I am not that far yet, but I believe you can access the mine tunnels trough the swamp and not troughout Marten's Bluff. If no-one lets you pass you should try to find another way in.

Mine tunnels walktrough:
http://www.volberding.com/Wizardry8/Wiz8MinesWalk.htm

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By the way, how do I get into the mine Tunnels ? Some T'rang trap keps me from invading those tunnels ...