There was a history of Tregembo Mine published some years ago in the Journal of the Trevithick Society.
Having the date, you could probably get more details of the accident from newspapers (Royal Cornwall Gazette is on 19th Century British Newspapers online, other papers at Cornish Studies Library, & Western Morning News & Western Daily Mercury (often the most detailed) at Plymouth Central Library.
My cousin worked at Tregembo for a long time, had a look over what remains there many years ago.