There are basically two types of aggregate mining.
Aggregates that are ready to be used without major blasting and
screening is the most prized. Geologically you would look for glacial,
marine, lake, stream, and eolian (wind) deposits that have not
yet become sedimentary rocks. These would be the cleanest deposits.
Other deposits may include poorly cemented sedimentary rocks, especially
for pebbles and gravel and hard consolidated rocks like igneous or
chemical sedimentary for road fill.
If they are looking just for sand deposits, they can mine them on an
ancient beach. For example, there are ancient sand dunes at Sand City in
California. The mining operation just bags the old sand and sells it as
play sand (coarser grained) and sand for concrete (finer grained).
Sand City, California