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Metallic Mineral Deposits (page 587) Mineral resources • Ore – refers to useful metallic minerals that can be mined at a profit and, in common usage, to some nonmetallic minerals such as fluorite and sulphur • To be considered of value, an element must be concentrated above the level of its average crustal abundance • Most nonmetallic minerals are generally not called ores, but rather they are called industrial minerals Magmatic Deposits • Some of the most important accumulations of metals are associated with magma that forms igneous rocks • Certain metals are enriched in certain magmas and further concentrated during cooling of the magma Magmatic Deposits: Gravitational Settling heavy minerals that crystallize early, settle and concentrate on the bottom of the magma chamber – Separation and non-mixing of liquid phases of a magma (e.g., Sudbury, Ontario and Voisey’s Bay, Newfoundland and Labrador) – Nickel deposits of Sudbury and Voisey’s Bay have similar origins, but different triggering mechanisms Hydrothermal Deposits: one of the most important ore deposits. These deposits are generated from hydrothermal (hot water) solutions. These solutions of hot water and salts (brines) dissolve small amounts of metallic elements. These brines are found around the magmas and they dissolve certain minerals (gold, silver, copper, lead, zinc and mercury) and form deposits in two ways. Hydrothermal Deposits • Hydrothermal Deposits associated with Igneous Activity – Among the best known and important ore deposits, generated from hot-water solutions – Majority originate from hot, metal-rich fluids that are remnants of late-stage magmatic processes – Move along fractures, cool, and precipitate the metallic ions to produce vein deposits Vein Deposits: hydrothermal fluid generated by cooling magmas rise to the surface through fractures, faults and other rock features. As these fluids pass through there fractures, they cool and solidify. These fluids are often silica (quartz) and calcite rich. When they solidify, they form veins rich in Au, Ag, Cu, Zn, Pb and other metallic elements. These deposits are referred to as vein or load deposits. See diagram on page 589. Hydrothermal Deposits • Hydrothermal Deposits associated with Igneous Activity – Can occur as disseminated deposits, which are distributed throughout the rock body, rather than concentrated in veins; called porphyry deposits = (low grade; large volume) Disseminated Deposits: these deposits are also formed through magmasourced hydrothermal fluids, however rather than having concentration of metals in veins, the metals are disseminated (spread out) through the rock. Most of world`s copper and molybdenum and lesser amounts of gold and silver are mined from disseminated deposits. These deposits are often referred to as a porphyry deposits because they are associated with porphyritic rocks. See page 589. Placer Deposits: minerals that deposited in streams, primarily gold. Think of Dawson City Yukon, gold panning. Sedimentary Deposits • Placer deposits – formed when heavy mechanically concentrated by currents • Examples include – Gold, Platinum, Diamonds metals Mineral Chalcopyrite Sphalerite Phyrite Galena Pentlandite Gypsum Halite Bauxite Borite Copper ore Zinc ore Fool`s gold Lead ore Nickel ore Evaporite Evaporite Aluminum ore Drilling mud CuFeS ZnS FeS PbS NiS Gyproc Rock salt Natural Mineral: gold, silver, copper occur naturally in nature. How do we find and then extract these mineral deposits? Some of the exploration techniques involved are: Seismic records, remote sensing, prospecting, observing drill core, cross sections, geologic mapping, magnetic surveys, gravity surveys, geochemistry. Once a deposit has been identified, there are two methods of extracting the minerals from the ground. Open Pit mining: Mineral extraction from the surface. The best example in Newfoundland is the Iron ore mines of Lab City. are Underground Mining: Ore is extracted by a series of tunnels in the ground accessing a particular deposit. A very local example would be the former Bell Island Iron ore mines. Once these deposits have been accessed, the minerals must be separated from the host rock. Some methods used are: Flotation, Gravity separation, Heap leaching, Pyromet, and Hydromet processes. Floatation involves mixing grounded ore with water, oil, and chemicals. The grounded ore becomes suspended in the water and is subjected to bubbles of air due to the presence of chemicals. This causes the minerals and/or elements of value to fl oat to the top where they can be scraped off. Gravity separation involves feeding grounded ore into a pulsating body of water, which serves to settle out the heavy material while floating away the light material. If the heavy material is what one wishes to retain, then it is taken from the bottom. If the light material is what one wishes to retain, then it is taken from the top. Heap leaching commences by placing the grounded ore as a layer onto impermeable material. The “heap” of material is irrigated with a liquid, which percolates down through dissolving away the valuable minerals and/or elements. Pyromet involves using heat to separate the minerals and/or elements of value from the ore. Heating separates the materials by density and the desired minerals and/or elements can be removed. Hydromet is a chemical method that involves oxidation and acid leaching to separate out the desired minerals and/or elements. Petroleum: A group of hydrocarbons that includes oil and gas. Where does it come from? It comes from organic matter contained in the tissue of living things. The vast majority of petroleum comes from the remains of simple, aquatic organisms (plankton algae ),which includes most of the plankton in the seas and large bodies of water. Most of this algae is eaten by scavenging organisms and aerobic (oxygen rich) organisms. This organic matter is said to have a low preservation potential in this type of environment. However in quiet water environments in lakes and seas where conditions are low in oxygen, large accumulations of plankton are “built up”. Because it is a still, quiet geological environment, the rock type formed is organic rich, fine grained sediments, such as shale and or fine grained carbonate rocks. These rocks are the principal sources of petroleum and are thus called Source Rocks. Different processes and temperature changes result in this organic matter forming gases and or liquid petroleum. As the hydrocarbon, rich, organic matter of source rock is subjected to higher temperatures ( through burial ) , the matter is transformed to a waxy substance called Kerogen, (kerogen is a mixture of organic matter in sediments from which petroleum is released). Then through a process called Cracking, the carbon-carbon bonds within the kerogen break forming oil and gas, (this usually occurs at temperatures above 50 degrees C. The three phases in the evolution of organic matter to petroleum. Include: (i) diagenesis (ii) catagenesis (iii) metagenesis Diagenesis: During diagenesis there is shallow burial of organic matter at near normal temperature and pressure as well as some decay. Methane, carbon dioxide, and water are released leaving behind the complex hydrocarbon called kerogen. Catagenesis: Deeper burial results in increased temperature and pressure. Petroleum is released from the kerogen – first oil is released and second gas is released. Metagenesis: The metagenesis phase involves even higher temperature and pressure verging on metamorphism. The only hydrocarbon that is released during this phase is methane. At this point the petroleum has matured enough to migrate to traps. So to form a petroleum deposit we need a petroleum trap. A Petroleum Trapis an environment that enables economically significant amounts of oil and gas to accumulate underground. To have a petroleum trap there must be a reservoir rock and a cap rock. A Reservoir Rock is a porous, permeable, rock that contains oil and or gas, such as sandstone. A reservoir rock requires high porosity and high permeability since it is the rock which petroleum moves through and is stored in. Note that porosity is the volume of pore spaces or holes between sediment grains and that permeability is the interconnectiveness of the pores, thereby allowing the movement of the petroleum. The porosity of a material is influenced by particle shape and size and the degree of sediment sorting. Large, rounded, well-sorted particles offer higher porosity, particularly if the amount of cement between them is limited. Usually, the higher the porosity and the larger the pore spaces, the higher the permeability. Examples of reservoir rocks are sandstone, dolomite, and conglomerate since they have both high porosity and permeability. A Cap Rock is a rock layer overlying the reservoir rock to halt the movement of oil and gas to the surface, such as shale. A cap rock is an impermeable rock that serves to trap petroleum from either escaping to the surface or spreading throughout the rock as opposed to being confi ned. Students should realize that petroleum exists within reservoir rock between sediment as opposed to being confi ned as a whole volume of liquid petroleum. “There are no ponds of petroleum in the ground”. There are two main types of petroleum traps. 1) Structural Traps: Petroleum traps that are due to structural deformation of the rock layer that contains the hydrocarbons. There are three types: a) anticline b) fault trap c) salt dome trap 2) Stratigraphic Traps: petroleum traps that are due to lateral or vertical changes in type that reflect patterns of sediment deposition. There are three types: a) pinch-out trap b) An unconformity trap c) Reef trap Common oil traps – anticline, fault, salt dome, and stratigraphic the rock The physical property density is used for the purpose of distributing petroleum in a reservoir. Water is denser than oil and oil is denser than gas. Therefore, when drilling into a petroleum trap, gas is encountered first followed by oil and then water. Note that some reservoirs may have all three components, whereas some reservoirs may only have two components or one component. There are two main ways oil is obtained from Earth. 1) Drilling eg.Hibernia 2) Surface extraction eg.open pit mining. The tar sands of Alberta. The Alberta oil sands, also called tar sands, are an example of petroleum being extracted directly from the surface (open-pit mining). Because the petroleum migrated towards the surface and volatiles (e.g. water) were lost to the atmosphere, the petroleum experienced increasing viscosity (i.e. thickness), thereby preventing it from spreading out and/or dissipating. The reservoir consisted of loose (unconsolidated) sediment that therefore, exhibited high porosity and permeability. In some instances, steam is injected directly into the tar sands to mobilize the hydrocarbons, which are then recovered from pumps much like conventional crude oil. The refining process involves three methods of refining petroleum. Include: (i) distillation (ii) cracking (iii) reforming Once petroleum is extracted from the sub-surface and is de-salted and de-watered, it enters a distillation column. Heat is added to the column to separate the petroleum into fractions based on boiling points (cracking). The bottom fraction is removed to another column for further separation. An image of a distillation column is shown below. Note some “fractions” in the image, which could include: asphalt; paraffin wax; lubricants; jet fuel; diesel; kerosene, and gasoline from the graphic. Source: http://upload.wikimedia.org/wikipedia/commons/thumb/3/3c/Crude_Oil_Distillation. png/225px-Crude_Oil_Distillation.png Reforming involves heat, pressure, and the use of catalysts (speed up reaction rates) to reform different hydrocarbon compounds. What are the environmental concerns of fossil fuels? How can we satisfy the human appetite for fossil fuels and its ever increasing effects on the environment?