If the near surface ore had a high amount of clay in it that...

If the near surface ore had a high amount of clay in it that could "constipate" the plant the clay problem can be reduced by using a trommel

For alluvial mining we have use a trommel on the front end of the plant in the past for removing the clay. Water jets blast the ore as it enters the trommel from the feed hopper, the water jets remove the clay material from the gravel which floats off leaving the higer grade particles to go through the jigs or sluice, in the case of MCO the ore would go from the trommel to the crusher.
When there is no clay problem we use a triple deck shaker, instead of a trommel.

As for why someone would not invest in this, one also has to compare MCO with its peers and there are some high grade UG miners trading at very low PEs currently.