By Leia Michele Toovey- Exclusive to Copper Investing News
Copper-containing rock hosts only a small percentage of copper. Most of the rock is unwanted material, typically referred to as gangue. Depending on to what minerals the copper is bound, the rock is processed in different ways in order to extract the valuable copper. There are two main copper ore types of interest, copper oxide ores and copper sulfide ores.
Both ore types can be economically mined, however, 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. It is important to note that sulfide ores are not as abundant as the oxide ores.
Copper oxide ores
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. Oxidized copper ore bodies may be treated several ways, with hydrometallurgical processes used to treat oxide ores dominated by soluble minerals such as copper carbonate minerals. These oxide ores are usually leached by sulfuric acid to liberate the copper minerals into a solution of sulfuric acid laden with copper sulfate in solution. The copper sulfate solution (the pregnant leach solution) is then stripped of copper via a solvent extraction and electrowinning (SX-EW) plant. Alternatively, the copper can be precipitated out of the pregnant solution through a process called cementation, where the copper is contacted with scrap iron. Copper produced through the cementation method is usually less pure than SX_EW copper.
Copper sulfide ores
The method used to separate copper in copper sulfide ores depends on the concentration of the copper. Higher concentrated ores can be separated via smelting; lower concentrated ores are separated via hydro-metallurgical processes. Some supergene sulfide deposits can be leached using a bacterial oxidation heap leach process to oxidize the sulfides to sulfuric acid, which also allows for simultaneous leaching with sulfuric acid to produce a copper sulfate solution. As with oxide ores, solvent extraction and electrowinning technologies are used to recover the copper from the pregnant leach solution. Secondary sulfides formed by supergene secondary enrichment are resistant to sulfuric leaching. When rich enough, native copper ore bodies may be treated to recover the contained copper via a gravity separation circuit. Supergene ores rich in sulfides may be concentrated using froth flotation.
Types of sulfide deposits
Massive sulfide deposits are formed via the flow of heated fluids (usually seawater) through sedimentary and/or igneous rocks. The circulation of the fluids is generally driven by volcanic activity. Fluids drawn down through sediments or igneous rocks towards the earth’s crust encounter rising temperatures. As these fluids are heated to become a hydrothermal fluid, any dissolved sulfates are reduced to sulfide or precipitated as calcium sulfate. As it is heated, the fluid also becomes depleted in magnesium, and this causes a drop in pH. What results is a heated, acidic fluid that reacts with the sold rocks in which it is contained. Various elements are leached from the rock and dissolved as complexes. This modified hydrothermal fluid rapidly reaches equilibrium with an assemblage of secondary minerals. The hot hydrothermal fluid becomes less dense and flows upwards. As it nears the earth’s surface, most often a seafloor, it cools. This causes precipitation of minerals such as pyrite chalcopyrite, sphalerite and galena that form massive sulfide deposits. The largest volcanogenic massive sulfide deposits are found in greenstone belts in Achaean cratons, such as those in South Africa and Canada. Massive sulfide deposits are a major source of many metals including lead, zinc, copper and silver.
Sediment-hosted deposits are found in oceans where spreading centers are buried beneath sediments shed from the nearby continents. While the process of mineral deposition is similar to that mentioned in regards to massive sulfide deposits above, there is an important difference. Instead of the hydrothermal fluids flowing directly into sea water from the oceanic crust, they must first pass through an overlying layer of sediments. When passing through the sediments, the fluid’s chemistry is substantially altered. Sediment-hosted massive sulfides have a wider range in mineralogy than volcanic-hosted deposits, reflecting the variation in the composition of sediments. When it comes to their mineral content, sediment-hosted massive sulfide deposits tend to have higher concentrations of lead, zinc and silver, and relatively smaller quantities of copper and iron than volcanic-hosted deposits. Two of the largest sediment-hosted massive sulfide deposits are the Sullivan in Canada and the Broken Hill in Australia.