July 22 (Reuters) - Canada’s main stock index opened lower on Thursday dragged by a dip in the materials index, with Iamgold Corp plunging 13% on cutting full-year production guidance. * At 09:31 a.m. ET (13:31 GMT), the Toronto Stock Exchange’s S&P/TSX composite index was down 17.95 points, or 0.09%, at 20,092.1. (Reporting by Susan Mathew in Bengaluru; Editing by Ramakrishnan M.) Our Standards: The Thomson Reuters Trust Principles.