Generally, the ores formed somewhere deep down tend to be sulfides. They eventually were pressed up in cracks of the rocks (“hydrothermal veins”) by geological events. Further up the ore experiences atmospheric impact, and in this “oxidation zone” these sulfides are exposed to the oxygen (and water) of the air, groundwater, and possibly large temperature changes.
In this oxidation or weathering zone, between the surface and the major ore body below, ores are oxidized to form various rock minerals, including copper oxides, sulfates, carbonates (produced in the presence of limestone), and God knows what else. The best known copper carbonates are green malachite and the more unstable deep-blue azurite (not to be confused with lapis lazuli). Copper silicates and turquoise – a rare, blue copper aluminium phosphate mineral, prized in itself – may also be present. Native copper occurs at the surface of ore bodies and sometimes deeper in the ore body.
Some of the compounds formed in the oxidation zone are water soluble. Rain leaches this stuff to below the groundwater level, where it precipitates again, thus enriching the ore concentration at this level.
Deep in the ground where reducing conditions prevail, copper sulfides like chalcopyrite, bornite and chalcocite dominate. These are now the major source of copper.
The very top of the deposit gets completely weathered off, leaving only useless stuff behind. This region is called “gossan” or “iron hat“, because quite often it is enriched with the stable iron hydroxide “limonite” (FeOOH) and therefore looks rusty. A gossan was easy to recognize and gave a clear hint that digging there might be worthwhile
There is often no sharp boundary to the host rock. Hydrothermal action may decompose the ore-less rock, and re-mineralization generates a corona, up to several 100 m wide, around the primary ore deposit that may also have some metal-bearing compounds mixed in