The New Adventures of Lassie S02 E15 Mrs. Lee's Secret

When Mrs. Lee opens a letter that Zoe, Harvey and Lassie bring her, she learns that she is the last living heir to a former mining concession. When Mrs. Lee lets them read her journal from the period, the kids and Lassie set out in search of the abandoned mine to discover the "magnificent treasure" that the journal hints is hidden there.