The Secret of Thunder Mountain (Ted Wilford #1), by Norvin Pallas (epub/Kindle/pdf)
When Old Goldie, a prospector rumored to have had a hidden gold mine, passes away, he leaves an envelope addressed to "T. A. F." It contains only a photograph of a small cabin.
Who was Old Goldie, and who is T.A.F.? Is there really a mine? Ted Wilford and his friend Nelson Morgan set out to answer these questions. If the mine exists at all, they soon realize, it must be on Thunder Mountain. From that point the race is on. A mining engineer, a secret code, a Native American, a rival reporter -- and most of the people from the two nearest towns -- are also looking for the mine. Whoever finds and claims it first may win a fortune. And some of them will stop at nothing to get it...