I've read your report at Sierra Topix; interesting stuff.
The door you entered has since been sealed off by the USFS. Probably just as well; it isn't really safe in there, even if it gives the appearance of being solid. Maybe word got around that people were going inside and the USFS got nervous.
The Pine Creek portal you found is probably the old 1500 level portal (1500 feet above the Pine Creek mill site), which was blasted shut, but I guess they haven't gone back to check it in quite a while. Deeper into that adit (tunnel) you will find a large metal "air-door" which was part of a system for controlling ventilation inside the mine (to clear out radon gas inside the mine). If the air door is closed you won't be able to proceed further. If you can hear air coming out, the door might be open. If you are able to go further into the adit, it is shored up with steel and timber, but possibly flooded in some places. Have fun!
