Ores are defined as chemical compounds of metals which occur naturally in the ground. Most have an igneous origin and have three distinct forms:

Veins - these ores have formed in bedrock cracks and fissures. The name 'Lode' is usually given to veins that are mined commercially for their content.

Beds - ores which have been laid down as layers either by deposition from solution or by injection of material between exisiting beds of rock. Sometimes these are referred to as Flats.

Disseminated ores - usually grains of ore sparsely dispersed in rock. These ores typically contain a lot of what miners call Gangue, unprofitable waste.

The most common ore deposits are:

Hydrothermal Deposits - precipitated from hot aqueous liquids or gases of igneous origin. They can be classified according to the depth of formation:

• Epithermal Deposits - ores that crystallise near the surface, usually under cool conditions. Examples include ores of antimony and silver.
• Mesothermal Deposits - ores that form at intermediate depths, usually under hot conditions. Examples include ores of lead and copper.
• Hypothermal Deposits - ores that form deep in conditions of high temperature and pressure. Examples include ores of tin and tungsten.

Furthermore, hydrothermal deposits can be grouped into three categories depending on when they solidified relative to the surrounding rock.

• Syngenetic ores are formed at the same time as the rock, often in sheets.
• Diagenetic ores are formed after the country rock, but before this has lithified.
• Epigenetic ores form after the surrounding rock has lithified.

Other types of ore deposits include:

Magmatic Deposits - formed as a result of concentration of metals by magmatic processes. The ore is typically an igneous rock. Many metals can exist in their native form. These ores are importants sources of industrial matals including Chromium, Platinum and Nickel.

Chemical Sediment Deposits - form as a result of metals precipitating out of surface water (lakes and oceans) to form ore minerals that become part of the sediment. If the sediment becomes lithified, the ore is a sedimentary rock. An important source of iron ore. Other metallic ores formed this way include those of lead, zinc, copper, gold, silver, cobalt and manganese.

Residual Deposits - form as a result of the chemical weathering of rock under unique conditions to form a metal-rich soil. For example, silica and iron are preferentially leached from certain soils to form an aluminum-rich soil called bauxite.

Mechanical Sediment (Placer) Deposits - form when ore minerals are weathered from a source rock and subsequently separated from other sediment by water-transport and deposition in soil, streams, and beaches. The ore may be unconsolidated sediment, a sedimentary rock, or a metamorphosed sedimentary rock. Examples include gold and tin alluvial deposits.