Best gold mine
At the arcade in any town, the goal is to get 1000 pts. If you do, the lady will tell you the location of the "best gold mine." The mine is in the hills above the west bank of Eldorado Creek.
OR
Choose Above Bonanza Creek #1, Then stake it right away. You should get lots of money.

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Extra money
If you sell your diary to the news lady, she will give you $20 in gold. Don't worry; she will return your diary.