The agreement gives Amcu full organisational rights and affirms the union's dominance in the Rustenburg platinum fields.
"We have full organisation rights and will now be able to implement formal structures and appoint shop stewards at Amplats," Amcu president Joseph Mathunjwa told City Press newspaper.
Last month, Cosatu general secretary Zwelinzima Vavi condemned the management of Impala Platinum (Implats), Amplats and Lonmin for giving Amcu preferential treatment as it sought members.
"[They] are manipulating the membership verification process and employing the age-old tactic of divide-and-rule," Vavi said.
Lonmin denied the allegation.
"Lonmin believes in freedom of association. There is no intimidation. We cannot decide who joins what union," spokeswoman Sue Vey said.
Amplats spokeswoman Mpumi Sithole said Amcu now represented 40 percent of its workers, who number just under 60,000.
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