"Solidarity has decided to boycott today's launch in protest, after having been informed that its recognition at Lonmin, together with that of other unions, is being revoked," he said in a statement on Wednesday.
He said Solidarity was a signatory to the Framework Agreement for a Sustainable Mining Industry, which provided for the forum's establishment.
"As signatory, the trade union is fully committed to all proposals contained in the agreement."
He said the suspension of the trade union's recognition at Lonmin was a result of the platinum mining company's introduction of a winner-takes-all principle.
The union was told on Wednesday that its recognition at Lonmin would be suspended in favour of the Association of Mineworkers' and Construction Union (Amcu).
"The undemocratic principle of majority recognition was one of the main causes that led to the events at Marikana and to other violent labour unrest," Du Plessis said.
"The suspension of the recognition of Solidarity and other minority trade unions by Lonmin again creates a climate for continued labour unrest and deprives members of the constitutional right to freedom of association."
He appealed to Lonmin to reconsider its decision.
"Should Lonmin not restore Solidarity's recognition in the workplace, it would be clear that the company is sacrificing sound labour relations in favour of the pressure being brought to bear on it by majority union, Amcu."
Police Minister Nathi Mthethwa said in a statement released at the forum's launch that the Marikana tragedy represented a watershed in the country's history.
"Following the incident we consistently reiterated a message that as government we were deeply saddened by the tragic incidents that led to the untimely deaths of many breadwinners, fathers, brothers, uncles and loved ones," he said.
"We urge all parties to play a constructive role and support the families and the nation at large in the process of healing," he said in a statement.
Mthethwa could not attend the event as he was at a Cabinet lekgotla. Forty-four people were killed during a wage-related strike at Lonmin's mines in Marikana in August last year.
On August 16 police shot dead 34 miners while trying to disperse them. Ten people, including two policemen and two security guards, were killed in strike-related violence the preceding week.
President Jacob Zuma appointed retired judge Ian Farlam to head a commission of inquiry into the Marikana deaths.
Mining companies, two trade unions – Uasa and the National Union of Mineworkers – business and government signed a pledge committing them to restoring peace and stability in the mining sector. – Sapa