TORONTO (Reuters) - Canada's main stock index dropped sharply at the open on Friday, weighed down by heavyweight mining and energy stocks following the release of unexpectedly soft U.S. retail sales data. The Toronto Stock Exchange's S&P/TSX composite index was down 107.62 points, or 0.86 percent, at 12,373.75 shortly after the open. (Reporting by Alastair Sharp; Editing by James Dalgleish)