However , a small amount of phosphate was produced in the 1970s , as a byproduct of iron mining in Missouri .
But since 1948 , all phosphate deposits mined in the US have been sedimentary deposits .
A small amount of phosphate was produced in the 1970s , as a byproduct of iron mining in Missouri .
But since 1948 , all phosphate deposits mined in the US have been sedimentary deposits .