Due to extensive mining activity by American mining corporations in the region in the late 1800's and early 1900's there are dozens of old mine shafts near Tumacácori.  These are not Spanish mines.  They may have been at one time but they have long since been worked and expanded by modern mining companies, removing any trace of earlier mining activity and any precious or valuable ores.  Regardless, old mine shafts are unstable and deadly.  There is no amount of treasure, imaginary or real, worth getting injured or killed over.