Seawater is a good example, and there have been scams run over the years that claimed to do exactly that. Currently, there are folks that are claiming to extract gold from fly ash (coal ashes), volcanic ashes, lakebed clays, smelter slag, and old mine tailings.
The latter two examples (slag and tailings) are actually possible, but unlikely to be cost-effective since the old-time miners generally did not leave enough behind to be worth processing, even at today's gold prices. But, always a new crop of investors who have a dream......! If it sounds too good to be true, it probably is.
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