JOHANNESBURG (Reuters) - South Africa's rand retreated on Thursday, surrendering early gains as investors remained cautious on commodity currencies despite OPEC's deal to curb crude output. * At 0645 GMT, the rand traded 0.3 percent weaker at 13.6575per dollar after an overnight close of 13.6150. * Organization of the Petroleum Exporting Countries (OPEC)agree to curb oil production, but lack of deal details stranglescommodity currency gains. * Locally, credit demand by business slows in August.Producer inflation data due at 0930 GMT. * Blue chip index up 1.4 percent at 0704 GMT. * Yield for the benchmark government bond due in 2026 up 1basis points to 8.65 percent. (Reporting by Mfuneko Toyana; Editing by Joe Brock)