The union had invited media representatives to attend a signing ceremony today in Muldersdrift, following four months of wage negotiations.
However in an urgent tweet published hours ago, AMCU said the signing ceremony had been postponed to a date "yet to be confirmed".
It said it apologised for any inconvenience and would communicate the new date once it was confirmed.
AMCU's opening demands had earlier been described as "outrageous" with the union seeking a 48% pay rise for workers.
Representatives from Sibanye Stillwater and Impala Platinum had told Mining Journal in July they believed employees were committed to finding a workable solution and were hopeful of reaching an agreement.
Sibanye Stillwater had been impacted by a five-month strike at its gold operations in South Africa, which ended in April when the company and AMCU agreed to a three-year deal.