Copper-containing rock, or copper ore, hosts only a small percentage of copper. Most of the rock is unwanted material, typically referred to as gangue.
Depending on which minerals the copper is bound to, 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 chalcopyrite, the sulfide ore mineral. This accounts for about 50 percent of copper production.
Copper sulfide ores are typically the most profitable types of ores to mine. That is because they have high copper content, and the copper can more easily be separated from the unwanted minerals. However, sulfide ores are not as abundant as oxide ores.
Copper ore types: Oxides
Copper oxide ores are not as attractive an exploration target as copper sulfide ores due to their lower grade. However, low-grade copper ores of this type can still be economically mined because they can be processed at a lower cost than copper sulfide ores.
Oxidized copper ore bodies may be treated several ways. For instance, hydrometallurgical processes are used to treat oxide ores dominated by soluble minerals, such as copper carbonate minerals. These oxide ores are usually leached using sulfuric acid to liberate the copper minerals into a solution of sulfuric acid laden with copper sulfate solution.
The copper sulfate solution (the pregnant leach solution) is then stripped of copper via a solvent extraction and electrowinning (SX-EW) process. Alternatively, the copper can be precipitated out of the pregnant leach solution through a process called cementation, whereby the copper is contacted with scrap iron. Copper produced through the cementation method is usually less pure than SX-EW copper.
Copper ore types: Sulfides
The method used to process copper sulfide ores depends on the concentration of copper within the ore — higher concentrated ores can be separated via smelting, while lower concentrated ores are separated via hydrometallurgical processes.
Some supergene sulfide deposits can be leached using a bacterial oxidation heap leach process to oxidize the sulfides to sulfuric acid. That also allows for simultaneous leaching with sulfuric acid to produce a copper sulfate solution.
As with oxide ores, SX-EW technologies are used to recover copper from a 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 contained copper via a gravity separation circuit. Supergene copper ores rich in sulfides may be concentrated using froth flotation.
Types of sulfide deposits
Massive sulfide deposits
Massive sulfide deposits containing copper ore 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 that causes a drop in pH.
What results is a heated, acidic fluid that reacts with the solid 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. That causes the precipitation of minerals such as pyrite chalcopyrite, sphalerite and galena, forming 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 with copper ore are found in oceans where spreading centers buried beneath sediments shed from nearby continents. While the mineral deposition process is similar to that of massive sulfide deposits, 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 copper ore 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 deposit, the site of a past-producing mine in British Columbia, Canada and the Broken Hill deposit, located underneath Broken Hill in New South Wales, Australia.
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This article first appeared on April 21, 2011 on Copper Investing News.