Board Thread:Fiction Universe Discussion/@comment-9141454-20150625191138/@comment-1073312-20150626142614

The problem with the method of sifting you described, is that drilling holes even into the ductile zone of the crust, the rock will reseal itself. Maybe some super-hard hoses could be used to hold open the holes, but the sheer pressure is enough to deform or destroy nearly any material you could manufacture (except maybe some forms of exotic materials).

Nanobots are another possibility yes, al least in the upper layers, But I can't see the usual kind being useful even deep in the crust due to the temperature and pressure of these regions. Perhaps they could built out of minerals stable in most of the crust and mantle. but then the weight of the nanobots and heat they would produce will also add to the conditions. Again, some weird materials have been proposed, made of "magmatter" (monopole atoms, from Orions Arm) or nuclear chemistry, that might work.

The thing is, asteroids alone are enormously useful. Many are composed of the smashed relics of planetary cores, form during the iron differentiation stage. The reason why many "Rare Earth Elements", are rare is because they are siderophile elements and bind with iron, sinking them to the core. In asteroids the abundances of these elements are far more common.

I personally want to try and think of some ways a civilisation might artificially induce geological processes, like ore genesis. Something simple like water for example, can control the viscosity and melting points of various melt, and also is pivotal to many metamorphic processes. The problem of course is timescales, which would want speeding up, and moving large masses of rock and fluid about. But I'm sure there is a way.