Ann: Heron's Regional Exploration results at Crosby Prospect, page-8

I don't suppose you'd accept he resigned for exactly the reasons stated, paraphrasing:
1) that he was aged and tired and had done enough
2) that the company was in a different phase, moving from exploration to production and that required different skills and a new lease of life to cover that phase.
(I've actually seen and experienced that as real reasons for long term directors an mangers to move on both successful and less successful ones).