Upvote:3
Let's go over what we do know:
The Seven Lost Cities were an old pre-Columbian myth. Spaniards and Portuguese believed there was an island called Antilla out in the Atlantic Ocean. This island had seven cities, populated by Christians who fled the Muslim conquest of Iberia (https://en.wikipedia.org/wiki/Antillia). The Caribbean islands that Columbus discovered got the name Antilles from the legend of Antilla. But the Spaniards found no cities of Christians there. Maybe the Seven Cities were yet further west, somewhere as yet undiscovered.
Francisco Cordoba discovered the Yucatan in 1517 and found gold there. Juan Grijalva was sent back in 1518 specifically to look for more, and he found much more than Cordoba did. Cortes was sent back in 1519 and found staggering amounts of gold in the city of Tenotchitlan. Pizarro's conquest of Peru in 1531-1533 was motivated by a quest for gold, based on rumors that turned out to be true. By the mid-1530s, undiscovered lands of gold were no longer myths, they were fact. The only question was where the next one would be found, and who would find it.
In 1536, Cabeza de Vaca and three comrades returned to Spanish civilization after eight years of being lost and reported on seeing and hearing signs and rumors of gold, silver, and copper in what is now the southwestern U.S. and northwestern Mexico. Viceroy Mendoza sent Friar Marcos de Niza to investigate. Marcos returned from a journey into Arizona and New Mexico. He said he found a city named Cibola. He only saw it from a distance and dared not enter, because some of his party had already been killed, but he described it as "grander than the city of Mexico." The natives with him told him that Cibola was the least of seven cities in that province. Marcos said he then went to the mouth of a valley and "I saw seven fair-sized settlements somewhat in the distance ... I obtained a report that in the valley there is much gold..." (Account of Fray Marcos de Niza, August 26, 1539, in Documents of the Coronado Expedition, 1539-1542 by Richard Flint and Shirley Cushing Flint, pp. 75-76.) This was in 1539.
With Mendoza's approval and aid, Coronado raised an army and went out in 1540 to find Cibola. After he got there and found no gold, he continued searching, following rumors of a land of gold all the way into Kansas.
Putting it together, it is unclear whether Marcos or someone before him morphed the seven lost Christian cities of Antilla myth into a rumor of seven cities of gold. I am unaware of any instance of the seven cities of gold rumor that predates him, but there could be one. If there is, I doubt it goes back further than 1517, so it's a little recent to be called a myth. Marcos is definitely the person who placed it in Cibola. And Coronado definitely was looking for them, even if he didn't write, "I'm looking for the Seven Golden Cities of Cibola" in his letters.
Also see Children of the Sun by David Carson for more about Ponce de Leon and the fountain of youth myth (p. 27, agrees with Peck that Ponce de Leon didn't sail looking for it); p.410 (the Seven Cities of Antilla), pp. 31-41 (the discovery of gold in Mexico), pp. 408-409 (Cabeza de Vaca's report), pp. 417-421 (Friar Marcos's journey and account), and pp. 420-421 (Coronado marches to Cibola). Full disclosure: It's my book.