Do archaeologists deliberately leave parts of sites untouched so that someone can come back later?
Yes.

From this article in National Geographic:


  The last step before digging is to divide the site into a grid to keep
  track of the location of each find. Then archaeologists choose sample
  squares from the grid to dig. This allows the archaeological team to
  form a complete study of the area. They also leave some plots on the
  grid untouched. Archaeologists like to preserve portions of their dig
  sites for future scientists to study—scientists who may have better
  tools and techniques than are available today.


(emphasis mine)