Article: With the arrival of Francisco Vázquez de Coronado, a Spanish conquistador, the first recorded history of encounter between Europeans and Native Americans in the Great Plains occurred in Texas, Kansas and Nebraska from 1540-1542. In that same time period, Hernando de Soto crossed a west-northwest direction in what is now Oklahoma and Texas. Today this is known as the De Soto Trail. The Spanish thought the Great Plains were the location of the mythological Quivira and Cíbola, a place said to be rich in gold.

Question: what did the spanish think the cities of Quivira and Cíbola were rich in?
gold