I go panning 2 or 3 times a year for fun. Biggest nugget I've found was a whopping 3 gram piece (actually considered large.) For mines, ore that produces 1 ozt (31.1 grams) per ton (1000kg) is considered high yield ore.

Hell, it's estimated that all the gold mined in all of human existence, if put in a single place, wouldn't fill an Olympic sized swimming pool.

To Op's statement, that's a great reference size!