Lone Star Heroes Book 1 — Patrick Dearen

The Big Bend of Texas is a mysterious place in 1869. Legend has it that there's a lost gold mine in the Chisos Mountains. Twelve-year-old Fish Rawlings and his cousin Gid have heard all about it. But when they discover a dying Indian in the desert, they have reason to believe it.
Suddenly the boys find themselves with a great secret. No one else knows the way to the last Chisos mines-but do they dare? To find it, they must cross a desert prowled by Apache warriors. They must ride a trail haunted by devil animals and Indian spooks. Even with the help of a young Apache boy, the journey won't be easy.
And what will they do if they succeed?