Answer: (a) Ores are minerals from which a metal can be extracted profitably; (b) gangue is the unwanted earthy impurity present in an ore.
(a) The compounds of a metal found in the earth’s crust are called minerals. A mineral from which the metal can be extracted easily and profitably is called an ore, e.g. bauxite for aluminium and haematite for iron.
(b) The ore as mined contains sand, clay, rock and other impurities along with it. These unwanted substances are called gangue, and they are removed during concentration of the ore.