Ben Moolman has resigned as chief operating officer and director of Lonmin, one of the world's largest platinum miners, citing personal reasons.
Moolman, who joined the mining company in 2015, will leave with effect from 5 April 2017.
Lonmin did not name a successor.
Chief executive Ben Magara said: "My colleagues and I would like to thank Ben for his contribution to Lonmin and wish him well in his future career."
The company's share price fell 4.29 per cent at 106p in morning trading.
The troubled South African-focused metal miner revealed "disappointing" performance in the first quarter of 2017 as production was weaker than expected and initiatives to improve it were taking longer than planned.
"Its not ideal that their main technical person is gone. It's another signal that Lonmin has some real problems," Peel Hunt analyst Peter Mallin-Jones said.
However, Sibonginkosi Nkosi, analyst at Momentum SP Reid Securities said Magara, who has experience in operations, can easily take on the function of chief operating officer.
In 2012, thousands of Lonmin workers went on strike at its Marikana mine after the company reportedly declined to meet with them to discuss pay and conditions. South African police were brought in to break up the occupation, and 34 miners ended up being killed in action widely reported as the "Marikana Massacre".
The longest sustained industrial action occurred between January and June 2014. The strike ended when Lonmin agreed to a minimum wage that included annual increases over the following three years, with a commitment to agree a new structure in 2016.
In October, the firm agreed to a new three-year deal with workers and unions over wages and working conditions, but in January Lonmin said the relationship between management and unions was "not working as effectively as we expected" while business improvement initiatives take longer than anticpated.