How did you phrase the question? If you couched it in terms of "permission", then you just open yourself up to an desk-bound bureaucrat's whims. Ie.: they can say "no", simply because they envision geeks with shovels leaving a mess. Ie.: there may be no real rule that addresses what you're asking, but they can morph ANYTHING they want to say "no". You'd have been better off to look it up for yourself (city codes are usually on-line somewhere at city web-sites). If there is nothing that specifically mentions detecting, then just go!
Personally, I don't ask or inquire about any city parks anywhere. If it's not posted, I consider myself no different than any other park user (like kite-flying, basket-ball, etc... none of them would "ask permission", right?) But if y ou absolutely feel you must "ask" a live person, don't phrase it in terms of permission, or "can I..." etc... Instead, phrase it like this: "Is there anything in any city law that addresses the subject of hobbyist metal detecing?" See the difference? Then the burden of proof is on them, to show something that actually prohibits it. 99.999% of the time, no city has anything (nor have they probably ever given the matter thought).
You may have actually gotten a "no", where, truth be told, you'd probably have been ignored and never given a second thought. Kind of like picking your nose: If you ask enough people, someone will tell you that you can't do it.