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A deposit is an accumulation of Earth materials, usually loose sediment or minerals, that is laid down by a natural agent. Deposits are all around you—the sand on the beach, the soil in your backyard, the rocks in a mountain stream. All of these consist of earth materials transported and laid down (that is, deposited) by a natural agent. These natural agents may include flowing water, ice, or gusts of wind (all operating under the influence of gravity), as well as gravity acting alone. For example, gravity alone can cause a rock fall along a highway, and the rock fall will form a deposit at the base of the slope. The agents of transport and deposition mentioned above are mechanical in nature and all operate in the same way. Initially, some force causes a particle to begin to move. When the force decreases, the rate of particle motion also decreases. Eventually particle motion ceases and mechanical deposition occurs.

Not all deposits form by mechanical deposition. Some deposits form instead by chemical deposition. As you may know, all naturally occurring water has some minerals dissolved in it. Deposition of these minerals may result from a variety of chemical processes; however, one of the most familiar is evaporation. When water evaporates, dissolved minerals remain behind as a solid residue. This residue is a chemical deposit of minerals.

Ocean water is very rich in dissolved minerals—that is why ocean water tastes salty. When ocean water evaporates, a deposit containing a variety of minerals accumulates. The mineral halite (that is, table salt) would make up the bulk of such a deposit. Large, chemically derived mineral deposits, which formed by the evaporation of ancient saline lakes, are currently being mined in several areas of the western United States. The Bonneville Salt Flats in Utah is a good example of an "evaporite" mineral deposit. Due to the arid climate, evaporite minerals are still being deposited today at Great Salt Lake in Utah.

The term "deposit" generally applies only to accumulations of earth materials that form at or near the earth's surface, that is, to particles, rocks, or minerals that are of sedimentary origin. However, ore deposits are an exception to this generality. The phrase "ore deposit" applies to any valuable accumulation of minerals, no matter how or where it accumulates. Some ore deposits do form by mechanical or chemical deposition (that is, they are of sedimentary origin).

For example, flowing streams deposit gold-bearing sand and gravel layers, known as placers. Placers, therefore, form by mechanical deposition. Some iron ores, on the other hand, form when subsurface waters chemically deposit iron in porous zones within sediments or rocks. However, many ore deposits do not form by either mechanical or chemical deposition, and so are not of sedimentary origin.

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