The Mail & Guardian must retract its story that Mbete was bribed by Gold Fields and apologise to her and the ANC, said the party on Tuesday.
"In their eagerness to discredit the national chairperson and the ANC, the paper published a story that is flimsy, unsubstantiated and is just an exercise in slander," the ruling party said in a statement.
"The M&G deliberately uses this headline ['Gold Fields bribed ANC Chair'] with an intention to be sensational, defamatory and slanderous."
The ANC statement was described as "completely over the top" by one of the authors and managing partner of the M&G Centre for Investigative Journalism, amaBhungane, Stefaans Brümmer.
"We stand by our story, which was clearly in the public interest. We reported on the findings of a top law firm, which was commissioned by Gold Fields itself," he said.
The newspaper reported on Friday that Gold Fields buried a New York law firm's finding that a R25-million share allocation to Mbete constituted bribery.
The law firm, which was commissioned by Gold Fields, found it had increased Mbete's cut in a contentious 2010 empowerment deal in response to an alleged threat by her representative.
'Seek legal counsel'
The ANC said on Monday that it and Mbete reserved the right to "seek legal counsel and recourse to address this libellous and defamatory act".
It said claims that Mbete was bribed were not proved or substantiated in the story.
"Nowhere in this story is it reported how, when, why and who bribed the ANC national chairperson," the ANC said.
"Of their own admission, the paper clearly states that it does not know how the national chairperson would have done this [sunk the deal]."
Brümmer said the ANC and Mbete were completely within their rights to seek legal advice, and should they bring action, the story would be "vigorously" defended.
'Threatened to scuttle'
"The story contains substantive details of the alleged threat by Ms Mbete's representative that the deal would be scuttled unless Ms Mbete's stake was substantially increased," he said.
"[It also contains details on how] Gold Fields in fact responded to the alleged threat by increasing her stake with an amount of about R25-million. The ANC's allegation is without merit."
The deal was reportedly hatched in 2010, in response to a mineral resources department requirement that the company secure an empowerment partner for its South Deep mine. The company was then applying for a new-order licence for the mine.
The M&G reported that Mbete's representative Brian Mosehla had "threatened to scuttle" the deal unless Mbete was given a bigger stake. – Sapa, additional reporting by Staff Reporter