CHAPTER 2 SOME BASIC GEOLOGIC CONCEPTS AND THOUGHTS ON THE EVOLUTION OF MINERAL DEPOSITS THROUGH TIME
| Jurisdiction | United States |
(Apr 1980)
SOME BASIC GEOLOGIC CONCEPTS AND THOUGHTS ON THE EVOLUTION OF MINERAL DEPOSITS THROUGH TIME
Department of Geology, University of Wyoming
Laramie WY 82071
INTRODUCTION
I hope to do three things in this discussion:
(1) Review some basic geologic concepts that I'm certain many of you have been exposed to before.
(2) Introduce the concept of geologic time.
(3) Take you on a tour through geologic time wherein we will discuss geologic processes that lead to the formation of some of our major mineral deposits.
I will assume you are familiar with some of the more common geologic terms such as basalt and granite, but they are defined in the glossary — just in case. The symbol m.y. means million years and the symbol b.y. means billion years.
PLANET EARTH MODEL
As shown in Figure I the earth is thought to have a partly liquid and partly solid core composed of metallic iron, metallic nickel, and some sulphur; a mantle containing a variety of silicate1 minerals but dominated by iron and magnesium silicates, and a thin outer crust. The crust is of great interest to us because it contains all of our available mineral deposits and is the surface that supports life, but it is really a thin surface skin. The crust comprises less than one percent by volume of
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the earth and approximately 4/10 of one percent by weight.
A simple model of the crust and outer mantle appears in Figure 2. The oceans are underlain by basalt and covered by layers of deep sea sediments. The continents are complex masses composed of three main rock types: igneous, sedimentary, and metamorphic rocks.
MAJOR ROCK TYPES
Igneous rocks are generated by melting of part of the mantle or by melting of pre-existing igneous, metamorphic, and/or sedimentary rocks. There are two main types of igneous rocks: basalt which is a rock rich in iron, magnesium, and calcium and granite which is a rock rich in potassium, silica, and sodium (Figure 3).
Basaltic magma is usually generated in the mantle and may rise to the surface to form basaltic lava flows (Figure 4) or may crystallize beneath the surface to form coarse-grained rocks.
Granitic magma may be derived by partial melting of the mantle, or by partial melting of rocks in the crust. Granitic magma also rises to the surface and forms lava flows but granitic magma tends to erupt or explode when it rises near the surface and forms widespread deposits composed of fragments of glass, rock, or minerals called tuffs. When granitic magma crystallizes at depth it forms the coarse-grained igneous rock granite.
Sedimentary rocks are of three basic types: clastic sedimentary rocks, chemically precipitated sedimentary rocks, and sedimentary rocks formed by organisms, plants or animals.
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Clastic sedimentary rocks are composed of rock and mineral fragments derived from weathered continental areas. These clasts are transported by streams and rivers and are deposited in rivers, lakes, and the sea. Figure 5 is an illustration showing how this works.
Chemically precipitated sedimentary rocks form by partial or complete evaporation of bodies of water — typical sedimentary rocks formed this way are salt, gypsum, and limestone. Figure 6 shows beds of the evaporite mineral trona (hydrated Na, sodium bicarbonate) which formed when large lakes dried up in Wyoming about 40 million years ago.
Sedimentary rocks formed by organisms are developed when the animal or plant causes the precipitation of protective coatings, for example, corals precipitate calcium carbonate to develop reefs of limestone.
Metamorphic rocks are formed when any of the above rocks are deformed and recrystallized during some geologic event. For example, the rocks may be compressed or squeezed together. These rocks are brought to deeper levels of the crust where they are subjected to high temperature and pressure. Examples are slate, schist, and gneisses (Figure 7).
MINERALS
Rocks are made up of minerals which are solid chemical compounds with a characteristic ordered internal structure. Well over two thousand minerals have been identified and more are being found each year, but the remarkable and happy fact is that very few of these minerals are common on the earth's crust. The geologist can identify most rocks if he knows about 20 minerals well. On the other hand, the economic or mining geologist is constantly
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searching for the less common minerals and he will be in trouble if he cannot identify hundreds of mineral species (Figure 8).
SETTING UP THE GEOLOGIC TIME SCALE
Almost all geologic work requires an understanding of geologic time and stratigraphy, which is the character and distribution of layers of sedimentary rocks with respect to geologic time. Geologists are basically historians concerned with the history of the earth. Just as the historian must know when a specific event in human affairs took place to put it in its proper perspective, a geologist must know the age of the rocks he studies. We cannot understand the history of the earth if we simply know about the evolution of rocks in one area. We must understand their evolution through time everywhere on the planet and this requires a time scale. A time scale allows us to compare rock A in New York City with rock B in China.
The geologic time scale was developed by studying stacks of sedimentary rocks like those of the Grand Canyon (Figure 9) and by trying to determine how long it took these sedimentary rocks to form. The study of sequences of sedimentary rocks is called stratigraphy. A simple way to look at stratigraphy is to consider it the manner in which sedimentary rocks are stacked one upon the other. We generally believe that the oldest rocks are at the bottom and the youngest rocks are at the top (Figure 5). This photograph of the Grand Canyon (Fig. 9) illustrates this principle very well. The great succession of sedimentary rocks (over 17,500 feet thick) is not severely deformed and the older rocks are at the base and the younger ones are at the top.
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If we study the rocks in the Grand Canyon and guess how long it took to deposit them, how can we compare these rocks with a similar stack of sedimentary rocks in Africa? Initially this was done by using fossils. Paleontologists, who are students of fossil life forms, determined that the fossils in the bottom of a stack of sedimentary rocks were different than those at the top of the stack. In other words, life forms on the planet could be shown to evolve or change with time. Even more helpful, the paleontologists discovered that certain fossils lived a relatively short time and thus if these fossils were found in rocks in Arizona and in rocks in Antarctica we knew the rocks were the same age. Ultimately the paleontologist and stratigraphers developed a time scale that showed how life forms developed through time (Figure 10). The trouble with this system was that it accurately demonstrated relative timing, but concrete dates were unknown. We had to guess how long it took sedimentary rocks to form and the guesses were not too good. It was like trying to tell time by the sun on a cloudy day — we needed a clock and we discovered one!
A clock was developed by scientists we know call...
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