where does your account settings say archive is located?

It sounds to me like it is somewhere other than the IMAP mail account you are attempting to archive mail from. That would account to mail being deleted as a part of the process.

Hi Matt, Thank you for your reply. I'm so sorry - I just didn't wait long enough. When I went into Thunderbird some time later, the achived email was there in Archives. I had thought just closing Thunderbird and waiting a minute or two would be enough time - but it was not. I tried then to find my question to say it had solved itself - but couldn't find it. Best wishes, Kitty