Lonmin said the Marikana mine reopened but no ore was produced after fewer than a third of staff turned up.
It also said a deadline for striking miners to return to work or face dismissal had been extended to Tuesday.
President Jacob Zuma declared a week of national mourning for those killed.
South African president has also called for a commission of inquiry to investigate the incident.
“Lonmin can confirm that work at its Marikana operations resumed today as significant numbers of employees returned to work,” the company said in a statement.
“Almost one third of the 28,000-strong workforce reported for their morning shifts.
“The company can also announce that those illegal strikers who did not return to work this morning will not be dismissed and have been allowed an extra day in light of current circumstances.”
Later, Lonmin executive vice-president for mining Mark Munroe said that “for all intents and purposes” no ore had been produced at the mine on Monday.
“By 07:00 tomorrow we expect workers to return to work. After that, Lonmin has the right to fire them,” he said.
Lonmin chief financial officer Simon Scott said the company wanted to “rebuild the trust of the workers”.
“We are aware that it will take some time for some trust to be regained,” he added.
Union officials quoted by Reuters said that at least 80% of the workforce was needed to bring platinum out of the shafts.
It remains to be seen whether more workers will report for duty on Tuesday.
A significant number have vowed to prolong their stay-away, saying that returning to work would be an insult to their dead colleagues.
The week of mourning began on Monday and a memorial service is planned for Thursday.
About 3,000 rock-drill operators (RDOs) walked out more than a week ago in support of demands for higher pay.
The strike was declared illegal by Lonmin, the world’s third-largest platinum producer, and the mine was shut.
Clashes between strikers, some holding clubs and machetes, and police culminated on Thursday when officers armed with automatic rifles and pistols fired dozens of shots.
In addition to those killed, at least 78 people were injured and some 250 people were arrested.
Those arrested were remanded in custody by a court in the Pretoria township of Ga-Rankuwa on Monday. Charges included murder, public violence and attempted robbery.
During the hearings, about 100 women appeared outside the court to appeal for leniency for the men.
While union leaders held meetings on Monday, about 1,000 workers gathered near the mine said they would not return.
Several accused Lonmin of insensitivity for expecting them to go back to work while they were still in mourning.
“They can fire us if they want, we are not going back to work. [President] Zuma must shut down that mine,” one worker told AFP news agency.
Correspondents at the scene said workers outside the mine were unarmed and in a calmer mood than on previous occasions.
The miners, who are currently earning between 4,000 and 5,000 rand ($484-$605) a month, say they want their salary increased to 12,500 rand ($1,512).
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