It's more than likely, that if you lost that much weight without doing resistance exercises, you would've lost some muscle mass -- I wouldn't like to speculate on the ratio, but (here i go anyway) I'd say 1/3 would be about the maximum.
Muscle gains and maintenance does depend on genetics.
I would say that if you continue to lose weight at any rate greater than 2lbs a week, you will continue to lose muscle mass off your upper body. You may even end up with arms like Lance Armstrong.
So, in other words, if you lift weights, and don't lose weight too quickly, you will most likely make muscle gains, especially if you haven't done weights for a while
I enjoy mixing cycling with upper body weights.