it's a matter of the cost of extraction, you can extract gold and other elements from seawater but it is very energy intensive and thus not cost effective at this time. But if the prices of precious metals were significantly higher then it would make sense to extract deep sea mud or refine commodities from seawater. A better question than where we will get enough rare earth elements is where will we get enough phosphorous?
extraction costs are dependent on the technology used.
recently there have been some VERY interesting advances made in 'filter' technology.
soon perhaps, coming to a bayou near you, sea water extraction of any element you chose...cheaper than dirt.
That's always the key factor. There is gold all over the place but it is not really cost effective to extract it in most places. They would have to be able to do it as cheap as the guys in Alaska. It's too bad you don't watch TV there are some great shows about gold mining.
where will we get enough phosphorous
Bat Shit
fun fact..there are more bats living in Austin Texas than there are people.
there MIGHT be more bats living in the USA than there are people too.
NOT a scarce resource..
(collection is rather tricky...perhaps an inflight removable diaper collected by drones needs to be invented?)
There is not enough natural manure to provide as much phosphorous as we mine, even including humanure. Most of the large guano deposits have been mined out, there were war fought over it.