Ores are also classified by the nature of their gangues, such as calcareous or basic (lime rich), or siliceous, or acidic (silica rich).
The minimum metal content required for a deposit to qualify as an ore varies from metal to metal.
Many nonferrous ores contain, as mined, as little as 1 % metal, and often much less.
Gold may be recovered profitably in ores containing only 5 parts per million (ppm) of the metal, whereas iron ores containing less than about 20 % metal are regarded as low grade.
There are many plants where minerals are recovered in secondary circuits, treating tailings, where the feed grades are much lower than would be economic on a mined ore.
Typical ore grades for tungsten ores are in the range of 0.5% ~ 1.5% WO3, but the Climax Molybdenum plant in the United States treats 45000 tonnes per day of tailings, containing less than 0.1% WO3, and is one of the two major producers of tungsten concentrate in the United States.
钨矿石的典型品位为0.5% ~ 1.5% WO3，但是美国的Climax钼厂每天处理45000吨尾矿，其中WO3含量低于0.1%，是美国两大钨精矿生产商之一。
Ores of economic value can be classed as metallic or non-metallic, according to the use of the mineral.
Certain minerals may be mined and processed for more than one purpose.
In one category the mineral may be a metal ore, i.e. when it is used to prepare the metal, as when bauxite (hydrated aluminium oxide) is used to make aluminium.
The alternative is for the compound to be classified as a non-metallic ore, i.e. when bauxite or natural aluminium oxide is used to make material for refractory bricks or abrasives.
Many non-metallic ore minerals associate with metallic ore minerals and are mined and processed together, e.g. galena, the main source of lead often associates with fluorite (CaF2) and barytes (BaSO4), both important non-metallic minerals.