Star fund manager Graham Birch will not return to BlackRock at the end of his nine-month sabbatical, leaving Evy Hambro in charge of BlackRock World Mining Trust.Birch has been away since the end of March and was due to return early this year.There was no indication Birch was going to call it a day at the top-performing fund when I spoke to Hambro before Christmas.'Graham is due to let the firm know what he wants to do at the beginning of next year. We'll wait and see,' he said.But the trust should be in safe hands. Hambro began his career in 1994 with the original Mercury Asset Management team, bought by Merrill Lynch in 1997, following an internship during the IPO roadshow in the winter of 1993. Merrill Lynch Investment Managers (MLIM) then merged with BlackRock in 2006.'I have been going to the board meetings and helping Graham write the reports and co-managing the portfolio now for years and years. It's business as normal,' the Newcastle University graduate told me.Hambro has been co-head of BlackRock's Natural Resources Equity team, responsible for managing the BlackRock Global Funds (BGF) World Mining Fund, BGF World Gold Fund, BlackRock Gold and General Fund, as well as the BlackRock World Mining Trust and BlackRock's Natural Resources Hedge Fund.He also has good pedigree. His father Peter co-founded Peter Hambro Mining, now Petropavlovsk plc, in 1994. 'It has been an honour working with Graham Birch, He has made a huge contribution to the team and we wish him all the very best for the future," he said today.'The natural resources team is well set for the future, with great depth of experience and resource. We have traditionally maintained strength in depth as demonstrated by the strong performance achieved for investors since Julian Baring left us in 1998.'Analysts at UK stockbroker Killik Capital said recently that BlackRock World Mining Trust is now 'an attractive switch candidate from JPMorgan Natural Resources' after the discount to net assets drifted out to 17%.Blackrock shares have outperformed over the past 12 months, up 137%, but some are disappointed by the size of the discount, which is better than the 28% seen a year ago, but not as good as the 10% traded in the spring.That's not Hambro's concern though. He looks after the assets in the portfolio and lets the board make decisions about managing the discount by buying back shares and so on.He blamed a bit of year-end profit-taking rather than any other factor.