A few words about the background of Skeleton Mountain.

The mission chapel at Tumacacori,

https://www.nps.gov/tuma/index.htm

just north of Tubac, served as the model for the chapel in the novel. Pictures of the interior show how thoroughly the walls have been chipped away and picked at by treasure seekers over the years.

The cemetery in the back also suggested the cemetery in the book.

The Superstition Mountains suggested Skeleton Mountain itself. Several good aerial views of the mountains can be found on Youtube; for instance:

https://www.youtube.com/watch?v=bf9c7...

https://www.youtube.com/watch?v=K6gP-...

The Peralta Stones also suggested the use of similar carvings in the novel.

https://en.wikipedia.org/wiki/Peralta...

Of course there are many legends and rumors of lost treasures and mines of all types in those mountains (and at the chapel); and there is no shortage of Seekers still going to look for them; and willing to buy a "genuine" map when it comes along.