The Human race has been extracting various resources from the Earth’s crust. These include minerals and rocks that are mined, hydrocarbon and ground water that are pumped up, and last but not the least, the heat of the rocks as geothermal energy. An ore deposit is a specific crustal segment, which is mined. The materials that are mined can be ores of metals, gemstones, non-metallic minerals having a wide variety of industrial applications, rocks used as building stones, coal, and oil shale. Conventionally, the study of ore deposits concerns with minerals from which metals (and gemstones) can be extracted. Therefore, the branch of Ore Geology does not include study of industrial minerals, coals, and oil shales, or for that matter, building stones. Having defined an ore deposit, let us see what it comprises. An ore deposit is composed of one or more ore bodies containing ore and gangue minerals. While the ore minerals are of interest for metal extraction, the gangues constitute a part of the rock that accompanies the ore. Though with little or no economic values, the chemistry of gangue minerals are often quite useful for genetic interpretation as they record the composition and other variables such as temperature and pH of the ore fluid and the nature of fluid–rock interaction, leading to hydrothermal alteration that accompanies mineralization.