The dominance by the upstart Association of Mineworkers and Construction Union (AMCU) comes after it gained worldwide notoriety last year in a violent strike that saw police shoot dead 34 miners.
Nearly a year later, workers en masse have abandoned the seemingly once-untouchable, ANC-allied National Union of Mineworkers (NUM), which shed 41,000 members countrywide in 2012.
The stakes are high, said mining analyst Peter Major from financial services group Cadiz.
With wage bargaining rights, cars and on-site offices among the victor's perks, the bitter, sometimes deadly rivalry has raised fears of further unrest after paralysing stoppages and bloodshed last year.
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AMCU now holds sway at the three largest platinum miners who employ a combined 84,000 workers northwest of Johannesburg.
The union represents 40 per cent at world number one producer Anglo American Platinum, 58 per cent at Impala Platinum's Rustenburg operations, and 70 per cent of workers at Lonmin.
AMCU's meteoric rise flipped the situation from just a year ago, when NUM had 65 per cent at Implats and 50 per cent at Lonmin.
The last time such radical changes were seen was during the rise of black unionism under apartheid in the early 1980s.
"This is the biggest upheaval since NUM was allowed in 1981," said Major, pointing to the year that union organisers were granted access to the country's mines.
AMCU has shrewdly used its newfound influence to strengthen its base, while capitalising on accusations that the NUM has abandoned workers by cosying up to the political elite.