XStrata Plc was able to secure a controlling stake equivalent to 51.12 percent in Sphere Minerals, while remaining rival China-backed Sin-Tang continues to tighten its grip in its shares in the company top up its holding.

After more than two months and revisions to its $514 million offer, Xstrata today said it has secured 51.12 per cent, which is likely to result in an automatic two-week extension as its bid becomes unconditional, the company said in a statement posted on its website.

The mining company's shareholdings originally at 18.99 percent, got a boost from the additional shares equivalent to 32.13 percent secured from Acorn Capital and Genesis, who agreed to a $3-a-share.

Xstrata said it would let its offer lapse if it did not secure more than 50 per cent of Sphere by 7pm AEDT tomorrow.

Sin-Tang, which has offered a rival proposal to match Xstrata has also increased its stake to 14.67 percent after snapping up 2 million shares on-market yesterday. It said it would not sell its shares to Xstrata.

Although based in Perth, Sphere is a West African iron ore producer with three large-scale projects in Mauritania.