JOHANNESBURG – A South African union gave Glencore Xstrata a seven-day ultimatum on Friday to rehire more than 1 000 workers fired at chrome mines for wildcat protests or face unspecified action.

The protests, launched in solidarity with a worker who said he was assaulted by a supervisor in an incident which has taken on racial overtones, are part of a wave of labour unrest that has shaken the mining sector in the world's top platinum producer.

"If management fails to respond to this memorandum within the timeframe prescribed, we reserve our right to pursue any other avenues permissible in terms of the laws," the Association of Mineworkers and Construction Union (AMCU) said.

This could include further strike action at other Glencore Xstrata operations in South Africa, which include coal mines.

The memorandum was presented to a Glencore executive near the company's Johannesburg office by the AMCU's charismatic leader Joseph Mathunjwa, who portrays himself as a Christian soldier fighting for South Africa's downtrodden miners.

About 150 of the fired mineworkers, clad in green AMCU shirts, chanted and danced in the office suburb of Melrose Arch as police looked on. The peaceful demonstration was the AMCU's first in Johannesburg.

Glencore sacked the workers early in June from mines near Steelpoort, northeast of Johannesburg in Limpopo province. A company spokesman said operations were running at minimal capacity while replacement workers are rehired.

Chromium is used to produce ferrochrome, a key ingredient in making stainless steel.

Mathunjwa said the incident with the supervisor and the subsequent dismissals smacked of racism, which he said was entrenched in the company's culture.

"Glencore Xstrata is still practising racism ... the central management office is a white-only office. The only black workers found there are only cleaners and tea ladies," he thundered into a microphone.

One protester waved a sign which read "Glencore Xstrata Stop Calling Black Workers Kaffir," a term offensive to black South Africans.

A spokesperson for Glencore said the company would investigate the allegations of racism.

The issue of race has arisen as South Africa's first black president Nelson Mandela, widely admired as a symbol of resistance to injustice and of racial reconciliation, is in a critical condition in hospital.

Black mineworkers have seen little improvement in their living conditions in the two decades since apartheid ended.

The AMCU has emerged as the dominant union on South Africa's platinum belt after poaching tens of thousands of workers last year in a vicious turf war with the National Union of Mineworkers, a key ally of the ruling ANC.

Labour unrest in South Africa's mines last year killed more than 50 people, cost producers billions in lost output and led to credit downgrades for Africa's biggest economy.

Wage negotiations due to start next month across the mining industry are expected to be among the toughest ever, as rising working militancy coincides with falling commodity prices and razor-thin company margins.