According to the myth "Bida, the Black Snake" the reason for the declinening of the trade of gold in Ghana was:
3. The death of Bida.
Bida, the black snake, and the kingdom of Ghana had an agreement. In return of prosperity and wealth Bida demanded every year the sacrifice of a virgin.
One year, the virgin elected to be sacrificed was Siya Yatabar, the most beautiful and pure virgin of the kingdom. But Siya was engaged to Maadi. Her lover promesed to save her and kill the snake, despite her oposition. When the moment of the sacrifice came, Maadi fulfilled his promese and killed Bida, who before dieing cursed the nation with no rain and no gold for seven bad years, seven bad months and seven bad days.
This myth explains the drought and the declination of gold minning that took place and was in part the reason for the decay of the Ghana empire.