Ahhh laddie, whilst that is a seductive hypothesis, and it is entertained by many a thinker in the halls of the Mito Chon Dria, (Land of the Outside Ice Box) it is also not true to a degree that not all of the source rock eroded. After all you are asking for a heck of a lot of erosion.
The odds are at least 50% that half the orebody that gave rise to the placer is still there.
Many, many placer's sources have been found and it has been found that much of the vein structure is still there.
On the other hand, an economic placer does not have to have an economic hard rock source.
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