What important natural resource in west africa did the songhai empire control?

The important natural resource in West Africa that the Songhai Empire controlled was salt.
This salt was predominantly found in the Sahara desert - there are still a lot of salt mines in this West African desert. The Songhai Empire heavily relied on salt trade, which brought most of its money into the empire - this is the reason why it was so powerful at the time - because of salt and gold. 

Communicating with the dead was common, it wasn’t seen as bizarre or weird,” explains murch. “it’s hard to imagine that now, we look at that and think, ‘why are you opening the gates of hell? ’