A Glencore unit, Piruto BV, "now holds a beneficial interest of 14.1 percent in Optimum Coal," it said in a statement.
Glencore did not make any statement on the transaction, nor was a value of the deal provided.
Optimum Coal had a 2010 net profit of 229.7 million rands, down 69 percent on sales of 3.4 billion rands.
Last week, Glencore posted a 57 percent jump in first half net profit to $2.45 billion, boosted by strong commodities demand and prices.
It also hinted at potential acquisitions during the debt crisis in the developed world, with the group's chief executive saying that Glencore was "remaining alert to the potential opportunities that such an environment uncovers in our end markets."
Also last week, the group announced a bid for Australian nickel miner Minara worth $1.07 billion.