British platinum miner Lonmin on Friday said chief executive Ian Farmer, off work since August owing to "serious illness", was stepping down permanently from his role at the troubled group.
Simon Scott would continue as interim head until a permanent successor was found, the world's third-largest platinum producer said in a statement. Scott would eventually revert to being solely its chief financial officer, it added.
Friday's announcement follows a turbulent end to the year for Lonmin, whose shareholders last month approved a rights issue to boost its finances after violent strikes disrupted work at its Marikana platinum mine in South Africa.
"Lonmin plc announces that Ian Farmer, who is being treated for a serious illness, has informed the board of his request to step down as chief executive officer (CEO) and as a director of Lonmin with immediate effect," the company said.
"The board has appointed an executive search agent to pursue the selection and engagement of Ian's successor as CEO. In the meantime, Simon Scott will continue in his role as acting CEO with the full support of the Lonmin board.
"Simon has requested that he should not be considered as a candidate for the role of CEO. Following the appointment of a new CEO, Simon will dedicate his time fully to his role as chief financial officer."
Farmer, 50, was chief executive for four years at the end of a career with Lonmin lasting more than a quarter of a century.
Lonmin chairman Roger Phillimore said of the British national: "We will greatly miss Ian's ability, commitment and drive which he has devoted to the company over a career spanning more than 26 years.
"He has been CEO for the last four years and the consistent improvement in Lonmin's operating performance over that period owes much to his leadership."
The end of Farmer's tenure was overshadowed by the violence at Marikana that left 46 people dead, including 34 who were killed by police gunfire on August 16.
The recently announced rights issue, worth $817 million (616 million euros), was aimed at reducing Lonmin's level of indebtedness and increasing its financial strength in the wake of the violence.
Swiss commodities giant Xstrata has called for a management shake-up at Lonmin, in which it is a major shareholder, citing serious financial problems at the mining group.