Which preposition to use with placer

of Occurrences 4%

Even the superficial placers of the present streams have derived much of their gold from them.

in Occurrences 2%

If the washings have made the richest placers in the world, what must the mother veins be?

on Occurrences 1%

When the rumored discovery in the year 1861 of extensive gold placers on Salmon river was confirmed, the intelligence spread through the states like wild fire.

Which preposition to use with  placer