produce useful concentrations of minerals, geologists can focus their search for these valuable substances. To give the layman an idea of how important minerals are, consider that 35 chemical elements are required to manufacture your car, 66 elements are needed to make a telephone, and about 40 elements to make a average house. Mineral resources may be divided into three groups:
1. abundant metals  1. industrial chemicals  1. coal 
2. less abundant metals  2. construction materials  2. petroleum 
3. scarce metals  3. fertilizers  3. natural gas 
4. water (fresh water)  4. nuclear 
5. ceramic & abrasive materials (uranium, thorium) 


Most metals in nature are not found as single elements, but are more commonly found as compounds combined with oxygen (oxides) or sulfur (sulfides). When these metallic minerals are found in concentrations large enough to be mined economically, they are called ores . The waste rock (usually made up of less valuable and more common minerals such as quartz) is collectively called gangue .

ABUNDANT METALS include: iron (Fe), aluminum (Al), titanium (Ti), magnesium (Mg), and chromium (Cr).

LESS ABUNDANT METALS include: copper (Cu), lead (Pb), zinc (Zn), tin (Sn), tungsten (W), molybdenum (Mo), and silver (Ag).

SCARCE METALS include: gold (Au), mercury (Hg), platinum (Pt), nickel (Ni), and uranium (U).

This list above is by no means complete, but includes the metals more commonly used in industry to make alloys, which are metals mixed together while in the molten state. Alloys allow man to "custom-design" metals for particular uses. Whereas pure copper is too soft to use as metal sculpting material, bronze (copper-tin alloy) is stronger; brass is a copper-zinc alloy. Steel is an iron alloy containing carbon (common steel), nickel (stainless steel), vanadium (machine steel), or tungsten (high-speed steel). Aluminum alloys are used for aircraft because they combine strength and lightness. Tin-lead alloy, known as solder, is used almost universally in the electronics industry to make strong and reliable electrical connections between components which are often very tiny. 24-karat (pure) gold is extremely soft and is not very suitable for jewelry; it is often alloyed with silver or platinum to give it more structural strength.


INDUSTRIAL CHEMICALS include: soil deposits, phosphates (compounds of phosphorus and oxygen, used to make laundry detergents), nitrates (compounds of nitrogen and oxygen, used to make gunpowder and explosives), sulfur (used in making synthetic fibers, papers, and pigments), and graphite (used as a dry lubricant).

CONSTRUCTION MATERIALS include: limestone and clay (used to make cement), sand, gravel, gypsum (used to make plaster), asbestos (for fireproof and heat-resistant materials).

CERAMIC & ABRASIVE MATERIALS include: kaolin (clay mineral used to make porcelain), quartz sand (for making glass), diamonds (below gem quality), corundum (aluminum oxide).

FERTILIZER CHEMICALS include: nitrates (also used as explosives), potash (potassium carbonate).


Fuels are chemical substances which provide heat energy through the process of combustion (the rapid combining of fuel with oxygen) or radioactive decay. COAL, PETROLEUM (OIL), and NATURAL GAS (METHANE) are known as fossil fuels , and are organic materials which are derived from the remains of animal and plant life which lived millions of years ago.

Most of our coal was formed during the Pennsylvanian period (about 310 to 280 million years ago), when huge swamps were common; the remains of giant trees and ferns eventually transformed into a dark rock composed of mostly carbon and hydrogen (hydrocarbon).

Petroleum is made up of the remains of sea creatures whose bodies sank to the ocean bottom when they died. The accumulated dead organic matter later underwent a chemical transformation into a liquid hydrocarbon which we call petroleum. Petroleum seeped into porous rock formations such as sandstone, shale, or fractured rock such as limestone, dolomite, or granite; it may reach the surface through natural oil seeps, or by drilling.

Natural gas is also a hydrocarbon, composed mostly of methane. Methane is also produced by the accumulation and chemical alteration of plant and animal remains (it is also known as swamp gas). Natural gas reservoirs are formed when methane is trapped in cavernous or porous rock formations by an overlying rock which is relatively impermeable. Natural gas often occurs along with petroleum (oil companies often burn off the gas in order to get the petroleum out more quickly or more safely).

Nuclear fuels are ores of radioactive metals such as uranium (U) and thorium (Th). These metals are usually found as oxides. Their radioactive energy is harnessed as heat - nuclear reactors use fuel rods made of uranium or some other radioactive metal to heat a steam turbine, whose blades drive a generator, which produces electricity. While nuclear fuels are extremely efficient (producing far more energy for the same weight as fossil fuels), the waste material left is toxic to all life forms, and their long half-life periods make their safe disposal a major technological problem.


The core and the mantle make up 99.6% of the earth's weight. The crust, the outer layer of the earth which averages only about 20 miles in thickness, makes up only 0.375% of the earth's weight, while the hydrosphere (including the oceans and groundwater) and the atmosphere together make up the remaining 0.025%. The abundant minerals which occur in the earth as a whole include: iron (Fe), oxygen (O), silicon (Si), magnesium (Mg), aluminum (Al), calcium (Ca), nickel (Ni), sodium (Na), titanium (Ti), and sulfur (S). However, because of technological limitations, the whole earth is not accessible to mining, so we must concentrate upon extracting the elements within the earth's crust.

O, Si, Al, Fe, Ca, Na, K, Mg are the 8 most abundant elements in the crust. Among the metallic elements in this list, aluminum (Al) is the most abundant (8% of the crust), followed by iron (Fe, 5% of the crust). The percentages of the non-metallic elements are much higher, especially oxygen and silicon, because they make up most of the rocks in the crust (as silicates).

To mine for minerals, it makes sense to look for concentrations of valuable minerals. Economics plays an important role in defining the market value of minerals, which depend upon: scarcity, the cost of mining, and the cost of refining. If high-grade, concentrated deposits become exhausted, it may be economically feasible to mine lower-grade deposits in the future, because the value of the mineral may offset the higher cost of mining.

Very briefly, some of the geological processes which can concentrate minerals into deposits are:

1. WATER: water from below the ground moves mineral constituents around. Hydrothermal water dissolves minerals in hot, watery solutions and later deposits them.

2. MAGMA: chemical alteration of rock by the heat of the magma.

3. SEDIMENTATION: chemical precipitation (forming solid from dissolved state).

4. MECHANICAL ACCUMULATIONS: harder, more resistant minerals are left as deposits, known as placers. Examples: magnetite (black mineral found in beach sand), diamond (found in beach sand in South Africa), and gold (found in river sediment which prospectors panned for).

5. BIOLOGICAL: certain species of bacteria are able to extract sulfur from metallic sulfides, or extract and fix metals into place. On the ocean floor, manganese nodules are very abundant, and are believed to be formed by microbes which concentrate and precipitate metallic minerals from seawater.

Copyright © 1988 by William K. Tong