While the
average content of gold and platinum-group elements in an iron asteroid is many thousands of times higher than their
average content in the crust of the Earth, that does not mean that these metals are easy to mine.
Unlike on Earth, where various processes have concentrated the precious metal into some deposits where they are much more abundant than on average, e.g. in alluvial deposits in river beds or in magmatic sulfide deposits, on such an asteroid the precious metals are dissolved in iron and spread uniformly through all the asteroid.
Therefore even if the total amount of precious metals is very great, a huge quantity of energy is required for its extraction. For instance, in order to produce one kilogram of gold more than four hundred tons of iron will have to be processed (e.g. by first producing hundreds of tons of some acid used to dissolve the iron and the other non-precious metals).
So it is very unlikely that we will see any such mining operation soon. There are other metals much easier to mine in such a place, e.g. iron, nickel, cobalt, germanium, chromium, gallium, manganese (but which are not worthwhile to be brought back to Earth).