This was tolf to me years ago, by a guy who was a huner and had travelled in rural Arizona. I have never read anthing like this, so here gos:
around about 1900 or so, a propsector was trekking through the Superstition mountains (Arizona), and came across a recent landslid-this exposed a narrow box canyon. He decided to follow the path, and came to a wider canyon-in it was an old spanish mission church. he entered the church, and it was filled with gold ornaments, candlesticks, chalices, etc. He wound up spending the night, and resolved to come back 9he marked the location on his map). Weeks later, the man returned, but could not find the path 9a flood had washed it away).
Sounds like a local myth
Yes it’s a myth unless you think you can trust the stories that people tell about finding this wealth. Stories about people finding the Lost Dutchman’s Gold Mine have been told for a long tome now. Variations like what you describe fit into the tales.
My story
This happened to me while on vacation and traveling in rural Arizona. I, too, had an experience like this, so here gos:
Around about 2000 or so, I was driving through the Superstition mountains (Arizona), and came across a sign recently opened souvenir store in a narrow box canyon. I decided to follow the path, and came to a wider canyon-there in the middle of a river bed was what seem to be an old tourist trap. I entered the store, and realized it was better than any trap I had ever seen: it was like being in a museum, filled with a bunch of Apache hunting gear, extremely rare Indian Baskets I’d only seen in museums, and get this–some of the best Indian ceramic pottery I’d ever seen in my life, and a bunch of really high-end Kachina dolls, etc. I stayed till closing spending only 10 bucks (they only took cash, and kept wondering if I was passing counterfeit bills, and couldn’t stop looking at my car), and I resolved to come back, figuring that for touristy stuff, it was like the bargain find of the century. I marked the location on my GPS. Weeks later, I returned, but could not find the store. I asked the gas station attendant up the and found it had been washed away in a flash flood, but that was back in 1900!
Yeah, it sounds like a local myth to me too. 
I think the Superstition mountains is the location that goes with a gold hoard that was inside a hollow mountain. The military is said to have plundered all the gold that was stored there.
For a really good story there’s the Egyptian ruins in the Grand canyon that had mummies of giants in it.
Another is the underground caverns in the desert with a large fresh water river. There’s enough to irrigate the whole southwest desert.