The earth's crust contains, on average, approximately 0,006 % copper. With this percentage, copper is number 23rd on the list of elements that comprise the earth's crust. Traces of copper can be found in nearly every rock. In massive or "pure" form, copper can be found in the Ural Mountains of New Mexico, USA. Like iron, copper has the tendency to bond easily with sulphur. As a result, both metals are often found together in ore as sulphur-bearing minerals.
The most important copper ores are copper pyrites (chalcopyrite) and glance copper (chalcosite). Additionally, the sulphur mineral bornite, the oxidised mineral malachite, azurite, and red copper ore (cuprite) are important with regard to mining. The copper content of copper pyrite (CuFeS2) and glance copper (Cu2S) are 34% and 79% respectively.
Based on data from 1991, the world's known copper reserves are estimated at 552 million tons, of which only 321 million tons are considered to be profitably extractable. Thus, the potential for current copper inventory are at least three times larger than that of the reserves. Reserves found in copper slate, such as in deep-sea nodules, cannot at this time be profitably extracted even with the most modern technology. Extraction technology is, however, constantly improving. The copper content of deep-sea nodules alone is estimated at 0.7 x 109 tons.