Once you've completed the Chapter 3 mission "The New South", there is a chance for a treasure hunter NPC to spawn in a few specific locations. We've marked these on the map above, with the green circle indicating the area where spawn chance is the highest percentage.
If you meet the treasure hunter, easily identified by his impressive beard, you'll need to take his treasure map. He won't be cooperative, so you need to taunt him, kill him or hogtie him to get it. Once you have the map, it's time for a bit of a road trip.
Your first stop is Cumberland Falls. Pulling a typical movie trope, the next map will be behind the waterfall. Go beneath the cascade to the right half of the falls to find a little cave, where the map is hidden.
The next map will take you to Barrow Lagoon. The islet in the center of the frozen lake is connected to the shore with a fallen log. Walk to the halfway point of this makeshift bridge to find the next map in a little hole.
This last map shows you the location of the treasure itself.
You need to go to a cave between Fort Wallace and Bacchus Station, just above the "R" in the "Fort Wallace" caption. To find the cave, climb the hill that gives you an overlook to the nearby bridge. You need to walk along the small ledge, then hop down a bit to find a cave entrance.
In here, you'll find your 3 gold bars worth $1,500.