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| Syllabus | |
| 1 History of Mining | |
| 2 Using Maps | |
| 3 Geological Exploration | |
| 4 Geophysical Exploration | |
| 5 Geochemical Exploration | |
| 6 Secondary Exploration | |
| 7 Open Pit Mining | |
| 8 *****Mid Term***** | |
| 9 Strip Mining & Placer Mining | |
| 10 Underground Mining | |
| 11 Permits and Laws | |
| 12 Environment Impact | |
| 13 Jobs and Tools | |
| 14 Mineral Economics | |
| 15 Reclamation | |
| 16**** Final Exam***** | |
| Resources | |
| Staff E-mail | |
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Lesson 3 Objectives To find a particular ore, one must begin by looking in regions where the ore is formed and/or concentrated. Therefore before exploration one must have an understanding of the geologic forces that form rocks and ore deposits. A rock is a mineral, or aggregate of minerals, that forms an essential part of the earth's crust. In other words, enough of a particular mixture of minerals exists, so that the rock can be named and recognized in many localities throughout the world. Rocks differ from minerals in that rocks are merely physical mixtures of minerals, while the minerals themselves are chemical compounds of fairly uniform composition. For most prospectors, the study of rocks is more complicated than the study of minerals. The study of rocks should not be overlooked because valuable minerals often are found associated with specific types of rocks. Rock structures can be indicative of ore deposits as well as the potential size of an ore deposit. Therefore, solid knowledge of ore deposits and structural geology is an important tool for seeking large ore deposits. Any rock can be classified as one of three types: igneous, sedimentary, or metamorphic. This method of classification is based on the mode of formation of the rock. Igneous rock is formed from a molten state. The sedimentary rocks are formed from sediments or erosion fragments deposited in lake and ocean beds. Metamorphic rocks form when great heat and pressure, caused by deep burial, alter the physical condition of sedimentary, igneous, or another metamorphic rock.
Igneous (fire formed) rocks are formed in deep-seated areas of the earth's crust. They may be fine grained, large grained or a combination of large and small grains. The grain size indicates the cooling rate of the rock. Fine grained (dense) igneous rocks form when rapid cooling occurs. Conversely, coarse-grained rocks cooled slowly and crystals grew large. Fundamentally, igneous rocks are classified as either intrusive or extrusive. Intrusive rocks originate from magmas (molten rock materials combined with gases) at depth in the earth. Intrusive rocks occur as massive structure or as in "injection" structure. This latter structure forms when the hot, liquid or plastic rock is injected into fractures in the surrounding solid rock. Intrusive action leads to the formation of batholiths, laccoliths, stocks, dikes, and sills. Weathering and erosion later expose these structures on the earth's surface. Extrusive rocks are formed by volcanic activity at the surface of the earth. These rocks cool rapidly. Examples of some common igneous rocks are: rhyolite, andesite, basalt, granite, diorite, gabbro. The formation of sedimentary rocks begins with the breaking down of other rocks into fragments. The forces of weathering and erosion, such as running water or freezing and thawing, accomplish this mechanical and chemical breakdown. Fragments are transported to and deposited in lake and ocean bottoms. Later, spaces between the fragments are filled with a cementing material or are eliminated by pressure. After some time passes, a massive rock layer results. Sedimentary rocks are classified based on the size of the particle of the sediment, or fragment. Shale (dense, fine particles), sandstone (particles distinguishable to the naked eye) and conglomerate (pebbles and gravel cemented together) are examples of sedimentary rocks. Metamorphic rocks are formed from previously existing igneous, sedimentary or possibly other metamorphic rocks. Great heat and pressure, yet not enough to completely melt the rock, alter the rocks original physical composition. Sometimes the process of metamorphism aligns the grains in parallel layers or bands. This layering is called foliation. When broken, a metamorphic rock usually breaks along the plane of foliation. Metamorphic rocks are classified based on their grain size and degree of foliation. Some examples of metamorphic rocks are: slate, schist, gneiss.
The ultimate source of ore deposits is deep in the igneous rocks in the earth. The original concentration takes place within bodies of magma through a process called differentiation. The concentration of some simple high temperature oxide minerals occurs within the liquid magma itself. The vast majority of minerals, however, are deposited in the surrounding rocks from the cooling liquid by replacement of wall rock and by the filling of the cavities. In addition, important deposits may be formed by the reworking of older deposits of magma origin, by weathering and erosion acting at or near the earth's surface. The cause of molten areas, or magma, is not fully known. It is probable tat several factors work together to form them. Radioactivity is an important contributor to the heat since it has been shown that igneous rocks in general are higher in content of radioactive minerals than are other rocks. Pressures and friction in zones of mountain building and release of pressure as a result of failure of the rock also are probable factors in the melting of rocks. Whatever the source of energies involved in its formation, the magma is very important in the filed of ore deposits since a great bulk of these deposits is found as a direct result from magma activity. The nature and location of deposition is influenced by factors outside the magma, such as structure, nature of rocks invaded, depth below surface, and abundance of ground water. The main source of mineral, however, is the magma itself and therefore, it must be of considerable size in order to have sufficient mineral present. The term "direct magma deposits" refers to deposits of mineral, which have formed within the body proper of magma, mass. As magma ceases to rise and encroach upon the rocks in which has been invading, it begins to cool. The rate depends upon such factors as total amount of heat present and the amount of overlying, insulating rocks present. Depending on their temperatures of formation in the magma solution, minerals begin to crystallize out and rise or sink depending on their specific gravities. By this method of differential crystallization, high temperature minerals, such as magnetite and chromite are formed. They separate into localized concentrations and upon solidification of the magma, become ore deposits. As cooling continues minerals continue to crystallize out of the magma solution, the remaining liquid becomes increasingly rich in low temperature minerals, such as quartz, and volatile minerals, such as water. The great bulk of metallic ore minerals are concentrated in this late-stage residual solution. Pegmatites are vein-like or dike-like formations that often contain very large crystals, some of which are valuable minerals. Usually pegmatites are closely associated with an igneous mass. They likely are the end stage of differentiation of cooling magma. The hydrothermal or water deposits, include high temperature gas and liquid magma solutions that react with the invaded rock. The most favorable host rock for hydrothermal deposits is limestone since it is readily replaced by the mineralizing solutions. The factors involved in dropping the metals from the solution to form ore are varied and complex. They include temperature, pressure, nature of the host rock, reaction with other solutions, concentration, rapidity of movement of solutions, and perhaps many others, all acting separately and/or in complex relationships with one another. In general, one can expect a certain order of mineralization ranging outward from high temperatures and pressures near the magma to low temperatures and pressures away from it. A large group of important mineral deposits is formed by action of surface agents upon earlier magmatic or hydrothermal deposits. Reworking of older material and reposition or reconcentration by mechanical, chemical or organic means form them. Sedimentary ore deposits may be divided into the following:
Metamorphic deposits result from rearrangement of minerals already existing in a rock, from heat and pressure. No outside minerals are added. Graphite, asbestos, talc, garnet, jade are examples of minerals which form economic deposits of this type. The formation of valuable mineral deposits result from a combination of factors, conditions, and events which go hand in hand to determine the areas in which metals, otherwise scattered through masses of rock somehow are gathered into concentrations greatly exceeding the average for rocks in general. Differentiation of large masses of rock material in a molten state is a major means of metal accumulation. Highly mobile and volatile solutions contain high concentrations of metal and carry them to some point where chemical and physical conditions are favorable for deposition. Hydrothermal and secondary deposits are found in areas where rocks have been "prepared" in advance by some kind of structural deformation. Areas of mountain building activity combine structural deformation and igneous activity. They are therefore, favorable places for ore deposits. There are two divisions of rock structures, which control ore formation, primary structures, and secondary structures. Primary structures are features such as bedding in sediments, igneous contacts, pillows in lavas, and other minor features that developed during the formation of the rock mass. Such structures might have had important local influence on the size, shape, or grade of a deposit. Bedding surfaces, igneous contact, or intergranular spaces might act as zones along which a solutions move to points of deposition. High permeability can be important for providing channels for solutions. Secondary structures develop after the formation of the rock. They are such features as faults, joints, and folds, and are of greater importance in control of ore deposition than are the primary structures. Faults - Faults are fractures in rock masses with major slippage. Tensional stresses, or pulling apart cause normal faults. Compression yields reverse or thrust faults. Strike-slip faults displace blocks horizontally, with little up or down movement. Joints - Different from faults, joints are cracks in which there has been no movement of the rock on side of the opening relative to the other side. Junctions of joints with faults or other joints can be areas of marked increase of width and grade of ore. Folds - The influence of folds in localization of ore is more or less indirect, their primary functions being that of confining and directing solutions to restricted channels or localizing fractures during fault movement. The common relationship between folds and ore is that a fold might cause deposition in conjunction with faults and other fractures. **The above information was adapted from Leo Mark Anthony and Michael Mark Anthony, Introductory Prospecting and Mining, pp. 69-93. Mining and Petroleum Training Service, University of Alaska, Soldotna, AK, 1997. Glossary: Illustrated Glossary of Geologic Terms A very complete geographic dictionary , many linked with pictures. http://www.geology.iastate.edu/new_100/glossary.v2.html Batholiths-A large, discordant, intrusive body of igneous rock. Differentiation -state of difference Dikes-A tabular igneous intrusion that cuts across the surrounding rock. Direct magma deposits- deposits of mineral, which have formed within the body proper of magma, mass Hydrothermal-water deposits that include high temperature gas and liquid magma solutions that react with the invaded rock Igneous-rock that is formed from a molten state Laccoliths-A concordant igneous intrusion with a flat floor and a convex upper surface, usually less than 8 km across and from a few meters to a few hundred meters thick at its thickest point. Magma-molten rock materials combined with gases)Metamorphic-A rock changed from its original form and/or composition by heat, pressure, or chemically active fluids, or some combination of them. Pegmatites- vein-like or dike-like formations that often contain very large crystals, some of which are valuable minerals. Sedimentary Rock-Rock formed from the accumulation of sediment, which may consist of fragments and mineral grains of varying sizes from pre-existing rocks, remains or products of animals and plants, the products of chemical action, or mixtures of these. Sills-A tabular igneous intrusion that parallels the planar structure of the surrounding rock. Stocks-A small batholith Resources: For further investigations This site includes short
description of panning for gold (in Co). Text description of post
hole prospecting This site has good interactive
units on minerals, minerals in everyday products, prospecting,
mining, reclamation. Focuses on Canadian minerals/mines.
Fun site. |
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