Rio Tinto Ltd/Plc said it would spend US$793 million as its share of the cost to extend the Kestrel coal mine in Australia, as it looks to internal growth to fight off an unwanted approach from rival BHP Billiton.

The total cost of the work at the Kestrel mine in the coal-rich Bowen Basin region of Queensland state, aimed at increasing productivity when existing sections of the mine start showing signs of depletion in about four years, was $991 million, Rio said on Monday.

Rio holds an 80 percent stake in the mine. Mitsui Kestrel Coal Investment, which is owned by Japan’s Mitsui Group (8031.T) holds the other 20 percent.

The work will allow for annual production of around 5.7 million tonnes of coal until 2031, Rio said.

“This represents a further 20-year commitment to the Bowen Basin and is a strong vote of confidence in the Asian coal market,” the company said.

The work will provide access to 112 million tonnes of high quality hard and semi-hard coking and thermal coal for export, it said.

Once coal seams are found, mining teams work to estimate the quantity, quality and depth of coal in the area.

After being mined, the coal is transported to handling facilities where it is sized, blended and loaded into trains according to customer specifications.

Finally, the coal is taken to power plants, mills or sold in the international market.

So far this year, Rio Tinto has spent $44 billion on new projects, including its $38.1 billion acquisition of Alcan in October

Rio is the subject of an unsolicited $120 billion takeover proposal from rival BHP Billiton Ltd/Plc (BHP.AX)(BLT.L).

Rio Chief Executive Tom Albanese this month stopped short of saying Rio was not for sale at any price, but reiterated that BHP’s three-for-one share proposal was well below what he thought Rio was worth.

After the Alcan acquisition, Rio would turn more toward organic growth, according to Albanese.

Rio shares were down 3.9 percent to A$131.90 by 0327 GMT. BHP was 3 percent down to A$40.80 in a broader market down 2.75 percent.