Types of ores
Ores or metallic rocks and minerals are classified into two categories depending on the amount of minerals per ton of rock, and the value of the mineral. For gold, for example, there is a low-grade gold ore (less than 2 grams of gold per tonne of ore) and high gold ore (from 6 grams per tonne of ore). For copper is 1% treated as low-grade ore, for iron is low-grade already 25% to 30%. Usually the surface of a mine site contains more than one metal, such as gold and copper in the Skourriesmine in Greece and the Conga mine in Peru.
The grade determines the which mining technique is profitable; in the case of low-grade gold, they will use open-pit mining, while in high-grade gold underground mining techniques are applied. The gradation determines the energy consumption and the amount of contaminated waste products, since there are more chemicals required when there are only low mineral concentrations present. But also the depth at which the sought-after metals can be found, plays a role. Gold ores to the surface can be detected earlier and can be mined easily (by surface mining), while later deeper soil layers will be drilled (using underground mining techniques).
Because of the high extraction rate in the last few decades, the most interesting ores (those with the higher concentrations) are already mined, while the soil layers with lower concentrations are left over. The increasing scarcity creates rising commodity, which also make it profitable to mine very poor mineral sites (or low-grade sites). For example, sites are already fully unearthed (via open-pit mining), even if there's less than 0,2 grams of gold per ton of ore is to be found. This causes more dangerous mining waster per gram of gold.