(Adds pricing, launch size) HONG KONG, Oct 15 (Reuters) - China launched on Thursday a U.S. dollar sovereign bond deal totalling $6 billion, according to a term sheet reviewed by Reuters. The bonds would be sold in three-, five-, 10- and 30-year tranches, the term sheet showed. The pricing was set at U.S. Treasuries plus 25 basis points for the three-year $1.25 billion tranche, 30 basis points for the 5 year $2.25 billion tranche, 50 basis points for the 10 year $2 billion tranche and 80 basis points for the $500 million 30 year tranche, the term sheet showed. It is the first time China has offered a bond deal to U.S.-based investors. (Reporting by Scott Murdoch in Hong Kong; Additional reporting by Tom Arnold in London; Editing by Jacqueline Wong and Toby Chopra)