So, considering that a Zoo has gates, maybe an entrance building, then open spaces and more buildings, what are the laws regarding libraries and "local units of government" - and how do they affect one's ability to OC and/or CC at a public zoo?
State of Michigan code 123.1102, sec. 2 says that no local unit of government can have any firearm or ammunition restrictions beyond what the state and feds have in place. In CADL vs. MOC, the lower courts basically ruled that because libraries are unrestricted (i.e. not an armory, jail or court) facilities and funded through taxpayer monies, they constitute a Local Unit and are therefore held to the pre-emption. CADL appealed, MSC refused to hear it, so the ruling stands. You can carry OC or CC, up to you. Libraries are not covered in MCL 750.234d.
That particular section does list seven places that you can't carry without a license: financial institutions, houses of religious worship, courts, theaters, sports arenas, day care center, hospitals and bars. Public zoos don't fall under any of that, regardless of construction.