The point is some of the ore rocks are rich enough that you could probably smash them with a sledgehammer and pan out the gold and make money.
It wouldn't take unimaginably sophisticated equipment.
If a hypothetical mining industry insider had spent 30 or 40 years in the industry he might be able to find the telltale signs of rich veins. He might have a talent for finding them.
He might hypothetically speaking even have retired knowing where some rich veins are located.
The rich veins need not be in mines his company was working on. He might have visited disused mines while in the area in order to test his hunch during his time off.
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