A significant constraint in developing a copper mine is the fact that copper-containing rock hosts only a small percentage of copper, most of the rock is unwanted material, typically referred to as gangue. In order a mine to be developed, there must be enough copper present in a deposit to make its extraction economical. There are numerous types of copper ore, but two of the types of ores, copper oxide ores and copper sulfide ores, are of primary economic interest.
The most common source of copper ore is the sulfide ore mineral chalcopyrite, which accounts for about 50 percent of copper production. Sulfide copper ores are the most profitable ores because they have high copper content, and the copper can be more easily separated from the unwanted minerals. Copper oxide ores are not as attractive of an exploration target as the copper sulfide ore types due to their lower grade, however low-grade copper oxide deposits can be economically extracted because they can be processed at lower cost than the copper sulfide ores.
Copper can be deposited in different geological environments; therefore there are a variety of types of copper deposits, each with its own broad characteristics- size, grade, etc. There are different types of deposits, with each type having a particular style of mineralization, and therefore basic value. Most of the world’s copper is mined from porphyry deposits, large size, low grade deposits that are normally mined via open pit methods. The next most common type of deposit mined for its copper is sedimentary deposits (there are a variety of types of sedimentary deposits). Other types of copper deposits include volcanogenic massive sulfide ore deposits, Iron oxide copper gold ore deposits (IOCG) mesothermal vein deposits, epithermal deposits and nickel-copper-PGE deposits.
Porphyry Copper Deposits
Porphyry deposits are the world’s most important sources of copper and molybdenum. Approximately 50-60 percent of the world’s copper, and 95 percent of molybdenum comes from porphyry deposits. Porphyry deposits are also major sources of silver, gold, and tin; and can host valuable by-products, including platinum, palladium, and tungsten.
Porphyry copper deposits are low grade, but high volume deposits. Grades for the different metals vary considerably, but generally average less than 1%. In porphyry Cu deposits copper grades range from 0.2% to more than 1% copper. Porphyry copper deposits always contain other metals, and the presence of these metals can greatly affect the economics of the deposit, and some deposits with lower concentrations of copper may be mined, particularly if they contain gold and/or molybdenum.
Sedimentary Copper Deposits
Sedimentary deposits all share a similar mode of deposition, beyond that there is a great deal of diversity in this group. Sedimentary deposits are created when copper-bearing fluid moves through strata, and for one reason or another precipitates. Precipitation is generally due to a chemical change, the main types of sedimentary deposits are copper-bearing sandstones and shales, red-bed deposits, Kuperschiefer and Sedimentary Exhalative.
In copper-bearing sandstones and shales copper is deposited when the copper precipitates from fluids circulating in the host rock. Copper-bearing sandstones can be found in Zambia.
Red-bed deposits are named for their color, which is a result of the oxidation of rocks after exposure to the atmosphere. Red-beds are created by either sedimentary or volcanic processes. The volcanic variety is of more economic importance than the sedimentary; sedimentary red-bed deposits are relatively small, and as a result are rarely mined. One example is the Paoli in Oklahoma. Kupferschiefer-type deposits are large, regionally extensive deposits that are similar to red-bed deposits.
Sedimentary Exhalative Deposits are formed when hydrothermal fluids enter a water reservoir, such as an ocean, and precipitate minerals. SedEx deposits are the world’s most important lead and zinc source, and a secondary source of copper.
Next Week we will look at the minor types of copper deposits, including VMS and IOCG