According to a review by Gun Digest, the slide is an aluminum allow while the frame/receiver is zinc alloy (Zamak). However, one poster claims he called ATI, which is the importer for GSG, and they reportedly told him the slide was "Zamak 5."
http://www.defensivecarry.com/forum/...ml#post2177045. Perhaps he got the slide and frame mixed up, but who can say? Neither ATI, GSG, or Sig give any details on their websites.