Glencore, the world's largest commodity trader has almost doubled its stake in South African takeover target Optimum Coal (OPT) to 23.9 per cent by buying shares at a higher price than the buyout offer it made last week.

Glencore acquired 24.6 million shares on the market yesterday at 38 rand ($5.34) each, taking its stake in South Africa's sixth largest coal producer to 23.9 per cent.

Late last month, the Swiss miner had acquired 14.1 per cent of Optimum and on 1 September offered to buy Optimum Coal at 34 rand a share, valuing the Johannesburg-based coal miner at $1.2 billion.

OPT was sold by BHP Billiton in 2008 for an undisclosed sum to local black economic empowerment company Optimum Coal Holdings (OCH), which was bankrolled by AMCI Capital - one of the world's largest energy-focused private equity fund.

OPT's operations include Optimum Collieries, a large open-cast and underground mining complex comprising four coal production areas, and Koornfontein Mines – both in the Mpumalanga coalfield.

Optimum Collieries is the third-largest open-cast coal mine in South Africa, having a life of 21 years with coal resources of 719 million tons and reserve base in excess of 257 million tons.