The National Union of Metalworkers of South Africa (Numsa) said that one of its shopstewards, Mahlomola Hlothoane, was murdered at his home on Friday evening.
Police have confirmed the incident, and said there have been no arrests yet.
Hlothoane was a shopsteward at Reagetswe Mining, a contractor to mining giant Impala Platinum (Implats). Numsa is currently in a standoff with Reagetswe and two other Implats contractors - Triple M Mining and Newrak - over wages.
Numsa accused the three contractors of paying employees as little as a third of what Implats permanent employees are paid for the same level of work. However, the contractors denied this in a joint statement last week.
Last week the Labour Court in Johannesburg granted Reagetswe Mining an interim interdict against strike action at its operations. Triple M CEO Sipho Xipu said Numsa did not pass the company's threshold for recognition, and it could therefore only negotiate wages with the Association of Mineworkers and Construction Union.
In a statement released on Sunday evening, Numsa secretary-general Irwin Jim condemned the "cold-hearted and brutal" murder of Hlothoane.
"Comrade Mahlomola was a Numsa shopsteward who assisted the union by co-ordinating recruitment and mobilising workers at Implats. He was murdered on Friday night at his home in Rustenburg by two unknown gunmen," said Jim.
The union said Hlotoane's murder was the second killing of a Numsa official organising at Implats operations. The statement said Hlothoane and Malibongwe Mdazo - murdered last year - were killed "for organising workers at Implats. The union called on the South African Police Service (SAPS) to bring the killers to justice.
"We demand that the SAPS must urgently find and arrest those who are responsible for this heinous crime. It is unacceptable that people are dying simply for exercising their constitutional right to join a union of their choice," the statement said.
Meanwhile, Xipu told Fin24 that an interim court order was granted in favour of Triple M in its Labour Court application against Numsa, and a final order was scheduled for 18 August.
"Employees are to return to work from the Friday night shift and the Saturday day shift," said Xipu.
Captain Aafie Botma confirmed that police in Phokeng were investigating the murder of a 45-year-old man residing in Lukavillage and employed by Reagetswe Mining who was allegedly shot by two unknown males.
"The incident occurred on Friday 24 June 2022, at the victim's rented place. Investigations into the matter are underway and no one has been arrested," Botma said.
Fin24 reached out to Reagetswe Mining for comment, but the company did not respond immediately.