Why did the Almoravids attack Ghana?

The region that makes up modern Mauritania and Senegal contained one of the few known, reliable sources of both gold and salt; the Almoravid home of the Sahara had neither. Ghana controlled trade in these commodities, including across much of West Africa, and as Almoravid Islam expanded it required increasing amounts of gold as it minted coins and as it funded the growth of its religious centers such as the zawaya.