The rocks could be made of ANYTHING. Rock dust is worth nothing except for the minerals it could help supply to plants that they wouldn't ordinarily get. One has to assume that commercial rock dust has been analyzed to see if it has any commercial worth. Maybe your garden is based on these same rocks. Maybe it's just ordinary limestone with sprinklings of arsenic and gold, for all one can tell.

The mineral part (the sand, silt, clay) of your soil is made from these rocks so that dust is what your soil is. Nutrient value? Maybe someday in the future but not this year or next.

Well, depending on where you are, rocks may not be the same at 20 feet as they are at 100 or 450. Your soil is possibly (probably) made up of former surface rock layers, unless glaciation brought in new material from elsewhere.

In any case, whether that dust is worth anything to the garden all depends on what they were drilling through. Rocks are not the same everywhere and at every depth - they change a lot. If it was limestone, that's powdered lime, which would raise the pH. If it's granite, that's more commonly used as a rock dust soil amendment.

You'd have to ask your drillers to really know.