Reuters reported Rio had priced the shares at $US15 to raise $459 million, despite earlier indicating shares could sell for $16-18.
Rio has retained a 48.3% stake in Cloud Peak.
Cloud Peak owns four mines in Wyoming and Montana – Spring Creek, Cordero Rojo, Antelope and half of Decker – which produce thermal product from the Powder River Basin.
A fifth operation, Jacobs Ranch, was sold to Arch Coal in March for $761 million.
According to Bloomberg, the four mines produced 97.1 million tons and had $1.24 billion in sales in 2008.
